EFFECTS OF PHOSPHATE IONS ON COPPER DISSOLUTION AND PASSIVATION

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-degrees-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-degrees-C, a protective base layer of Cu(II) compounds was formed while between 30 and 60-degrees-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.