Pitting and electrodeposition

A steel electrode was polarized in sodium chloride solution in order to obtain a lot of deep pits. The pitted specimen was then polished so as to remove the passive layer, but to preserve the cavities. The prepared electrode was then cathodically polarized in copper sulfate solution to obtain copper crystallisation. Microscopic examination shows that the cavities were surrounded by rings where the copper crystallite density is higher than on the surface far from the pits. Some hypotheses are proposed to explain this inhomogeneous deposit.