Chromate passivation of hot dipped Zn25Al alloy coatings

A low concentration chromate passivation treatment has been successfully applied to a new type of hot dipped Zn2 5Al alloy coating, and the corrosion resistance of the chromate passive film has been assessed using the copper accelerated acetic salt spray (CASS) test, electrochemical measurements, and sea water immersion testing. The results showed that the corrosion resistance of the Zn2 5Al alloy coating was significantly better after the chromate passivation treatment. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses showed that the composition (at.-%) of the low concentration chromate passive film was: 5·5S–3·4Na–11·8C–7·9Ti–41·6O–1 3·7Cr–16·0Zn. Aluminium was not found in the film, which is attributed to the dissolution behaviour of the Zn2 5Al alloy coating in acidic chromate solution.