Experimental and CFD investigations of impact of rotating cage fluid dynamics on the sweet/sour corrosion product formation

The internal corrosion in carbon steel pipelines in mixed CO2/H2S environments are directly affected by the fluid flow transported. Turbulent flow affects mass transfer, the diffusion of chemical species, and the corrosion products formation, resulting in higher corrosion rates. This work used the rotating cage (RC) autoclave to investigate the correlation between fluid dynamics and the formation of corrosion products mechanisms. Samples of API X65 carbon steel were tested in CO2/H2S environments at 120°C and at 500 and 800 rpm, and analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) after the tests. The corrosion rate (CR) was obtained by mass loss. The distribution of shear stress in the samples (τw) was obtained by Computational Fluid Dynamics (CFD), using a system similar to the one in the laboratory. The results showed that increasing the velocity of a turbulent flow in a pipeline carrying fluids containing CO2 and H2S, simulated by increasing the rotation in the tests, can promote an increase in the corrosion rate due to the mechanical removal of FeS films and the difficulty of precipitating a protective FeCO3 layer. Although the FeS film has faster formation kinetics than FeCO3, it is less resistant to the effects of turbulent flow. Therefore, lower shear stresses, resulting from less rotation, favor the permanence of FeS on the surface. On the other hand, higher shear stresses impair the adhesion of FeS, which is naturally fragile and brittle, promoting the precipitation of a poorly protective FeCO3 film and, consequently, increasing the corrosion rate.