**2. Organic corrosion inhibitors**

Organic corrosion inhibitors are preferred due to its environmental friendly and effectiveness at wide range of temperatures [53–59]. The efficiency of an organic inhibitor depends on the size of the organic molecule, aromaticity, type, and number of bonding atoms or groups in the molecule (either π or σ), nature and surface charge, the distribution of charge in the molecule and type of aggressive media. The presence of polar functional groups with S, O, or N atoms in the molecule, heterocyclic compounds and pi electrons present in the molecule also increases the efficiency of these organic corrosion inhibitors. The adsorption of the molecule on the metal surface depends on the polar function of the molecule. The organic compound that contains oxygen, nitrogen and/or sulfur blocked the active corrosion sites by adsorbing on the metallic surface.

The organic compounds act as cathodic, anodic, and mixed inhibitors. The organic molecules exhibiting a strong affinity for metal surfaces show good inhibition efficiency. The organic compounds adsorb on the surface of the metal to form a protective film which displace water from the metal surface and protect it against corrosion. The organic inhibitors adsorb onto the metallic surface depend on the chemical structure of the inhibitors, type of environment, and surface charge of the metal. The inhibitor adsorption on metal surfaces occurs through donor-acceptor interactions (physical, chemical, or mixed type) [60–63]. The organic inhibitors are adsorbed on metal surface either by physisorption or chemisorptions mechanism. The physisorption occurs through an electrostatic interaction between the metal surface and inhibitor's charged molecule [64–66]. The chemisorptions mechanism occurs through charge transfer share between the adsorbed inhibitor molecules and the metal surface.

The mechanism of organic corrosion inhibition takes place in two steps. The first step involves the transfer of the corrosion inhibitors over the metallic surface. In the second step interactions between metal surface and adsorbed inhibitor molecules occur [67–69]. In physisorption, the adsorbed inhibitor molecules are not in direct contact with the metallic surface. A layer of solvents which are already adsorbing on the surface molecules separated the inhibitor molecules [70, 71].

The mechanism of organic corrosion inhibition in aqueous medium can be occurs as follows [72].

$$\text{M} + \text{nH}\_2\text{O} \leftrightarrow \text{M}(\text{H}\_2\text{O})\_{\text{nads}}\tag{1}$$

$$\text{M(H}\_2\text{O)}\_{\text{n }\text{ads}} + 2\text{X}^- \leftrightarrow \text{M} \left[ (\text{H}\_2\text{O})\_\text{n} (\text{X})\_2^{-} \right]\_{\text{ads}} \tag{2}$$

$$\mathbf{M} \Big[ (\mathbf{H}\_2 \mathbf{O})\_\mathbf{n} (\mathbf{X})\_2^{-} \Big]\_{\text{ads}} \leftrightarrow \mathbf{M} \Big[ (\mathbf{H}\_2 \mathbf{O})\_\mathbf{n} (\mathbf{X})\_2 \Big]\_{\text{ads}} + 2\mathbf{e}^- \tag{3}$$

$$\mathbf{M} \left[ (\mathbf{H}\_2 \mathbf{O})\_\mathbf{n} (\mathbf{X})\_2 \right]\_{\text{ads}} \leftrightarrow \mathbf{M}^{2+} + \mathbf{O} \mathbf{H}^- + 2\mathbf{X}^- + \mathbf{H}^+ \tag{4}$$

In the presence of organic corrosion inhibitor (INH), the following mechanism operates [73, 74].

$$\text{M} + \text{nH}\_2\text{O} \leftrightarrow \text{M}(\text{H}\_2\text{O})\_{\text{nads}}\tag{5}$$

$$\mathbf{M} (\mathbf{H}\_2 \mathbf{O})\_\mathbf{n} \text{ }\_\text{ads} + 2\mathbf{X}^- \leftrightarrow \mathbf{M} \left[ (\mathbf{H}\_2 \mathbf{O})\_\mathbf{n} (\mathbf{X})\_2^{-} \right]\_\text{ads} \tag{6}$$

$$\mathbf{M} \Big[ \left( \mathbf{H}\_2 \mathbf{O} \right)\_\mathbf{n} \big( \mathbf{X} \big)\_2^{-} \big]\_{\text{ads}} + \text{INH} - \mathbf{H}^+ \leftrightarrow \mathbf{M} \Big[ \left( \mathbf{H}\_2 \big)\_\mathbf{n} \big( \mathbf{X} \big)\_2 \text{INH} - \mathbf{H} \Big]^-\_{\text{ads}} \tag{7}$$

The effect of organic compounds as corrosion inhibitors and adsorption on metal surfaces depends on various factors. Temperature is one of the factors. The increase in temperature results in a decrease in the inhibition of corrosion due to adsorption on metallic surface decreases [75, 76]. Then inhibitor concentration is another parameter for the effectiveness of the organic inhibitor molecules. With increasing inhibitor concentration, inhibition efficiency of compounds increases. However, after certain concentration, there is no further enhancement of inhibition. The effectiveness of the organic inhibitor also depends on the electronic structure and molecular size of the molecule [77, 78]. The organic compound with high molecular size generally shows a better corrosion inhibitor than compound with lower molecular size. But, too big molecular size compound show a decrease in adsorption on metallic surfaces due to a decrease in solubility. The molecular electronic structure is one of the aspects of corrosion inhibitors. The planar geometry organic compound acts as better corrosion inhibitor than with vertical geometry of the compounds. The aromatic organic compounds having conjugation in the form behave as effective corrosion inhibitors The aromatic compound containing electron-donating substituent such as –OCH3, –CH3, –OH, –NH2, –NHR, and –NR2 increases the organic corrosion inhibitor molecules, whereas, electron-withdrawing substituent such as –CN, –COOH, –COOC2H5, and –NO2 decreases inhibitor molecules [79]. The effect of aromatic substituted substituent on corrosion inhibition can be determined by Hammett substituent constant (σ) values. The corrosion inhibition efficiency of substituted aliphatic linear and cyclic compounds can be determined by Taft constant (σ\*) values [80]. The Hammett equation is shown below

$$\log \, \, \, \frac{\mathbf{K}\_{\text{R}}}{\mathbf{K}\_{\text{H}}} = \frac{\mathbf{K}\_{\text{R}}}{\mathbf{K}\_{\text{H}}} = \rho \sigma \,\, \, \, \tag{8}$$

whereas KH and KR are equilibrium constants for non-substituted and substituted compounds. σ is the Hammett constant, ρ is its magnitude depending on the nature of metal-inhibitor interactions.
