Effect of Co and Fe contents on the microstructure and corrosion behaviour of heat-treated Ni-Fe-Co superalloys in 3.5 wt% NaCl aqueous solution

The effect of chemical composition on the corrosion behaviour of heat-treated Ni-Fe-Co superalloys has been investigated in aerated 3.5 wt% NaCl aqueous solution. Nickel-base alloys with varying amounts of Fe (5–20 wt%) and Co (30–45 wt%) were fabricated via spark plasma sintering. The alloys were heat-treated at 1000 ^∘C in a muffle furnace for 1 hour, and then quenched in distilled water for 5 minutes. Corrosion resistance of the alloys was analysed by measuring potentiodynamic polarization. Corrosion parameters, i.e. E_corr, i_corr, E_pass and i_pass, were compared for different Co and Fe contents to establish a relationship between the alloy composition and corrosion properties. The results showed that the corrosion resistance of the alloys improved with increasing Co and decreasing Fe content. Accordingly, corrosion rate and corrosion current density (i_corr) decreased with increasing Co content and the imposition of heat treatment. Ni-5Fe-45Co (wt%) showed the highest resistance to corrosion with corrosion rates of 0.0145 and 0.00294 mm/year in as-sintered and heat-treated conditions, respectively. Generally, heat treatment improved the grain homogeneity of the microstructures. Characterisation of surface morphologies of as-sintered and heat-treated corroded alloys was conducted using X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS).