Galvanic Corrosion in the Copper/Zinc System: II. Effect of NH 4 + -ion

The effect of increasing concentrations of NH⁺ ₄-ion on the open circuit potentials of Zn, Cu and three Cu–Zn alloys in 0.1 M Na₂SO₄ was investigated. The effect of the NH₄ ⁺-ion additions on potential distribution and the corrosion currents developing between Zn anodes and Cu or Cu-Zn alloy cathodes was determined. The additive increased the polarisability of the Cu (alloy) cathodes and flattened the S-shaped distribution curves. The additive caused a marked change in the distribution of corrosion attack on the Zn plates.

The galvanic corrosion currents were practically independent of the NH₄ ⁺-ion content of the solution, but were considerably lower than those calculated from the loss in the weight of the Zn anode. This is attributed to the formation, on the surface of the Zn anode, of numerous microgalvanic cells resulting from the reduction of copper ammine complexes.