The Mechanism of Accelerated Oxidation of Fe-Cr Alloys in Water Vapor Containing Atmosphere

Abstract

Although oxidation rate of metals and alloys is often accelerated by the presence of water vapor, no clear explanation is given of the acceleration mechanism. The object of this investigation is to evaluate the magnitude of increase in oxidation rate and to elucidate the acceleration mechanism by the presence of water vapor. Fe-Cr alloys containing 5-30%Cr were oxidized in a stream of O₂ and O₂-10vol% H₂O at 1023 and 1173K. Sulfur decoration method was applied to detect defects in the scale which allow gas penetration to the scale/alloy interface. A scale on alloys containing 5-15%Cr cracks sometimes to allow gas penetration to an alloy surface, developing an inner scale layer of a spinel type. In dry O₂, however, the cracks are filled with a newly formed oxide in them and gas penetration stops. On the other hand, gas penetration was observed throughout oxidation in wet O₂ and the mass gain was greater an order of magnitude than that in dry O₂. Fe-20%Cr alloy is fairly oxidation resistant in dry O₂. It is covered with a Cr rich scale mainly consisting of Cr₂O₃ and readily healed though the cracks are sometimes generated in the scale. In wet O₂ this alloy, though covered with a Cr rich scale at first, produced sooner or later an inner scale layer of a spinel type similarly to lower Cr alloys because of significant gas penetration. Fe-30Cr was also covered with a Cr rich oxide scale. This scale was maintained even in wet O₂ in spite of considerable gas penetration. These results lead to the conclusion that the accelerated oxidation in wet atmosphere is caused by oxidant gas penetration through defects in a scale to an alloy surface, and that 20%Cr is not sufficient for Fe-Cr alloy to be oxidation resistant in wet O₂. The dissociation mechanism is considered to have, if any, only minor significance in accelerated oxidation.