Adsorption and Corrosion Inhibitive Properties of Expired and Unexpired Benzhexol on Mild Steel in HCl Solution

Abstract

This study evaluated the adsorption and corrosion-inhibitive properties of expired and unexpired Benzhexol for mild steel in 1.0 M HCl using weight-loss measurements, potentiodynamic polarisation, scanning electron microscopy, and thermodynamic analysis. The weight-loss results showed that both forms of Benzhexol reduced mild-steel corrosion, with inhibition generally improving as inhibitor concentration increased. Potentiodynamic polarisation measurements indicated suppression of both anodic and cathodic reactions, identifying the inhibitors as mixed-type inhibitors. Adsorption data were assessed using Langmuir and Freundlich models, with the Langmuir isotherm providing the best overall fit. At 303 K, the calculated free energies of adsorption ranged from −9.60 to −31.92 kJ/mol. Activation-energy values for the inhibited systems ranged from 5.56 to 39.92 kJ/mol, compared with 8.62 kJ/mol for the blank. Most inhibited systems showed activation energies above the blank value; however, unexpired Benzhexol at 1000 mg/L gave a lower value of 5.56 kJ/mol. The negative free-energy values indicated spontaneous adsorption, while the overall thermodynamic and activation-energy trends were consistent with predominantly physical adsorption. Scanning electron microscopy further showed that the presence of expired and unexpired Benzhexol reduced acid-induced surface damage. Overall, the findings demonstrate that both expired and unexpired Benzhexol can inhibit mild-steel corrosion in hydrochloric acid under the investigated conditions, while the expired drug also provides a potential route for waste valorisation.