Research paper

Surface and Coatings Technology

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

Masataka Ijiri,  Shotaro Fujii,  Ryunosuke Yamada,  Toshihiko Yoshimura,  Yoshiaki Morisada,  Hidetoshi Fujii,  Reza Hashemi, Maziar Ramezani,  Shunta Matsuoka,  Shoichi Kikuchi

527

133389

1

11

Elsevier

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 evalu
ated 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-Mg17Al12 
phase exhibited limited phosphate film formation during MC-MFC treatment, resulting in deteriorated surface 
stability in chloride-containing environments.