A Study of Early-Stage Corrosion Behavior of AZ91 Alloy and MAO-Coated Alloy in 3.5% NaCl Solutions

The early-stage (1 h) corrosion behavior of AZ91 alloy before and after microarc oxidation treatment in a 3.5% NaCl solution was revealed using open circuit potential, potentiodynamic polarization, electrochemical impedance spectroscopy, and the observations of corroded surfaces at different immersion time (5, 25 and 45 min). The coating offers excellent corrosion resistance for the substrate for 1 h immersion corrosion by serving as a physical barrier. For the alloy, pitting corrosion initiates easily, but propagates difficultly, due to the formation of oxides/hydroxides in the pits. By comparison, the localized corrosion of the coated alloy proceeds continuously via the through pores, and the newly formed corrosion products inside the pores are easily damaged. Based on the electrochemical results, the alloy exhibits quasi-uniform corrosion, and the coated alloy reveals localized corrosion, both of which are under cathodic control.