# Effect of Annealing Time on Corrosion Behaviours of Zr56Cu19Ni11Al9Nb5 in Hank Solution

## TL;DR

This study examines how different annealing times affect the corrosion resistance of a zirconium-based alloy in a simulated body solution.

## Contribution

The study reveals that optimal annealing time improves corrosion resistance by forming a homogeneous nanocrystalline microstructure.

## Key findings

- Annealing for 5 minutes maximizes corrosion resistance with high corrosion potential and low corrosion current density.
- Prolonged annealing (30 minutes) reduces corrosion resistance due to inhomogeneous crystal growth.
- Localized corrosion products include ZrO2, Cu2O, CuO, Ni(OH)2, Al2O3, and Nb2O5.

## Abstract

The microstructures of the as-cast and annealed Zr56Cu19Ni11Al9Nb5 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 Zr2Cu and Zr2Ni. 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 Ecorr at −0.007 VSCE, versus saturated calomel electrode (SCE), i.e., 0.234 VSHE, versus standard hydrogen electrode (SHE), the smallest corrosion current density icorr at 2.20 × 10−7 A·cm−2, the highest pitting potential Epit at 0.415 VSCE (i.e., 0.656 VSHE), the largest passivation region Epit–Ecorr at 0.421 VSHE, the largest arc radius, and the largest sum of charge transfer resistance and film resistance Rct + Rf at 15489 Ω·cm2. Annealing for 30 min led to the lowest corrosion resistance, with low Ecorr at −0.069 VSCE (i.e., 0.172 VSHE), large icorr at 1.32 × 10−6 A·cm−2, low Epit at −0.001 VSCE (i.e., 0.240 VSHE), small Epit − Ecorr at 0.068 VSHE, the smallest arc radius, and the smallest Rct + Rf at 4070 Ω·cm2. 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 ZrO2, Cu2O, CuO, Ni(OH)2, Al2O3, and Nb2O5.