NICKEL PLATING, the technical process of applying to the surface of metals b. or m. a 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, and give it a more beautiful appearance. First obtained by Böttger in 1842 and industrially carried out in the United States since 1860, nickel plating has now become one of the most widely adopted methods of metal coating in the industry.
The existing numerous methods of nickel plating can be divided into two main groups: contact methods and methods electroplating; at the present time, the latter are especially often resorted to. Nickel film applied to surfaces 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 from alloys such as Britannia-metal, 4) aluminum and aluminum alloys. Nickel films provide quite satisfactory protection of iron from rusting indoors.
However, they are inadequate 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 kitchen utensils become stained. In those cases where it is required completely reliable protection from the effects of bad weather and at the same time the elegant appearance of the nickel-plated surface, on iron, etc. b. a double film was applied - zinc and then nickel. This method of double coating (zinc and then nickel) is also used for the so-called. corset steel. If it is necessary to obtain especially resistant films, for example, 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, cannot be made of pure nickel, but there is not enough support with a nickel film over iron or copper, they are lined with a layer of lead of several mm, and over it with a layer of nickel in 1-2 mm. Rusting of iron and steel nickel-plated products is due to the presence of electrolyte remaining in the fine pores of the nickel film. This phenomenon is eliminated if the products are kept in oil at 200 ° C before nickel plating, after cooling, degrease, slightly copper, then otickel in a nickel citrate bath with a weak current and finally dry in a cabinet at 200 ° C; then moisture is removed from the pores, which are clogged with oil 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 cover the products with a thin film of nickel by contact or electrolytic methods, and then immerse them in a bath with molten zinc or tin (Vivien and Lefebvre, 1860 g .). 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 serves for hot coating of iron sheets. Resistance of surfaces of aluminum and its alloys against salt and sea water m. b. achieved by galvanic deposition on them, after cleaning them with a sand jet, successive layers: nickel with a thickness of 6 microns, copper in 20 microns and then again nickel in 50 microns, after which the surface is polished. The resistance of aluminum against 15% soda alkali is achieved by a 40 µm thick nickel film. In some cases, a coating is used not with pure nickel, but with an alloy, for example, nickel-copper; for this, 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 article to a red-hot heat.
Contact nickel plating... Steel objects, according to F. Stolba (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 is formed from the basic nickel salt. Nickel plating lasts about 1 hour. After that, the item is rinsed in water with chalk, and the bath, after filtration and the addition of nickel salt, can be used again. The resulting nickel film is thin but holds firm. 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. In order to avoid rusting of objects, they are kept for 12 hours in lime milk. 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 sulphate nickel-ammonium salt.
Electroplated nickel plating... Depletion of the nickel bath m. B. Preventatively easy dissolution of nickel anodes. Rolled, and especially of pure nickel, anodes are difficult to dissolve and therefore, in technical nickel plating, nickel bars containing up to 10% iron are used as anodes. However, such anodes lead to deposition on the object of iron, and the presence of iron in the nickel film entails whole line vices of nickel plating. As indicated by Kalgane and Gammauge (1908), it is impossible to obtain a precipitate completely free of the latter with anodes with iron. But the nickel deposit will already contain only 0.10-0.14% iron, if the iron content in the anodes is reduced to 7.5%; the iron content in the sludge can be further reduced by enclosing the anodes in cloth bags, while the rotation of the electrodes leads to an increased iron content in the sludge and to a decrease in its yield. The presence of iron in the nickel film leads to the deposition of deposits with a gradually decreasing iron content and therefore heterogeneous in terms of mechanical properties at different depths; K. Engemann (1911) considers this inhomogeneity to be the only reason for the easy splitting off of nickel films. The presence of iron m. B. the cause of a number of other defects in nickel plating (see table), for example, the ease of rusting of films.
