Stress Corrosion Cracking of ASTM A517 Steel in Liquid Ammonia: Environmental Factors

Stress corrosion cracking (SCC) of carbon and alloy steels in liquid ammonia has occurred spasmodically over the last twenty years. At the present time, U. S. Department of Transportation regulations provide effective safeguards to prevent this problem through the required use of postweld heat treatment and the use of 0.2% water as an inhibitor. In view of the increasing use of ammonia as a direct fertilizer and as a chemical intermediate necessitating the increasing use of steel for transport and storage tanks, a research program was undertaken to identify the environmental factors that control SCC of steels in ammonia and also to evaluate present and potential palliative procedures to inhibit SCC of steels in service. In this program, numerous accelerated SCC tests were conducted on ASTM A517 Grade F steel, and the results may be summarized as follows: (1) Oxygen contamination in ammonia is the primary cause of SCC. The simultaneous presence of nitrogen contamination markedly increases susceptibility to SCC; (2) as little as 0.01 ppm oxygen will cause SCC; (3) water additions as low as 0.08 Wt% are effective in inhibiting SCC in ammonia contaminated with up to 200 ppm oxygen in the liquid phase. Hydrazine additions at the 0.025% level were also found to be effective; and (4) the mechanism accounting for SCC of steel in ammonia is believed to be the rupture of the corrosion-product film at sites of slip-step emergence caused by stress.