Saturated vapor pressure of common VPCIs
Substance Temperature,C Vapor pressure,mm Hg melting point, C
Morpholine 20 8.0
Benzylamine 29 1.0
Cyclohexylamine carbonate 25.3 0.397
Diisopropylamine nitrite 21 4.84 x 10-3 139
Morpholine nitrite 21 3 x 10-3
Dicyclohexylamine nitrite 21 1.3 x 10-4 179
Cyclohexylamine benzoate 21 8 x 10-5
Dicyclohexylamine caprylate 21 5.5 x 10-4
Guanadine chromate 21 1 x 10-5
Hexamethyleneimine benzoate 41 8 x 10-4 64
Hexamethyleneamine nitrobenzoate 41 1 x 10-6 136
Dicyclohexylamine benzoate 41 1.2 x 10-6 210

Vapor phase corrosion inhibitor (VPCI), also called volatile corrosion inhibitor (VCI), are compounds that are transported in a closed system to the site of corrosion by volatilization form a source.

Vapor phase corrosion inhibitor for boilers

In boilers, volatile basic compounds such as morpholine or octadecylamine are transported with steam to prevent corrosion in a condenser tubes by neutralizing acidic carbon dioxide. Compounds of this type inhibit corrosion by making the environment alkaline.

Vapor phase corrosion inhibitor for closed space

In closed vapor spaces, such as shipping containers, volatile solids such as the nitrite, carbonate, and benzoate salts of dicyclohexylamine, cyclohexylamine, and hexamethylene-imine are used. The mechanism of inhibition by these compounds is not entirely clear, but it appears certain that the organic portion of the molecules merely provides volatility.

Corrosion inhibition mechanism

On contact with a metal surface, the inhibitor vapor condenses and is hydrolyzed by any moisture present to liberate nitrite, benzoate, or bicarbonate ions. Because ample oxygen is present, nitrite and benzoate ions are capable of passivating steel as they do in aqueous solution. The mechanism for carbonate my not be the same, and hence the organic amine portion of the VPI may serve to aid inhibition by adsorption and by providing alkalinity.

It is desirable for a VPCI to provide inhibition rapidly and to have a lasting effect. Therefore, the compound should have a high volatility to saturate all of the accessible vapor space as quickly as possible, but at the same time it should not be too volatile, because it would be lost rapidly through any leaks in the package or container in which it is used. The optimum vapor pressure of VPI then would be just sufficient to maintain an inhibiting concentration on all exposed metal surfaces.

Implications of vapor pressures

Vapor pressures and other properties of some VPCIs are given in the table above.

Note that the vapor pressure of cyclohexylamine carbonate is 2000 times higher than that of dicyclohexylamine nitrite, thus making it a better choice for containers that are opened occasionally because a more volatile inhibitor will re-saturate the vapor space rapidly. Dicyclohexylamine nitrite is advantageous for once-opened containers that may be stored for extended periods.

The amount of VPI required depends on conditions, but 2.2 kg per 100 m2 of surface has been suggested for dicyclohexylamine nitrite and 2.2 kg per 30 m2 surface for cyclohexylamine carbonate.

Precautions

VPCIs attack nonferrous metals to varying degrees. It is suggested to evaluate several commercially available VPCIs for any particular application. Compatibility of the amines and nitrites with the copper alloys should especially be considered.

See also

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