Under-deposit CO2 corrosion occurs when solids like sand, corrosion products, wax or a variety
of other particles deposit on the bottom of pipelines, forming a bed or layer of solids. Underdeposit
CO2 corrosion is different from “regular” CO2 corrosion as the aqueous solution beneath the solids is chemically and physically different from the “bulk” solution in the pipe. The
presence of solids can also limit inhibitor access to the metal surface which can initiate severe
localized corrosion.
Initial testing was directed towards defining and modeling the fundamental mild steel CO2
corrosion mechanisms occurring in the presence of clean inert solid deposits with different
particle sizes and shapes (SiO2 powder, glass beads (GB) and sand). Experiments were
conducted in a glass cell at a total pressure of 1 bar for temperatures of 25°C and 80°C. The mild
steel sample was immersed in 1 wt% NaCl electrolyte for one day. Corrosion behavior was
characterized by electrochemical techniques, including LPR (linear polarization resistance), EIS
(electrochemical impedance spectroscopy) and potentiodynamic sweeps. Steel surface
morphologies have been analyzed by using SEM (scanning electron microscope) and EDX
(energy dispersive x-ray).
Keywords: solids, under-deposit, CO2 corrosion, mild steel, pipelines