| Vice | Cause of occurrence | Control measure |
| No deposition of nickel, no gassing | Power supply does not work | Checking and renewing the energy source |
| The wires are wired wrong | Switching wires | |
| The bath is too cold | Heating the bath to a temperature above 15 ° С | |
| The bath is too sour | An aqueous solution is added ammonia or an aqueous suspension of nickel carbonate with continuous stirring and frequent testing on Congo paper | |
| Bath contains zinc | The bath is made alkaline by means of nickel carbonate, stirred for several hours, filtered and acidified with 10% sulfuric acid | |
| Incomplete coverage of the item with a nickel film | Insufficient current | Items are suspended at equal distances from the anodes, the bath is heated up to at least 20 ° С |
| Very deep concavities of the surface of the object | Small auxiliary anodes are installed, inserted into the recesses of the object | |
| Bath alkalinity | Careful acidification of the bath with 10% sulfuric acid with stirring and constant testing with litmus paper | |
| Easy peeling off white or yellow-nickelpolishing films | Contamination of the surface of objects with oxides and grease | Additional cleaning of the surface of objects |
| Too much voltage (above 4 V) | Increase the number of nickel-plated items or lower the voltage to 2.5-3 V | |
| Too much acidity in the bath | Neutralization with ammonia or aqueous suspension of nickel carbonate | |
| Nickel bath poverty | Removing some of the electrolyte and adding nickel salt until the bath turns to a normal green color | |
| Inappropriate viscosity and surface tension of the bath | Adding glycerin or amyl alcohol or herbal decoctions or other colloids | |
| Release of hydrogen ions | Addition of oxidizing agents or hydrogen scavengers; unbalanced AC application | |
| Inappropriate surface preparation of objects | Roughing surfaces, mechanically or chemically, coating them with a thin layer of nickel from a hot solution of nickel chloride or a cold concentrated solution of ethyl sulphate nickel | |
| Nickel film lag or rupture when objects are bent and stretched | The presence of capillary electrolyte layers | Drying and heating of objects up to 250-270 ° С |
| Insufficient workability of sheets coated with a thick nickel layer | Probably the same | Rinsing, drying without air access and finally heating to a weak red-hot heat |
| Dimpled surface and film riddled with countless pores | Dust and fiber particles floating in the bath | The bath is boiled, filtered and the correct reaction is set in it |
| Gas bubble formation | Tapping on the current-carrying rod. Bubbles are removed; establish a slightly acidic reaction | |
| Roughness and unevenness of the surface | Evolution of hydrogen | The introduction of hydrogen-binding free chlorine in gaseous form from an occasionally passed stream or in an aqueous solution; with slightly less success chlorine m b. replaced with bromine; the addition of a cobalt chloride solution is highly recommended |
| Insufficient flexibility of the film | High bath resistance | Sodium Salt Supplement |
| Yellowness of the film; the surface becomes matte, and then becomes yellow and dark yellow | The presence of iron impurities in the bath, the content of which increases in old baths | Avoid old baths, do not move baths too much, work with weak currents |
| Film blackness, dark streaks in lagging areas at correct current density | Content of foreign metals in the bath (up to 1%) | Removal of foreign metals |
|
Lack of conductive salts |
Adding conductive salts in the amount of 2-3 kg per 100 l of the bath: ammonia, potassium chloride and sodium chloride give an increase in conductivity by 84.31 and 18%, respectively | |
| Bath Poor Nickel Salt | Nickel salt addition | |
| Surface tanning | Too high conductivity of the bath due to its excessive strength | Control of bath concentration (e.g. constant density at 5 ° Vẻ) and current density |
| Formation of stripes | Dirt produced by the polishing wheel in small depressions | Elimination is difficult; achieved to a certain extent by instant immersion in a cauldron of liquor or by mechanical wiping of objects |
| Changes in concentration and occurrence of fluid flows | Decrease in current density and increase in bath temperature | |
| Staining | Insufficient cleaning of finished nickel-plated products | Thorough rinsing in running water of products after nickel plating, then immersion in boiling water clean water, shaking off products and drying in heated sawdust |
| Unstable adhesion of the nickel film to the iron | The presence of rust | Thorough removal of rust. Electroplating of an intermediate layer from a cyanogen bath, after which the film thickens in an acid bath |
