Stress Corrosion Cracking Behavior of ASTM A517 Grade F Steel in Liquid Ammonia Environments

The stress corrosion cracking (SCC) characteristics of ASTM A517 Grade F steel in liquid ammonia environments have been investigated. Acceleration of the cracking process was accomplished at 150 psig by the use of a dynamic slow strain rate testing procedure capable of producing cracking in as little as 24 hours. Test results confirm previous research findings that (1) uncontaminated ammonia does not cause cracking; (2) air contamination in ammonia causes cracking, and (3) 0.2 wt % water inhibits cracking in air contaminated ammonia. For the degree of contamination used, [approximately 2 atmospheres partial pressure of air at 25 C (77 F)], a limit for the effectiveness of water as an inhbitor was observed at 0.08%. Electrochemical hydrogen permeation studies and impressed current SCC experiments indicate conclusively that the mechanism of cracking is anodically controlled and cannot be a result of embrittlement by hydrogen.