*Agricultural Machinery Corrosion DOI: http://dx.doi.org/10.5772/intechopen.108918*


#### **Table 4.**

*Effect Ni-composite and nanocomposite coating on improving the steel surface.*

movement or diffusion of ions to the metallic surface, and increasing the electrical resistance of the metallic surface [69]**.**

Monticelli mentioned that the standardized definition of a corrosion inhibitor is a "chemical substance that, when presented in the corrosion system at a suitable concentration, decreases the corrosion rate, without significantly changing the concentration of any corrosive agent" [70]. Abbout [71] proposed several characteristics that need to be possessed by an effective corrosion inhibitor, namely, stability at a certain temperature, low cost, and, more importantly, adherence to environmental laws and standards [71].

The corrosion inhibitors can be chemicals either synthetic or natural and could be classified by the chemical nature as organic or inorganic, the mechanism of action as anodic, cathodic, or an anodic-cathodic mix, and by adsorption action, or as oxidants or not oxidants [70].

The cathodic corrosion inhibitors prevent the occurrence of the cathodic reaction of the metal. These inhibitors have metal ions able to produce a cathodic reaction due to alkalinity, thus producing insoluble compounds that precipitate selectively on cathodic sites. Deposit over the metal a compact and adherent film, which restricting the diffusion of corrosion action in these areas. Thus, increasing the impedance of the surface and the diffusion restriction of the reducible species, that is, the oxygen diffusion and electrons conductive in these areas. These inhibitors cause high cathodic inhibition [72].

The label "green," or sometimes "eco-friendly," is commonly added to the noun inhibitor if the molecule, in addition to meeting the previous definition of inhibitor, also has biocompatibility in nature. Part of this class of inhibitors is complex matrices like plant extracts due to their biological origin and, by extension, their single components, such as flavonoids, alkaloids, polyphenols, and glycosides [73]**.**

The relative simplicity of many extraction procedures, combined with the plenty of vegetable sources available even at very low prices, made the exploitation of plant extracts a new "trend" in corrosion science. This "green turn" must surely receive praise, but it is to be worth noting also the other side of the medal. A critical revision of the open literature concerning green corrosion inhibitors brings up a constructive critique [74]. **Table 5** summarizes different types of green inhibitors.


**Table 5.**

*The different types of green inhibitors.*
