Cut-edge corrosion of a Zn–55Al-coated steel: A comparison between sulphate and chloride solutions

Ex situ observations and in situ measurements of current and pH distributions over cut-edge coated steels revealed that Zn–55Al coatings are sacrificial only in chloride solutions, because of an activation of both Zn- and Al-rich phases present in the coating. In sulphate solutions, an efficient cathodic protection occurs only for short times of immersion, because only Zn-rich phases are active in this medium. The low solubility and strong acidification induced by Al(OH)SO4 allows only short distance cathodic protection, but cannot prevent red rust precipitation on the steel surface.

► In situ measurements of current and pH distributions over cut-edge of Zn–55Al coated steel in sulphate and chloride media.
► Zn–55Al coating is sacrificial in both solutions but only for short times in sulphate solution.
► Differences are explained in the light of thermodynamic stability of the solid products formed during coating dissolution.