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27PROPERTIES OF INORGANIC CORROSION INHIBITORS
Sidikov S.S.
Sidikov Sanjar Siroj ugli - Teacher, DEPARTMENT VOCATIONAL TRAINING, BUKHARA ENGINEERING-TECHNOLOGICAL INSTITUTE, BUKHARA, REPUBLIC OF UZBEKISTAN
Abstract: this article deals with the properties of inorganic corrosion inhibitors and consumption of corrosion inhibitors in the oil industry It also analyzes their contribution to the delayed spread of corrosion.
Keywords: inhibitors, inorganic corrosion inhibitors, special substances, corrosion, chemical reaction, protection.
The ability to slow down the corrosion of metals in corrosive environment have many inorganic substances. The inhibitory effect of these compounds is due to the presence of cations (Ca2 +, Zn2 +, Ni2 +, As3 +, Bi3 +, Sb3 +) or anions (CrO2-, Cr 02-, NO-, SiO2-, PO3-) in them.
Shielding cathodic corrosion inhibitors are compounds that form insoluble compounds on microcathodes, which are deposited as an insulating protective layer. For iron in an aqueous medium, such compounds can be ZnSO4, ZnCl2, and more often Ca (HC03) 2. Calcium bicarbonate Ca (HC03) 2 is the cheapest cathodic shielding inhibitor used to protect steel against corrosion in water supply systems. Calcium bicarbonate in an alkaline medium forms insoluble CaCO3 compounds, which are deposited on the surface, isolating it from the electrolyte.
Anodic inorganic corrosion inhibitors form thin (~ 0.01 ^m) films on the metal surface, which inhibit the transition of the metal into the solution. The group of anodic corrosion inhibitors includes chemical compounds - film-forming agents and oxidizing agents, often called passivators.
Cathode-anodic inorganic inhibitors, for example, KJ, KBr in acid solutions, inhibit equally the anodic and cathodic processes due to the formation of a chemisorption layer on the metal surface. Film-forming inhibitors protect the metal by creating phase or adsorption films on its surface. These include NaOH, Na2C03 and phosphates. The most common phosphates are widely used to protect iron and steel in the system of household and municipal waste.
In the presence of phosphates, a protective film is formed on the surface of the iron. It consists of iron hydroxide compacted with iron phosphate. For a more protective effect, phosphates are often used in a mixture with polyphosphates.
Passivators inhibit the anodic dissolution of the metal due to the formation of oxides on its surface. This reaction can proceed only on metals prone to passivation.
Passivators are good, but dangerous inhibitors. At an incorrectly chosen concentration, in the presence of Cl-ions or with an inappropriate acidity of the medium, they can accelerate metal corrosion, and in particular, cause very dangerous pitting corrosion.
Inhibitors-passivators cause the formation of a protective film on the metal surface and contribute to the transition of the metal to a passive state. The most widely used passivators are used to combat corrosion in neutral or close to them environments, where corrosion proceeds mainly with oxygen depolarization. The mechanism of action of such inhibitors is different and is largely determined by their chemical composition and structure. There are several types of passivating inhibitors, for example, inorganic substances with oxidizing properties (nitrites, molybdates, chromates).
The latter are able to create protective oxide cover on the surface of a corrosive metal. In this case, as a rule, the potential is shifted towards positive values to a value corresponding to the release of oxygen from water molecules or hydroxyl ions. In this case, the formed
oxygen atoms that block the most active centers of the metal surface and create an additional potential jump, slowing the dissolution of the metal, are chemisorbed on the metal. The resulting chemisorption layer is close in composition to the surface oxide.
A large group consists of passivators, which form hardly soluble compounds with ions of a corrosive metal. The salt precipitate formed in this case, if it is sufficiently dense and adheres well to the metal surface, protects it from contact with aggressive media. Such inhibitors include polyphosphates, silicates, alkali metal carbonates.
Chromates and bichromates of sodium and potassium are used as inhibitors of corrosion of iron, galvanized steel, copper, brass and aluminum in industrial water systems.
The oxide cover consists of 25% Cr203 and 75% Fe203. Nitrites are used as corrosion inhibitors for many metals (except for zinc and copper) at a pH of more than 5. They are cheap and effective in the presence of rust. The protective effect of nitrite is the formation of a surface oxide cover. [1, 121]
The protective effect of nitrite is the formation of a surface oxide film. Silicates are a mixed-effect corrosion inhibitor, reducing the rates of both cathodic and anodic reactions. The action of silicates is to neutralize carbon dioxide dissolved in water and to form a protective film on the metal surface.
The cover does not have a permanent composition. In structure, it resembles a silicic acid gel in which iron compounds and hardness salts are adsorbed. Its thickness is usually about 0.002 mm.
Polyphosphates are water-soluble compounds of metaphosphates of the general formula (MeP03) n. The protective effect of polyphosphates is the formation of an impermeable protective film on the metal surface. In aqueous solutions, slow hydrolysis of polyphosphates occurs, resulting in the formation of orthophosphates. In the presence of Ca2 + and Fe3 +, an impermeable protective film is formed on the surface.
Sodium hexametaphosphate is the most widely used in industry. Phosphates and polyphosphates are used as inhibitors for steel corrosion in water and refrigerated brines. Great effect is achieved by the joint use of phosphates and chromates.
References
1. Gabitov A.I. Results and prospects in the theory and practice of the fight against corrosion. State Publishing House of Scientific and Technical Literature "Reagent", 1998. 121 p.
