The presence of halides, particularly chlorides, has resulted in numerous in-service cracking failures of insulation-covered 18-8 austenitic stainless steels. These failures were first discovered in thermally insulated piping in petrochemical plants which had been built in the 1940s. The elimination of all halide sources during welding and installation, and the prevention of halide contamination during subsequent service, have proven to be extremely difficult. Some failures have been noted even in the presence of very low levels (10 ppm) of chlorides.
These failures have been associated with stress corrosion cracking (SCC), an electrochemical reaction, which produces a fine network of transgranular cracks
on the surface of the insulation-covered 18-8 stainless steel. Depending on conditions, failure by SCC may occur in as little as a few days or weeks.
Four conditions are necessary for this SCC to develop:
(1) An 18-8 austenitic stainless steel (such as 304, 304L, 316,316L, 317,321,347.
(2) The presence of halides (particularly the chlorides)
(3) The presence of tensile stresses (elastic or plastic, residual or applied).
(4) The presence of an electrolyte (water). During SCC, the halide ions dissolve the passive protection layer on the 18-8 stainless steels; localized corrosion cells then become active.
Austenitic stainless steels with higher nickel, chromium, and molybdenum contents have been developed for enhanced resistance to the SCC problem which has plagued the insulation-covered 18-8 stainless steels.
Among the potential trouble-makers are the inks of several types of metal marking pens with high available halogen content, as well as perspiration from the worker’s hands. Clean cotton gloves should be worn when working with stainless steel.