Effect of Anhydrous Ammonia Composition on its Ability to cause Stress Corrosion Cracking in Steels

Stress corrosion tests have been performed on pressure vessel steels in ammonia liquid and vapour, using the slow strain rate test and pre-cracked Wedge Opening Load (WOL) specimens. The materials. tested were BS 1501–224 Grade 30 weld imd parent plate, BS 1501–151 Grade 26 parent plate, STE 47 weld and parent plate and DIN 17135–ASt 52. Discriminant analysis was used to relate the severity of cracking in the slow strain rate test to the concentrations of oxygen, carbon dioxide and water present in the ammonia. The WOL specimens were not effective in predicting stress corrosion cracking in the liquid ammonia environment.

Oxygen promoted cracting at a level of ≥5 ppm in commercial grade ammonia, but may be as low as 1 ppm when carbon dioxide and water are very low. Water, hydrazine, ammonium carbonate, bicarbonate and carbamate were effective inhibitors whereas an oil film was not. All of the steels tested suffered cracking but welded BS 1501–224 Grade 30 and STE 47 were the most susceptible. Cracking was mainly transgranular and the mechanism was thought to be passivation film rupture.