To improve red rust resistance at the initial stage of corrosion in exposed steel parts of Zn-11 mass%Al-3 mass%Mg-0.2 mass%Si coated steel sheets, we investigated the mechanism and effects of applying a chemical conversion coating including a corrosion inhibitor to the zinc alloy coating.

A pure-water-spray cyclic corrosion test revealed effects of combining a zinc alloy coating and chemical conversion coating. Red rust resistance in the exposed part of the steel substrate at the initial stage of corrosion is improved by application of a phosphoric-acid-containing chemical conversion coating. When the phosphoric-acid-containing chemical conversion coating was applied, Mg, Zn, and P of the coating components moved farther from the coating layer - steel substrate interface. Because the Mg^2＋, Zn^2＋, PO₄³⁻, CO₃²⁻, and OH⁻ compound protected the steel substrate, the steel substrate corrosion was suppressed even when the water film became thin or island-shaped and when the sacrificial anticorrosive action of the coating worked only with difficulty.

As the electrical conductivity of the water film increased, the increase in the sacrificial protection distance to the steel substrate by the coating component probably also contributed to suppression of the steel substrate corrosion when the water film became thick.