Abstract
CWT (combined water treatment) was introduced in Japan in 1990 and over 50 power generation boilers are now in operation. However, the effect of oxygenated treatment on the steam oxidation of the ferritic-martensitic steels and austenitic stainless steels that are used for superheaters and reheaters is currently far from clear. In this study, laboratory tests were used to examine the effect of the oxygen level of the feed water on the scale growth and the scale exfoliation propensity of T91 ferritic-martensitic steel and 300-series austenitic stainless steels, as represented by TP316H and TP347H (coarse- and fine-grained, respectively). The oxygen level of the feed water had little effect on the steam oxidation rates of all the steels tested. Hematite (Fe2O3) formed in the outer layer of the oxide scales on both the ferritic and austenitic steels and is considered to have been encouraged in the simulated CWT atmosphere. The adhesion strength of the oxide scale formed on T91 in the simulated CWT atmosphere, that is, scale in which hematite was present, was lower than that of the oxide scale formed in the simulated AVT (all volatile treatment) atmosphere. The oxidation rate of fine-grained TP347H was confirmed to be slower than that of coarse-grained TP316H. Hematite significantly influenced the scale exfoliation of the austenitic steels and the critical oxide thickness for exfoliation decreased with increasing proportion of hematite in the outer scale.
