Galvanic galvanizing of metal: technology, equipment

Galvanic galvanizing is an efficient and inexpensive, and therefore widespread, method for protecting ferrous metals from corrosion. Most often it is used in the manufacture of hardware and fasteners, as well as steel mesh.

Methods of zinc coating

Anticorrosive 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 required properties, the dimensions of the product, the conditions for its further operation.

The simplest and most technological, but insufficiently resistant to the mechanical effects of the protective layer, is cold galvanizing with the help of soils containing high-dispersed zinc powder in large quantities.

In terms of the volume of zinc production, the second place is occupied by hot-dip galvanizing. The coating obtained in this way, quality and durable, but environmentally unsafe, because zinc melt is used, and even at a temperature of just below 500 ° C, a large amount of electricity, chemical methods of surface preparation is needed.

Very similar to hot-dip galvanizing is a more technologically, but less efficient, method of thermal diffusion application of the protective layer. It is used when high requirements are imposed on the thickness and appearance of the coating.

Another method of galvanizing is gas-thermal spraying, which is used to protect bulky items and structures that can not be placed in a bathtub.

Galvanic galvanizing is devoid of many disadvantages of other coating methods and has its positive aspects.

Advantages of galvanic galvanizing

The method of depositing a zinc coating by electrolysis is most common.

The main advantage, due to which galvanic galvanizing of metal is applied , is a high degree of protection of the surface of the material from corrosion. A thin layer of zinc extends the service life of the products several times, and therefore reduces the costs of their maintenance and replacement.

The coating is smooth, without streaks and drops, the shape and size of the product are preserved. You can apply it 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 very process of applying zinc coating requires low costs, and galvanic aggregates have high productivity.

Disadvantages of galvanic galvanizing

The method of applying a protective coating with electrolysis is not devoid of shortcomings.

The main disadvantage is the low adhesion of zinc to the metal, because of which the surface of the product must be carefully cleaned.

The disadvantage is also the formation of toxic waste in the process of coating, requiring serious cleaning.

Failure to comply with the regimes may lead to the filling of the base metal with hydrogen, which leads to the fragility of the product itself and to a deterioration in the quality of the coating.

Operating principle

Galvanic galvanizing is based on the principle of protective action, which is determined by the difference in the electrochemical potentials of zinc and iron. Since zinc has a smaller electrochemical potential, the coating from it is a protective protection for ferrous metals. That is, in a damp environment of electrochemical corrosion, it is he.

Oxidation of iron produces oxides having a larger volume than the original metal. The oxide film becomes loose and passes oxygen to the metal not yet oxidized. And on zinc during oxidation, the film forms thin and dense, it does not let oxygen into the metal, protecting not only the coating, but also the underlying metal beneath it.

Types of galvanic galvanizing

Galvanic galvanizing is a technology that is an electrolysis, that is, electrochemical oxidation-reduction processes in an electrolyte under the action of a constant electric current.

According to the composition of the electrolyte, the galvanizing is divided into three types: acidic, cyanide and alkaline.

The most commonly used method is zinc coating in weakly acid electrolytes, especially for cast iron and steel parts of complex configuration. Products made of carbon and alloy steels with this kind of zinc coating are less exposed to hydrogen embrittlement, and the appearance is excellent, with a remarkable decorative effect in a wide range of colors.

Additional protection for galvanizing

The protective action of zinc coatings depends on its thickness, which when galvanically applied is only 5 μm, and the nature of the electrolyte.

In some cases, the protective properties of the zinc coating are increased 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 the protective properties as the decorative properties, because as a result the luster of the coating increases, and it can be colored in different colors.

When phosphating (treatment in phosphoric acid salts) of galvanized products, a phosphate film forms on the surface of the film. After phosphating, paintwork may still be applied.

Stages of galvanic galvanizing

At the production galvanic 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 from rust, scales, process grease, coolant residues or paint and degreased in alkaline solutions. Then the electrolytic degreasing takes place.

After it is etched in an aqueous solution of hydrochloric acid, in which the surface is finally cleaned without disturbing the surface layer and its pickling takes place - activation before the application of the zinc coating. Only then is the actual zincification.

