Nickel plating- applying a nickel coating to the surface of products (thickness, as a rule, from 1-2 to 40-50 microns).

Nickel plating of metals at home is a completely feasible process.

The item must be prepared before nickel plating. Treat it with sandpaper to remove the oxide film, wipe with a brush, rinse thoroughly with water, degrease in a hot soda solution and rinse again.

There are two methods of nickel plating: electrolytic and chemical.

Electrolytic nickel plating of metals at home

Before nickel plating, pre-plate the metal object.

Prepare electrolyte (30 g nickel sulfate, 3.5 g nickel chloride and 3 g boric acid per 100 ml of water) and pour this electrolyte into the container. Nickel plating requires nickel electrodes - anodes. Dip them into electrolyte. Hang the part between them on a wire. Connect the wires that come from the nickel plates together and connect them to the positive pole of the current source, and the part to the negative pole; include a rheostat in the circuit to regulate the current, and a milliammeter (tester). Source direct current with a voltage of no more than 6 V.

Turn on the current for about twenty minutes. Remove the part, rinse and dry it. It is covered with a grayish matte layer of nickel. In order for the coating to acquire its usual shine, it must be polished.

The disadvantages of electrolytic nickel plating are the uneven deposition of nickel on the relief surface and the impossibility of coating narrow and deep holes, cavities, etc.

Electroless nickel plating

In addition to the galvanic method, you can use the following, very simple method to coat polished steel or iron with a thin but very durable layer of nickel.

Take a 10% solution of pure zinc chloride and gradually add it to a solution of nickel sulfate until the liquid turns bright green, then it is slowly heated to a boil, preferably in a porcelain vessel. The turbidity that may appear does not have any effect on nickel plating, which is carried out as follows: when the above-mentioned liquid is brought to a boil, the object to be nickel-plated is dipped into it, and the latter must first be thoroughly cleaned and degreased. The item is boiled in the solution for about an hour, adding distilled water from time to time as it evaporates. If during boiling it is noticed that the color of the liquid instead of bright green has become weak green, then add nickel sulfate little by little until the original color is obtained.

After the specified time has passed, the object is removed from the solution, washed in water in which it is not dissolved. a large number of chalk and dry thoroughly. Polished iron or steel, coated with nickel in this manner, holds this coating very firmly.

Properties and applications of the coating. The basis of the chemical nickel plating process is the reduction of nickel from aqueous solutions of its salts with sodium hypophosphite. Industrial Application obtained methods for the deposition of nickel from alkaline and acidic solutions. The deposited coating has a semi-shiny metallic look, fine-crystalline structure and is an alloy of nickel and phosphorus. The phosphorus content in the sediment depends on the composition of the solution and ranges from 4-6% for alkaline solutions to 8-10% for acidic solutions.

In accordance with the phosphorus content, the physical constants of the nickel-phosphorus deposit also change. Specific gravity it is 7.82-7.88 g/cm 3 , melting point 890-1200°, electrical resistivity is 0.60 ohm mm 2 /m. After heat treatment at 300-400°, the hardness of the nickel-phosphorus coating increases to 900-1000 kg/mm ​​2. At the same time, the adhesion strength increases many times over.

The indicated properties of nickel-phosphorus coating also determine its areas of application.

It is advisable to use it for coating parts with complex profiles, the inner surface of tubes and coils, for uniform coating of parts with very precise dimensions, for increasing the wear resistance of rubbing surfaces and parts exposed to temperature influences, for example, for coating molds.

Parts made of ferrous metals, copper, aluminum and nickel are subjected to nickel-phosphorus coating.

This method is not suitable for depositing nickel on metals or coatings such as lead, zinc, cadmium and tin.

Nickel precipitation from alkaline solutions. Alkaline solutions are characterized by high stability, ease of adjustment, lack of tendency to rapid and instant precipitation of powdered nickel (self-discharge phenomenon) and the possibility of their long-term operation without replacement.

The nickel deposition rate is 8-10 microns/hour. The process proceeds with intense release of hydrogen on the surface of the Parts.

Preparation of the solution consists of dissolving each of the components separately, after which they are poured together into working bath, with the exception of sodium hypophosphite. It is added only when the solution is heated to operating temperature and the parts are prepared for coating.

Preparing the surface of steel parts for coating has no specific features.

After heating the solution to operating temperature, it is adjusted with a 25% ammonia solution until stable of blue color, add sodium hypophosphite solution, hang the parts and begin coating without preliminary treatment. The solution is adjusted mainly with ammonia and sodium hypophosphite. With a large volume of nickel plating bath and a high specific loading of parts, the solution is adjusted with ammonia directly from a cylinder with gaseous ammonia, with a continuous supply of gas to the bottom of the bath through a rubber tube.

For ease of adjustment, a solution of sodium hypophosphite is prepared with a concentration of 400-500 g/l.

A solution of nickel chloride is usually prepared for adjustment together with ammonium chloride and sodium citrate. For this purpose, it is most advisable to use a solution containing 150 g/l nickel chloride, 150 g/l ammonium chloride and 50 g/l sodium citrate.

The specific consumption of sodium hypophosphite per 1 dm 2 of the coating surface, with a layer thickness of 10 μm, is about 4.5 g, and nickel, in terms of metal, is about 0.9 g.

The main problems during the chemical deposition of nickel from alkaline solutions are given in Table. 8.

Nickel precipitation from acidic solutions. Unlike alkaline solutions, acidic solutions are characterized by a wide variety of additives to solutions of nickel and hypophosphite salts. So, sodium acetate, succinic, tartaric and lactic acids, Trilon B and other organic compounds can be used for this purpose. Among the many compositions, below is a solution with the following composition and precipitation mode:

The pH value should be adjusted with a 2% sodium hydroxide solution. The nickel deposition rate is 8-10 microns/hour.

Overheating the solution above 95° can lead to self-discharge of nickel with the instantaneous precipitation of a dark spongy sediment and the solution splashing out of the bath.

The solution is adjusted according to the concentration of its constituent components only until 55 g/l of sodium phosphite NaH 2 PO 3 accumulates in it, after which nickel phosphite can fall out of the solution. Once the specified concentration of phosphite is reached, the nickel solution is drained and replaced with a new one.

Heat treatment. In cases where nickel is applied to increase surface hardness and wear resistance, the parts are subjected to heat treatment. At high temperatures, the nickel-phosphorus deposit forms a chemical compound, which causes a sharp increase in its hardness.

