On the zinc and copper dissolution in phosphate solutions

The copper and zinc dissolution in phosphate solutions is studied over wide pH range (from 4.5 to 11.7). The cathodic reaction rates are shown to depend on the pH of the solutions; they are minimum at a pH of 9.5 for zinc and 11.7 for copper. Phosphate directly participates in the cathodic process on zinc because of the reduction of phosphate to phosphite anions. Thermodynamic calculations, corroborated by electrochemical studies and analyses of the electrode surfaces by different methods, showed that zinc passivates itself by the deposition of either ZnO, Zn(OH)(2), Zn(H2PO4)(2), and ZnHPO4, in the solutions of low acidity, or ZnO, Zn(OH)(2), ZnHPO4, and Zn-3(PO4)(2) at a pH of 9.5 to 11.7. On copper, the passive films are formed of CuH2PO4 and CuHPO4 in weakly acid phosphate solutions or of CuO, Cu(OH)(2), CuHPO4, and Cu-3(PO4)(2) at a pH of 9.5 to 11.7.