THE BEHAVIOR OF CU-NI-X (X=10 TO 40 WT-PERCENT) 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 co mposition 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 electrochemi cal reactions occurring both at the metal/oxide and at the oxide/elect rolyte interfaces. Interesting behaviour of Cu-Ni-x alloys is observed in the presence of chloride ions, where passive layers formed on thes e materials suffer localized breakdown. It was found that a general re lationship E(c) = a + b log c(NaCl) is established in all cases, howev er, with constants a and b being strongly dependent on the nickel cont ent in the alloy and the chloride concentration range. A critical chlo ride concentration, c(crit), exists below which the resistance to loca lized corrosion increases with decreasing nickel content, and above wh ich it increases with increasing nickel content. The results are discu ssed in terms of segregation of the alloying element (Ni), formation o f charged solutes and their subsequent complexing with mobile;cation v acancies, ie according to the Solute Vacancy Interaction Model (SVIM). (C) 1997 Elsevier Science Ltd.