Intergranular Corrosion in a Martensitic Stainless Steel Detected by Electrochemical Tests

Quenched and tempered martensitic stainless steel UNS S41000 was tested electrochemically for susceptibility to sensitization in specimens quenched from 975°C (1,248 K) and tempered for 2 h at different temperatures between 300°C (573 K) and 700°C (973 K). Besides an oxalic acid etch test, the following tests were performed using a potentiostat and 1 N sulfuric acid (H₂SO₄) solution: potentiokinetic polarization, potentiostatic etch, and electrochemical potentiokinetic reactivation in the double-loop version (DL-EPR). Tested surfaces were observed metallographically. The maximum susceptibility to intergranular corrosion was observed in the condition tempered at 550°C (823 K), and a sensitized structure was detected. 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.