Corrosion inhibition performance of tannins for mild steel in hydrochloric acid solution

Abstract

The corrosion inhibition behavior of commercial hydrolysable tannin (tara tannin), condensed tannin (black wattle tannin) and complex tannin (bayberry tannin) for mild steel in hydrochloric acid were investigated by means of gravimetric and electrochemical measurements. The inhibition efficiency increased with the increase of tannin concentration, and the inhibition performance of tannin followed hydrolysable tannin > complex tannin > condensed tannin. The pH of bulk solution has a significant impact on the inhibition behavior of tannin. Tannins exhibited high inhibition performance at low pH (0.3), but the corrosion rate slightly increased with a pH ≥ 2 for hydrolysable tannin and a pH > 3 for condensed tannin and complex tannin. Surface analysis revealed the formation of a protective compact inhibition film at pH 0.3, whereas a loose cracked film at pH 5.0 was revealed on the mild steel surface. The loose cracked film was deposition of ferric-tannates characterized by FTIR. Polarization experiments showed tannins act as mixed-type inhibitors with predominant anodic depression for hydrolysable tannin and cathodic depression for condensed tannin and complex tannin. EIS results indicated the increase of charge transfer resistance in the presence of tannins in 0.5 M HCl, supporting the formation of a protective film on a mild steel surface. Quantum chemical calculations predicted the higher inhibition performance of hydrolysable tannin based on the electronic properties of the molecular model. Molecular dynamic simulation revealed a nearly flat configuration for the hydrolysable tannin model molecule on a metal surface with more negative adsorption energies, as confirmed by the experiments.