mi̇lli̇ geli̇rden önemli̇ bi̇r kaybimiz!

Transkript

MİLLİ G
GEL
E Lİİ RD EN Ö
ÖN
NEML
EM L İ B İ R KAYB
KAY B IMIZ!
IMIZ!
“KOROZYON DERGİSİ”
Derginin amacı, korozyonu önlemenin bilimsel ve teknolojik altyapısına ilişkin gelişmelerin izlendiği ve bunların özümlenmesi ve uygulamaya aktarılması imkân ve mekanizmalarının tartışılarak değerlendirildiği bir forum olarak etkili olabilmektir.
“Korozyon” Dergisi’nde konu ile ilgili olmak üzere Türkçe ve
İngilizce’de özgün makaleler, çeviri yazıları, öncelikle uygulama alanına hitap eden araştırma sonuçları, uygulama alanında edinilen bilgi ve deneyimlerin konu alındığı yazılar, tarama ve tanıma yazıları,
konferans raporları ve kitap eleştirileri yayınlanır.
“Korozyon” Dergisi’ne gönderilecek yazılar 14 daktilo sayfasını geçmeyecek şekilde aşağıdaki düzende hazırlanmalıdır: (1) Makale
başlığı (Türkçe ve İngilizce), (2) Yazar(lar)ın ad(lar)ı, (3) Yazar(lar)ın
50 kelimeyi geçmeyen kısa özgeçmiş(ler)i, (4) 100 kelimeyi geçmeyen Türkçe ve İngilizce özetler, (5) Makale, (6) Kaynakça. Yazarların
makale yazım kuralları olarak “Korozyon” Dergisi’nde daha önce yayınlanmış yazıları örnek almaları önerilir.
JOURNAL OF CORROSION
The objective of the journal is to provide an effective forum for the investigation of developments related to the scientific and technological infrastructure of corrosion prevention, as also for the discussion
and evalution of adaption and application possibilities and procedures for these developments.
The journal publishes original articles, translations, research results,
especially related to practical applications, papers treating information from case-studies, survey and review articles, conference reports and book reviews in English and Turkish on the subject or corrosion.
Contributions to the journal should not excead 14 typed pages in
length and be prepared in the manner given below: (1) Title of article
(Turkish and English) (2) Name(s) of author(s) (3) Biographical data
of author(s) not exceed 50 words (4) Turkish and English summaries
not to exceed 100 words each (5) Text of the article and (6) References. Authors may consult articles published in back issues of the
journal for guidance in the preparation of their manuscripts.
“KOROZYON” DERGİSİ yılda iki defa olmak üzere Korozyon Derneği
tarafından yayınlanır ve ücretsiz olarak dağıtılır.
Yazışma adresi : Korozyon Derneği, Orta Doğu Teknik Üniversitesi,
Metalurji ve Malzeme Mühendisliği Bölümü, 06531 ANKARA
Tel : (90-312) 210 25 29
SAHİBİ / OWNER
KOROZYON DERNEĞİ
THE CORROSION ASSOCIATION
•••
YAYIN YÖNETMENİ
PUBLISHING DIRECTOR
Mustafa Doruk
Orta Doğu Teknik Üniversitesi, Ankara
•••
YAYIN KURULU
PUBLISHING BOARD
Mustafa Doruk
Orta Doğu Teknik Üniversitesi , Ankara
Saadet Üneri
Ankara Üniversitesi, Ankara
Ali Fuat Çakır
İstanbul Teknik Üniversitesi, İstanbul
Semra Bilgiç
Ankara Üniversitesi, Ankara
Necil Kurtkaya
Elek. Y. Muh.
Oktay Akat
AKAT Mühendislik A.Ş. Ankara
•••
YAYIN DANIŞMA KURULU
PUBLISHING ADVISORY BOARD
Hayri Yalçın
Gazi Üniversitesi, Ankara
Mehmet Erbil
Çukurova Üniversitesi, Adana
Ahmet Çakır
Dokuz Eylül Üniversitesi, İzmir
Mustafa Ürgen
İstanbul Teknik Üniversitesi, İstanbul
Melike Kabasakaloğlu
Gözen Bereket
Osmangazi Üniversitesi, Eskişehir
Timur Koç
Kadri Aydınol
Orta Doğu Teknik Üniversitesi, Ankara
Fatma Erdem
Türkiye Şeker Fab. A.Ş., Ankara
Vedat Yalçın
Noksel Çelik Boru Sanayi A.Ş.
Yves M. Günaltun
Petroleum Institute of Abu Dhabi
Abu Dhabi
Kemal Nişancıoğlu
Norwegian Institute of Technology
NTH-Trondheim, Norway
M.Tettamanti
De Nora S.p.A., Milano, İtaly
H.M. Shalaby
Kuwait Institute for Scientific Research, Kuwait
“JOURNAL OF CORROSION” is published two times a year by the
Corrosion Association in Turkey, Department of Metalurgical and
Materials Engineering, Middle East Technical University,
06531 ANKARA/TURKEY
Tele-Fax : (90-312) 210 25 18, E-mail : [email protected]
www.korozyondernegi.org.tr
Dizgi ve Baskı : Poyraz Ofset - İvedik O.S.B. 2. Matbaacılar Sitesi 1534. (578.) Sk. No. 9 ANKARA Tel : (312) 384 19 42 - 15 • Fax : (312) 384 18 77
KOROZYON DERNEĞİNDEN HABERLER
Derneğimizin üyesi olduğu Avrupa Korozyon Federasyonu (EFS)’ nin 2013 Eylül ayında Portekiz’in
Estroil kentinde yapılan toplantısında yönetim ve
danışma kurullarına 01.01.2004 - 31.12.2016 çalışma dönemi için yeni üyeler atandı. Derneğimiz
kurucu üyelerinden Prof.Dr.A.Fuat Çakır EFS Yöneticiler Kuruluna (BoA) en yüksek oyu alarak ikinci defa, derneğimiz üyesi ve Çukurova Üniversitesi öğretim üyesi Prof. Dr. Tunç Tüken ise EFC Bilim
ve Teknoloji Danışma Komitesi (STAC)’ ne seçildiler. Prof. Çakır ve Prof. Tüken’ i kutlar, üstlendikleri
bu önemli görevde başarılı olmalarını dileriz.
Bilindiği gibi, Avrupa Korozyon Federasyonu çatısı altında çeşitli ilgi alanlarında etkinlik gösteren 21
çalışma grubu bulunmaktadır. Çalışma gruplarının
görevi, korozyon ve önlenmesi konularında bilimsel ve teknolojik gelişmeleri izlemek, yayın, araştırma ve diğer etkinlikler için öneri ve girişimlerde bulunmak olarak özetlenebilir. Federasyon son dönemde aldığı bir kararla, grup çalışmalarına katılımın daha geniş bir tabana yayılması ve böylece üye
kuruluşların bilgi paylaşım sürecine etkili olarak katılmalarını sağlamak yönünde yararlı bir adım attı.
Bu çerçevede Derneğimiz 17 araştırma grubuna
katılmak üzere üyelerimizden isimler önerdi (EFC
araştırma gruplarına önerilen üyelerimizin tam listesi derneğimiz web sayfasında görülebilir). Önerilen
üyelerin olanaklar ölçüsünde grup toplantılarına katılmaları, ve bilgi alış verişi için tüm iletişim kanallarından yararlanmaları beklenmektedir.
Bilindiği gibi, Derneğimiz her iki yılda bir uluslar arası düzeyde korozyon sempozyumu düzenlemektedir. Bunlardan ‘XIII. Uluslar arası Korozyon
Sempozyumu 15-17 Ekim 2014 tarihinde Elaziğ’ da
yapılacaktır Derneğimiz onur üyesi Prof.Dr.Saadet
Üneri’ nin adı ile sunulacak bu etkinliğin organizasyonunu Fırat Üniversitesi üstenecektir.
Sanayide görevli ve korozyon sorunu ile er geç
tanışmaya aday mühendis ve teknik elemanlar için
eğitim kursları düzenlemek Derneğimizin başlıca
amaçlarındandır. Bu çerçevede 17 – 20 Aralık 2012
tarihlerinde gerçekleştirilen eğitim seminerine MAN
Türkiye AŞ.’ den 36 kişi katıldı. Korozyonun temel
ilkeleri, korozyonun türleri, korozyonu önlemede temel yaklaşımlar ve atmosferik korozyon konularının
işlendiği seminerin sonunda uygulanan sınavda ba-
NEWS FROM THE CORROSION ASSOCIATION
At the meeting held in September, 2013 at Estroil/Portugal the European Federation of Corrosion (EFC) re-elected the members for its administrative and advisory bodies for the term 01.01.2014
– 31.12. 2016. Prof.Dr.A.Fuat Çakır , one of the founding members of our association has been elected to the Board of Administrators (BoA) for the second time having highest number of votes. Also
one of our members, Prof.Dr.Tunç Tüken from Çukurova University has been elected to the Science and
Technology Advisory Committee (STAC). We wood
like to congratulate Prof. Çakır and Prof. Tüken and
wish much success them during performing this important mission.
As known, there are 21 working parties operated by European Federation of Corrosion that work
in different areas. Missions of these working groups
could be summarized as to monitor recent scientific
and technological developments regarding corrosion and its prevention, provide feedbacks and suggestions to publications, research and other type
of activities. . Recently the federation took the necessary steps to encourage participation of scientists from different background to working groups
to enhance exchange of information and knowledge in more convenient ways with member societies. In line of this initiative, our association elected
and proposed several of our members to participate 17 working group in the federation. (You can find
full list of proposed members from our association
website). It is expected from our proposed members to use all means to attend respective working
party meetings and exploit all communication channels for information exchange as much as they can.
As known, our association has been organizing
international corrosion symposium, once two years.
The next one, 13rd International Corrosion Simposium will be held on 15-17 October 2014 in Elazig for
the name of our honorary member Prof.Dr.Saadet
Uneri. The Fırat University will support the activity as
the host organization.
Our association organizes courses and training
about corrosion and its impacts for industrial professionals and engineers regularly who will face such
problems early or late in future. In line of this effort,
our association organized a seminar at MAN Türkiye A.Ş. during 17-20 December 2012. 36 people attended to this seminar in which specific topics such
as basic principles of corrosion, types of corrosion,
basic measures to prevent corrosion and atmosp-
şarılı olanlara katılım belgesi verildi.
Uygulama alanında karşılaşılan korozyon sorunlarını tanımlamak ve giderilmeleri için çözüm önermek derneğimizin bugüne değin başarı ile sürdürdüğü etkinlik alanlarındandır. Uluslar arası bir şirketin istemi üzerine derneğimiz tarafından görevlendirilen bir ekip Libya’ nın Sitre kenti yakınında, sahilde stoklanmış çelik saçların uğradığı korozyonu değerlendirerek kapsamlı bir rapor hazırlamıştır. Aşağıdaki resimler görevli ekibimizce yerinde yapılan
araştırmadan kareler sunmaktadır.
Gerek eğitim, gerekse uygulama alanı için dilimizde yazılmış eserlerin önemi ve gerekliliği yadsınamaz. Derneğimiz korozyon konusunda dilimizde
mevcut yayınların çoğaltılması ve çeşitlendirilmesi için atılımlarını ara vermeden sürdürmeye özen
göstermiştir. Korozyon Derneği yayını olarak yaşama geçirilen Prof.Dr.Saadet Üneri’nin ‘Korozyon ve
Önlenmesi’, ve Prof.Dr.Mehmet Erbil’ in ‘Korozyon:
İlkeler ve Önlemler’ adlı eserlerini, Prof.Dr.Mustafa
Doruk’ un yazdığı ve yakın gelecekte edinilebilecek
‘Metalik Malzemeler ve Korozyon’ adlı kitap izleyecektir. Derneğimiz için önemli saydığımız yayınlardan biri de‘Korozyon Dergisi’ dir. Derginin düzenli
yayınını sağlamak için, mevcut kısıtların giderilmesi
yönündeki çabalarımızı artırmamız gerekmektedir.
İcten esenlik ve başarı dileklerimizle,
heric corrosion were covered. At the end of seminar an evaluation test was performed to certify successful participants.
Another principal mission of our association is
to analyze corrosion failures in practice and propose solutions to these problems. By a request of an
international company, an inspection team formed
within our association for the assessment of corrosion of the tank steel plates stored in a coastal area
due the atmospheric conditions and other effects
near Sirte, Libya The photos below show the on-site
investigation by scientists assigned by the association for this particular task.
Publications written in our language are important sources for education as well as for the assessment and solution of corrosion problems. Our association continues its effort to increase and diversify
such sources with due diligence. “Corrosion and
Its Prevention” by Prof. Dr. Saadet Üneri and “Corrosion: Principals and Prevention” by Prof. Dr. Mehmet Erbil, will follow a new book entitled “Metallic
Materials and Corrosion” by Prof. Dr. Mustafa Doruk
which will be available in near future. Another significant publication of our association is the “Corrosion Journal.” Obviously, we have to intensify our efforts to overcome limitations in order to provide its
regular publication.
With sincere wishes of success,
INHIBITION BEHAVIOR OF BENZONITRILES
ON MILD STEEL IN HCl SOLUTION
ABSTRACT
The aim of this study is to investigate the inhibition efficiency of benzonitriles with functional amine groups in different positions, for mild steel corrosion
in 0.5 M HCl solution. For this purpose,
electrochemical impedance spectroscopy (EIS) and potentiodynamic measurements were realized. The obtained
experimental results were evaluated by
the basis of polarization curves, polarization resistance and capacitance values. The surface analysis was also carried out by scanning electron microscopy technique. The results show that all
these inhibitors have a good inhibition
effect on mild steel in 0.5 M HCl solution. Also, the position of amine group
affects the inhibitor efficiency.
BENZONİTRİLLERİN HCl ÇÖZELTİSİNE BIRAKILMIŞ KARBON
ÇELİĞİ ÜZERİNDEKİ İNHİBİTÖR
ETKİNLİĞİ
Bu çalışmada, 0,5M HCl çözeltisi
içinde yumuşak çeliğin korozyonu üzerine, molekülünün farklı konumlarında
amin fonksiyonel grubu bulunan benzonitrillerin inhibitor etkinliği araştırılmıştır.
Bu amaçla, elektrokimyasal empedans
spektroskopisi (EES) ve potansiyodinamik ölçümler ve taramalı elektron mikroskobu tekniği ile yüzey analizleri yapılmıştır. Elde edilen veriler, polarizasyon
eğrileri, polarizasyon direnci ve çift tabaka kapasitesi bazında değerlendirilmiştir. Sonuçlar, çalışılan inhibitörlerin 0,5 M
HCl çözeltisinde yumuşak çelik üzerinde
iyi bir inhibisyon gösterdiğini ve inhibitörlerin etkinliğinde amin gruplarının molekül yapısındaki konumlarının da önemli olduğunu göstermiştir.
4
KOROZYON, 20 (1-3), 2013
1. INTRODUCTION
It is well known that mild
steel is widely used material in
a variety of industrial applications. The use of inhibitor in
order to protect metal from corrosion is still in the foreground
of many researchers. The effect
of inhibitor depends on the molecular structure, size and the
number of electron richfunctional groups. Furthermore, the
surface state and surface potential of used metal can play
important role for the inhibition
efficiency. Mostly used acid corrosion inhibitors are functionalized organic compounds. Benzonitrile compounds could be
good inhibitor due to their molecular structure. Also, the number and position of functional
groups should be considered in
aspect of their activating effect
on the aromatic ring. This issue
should be taken into account for
discussion of benzonitrile compounds as corrosion inhibitors.
2. EXPERIMENTAL
Mild steel samples (MS)
were cylindrical rods measuring
0.8 cm in the radius (0.502cm2
exposure surface area). The
working surface area was polished mechanically withSiC
paper to a 1200 grit finish, then
G. SIĞIRCIK
T. TÜKEN
M. ERBIL
degreased with 1:1 ethanol/water mixture and washed with distilled water, finally dried at room
temperature. The corrosive test
solution (0.5 M HCl) was prepared by dilution of analytical
grade 37% HCl with distilled
water. The concentration range
of employed inhibitors 2-aminobenzonitrile (2-AB), 3-aminobenzonitrile (3-AB) was 5.10-4 to
1.10-2 M in 0.5 M HCl.
The electrochemical cell
consisted of a three electrode
set up where the auxiliary electrode was a platinum sheet (2
cm2 surface area) and Ag/AgCl
(3M KCl)electrode was used as
the reference. All the potentials
given in this paper are referred
to this electrode. The EIS measurements were obtained at
instantaneous open circuit potential, in a frequency range of
1 mHz-100 kHz, peak to peak
perturbation voltage was 14 mV.
The polarization curves were
recorded with a scan rate of 2
mV/s, where the initial potential
was the corrosion potential value reached after 1 h of exposure
time. The surface morphology
of the mild steel samples after 6
days immersion in HCl solution
with and without inhibitor was
investigated by SEM.
3. RESULT AND DISCUSSION
3.1. Electrochemical Impedance
Spectroscopy (EIS)
The corrosion behavior of mild steel in 0.5 M
HCl solution with and without inhibitors was investigated by EIS at 25oC.In Fig. 1 and 2 the EIS results were given for two different inhibitors. As can
be seen from these figures, the plots of mild steel
yield a slightly depressed semicircle. The real impedance at lower and higher frequencies at Nyquist
plot is handled as polarization resistance.
Electrochemical equivalent circuit used for modeling mild steel/solution interface is given in Fig. 3.
CPE is the constant phase element, Rs and Rp are
solution resistance and polarization resistance, respectively.
-75
-1500
103
-50
102
-25
101
0
|Z|
Z''
104
-1000
-500
0
0
500
1000
Z'
1500
phase angle
-2000
25
100
10-3 10-2 10-1 100 101 102 103 104 105
Frequency (Hz)
2000
Figure 1. The EIS result of mild steel in 0.5 M HCl solutions (-) and containing 5.10-4(♦), 1.10-3(◊), 5.10-3(●), 1.10-2 M(○)2-AB. (solid lines show fitted results)
Şekil 1. 5.10-4(♦), 1.10-3(◊), 5.10-3(●), 1.10-2 M(○) 2-AB içeren 0,5 M HCl (-) çözeltisine bırakılan karbon çeliğiyle elde edilen EES sonuçları
(koyu çizgiler fitted sonuçları göstermektedir).
-75
-1500
103
-50
102
-25
101
0
|Z|
Z''
104
-1000
-500
0
0
500
1000
Z'
1500
2000
phase angle
-2000
25
100
10-3 10-2 10-1 100 101 102 103 104 105
Frequency (Hz)
Figure 2. The EIS result of mild steel in 0.5 M HCl solutions (-) and containing 5.10-4(♦), 1.10-3(◊), 5.10-3(●), 1.10-2 M (○)3-AB. (solid lines show fitted results)
Şekil 2. 5.10-4(♦), 1.10-3(◊), 5.10-3(●), 1.10-2 M(○) 3-AB içeren 0,5 M HCl (-) çözeltisine bırakılan karbon çeliğiyle elde edilen EES sonuçları (koyu çizgiler fitted
sonuçları göster-mektedir).
KOROZYON, 20 (1-3), 2013
5
Rs
CPE
Rp
Figure 3. The equivalent circuit used to identify and fit the EIS results.
Şekil 3. EES sonuçlarını belirleme ve fit etmede kullanılan eşdeğer devre.
The double layer capacitance value (Cdl) calculated by using the following equation:
Cdl=
1
2πƒmaxRp
(
Z”
Z’ - Rs
)
wherefmaxis the frequencyat which the imaginary
component ()of the Nyquist plot is maximum, is the
reel impedance at the same point. The surface coverage factor was calculated by using the following
equation:
C0dl and Cdl are the double layer capacitances of
the surfaces obtained in the solutions without and
with inhibitor, respectively. The inhibition efficiency
was calculated from the surface coverage factor
(IE%a).
