Corrosion Performance of API 5L X65 Steel in a Low-temperature Saline Environment Containing CO₂

Abstract

This paper focuses on the corrosion performance of API 5L X65 steel in de-aerated 3.5wt% NaCl solution containing CO2 at 10°C. It is generally acknowledged that iron carbonate scale formation is noticeable above 20°C, while tenacious protective scales form above 60°C. This study investigates this phenomenon further by identifying the degree of severity of CO₂ corrosion of X65 carbon steel at temperatures lower than 20°C. Electrochemical testing and surface examination indicate that cementite (Fe₃C) dominates the layer formation, leading to higher corrosion rates, since it is responsible for the formation of localized anodes and the local acidification of the Fe₃C/steel substrate interface. The formation of some loose iron carbonate scale (FeCO₃) was also observed; however, the very low temperature prevented any strong precipitation and adhesion. Finally, in addition to general corrosion, localized corrosion in the form of pits was also observed. Although the extent of pitting was very limited, the localized damage of the steel seemed to be more severe than the general corrosion damage.

Introduction

Carbon dioxide, CO₂, is usually present in produced fluids and although it does not generally by itself cause catastrophic failure, its presence in contact with an aqueous phase can nevertheless result in significant metal degradation of the equipment used in the production and transport of oil and gas. In 1975, de Waard and Milliams proposed the first CO₂ (“sweet”) corrosion model which was later revised by Nešic et al. ^{1-2, 3-5}

The main product of CO₂ corrosion is iron carbonate, FeCO₃, which deposits on the surface of the steel and, depending on the conditions, can be protective or non-protective towards the surface of the steel. Since the inception of de Waard and Milliams’ model, the initiation, growth and stability of corrosion product films have been studied and many factors influencing the role of the formed scales and thus the corrosion rate of carbon steel have been investigated, including temperature, partial pressure, pH, and steel microstructure, as well as the presence of salts, organic acids and inhibitors. ^6-14