Effect of microstructure on KCl corrosion attack of modified AISI 310 steel

The effect of microstructure on KCl corrosion attack was studied using a specifically chosen modified AISI 310 austenitic steel in a 15% (v/v) H2O (g)+ 5%(v/v) O2 (g) + N2 (g) (balance) atmosphere at 600°C for 168 h. The material was a targeted choice as it allows investigation of different microstructures i.e. as-received (without sigma phase) and heat-treated (29% σ-phase per area) microstructures. The corrosion attack was studied with light optical, scanning electron and transmission electron microscopy as well as X-ray diffraction. The heat-treated sample showed a corrosion attack that was 5 times higher than the as-received sample. In the heat-treated sample, the σ-phase was selectively attacked. At the corrosion front, chlorine (but not potassium) was detected in the selectively attacked σ- phase but not in the unattacked adjacent matrix. Therefore, the corrosion attack was propagated by preferential σ-phase attack by chlorine species.