The inhibition efficiency was also calculated from
the polarization resistance (IE%b) by using the following equation:
where and are the polarization resistancesobtained in the solutions without and with inhibitor, respectively. The EIS results were listed in Table 1.
Table 1. EIS results.
Çizelge 1. EES sonuçları
Inhibitor
C (M)
Rp (Ω)
Cdl(μF)
n
IE%a
IE%b
Blank
-
92
90.09
0.92
-
-
2-AB
5.10-4
215
59.50
0.91
34.0
57.2
1.10-3
274.2
54.22
0.91
39.8
66.4
5.10
869.9
29.82
0.86
66.9
89.4
1.10
1329
25.61
0.87
71.6
93.1
5.10-4
512.1
48.16
0.86
46.5
82.0
1.10
778.3
34.48
0.88
61.7
88.2
5.10
1312
28.38
0.87
68.5
92.9
1.10
1667
24.46
0.84
72.8
94.5
-3
-2
3-AB
-3
-3
-2
6
KOROZYON, 20 (1-3), 2013
As it is seen from Table 1 with the addition of
inhibitors the polarization resistances of mild steel
have increased. The values which are related to
open surface area have decreased.
3.2 Potentiodynamic polarization
measurements
The potentiodynamic polarization curves of mild
steel in 0.5 M HClsolution without inhibitor and different range of inhibitor concentration were shown
in Fig. 4 and 5. As it can be seen from figures, both
anodic and cathodic current values decreased with
addition of inhibitors.
hibitor reduces the corrosion rate by covering active centers on the metal surface. So, it is important
to determine surface coverage factor, θ. The lineer
relationships of C/θ versus C (in Fig. 6) suggest
that the adsorption of 2-AB, 3-AB on the mild steel
obeys the Langmuir adsorption isoterm. This isotherm means that the adsorption of the molecule
on metal surface is monolayer. Langmuir isoterm
can be expresses by the following equation:
where Cinh is inhibitor concentration and Kads is
the adsorption equilibrium constant.
C
C
(10-3)
(10-3)
(a)
Figure 4. The potentiodynamic polarization curves of mild steel in 0.5 M
HCl(■) and containing 5.10-4(◊), 1.10-3(♦),5.10-3(○), 1.10-2 M (●)2-AB.
Şekil 4. 5.10-4(◊), 1.10-3(♦),5.10-3(○), 1.10-2 M (●)2-AB.içeren 0.5 M
HCl(■) çözeltisine bırakılan karbon çeliğinin potansiyodinamik eğrileri.
C (10-3)
C
(10-3)
(b)
C (10-3)
Figure 6. Langmuir adsorption plots for mild steel in 0.5 M HCl containing
different concentrations of 2-AB (a), 3-AB (b).
Şekil 6. Çeşitli miktarlarda 2-AB (a), 3-AB (b) içeren 0,5 M HCl çözeltilerine
bırakılan karbon çeliği için Langmuir yüzerme eğrileri,
Figure 5. The potentiodynamic polarization curves of mild steel in 0.5 M
HCl(■) and containing 5.10-4(◊), 1.10-3(♦),5.10-3(○), 1.10-2 M (●)3-AB.
Şekil 4. 5.10-4(◊), 1.10-3(♦),5.10-3(○), 1.10-2 M (●)3-AB içeren 0.5 M
HCl(■) çözeltisine bırakılan karbon çeliğinin potansiyodinamik eğrileri.
3.3 Adsorption isotherm and
thermodynamic parameters
The inhibition efficiency of inhibitors is related
their adsorption ability on the metal surface. An in-
)
The free energy of adsorption) of the inhibitors
on mild steel surface can be determined using the
following equation:
The calculated and results were listed in Table 2.
The values of there calculated as -29.23 and -32.93
KOROZYON, 20 (1-3), 2013
7
kj/mol for 2-AB and 3-AB, respectively. The negative value of free energy of adsorption indicates
spontaneous adsorption of inhibitor molecules on
mild steel surface. These values show that the adsorption is mostly physical adsorption.
Table 2. The andvalues of 2-AB and 3-AB on mild steel in 0.5 M HCl.
Çizelge 2. 2-AB and 3-AB içeren 0.5 M HCl çözeltisine bırakılan karbon
çeliği için ve değerleri.
(kj/mol)
2-AB
0,240 x 10
4
3-AB
1,067 x 10
4
-29.23
-32.93
3.4 Scanning electron microscopy studies
The SEM images of mild steel in the absence
and presence of 1.10-2 M 2-AB and 3-AB after 6 days
immersion time are given in Fig. 7. The surface of
mild steel was strongly damaged in the absence of
inhibitor due to metal dissolution in corrosive solution. However, the SEM images of mild steel in the
presence of inhibitors are very different (Fig. 7b and
c). As long as the inhibitor molecules covered the
surface, the corrosion rate reduced significantly.
Thus, there was less much damage on the mild
steel surface.
b
a
c
Figure 7. The SEM images of mild steel in the absence (a) and presence of 1.10-2 M 2-AB (b) and 3-AB (c) after 6 days immersion time.
Şekil 7. 1.10-2 M 2-AB (b) ve 3-AB (c) içeren çözeltilere bırakılmş karbon çeliğinden elde edilen SEM görüntüleri.
4. CONCLUSION
(1) Both 2-AB and 3-AB have good corrosion inhibition efficiency for mild steel in 0.5 M HCl solution.
(2) The potentiodynamic polarization results show
that these compounds which inhibit both anodic
metal dissolution and also cathodic hydrogenevolution reaction have mixed type inhibitor
properties.
(3) The adsorption isotherm of 2-AB and 3-AB molecules on the mild steel in 0.5 M HCl solution
obey Langmuir adsorption isotherm with high
correlation coefficient.
(4) SEM images show that these inhibitor molecules
form a good protective film on the metal surface.
8
KOROZYON, 20 (1-3), 2013
REFERENCES
1. I. Ahamad, R. Prasad, M.A. Quraishi, Corrosion Science 52
(2010) 933–942.
2. R. Solmaz, G. Kardaş, M. Çulha, B. Yazıcı, M.
Erbil,ElectrochimicaActa 53 (2008) 5941–5952.
3. R. Agrawal, T.K.G. Namboodhiri, Journal of Applied Electrochemistry 27 (1997) 1265-1274.
4. S. Ghareba, S. Omanovic,ElectrochimicaActa 56 (2011)
3890–3898.
5. K.F. Khaled,ElectrochimicaActa 48 (2003) 2493–2503.
6. R.A. Prabhu,T.V. Venkatesha, A.V. Shanbhag, G.M. Kulkarni,
R.G. Kalkhambkar,Corrosion Science 50 (2008) 3356–3362.
AUTHORS
Gökmen SIĞIRCIK, Tunç TÜKEN, Mehmet ERBIL
Cukurova University, Science & Arts Faculty,
Chem. Dept., 01330, Adana, Turkey
Yazarlarla iletişim için:
[email protected], [email protected], [email protected]
SYNERGISM CAUSED BY A BLEND OF NITRITE
BASED INHIBITORS AND VACCINIUM MYRTILLUS
(BLUEBERRY) PLANT EXTRACT ON BOILER
STEEL IN DE-AERATED WEAK ACID
SUMMARY
This article presents results of an
interdisciplinary project focusing on the
development of a hybrid type of inhibitors composed of inorganic and plant
based natural substances. Hence use
was made of total plant extract Vaccinium myrtillus (VM) mixed with nitrite
based inorganic inhibitors with additional chemicals, commercially named
as Technophos (TP), at different ratios
in fully de-aerated M blank solution. The
effects of the presence of TP (200-1000
ppm), VM (20-100 ppm ) and their synergistic mixture (TP+VM) on EN 10204
boiler steel in fully de-aerated solution
sequentially at temperatures of 25-80
on the ability to act as corrosion inhibitors were investigated by Tafel extrapolation, Potentiodynamic anodic polarization and optical microscopy methods.
The aim of this work was to study the
inhibiting efficiency of Vaccinium myrtillus in presence and absence of TP in
mildly acidic solution.
Corrosion efficiency (IE%) of inhibitors was estimated by Tafel extrapolation technique. Results indicated an increase in IE% upon the addition of TP in
blank solution at varying concentration
between 200–1000 ppm with increasing temperature beyond 25. Results
obtained for VM extract also revealed
some protection at concentrations and
temperatures used in blank solution.
However no systematic approach could
be made regarding IE% with increasing
concentration and temperatures. Highest IE% of 82% was recorded at 40 for
100 ppm VM. Additions of (TP+VM) resulted in a 100 fold decrease in corrosion current density (icorr) at all temperature except 25 when compared with no
inhibitor. A corresponding increases in
inhibition efficiency as high as 94-99%
was recorded at different mixtures of
(TP+VM) concentrations.
Potentiodynamic anodic polariza-
tion in de-aerated blank solution was
performed at all temperatures studied.
Effectiveness of inhibitors either single
or mixed state was evaluated for polarization curve of 40oC, since it was most
explicitly displayed among others with
regard to active-passive transition region, critical current density (icrit), passive current (ip) and passivation potential (Epp). Addition of inhibitors increased
ip while decreasing icrit. The highest decrease in icrit was found with a blend of
(TP+VM). A competitive adsorption between inhibitor molecules on an active
surface involving formation of a chelating complex was proposed to explain
these changes. Thermodynamic, kinetic
and adsorption characteristics of TP
and VM were determined. Adsorption
of (TP+VM) on EN 10204 boiler steel
surface was found to obey Langmuir
adsorption isotherm at temperatures40
studied.
NİTRİT ESASLI İNHİBİTÖR VE
MERSİN YAPRAĞI (BLUEBERRYVM) ÖZÜTÜ KARIŞIMININ
HAVASIZ ZAYIF ASİTLERDEKİ
KAZAN ÇELİKLERİ ÜZERİNDE
SİNERCİK ETKİSİ
Bu makalede inorganik ve bitki esaslı doğal madde karışımından meydana
gelen hibrit türü korozyon önleyicilerin
geliştirilmesi üzerine yürütülen disiplinler arası bir projenin sonuçları verilmiştir.
Çalışma içinde başka kimyasallar da bulunan esaslı, ticari adı Technophos (TP)
olan inorganik inhibitörü ve mersin yaprağı özütünün farklı oranlardaki karışımının havası tamamen giderilmiş M çözeltisi içindeki korozyon önleyici özellikleri üzerine yürütülmüştür. TP (200-1000
ppm), VM (20-100 ppm ) ve sinercik karışımların (TP+VM) 25-80 lerdeki havasız
çözeltilerde EN 10204 kazan sacı üzerindeki korozyon koruyucu olarak etkileri Tafel ekstrapolasyonu, potansiyodina-
A. TURHAN
B. KARAHAN
A. ALBAYRAK
H. EKINCI
A. ÇAKIR
mik anodik polarizasyon ve optik mikroskop metotları ile araştırılmıştır. Çalışmanı amacı VM nin TP içeren ve içermeyen
hafif asidik çözeltilerdeki koruyucu etkinliğini araştırmaktır.
İnhibitörlerin koruyucu etkinliği (%IE)
Tafel eksrapolasyon tekniği ile hesaplanmıştır. Sonuçlar 25 oC üzeri sıcaklıklarda
referans çözeltiye yapılan 200-1000 ppm
arasındaki TP ilavelerinde %IE nin arttığını göstermiştir. VM ekstresi ile elde edilen sonuçlar referans çözeltide kullanılan bazı konsantrasyon ve sıcaklıklarda
korumanın olduğunu göstermiştir. Ancak
konsantrasyon ve sıcaklık artışlarında
%IE ile ilgili sistemli bir yaklaşım yapmak
mümkün değildir. En yüksek %IE (%82),
koruyucu etkinliği 40 oC ve 100 ppm VM
de kaydedilmiştir. 25 oC hariç tüm sıcaklıklarda yapılan (TP+VM) ilaveleri, inhibitörsüzlerle karşılaştırıldığında, korozyon
akım yoğunluğunda (ikor) 100 kat azalma
yaratmıştır. Farklı (TP+VM) konsantrasyonlarında %94-99 gibi yüksek koruma
etkinliği kaydedilmiştir.
Çalışılan tüm sıcaklıklardaki havasız referans çözeltilerde i potansiyodinamik anodik polarizasyon çalışmaları yürütülmüştür. İnhibitörlerin tekil veya karışık haldeki koruyucu etkinliği, aktif-pasif
geçiş bölgesi, kritik akım yoğunluğu (ikr),
pasif akım (ip) ve pasif potansiyel (Epp)
değerleri bakımında en belirgin ve açık
olduğundan, 40 oC deki polarizasyon
eğrileri ile değerlendirilmiştir. İnhibitörlerin katkısı ikr değerini azaltırken ip değerini artırmıştır. ikr değerindeki en yüksek artış (TP+VM) karışımında gözlenmiştir. Bu değişiklikleri açıklamak için inhibitör molekülleri arasında şelat (chelating) oluşumuna yol açan soğrulma yarışı önerilmiştir. TP ve VM nin termodinamik, kinetik ve soğrulma özellikleri belirlenmiştir. (TP+VM) nin EN 10204 kazan
çelik yüzeyindeki soğrulmasının ≥40 oC
sıcaklıklarda Langmuir soğrulma teorisine uyduğu bulunmuştur.
KOROZYON, 20 (1-3), 2013
9
1 . INTRODUCTION
Use of inhibitors is a significant means to prevent corrosion of structural materials occurring under aqueous conditions. In cases where other preventive measures such as design, materials selection, coatings etc., are likely to fail to protect metals
and alloys from corrosion, altering the environment
in such cases by the use of corrosion inhibitors
becomes the only possible means of corrosion
prevention. Due to some restriction imposed on
structural materials owing to their inherent and unavoidable properties usage of inhibitors becomes
indispensable.
Application of inhibitors ranges from chemical to
production and manufacturing industries, including
utilities such as gas distribution, drinking water and
sewage systems etc. Cost of corrosion prevention
is negligible small when compared to the amounts
spend to invest on industrial installations. Therefore
every penny of the investment spend on corrosion
prevention is well worth for considering. However
the extent of investments, expected to be made in
USA alone in 2012, amounts to one trillion dollars,
which underlines the importance of corrosion prevention alone. The number of published articles in
corrosion literature also indicates the ever growing
interest in corrosion inhibitors.
Types of inhibitors and mechanism of inhibition
are well documented in detail in corrosion literature
where their classification and vivid account of mode
of protections are featured 1-5. The most prominent
aspect of corrosion prevention by inhibitors depends solely on the interaction between molecules
of inhibitors and the surface of metallic materials resulting either in a two dimensional adsorption (chemisorption or physisorption) layer or a three dimensional oxide layers which are closely related to the
chemical and molecular structures of inhibitor ions.
Chemisorption types of inhibitors, mostly organic compounds, contain heteroatoms like N, S,
P and O atoms, capable of forming coordinate covalent bond with metals due to their free electron
pairs. Chemisorption takes place when the free
electrons of these atoms are donated to the metal
cations on the surface and forms a strong coordination bond resulting in high efficacy of inhibition.
Easy donation of electrons by these heteroatoms
induces high rate of inhibition. The increasing order of inhibition efficacy have been reported to follow the sequence O < N < S < P 1, 3, 6. This is the
reverse order of electronegativity of these atoms.
Accordingly S atoms are less electronegative than
N atoms which means to say that S atoms are less
effective in drawing electrons to themselves, therefore are better electron donor resulting in improved
10 KOROZYON, 20 (1-3), 2013
inhibition. Therefore heteroatoms form active centres of the organic inhibitors for the process of adsorption on the metal surface. The strength of chemisorptions bond depends on the electron density
on the donor atom of the functional group as well as
polarizability of the group. It is stated that replacement of H atom in the aromatic rings by some
functional groups such as -NH2, -NO2 and –COOH
improves inhibition owing to the high electron density of the heteroatoms of the functional group 7.
According to H. Wang as cited in reference [8] the
compounds containing both nitrogen and sulphur
found in some synthetic organic compounds such
as mercapto-triazol were reported to provide excellent inhibition, compared with compounds containing only nitrogen or sulphur atoms.
There has been a number of works where the
presence of functional groups, such as HC=N,
N=N, -CHO, R-OH, C=C, etc., in the inhibitor molecule, the aromaticity, molecular and chemical structures and electron density at the donor atoms were
reported to influence the adsorption of the inhibitor
molecule over corroding metal surface promoting
effective inhibition 6, 9-15. Upon the attachment of the
organic inhibitors, the electron density in the metal
at the point of attachment changes which in return
results in the deceleration of anodic and cathodic
reactions. Thus electrons produced at the anode
are consumed at the cathode side. As far as structural characteristics are concerned, the inhibition
efficiency of straight chain amines was claimed to
increase with the increasing number of carbon atoms in the chain up to 10 3. Increasing length of carbon chain beyond 10 was told to be indifferent in
terms of inhibitor efficiency, due to the decreasing
solubility of the organic inhibitors. A vast number
of works devoted in corrosion prevention of metallic materials has been published ever since the
considerable efforts has been deployed to develop
chromate-free inhibitors for the last couple of decades.
In vast majority of the inhibitors literature it has
been almost unanimously agreed that the inhibition mechanisms are largely associated with the
ability of adsorption of inhibitor’s molecules on the
protected metal surface which in return grossly depends on the molecular structure of the inhibiting
components. It has also been agreed that although
considerable attention was devoted to developing chromate-free inhibitors either inorganic or organic, finding a suitable replacement was not fully
achieved 16. However efforts were made to develop
more effective and efficient inhibitors by combining organic and inorganic components to provide
environmentally friendly, multi-functional corrosion
inhibitors 16-18. High-throughput screening (HTS)
methods introduced by S.R. Taylor et al.19 to test
combination of inhibitors (synergistic effect) have
revealed that the synergistic combinations of nonchromate inhibitors have exhibited better corrosion
inhibition properties exceeding those of chromate.
The synergistic inhibition effects between plant extracts and inorganic inhibitors, though very rare in
inhibitor’s literature, was reported to improve corrosion of EN 10204 boiler steel in de-aerated 10-4 M
solution 18. Synergistic effects produced by combination of the rare earth and organic inhibitor components were also reported recently to mitigate stress
corrosion cracking (SCC) of high strength steels
and filiform corrosion 19. The results of screening
methods, as reported in a review paper by G. Gece
20
, have indicated many structural similarities of
drugs and corrosion inhibitors, such as carbocyclic
and heterocyclic systems existing ubiquitously in
both structures. It was also indicated that drugs as
classified in this work contain heteroatoms containing lone pair of electrons as well as aromatic rings
with delocalised Π-electron systems acting as active adsorption centres.
Corrosion, as an undesirable phenomenon, can
simply be defined as a degradation of metallic materials causing to lose their integrity by the anodic
metal dissolution reaction with the surrounding environmental conditions. Oxidation of metal atoms
occurs in almost every type of corrosion and results
in formation of metallic ions which either dissolve
in aqueous environment or form corrosion compounds. Formation of corrosion compounds might
take place both in aqueous solution or on metallic surfaces with or without any capacity to protect
the surface from further corrosion. Preventing metal
atoms from getting oxidised in some ways would
bring about the protection of metallic materials to
some extent. As one of the most significant corrosion preventive measures, the use of inhibitors are
highly regarded among others due to the applicability in situ conditions without any interruption of the
ongoing processes. Inhibitors, when used in small
amounts in aqueous environment, reduce the rate
of corrosion and/or oxidation of materials exposed
to that environment. This is to say that the inhibitors act as an anticorrosive or antioxidant agents
by forming a protective films of some forms on the
metal surface. In this regards corrosion scientists
have long been after finding suitable chemicals
with a high antioxidant activity reconciled by their
eco-friendly attributes. This is just the case where
plant extracts with naturally occurring constituents
raised scientist’s interest not just in domains such
as medicine, nutrition, flavouring, beverages, dye-
ing, repellents, fragrances and cosmetics as stated
in 22 but also in corrosion prevention 3, 21, 23, 24. Therefore intensive efforts motivated by the desire to replace toxic inhibitors used for corrosion prevention
has been going on for the last couple of decades.
