Materials Transactions, Vol. 51, No. 9 (2010) pp. 1671 to 1676
#2010 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Novel Technique for the Application of Azole Corrosion Inhibitors
on Copper Surface
Faiza M. Al Kharafi, Nouria A. Al-Awadi, Ibrahim M. Ghayad* ,
Ragab M. Abdullah and Maher R. Ibrahim
Chemistry Department, Faculty of Science, Kuwait University, Kuwait
A new method was proposed for the application of azole corrosion inhibitors on the surface of copper. This method depends on the vacuum
pyrolysis of the inhibitor in the presence of copper specimens. Three azole inhibitors namely; benzotriazole (Azole (1)), N-[Benzotriazol-1-yl(phenyl)-methylene]-N-phenyl-hydrazine (Azole (2)) and N-[Benzotriazol-1-yl-(4-methoxy-phenyl)-methylene]-N-phenyl-hydrazine (Azole
(3)) were tested. After pyrolysis copper samples were electrochemically tested in sulfide polluted salt water and compared to the behavior of
copper tested in the sulfide polluted salt water containing dissolved benzotriazole. Results showed that copper specimens treated in the presence
of Azoles (2) and (3) exhibit excellent corrosion resistance. Those samples could resist the poisoning effect of sulfide ions. Azole (1) shows good
resistance at low sulfide concentration and failed at the high concentration. Surface investigation support the results of electrochemical tests.
[doi:10.2320/matertrans.M2010141]
(Received April 21, 2010; Accepted June 11, 2010; Published August 25, 2010)
Keywords: copper, corrosion inhibitors, pyrolysis, azoles
1.
Introduction
Azoles and in particular Benzotriazole (C6 H5 N3 , BTAH)
have long been known as inhibitors for the corrosion of
copper and many of its alloys.1–16) The remarkable inhibiting
efficiency of BTAH is attributed to the formation of a
protective film of Cu(I)BTA on the copper surface.1–16)
However, benzotriazole loses its inhibition efficiency in
environments polluted with sulfide ions. An example of these
environments is the formation waters in sour oil and gas
wells.17) These waters are heavily contaminated with dissolved sulfides which are mostly in the form of HS ions in
nearly neutral media.18) In sulfide polluted environments,
sulfide ions compete for Cu(I) ions under a much stronger
driving force than BTAH, consequently, sulfide ions can
extract the Cu(I) ions from the Cu(I)BTA complex. This
leads to the break down of the protective Cu(I)BTA film and
occurrence of corrosion on the bare areas.18)
To the date of this work, there are not known inhibitors
which can resist the poisonous effect of sulfide ions. This
dilemma forced the authors to think of a new method of
application of the azole corrosion inhibitors on the copper
surface in particular and metals in general. This method
depends on the vacuum pyrolysis of the inhibitor in the
presence of copper specimens. Such method would cover
the copper surface with a compact protective layer of the
inhibitor.
2.
Experimental
Electrodes were prepared from Cu (99.9 mass%) obtained
from Goodfellow Corporation. Copper disc specimens of
1 cm diameter and 2 mm thickness were polished using SiC
papers successively up to 2400 grits to acquire a mirror-like
finish. A conventional three-electrode cell was used with a
Ag/AgCl reference electrode, E ¼ 0:197 V SHE, and a Pt
*Corresponding
author, E-mail: ighayad@yahoo.com. Present address:
P.O. Box: 5969 Sfat 13060, State of Kuwait
sheet counter electrode. Solutions were prepared using
double distilled water, BTAH (Azole (1) was purchased
from Sigma-Aldrich, Azoles (2) and (3) were prepared as
described previously19) while Na2 S and NaCl were purchased
from Fluka Chemicals.
Potentiodynamic polarization curves were measured on
the Cu electrode in 3.5 mass% NaCl containing sodium
sulfide and BTAH at a voltage scan rate of 5 mV s1 . The
potential was controlled using a Gamry Instruments potentiostat which was also used in measuring potentiostatic
polarization curves. Measurements were performed at
25 1 C while the electrolyte was stirred using a magnetic
stirrer. The surfaces of the electrodes were examined using
JEOL Ltd., JSM-6300 scanning electron microscope (SEM).
