Corrosion of Binary Fe-Si Alloys in Reducing Oxidizing and Sulfidizing-Oxidizing Atmospheres at 700 A°C

The corrosion behavior of three Fe-Si alloys containing approximately 5, 9 and 13 at.% Si was studied at 700 A degrees C in an H-2-CO2 gas mixture providing 10(-20) atm O-2 as well as in an H-2-H2S-CO2 gas mixture providing the same oxygen pressure coupled to an S-2 pressure of 10(-8) atm. All the alloys followed complex kinetics which were mostly linear for Fe-5Si, but showed one or two parabolic stages for the other two alloys. Simple oxidation produced essentially two-layered scales in which Si was confined to the alloy consumption zone in the form of silicon oxide and iron-silicon double oxide. Corrosion in the oxidizing-sulfidizing gas mixture produced scales composed of a thick external zone of pure FeS followed by an internal region containing complex mixtures of FeS with Si and Fe oxides. Internal oxidation of silicon was only observed for the oxidation of Fe-5Si in both environments. The extent of corrosion decreased in both gas mixtures with an increase in the Si content of the alloys. Finally, the addition of sulfur produced a significant increase of the overall mass gains for each alloy.