The change in microhardness depending on the heating temperature is shown in Fig. 13. As can be seen from the diagram, the greatest increase in hardness occurs in the temperature range of 400-500°. When choosing a temperature regime, it should be taken into account that for a number of steels that have undergone hardening or normalization, high temperatures are not always acceptable. In addition, heat treatment carried out in air causes the appearance of tarnished colors on the surface of parts, turning from golden yellow to purple. For these reasons, the heating temperature is often limited to 350-380°. It is also necessary that the nickel-plated surfaces be clean before placing them in the oven, since any contamination is revealed very intensively after heat treatment and can only be removed by polishing. Heating time is 40-60 minutes. is sufficient.

Equipment and accessories. The main task in the manufacture of equipment for chemical nickel plating is the selection of bath linings that are resistant to acids and alkalis and are thermally conductive. For experimental work and for coating small parts use porcelain and steel enameled bathtubs.

When coating large products in baths with a capacity of 50-100 liters or more, enameled tanks with enamels that are resistant to strong nitric acid are used. Some factories use steel cylindrical baths lined with a coating consisting of glue No. 88 and powdered chromium oxide taken in equal weight quantities. Chromium oxide can be replaced with micro-emery powders. The coating is carried out in 5-6 layers with intermediate air drying.

At the Kirov plant, lining cylindrical baths with removable plastic covers is successfully used for this purpose. If it is necessary to clean the baths, the solutions are pumped out, and the covers are removed and treated in nitric acid. Carbon steel should be used as material for pendants and baskets. Insulation of individual sections of parts and suspensions is carried out with perchlorovinyl enamels or plastic compound.

To heat the solution, electric heaters with heat transfer through a water jacket should be used. Heat treatment of small parts is carried out in thermostats. For large products, shaft furnaces with automatic temperature control are used.

Nickel plating of stainless and acid-resistant steels. Nickel plating is carried out to increase surface hardness and wear resistance, as well as to protect against corrosion in those aggressive environments in which these steels are unstable.

For the adhesion strength of the nickel-phosphorus layer to the surface of high-alloy steels, the method of preparation for coating is decisive. So, for stainless steel grade 1×13 and it similar preparation surface treatment consists of its anodic treatment in alkaline solutions. The parts are mounted on carbon steel hangers, using internal cathodes if necessary, and hung in a bath with a 10-15 percent solution caustic soda and perform their anodic treatment at an electrolyte temperature of 60-70° and an anodic current density of 5-10 A/dm 2 for 5-10 minutes. until a uniform brown coating without metal gaps forms. The parts are then washed in cold running water, decapitated in hydrochloric acid (specific gravity 1.19), diluted by half, at a temperature of 15-25° for 5-10 seconds. After washing in cold running water, the parts are hung in an electrochemical nickel plating bath in an alkaline solution and coated in the usual manner to a given layer thickness.

For parts made of acid-resistant steel type IX18H9T, anodic treatment must be carried out in a chromic acid electrolyte with the following composition and process mode:

After anodic treatment, the parts are washed in cold running water, pickled in hydrochloric acid, as indicated for of stainless steel, and hung in a nickel plating bath.

Nickel plating of non-ferrous metals. To deposit nickel onto a previously deposited layer of nickel, the parts are degreased and then pickled in a 20-30% solution of hydrochloric acid for 1 minute, after which they are hung in a bath for chemical nickel plating. Parts made of copper and its alloys are nickel-plated in contact with a more electronegative metal, such as iron or aluminum, using wire or pendants made of these metals for this purpose. In some cases, for a deposition reaction to occur, it is enough to briefly touch the iron rod to the surface of the copper part.

For nickel plating of aluminum and its alloys, parts are etched in alkali, brightened in nitric acid, as is done before all types of coatings, and subjected to double zincate treatment in a solution containing 500 g/l caustic soda and 100 g/l zinc oxide, at a temperature 15-25°. The first immersion lasts 30 seconds, after which the contact zinc deposit is etched off in dilute nitric acid, and the second immersion is 10 seconds, after which the parts are washed in cold running water and nickel-plated in a bath with an alkaline nickel-phosphorus solution. The resulting coating is very weakly bonded to aluminum, and to increase the adhesion strength, the parts are heated by immersing them in lubricating oil at a temperature of 220-250° for 1-2 hours.

After heat treatment, the parts are degreased with solvents and, as necessary, wiped, polished or subjected to other types of mechanical treatment.

Nickel plating of cermets and ceramics. The technological process of nickel plating of ferrites consists of the following operations: parts are degreased in a 20% solution soda ash, washed with hot distilled water and etched for 10-15 minutes. in an alcoholic solution of hydrochloric acid with a component ratio of 1:1. Then the parts are washed again with hot distilled water while simultaneously cleaning the sludge with hair brushes. A solution of palladium chloride with a concentration of 0.5-1.0 g/l and a pH of 3.54:0.1 is applied to the surfaces of the parts to be coated. After air drying, the application of palladium chloride is repeated again, dried and immersed for preliminary nickel plating in a bath with an acidic solution containing 30 g/l nickel chloride, 25 g/l sodium hypophosphite and 15 g/l sodium succinate. For this operation, it is necessary to maintain the solution temperature within 96-98° and pH 4.5-4.8. Then the parts are washed in distilled water hot water and nickel-plated in the same solution, but at a temperature of 90°, until a layer 20-25 microns thick is obtained. After this, the parts are boiled in distilled water, copper plated in a pyrophosphate electrolyte until a layer of 1-2 microns is obtained, and then subjected to acid-free soldering. The adhesion strength of the nickel-phosphorus coating to the ferrite base is 60-70 kg/cm2.

In addition, they undergo chemical nickel plating different kinds ceramics, such as ultraporcelain, quartz, steatite, piezoceramics, tikond, thermokond, etc.

Nickel plating technology consists of the following operations: parts are degreased with alcohol, washed in hot water and dried.

After this, for parts made of tikond, thermokond and quartz, their surface is sensitized with a solution containing 10 g/l tin chloride SnCl 2 and 40 ml/l hydrochloric acid. This operation is performed with a brush or by rubbing with a wooden washer moistened with a solution, or by immersing parts in the solution for 1-2 minutes. Then the surface of the parts is activated in a solution of palladium chloride PdCl 2 2H 2 O.

For ultraporcelain, a heated solution with a PdCl 2 ·2H 2 O concentration of 3-6 g/l and an immersion duration of 1 second is used. For tikond, thermokond and quartz, the concentration decreases to 2-3 g/l with an increase in exposure from 1 to 3 minutes, after which the parts are immersed in a solution containing calcium hypophosphite Ca(H 2 PO 2) 2 in an amount of 30 g/l, without heating, for 2-3 minutes.