The structure of the inhibitor deposited layer was determined
using 1 H-NMR technique performed by Bruker AVANCE II
600 MHz NMR for solutions and solids.
In the vacuum pyrolysis experiments, 3 polished specimens of copper were placed in a pyrex glass tube (15 mm
diameter) together with about 0.3 g of the azole inhibitor.
Three azole inhibitors namely; bnezotriazole (Azole (1)),
N-[Benzotriazol-1-yl-(phenyl)-methylene]-N0 -phenyl-hydrazine (Azole (2)) and N-[Benzotriazol-1-yl-(4-methoxy-phenyl)-methylene]-N0 -phenyl-hydrazine (Azole (3)) were tested.
The chemical structures of the tested azoles are illustrated in
Fig. 1. The open part of the tube was narrowed to 3 mm
diameter. The assembly was placed in liquid nitrogen to
freeze the azole inhibitor inside and then connected to a
vacuum pump which allows the pressure inside the tube to
reach about 1.33 Pa. The tube was then sealed by heating its
neck till melt and close. The tube is finally placed in the
pyrolyser which adjusted to the desired temperature (200 C).
After vacuum pyrolysis for the desired time (30 min); the
glass tube was broken and the copper specimens were taken
out and corrosion tested. In most instances, both sides of
copper specimens are covered with the inhibitor. Under such
condition, one side is repolished to make electrical connection through this side while the other side, covered with
inhibitor, is exposed to the testing solution.
1672
F. M. Al Kharafi, N. A. Al-Awadi, I. M. Ghayad, R. M. Abdullah and M. R. Ibrahim
(A)
(B)
(C)
Fig. 1 Chemical structure of the tested azoles: (A) Benzotriazole (Azole (1)). (B) N-[Benzotriazol-1-yl-(phenyl)-methylene]-N0 -phenylhydrazine (Azole (2)). (C) N-[Benzotriazol-1-yl-(4-methoxy-phenyl)-methylene]-N0 -phenyl-hydrazine (Azole (3)).
2.5
(2)---+ 0.01 mol L-1 BTAH
(3)---+ ---+ 0.001 mol L-1 HS(4)---+0.001 mol L-1 HS-
1.0
0.5
(2)
(3)
Eb
0.0
(1)
(4)
-0.5
-1.0
10-9
2.0
Potential, E / V (Ag/AgCl)
Potential, E / V (Ag/AgCl)
(1) 3.5 mass% NaCl
Azole (2)
1.5
1.0
0.5
Azole (1)
Azole (3)
0.0
-0.5
-1.0
10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
Current, I / A
Current, I / A
Fig. 2 Effect of sulfide ions on the polarization curves of copper in
3.5 mass% NaCl with and without pretreatment in 102 mol L1 BTAH at
25 C, see text.
3.
Results and Discussion
3.1 Electrochemical tests of copper before pyrolysis
Figure 2 illustrates the effects of BTAH and of sulfide ions
on the polarization curves of copper, measured at 5 mV s1 .
Curve (1) refers to the blank solution (3.5% NaCl), curve (2)
was measured after the electrode was pretreated in the
presence of 0.01 mol L1 BTAH for 1 h while curve (3) was
measured after injection of 103 mol L1 HS following the
above pretreatment. Curve (4) was obtained in the presence
of 103 mol L1 HS in the blank electrolyte. It is added for
the sake of comparison with curve (3) which was measured
after pretreatment with BTAH. The presence of BTAH
decreases the rate of anodic dissolution of copper by about
four orders of magnitudes (compare curves 1 and 2). A
passive region appears in the anodic branch of the curve,
which is attributed to the formation of a protective film of the
Cu(I)BTA complex. It extends about 500 mV and ends at the
break down potential, E , beyond which the current increases
rapidly with potential. The sulfide ions diminish the passivity
caused by BTAH by increasing the current in the passive
region by about three orders of magnitude (compare 3 and 1).