Studies on plant extracts as antioxidant agent
has started as early as, if not earlier than, works on
anticorrosive activity of natural plant extracts. The
use of vegetal tannins, for example, was reportedly
disclosed since 1936 25. Total equivalent antioxidant
capacity (TEAC) of plant extracts are generally related to their phenolic contents whose determination were reported to depend on the methods of extraction and chemicals used thereby 26. Antioxidant
capacity of some medicinal plants was ascribed to
the contribution made by phenolic and flavonoid
compounds and strong correlation as high as was
determined between antioxidant activity and the
contents of flavonoid and phenolic compounds
22
. However total phenolic contents were reported
not to have necessarily incorporated in all the antioxidants found in plant extracts 27, 28. Accordingly it
was stated that same aqueous extracts with a higher phenolic content than some others may have
lower antioxidant activity. In view of these findings
antioxidant (components inhibiting oxidation) activity of plant extracts may not always be associated
with their phenolic contents.
A number of factors were found to influence the
concentration of the active constituent’s particularly
phenolic compounds present in the plant extracts.
Time and period of collection, geographical origin
and climatic conditions, method of extraction, type
of the solvents used for extraction, part of the plants
such as leaves, bark, flowers, fruits used for extraction are a few of the noticeable factors to mention
here 29, 30. According to Marcus et al. cited in 29 the
influence of these factors could be such dominant
that even lead to the absence of active constituents
in the same plant collected from different regions. A
positive correlation between the polyphenolic contents and solvent’s dielectric constant (R=0.728,
P<0.05) was reported to exist indicating how significant influence could the dielectric constant of
the solvent chosen for extraction play on the total
phenolic contents and subsequently on the antioxidant activity of the plant extract 29.
Plants belong to the world’s most precious
legacy and mankind enjoys much goodness provided by the plant world. Now they are exploited
for their extracts for a variety of reasons, corrosion
prevention being just one of them. Therefore the
extracts rich in polyphenolic compounds have now
been studied for their activity to prevent or mitigate
corrosion of metals. However there are a number
KOROZYON, 20 (1-3), 2013 11
of factors that influence the concentration of the
constituents, phenolic compounds in particular.
Extracts when used as prospect echo-friendly inhibitor; question is generally raised as to how the
main constituents of plant extracts can be associated with their chemical and structural properties.
However there are limited amount of inhibitor literature where structural and chemical properties of
the components existing as the main ingredients in
natural extracts were investigated16, 21.
A number of structurally-related compounds
some with others without similar substructures attached were tested for their capacity to inhibit corrosion of high strength aluminium alloys, namely
AA2024 and AA7075 16. Among the functional
groups tested, –SH (thiol) group, besides the orthoand para- positions to a carboxylate on a monoaromatic ring and substitution of N for C in certain
position of aromatic ring were found to display a
high inhibitive activity, while hydroxyl group with
slight and carboxylate little or no capacity to inhibit
on their own. –SH (thiol) group was found to be the
most effective to inhibit aluminium alloys, unless
the inhibiting capacity of an aromatic component
is disrupted by substituting for N in the ring. An argument was put forth saying that this interruption
would be caused by the remaining N on the ring
withdrawing electrons from the thiol group and reducing its activity.
Some phenolic compounds such as (1) o-aminophenol, (2) catechol, (3) salicaldehyde and (4)
salicylic acid was investigated for their inhibition efficiency tested on carbon steel in HCl acid with and
without some potassium salts 21. This is one of the
few works where inhibition efficiency of the inhibitors was investigated in association with chemical
structure of inhibiting molecules. Thermodynamic
parameters of adsorption process such as , and
were determined to assess the inhibition efficiency.
All parameters found indicated a spontaneous physisorption with decreasing inhibition efficiency in
the order: 1>2>3>4. The negative values of and
indicated exothermic nature of the adsorption process. Kinetic parameters for the adsorption process
also indicated same inhibition mechanism for the
inhibitors since Ea increases in with the presence
of inhibitors . In this work finding were based on
the adsorption capacity of the inhibitors explained
in terms of electronegativity and electron donating
capacity of the inhibitors molecules. According to
the functional groups accommodated in inhibitor
molecules, the inhibition efficiency of the inhibitors
as determined by weight loos and electrochemical
technique were shown to decrease in the following order: -NH2 > -OH > -CHO > -COOH. It is
12 KOROZYON, 20 (1-3), 2013
obvious from the findings that, compounds 1 and
2 containing electron donating groups (-NH2, -OH)
with lone pairs on the atoms next to the π-system
activate the aromatic ring through a resonance
donating effect. Thus, electron density on the ring
is increased and the compound becomes more
nucleophilic leading to an increase in the inhibition efficiency. Compounds 3 and 4 however, as
they contain electron withdrawing groups (-CHO,
-COOH) with electronegative atoms next to the
π-system, deactivate the aromatic ring through a
resonance withdrawing effect. These electron withdrawing groups by removing electron density from
the π-system make the compounds less nucleophilic therefore they have lower inhibition efficiency
compared to compounds 1 and 2. Among these
substituents, NH2 is the most electron donating
and COOH is the most electron withdrawing one
which correlates well with the order of the inhibition efficiency. A special effort has been put forth to
explain inhibiting mechanisms in association with
the chemical and molecular structure of some plant
extracts, hypericum perforatum, vaccinium myrtyllus (blueberry) in particular.
Computer modelling techniques are powerful tools for studying the mechanism of corrosion
and foretelling molecular structures that are better
as corrosion inhibitors. Researchers have begun
to use theoretical data in their studies to support
their experimental results as well as to find a solution by consuming lesser chemicals. The geometry
of the inhibitor in its ground state and energy of its
molecular orbital (HOMO-LUMO) calculated using
computational methodologies were shown to be
well correlated with inhibitor’s activity. According
to the frontier molecular orbital theory, high EHOMO
values indicate that the molecule has a tendency
to donate electrons to acceptor molecules with
unoccupied molecular orbital whereas low ELUMO
values mean that the molecule has a tendency to
accept electrons. K.F. Khaled studied the inhibition
performance of triazole derivatives (triazole, aminotriazole and benzotriazole) on mild steel in 1M HCl
both experimentally and computationally 31. They
found out that aminotriazole was the best inhibitor among these three. According to the quantum
chemical parameters for triazole derivatives, max.
charge on N-atoms was calculated to be highest
for aminotriazole which enhanced the stronger adsorption possibility of it on iron surface. Actually, it
is well known that, the more negative the atomic
charges of the adsorbed centre, the more easily the
atom donates its electrons to the unoccupied orbital of the metal. Also, highest EHOMO value of aminotriazole enhanced the assumption that it would
adsorb better on iron surface. The negative sign
of EHOMO is generalized as an indicator that adsorption is physisorption. In addition to this, adsorption
power in other words inhibitor efficiency was correlated with dipole moments such that as dipole
moment decreases, inhibitor efficiency increases.
According to the experimental results, inhibition
efficiency of the aminotriazole was the best with
90.2% at a concentration of 10-2M and ΔGads value
is -14.323KJ/mol indicating that adsorption mechanism was typical of physisorption. In another study,
the interaction between L-tryptophan molecule
and iron surface was investigated using computational modelling. L-tryptophan molecular structure
was optimized and probable negative and positive
charge centres were found within the molecule. It
was stated that negative charge centres can offer
electrons to the iron atoms to form coordinate bond
and positive charge centres can accept electrons
from iron atoms to form back-bonding. This dual
interaction was assumed to be the reason of the
excellent corrosion inhibition.
1.1
Nomenclatures related to the
contents of plant extracts
Aromaticity: An aromatic compound contains;
i) delocalized conjugated π-system, ii) coplanar
structure with contributing atoms arranged in one
or more ring and iii) 4n + 2 number of π electrons
(Hückel’s Rule). The positions of the 6 p-orbitals
of benzene is shown on the left figure. Since they
are out of the plane, these orbitals can interact with
each other and become delocalized.
Phenolics: Phenol is an organic compound with
the chemical formula C6H5OH.
The molecule consists of a phenyl group (-C6H5) bonded to a
hydroxyl group (-OH). There is
an interaction between the delocalised electrons in the benzene
ring and one of the lone pairs on
the oxygen atom. The donation
of the oxygen’s lone pair into the
ring system increases the electron density around
the ring. That makes the ring much more reactive
than it is in benzene itself. It also helps to make the
-OH group’s hydrogen a lot more acidic than it is in
alcohols.
Heterocyclic Aromatic Compounds: In heterocyclic compounds an element other than carbon
is present in the ring. Heterocyclic compounds
containing nitrogen, oxygen, or sulfer are by far
the most common and they are quite commonly
encountered in nature. Pyridine, pyrrole, furan and
thiophene are examples of heterocyclic aromatic
compounds.
Steric effects: The influence of the spatial configuration of reacting substances upon the rate, nature, and extent of reaction. Steric effects arise from
the fact that each atom within a molecule occupies
a certain amount of space. If atoms are brought too
close together, there is an associated cost in energy due to overlapping electron clouds and this
may affect the molecule’s preferred shape (conformation) and reactivity.
Flavonoids: Flavonoids are polyphenolic compounds found in plants and are categorized according to their chemical structures into flavonols,
flavones, flavanones, isoflavones, catechins, anthocyanidins and chalcones. The flavonoids have
drawn attention because of their potential beneficial
effects on human health. They have been reported
to have antiviral, anti-allergic, antiplatelet, anti-inflammatory, antitumor and antioxidant activities.
2. EXPERIMENTAL
2.1 Inorganic inhibitor
The inorganic nitrite based inhibitor, commercially named as Technophos (TP), was provided
by Günsu A.S, manufacturers of household and
industrial cleaning products and water treatment
chemicals, in Antalya-Turkey. Inhibition efficiency of
TP was studied earlier by this group and its inhibition characteristics with and without Hypericum
Perforatum (HP) has been reported [18]. Inhibiting
attributes of TP in combination of blueberry was
studied and reported in this work.
KOROZYON, 20 (1-3), 2013 13
2.2 Preparation of plant extracts and
Determination of their total phenolic
and flavonoid contents
Vaccinium myrtillus (blueberry) plant powder
(15 g) provided by a regional company in Izmir was
extracted with ethanol in a Soxhlet extractor for 16
h. The extraction solvent was evaporated in an oven
at 25-30oC to dryness. Crude extract was evaluated
for their total phenolic and flavonoid contents as
well as for DPPH radical scavenging activity as de-
scribed in 18 . Four different plants selected from the
Turkish species was considered as a candidate of
prospect inhibitors, but only blueberry extract was
studied and reported for their inhibitive attributes in
this work. Some characteristic of the selected plant
extracts are given in Table 1. Main ingredients of
the plant extracts studied in this work and some
other are given in Table 2. Structure of some main
ingredients of the studied plant extracts are shown
in Table 3.
Table 1. Antioxidant activity of plant extracts in association with total flavonoid and phenolic contents selected for corrosion studies.
Çizelge 1. Korozyon çalışmaları için seçilen bitki özütlerinin toplam flovanoid ve fenolik içerikleri ile ilgili antioksidan aktiviteleri
Plant Extract
Extraction
Yield %
Total flavonoid
contents
(QEmg /100mg)
Total phenolic
contents
(GAE mg/100mg)
Radical scavenging
activity,
DPPH %
Rosmarinus
Officinalis
30
2.55
13.14
4.80
Olea europea
32
4.31
16.98
12.97
Vaccinium
Myrtillus
31
24.51
46.33
48.50
Hypericum
Perforatum
33
26.58
50.58
57.46
Main
ingredients
Extracts
Table 2. Main ingredients of plant extracts studied.
Çizelge 2. Çalışılan bitki özütlerinin ana içerikleri
1.
2.
3.
4.
5.
1. ST. John’s
wort (Hypericum
perforatum)
2. Blueberry
(Vaccinium
myrtyllus)
3. Mimosa
(Acacai Mearnsii)
4. Quebracho
Red Wood
(Schinopsis
Lorentzii)
1.a. Protohypericin1
1.b.Protopsedohypericin
1.c. Hypericin1
1.d. Pseudohypericin1
1.e. Quercetin1
1.f. Quercitrin1
1.g.Isoquercitrin1
1.h. Hyperoside1
1.i Rutin1
1.j. Hypuercitrinerforin
1.k. 8-Biapigenin1
1.l Tannic acid1
2.a Gallic acid2
2.b Cafeic acid2
2.c Coumaric acid2
2.d Ferulic acid2
2.e Catechin2
2.f Epicatechin2
2.g Quercetin2
2.h Kaempferol2
2.i Delphinidin2
2.j Cyanidin2
2.k Petunidin2
2.l Peonidin2
2.m Malvidin2
3.a C-glycosylflavones3
3.b Isoorintin3
3.c Rhamnosylorientin3
3.d Hydroxymaysin3
3.e Cassiaoccidentalin3
3.f Quercitrin4
3.g Myricitrin4
3.h Catechin4
3.i Gallocatechin4
3.j Mearnsitrin4
3.k Quercetin4
3.l Myricetin4
4.a Catechin5
4.b Epicatechin5
4.c Gallocatechin5
4.d Epigallocatechin5
4.e Fisetinidol5
4.f Gallic acid5
4.g Chlorogenic acid5
S.H. Hansen, A. G. Jensen, C. Cornett, I. Bjornsdottir, S. Taylor, B. Wright, I.D. Wilson, High-performance liguid chromatography online coupled to highfield NMR and Mass spectrometry for structure elucidation of constituents of Hypericum Perforatum,Anal. Chem., 71, (1999), 5235-5241.
K. Riihinen, L. Jaakola, S. Karenlampi, A. Hohtola, Organ-specific distribution of phenolic compounds in bilberry (Vaccinium myrtillus) and ‘northblue’
blueberry (Vaccinium corymbosum x V. angustifolium), Food Chemistry, 110, (2008) 156–160.
L.M. de M. Camargo, J. Fe´ re´zou, L. W. Tinoco, C. R. Kaiser, S. S. Costa, Flavonoids from Mimosa xanthocentra (Leguminosae: Mimosoideae) and
molecular modeling studies for isovitexin-200-O-a-L-rhamnopyranoside rotamers, Phytochemistry Letters, (2012).
A.M. MacKenzie, The flavonoids of the leaves of Acacia mearnsii Phytochemistry, Volume 8, Issue 9, September 1969, Pages 1813-1815].
P.B. Venter, M. Sisa, M. J. van der Merwe, S. L. Bonnet, J. H. van der Westhuizen, Analysis of commercial proanthocyanidins. Part 1: The chemical composition of quebracho (Schinopsis lorentzii and Schinopsis balansae) heartwood extract Original Research Article Phytochemistry, 73, 2012, 95-105.
14 KOROZYON, 20 (1-3), 2013
Table 3. Chemical structures of the main ingedients fort he studied plant extracts.
Çizelge 3. Çalışılan bitki özütlerinde bulunan ana içeriklerin kimyasal yapıları.
Some compounds present in the mentioned plant extracts
Phenolic
Compounds
Structure
Cafeic Acid
(R1=OH, R2=H)
p-Coumaric Asit (R1=H, R2=H)
Ferulik asit
(R1=OCH3, R2=H)
Gallic Acid
(R1=OH, R2=OH)
Chlorogenic acid (ester formed between caffeic acid
and L-quinic acid)
Kaempferol (flavonol) (R1=R2=H)
Quercetin (flavonol) (R1=OH, R2=H)
Quercitrin (flavonol-glycoside)
(R1=OH, R2=H, R3=Rha)
Flavonol backbone
Rutin (flavonol-glycoside)
(R1=OH, R2=H, R3=Rutinose)
Flavonoids
Hyperoside (flavonol-glycoside)
(R1=OH, R2=H, R3=Gal)
Catechin (flavan-3-ol)
Cyanidin
Catechin
(R1=OH, R2=H)
Delphinidin (R1=R2=OH)
Anthraquinones
Peonidin
(R1=OCH3, R2=H)
Petunidin
(R1=OH, R2=OCH3)
Malvidin
(R1=R2=OCH3)
Anthocyanidin
backbone
Hypericin
Hypericin
2.3 Characterization of plant extracts
using FTIR analysis
The infrared spectra of the plant extracts were
recorded with a Perkin Elmer Spectrum BX instru-
ment equipped with ATR apparatus in the spectra
range between 4000 and 650 cm-1 with a resolution
of 4 cm-1 and 25 scans per sample. FTIR analysis
of hypericum perforatum, vaccinium myrtyllus (blueberry) are given in Fig.1
KOROZYON, 20 (1-3), 2013 15
0,35
0,30
4000
3500
3000
2500
2000
1500
1000
500
1500
1000
500
Vaccinium Myrtillus, VM
0,25
0,20
0,15
0,10
Absorbance
0,05
0,00
Hypericum Perforatum,HP
0,125
0,100
0,075
0,050
0,025
0,000
4000
3500
3000
2500
2000
Wavenumber (cm
-1
)
Figure 1. FT-IR absorbance spectra of VM and HP extracts.
Şekil 1. VM ve HP özütlerinin FT-IR abzorbans spektrumları
Table 4. FT-IR absorbance spectra of VM and HP extracts and their identifications
Çizelge 4. VM ve HP özütlerinin FT-IR abzorbans spektrumları ve tanımlayıcı özellikleri.
Wavenumber
(cm-1)
Vaccinium Myrtillus,VM
(Bluberry)
Wavenumber
(cm-1)
Hypericum Perforatum, HP
(St. John’s wort)
3000-3700
O-H stretching (Phenolic)
3000-3700
O-H stretching (Phenolic)
2937
C-H stretching (Aromatic)
2937
C-H stretching (Aromatic)
2832
C-H stretching (OCH3)
2855
C-H stretching (cyclic)
1705
(C=O)OH stretching
1714
(C=O)OR stretching
1646
C=O stretching (Ketone)
1648
C=O stretching (Ketone)
1597
C=C stretching(Aromatic)
1597
1457
1435
1341
C-O stretching (Phenolic)
1369
1213
C-H in plane bending (Aromatic)
or C-O stretching (Phenolic)
1271
C-H in plane bending (Aromatic) or
1000
1050
700-900
O-H bending (Phenolic) or CH
out of plane bending (Aromatic)
700-900
O-H bending (Phenolic) or CH out of
plane bending (Aromatic)
Both Bluberry and St Johns wort plant extracts
show similar characteristic absorption peaks as
shown in Table 4. However, the intensity of absorption due to C=C (1597 cm-1) in the aromatic rings is
16 KOROZYON, 20 (1-3), 2013
much lower in VM compare to HP (Fig. 1).From this
result better inhibition efficiency is expected from
HP as it has more π-electrons.
2.5 Potentiodynamic polarization studies
Electrochemical experiments were carried out in
the conventional three-electrode cell with a graphite counter electrode (CE) and a saturated calomel
electrode (SCE) coupled to a fine lugging capillary
as the reference electrode (RE). To minimize ohmic
contribution in the cell circuit, the lugging capillary
was kept close to working electrode (WE).
All polarization experiments were performed
using Gamry reference 3000 potentiostat/galvanostat corrosion measurement system according to
ASTM G59 norm 32 and G102 norm 33. Before potentiodynamic polarization tests the electrode was
immersed in the test solution under open circuit
condition and open circuit potential (OCP) was
measured for 55 min. until a steady state was attained. Potentiodynamic polarization curves in uninhibited and inhibited solution were carried out in
test solution at 25, 40, 60 and 80 ±1°C and Arrhenius plots were obtained by measuring corrosion
current density at these temperature.