Ultra porcelain parts with an activated surface are hung for 10-30 seconds. into a pre-nickel plating bath with an alkaline solution, after which the parts are washed and hung again in the same bath to build up a layer of a given thickness.

Parts made of tikond, thermokond and quartz after treatment in calcium hypophosphite are nickel-plated in acidic solutions.

Chemical deposition of nickel from carbonyl compounds. When nickel tetracarbonyl vapor Ni(CO) 4 is heated at a temperature of 280°±5, a reaction of thermal decomposition of carbonyl compounds occurs with the deposition of metallic nickel. The deposition process occurs in a hermetically sealed container at atmospheric pressure. The gas environment consists of 20-25% (by volume) nickel tetracarbonyl and 80-75% carbon monoxide CO. The admixture of oxygen in the gas is permissible no more than 0.4%. To ensure uniform deposition, gas circulation should be created with a supply speed of 0.01-0.02 m/sec and reversing the supply direction every 30-40 seconds. . Preparing parts for coating involves removing oxides and grease. The nickel deposition rate is 5-10 μ/min. Precipitated nickel has a matte surface, a dark gray tint, a fine-crystalline structure, a Vickers hardness of 240-270 and relatively low porosity.

The adhesion strength of the coating to the metal of the product is very low and to increase it to satisfactory values, heat treatment at 600-700° for 30-40 minutes is necessary.

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We temporarily do not apply vacuum coatings

Due to the modernization of the vacuum coating section, we are temporarily not performing vacuum coating work.

ISO 9000 Certificate

The quality management system at our enterprise complies with ISO 9000

Application of titanium nitride

We vacuum-spray titanium nitride (TiN) onto products with dimensions up to 2500x2500x2500 mm.

Brass plating and bronzing

It became possible to carry out work on decorative application brass and bronze

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In connection with the long-awaited expansion of production, we moved to a new site in Balashikha. For your convenience, it is now possible to pick up/delivery parts using our vehicles!

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N - Nickel plating

• Coating codes: N, N.b., Khim.N.tv, Khim.N, N.m.ch.
• Processed steels: any, including aluminum and titanium alloys
• Product dimensions: up to 1000x1000x1000 mm. Weight up to 3 tons.
• Coating of products of any complexity
• Quality control department, quality certificate, work within the framework of the state defense order

general information

Nickel plating is a process of electroplating or chemical deposition of nickel with a thickness of 1 micron to 100 microns.
Nickel coatings have high corrosion resistance, high hardness and good decorative properties.

Nickel melting point: 1445° C
Microhardness of nickel coatings: up to 500 HV (chemical 800 HV)

The applications of nickel-plated parts depend on whether the nickel coating is used as a finishing coating, or whether the nickel coating acts as a sublayer (substrate) for the application of other electroplating coatings.
Nickel coatings can be applied to almost all metals.

The main areas of application of galvanic and chemical nickel plating:

Use of nickel as an independent coating

• For decorative purposes.
  Nickel coatings have a good mirror shine and practically do not fade in air. The coatings withstand operation well in atmospheric conditions due to their high corrosion resistance. Nickel is often used to coat decorative items, fences, equipment and tools.
• For technical purposes.
  For corrosion protection electrical contacts or mechanisms operating in a humid environment, as well as as a coating for soldering. The black nickel plating process has become widespread in the optical industry.

Use of nickel in combination with other electroplating coatings

• When applying multilayer protective and decorative coatings.
  Typically in combination with copper and chromium (copper plating, nickel plating, chromium plating) and other metals as an intermediate layer to increase the gloss of the chrome plating, as well as for corrosion protection and to prevent copper from diffusion through the pores of the chromium to the surface, which can lead to short-lived time for red spots to appear on the chrome plating.

Examples of nickel-plated parts

Nickel plating technology

During electrochemical deposition of nickel on the cathode, two main processes occur: Ni 2+ + 2e - → Ni and 2Н + + 2е - → Н 2.

As a result of the discharge of hydrogen ions, their concentration in the near-cathode layer decreases, i.e., the electrolyte becomes alkalized. In this case, basic nickel salts can form, which affect the structure and mechanical properties of the nickel coating. The release of hydrogen also causes pitting - a phenomenon in which hydrogen bubbles, lingering on the surface of the cathode, prevent the discharge of nickel ions in these places. Pits form on the coating and the sediment loses its decorative appearance.

To combat pitting, substances are used that reduce the surface tension at the metal-solution interface.

During anodic dissolution, nickel is easily passivated. When passivating anodes in the electrolyte, the concentration of nickel ions decreases and the concentration of hydrogen ions quickly increases, which leads to a drop in current efficiency and deterioration in the quality of the deposits. To prevent passivation of anodes, activators are introduced into nickel plating electrolytes. Such activators are chlorine ions, which are introduced into the electrolyte in the form of nickel chloride or sodium chloride.

Nickel sulfate electrolytes are most widely used. These electrolytes are stable in operation, with correct operation they can be used for several years without replacement. Composition of some electrolytes and nickel plating modes:

Sodium sulfate and magnesium sulfate are introduced into the electrolyte to increase the electrical conductivity of the solution. The conductivity of sodium solutions is higher, but in the presence of magnesium sulfate, lighter, softer and more easily polished deposits are obtained.

Nickel electrolyte is very sensitive to even small changes in acidity. To maintain the pH value within the required limits, it is necessary to use buffer compounds. Boric acid is used as such a compound that prevents a rapid change in the acidity of the electrolyte.

To facilitate the dissolution of the anodes, sodium chloride salts are introduced into the bath.

To prepare nickel sulfate electrolytes, it is necessary to dissolve all components in separate containers in hot water. After settling, the solutions are filtered into a working bath. The solutions are mixed, the pH of the electrolyte is checked and, if necessary, adjusted with a 3% sodium hydroxide solution or a 5% sulfuric acid solution. Then the electrolyte is adjusted with water to the required volume.

If there are impurities, it is necessary to work on the electrolyte before using it, since nickel electrolytes are extremely sensitive to foreign impurities, both organic and inorganic.
Defects during the operation of bright nickel plating electrolyte and methods for eliminating them are given in Table 1.

