Hydrogen Induced Stress Cracking on Superduplex Stainless Steel Under Cathodic Protection

The optimized and secure operation of oil and gas floating production units depends essentially on the performance of their devices, components and structures. Rigid pipelines are key equipment used in the offshore industry commonly employed as flowlines and risers. Carbon steel such as API 5L X65 is the material of choice for those applications due to its low relative cost and availability. However, for the Brazilian pre-salt it seems unlikely that carbon steels can be applyed, since the oil is contaminated by high concentrations of CO₂, which causes generalized corrosion. Therefore, operators in Brazil should consider an alternative solution, such as lined or clad pipes as well as corrosion resistant alloys (CRA). Duplex and super duplex stainless steels (SDSS) have emerged in the last decade or so, as an alternative material for harsh environments. Nevertheless, according to recent studies, SDSS when cathodically protected against corrosion are prone to hydrogen induced stress cracking (HISC). The aim of this investigation is to evaluate through fracture toughness measurements the susceptibility of welded SDSS samples to HISC for two different levels of cathodic protection. For fracture toughness evaluation the step loading test method was selected. This practice is believed to be more realistic because samples are exposed to hydrogen during the entire tests instead of simple hydrogen pre-charging before performing the test in air, as recommended by some procedures. Fracture toughness values are given in terms of both CTOD and J-integral for crack initiation and maximum stress for SENB specimens. The results given here indicates that SDSS are quite susceptible to HISC especially in the heat affect zone even for potentials as negative as −650 mV_sce.