We investigated the electrochemical properties of compounds and chlorides in the films formed on the surfaces of Mg alloys using cavitation for phosphating. The compounds formed by this treatment were Mg3(PO4)2 and Mg(OH)2. In addition, electrochemical measurements and combined cycle tests were carried out to investigate the chloride corrosion resistance of the films formed after surface treatment. Regarding the anodic polarization curves, each surface-treated sample exhibited passivation–depassivation behavior. In cathode polarization curves, the corrosion rate for each surface-treated sample was approximately 1/10 that of the corresponding untreated sample. In the combined cycle test, the amount of surface oxidation caused by chlorides was less for each treated sample than for the corresponding untreated sample. In addition, when droplets containing chlorides were dropped onto a surface and the angle of the droplets that adhered to the surface was measured, the angle for the untreated sample remained constant even after the droplets were dropped several times. However, when several droplets were dropped onto each of the cavitation-treated samples, some areas were highly hydrophobic and some areas were less hydrophobic than the untreated samples. The results revealed that, although the coating formed on Mg alloys by phosphating using cavitation showed a protective effect in the early stages when chlorides were attached, the protective effect decreased over time.