Abstract
The present study represents an investigation of the effect of thiourea, and molecularly related compounds, on the anodic behaviour of nickel in chloride solutions which were either O[2]-free or in equilibrium with in the atmosphere. The results have been presented in the form of E[H]/i plots and Tafel relationships, and it has been shown that the action of thiourea at a nickel anode is concentration dependent. A mechanism has been suggested to explain the effects of thiourea concentration in terms of thiourea adsorption by the nickel electrode, and further experiments in the presence of both thiourea and hydrogen sulphide, and also a three stage experiment have produced results which support this theory. A similar sequence of experiments has been carried out using thioacetamide, and thiosemicarbazide, and almost identical results obtained. Electron microscopy has been used in an attempt to relate the metallographic aspects of nickel dissolution with the electrochemical results. It has been shown that additions of thiourea - and molecularly related compounds -significantly alter the type of attack on nickel in chloride solutions, producing crystallographically orientated etch pits. Although this work is directed towards a better understanding of the anodic behaviour of nickel, an extensive literature survey of the effects of sulphur containing organic compounds on the cathodic electrodeposition of nickel has indicated that the effects of concentration noted during the present studies are reflected in the effects of thiourea concentration on the properties of nickel electrodeposits. A comparison of these results, Maurer's(1) results, and those of the present work, indicates a possible grouping of thio compounds with respect to their effects at nickel electrodes, depending on the electron bonding and hence the reactivity of the sulphur atom (or atoms) in the molecule. Some observations have been made on the mechanism of thiourea action as an inhibitor and the reported abnormal effects of thiourea on the o scale of potentials have been explained in terms of the incapability of the o scale to account for strong chemisorptive links in the corroding metal/inhibitor system.