The potential was increased at a rate of
0.17mV/s (0.6 V/h) for Tafel extrapolation measurement curves and changed within a potential
range of ± 30 mV around OCP. Corrosion potential (Ecorr) and corrosion current density (icorr) were
measured within a selected range of ±15 mV on
Tafel curve. Potentiodynamic Anodic Polarization
curve measurement was obtained at a scan rate of
1 mV/s starting from cathodic potential (Ecorr -100
mV) going to anodic direction 1500 mV. The aim of
potentiodynamic anodic polarization was to enable
inhibitors molecule interacts directly on the bare
surface for comparison purposes. In order to check
the reproducibility and consistency of the results,
each experiment was repeated at least two or more
times and the average of repetitions were recorded
correspondingly.
Inhibition efficiencies, %IE, and polarization resistance, Rp were calculated using the equations
(1) and (2):
(1)
-
(2)
where and are corrosion current densities in
absence and presence of inhibitor, and are anodic
and cathodic Tafel constants respectively.
3.
RESULT AND DISCUSSION
3.1 Tafel extrapolation measurements
Kinetic of corrosion reactions occurring on mild
steel surfaces in solution at various concentrations
of TP and VM were studied through polarization
measurements.
3.1.1 Effect of TP concentration and
temperature on inhibition efficiency
Corrosion inhibition of TP was evaluated at various temperatures and concentrations by Tafel extrapolation technique insolution. The electrochemical parameters and the corresponding inhibition
efficiency obtained from Tafel polarization measurements are given in Table 5.
Electrochemical parameters shown in Table
5 indicate an increase in inhibition efficiency as a
function of temperature as compared to blank solution. This increase remained rather low at 25oC and
varied between 11-40%. At higher temperatures
there was a remarkable increase varying between
80-95% at different concentrations. Change in inhibition efficiency with increasing temperature is given in Fig.2. The highest inhibition efficiency of 40%
at 25oC was obtained at 800 ppm TP, while at higher
temperatures 200 ppm seems to suffice to obtain a
high IE% around 95%. The significant increase in
IE% of TP with temperature beyond 25oC indicates
that the deployment of TP in blank solution at temperatures ≥ 40oC is beneficial to decrease the rate
of corrosion reaction.
100
80
IE%
2.4 Electrolyte and Specimen preparation
The corrosion tests were performed in a mixture
of 10-4 M H2SO4 (Merck) and 0.25 M K2SO4 solution (Sigma Aldrich) named as a blank solution with
pH=4.62. However addition of TP has increased pH
value of blank solution from 4.62 to 9-11 depending
on the amount of addition. Preparation of the electrolytes, the test specimens and test setup were
carried out as described previously 18. The chemical composition (wt%) of working electrode used
for the experiments was C:0.1, Si:0.22, Mn:0,44,
P:0.012, S:0.012 and Fe: balance.
60
40
200 ppm
400 ppm
600 ppm
800 ppm
1000 ppm
20
0
20
30
40
50
60
70
80
o
T, C
Figure 2. Inhibition efficiency of TP as a function of temperatures at different concentrations in blank solution.
Şekil 2 Ana çözeltideki farklı konsantrasyonlarda sıcaklığın fonksiyonu
olarak TP nin koruyucu etkinliği.
KOROZYON, 20 (1-3), 2013 17
Table 5. Kinetic parameters derived from Tafel polarization plots and inhibition efficiencies of mild steel in solution containing 200-1000 ppm TP at different
temperature..
Çizelge 5. Tafel polarizasyonu kinetik parametreleri ve farklı sıcaklıklardaki ana çözelti içinde 200-1000 ppm arasındaki konsantrasyonlarda TP nin imalat
çeliği üzerindeki koruyucu etkin
T
(oC)
βa
Concentration
of TP, ppm
βc
(mV/dec) (mV/dec)
Ecorr
OCP
icorr
x10-4
(mV)
(mV)
(mA/
cm2)
Rp
IE%
(Ω cm2)
Blank
6.55
7.65
-772
-771
2.00
7661.10
....
200
13.50
12.35
-595
-587
1.68
16670.10
16.00
400
12.15
10.90
-612
-604
1.78
14055.00
11.00
600
14.45
11.05
-511
-502
1.46
18686.70
27.00
800
10.25
9.10
-614
-605
1.21
17370.00
39.50
1000
20.00
12.55
-506
-494
1.63
33483.00
18.50
Blank
16.97
28.80
-782
-777
40.90
1133.64
....
200
11.60
10.45
-648
-639
1.62
14735.22
96.00
400
10.25
9.20
-626
-615
3.76
5598.99
90.80
600
17.85
13.45
-632
-625
4.20
7929.99
89.70
800
11.15
9.85
-607
-597
3.21
7074.45
92.15
1000
18.05
14.40
-605
-595
6.00
5796.68
85.30
Blank
11.60
15.50
-787
-783
790.92
....
200
7.95
7.60
-676
-667
0.65
25956.31
98.91
400
7.80
8.40
-636
-629
1.10
15965.12
98.16
600
9.75
9.50
-647
-640
3.17
6590.89
94.71
800
9.40
8.25
-633
-622
3.27
5834.38
94.55
1000
8.30
8.20
-647
-640
2.14
8369.51
96.43
Blank
13.35
16.10
-797
-795
94.00
337.13
....
200
12.25
13.20
-700
-696
10.30
2678.49
89.00
400
18.55
12.35
-660
-652
24.30
1324.80
74.10
25
40
60
80
60.00
600
13.30
11.50
-688
-680
7.80
3433.28
91.70
800
10.75
10.55
-678
-671
11.70
1976.06
87.56
1000
11.10
9.90
-700
-692
7.40
3070.52
92.13
Corrosion of metals in near neutral and alkaline
solution oxygen reduction reaction can occur as
shown in equation (3).
4O2+2H2O+4e → 4OH
-
-
(3)
Nitrite as an oxidizing inhibitor has no direct effect on anodic oxidation of iron; rather it involves
primarily in the cathodic reactions as shown in
equations (3.1) which increase the pH of solution
near electrode surface and accelerates anodic metal dissolution accordingly 34
NO-2+8H++6e-→NH+4+2H2O
(3.1)
The consumption of protons in equation (3.1)
18 KOROZYON, 20 (1-3), 2013
increases pH and promotes the formation of oxide
film on the surface according to reaction (3.2) 35, 36.
-
(3.2)
-
At the defected sites of a passive film where anodic metal dissolution takes place, hydrolysis of is
expected to produce ferrous hydroxide according
to the following reaction (4) 37.
-
-
-
(4)
Acidity caused by the reaction (4) in the anodic
sites is counterbalanced by the presence of ions
in the solution and pH is shifted to 9. Nitrite ions
was reported to perform better in solution with pH
higher than 6. Ferrous hydroxide thus produced by
the reaction (4) was reported to have converted to a
more protective iron oxide according to the following reaction (5):
-
-
-
(5)
Passivation and then adsorption of NO2- in the
metal oxide layer was presumed as a mechanism
of corrosion prevention which was facilitated by
B4O-27.The oxidizing power of NO2- in forming a passive ferric oxide layer was further proved by the XPS
data from NO2- induced film and reported to have
composed largely of γ-Fe2O3 38. As regard to the
formation of a passive oxide layer, seemingly acts
as an anodic-active inhibitor that prevents local corrosion at defected sites of the film. The increasing
efficiency of with increasing temperature ≥40O C,
as shown in Fig. 2, complies with the assumption
that increase in anodic dissolution process with increasing temperature accelerate the rate of film formation according to the reaction (5). This process
of film formation by nitrite ions in slightly alkaline
solution may not ignore the adsorption of ions on
the metal as postulated by Kuznetsov 34.
3.1.2 Effect of VM concentration and
temperature on inhibition efficiency
As regards to the corrosion inhibition of most
organic component there is general agreement
on the mechanism of inhibitory action controlled
by adsorption mechanism. Some of the concepts
largely gained acceptance as regards to adsorption mechanism.
The electrochemical test measurements indicated that the extract inhibit the corrosion processes
by blocking the available cathodic and anodic sites
of the metal surface through adsorption of the extract chemical constituents on the metal/solution
interface 39. According to D. Schweinsgberg et al.,
as reported in 39, this phenomenon could take place
via (i) electrostatic attraction between the positively
charged protonated nitrogen atom and negatively
charged mild steel surface (cathodic sites) (ii) dipole-type interaction between unshared electron
pairs of oxygen atom or p electrons-interaction with
the vacant, low energy d-orbitals of Fe surface atoms (anodic sites) and (iii) a combination of all of
the above (mixed type).
According to Martinez and Hucovic, as cited
in40, corrosion inhibition by organic components
is brought about by two means: (i) the available
reaction area is decreased by absorption causing
so-called geometric blocking effect; and (ii) the activation energy on anodic and/or cathodic reaction
during the course of corrosion inhibition process is
modified by the adsorption. In cases where geometric blocking effect is stronger than energy effect, no shift in the corrosion potential should be
observed.
The polarization parameters obtained with addition of varying VM concentrations are given in
Table 6. VM addition caused significant decrease in
IE%, when 100 ppm VM was added in blank solution at 25oC. There is hardly any change in between
blank solution and VM at three concentrations (20,
60, 100 ppm) tested at all temperatures as seen in
Table 6. Since the displacement in Ecorr<<85 mV
VM can be regarded as mixed type of inhibitors as
reported in 41. However addition of TP at all temperatures tested shifted Ecorr in anodic direction more
than 100 mV signifying the anodic character of TP
(See Table 5). Table 6 also indicates some small
changes with no noticeable trend in βa and βc upon
addition of VM into the blank solution which was in
line with small changes in Ecorr indicating VM as a
mixed type of inhibitors, similar to some studies on
plant extracts 42, 43. A dark blue film was produced
during the electrochemical polarization studies in
solution of VM extract alone at all temperatures and
concentrations tested. According to Brouillard and
Favre as reported in 44 similar film formation in solutions containing natural polyphenolic compounds
with a catechol group in their B-ring was reported
to form a complex with di- and thrivalent ions. A noticeable increase up to 3 to 4 folds in βa and βc was
recorded upon the addition of 100 ppm VM into
blank solution at 25 which resulted in an increase
in corrosion current density almost 40 folds. This
increase was accompanied by the peeling off the
dark blue deposits (a complex film) formed on the
surface during the time interval of the Tafel polarization. VM addition at concentration greater than
60 ppm was assumed to form a loosely bond complex film on the surface with a high internal stresses
causing the complex to peel off. Formation of this
dark blue complex film taking place indiscriminately all over the surface, at anodic and cathodic sites, could also account for the mixed type of
character of VM. Solution temperatures at 40 and
60 seemed to cause a stress relaxation of the film
and increased the adherence to the surface providing better protection as indicated in increased
IE%. These results seemed to be in conformity with
some other studies of plant extract where increase
in temperature was reported to have caused an increase in IE% 45, 46. However this argument didn’t
seem to apply for 80 where all concentration of
VM provided smaller IE% than obtained for other
temperatures. In line with these βa and βc increased
at 80 at all VM concentrations tested with respect
KOROZYON, 20 (1-3), 2013 19
Table 6. Kinetic parameters derived from Tafel extrapolation plots, inhibition efficiencies and synergistic parameters of mild steel in blank solution in
the absence and presence of TP, VM and mixture of TP and VM at different ratio of concentrations.
Çizelge 6. TP ve VM içermeyen, her birini tek olarak ve farklı konsantrasyon oranlarındaki karışımlarını içeren dört farklı sıcaklıktaki ana çözeltide
imalat çeliğinin Tafel polarizasyonu kinetik parametreleri, özütlerin koruyucu etkinlikleri ve sinercik parametreleri
T(oC)
25
40
60
80
Ecorr
icorr
x10-4
Rp
(mV/dec) (mV)
(mA/
cm2)
(Ω cm2)
-771
2.00
7661.10
....
12.55
-494
1.63
33483.00
18.50
5.45
6.20
-772
0.77
16356.02
61.50
60
4.80
5.40
-772
0.45
24575.06
77.55
TP,
VM,
βa
βc
ppm
ppm
(mV/
dec)
Blank
Blank
6.55
7.65
1000
0
20.00
0
20
0
%IE
Sθ
0
100
16.60
22.70
-769
39.70
1048.71
-1885.00
1000
20
8.50
8.50
-476
0.15
12639.60
92.70
4.56
1000
60
20.20
23.30
-568
5.10
9212.00
-155.00
0.32
1000
100
8.70
9.50
-446
0.21
93898.41
89.50
220.00
Blank
Blank
16.97
28.80
-777
40.90
1133.64
....
400
0
10.25
9.20
-615
3.76
5598.99
90.80
0
20
23.50
37.10
-791
33.70
1853.73
17.60
0
60
8.30
10.40
-764
8.60
2330.65
78.97
0
100
8.30
9.10
-753
7.43
23368.07
81.83
400
20
6.62
6.87
-648
0.53
27620.59
98.70
5.83
400
60
5.45
6.35
-614
0.31
40948.11
99.23
2.42
400
100
6.60
7.10
-630
0.80
18565.12
98.00
1.23
Blank
Blank
11.60
15.50
-783
60.00
709.92
....
600
0
9.75
9.50
-640
3.17
6590.89
94.72
0
20
10.20
13.15
-795
24.40
1022.25
59.33
0
60
19.18
28.53
-769
66.10
753.43
-10.10
0
100
8.80
10.20
-784
13.30
1542.35
77.83
600
20
4.40
4.20
-637
0.15
62204.00
99.75
8.58
600
60
7.30
6.25
-636
0.49
30083.85
99.18
7.08
600
100
4.60
7.80
-601
0.17
73907.39
99.72
4.18
Blank
Blank
13.35
16.10
-795
94.00
337.13
....
200
0
12.25
13.20
-696
10.30
2678.50
89.00
0
20
14.90
19.00
-794
75.35
481.56
19.89
0
60
20.75
29.60
-794
88.00
601.91
6.38
0
100
21.20
28.60
-784
131.00
403.56
-39.36
200
20
6.00
6.35
-674
0.47
28501.40
99.50
110.00
200
60
9.45
10.70
-691
3.26
6683.88
96.53
2.75
200
100
12.75
13.50
-651
5.65
5039.32
94.00
16.27
20 KOROZYON, 20 (1-3), 2013
to the blank solution and caused increase in icorr.
Results of VM addition indicated that VM interact
on anodic and cathodic part of corrosion reactions
and this interaction could be equally effective on
both corrosion reactions resulting in an increase
in IE% with some exceptions at certain concentrations and temperatures. As indicated in Table 5 and
Table 6, the inhibition efficiency of the TP and VM
was significantly different when tested separately.
Therefore it was interesting to study the synergistic
affect when a blend of TP and VM were used at different temperature and ratio of mixture.
sorbing VM and is measured as surface coverage
for TP in combination with VM. It was noted that the
effect would be synergistic if Sθ > 1, antagonistic if
Sθ<1 45, 48, 49. The value of Sθ more than unity given
in Table 6 suggests an enhanced inhibition efficiency caused by the addition of VM at selected ppm
concentration into the moderate TP concentration
selected for synergistic study. At 25oC, addition of
60 ppm VM into 1000 ppm TP containing solution
yielded antagonistic synergy effect (0.32) with the
worst IE% (-155.00) while 20 and 100 ppm VM addition gave a synergistic parameter 4.56 and 220
respectively. Synergistic parameters calculated at
other selected moderate concentration of TP were
found remarkably high, going as high as 110 for a
mixture of 200 ppm TP+20 ppm VM at 80oC with
IE% of 99.50. Synergistic effect observed for mixture of inhibitors caused enormous increase down
to the range of nano-Amps. Further decrease in icorr
caused by synergy at 60oC in a blend of inhibitors at
VM/TP ratio of 20/600 ppm with the highest IE% of
all (99.75) is displayed in Fig. 3 together with icorr obtained by the individual use of VM and TP inhibitors.
Although there seems to be no systematic change
concerning the decrease in current densities with
increasing temperature, by judging from the Table 6
it is faire to say that under the synergistic condition
the decrease in icorr accompanied by the increase
in IE% irrespective of the temperature. Increase
in IE% with increasing temperature was in good
agreement with the results reported by Oguzie et.
al. who attributed this increase to chemisorption 46.
IE% with the highest and lowest values, as
shown in Table 7, obtained at 60 and 25 respectively will have to be tested at all other temperatures
with the corresponding VM/TP ratios of %3.28 and
%6 to reach a firm conclusion regarding to the appropriate ratio of inhibitors needed for protection.
Inhibition of nitrite ions was attributed to the
formation of ferric oxide film formed as a result of
equation (5). A protective mechanism was put forward to explain the synergistic effects caused by
the VM molecules: Presence of VM in nitrite containing solution seemed to increase the integrity
of the passive films by adsorbing on the tiny local
areas in the film and provide extra protection. Ionic
dissolution at these small active sides might attract the extract’s negatively charged molecules in
3.1.3 Synergistic effect between TP and VM
Synergistic effects between VM and TP inhibitors were studied in 10-4M H2SO4+0.25M K2SO4 at
various temperatures. Inhibition efficiency of TP and
VM were studied individually by Tafel extrapolation
technique and the electrochemical parameters thus
obtained are summarized in Table 5 and Table 6
successively.
There was a significant difference between the
inhibition efficiency of the VM and TP when tested
separately as indicated by Table 5 and Table 6. Inhibition efficiency of TP was found to increase with
increasing temperature and remained more or less
constant around 80-100%. For each temperature
a TP concentration with moderate inhibition efficiency was chosen to study the synergistic effect
of VM at concentration 20, 60 and 100 ppm. The
optimum concentration and the results of the synergistic studies together with inhibition efficiency and
synergistic parameter derived all from polarization
studies conducted at different temperatures are
summarized in Table 6.
Synergistic effect generally observed for corrosion reaction, when mixed inhibitors are used,
causes an increase in inhibition efficiency greater
than that obtained by the use of individual inhibitors. This is related to ion pair interactions between
organic cations and the anions 47. Synergistic effect
is evaluated by denoting a synergistic parameter
calculated according to the equation (6) as follows:
1  è 1  è 2  è 1è 2
(6)
1  è '1 2
where is the surface coverage (IE%/100) by the
adsorbing TP ions, is the surface coverage by adSè 
Table 7. VM/TP and IE% presented with the increasing order of VM/TP values.
Çizelge 7. Artan VM/TP değeriyle KE% koruyucu etkinliğinin değişimi
VM/
TP
IE%
%2
%2.5
%3.28
%5
%6
92.70
99.23
99.75
94.0
-155.0
%10
%10
%10
%15
%16.67
%25
%30
89.5 99.18
99.72
98.70
99.72
98.0
96.53
KOROZYON, 20 (1-3), 2013 21
dynamic characteristics indicating chemisorptions
type of adsorption.
-0,55
-0,60
E(V)
-0,65
-0,70
-0,75
-0,80
Blank
600 ppm TP
20 ppm VM
600 ppm TP
+20 ppm VM
10-8
10-7
10-6
Im(A)
10-5
10-4
Figure 3. Synergistic effect of 600 ppm TP and 20 ppm VM recorded by
Tafel polarization of mild steel at 60oC.
Şekil 3. 600 ppm TP ve 20 ppm VM konsantrasyonlarındaki özütlerin, 60oC
de Tafel ekstrapolasyonu ile ölçülen, imalat çeliği üzerindeki koruyucu
etkinliği.
the vicinity and decrease the ionic diffusion. Since
a surge of ionic flow from these tiny areas is not
assumed, adsorption of abstract’s molecules was
anticipated to repair the film at the defected sites
and reduce icorr further. This is in conformity of the
protective mechanism explained in association with
the passive film formation owing to the nitrite ions.
The lack of full protection by VM alone indicates its
subsidiary protective nature besides nitrite ions under the conditions tested. This is further confirmed
by a full potentiodynamic polarization where critical
current density decreased while ipass, passive current density increased, as will be explained later. Increase in IE% under synergistic conditions showed
the increasing adsorption characteristics at high
temperature which was further proved by thermo-
3.1.4 Kinetic/Thermodynamic Parameters
Kinetic (Ea,k) and thermodynamic parameters
(S0a , H0a) play important role in understanding the
inhibitive and adsorption mechanism of corrosion
inhibitors. Evaluating the temperature dependence
of inhibition efficiency is possible by comparing the
apparent activation energy, Ea, of the corrosion process in the absence and presence of inhibitors.
