Corrosion resistance of the protective metal coatings applied to dip, largely determined by its chemical composition, forming conditions, and depends on the structure of the obtained coating.
For long term protection against corrosion of steel parts, especially in hostile environments, it is currently mainly used zinc coatings applied by dipping into the melt.
It should be noted that to achieve the desired performance properties of the steel strip (resistance to corrosion, and durability of protection against corrosion after plastic deformation) are applied protective coating based on zinc and aluminum.
In the presence of moisture phase film due to the large potential difference zinc coating with respect to the steel base carries cathodic protection, which protects the base from corrosion in the areas of the coating and flaws in the cut edges. In the first stage of the corrosion process of zinc coating on the surface of most aggressive media are formed fairly dense zinc corrosion products, the presence of which inhibits the further process of the destruction of the coating.
Aluminum, due to the dense and inert in most aggressive media the surface oxide film has a sufficiently high resistance to corrosion.
Using various combinations of aluminum and zinc can be molten to form coatings with desired performance properties.
Formation of coatings on steel dip aluminum-based and zinc characterized by the formation at the interface between the phases (solid and liquid) complex composition of the diffusion layers. Depending on the composition and mode of formation of the melt coating diffusion layer may vary in thickness and composition. The surface layer of the coating composition and structure will be determined by the phase diagram of the alloy and the cooling mode, the product after removing it from the melt. It should be noted that the performance properties of such coatings are defined by their structure and chemical composition included in the coating phase.
It is known that the introduction of aluminum in the molten zinc resulting in significant changes in the structure of the coating formed therein. This applies to both the thickness and composition of the diffusion layer, and the phase composition of the surface layer.
Currently, the best known are the following protective coatings based on aluminum and zinc.
In hot aluminizing steel formed on the surface of a diffusion layer composed mainly of intermetallic compounds GeA1z; RegA15, after removing the steel product from the melt during cooling forms a surface coating layer composed of aluminum, the alloying elements iron and solid solution in the aluminum. At low speeds the cooling of the product after coating can be formed by a diffusion layer is sufficiently thick, and the inclusion of intermetallic compounds can be formed in the surface layer of the coating, which reduces its performance properties. Aluminium coatings have excellent corrosion resistance in most corrosive environments except alkaline. This is especially noticeable in the industrial and marine atmospheres.
Two types of aluminum coatings applied to steel products dip, including on continuous lines.
Type 1 aluminum melt contains up to 10% of silicon introduced to reduce the thickness of the diffusion layer. This ensures a good deformability of bases without coating delamination. The aluminide coating containing silicon, good resistance to high temperatures (up to 900 ° C) and aggressive environments.
Type 2 aluminum melt contains small additions of manganese to reduce the harmful effect of iron in the coating. Such coatings are sufficiently developed diffusion layer, which limits the possibility of deformation of the substrate. This type of coating is mainly applied to the products and semi-finished product used in its original state or after a minor deformation.
When hot-dip galvanizing steel is formed on the surface of a rather complex phase of the coating composition. Metallographic studies allow to distinguish at least five phase coating layers. First, on the surface of steel forming intermetallic phases rich in iron (r- a- and phase), then the coating formed during the crystallization phase and T 6 ,, comprising crystals of intermetallic compounds of zinc and iron in the zinc matrix surface coating layer is a solid solution of iron in zinc (t | — phase). Galvanizing bath is alloyed with cadmium, tin, aluminum (in small amounts) to impart gloss and to improve the corrosion properties of the coating. To improve melt flow the liquid-injected therein microadditive nickel and vanadium. Currently, hot dip galvanizing is the primary means of long-term protection of steel structures and steel sheet strip and fasteners against corrosion.
The coating Galfan
French Research Center has developed a metallurgical Galfan coating that is formed on steel products by immersion in a zinc melt containing 5% aluminum microaddings cerium and lanthanum. This corresponds to the eutectic composition and has a melting point of 380 ° C.
Galfan coating process subject application modes is thinner compared to galvanizing diffusion layer and thus has better formability. The surface layer at a sufficiently high rate of cooling of the product after extraction from the molten eutectic has a structure in connection with this coating is characterized by relatively high corrosive properties.
The company "Betlhem Steel" alloy developed for applying coatings to steel called Galvalyum containing 55% aluminum, 43.4% zinc, 1.6% silicon. This coating differs sufficiently thin diffusion layer and a complicated structure of the surface layer. Silicon melt restrains exothermic reaction in the aluminum-zinc-iron and inhibits the growth of the diffusion layer thickness of the coating. On a surface coating formed dendritic structure in which the carcass consists of aluminum-rich crystals and the matrix — rich alloy of zinc with inclusions of silicon. At high speeds, the cooling of the product after taking it out of the melt is formed by coating with high performance. The coating combines high protective properties of aluminum and protective properties of zinc. The protective properties of the coating most fully manifested in the marine atmosphere.
Filed  in the Table. 1 shows the main characteristics of the coatings applied by immersion in a molten zinc-based and aluminum. Line apply protective metal coatings on steel wire.
Designed and developed node for coating elongated steel products, in particular wire, without bending . The coating is done by moving the wire cleaned and prepared surface through horizontal channels coating chamber, in which the melt level located above the channels 30-40 mm. A special pneumatic system makes the movement of the melt from the crucible furnace and the filling of the chamber, as well as providing conditions retention of the melt from flowing through the horizontal channels in the coating.
