High temperature oxidation behavior of 2.25%Cr-1%Mo steel boiler tubes in long-term exposure to superheated steam

The ferritic 2.25%Cr-1%Mo steel tube (JIS STBA 24), which is extensively used for superheaters and reheaters in fossil fuel-fired power plant boilers, was subjected to long-term steam oxidation tests of up to 3.6 x 10(7) s (10000 h) to clarify its oxidation behavior in high-temperature steam atmosphere. An equation was also derived to estimate the oxidation temperature of the material from the scale thickness. The validity of this equation was tested with a 2.25%Cr-1%Mo steel reheater tube used in an actual boiler for 3.6 x 10(8) s (100000 h). The results of this study are as follows: (1) It was revealed that the scale which formed on the 2.25%Cr-1%Mo steel tube exposed in steam at 823 to 898 K for 1.08 x 10(6) to 3.6 X 10(7) s comprises two distinctively characterized layers: the outer scale composed of Fe3O4 (including a slight amount of Fe2O3) and the inner scale composed of (Fe, Cr)(3)O-4, each having almost identical thickness. It was also found that the outer scale grows as a result of the outward diffusion of Fe ions and the inner scale as a result of the inward diffusion of O ions. (2) The metal temperature of boiler steel tube, T (K), can be expressed as a function of the scale thickness d (m) and the cumulative service time t (s) by the following equation: T=-381000/(R . ln(d(2)/1.66 x 10(8) . t)) where R is the gas constant (8.314 J . mol(-1). K-1). The estimated temperature of the 2.25%Cr-1%Mo steel tube, which had been used in an actual boiler for 3.6 x 10(8) s (100000 h), agreed well with the service temperature of the steel tube measured with a thermocouple.