Thermal diffusion zinc coating

For the first time this method was applied in England at the beginning of the 20th century and received the name “sherardization” (by the name of the inventor, Sherard Cowper-Coles).

Thermal diffusion zinc coating is anodic with respect to ferrous metals and protects steel electrochemically. It has a strong adhesion (adhesion) with the base metal due to the mutual diffusion of iron and zinc in the surface intermetallic phases of Zn-Fe, so the coating is little susceptible to flaking or chipping during impact, mechanical loads and deformations of the processed products.

The advantage of thermal diffusion coating technology compared to electroplating is not only in its superior corrosion resistance, but also in that it does not cause hydrogen embrittlement of the metal.

Thermal diffusion zinc coating exactly repeats the contours of products, it is uniform in thickness over the entire surface, including products of complex shape and threaded connections.

The essence of the technology of thermal diffusion zinc coating is that the anti-corrosion coating is formed by saturating the surface of metal products with zinc in a powder medium at a temperature of 290-450 ° C, and the choice of temperature depends on the type of steel products, steel grade, and requirements of manufacturers of parts. This technology allows to obtain any thickness of the coating in the range from 6 to 100 microns at the request of the customer without changing the process. The process takes place in a closed container with a special saturating mixture added to the parts to be processed. Passivation (finishing parts) is designed to prevent the formation of yellow or white corrosion products on surfaces exposed to high humidity atmospheres, salt water, marine atmospheres or condensation and drying cycles.

Protection of metals against corrosion ensures the long-term functioning of various parts, structures and structures. About 10% of the annual output of steel and steel products produced annually lose their technical characteristics due to corrosion, which is estimated at tens of billions of dollars. One of the most common ways to protect metals from corrosion is zinc coating. The choice of zinc is not accidental and is explained by the high negative value of the redox potential of the Zn ²⁺ / Zn pair. This metal protects the base metal (iron) anode, that is, zinc dissolves in the “Fe-Zn” galvanic couple. Electrochemical alternative to zinc is cadmium , but its use in many countries is prohibited due to its high toxicity. In the Russian Federation, cadmium plating has limited use and is possible, as a rule, only by special permission. The thickness of the protective zinc coating is selected depending on the purpose of the product and the conditions of its operation. Analysis of the conditions for the use of various metal products shows that a protective ( anti-corrosion ) coating for these conditions should not only have increased corrosion resistance, but also be resistant to abrasive wear and have a high degree of adhesion to the surface of the protected product.

Galvanic (electrolytic) coatings . Coatings are applied to the surface of products in electrolyte solutions under the action of electric current. The main components of these electrolytes are zinc salts. The galvanic method of protecting metal products has low corrosion resistance (about 140 hours in the neutral salt mist chamber), does not allow to apply coatings on products of complex configuration, when preparing the surface for galvanizing on high-strength products, causes hydrogen embrittlement , does not allow dismantling metal parts, has a low degree of adhesion with the surface. This type of protection is rather a decorative function.

Metallization coatings . Coatings are applied by spraying with a stream of air or hot gas of molten zinc. Depending on the method of spraying using zinc wire (rod) or zinc powder. In industry, using flame spraying and electric arc metallization .

Zinc-rich coating. These coatings are compositions consisting of a binder and zinc powder. Various synthetic resins (epoxy, phenolic, polyurethane, etc.), varnishes, paints, polymers are used as binders. Show the properties of paint rather than metal coatings.

Hot galvanizing . As a rule, hot zinc is used to protect against corrosion of large metal structures. Galvanizing of small and threaded parts does not provide the required quality of galvanized surface. After degreasing, washing, pickling and re-washing, the parts in the drum are dipped into a bath (usually ceramic) with molten zinc. Rotation of the drum provides a flow of zinc mass relative to the parts to fill all the pores and microcracks. The drum is then removed from the bath and spun to remove excess zinc by centrifugation . Flows are formed on the parts, excess zinc remains on the internal threads, the threads have to be machined, thus removing the protective zinc layer, which further causes corrosion. This method is not applicable for small parts, such as metric fasteners. Not applicable for high strength and alloy steel parts.

Thermal diffusion zinc coatings can protect parts from any steel grades, including high-strength and cast iron, from corrosion without changing the properties of the base metal, parts of a complex configuration with holes, assembled parts, welded and threaded. Limit the size of parts to container size.

