Nanoscopic view on the initial stages of corrosion of hot dip galvanized Zn–Mg–Al coatings

• The initial corrosion attack of Zn–Mg–Al coating has been investigated.
• Nanoscopic monitoring enabled identification of different corrosion sub-processes.
• Substantial anodic and cathodic activity dominated the initial corrosion.
• Anodic dissolution formed micro-cracks/pits predominantly at eutectic phases.

The onset and initial progress of atmospheric corrosion of ternary alloyed Zn–Mg–Al coatings induced by an aqueous sodium chloride environment was studied in detail by means of surface sensitive techniques, X-ray photoelectron spectroscopy and Auger electron spectroscopy. Two fundamental processes were found to be involved in the initial corrosion attack: (i) dissolution of the surface layer consisting of MgO and (ii) preferential anodic dissolution of both, binary and ternary eutectic phases constituting the coating, which led to the formation of characteristic cracks/holes and was followed by a predominant deposition of zinc corrosion products around anodic regions.