SCC of Carbon Steels in Ammonia - Crack Growth Studies and Means to Prevent Cracking

Abstract Stress corrosion crack growth of carbon steel in ammonia was studied with compact tension specimens. The growth of stress corrosion cracks increased with stress intensity factor and followed approximately the square root of the exposure time. The reduction of crack growth rate with time is mainly caused by a crevice corrosion attack in the outer part of the crack. A model for SCC crack growth in ammonia was developed based on the present experiments. The crack growth rates obtained are in good harmony with practical experience. Experiments with welded specimens showed that residual stresses in welds are important for SCC of carbon steel in ammonia, while microstructure variations in the weld and HAZ seem to have little effect. Electrochemical studies showed that SCC of carbon steel in ammonia was prevented by cathodic polarization. With high anodic polarization SCC occurred in ammonia with 2000 ppm water and in ammonia without oxygen. SCC did not occur in these environments without polarization. Coupling to zinc, aluminium and magnesium anodes gave cathodic protection and prevented SCC. The low conductivity in ammonia prohibits the use of traditional sacrificial anodes. Aluminium and zinc metal spray both prevented cracking. Aluminium spray has a low potential margin and a high consumption rate. Zinc spray has a large potential margin and a low consumption rate in the range 0.02 mm/year, allowing about 10 years' lifetime.