Kinetic (Ea and k) and thermodynamic parameters (S0a , H0a) obtained are summarized in Table
8. Values of Ea for mild steel in blank solution in
absence and presence of both TP and VM were determined from the slope of logarithm of the corrosion rate Icorr (mm/y) versus 1/T plots according to
the Arrhenius equation (7) as shown in Fig. 4 and
Fig. 5 respectively.
-
(7)
where Ea is the apparent activation corrosion energy; R is the universal gas constant; k is the Arrhenius pre-exponential factor, T is the absolute
temperature and Icorr is corrosion rate obtained
from Tafel extrapolation technique according to the
equation (8) 32.
(8)
-
where Icorr is corrosion rate, EW and r are equivalent
weight and density of working electrode respectively.
The thermodynamic parameters were measured
according to the modified Arrhenius equation (9) as
given below 50, 51, 52;
Table 8. Kinetic (Ea-k and thermodynamic S0a, H0a) parameters for mild steel in 10-4M H2SO4+0.25M K2SO4 at various temperatures in the absence and the presence of different concentrations of TP and VM.
Çizelge 8. TP ve MV katkısız ve değişik konsantrasyon değerlerinde farklı sıcaklıklardaki 10-4M H2SO4+0.25M K2SO4
çözeltisi içinde çeliğin kinetik (Ea-k) ve termodinamik (S0a , H0a) parametreleri.
Inhibitor [C]inh, ppm
Blank
TP
Ea
(kJ mol-1)
K
ΔH0
(kJ mol-1)
ΔS0
(kJ mol-1 K-1)
0
27x10-5
49
46
-0.13
200
6.20
21
18
-0.24
400
424.00
31
28
-0.20
600
15.60
22
19
-0.23
800
639,00
32
29
-0.20
1000
1.70
20
VM
60
100
22 KOROZYON, 20 (1-3), 2013
16
13
-0.25
8
62
60
-0.10
11
6x10
84
82
-0.03
57,00
20
18
-0.22
2x10
where h is the Planck’s constant and NA is the Avogadro’s number; S0a is activation entropy; H0a is
activation enthalpy. Log (Icorr / T) versus 1/T plot
gave a straight lines with a slope of logR/NA.h+
S0a/2.303xR.H0a and an intercepts with log (Icorr/T)
- axis, Values were obtained from the slope and S0a
values were obtained from the intercept.
In many studies the increase in Ea upon the addition of inhibitors were interpreted as a indication of
physical adsorption behaviour of the inhibitor while
a decrease in Ea is assumed as indicative of chemisorptions of the inhibitor 46, 53, 54. In accordance with
this the decrease in the inhibition efficiency with
increasing temperature was attributed to a higher
value of Ea, which when compared to an uninhibited solution was interpreted as an indication for an
electrostatic character of the inhibitor’s adsorption,
namely physisorption 55. According to Zerga et al,
cited in 55, lower value of Ea in an inhibited solution when compared to that of an uninhibited one
was reported to show a strong chemisorption bond
between the inhibitor and the metal. Noor and AlMoubaraki in their study on mild steel in 1-methyl4[4’(-X)-styryl pyridinium iodides and hydrochloric
acid systems reported an increase in both Ea and
H0a and suggested a comprehensive (physisorption and chemisorption) adsorption taking place on
mild steel surface56. According to Riggs and Hurd,
cited in 56, the decrease in apparent activation energy at higher level of inhibition ascribed to a shift
of the net corrosion reaction from that on the uncovered surface to one involving the adsorbed sites
directly.
-0,8
-1,0
-1,2
log Icorr(mm / y)
-1,4
-1,6
-1,8
-2,0
-2,2
-2,4
-2,6
-2,8
-3,0
-3,2
Blank
200 ppm TP
400 ppm TP
600 ppm TP
800 ppm TP
1000 ppm TP
0,0028
0,0029
0,0030
0,0031
(1/T) K-1
0,0032
0,0033
0,0034
Figure 4. Arrhenius plots for mild steel in blank solution in the absence
and presence of TP.
Şekil 4. TP katkısız ve değişik konsantrasyon katkılı ana çözelti içindeki
imalat çeliğinin Arrhenius eğrisi.
log Icorr (mm / y)
(9)
-
-0,8
-1,0
-1,2
-1,4
-1,6
-1,8
-2,0
-2,2
-2,4
-2,6
-2,8
-3,0
-3,2
-3,4
Blank
20 ppm VM
60 ppm VM
100 ppm VM
2,8x10-3 2,9x10-3 3,0x10-3 3,1x10-3 3,2x10-3 3,3x10-3 3,4x10-3
1/ T ( K -1)
Figure 5. Arrhenius plots for mild steel in blank solution in the absence
and presence of VM.
Şekil 5. VM katkısız ve değişik konsantrasyon katkılı ana çözelti içindeki
imalat çeliğinin Arrhenius eğrisi.
The values of Ea decreased with addition of TP
at all concentrations studied but the decrease was
indiscriminate as seen in Table 8. The decrease in
corrosion activation energy, which indicates the endothermic nature of the reaction, may be interpreted as chemisorptions as indicated in [55]. This was
supported by an increase in inhibition efficiency of
TP with increasing temperature as seen in Table 5.
The lower value of Ea in an inhibited solution when
compared to that of an uninhibited one shows that
strong chemisorptions bond between the inhibitor
and the metal is highly probable 57. In this study results regarding to the use of TP is in good agreement with the works reported in the literature 46, 53,
56
. Individual polarization studies with VM added in
blank also showed a increase in Ea at concentrations used except 100 ppm indicating adsorption
of VM by physisorption. However at high concentration of 100 ppm VM adsorption mechanisms
seemed to be chemisorption. There are similar
studies carried out on plant extract indicating a decrease in Ea 46, 58, 59 in weak acidic solution with increasing concentrations.
The positive value of H0a indicates that the adsorption of inhibitor takes place on the basis of an
endothermic mechanism 60, 61, whose process was
attributed to chemisorption implying that the dissolution of steel is difficult 5, 47. In an exothermic process, physisorption is distinguished from chemisorptions by considering the absolute value of H0a
for the physisorption process which is lower than
40 kJ mol−1 while that for chemisorptions process
approaches 100 kJ mol−1 56, 62. Results found in this
study showed that all H0a values found in presence
of both TP and VM were positive indicating endothermic reaction of the process as mentioned elseKOROZYON, 20 (1-3), 2013 23
where 46, 47.
It is generally accepted that the large and negative values of -S0a in the uninhibited and inhibited
systems indicate that the activation complex in the
rate determining steps represents association rather than dissociation step. This means a decrease
in disordering going from reactants to the activated
complex as inhibitor molecules becomes orderly
adsorbed on the metal surface in the absence and
presence of inhibitors in acidic solution 5, 37. As a
result, adsorption on the surface was accompanied
by a decrease in entropy 32, 36. In this work negative
value of activation entropy, S0a were obtained in
both TP and VM inhibited and uninhibited solution
and protection mechanism obeyed endothermic
process.
3.1.5 Adsorption Consideration
Corrosion inhibition provided by plant extracts
as organic inhibitors are caused by adsorption of
inhibitor molecules on metal surface. Adsorption
mechanisms can be due to physisorption or chemisorptions. Chemisorptions of inhibitor molecules
on metals is slow and involves interaction forces
stronger than the forces in physisorption 41.
The adsorption of an organic adsorbate at a
metal/solution interface can be represented as a
substitutional adsorption process between the organic molecules in aqueous solution I(sol) and the
water molecule on the metallic surface H2O(ads) according to the following equation (10) 7, 42, 63;
(10)
where water molecules adsorbed on the surface
exchange with organic molecules.
Adsorption isotherms are the best method to
describe the surface coverage and performance of
the inhibitors. In order to evaluate the adsorption
process of TP and VM on mild steel surface Temkin,
Frumkin, Freundlich and Langmuir adsorption isotherms were tested in order to clarify the adsorption
process of VM on the passive film formed on the
test material according to the following equations
41, 42
.
Temkin :
Frumkin :
Freundlich:
Langmuir :
-
where θ is the surface coverage, K is the adsorption–desorption equilibrium constant, C is the
concentration of inhibitor VM and g is the adsorbate parameter. Among these isotherms tested the
Langmuir isotherm was found to fit the adsorption
data best. Correlation coefficients (R2) obtained for
the isotherm tested are given in Table 9. As can be
seen from this table the highest r2 was measured
for Langmuir isotherm and was very close to 1.
Therefore adsorption of VM follows the Langmuir
isotherm with a slope shown in Fig. 6.
Standard adsorption free energy for inhibitors
can be used to evaluate corrosion behaviour in
presence of inhibitors and can be calculated according to the following equation (15);
(15)
-
where 55.5 is the molar concentration of water, R
is universal gas constant. This formulation could
only be used for inhibitors with a known molecular
weight. This equation is not applicable in this work
or any other works done with total plant extract
since its molecular weight is not known. This point
raised questions in a number of other studies 4, 43.
L. Tang et al. and S.A Umoren et. al. were evaluated adsorption mechanism by adsorption–desorption equilibrium constant. It was reported that the
high values K obtained from Langmuir adsorption
isotherm was attributed to a strong adsorption of
inhibitor ions on the steel surface at localised sites
7, 43, 35
. Langmuir isotherm corresponding to equation (14) was based on the basic assumptions that
(i) molecules are adsorbed at fixed number of welldefined localized sites, (ii) each site can hold one
adsorbate molecule, (iii) all sites are energetically
Table 9. The linear regression coefficient of R2 for Temkin, Frumkin, Freundlich and Langmuir adsorption isotherm.
Çizelge 9. Temkin, Frumkin, Freundlich and Langmuir adsorpsiyon isotermlerinde R2 doğrusal regresyon kat
R2
T(oC)
Langmuir
Temkin
Frumkin
Freundlich
25
0,4169
0,0768
0,2448
0,2752
40
0,9949
0,6714
0,8414
0,6266
60
0,9980
0,6398
0,8458
0,5992
80
0,9833
0,0288
0,2843
0,0892
24 KOROZYON, 20 (1-3), 2013
(11)
(12)
(13)
(14)
3,2
0,8
A
0,7
B
3,0
0,6
2,8
C
0,4
2,6
0,3
log
log( C)
0,5
0,2
2,4
2,2
0,1
y= 18,192x-17,75
2
R =0,0798
0,0
y=70,982x-67,949
2
R =0,5431
2,0
-0,1
0,986
-0,001
0,988
0,990
0,992
0,994
0,986
0,996
0,988
0,990
0,992
0,994
0,996
D
C
1,0
-0,002
0,8
C/ (g/l)
log
-0,003
-0,004
0,4
-0,005
-0,006
0,6
y= -0,0019x-0,0039
2
R =0,0762
-0,8
-0,7
-0,6
-0,5
y=1,005x+0,00017
2
R =0,9999
0,2
-0,4
-0,3
-0,2
-0,1
0,0
log C
0,1
0,2
0,4
0,6
0,8
1,0
C (g/l)
Figure 6: (A) Temkin, (B) Frumkin, (C) Freundlich and (D) Langmuir isotherms for the adsorption of VM on the surface of mild steel in 600 ppm TP containing solution at 60oC.
Şekil 6. 600 ppm TP içeren 60 oC çözeltideki VM nin imalat çelik yüzeyine adsorpsiyonunda (A) Temkin, (B) Frumkin, (C) Freundlich ve (D) Langmuir izotermleri.
equivalent, (iv) there is no interaction between molecules adsorbed on neighbouring sites 45.
In this study results of adsorption isotherms
showed that the best inhibition efficiencies were observed for 600 ppm TP at different concentration of
VM used at 60oC. Hence the highest regression coefficient R2 values obtained for Langmuir isotherm
was ~1. Langmuir isotherm of adsorption model is
shown only for 60oC in Fig. 6D since the Langmuir
plots at 40 and 80oC looked also similar to at 60oC.
3.2 Potentiodynamic anodic
polarization measurements
Acidic solutions, HCl and H2SO4 in particular, are
mostly used corrosive media as reported in literature
regarding to the use of inhibitors. In such cases the
surface of the materials under conditions remains
active and free of the air formed films. Therefore
adsorption of the inhibitors could take place on the
bare surface and protection mechanisms provided
by the adsorbing molecules can be simplified and
explained more judiciously. However air formed film
remains on the surface when tests are conducted
in mildly acidic and alkaline solutions. Thus the adsorption mechanisms under these conditions are
more complex to judge compared to alkaline solutions. Because of this complexity potentiodynamic
polarization tests were undertaken to dissolve the
air formed film on the surface and let the adsorbent
molecules face directly to the unprotected surface
as it is the case in acidic solutions. Potentiodynamic polarization experiments were conducted in
blank, without and with TP and HP as well as with
TP and VM additions. The results are given in Fig 7a
KOROZYON, 20 (1-3), 2013 25
(a)
Blank
Blank+400 ppm TP
Blank+60 ppm HP
Blank+400 ppm TP
+60ppm HP
1,5
1,0
Blank+60 ppm VM
Vf(v)
Vf(V)
0,5
0,0
-0,5
-0,5
-1,0
-1,0
-5
10
Blank+400 ppm TP
+60 ppm VM
0,0
-6
Blank+400 ppm TP
1,0
0,5
10
(b)
Blank
1,5
-4
-3
10
10
-2
10
-1
10
0
10
Im (A)
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
Im(A)
Figure 7. Potentiodynamic polarization in blank solution without and with additions of TP, HP, TP+HP in (a) and TP, VM and TP+VM in (b).
Şekil 7. Ana çözeltide katkısız ve (a) TP, HP, TP+HP katkılı ve (b) TP, VM ve TP+VM katkılı durumlarda potansiyodinamik polarizasyon eğrileri.
and 7b in the corresponding order. Mixtures of both
TP-HP and TP-VM increased Ecorr indicating anodic
character of the mixtures. TP addition to the blank
also increased corrosion potential, Ecorr, into anodic
direction. Potentiodynamic polarization in blank solution revealed an active polarization region before
reaching the critical current density. Thereafter passivation has taken place following a distinct activepassive transition and specimen remained passive
over a range of 500 mV potential as seen in Fig.
7 before pitting process has taken place. Potentiodynamic polarizations curve for blank ( ) and HP
() solutions run very smoothly without any ripples,
where addition of TP(ο) and TP+HP () in Fig. 7a
and VM() and TP(ο) in Fig. 7b displayed ripples
at passive region indicating some disturbances of
the film in passive region. Polarization curve for HP
alone was very smooth while that of VM displayed
ripples. However passive current density, ip, in both
cases increased with respect to the blank solution
whereas critical current density decreased [64].
Passive current density, ip, runs very smooth in case
of HP indicating the stability of the film in passive
state, while striation in ip in case of VM shoved the
disturbed state of the passive film. The striation in ip
was assumed to have created due to the competitive mechanisms of adsorption between TP+HP
and VM alone. Chelating of Fe+2 with some ingredients having high electron density, such as catechin and quercetin present in the extracts, forming
a complex molecules with a closed ring was supposed to intervene the state of passivity by interacting with adsorption mechanism during the transition state. Dark green colouring of the test solution
observed during crossing the transition stage of the
potentiodynamic polarization curve could be ascribed to the formation of complex molecules. This
26 KOROZYON, 20 (1-3), 2013
effect was attributed to the presence of some phenolic acids in extract. E.E. Oguzie et al. found similar results where the instability of the passive state
was attributed to the formation of chelates with the
freshly formed Fe2+ ions on a corroding surface 65.
This effect was profoundly displayed for the mixture of TP+VM solution as indicated by curve () in
Fig. 7b. The increase in ip recorded for synergistic
mixture could also be attributed to the deficiency
of dissolved ions during the transition stage disabling the formation of passive films. High critical
current density was thought to dissolve Fe+2 ions
high enough to form the passive films as it reached
to the state of saturation. Thus the decrease in critical current density, icr, creates Fe+2 ions not enough
to promotes passive film formation resulting in high
ip.Critical parameters recorded for potentiodynamic
polarizations curve are shown in Table 10.
4. CONCLUSION
A screening test for metal extracts to differentiate their active ingredient is required to evaluate
their inhibition characteristics as quickly and efficiently as possible. Measuring the capacity of
electron density available for the interaction with
the metal surface to take place could stimulates
scientists and industrialists for their future works on
plant extracts. FTIR spectra could be a simple tool
to look into the matter.
It is advised to prepare a database consisting of
the main characteristics of the common ingredients
found in the extracts; some parameters such as dielectric constant of solvents used for extraction and
the characteristics of the known flavonoids with a
high antioxidant activity could be used for a computer modelling in conjunction with FTIR analysis.
Table 10. Some critical attributes obtained for potentiodynamic polarization curves.
Çizelge 10. Potansiyodinamik polarizasyon eğrilerinde bazı kritik özellikler.
Anodic Polarization
OCP
(mV)
icrit
(mA/cm2)
Blank
Blank + 400 ppm TP
Blank + 20 ppm HP
Blank + 60 ppm HP
Blank + 100 ppm HP
Blank+400 ppm TP+20 ppm HP
-799
-620
-785
-790
-780
-627
-633
-634
316
272
304
313
282
233
221
225
181
937
651
581
7214
706
747
511
667
661
571
571
***
750
744
488
Blank+ 20 ppm VM
Blank+ 60 ppm VM
Blank+ 100 ppm VM
Blank + 400 ppm TP + 20 ppm VM
Blank+ 400 ppm TP + 60 ppm VM
Blank+ 400 ppm TP + 100 ppm VM
-793
-787
-783
-635
-631
-620
262
205
157
246
114
60
300
364
950
377
15150
24040
625
651
720
429
***
***
Concentration (40oC)
The inhibition efficiency (IE%) of nitrite based
inorganic inhibitor (TP) and blueberry (VM) plant
extract was studied in de-aerated solutions in
separate and mixed states and the following results
were found:
 Presence of TP in blank solution increased IE%
(80-95%) at all temperatures (40, 60 and 80) except 25. βa and βc decreased at all temperatures
indicating a decrease in current density. Protection provided by TP is ascribed to the formation
of passive films composed of by accelerating
anodic metal dissolution primarily which then
promotes the formation of oxide film.
 Results of electrochemical studies with VM indicated that addition VM interacted on anodic and
cathodic part of corrosion reactions and displayed the character of a mixed type inhibitors.
A correlation between the inhibition efficiencies
and βa and βc were found to have inversely related to each others. Small changes in βa and βc
with no noticeable trend upon the concentration
was accompanied by small changes in Ecorr indicating VM as a mixed type of inhibitors
 Individual polarization studies with VM added
in blank also showed a increase in Ea at concentrations used except 100 ppm indicating
adsorption of VM by physisorption. However at
high concentration of 100 ppm VM adsorption
mechanisms seemed to be chemisorption.
 IE% of TP+VM blend was found to be greater
ipass
(mA/ cm2)
Epp
(mV)
than that of individual TP and VM on mild steel
in solution. However the synergistic effect was
found to occur at all temperatures and mixtures
of TP and VM concentrations except (1000 ppn
TP+60 ppm VM) where an antagonistic effect
(Sθ=-155) was observed. The highest synergistic parameter (Sθ=220) corresponding to an inhibition efficiency of (89.50%) was obtained at
25oC with (1000 ppm TP+ 100 ppm HP) blend.
This result complies with nature of physisorption. Thermodynamic and kinetic parameters
indicated that adsorption of (TP+HP) blend in
solution is chemisorptions and follows the Langmuir isotherm. The best inhibition efficiencies
were observed for 600 ppm TP at different concentration of VM used at 60oC though IE% found
at other temperatures were very close to this efficiency.
