Effect of Linoleate on Electrochemical Behavior of Stainless Steel in Phosphate Buffer

Electrochemical behavior of a high-purity, austenitic, low-carbon, stainless steel (SS) surface in the presence of a sodium salt of linoleic acid (C₁₇H₃₁COONa), sodium linoleate (LA), was studied in a phosphate buffer solution (pH 7.0) using cyclic voltammetry (CV), potentiodynamic linear polarization (LP), and electrochemical impedance spectroscopy (EIS) techniques. Impedance spectra were interpreted in terms of an equivalent electrical circuit (EEC) based on a possible physical model with circuit elements representing the electrochemical properties of the investigated system. Adsorption of LA onto the SS surface resulted in an almost saturated coverage and high inhibition efficiency toward corrosion after a “threshold” LA concentration in the bulk solution was reached. LA acted as a mixed-type inhibitor, and its inhibitory effect was explained on the basis of a self-assembled blocking mechanism where the negatively charged carboxylate groups of molecules were bound strongly to the SS surface. This formed a dense-ordered overlayer with the linear hydrocarbon hydrophobic tails oriented toward the solution, which provided a hydrophobic core that served as a barrier to diffusion of ions and electrons. These conclusions were supported by a very low capacitance value of ≈ 2 μF/cm² determined by EIS measurements. Adsorption of LA onto the SS surface was described with a Langmuir adsorption isotherm. The calculated free energy of adsorption suggested a very strong irreversible adsorption of LA molecules through chemisorption.