Effect of silicon on the steam oxidation resistance of a 9% Cr heat resistant steel

The steam oxidation of 9Cr-0.5 Mo-1.8W steels containing 0.06 to 0.49% Si was investigated at 500degrees, 550degrees, 600degrees, 650 and 700degreesC. The steam oxidation rate of the steel decreased with increasing silicon content. The effect of silicon was most remarkable at 700degreesC. At 500degrees, 550degrees and 600degreesC, the effect was almost the same, and was smaller than that at 700degreesC. At 700degreesC, the formation of a protective amorphous-SiO2 film reduced the oxidation rate considerably. On the other hand, at 600degreesC or less, silicon dissolved in the Fe-Cr spinel lattice with no evidence of SiO2. At 650degreesC, although amorphous SiO2 was observed, as at 700degreesC, at the scale-metal interface, the effect of silicon was the least within the test-temperature range. Thus, 650degreesC was a peculiar temperature for the effect of silicon on the steam oxidation of 9% Cr steels. The relatively small effect of silicon at 650degreesC is attributed to the formation of metastable FeO.