Electrochemical studies on the corrosion of a range of zinc alloy coated steel in alkaline solutions

Abstract

The corrosion behaviour of a range of commercial zinc and zinc alloy coatings in alkaline solutions was studied by means of polarization resistance and potentiodynamic polarization techniques. Coatings included Zn, ZnFe and ZnAl produced by hot dip galvanizing, and Zn, ZnNi and ZnCo produced by electroplating. A number of samples had received subsequent chromate conversion treatments. Corrosion resistance of the coatings was found to be dependent on the alkalinity of the solutions and their chloride contents. Corrosion behaviour of these coatings changes from the passive state to active dissolution with increasing pH and in the presence of chloride ions. The zinc-nickel coatings had poor corrosion resistance which was attributed to microcracking of the coatings in these environments. In contrast, it was found that pure zinc and zinc-cobalt alloy coatings with specific chromate conversion treatments provided the best resistance to corrosion in these environments. However, the quality of the coatings was found to be important since it was noticed that pitting could be initiated at defects in the as-produced coatings.