Internal CO2 Corrosion of Mild Steel Pipelines under Inert Solid Deposits

Underdeposit 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 corrosion results in an area beneath the solids where it is chemically or physically different than the bulk solution in the pipe.  This difference can lead to the generation of a galvanic corrosion cell.  The presence of solids can also limit inhibitor coverage of the metal surface which can either affect corrosion inhibitor performance or affect corrosion behavior beneath the deposit thus initiating severe localized corrosion.

CO₂ corrosion behavior of carbon steel in the presence of clean inert solid deposit with different particle size and shape (SiO₂ powder, glass beads and sand) was initially investigated. Tests were conducted in a glass cell at CO₂ pressure 1 bar and temperature 25⁰C and 80⁰C.  Both short term (24 hours) and relative long term (7 days) corrosion behavior of mild steel was characterized by electrochemical methods, including LPR, EIS and potentiodynamic sweeps. Steel surface morphologies have been analyzed by SEM and EDX. The process and mechanisms of underdeposit corrosion mild steel are discussed.

Key words: solids, underdeposit, CO₂ corrosion, mild steel, pipelines