A Study on the Effect of Surface Finish on Corrosion of Carbon Steel in CO2 Environment

Standard surface finish used in laboratory corrosion testing may not represent the 'as-delivered‟ internal pipe surface condition, which typically varies from 20 μm to 50 μm. Surface roughness is known to affect the hydrodynamic and mass-transfer boundary layer, thus influencing the corrosion mechanism and rate. The objective of this study is to investigate the effect of various grit surface finishes of 60, 240, 400, 600, 800 and 1200 used in laboratory corrosion testing in predicting field corrosion behaviour.
The effect of the surface roughness on the corrosion rate is investigated by varying the specimen surface finishing using SiC abrasive paper of 60, 240, 400, 600, 800 and 1200 grit. The specimen used is carbon steel BS 970 (070M20), tested in stagnant and turbulent CO2 environment at pH 5.5 and 3 wt% NaCl. The corrosion rate is measured by the Linear Polarization Resistance (LPR) Long Term and Custom Sweep, Weight Loss and Electrochemical Impedance Spectroscopy (EIS) method.
Surface profiles produced by 400, 600 and 800 grit samples fall within the range of 20 μm to 50 μm, which resemble the 'as-delivered‟ internal pipe surface roughness. Corrosion rates in turbulent flow are higher than static condition for all surface finish. For both static condition and turbulent flow, the average corrosion rates of 400, 600 and 800 grit finish samples obtained differ within 18% and 15%, respectively.
In conclusion, based on surface profiles and corrosion rate measured, the practice of 400, 600 and 800 grit surface finish in laboratory testing is acceptable to represent the 'as-delivered‟ pipe condition.