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 newmethod was proposed for the application of azole corrosion inhibitors on the surface of copper. This method depends on the vacuumpyrolysis 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 ofcopper tested in the sulfide polluted salt water containing dissolved benzotriazole. Results showed that copper specimens treated in the presenceof Azoles (2) and (3) exhibit excellent corrosion resistance. Those samples could resist the poisoning effect of sulfide ions. Azole (1) shows goodresistance 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 (C6H5N3, BTAH)have long been known as inhibitors for the corrosion ofcopper and many of its alloys.1–16) The remarkable inhibitingefficiency of BTAH is attributed to the formation of aprotective film of Cu(I)BTA on the copper surface.1–16)

However, benzotriazole loses its inhibition efficiency inenvironments polluted with sulfide ions. An example of theseenvironments is the formation waters in sour oil and gaswells.17) These waters are heavily contaminated with dis-solved sulfides which are mostly in the form of HS� ions innearly neutral media.18) In sulfide polluted environments,sulfide ions compete for Cu(I) ions under a much strongerdriving force than BTAH, consequently, sulfide ions canextract the Cu(I) ions from the Cu(I)BTA complex. Thisleads to the break down of the protective Cu(I)BTA film andoccurrence of corrosion on the bare areas.18)

To the date of this work, there are not known inhibitorswhich can resist the poisonous effect of sulfide ions. Thisdilemma forced the authors to think of a new method ofapplication of the azole corrosion inhibitors on the coppersurface in particular and metals in general. This methoddepends on the vacuum pyrolysis of the inhibitor in thepresence of copper specimens. Such method would coverthe copper surface with a compact protective layer of theinhibitor.

2. Experimental

Electrodes were prepared from Cu (99.9mass%) obtainedfrom Goodfellow Corporation. Copper disc specimens of1 cm diameter and 2mm thickness were polished using SiCpapers successively up to 2400 grits to acquire a mirror-likefinish. A conventional three-electrode cell was used with aAg/AgCl reference electrode, E ¼ 0:197V SHE, and a Pt

sheet counter electrode. Solutions were prepared usingdouble distilled water, BTAH (Azole (1) was purchasedfrom Sigma-Aldrich, Azoles (2) and (3) were prepared asdescribed previously19) while Na2S and NaCl were purchasedfrom Fluka Chemicals.

Potentiodynamic polarization curves were measured onthe Cu electrode in 3.5mass% NaCl containing sodiumsulfide and BTAH at a voltage scan rate of 5mV s�1. Thepotential was controlled using a Gamry Instruments poten-tiostat which was also used in measuring potentiostaticpolarization curves. Measurements were performed at25� 1�C while the electrolyte was stirred using a magneticstirrer. The surfaces of the electrodes were examined usingJEOL Ltd., JSM-6300 scanning electron microscope (SEM).The structure of the inhibitor deposited layer was determinedusing 1H-NMR technique performed by Bruker AVANCE II600MHz NMR for solutions and solids.

In the vacuum pyrolysis experiments, 3 polished speci-mens of copper were placed in a pyrex glass tube (15mmdiameter) 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-hydra-zine (Azole (2)) and N-[Benzotriazol-1-yl-(4-methoxy-phen-yl)-methylene]-N0-phenyl-hydrazine (Azole (3)) were tested.The chemical structures of the tested azoles are illustrated inFig. 1. The open part of the tube was narrowed to 3mmdiameter. The assembly was placed in liquid nitrogen tofreeze the azole inhibitor inside and then connected to avacuum pump which allows the pressure inside the tube toreach about 1.33 Pa. The tube was then sealed by heating itsneck till melt and close. The tube is finally placed in thepyrolyser which adjusted to the desired temperature (200�C).After vacuum pyrolysis for the desired time (30min); theglass tube was broken and the copper specimens were takenout and corrosion tested. In most instances, both sides ofcopper specimens are covered with the inhibitor. Under suchcondition, one side is repolished to make electrical con-nection through this side while the other side, covered withinhibitor, is exposed to the testing solution.

