Each year, about a quarter of all metal produced in the world is lost due to the development and occurrence of corrosion processes. The costs associated with the repair and replacement of equipment and communications of chemical industries are often several times higher than the cost of materials required for their manufacture. Corrosion is called spontaneous destruction of metals and various alloys under the influence of the environment. However, you can protect yourself from these processes. There are various methods of protection against corrosion, as well as 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 the method of struggle in this case depends not only on the characteristics of the metal itself, but also on its operating conditions. Corrosion protection methods are selected in accordance with certain factors, however, a number of difficulties often arise here. A particular problem involves choosing an option for a multicomponent medium with parameters that change during the process. In the chemical industry, this is quite common. The methods of corrosion protection used in practice are divided according to the nature of their effect on the environment and metal.
Environmental impact
Even in the Middle Ages, special substances became known that were introduced in relatively small quantities, 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 you count dozens of their manufacturers. Metal corrosion inhibitors are quite widespread 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 units.
The properties
Corrosion inhibitors can be organic and inorganic in nature. They can protect against exposure to liquids 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 corroding metal surface, which led to the appearance of the name passivators. In this capacity, oxidizing agents of inorganic origin are traditionally used: nitrates, chromates and molybdates. They are easily restored on the cathode surfaces, which is why they become similar to depolarizers, reducing the speed of the anodic transition to a solution containing corroding metal ions.
Anode inhibitors are also considered 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 passivator. These substances lead to the adsorption of oxygen on metal surfaces. In addition, they cause inhibition of the anodic dissolution process due to the formation of protective films, which consist of difficultly soluble products of the interaction of the inhibitor and metal ions passing into the solution.
Features
Anodic metal corrosion inhibitors are usually classified as dangerous, because under certain conditions they turn from moderators to initiators of the destructive process. To avoid this, it is necessary that the corrosion current in density be higher than that at which the absolute passivation of the anode sections is formed. The concentration of the passivator should not decrease below a specific value, otherwise passivation may not occur, or it will be incomplete. The latter option is fraught with great danger, because it becomes the reason for the reduction of the anode surface, an increase in the depth and rate of destruction of the metal in small areas.
Requirements
It turns out that effective protection can be ensured 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 quite sensitive to the pH level 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 in comparison with the anode ones. Their action is based on the fact that local alkalization of the medium leads to the formation of insoluble products on the cathode sections that isolate part of the surface from the solution. Such a substance can be, for example, calcium bicarbonate, which releases calcium carbonate in an alkalized medium in the form of a precipitate that is difficult to dissolve. A cathodic corrosion inhibitor, the composition of which depends on the environment of use, does not lead to an increase in destructive processes even with insufficient content.
Varieties
In neutral environments, inorganic substances often act as cathodic and anodic inhibitors , but they are not able to help in strongly acidic solutions. Organic substances in which the molecules contain specific or polar groups, for example amines, thiourea, aldehydes, carbonate salts and phenols, are used as moderators in the production of acids.
According to the mechanism of action, these corrosion inhibitors are adsorptive. After adsorption on the cathode or anode sites, they greatly complicate the discharge of hydrogen ions, as well as the metal ionization reaction. To a large extent, the protective effect is based on temperature, concentration, the form of the acid anion, as well as the concentration of hydrogen ions. They are most often added in small quantities, because the protective effect of a number of organic inhibitors in high concentrations can be even dangerous.
For example, an organic compound called Penta-522 is oil-soluble in water. It is able to provide a degree of protection of more than 90% at a flow rate of only 15-25 grams per ton. The corrosion inhibitor manufactured under the brand name "Aminkor" is a product of the esterification of carboxylic acids, which is non-volatile, has no unpleasant odor, and is non-toxic. Its dosage is determined only after setting how corrosive the real environment is.
Impact on metal
This group of protection methods involves the use of a variety of coatings. These are paintwork, metal, rubber and other types. They are applied in various ways: by spraying, electroplating, gumming and others. You can consider each of them.
Gumming is understood to mean corrosion protection by means of rubber coatings, which is often required in chlorine production. Rubber compounds have high chemical resistance and provide reliable protection for tanks, bathtubs, and other chemical equipment from aggressive environments and corrosion. Gumming can be cold as well as hot, which is carried out by the method of vulcanization of epoxy and fluoroplastic mixtures.
It is important not only to choose correctly, but also to apply a corrosion inhibitor. Manufacturers usually give fairly clear guidance on this. At the moment, in addition to galvanic deposition, a method of high-speed spraying has become quite widespread. With its help, a fairly wide range of tasks is solved. Powder materials can be applied to obtain coatings having various properties.
Equipment protection
Issues related to the protection of chemical equipment are quite specific, and therefore require very careful study. The choice of material to obtain a high-quality coating requires an analysis of the surface condition, composition of the medium, operating conditions, degree of aggressiveness, temperature conditions, and so on. Sometimes in "uncomplicated environments" there is a critical parameter that makes it difficult to choose the type of coating, for example, steaming even every few months in a propane tank. That is why each aggressive environment requires the selection of such a foaming agent and such components for coating, which are characterized by resistance to the reagent.
Special opinion
Experts say that it is impossible to compare gas-thermal spraying methods with each other and all the more so to say that one of them is better than the other. Any of them has certain advantages and disadvantages, and the resulting coatings have different properties, which indicates their ability to solve some of their problems. The optimal composition with which corrosion inhibitors should be characterized, as well as the method of their application, are selected depending on the specific case.
At chemical plants, this method is most often used in the process of ongoing repairs. Even if acid corrosion inhibitors are used, it is first necessary to qualitatively prepare the metal surface. This is the only way to guarantee quality coverage. Before direct application of the paint material, it is possible to use blasting, with the help of which a sufficiently rough surface is obtained.
Every year more and more new developments appear on the market, and there is a considerable choice. However, chemists should decide what will be more profitable - to conduct timely equipment protection or complete replacement of all structures.