article · ACS Omega
Cysteine functions as an environmentally friendly corrosion inhibitor for mild steel exposed to acidic environments, including hydrochloric, phosphoric, and sulphuric acid solutions. Testing demonstrates that cysteine molecules adsorb directly onto the mild steel surface to establish a protective layer or form insoluble compounds, effectively blocking active corrosion sites. The inhibition efficiency improves when increasing the concentration of cysteine and reducing temperature. Across testing in one molar acid solutions at a concentration of 0.01 molar cysteine, the inhibitor achieved efficiency levels of 97.3 per cent in hydrochloric acid, 89.7 per cent in phosphoric acid, and 84.4 per cent in sulphuric acid. Electrochemical measurements confirmed an increase in charge-transfer resistance and classified cysteine as a mixed-type inhibitor following a Langmuir adsorption model.
Corrosion of industrial metals in acidic settings creates significant operational challenges and financial costs globally. Identifying biodegradable, eco-friendly inhibitors like cysteine provides an alternative to hazardous chemical treatments. Demonstrating high protection rates across multiple acid types provides baseline evidence for developing safer industrial anti-corrosion treatments for steel components during acidic processing.
This research provides early-stage laboratory evidence that cysteine can serve as a green chemical additive to protect mild steel from acid-induced degradation. Potential users include industrial operators managing acid cleaning, pickling, or processing facilities. However, the work remains at an early experimental stage, consisting of bench-scale electrochemical and surface characterisations without industrial pilot testing or formulated product validation.
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The corrosion of iron in acidic environments has a negative impact on global industry. Herewith, the inhibitory effect of cysteine (Cys.) on mild steel (MSL) corrosion in different acidic solutions (1 M HCl, 1 M H<sub>2</sub>SO<sub>4</sub>, and 1 M H<sub>3</sub>PO<sub>4</sub>) was investigated through weight loss, potentiodynamic polarization (PDP), electrochemical impedance spectroscopy, scanning electron microscopy (SEM), and theoretical calculations. The measurement results indicated that the adsorption of Cys. molecules on the metal surface caused corrosion inhibition. As a result, a protective layer or insoluble compound, or both, is obtained, blocking the active sites, preventing corrosion. The effectiveness (IE %) of the Cys. was enhanced by increasing concentration and lowering temperature. The maximum IE % of inhibition at 1 × 10<sup>-2</sup> M of Cys. obtained are 97.3, 89.7, and 84.4% in HCl, H<sub>3</sub>PO<sub>4</sub>, and H<sub>2</sub>SO<sub>4</sub> solutions, respectively. At the same inhibitor concentration, the double-layer capacity decreased, and the charge-transfer resistance increased from 17.17 to 188.5, 3.564 to 31.91, and 1.325 to 8.715 Ω cm<sup>2</sup> in HCl, H<sub>3</sub>PO<sub>4</sub>, and H<sub>2</sub>SO<sub>4</sub> solutions, respectively. Adsorption and PDP studies confirmed that it obeys the Langmuir adsorption isotherm and acts as a mixed-type inhibitor of physicochemical nature. The corresponding thermodynamic and kinetic parameters were also calculated and discussed. Moreover, the inhibitory effect on the surface was inspected by SEM. The findings demonstrated that the order of IE % using Cys as anticorrosion agent for MSL is HCl > H<sub>3</sub>PO<sub>4</sub> > H<sub>2</sub>SO<sub>4</sub> solutions.
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DOI: 10.1021/acsomega.3c10522
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