On CO₂ corrosion resistance of low carbon steels in the formation water chemistry: The impact of Cr content as an alloying element

One of the major causes of pipeline failure in the oil and gas industry is CO₂ corrosion and scaling in production steel wells. Generally speaking, the addition of Cr as a constituent alloying element in steels is known to improve their corrosion resistance when subjected to aqueous condition. However, adding alloying elements in high quantities increases the material's cost, making it unsuitable for low-yield fields. Thus, it is proposed that a particular quantity of Cr could be added to the low-alloy steel for enhancing its corrosion resistance and making it more cost-effective. During the last few decades, researchers studied the effect of Cr on CO₂ corrosion behavior of mild steel and how Cr enrichment in the corrosion products may influence the protectiveness of the underlying material. However, limited investigations have been focused on determining the impact of Cr on corrosion behavior of steels in CO₂-saturated environments comparable with realistic conditions, such as formation water chemistry containing Ca²⁺ ions.
The goal of this research is to comprehensively understand the effect of Cr as an alloying element on CO₂ corrosion behavior and scaling of low-alloy steels. Conventional L80 steels with various content of Cr were electrochemically subjected to CO₂-saturated simulated formation water chemistry. DC polarization and AC impedance techniques were used to assess the steels' electrochemical and corrosion behavior. The results of characterization of the corrosion scale obtained by employing ex-situ high-resolution electron microscopy and X-ray Diffraction applied on the so-called “end-product” are presented. Furthermore, the results of the X-ray (micro-) computed tomography for determining the mode of corrosion by evaluating the surface of the material underneath of the corrosion scale and visualizing the scale morphology in 3D are thoroughly discussed in relation to the CO₂-electrochemical response of steels with different content of Cr.