Nickel plating of steel. Secrets of nickel plating metal objects at home

The most widespread are chemical coatings with nickel, copper, silver, palladium, cobalt and less commonly with 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.

Basically, the technological process of 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 electroless nickel plating the parts are washed in a catcher, then in a cold flow 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 the strength of adhesion 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 inner surface bathtubs due to parts touching the walls or bottom of the bathtub 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 of acetic acid or sodium hydroxide.

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 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; hypophosphite content is too high (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. The inner surface of steel containers is 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 electric heaters rod type 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.

Nickel is widely used in mechanical engineering and instrument making, as well as in various industries. In the food industry, nickel replaces tin coatings, and in the field of optics it has spread thanks to the procedure of black nickel plating of metal. Parts made of non-ferrous metals and steel are coated with nickel to increase the resistance of products to mechanical wear and protection against corrosion. The presence of phosphorus in nickel makes the film close in hardness to the chromium film!

Nickel plating procedure

Nickel plating is the application of a nickel coating to the surface of a part, which usually has a thickness of 1 to 50 microns. Nickel coatings come in shiny or matte black, but regardless of this, they provide reliable protection metal in aggressive environments (acids, alkalis) and at elevated temperatures.

Before the nickel plating process, the product must be prepared. It is treated with sandpaper to remove the oxide film, wiped with a brush, washed with water, degreased in a hot soda solution and washed again. Nickel coatings can lose their initial shine over time, so they often cover the nickel layer with a more stable layer of chromium.

Nickel, which is applied directly to steel, is a cathodic coating and protects the material solely mechanically. Discontinuity protective coating promotes the formation of corrosive vapors in which steel acts as a soluble electrode. As a result, corrosion forms under the coating, destroying the steel substrate and causing peeling of the nickel coating. To prevent this, the metal should always be coated with a thick layer of nickel.

Nickel coatings can be applied to iron, copper, their alloys, as well as tungsten, titanium and other metals. Metals such as lead, cadmium, tin, lead, antimony and bismuth cannot be plated using electroless nickel plating. When nickel plating steel products, it is customary to apply an underlayer of copper.

Nickel coatings are used in various industries for special, protective and decorative purposes, and also as a sublayer. Nickel plating technology is used to restore worn auto parts and machine parts, coatings of chemical equipment, medical instruments, measuring instruments, household items, parts that are used with light loads under conditions of dry friction or exposure to strong alkalis.

Types of nickel plating

There are two types of nickel plating known in practice - electrolytic and chemical. The latter method is somewhat more expensive than the electrolytic method, but it can provide the ability to create a coating of uniform quality and thickness on any surface area if the condition for access to the solution is met.

Electrolytic nickel plating

Electrolytic coatings are characterized by some porosity, depending on the thoroughness of the preparation of the base and the thickness of the protective coating. To organize high-quality protection against corrosion, a complete absence of pores is required; for this, it is customary to first copper-plate a metal part or apply a multi-layer coating, which is more reliable than a single-layer coating even with the same thickness.

To do this, you need to prepare an electrolyte. Take 30 grams of nickel sulfate, 3.5 grams of nickel chloride and 3 grams of boric acid per 100 milliliters of water, pour this electrolyte into a container. Nickel plating of steel or copper requires nickel anodes, which must be immersed in an electrolyte.

The part should be suspended on a wire between the nickel electrodes. The wires coming from the nickel plates must be connected together. The parts are connected to the negative pole of the current source, and the wires are connected to the positive pole. Then you need to include a rheostat in the circuit to regulate the current and a milliammeter. Select a DC source that has a voltage of 6 V or less.

The current must be turned on for approximately twenty minutes. Then the part must be removed, washed and dried. The product is coated with a matte layer of grayish nickel. In order for the protective coating to become shiny, it must be polished. However, when working, remember the significant disadvantages of electrolytic nickel plating at home - uneven deposition of nickel on the relief surface and the inability to coat deep and narrow holes, as well as cavities.

Electroless nickel plating

In addition to the electrolytic method, you can use another, very simple method to coat iron or polished steel with a thin but durable layer of nickel. It is customary to take a 10% solution of zinc chloride and gradually add nickel sulfate to the solution until the liquid turns bright green. After this, the liquid must be heated to a boil; it is better to use a porcelain vessel for this.

