SULFIDATION BEHAVIOR OF NICKEL ALUMINIDES

The sulfidation behaviour of four nickel aluminium alloys containing 2 5 to 45 at.% Al was studied over the temperature range of 750 to 950 d egrees C in a gas mixture of H-2-H2S (0.1 to 10 vol.%). The sulfidatio n kinetics were determined using a continous weight gain system. The c orrosion products were examined by SEM, EDX and XRD. Sulfidation in H- 2-H2S gas mixtures formed bilayered scales consisting of an outer laye r of Ni3S2 and an inner layer of NiAl3,5S5,5 on all alloys regardless of the different aluminium contents. In H-2-H2S gas mixtures the sulfi dation kinetic generally followed the parabolic rate law for all alloy s. The influence of aluminium content on corrosion rate was relatively low. The influence of low oxygen partial pressure on sulfidation was investigated in H-2-H2S-H2O mixtures. In these atmospheres the corrosi on mechanism is completely different. Severe attack by rapid internal oxidation destroyed all the alloys except Ni25Al (25 at.%Al). The inte rnal oxidation zone consisted of a mixture of gamma-Ni3Al and Al2O3 On the alloys containing 36 and 45 at.% Al local attack occurred, fast g rowing pocks were observed after an incubation period.Nickel aluminide s show this corrosion phenomena only in H-2-H2S-H2O mixtures. An inter ruption of the H2S gas flow stops the running internal oxidation. In f lowing H-2-H2O atmospheres no internal oxidation was observed. These f acts prove that H2S is necessary for starting and maintaining the inte rnal oxidation of the nickel aluminides.