They also shift the polarization curve much closer to that of
the unprotected copper. Furthermore, they lower both the
Fig. 3 Potentiodynamic polarization curves of copper in 3.5 mass% NaCl
containing 0.001 mol L1 HS . Prior to testing, copper was pyrolyzed in
Azole (1), Azole (2) or Azole (3) at 200 C for 30 min.
breakdown potential E , and the free corrosion potential by
hundreds of mV.
3.2 Electrochemical tests of copper after pyrolysis
Figure 3 represents the polarization curves of copper
tested in 0.001 mol L1 sulfide polluted salt water. Prior to
testing, copper was pyrolyzed in Azole (1) (Benzotriazole),
Azole (2) and Azole (3) at 200 C for 30 min. The application
of Azoles (2) and (3) result in the increase of their inhibiting
efficiency to a very large extent. A wide range passive region
of about 2 V is observed for these inhibitors. A very low
passive currents (<109 A) are observed. These extremely
low currents indicate a 100% protection of copper. The
polarization curve of copper treated with Azole (1) (benzotriazole) differs clearly from those obtained in the presence of
Azole (2) and Azole (3). Although the treated copper could,
to some extent, resist the 103 mol L1 sulfide solution, the
passive current obtained under this condition is 105 A
which is fairly high compared to the one obtained in the
presence of the other two inhibitors (>109 A).
Figure 4 represents the polarization curves of copper
tested in 0.01 mol L1 sulfide polluted salt water. Prior to
testing, copper was also pyrolyzed in Azole (1) (Benzotriazole), Azole (2) and Azole (3) at 200 C for 30 min. Copper
treated in Azoles (2) and (3) still show good passivation but
Novel Technique for the Application of Azole Corrosion Inhibitors on Copper Surface
10 -1
2.0
Azole (1) + 0.01 mol L-1 HS
10 -2
1.5
-
10 -3
1.0
10 -4
Current, I / A
Potential, E / V (Ag/AgCl)
1673
0.5
Azole (1)
Azole (2)
Azole (3)
0.0
-0.5
Azole (I) + 0.001 mol L-1 HS-
10 -5
10 -6
Azole (3) + 0.01 mol L-1 HS-
10 -7
10 -8
Azole (2) + 0.01 mol L-1 HS
10 -9
-1.0
-
10 -10
-1.5
10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1
10 -11
0
2000
Current, I / A
4000
6000
Time, t / s
Fig. 4 Potentiodynamic polarization curves of copper in 3.5 mass% NaCl
containing 0.01 mol L1 HS . Prior to testing, copper was pyrolyzed in
Azole (1), Azole (2) or Azole (3) at 200 C for 30 min.
Fig. 5 Potentiostatic polarization curves of copper in sulfide polluted salt
water. Prior to testing copper was pyrolyzed in Azole (1), Azole (2) or
Azole (3) at 200 C for 30 min.
Table 1 Variation of corrosion parameters (corrosion current, Icorr , corrosion potential, Ecorr , and percentage Inhibition Efficiency, E) of
copper subjected to vacuum pyrolysis in the presence of different azoles. Treated copper specimens were tested in 3.5 mass% NaCl
containing either 0.001 or 0.01 mol L1 sulfide ions.
Blank
(Cu-only)
CHS /mol L1
Icorr /A
Azole (2)
0.01
0.001
0.01
0.001
0.01
0.001
0.01
1:39 108
3:18 105
1:1 1011
4:24 109
7:45 1012
9:6 109
99.991
79.351
100.00
99.997
100.00
99.994
0:927
0:349
0:954
0.0167
0:397
0.119
0:205
to a lower extent than that obtained in the 0.001 mol L1
sulfide solution. The corrosion potentials are shifted towards
more negative values of 0:4 and 0:2 for Azole (2) and
Azole (3), respectively compared to 0:00 and 0.12 V
obtained in the 0.001 mol L1 sulfide solution. Passive
current are also increased by about three orders of magnitude
(107 A) compared to those obtained in the 0.001 mol L1
sulfide solution (1010 A). On the other hand, copper
treated in Azole (1) could not resist the 102 mol L1 sulfide
solution. Polarization curves are quite similar to those
obtained in the sulfide solution containing dissolved benzotriazole (see Fig. 2).
