Protection of Steel Against Corrosion in 4% HCl Solution Using Wild Oregano Extract

In this study, the potential use of wild oregano extract as a green corrosion inhibitor for two types of steel in 4% HCl solution was investigated. Four solutions of 4% HCl were prepared: one without inhibitor (solution 1) and three with the addition of wild oregano extract at different concentrations (0.5, 1.0, and 1.5 g/L). Steel samples 1 and 2, with dimensions of 30 mm × 30 mm × 2 mm, were immersed in the prepared solutions for 2, 4, 6, 8, 24, 72, and 168 hours. Based on the weight loss of the steel samples before and after immersion, the negative mass corrosion index (Km⁻), corrosion rate (π), and inhibition efficiency (z) were calculated. The obtained results indicate that the wild oregano extract exhibits corrosion inhibition properties and effectively slows down the corrosion of both steel types. For steel 1, the protection efficiency ranged from 83.78% to 97.56%, while for steel 2 it reached up to 79.68%. The optimal concentration of wild oregano extract in 4% HCl was determined to be 1 g/L, as at this concentration the protection efficiency for steel 1 ranged from 79.89% to 97.56%, and for steel 2 from 40.25% to 79.49% across all immersion times. The average corrosion rate for steel 2 in pure 4% HCl was 1.0836 mm/year, while for steel 1 it was 4.1359 mm/year. This indicates that steel 2 is more resistant and exhibits faster self-passivation than steel 1, which explains the somewhat lower, though still significant, inhibitory effect of wild oregano extract. The obtained results are in good agreement with those derived from electrochemical measurements of corrosion rate, including electrochemical impedance spectroscopy (EIS) and Tafel polarization plots. These findings suggest that wild oregano extract can serve as an effective and environmentally friendly corrosion inhibitor in acidic environments such as HCl solutions.