The electrolytic bath for nickel plating is composed of Ch. way from double nickel-ammonium salt, and weak acids are added to eliminate basic salts. More acidity in 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 in sulfate baths the precipitation is harder, whiter and more stable than in chloride baths. The high resistance of the nickel bath can be advantageously reduced by the addition of various conductive salts - especially ammonia and sodium chloride - and by heating. The 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 by soda. For the whiteness and smoothness of the films, a large number of proposals have been made to add various organic acids (tartaric, citric, etc.) and their salts to the nickel bath, for example, acetic, citric and tartaric acid salts of alkali and alkaline earth metals (Keith, 1878, p. ), propionic nickel, boric tartrate 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, lactic acid per 1 liter of the 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 precipitate has good properties also with a simple bath of nickel-ammonium sulfate, but under the condition of the alkalinity of the solution, which is achieved by adding ammonia. Very good precipitates are obtained from a neutral solution of fluoric-boric acid nickel at room temperature (at temperatures above 35 ° C, the solution decomposes with the formation of an insoluble basic salt) and a current density of 1.1-1.65 A / dm 2 ... Here are some bath recipes. 1) 50 hours of sodium bisulfite, 4 hours of nitric oxide nickel and 4 hours of concentrated ammonia are dissolved in 150 hours of water. 2) 10-12 hours of nickel sulfate, 4 hours of double nickel-ammonium sulfate salt, 1-3 hours of boric acid, 2 hours of magnesium chloride, 0.2-0.3 hours of ammonium citrate, topped up to 100 hours . (total) water. Current density 1.6 A / dm 2 deposits the film at a speed of 2 microns / h; increasing the temperature to 70 ° C, it is possible to reduce the resistance of the bath two to three times and thus accelerate the nickel plating. 3) An electrolyte 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-porosity of the sediment, since it reduces the release of hydrogen.
Obtaining nickel films special kind ... 1) A white film for zinc, tin, lead and britain 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 must be kept neutral. 2) An opaque 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; bath d. b is concentrated up to 10 ° Vẻ; voltage from 2 to 2.5 V. 3) A black film is obtained on surfaces thoroughly degreased or coated with a thin layer of white nickel by electrolysis in a bath of 60 g of double nickel-ammonium sulfate salt, 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 of 9 g of double nickel-ammonium sulfate salt 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 sulphate nickel 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 a possibly small 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 l 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 in degreasing, then cleaning with a pumice stone and finally immersion in a 3% aqueous solution of potassium cyanide; after electrolysis in a nickel bath, the products are washed cold water... 2) After rinsing with a 2% solution of potassium cyanide, the products are immersed in a solution of 1 g of ferric chloride (ferrochloride) in 0.5 liters of water and technical hydrochloric acid until the surface turns silver-white, and then nickel for 5 minutes. at 3 V. some ferric chloride) and 38% nitric acid, new washing 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: etching of the product with boiling potassium alkali, brushing in milk of lime, 0.2% cyano-potassium bath, bath of 1 g of iron in 500 g of hydrochloric acid and 500 g of water, rinsing, 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 sediment. The iron bath serves to roughen the surface of the aluminum and thereby 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 precipitate is obtained, which has a high gloss after polishing with stearic oil and Viennese lime. 6) A hot bath (60 ° C) is composed of 3400 g of double nickel-ammonium sulfate salt, 1100 g of ammonium sulfate and 135 g of milk sugar in 27 liters of water. 7) 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 outer flame of a Bunsen burner: the nickel film turns blue, gets a black sheen and remains unharmed; silver does not change in a flame, but turns black when treated with a dilute solution of ammonium sulfide; 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 carried out using the so-called. ferroxyl test and with particular convenience using ferroxyl paper coated with agar-agar gel with ferruginous synergistic potassium and sodium chloride. Applied wet on the test surface and after 3-5 minutes. fixed in water, this paper gives a documentary image of the smallest pores, which can be used. persistent.
