THE HIGH-TEMPERATURE CORROSION OF FE-NB ALLOYS IN A H2/H2O/H2S GAS-MIXTURE

The corrosion of Fe-Nb alloys containing up to 40 wt.% Nb has been studied over the temperature range 600-980-degrees-C in a mixed gas of constant composition having sulfur and oxygen pressures ranging from 10(-8) to 10(-4) atm. and from 10(-27) to 10(-18) atm., respectively. All alloys were two-phase, consisting of an Fe-rich solid solution and an intermetallic compound, Fe2Nb. The scales formed on the Fe-Nb alloys were duplex, consisting of an outer layer of iron sulfide (FeS) and an inner complex layer of FexNbS2(FeNb2S4 or FeNb3S6), FeS and unreacted Fe2Nb. No oxides were detected at any temperature. The addition of Nb reduced the corrosion rate. The corrosion kinetics of Fe-Nb alloys followed the parabolic rate law, regardless of alloy composition and temperature. Platinum markers, attached to the original alloy surfaces, were always located at the interface between the inner and outer scales.