Effect of Annealing Time on Corrosion Behaviours of ZrCuNiAlNb in Hank Solution.

The microstructures of the as-cast and annealed ZrCuNiAlNb were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), their microhardness values were tested, and their corrosion behaviours in Hank solution were studied. XRD results and SEM analysis showed that the as-cast sample was amorphous, and crystallisation occurred in the samples annealed at 923 K for 5-30 min with crystals of ZrCu and ZrNi. Microhardness gradually increased and then levelled off, due to higher crystallisation degree with longer annealing time. Passivation occurred for all the samples in Hank solution. Prolonged annealing time leads to the initial rise and then a drop in corrosion resistance. Annealing for 5 min resulted in the highest corrosion resistance, with high corrosion potential E at -0.007 V, versus saturated calomel electrode (SCE), i.e., 0.234 V, versus standard hydrogen electrode (SHE), the smallest corrosion current density i at 2.20 × 10 A·cm, the highest pitting potential E at 0.415 V (i.e., 0.656 V), the largest passivation region E-E at 0.421 V, the largest arc radius, and the largest sum of charge transfer resistance and film resistance R + R at 15489 Ω·cm. Annealing for 30 min led to the lowest corrosion resistance, with low E at -0.069 V (i.e., 0.172 V), large i at 1.32 × 10 A·cm, low E at -0.001 V (i.e., 0.240 V), small E - E at 0.068 V, the smallest arc radius, and the smallest R + R at 4070 Ω·cm. When the annealing time was appropriate, the homogeneous microstructure of nanocrystals in an amorphous matrix resulted in improved passivation film, leading to the rise of corrosion resistance. However, if the annealing time was prolonged, the inhomogeneous microstructure of larger crystals in an amorphous matrix resulted in a drop in corrosion resistance. Localised corrosion was observed, with corrosion products of ZrO, CuO, CuO, Ni(OH), AlO, and NbO.