Oxidation Mechanism of Steels in Liquid–Lead Alloys

Abstract

The oxidation mechanism of steels in liquid–lead alloys (lead or lead–bismuth) was studied. Parametric dependencies of oxidation, including oxygen-concentration effects, oxidation-rate constant and corrosion-rate effects, are analyzed. An oxidation model is developed based on the assumptions that the chemical reactions are at equilibrium locally, and scale removal is due to mass-transfer corrosion. The model shows that outward-iron diffusion in the solid phase (oxide layer) controls the oxide growth and mass-transfer rate in the flowing-boundary layer determines the corrosion-product transport in the liquid phase (liquid–lead alloy). The oxide thickness depends on both the parabolic oxide-growth-rate constant and the mass-transfer-corrosion rate. For long-term operation, the outer layer of a duplex-oxide layer can be completely removed by flowing lead alloys and it is expected that a pure-chromium-oxide layer forms underneath the Fe–Cr spinel if iron is heavily depleted. The oxide thickness and steel weight change are very different from those of the pure parabolic law and they are classified into distinct and universal categories. The model is validated partially by application to interpreting the measured oxide behavior of several steels in a lead-bismuth eutectic-test loop.