1Engineering Department, Toray Industries Ltd., Nagoya
Some electrochemical aspects of the stress corrosion cracking (SCC) behavior of HT 80 steel in a liquid ammonia (99.9%) at 30°C have been studied. The measurement of electrode potential was made in reference to the Pt electrode. The results obtained are as follows:
(1) In liquid NH3 and NH3+0.5 wt% H2O, the ``as-dipped'' specimens exhibited a passive potential regardless of air contaminations, and the ``cathodic treated'' specimens showed an active potential except in a highly air-contaminating condition. The anodic polarization curves for steel showed no active-passive behavior in the above environments.
(2) In liquid NH3, SCC was observed when ever the potential was in an unstable active-passive region, and the potential suddenly shifted from the active to the passive direction by the addition of air but not by the addition of water or N2 gas. It is considered that the SCC is inhibited by the addition of O2.
(3) In a liquid NH3 in which SCC occurred in a field tanks, cracking was not observed in the dynamic slow strain rate testing procedure. This seems to be due to the effect of concentrated impurities in the field to assist the cracking.
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