The high temperature aqueous corrosion of ferritic steels: Effect of heat flux on corrosion and magnetite deposition

Abstract

A length of 9Cr-1Mo ferritic steel tube has been exposed to a wide range of heat fluxes (0–1389 kW m −2) under realistic steam generator conditions (355°C, 17.6 MPa, subcooled boiling to 15% steam quality) for 3906 h. Deposition of magnetite on to the tube surface was found at first to increase rapidly with heat flux. At higher heat fluxes the rate of increase slowed down and a maximum was attained at ∼ 1200 kW m −2. The results can be understood in terms of concentration and dryout processes beneath growing steam bubbles. The rate of corrosion was found to be inversely proportional to the deposited oxide thickness and thus consistent with a reaction controlled by diffusion of water molecules through the deposited oxide layer. Particles of metallic iron, observed in the middle oxide layer at high heat fluxes, are thought to be a consequence of the limited diffusion routes for H 2O through the outer layer and the formation of hydrogen within the oxide.