Sulfide Induced Intergranular Corrosion of Copper in Salt Water Containing Benzotriazole

This paper investigates the effect of pretreatment with benzotriazole on the corrosion of copper in sulfide polluted salt water. The treatment of copper in the presence of benzotriazole under a mild oxidizing potential reveals the remarkable inhibition efficiency of BTAH reflected in a passive current of less than 0.1 μA cm−2 after one hour. Upon injection of 10−3 M HS−, a rapid increase in current to about 200 μA was obtained. The current jump seen after injection of sulfide ions is due to localized intergranular corrosion as shown by the scanning electron microscopy (SEM) examination. Copper suffered only localized corrosion occurred at the grain boundaries while the grain surfaces remained well protected. Under this condition, sulfur was detected using energy dispersive spectroscopy (EDS) at the grain boundaries, but not on the grain surfaces. X-ray photoelectron spectroscopy revealed the presence of sulfide ions and BTAH on the corroded surface. [DOI: 10.1380/ejssnt.2011.306]


I. INTRODUCTION
Copper and many of its alloys have favorable combinations of mechanical, thermal and electrical properties.Hence they are manufactured in various forms and utilized in many industries, particularly in marine environments.The corrosion of copper and many of its alloys in such environments has been studied.The stability of copper and its alloys in such environment is attributed to the formation of a protective film of corrosion products which contains Cu 2 O, Cu 2 (OH) 3 Cl and other products [1][2][3][4][5].
It is now widely recognized that many natural water bodies and industrial waste water streams are polluted with hydrogen sulfide.For example, formation waters in sour oil and gas wells are heavily contaminated with hydrogen sulfide which promotes metallic corrosion [6].Sulfide ions are known to promote the corrosion of copper [7][8][9][10][11][12].This is believed to occur via an initial adsorption step, i.e.HS − + Cu = Cu.HS − (ads) (1) Cu.HS − (ads) = CuS (s) + H + + 2e − (2) where Cu.HS − (ads) refers to an adsorbed HS − ion on the copper surface.Sulfide attack on copper and its alloys is particularly relevant to the proposed use of copper canisters for the disposal of high level nuclear waste deep in granite environment [10,11] and the rapid failure of copper nickel condenser tubes in wet hydrogen sulfide atmosphere [12].
Benzotriazole (C 6 H 5 N 3 , BTAH) has long been known as an inhibitor for the corrosion of copper and many of its alloys.The remarkable inhibiting efficiency of BTAH is attributed to the formation of a protective film of Cu(I)BTA on the surface, which inhibits the anodic corrosion reaction [13][14][15][16][17][18][19][20].This mechanism of inhibition is well accepted in unpolluted media.Benzotriazole loses its inhibition efficiency when the medium is contaminated with sulfide ions [21][22][23].Most work dealt with the effect of sulfide pollution on the polarization curves of copper * Corresponding author: faizaalkharafi@yahoo.com Current / A cm -2 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 - in the salt water with very little attention to the surface characterization of corroded copper surface.The objective of this work is to assess the efficiency of pretreatment of copper with BTAH on its resistance to sulfide attack.The work focused on the surface morphology of copper pretreated in benzotriazole prior to the injection of sulfide ions.

II. EXPERIMENTAL
Electrodes were prepared from electrolytic copper (99.9%) in the form of rods having 0.96 cm diameter.The working electrode was the cross sectional area of the rod while the immersed length of the rod was coated with a protective adhesive so that the cross sectional area is only exposed to the solution.Electrical contact to the external circuit was made through the rod.The working electrodes were polished using SiC papers successively up to 2400 grits, followed by 0.3 micron alumina to acquire a mirror-like finish.
Solutions were prepared using deionized water, Analar Na 2 S and NaCl from Fluka, BTAH from Aldrich.Mea-e-Journal of Surface Science and Nanotechnology Volume 9 (2011)  surements were performed in 3.5% NaCl containing 10 −2 M BTAH in the absence and in the presence of 10 −3 M Na 2 S. In view of the values of pK1 and pK2 of H 2 S (7 and 14, respectively), the predominant sulfide ions in the electrolyte is HS − at pH values from 9 to 12 at 25±1 • C. Measurements were performed while the electrolyte was open to air and stirred using a magnetic stirrer.Potentiodynamic and potentiostatic polarization curves were measured on the Cu electrodes using a conventional three-electrode cell was used with a Ag/AgCl reference electrode, E = 0.197 V SHE, and a Pt sheet counter electrode.The potential was controlled using a Gamry potentiostat.

