Inhibitors of corrosion. Methods of protection against corrosion

Every year, about a quarter of all metal produced in the world is lost due to the development and flow of corrosion processes. The costs associated with the repair and replacement of equipment and communications of chemical industries often exceed the cost of materials required to produce them. Corrosion is usually called the spontaneous destruction of metals and various alloys under the influence of the environment. However, you can protect yourself from these processes. There are different ways to protect against corrosion, like the types of exposure. In the field of chemical production, the most common types of corrosion are gas, atmospheric and electrochemical.

Way out

The choice of method of struggle in this case depends not only on the characteristics of the metal itself, but also on its operating conditions. Methods of protection against corrosion are selected in accordance with certain factors, but here, too, there is a number of difficulties. A special problem is associated with the choice of an option for a multicomponent medium with parameters that vary in the course of the process. In the chemical industry, this occurs quite often. Methods of protection against corrosion, used in practice, are divided according to the nature of their effect on the environment and metal.

Impact on the environment

Even in the Middle Ages special substances that were introduced in relatively small quantities became known, which made it possible to reduce the aggressiveness of the corrosive medium. For these purposes, it was customary to use oils, resins and starch. Over the past period, more and more corrosion inhibitors have appeared. At the moment, only in Russia can count dozens of their producers. Metal corrosion inhibitors are widely spread due to their affordable cost. They are most effective in systems where there is a constant or little-updated volume of corrosive medium, for example in tanks, tanks, cooling systems, steam boilers and other chemical aggregates.

Properties

Corrosion inhibitors can have an organic and inorganic nature. They can protect from the effects of liquid media or gas. In most cases, corrosion inhibitors in the oil industry are associated with inhibition of anodic and cathodic processes of electrochemical damage, the formation of passivating and protective films. You can consider the essence of this.

Anodic corrosion inhibitors act on the basis of passivation of the anodic sections of the corrosive surface of the metal, which was the reason for the appearance of the name of passivators. As such, oxidizers of inorganic origin are traditionally used: nitrates, chromates and molybdates. They are easily recovered on cathode surfaces, which makes them become similar to depolarizers, reducing the rate of anodic transition into a solution containing ions of a corrosive metal.

Anodic retarders are also considered to be some compounds that are not characterized by the presence of oxidizing properties: polyphosphates, phosphates, sodium benzoate, silicates. Their action as inhibitors is manifested exclusively in the presence of oxygen, which is assigned the role of a passivating agent. These substances lead to the adsorption of oxygen on metal surfaces. In addition, they cause the anodic dissolution process to slow down due to the formation of protective films, which consist of hardly soluble reaction products of the inhibitor and metal ions passing into the solution.

Features

Anodic inhibitors of corrosion of metals are usually classified as dangerous, because under certain conditions they transform from moderators into initiators of the destructive process. To avoid this, it is necessary that the corrosion current in density is higher than that at which absolute passivation of the anode regions is formed. The concentration of the passivator should not fall below a specific value, otherwise passivation may not occur, or it will be incomplete. The latter option is fraught with great danger, because it causes a reduction in the anode surface, an increase in the depth and speed of destruction of metal in small areas.

Requirements

It turns out that it is possible to provide effective protection if the concentration of the anode inhibitor is maintained above the maximum value in all areas of the product that is being protected. These substances are sensitive enough to the pH of the medium. Most often, chromates and nitrates are used in heat exchangers and to protect the surface of pipes.

Cathodic Inhibitors

According to the protective effect, these substances are less effective than anodic ones. Their action is based on the fact that local alkalinization of the medium leads to the formation of insoluble products on the cathode sections, which isolate part of the surface from the solution. As such a substance, for example, calcium bicarbonate can emit, liberating calcium carbonate in an alkaline medium in the form of a precipitate that is difficult to dissolve. A cathodic corrosion inhibitor whose composition depends on the environment of use does not lead to an increase in destructive processes, even if the content is insufficient.

Varieties

In neutral media, inorganic substances often act as cathodic and anodic inhibitors , but in strongly acidic solutions they can not help. As retarders in the production of acids, organic substances are used in which molecules contain specific or polar groups, for example amines, thiourea, aldehydes, carbonate salts and phenols.

By the mechanism of action, these corrosion inhibitors are characterized by an adsorption character. After adsorption on cathode or anode sites, they greatly impede the discharge of hydrogen ions, as well as the ionization reaction of the metal. To a large extent, the protective effect is based on temperature, concentration, the form of the acid anion, and also the concentration of hydrogen ions. They are often added in small amounts, because the protective effect of a number of organic inhibitors in high concentrations may even be dangerous.

For example, the organic composition called Penta-522 is an oil-soluble. It is able to provide a degree of protection of more than 90% with a consumption of only 15-25 grams per ton. The corrosion inhibitor produced under the trade mark "Aminkor" is a product of esterification of carboxylic acids, which is not volatile, does not have an unpleasant odor, is non-toxic. Its dosage is determined only after setting how corrosive the real environment is.

Effects on metal

This group of methods of protection involves the use of a variety of coatings. These are paint, metal, rubber and other types. They are applied in various ways: by spraying, galvanic, gumming and other. You can consider each of them.

Under gumming, it is customary to understand the protection against corrosion through rubber coatings, which is often required in chlorine production. Rubber mixtures have increased chemical resistance and provide reliable protection of containers, baths, as well as other chemical equipment from corrosive media and corrosion. Gumming is cold, and also hot, which is carried out by the method of vulcanization of epoxy and fluoroplastic mixtures.

It is important not only to choose, but also to apply a corrosion inhibitor. Manufacturers usually give fairly clear instructions on this matter. At the moment, in addition to galvanic deposition, the method of high-speed spraying has become quite widespread. With its help a fairly wide range of problems is being solved. Powder materials can be applied to produce coatings having different properties.

Protection of equipment

Issues related to the protection of chemical equipment are quite specific, and therefore require very careful consideration. The choice of material to obtain a high-quality coating requires an analysis of the state of the surface, the composition of the medium, the operating conditions, the degree of aggression, temperature conditions and other. Sometimes in "uncomplicated environments" there is a critical parameter that complicates the choice of the type of coating, for example, steaming even once in a few months of propane capacity. That is why each aggressive medium requires the selection of such a film former and such coating components that are characterized by resistance to the reagent.

Special opinion

Specialists say that it is impossible to compare the gas-thermal spraying methods among themselves and even more to say that one of them is better than the other. Each of them has certain advantages and disadvantages, and the resulting coatings have different properties, which indicates their ability to solve some of their tasks. The optimum composition, which must be characterized by corrosion inhibitors, as well as the method of their application, are selected as the case may be.

At chemical plants, this method is used most often in the process of ongoing repairs. Even if acid corrosion inhibitors are used, the surface of the metal should be prepared first. This is the only way to guarantee a quality coating. Before direct application of paint and varnish material, it is possible to use a jet blasting process, which allows obtaining a sufficiently rough surface.

Every year more and more new developments appear on the market, and there is a considerable choice. However, chemists should decide that it will be more profitable to conduct timely protection of equipment or complete replacement of all structures.