Excimer laser surface modification of AZ31B-H24 for improved corrosion resistance

Corrosion is an issue in Mg alloys resulting from chemical, and hence electrochemical, heterogeneity of alloy microstructures. In Mg alloy AZ31B-H24 (UNS M11311), Al₈Mn₅ intermetallic particles are dispersed throughout the matrix, forming microgalvanic couples. Laser surface modification was performed in Ar and N₂ environments with a pulsed excimer laser (λ = 248 nm, 25 ns full width at half maximum) with the aim of imparting chemical homogeneity on corrosion resistance. The results show the intermetallic particles can be partially or completely dissolved during lasing, resulting in a localized enrichment of Al and Mn in the matrix adjacent to the Al₈Mn₅ particles. In addition, thin oxide and nitride films were formed during processing, dependent on the processing gas used, not previously reported. Improved corrosion resistance at open circuit potential was observed for laser processed AZ31B, verified by electrochemical impedance spectroscopy and mass loss after immersion in 0.6 M NaCl. All processed samples showed a reduction in the H₂ evolution reaction rate, as well as a reduction in growth of filiform-like corrosion surface coverage. The improvement was attributed to homogenization instead of film formation. Material characterization was performed by optical and electron microscopy, grazing incidence x-ray diffraction, and white light interferometry.