Potentiodynamic polarization curves were also used to
deduce the corrosion parameters of copper namely; Corrosion potential (Ecorr ) corrosion and current (Icorr ) which in
turn used to calculate the inhibitor (azole) percentage inhibition efficiency (E) according to the following equation:20)
E¼
Azole (3)
1:54 104
E (%)
Ecorr /V
Azole (1)
Icorr (Blank) Icorr (Azole)
100
Icorr (Blank)
ð1Þ
Table 1 summarizes the results. Extremely low corrosion
currents and very high inhibition efficiencies reach as high as
100% were obtained for copper pyrolyzed in the presence
of Azoles (2) and (3). On the other hand considerable low
inhibition efficiency (79%) was obtained for copper pyrolyzed in the presence of Azole (1) and tested at 0.01 mol L1
HS . The shift of Ecorr towards more anodic values compared
to the untreated copper is another evidence of the enhancement of corrosion resistance of copper subjected to vacuum
pyrolysis in the presence of azole inhibitors.
Figure 5 represents potentioststatic polarization curves
of copper tested at 0.5 V (Ag/AgCl) in 0.01 mol L1 sulfide
polluted salt water. Prior to testing, copper was pyrolyzed
in Azole (1), Azole (2) and Azole (3) at 200 C for 30 min.
Steady state currents of 102 , 1010 and 108 A were
obtained for copper pyrolyzed in Azole (1), Azole (2) and
Azole (3), respectively. The extremely low current shown by
copper pyrolyzed in Azoles (2) and (3) again show a 100%
inhibiting efficiency which supports the results of potentiodynamic tests. On the other hand, the high steady current
shown by Azole (1) indicates its failure to resist sulfide
attack. Copper pyrolyzed in Azole (1) was also tested in
0.001 mol L1 sulfide solution. Under this condition, it shows
a steady current of 105 A. This low steady current indicates
that Azole (1) could resist acceptably the low concentrations
of sulfide ions.
3.3 Surface characterization
Figure 6(a) and (b) represents the surface of copper
subjected to vacuum pyrolysis at 200 C in the presence of
Azoles (2) and (3). Images show that copper surface is
covered with a black layer of the inhibitor. However, Azole
(2) shows some degree of roughness while Azole (3) show a
smooth surface. Figure 7(a) and (b) illustrates cross sections
of copper samples subjected to vacuum pyrolysis in the
presence of Azole (2) and Azole (3). Images differentiate two
distinct layers; one represents the copper metal while the
other represents a compact layer of the inhibitor deposited
on the copper surface. The thickness of this layer (in its
maximum) was shown to be 128 and 59 mm for the Azole (2)
1674
F. M. Al Kharafi, N. A. Al-Awadi, I. M. Ghayad, R. M. Abdullah and M. R. Ibrahim
(a)
(b)
Fig. 6 SEM micrographs of copper pyrolyzed at 200 C for 30 min in Azole (2) (a) and Azole (3) (b).
(a)
(b)
Fig. 7
SEM micrograph of the cross sectional area of copper pyrolyzed in either Azole (2) (a) or Azole (3) (b) for 30 min at 200 C.
and Azole (3) respectively. These results show that the layer
formed in the presence of Azole (2) to be thicker than that
formed in the presence of Azole (3) which may account for
the slightly lower current obtained for copper treated in the
presence of Azole (2) (1010 A) compared to the one
obtained in the presence of Azole (3) (108 A).
