Investigation into the Corrosion Mechanisms and Effective Inhibitor Additions for Zinc-Magnesium-Aluminium (ZMA) Alloys

Abstract

The use of zinc-magnesium-aluminium (ZMA) alloys is increasing in popularity within the coated steel industry due to the potential advantages it can offer. As such, a collaboration of work involving three differing ZMA alloy coated steels were studied using the scanning vibrating electrode technique (SVET) and novel in situ time-lapse optical microscopy whilst immersed in a 1% NaCl electrolyte, facilitating the investigation of each microstructure. The collation of data demonstrated preferential corrosive attack of the eutectic phases amongst each of the alloy compositions, and more specifically the MgZn2 lamellae, succeeded by the corrosion of Zn-rich dendrites. The use of SVET enabled an estimation of mass loss from each alloy, and when correlated with the time-lapse data, revealed a reduction in magnitude of corrosion with increasing content of Mg and Al alloy additions. The improvement in corrosion performance was likely a result of increased levels of both MgO and Al2O3 existing at the alloy surface, thereby restraining the corrosion event kinetics. A corrosion inhibitor in the form of Na3PO4 introduced to 1% NaCl electrolyte at varying concentrations exhibited marked effects on the corrosion of the selected ZMA alloy, as evidenced by both SVET and time-lapse microscopy data sets. Approximations based on the measured SVET data showed that sodium phosphate additions diminished the corrosion activity, the extent of which was dependant on the chosen concentration. The time-lapse images establish suitable correlation with the estimated mass loss results at pH 7, whereby images indicated phosphate precipitation at anodic sites, and thus their passivation. Based on these observations, it can be realised that sodium phosphate inhibitor additions have an appreciable effect on corrosion behaviour within this particular alloy system.