ABSTRACT

In the previous study by the same authors, a localized corrosion mechanism related to the presence of pyrite corrosion product was proposed by observing and replicating severe localized corrosion in sour environments. In the present study, in order to further validate this localized corrosion mechanism, a comprehensive mechanistic study of localized corrosion was designed and conducted. The experimental design was based on two hypotheses addressing the cause of this type of localized corrosion: an electrochemical galvanic effect and a chemical effect. Thus, novel experiments involving deposition of pyrite particles onto the bare steel surface and onto steel covered by a thin electrically insulating mesh surface were conducted in an aqueous H2S solution. It was found that no localized corrosion was observed when the physical contact between pyrite particles and the steel underneath was eliminated by using an insulating mesh. Moreover, the experiments were also performed in aqueous CO2 solution for further validation. Based on the experimental observations, the electrochemical galvanic hypothesis was proven to be the key mechanism in this type of localized corrosion.

INTRODUCTION

The role of a corrosion engineer in the oil and gas industry is to assure integrity and performance of assets through prediction, mitigation, monitoring, and inspection, from casing strings through to the platforms and transportation pipelines, from time of drilling to the time when the field is abandoned. Among many risk factors, localized corrosion of steel is generally considered to be a great risk to asset integrity by causing catastrophic upstream assets failures. On the other hand, it is notoriously difficult to predict, control, and detect localized corrosion, particularly in sour fields. In addition to those practical challenges in the field, the mechanisms and causes of H2S localized corrosion are unclear. In order to control localized corrosion in sour environments, it is critical to thoroughly understand how it occurs.

The authors’ previous experimental study1,2 has proven that pyrite plays an important role in the initiation of the localized corrosion; however, a more comprehensive mechanistic study on this type of localized corrosion is needed. To bypass the complicated transformation steps between different iron sulfides, pyrite - being a thermodynamically stable iron sulfide, was directly deposited onto the steel surface and the subsequent corrosion process of the steel was studied2. Serious localized corrosion was observed when pyrite particles were deposited onto a mild steel surface in an aqueous H2S environment, suggesting that the formation of pyrite as a corrosion product in sour environments may initiate localized corrosion. Therefore, it is important to understand the mechanism of this type of localized corrosion caused by pyrite. Moreover, this understanding of localized corrosion can be incorporated into the prediction models of localized corrosion in sour environments.

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