ABSTRACT

Super duplex stainless steels (SDSS) combine excellent corrosion resistance especially to the localized forms of corrosion with medium to high mechanical strength. This unique combination of properties has made these alloys very successful in the oil and gas industry. However, in some specific scenarios, even these steels have to be protected against corrosion, which is usually accomplished by cathodic protection. The synergic effect of hydrogen produced during cathodic protection with service mechanical loads may produce the embrittlement phenomenon known as hydrogen induced stress cracking (HISC). Indeed, documented cases of failure due to HISC have somehow deteriorated the image of the SDSS and raised some concerns. The aim of this investigation is to evaluate through fracture toughness tests the susceptibility of SDSS to HISC and more specifically to determine the effect of the cathodic protection potential and the stress intensity factor rate (K-rate) on the results produced. Within the range of parameters studied here, the degradation of fracture toughness due to HISC is strongly dependent on the testing parameters employed, especially the cathodic potential with a less pronounced effect of the K-rate. The results also suggest that the issue of HISC might not be a material’s problem but instead can be mitigated by the optimization of cathodic protection design.

INTRODUCTION

Oil and gas floating production units fundamentally depend on the performance of their devices, components and structures. Rigid pipelines are essential equipment used in the offshore industry, commonly employed as flow-lines and risers. Carbon steels such as API1 5L X65 are the material of choice for such applications due to their low relative cost and availability. However, in the case of the Brazilian pre-salt, it seems unlikely that carbon steels can be applied, since the oil contains high concentrations of CO2, which causes generalized corrosion.1-3 Therefore, operators in Brazil are compelled to consider alternative solutions, such as lined or clad pipes as well as corrosion resistant alloys like duplex and super duplex stainless steels.

Super duplex stainless steels strategically combine the properties of ferritic and austenitic steels. The superior corrosion resistance of SDSS, especially for the localized forms of corrosion, is attributed to the use of high amounts of Cr and Mo.4 The addition of Ni and N, on the other hand, stabilizes the microstructure at the ideal volume fraction of austenite and ferrite phases, thus providing the best combination of mechanical properties and corrosion resistance.2,5-7 Applications for SDSS include sea water systems, flow-lines, risers, pressure vessels and pipelines in general, where optimized mechanical properties are required along with high corrosion resistance in different media.5, 7, 8

Although SDSS present excellent corrosion resistance in general, in some cases to operate satisfactorily these alloys need to be cathodically protected against corrosion. That is the case for environments containing concentrated chloride solutions (severe salinity) and high temperature beyond their critical pitting/crevice temperature (CPT/CCT). Even when the operating conditions are not particularly severe, equipment and components made of SDSS may still be under cathodic protection due to occasional electric contact (galvanic coupling) with other structures that must be protected against corrosion.7,9-11

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