Localized corrosion behavior in aluminum-zinc alloy coatings investigated using the scanning reference electrode technique

The scanning reference electrode technique (SRET) has been used to determine the influence of Al content on the kinetics of localized corrosion processes occurring on the intact surface of Al-Zn alloy-coated steel. Three commercial hot-dipped coatings were investigated: Galvatite (0.15%Al-Zn), Galfan (4.3%Al-Zn), and Galvalume (55%Al-Zn). All three exhibited strongly localized patterns of anodic metal dissolution when freely corroding in aerated aqueous chloride electrolyte. The currents emerging from discrete anodic sites and the mean active lifetime of the same sites decreased markedly with increasing Al content in the coating. Conversely, the total number of discrete anodic sites becoming initiated over a 24-h period was greater for the coatings with higher Al content. Anodic sites on the 0.15%Al and 4.3%Al coatings showed metal dissolution but no evidence of dealloying. However, anodic sites on the 55%Al coating exhibited local dezincification and porosity development. In this paper, corrosion behavior is discussed in relation to coating composition, and mechanisms are proposed for the initiation, propagation, and termination of localized anodic activity.