CUI in Austenitic and Duplex Stainless Steels

CUI damage in austenitic and duplex stainless steels is a form of external chloride stress corrosion cracking (ECSCC). As with all forms of SCC, cracking occurs when a susceptible metallurgy is exposed to the combined action of a corrosive environment and an applied/residual tensile stress. Susceptible materials include Type 300-series austenitic stainless steels. Duplex stainless steels, though more resistant than austenitic stainless steels, are not immune. A corrosive environment occurs when chlorides concentrate under the insulation at the surface of the austenitic stainless or duplex steel when the insulation becomes wet. Residual cold work from fabrication or residual welding stresses provide the tensile stresses necessary promote cracking.
Most CUI damage in austenitic stainless steels occurs at metal temperatures between 140°F and 350°F (60°C and 175°C) although exceptions have been reported at lower temperatures. Below 120°F (50°C), it is difficult to concentrate significant amounts of chlorides; while above 350°F (175°C), water is not normally present and CUI damage is infrequent. It should be noted that even austenitic stainless steel piping that normally operate above 500°F (260°C) can suffer severe ECSCC during start up after insulation gets soaked from deluge system testing or rain during
downtime.
Typically, CUI damage in austenitic and duplex stainless steels goes unnoticed until insulation is removed or a leak occurs.
CUI damage in duplex stainless steels occurs at higher temperatures than observed for austenitic stainless steels.  It is experienced that SCC of duplex stainless steels does not occur until about 285°F (140°C) at very high chloride concentration levels. In general, there have been few reported cases of cracking in the industry. Those that have been reported have been under severe conditions where SCC could be predicted. Some of the failures reported have been on offshore facilities, and were attributed to ECSCC on relatively hot equipment.