Effect of Sulfide-Containing Solutions on the Corrosion of Cu-Ni Alloys; Electrochemical and Surface Studies

The copper-nickel alloy (CuNi 90/10) is extensively used in different oil industries and marine structures. Some of the main usage of this class of alloys is piping in seawater and heat exchangers. In several occasions, the alloy is exposed to sulfide-containing solutions of chloride and sulfate. There is limited information available on the effect of sulfide on the corrosion rate of Cu-Ni alloy and the interrelation between the surface structure of the films formed and the concentration of metal ions dissolved in the electrolyte side. A systematic study is required to understand the role of sulfide both in promoting film formation and dissolution of the alloy in presence of sodium chloride, sodium sulfate and sulfuric acid. We found that the presence of sulfide ions with a concentration of 20 ppm resulted in a relative increase in the rate of corrosion compared to sulfide-free electrolytes. The rates of corrosion are 3.98, 3.13, 5.30, 9.30, 13.1 mm year^-1 in 0.1 M H₂SO₄, 0.1 M Na₂SO₄, 0.58 M NaCl, 0.1 M H₂SO₄ + 0.58 M NaCl, and 0.58 M NaCl + 0.1 M Na₂SO₄, respectively [1]. The corresponding rates of corrosion in the same electrolytes containing 20 ppm sulfide ions are: 6.22, 14.52, 20.36, 15.35, and 23.75 mm year^-1, respectively. The rate of corrosion of the alloy is highest in case of 0.58 M NaCl, and 0.58 M NaCl + 0.1 M Na₂SO₄ containing 20 ppm sulfide. Electrochemical impedance spectroscopy (EIS) measurements indicate that relatively thicker passive films are formed over the alloy surface in these electrolytes as indicated in Figure 1. Moreover, the film thickening increases with time of exposure of the electrode in these electrolytes. This finding is ascertained from the systematic experiments and data obtained from FE-SEM images (Figure 2), XPS analyses of the films morphologies and structures, and electrolyte composition determined from ICP-MS measurements. The alloy was exposed to different time intervals (namely 30 s, 30 min and 60 min, respectively) of anodizing potential of +0.15 V vs. Ag/AgCl in different electrolytes. The concentration of Cu and Ni ions in the electrolyte side progressively increases with time of exposure in all electrolytes studied. The relative concentration is lowest in Na2SO4 solution containing sulfide, and is highest in Na₂SO₄ + NaCl. The composition of the passive film reflects that beside oxides/hydroxides, metallic sulfides are also formed. The protective layer is formed mainly of Cu₂O and NiO₂with ratios that differ according to the type of electrolyte. A possible mechanism for the corrosion of Cu-Ni alloy in presence of sulfide is as follows [2]:

O₂ + 2H₂O + 4e^-  =  4OH^-

S^2- + H₂O  =  HS^- + OH^-

Cu + HS^-  =  Cu(HS^-)_ads

Cu(HS^-)  =  Cu(HS) + e^-

Cu(HS)  =  Cu⁺ + HS^-

2Cu⁺ + HS^- + OH^-  =  Cu₂S + H₂O

 [1] F.M. Al-Kharafi, A. Abdel Nazeer, R.M. Abdullah, A. Galal, ECS Transactions, 58 (xx) 2013.

[2] K. Rahmouni, M. Keddam, A. Srhiri, H. Takenouti, Corros. Sci., 47, 3249-3266, 2005.