EIS evaluation of the effect of neutralizing and inhibitor compounds on corrosion process of 1018 carbon steel in acid solutions typical of atmospheric distillation plants

The effect of monoethanol amine (MEA) as neutralizing and a corrosion inhibitor (IC) on carbon steel corrosion process in typical environments of petroleum refining distillation plants was evaluated using electrochemical impedance spectroscopy (EIS). The EIS analysis showed that the carbon steel corrosion process in HCl solution is favored by the presence of H₂S reaching maximum activity at 500 ppm dosage. The EIS analysis using a Randles circuit allows to identify that under the experimental conditions considered here (pH = 2) all MEA dosages result in a contrary effect accelerating the corrosion process of the carbon steel in sour medium (0.05 M HCl + 500 ppm of H₂S). However, an important inhibition effect was observed when IC compound was added to the same sour solution. This fact could be associated to the formation of a passive film composed by corrosion products and IC molecules on steel surface. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) confirmed the EIS results obtained in presence and in absence of MEA and IC compounds.