Effect of Ni on the corrosion resistance and passivation characteristics of Ni-Mn-Ga-Gd shape memory alloys in sodium chloride solutions

This paper investigates the effect of different Ni contents on the corrosion and passivation behavior of Ni54+xMn25Ga20.9-xGd0.1 (x = 0, 1, 2, 3, 4) shape memory alloy in a 3.5 wt% NaCl solution. Using scanning electron microscope, electrochemical workstation, and X-ray photoelectron spectroscopy. The results indicate that the corrosion resistance of single-phase alloys initially increases and then decreases as the Ni content ranges from x = 0 to 3, with the best corrosion resistance observed at x = 2. The passive film of the alloy mainly consists of NiO, Ni(OH)2, and Ga2O3. Both NiO and Ga2O3 positively affect the corrosion resistance of the passive film. In the range of 54 to 57 at% Ni content, the alloy remains a single-phase alloy. The elements Ni and Ga work together in the passivation film and at a Ni content of 56 at% to reach equilibrium, which provides the best corrosion resistance for single phase alloys. When the Ni content reaches 58 at%, the second phase appears in the form of a nickel-rich phase. Typically, intergranular corrosion preferentially corrodes the second phase, but in this study, the second phase is a nickel-rich phase with some corrosion resistance, which improves the overall corrosion resistance of the alloy.