Afterwards, 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 different products, additional operations can be performed. For example, strip before galvanizing unwind, weld ends, rule, and after it oil and coil.

Galvanizing equipment

Galvanic line - this is a certain sequence of washing and technological baths, in which one or multi-layer zinc coating with the required functional properties is created.

In accordance with the volume of production, equipment with various 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 galvanizing line consists not only of baths. It includes transport systems of various designs, necessarily equipment for wastewater treatment, ancillary equipment, which include galvanizing devices, heaters, heat exchangers, cathode and anode rods.

Additional equipment includes ventilation systems, drying chambers and cabinets, refrigeration equipment, filter units, equipment for obtaining demovods, pumps.

Electrolytes for galvanizing

For galvanic 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 borofluoride, sulfate and chloride solutions.

Complex complex acidic and alkaline electrolytes contain zinc in complex ions with positive and negative charges. These are ammonia, pyrophosphate, cyanide and other solutions.

On the basis of which electrolyte is used, the precipitation rate firstly depends, and then the quality of zinc precipitation on the product (cathode).

Of complex electrolytes, zinc settles on the cathode with high ion scattering. As the current density increases, the yield of the metal decreases and the yield of hydrogen increases.

Therefore, galvanizing in complex electrolytes is performed at low current density, and the coating turns out to be very high-quality, fine-grained and uniform.

In weakly acidic simple electrolytes, galvanic galvanizing, including at home, is carried out at a high current density, which is faster 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 shape.

Galvanic Galvanizing

The direct galvanizing process takes place in the bath with the electrolyte. Black metal products are dropped into it, to which electric current (cathode) is fed through special electrodes, and pure zinc in the form of balls or plates, laid in special mesh sections (anode).

In the process of electrochemical reaction under the influence of electric current density from 1 to 5 A / dm zinc dissolves in the electrolyte, then its ions settle on the cathode, forming a galvanic coating 4-25 microns thick.

It is with the use of this technology is obtained fasteners (bolts and nuts) with a uniform and shiny coating.

Automated line for galvanic galvanizing

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 different purposes and configurations.

The automatic line in general consists of a set of technological galvanic baths, modular rectifiers, loading / unloading stand, transport equipment, equipment for exhaust ventilation, water supply and discharge of sewage, metal frame with a service ladder.

Galvanic baths can be made of stainless steel, steel lined with polymeric materials or rubber. Modern baths, 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.

Communication of water supply and sewage systems, and in most cases ventilation is located under the baths and also made of polypropylene.

The dimensions of the line are determined by its performance and the dimensions of the galvanic baths.

Galvanic 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 tanks made of engineering polymers.

Electrolytic zinc coating at home

Galvanic galvanizing, produced by hand, 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. The craftsmen etch zinc in storage sulfuric acid and get the ZnSO ₄ electrolyte, but this process is dangerous, since explosive hydrogen and heat are released during the reaction. In the electrolyte, in no case should there be a precipitate of undissolved crystals of salt.

Pure zinc can be bought at a chemical store or on the radio market, but can be obtained from salt batteries or fuses that have been preserved since the times of the Soviet Union.

The galvanic bath can be a glass or plastic container. It supports tripods for the anode and cathode. The anode is a zinc plate, to which a plus is connected from the power source. The larger the anode, the more uniform the coating will be on the cathode, the product to which the protective coating will be applied. Anodes can be several, they can be placed around the cathode at the same distance, so that its surface is covered with zinc evenly and simultaneously from all sides. A "minus" of the power supply is connected to the cathode.

Even when galvanizing is performed at home, the technology necessarily includes thorough cleaning and degreasing of the part, as well as its activation in the acid solution.

A power source is a car battery with an incandescent light bulb or another consumer in the circuit, so that the current in the circuit is lower, or a power supply with a constant output voltage. The main thing is that there should not be a rapid boiling of the electrolyte in the process of galvanizing.

The actual zincification occurs when the anodes and cathode are lowered into the electrolyte and the electrical circuit is closed. The longer the process lasts, the thicker the zinc layer is on the product.

With the help of galvanic galvanizing, the protective coating on the products becomes precise, uniform and smooth, with a decorative effect. It is used both in industry and at home, despite the fact that waste water treatment from environmentally hazardous waste is required.