*Corresponding author, E-mail: ighayad@yahoo.com. Present address:

P.O. Box: 5969 Sfat 13060, State of Kuwait

Materials Transactions, Vol. 51, No. 9 (2010) pp. 1671 to 1676#2010 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

3. Results and Discussion

3.1 Electrochemical tests of copper before pyrolysisFigure 2 illustrates the effects of BTAH and of sulfide ions

on the polarization curves of copper, measured at 5mV s�1.Curve (1) refers to the blank solution (3.5% NaCl), curve (2)was measured after the electrode was pretreated in thepresence of 0.01mol L�1 BTAH for 1 h while curve (3) wasmeasured after injection of 10�3 mol L�1 HS� following theabove pretreatment. Curve (4) was obtained in the presenceof 10�3 mol L�1 HS� in the blank electrolyte. It is added forthe sake of comparison with curve (3) which was measuredafter pretreatment with BTAH. The presence of BTAHdecreases the rate of anodic dissolution of copper by aboutfour orders of magnitudes (compare curves 1 and 2). Apassive region appears in the anodic branch of the curve,which is attributed to the formation of a protective film of theCu(I)BTA complex. It extends about 500mV and ends at thebreak down potential, E�, beyond which the current increasesrapidly with potential. The sulfide ions diminish the passivitycaused by BTAH by increasing the current in the passiveregion by about three orders of magnitude (compare 3 and 1).They also shift the polarization curve much closer to that ofthe unprotected copper. Furthermore, they lower both the

breakdown potential E�, and the free corrosion potential byhundreds of mV.

3.2 Electrochemical tests of copper after pyrolysisFigure 3 represents the polarization curves of copper

tested in 0.001mol L�1 sulfide polluted salt water. Prior totesting, copper was pyrolyzed in Azole (1) (Benzotriazole),Azole (2) and Azole (3) at 200�C for 30min. The applicationof Azoles (2) and (3) result in the increase of their inhibitingefficiency to a very large extent. A wide range passive regionof about 2V is observed for these inhibitors. A very lowpassive currents (<10�9 A) are observed. These extremelylow currents indicate a 100% protection of copper. Thepolarization curve of copper treated with Azole (1) (benzo-triazole) differs clearly from those obtained in the presence ofAzole (2) and Azole (3). Although the treated copper could,to some extent, resist the 10�3 mol L�1 sulfide solution, thepassive current obtained under this condition is �10�5 Awhich is fairly high compared to the one obtained in thepresence of the other two inhibitors (>10�9 A).

Figure 4 represents the polarization curves of coppertested in 0.01mol L�1 sulfide polluted salt water. Prior totesting, copper was also pyrolyzed in Azole (1) (Benzotri-azole), Azole (2) and Azole (3) at 200�C for 30min. Coppertreated in Azoles (2) and (3) still show good passivation but

(A) (C)(B)

Fig. 1 Chemical structure of the tested azoles: (A) Benzotriazole (Azole (1)). (B) N-[Benzotriazol-1-yl-(phenyl)-methylene]-N0-phenyl-

hydrazine (Azole (2)). (C) N-[Benzotriazol-1-yl-(4-methoxy-phenyl)-methylene]-N0-phenyl-hydrazine (Azole (3)).

Current, I / A

10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1

Pot

entia

l, E

/ V

(A

g/A

gCl)

-1.0

-0.5

0.0

0.5

1.0

(1) 3.5 mass% NaCl

(2)---+ 0.01 mol L-1 BTAH(3)---+ ---+ 0.001 mol L-1 HS-

(4)---+0.001 mol L-1 HS-

Eb

(2)

(1)

(3)

(4)

Fig. 2 Effect of sulfide ions on the polarization curves of copper in

3.5mass% NaCl with and without pretreatment in 10�2 molL�1 BTAH at

25�C, see text.

Current, I / A

10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3

Pot

entia

l, E

/ V

(A

g/A

gCl)

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

Azole (2)

Azole (3) Azole (1)

Fig. 3 Potentiodynamic polarization curves of copper in 3.5mass% NaCl

containing 0.001mol L�1 HS�. Prior to testing, copper was pyrolyzed in

Azole (1), Azole (2) or Azole (3) at 200�C for 30min.

1672 F. M. Al Kharafi, N. A. Al-Awadi, I. M. Ghayad, R. M. Abdullah and M. R. Ibrahim

to a lower extent than that obtained in the 0.001mol L�1

sulfide solution. The corrosion potentials are shifted towardsmore negative values of ��0:4 and �0:2 for Azole (2) andAzole (3), respectively compared to �0:00 and 0.12Vobtained in the 0.001mol L�1 sulfide solution. Passivecurrent are also increased by about three orders of magnitude(�10�7 A) compared to those obtained in the 0.001mol L�1

sulfide solution (�10�10 A). On the other hand, coppertreated in Azole (1) could not resist the 10�2 mol L�1 sulfidesolution. Polarization curves are quite similar to thoseobtained in the sulfide solution containing dissolved benzo-triazole (see Fig. 2).

