Effect of Acridine and Myristic Acid on CO₂ Corrosion During Intermittent Oil/Water Wetting

The effect of a paraffinic model oil containing select surface-active compounds (myristic acid and acridine) on CO₂ corrosion with and without intermittent wetting has been studied. Observations have shown that the presence of myristic acid in the oil phase has no effect on corrosion behavior due to its lack of partitioning in the water phase. However, after direct contact between the oil phase containing myristic acid and the metal surface, there was a significant decrease in the corrosion rate. This phenomenon gradually diminished at pH 4.0 but was more persistent at pH 6.5. The presence of acridine in the oil phase was shown to have a strong inhibitive effect at pH 4.0, even during the partitioning step. However, there was no inhibitive effect conferred by the presence of acridine on the corrosion rate at pH 6.5. An experimental methodology was developed that facilitated improved simulation of the effect of intermittent oil/water wetting on CO₂ corrosion. The electrochemical current response during the oil/water intermittent wetting cycles showed that persistency of model oil (without surface active compounds) on the mild steel surface is only a matter of seconds. Corrosion rate measurements showed that the presence of myristic acid renders the oil layer more persistent after intermittent wetting as compared to one-time direct contact.

Pipelines are the most effective way to transport oil and natural gas, particularly for their bulk transmission over long distances^1,2. Corrosion in oil and gas pipelines occurs because of the presence of dissolved corrosive gases, such as CO₂ and/or H₂S, in reservoir derived brine, and contact between this brine and the steel surface. The oil phase by itself does not cause corrosion and can even inhibit corrosion ^3,4,5,6. In fact, many surface-active compounds naturally exist in crude oils⁷. Crude oils can be defined as naturally occurring liquid mixture of hydrocarbons (83 wt.%–87 wt.% of carbon, 10 wt.%–14 wt.% of hydrogen) containing derivatives of nitrogen (0.1 wt.%–2.0 wt.%), oxygen (0.05 wt.%–1.5 wt.%), sulfur (0.05 wt.%–6.0 wt.%), metals (less than 1000 ppm), and other elements ^8,9. Some chemical compounds native to crude oils identified as surface-active compounds have an ability to preferentially adsorb at steel-water, steel-oil, and oil-water interfaces, thereby altering the wetting and corrosion properties of steel surfaces¹⁰. Therefore, if the water phase in a pipeline was entirely entrained by the hydrocarbon phase, instead of flowing at the bottom, no corrosion problems are expected to occur. Therefore, it is important to determine whether the pipe internal surface is wetted by oil or water depending on the fluid properties and the operating flow rates¹⁰. This knowledge can help decrease economic costs and mitigate the potential for adverse environmental impacts caused by leakage from corroding tubular steels.