The influence of heat treatments on the corrosion behaviour of nickel-based alloy 718

The corrosion resistance of the nickel-based alloy 718 in chloride environment depends on its microstructure and the influence of the different phases is still uncertain in the literature. In this study, the effect of heat treatments on the corrosion behaviour of alloy 718 in aerated 25% NaCl solution at room temperature was investigated, using chronoamperometric method applying 1.3 V anodic potential. The alloy was solubilized and aged in different conditions in order to obtain different grain sizes and δ phase volume fractions. The microstructure and the surface of the samples were characterized using scanning electron microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy. The microstructure was also observed by transmission electron microscopy. This work showed that the presence of a passive Al-rich oxide film at the surface increases the pitting corrosion resistance of alloy 718 after aging heat treatment. This Al-rich oxide layer results from the dissolution of γ’ precipitates formed during aging.