Some Electrochemical Aspects of the Stress Corrosion Cracking of Steels in Liquid Ammonia Environments

Some electrochemical aspects of the stress corrosion cracking (SCC) behavior of ASTM A517 Grade F steel in liquid ammonia environments at 25 C have been studied. Electrode potentials were measured during SCC tests by means of thallium/saturated thallous chloride/ammonia reference half cell. The effects of controlled additions of oxygen and nitrogen (species in combination known to cause SCC), as well as the inhibitive properties of water, were investigated. The test results indicate that an oxygen-bearing film forms on the metal surface after which passive-range potentials near +400 mV are observed. Under film-free conditions, potentials near -150 mV were measured. From coulometric measurements, film thicknesses were estimated at 3 to 5 monolayers coverage. SCC in this system can best be described by a film rupture model in which crack initiation occurs at local sites of film breakdown due to straining. The role of water as an inhibitor is to assist in the repassivation of the surface films, as indicated by the fact that water additions promote more noble potentials (+500 mV).