08332 Roles of Passivation and Galvanic Effects in Localized CO2 Corrosion of Mild Steel

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Product Number: 51300-08332-SG

ISBN: 08332 2008 CP

Author: Jiabin Han, Yang Yang, Srdjan Nesic, and Bruce N. Brown

Publication Date: 2008

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A galvanic mechanism of localized CO2 corrosion was explored to explain a “mesa” type of localized CO2 corrosion. Localized corrosion of mild steel in a deaerated environment dominated by CO2 can be caused by local defects in the corrosion product film/scale covered surface. After the film is locally damaged, the bare surface, usually small in relation to the film covered surrounding area, corrodes one or more orders of magnitude faster than the film protected area. It was found that the open circuit potential (OCP) of the two surfaces is different, a higher OCP at the film covered surface (cathode) and a lower OCP at the bared surface (anode). The OCP difference between the anode and cathode drives the localized corrosion. The causes for this OCP difference between these surfaces were investigated. Studies using potentiodynamic sweeps have shown the passivation of the carbon steel surface, which was developed at higher pH under the FeCO3 film. Cyclic polarization experiments reconfirmed the passivation phenomenon. The nature of the passivation was further explored by a depassivation experiment. The passive film dissolution or depassivation may help detach FeCO3 film from substrate steel bulk and cause it to be removed more easily. These results have been combined into a 2-D galvanic mechanism, which can be used to explain the “mesa” localized CO2 corrosion for mild steel.