Thiourea and N -Methylthiourea as Corrosion Inhibitors for Steel in Phosphoric Acid

The effect of thiourea (TU) and N-methylthiourea (MTU) on the corrosion of mild steel in 0.5 M H3PO4 was examined using potentiodynamic polarization curve measurements, electrochemical impedance spectroscopy techniques and quantum chemical calculations. Results obtained revealed that MTU is more effective than TU in 0.5 M H3PO4. Theoretical fitting of different adsorption isotherms such as Langmuir, Flory–Huggins, Temkin, and the kinetic-thermodynamic models was tested. The thermodynamic activation parameters were calculated. Quantum chemical parameters were calculated and explained. The data explained that the inhibition of mild steel by TU in 0.5 M H3PO4 take place through chemical adsorption mechanism, while physicochemical adsorption mechanism is suggested for MTU.