Hiroshi Imagawa1 and Kouji Nakamura2
1Engineering Department, Toray Industries Ltd., Nagoya
The relation between the corrosion cracking of ammonia spherical storage tanks and the quality of liquid ammonia, which changed with the manufacturing processes, was studied by means of the analysis of impurities in the liquid ammonia and corrosion cracking tests for WOL type specimens 80 kg/mm2 high strength steel. The results obtained are as follows:
(1) Branched corrosion cracks were observed within one year on the base metal near its welding line and on the base metal near its welding line in the liquid zone of the tank when it stored the liquid ammonia at atomospheric temperature.
(2) Corrosion cracking of ammonia storage tanks has become more serious after the improvement of the manufacturing process of ammonia. The liquid ammonia of the old plant contained almost the same amount of CO3--, but about ten times as much oil as that of the new plant.
(3) The liquid ammonia contained water, oil, carbonate and chloride as impurities. After evaporation they were concentrated in the residue, in which CO3-- and Cl- were contained about 22000 ppm and 10∼220 ppm respectively.
(4) Corrosion cracking of quenched specimens occurred in liquid NH3+5 wt%NH4CO2NH2 or liquid NH3+5 wt%NH4HCO3 within a week at 30°C. On the other hand, the number of cracks was reduced by adding air, oil or NH4Cl to the former environment.
(5) The carbonate is believed to be the most important contaminant for the corrosion cracking of the steel in the liquid ammonia.
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