Acknowledgements
Authors would like to express their appreciation and thanks to The Ministry of Science, Industry and Technology and Günsu A.Ş, manufacturers
of household and industrial cleaning products and
water treatment chemicals, in Turkey for their support provided by within the frame of research projects No (STZ005282010-1). Authors also thank to
the Department of Metallurgical and Materials Engineering at Dokuz Eylül University for letting use
the laboratory facilities. Our special thanks go to
KOROZYON, 20 (1-3), 2013 27
Dr. Iskender Ince, Ege University, Argefar Research
Centre for preparing the plant extracts used in this
work.
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AUTHORS
Alper Turhan, Buğra Karahan, Aylin Albayrak, Ahmet Çakır
Dokuz Eylul University, Faculty of Engineering,
Department of Metallurgical and Materials Engineering,
35160 Buca-Izmir/TURKEY
Hakan Ekıncı
Gunsu A.Ş., OSB 1. Kısım, Ataturk Bulvarı No.1
Antalya/ TURKEY
Yazarlarla iletişim için: [email protected]
KOROZYON, 20 (1-3), 2013 29
EFFECT OF MgO PARTICLE SIZE ON CORROSION
BEHAVIOUR OF ALUMINUM –MgO COMPOSITES
IN AQUEOUS 3.5% M NaCl SOLUTION
ABSTRACT
The effect of MgO particle size on corrosion behaviour of Aluminum-MgO metal
matrix composites (MMC) was investigated. Two different MMC composites,
with mean diameters of 180 and 250 μm
MgO, were prepared by vacuum infiltration technique. The corrosion behaviour
of the composites was examined using dynamic polarisation, Tafel and impedance
techniques in 3.5 % NaCl aqueous solution. The microstructures of the composites were investigated by SEM and optic
microscopy. Results show that the MgO
reinforcement is effective in improving the
corrosion resistance of MMCs compared
to Al matrix.
ALÜMİNYUM-MgO KOMPOZİT
MALZEMESİNİN KOROZYON
DAVRANIŞI ÜZERİNE %3,5 M NaCl
ÇÖZELTİSİNDE MgO PARTİKÜL
BÜYÜKLÜĞÜNÜN ETKİSİ
Alüminyum-MgO kompozit (MMC)
malzemesinin korozyon davranışı üzerine MgO partikül büyüklüğünün etkisi incelendi. Çapları 180 and 250 μm MgO içeren iki farklı MMC kompoziti vakum infiltrasyon tekniği ile hazırlandı. Bu kompozitlerin korozyon davranışı %3,5 NaCl çözeltisinde dinamik polarizasyon, Tafel ve empedans teknikleri kullanılarak çalışıldı. Kompozitlerin mikro yapısı SEM ve optic mikroskop ile incelendi. Sonuçlar MgO takviyesinin MMC’lerin korozyon direncini al matriksle karşılaştırıldığında etkin şekilde artırdığını gösterdi.
1. INTRODUCTION
Research on the mechanical and corrosion properties of
aluminium matrix composites
(MMCs) is still at the development
stage, but the outlook is very
30 KOROZYON, 20 (1-3), 2013
promising. In the next 20 years,
these materials are expected to
take over the conventional material such as Al base alloys. In recent
years the aerospace, military and
the automotive industries have
developed new composite materials to achieve good mechanical strength/density and stiffness/
density ratio5, 6.
The developing metal matrix
composites have been significantly increased7. Aluminum alloy
matrix composites attract much
attention due to their strength,
lightness, moderate casting temperature, etc.8,9. Various kinds of
these materials, for example SiC,
Al2O3 and MgO, are used to reinforce aluminum alloy matrices.
Attractive properties of these materials such as high hardness, refractoriness, high strength, resistance, etc. make them suitable for
use as reinforcement in matrix of
composites10–17.
The incorporation of a second
reinforcing phase into aluminum
can enhance the physical and
mechanical properties of materials and alter their corrosion
behaviors. The presence of reinforcement may or may not increase the material’s susceptibility to corrosion, depending on the
metal-reinforcement combination
as well as processing parameters
involved18.
The addition of reinforcement
A. AYTAÇ
A. E. SANLI
F. GÜL
M. USTA
particles could significantly alter
the corrosion behaviour of these
materials. There has been significant amount of research on the
evaluation and optimization of the
mechanical behaviour of MMCs19,
20
. Instead, published literature on
the corrosion of aluminium-based
composites is rather limited and
often contradictory. This is due
to the fact that there is a variety
of aluminium alloy matrix and reinforcement type combination,
which may exhibit completely different corrosion behaviour.
One of the main obstacles to
the use of MMCs is the influence
of reinforcement on corrosion resistance. This is particularly important in aluminium alloy based
composites, where a protective
oxide film imparts corrosion resistance. The addition of a reinforcing phase could lead to further
discontinuities in the film, increasing the number of sites where corrosion can be initiated and rendering the composite liable to severe
attack 21-23.
The aim of this work was to investigate corrosion behaviour of
infiltrated aluminium matrix composites containing high volume
fraction of MgO particles with 180
and 250-μm particle sizes by the
use of dynamic polarization and
impedance techniques in 3.5%
NaCl aqueous solution. The microstructure of the MMCs was
Table 1. Chemical composition of Al alloy (wt.%)
Çizelge 1. Alüminyumun alaşımının kimyasal bileşimi (ağ..%)
Si
Fe
Cu
Mn
Mg
Cr
Ni
Ti
Pb
Al
9.42
0.38
0.05
0.431
0.36
0.015
0.04
0.10
0.01
Balance
investigated by means of optical microscopy and
scanning electron microscopy (SEM). The specimens were also subjected to energy dispersive Xray analysis (EDAX).
2.
EXPERIMENTAL
2.1. Preparation of composites
MgO particulate-reinforced aluminium alloy
were fabricated by the vacuum infiltration technique, two different size of MgO powder (180 and
250 μm) and Al were incorporated. The chemical
composition of Al alloy and MgO particles used in
this work determined by spectrographic analysis,
are given in Table 1 and 2, respectively.
Table 2. Chemical composition of MgO particles (wt.%)
Çizelge 1. MgO partiküllerinin kimyasal bileşimi (ağ..%)
MgO
SiO2
CaO
Fe2O3
Al2O3
96
1-3
1-2
0,3
0,1
The MgO reinforced Al alloy composite specimen was prepared by the use of a steel tube with
8 mm inner and 10 mm outer diameter and 300
mm length. The bottom of steel tube is closed by
stainless steel filter and Al foil. MgO reinforcement
particle powder was poured into steel tube. Special
vibration equipment was used to obtain homogeneously compact particle. Two stainless steel filter
and alumina mat were used to prevent leakage of
molten metal into the vacuum unit23.
Abrasive grade MgO particles with mean diameters of 180 and 250 μm were used as the reinforcement. The matrix of composites was Al alloy,
while the volume fraction of the MgO particles was
50±5%. The composites were designated as C180
and C250, respectively.
2.2. Microstructure and Surface Morphology
The Al alloy and MMCs were examined with
optical microscopy before and after the corrosion
test to determine the morphology. Specimens for
microscopic observations were prepared by polishing down with emery papers with successively
increasing grits up to 1200 grit and finally the surface was mirror finished by 3 μm diamond paste.
The microstructure of the MMCs was investigated
by means of optical microscopy and scanning
electron microscopy (SEM). The specimens were
also subjected to energy dispersive X-ray analysis
(EDAX).
2.3. Polarization Measurement
Potentiodynamic polarization measurement and
electrochemical impedance measurements were
carried out in a conventional three electrodes electrochemical cell. The counter and reference electrodes were a platinum plate (2cm2) and Ag/AgCl
electrode, respectively. Working electrodes (1.00
mm diameter) was sealed in Teflon holders and the
surface of the of Al specimen electrodes was mechanical polished to a mirror finish prior to each experiment with successive grades of emery papers
down to 1200 grit. The electrodes were then rinsed
with acetone, distilled water and dried at room temperature.
The polarization measurements were carried
out in a 3.5 wt % NaCl solution at 25 oC in a Pyrex
glass cell exposed to atmospheric air. A Volta Lab
40(PGZ 301 Dynamic-EIS Voltammetry) potentiostat was used in all experiments. The potential vs.
current density curves were recorded with a scan
rate of 5 mV s-1. The exposed area of the samples
was 1.13 cm2.
2.4. Impedance Measurement
The impedance measurements were carried out
in the frequency region of 50 mHz to 20 kHz, taking
five points per decade for different time. The real
(Z’) and imaginary (Z”) components of the impedance spectra in the complex plane were analyzed
using the Circular Regression analyzer of the Volta
Lab 80 potentiostat. The EIS measurements were
conducted after 20 min immersion in experimental
solution to ensure a system to be in equilibrium.
The samples were prepared in the same way for
microstructure evaluation and polarization measurement.
3.
RESULTS AND DISCUSSION
3.1. Microstructure
The light microscopy photographs of the Al Alloy, C180 and C250 after the polishing and corrosion in 3.5 % NaCl taken by the metal microscope
KOROZYON, 20 (1-3), 2013 31
Figure 1. The metal microscope photographs of Al alloy before(a) and after in 3.5 wt% NaCl solution for 20 days (b) the corrosion.
Şekil 1. Al alaşımının korozyondan (a) önce ve (b) %3,5 NaCl çözeltisinde 20 gün sonra metal mikroskobu fotoğrafları.
Figure 2. The metal microscope photographs of C180 composite before(a) and after in 3.5 wt% NaCl solution for 20 days (b) the corrosion.
Şekil 2. C180 kompozitinin korozyondan (a) önce ve (b) %3,5 NaCl çözeltisinde 20 gün sonra metal mikroskobu fotoğrafları.
are given Figures 1, 2 and 3. The matrix contained
geometrically shaped MgO particles. The C180
composite presented a microstructure similar to
the C250 material, with MgO particles. After the material surfaces tested in contact with 3.5 wt% NaCl
solution for 20 days, the light surface structure of
the Al alloy and MMCs became darker than before
and show a significantly more altered surface (Figure 1, 2 and 3). The surface is grey/black with some
particles which can be easily detached. This nonuniform black film is consistent with Aluminum and
white MgO particles that reported by Williams and
McMurray24.
32 KOROZYON, 20 (1-3), 2013
In order to understand the surface structure
of MMCs before and after in contact with 3.5 wt%
NaCl solution for 20 days, SEM pictures and EDAX
analysis of the C180 are given in Figures 4 and
5. From the figure 4, EDAX analysis examined on
white region (1) of specimen showed that high Mg
content. EDAX analysis of the black region (2) of
this specimen showed that this region was bulk layer containing more amounts of Al. Figure 5 show
the morphology of the material surfaces tested in
contact with 3.5wt% NaCl solution for 20 days. In
figure 5 EDAX analyses of the black region (1) note
that the Al2O3 protective layer is completely covered
Figure 3. The metal microscope photographs of C250 composite before(a) and after in 3.5 wt% NaCl solution for 20 days (b) the corrosion.
Şekil 3. C250 kompozitinin korozyondan (a) ona ve (b) %3,5 NaCl çözeltisinde 20 gün sonra metal mikroskobu fotoğrafları.
Figure 4. a) SEM picture and b) EDAX analysis for the region of 1 and 2 of the C180 after the polishing.
Şekil 4. Parlatma işleminden sonra C180’in a) SEM fotoğrafları ve b)EDAX analizleri.
KOROZYON, 20 (1-3), 2013 33
Figure 5. a) SEM picture and b) EDAX analysis for the region of 1 and 2 of the C180 after the corrosion in 3.5 % NaCl (20 days).
Şekil 5. 20 gün %3,5 NaCl çözeltisinde korozyon ardından C180’in a) SEM fotoğrafları ve 1 ve 2 numaralı bölgelerin EDAX analizleri.
the surface. The region (2) of specimen showed
that high Al2O3 content. These results showed that
the MgO provides sacrificial protection to the Al
substrate. The C250 were shown similar results by
SEM and EDAX analyses.
3.2. Effect of MgO particle size on corrosion
resistance of MMC
Polarisation curves for matrix, C180 and C250
are given in Figure 6 .It is seen that the area between the forward and reverse scans for the matrix
is quite large. This shows the fact that passive layer
film is loose and easily removed from the surface
making the material prone to the corrosion process.
This area decreases for the composites indicating a
compact and strong protecting passive film.
34 KOROZYON, 20 (1-3), 2013
Figure 7 shows the Tafel plots of the materials
investigated. It is seen that the corrosion potential
shows an anodic shift from C180 to C250 composite. The corrosion potential (Ecorr), corrosion current
density (icorr) and  a,  c slopes extrapolated from
the Tafel curves and the electrochemical parameters obtained from the polarization curves are given
Table 3.
According to the corrosion potentials, currents
and cathodic Tafel slopes, the C180 and C250 display different corrosive properties. The corrosion
potential of C250 is at more negative value, which
indicates a higher corrosion rate as seen from higher corrosion current. This behavior of C250 can be
attributed to the size of MgO particles.
MgO is sparingly soluble in water and the pH
Figure 7. The Tafel slops of C180 and C250 in 3.5% NaCl at 25oC with
scan rate of 5mVs-1.
Şekil 7. C180 and C250 kompozitlerin %3,5 NaCl çözeltisinde 25oC,
5mVs-1 tarama hızında elde edilen Tafel eğrileri.
Figure 6. Cyclic polarization of Al matrix, C180 and C250 in 3.5% NaCl
at 25oC.
Şekil 6. Al, C180 ve C250’nin %3,5 NaCl çözeltisinde 25oC alınan döngüsel polarizasyon eğrileri.
value of its saturated solution is around 12. The
C180 composite supported by the smaller sized
MgO particles gives the following reaction with water much easily and the surface becomes saturated
by Mg(OH)2. The coverage of Al surface with MgO
is much wider and the contact surface of these alloys with water is much larger. The oxide layer may
therefore not be totally protective, but it can nevertheless provide some degree of barrier protection25-27.
MgO + H2O → Mg(OH)2 ↔ Mg2+ + 2 OH
Figure. 8. Variation of corrosion potential of Al matrix, C180 and C250
with time in 3.5% NaCl at 25oC.
Şekil 8. Al matriks, , C180 ve C250 kompozitlerin %3,5 NaCl çözeltisinde
25oC’de zamana bağlı korozyon potansiyelleri.
If we consider that the potential shows 60 mV cathodic shift per one unit increase in pH the reaction
(1) is expected to take place at 250 mV more negative values than the C180. The higher pH values of
the surface film will alter the hydrogen evolution
mechanism and one finds differing Tafel slopes27.
Open circuit potentials for these composites are
(1)
Table 3. The electrochemical parameters obtained from the polarization curves.
Çizelge 3. Polarizasyon eğrilerinden elde edilen elektrokimyasal parametreler.
Electrode
icorr
-Ecorr (V)
(

-2
A.cm
)
 a (mV/
 c (mV/
Rp (kohm.cm2 )
C
(  F cm2)
dec)
dec)
C 180
0.925
0.24
44
99
20.5
4.0
C 250
1.170
0.55
185
92
12.5
8.3
KOROZYON, 20 (1-3), 2013 35
Figure 9. The initial Nyquist diagram of the Al matrix, C180 and C250 at
3.5 % NaCl medium
Şekil 9. Al matriks, C180 ve C250’nin %3,5 NaCl çözeltisinde 25oC’de
elde edilen başlangıç Nyquist eğrileri.
Figure 10. The Nyquist diagram of the matrix, Al matrix, C180 and C250
after 60 min immersion at 3.5 % NaCl.
Şekil 10. Al matriks, C180 ve C250’nin %3,5 NaCl çözeltisinde 25oC’de
elde edilen 60. dakikadaki Nyquist eğrileri.
given in figure 8. At the beginning it started from
a more anodic potential and then fluctuated until
a stable value was attained. The initial potential
of Al matrix, C180 and C250 was -1.06, -0.90 and
-0,82 orders. This initial phase was probably due
to the activation of the sacrificial protection, which
requires both the penetration of electrolyte to the
surface and the dissolution of the MgO from the
particle surface.
e are the electrical permittivity of the vacuum and
the oxide layer. According to the table 3 the capacity values decrease from C250 to C180. We can
therefore conveniently conclude that the aluminum
oxide layer on the surface thickens by the time.
These results demonstrate that MgO dissolution occurs in the beginning under the conditions
of our experiments. Nevertheless, cathodic activation of the C250 is clearly observed in chloride by
an increasing corrosion rate between potentiostatic
pulses. The corrosion current increased from 0.24
 A/cm2 to 0.55  A/cm2 . The existence of small
size of MgO contributes to better corrosion resistance and decrease of contact area between the
substrate and corrosion media.
3.3. Effect of MgO particle size on
Impedance Measurements of MMC
In order to prove the surface dissolution the
impedance measurements of the three electrodes
were taken at the time of the immersion and 15 and
60 minutes after the immersion. Figure 9 and 10
shows the Nyquist diagrams of the Al matrix, C180
and C250 obtained at open circuit potential (OCP).
The impedance curves obtained at the time of
the immersion shows uncompleted arcs at higher
frequencies and a Warburg impedance at lower frequencies. This can be explained as follows: At the
initial state there is a barrier film formed in air upon
the surface. This gives an uncompleted arc. The
hydrogen evolution as a result of reaction (1) and
diffusion controlled removal of OH- ions from the
surface result a Warburg impedance. The change
of the polarization resistance (Rp), which was taken
as the resistance of the surface film, found by the
circular regression analysis of the Volta Lab80 software and the electrolyte/electrode interphase (double layer capacitance, C) with time are shown in
Table 3. The relation between the capacity and the
thickness of the aluminum /aluminum oxide layer is
C
 0
4d
(228)
The thickness of the oxide is shown as d. e0 and
36 KOROZYON, 20 (1-3), 2013
Table 4. The change of the polarization resistance and the double layer
capacitance with time.
Çizelge. Tamana bağlı polarizasyon direnci ve çift tabaka kapasindeki
değişim.
60 min
Double layer
capacitance
 F cm2
60 min
Matrix
8.3
3.84
C 250
12.5
4.0
C 180
20.5
8.3
Polarization
resistance
Rp (kohm.cm2 )
4. CONCLUSIONS
The corrosion behaviour of infiltrated aluminium
matrix composites containing high volume fraction
of MgO particles with 180 and 250-μm particle sizes
by the use of dynamic polarization and impedance
techniques was investigated. According to the results obtained\ the following conclusions can be
drawn:
1. According to the surface morphology of the
material surfaces tested in contact with 3.5wt%
NaCl solution for 20 days, results showed that
the surface is grey/black with some particles.
The original Al2O3 protective layer is covered the
MMCs. These results showed that the MgO provides sacrificial protection to the Al substrate.
2. The corrosion behaviour of MMCs was studied and compared using Tafel polarization and
electrochemical impedance spectroscopy techniques. The C180 composite supported by the
smaller sized MgO particles. Since the surface
of MgO is much wider for C180 than C250, so
C180 was given more interaction with water. According to the corrosion potentials, the C180
and C250 display different corrosive properties. The corrosion potential of C250 is at more
negative value, which indicates a higher corrosion rate as seen from higher corrosion current.
The corrosion resistance order of MMCs in 3.5%
NaCl was C250>C180.
3. In conclusion, the analysis of potentiodynamic
polarization, EIS and appropriate equivalent circuit models reveal that the MMCs corrosion is
protected with 180 μm of MgO particles.
