ABSTRACT

Sour (H2S –containing) environments present a major problem for oil and gas industries: of particular concern is underdeposit corrosion (UDC) which is a type of accelerated localized corrosion observed under deposits (in this case typically iron-sulfides), creating a risk to asset integrity. The mechanism of UDC is not fully understood and there is no consensus on the methods used to evaluate UDC in pipelines. In this work, both planar electrode and 1-dimensional (1D) artificial pit experiments were performed (where the artificial pit acts as an anode in a three-electrode electrochemical cell, simulating an actively dissolving pit). In the 1D case the dissolution kinetics can be measured directly as a function of system variables such as chloride ion concentration, pH values and the existence of different types of deposit at the pit cavity opening. The effects of deposit chemistry and morphology on the electrochemical dissolution behavior are discussed in terms of a transport-controlled model for pit propagation.

INTRODUCTION

With the continued need for fossil fuels, the extraction of oil and gas from significantly sour fields is increasing globally. A major problem in sour (H2S –containing) gas environments is underdeposit corrosion (UDC) which is a type of localized corrosion observed under deposits, in this case typically iron-sulfide scales.1–4 This presents several challenges to system integrity, including an accelerated rate of localized corrosion, and potential catastrophic-premature failure in fluid handling equipment. This creates a risk to asset integrity, leading to financial losses, and significant risk to both humans and the environment.

In oil and gas field brines, in addition to the abundant sand from the formation, there exists several other reactive components mainly carbon dioxide (CO2) and hydrogen sulfide (H2S), which react with the pipeline steel to give iron carbonate (FeCO3) and iron sulfide (FexSy) products respectively, along with oxides. Since H2S is an efficient scale-forming agent, iron sulfide, which is a much less soluble scale than the formed FeCO3, is formed in different phases and stoichiometry.5 These phases are different in properties and can provide different degrees of protection to the bare metal according to the film growth, structure, reactivity and stability.6 The study of the chemical and physical characteristics of the solid deposits and their influence on the bare metal will be helpful for the development of new corrosion inhibitors and mitigation strategies.3,7

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