O’zbekistоn respublikasi оliy va o’rta maxsus ta`lim vazirligi

7.3 Corrosion-mechanical wear


Corrosion-mechanical wear occurs as a result of mechanical action, accompanied by the chemical interaction of materials with the environment.
The surface layers of the material, activated by friction, increase their capacity for adsorption, diffusion and chemical reactions. In this case, if the activity of the environment is so high that it has a decisive influence on the processes of destruction of friction surfaces, then corrosion-mechanical wear takes place. In this case, the destruction of the body surface occurs as a result of mechanical wear (abrasive, fatigue, adhesive), activated by chemical or electrochemical interaction of the material with the environment. Chemical interaction occurs when the friction surface comes into contact with dry gases or non-conductive aggressive liquids. Electrochemical interaction - when metals come into contact with electrolytes, for example, aqueous solutions of acids, alkalis, salts, molten salts. In this case, two main processes are observed: anodic (direct transition of metal atoms into solution in the form of ions) and cathodic (assimilation of excess electrons by atoms or ions of the solution). The films formed on the contact surfaces, depending on the composition, can be easily removed, and then setting occurs, or they act as wear inhibitors, reducing the wear rate.
O-rings of mechanical seals of reactors, centrifuges, plain bearings of reactors, pumps, spray discs of centrifugal dryers, reactor mixers, wheels and casings of centrifugal pumps, screws, bushings of mixers and granulators are most susceptible to corrosion-mechanical wear.
The intensity of corrosion-mechanical wear depends primarily on the composition of the environment and the corrosion resistance of the contacting materials in it.
To reduce the amount of corrosion-mechanical wear, it is necessary to increase the corrosion resistance of tribological materials with a simultaneous increase in antifriction characteristics.
An increase in the corrosion resistance of materials can be achieved by introducing inhibitors into an aggressive environment and by special treatment of friction surfaces. Metal materials are subjected to heat treatment in order to increase hardness and improve corrosion resistance. To reduce the tendency of corrosion-resistant steels to seize and seize, nitriding and chromium plating are used. A good effect of increasing wear resistance is given by coatings, incl. based on silicates.
For the manufacture of friction units operated in aggressive environments, parts are used from high-alloy stainless steel grades: 14Kh17N2, 20KhVN4G9, 12Kh18N10, 08Kh17N15M3T, etc., paired with soft anti-friction materials such as carbon graphites, composites based on fluoroplast-4, etc.
A variation of corrosion-mechanical wear is oxidative wear, in which the main contribution to wear is made by the chemical reaction of the material with oxygen or an oxidizing environment. During oxidative wear, oxide films are formed on the contacting surfaces, which are destroyed during friction. Wear products are particles of metal oxides. This type of wear, in contrast to other types of corrosion-mechanical wear, occurs in the absence of an aggressive medium in conditions of friction without lubrication or with a lack of it. At the same time, the wear surfaces have a low roughness, which is explained by the obstruction of oxides to the adhesive setting of materials on the spots of actual contact.
The intensity of oxidative wear increases with increasing temperature in the contact zone and as a result of plastic deformation of the surface layers.
Oxidative wear is mainly exposed to gauges, parts of hinged and bolted suspension devices of machines operating without lubricant, metal wheels of friction gears and variator cups, dry friction bearings, plates and cylinders of centrifugal blowers, etc.
To reduce the intensity of oxidative wear, it is necessary to reduce the temperature of frictional heating of parts by selecting the optimal composition of the lubricant. A good result is provided by an increase in the hardness of the surfaces of friction parts.
Another type of corrosion-mechanical wear is hydrogen wear , which occurs due to the absorption of hydrogen by a rubbing metal surface. The latter is formed during the destruction of the lubricant of the associated plastic parts and as a result of tribochemical reactions.
Hydrogen wear is manifested in the transfer of metal particles to the mating surface. For example, it has been established that steel bandages are smeared on a brake shoe, and steel particles are transferred from the surface of the crankshaft to a soft babbitt bearing.
The main means of combating hydrogen wear at present is the selection of lubricants and contacting polymeric materials.