The behavior of corrosion layers on Pb in solutions is shown to resemble that of perm‐selective precipitation membranes, prepared by interdiffusion of Pb+2 ions and ions into cellophane foil. Diffusion potentials in these precipitation membranes follow, over a wide pH‐range, the equation and can reach values of several hundred mV. The lower limit of that electrode potential range where (and/or ) forms in the interior of the corrosion film, underneath a perm‐selective layer, that is, −0.4V (or +0.5V) vs. , can be predicted theoretically in a proper manner by taking into account the diffusion potential in the film.