Effect of Chromium Content on the Oxidation Behaviour of Ferritic Steels for Applications in Steam Atmospheres at High Temperatures

Abstract

Environments containing water vapour are common in many industrial processes, such as power generation systems. Hence, long-term oxidation (1000 h) of P-91 and AISI 430 was studied at 650 and 800°C, in 100% H 2 O atmosphere. The oxidation resistance of the AISI 430 is better than that of the P-91, due to the formation of protective phases on the surface. At 650°C, a scale composed of Fe 3 O 4 , Fe 2 O 3 and (Fe,Cr) 3 O 4 is formed on P-91, although at 800°C the scale is mainly composed of Fe 3 O 4 and (Fe,Cr) 3 O 4 . On the other hand, on AISI 430 the scale is composed mainly of (Fe,Cr) 2 O 3 at 650°C, and at 800°C a layer of Cr 2 O 3 is formed and remains owing to the higher diffusion rate of Cr at this temperature than at 650°C, the latter of which compensates the Cr depletion by the degradation of the chromia scale.

Analysis of High Temperature Steam Oxidation of Superheater Steels Used in Coal Fired Boilers

“…Shibli and Starr [32] indicated that 10-11 % of chromium in material allows the protective, external chromia (Cr 2 O 3 ) to be formed; on the other hand Sanchez et al [31] have shown that such level suppose to be around 11-12 %.…”

“…The reduced impact of Cr is a result of slower diffusion of metal ions at 600°C as the diffusion coefficients are temperature dependent [29,30]. It could be also explained as a result that neither of those two steels has sufficient amount of chromium which can effectively suppress the non-protective oxide growth at higher temperatures [31].…”

Comparative Study on High Temperature Oxidation of T92 Steel in Dry and Wet Oxyfuel Environments

“…A number of modified 9-12 % Cr ferritic/martensitic steels with higher creep strength have been developed and are currently being used for critical components in coal-fired power plants such as heat exchangers, headers, steam lines and exhaust gas systems [5][6][7]. Although these steels have excellent oxidation resistance during exposure in oxygen or air [7], their oxidation rates are significantly enhanced in environments containing water vapour as those prevailing in power plant service conditions [6][7][8][9][10][11][12][13]. Therefore, superheater tubes of modified 9-12 % Cr steels are limited for use up to a maximum working temperature of 620°C in the air-fired power plants due to their limitations to oxidation and creep [11,12].…”

Impact specimen geometry on T23 and TP347HFG steels behaviour during steam oxidation at harsh conditions

“…7 Both oxidation kinetics and the scale morphology are reported to be strongly dependent on the steel composition, surface finish and test conditions: metal temperature, steam flow and chemistry. 8 Among highlighted factors, chromium levels are of crucial importance 9 ; however, levels seems to have less impact below 570°C. Therefore for components operating at temperatures below 570°C the steels with lower Cr levels may be employed.…”

“…Shibli and Starr 11 indicated that in temperature range of 600-650°C, 10-11% of chromium in the base metal allows protective, external chromia (Cr 2 O 3 ) to be formed; in contrast Sanchez et al 9 have shown that such an oxide forming with Cr content greater than 11-12% for steels exposed to steam at temperatures above 600°C. Quadakkers et al 12 concluded that the scale formed on ferritic and austenitic steels differs with the chromium content in the base material; it is recognised that scale changes in following manner: with increasing Cr content it changes from haematite/magnetite to magnetite/(Fe, Cr) 3 Viswanathan et al 6 proposed the following ranking of the high-temperature resistant steels, due to their chromium levels and their relation with protective properties of the scale: IN740, Haynes 230, HR120, HR6W, HR3C, TP347HFG, Super 304H, T92, T91 and finally T23.…”