Thermal spraying of metal coatings is the process of applying molten metal to a substrate. Both the application methods used and the metals themselves can vary, but the key is that the spraying process leaves a thin, corrosion and abrasion resistant coating that maintains the properties of the structure.
The process of gas-thermal spraying of metals is an excellent means of corrosion protection for steel. This coating method competes successfully with both relatively inexpensive limited life coatings and expensive organic coatings such as epoxy, polyurethane and urea systems.
General process of gas thermal spraying
The general process of thermal spraying consists of deposition of highly effective corrosion-resistant aluminum, zinc, aluminum-zinc materials by a directed high-speed and high-temperature gas stream. Coatings deposited on the surface of steel metal products are formed from molten and solidified particles of the sprayed metal.There are several methods of metallization, differing from each other in the type of raw materials, the method of melting the sprayed metal, the design of apparatus and the source of applied energy. But all of them are based on a common basic scheme, including the following operations: feeding metal to the melting point, heating the metal to melting, dispersing the sprayed metal, giving metal particles a significant speed, the impact of fast-moving particles on the surface of the metallized product, cooling of the fixed particles and the coating as a whole.Stages and method of sputtering
What happens in the process of gas dynamic spraying? It is based on the creation of high-temperature gas streams (the temperature of the gas jet is controlled from 1000 to 3000 ˚C depending on the operating modes) moving at a speed of more than 1000 m/sec. At the same time, the sprayed material, whether it is a rod, wire or powder, passes through three stages: the first - heating, melting; the second - the formation of a stream of atomized particles, their directed accelerated movement, chemical and energy exchange with the environment; the third - interaction with the substrate material and the formation of a coating (up to 500 microns thick).In the process of application of the arc spraying method, the wire dosing is more precise and the coating adheres well to the surface. This method is usually used for large areas.The gas-thermal spraying method is well suited for hard-to-reach areas. Good adhesion of the applied coating to the protected surface (not less than 90÷110 kg/sq.cm) is caused by high temperature and speed of the molten metal jet, contributing to diffusion penetration of fusible metals into the treated steel surface.When metallizing by hand, parallel overlapping strips are successively applied to the surface to be treated. To reduce the thickness variation, the coating is applied in several layers. Each subsequent pass is applied in a direction perpendicular to the previous one. This application mechanism is similar to airless painting.Characteristics of sprayed coatings
Important characteristics of sprayed coatings are porosity and gas permeability.As the distance to the surface to be protected increases, the porosity increases. This is due to the fact that as the distance increases, the temperature of the particles at the moment of impact with the substrate and their velocity decrease. However, it should be noted that when applying from small distances corresponding to the formation of a dense coating with low porosity, overheating and deformation of the substrate may occur. In addition, when spraying at short distances, there are difficulties associated with obtaining an even coating surface.As the thickness of coatings (except aluminum) increases, their gas permeability decreases. In order to obtain a quality coating, it is necessary to apply the optimum thickness of metallization coating.To protect structures designed for a long service life or equipment operating in highly aggressive environments, metallization with subsequent coating with paint and varnish materials is used. This dramatically increases the service life of the coating and eliminates one of its disadvantages - the presence of pores in the upper layers. Thanks to the impregnation, the micropores are plugged and corrosion-causing substances do not penetrate deep into the metallization layer.Zinc and aluminum coatings
Currently, zinc and aluminum coatings are most widely used as anticorrosive coatings for various structures. The use of these metals is due to the relatively low melting point and electrochemical nature of protection.Zinc (metallization) coating in relation to steel, in all cases behaves as an anode and has the same protective properties as zinc coatings obtained by other methods. Zinc coatings are effective in media with pH=6÷11, in alkaline solutions with pH>12 and acids they are unstable. Corrosion resistance of metallization zinc coating is essentially similar to the resistance of homogeneous metal, its durability is a function of coating thickness.Zinc coatings (thickness 80÷360 microns) are usually used to protect steel structures (bridges, towers, water storage tanks, pipelines, various mining equipment, gas tanks, washing equipment, blowers, pumps, railings, etc.) from atmospheric effects, as well as from the action of fresh and salt water.Aluminum coatings are corrosion-resistant both in normal atmospheric conditions and in the environment containing sulfuric gases, vapors and gases at high temperatures, in hot and soft water, in ammonia solutions and many acids. Protection of steel structures by aluminum in oxidizing environments is mainly due to the clogging of coating pores by a film of aluminum oxide.Advantages and disadvantages of the method
In many cases, sprayed aluminum can be left unprotected without the use of reinforcing materials. With the use of reinforcing materials, zinc plating is a good long-term system.Using an aluminum alloy with 5% magnesium is an excellent alternative to paint coating. This alloy is an effective coating for offshore platforms and parts that are on the deck of a ship.Thermally sprayed aluminum is good to use for factory equipment that operates at high temperatures. Unlike surface coating with paint, thermally sprayed products can be used almost immediately after the job is done. If the coating is applied to individual parts, there will be less damage during installation. The thermal method allows the coating to be sprayed on any part of the product, and makes it possible to carry out assembly, installation and repair in a short time.The main advantage of the gas-thermal method of coating is the possibility of spraying metal in a wide range of temperatures. For example, paintwork materials are usually applied at positive temperatures, usually from plus 5°C to plus 30°C. There are paints and varnishes, the application of which is possible at negative temperatures (up to minus 10÷15 ° C), but even the paint manufacturers themselves recommend applying the coating in milder conditions for effective protection.There is practical experience in applying gas-thermal coatings at temperatures up to plus 40°C (Alma-Ata, Kazakhstan). And in winter time - up to minus 25°C in the city of Kogalym, Khanty-Mansi Autonomous Okrug. This coating “withstood” the winter at temperatures up to minus 60°C, and cracking occurred in the tanks with paint coating.Another advantage of gas-thermal coatings is the possibility to apply them in the zone of variable wetting. Diverting water for a small amount of time allows time to prepare the surface and apply a coating that does not require time to form.The efficiency of the gas-thermal method depends on the quality of surface preparation, its degree of cleanliness and roughness. To obtain a quality coating it is necessary to abrasive blast clean the surface not lower than Sa 2½ - Sa 3 according to ISO 8501-1, surface profile within 30-60 microns. This is one of the “inconveniences” of gas-thermal coatings, as other types of coatings can also be applied to less prepared surfaces.Research on corrosion by the American Welding Society
The results of this study are as follows:- Aluminum coatings 0.08-0.15 mm thick, both with and without the use of sealants, provide complete corrosion protection of the base metal for 19 years in seawater and in harsh marine and industrial atmospheres;
- The zinc coating without sealants shall be 0.30 mm thick to provide full surface protection in seawater for 19 years. In harsh marine and industrial atmospheres, a zinc coating thickness of 0.23 mm without sealant and 0.08-0.15 mm with sealant shall provide corrosion protection for 19 years;
- When exposed to harsh marine atmospheres, apply 1 coat of wash solution and add 1-2 coats of vinyl aluminum. This will improve the appearance of the product and extend the life of the zinc coating by almost 100%. Using sealer material along with aluminum improves the appearance, both with and without sealer material, no rust appears on the base metal for 19 years. A thin layer of heat-dusted aluminum is more effective, it has less tendency to blister, and therefore has a longer life.
In cases where physical damage such as scratches occurs after the aluminum coating is used, corrosion does not progress, meaning galvanic protection is necessary.Author: Svetlana Kazantseva,
according to the materials of PTO LLC “Promzaschita”.
The article was published in the magazine
“CLEANING. COLORING” (NO. 2, 2007).