Corrosion product formation on Zn55Al coated steel upon exposure in a marine atmosphere

► Corrosion product formation on Zn55Al coated steel in a marine environment. ► Selective corrosion of the zinc rich interdendritic areas of the coating. ► Corrosion products consisted of a mixture of zinc and zinc aluminium corrosion products. The formation of corrosion products on Zn55Al coated steel has been investigated upon field exposures in a marine environment. The corrosion products consisted mainly of zinc aluminium hydroxy carbonate, Zn 0.71Al 0.29(OH) 2(CO 3) 0.145· xH 2O, zinc chloro sulfate (NaZn 4(SO 4)Cl(OH) 6·6H 2O), zinc hydroxy chloride, Zn 5(OH) 8Cl 2·H 2O and zinc hydroxy carbonate, Zn 5(OH) 6(CO 3) 2 were the first three phases were formed initially while zinc hydroxy carbonate Zn 5(OH) 6(CO 3) 2 was formed after prolonged exposure in more corrosive conditions. The initial corrosion product formation was due to selective corrosion of the zinc rich interdendritic areas of the coating resulting in a mixture of zinc and zinc aluminium corrosion products.