Effect of Cementite on the Corrosion Inhibition of Carbon Steel with Tetradecyl Phosphate Ester in CO₂ Environment

Despite extensive use of corrosion inhibitors (CIs) to mitigate internal pipeline corrosion, few studies address how corrosion residue or products affect their performance. In the present work, in-house synthesized CI model compound, tetradecyl phosphate ester (PE-C14), was used to investigate the effect of cementite (Fe₃C) on corrosion inhibition performance on C1018 steel with ferritic-pearlitic microstructure, on which residual Fe₃C skeleton tended to stay on the surface during pre-corrosion. A 40 μm Fe₃C skeleton was exposed and maintained on specimen surface after 2-day pre-corrosion under studied condition. The corrosion rata (CR) evolution results indicated that the residual Fe₃C skeleton decreased significantly the inhibition efficiency (IE) of PE-C14, with 300 ppm_w being insufficient to obtain an acceptable IE. Some caveats related to the testing protocols are discussed. Under condition of maintained Fe₃C, from the polarization curves, corrosion rate and limiting current of H⁺ reduction reaction were significantly accelerated due to the Fe₃C layer supporting excessive cathodic reaction. With a ca. 40 μm Fe₃C layer, the retardation on anodic reaction with 300ppm_w PE-C14 is very limited compared with that on bare surface. After adding PE-C14, a non-uniform corrosion morphology was observed on specimens with 2-day pre-corrosion, which could be postulated to promote localized attack. Two main limits to the above results are identified: the experiments were conducted in close medium where the inhibitor concentration of the bulk could easily be depleted and the obtained data can only be applied in case a 40 μm thick layer of cementite is formed. These two factors have a significant effect on the outcome and further tests and work are ongoing to address these limitations.

Internal corrosion of pipelines associated with oil and gas production and refinery has always been a challenge for corrosion engineers.¹ Over the past decades, corrosion engineers have made significant progress in developing mitigation approaches to protect these carbon steel pipelines by using corrosion inhibitors (CIs), corrosion resistant materials, and various cleaning techniques. Among all these mitigation strategies, corrosion inhibitors are considered as the first choice in handling the internal corrosion of pipelines.