Iron-group metals hardly precipitate in Zn sulfate solutions by cementation with Zn dust, although the less noble Cd than iron-group metals can precipitate easily. The partial polarization curves of irongroup metals indicated that the potential, at which an appreciable deposition of iron-group metals began, was polarized apparently in Zn sulfate solution nearly to reach the equilibrium potential of Zn. This fact was directly responsible for their extremely small deposition rate. According to the hydroxide suppression mechanism proposed for anomalous codeposition of iron-group metals with Zn, the minimum overpotential necessary to initiate iron-group metal deposition was composed of an inherent deposition overpotential, η°, and an extra-overpotential, ηInh, appearing due to inhibitory Zn hydroxide adsorbed on the cathode. Consequently, it was predicted that any countermeasurs to reduce either η° or ηInh might promote the precipitation rate of iron-group metals. This prediction was confirmed ernpirically by the precipitation behavior of iron-group metals in Zn ion-free sulfate solution, in Zn sulfate solution at elevated temperature or in Zn sulfate solutions containing catalytic chloride or thiocyanate ions. Further the mechanism, which permited the high removal rate of iron-group metals in conventional cementation by Zn dust with arsenous oxide in the presence of cupuric ions, was briefly discussed.
