Effect of Metastable Phase Content on Corrosion Behavior of Zn-22Al Alloy

The effect of heat treatment on the microstructure and corrosion behavior of Zn-22Al alloy was investigated.The results show that, after isothermally heat treatment below the eutectoid point(277 ℃)for 10 h, the slower the cooling rate, the better the corrosion resistance of the alloy in NaCl (3.5wt%)aqueous solution.The alloy only consists of α₁ phase and η phase in equilibrium state, and the cooling rate does not affect the phase composition and phase content of the alloy. The increase of corrosion resistance of the alloy is due to the low residual stress, defects and segregation in the alloy at a slower cooling rate.When the isothermal treatment temperature is higher than the eutectoid transition point, the faster the cooling rate is, the better the corrosion resistance of the alloy is. When the cooling rate is fast, there is a metastable α₂ phase in the alloy, and the volume fraction of metastable phase decreases with the cooling rate slowing down. Because the potential difference between α₂ phase and α₁, η phase are smaller than that between η phase and α₁ phase, the larger the volume fraction of metastable α₂ phase, the better the corrosion resistance of the alloy.