Potentiodynamic polarization curves were also used todeduce the corrosion parameters of copper namely; Corro-sion potential (Ecorr) corrosion and current (Icorr) which inturn used to calculate the inhibitor (azole) percentage in-hibition efficiency (E) according to the following equation:20)

E ¼Icorr (Blank) � Icorr (Azole)

Icorr (Blank)� 100 ð1Þ

Table 1 summarizes the results. Extremely low corrosioncurrents and very high inhibition efficiencies reach as high as100% were obtained for copper pyrolyzed in the presenceof Azoles (2) and (3). On the other hand considerable lowinhibition efficiency (79%) was obtained for copper pyro-lyzed in the presence of Azole (1) and tested at 0.01mol L�1

HS�. The shift of Ecorr towards more anodic values comparedto the untreated copper is another evidence of the enhance-ment of corrosion resistance of copper subjected to vacuumpyrolysis in the presence of azole inhibitors.

Figure 5 represents potentioststatic polarization curvesof copper tested at 0.5 V (Ag/AgCl) in 0.01mol L�1 sulfidepolluted salt water. Prior to testing, copper was pyrolyzedin Azole (1), Azole (2) and Azole (3) at 200�C for 30min.Steady state currents of 10�2, 10�10 and 10�8 A wereobtained for copper pyrolyzed in Azole (1), Azole (2) andAzole (3), respectively. The extremely low current shown bycopper pyrolyzed in Azoles (2) and (3) again show a 100%inhibiting efficiency which supports the results of potentio-dynamic tests. On the other hand, the high steady currentshown by Azole (1) indicates its failure to resist sulfideattack. Copper pyrolyzed in Azole (1) was also tested in0.001mol L�1 sulfide solution. Under this condition, it showsa steady current of 10�5 A. This low steady current indicatesthat Azole (1) could resist acceptably the low concentrationsof sulfide ions.

3.3 Surface characterizationFigure 6(a) and (b) represents the surface of copper

subjected to vacuum pyrolysis at 200�C in the presence ofAzoles (2) and (3). Images show that copper surface iscovered with a black layer of the inhibitor. However, Azole(2) shows some degree of roughness while Azole (3) show asmooth surface. Figure 7(a) and (b) illustrates cross sectionsof copper samples subjected to vacuum pyrolysis in thepresence of Azole (2) and Azole (3). Images differentiate twodistinct layers; one represents the copper metal while theother represents a compact layer of the inhibitor depositedon the copper surface. The thickness of this layer (in itsmaximum) was shown to be 128 and 59 mm for the Azole (2)

Current, I / A

10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1

Pot

entia

l, E

/ V

(A

g/A

gCl)

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Azole (1)Azole (3)

Azole (2)

Fig. 4 Potentiodynamic polarization curves of copper in 3.5mass% NaCl

containing 0.01mol L�1 HS�. Prior to testing, copper was pyrolyzed in

Azole (1), Azole (2) or Azole (3) at 200�C for 30min.

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 3.5mass% NaCl

containing either 0.001 or 0.01mol L�1 sulfide ions.

Blank

(Cu-only)Azole (1) Azole (2) Azole (3)

CHS�/mol L�1 0.01 0.001 0.01 0.001 0.01 0.001 0.01

Icorr/A 1:54� 10�4 1:39� 10�8 3:18� 10�5 1:1� 10�11 4:24� 10�9 7:45� 10�12 9:6� 10�9

E (%) 99.991 79.351 100.00 99.997 100.00 99.994

Ecorr/V �0:927 �0:349 �0:954 0.0167 �0:397 0.119 �0:205

Time, t / s

0 2000 4000 6000

Cur

rent

, I /

A

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Azole (2) + 0.01 mol L-1 HS-

Azole (3) + 0.01 mol L-1 HS-

Azole (I) + 0.001 mol L-1 HS-

Azole (1) + 0.01 mol L-1 HS-

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 30min.

