ABSTRACT
The alloys designed to withstand high-temperature degradation by oxidation during their service must meet two essential requirements. Primarily they must form a surface compound (oxide, sulfide, halide, etc.) that thickens only at a slow rate, and secondly the respective compound layer must remain adherent to the alloy substrate under service conditions. It is also well known that all of the technologically important high-temperature alloys used in an oxidizing environment (oxygen as oxidant) receive protection from degradation by the formation and maintenance of a thermodynamically stable, coherent, crack-free, well-adherent, oxide scale of low diffusivity, such as chrornia (Cr20 3) or alumina (Al20 3). Cr20 3forrning alloys generally contain less than 2-3% AI and 15% Cr or more depending on the base alloy. On the contrary, Al203-forrning alloys usually contain more than 5% AI with a substantial amount of chromium. On the basis of low volatility, relative chemical inertness, and slow-growth characteristics, Ah03 is often the scale of choice.
