The effect of fluid flow on CO2 corrosion of high-strength API carbon steels

Abstract

This paper presents and discusses the outcomes of a recent study on the corrosion resistance of API carbon steels (T95 and Q125). The scope includes experimental investigation as well as theoretical modeling. To predict CO2 corrosion under dynamic conditions, an existing model is improved and validated using experimental measurements.Corrosion tests were conducted under static and dynamic conditions. API casings were cut and machined to prepare test specimens used for measuring Average Corrosion Rate (ACR) and Load-Carrying Capacity (LCC). The dynamic test condition was simulated using the flow between two concentric cylinders (Couette type flow). The test solution (2% wt. NaCl) was saturated with a gas mixture containing 50% CO2 and 50% methane. Tests were carried out at 41.37 MPa with varying temperature and the speed of the inner cylinder, and subsequently, changing the wall shear stress (τw). The ACR was determined by applying the Weight Loss (WL) method. A digital microscope was utilized to inspect the morphology of the corrosion scale (product). The change in LCC of the specimens was compared with the theoretically predicted change in LCC to identify the presence of localized corrosion and/or intergranular attack. In addition to LCC, the ultimate tensile strength (UTS) of corroded specimens is determined and compared with that of uncorroded specimens.Results show that the CO2 corrosion of API steels is sensitive to the fluid velocity regardless of temperature. The corrosion of Q125 is more affected by fluid flow than that of T95. Below critical shear stress, CO2 corrosion was strongly affected by fluid flow. Under a static condition, Q125 demonstrated higher corrosion resistance than T95 but under a dynamic condition, T95 exhibited higher corrosion than Q125. At high temperature (71 °C), the occurrence of localized corrosion was observed. Corrosion model predictions show reasonable agreement with measurements.