Impurity effects on the macromorphology of electrodeposited zinc II. Causes, appearances and consequences of spongy zinc growth

Abstract

The effects of minor impurity components in zinc electrodes (metals which are more electropositive than zinc or which exhibit rather low hydrogen overpotentials) were investigated to obtain information regarding the initiation and development of the spongy growth of zinc and its suppression in the electrowinning of zinc from binary chloride systems. Several types of metallic impurity result in the appearance of spongy zinc: (a) impurities such as copper which grow independently and locally and further perturb the growth surface; (b) impurities such as nickel and cobalt which are codeposited as alloys with zinc and, after attaining a critical content in the growth surface, affect either the mossy growth or dissolution of zinc with the evolution of hydrogen; (c) ionic species such as iron which neither codeposit locally nor alloy with zinc but have a catalytic effect on the spongy growth of pure zinc; (d) impurities such as molybdate which create dense compact bright zinc deposits and suppress the detrimental effects of other impurities, and in particular neutralize the synergic effects of iron, nickel and cobalt.