Stress corrosion cracking of alloy 600 in 0.1M NaHSO4 and NaOH solution

Stress Corrosion Cracking (SCC) of alloy 600 has been studied in 0.1M NaHSO4 of 360 .deg. C and 1, 10 and 40% NaOH of 315 .deg. C. SCC test was performed using C-ring specimen at potential of O and +200mV above corrosion potential. Stress at the apex of C-ring specimen was controlled to be 150 and 250% of room temperature yield strength. The tubing material were systematically heat treated to control degree of sensitization and intergranular carbide distribution. Degree of sensitization was measured with modified Huey test. The SCC rate of alloy 600 was higher that 3.4x10-7(mm/sec) for stress of 250% of yield strength irrespective of microstructure. SCC rate of HTMA, HTMA+SEN, HTMA+TT were 1.8x10-8, 3.1x10-8 and 2.6x10-9(mm/sec), respectively in 0.1M NaHSO4 for stress of 150% yield strength at the apex of C-ring. SCC resistance increased with a following sequence in 0.1M NaHSO4: HTMA+SEN, HTMA and HTMA+TT. No SCC was found on specimen exposed to 1% NaOH for 35 days irrespective of applied potential and stress. The SCC resistance of alloy 600 increased with a following sequence in 10 and 40% NaOH solution: HTMA, HTMA+SEN and HTMA+TT. SCC rate increased with both NaOH concentration and stress at the apex of C-ring. Weight loss in modified Huey test increased with a following sequence: HTMA+TT, HTMA, HTMA+SEN. In 0.1M NaHSO4, SCC rate was enhanced as alloy 600 become sensitization while in NaOH solution SCC rate was accelerated as alloy 600 has less intergranular carbide. This suggest that microstructural dependency of SCC rate depends on environment