Thermodynamic and kinetic analyses of time-temperature-sensitization diagrams in austenitic stainless steels

Abstract

The development of a chromium-depleted zone, the cause of sensitization in austenitic stainless steels, is analyzed by integrating a thermodynamic and a kinetic model for the grrin boundary precipitation of (Cr,Fe) 23C 6. The thermodynamic model utilizes the Kohler extrapolation formula for the Gibbs free energies of austenite and (Cr,Fe) 23C 6 and provides the matrix concentrations at the carbide-matrix interface as a function of alloy chemistry and temperature. The kinetic model describes the complete space-time evolution of the chromium concentration and takes account of the effects of alloy chemistry, temperature and the grain size. Calculations at temperatures between 773 and 1173 K indicate that the onset of sensitization and the subsequent restoration of the corrosion resistance can be related to respectively a fall below and a rise above the average chromium content of 12 wt.% in a region 35 nm thick adjacent to the grain boundary for the type 304L and in a region 31 nm thick for type 316L austenitic stainless steels. This finding is in very good agreement with 0.010 inch penetration per year time-temperature-sensitization diagrams for the two austenitic stainless steels.