The Effect of Microstructural Refinement on the Localized Corrosion of Model Zn-Al-Mg Alloy Coatings on Steel

Abstract

A combination of in-situ Scanning Vibrating Electrode Technique (SVET) and time-lapse immersion optical microscopy (TLM) is used to investigate the effect of microstructural refinement on patterns of localized corrosion affecting zinc-aluminum-magnesium (ZAM) galvanized coatings on steel. Model ZAM coatings comprising Zn-2.7 wt% Al-1.5 wt% Mg are produced on 0.7 mm mild steel sheet by hot dipping, and the resulting coating microstructure is systematically refined by increasing the cooling (solidification) rate from 5°C.sec−1 to 1000°C.sec−1. The intact ZAM coated surface is immersed in 0.17 M aqueous NaCl, and SVET and TLM are used to follow the resulting localized corrosive attack. TLM shows that corrosion initiates preferentially within MgZn2 and spreads laterally over the ZAM surface by preferentially following MgZn2 rich phases. In coarse microstructures, large primary zinc grains tend to deflect and constrain lateral spreading whereas in fine microstructures the smaller primary zinc grains do not. Consequently, lateral spreading rate increases with microstructural refinement. SVET shows that global corrosion rates are similar for all the ZAM coatings but that increased lateral spreading results in lower rates of through-coating penetration for the refined microstructures. These findings are explained in terms of the lateral diffusion of aggressive anolyte species.