To control the movement of the melt, maintaining its target level and to ensure the retention of the melt from flowing through the channels of the chamber coating system designed automatic control unit coating circuit diagram is shown in Figure 1.
In addition, this system makes control and management of the basic parameters of the process of coating: pressure displacing melt from the crucible; vacuum coating chamber; the melt level in the coating chamber; temperature of the melt in the crucible; temperature of the melt in the chamber; speed of movement of the wire, the number of the processed wire.
The automated system is basically designed based on the known technical solutions and equipment is, however, some devices are created at inventions.
To control the pressure in the crucible and the melt in the vacuum chamber coating used sensors from "Motorola", Which analyzes the signals from the analog-to-digital converter control unit (SCU). As the liquid level sensor using a float made of refractory material, which monitors the movement of a specially designed motion sensor based on Hall sensors. The signals from the level sensors come in the SCU. Pressure control of the melt in the crucible and the vacuum in the chamber by means of proportional pressure regulators. SCU is a self-contained unit, established on the basis of the microcontroller, which allows for pre-recorded in a program to produce the original mathematical and logical processing of signals from the sensors and control actuators. The data processed by the SCU (pressure, vacuum, melt level) are reflected on the display, placed on the operator panel. In addition, the SCU to exchange data with a PC. For a personal computer created software to communicate with the SCU, control temperature parameters, displaying data about pressure, vacuum, liquid level. Software allows acquisition and recording of incoming data to the computer in real time. The accumulated data can be displayed graphically or in tabular form. Fig. 2 shows the operating menu.
Node coating consists of a crucible furnace with a crucible resistance of 250 kg aluminum. The lid of the furnace is sealed coating chamber having a vertical passage, plunged into the melt crucible and two horizontal conduit for passing the processed wire. The horizontal channel chamber is filled with the melt, the melt volume in the working chamber depending on the desired level of 25-30 kg as aluminum.
Construction assembly coating without any major changes ensures retention of the melt from flowing through the inlet and outlet diameter of 30.0 mm and thus allows the processing of the entire range of diameters existing wire. Economic expediency, apparently cause the wire diameter of 1.0 mm to apply a coating unit smaller and smaller volumes of the melt. It should be noted that for the same design and size coating unit for different wire diameters needed for high power devices unwinding, transport and winding. Conditionally accepted that, for the application of protective coating on the wire should be used two modifications unwinding devices, transportation and winding. The first modification to the wire diameter of 1.0 to 2.5 mm, the second-from 2.5 to 6, Omm. Moreover, increasing the wire diameter will transform node cleaning and surface preparation.
An experimental obrazetslinii protective coatings of melts based on zinc and aluminum wire with a diameter of 1.0 to 2.5 mm. The maximum capacity of the line about 0.5 thousand. Tons a year in three-shift operation. The maximum speed of the wire is 30 m / min. The coating of the wire is carried out by single-stranded scheme, but construction of the unit coating process allows for multi-line wire circuit. The length of the line, depending on the design and type of winding and unwinding units is from 12.0 to 15.0 meters. The main and auxiliary technological equipment, power and control cabinets, as well as the operator control panel production facilities occupy about 75 m2. General view of the line is shown in Figure 3.
The main technical and economic characteristics of the line are shown in Table. 2. Diagram of the process of applying protective coatings on the metal line is shown in Figure 4.
The established lines can be used by any of the existing methods of cleaning and surface preparation prior to coating the wire with the proviso that the rate of cleaning and surface preparation must match the speed of coating formation. Surface preparation can be carried out by chemical means, by machining abrasive elements or a stream of particles, acupressure high energy (plasma and micro-cleaning) by thermochemical treatment in a reducing atmosphere protective. Preparation of the surface largely determines not only the quality of the coating, but the performance of the line. If the approach in this regard, it should be noted that the single-stranded line capacity of about 2.0 ths. Tons per year, occupying a small production space, with low power consumption can be grouped as the machines in the machine-building enterprise. The advantages of such an approach in the establishment of the required capacity in the field of application of protective metal coatings should include the following: ease of maintenance and operation; carrying out routine maintenance and repair of the line without stopping the entire production (work performed on one line); opportunity within simultaneously by spray coating on the wire of different diameter and flexibly adjust the amount of the coated wire of various diameters; opportunity in the proceedings at the same time to apply coatings of different chemical composition (eg, zinc, aluminum, aluminum-zinc compound) and the flexibility to adjust the amount of output wire with different coatings.
The chosen conceptual approach in the design of individual components and the whole line will allow its use not only in a large steel industries, but also in small enterprises specializing in the processing of steel wire. The approach in the design of lines in the overall project provides a set of standard components and assemblies for cleaning and preparing the surface of the wire; coating; unwinding and winding wire not only different diameter, but of different weight and type of the coils and for conveying wire coils of different diameter, depending on production requirements. In general, the draft provides for the selection of the line number of combinations of different coiling, conveying and winding components and assemblies, meeting the requirements of virtually any production. To create a link on the individual task enough to pick up the database required components and assemblies, and assemble them. A general view of one of the possible modifications of the line of application of protective metal coatings on steel wire (the project) is shown in Figure 5.
Thus, developed a line of application of protective metallic coatings on steel wire dip on the basis of aluminum and zinc in the presence of a set in the general draft standard components that meet the requirements of almost any steel wire production, due to the small space requirements, with sufficient automation of the process can naturally fit in any manufacturing.