Cathodic self-healing at cut-edges: The effect of Zn2+ and Mg2+ ions

At the cut edge of galvanised steel sheet a small area of exposed zinc has to protect a comparatively large steel area. This should lead to a short system lifetime, if the oxygen reduction on the steel were not quickly inhibited. This “cathodic self-healing” is widely assumed to be due to the formation of zinc corrosion product precipitates. It will be shown that, at least in the early stages of corrosion, this is rather due to a direct modification of the iron oxide by zinc cations, enhanced by magnesium cations, causing an inhibition of oxygen reduction directly on the steel.

► Self-healing effect by zinc cations at the cut edge correlated with the iron oxide.
► This is due to a significant stabilisation of the Fe3+ states in the passive film.
► The presence of Mg cations enhances these effects at low Zn2+ concentrations.
► This explains the positive effect of Mg in ZnMg at short exposures.