Table 1. Defects during operation of sulfuric acid electrolytes of nickel plating and methods for their elimination

When nickel plating, hot-rolled anodes are used, as well as non-passivable anodes. Anodes in the form of plates (cards) are also used, which are loaded into covered titanium baskets. Card anodes promote uniform dissolution of nickel. To avoid contamination of the electrolyte with anode sludge, nickel anodes should be enclosed in fabric covers, which are pre-treated with a 2-10% solution of hydrochloric acid.
The ratio of the anodic surface to the cathodic surface during electrolysis is 2:1.

Nickel plating of small parts is carried out in bell and drum baths. When nickel plating in bell baths, an increased content of chloride salts in the electrolyte is used to prevent passivation of the anodes, which can occur due to a mismatch between the surfaces of the anodes and cathodes, as a result of which the concentration of nickel in the electrolyte decreases and the pH value decreases. It can reach such limits that the deposition of nickel stops altogether. A disadvantage when working in bells and drums is also the large carryover of electrolyte with parts from the baths. The specific loss rates range from 220 to 370 ml/m2.

For the protective and decorative finishing of parts, shiny and mirror nickel coatings obtained directly from electrolytes with shine-forming additives are widely used. Electrolyte composition and nickel plating mode:

Nickel sulfate - 280-300 g/l
Nickel chloride - 50-60 g/l
Boric acid - 25-40 g/l
Saccharin 1-2 g/l
1,4-butynediol - 0.15-0.18 ml/l
Phthalimid 0.02-0.04 g/l
pH = 4-4.8
Temperature = 50-60°C
Current density = 3-8 A/dm2

To obtain shiny nickel coatings, electrolytes with other brightening additives are also used: chloramine B, propargyl alcohol, benzosulfamide, etc.
When applying a shiny coating, intensive mixing of the electrolyte with compressed air is necessary, preferably in combination with swinging the cathode rods, as well as continuous filtration of the electrolyte,
The electrolyte is prepared as follows. In distilled or deionized hot (80-90°C) water, dissolve nickel sulfate, nickel chloride, and boric acid with stirring. The electrolyte brought to the working volume with water is subjected to chemical and selective purification.

To remove copper and zinc, the electrolyte is acidified with sulfuric acid to pH 2-3, large area cathodes made of corrugated steel are hung and the electrolyte is processed for 24 hours at a temperature of 50-60 ° C, stirring with compressed air. Current density 0.1-0.3 A/dm2. Then the pH of the solution is adjusted to 5.0-5.5, after which potassium permanganate (2 g/l) or a 30% solution of hydrogen peroxide (2 ml/l) is introduced into it.
The solution is stirred for 30 minutes, 3 g/l of activated carbon treated with sulfuric acid is added, and the electrolyte 3-4 is mixed using compressed air. The solution settles for 7-12 hours, then filters into the working bath.

Brightening agents are introduced into the purified electrolyte: saccharin and 1,4-butynediol directly, phthalimide - pre-dissolved in a small amount of electrolyte heated to 70-80 ° C. The pH is adjusted to the required value and work begins. The consumption of brightening agents when adjusting the electrolyte is: saccharin 0.01-0.012 g/(Ah); 1,4-butindiol (35% solution) 0.7-0.8 ml/(Ah); phthalimide 0.003-0.005 g/(Ah).
Defects during the operation of bright nickel plating electrolyte and methods for eliminating them are given in Table 2.

Table 2. Defects during operation of bright nickel plating electrolyte and methods for eliminating them

Multilayer nickel plating is used to increase the corrosion resistance of nickel coatings compared to single-layer coatings.
This is achieved by sequential deposition of nickel layers from several electrolytes with different physical and chemical properties of the coating. Multilayer nickel coatings include: bi-nickel, tri-nickel, sil-nickel.

The corrosion resistance of bi-nickel coatings is 1.5-2 notches higher than single-layer coatings. It is advisable to use them instead of single-layer matte and shiny nickel coatings.

To achieve high corrosion resistance, the first layer of nickel (matte or semi-shiny), constituting at least 1/2 - 2/3 of the total coating thickness, deposited from a standard electrolyte, contains virtually no sulfur. The second layer of nickel is deposited from a bright nickel plating electrolyte; sulfur contained in organic brighteners is part of the nickel coating, while the electrode potential of the second shiny layer shifts by 60-80 mV towards electronegative values ​​relative to the first layer. Thus, the shiny nickel layer becomes the anode in the galvanic couple and protects the first layer from corrosion.

Three-layer nickel plating has the highest corrosion resistance. With this method, after deposition of the first layer of nickel from the same electrolyte as in double-layer nickel plating, a middle layer of nickel is deposited from the electrolyte, which contains a special sulfur-containing additive, ensuring the inclusion of a large amount of sulfur (0.15-0.20%) in composition of the nickel intermediate layer. Then the third one is applied upper layer from electrolyte to obtain shiny coatings. At the same time, the intermediate layer, acquiring the most electronegative potential, protects the nickel layers in contact with it from corrosion.

In the automotive industry, two-layer nickel plating of the sil-nickel type is used. The first layer of nickel is applied from a bright nickel plating electrolyte. The parts are then transferred to a second electrolyte, where sil-nickel is deposited. Non-conducting highly dispersed kaolin powder is added to the composition of this electrolyte in an amount of 0.3-2.0 g/l. Temperature 50-60°C, current density 3-4 A/dm2. The process is carried out without continuous filtration. To ensure uniform distribution of kaolin particles throughout the entire volume of the electrolyte, intensive air mixing is used. The sil-nickel layer increases the wear resistance of the coating and has high corrosion resistance.

Sil-nickel is used as the last layer before chrome in a protective and decorative coating. Due to the high dispersion of inert particles, a thin layer of sil-nickel (1-2 microns) does not change decorative look shiny nickel-plated surface, and with subsequent chrome plating allows you to obtain microporous chromium, which increases the corrosion resistance of the coating.

Removal of defective nickel coatings is carried out by anodic dissolution of nickel in an electrolyte consisting of sulfuric acid diluted to a density of 1.5-1.6.103 kg/m 3. Temperature 15-25°C, anode current density 2-5 A/dm 2.

Along with electrolytic nickel plating, the chemical nickel plating process is widely used, based on the reduction of nickel from aqueous solutions using a chemical reducing agent. Sodium hypophosphite is used as a reducing agent.
Electroless nickel plating used for nickel coating of parts of any configuration. Chemically reduced nickel has high corrosion resistance, great hardness and wear resistance, which can be significantly increased by heat treatment (after 10-15 minutes of heating at a temperature of 400°C, the hardness of chemically deposited nickel increases to 8000 MPa). At the same time, the adhesion strength also increases. Nickel coatings reduced with hypophosphite contain up to 15% phosphorus. The reduction of nickel by hypophosphite proceeds according to the reaction NiCl 2 + NaH 2 PO 2 + H 2 O → NaH 2 PO 3 + 2HCl + Ni.

