Effect of mechanochemical multifunction cavitation treatment on the surface microstructure and phosphate film formation on magnesium alloys with various aluminum contents

The effects of mechanochemical multifunction cavitation (MC-MFC) treatment, which generates cavitation bubbles containing a small amount of phosphoric acid during chemical conversion, on the surface characteristics of high-strength Mg alloys with various Al contents (AZ31, AZ61 and AZ91) were investigated to suppress the formation of pits on the surface of the alloys. Gold-colored films were formed on all of the investigated alloys, with darker coloration corresponding to a higher Al content. The surface roughness of AZ61 and AZ91 increased substantially after the treatment, and the formed films were thinner than those on AZ31. Analyses using scanning electron microscopy with electron-probe microanalysis revealed precipitates in regions where phosphate films were absent, and their size increased with increasing Al content. Fourier transform infrared analysis revealed a decrease in phosphate-related peak intensity, indicating nonuniform film formation. Surface wettability evaluated by contact angle measurements suggested increased susceptibility to localized surface degradation in chloride-containing environments after MC-MFC treatment. Observations of untreated alloys revealed network-like Mg–Al–Zn compounds whose fraction increased with increasing Al content, reducing the reactivity with phosphoric acid and hindering film formation. Consequently, alloys containing a larger amount of β-Mg 17 Al 12 phase exhibited limited phosphate film formation during MC-MFC treatment, resulting in deteriorated surface stability in chloride-containing environments. • MC-MFC formed uniform phosphate films on AZ31 but discontinuous films on AZ61 and AZ91. • β-Mg 17 Al 12 slowed phosphate formation due to high stability / low Mg 2+ dissolution. • Optimal phosphate concentration (0.02–0.04 mass%) promoted uniform film formation. • Phosphate film formation on β-Mg 17 Al 12 was suppressed by limited Mg 2+ supply, stable oxides, and local pH increase.