The influence of alloy diffusion processes on the corrosion of ferritic and austenitic stainless steels

Abstract

Sulphidation has been studied in austenitic steels AISI 310 and 447 and in ferritic steel AISI 446 at temperatures between 550°C and 700°C. The kinetics of mass gain change at about 650°C from a linear, spalling-controlled type at lower temperatures to a diffusion-controlled type above this temperature. At the higher temperatures internal sulphidation is observed. The explanation for the mass gain rate constants measured is that diffusion in the alloy is rate controlling. This is substantiated by a new high diffusivity-path-enhanced diffusion model described in a parallel paper. The high diffusivity paths are shown to be intergranular and intragranular sulphides, in ferritic and austenitic alloys, respectively. These sulphides nucleate on existing carbides. In cases where the aggressiveness of the atmosphere is not high, as is the case here, this alloy diffusion controlled corrosion mechanism can be suppressed by using austenitic as opposed to ferritic matrices and reducing the concentration of relatively unstable carbides like M 23C 6 in the material.