COMPARISON OF THE ROTATING CYLINDER AND PIPE-FLOW TESTS FOR FLOW-SENSITIVE CARBON-DIOXIDE CORROSION

The effects of various hydrodynamic parameters on the corrosion rate of low-carbon steel in carbon dioxide (CO2) environments were studied, Two different flow geometries, rotating cylinder (RC) and pipe flow, were studied simultaneously in the same electrolyte within a glass loop. Comparisons were made over a wide range of parameters: temperature (T) = 20 degrees C to 80 degrees C, pH = 4 to 6, CO2 partial pressure (P-CO2) = 0 bar to 1 bar (0 kPa to 100 kPa), velocity (upsilon) = 0 m/s to 13 m/s. The hydrodynamic conditions studied covered the range from static to highly turbulent flow. The corrosion process was monitored using polarization resistance, potentiodynamic sweep, and electrochemical impedance methods. The comparison of the two flow geometries was carried out in terms of hydrodynamics, mass transfer and CO2 corrosion. The measured mass transfer rates agreed well with published correlations for the RC and straight pipe (SP) flow. In the case of CO2 corrosion, it was possible to achieve good agreement between corrosion rates in the two flow geometries at low temperatures by having the same water chemistry and mass-transfer conditions. This conclusion was valid for cases where no protective corrosion products, scale, or inhibitor films were present. However, at higher temperatures, films with a certain degree of protectiveness were observed, In those cases, lower corrosion rates were obtained on the SP specimen because of more dense and protective films.