Galvanizing is an effective and inexpensive, and therefore a common way to protect ferrous metals from corrosion. Most often it is used in the manufacture of hardware and fasteners, as well as steel mesh.
Zinc coating methods
Anti-corrosion galvanizing is carried out in various ways, and the service life of the coating depends on the thickness of the protective layer.
The method of coating depends on its necessary properties, product dimensions, conditions for its further operation.
The simplest and most technological, but not sufficiently resistant to mechanical influences of the protective layer, is cold galvanizing with the help of primers, in which a large amount of highly dispersed zinc powder is contained.
In terms of galvanizing production, hot galvanizing takes the second place. The coating obtained in this way is high-quality and durable, but environmentally unsafe, since zinc melt is used, and even to maintain its temperature a little less than 500 ยฐ C, a large amount of electricity is needed, chemical methods of surface preparation.
Very similar to hot dip galvanizing is a more technological, but less productive method of thermal diffusion deposition of a protective layer. It is used when high demands are placed on the thickness and appearance of the coating.
Another method of galvanizing is gas-thermal spraying, which is used to protect large-sized products and structures that simply cannot be placed in the bath.
Galvanizing is free from many of the disadvantages of other coating methods and has its positive aspects.
Benefits of Galvanizing
The method of applying a zinc coating by electrolysis is the most common.
The main advantage, because of which galvanizing of metal is used, is a high degree of protection of the surface of the material from corrosion. A thin layer of zinc increases the service life of products several times, which means it reduces the cost of their maintenance and replacement.
The coating is smooth, without drips and drops, both the shape and size of the product are preserved. It can be applied to objects of any, even the most complex form.
Smooth and shiny decorative coatings do not require additional processing in most cases.
In addition, the process of applying a zinc coating requires low costs, and galvanic units have high performance.
Disadvantages of galvanizing
The method of applying a protective coating by electrolysis is not without drawbacks.
The main disadvantage is the low adhesion of zinc to metal, due to which the surface of the product must be carefully cleaned.
Another disadvantage is the formation of toxic waste in the coating process that requires serious cleaning.
Failure to comply with the regimes can lead to the filling of the base metal with hydrogen, which leads to the fragility of the product itself and to a violation of the quality of the coating.
Operating principle
Galvanic galvanizing is based on the principle of protective action, which is determined by the difference in electrochemical potentials of zinc and iron. Since zinc has a lower electrochemical potential, the coating from it is a protective tread for ferrous metals. That is, in a humid environment, it is he who undergoes electrochemical corrosion.
During the oxidation of iron, oxides are formed having a larger volume than the initial metal. The oxide film becomes loose and transmits oxygen to the unoxidized metal. And on zinc, during oxidation, a film is formed thin and dense, it does not pass oxygen into the metal, protecting not only the coating, but also the base metal underneath.
Types of galvanizing
Galvanizing is a technology representing electrolysis, i.e., electrochemical redox processes in an electrolyte under the influence of direct electric current.
According to the composition of the electrolyte, galvanizing is divided into three types: acid, cyanide and alkaline.
The most commonly used method is the application of zinc coating in slightly acidic electrolytes, especially for cast iron and steel parts of complex configuration. Products made of carbon and alloy steels with this type of galvanizing are less susceptible to hydrogen embrittlement, and the appearance is excellent, with a wonderful decorative effect in a wide range of colors.
Extra galvanizing protection
The protective effect of zinc coatings depends on its thickness, which, when galvanically applied, is only 5 microns, and the nature of the electrolyte.
In some cases, the protective properties of the zinc coating are enhanced by passivation, phosphating or painting.
Passivation (chromating) - chemical treatment of products in solutions with chromic acid or its salts, as a result of which chromate films are formed on the surface. This process enhances not so much protective properties as decorative, because as a result, the gloss of the coating increases, and it can be painted in various colors.
When phosphating (processing in salts of phosphoric acid) of galvanized products, a phosphate film forms on the surface. After phosphating, a paintwork may still be applied.
Stages of galvanizing
In production, galvanizing consists of several technological processes, each of which is completed by rinsing with water in a flow bath or brush-washing method.
First, the products are thoroughly cleaned of rust, scale, technological grease, residual coolant or paint and degreased in alkaline solutions. Then electrolytic degreasing takes place.
After it, they are etched in an aqueous solution of hydrochloric acid, in which the surface is finally cleaned without disturbing the surface layer and it is decapitated - activated before applying the zinc coating. Only then is galvanizing proper.
After it, if necessary, the products are clarified and cleaned of the oxide film in an aqueous solution of nitric acid, then phosphating, passivation and drying.
When galvanizing various products, additional operations can be performed. For example, the strip before galvanizing is unwound, the ends are welded, straightened, and after it is oiled and wound.
Galvanizing Equipment
A galvanic line is a certain sequence of washing and technological baths in which a single or multi-layer zinc coating with the required functional properties is created.