Combined coatings are a combination of zinc coating, paint or polymer. In world practice, such coatings are known as “duplex systems”. In such coatings, the electrochemical protective effect of the zinc coating is combined with the waterproofing protective effect of the paintwork or polymer. It should be noted that galvanic and metallization zinc coatings do not contain intermetallic compounds (phases) and consist of zinc of the appropriate chemical composition. Hot-zinc coatings obtained by hot-dip galvanizing (from a zinc melt) and diffusion coatings applied from zinc-based powder mixtures have a similar formation mechanism — diffusion. However, the diffusion of zinc into the metal is different: with the use of a hot zinc coating, diffusion is 0.1–3%, thermal diffusion galvanizing is 50–70%. In accordance with the phase diagram of the Fe – Zn system, a number of similar phases (intermetallic compounds) are present in the structure of these coatings. However, the general structure of these coatings is still different, as well as their properties.

The coating process is a closed technological cycle, divided into several operations:

1st stage: preliminary mechanical cleaning using a shot blasting unit; 2nd stage: loading the cleaned parts into the container; adding a saturating mixture; 3rd stage: carrying out a thermal diffusion process, providing a zinc layer on the surface to be coated; 4th stage: unloading parts from the container while simultaneously cleaning them from saturating residue and passivating. 5th stage: cooling of finished products. Parts intended for applying the following types of coatings (paints, gluing, plasticizing, etc.) are usually passivated 1 time. In all other cases, two passivation operations are carried out, with intermediate washing of parts. To obtain a high-quality anti-corrosion coating, all technological steps are equally important and are equal components of the technological process. It should be noted that the coating technology does not make an exception for any type of parts that, by their size, weight and configuration, are included in the technological container of the equipment adopted at the moment. Equipment designed to process parts of a different size is made to order. Accordingly, at the stage of development of technical specifications, the overall size of the container and furnaces is determined, as well as the capacity of the remaining units in the line, the throughput capacity of the shot-blaster, passivation units and drying. The stage of surface preparation includes ultrasonic, shot-blasting or sandblast cleaning of parts, which is especially important for metal products that have scale on themselves after heat treatment during production.

The first is the ability of the process to obtain any thickness of the coating on request. The second is the lack of gluing parts. This is one of the most negative moments taking place in hot zinc and in electroplating. The third advantage concerns the external and internal shape of the parts. The Russian market showed that the majority of fasteners and any other auxiliary parts used to be painted, plasticized or simply remained uncoated, as they had threaded connections, hollow and blind holes, complex joints, welds. The advantage is also the absence of any zinc fluxes in the recesses or joints. The fourth advantage relates to the possibility of subsequent processing of parts by different types of paints, plasticizers, etc. Almost all types of industrial paints fit well to the thermal diffusion coating. High adhesion increases corrosion resistance, blistering and peeling of paints from the surface is practically excluded. The service life of parts with double coating increases, which leads to significant savings in their operation. The fifth advantage is the ecological purity of the process (if there is no need to degrease the product).

In December 2003, the Department of Electrification and Power Supply of Russian Railways OJSC issued an instruction on the use of thermal diffusion galvanizing of parts and structures of the contact network. This manual applies to protective zinc coatings applied by thermal diffusion galvanizing on threaded parts, valves, contact network designs and other products made of carbon and low carbon steel, including increased strength, cast iron and non-ferrous metal parts of the contact network, including cast-iron porcelain insulators. Since January 2008, GOSTs have come out for high-strength fasteners for metal structures, in which there is an indication that a thermal diffusion coating is used to protect high-strength bolts, nuts and washers from corrosion.

• Galvanizing
• Metallization
• Thermal spraying

• GOST 9.316-2006 "Unified system of protection against corrosion and aging. Thermal diffusion zinc coatings. General requirements and methods of control.
• GOST 52643-2006 “High-strength bolts and nuts and washers for metal structures. General technical conditions.

• PROTECTIVE ZINC COATINGS: COMPARATIVE ANALYSIS OF PROPERTIES, RATIONAL APPLICATIONS. E. Proskurkin SE "NITI", Ukraine magazine "National Metallurgy" September, October 2005 http://www.nmet.ru/a/2005/10/03/153.html