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AUTHORS
Aylin AYTAÇ,
Gazi University, Faculty of Science, Department of Chemistry,
06500 Teknikokullar, Ankara, Turkey
A. Elif SANLI
Turgut Ozal University, Engineering Faculty, Department of
Electric-Electronics Engineering, Ankara, Turkey
Ferhat GÜL, Metin USTA
Gazi University, Technical Education Faculty,
06500 Teknikokullar, Ankara, Turkey
Department of Materials Science and Engineering,
Gebze Institute of Technology, Gebze, Kocaeli 41400, Turkey
KOROZYON, 20 (1-3), 2013 37
PASSIVATION OF NANO-SILVER ELECTRODE SUPPORTED
ON CARBON FIBER USING FOR APPLICATION OF DIRECT
BOROHYDRIDE/PEROXIDE FUEL CELLS
ABSTRACT
In this study, nano-silver deposited
carbon fiber catalyst (nano-Ag/CF) prepared by electroless deposition method.
Electrocatalytic activities of nano-Ag/CF
was investigated during the oxidation
of sodium borohydride in alkaline solutions and compared with metallic silver
electrode. The nano-Ag/CF catalyst was
characterized by X-ray diffraction (XRD),
scanning electron microscopy (SEM)
and cyclic voltammetric analysis. The
results of cyclic voltammetry show that
nano-Ag/CF was deactivated in alkaline
sodium borohydride solution caused by
the carbonate formation. This formation
of silver carbonate was detected on the
surface by the X-ray diffraction. Preliminary tests on a single cell of a direct borohydride/peroxide fuel cell with metallic
Ag and nano-Ag catalysts indicate that
metallic Ag with the power density of 5.1
mW.cm-2 provides higher performance
than nano-Ag/CF (2.2 mW.cm-2).
DİREK BORHİDRÜR/PEROKSİT
YAKIT HÜCRESİ UYGULAMALARI
İÇİN KULLANILAN KARBON LİF
DESTEKLİ NANO-GÜMÜŞ ELEKTRODUN PASİFLEŞMESİ
Bu çalışmada karbon lif destekli nano-gümüş elektrot (nano-Ag/CF)
akımsız çöktürme yöntemi ile hazırlandı.
Nano-Ag/CF katalizörün elektrokatalitik
aktifliği alkali çözeltide sodyum borhidrürün oksidasyonu boyunca incelendi ve
metalik gümüşle karşılaştırıldı. Nano-Ag/
CF katalizör X-ışını kırınımı (XRD), taramalı electron mikroskobu (SEM) ve dönüşümlü voltametri yöntemi ile karakterize edildi. Dönüşümlü voltametri sonuçları, alkali sodium borhidrür çözeltisinde karbonat oluşumu ile nano-Ag/CF
katalizörün aktifliğini kaybettiğini gösterdi. Yüzeyde gümüş karbonat oluşumu
X-Işınları kırınımı yöntemi ile tespit edildi.
38 KOROZYON, 20 (1-3), 2013
Ag ve nano-Ag katalizör kullanılarak tek
hücreli doğrudan borhidrür/peroksit yakıt hücresilerinin ön testleri metallic Ag
ün 5.1 mW.cm-2 güç yoğunluğu ile nanoAg’den (2.2 mW.cm-2) daha yüksek performans gösterdiğini göstermiştir.
1. INTRODUCTION
The use of aqueous solution
of NaBH4 as a hydrogen carrying medium and a fuel for fuel
cells is relatively a new development which would reduce the
problem of pressure storage
vessels containing hydrogen
externally. Another advantage
of using the borohydride in the
fuel cells is that the theoretical open circuit potential of the
DBFC is higher than that of the
DMFC and PEMFC 1, 2. The main
issues for DBFCs are the hydrogen generation and incomplete
electro-oxidation of BH4- at the
anode that results in the hydrogen evolution. Generation of the
hydrogen not only reduces the
efficiency but also causes problems in the design and safety of
the cell 1, 3.
In recent years the use of hydrogen peroxide instead of oxygen as an oxidant is an attractive
choice for the fuel cell applications and is comparatively safe,
stable, easily handling and nontoxic. DBFC employing H2O2 is
known as direct borohydride/
peroxide fuel cell (DBPFC) and
also operates at higher voltages
A. AYTAÇ
A. E. SANLI
compared to the DBFC fed with
oxygen. A DBPFC provides a
theoretical cell potential of 2.11
V by using the suitable anode
that prevents the hydrolysis of
borohydride and the cathode
that leads to the direct reduction of H2O2 without the oxygen
generation 4-7.
For DBFC: BH4- + 2O2
For DBPFC: BH4- + 4H2O2
2
-
+ 2H2O
2
-
E0cell= - 1.64 V
+ 6H2O E0cell= - 2.11 V
DBPFCs as power sources
have attracted increasing attention for the air ties applications
such as space and underwater
applications. Great efforts have
been devoted to the development of fuel cell electrocatalysts
with a focus in increasing their
electrocatalytic activity and reducing the noble metal content.
The development of DBPFC
is prevented by the poor anodic
efficiency of borohydride and
the hydrolysis of borohydride.
In recent years most researches
focus on developments of the
anode electrocatalyst having a
low catalytic activity for the hydrolysis of borohydride. The restriction of the hydrolysis reaction is a key point in increasing
the cell efficiency. The anode
catalyst should be inactive towards the hydrolysis of borohydride. Au and Ag as inactive catalysis towards the hydrolysis of
borohydride, are the most active catalysis but have
relatively low activities8-13. Au shows better electrocatalytic activity as an anode material if its particles
are nanosized14-16. In another study Ponce-de-Leon
et al. deposited Au on titan dioxide by the ion exchange method. During the oxidation of borohydride, nanosized Au/TiO2 electrode had promising
catalytic effect and higher electrical charge compared to the commercial Au/C17. In our previous
study, it was verified that according to the following
reaction, Ag oxides (Ag2O) has a catalytic effect on
the oxidation of borohydride as follows 8.
Ag2O + BH4- + 6OH- → 2Ag + BO2- + 5H2O +6eIn the case of nano-Ag, Chatenet et al investigated the electrochemical behavior of nano silver
catalysis. For this purpose they used the commercial carbon supported Ag (Ag/C from E-Tech) and
nano dispersed on carbon platinum (Pt/C) Pt-Ag
binary metal alloys. With Ag/C electrode, borohydride oxidation reaction (BOR) onset was shifted to
the negative with respect to the bulk Ag. This was
explained in the positive particle size effect and the
easier oxidation of nano-Ag particles to the silver
oxides than the metallic Ag 18.
In this study we aim to investigate the nano-Ag
deposited on carbon fiber as an anode catalyst in
the DBPFCs. Taking into account that there are no
previous comparison studies on the carbon support materials in DBPFCs, we used the carbon fiber as the support material for the deposition of
the nano-Ag particles in order to make comparison
with metallic Ag catalyst. Carbon fiber (CF) known
as graphite fiber was being used as a catalyst support material because of the unique structure of
the carbon fiber. Moreover electroless deposition
technique used in this study is a simple and cheap
method compared to the other synthesis methods
to make the nano phase. In our previous paper we
described the catalyst preparation method by the
electroless deposition technique and the effect of
the catalyst prepared on the performance of the
fuel cell 19. The nano-Ag/CF was electrochemically
investigated in the basic borohydride solution and
tested in the Direct Borohydride/peroxide fuel cell.
The Results obtained, show that nano-Ag deposited on the graphite was deactivated in the oxidizing media20, 21
2. EXPERIMENTAL
2.1 Preparation and characterization of nano-Ag/CF
The nano-Ag catalyst was deposited onto the
carbon fiber by using the electroless deposition
technique. This technique was described else-
where in details19. An area of 4.00 cm2 (0.0339
grams) of carbon paper was immersed into 10 ml,
0.4M HCl and the slurry was sonicated for 30 min
(70oC). Then, 0.3980 g of AgNO3 was added to 0.4
M, 10 mL HCl solution and the mixture was stirred.
Then, 5 ml of ammonia was added in 1 ml portions
with stirring to reach a final pH of 11. 1.0M NaBH4
was prepared in alkaline solution and slowly added
to the baker. It was assumed that, at this point, all
the metal ions which were dissolved in the solution
had been reduced and deposited on the substrate.
Carbon fiber was washed with distilled water in order to prevent reduction of the oxidation reduction
reaction (ORR) activity and was dried afterwards.
The carbon electrodes were characterized using a
field emission scanning electron microscope (SEM,
Joel). For the elemental analysis of the samples Xray diffraction (RIGAKU, D/MAX-2200 Model) was
used.
2.2. Electrochemical characterization
Cyclic voltammetry tests were performed in a
three compartment glass cell. The glassy carbon
pellet electrode was placed into a Teflon holder
that was 0.40 cm2 diameter with the edge surfaces
attached with Ag deposited carbon fiber. Pt plate
(0.5 cm2) was used as a counter electrode and
SCE (Saturated Calomel Electrode) in a Luggin
capillary compartment, as a reference electrode.
All the potentials in this manuscript are relative to
that of the SCE reference electrode. During the experiments the electrochemical cell was filled with
0.1MNaBH4+1M NaOH solution as fuel All electrochemical experiments were carried out with the use
of a Gamry Instrument potentiostat at room temperature (25±3oC). The cell was cycled between -1.2
and 1.2 V at a 100 mV/s sweep rate. The voltammograms were reproducible from the third scan on.
2.3 Preparation and construction of the single cell
In this study metallic Ag electrode and nano-Ag/
CF electrode was tested as the anode catalysis in
the fuel cells in order to make a comparison. The
100 mg/cm2 of Pt/C (5%, from Aldrich) was loaded
for cathodes. Pt/C powders were mixed with Nafion
solution (5%, from Aldrich) and ethanol solution in
the ultrasonic bath. The resulting ink was applied
on the carbon paper and dried at room temperature. The anodic side of the cell was prepared as
described above. For these fuel cells, the membrane electrode assembles (MEAs) were manufactured by assembling three compounds of the anode, cathode and nafion-117 membrane (Aldrich)
by the hot-pressing at 150 oC and under the presKOROZYON, 20 (1-3), 2013 39
The micrographs presented in Figure 1 displays
the morphology of the nano-Ag/CF by SEM. It is
observed from this figure that the carbon fiber surface was deposited by nano-Ag articles which are
highly dispersed and homogen in shape. The XRD
diffractogram of Ag nanoparticles on CF electrode
prepared at optimum condition is shown in Figure
2. Typical XRD patterns of the sample are shown
where the peaks at 38.18°, 44.39°, 64.58°, and
77.54° are assigned as the (111), (200), (220), and
(311) reflection lines of fcc Ag particles (JCPDS file,
No. 04-0783). The peak around the 26.603o in the
pattern was indexed to graphite.
Figure 1. SEM images of surface of the nano-Ag supported CF catalystby
the electroless deposition technique.
Şekil 1. Akımsız çöktürme yöntemi ile hazırlanan CF destekli nano-Ag
katalizörün SEM fotoğrafı
sure of 1.2 psi.
In this study, two fuel cells constructed with
metallic Ag powder and nano-Ag/CF were tested.
The cell with metallic Ag was called as MC (Metallic cell) and the cell constructed with nano-Ag/CF
was called NC (nano cell). Experiments were carried out in a single cell made from plexy glass with
two containers for anolite and catholite described
before19. The cell performances were evaluated by
measuring the current density against cell voltage
(I-V) characteristics using Gamry Instrument. For
the evaluation of the cell performance, the basic
borohydride solution as the fuel and the acidic peroxide solution as the oxidant were fed to each compartment and the I-V characteristics were measured
as well.
3.
RESULT AND DISCUSSIONS
3.1. Characterization of the nano-Ag/CP electrode
Figure 2. XRD pattern of the nano-Ag supported carbon fiber (CF) deposited by the electroless deposition technique ( ■ Graphide ▼ Ag)
Şekil 2. Akımsız çöktürme yöntemi ile hazırlanan CF destekli nano-Ag
katalizörün XRD analizi ( ■ Graphide ▼ Ag)
40 KOROZYON, 20 (1-3), 2013
3.2. Electrochemical Investigation of
nano-Ag/CF Electrode in 1M NaOH solution
Figure 3a and b shows multi-cycles of the metallic Ag and nano-Ag/CF electrodes, respectively.
The overall oxidation process involves a successive
formation of Ag2O and AgO according to the reactions:
2Ag + 2OH-  Ag2O + H2O + 2eAg2O + 2OH-  2AgO + H2O + 2e-
(1)
(2)
A competing process to Ag2O formation is a Ag
dissolution according to the reaction
Ag + 2OH-
 [Ag(OH)2]- + e-
(3)
The characteristic anodic and cathodic peaks
correspond to the formation and reduction of the
Ag2O and AgO phases according to reactions (1)
and (2), respectively. The observed increase of
current peaks of the cyclic voltammogram for the
latter cycle is caused by the increase of the surface roughness during the first oxidation–reduction
cycle 15,16.
In Figure 3b pair of redox peaks appearing in
the first cycle at 0.37 and -0.11 mV/SCE was assigned to Ag/Ag+ redox couple and a pair of redox
peaks appearing at 0.80 and 0.25 mV/SCE was
also assigned Ag+/Ag2+ redox couple, respectively,
in alkaline media recorded at a potential sweep rate
of 100 mVs−1 Both cathodic and anodic peaks of
nano-Ag/CF electrode were decrease after the 50
subsequent cycles. But both cathodic and anodic
peaks of metallic Ag were stabilized after the 50
subsequent cycles.
The degradation mechanism for nano-Ag catalyst can be characterized by the first order kinetics. It has been shown in Figure 4 that the inherent
degradation of the current density for the catalyst
is defined by the first order kinetics approximately:
η(t) = η(0).e-γt
(a)
(b)
Figure 3. Cyclic voltammograms of a) metallic Ag electrode and b) nano-Ag deposited carbon fiber in 1M NaOH solution at scan rate of 100 mV/s
Şekil 3. 1M NaOH içinde 100 mV/s tarama hızında elde edilen a) metalik Ag ve b) karbon destekli nano-Ag’ün döngüsel voltamogramları
where η(t) is the level of current density at time
t, η(0) is the initial current density, and γ (>0) is the
rate of density degradation. As shown in Figure 4,
the current density steadily decreases with the time
at the potential of 0.37V. The surface area of electrocatalyst was decrease during the subsequent
cycles.
Figure.4. Degradation path of the current for nano-Ag/CF at the potential
of 0.37 V (solid line represents linear fit for current data. The original data
are shown as circles).
Şekil 4. 0,37 V potansiyel altında nano Ag(CF için akım çöküş eğrisi (kalın
çizgi. Akım değerleri için doğrusal bağlanım eğrisi. Özgün değerler dairesel noktalarla gösterilmiştir).
To find out the reason of the deactivation of the
nano-Ag/CF, each electrodes surface was analyzed
with X-ray diffraction before and after the 50 subsequent cycles in 1 M NaOH. Figure.5a and b demonstrates the XRD results of the metallic Ag surface
before and after 50 cycles treatment. XRD analysis
detected the presence of Ag2O on the surface after the 50 subsequent cycles. In Figure5b there are
two broad peaks centered at 37.30 and 44.30 which
are related to Ag2O and Ag. These data confirmed
that the oxide layer (Ag2O) formed at 0.3 V shown
at Figure.3a and grew consistently after the subse-
quent cycles. It’s known from literature that Ag2O
has a catalytic effect towards the oxidation of borohydride by 7 electron transfer mechanism. Despite
of its low activity, Ag2O provides high fuel efficiency
by preventing the hydrolysis of the borohydride8, 10,
15
.
The XRD analysis of the nano-Ag/CF was performed in order to determine the surface characteristic. The XRD result after the 50th cycles is depicted
in Figure 6. XRD result detected the silver carbonate on the electrode surface. The peaks at, 32.594°,
33.666°, 38.252° and 57.915° belong to the diffraction peaks of Ag2CO3.
The nano-Ag particles on graphite show different behaviors compared to that of metallic silver.
The electrochemical properties of nanoparticles are
very different from those of the metallic material.
Besides size dependency of the catalytic activity,
catalyst support materials and the metal-support interaction should also be understood. In the case of
nano-Ag particles on carbon fiber, Ag2O can be further oxidized to silver carbonate. Carbonate formation is responsible for the deactivation of the nanoAg/CF catalyst. It is suggested that the formation
and accumulation of carbonate reduces the active
silver surface for the catalytic reactions and deactivates the catalyst. Joeng et al. and Bukhtiyarov et
al. studied the silver nanoparticles and observed
the deactivation of the nano-Ag catalyst on graphite, in highly oxidizing media at low temperatures
(T< 420 K), caused by the carbonate formation.
The activity of the silver increased after the carbonate was removed from the surface at >420 oC 23, 24.
In our system, the electrolytic and electrode conditions with the high OH- concentration, graphite
support material and the low temperature are also
suitable for the formation of carbonate.
KOROZYON, 20 (1-3), 2013 41
Figure 5. The XRD pattern of metallic Ag a) before and b) after 50 subsequence cycles treatment in 1M NaOH+0.1M NaBH4
Şekil 5. 1M NaOH+0.1M NaBH4 çözeltisinde metalik gümüşün ard arda 50 döngüden a) önce b) sonra elde edilen XRD analizleri
Figure 7a and b shows the cyclic voltammograms of metallic Ag and nano-Ag/CF catalyst in
1M NaOH+0.1M NaBH4 solution at a potential
sweep rate of 100 mVs-1. As shown from figue 7,
electrochemical activity of the metallic Ag after 50
subsequence cycles remained almost stable but
nano-Ag/CF catalyst was deactivated due to the
formation of carbonate.
Figure 6. XRD pattern of the nano-Ag/CF catalyst after 50th cycles treatment in
1M NaOH+0.1M NaBH4 solution.
Şekil 6. 1M NaOH+0.1M NaBH4 çözeltisinde nano-Ag/CF katalizörün ard arda 50
döngüden sonra elde edilen XRD analizi
42 KOROZYON, 20 (1-3), 2013
3.3 The performance tests of the fuel cells
We examined the anodic behavior of the peroxide on two different types of catalysts that are metallic Ag and nano-Ag/CF. Figure 8 shows the performance changes on the I-V polarization curves
for MC (cell constructed with Ag) and NC (cell con-
(a)
(b)
Figure 7. The cyclic voltammograms of a) metallic Ag and b) nano-Ag/CF catalyst in 1M NaOH+0.1M NaBH4 solution at a potential sweep rate of 100 mVs-1
Şekil 7. 1M NaOH+0.1M NaBH4 çözeltisinde 100 mVs-1 tarama hızında elde edilen a ) metalik Ag ve b) nano-Ag/CF katalizöre ait döngügel voltamogramlar
Figure 8. The performance curves of NC (▬) and MC (−0−) before loading
Şekil 8. Potansiyel yüklemeden önce NC (▬) and MC (−0−)’ın performans eğrileri
Figure 9. The polarization curves of MC (−0−) and NC (▬).
Şekil 9. MC (−0−) and NC (▬)’in polarizasyon eğrileri
KOROZYON, 20 (1-3), 2013 43
solution. Under aggressive conditions like oxidizing
media, the formation of the carbonate as a passive
layer induces the negative effect on anode catalyst
and performance of fuel cell. On graphite support,
the final stage of the oxidation of Ag (Ag2O/AgO)
can be further oxidized to form silver carbonate.
Figure 10. The performance curves of NC before and after loading of 1
mA for 2 hours
Şekil 10. 2 saat 1 mA akım yüklemesinden once ve sonra alınan performans eğrisi
structed with nano-Ag) before potential loading.
Maximum power densities were found to be 4.5
mW.cm-2 for MC and 5.1 mWcm-2 for NC. It’s shown
that before potential loading, the power density of
NC is higher than that of MC.
The electrochemical performance of MC remained stable at 4.5 mW.cm-2 after loading. However, the performance of NC after loading of 1 mA for
2 hours showed the deactivation caused by the formation of carbonate was as expected and reduced
to 2.2 mW.cm-2 from 5.1 mW.cm-2. It is interesting
to note that the performance of NC, apparently decreased at the high current density regions due to
the mass transfer limitation at Figure 9. This decline
in electrochemical performance after loading is due
to the degradation of the electrode surface.