In this case, a characteristic haze appears, but it does not have any effect on the process of nickel plating of parts. When you bring the liquid to a boil, you should lower the object that is to be nickel-plated into it. Pre-clean the part and degrease it. The product should boil in the solution for about an hour; add distilled water from time to time as it evaporates.

If you notice during boiling that the liquid has changed color from bright green to faint green, then you need to add a little nickel sulfate to obtain the original color. After the specified time has passed, remove the product from the solution, rinse in water with a little chalk dissolved in it, and dry thoroughly. Steel or polished iron, plated in a similar way nickel plating, this protective coating holds very firmly.

The chemical nickel plating procedure is based on the reduction reaction of nickel from an aqueous solution of its salts using sodium hypophosphite and other chemical reagents. Solutions that are used for chemical nickel plating are acidic with a pH level of 4-6.5 and alkaline with a pH level above 6.5.

It is advisable to use acidic solutions for coating ferrous metals, brass and copper. Alkaline ones are intended for stainless steels. An acidic solution, compared to an alkaline solution, gives a smoother surface on a polished part. Quite a few more important feature acidic solutions are considered to have a lower probability of self-discharge when the threshold is exceeded operating temperature. Alkaline solutions guarantee more reliable adhesion of the nickel film to the base metal.

All aqueous solutions for do-it-yourself nickel plating are universal, that is, suitable for all metals. For chemical nickel plating, distilled water is used, but you can also use condensation from a household refrigerator. Chemical reagents are suitable clean - with the designation “C” on the label.

The sequence of making the solution is as follows. All chemical reagents, with the exception of sodium hypophosphite, must be dissolved in water using enamel containers. Then heat the solution to operating temperature, dissolve sodium hypophosphite and place the parts in the solution. Using one liter of solution, you can nickel-plate parts that have a surface area of ​​up to 2 dm2.

Black coatings

Black nickel coatings are used with special and decorative purpose. Their protective properties are very low, so they are usually applied to a sublayer of ordinary nickel, zinc or cadmium. Steel products must be pre-galvanized, and copper and brass must be nickel-plated.

Black nickel plating is hard but brittle, especially when thick. In practice, they stop at a thickness of 2 microns. The nickel bath for applying such coatings, as a rule, contains a large amount of thiocyanate and zinc. The coating contains about half of nickel, and the remaining 50% consists of sulfur, nitrogen, zinc and carbon.

Baths of black nickel plating of aluminum or steel are usually prepared by dissolving all the components in warm water and filtering with filter paper. If difficulties arise when dissolving boric acid, then it is separately dissolved in water that is heated to 70 degrees Celsius. Achieving deep blacks depends on the right choice current density values.

Nickel plating baths

In workshops, a bath is widely used, which consists of 3 main components: boric acid, sulfate and chloride. Nickel sulfate is a source of nickel ions. Chloride significantly affects the performance of nickel anodes; its concentration in the bath is not precisely standardized. In chloride-free baths, strong passivation of nickel occurs, after which the nickel content in the bath decreases, and the result is a decrease in current efficiency and a decrease in the quality of coatings.

Anodes in the presence of chlorides dissolve in sufficient quantities for the normal process of nickel plating of copper or aluminum. Chlorides increase the conductivity of the bath and its functioning when contaminated with zinc. Boric acid helps maintain pH at a certain level. The effectiveness of this action depends to a large extent on the concentration of boric acid.

The chloride can be sodium, zinc or magnesium chloride. Watts sulphate baths are widely used, which contain electrically conductive salts as additives, which increase the electrical conductivity of the baths and improve appearance protective coatings. The most commonly used among these salts is magnesium sulfate (about 30 grams per liter).

Nickel sulfate is most often administered in a concentration of about 250-350 grams per liter. Recently, there have been trends towards limiting nickel sulfate - less than 200 g/l, which helps to significantly reduce solution losses.

The concentration of boric acid is 25-40 grams per liter. Below 25 g/l the tendency towards rapid alkalization of the bath increases. And exceeding the permissible level is considered unfavorable due to the possible crystallization of boric acid and the deposition of crystals on the walls of the nickel bath and anodes.

The nickel bath operates in different temperature ranges. However, nickel plating technology at home is rarely used at room temperature. Nickel often flakes off from coatings applied in cold baths, so the bath must be heated to at least 30 degrees Celsius. The current density is chosen experimentally to avoid burning of the coatings.

