Influence of H₂S on the pitting corrosion of 316L stainless steel in oilfield brine

Highlights

• Impact of H₂S partial pressure on pitting of 316L SS in oilfield brines.

• Pitting most severe at intermediate partial pressures of H₂S.

• pH of pit solution controls solubility of metal sulphides and availability of free H₂S.

• Metal sulphide formation proposed to limit pit development at higher partial pressures.

Abstract

Coupon immersion tests were performed on 316L stainless steel in a simulated oilfield environment to evaluate the effect of H₂S partial pressure on pit depth and density. Pitting was most significant at intermediate partial pressures of H₂S, for which free H₂S in the pit solution is maximised. Inhibition of pitting at higher partial pressures is attributed to blocking of the pit surface by metal sulphide phases. The key role of pH in the pit solution is to determine the solubility of metal sulphides and the availability of free H₂S to adsorb on the reacting pit surface and sustain activity.

Introduction

In the oil and gas industry, corrosion resistant alloy (CRA) pipelines, linings, and fixtures are deployed for operating conditions considered too severe for application of carbon steel with chemical inhibition, or where the CRAs provide the most practical economic choice for system management. The environments tend to be mildly acidic, with a high concentration of chlorides, and may contain H₂S. In combination with an elevated temperature, these conditions are highly corrosive and capable of inducing stress corrosion cracking (SCC). As such, materials specified for service must be shown to be resistant to SCC through laboratory qualification testing according to standards such as NACE MR0175/ISO 15156 [1]. Tests for resistance to SCC typically involve exposing specimens of the stressed alloy to conditions broadly comparable to or more conservative than those encountered in service and performing post-test analysis to detect evidence of SCC.

The potential limitations of this approach are the relatively short timescale and small-scale nature of most testing, the idealised and variable surface preparation of test specimens in some cases, and uncertainty as to whether the laboratory environmental exposure conditions are sufficiently representative of service environments. While service experience of alloy performance can compensate to an extent, the complex interplay of variables, such as salt composition and concentration, gas pressure and composition, temperature, active electrochemical processes, and stress mode, can challenge confidence in applying laboratory test data to materials selection and design for new, often more aggressive, oilfields. Confidence is undermined by the lack of fundamental understanding of the cracking mechanism and of the relative significance of different variables in controlling the cracking process. The consequences can be over-conservatism and over-specification of material in design or, conversely, and more concerningly, under-specification, resulting in catastrophic failure of in-service components. Improved understanding of the underlying mechanisms would lead to more reliable interpretation and application of test results, leading to reduced costs and safer operation.

In chloride solution, stress corrosion cracking of austenitic stainless steels such as 316L stainless steel (SS) is usually preceded by pitting corrosion, both in aerated solution [2,3] and in sour (H₂S-containing) environments [[4], [5], [6], [7]]. However, pits do not necessarily lead to the formation of cracks; whether they do so will depend on pit size, pit geometry and pit growth rate. The associated uncertainty has resulted in a lack of guidance on the interpretation of short-term tests for SCC of CRAs in sour environments in which the material exhibits corrosion pits but without cracks. At NACE International meetings on standardisation, this has led to discussion of a threshold pit size for acceptance or rejection of a material, but no consensus. In the absence of detailed insight, such a concept has been excluded from current standards and it is left to the end user to define their acceptance criteria.

To allow material selection decisions to be made on a more informed basis, better understanding of the pitting mechanism on 316L SS in sour conditions is required. Pits initiate at inhomogeneities such as inclusions and physical defects, with the latter becoming more significant for low sulphur containing steels with less refinement of the surface finish [7,8]. Ferrite stringers could also provide a site for localised attack. Sophisticated surface analytical techniques such as nanoSIMS and XPS have shown that exposure to H₂S-saturated brine leads to a high uptake of both sulphide and chloride into the oxide layer [[9], [10], [11], [12], [13], [14]]. It has been proposed [12] that sulphide and chloride in the film may act synergistically, whereby the integration of sulphide into the passive film facilitates further infiltration by chloride and enhances initiation of pits at surface inhomogeneities. This would explain the more ready breakdown of the passive film in H₂S-containing solutions. However, the role of H₂S in sustaining propagation of pits in stainless steels has been less well characterised since the introduction of the concept proposed by Mat and Newman [15] and by Marcus [16] of sulphur species retarding repassivation and sustaining activity of the pit surface. Pit propagation in 316L SS has been studied in detail for many decades but the focus has been predominantly on aerated chloride environments. These conditions are very different to those encountered in sour oil and gas wells, which are essentially anoxic and contain hydrogen sulphide. As such, the well-developed, classical pitting models based on differential aeration and development of an acidic metal salt solution sustaining active corrosion in the pit [17] are not sufficient to predict the response of corrosion resistant alloys in these anoxic conditions.

To understand in greater detail the impact of H₂S on pit development in 316L SS, corrosion coupon exposure tests were conducted in simulated oilfield brines for several partial pressures of H₂S (balance CO₂) at two pH values. Supporting thermodynamic analysis was undertaken to rationalise the observations in the context of H₂S depletion in the pit, as proposed by Mat and Newman [15]. Test conditions were selected to be close to the pass/fail boundary with respect to resistance to SCC, guided by previous studies on SCC of 316L SS in H₂S environments [7].