Comparison of rotating cylinder and loop methods for testing CO 2 corrosion inhibitors

Abstract

A study was carried out to investigate the effect of various hydrodynamic parameters on the corrosion rate of low carbon steel in CO₂ environments in the presence of inhibitors. Two different flow geometries, rotating cylinder and pipe flow, were studied simultaneously in the same electrolyte within a glass loop. Comparisons were also carried out at room temperature, pH 4-6, partial pressure of CO₂ 1 bar, and velocity 0-13 m S⁻¹. The hydrodynamic conditions studied cover the range from stagnant to highly turbulent flow. The corrosion process was monitored using polarisation resistance, potentiodynamic sweep, and electrochemical impedance. Comparison of the two flow geometries was carried out in terms of the hydrodynamics, mass transfer rates, CO₂ corrosion rates, and corrosion mechanisms. The measured mass transfer rates were found to agree well with previously published correlations for the rotating cylinder and straight pipe flow. In the case of CO₂ corrosion without inhibitors, it was possible to achieve good agreement between corrosion rates in the two flow geometries by ensuring the same water chemistry and mass transfer conditions. This conclusion is valid for the case when no protective corrosion product or scale is present. With inhibitors present, the performance of both amine and imidazoline based inhibitors measured using the rotating cylinder was identical to that in the straight pipe flow geometry. The performance of the inhibitors was not significantly affected by flowrate in the range 2–10 m s⁻¹ (corresponding to shear stresses of 13 and 222 Pa respectively).