The sodium bath works reliably over a wide pH range. Previously, the pH was maintained at a level of 5.4-5.8, citing less aggressiveness and higher hiding power of the bath. However, high pH values ​​provoke a significant increase in stress in the nickel coating. Therefore, in most baths the pH is 3.5-4.5.

Subtleties of nickel plating

The adhesion of the nickel film to the metal is relatively low. This problem can be solved using heat treatment nickel films. The low-temperature diffusion procedure consists of heating nickel-plated products to a temperature of 400 degrees Celsius and holding the parts for one hour at this temperature.

But remember that if parts that are plated with nickel have been hardened (fish hooks, knives and springs), then at a temperature of 400 degrees they can be released, losing hardness - their main quality. Therefore, low-temperature diffusion in such a situation is carried out at a temperature of approximately 270-300 degrees with a holding time of up to 3 hours. Such heat treatment can also increase the hardness of the nickel coating.

Modern nickel baths require special equipment for nickel plating and mixing of the aqueous solution to intensify the nickel plating procedure and reduce the risk of pitting - the appearance of small depressions in the coating. Stirring the bath entails the need to create continuous filtration to remove contaminants.

Mixing using a moving cathode rod is not as effective as using compressed air for this purpose, and among other things, requires a special ingredient that prevents foaming.

Removing Nickel Plating

Nickel coatings on steel are usually removed in baths of dilute sulfuric acid. Add to 20 liters cold water 30 liters of concentrated sulfuric acid in portions with constant stirring. Make sure that the temperature does not exceed 60 degrees Celsius. After cooling to room temperature baths, its density should reach 1.63.

In order to reduce the risk of seeding of the material from which the substrate is made, glycerin is added to the bath in an amount of 50 grams per liter. Bathtubs are usually made from vinyl plastic. The products are hung on the middle rod, which is connected to the plus of the current source. The rods on which the lead sheets are attached are connected to the minus of the current source.

Make sure that the bath temperature does not exceed 30 degrees, as the hot solution has an aggressive effect on the substrate. The current density should be 4 A/dm2, but voltage variations within 5-6 Volts are allowed.

Add via certain time concentrated sulfuric acid to maintain a density of 1.63. To prevent bath dilution, immerse items in the bath after pre-drying. Controlling the process is not particularly difficult, because the current density drops sharply at the moment of nickel removal.

Thus, nickel plating is the most popular electroplating process. Nickel plating is characterized by hardness, great corrosion resistance, reasonable nickel plating costs, good reflectivity and electrical resistivity.

Nickel is widely used in instrument making and mechanical engineering, as well as in other various industries. IN food production Nickel replaces tin coatings, and in the field of optics it is known for its black nickel plating process. Nickel is used to treat products made from steel and non-ferrous metals to protect against corrosion and increase the resistance of parts to mechanical wear. The phosphorus content in nickel makes it possible to produce a film similar in hardness to a chromium film.

Nickel plating process

The nickel plating procedure involves applying a nickel coating to the surface of the product, which, as a rule, has layer thickness 1-50 microns. Nickel coatings can be matte black or shiny, but regardless of this, they create reliable and durable protection of the metal from aggressive influences (alkali, acid) and at high temperatures.

Before nickel plating, the product must be prepared. Preparation stages:

• the part is treated with sandpaper to remove the oxide film;
• brushed;
• washed under water;
• degrease in a warm soda solution;
• washed again.

Nickel coatings can lose their original shine over time, so very often the nickel layer is coated with a more durable layer of chromium.

Nickel applied to steel is a cathodic coating that only protects the metal mechanically. The weak density of the protective layer contributes to the appearance of corrosion pores, where the steel part is the soluble electrode. As a result, corrosion occurs under the coating, it destroys the steel substrate and causes peeling of the nickel layer. To prevent this, the metal must always be treated with a thick layer of nickel.

Nickel coatings are applied to:

• copper;
• iron;
• titanium;
• tungsten and other metals.

Cannot be processed using nickel plating metals such as:

When nickel plating steel parts, it is necessary to make an underlayer of copper.

Nickel coatings are used in various fields industry for special, decorative and protective purposes, and is also used as a sublayer. The nickel plating technique is used to restore worn parts and spare parts for cars, coatings of medical instruments, chemical equipment, household items, measuring instruments, parts that are subject to light loads under the action of strong alkalis or dry friction.

