Effect of H₂S on the Performance of Welded 13% Cr Steel in Supercritical CO₂

Welded 13% Cr steel specimens were tested at 80°C in de-aerated NaCl solution (1000mg/L) for 30 days purged with (i) ∼10MPa CO₂ and (ii) a mixture of ∼9.999MPa CO₂ and ∼0.001MPa H₂S. One set of specimens was submerged in the solution and the other was suspended above it. Two sets of specimens were prepared: (i) for corrosion test with no applied stress, and (ii) for stress corrosion cracking (SCC) test using a four-point method by applying 100% of 0.2% proof stress. For four-point testing, specimens were extracted transverse to the weld and were tested with the weld root in tension. Post-test examination of the specimens revealed evidence of corrosion. The strained specimens tested in H₂S/CO₂ showed very fine features. These sub-micron features were narrow in the vapor space, but wider features were observed in the region close to the fusion line in the specimen tested in the aqueous phase.

Carbon steels such as API 5L X65 are widely used oil and gas exploration, production and transportation service. However, these steels tend to corrode in the presence of wet CO₂ and corrosion is more pronounced in the presence of dissolved salts and acids.^1-5 Other metals, alloys and polymers also degrade in the presence of high pressure gaseous and supercritical CO₂.^6-12 The corrosion rate of carbon steels in some aqueous environments have been reported to be more than a few millimeters per year.^9-10 The situation could be further exacerbated by H₂S where cracking can be an issue for high strength steels.¹³ The interplay between CO₂ and H₂S is rather complex.¹⁴ Nonetheless, the high corrosion rates observed in the case of carbon steels is unacceptable. Such high corrosion rates necessitate the use of corrosion resistant alloys (CRAs).¹⁵ However, the cost of some of the CRAs are prohibitive and often a compromise is required to allow CapEx to be manageable. Work carried out on steels with different Cr contact suggest that 13% Cr supermartensic steel provides a cost-effective compromise between carbon steel and CRAs.^16-18 There are several publications elucidating the effect of 13% Cr stainless steels in H₂S/CO₂, however, the behavior of welded steel has not been explored in detail, particularly in supercritical CO2. This paper addresses the above knowledge gap and explores the behavior of welded 13% Cr steel in supercritical CO₂ containing H₂S.