Superimposed a.c. is found to favor the dissolution of lead while d.c. helps the formation of lead dioxide. The role of a.c. is thus similar to that of a "forming agent," like nitrate, the use of which tends to boost the formation of lead dioxide. This view is supported by measurements of anode potentials and the time needed for passivation. The porous' nature of the surface film, indicated by the resistance measurements, explains the continued formation of lead dioxide. A probable mode of attack of the lead surface is given based on the microscopic examination of the corroded surface.
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Vijayavalli et al. (1963) studied this question.