*11.5.4.4 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 the schematic representation vapor phase inhibitors.

Examples,

*Corrosion Inhibitors*

metal complexes.

solutions [45].

*11.5.3.3 Alkaline inhibitors*

*11.5.3.4 Neutral inhibitors*

*11.5.4 Based on mode of protection*

used as an inhibiting pigment for paints.

*11.5.4.1 Chemical passivators*

*11.5.4.2 Adsorption inhibitors*

pickling inhibitors [47–49].

*11.5.4.3 Film forming inhibitors*

aldehydes.

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

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

Substances which usually have a sufficiently high equilibrium potential (redox or electrode potential) and sufficiently low over potential decrease corrosion rate on

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

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

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

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

attainment of passivity and are called chemical passivators [46].

**18**

voyages [50].

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