Stress Corrosion Cracking of Carbon Steel Storage Tanks for Anhydrous Ammonia

Keywords: Ammonia safety, Ammonia, Stress corrosion.

The combined effect of water and oxygen on stress corrosion cracking of carbon-manganese steel in liquid and gaseous ammonia has been studied in detail. The highest susceptibility to SCC was found in liquid ammonia with 3-10 ppm oxygen and a water content lower than 100 ppm. SCC can occur in ammonia with an oxygen content down to 0.5 ppm when the water content is very low. A borderline for safe operation is given as function of oxygen and water content.

It has been demonstrated that SCC can occur in a cooled vapour phase when the ammonia is inhibited in the liquid phase by water additions up to 0.2%. The SCC susceptibility in the vapour phase is dependent upon liquid phase composition and the degree of cooling.

Electrochemical studies showed that SCC of carbon steel in ammonia was prevented by cathodic polarisation. With high anodic polarisation SCC occurred in ammonia with 2000 ppm water and in ammonia without oxygen, environments where SCC did not occur without polarisation. Coupling to zinc, aluminium and magnesium anodes gave cathodic protection and prevented SCC. The low conductivity of 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.

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 decreased with 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. In this model the crack depth is proportional to the square of the stress intensity factor and the square root of the exposure time. The crack growth rates obtained by the model are in good harmony with practical experience.

The effect of temperature on SCC of carbon steel in ammonia was studied with parent material compact tension specimens and bend specimens with welds. The experiments were performed at -33^oC, where many refrigerated ammonia storage tanks operate. The results were compared with results from experiments at 18^oC. It is much more difficult to initiate stress corrosion cracks at -33^oC than at 18^oC, and crack growth is slower at low temperature. The ratio between maximum crack depth in comparable experiments at -33^oC and 18^oC was 1:3 for CT specimens and 1:20 for bend specimens. Practical experience shows also that SCC of carbon steel in ammonia can occur at low temperature, but to a much lesser extent than at ambient temperature.

Stress corrosion cracking of welded carbon steel in liquid ammonia was studied with bend specimens made from carbon steel plates welded with different welding electrodes. Three electrodes commonly used for construction of ammonia storage tanks and one recently developed low strength electrode were studied at ambient temperature. The low strength electrode was the least susceptible to stress corrosion cracking of the four electrodes investigated, while the welding electrode containing nickel seemed to have a somewhat higher susceptibility than the other electrodes. The susceptibility to SCC increases with the strength of the weld metal. The high susceptibility of the nickel containing electrode remained after heat treatment, indicating a possible detrimental effect of nickel.

Recommendations for design and operation of ammonia tanks based on the present experiments and practical experience are given.

L Lunde and R Nyborg, Institutt for Energiteknikk, Kjeller, Norway.

52 pages, 23 figures, 3 tables, 41 refs.