*11.5.3.2.1 Organic inhibitors*

Organic compound containing oxygen, nitrogen, sulfur with multiple bonds have been reported as good corrosion inhibitors. Many organic inhibitors such as amines, aldehydes, alkaloids, nitro and nitroso compounds have been studied and tried as corrosion inhibitors [39]. Organic inhibitors can be anodic, cathodic and mixed type based on its reaction at the metal surface and potential. These are effective depending upon its size, carbon chain length, aromaticity, conjugation and nature of bonding atoms [40].

#### *11.5.3.3 Alkaline inhibitors*

Metals, which form amphoteric oxides, are prone to corrosion in alkaline solutions. Many organic compounds are often used as inhibitors for metals in basic solutions [41]. Compounds such as thiourea, substituted phenols, naphthol, β-diketone, etc., have been used as effective inhibitors in basic solutions due to the formation of metal complexes.

#### *11.5.3.4 Neutral inhibitors*

Inhibitors which are effective in acidic solutions do not function effectively in neutral solutions, since the mechanism is different in the two solutions [42–44]. In neutral solutions, the interaction of inhibitors with oxide covered metal surface and prevention of oxygen reduction reaction at the cathodic sites takes place. Such inhibitors protect the surface layers from aggressiveness. Some surface active chelating inhibitors have been found to be efficient inhibitors in near-neutral solutions [45].

#### *11.5.4 Based on mode of protection*

## *11.5.4.1 Chemical passivators*

Substances which usually have a sufficiently high equilibrium potential (redox or electrode potential) and sufficiently low over potential decrease corrosion rate on attainment of passivity and are called chemical passivators [46].

Example, nitrites are used as inhibitors for antifreeze cooling waters. Chromates are mostly used as inhibitors for recirculating cooling waters. Zinc molybdate is used as an inhibiting pigment for paints.

#### *11.5.4.2 Adsorption inhibitors*

These represent the most widely used class of inhibitors. In general, they are organic compounds which get adsorbed on the metal surface and provide a blanketing effect over the entire surface, that is, both in cathodic and anodic cases. Generally they effect both cathodic and anodic reactions equally, but in many cases the effect may not be equal. These are commonly used in the acid pickling of hot rolled products in order to remove the black mill scale and are thus known as pickling inhibitors [47–49].

Examples: Compounds containing lone pairs of electrons such as nitrogen atoms in amines, quinolines, sulfur atoms in thio compounds and oxygen atoms in aldehydes.

#### *11.5.4.3 Film forming inhibitors*

In contrast to the adsorption inhibitors which form the straight forward adsorbed film of the inhibiting species, many substances called film forming inhibitors, appear to stop corrosion by forming a blocking or a barrier film of a material other than the actual inhibiting species itself. Such materials tend to be specific either to the cathode or to the anode. Zinc and calcium salts are the most common examples of cathodic film forming inhibitors. Benzoate is the common example of anodic film forming inhibitors, which inhibit corrosion during voyages [50].

**19**

**Figure 14.**

*Corrosion Inhibitors*

Examples,

*11.5.4.6 Synergistic inhibitors*

*11.5.4.7 Precipitation inhibitors*

silicate, zinc-tannins, and zinc-phosphates.

*Schematic representation of vapor phase inhibitors.*

water softeners to prevent the occurrence of rust [57, 58].

*DOI: http://dx.doi.org/10.5772/intechopen.80542*

the schematic representation vapor phase inhibitors.

*11.5.4.5 Volatile inhibitors or vapor phase inhibitors*

Atmospheric corrosion of metals in closed spaces as in parcels during storage and shipment can be prevented by the use of certain substances called vapor phase inhibitors also called as volatile inhibitors [51–56]. These are substances of low but significant vapor pressure. The vapor comes in contact with the surface of the metal and the adsorption of the inhibitor takes place. The moisture then hydrolyses it and releases protective ions which have corrosion inhibiting properties. **Figure 14** shows

Dicyclohexylamine chromate and benzotriazole for protecting copper.

environment to the site of corrosion by volatilization from a source **(Figure 15).** If the corrosion product is volatile, it volatilizes as soon as it is formed, thereby leaving the underlying metal surface exposed for further attack. This causes rapid and continuous corrosion leading to excessive corrosion. Example, Molybdenum oxide (MoO3), the oxidation corrosion product of molybdenum is volatile. In closed vapor process (shipping containers), volatile solids such as salts of dicyclohexylamine, cyclohexylamine and hexamethylene amine are used as volatile corrosion inhibitors [52].

Volatile corrosion inhibitors (VCIs) are compounds which transferred in a closed

These are single inhibitor which is used in cooling water systems. More often, a combination of inhibitors (anodic and cathodic) is used to obtain better corrosion protection properties [56]. Examples include chromate-phosphates, polyphosphate-

These are compounds that forms precipitates on the metal surface, thereby providing a protective film. The most common inhibitors of this category are the silicates and the phosphates. For example, Sodium silicate, is used in many domestic

Phenyl thiourea and cyclohexylamine chromate for brass. Dicyclohexylamine nitrite for ferrous and nonferrous part.

*11.5.4.4 Vapor phase inhibitors*
