Abstract
In the oil and gas industry, long-distance transmission of produced hydrocarbons is usually carried out in large-diameter steel pipelines. However, water co-produced with the oil and gas, combined with CO2 or H2S, can cause severe corrosion of internal pipeline surfaces. A widely applied, economically effective method of corrosion control is to inject corrosion inhibitors (CIs). Adsorption of the organic molecules on surfaces through heteroatom functionalities, containing nitrogen, oxygen, sulfur and/or phosphorus, can markedly change the corrosion resistance characteristics of the exposed metal, typically mild steel for pipelines. Among these organic compounds, heterocyclic molecules containing nitrogen atoms have been demonstrated to be excellent corrosion inhibitors (CIs) for many alloys in various aggressive media. In this research, surface saturation concentrations, as well as inhibition efficiencies of tetradecyltetrahydropyrimidinium (THP-C14), were determined at 25, 55 and 80°C by monitoring linear polarization resistance (LPR) until the corrosion of the steel specimens stabilized. Potentiodynamic sweeps were obtained at the end of each test. The five most frequently used adsorption isotherms, which correlate coverage and inhibitor concentration using different assumptions, were reviewed and adapted to fit the corrosion inhibition behavior. These isotherms were evaluated based on their goodness of fit and the relevance of their underlying assumptions. In addition, the equilibrium constants of adsorption/desorption (KAD) at 25, 55 and 80°C were determined by fitting the steady state experimental data. Finally, the changes of activation energy with and without inhibitor at 25, 55 and 80°C were calculated and compared.
