**4.1 Organic inhibitors**

Organic inhibitors act through forming a film on the surface of the metals and they can act as anodic, cathodic, or mixed inhibitors. The formation of this protective film happens with the help of strong interactions, such as π-orbital adsorption, chemisorption, and electrostatic adsorption that prevent the corrosive species from attacking the metal surface [33]. This adsorption is usually one molecular layer thick and does not penetrate into the bulk of the metal itself [34]. Physicochemical properties, such as functional groups, steric factors, aromaticity, π-orbital character of donating electrons, electron density at the donor atoms, and the electronic structure of the molecules govern the adsorption process [35, 36]. The corrosion inhibition efficiency of an organic inhibitor relies on its adsorption

**83**

*4.3.1 Plant extracts*

*Green Corrosion Inhibitors*

**4.2 Inorganic inhibitors**

chromates (CrO4

and nitrate (NO2

**4.3 Green corrosion inhibitors**

*DOI: http://dx.doi.org/10.5772/intechopen.81376*

pyridines, fatty amides, imidazolines, and 1,3-azoles [39].

<sup>2</sup><sup>−</sup>), molybdate (MoO3

through their incorporation into the oxide layer [39].

ability and mechanical, structural, and chemical characteristics of the adsorption layers formed under a specific environment [37]. An efficient organic inhibitor will usually contain polar functional groups with S, O, or N atoms in the molecule and a hydrophobic moiety that will repel the aqueous corrosive species away from the metal surface. However, the polar head is considered to be responsible for establishing the adsorption layer [38]. Some chemical families of organic inhibitors are

Inorganic inhibitors are those inhibitors in which the active substance is an inorganic compound. The addition of electropositive metal salts to a corrosive medium is one of the simplest ways to improve the passivity of a metal. However, the protective metal ion must have a redox potential more positive than the one to be protected and potentially more positive than that required for discharging protons so that the protective metal ion can be discharged on the surface of the metal in need of protection. Cathodic depolarization by overvoltage reduction and subsequent formation of an adherent deposit take place through the deposition of the protective metal on the surface of the metal susceptible to corrosion. Some of the metals that serve this purpose are palladium (Pd), platinum (Pt), iridium (Ir), rhodium (Rh), mercury (Hg), and rhenium (Re). Many inorganic anions, such as

<sup>−</sup>), silicates (SiO4

<sup>−</sup>) as well provide passivation protection to the metal surfaces

Environmental friendliness, cost, availability, and toxicity are some factors that should play an extremely important role when it comes to choosing an inhibitor for a particular condition [40]. The toxicity, biodegradability, and bioaccumulation of conventional corrosion inhibitors discharged into the environment are matter of huge concern. Even though the environmental implications of commercial corrosion inhibitors are not fully understood, it is not unknown that their chemical components have hazardous impact [41]. Inorganic inhibitors, for example, arsenates, phosphates, chromates, and dichromates not only have shown promising inhibition efficiency but also have been proved intolerant as well due to the threat they pose to our social health in the long run [42]. Likewise, the ecological and health risks associated with the organic inhibitors have pushed us towards finding or using nontoxic or green corrosion inhibitors that would impart maximum protection to the

Corrosion inhibitors are extensively used for the protection of metals and equipment and they are required to be acceptable, non-toxic, and eco-friendly due to environmental concerns. The cost and harmful effect associated with the commercial organic and inorganic inhibitors have raised considerable awareness in the field of corrosion mitigation. Thus, corrosion scientists and engineers are more inclined towards the implication of green corrosion inhibitors that are inexpensive, readily available, environmentally friendly and ecologically acceptable, and renewable.

Umoren et al. investigated the inhibition efficiency (IE) of gum arabic (GA) in absence and presence of halide ions on mild steel in 0.1 M H2SO4 at different

metallic structures but have least impact on mankind and nature [36].

Several classes of such inhibitors have been discussed briefly below.

<sup>4</sup><sup>−</sup>), phosphate (H2PO3

<sup>−</sup>),

ability and mechanical, structural, and chemical characteristics of the adsorption layers formed under a specific environment [37]. An efficient organic inhibitor will usually contain polar functional groups with S, O, or N atoms in the molecule and a hydrophobic moiety that will repel the aqueous corrosive species away from the metal surface. However, the polar head is considered to be responsible for establishing the adsorption layer [38]. Some chemical families of organic inhibitors are pyridines, fatty amides, imidazolines, and 1,3-azoles [39].
