Effect of FeCO_3 Supersaturation and Carbide Exposure on the CO_2 Corrosion Rate of Carbon Steel

摘要

The pH stabilization technique is a widely used corrosion protection method for multiphase gas pipelines with glycol as hydrate inhibitor. It implies to increase the pH by addition of HCO3 in order to enhance the formation of protective iron carbonate films. The protection mechanism at ~20°C is of concern because the conditions for precipitating protective corrosion film are less favorable compared to higher temperatures due to the increasing solubility of FeCO_3 with decreasing temperature. The scope of the ongoing work is to investigate whether corrosion mitigation of pipelines at -20°C relies on the formation of protective corrosion films or if the corrosion rate is sufficiently lowered by the elevated pH. This paper discusses the corrosion rate and corrosion potential observed on carbon steel exposed to varying concentrations of HCO3 and Fe~(2+) at 20°C in a 1wt% NaCI and 50wt% glycol solution purged with CO_2 at 1 atm partial pressure. The objective was to promote protective FeCO_3 films by high iron and bicarbonate concentrations and study the effect of supersaturation and variations in iron and bicarbonate concentration. Protective films did not form despite high supersaturation and long exposure times. The reason for this is discussed in light of exposed iron carbide (Fe_3C) and prerequisites for iron carbonate growth.