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Fundamentals of Corrosion
Dicyclohexylamine nitrite is a widely used vapor inhibitor that is often
impregnated in the waxed paper or cardboard used for wrapping and packaging. One gram of this inhibitor saturates about 550 M 3 of air and the protection to steel is provided over years. Cyclohexylamine carbonate has a
higher vapor pressure and is used in packages that must be opened and
closed repeatedly.
The mechanism of inhibition is not the same for all vapor-phase inhibitors. Nitrite ions and benzoate ions in association with oxygen present passivate the steel surface. Carbonate provides alkalinity to the environment
and the organic amine portion of the inhibitor effectively provides protection through adsorption.
While the vapor-phase inhibitors, in general, are effective in the prevention of corrosion in steel, they accelerate the corrosion of some nonferrous metals. The vapor-phase inhibitors based on nitrobenzoate organic
compounds have been reported to protect ferrous, copper, and other alloy
systems.
10.3 Inhibition in Acid Solution
The inhibition of corrosion in acid solutions can be accomplished using a
variety of organic compounds. Among those used for this purpose are triple-bonded hydrocarbons, acetylenic alcohols, sulfoxides, sulfides, and mercaptans; aliphatic, aromatic, or heterocyclic compounds containing nitrogen;
and many other families of simple organic compounds and of condensation products formed by the reaction between two different species such as
amines and aldehydes.
Incorrect choice or the use of organic inhibitors in acid solutions can lead
to corrosion stimulation and/or hydrogen penetration into the metal. In general, stimulation of corrosion is not related to the type or structure of the
organic molecule. Stimulation of iron’s corrosion has been found with mercaptans, sulfoxides, azoles, and triazole derivatives, nitrites, and quinoline.
This adverse action depends on the type of acid. For example, bis(4-dimethylamino-phenylantipyrilcarbinol and its derivatives at a 10 −4 concentration
inhibited attack of steel in hydrochloric acid solutions but stimulated attack
in sulfuric acid solutions. Much work has been done studying the inhibiting and/or stimulating phenomena of organic compounds on ferrous as well
as nonferrous metals. Organic inhibitors have a critical concentration value,
below which inhibition ceases and stimulation begins. Therefore, it is essential that when organic inhibitors are used, constant monitoring of the solution should take place to ensure that the inhibitor concentration does not fall
below the critical value.
Fundamentals of Corrosion
Dicyclohexylamine nitrite is a widely used vapor inhibitor that is often
impregnated in the waxed paper or cardboard used for wrapping and packaging. One gram of this inhibitor saturates about 550 M 3 of air and the protection to steel is provided over years. Cyclohexylamine carbonate has a
higher vapor pressure and is used in packages that must be opened and
closed repeatedly.
The mechanism of inhibition is not the same for all vapor-phase inhibitors. Nitrite ions and benzoate ions in association with oxygen present passivate the steel surface. Carbonate provides alkalinity to the environment
and the organic amine portion of the inhibitor effectively provides protection through adsorption.
While the vapor-phase inhibitors, in general, are effective in the prevention of corrosion in steel, they accelerate the corrosion of some nonferrous metals. The vapor-phase inhibitors based on nitrobenzoate organic
compounds have been reported to protect ferrous, copper, and other alloy
systems.
10.3 Inhibition in Acid Solution
The inhibition of corrosion in acid solutions can be accomplished using a
variety of organic compounds. Among those used for this purpose are triple-bonded hydrocarbons, acetylenic alcohols, sulfoxides, sulfides, and mercaptans; aliphatic, aromatic, or heterocyclic compounds containing nitrogen;
and many other families of simple organic compounds and of condensation products formed by the reaction between two different species such as
amines and aldehydes.
Incorrect choice or the use of organic inhibitors in acid solutions can lead
to corrosion stimulation and/or hydrogen penetration into the metal. In general, stimulation of corrosion is not related to the type or structure of the
organic molecule. Stimulation of iron’s corrosion has been found with mercaptans, sulfoxides, azoles, and triazole derivatives, nitrites, and quinoline.
This adverse action depends on the type of acid. For example, bis(4-dimethylamino-phenylantipyrilcarbinol and its derivatives at a 10 −4 concentration
inhibited attack of steel in hydrochloric acid solutions but stimulated attack
in sulfuric acid solutions. Much work has been done studying the inhibiting and/or stimulating phenomena of organic compounds on ferrous as well
as nonferrous metals. Organic inhibitors have a critical concentration value,
below which inhibition ceases and stimulation begins. Therefore, it is essential that when organic inhibitors are used, constant monitoring of the solution should take place to ensure that the inhibitor concentration does not fall
below the critical value.