Sulfidation behaviour of nickel aluminides

Abstract

AbstractThe sulfidation behaviour of four nickel aluminium alloys containing 25 to 45 at.% Al was studied over the temperature range of 750 to 950°C in a gas mixture of H2‐H2S (0.1 to 10 vol.%). The sulfidation kinetics were determined using a continous weight gain system. The corrosion products were examined by SEM, EDX and XRD.Sulfidation in H2‐H2S gas mixtures formed bilayered scales consisting of an outer layer of Ni3S2 and an inner layer of NiAl3,5S5,5 on all alloys regardless of the different aluminium contents. In H2‐H2S gas mixtures the sulfidation kinetic generally followed the parabolic rate law for all alloys. The influence of aluminium content on corrosion rate was relatively low.The influence of low oxygen partial pressure on sulfidation was investigated in H2‐H2S‐H2O mixtures. In these atmospheres the corrosion mechanism is completely different. Severe attack by rapid internal oxidation destroyed all the alloys except Ni25Al (25 at.%Al). The internal oxidation zone consisted of a mixture of γ‐Ni3Al and Al2O3. On the alloys containing 36 and 45 at.% Al local attack occurred, fast growing pocks were observed after an incubation period.Nickel aluminides show this corrosion phenomena only in H2‐H2S‐H2O mixtures. An interruption of the H2S gas flow stops the running internal oxidation. In flowing H2‐H2O atmospheres no internal oxidation was observed. These facts prove that H2S is necessary for starting and maintaining the internal oxidation of the nickel aluminides.