Performance of API 5L X65 in 138 MPa Supercritical CO₂

This paper reports the performance of welded API 5L X65 in an aqueous solution pressurized with 138 MPa supercritical CO₂. Welded API 5L X65 carbon steel specimen was prepared and exposed to 3.5 wt.% NaCl solution pressurised with 138MPa CO₂. The sample was stressed to 100% of 0.2% proof stress using a four-point method. The specimen were extracted transverse to the weld and was tested with the weld root in tension. The test ran at 40°C for 30 days. Post-test visual examination indicated a scale on the surface which was confirmed by microstructural characterization. The SEM micrographs indicated corrosion damage, however, no cracks were seen on the tensile surface of the specimen.

Carbon steels and low alloy steels are the workhorse of several industries where properties such as strength, fracture toughness and weldability play a key role. In addition to these properties, carbon steels are also the most cost effective materials is several applications. Of particular interest is the API 5L X65 which is widely used in oil and gas exploration, production and transportation service. In recent years, with the advent of carbon capture and storage (CCS), these steels are also considered for transportation of captured CO₂. However, these steels corrode in wet CO₂ and corrosion is more pronounced in the presence of dissolved salts and acid gases.^1-5 Other metals, alloys and polymers also degrade in high pressure CO₂.^6-16 Polymers are also prone to suffer from rapid gas depressurization (RGD) when removed from high pressure CO₂. In the case of carbon steel, the corrosion rate in some aqueous CO₂-containing environments have been reported to be a few millimeters per year.^8,10 Such high corrosion rates are often acceptable for the design engineers and if carbon steel cannot be used then this would have serious impact on the economics of the operations involved. To ensure material integrity, some recent publications also elude to CO₂ specification.¹⁷ Given the variability in literature data for carbon steel in different environments, the ability to understand the corrosion mechanisms in high pressure CO₂ (and mitigate such effects) could offers a tool for engineers and scientists to ensure cost-effective CCS operations. If future cost-effective transport requires handling of very high pressure CO₂ (>100MPa) then data is required on materials performance. This paper addresses the above knowledge gap and explores the behavior of welded API 5L X65 steel in 138MPa supercritical CO₂.