III. RESULTS AND DISCUSSION
Figure 1 illustrates the effects of BTAH and of sulfide ions on the polarization curves of copper.Curve (a) refers to the blank solution (3.5% NaCl), curve (b) was measured after the electrode was pretreated in the presence of 10 −2 M BTAH for 1 h while curve (c) was measured after injection of 10 −3 M HS − following the above pretreatment.The presence of BTAH decreases the rate of anodic dissolution of copper by about four orders of magnitudes (compare curves a and b).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 for about 500 mV and ends at the break down potential, E b , 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 c and b).They also shift the polarization curve much closer to that of the unprotected copper.Furthermore, they lower both the breakdown potential E b , and the free corrosion potential by hundreds of mV.The corrosion parameters; corrosion potential (E corr ), corrosion current (I corr ) , β a , β c and polarization resistance (R p ) are summarized in Table I.The lowest I corr as well as the highest R p is given by copper treated in 3.5% NaCl containing 0.01 M BTAH indicating the high effieiciency of BTAH to inhibit corrosion of copper under such condition.
Potentiostatic current transients were measured in the BTAH inhibited electrolyte at a potential of 0.0 V (Ag/AgCl), before and after injection of sulfide ions.This potential was chosen to be in the passive region.Figure 2 illustrates the results.The remarkable inhibiting efficiency of BTAH is reflected in a passive current of less than 0.1 µA cm −2 in the presence of 10 −2 M BTAH after one hour (curve a).Upon injection of 10 −3 M HS − , a rapid increase in current to about 200 µA was obtained (curve b).The magnitude of this sudden increase in current upon injection of sulfide ions is taken as a measure of the intensity of sulfide attack.The exceedingly low currents (∼0.1 µA) shown before the injection of sulfide ions (curve a) point to a well protected surface.The current jump seen after injection of sulfide (curve b) is due to localized intergranular corrosion occurred only at the grain boundaries as shown below.
Figure 3(b) shows the morphology of copper surfaces treated at 0.0 V (Ag/AgCl) for 1 h in the salt solution containing 10 −2 M BTAH followed by injection of 10 −3 M HS − and further potentiostating for another hour.Only localized intergranular corrosion occurred at the grain boundaries with minor attack on the grains surfaces.The morphology of the grains surfaces resembles that of the untreated copper surface (Fig. 3(a)).A closer look at the topography of the corroded surfaces given in Fig. 3(b) reveals a moderate degree of intergranular attack.The occurrence of intergranular attack explains the increase in current upon injection of HS − ions (curve b in Fig. 2).A previous work of the authors documented the occurrence of extensive corrosion on the grain surfaces in addition http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) to the grain boundary attack [11].Benzotriazole imparts protection on the grain surfaces but not the grains boundaries.
The corrosion products were characterized using energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS).EDS results were obtained from regions inside and outside the grain boundaries in Fig. 3(b).Figure 4(a) shows EDS spectra on the grain surface while Fig. 4(b) shows EDS spectra inside the grain boundary.A weak sulfur signal appears in the EDS spectrum recorded off the grain boundary while no sulfur signals were detected on the grain surface.
Figures 5(a ing the cleanness of the surface.Figures 5(c)-(f) illustrates the XPS spectra obtained from the copper surface that was imaged in Fig. 3(b).Copper signal was detected at 935.1 (Fig. 5(c)) corresponding to Cu(1)BTA surface polymer [24].An oxygen signal was detected at 531.1 (Fig. 5(d)) which can be associated with water trapped beneath the surface polymer layer during Cu(1)BTA film formation [24].The spectra show a sulfur signal at a binding energy of 162.8 eV (Fig. 5(e)).This is characteristic for copper sulfide [4,25].Figure 5(f) shows nitrogen signals at binding energies of 399.05 eV (N 1s), 396.15 eV (N 1s A), 400.14 eV (N 1s B) and 397.76 eV (N 1s C).These signals correspond to C-N, N-H and NH ads [25] which indicates the presence of BTAH on the corroded surface, even after injection of sulfide ions.
The results show that the pretreatment in the presence of BTAH leads to the development of a Cu(I)BTA film on the copper surface.The first step in the process is chemisorption followed by polymeric formation with copper ions provided by the Cu 2 O layer covering the copper surface in alkaline medium, i.e. [26] n(BTA-H) ads + nCu ←→ [Cu(BTA)] n + nH + + ne − .(3) The copper surface was subjected to localized intergranular corrosion after injection of sulfide ions.This can be related to the fact that the grain boundary region is often less homogeneous and more rich in impurities than the grain surfaces due to segregation of impurities to the grain boundaries [27][28][29].As a result, the Cu(I)BTA film http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology on the grain surface is much stronger than that which forms at the grain boundaries.The breakdown of the protective film at the grain boundary is attributed to the ability of sulfide ions to compete BTA and replaces it in the Cu(I)BTA film, i.e.where "aq" represents aqueous phase and "f" represent the film.

IV. CONCLUSIONS
Intergranular corrosion of copper was documented in sulfide polluted salt water in the presence of benzotriazole.The pretreatment of copper in the presence of BTAH leads to the development of a Cu(I)BTA film on the copper surface.After injection of sulfide ions, the copper surface suffered from localized intergranular corrosion while the grains surfaces remained well protected.copper sulfide was detected only at the corroded grain boundaries while some BTAH remains on the surface.The breakdown of the protective film at the grain boundary is attributed to the ability of sulfide ions to compete BTA and replaces it in the Cu(I)BTA film.

FIG. 1 :
FIG. 1: Effect of sulfide ions on the potentiodynamic polarization curves of copper in 3.5% NaCl with and without pretreatment in 10 −2 M BTAH.Measurements performed at a scan rate of 5 mV s −1 at 25 • C.

FIG. 3 :
FIG. 3: SEM micrographs of copper potentiostated at 0.0 V (Ag/AgCl) and 25 • C for 1 h in 3.5% NaCl in the presence of 10 −2 M BTAH followed by injection of 10 −3 M HS − and further potentiostating for another hour.

FIG. 4 :
FIG. 4: EDS spectra of (a) the grain surface region and (b) the grain boundary region (shown in Fig. 3(b)) of copper potentiostated at 0.0 V (Ag/AgCl) and 25 • C for 1 h in 3.5% NaCl containing 10 −2 M BTAH before the injection of 10 −3 M HS − and further potentiostating for another hour.

TABLE I :
Variation of corrosion parameters of copper (Ecorr, Icorr, βa, βc, and Rp) treated in 3.5% NaCl in the absence and in the presence of BTAH and sulfide ions.