The scaling behavior of an iron-5% aluminum alloy in sulfur vapor

Abstract

The scaling behavior of an iron alloy containing 5 wt. % aluminum has been assessed (involving kinetic, metallographic, x-ray diffraction, and electron probe microanalysis studies) in sulfur vapor at 10 ton in the range 500–700°C. For comparison purposes similar experiments were also performed with pure iron. The reaction kinetics observed with the alloy were linear after an initial short period of exposure during which the kinetics were parabolic. Parabolic kinetics, however, were exhibited over the entire period of sulfidation by the pure iron, and the overall rate of sulfidation of the iron was an order of magnitude greater than that of the alloy at all temperatures. Duplex scales were formed both on the alloy and the iron after prolonged periods of exposure. However, whereas the nature and composition of the outer layer of scale formed from both materials was similar, the inner layer of scale on the alloy was significantly different from that grown on the pure iron, both morphologically and in composition containing Al₂S₃ in addition to FeS. The probable mechanism of sulfidation of the alloy is discussed in relation to current theories regarding the behavior of pure iron.