L-histidine was analyzed in this work as a corrosion inhibitor for AISI 1018 carbon steel in chloride solution, without (pH 7) and with carbon dioxide saturation (pH 4). Mass loss methods and electrochemical tests were used. In a medium with pH 4 saturated with CO₂, the inhibition efficiency values (IE%) did not reach 34%. For the pH 7 condition, an inhibition efficiency of up to 89% was observed. The values corroborated with the quantum chemistry and Density Functional Theory (DFT) calculations, showing that the histidine molecule is responsible for the excellent inhibition potential. A Fourier transform infrared spectroscopy (FTIR) study and a thermogravimetric study (TGA) of the amino acid were also carried out. The substrate surface was analyzed by scanning electron microscopy and atomic force microscopy.

The amino acid L-histidine, which has an imidazole ring, was investigated as a corrosion inhibitor for AISI 1018 carbon steel in chloride solution based on the effectiveness of inhibitors containing imidazole in their composition. A neutral environment was chosen for this study due to the scarcity of research on this amino acid in this environment type. Concentrations of 250, 500, and 1000 ppm were evaluated. Various methods were used to determine inhibition effectiveness, including mass loss, open circuit potential, linear potentiodynamic polarization, and electrochemical impedance spectroscopy. For mass loss, the inhibition efficiency varied from 83 to 88% according to the increase in concentration. For the electrochemical tests, the efficiency variation ranged from 62 to 90% with increasing amino acid concentration. Furthermore, a simulation analysis using quantum chemical calculations within the scope of Density Functional Theory (DFT) revealed that histidine’s nucleophilic character is crucial for its corrosion inhibitory capacity in an aqueous medium at pH 7. The inhibition efficiency increased with increasing concentration in a neutral medium, following the Langmuir isotherm for the adsorption of L-histidine. Additional studies were carried out using Fourier transform infrared spectroscopy (FTIR) and thermogravimetry (TGA). Analysis of the substrate surface by scanning electron microscopy (SEM) showed greater preservation with the addition of L-histidine, confirming its adsorption on the steel. Atomic Force Microscopy (AFM) also demonstrated an improvement in surface roughness in the presence of amino acids compared to the medium without an inhibitor.

Copper and its alloys have characteristics of electrical and thermal conductivity and good toughness. As copper has a positive value for standard reduction potential, it does not react with water, but with oxidizing acids. A useful method to protect metals and alloys is the addition of corrosion inhibiting substances in the solution, which has a high inhibition efficiency.However, many inhibitors are undesirable because of their toxicity to the environment and their high cost. Studies have been developed in order to investigate plant extracts as corrosion inhibitors. The sapindus saponaria L plant may have some technological interest in relation to corrosive processes.For this purpose, the techniques of potentiodynamic polarization of cyclic and linear scanning, chronoamperiometry, gravimetric by mass loss immersion, flame atomic absorption spectrometry and scanning electron microscopy were used. In cyclic voltammetry with the cathodic region extract, there is some interaction with the species in solution. From the polarization curves, linear voltammetry, the efficiency was 63.6%, where the extract behaved as a mixed type inhibitor. Determining the values of the corrosion rate in the absence and in the presence of the extract, the value of 59.8% of inhibitory efficiency in terms of mass loss immersion and 80.0% in terms of copper ions content in solution from flame atomic absorption were found.The micrographic images show that in the presence of the extract, on the copper surface there were no significant changes and the formation of corrosion products did not occur. It is concluded that the extract of sapindus saponaria L is a reasonable inhibitor of copper corrosion in HNO3 1 mol/L.