Aqueous corrosion characteristics of Mg-4Ni-xAl alloys in acid-chloride solution

Aqueous corrosion properties of Mg-4Ni-xAl alloys in acid-chloride solution(pH 5.4, 200ppm Cl^-) were studied as a function of Al composition. Three kinds of Mg alloys, namely, NA40, NA41 and NA42, were prepared by a new alloying method, RCM(Rotation-Cylinder Method). The corrosion behavior for the prepared alloys was investigated at room temperature by polarization tests, pH measurement, hydrogen evolution test, galvanic corrosion tests and surface analyses(SEM, EPMA, XRD). The results of the polarization test indicated that with increasing Al content in the Mg-4Ni-xAl alloys, (a) the corrosion potential had more active potential but corrosion rate decreased, (b) segregation of the α-Mg/Mg₂Ni eutectic phase at grain boundary was reduced. Generally, Mg₂Ni has more noble potential than the α-Mg. Due to the addition of Al, micro-galvanic reactions (a) between the Mg matrix and the α-Mg/Mg₂Ni eutectic phases and (b) between the α-Mg and Mg₂Ni in the eutectic phase itself reduced, which decreased the corrosion rate of Mg alloys. Galvanic corrosion tests were also done for consideration of Mg-4Ni-xAl alloys application as structural materials, they were compared to AZ91D Mg alloy coupled with low carbon steel.