Materials Problems in Fluidized-Bed Combustion Systems. IV. Corrosion Chemistry in Low-Oxygen Activity Atmospheres. Interim Report

摘要

This report presents the results of an investigation to evaluate the factors controlling the corrosion behavior of iron, nickel, and cobalt-base high temperature alloys in atmospheres representative of gasified coal. Major factors investigated include alloy composition and surface condition, atmosphere composition, exposure time and temperature, and variable exposure parameters. Thermodynamic phase stability diagrams for the major alloy elements (Fe, Ni, Co, Cr, Al, Mn) with the major oxidants in the gas (C, O, S, N) were derived for use in selecting test conditions and aiding in the interpretation of test results. A transition in behavior from the formation of protective oxide scales to non-protective oxides with overlaying sulfide scales or molten slags was found for all alloys with increasing P/sub sub 2 / or decreasing P/sub O sub 2 / in H sub 2 /H sub 2 O/H sub 2 S gas mixtures. The failure mode was observed to be the formation of a sulfide scale over the surface of an oxide scale. The nature of this transition from protective to nonprotective behavior has been studied as a function of material and environmental variables. Both thermodynamic and kinetic factors were found to be important. The critical thermodynamic factors are the relative stability of the base metal (Fe, Co, Ni) sulfide for a given sulfur pressure and temperature, and the activity (concentration) of base metal and sulfur ions in the oxide scale. The important kinetic factors are the rates at which base metal ions diffuse or migrate outward to the surface of the oxide scale and the sulfur ions diffuse inward to the scale/metal interface. These factors are controlled by the sulfur activity in the gas in which the oxide is formed. (ERA citation 05:025272)