Effect of a high heat flux on the corrosion of 2 1/4 Cr-1 Mo steel in superheated steam

A series of corrosion tests was conducted to determine the effect of heat flux on the corrosion of 21/4 Cr-1 Mo steel in superheated steam. The tests were conducted with a constant heat flux of (126 kW/m2) 40,000 Btu hr-1 ft-2 and surface temperatures varying from 950 to 10000F (510 to 5410C) on each specimen. Specimens were exposed for varying lengths of time, ranging from 500 to 6000 hr. Essentially all the corrosion product oxide remained on the steel during the first 2000 hr, but after that time, exfoliation of the oxide began. The percentage of corrosion product oxide lost from the specimen increased with exposure time; about 33 percent was missing after 6000 hr. After an initial period of rapid corrosion, the average corrosion rate was constant at 1.8 mils/year (45 μm/year). In contrast, isothermal specimens exposed to superheated steam at 9300F (4990C) in the same test facility corroded at a decreasing rate throughout the test. Metallographic sections showed that, with heat transfer across the specimen-steam interface, both cracks and porosity developed in the oxide, even on short exposure, a face that was undoubtedly responsible for the early exfoliation of the oxide and the constant corrosion rate