During the pitting of magnesium and other metals in halide solutions.a film of halide salt of the metal forms on the corroding surface. This @&it film plays a major role as a diffusion barrier for water and prevents repassivation by oxide. Properties of magnesium chloride layers were determined on corroding high-purity magnesium shielded electrodes or one-dimensional pits. Electrical transient experiments showed that the salt film thickness is proportional to applied anodic potential and that conduction is by the high field mechanism. Water diffusion rate decreases with film thickness.