Novel Technique for the Application of Azole Corrosion Inhibitors on Copper Surface 1673

and Azole (3) respectively. These results show that the layerformed in the presence of Azole (2) to be thicker than thatformed in the presence of Azole (3) which may account forthe slightly lower current obtained for copper treated in thepresence of Azole (2) (10�10 A) compared to the oneobtained in the presence of Azole (3) (10�8 A).

Figure 8 shows the surface of copper potentiostated in0.01mol L�1 sulfide polluted salt water after removal ofthe deposited layer of Azole (2) (a) and Azole (1) (b). Thesurface of copper treated in Azole (2) shows very goodappearance with no corrosion attack. On the other hand thesurface of copper treated in Azole (1) suffers from severcorrosion attack. These images are in full agreement withthe electrochemical tests which show excellent corrosionresistance for Azole (2) and poor corrosion resistance forAzole (1).

It was interesting to determine the chemical structure ofthe deposited inhibitor layer. Azole (3) was taken as example.After pyrolysis and corrosion testing of Azole (3), coppersamples were taken and immersed in chloroform to dissolvethe coat and then analyzed using 1H-NMR. Figure 9(a)

represents the 1H-NMR chart of Azole (3) before pyrolysiswhile Fig. 9(b) shows the 1H-NMR chart of the dissolvedcoating. It is clear that the structure of the coating is nearlythe same as that of Azole (3) before pyrolysis.

Surface investigations revealed that the deposited inhibitorlayer is distinct and differentiated from the metal surface(see Fig. 7). 1H-NMR charts shows that the structure ofthe coating resembles that of the mother material beforepyrolysis. These observations suggest that there is nochemical interaction between the metal and the inhibitorand consequently look to the vacuum pyrolyisis of azoles as akind of coating rather than chemisorption. The process lookslike hot-melt dip coating. When azole reaches a temperaturehigher than its melting point it transfers into liquid whichspreads and adheres to the surface of copper. The adheredlayer behaves like a barrier hydrophobic coating thateffectively prevent electrolyte from contacting the metalsurface and excellently resist corrosion attack. The differencein efficiency between different azoles can be related to theproperties of the formed barrier coating. Azole (1) (benzo-triazole) is weakly soluble in water while Azoles (2) and (3)

(a) (b)

Fig. 6 SEM micrographs of copper pyrolyzed at 200�C for 30min 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 30min at 200�C.

1674 F. M. Al Kharafi, N. A. Al-Awadi, I. M. Ghayad, R. M. Abdullah and M. R. Ibrahim

are not soluble at all. Depending on the afore mentionedfacts, it is expected that the formed barrier coating by Azole(1) allows some penetration of the electrolyte to the coppersurface and hence enables corrosion to occur. On theother hand, barrier coatings formed by Azoles (1) and (3)acts as perfect insulators preventing the penetration of theelectrolyte.

4. Conclusions and Future Outlook

The present work explores the application of azoles, someof which are new corrosion inhibitors for copper, using newmethodology depends on the vacuum pyrolysis of azoleinhibitor in the presence of copper specimens. The promisingresults obtained in this work encourages further application

Aromatic Hs

NH

OCH3

Solvent

(a)

SolventOCH3

Aromatic Hs

NH

(b)

Fig. 9 1H-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.

(a) (b)

Fig. 8 SEMmicrographs of copper surface potentiostatic testing in 3.5mass% NaCl containing 0.01mol L�1 sulfide ions after removal of

the deposited layer of Azole (2) (a) and Azole (1) (b).

Novel Technique for the Application of Azole Corrosion Inhibitors on Copper Surface 1675

of this new methodology to other azoles and also to othermetals. This also encourages investigation of this method tolarge scale application.

Vacuum pyrolysis of Azole (2) and Azole (3) on copperleads to the deposition of a compact layer of the inhibitoron the copper surface. The thickness of this layer (in itsmaximum) was shown to be 128 and 59 mm for the Azole (2)and Azole (3) respectively. Surface investigations revealedthat the deposited layer is distinct and differentiated from themetal surface. This layer could resist the poisoning effect ofsulfide 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 bythe Research Administration of Kuwait University, GrantNumbers SC06/07 and GS01/03. They also acknowledge theuse of the scanning electron microscope (SEM).

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1676 F. M. Al Kharafi, N. A. Al-Awadi, I. M. Ghayad, R. M. Abdullah and M. R. Ibrahim