High Temperature Corrosion of Ni-Based Alloys SCA425+ and IN792

This paper reports the high temperature corrosion of two Ni-base superalloys: a newly developed alloy, SCA425+, and the extensively used IN792. The composition of the two materials is quite similar, but SCA425+ contains more Cr and Al (17.1 and 10 at.% compared to 13.9 and 7.4 at.% in IN792). The results from exposures at 900 °C in SO₂ (3,000 ppm) + O₂ (69 vol%) + H₂O (31 vol%) mixed gas for 260 h using 65 h/cycle are compared with those obtained from tests in laboratory air. The microstructure of the formed oxide scales was studied using several techniques, such as XRD, SEM, FIB, EDX, STEM and XPS. It is shown that in IN792 severe internal oxidation takes place after both types of exposures. On the other hand, the newly developed SCA425+ has the tendency to form an alumina layer proving that it has more potential to be used in the aggressive environments. Surprisingly the mass gains for SCA425+ alloy exposed in SO₂-rich environment are lower than in laboratory air. The reason for this behavior is discussed.