Sodium Heated Steam Generator Tubes: Effect of Heat Flux on the Deposition of Magnetite from Solution and Corrosion of the Underlying Steel

Abstract

Deposition of magnetite on two chromium ferritic steels has been studied in high temperature water (350 C) at high heat flux (0 to 860 kW/m²) using a sodium heated test section. Corrosion of the steels, which occurs at the same time as magnetite deposition, was also measured. Magnetite deposition was found to increase in proportion to (heat flux). This behavior is attributed to enhancement of soluble iron levels in the evaporating microlayer beneath developing steam bubbles. A triple oxide layer (in contrast to the normal double layer) was observed under boiling conditions. The additional layer (magnetite) is believed to form from outwardly diffusing Fe²⁺ (originating from corrosion of the metal) reacting with inwardly diffusing H₂O or O²⁻. Under boiling conditions, Fe²⁺, ions are largely prevented from reaching the water phase by the rapid formation of an outer layer. Both steels were found to corrode at similar rates. The corrosion rate was independent of heat flux up to ∼600 kW/m² but decreased at higher heat fluxes. This change is thought to be a result of diffusion through the outer layer becoming rate-controlling, in contrast to the situation at lower heat fluxes where control is exercised by the inner layer.