| Abstract: | The corrosion behaviour of a binary Ni-15 Cr alloy and a ternary Ni-15 Cr'1 Zr alloy has been examined when exposed to a bioxidant O2:SO2 atmosphere at 850°C. The patterns of scaling exhibited by the two alloys, especially in the early stages of reaction, have been studied using optical and scanning electron microscopy and EDAX analysis. It has been established that the nucleation of Cr2 O3 on, and its subsequent growth over the sample surface was much more rapid with the ternary alloy than the binary material. Furthermore the steady-state scale formed on the ternary alloy was single-layered and contained no NiO, in contrast to the anticipated duplex-layered scale developed on the binary material. It is suggested that the pre-existing intermetallic network in the as-cast microstructure of the Ni-15 Cr-1 Zr alloy is a key factor in promoting the rapid formation of the thin protective layer of Cr2O3, free from NiO. These features are responsible for the reduced rate of corrosion of the Zr'bearing material, relative to that exhibited by the binary alloy. The observations are discussed in the light of the published literature concerning the effects of rare earth/reactive metal and inert oxide additions to chromia-forming alloy systems. |
