Chemical Inhibitors

Inhibitors, Chemical

 

substances that slow various chemical reactions. Inhibitors are widely used to inhibit or retard various undesirable processes—for example, the corrosion of metals; the oxidation of fuels, lubricants, and foods; polymerization; and polymer aging. A unique feature of inhibitors is their effectiveness in low concentrations (from thousandths of 1 percent to several percent). Inhibition effectiveness depends on the nature of the inhibitor as well as on the nature of the reaction being inhibited and on the quantity of the inhibitor, the time of its introduction into the reaction mixture, temperature, and the presence of other substances affecting the efficiency of the inhibitors.

Corrosion inhibitors are introduced into the corrosion-active medium in order to protect metals. They are among the most effective means of corrosion control and are widely used in metal pickling to remove scale, in the protection of power-plant equipment, in oil extraction and refinement, in industrial and consumer water supply, in the cooling systems of industrial installations and transportation devices (internal combustion engines), in the protection of machines against atmospheric corrosion, in water testing, and in other fields. Inhibitors are widely used to conserve machine parts and instruments and to protect them during transport and storage between periods of use. The protective action of corrosion inhibitors is the result of their ability to change the kinetics of electrochemical reactions, which are responsible for the corrosion processes. Depending on which type of electrochemical reaction is more strongly retarded by the corrosion inhibitors, they are divided into anodic, cathodic, and mixed inhibitors. The anodic corrosion inhibitors include, for example, such oxidants as chromates and nitrites, which are widely used in industry (aviation, chemistry, petroleum refining, and others). The action of these inhibitors causes the metal to transfer to a stable, passive state. The cathodic corrosion inhibitors include arsenic and bismuth salts as well as various organic compounds that increase the hydrogen overvoltage on the metal. Cathodic inhibitors also include substances that absorb cathodic depolarizers. Specifically, hydrazine and sodium sulfite, which bind the oxygen dissolved in water, are used to protect boiler equipment. Depending on the nature of the corrosive medium, there are corrosion inhibitors for acid, neutral, and alkaline media, as well as atmospheric corrosion inhibitors. To protect against atmospheric corrosion, the so-called volatile inhibitors are used extensively, the vapors of which are adsorbed at the surface of the metal. A widespread and effective method of applying atmospheric corrosion inhibitors is incorporating them into packaging materials. Dicyclohexylammonium nitrite, cy-clohexylammonium carbonate, and mixtures of urea or hexame-thylenetetramine (urotropine) with sodium nitrite are used to protect ferrous metals. Salts of nitrobenzoic and dinitrobenzoic acids with amines are used to protect ferrous metals in combination with nonferrous metals. In order to prevent metal corrosion, inhibitors are also added to fuels, oils, lubricants, and polymeric coatings. Oxidized petroleum products, nitrated oils, sulfonates, amines, nitrites, and other substances are added to oils and lubricants; chromates and nitrated oils, among other substances, are added to polymeric coatings.

Inhibitors of oxidation reactions slow down the oxidation by molecular oxygen. They are added to fuels, oils, and lubricants in order to retard their oxidation during storage and use. The presence of certain metals in fuels and oils exerts a catalytic action on oxidation and decreases the effectiveness of the inhibitors. In order to eliminate this effect, so-called metal deactivators, such as salicylidenes, are added to the fuels, which together with metals form unreactive complexes. Phenols, aromatic amines, and some sulfur compounds are primarily used as inhibitors of oxidation reactions. For example, phenyl-N-amino-phenol, 2, 4-dimethyl-6-t-butylphenol, and 2, 6-di-t-butyl-4-methylphenol (commercial name, Topanol-O) are added to gasolines. The chemical inhibitors added to lubricating oils for the same purpose usually have complex chemical structures and, in addition to increasing the stability to oxidation, also improve a number of the operating properties of these oils.

Food oxidation inhibitors are natural or synthetic substances that retard the oxidation of fats and fat-containing products. Examples of such inhibitors are such spices known since antiquity as thyme, sage, and caraway, which impart a pleasant taste to the food products and inhibit their oxidation (rancidity). Natural inhibitors, of which the tocopherols are the best known, are contained in many natural fats and oils, but these inhibitors are lost during refining. For this reason, chemical inhibitors are frequently added to the refined fats and oils. The substances used for this purpose include, for example, natural products, such as raw cottonseed or soybean oil (in concentrations of up to 5 percent), or synthetic products, such as the nontoxic esters of gallic acid, for example, ethyl and propyl gallates and ionol (3, 5-di-t-butyl-4-hydroxytoluene), and numerous phenols and amines (for example, for the protection of linseed oil from oxidation). The action of chemical inhibitors in these cases may be reinforced by other substances, such as citric and ascorbic acids.

Polymerization inhibitors inhibit or retard the polymerization of monomers (as well as of oligomers) during storage or distillation. Polymerization inhibitors should protect not only against spontaneous polymerization but also against oxidation by atmospheric oxygen. Polymerization inhibitors include sulfur, phenols, tannin, rosin, and copper salts.

REFERENCES

Altsybeeva, A. I., and S. Levin. Ingibitory korrozii metallov, Spravochnik. [Leningrad] 1968.
Bregman, J. Ingibitory korrozii. Moscow-Leningrad, 1966. (Translated from English.)
Rozenfel’d, I. L. “Letuchie ingibitory korrozii.” In the collection Korroziia i zashchita ot korrozii, vol. 1. Moscow, 1971. [Itogi nauki. Ser. khimiia (collection 34)].
Putilova, I. N., S. A. Balezin, and V. P. Barannik. Ingibitory korrozii metallov. Moscow, 1958.
Kuliev, A. M. Prisadki k smazochnym maslam. Moscow-Leningrad, 1964.
Aksenov, A. F. Aviatsionnye topliva, smazochnye materialy i spetsial’nye zhidkosti, 2nd ed. Moscow, 1970.

V. V. SCHCHEKIN and I. L. ROZENFEL’D

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