Abstract
Some Preventive methods for corrosion cracking were studied for 80 kg/mm2 high strength steel (HT 80) in an accelerated corrosion cracking environment (liquid ammonia saturated with NH4CO2NH2) at 30°C and in field liquid ammonia by means of a constant load or strain method using quenched specimens. The results obtained are as follows:
(1) In the accelerated environment, corrosion cracking of HT 80 is prevented when the specimen is in contact with Mg or Zn, but accelerated when it is in contact with Al. However, it is difficult to prevent the corrosion cracking of WOL-type specimens in liquid ammonia in a field pipe line, even if they are in contact with Mg or Zn. In this case Mg and Zn are covered with corrosion products.
(2) Corrosion cracking of HT 80 is prevented by the addition of more than 0.01 wt% refrigerator oil or more than 0.0005 wt% rape seed oil to the accelerated environment.
(3) Corrosion cracking of HT 80 is prevented by keeping the temperature of the accelerated environment below 10°C. Anodic polarization curves of HT 80 in the above-mentioned environment show that the passive current density decreases and the width of passive region increases as the temperature drops. These experimental results suggest that the field liquid ammonia tanks should be kept at low temperatures. The number of corrosion cracks in the tanks is certainly decreased markedly by keeping the storage temperature at 5°C.
(4) Recently corrosion cracking of many ammonia storage tanks has been found out in Japan. This phenomenon seems to come from the decrease of the amount of machine oil entrained by the ammonia. The decrease of the oil amount is due to the change of compressor’s types employed in ammonia manufacturing plants.
