Understanding pitting corrosion behavior of AZ91 alloy and its MAO coating in 3.5% NaCl solution by cyclic potentiodynamic polarization

The pitting corrosion behavior of AZ91 alloy before and after micro-oxidation treatment in 3.5% NaCl solution was investigated by cyclic potentiodynamic polarization (CPDP) and optical and SEM observations of corroded surfaces at different polarization potentials. The CPDP results show that both the alloy and the MAO-coated alloy suffer from pitting corrosion and it is difficult for pits to stop growth once the pits initiates. It is revealed that the air-formed MgO film on AZ91 alloy, the MAO coating along with corrosion products (mainly Mg(OH)2) formed during CPDP can significantly influence the kinetics of the redox reactions of Mg, and further influence the propagation behavior of pitting corrosion. The optical and SEM images show that the corrosion products on AZ91 alloy are dense and protective but on the MAO-coated corrosion products are very loose. Such observations support the analyses of CPDP results that pits on AZ91 alloy spread to the width whereas pits on MAO-coated alloy propagate to the depth. Overall, pitting corrosion on the MAO-coated alloy can be very severe once the coating ruptures and post-treatments are necessary to provide a promising corrosion protection for the Mg alloys.