In accordance with the volume of production, equipment with varying degrees of mechanization is used. At large enterprises, mechanized lines with automatic control are installed. There are lines with partial or full manual control, as well as mini-lines.
The galvanizing line does not only consist of bathtubs. It includes transport systems of various designs, necessarily equipment for wastewater treatment, auxiliary equipment, which includes devices for galvanizing, heating elements, heat exchangers, cathode and anode rods.
Additional equipment includes ventilation systems, drying chambers and cabinets, refrigeration equipment, filter units, demodulating equipment, pumps.
Electrolytes for galvanizing
For galvanizing, depending on the purpose of the product, electrolytes are used, which are divided into two main groups.
Electrolytes in which zinc is in the form of simple hydrated ions are called simple acidic. These are hydrogen fluoride, sulfate and chloride solutions.
Complex complex acidic and alkaline electrolytes contain zinc in complex ions with a positive and negative charge. These are ammonia, pyrophosphate, cyanide and other solutions.
First of all, the deposition rate depends on which electrolyte is used, and then the quality of zinc deposits on the product (cathode).
Of the complex electrolytes, zinc deposits on the cathode with high ion scattering. With increasing current density, the metal yield decreases and the hydrogen yield increases.
Therefore, galvanizing in complex electrolytes is carried out at a low current density, and the coating is very high quality, fine-grained and uniform.
In mildly acidic simple electrolytes, galvanic galvanizing, including at home, takes place at a high current density, with a greater speed than with complex solutions. The appearance of the products is good, but the coating is not very high quality and is suitable only for products of a fairly simple form.
Galvanizing
The process of direct galvanizing takes place in a bath with electrolyte. Products made of ferrous metal are lowered into it, to which electric current (cathode) is supplied through special electrodes, and pure zinc in the form of balls or plates placed in special mesh sections (anode).
During the electrochemical reaction, under the influence of an electric current with a density of 1 to 5 A / dm, zinc dissolves in the electrolyte, then its ions settle on the cathode, forming a galvanic coating 4โ25 ฮผm thick.
It is with the use of this technology that fasteners (bolts and nuts) with a uniform and shiny coating are obtained.
Automated galvanizing line
The modern galvanizing galvanizing line is a fully automated line on which all stages of coating are carried out, including welding and high-quality degreasing of products for various purposes and configurations.
An automatic line generally consists of a set of technological galvanic bathtubs, modular rectifiers, a loading / unloading stand, transport equipment, equipment for exhaust ventilation, water supply and drainage of sewage, a metal frame with a maintenance ladder.
Electroplating baths can be made of stainless steel, steel, lined with polymeric materials or rubber. Modern bathtubs welded from sheet polymers are increasingly replacing metal containers. The choice of bath material depends on the composition and concentration of the electrolyte and the operating temperature.
Communications of water supply and sewage systems, and in most cases ventilation, are located under the bathtubs and are also made of polypropylene.
The dimensions of the line are determined by its performance and dimensions of the galvanic baths.
Galvanizing occurs with the formation of wastewater with a high concentration of heavy metal ions. Therefore, they are defended, filtered, neutralized, chemical precipitation, sorption, and other processes are used in containers made of engineering polymers.
Electrolytic galvanizing at home
The galvanic galvanizing, made with your own hands, begins with the selection of materials. The electrolyte can be a solution of zinc chloride and hydrochloric acid in distilled water. This is the so-called soldering acid, which is most often used at home. Craftsmen etch zinc in battery sulfuric acid and produce ZnSO 4 electrolyte, but this process is dangerous because explosive hydrogen and heat are released during the reaction. In the electrolyte, in no case should there be a precipitate from undissolved salt crystals.
Pure zinc can be bought at a chemical store or on the radio market, or can be obtained from salt batteries or fuses that have been preserved since the Soviet Union.
A plating bath can be a glass or plastic container. Tripods for the anode and cathode are installed in it. An anode is a zinc plate to which a plus is connected from a power source. The larger the anode, the more uniform the coating on the cathode, the product on which the protective coating will be applied. There can be several anodes; they can be placed around the cathode at the same distance so that its surface is coated with zinc evenly and simultaneously from all sides. The minus of the power source is connected to the cathode.
Even when galvanizing is carried out at home, the technology necessarily includes thorough cleaning and degreasing of the part, as well as its activation in an acid solution.
The power source is a car battery with an incandescent lamp of low power or another consumer in the circuit, so that the current in the circuit is lower, or a power supply with a constant voltage at the output. The main thing is that there should be no rapid boiling of the electrolyte during the galvanizing process.
Galvanizing itself occurs when the anodes and cathode are lowered into the electrolyte and the circuit is closed. The longer the process lasts, the thicker the layer of zinc is obtained on the product.
Using galvanic galvanizing, the protective coating on the products becomes accurate, uniform and smooth, with a decorative effect. It is used both in industry and at home, despite the fact that sewage treatment from environmentally hazardous waste is required.