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

Abstract

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 modified DL-EPR test were compared to intergranular stress corrosion cracking (IGSCC) susceptibilities determined in deaerated 0.01 M sodium tetrathionate (Na2S4O6) under deformation at a constant strain rate of 0.93 x 10(-6)/s. By analyzing the effects of solution concentration, temperature, and scan rate on the electrochemical response in the EPR tests and the morphologies, the optimal modified DL-EPR test condition for alloy 600 was obtained in 0.01 M sulfuric acid (H2SO4) + 10 ppm potassium 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 general and pitting corrosion occurred, in addition to intergranular corrosion. The modified test, however, was highly discriminating because of its highly selective corrosion attack at grain boundaries. IGSCC occurred in samples of alloy 600 aged for < 20 h, and susceptibility to IGSCC (S-IGSCC) increased with decreasing aging times up to 1 h, showing 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 zone produced greater S-IGSCC. It was demonstrated that DOS measured by the modified DL-EPR test was correlated more closely with S-IGSCC than was DOS measured by the standard EPR test. This correlation resulted from the fact that the modified EPR test selectively attacked the more deeply chromium-depleted regions in comparison to the standard EPR test.