Mixed potential measurements in the elucidation of corrosion mechanisms—II. Some measurements

Abstract

The behaviour of the mixed potential as a function of oxidant concentration and agitation is examined for a variety of corrosion reactions. Using the theory developed in a previous paper, this information is used to elucidate the mechanism of each reaction. The dissolution of mercury, copper and silver in the presence of ferric ions was studied. For the Fe(III)-CuSO 4 2− system, it was shown that the anodic dissolution half-reaction was activation controlled and the cathodic half-reaction was diffusion controlled. Changing the anion to Cl − made both half-reactions diffusion controlled. For the Fe(III)AgNO 3 − system, the anodic half-reaction is diffusion controlled the ferric reduction being activation controlled. In the attack of ferric ions on mercury in nitrate solution, both half-reactions are activation controlled. The metal oxidation reaction in which silver is oxidized to silver iodide by the triiodide ion was considered. It was shown that the ir drop in the AgI layers had a negligible effect on the corrosion potential. An examination of the behaviour of the corrosion potential in the Cu(II)Zn metal displacement reaction enabled estimates to be made of the anodic area at various times during the reaction. It was shown that in the first stage of the reaction the anodic area is fairly constant and equal to the geometric area of the zinc. As the second stage is entered, the anodic area suddenly drops to about 3 per cent of the geometric area, at which level it remains constant.