Duplex and superduplex stainless steels contain approximately 50% ferrite, a phase which is inherently susceptible to hydrogen embrittlement. A considerable body of published data exists on this subject but until recently few practical problems had been encountered.
From 1996 onwards, a number of major failures have occurred in duplex and superduplex components operating subsea under cathodic protection applied by aluminium alloy anodes, at -1.0 to -1.1V, leading to considerable loss of production. In these cases, cracking had occurred progressively over a period of time by a hydrogen induced or hydrogen embrittlement stress cracking (HISC or HESC) mechanism, which does not require bulk charging, and hence can occur after a relatively short period of exposure.
It has become clear that the fitness-for-purpose of ferritic-austenitic materials under cathodic protection, at potentials around -1.0V and more negative, is dependent upon the microstructure of the material and is encouraged by high stress and strain levels. Limiting conditions for avoiding cracking in a coarse superduplex material were established at TWI as part of a failure investigation. The study showed that cracking can occur at around -1.1V when the stress reaches the yield stress, even if this is only achieved locally at a stress concentrator.
Fine grained materials apparently show substantially greater resistance to cracking than coarse material. It is also known that there is little or no susceptibility to cracking at potentials more positive than around -0.75V. A more comprehensive understanding of the phenomenon has been developed at TWI as part of Group Sponsored Projects '5699 and 13827 which deal with Cathodic protection of duplex and super duplex stainless steel' and 13827. Please contact Paul Woollin for details. DNV has developed a recommended practice for design of duplex stainless steel subsea equipment exposed to cathodic protection to avoid HISC/HESC, designated DNV-RP-F112.
Further information
FAQ: Can I weld duplex or superduplex stainless steel to carbon steel and, if so, what consumable should I use?