Electrochemical investigations of naphthenic acid corrosion for carbon steel and the inhibitive effect by some ethoxylated fatty acids

The electrochemical behavior and corrosion of carbon steel in various types of naphthenic acid solutions have been studied by using cyclic voltammetry technique. The data reveal that the anodic excursion spans of carbon steel in naphthenic acid solution are characterized by the occurrence of a well-defined anodic peak (peak A), followed by a passive region. The passivation may be related to the formation and precipitation of oxide film on the electrode surface. The data reveal that increasing naphthenic acid concentration enhances the anodic peak current density (I_PA) and shifts its peak potential towards more positive values. The corrosion rate of carbon steel depended critically on the naphthenic acid types. The rate of the corrosion process increased with increasing in molar mass of naphthenic acid, reaching a maximum value in the presence of CHPA acid (C₉), and then decreased.

Potentiodynamic polarization measurements indicated that ethoxylated fatty acid derivatives have a strong effect on the corrosion behavior of the carbon steel in naphthenic acid solutions. The inhibition efficiency of these compounds was found to vary with their nature and concentration. The order of effectiveness of the inhibitors was OL(EO)₈₀ > OL(EO)₄₀ > OL(EO)_20. The adsorption of ethoxylated fatty acid on the carbon steel surface in naphthenic acid solutions was found to obey Frumkin isotherm and kinetic-thermodynamic model.