Corrosion Inhibition of Carbon Steel in Cooling System Media by Non-Toxic Linear Sodium Octanoate Available to Purchase

Sodium octanoate CH₃-(CH₂)₆-COONa was found to be one of the straight chain aliphatic carboxylates that show good inhibition characteristics towards mild steel. However, as the previously tested concentrations were too high to be applied in the water treatment of secondary circuits, lower concentrations have been investigated in this research. The tested concentrations ranged from 10 to as high as 700 ppm. Potentiodynamic polarization and weight loss tests have been applied at room temperature to study the metal corrosion behaviour in the presence and absence of the tested corrosion inhibitor. The weight loss test simulated the dynamic condition of cooling water as water available at site ‘city water’ was allowed to circulate via pumps through a corrosion test rack in which carbon steel specimens were mounted. Results obtained from both tests were in good agreement. Results revealed that the concentration should not fall below 200 ppm for the tested inhibitor to be effective. The adsorption of inhibitor on the carbon steel surface was found to obey the Langmuir adsorption isotherm model. Thermodynamic parameters of adsorption of the carboxylate have been determined and discussed. The protective film formed due to the presence of the inhibitor at high concentration has been analyzed by Fourier transform infrared (FTIR) spectra. FTIR spectra revealed that octanoate was adsorbed on the steel surface via its functional group.