Investigating Pitting Corrosion on X65 Carbon Steel in Sweet Oil and Gas Environment

Low carbon steel is widely applied for engineering purposes such as construction of pipelines, vessels, refinery components amongst others. The set-back in low carbon
steel is in its corrosion performance which takes a toll on operations, economy as well as health and safety. Pitting corrosion ranks among the highest level of corrosion defects and it is inevitable due to the effect of nature, design and operation parameters. Pitting is hard to predict while pitting in sweet (CO2) environments remains the most dangerous form of localised corrosion. Pitting in passive materials is generally well understood but pitting in actively corroding materials has received much less attention and remains poorly understood to date. Pitting can progress beneath films formed on metal surfaces while cations such as calcium are known to influence pitting kinetics and there are no well established models to describe such processes.

A comprehensive study of pitting in API-X65 carbon steel was conducted by examining how pits of predetermined depth behave in a calcium-free and calcium-rich environment. Reproducible stress-free pits with depths of approximately 70 μm were generated using a novel potentiostatic polarisation approach. The relationship between general and pitting corrosion was established using profilometry data and electrochemical results. Pit growth on steel samples
was investigated in a calcium-rich environment to understand how Ca2+ influences pitting kinetics and how the different CaCO3 polymorphs influence pitting in low carbon steel under conditions typical to crude oil storage and other oil & gas exploration settings. The investigation was extended to include the use of an artificial pit to understand how the galvanic trend influences pitting under mixed carbonate (CaxFe(1−x)CO3) film conditions and also to reveal events within an actively corroding pit. Robust analytical tools such as XRD, SEM, Raman and white light interferometry were utilised to examine formed products and reveal details of pitting recorded beneath formed films.

It was observed that pseudo-passivation occurs in a calcium-rich environment and that the pitting trend relates to the nature of the CaCO3 polymorph formed on the steel surface. It was as also revealed that the presence of calcium increases galvanic current between film-covered and compromised spots in low carbon steel and that the magnitude of galvanic current recorded in Ca2+ systems relates to pitting threshold recorded on the metal. Corrosion products that form on steel surfaces were also
detected inside actively corroding pits.