Composition of corrosion products formed on Zn-Mg, Zn-Al and Zn-Al-Mg coatings in model atmospheric conditions

Formation of corrosion products on hot-dip galvanised steel (HDG), Zn-5Al, Zn-11Al-3Mg-0.2Si, Zn-16Mg and Zn-1.5Al-1.5Mg with pre-deposited NaCl was followed in humid air at 20 °C. The alloyed coatings showed an improvement in mass loss by a factor of 4-7 to HDG. Corrosion products on the alloyed coatings contained twice as much carbonates than those formed on HDG. Magnesium dissolved preferentially, and aluminium-enriched phases were the most stable. Magnesium buffered the pH at cathodic sites, thus hindering the formation of zinc oxide and inhibiting the oxygen reduction. Magnesium products at the metal/corrosion product interface might also have an inhibiting effect.