Question

Duplex stainless steels (22Cr duplex and 25Cr super duplex) are susceptible to hydrogen induced stress cracking (HISC) under cathodic protection (CP).  Industry subsea failures have resulted from HISC under load-control scenarios where local strain levels exceed a critical strain threshold.  Load-control situations occur in the absence of restraints on continuous material straining, such as for thermal growth and free spans.  Residual stress levels from fabrication and installation, coating system integrity, and defective material microstructures contribute to initiation of HISC.

What can be done to avoid HISC?

Answer 1

In order to avoid CP-induced HISC in subsea use of 22Cr duplex and 25Cr super duplex, design conditions should include each of the following mitigators:

• Avoidance of all possible load-control scenarios on duplex components
• Use of finite element models that limit local strains to less than 0.50 percent
• Control of peak loading to less than 80 percent of the nominal material yield strength

Further, coatings should be applied to all subsea duplex materials.  The required coating system should be durable, of high dielectric properties, and capable of shielding critical areas of the duplex material from CP.  The coating system should consist of a corrosion protection undercoat and a thick film insulation coating.  The undercoat should be fusion bonded epoxy for flowlines or a three layer epoxy for fabricated components.  The thick film insulation coating should be either syntactic polyurethane foam or other insulating material such as polypropylene.