Localized Corrosion of Mild Steel under Silica Deposits in Inhibited Aqueous CO₂ solutions

Corrosion experiments were conducted on API 5L X65 mild steel specimens covered by a silica deposit in inhibited 1 wt% NaCl aqueous solutions saturated by CO₂ at pH 5.0 and 25 °C. It was observed that a generic imidazoline based inhibitor was not able to reduce the general corrosion rate of the steel underneath the silica sand particles and local acceleration of corrosion (pitting) was found in those areas. A pitting penetration rate was calculated using Infinite Focus Microscopy (IFM) measurements. After examining many possible mechanisms, it was concluded that the imidazoline inhibitor was not depleted globally by adsorption on the sand surface and that it did diffuse through the sand deposit to reach the steel surface. However, locally - immediately underneath each sand grain, the inhibitor preferentially adsorbed on the silica surface, leaving the steel beneath it unprotected. This led to formation of galvanic cells and to accelerated localized corrosion at those locations.