Types of nickel plating

In practice there is two types of nickel plating:

• Chemical;
• Electrolytic.

The first option is slightly more expensive than the electrolytic one, but it can provide the opportunity to create a uniform coating in thickness and quality on any areas of the product if the conditions for the solution to be accessible to them are created.

Electrolytic nickel plating at home

Electrolytic nickel plating is characterized by low porosity; it depends on the thickness of the protective layer and the thoroughness of the preparation of the base. To create high-quality anti-corrosion protection, an absolute absence of pores is necessary, for which it is customary to first copper-plate a metal part or apply several layers of coating, which is much stronger than a single-layer coating even with the same thickness.

Why at home? you need to prepare the electrolyte. Requires 3.5 g. Nickel chloride, 30 g. nickel sulfate and 3 gr. boric acid per 100 ml. water, pour this electrolyte into a container. Nickel plating of copper or steel will require nickel anodes, which must be immersed in an electrolyte.

The part is suspended on a wire between nickel electrodes. The wires that come from the nickel plates need to be connected together. The parts are connected to the negative pole of the voltage source, and the wires are connected to the positive pole. Afterwards, you need to connect a rheostat to the circuit and a milliammeter to regulate the voltage. You will need a DC source with a voltage of no more than 6 Volts.

The current needs to be turned on for about 20 minutes. Afterwards the part is removed, washed and dried. The part is coated with a matte layer of gray nickel. To make the protective layer shine, it needs to be polished. But when working, do not forget about the significant disadvantages of electrolytic coating at home - the inability to cover narrow and deep holes and uneven deposition on a relief nickel surface.

Chemical nickel plating at home

In addition to the electrolytic method, there is another, rather simple option for coating polished steel or iron with a durable and thin nickel layer. It is necessary to add a 10% solution of zinc chloride and slowly add it to the nickel sulfate solution until the solution won't be bright green. Then the liquid must be brought to a boil; it is advisable to take a porcelain container for this.

In this case, a characteristic haze is formed, but it does not affect the nickel plating of products. When you bring the solution to a boil, you need to lower the product that is being nickel-plated into it. It must first be degreased and cleaned. The part should boil in the liquid for about an hour; periodically add distilled water as the solution decreases.

If during boiling you see that the solution has changed color from bright to faint green, then it is necessary add a little nickel sulfate to get the original color. After the specified time, remove the part from the liquid, rinse in water with a little chalk, and dry thoroughly. Polished iron or steel coated in this way retains this protective layer quite well.

The chemical coating process is based on the reaction of converting nickel from an aqueous solution of its salts using sodium hypophosphite and other chemical elements. Solutions used for chemical coating can be alkaline with a pH greater than 6.5 and acidic with a pH of 4-6.5.

Acidic solutions are best used for processing copper, brass and ferrous metals. Alkaline ones are used for stainless steel. An acidic solution, unlike an alkaline one, creates on a polished product smoother surface. Another important feature of acidic solutions is the lower chance of self-discharge with increasing operating temperature. Alkaline substances guarantee stronger adhesion of the nickel film to the metal base.

Any aqueous solutions for nickel plating are considered universal, namely suitable for any metal. For the chemical coating, distilled water is used, but you can also use condensation from a regular refrigerator. Chemical reagents Clean ones are suitable - marked “C” on the package.

Stages of preparing the solution:

• All chemicals, except sodium hypophosphite, must be dissolved in water in an enamel container.
• Then heat the liquid to boiling, dissolve sodium hypophosphite and place the product in the solution.
• Using a liter of solution, you can coat parts with an area of ​​up to 2 square meters with nickel. dm.

Nickel plating baths

Workshops often use a bathtub consisting of three main elements:

• chloride;
• sulfate;
• boric acid.

Nickel sulfate is a source of nickel ions. Chloride significantly affects the performance of anodes; its proportion in the bath is not precisely specified. In chloride-free baths, significant passivation of nickel occurs, after which the amount of nickel in the bath decreases, and as a result, a decrease in the quality of coatings and a decrease in current efficiency.

Anodes with chlorides dissolve in required quantity for sufficient progress of nickel plating of aluminum or copper. Chlorides increase the performance of the bath when contaminated with zinc and its conductivity. Boric acid maintains pH at the required level. The efficiency of this process depends mainly on the amount of boric acid.

