Impurity effects on the macromorphology of electrodeposited zinc I: Theoretical considerations and a review of existing knowledge

A comprehensive survey of the existing literature on the spongy electrogrowth of zinc is given and discussed. Some conclusions relating to the appearance of mossy zinc and its features are also drawn. It is shown that spongy zinc deposits represent natural crystallization at lower current densities and consist of small microcrystals of the same lattice characteristics as those of the compact electrogrowth modes. Some impurities and additives enhance whereas others rather retard or even suppress the mossy growth of zinc, creating various induction periods for spongy electrogrowth (the synergetic amplification and counteraction effect). It is shown that spongy deposits grow whenever the exchange current approaches or exceeds the limiting current and hence the appearance of mossy zinc is a feature of chloride rather than of sulphate or alkaline media. The spongy growth appears to be mass transport controlled. The situation becomes even more complex because the mossy deposit frequently accompanies black zinc oxide as excess metallic zinc, while the crystalline forms at and close to the limiting current appear to be of the same nature even under microscopy analysis.