Study of Caffeine as an Anticorrosion for API 5LX52 Steel in Geothermal Environment Simulation

Abstract

The research objective was to study the thermodynamics and kinetics of caffeine as anticorrosion in geothermal environment simulations. The weight loss method following the ASTM G31, and the Electrochemical Impedance Spectroscopy (EIS) method following the ASTM G59-91. The data obtained from this study are the value of polarization resistance (Rp), solution-electrode interface (Cdl), the electrolyte resistance (Rs), corrosion current density (icor), corrosion rate (CR), and inhibition efficiency. The results obtained are caffeine has a higher inhibition efficiency at lower temperatures. The caffeine inhibitor efficiency of 90% was achieved at a caffeine concentration of 20 ppm in a 3.5% of NaCl + 500 ppm of H2S solution at a temperature of 70C, pH 4, and stirring speed of 250 rpm. The ∆Gads, ∆So, and Qads prices for caffeine are negative, while the Ea and ∆Ho prices for caffeine are positive. This shows that the adsorption strength of the caffeine molecule with iron (Fe) is good in the test environment. The nitrogen (N) atom in caffeine is considered to be alkaline as in ammonia (NH3) which can accept a proton to give NH4+. The minimum Cdl value that occurs at a caffeine concentration of 20 ppm, indicating a double layer at the steel interface with the solution, is considered an electric capacitor. This happens because there are four N atoms in the caffeine molecule. This research is useful for manufacturing green inhibitors in hydrogen sulfide environments such as geothermal environments.