At the same time, hydrolysis of sodium hypophosphite occurs. Degree beneficial use HPP is taken about 40%.

The reduction of nickel from its salts by hypophosphite occurs spontaneously only on iron group metals that catalyze this process. To coat other catalytically inactive metals (for example, copper, brass), it is necessary to contact these metals in solution with aluminum or other metals that are more electronegative than nickel. For this purpose, surface activation is used by treatment in a solution of palladium chloride (0.1-0.5 g/l) for 10-60 s. On some metals, such as lead, tin, zinc, cadmium, a nickel coating does not form even when using the contact and activation method.
Chemical deposition of nickel is possible from both alkaline and acidic solutions. Alkaline solutions are characterized by high stability and ease of adjustment. Composition of the solution and nickel plating mode:

Nickel chloride - 20-30 g/l
Sodium hypophosphite - 15-25 g/l
Sodium citrate - 30-50 g/l
Ammonium chloride 30-40 g/l
Ammonia aqueous, 25% - 70-100 ml/l
pH = 8-9
Temperature = 80-90°C

Coatings obtained in acidic solutions are characterized by less porosity than those obtained from alkaline solutions (at a thickness above 12 microns, coatings are practically non-porous). The following composition (g/l) and nickel plating mode are recommended for acidic solutions of chemical nickel plating:

Nickel sulfate - 20-30 g/l
Sodium acetate - 10-20 g/l
Sodium hypophosphite - 20-25 g/l
Thiourea 0.03 g/l
Acetic acid (glacial) - 6-10 ml/l
pH = 4.3-5.0
Temperature = 85-95°C
Deposition rate = 10-15 µm/h

Chemical nickel plating is carried out in glass, porcelain or iron enameled baths. Carbon steel is used as the suspension material.
IN Lately a nickel-boron alloy is chemically coated using boron-containing compounds as a reducing agent - sodium borohydride and dimethyl borate, which have a higher reducing ability compared to hypophosphite.
The resulting nickel-boron alloy coatings have high wear resistance and hardness.

To estimate the cost of work, please send a request by email[email protected]
It is advisable to attach a drawing or sketch of the products to your request, as well as indicate the number of parts.

In the price section it is indicated cost of nickel plating of products

NICKEL PLATING, the technical process of applying to the surface of metals b. or m. thin film of metallic nickel or nickel alloys; the purpose of this application is to reduce metal corrosion, increase the hardness of the outer layer, increase or change the reflectivity of the surface, give it more beautiful view. First discovered by Boettger in 1842 and industrialized in the United States since 1860, nickel plating has now become one of the most widely adopted metal plating methods by industry.

The existing numerous methods of nickel plating can be divided into two main groups: contact methods and methods galvanic; nowadays the latter are especially often resorted to. The application of nickel film is used on the surfaces of various metals, and in accordance with the nature of nickel plating they can be divided into groups: 1) copper, brass, bronze, zinc, 2) iron, 3) tin, lead and alloys such as Britannia metal, 4 ) aluminum and aluminum alloys. Nickel films provide quite satisfactory protection of iron from rusting in interior spaces.

However, they are not sufficient in the open air; In addition, hot fats, vinegar, tea, mustard act on polished nickel-plated surfaces, as a result of which nickel-plated tableware and kitchenware become stained. In cases where it is required reliable protection from exposure to bad weather and at the same time an elegant appearance of the nickel-plated surface on iron. a double film is applied - zinc and then nickel. This method of double plating (zinc and then nickel) is also used for the so-called. corset steel. If it is necessary to obtain particularly resistant films, such as on wires, nickel and platinum are deposited simultaneously, the content of the latter being gradually increased from 25% to 100% and, finally, the object is calcined in a stream of hydrogen at 900-1000°C. Large products, for example, boilers for cooking, centrifuge drums or fans, if due to economic conditions they cannot be made of pure nickel, but are not resistant enough to the nickel film on iron or copper, are lined with a layer of lead of several mm, and over it with a layer of nickel in 1-2 mm. The rusting of nickel-plated iron and steel products is explained by the presence of an electrolyte remaining in the thin pores of the nickel film. This phenomenon is eliminated if the products are kept in oil at 200°C before nickel plating, degreased after cooling, lightly copper-plated, then nickel-plated in a nickel citrate bath with a low current and finally dried in a cabinet at 200°C; then moisture is removed from the pores, which are clogged with the oil contained in them.

There are a number of proposals to impose double protective films on cast iron, iron or steel sheets, wires and strips in the reverse order of the above, i.e., first coat the products with a thin film of nickel by contact or electrolytic method, and then immerse them in a bath of molten zinc or tin (Vivien and Lefebre, 1860 .). It is also proposed to add a certain amount of nickel to an alloy of 25-28 kg of zinc, 47-49 kg of lead and 15 kg of tin, which is used for hot coating of iron sheets. Resistance of aluminum and its alloys surfaces against salt and sea water. achieved by galvanic deposition on them, after cleaning them with a sand jet, of successive layers: nickel 6 microns thick, copper 20 microns and then nickel again 50 microns thick, after which the surface is polished. The resistance of aluminum against 15% sodium alkali is achieved by a nickel film 40 microns thick. In some cases, coating is not applied with pure nickel, but with an alloy, for example nickel-copper; for this purpose, electrolysis is carried out in a bath containing cations in the ratio of the required alloy; the deposited film is then converted into an alloy by heating the product to red-hot heat.

Contact nickel plating. Steel objects, according to the instructions of F. Stolb (1876), after polishing and proper degreasing, are boiled in a bath of 10-15% aqueous solution of pure zinc chloride, to which nickel sulfate is added until a green turbidity from the basic nickel salt is formed. Nickel plating lasts about 1 hour. After this, the item is rinsed in water with chalk, and the bath, after filtering and adding nickel salt, can be used again. The resulting film of nickel is thin but strong. To increase the temperature of the bath, it was proposed either to carry out the process under pressure (F. Stolba, 1880) or to use a bath with a concentrated solution of zinc chloride. To prevent items from rusting, they are kept in lime milk for 12 hours. A more complex bath for iron objects, previously copper-plated in a bath of 250 g of copper sulfate in 23 liters of water with a few drops of sulfuric acid, contains 20 g of tartar, 10 g of ammonia, 5 g of sodium chloride, 20 g of tin chloride, 30 g of nickel sulfate and 50 g of double sulfate nickel-ammonium salt.

