Effect of laser cleaning on the growth and properties of micro-arc oxidation layers of AZ31 magnesium alloy

The ceramic coating was prepared on surface of AZ31 Magnesium alloy by laser cleaning (LC) combined with micro-arc oxidation (MAO) to improve its corrosion resistance. The voltage and current were collected during the MAO process. The laser confocal, SEM/EDS, XRD and XPS were adopted to characterize the morphology, structure and phase compositions of the MAO coating. The results showed that the laser crater morphology formed on the AZ31 magnesium alloy substrate after LC treatment had a certain influence on the discharge mode and coating growth mechanism during the MAO process. The arc was preferentially generated around the laser crater due to the tip effect, resulting in the formation of MAO coating on the periphery of the laser crater first, and then gradually inside. At the same time, the oxides on surface of the LC treatment substrate increased, which would lead the generation of MAO coating more uniform and dense. Moreover, the corrosion resistance of the MAO coatings with and without LC treatment were tested by electrochemical and salt spray corrosion experiments. The self-corrosion current density of LC(20 W)/MAO coating was 1.096 x 10- 8 A & sdot;cm- 2, much lower than that of MAO coating without LC treatment (3.882 x 10- 7 A & sdot;cm- 2) and the alloy substrate (4.499 x 10- 5 A & sdot;cm- 2). Furthermore, there were no obvious corrosion traces for the LC(20 W)/MAO coating after 192 h of salt spray. The laser cleaning treatment combined with micro-arc oxidation provides a great important method for magnesium alloy to improve its corrosion resistance.