As a chloride, you can choose magnesium, zinc or sodium chloride. Watts sulphate baths are widely used, containing electrically conductive salts as additives that increase the electrical conductivity of the baths and increase the attractive appearance of the protective layer. The most commonly used among these salts is magnesium sulfate (about 30 g per 1 liter).

As a rule, nickel sulfate is added in a ratio of approximately 220-360 gr. for 1 l. Today, there are trends towards reducing nickel sulfate - less than 190 g/l. This helps to significantly reduce solution losses.

Adding boric acid approximately 25-45 g. for 1 l. If it is less than 25 g/l, then the processes of alkalization of the bath increase. And exceeding this limit is unfavorable due to the probable crystallization of boric acid and precipitation of crystals on the anodes and walls of the bath.

The nickel bath can operate in a different temperature range. But the nickel plating technique at home is not often used at room temperature. Nickel often comes off from coatings applied in cool baths, so the bath must be heated to at least 32 degrees. Current Density selected experimentally so that the protective layer does not burn.

A sodium bath works well over a wide pH range. Once upon a time, the pH was maintained at 5.3-5.9, citing the weak aggressiveness and better hiding properties of the bath. But high pH values ​​provoke a significant increase in stress in the nickel layer. Therefore, in many baths the pH is 3.4-4.6.

The adhesion of the nickel film to the metal is relatively low. This problem is solved by heat treatment of nickel films. The process of low-temperature diffusion is based on heating nickel-plated parts to a temperature of 400 degrees. and holding the products for an hour at a given temperature.

But do not forget that if the nickel-plated products were hardened, then at 400 gr. They may lose strength– their main quality. Therefore, low-temperature diffusion in these cases is done at a temperature of about 260-310 degrees. with a holding time of three hours. This heat treatment can also increase the strength of the nickel coating.

Baths require special equipment for plating with nickel and mixing the water solution to intensify the nickel plating process and reduce the likelihood of pitting - the appearance of small depressions in the protective layer. Stirring the bath entails the need for constant filtration to remove contaminants.

Mixing using an active cathode rod is not as effective as using compressed air, and in addition, it requires a special substance to prevent the formation of foam.

Removing Nickel Plating

Nickel coatings on steel are usually cleaned in bathtubs with diluted sulfuric acid. Add to 25 l. chilled water in parts of 35 l. concentrated sulfuric acid, while constantly stirring. Make sure that the temperature does not exceed 55 degrees. After the liquid has cooled to room temperature, its density should be 1.64.

To reduce the likelihood of etching of the metal from which the substrate is made, glycerin is added to the bath in a proportion of 50 g. for 1 l. Bathtubs are most often made of vinyl plastic. The parts are hung on the middle handrail connected to the plus of the voltage source. The handrails where the lead sheets are attached are connected to the negative side of the power supply.

Make sure that the bath temperature is no more than 32 degrees, because the hot solution has an aggressive effect on the substrate. The current density should be about 4.1 A/dm. kv., but current change is possible in the range of 4.5-6.2 Volts.

After some time, add sulfuric acid to maintain a density of 1.64. To avoid dilution of the bath, immerse parts only after they have been pre-dried.

Today, nickel plating is the most popular electroplating process. Nickel coatings are characterized by high corrosion resistance, hardness, inexpensive nickel plating cost, electrical resistivity and excellent reflective capabilities.

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. Methods for deposition of nickel from alkaline and acidic solutions have received industrial application. The deposited coating has a semi-shiny metallic appearance, a 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 a 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 deposition 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 temperature regime It should be taken into account that for a number of steels that have undergone hardening or normalization, high temperatures 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, porcelain and steel enameled baths are used.

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. Thus, for stainless steel grade 1×13 and similar surface preparation consists of its anodic treatment in alkaline solutions. The parts are mounted on carbon steel suspensions, using, if necessary, internal cathodes, suspended in a bath with a 10-15 percent caustic soda solution and subjected to 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. Technological process Nickel plating of ferrites consists of the following operations: parts are degreased in a 20% soda ash solution, 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 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 matte surface, dark gray tint, fine-crystalline structure, hardness 240-270 Vickers 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.

Nickel plating at home is a simple process. After it metal surface becomes protected from corrosion for a long time. The material is used in mechanical engineering, in the food industry, and in optical production.

