STRUCTURE AND OXIDATION BEHAVIOR OF THE SCALE FORMED ON AL-CONTAINING FERRITIC STAINLESS STEEL

Abstract

The oxidation scales formed on Al-containing ferritic stainless steel exposed to oxidizing environment having different oxygen partial pressure at 850–1200°C have been characterized using TEM, SEM, X-ray diffraction and other microstructural techniques. The scales formed above 1000°C are composed of alpha alumina and the surface of these scales is generally smooth. The scales formed at 850–950°C are predominantly composed of theta alumina and delta alumina which are called transition alumina. These forms of alumina grow as whisker morphology which are 3μm in length, 0.5μm in width and 0.1μm in thickness. These whiskers consist of theta alumina crystal and delta alumina single crystal. The oxidation rates of these scales are much faster than those of scales which are composed of alpha alumina. These facts mean the growth mechanism of the transition alumina formed on Al-containig ferritic stainless steel is quite different from that of the alpha alumina in the scales. Once the whiskers formed, these whiskers can be transformed to the alpha alumina crystal structure without changing the morphology by heating above 1000°C. The oxidation rates of the alpha alumina whiskers are slower than that of the transition alumina whiskers.