Electroplated nickel plating. Depletion of the nickel bath m.b. preventable by fairly easy dissolution of nickel anodes. Rolled anodes, and especially those made of pure nickel, are difficult to dissolve, and therefore, during technical nickel plating, nickel bars containing up to 10% iron are used as anodes. However, such anodes lead to the deposition of iron on the object, and the presence of iron in the nickel film entails a number of defects in nickel plating. As indicated by Kalgan and Hammoge (1908), it is impossible to obtain a sediment completely free of the latter with anodes containing iron. But the nickel sediment will contain only 0.10-0.14% iron if the iron content in the anodes is reduced to 7.5%; The iron content of the sediment can be further reduced by enclosing the anodes in fabric bags, while rotating the electrodes leads to an increased iron content in the sediment and a decrease in its yield. The presence of iron in the nickel film leads to the deposition of sediments with a gradually decreasing iron content and therefore heterogeneous in relation to mechanical properties at different depths; K. Engeman (1911) considers this heterogeneity to be the only reason for the easy detachment of nickel films. The presence of iron may. the cause of a number of other defects in nickel plating (see table), for example, the ease of rusting of films.

The electrolytic bath for nickel plating is compiled in Chap. way from a double nickel-ammonium salt, and weak acids are added to eliminate the basic salts. Higher acidity of the bath leads to harder films. It must be borne in mind that technical nickel sulfate is not suitable for baths, since it often contains copper; it should be removed by passing hydrogen sulfide through an aqueous solution of vitriol. Chloride salts are also used, but with sulfate baths the sediments are harder, whiter and more persistent than with chloride baths. It is advantageous to reduce the high resistance of a nickel bath by adding various conductive salts - especially ammonia and sodium chloride - and heating. Neutralization of excess sulfuric acid in old solutions is successfully carried out with nickel carbonate, which is obtained from a warm aqueous solution of nickel sulfate precipitated with soda. For the whiteness and smoothness of films, a large number of proposals have been made to add various organic acids (tartaric, citric, etc.) and their salts, for example, acetic, citric and tartrate salts of alkali and alkaline earth metals to the nickel bath (Kate, 1878). ), nickel propionate, borate-tartarate salts of alkali metals. If it is necessary to obtain thick nickel deposits, it is proposed to add boric, benzoic, salicylic, gallic or pyrogallic acids, and in addition 10 drops of sulfuric, formic, and lactic acid per 1 liter of bath to prevent polarization on the product. As Powell (1881) pointed out, the addition of benzoic acid (31 g per bath of 124 g of nickel sulfate and 93 g of nickel citrate in 4.5 liters of water) eliminates the need to use chemically pure salts and acids. The nickel deposit has good properties also with a simple bath of nickel-ammonium sulfate, but subject to the alkalinity of the solution, which is achieved by adding ammonia. Very good precipitates are obtained from a neutral solution of nickel fluoride-borate at room temperature(at temperatures above 35°C, the solution decomposes to form an insoluble basic salt) and a current density of 1.1-1.65 A/dm 2 . Here are some bath recipes. 1) 50 parts of sodium bisulfite, 4 parts of nickel nitrate and 4 parts of concentrated ammonia are dissolved in 150 parts of water. 2) 10-12 parts of nickel sulfate, 4 parts of double nickel-ammonium sulfate, 1-3 parts of boric acid, 2 parts of magnesium chloride, 0.2-0.3 parts of ammonium citrate, added to 100 hours .(total) water. Current density 1.6 A/dm 2 deposits film at a rate of 2 µm/h; By raising the temperature to 70°C, you can reduce the resistance of the bath by two to three times and thereby speed up nickel plating. 3) An electrolyte consisting of 72 g of double nickel-ammonium sulfate salt, 8 g of nickel sulfate, 48 g of boric acid and 1 liter of water is especially favorable for the softness and non-porousness of the sediment, since it reduces the evolution of hydrogen.

Obtaining nickel films of a special type. 1) A white film of zinc, tin, lead and british metal is obtained in a bath of 20 g of double nickel-ammonium sulfate salt and 20 g of nickel carbonate, dissolved in 1 liter of boiling water, and neutralized at 40 ° C with acetic acid; the bath should be kept neutral. 2) A matte white film is obtained in a bath of 60 g of double nickel-ammonium sulfate salt, 15 g of recrystallized nickel sulfate, 7.4 g of ammonia, 23 g of sodium chloride and 15 g of boric acid per 1 liter of water; the bath should be concentrated to 10° Bẻ; voltage from 2 to 2.5 V. 3) A black film is obtained on surfaces that are thoroughly degreased or coated with a thin layer of white nickel by electrolysis in a bath of 60 g of double nickel-ammonium sulfate, 1.5 g of ammonium thiocyanate and about 1 g of sulfate zinc per 1 liter of water 4) A black film is also obtained in an electrolyte from 9 g of double nickel-ammonium sulfate in 1 liter of water, followed by the addition of 22 g of potassium thiocyanate, 15 g of copper carbonate and 15 g of white arsenic, previously dissolved in ammonium carbonate; The depth of the black tone increases with the arsenic content in the solution. 5) A deep blue film is obtained in a bath of equal parts of double and simple nickel sulfate salts, brought to 12° Bẻ, and 2 hours of ammonia decoction of licorice root are added per liter; electrolysis lasts 1 hour at 3.5 V, and then another 1/2 hour at 1.4 V. 6) The brown film is obtained as follows: electrolysis at a voltage of 0.75-1 V is carried out in a bath of 180 g of double nickel-ammonium sulfate salt and 60 g of nickel sulfate, dissolved in the smallest possible amount of boiling water, added to 50 cm 3 and then mixed with solutions of 30 g of nickel sulfate and 60 g of sodium thiocyanate, each in 0.5 liters of water, after which the solution is added to 4, 5 l. The resulting black film is given a brown tint by immersing the product for a few seconds in a bath of 100.6 g of iron perchlorate and 7.4 g of hydrochloric acid in 1 liter of water: after washing and drying, the surface of the product is varnished to fix the tone.

