Natural oils as corrosion inhibitors for stainless steel in sodium hydroxide solutions

We used electrochemical techniques (galvanostatic and potentiodynamic anodic polarization methods and also electrochemical impedance spectroscopy) to study corrosion inhibition by some natural oils (parsley, lettuce, sesame, arugula, and sweet almond oils) on the corrosion of type 304 stainless steel in 0.1 M NaOH solution. We show that the inhibition efficiency increases as the concentration of these oils increases. The inhibiting effect was attributed to adsorption of the major components of these oils on the stainless steel surface. The adsorption process is described by a Langmuir isotherm. It was found that incorporation of chloride ions in a 0.1 M NaOH solution accelerates pitting corrosion of the stainless steel as a result of shifting the pitting potential toward more negative values. Addition of the natural oils under study to sodium hydroxide solutions containing chloride ions shifts the pitting potential toward more positive values, indicating increased resistance of the steel to pitting corrosion. As the concentration of the oils under study in the solution increases, the charge transfer resistance increases while the capacitance of the double layer decreases.