Electrochemical Investigation of Localized CO₂ Corrosion on Mild Steel

Localized CO₂ corrosion on mild steel is always associated with the partial breakdown of a protective corrosion product scale such as iron carbonate. The scale breakdown can happen for a variety of reasons many of them related to fluid flow. It is hypothesized that following the scale damage, a galvanic effect is established between the scale covered surface (cathode) and the scale free surface (anode) leading to propagation of localized attack.

To test this hypothesis, in a series of laboratory experiments, an iron carbonate scale is formed by a repeatable process. Subsequently, in the so called “scale removal tests” the breakdown of the scale under flowing conditions is investigated. The results show that the iron carbonate scale can be partially removed by mechanical stresses, chemical dissolution or by both mechanisms acting simultaneously. In another series of experiments, a newly developed “artificial pit” test is used to investigate the propagation of localized CO₂ corrosion via a galvanic coupling. The artificial pit is composed of a large cathode covered by protective iron carbonate scale, and a small bare steel anode. The two are electrically isolated and connected by a zero resistance ammeter to measure the galvanic current during the tests. The results have confirmed the galvanic mechanism for localized CO₂ corrosion propagation. It has been demonstrated that pits will propagate only if the conditions are just right: the solution is neither undersaturated nor heavily supersaturated with respect to iron carbonate, i.e. they are in the so called “grey zone”.