Silicon Contamination During Alloy Oxidation in Water Vapour at 650 °C

Abstract

A model Si-free alloy, Fe-30Cr (wt.%), was exposed to Ar-10H2O mixture (in vol.%) at 650 °C, forming a protective Cr2O3 scale, which contained silica. Liquid water used to form water vapour in a bubbler was either deionised or distilled. In both cases, low-level water impurities of Si, F, Cl, S, and Ca matched those detected in the Cr2O3 scales by TEM/EDX and ToF–SIMS. Consideration of water droplet and residual solute particle sizes, together with Stokes’ Law shows that entrainment of Si in the gas stream is likely. Calculation of mass transfer rates from the gas to the growing chromia scale show that the amount of silica in the scale is accounted for. Many laboratory experiments could be affected in this way.