Formation of chromium rich oxide scales for protection against metal dusting

Metal dusting is a disintegration of metals and alloys into graphite and me tal particles, caused by strongly carburizing gas mixtures mainly in the te mperature range 400-700 degrees C. Protection of steels against metal dusti ng is possible through the formation of dense chromium rich oxide scales bu t it is not guaranteed that such scales are formed at low temperatures, eve n on high Cr-steels. Surface analytical studies have been conducted on the formation and composi tion of the oxide scales on 9-20%Cr steels. The growth of oxide films was f ollowed by AES for 3 hours at 10(-7) mbar O-2 great differences were observ ed in dependence on surface finish. On ground samples, Mn and Si appeared e arly and Cr-rich oxide was formed, whereas on chemically etched samples Fe- rich oxides grew. After long term exposures (240 h) under metal dusting conditions, i.e. in C O-H-2-H2O mixtures at 600 degrees C, thin Cr-rich scales were observed on g round steels which were impermeable to carbon whereas on chemically etched steels thick Fe-rich scales had grown and carbon penetration was detectable . Accordingly, the oxide formation on Cr-steels at relatively low temperatu res strongly depends on the surface treatment. Any surface working such as grinding and sand-blasting etc. introduces dislocations and causes a fine-g rained microstructure near the surface, and the dislocations and grain boun daries act as rapid diffusion paths for supply of Cr to the surface in the first minutes of exposure, which leads to the formation of a protective oxi de scale.