A STUDY OF THE ELECTROCHEMICAL PROPERTIES OF NIRESIST IN 3-PERCENT SODIUM-CHLORIDE SOLUTION

In this work, the electrochemical properties of NiResist in 3% NaCl so lution at 25-degrees-C have been investigated using rotating disc volt ammetry. The value of the corrosion potential in aerated and deaerated solutions and the dependence on electrode rotation speed showed that oxygen reduction is the cathodic process and that this is under kineti c control rather than mass transport control. It is also shown that ox ygen reduction will occur preferentially, but not exclusively, on the graphite areas of the NiResist electrodes. The anodic process is found to have two Tafel regions. At low current densities or low overpotent ials, a slope of ca 70 mV decade-1 is observed and this is explained i n terms of a mechanism involving chloride ion adsorption at low pH. At high current densities, or high overpotentials, a slope of ca 49 mV d ecade-1 is found and a mechanism with only hydroxide ions is proposed to explain this. The changeover in surface pH required for this system of two mechanisms is suggested to arise from the reduction of a passi ve oxide layer.