Effects of Water and Air on Stress Corrosion Cracking of High Strength Steel in Liquid Ammonia

Effects of water and air on the anodic polarization and the stress corrosion cracking (SCC) of a quenched high strength steel in liquid ammonia saturated with NH4CO2NH2 (about 0.9 wt%) were investigated by using the potentiodynamic and constant strain methods. The results obtained are as follows:(1) In the above-mentioned environment without water, the steel showed different anodic polarizations and SCC behaviors in the same content of air, according to the kind of gases (N2, Ar or NH3) with which the air in the test vessel was replaced before filling it with liquid ammonia. SCC of the steel was observed and the air inhibited corrosion and SCC in those environments.(2) The critical current density of anodic polarization curves increased with increases in water content and temperature and decreased with an increase in air content, respectively. The passive current density decreased with water content and increased with temperature. SCC of steel tended to occur when the critical current density and the passive current density of the anodic polarization curves were high.(3) In the liquid with more than 0.3 wt% water, the steel showed a passive potential and did not corrode in presence of air. On the other hand, the steel showed an active potential in the presence of a small amount of air.(4) In the liquid with water (maximum 1.0 wt%), SCC of specimens showing an active potential was prevented by addition of 1.0 wt% water, but it was liable to occur when the contents of air and water were low. And the test results of SCC of specimens showing a passive potential were lacking in reproducibility.(5) The amount of water neccessary to prevent SCC in a liquid ammonia tank seems to vary with CO3−− and O2 contents in the liquid ammonia and its temperature.