Figure 8 shows the surface of copper potentiostated in
0.01 mol L1 sulfide polluted salt water after removal of
the deposited layer of Azole (2) (a) and Azole (1) (b). The
surface of copper treated in Azole (2) shows very good
appearance with no corrosion attack. On the other hand the
surface of copper treated in Azole (1) suffers from sever
corrosion attack. These images are in full agreement with
the electrochemical tests which show excellent corrosion
resistance for Azole (2) and poor corrosion resistance for
Azole (1).
It was interesting to determine the chemical structure of
the deposited inhibitor layer. Azole (3) was taken as example.
After pyrolysis and corrosion testing of Azole (3), copper
samples were taken and immersed in chloroform to dissolve
the coat and then analyzed using 1 H-NMR. Figure 9(a)
represents the 1 H-NMR chart of Azole (3) before pyrolysis
while Fig. 9(b) shows the 1 H-NMR chart of the dissolved
coating. It is clear that the structure of the coating is nearly
the same as that of Azole (3) before pyrolysis.
Surface investigations revealed that the deposited inhibitor
layer is distinct and differentiated from the metal surface
(see Fig. 7). 1 H-NMR charts shows that the structure of
the coating resembles that of the mother material before
pyrolysis. These observations suggest that there is no
chemical interaction between the metal and the inhibitor
and consequently look to the vacuum pyrolyisis of azoles as a
kind of coating rather than chemisorption. The process looks
like hot-melt dip coating. When azole reaches a temperature
higher than its melting point it transfers into liquid which
spreads and adheres to the surface of copper. The adhered
layer behaves like a barrier hydrophobic coating that
effectively prevent electrolyte from contacting the metal
surface and excellently resist corrosion attack. The difference
in efficiency between different azoles can be related to the
properties of the formed barrier coating. Azole (1) (benzotriazole) is weakly soluble in water while Azoles (2) and (3)
Novel Technique for the Application of Azole Corrosion Inhibitors on Copper Surface
1675
(b)
(a)
Fig. 8 SEM micrographs of copper surface potentiostatic testing in 3.5 mass% NaCl containing 0.01 mol L1 sulfide ions after removal of
the deposited layer of Azole (2) (a) and Azole (1) (b).
(a)
OCH3
Aromatic Hs
Solvent
NH
Aromatic Hs
(b)
OCH3
Solvent
NH
Fig. 9 1 H-NMR-Charts of Azole (3): (a) Adhered layer of Azole (b) Before pyrolysis (3) formed on the surface of copper after pyrolysis.
Prior to NMR analysis, this layer was dissolved in chloroform.
are not soluble at all. Depending on the afore mentioned
facts, it is expected that the formed barrier coating by Azole
(1) allows some penetration of the electrolyte to the copper
surface and hence enables corrosion to occur. On the
other hand, barrier coatings formed by Azoles (1) and (3)
acts as perfect insulators preventing the penetration of the
electrolyte.
4.
Conclusions and Future Outlook
The present work explores the application of azoles, some
of which are new corrosion inhibitors for copper, using new
methodology depends on the vacuum pyrolysis of azole
inhibitor in the presence of copper specimens. The promising
results obtained in this work encourages further application
1676
F. M. Al Kharafi, N. A. Al-Awadi, I. M. Ghayad, R. M. Abdullah and M. R. Ibrahim
of this new methodology to other azoles and also to other
metals. This also encourages investigation of this method to
large scale application.
Vacuum pyrolysis of Azole (2) and Azole (3) on copper
leads to the deposition of a compact layer of the inhibitor
on the copper surface. The thickness of this layer (in its
maximum) was shown to be 128 and 59 mm for the Azole (2)
and Azole (3) respectively. Surface investigations revealed
that the deposited layer is distinct and differentiated from the
metal surface. This layer could resist the poisoning effect of
sulfide ions. On the other hand, layer deposited by Azole (1)
failed to resist the poisoning effect of sulfide ions.
Acknowledgements
The authors acknowledge the support of this work by
the Research Administration of Kuwait University, Grant
Numbers SC06/07 and GS01/03. They also acknowledge the
use of the scanning electron microscope (SEM).
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