Abstract
The combined effect of water and oxygen on stress corrosion cracking (SCC) of carbon-manganese steel in liquid and vaporous ammonia has been studied in detail. The highest susceptibility to SCC was found in liquid ammonia with 3 to 10 ppm oxygen and a water content lower than 100 ppm. SCC can occur in ammonia with an oxygen content down to 0.5 ppm when the water content is very low. A borderline for safe operation is given as a function of oxygen and water content. It has been demonstrated that SCC can occur in a cooled vapor phase when the ammonia is inhibited in the liquid phase by water additions of up to 0.2%. The SCC susceptibility in the vapor phase is dependent upon the liquid phase composition and the degree of cooling. SCC has been found in different carbon-manganese steels and in a nickel steel tested at −33 C. It is suggested that the fact that no cracking has been experienced in storage tanks operating at −33 C is a result of the lack of oxygen in the environment.
