Intergranular corrosion in a martensitic stainless steel detected by electrochemical tests

Quenched and tempered martensitic stainless steel UNS S41000 was tested ele ctrochemically for susceptibility to sensitization in specimens quenched fr om 975 degrees C (1,248 K) and tempered for 2 h at different temperatures b etween 300 degrees C (573 K) and 700 degrees C (973 K). Besides an oxalic a cid etch lest, the following tests were performed using a potentiostat and 1 N sulfuric acid (H2SO4) solution: potentiostat polarization, potentiostat ic etch, and electrochemical potentioknetic reactivation in the double-loop version (DL-EPR). Tested surfaces were observed metallographically. The ma ximum susceptibility to intergranular corrosion was observed in the conditi on tempered at 550 degrees C (823 K), and a sensitized structure was detect ed. For lower tempering temperatures, steel was less sensitized, or not at all, and for higher tempering temperatures, steel was less sensitized. All tests except for the oxalic acid etch were able to evaluate quantitatively different degrees of sensitization as a function of tempering temperature, The DL-EPR method was the best to discriminate degrees of sensitization.