Formation of a phosphoric acid compound film on an AZ31 magnesium alloy surface using cavitation bubbles

Abstract

Cavitation processing with chemicals was used to improve the corrosion resistance of an AZ31 magnesium alloy surface. This technique improves corrosion resistance by the formation of a phosphoric acid compound film on the surface of the Mg alloy. However, the processing conditions affect the state of the film; therefore, the effect of the distance from the nozzle to the specimen surface (stand-off distance) on the cavitation state was investigated. This technique employs phosphoric acid and water for processing; therefore, immersion tests were performed with various aqueous concentrations of phosphoric acid to evaluate corrosion resistance. Cavitation processing employs water jet peening (WJP) and multifunction cavitation (MFC). The stand-off distance at which the phosphoric acid-based film was easily formed on the Mg alloy surface was specified for each processing condition. In addition to the formation of a phosphoric acid compound film, the hardness and compressive residual stress at the surface were improved after each process. A film was formed on the surface during the immersion test; however, the thickness of the film was negligible. These results confirmed that cavitation processing is effective for the formation of a phosphoric acid compound film on Mg alloy surfaces.