Structural elements made of ferrous or non-ferrous metals are protected from corrosion and are less subject to wear. If phosphorus is present in the nickel solution, the surface film becomes stronger and the hardness index approaches that of a chrome-plated surface.

About the execution process

Nickel plating is a popular part of technology and good decision for coating the processed product. Applied to the part thin layer liquid nickel, adjustable thickness ranging from 0.8 microns to 0.55 micrometers. Nickel plating of metal also serves as a decorative coating.

This process will ensure the formation of a durable film, which, in turn, will help protect the product from alkalis and acids, and atmospheric agents. For the production of plumbing products, coating pipes, taps, adapters and other parts is an ideal solution.

Protection from external influences by this method is recommended for:

1. Metal products intended to be used outdoors.
2. Vehicle bodies.
3. Tools and equipment that dental clinics are equipped with.
4. Metal parts if their operation is planned in an aquatic environment.
5. Steel or aluminum structures, performing the functions of a fence.
6. Products whose operation will interact with chemical media.

In total, several unique methods of performing work are practiced. They have found application both in production and in everyday life. In any case, the process of carrying out this work in personal workshops is of interest, because there is no need to perform complex technological operations.

These methods include:

• chemical nickel plating;
• electrolytic coating.

Electroplating parameters:

Evaluation criterion	Type of product coating
	galvanic	chemical
Required temperature to melt the material	1450 0 C	890 0 C
Material resistivity limit, OM x m	Approximately 8.5 * 10 -5	Approximately 60 *10 -5
Susceptibility to creating magnetism	37	4
Vickers hardness	250	550
Longitudinal deformation indicator in %	From 10 to 30	From 3 to 6
Characteristics of strength during adhesion to the surface of the material	From 35 to 45	From 35 to 50

Carrying out work

Applying a thin film of material to the surface to be treated helps create shine and protect against temperature changes and aggressive influences of external environments.

Before performing the task itself, the metal should be carefully prepared so that the nickel adheres to surface layer was thorough.

The preparation technology is:

1. Processed with fine-grained sandpaper.
2. Wipe the surface with a brush and stiff bristles or metal wire.
3. Washing with water.
4. Degreasing in soda ash solution.
5. Washing clean water again.

Since a surface treated with nickel often quickly loses its ability to reflect light and becomes dull, it is chrome-plated. This coating ensures reliability during product operation.

The composition used when applied to a steel surface provides cathodic protection of the material. Therefore, nickel plating of steel guarantees reliability during operation of the product. If the surface is not partially protected by layers of nickel, then rust will soon appear, and the layer of hardened nickel will gradually peel off. It is recommended to cover the metal with a thick nickel coating.

Coating can be applied to copper and iron surfaces, or alloys based on them. Titanium or tungsten and other metals can also be treated with nickel. Plating materials such as lead, bismuth, tin or cadmium is not recommended. Before coating a steel surface, the latter should be treated with a thin layer of copper.

Electrolytic nickel plating

It is also called galvanic nickel plating. This method is considered inexpensive, so it is most often used. The coatings are porous and directly depend on the preparation of the base and the thickness of the protective coating layer. To this work was produced with proper quality, the percentage of pores should be reduced. For these purposes, preliminary copper plating of the part or multilayer coating is used.

Electrochemical nickel plating of bases is carried out in the following stages:

• Nickel plating electrolyte is prepared according to the described scheme. To do this, for 200 ml of water you need to prepare 60 grams of nickel sulfate, 7 grams of nickel chloride, 6 grams of boric acid. Thoroughly dilute all components in water in a designated container. To coat a steel or copper surface, use nickel anodes dipped directly into the electrolyte.
• Next, fix the part on a wire and place it between the nickel plates, and the wires passing through the nickel plates need to be connected. The parts are connected to a negative electrical charge, and the wires to a positive one.
• This is followed by connecting the rheostat and microammeter to the current source control circuit. To ensure such an action, it is necessary to select current sources with a voltage rating of no more than 6 V. The effect of current on the product should last no more than 20 minutes.
• Afterwards, the product to be treated must be washed and dried. The result is a matte grayish finish.
• To ensure shine, it is necessary to polish the surface layer.