Nickel plating of aluminum and its alloys. Several processes have been proposed. 1) Surface preparation of aluminum products consists of degreasing, then cleaning with pumice and finally immersing in a 3% aqueous solution of potassium cyanide; After electrolysis in a nickel bath, the products are washed cold water. 2) After washing with a 2% solution of potassium cyanide, the products are immersed in a solution of 1 g of ferric chloride (ferrochloride) per 0.5 liters of water and technical hydrochloric acid until the surface becomes silver-white, and then nickel-plated for 5 minutes. at a voltage of 3 V. 3) Polishing products, removing the polishing compound with gasoline, holding for several minutes in a warm aqueous solution of sodium phosphate, soda and resin, washing, immersing for a short time in a mixture of equal parts of 66% sulfuric acid (containing some ferric chloride) and 38% nitric acid, a new wash and electrolysis in a bath containing nickel salt, bitter salt and boric acid; voltage 3-3.25 V. 4) According to J. Kanak and E. Tassilli: pickling the product with boiling potassium alkali, brushing in lime milk, 0.2% cyanide bath, bath of 1 g of iron in 500 g of hydrochloric acid and 500 g of water, washing, nickel plating in a bath of 1 liter of water, 500 g of nickel chloride and 20 g of boric acid at a voltage of 2.5 V and a current density of 1 A/dm 2, finally polishing the matte gray residue. The iron bath serves to roughen the surface of the aluminum and thus contributes to the strength with which the film is held on the metal. 5) According to Fischer, a nickel plating bath is made up of 50 g of nickel sulfate and 30 g of ammonia in 1 liter of water at a current density of 0.1-0.15 A/dm 2, in 2-3 hours a thick deposit is obtained that has a high gloss after polishing with stearic oil and Vienna lime. 6) A hot bath (60°C) is made up of 3400 g of double nickel-ammonium sulfate, 1100 g of ammonium sulfate and 135 g of milk sugar in 27 liters of water. 7) The cold bath contains nickel nitrate, potassium cyanide and ammonium phosphate.

Nickel film inspection. Recognition of the composition of a metal film on an object, according to L. Loviton (1886), can be done by heating the object in the external flame of a Bunsen burner: the nickel film turns blue, receives a black sheen and remains unharmed; silver does not change in the flame, but turns black when treated with a dilute solution of ammonium sulphide; finally the tin coating quickly turns from gray-yellow to gray and disappears when treated with the specified reagent. Checking the quality of the nickel film on iron and copper in relation to pores and flaws can be done using the so-called. ferroxyl test and with particular convenience using ferroxyl paper coated with agar-agar gel with ferrous potassium chloride and sodium chloride. Applied wet to the test surface and after 3-5 minutes. fixed in water, this paper gives a documentary image of the smallest pores, which can. saveable.

Nickel recovery from old products. Removal of nickel coating from products made of iron and other non-amalgamated metals is carried out in the following ways: a) mercury vapor under vacuum or under ordinary pressure; b) heating the scraps with sulfur, after which the metal layer is easily removed with hammers; c) heating scraps with substances that release sulfur when high temperature) upon sudden cooling, the nickel film comes off; d) treatment with sulfuric or nitric acid heated to 50-60°C; iron goes into solution, and nickel remains almost undissolved; however, despite its simplicity, this method is of little use, since the resulting nickel still retains a significant iron content, which is not removed even by repeated treatment with acid (T. Fleitman); e) prolonged heating with access to air or water vapor, after which the trimmings are subjected to mechanical shock and the nickel bounces off; f) electrolytic dissolution: an iron object coated with nickel is made an anode in a bath containing ammonium carbonate; if the coating consists of a nickel alloy, then it is necessary to regulate the voltage, and at 0.5 V copper is deposited, and at a voltage greater than 2 V - nickel; during this process the iron is not corroded; g) iron or steel scraps are made into an anode in a bath of an aqueous solution of sodium nitrate, while the cathode consists of a coal stick; voltage should not exceed 20 V; h) nickel is removed from zinc mugs by electrolysis of objects made with an anode in 50° sulfuric acid; an acid of this concentration has the property of dissolving only nickel, silver and gold, but not other metals, if current is flowing; voltage applied 2-5 V; iron sheets on which nickel is deposited in the form of dust serve as cathodes; zinc does not dissolve, even if the mugs remain in the electrolyte for a long time.

Found the most widespread chemical coatings nickel, copper, silver, palladium, cobalt and less commonly tin, chromium and other metals.

Chemical nickel plating. The reduction of nickel ions from solutions occurs due to the oxidation of hypophosphite according to the total reaction

H 2 PO - 2 + H 2 O + Ni 2+ = H 2 PO - 3 + 2H + + Ni.

In this case, recovery can proceed as follows:

NiCl 2 + NaH 2 PO 2 + H 2 O = Ni + 2HCl + NaH 2 PO 3

NaH 2 PO 3 + H 2 O = NaH 2 PO 3 + H 2

or H 2 PO - 2 = PO - 2 + 2H +

(decomposition of hypophosphite)

Ni 2+ +2H = Ni + 2H +

(nickel reduction).

The released hydrogen also reduces phosphite to phosphorus, so the nickel coating contains 6 - 8% phosphorus, which largely determines its specific properties (Table 24).

24. Properties of chemical and electroplating nickel

Despite the fact that nickel, precipitated chemically, has significant corrosion resistance, it cannot be used for protection against corrosion in nitric and sulfuric acid environments. After heat treatment, such nickel has a hardness of HV 1000-1025.

Mostly technological process Nickel plating comes down to the following. Parts made of steel, copper and its alloys are prepared in the same way as for galvanic coating.

Nickel plating is carried out in a solution of the following composition (g/l):

Nickel sulfate 20

Sodium hypophosphite 25

Sodium acetate 10

Thiourea (or maleic anhydride) 0.003 (1.5 - 2)

Temperature 93 ± 5°C, deposition rate 18 µm/h (at 90°C and loading density 1 dm 2 /l), pH = 4.1 ÷ 4.3.

Parts must be shaken during the nickel plating process. It is allowed to replace thiourea with maleic anhydride in the amount of 1.5 - 2 g/l.

To initiate nickel deposition on parts made of copper and its alloys, it is necessary to ensure their contact with steel or aluminum. The process is carried out in porcelain or steel containers lined with polyethylene film, as well as in silicate glass containers.

For rapid deposition and high loading densities of simple-profile parts, it is recommended to use a solution of the following composition (in g/l):

Nickel sulfate 60

Sodium hypophosphite 25

Sodium acetate 12

Boric acid 8

Ammonium chloride 6

Thiourea 0.003

Solution temperature 93 ± 5°C, sedimentation rate 18 µm/h (at 90°C and loading density 3 dm 2 /l), pH = 5.6 ÷ 5.7.

