CORROSION OF LOW-ALLOYED STEELS IN LIQUID-AMMONIA

Corrosion behaviour and stress corrosion cracking of the base material, the welding and the heat affected zone in liquid ammonia at room temperature of the low-alloyed steels 13 Cr Mo 4 4 (W.Nr. 1.7335) and 15 Mo 3 (W.Nr. 1.5415), as well as the un-alloyed steel StE 460 (W.Nr. 1.8905) for comparison were investigated. According to polarization curves, the corrosion current density in the passive state usually was lower for the welding than for the base steel. In the active state the welding of the low-alloyed steel corroded faster than the weldings of the unalloyed steel. In the region between the welding and the heat affected zone of the base steel corrosion was nonuniform. CERT-experiments revealed susceptibility to stress corrosion cracking in the orders StE 460 < 15 Mo 3 < 13 Cr Mo 4 4, and base steel < welding < transition zone. The highest susceptibility was observed at electrode potentials in the middle of the passive region. Additions of water exceeding about 100 ppm favoured stress corrosion cracking. The susceptibility decreased again at concentrations greater-than-or-equal-to 1000 ppm. Oxygen increased the susceptibility to stress corrosion cracking only at high concentrations not occurring in practice. The presently investigated low-alloyed steels are not superior to the unalloyed steel from the viewpoint of stress corrosion cracking.