Microstructure and intergranular corrosion resistance evaluation of AISI 304 steel for high temperature service

AISI 304 austenitic stainless steel is susceptible to intergranular corrosion when aged in the 450–850 °C temperature range due to chromium depletion at grain boundaries. However, some standard specifications recommend this material for high temperature service in this range (500 °C–800 °C). It is believed that the AISI 304 is able to recover its intergranular corrosion resistance after a short period of time, due to bulk chromium diffusion. The kinetics of this so called healing or desensitization process depends on the service temperature, carbon content and also on prior deformation. In the present work we analyzed the kinetics of healing at 650 °C and 750 °C of an AISI 304 steel used to produce a tubular component for high temperature service. As the initial material contained δ ferrite islands, the stability of this phase under aging at 750 °C was also investigated. The degree of sensitization was evaluated by means of electrochemical potentiodynamic reactivation tests. The results show that the material is healed after 48 h at 750 °C, but not at 650 °C. The δ ferrite was stable for periods up to 200 h and the hardness decreased with sensitization at 650 °C and 750 °C.