Effects of Phosphate Ions on Copper Dissolution and Passivation

Abstract

The dissolution and passivation of copper was studied in the presence of phosphate and phosphate + chloride in aqueous solution at pH 8 for temperatures ranging from 5 to 60°C. The copper oxidation current increased with (i) the concentration of phosphate ions in chloride‐free solutions, (ii) the concentration of chloride ions in solution containing phosphate ions, and (iii) the temperature in both solutions. Two passivation mechanisms are postulated. Below 30°C, a protective base layer of Cu(II) compounds was formed while between 30 and 60°C a porous layer of a Cu(I) compound was formed first on the electrode surface followed by the growth of Cu(II) passive film. The presence of Cu(II) compound was necessary to slow down the copper dissolution. High [phosphate]/[chloride] ratio was favorable for the formation of a thin, compact, passive layer while a high [chloride]/[phosphate] ratio led to the precipitation of a thick porous protective deposit on the copper surface.