Effect of Corrosion Products on the Atmospheric Corrosion of Electrodeposited Zinc and Zinc Alloy Coatings

The role of the corrosion products in the inhibition of Zn alloy coating corrosion has not been considered fully to date. Atmospheric corrosion data on Zn, Zn-Fe, Zn-Co, and Zn-Ni electrodeposits exposure to marine and urban test sites of a humid, tropical climate during a 3-year period revealed that Zn-Ni and Zn-Co coatings are more resistant than Zn and Zn-Fe. Corrosion products formed on these samples have been characterized by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). Sodium chlorhydroxysulfate (NaZn4Cl[OH]6SO4·6H2O) and zinc hydroxychloride (Zn5[OH]8Cl2·H2O) were determined to be the main constituent compounds of corrosion films formed in both test sites. It is assumed that the amorphous structure and properties of the oxide phase may play an important role in determining coating corrosion resistance.