The Influence of Rotating Cage Geometry and Baffles on the Wall Shear Stress Equation in Sweet/Sour Corrosion

Flow-accelerated corrosion has been studied by means of computational fluid dynamics (CFD) techniques and rotating cage (RC) autoclave tests according to ASTM G170. This study evaluated through CFD and laboratory tests the influence of RC geometry and baffles on the wall shear stress (τ_w) proposed by the aforementioned standard. The experimental were performed at 120 °C and 2 angular velocities (500 and 800 rpm) during 25 h in a solution with 600 mg/L of Cl^- ions and 10^-3 mol/L of sodium thiosulfate. Corrosion rates were obtained by ASTM G31 and the surfaces were evaluated by OM and XRD. Corrosion rate values and XRD analyses showed divergence between samples in different positions in the RC and between distinct regions in the same sample. The numerical simulations and the experimental tests showed a great influence of the baffles on τ_w. The conservatism in the values obtained with this technique is caused by excessive corrosion close to the samples edges and the non-uniformity of τ_w on the internal samples surfaces. Thus, the main conclusion of the study is related to the importance of better knowledge on experimental methodology and mathematical approximations for updating the criterion for the correct τ_w calculation on sample surfaces.

Keywords:
Flow accelerated corrosion; rotating cage autoclave; CFD; API X65 steel; CO₂/H₂S