In front of everyone positive qualities production of this operation, there is significant drawback, which needs to be remembered. During electrolytic processing metal product, the coating turns out to be uneven, that is, the shells are not filled, and in places of protruding roughness the nickel-plating layer flows off.

Chemical method

This method is considered expensive relative to the electrolytic method. The result is a fairly strong and thin base of the applied layer.

Nickel plating of parts is carried out as follows:

1. Take a 10% solution of zinc chloride and dilute it in small portions in a solution of nickel sulfate until a bright green tint is obtained.
2. Next, using a porcelain vessel, the resulting mixture should be heated until boiling. There is no need to be afraid that the result will be muddy; this will in no way affect the quality of the planned work.
3. For nickel plating, you should lower the part, previously cleaned of dust and degreased with soda solution, into a boiling solution.
4. The boiling process should last at least an hour, but as the liquid evaporates, distilled water must be gradually added to the container. If saturated green color will become lighter, this means that it is necessary to add a small portion of nickel sulfate.
5. After the boiling time has passed, remove the part and rinse it in water with chalk dissolved in it.
6. Dry thoroughly in the open air.

Products made of ferrous metal coated with this method are durable and reliable during operation.

Analysis of the chemical application of the protective layer shows that the ongoing process underlies the recovery of nickel from salt liquid using sodium hypophosphite and other elements. Solutions can be either alkaline or acidic.

The purpose of acid compositions is better suited for processing non-ferrous or ferrous metals. Alkalis are intended for application to stainless steel surfaces.

The acid provokes a decrease in discharge with increasing temperature, but the surface is obtained with a lower roughness index. When using this composition, good adhesion of the coating to the surface is ensured.

A water-based solution for nickel plating, used for all metals. You can use not only distilled water, but also condensation formed in the refrigerator. It is better to use clean chemicals with the letter “C” on the packaging.

To obtain a solution, initially all ingredients are diluted in water, and then sodium hypophosphite is added. One liter of solution is enough for nickel plating of a surface area of ​​10x10 cm2.

About black coating

Black nickel plating simultaneously serves two purposes:

• decorative coating;
• specialized purpose.

In this case, the protection properties of the metal are not sufficiently ensured; based on this conclusion, intermediate layers of zinc, cadmium or nickel should be applied. In this case, steel must be galvanized, and non-ferrous metals must be nickel-plated. The thickness of the coating is quite thick, up to 2 microns, so it is fragile. For baths containing nickel solution, a significant amount of thiocyanate and zinc is added.

The composition is about 50% of the element nickel, and the remainder contains carbon, zinc, nitrogen and sulfur.

Nickel plating of aluminum or steel structures is carried out by preparing baths with dissolution of all components, followed by filtering. With boric acid, problems usually arise when dissolving it, but it can be diluted separately in water at temperatures up to 700C. Rich nickel plating with this color is directly proportional to the supplied current density.

About nickel plating baths

In home workshops, nickel plating baths use three components: sulfate, boric acid and chloride. Sulfate - plays the role of a source of formation of nickel ions. For the functioning of nickel anodes, chloride has a significant influence, and the percentage of concentration is not taken into account.

If there is not enough chloride in the bath, then the release of nickel is small, the output current decreases, and the quality of the resulting coating leaves much to be desired.

The anodes are dissolved almost completely to allow the coating process to take place on aluminum or copper products. Chloride helps to increase the conductivity of baths at high concentrations of zinc. Boric acid solution provides normal level acidity.

Video: chemical nickel plating.

About chrome plating of plastic

Chrome plating of plastic at home is done as follows:

1. To coat the plastic, it is necessary to attach structural elements or parts to the transformer.
2. Take a brush, also attached to the transformer, and fill it with electrolyte.
3. Apply a layer of electrolyte to the previously prepared surface, using movements up and down.
4. If necessary, the application of the layer must be repeated.

In order for the coating layer to lay down well, the process should be repeated at least 30 times.

After processing, the surface of plastic parts must be dried and washed with water. Chrome plating of surfaces will look attractive if you rub the product with a piece of felt, this will add shine to the coating.

It is not always possible to chrome plating plastic products, so nickel-based solutions are preferred.

Chrome plating of plastic products is quite labor-intensive and expensive; for example, the price of a transformer is considerable. So the best solution will contact a specialized organization.

When performing any of the work on coating products, chemical processes, so Chemist's Handbook 21 will come in handy.