A Study of the Anodic Behaviour of Nickel in Chloride Solutions Containing Thiourea

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Abstract
The anodic behaviour of nickel has been studied in chloride solutions containing various concentrations of thiourea, thioacetamide and thiosemicarbazide and it has been shown that the activation overpotential is a minimum at a critical concentration of the organic compound. In the case of thiourea the same critical concentration appears to have the maximum effect in lowering cathode polarization during electrodeposition of nickel. The results have been interpreted in terms of the actual and relative concentrations of the sulphur compound and its decomposition products (the sulphide and bisulphide ions). The sulphur anions are considered to facilitate egress of nickel ions, whereas the undecomposed sulphur compounds inhibit dissolution, presumably by forming a chemisorbed layer. The nature of the corroded surface of the nickel has been studied by electron microscopy and it has been shown that the presence of the sulphur compounds modifies the characteristic hemispherical shape of pits which occur in chloride solutions.