Nickel recovery from old products... Removal of nickel coating from 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 scrap with sulfur, after which the metal layer is easily removed with hammers; c) heating cuttings with substances that give off sulfur when high temperature) upon sudden cooling, the nickel film falls 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 nickel obtained still retains a significant iron content, which is not removed even during repeated acid treatment (T. Fleitman); e) prolonged heating with the access of air or water vapor, after which the cuttings are subjected to mechanical shock and the nickel rebounds; f) electrolytic dissolution: an iron plated object is made 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 by 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 there is a current; voltage is applied 2-5 V; iron sheets are used as cathodes, on which nickel is deposited in the form of dust; zinc does not dissolve, even if the mugs remained in the electrolyte for a long time.
Plating non-ferrous metal and steel parts with nickel increases their resistance to corrosion processes and mechanical wear. Nickel plating at home is available to everyone and is characterized by a simple technology.
1 Nickel plating of metal surfaces - the basics of technology
Nickel plating consists in applying a thin nickel coating to the surface of the workpiece, the thickness of which, as a rule, is 1–50 µm. The parts are subjected to this operation in order to protect them or to obtain the characteristic (matte black or shiny) appearance of the nickel-plated surface. The coating, regardless of the shade, reliably protects metal objects from corrosion in the open air, in solutions of salts, alkalis, weak organic acids.
As a rule, nickel-plated parts made of steel or such metals and alloys from them as copper, aluminum, zinc, less often titanium, manganese, molybdenum, tungsten. Do not use chemical nickel plating on the surfaces of products made of lead, tin, cadmium, bismuth, antimony. Nickel coatings are used in various industrial sectors for protective, decorative and special purposes or as an underlayer.
This technology is used to restore the surface of worn parts of various mechanisms and cars, coating of measuring and medical instruments, household items and products, chemical equipment, parts operated under light loads under conditions of exposure to strong alkali solutions or dry friction. There are 2 methods of nickel plating - electrolytic and chemical.
The second is somewhat more expensive than the first, however, it allows you to obtain a coating uniform in thickness and quality on the entire surface of the part, provided that the solution is provided access to all its areas. Nickel plating at home is a feasible task. Before starting work, the product is thoroughly cleaned of dirt and rust (if any), treated with fine sandpaper to remove the oxide film, washed with water, then degreased and washed again.
2 Secrets of Increasing the Resistance and Life of Nickel Plating
Before nickel plating, it is advisable to perform copper plating of the product (to cover it with a copper sublayer). This technology is used in industry, as a separate process, and also as a preparatory process before silvering, chrome plating, nickel plating. Copper plating, prior to the application of other layers, allows you to level out surface defects and ensures the reliability and durability of the external protective coating... Copper adheres very strongly to steel, and other metals are deposited on it much better than on pure steel. In addition, nickel coatings are not continuous and have through pores (up to the substrate metal) per 1 cm2:
- several dozen - for single-layer nickel coatings;
- several - for three-layer.
As a result, the metal of the substrate underneath the nickel undergoes corrosive processes, and conditions arise that provoke peeling of the protective coating. Therefore, even with preliminary copper plating, multilayer nickel plating, and especially with a single layer on pure steel, it is necessary to treat the surface of the protective nickel coating with special compounds that close the pores. At self-processing at home, the following methods are possible:
- wipe the coated part with a mushy mixture of water and magnesium oxide and immediately immerse it for 1-2 minutes in a 50% hydrochloric acid composition;
- wipe the surface of the part 2-3 times with an easily penetrating lubricant;
- Immediately after processing, immerse the still not cooled product in fish oil (non-vitaminized, preferably old, which is already unsuitable for its intended purpose).
