Formation of Mg,Al-hydrotalcite conversion coating on Mg alloy in aqueous HCO3−/CO32− and corresponding protection against corrosion by the coating

An environmentally clean method is studied for synthesizing a conversion coating on Mg alloy in aqueous HCO3-/CO32- at 50 °C. A precursor layer was firstly formed on AZ91D sample. The precursor layer transformed into a crystallized Mg,Al-hydrotalcite when the sample was continuously immersed in the bath until the bath changed from acidic to alkaline. A rapid conversion treatment was, therefore, developed: it involved maintaining the pH under 6 for precursor layer formation and then increasing it to 11.5 to form crystallized Mg,Al-hydrotalcite coating. No corrosion spot on the Mg,Al-hydrotalcite-coated sample was observed after a 72-hour salt spray test.