Corrosion inhibition of AZ91D magnesium alloy and its micro-arc oxidation coating by three carboxylate-based inhibitors in NaCl solution

The corrosion inhibition performance of sodium fumarate, sodium 2,6-pyridinedicarboxylate, and sodium benzoate was investigated on both bare AZ91D magnesium alloy and micro-arc oxidation (MAO)-treated AZ91D magnesium alloy in 3.5 wt.% NaCl solution. All three inhibitors effectively enhanced the corrosion resistance of the substrates, with high corrosion inhibition efficiency and charge transfer resistance. The inhibition mechanism was attributed to the formation of an adsorption film between the Mg 2+ and COO − , which suppressed the generation of corrosion products such as Mg(OH) 2 , further forming a protective film mainly composed of MgO. Among the three inhibitors, sodium fumarate demonstrated better inhibition performance with a more complete protective layer and fewer corrosion products formed, which may be due to its dual carboxyl groups and unsaturated and flat molecular structure. For the MAO-treated magnesium alloy, the inhibitors provided sequential protection by first adsorbing onto the surface and within the pores of the MAO coating and subsequently protecting the underlying alloy after coating degradation. This study offers valuable insights into the selection of carboxylate corrosion inhibitors for magnesium alloys and supports the development of anticorrosion strategies based on micro-arc oxidation coatings combined with corrosion inhibitors.