The effect of sulfur segregation on the adherence of the thermally-grown oxide on NiAl -: II:: The oxidation behavior at 900°C of standard, desulfurized or sulfur-doped NiAl(001) single-crystals

In Part II of this study, the characterization by TGA, SEM and AFM of the oxidation behavior at 900degrees C of NiAl(001) samples with various sulfur concentrations is reported. The formation of interfacial cavities is observed for all samples. A constant ratio of the oxide thickness to cavity depth is found showing that the formation of cavities at the metal-oxide interface is not due to sulfur but only to vacancy injection during the cationic growth of transient theta-alumina. It is also observed that the presence of sulfur in the alloy decreases the oxidation rate of the nickel aluminide and, consequently, lowers the formation of interfacial cavities. This effect is interpreted as an indirect evidence of the control of the transient-alumina growth by the aluminium diffusion in the alloy, also advanced as an explanation for the interfacial nucleation of alpha alumina. These results are combined with those presented in Part I-1 to propose a model that explains how sulfur, present in small quantities in the alloy, has a deleterious effect on the oxide adherence. The indirect role of the cavities formed during the growth of the transient alumina is to create transitory conditions for the rapid segregation of sulfur at the interface. The segregated sulfur remains as a vestige of the initial stages of growth after the transformation of the scale into mature alumina and weakens its adherence.