Cases of Stress Corrosion Cracking (SCC) in steel vessels using NH3 have been reported by members of the ammonia refrigeration industry. When NH3 is used as a refrigerant, it is kept at -33° C (-28° F). While it was once believed that NH3 SCC)could not occur at such low temperatures, both practical experience and experimental studies have shown otherwise. The Industrial Refrigeration Consortium (IRC) has developed classes and issued technical bulletins for its members to address the causes and prevention of NH3 SCC.
The IRC has identified non-stress-relieved (non-annealed) welds and areas exposed to condensing NH3 vapor as “hot spots” for SCC. In one incident, an SCC attack formed a hole through a high-pressure receiver line from an NH3 refrigeration system. The hole initiated on the inside of the vessel and grew completely through the weld and heat-affected zones to the exterior surface. After the leak occurred, the interior surface of that pressure vessel was examined with a liquid penetrant, and this showed that several SCC 5 to 10 cm long cracks were present, all oriented perpendicular to the weld line.
Other cases of failure have been reported by the International Institute of Ammonia Refrigeration:
High-Pressure Receiver, England
In the 1980’s a high-pressure NH3 receiver made of 10-mm-thick steel failed after just 3 months of service. The vessel had been constructed according to BS5500, the relevant British standard, which like U.S. nurse tanks does not require post-weld heat treatment for stress relief. NH3 was discovered leaking out of a crack oriented perpendicular to a weld seam. Upon further examination, several cracks transverse to the weld seam were found, and the yield strength of the BS1501-151-430 steel was determined to be 332 MPa. No injuries were reported in the incident, but high-pressure vaporous NH3 was released to the atmosphere.
Shell Condenser, France
An ice cream shell and tube condenser were made of A48CP steel (European standard NF A 36- 205) instead of the recommended lower strength A42CP steel and failed by SCC within a few months of service. (There are many steel classification systems. A reader who is interested in converting a designation to another system can obtain “translations” on the Internet.) A48CP steel has a yield strength of 285 MPa. However, the condenser was not stress-relieved and thus had a measured yield strength of about 365 MPa.
High-Pressure Receiver, Holland
A cold-formed pressure receiver began leaking shortly after it was put into service due to the formation of transverse weld cracks near the end cap of the vessel. The vessel had not been post-weld stress-relieved.
Low-Pressure Shell and Tube Evaporator, England
A low-pressure evaporator vessel experienced SCC along a weld at the normal liquid level where liquid and vaporous NH3 coexist. The evaporator was constructed of steel with a yield strength of 370 MPa and was not post-weld heat treated. Unlike the previous cases, this failure occurred in an environment of low pressure. Thus, the tension stress contributing to the SCC was dominantly from residual stress, not for stress induced by working at high pressure.
Suction Pipe on Freezer, U.S.
A freezer suction pipe failed due to thermal shock, which was aided by SCC of welds. The cracks caused a section of the pipe to weaken to the point that it failed. Contributing factors included NH3 condensate (comparable to the vapor area of nurse tanks) coupled with the presence of residual stresses due to pipe fabrication welding.
SOURCE: Federal Motor Carrier Safety Administration Study Report