High Temperature Oxidation Behaviour of Si-containing 11%Cr Stainless Steels in Water Vapor Containing Atmosphere

Abstract

It is widely known that the presence of water vapor in an oxidizing atmosphere changes the oxidation behaviour of stainless steels. In the present work, the influence of water vapor on the oxidation behaviour of SUS430 (17%Cr) steel and the effect of Si addition to 11%Cr steels were studied. Especially, an accelerated oxidation accompanied with the formation of reddishbrown scale (α-Fe₂O₃) was investigated in a temperature range of 500 to 700°C in which ferritic stainless steels usually suffer a slight attack.
Such scale was observed on SUS430 steel in a high H₂O atmosphere (in Ar+0.3%O₂+20%H₂O) and not in low H₂O atmospheres (in Ar+0.3%O₂+2%H₂O and in air). In the former atmosphere more Cr-rich initial scale is formed than in the latter. As a result, the substrate becomes more Cr-depleted, which permits the growth of Fe-rich scale after the breakdown of the initial Cr-rich scale. According to the oxidation study of various Fe-Cr binary alloys, more than 18%Cr is necessary to minimize the occurrence of such scale. Addition of more than 1% Si to 11%Cr ferritic steel improved its oxidation resistance in the high H₂O atmosphere. 11%Cr steel with 2%Si showed protective oxidation over the temperature range of 500 to 900°C and was insensitive to the concentration of water vapor. The addition of silicon promoted the formation of Cr-rich scale probably due to the oxygen gettering effect suggested by Wagner.