During the pitting of magnesium in chloride solution a film of magnesium chloride forms on the corroding surface. This salt film is apparently a diffusion barrier for water and prevents repassivation by oxide. Properties of magnesium chloride layers were determined on corroding, high purity magnesium shielded electrodes (one-dimensional pits). Electrical transient experiments showed that the salt film thickness is proportional to applied anodic potential and the conducton is by the high field mechanism.