Corrosion protection and surface potential distribution of zinc coated steels

Surface potential distribution of Zn and Zn-Al alloy coated steels with a large scratch was measured under a thin electrolyte layer using the Kelvin sensor in order to study the sacrificial corrosion protection effect due to the Zn and Zn-Al alloy coatings. The measurement was conducted under the thin electrolyte layer containing NaCl and MgCl₂ at 25°C and at 80%RH or 98%RH. The scratched part, where the steel substrate was directly exposed to the electrode layer, showed less noble potential as the electrolyte layer becomes thicker and the concentration of chloride ion becomes higher, because the couple current flowing between the scratched part and the coating through the electrolyte layer becomes larger. When the electrolyte layer was very thin, the surface potential of steel coupled with Zn coating shifted to noble direction with the distance from the Zn anode, indicating that the sacrificial corrosion protection effect is decreased. It was found that the Kelvin sensor is a very useful tool for studying the sacrificial corrosion protection effect under thin electrolyte layer.