The behaviour of Cu-Ni-x (x=10 to 40 wt%) alloys in alkaline solutions containing chloride ions

An electrochemical study in combination with microscopic investigation and photopotential measurements was performed for Cu-Ni-x alloys (x = 10 to 40 wt%) and Cu and Ni metals in slightly alkaline solution, pH = 9.2. Cu-Ni-x alloys behave similarly to copper metal. The general composition of the passive layer can be described by an outer CuO/Cu(OH)(2) layer overlying a Cu2O barrier layer, which contains incorporated nickel cations. The results are discussed with respect to electrochemical reactions occurring both at the metal/oxide and at the oxide/electrolyte interfaces. Interesting behaviour of Cu-Ni-x alloys is observed in the presence of chloride ions, where passive layers formed on these materials suffer localized breakdown. It was found that a general relationship E(c) = a + b log c(NaCl) is established in all cases, however, with constants a and b being strongly dependent on the nickel content in the alloy and the chloride concentration range. A critical chloride concentration, c(crit), exists below which the resistance to localized corrosion increases with decreasing nickel content, and above which it increases with increasing nickel content. The results are discussed in terms of segregation of the alloying element (Ni), formation of charged solutes and their subsequent complexing with mobile;cation vacancies, ie according to the Solute Vacancy Interaction Model (SVIM). (C) 1997 Elsevier Science Ltd.