Effect of aluminum on the corrosion characteristics of Mg-4Ni-xAl Alloys

Corrosion properties of Mg-4Ni-xAl alloys were investigated using electrochemical techniques, pH measurement, hydrogen evolution test in acid-chloride solution (pH 5.4. 200 ppm Cl), and surface analysis. The Mg-4Ni-xAl alloys were prepared by a new alloying method called rotation-cylinder method (RCM). Mg dissolution was closely related to the pH value of the test solution. At pH > 11, the surface of Mg alloys was covered by magnesium hydroxide (Mg[OH]₂) film, which influenced the corrosion potential and corrosion rate of Mg alloys. In this case, the corrosion potential was shifted toward the anodic side and corrosion rate decreased. At pH < 11, however, corrosion rate continuously increased as a function of time. With increasing Al content in the Mg-4Ni-xAl alloys from 0 to 2 wt%, (1) the corrosion potential had more active potential, but the corrosion rate decreased, and (2) the segregation of the α-Mg/Mg₂Ni eutectic phases at the grain boundary was reduced. Generally, Mg₂Ni had more noble potential than the α-Mg. As a result of the addition of Al, microgalvanic reactions (1) between the Mg matrix and the α-Mg/Mg₂Ni eutectic phase and (2) between the α-Mg and the Mg₂Ni in eutectic phase decreased, which reduced the corrosion rate of Mg alloys.