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

摘要

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_(0.71)Al_(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_2H_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)_5(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.