Corrosion and Coating of Advanced Materials

Extract

Most metals are thermodynamically unstable and tend to change to more stable phases by reaction with their service environment. These oxidation or corrosion processes are usually controlled by kinetic factors, and the rate often increases with increasing temperature. Degradation by these processes is an important, often life-limiting, factor for critical components in thermal-power systems and other high-temperature process plants, and it can have considerable economic and technical consequences. Corrosion can be particularly severe, for example, in

Gas-turbine blades and vanes, Gas coolers in coal-gasification systems, Superheaters and reheaters in coaland oil-fired boilers, In-bed tubes in fluidized-bed coal combustion systems, Components in high-temperature fuel cells, Components in steam-reforming plants, Components in incinerator plants, and Exhaust systems (e.g., automobiles).

Although there is considerable awareness of such problems and much attention is given to the materials requirements for high-temperature systems, failures still occur and can be catastrophic (fire, explosion, burst tube, etc). Plant conditions and control are particularly important since damage can occur due to overheating and overstressing when a material is taken beyond its physical, mechanical, and chemical limits or due to so-called dew-point corrosion when the system is cooled and acidic vapors condense to electrolytically corrode unprotected surfaces.

The oxidation of metals and alloys in oxygen at high temperatures is now reasonably well-understood. Indeed, alloys have been successfully developed to resist environmental attack in such atmospheres. The oxidation resistance of these materials is due to the presence of alloying elements—such as chromium, aluminum, and silicon—which can react with oxygen to form stable, protective, external oxide scales on the component surface, thereby inhibiting further high-temperature degradation.