**4. Mechanism of action of green corrosion inhibitors**

During corrosion, metal ions migrate into the solution in the active regions (anodic site) and transfer electrons from the metal to the acceptor at less active regions (the cathode); the cathodic process requires the presence of an electron acceptor functioning as oxygen, oxidizing agents or hydrogen ions. Green corrosion inhibitors have adsorbing properties and are known as site blocking elements [85]. They can minimize the corrosion rate through adsorption of active species onto the metal/alloy surface when added to many industrial systems by:


In fact, many researchers have postulated several theories to explain the mode of action of green corrosion inhibitors. For example, the active constituent derived from natural inhibitors varies from one plant species to another. The best sources of green inhibitors are natural products because they contain polar compounds with multiple "heteroatoms" similar to organic inhibitors. These heteroatoms present in the plant extracts act as an active center and adsorb on metallic surface by creating a film that denies access to corrosive agent. Non-polar compounds with aromatic rings, aliphatic chains, heterocyclic rings, and functional moieties are abundant in plant extracts. These compounds can be effectively adsorbed on the mineral surface and thus protect it from corrosion without harming the environment like inorganic compounds [86].

There are several methods to identify the inhibitory mechanism of green corrosion inhibitors. Electrochemical techniques such as electrochemical impedance

#### *Controlling Corrosion Using Non-Toxic Corrosion Inhibitors DOI: http://dx.doi.org/10.5772/intechopen.109816*

spectroscopy and potentiodynamic polarization analysis have been successfully implemented and provide valuable information on the corrosion rate and the mechanism of corrosion protection. These methods are briefly described in this section.

Potentiodynamic polarization is an electrochemical method used to determine green corrosion inhibitors performance, instantaneous corrosion rates and to elucidate the corrosion prevention mechanism. This method relies on changing the current or potential across a sample under study and recording the corresponding potential or current change. This can be facilitated using either a direct current source or an alternating current source. In most studies, a conventional three-electrode cell is used for the measurement, consisting of a counter electrode (Pt or graphite), a reference electrode (calomel or Ag/AgCl), and a working elecrode (metal substrate) immersed in the test solution [87]. The reference electrode measures and controls the system's voltage (V) and the counter electrode measures and controls current (*I*). The open circuit potential (*E*ocp) of a metal changes when electrochemical reactions occur. Once equilibrium is reached, a steady value is measured, and then the potentiodynamic polarization curve is performed by providing ranges of potential values. The plots are then used to calculate the corrosion potential (*E*corr) and the corrosion current density (*i*corr). Additionally, different concentrations of green inhibitors and experimental temperatures can be used to examine their different effects on corrosion prevention performance [88].

Electrochemical impedance spectroscopy is one of the best and powerful analytical tools for following in situ electrochemical progression with insight into the physical phenomena acting at the metal-electrolyte interface, providing valuable information on the surface properties and electrode kinetics via impedance diagrams. In this technique, an AC voltage (in the case of potentiostatic EIS) or current (in the case of galvanostatic EIS) is applied to the system under study to receive a response in the form of AC current (voltage) or voltage (current) as a function of the frequency. This technique can be implemented in a three-electrode cell, similar to Potentiodynamic polarization. Electrochemical impedance spectroscopy is usually used to determine resistance and current flow values, both when green corrosion inhibitor is present in the solution and when it is not. The reported result is usually a Nyquist diagram, with the real part of the impedance (Z′) on the X-axis and the imaginary part (Z″) on the Y-axis [89].

The two main adsorption mechanisms are physisorption and chemisorption. It has been recommended that physisorbed molecules attach to the surface at the cathodes and basically retard metal dissolution by the cathodic reaction, whiles chemisorbed molecules shield anodic areas and reduce the inherent reactivity at the sites where they are attached. It is acknowledged that the values of the standard free energy of adsorption ΔGo ads in aqueous solution are −20 kJ.mol−1 or lower establishes the physisorption process. While, those around −40 kJ.mol−1 or more negative include charge sharing or transfer of electrons from inhibitory molecules to the metal surface, point toward coordinate or covalent bond. It is important to signal that both mechanisms can take place together on the same metal surface. Isotherm equations were used to validate the adsorption mechanism, and to ascertain the closest equation that relates the dosage of inhibitors to the adsorbed concentration at saturation. Empirical equations functioning as hyperbolic, exponential, logarithmic, and power are complicated to relate to the given adsorption mechanisms. There are many mathematical equations called adsorption models that estimate the adsorbate amount in the absorbent at constant temperature. Most of the time, green inhibitors obey the Langmuir isotherm model, but some also adhere to the Freundlich and Frumkin isotherms [7].

However, the study of the precise mechanism of the adsorption process is complex because most of the constituents moderate the corrosion reactions in many ways, which makes it difficult to assign the credit for corrosion mitigation to a particular constituent. Moreover, the nature of the adsorption of an inhibitor onto a metal surface is largely governed by characteristics such as chemical and electronic properties of the inhibitor, temperature, type of electrolyte, steric effects, and the nature and charge of the surface of the metals [90]. The negative surface charge will enhance the adsorption of the cation while the adsorption of the anion with the positive surface charge is preferred.

Simulation and computational modeling backed by wet experimental results would help to better understand the mechanism of inhibitor action, their adsorption patterns, and the inhibitor metal surface interface and aid the development of designer inhibitors with an understanding of the time required for the release of self-healing inhibitors. This is achieved by density functional theory (DFT) which is based on quantum chemical calculations that have emerged as potential tools for studying metal-inhibitor interactions between inhibitors and metallic surfaces [91]. Monte Carlo simulation is also well known as a traditional and powerful method if computational complexity and time are not limiting [92].
