Enhancing the corrosion resistance of magnesium alloy AZ91D in 3.5 per cent NaCl solution by cerate conversion coatings

Abstract

PurposeTo develop new eco‐environmentally friendly surface treatments based on cerate compounds as alternatives to the process involving toxic chromates for the corrosion protection of magnesium alloys.Design/methodology/approachA treatment process in which a surface was alkaline‐etched prior to ceria treatment is proposed. The process involves cleaning, etching in potassium hydroxide followed by treatment in ceria conversion coatings. The effect of surface preparation prior to ceria treatment on the corrosion resistance of AZ91D in 3.5 per cent NaCl solution was measured using AC impedance spectroscopy and DC polarization techniques. Surface examination was performed by scanning electron microscopy, energy dispersive X‐ray.FindingsIt was shown that the ceria treatment can be used as a localized corrosion inhibitor for alloy AZ91D in NaCl solution. The level of inhibition strongly depended on the cerium concentration. Moreover, ceria treatments improved the pitting corrosion resistance due to the formation of protective oxide films which act as a barrier to oxygen diffusion to the metal surface. According to the EIS and polarization measurements, alkaline etching in KOH is more effective in reducing the pitting corrosion of AZ91D than was HCl. It was shown that surface treatment in alkaline solution (KOH) prior to ceria treatments played an important role in inhibiting the active surface sites, rejecting the chloride ions from the surface and forming uniformly distributed oxide film.Originality/valueCeria conversion coatings seem very promising as alternatives to toxic chromating for the corrosion protection of magnesium alloys in NaCl solution.