INTERFACIAL PHENOMENA IN THE OXIDATION AND HIGH-TEMPERATURE CORROSIONOF METALS AND ALLOYS

Metallic materials for application at high temperatures must form a sl owly growing, dense oxide scale for their own protection. Up to about 1000-degrees-C, chromia-forming, at higher temperatures, alumina-formi ng Fe, Ni or Co based materials are used. In the present paper it is d emonstrated how intact homogeneous scales on materials can be studied by AES sputter profiling and SNMS, but the main objective is to show p henomena at surfaces and interfaces which occur in complex atmospheres . In complex atmospheres, the formation of a protective scale is aggra vated if several aggressive components of the gas atmosphere attack th e free metal surface. This was demonstrated by AES for oxidizing and c hlorinating atmospheres, in which chlorides and oxides grow simultaneo usly on the free metal surface. The chlorides are overgrown after some minutes by the stable oxide, but the formation of a dense, protective oxide scale is prevented. Similar effects have been observed by AES i n oxidizing and nitriding atmospheres and also in oxidizing and sulfid izing atmospheres. The structure and the composition of the interface oxide/metal is decisive for the adherence of oxide scales. Stresses in the oxide scale can initiate detachment, diffusion processes in the a lloy can lead to formation of voids and cavities beneath the oxide lay er; both processes are favoured by segregation of sulfur from the allo y to the nascent metal surface as was shown by AES. For the investigat ion of the interface, this has to be laid bare by in-situ bending of t he sample; this causes physical removal of the oxide scale. In additio n to the surface of the oxide and the interface oxide/metal, grain bou ndaries of the alloys are also of interest; at these grain boundaries, dissolved non-metal atoms such as C, O, N, S ... can penetrate into t he alloy. The grain boundaries can be analyzed by AES if the samples a re fractured inside the UHV system. It can be shown, for example, that oxygen can penetrate into the grain boundaries of the intermetallic c ompounds NbAl3 and NiAl; this oxygen penetration leads to grain bounda ry oxidation or even disintegration of the material into fine oxidized metal particles. These examples should demonstrate that the applicati on of surface analytical methods, especially of AES, can provide valua ble information for improved understanding and control of the high tem perature corrosion of metallic materials.