After chemical nickel plating, the parts are washed in a catcher, then in running cold and hot water, dried at 90 ± 10°C for 5 - 10 minutes and heat treated at 210 ± 10°C for 2 hours (in order to relieve internal stresses and increase adhesion strength to the base). Then, depending on the operating conditions, the parts are varnished, treated with a hydrophobic liquid (GKZh, etc.) or submitted for assembly without treatment.

The main reasons for poor-quality coating during chemical nickel plating are:

1) spontaneous deposition of nickel in the form of black dots due to poor cleaning of the baths, the presence of traces of nickel or other centers of crystallization on the bottom and walls of the bath, as well as due to overheating of the solution;

2) the presence of uncovered areas on parts of complex configuration due to the formation of gas bubbles and uneven washing of parts with the solution;

3) partial deposition of nickel on the inner surface of the bath due to parts touching the walls or bottom of the bath during the nickel plating process;

4) decrease in the acidity of the solution (cracking, brittle coating);

5) increase in the acidity of the solution (the coating is rough and rough).

The pH value is adjusted by adding a 10% solution acetic acid or caustic soda.

Silicon parts are nickel-plated in alkaline solutions of the following composition (in g/l):

Nickel chloride 30

Sodium hypophosphite 10

Sodium citrate 100

Ammonium chloride 50

The deposition rate is 8 µm/h, pH = 8÷10 (due to the introduction of NH 4 OH).

The procedure for chemical nickel plating of ceramics: degreasing in alkaline solutions and chemical etching of the surface (a mixture of sulfuric and hydrofluoric acids), sensitization in a solution (150 g/l) of sodium hypophosphite at 90°C, nickel plating in an alkaline bath. The thickness of the coatings of parts, depending on their operating conditions, is indicated in table. 25.

25. Coating thickness values ​​depending on operating conditions

Thus, at pH = 5.5, sediments contain 7.5% phosphorus, and at pH = 3.5, 14.6%. An increase in coating hardness to 1100-1200 kgf/mm 2 at 200-300°C is caused by the release of the Ni 3 P phase, which crystallizes in a tetragonal system with a crystal lattice constant a = b = 8.954. 10 -10 m and c = 4.384.10 -10 m. The maximum hardness of nickel corresponds to 750°C. The elastic modulus is 19,000 kgf/mm 2. The tensile strength is 45 kgf/mm 2 (at 20°C) and 55 kgf/mm 2 after heat treatment at 200°C for 1 hour. The friction coefficient of the coating (at a load > 10 kgf) after application is the same as and shiny chrome. The specific wear of the nickel coating at 100°C is 2.10 -3 mm 3 /m.

When stirring the acidic solution, the shine of the sediments and the rate of deposition increases. If the deposition process is interrupted for a few minutes, the parts can be loaded into the bath without additional activation. During a long break (24 hours), parts should be stored in a cold nickel plating solution and then transferred to a working bath.

The lower the pH of the solution, the lower the rate of metal deposition. In addition, the rate is a function of the ratio Ni 2+ : H 2 PO - 2 . For a normal acidic bath, it should range from 0.25 to 0.60 (for an acetate-buffered bath, 0.3 to 0.4).

In the presence of ammonium salts, the deposition rate decreases. In newly prepared solutions, the deposition rate is initially high, and then decreases as it ages. Thus, in acetate and citrate solutions it decreases from 25 to 2 - 5 µm/h. The most optimal deposition rate is ~ 10 µm/h.

The gloss of the coating is determined by the quality of preparation base surface which should be polished. In alkaline baths, coatings are more shiny than in acidic ones. Coatings containing<= 2% фосфора — матовые, 5% фосфора — полублестящие и =>10% phosphorus - very shiny, but with a yellowish tint. The spread in coating thickness of 30 microns, even on parts of complex configurations, is, for example, no more than 1-2 microns. When the bath is operated at a constant pH value, the amount of phosphorus in the coating is proportional to the concentration of hypophosphite in the bath.

The normal phosphorus content in the coating is 5 - 6%. The higher the ratio of H 2 PO 2:Ni 2+, the higher the phosphorus content. On low-carbon steels, the adhesion of nickel coatings is very high (2200 - 4400 kgf/cm2), but deteriorates if the solution temperature drops to 75°C. Adhesion on steels alloyed with Al, Be, Ti, and copper-based alloys depends on the method of surface treatment and is improved by subsequent heat treatment at 150-210°C.

The first sign of a violation of the stability of the solution composition is the formation of white foam due to excessive hydrogen evolution throughout the entire volume of the bath. A very fine black Ni-P suspension then appears, which accelerates the decomposition reaction of the solution.

The reasons for premature decomposition of the solution may be: too rapid introduction of alkali and hypophosphite (a dilute aqueous solution should be added with vigorous stirring); local overheating; too much high content hypophosphite (need to lower pH and temperature); adding palladium to a solution with parts activated in PdCl 2, incorrect ratio of the total area of ​​​​the parts to the volume of the solution.

The level of the solution in the bath must be maintained constant, since lowering it due to evaporation leads to concentration of the solution. During the coating process, heaters (steam, thermal electric heating, etc.) should not be turned off.

Unlike hydrazine, sodium hypophosphite has an important advantage, since the sediment contains 8 to 10 times less gases. The addition of sodium thiosulfate helps reduce the porosity of nickel. Thus, with a thickness of 20 microns it decreases from 10 to 2 pores/cm2. When choosing a material for a bath, it should be taken into account that solutions evaporate at a temperature approximately equal to the boiling point and are highly sensitive to various contaminants. In addition, the material must be resistant to HNO 3, since nickel deposits must be periodically removed from the walls of the bath. Bathtubs with a volume of 20 liters are made of Pyrex, and larger ones are made of polished ceramic. Inner surface steel containers are coated with glassy enamel. Baths made of corrosion-resistant steel must be passivated with concentrated nitric acid for several hours. To prevent the formation of galvanic couples between the steel bath and the parts being coated, its walls must be lined with glass or rubber. Polyethylene liners are used as lining in small-capacity baths.

After each unloading of parts, rod-type electric heaters must be etched in HNO 3.

Defective coatings from parts made of steel, aluminum and titanium should be removed in concentrated nitric acid at a temperature not exceeding 35°C, from parts made of corrosion-resistant steel in a 25% solution of HNO 3, and from brass and copper - by anodic dissolution in H 2SO4.

In order to improve the stability of the solution composition, foreign companies recommend adding chromium salts. The porosity of coatings obtained in a solution containing 10 g/l K 3 Fe(CN) 6 and 20 g/l NaCl is determined within 10 minutes. Pores are completely absent at a coating thickness => 100 microns.