Secondary Plant Metabolites of Natural Product Origin—<i>Strongylodon macrobotrys</i> as Pitting Corrosion Inhibitors of Steel around Heavy Salt Deposits in Gabu, Nigeria

Abstract

Investigation into the Inhibition of pitting corrosion in mild steel around heavy salt deposits by some selected secondary plant metabolites—alkaloid extract (AESML), saponin extract (SESML and flavonoid extract (FESML) of natural product origin— Strongylodon macrobotrys was successfully completed with the aid of electrochemical impedance spectroscopy, potentiodynamic polarization, gravimetric and gasometric experimentation. The research proved that the selected secondary plant metabolites were excellent inhibitors of mild steel in the salt water environment as inhibition efficiency was recorded at 99.2%, 92.6% and 84.7% for AESML, SESML and FESML. The inhibitors showed higher inhibition at lower temperature due to frequent scale redeposition from agitation in temperature rise and loss in collision of the molecules. The potentiodynamic polarization result confirmed the reduction in the loss of electrons at the anode by the inhibitors that would have trigger oxidation reaction that causes the anode to corrode. Charge transfer resistance reflected the maximum inhibition efficiency obtained for mild steel at maximum concentration and the decrease in double layer capacitance is due to the decrease of the area where electrolyte is present due to the formation of inhibitor film. Thermodynamic investigation shows that the inhibitor has the potential of increasing the energy of the intermediate, reducing both the number of collisions, and number of particles that have enough energy to react and also number of corrosion reaction particles with the correct orientation. The adsorption isotherm consideration shows physical adsorption mechanism with binding constant increasing with increasing temperature.