Excimer Laser Processing of Cast Mg-Al-Zn (AZ91D) and Mg-Al (AM60B) Alloys for Improved Corrosion Resistance.

Poor intrinsic corrosion resistance of Mg alloys is often attributed to recommended heterogeneous microstructures containing more electrochemically noble secondary phases resulting in the preferential dissolution of Mg. The investigation herein demonstrates the laser (excimer laser, λ = 248 nm, 25 ns) processing of cast Mg-Al-Zn (AZ91D) and Mg-Al (AM60B) alloys mitigates the impact β-Mg₁₇Al₁₂ and Al-Mn intermetallic particles have on the corrosion resistance. Control of the laser fluence reduced surface defects commonly formed while improving the corrosion lifetime. Scanning electron microscopy of laser processed specimens revealed significant dissolution of the β phase and Al-Mn intermetallic particles. The corrosion response showed that an order of magnitude reduction in H₂ evolution reaction rate was achieved for all times of immersion. Electron impedance spectroscopy showed a 10-fold increase in the estimated polarization resistance with laser processed surfaces, exhibiting a corrosion lifetime of the order of 60 h. The impedance data were verified by 72 h time lapse images of the fully immersed samples exhibiting minimal filiform corrosion coverage for both processed alloys in a quiescent 0.6 M NaCl solution. [ABSTRACT FROM AUTHOR]