Cut Edge corrosion inhibition by chromate in primer of prepainted 55% Al-Zn alloy coated steel

The effect of chromate in primer layers on the cut edge corrosion of prepainted 55% Al-Zn alloy coated (galvalume) steels has been examined under simulated atmospheric corrosion conditions in a marine environment. The chromate released from the primer layers in effect prolonged and expanded the sacrificial protection of the 55% Al-Zn layers. The corrosion potentials, measured by a Kelvin probe, indicated that the cut edges were maintained at low potentials. The galvanic current measured for the 55% Al-Zn/steel couple specimen with the chromate-containing primer was lower than that of the specimen with the chromate-free primer. This concurs with the results of conventional polarization measurements, indicating that the cathodic oxygen reduction on steel substrates is inhibited by the presence of chromate. The corrosion potential variations across the interface between 55% Al-Zn layers and steel substrates were measured using Kelvin probe force microscopy (KPFM). Almost negligible variations in the KPFM potential were found at the interface in the cut edges with the chromate-containing primer. Our findings indicate that the prolonged and expanded sacrificial protection effect of the 55% Al-Zn layers is likely due to the release of chromate from the primer layers, which makes it possible for the 55% Al-Zn layers to dissolve slowly.