Investigation Of Indole And Its Derivates As Inhibitors For Mild Steel Corrosion In Sulfuric Acid Medium

The employment of inhibitor is one of the most practical and economic method toprevent the metal corrosion among many different approaches. The best well known arebenzotriazole type inhibitors, which have been proved to be very effective inhibitors formetal corrosion in different aggressive electrolyte. In present research, indole and fourindole’s derivates have been selected as the corrosion inhibitors for mild steel corrosionin0.1M H2SO4solution. These five compounds are indole, indole-3-carboxylicacid(ICA), indole-3-acetic acid(IAA),3-indolepropionic acid(IPA), and3-indolebutyricacid(IBA), respectively. In order to evaluate these inhibitors’ inhibiting effect andmechanism, weight loss measurement, electrochemical impedance spectroscopy (EIS),potentiodynamic polarization techniques have been performed, and the corrosionparameters are calculated according to the raw data. In addition, the scanning electronmicroscope (SEM) technology is used to provide the information about the surfacemorphologies of steel surface after immersion in uninhibited and inhibited acidsolutions, showing inhibitors’ performance on corrosion protection directly.These five inhibitors exhibit good inhibition property in weight loss measurementsin0.1M H2SO4solution. The inhibition effect is enhanced with the increasing inhibitorconcentration, this is probably attributed to the formation of a protective film on themetal surface, and this protective film tends to be more complete and compact atrelative higher concentration. At a given inhibitor’s concentration, corrosion rates areincreased along with the increase of temperature, while the effect of temperatures on theinhibition efficiency is depended on the nature of different organic compounds.EIS experiment shows one capacitive arc which is a semicircle in Nyquist plots foreach inhibitor. The diameter of the semicircle represents the value of charge-transferresistance and is enlarged with the increase of inhibitor concentration, leading to theincrease in the inhibition efficiency. Meantime, the double layer capacitance valuesshow a decreasing trend, suggesting that the water molecules on metal surface aregradually replaced by the inhibitor molecules during the interaction process, which havelarger size and lower dielectric constant. As a consequence, the inhibitor molecules forman adsorptive film onto metal surface, suppressing the steel dissolution by limiting thecontact between Fe and H+.Potentiodynamic curves indicate that both the anodic and cathodic reactions can be retarded in the inhibited acid solution, but the presence of inhibitor does not modify thecorrosion reaction mechanisms. Inhibitor molecules act simply by blocking thecorrosion active sites on the steel surface and inhibiting the corrosion. What’s more, theshift value of the corrosion potential shows that all these five inhibitors are mixed typeinhibitors.SEM micrographs show that the metal surface is well protected and less damagedin the inhibited acid solution.The adsorption isotherm shows that five inhibitors obey Langmuir adsorptionmodel, and the adsorption free energy Gadsimplies that the adsorption of inhibitormolecules on the steel surface takes place spontaneously, and is the outcome ofcombined physisorption and chemisorption.