PREDICTING SUSCEPTIBILITY OF ALLOY-600 TO INTERGRANULAR STRESS-CORROSION CRACKING USING A MODIFIED ELECTROCHEMICAL POTENTIOKINETIC REACTIVATION TEST

Modified double-loop electrochemical potentiokinetic reactivation (DL- EPR) tests were applied to evaluate the degree of sensitization (DOS) for alloy 600 (UNS N06600) aged at 700 degrees C. Results of the modif ied DL-EPR test were compared to intergranular stress corrosion cracki ng (IGSCC) susceptibilities determined in deaerated 0.01 M sodium tetr athionate (Na2S4O6) under deformation at a constant strain rate of 0.9 3 x 10(-6)/s. By analyzing the effects of solution concentration, temp erature, and scan rate on the electrochemical response in the EPR test s and the morphologies, the optimal modified DL-EPR test condition for alloy 600 was obtained in 0.01 M sulfuric acid (H2SO4) + 10 ppm potas sium thiocyanate (KCNS) at 25 degrees C and at a scan rate of 0.5 m V/ s. The standard DL-EPR test, performed under conditions of 0.5 M H2SO4 + 0.01 M KCNS at 30 degrees C and a scan rate of 1.67 m V/s, provided very poor discriminating power for various DOS of alloy 600 because g eneral and pitting corrosion occurred, in addition to intergranular co rrosion. The modified test, however, was highly discriminating because of its highly selective corrosion attack at grain boundaries. IGSCC o ccurred in samples of alloy 600 aged for < 20 h, and susceptibility to IGSCC (S-IGSCC) increased with decreasing aging times up to 1 h, show ing maximum S-IGSCC in the sample aged for 1 h. S-IGSCC for the alloy was found to be associated closely with the chromium-depleted profile across grain boundaries. The deeper and narrower chromium-depleted zon e produced greater S-IGSCC. It was demonstrated that DOS measured by t he modified DL-EPR test was correlated more closely with S-IGSCC than was DOS measured by the standard EPR test. This correlation resulted f rom the fact that the modified EPR test selectively attacked the more deeply chromium-depleted regions in comparison to the standard EPR tes t.