Inhibition of Acid Corrosion of Steel by Some S-Alkylisothiouronium Iodides

Five selected S-alkylisothisothiouronium iodides have been studied as acid corrosion inhibitors at 30°C for steel in 0.5 M H₂SO₄ using gasometry, mass loss, and direct current (DC) polarization techniques. All of the data reveal that the compounds act as inhibitors in the acid environments; furthermore, polarization curves show that the compounds act as mixed-type inhibitors. It was found that the inhibition efficiency increases with the increase of the length of the additive alkyl chain. Langmuir's adsorption isotherms fit the experimental data for the studied compounds. Thermodynamic parameters were obtained from experimental data of the temperature studies of the inhibition process at five temperatures ranging from 30 to 70°C. It was observed that the activation energy is slightly increased with the increase of the additive alkyl chain. On the other hand, the sudden large increase of the inhibition behavior of S-hexylisothiouronium iodide was attributed to a different adsorption process.