The performance tests confirmed that the fuel
cell performance decreases as a result of CO3-2 formation at Figure 10. For the Ag2CO3, the negative
influence on the fuel cell is irreversible. XRD graphs
showed the presence of the carbonate species
on the graphite. Although the graphitized carbon
supported catalyst such as carbon fibers, showed
higher resistance to carbon oxidation than the conventional catalyst, in the basic media nano-Ag/
graphite oxidized to Ag2CO3 23,24.
4. CONCLUSIONS
The nano-Ag particles on graphite show different behaviors compared to that of metallic silver.
The electrochemical properties of nanoparticles are
very different from those of the metallic material.
Besides size dependency of the catalytic activity,
catalyst support materials and the metal-support interaction should also be understood. Although metallic Ag as an anode catalyst is promising for the
application of borohydride fuel cell, the nano-Ag/
CF catalyst becomes deactivated after the potential loading in the electrochemical applications because of the highly basic nature of the borohydride
44 KOROZYON, 20 (1-3), 2013
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Levent Aksu, J. Power Sources, 195 (2010) 2604.
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Northwood, Elod L. Gyenge, J. Power Sources 158 (2006)
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AUTHORS
Aylin AYTAÇ
Gazi University, Faculty of Science,
Department of Chemistry, 06500 Teknikokullar, Ankara, Turkey
A. Elif SANLI
Turgut Ozal University, Engineering Faculty,
Department of Electric-Electronics Engineering, Ankara, Turkey
OKSİTLENMENİN YOL AÇTIĞI KIZILÖTESİ
FOTODEDEKTÖR KARARSIZLIKLARININ İNCELENMESİ
ÖZET
Oksitlenmenin yol açtığı Kızılötesi
(KÖ) fotodetektör kararsızlıkları Townsend boşalma modunda tek gaz boşalma aralıklı KÖ görüntü çevirici sistemde
deneysel olarak D (9-12 mm) lik yarıiletken elektrot çapları için araştırıldı. Bilindiği üzere hava oksijence oldukça aktif
bir ortamdır. Elektrotlar arasında oluşturulacak olan plazma ortamının oksijence aktif olması katot olarak kullanılan GaAs fotodetektörün yüzeyinde oksitlenmelere yol açmaktadır. Bu ise kararlı olan sistem karakteristiklerini kararsız hale getirmektedir. Bu kararsızlıkların
tespiti KÖ görüntü çevirici olarak kullanılan sistemin optimizasyonunda son derece önemlidir.
AN INVESTIGATION ON THE
EFFECT OF OXIDATION
ON IR PHOTODETECTOR
INSTABİLITIES
The IR photodetector instabilities
due the oxidation are explored experimentally under the Townsend discharge
mode in a single gap gas discharge IR
image converter system with the semiconductor electrode diameter of D (9-12
mm). As it is known, air is an active media open to any oxidation. Such an oxygen-active media between the cathodes
inside the plasma leads to the oxidations
on the surface of GaAs photodetector, which is used as the cathode. This
situation leads to the instabilities over
a stable system characteristics. The
identification of these instabilities is very
important for the optimization of the IR
image converter system.
1. GİRİŞ
Townsend tipi boşalmaların
dinamik özelliklerini incelemeye ilgi, gaz boşalma fiziği alanındaki bilginin artırılmasına ve
teknik sistemlerde bu tip boşalmanın kullanımı ile bağlantılı
pratik problemleri çözmeye yardım etme ihtiyacından kaynaklanmaktadır. Ele alınan görüntü
çeviricideki boşalmanın dinamik
karakteristikleri, fotodedektöre
ya pulslu KÖ ya da pulslu besleme voltajı uygulayarak incelenebilir. Yarı iletken GaAs (SI-GaAs)
daki lineer olmayan elektronik
taşıma özellikleri ile ilgili ayrıntılı
bilgilere deneylerimiz sonucunda ulaşılmıştır1.
KÖ çeviricinin avantajları ve
uygulama alanları şu şekilde sıralanabilir:
 1,1 μm ile 11 μm aralığındaki
spektral duyarlılığı,
 Uygulama alanına bağlı olarak nanosaniye sınırına inebilen yüksek zamansal çözünürlüğü,
 mm başına 16 çizgiye kadar
çıkan uzaysal çözünürlüğü,
 Düşük maliyetli deney seti.
Bunların sonucu olarak, KÖ
görüntü çevirici aşağıda sıralanan uygulama alanlarında kullanılabilir:
 Hızlı uzaysal çözünürlükteki
termografik ölçümler,
 KÖ lazer ışığı profillerinin
analizi,
 Hasarsız test etme,
 Oksitlenme (paslanma) süreçlerinin belirlenmesi,
 Lazer ışığı kaynağının görüntülenmesi.
H. Y. KURT
S. ÇETİN
A. YURTSEVEN
2. DENEYSEL SİSTEM
Gazlardaki elektriksel boşalma kuvvetli bir dengesizlik sürecidir. Bunun özellikleri çok
çeşitlidir gaz içeriğine ve gazın basıncına; boşalma sisteminin geometrisine elektriksel güç
besleme moduna v.b. ne bağlıdır. Örneğin, bir dc ya da yüksek frekanslı voltaj kaynağı tarafından beslenen veya bir mikrodalga elektromanyetik alan
tarafından uygulanan boşalmaların kararlılığı oldukça farklı
olabilir. Boşalma süreçlerinin bu
özellikleri pratikte ve ayrıca desen oluşum deneylerinde etkin
olarak kullanılmaktadır2.
Böyle bir boşalma çığ mekanizması dolayısıyla gaz hacmindeki çok sayıda yüklü taşıyıcıların çoğalması ve elektrot süreçleri tarafından desteklenmektedir. Büyük çaplı yarıiletken katotlu tek boşalma aralıklı plazma sisteminin şeması Şekil 1
de gösterilmiştir. Sistemin özelliklerini belirleyen başlıca iki kısım yarıiletken ve gaz tabakasıdır. Yüksek dirençli (ρ-108 Ωcm)
SI GaAs katodun çapı 36 mm ve
kalınlığı 1 mm dir3.
GaAs ın aydınlatılan kısmına saydam iletken vakum evaperasyonlu Au tabaka kaplanmıştır. GaAs oda sıcaklığında
(4) 1,42 eV luk bir band aralıklı direk yarıiletkendir. Radyasyon
soğurulduğu zaman elektronlar
uyarılır ve valans banttan iletKOROZYON, 20 (1-3), 2013 45
1
2
3
4
5
6
UV-
+
R1
=
U0
Şekil 1: Tek gaz boşalma aralıklı plazma sistemi: 1- yarısaydam Au kontak; 2- GaAs katot; 3- gaz boşalma aralığı; 4- mika; 5- saydam iletken
SnO2 kontak; 6 - düz cam disk
Figure1: Single gap gas discharge plasma system: 1-semitransparent Aulayer; 2-GaAs cathode; 3-gas discharge gap; 4-mica foil; 5- transparent
conductive SnO2 contact; 6- flat glass disc;
3,5x10
-5
3,0x10
-5
2,5x10
-5
2,0x10
-5
1,5x10
-5
1,0x10
-5
5,0x10
-6
200
3. SONUÇ VE TARTIŞMA
Küçük aktif hacimli gaz boşalma sistemleri son
yüzyılda büyük bir ilgiye sahiptir. Direnç dağılımlı bir
yarıiletken kullanmak akım dağılımını önemli ölçüde
değiştirmektedir. İletilen akımın değeri ve boşalmanın tipi yarıiletken katodun direnç dağılımının homojenliği tarafından belirlenir. Bu durum ışığa duyarlı
yarıiletken maddelerdeki akım yoğunluğunun uzaysal dağılımının görüntülenmesi ile ilgili bazı pratik
uygulamalarda istenen bir durumdur 4,5. Çünkü bu
çözünürlüğü artırır, güvenilir operasyonun tekniksel
realizasyonunu kolaylaştırır ve kullanılan materyalin
kararlılık bölgesini genişletir. Fakat teknolojik plazma sistemleri, 6,7 katot çapının katot ve anot arasındaki mesafeden çok daha büyük olduğu boşalma
hücresini sıklıkla kullanır. Şu anda, değişik gazlarda
ve değişik boşalma aralıklı geometrilerde homojen
boşalmayı optimum şartlarla elde etmek için kapsamlı araştırma mevcuttur 8,9.
Bu bakımdan, en elverişsiz faktör yarıiletken katodun oksijence aktif plazma ortamıyla etkileşiminin sonucunda ortaya çıkan oksitlenme sonucu
enine doğrultuda boşalma homojenliğinin kaybına yol açan kararsızlıkların gelişimidir. Bu şartlar altında, enine kararsızlıklar oldukça düşük boşalma
akım yoğunluklarında bile gelişebilir 10,11. Bu osilasyon kararsızlıklarının özellikleri başlıca elektrik akımının yoğunluğu tarafından belirlenir. Şekil 2 plaz-
5,0x10
-5
4,0x10
-5
D = 9 mm, Hava
3,0x10
-5
A2
2,0x10
-5
1,0x10
-5
p = 70 Torr, d1= 50 m, d2 = 240 m,
p = 70 Torr, d1= 50 m, d2 = 240 m
D = 9 mm, Hava
A2
I(Amper)
I (Amper)
kenlik bandına geçişler yaparlar. İç fotoetki materyalin direncini düşürür. Anot saydam iletken SnO2
nin ince bir tabakası ile kaplanmış (30 mm çaplı ve
2 mm kalınlıklı) cam disktir. Yarıiletken katodun karşı yüzeyi, merkezinde dairesel bir boşluk (3) bulunan yalıtkan mika tabakası (4) ile düz anottan ayrılmıştır. Cam disk ve yarıiletken plaka (yani hem katot hemde boşalma aynı alanları işgal eder) arasındaki aktif elektrot alanları D gaz boşalma aralığıdır;
bunun genişliği tipik olarak 45 ve 330 μm arasındaki bölgede değiştirilmiştir. Yalıtkandaki (4) dairesel
boşluğun tipik çapları (D; yani aktif elektrot alanları)
5,9,12,18,22 mm dir. Au ve SnO2 elektrot dış elektrik devreye bağlantılıdır ki bu yüksek voltaj bir dc Uo
kaynağı ve seri R1 direncinden ibarettir. Hücremizde polyosteren anot ile katodun kısa devre olmasını önlemek amacıyla kullanılmaktadır. Hücreden
geçen akım, hücreye seri olarak bağlı (10 kΩ ±1)
dış sınırlayıcı direnç boyunca voltaj düşüşünü ölçerek elde edildi.
a)
400
600
800
1000 1200 1400 1600
V (Volt)
b)
200 250
900
1000
1100
1200
1300
1400
1500
V (Volt)
Şekil 2a. Kuvvetli aydınlatma şiddeti altında (A2) oksitlenme öncesi yarıiletken katodun akım- voltaj karakteristiği (AVK); b) Oksitlenme sonrası yarıiletken katodun akım voltaj karakteristiği.
Figure 2a. Current-voltage characteristics (CVC) of the semiconductor cathode before the oxidation under strong illumination (A2) b) Current-voltage characteristics (CVC) of the semiconductor cathode after the oxidation under strong illumination (A2).
46 KOROZYON, 20 (1-3), 2013
I (Amper)
4,0x10
-5
3,5x10
-5
3,0x10
-5
2,5x10
-5
2,0x10
-5
1,5x10
-5
1,0x10
-5
5,0x10
-6
Şekil 4, Şekil 3 deki histerezis grafiğine ait ve değişik voltajlar için elde edilen akım-zaman grafiklerini göstermektedir. Şekilden 4 den de görüleceği
üzere AVK da kararlı
p = 66 Torr, D = 12 mm
d1= 50 m, d2 =320 m, Hava
A1
A1
800
mm, p = 66 Torr, interelectrode distanced d1
= 50 μm; d2 =320 μm and illumination intensity
1000
1200
1400
1600
V (Volt)
Şekil 3. d1 = 50 μm; d2 = 320 μm, D =12 mm, p = 66 Torr, A1 zayıf aydınlatma seviyesi için geri dönüşüm davranışı.
Figure 3. The hysteresis curve in the cases of d1 = 50 μm; d2 = 320 μm, D
=12 mm, p = 66 Torr under weak illumination intensity A1
3,2x10
-5
2,8x10
-5
2,4x10
-5
2,0x10
-5
1,6x10
-5
5,0x10
-6
p = 66 Torr, D = 12 mm
d1= 50 m, d2 =320 m, Hava, A1
V = 1501 Volt
V = 1502 Volt
V = 1503 Volt
I (Amper)
I (Amper)
ma ile yarıiletken katodun etkileşimi sonucu oksitlenmenin AVK da meydana getirdiği değişimi göstermektedir. Şekilden görüleceği üzere kararlı olan
AVK da kararsızlıklar tespit edilmiştir (Şekil 2b). Bu
nedenle deneylerde doğru ölçüm alabilmek için oksitlenmiş yarıiletken yüzeyinin temizlenmesi gerekmektedir12.
Şekil 3 de ise elektrotların gaz doyumuna bağlı
olarak geri dönüşüm davranışı gözlenmektedir. Şekil 3 ileri ve geri beslem altında voltaja bağlı olarak akımdaki değişmeyi göstermektedir. Dikkat edilirse sistem ileri ve geri beslem altında aynı davranışı göstermektedir buda sistemin kararlı çalışmasının bir göstergesidir ve teknolojik uygulamalar için
son derece önemlidir.
olan bölgelere ait olan akım-zaman grafikleri daha kararlı bir davranış sergilerken, AVK da kararsız bölgeye ait voltajlarda akım- zaman grafiği
da kararsız durum sergilemektedir. Şekil 5 zımpara tozu ile işlenmiş (b) ve işlenmemiş yüzeylerin(a)
görüntülerini göstermektedir. Profil grafiklerinden
anlaşılacağı üzere işlenmiş yüzeyin ışığa duyarlılığı 24 kat artmıştır. Çünkü işlenmiş yüzey ışığın soğurulmasını artırır; ışık bu işlem sürecince oluşan
homojensizlik merkezlerinden (ki bunlar yansıma
merkezleri gibi davranır) çok sayıda yansımaya uğrar. Gelen ışığın dalga boyu yansıtıcı merkezlerin
boyutlarına yakın ise maksimum yansıma meydana gelir. KÖ bölgesinde zımpara tozu ile işlenmiş
yarıiletken ile sistemin ışığa duyarlılığının artışını
doğrulamak ve görüntüleri karşılaştırmak için gaz
ortamında boşalma ışımasını dijital CCD kamera ile
kaydettik (Şekil 5).
Yarıiletken işlendiği zaman profilden açıkça görüleceği üzere yarıiletkenin ışığa duyarlılığı artmaktadır. Çapı D = 20 mm ve kalınlığı L = 1 mm olan
disk şeklindeki yarıiletken GaAs plakanın bir yüzeyinde metalik Au kontağı evaperasyon yöntemi ile
oluşturulmuştur. Bundan sonra bu plaka iki metal
elektrot arasında sıkıştırılmıştır. Plakanın serbest ka-
3,0x10
-5
2,0x10
-5
p = 66 Torr, D = 12 mm
d1= 50 m, d2 =320 m, Hava, A1
V = 1266 Volt
V = 1295 Volt
V = 1390 Volt
-5
1,0x10
-6
5,0x10
0
30
60
90
Zaman (s)
120
150
0
30
60
90
120
150
Zaman (s)
Şekil 4. Elektrot çapı D = 12 mm, p = 66 Torr, elektrotlar arası mesafe d1 = 50 μm; d2 =320 μm, A1 aydınlatma seviyesi için akım – zaman karakteristiği
Figure 4. Current-voltage characteristics for the semiconductor electrode diameter D = 12 mm, p = 66 Torr, interelectrode distanced d1 = 50 μm; d2 =320
μm and illumination intensity
KOROZYON, 20 (1-3), 2013 47
20
U = 850 Volt
a)
15
10
5
0
0
50
100
150
200
250
piksel
b)
200
U = 850 Volt
b)
160
120
80
40
0
0
50
100
150
200
250
piksel
Şekil 5. Tek gaz boşalma aralıklı plazma sisteminden elde edilen
gaz boşalma ışımasının görüntüleri: a) yarıiletken katodun cilalanmış; b) zımpara tozu ile işlenmiş yüzeyi ve bu görüntülere ait yüzeyin çap boyunca profilleri.
Figure 5. Discharge light emission patterns from the single gap gas
discharge plasma system: a) The polished semiconductor cathode b) Emeried semiconductor surface and the light emission profiles along the diameter.
lan yüzeyi tarafından halka şeklinde elektrot yapılmıştır. Işık yarıiletkenin serbest kalan yüzeyine (halka tarafından boş kısmı) normal doğrultuda düşürülmüştür 13.
Deneyler, önceden cilalanmış yüzeyin farklı zımpara tozu ile işlenmesinin KÖ bölgesinde ışığa duyarlılığın arttırdığını göstermektedir. Genişletilmiş
alan üzerindeki yüksek enerjili parçacıkların akışının
içine katodun alınan yüzeyi üzerindeki kısmen düşük güçlü fotonu değiştirerek ve yükselterek düzlemsel gaz boşalma sistemi son derece etkili enerji
değiştirici olabilir. Sistemin katodunu uyarmak için
KÖ ışık kullanarak uygulanan boşalma voltajının artırılması ayrıca gösterilmiş ve etkili ikinci elektron
yayınım katsayısının değişimine bağlı olarak açıklanır. Bunun değeri; elektrot yüzeyinin şartlarına ve
gaz boşalma plazmasındaki iyon bileşenlerinin birleşimine bağlıdır. Bu yüzden son derece parlak UV
ve görünür kaynak oluşabilir. Bu UV ışık kaynağının
düşük fiyatı ve yüksek gücünün olması bu çalışmanın ilginç olmasını sağlar. Aynı zamanda geleneksel UV lambaları için çok faydalı bir alternatiftir. Bu
cihaz KÖ ışık ile kontrol edilen UV radyasyonun hızlı kaynağının bir uygulaması olabilir. Glow boşalma
ışık yayınımının özelliği; yayılan yüzeyin büyük alanlı ışık kaynağının gelişimi ve UV radyasyonun yüksek uzaysal homojenliği için ümit vermesidir.
48 KOROZYON, 20 (1-3), 2013
4. SONUÇLAR
Gazlardaki elektriksel boşalma kuvvetli bir dengesizlik sürecidir. Bunun özellikleri çok çeşitlidir ve
gaz içeriğine ve gazın basıncına; boşalma sisteminin geometrisine elektriksel güç besleme moduna
v.b ne bağlıdır. Duyarlılık ve çözünürlüğü arttırmak
için yarıiletken katotlu gaz boşalma sistemlerinin
özelliklerini detaylı olarak anlamak ve optimum boşalma şartlarını belirlemek gerekmektedir. Bu özellikler, gaz boşalma sistem hücrelerinin tasarlanması için ana özelliklerdir. Bu çalışmada yarıiletken yüzeyinde plazmanın oluşturduğu oksitlenmenin sistem karakteristiklerinin kararlılığını olumsuz yönde
etkilediği ve AVK da osilasyonlara yol açtığı gösterilmiştir. Buna karşın yarıiletken yüzeyinin zımpara
tozu ile zımparalanması sistemin KÖ ışığa duyarlılığını artırmıştır.
TEŞEKKÜR
Bu çalışma Gazi Üniversitesi BAP 05/2012-47
ve BAP 05/2012-72 kodlu projeler tarafından desteklenmiştir.
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YAZARLAR
Hilal Yücel KURT, Sadık ÇETİN, Adem YURTSEVEN
Gazi Üniversitesi, Fen Fakültesi, Fizik Bölümü,
06500, Teknikokullar/ANKARA
Yazarlarla iletişim için: e-mail: [email protected]

mi̇lli̇ geli̇rden önemli̇ bi̇r kaybimiz!

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