The Role of Iron Sulfide Polymorphism in Localized H₂S Corrosion of Mild Steel

Abstract

Localized corrosion in sour fields is a challenge persisting in the oil and gas industry since it has frequently been seen as a cause for catastrophic failures of upstream pipelines. Hence, prediction and mitigation of H₂S localized corrosion of mild steel is of key importance for integrity management. However, our current understanding of H₂S localized corrosion mechanism(s) from numerous studies in both in the laboratory and the field is far from being conclusive. Especially, the environmental conditions that may cause localized H₂S corrosion are unclear. Therefore, defining an experimental condition in the laboratory that can replicate localized corrosion in a sour environment is critical to our understanding of mechanisms of localized corrosion. The focus of the present research was to explore environmental conditions leading to localized H₂S corrosion. It was found that severe localized corrosion was repeatedly observed in experiments, when there was a simultaneous formation of greigite and/or pyrite. Based on those experimental results, a hypothesis for a mechanism of H₂S localized corrosion was proposed.

Introduction

Corrosion caused by the presence of H₂S and CO₂ in produced fluids is frequently encountered in pipelines during the production of oil and gas. Compared to general CO2 and H₂S corrosion^1-3, localized H2S corrosion is much less understood and less studied. This poses a key challenge for integrity management in the oil and gas industry.

In open literature, H₂S localized corrosion has been usually associated with multiple risk factors, such as the presence of elemental sulfur^4-8, the presence of polysulfides^9-11, high salinity^12-14, flow velocity¹⁵, a change in local water chemistry at steel surface16, and metallurgy. In addition, corrosion and scaling mitigation strategies, such as corrosion inhibitors, alcohol and glycols, and pH stabilization, used in sour systems in the oil and gas industry, can greatly decrease the uniform corrosion, while increasing the probability for localized corrosion. Kvarekval et al.¹⁷ have showed very strong evidence of this with examples of severe localized corrosion.