Understanding the influence of solution's pH on the corrosion of tin in saline solution containing functional amino acids using electrochemical techniques and molecular modeling

In this work, the effect of pH on the corrosion of fin in stagnant 2 wt.% NaCl solution without and with three functional side chain amino acids, namely Threonine (Thr), Asparagine (Asn) and Glutamine (Gin), was investigated. This study was carried out using potentiodynamic polarization and electrochemical impedance spectroscopy. The influence of solution's pH on protonation state of tested compounds and chemical nature of the metal surface (Sn or SnO2) was considered during the theoretical studies using DFT/B3LYP method and Monte Carlo/SA simulations. The obtained results revealed that the pH variation had affected significantly the speed and the corrosion mechanism of fin, as well as, the inhibitory performance of tested biomolecules. Higher inhibition efficiencies were obtained at pH 5 as compared to pH 2. The maximum value was obtained for Gin and Asn at pH 2 and at pH 5, respectively. Thr and Asn were shown a dually effect against the corrosion of fin, which can act as corrosion accelerators at pH 2 and as corrosion inhibitors at pH 5. The relationship between calculated molecular electronic structure parameters and observed inhibition efficiency was discussed. The capacity of tested compounds to form hydrogen bonds with metallic surface has explained the accurate inhibition effectiveness recorded at pH 5 against pH 2. A mechanism of the effect of tested amino acids against fin corrosion was proposed.