In the last two cases, excess grease (fat) is removed from the surface after a day with gasoline. In the case of processing large surfaces (moldings, bumpers of cars), fish oil is used as follows. In hot weather, they wipe the part with it 2 times with an interval of 12-14 hours, and after 2 days the excess is removed with gasoline.
3 Electrolytic nickel plating at home
This method requires the preparation of an electrolyte, the composition of which is as follows:
- 140 g of nickel sulfate;
- 50 g of sodium sulfate;
- 30 g of magnesium sulfate;
- 5 g table salt (sodium chloride);
- 20 g boric acid;
- 1000 g of water.
The chemicals are dissolved separately in water, the resulting solutions are filtered and then mixed. The prepared electrolyte is poured into a container. For galvanic nickel plating, nickel electrodes (anodes) are required, which are dipped into an electrolyte bath (one electrode is not enough, since the resulting coating will be uneven). A part is suspended between the anodes on a wire. Copper conductors coming from the nickel plates are connected in one circuit and connected to the positive terminal of the source direct current, the wire from the part to the negative.
To control the current strength, a resistance (rheostat) and a milliammeter (device) are included in the circuit. The voltage of the current source must be no more than 6 V, the current density must be maintained at the level of 0.8–1.2 A / dm2 (surface area of the product), the electrolyte temperature is room temperature 18–25 oC. The current is supplied for 20-30 minutes. During this time, a nickel layer with a thickness of about 1 μm is formed. Then the part is taken out, properly washed with water and dried. The resulting coating will be matte gray. To make the nickel layer shine, the surface of the part is polished.
If there is no sodium sulfate and magnesium, then they take more nickel sulfate, bringing its amount in the electrolyte to 250 g, as well as boric acid - 30 g, sodium chloride - 25 g. Nickel plating in this case is carried out at current density values within 3-5 A / dm2, the solution is heated to 50–60 oC.
Disadvantages of the electrolytic method:
- on embossed, uneven surfaces nickel is deposited unevenly;
- impossibility of coating in deep and narrow cavities, holes and the like.
4 Chemical nickel plating of products at home
All compositions for chemical nickel plating versatile - suitable for processing any metal. Prepare solutions in a specific sequence. All chemical reagents (excluding sodium hypophosphite) are dissolved in water. The dishes must be enameled. Then the solution is heated, bringing its temperature to the working temperature, after which sodium hypophosphite is dissolved. The detail is hung in liquid composition, the temperature of which is maintained at the required level. In 1 liter of the prepared solution, it is possible to nickel the product, the surface area of which is up to 2 dm2.
The following solution compositions are used, g / l:
- Sodium succinic acid - 15, nickel chloride - 25, sodium hypophosphite - 30 (acidity of the solution pH - 5.5). The working temperature of the mixture is 90–92 ° С, the rate of coating build-up is 18–25 µm / h.
- Nickel sulfate - 25, sodium succinic acid - 15, sodium hypophosphite - 30 (pH - 4.5). Temperature - 90 ° С, speed - 15–20 μm / h.
- Nickel chloride - 30, glycolic acid - 39, sodium hypophosphite - 10 (pH - 4.2). 85–89 ° C, 15–20 μm / h.
- Nickel sulfate - 21, sodium acetate - 10, lead sulfide - 20, sodium hypophosphite - 24 (pH - 5). 90 ° С, up to 90 μm / h.
- Nickel chloride - 21, sodium acetate - 10, sodium hypophosphite - 24 (pH - 5.2). 97 ° С, up to 60 μm / h.
- Nickel chloride - 30, acetic acid - 15, lead sulfide - 10-15, sodium hypophosphite - 15 (pH - 4.5). 85–87 ° C, 12–15 μm / h.
- Nickel chloride - 30, ammonium chloride - 30, sodium succinic acid - 100, ammonia (25% solution) - 35, sodium hypophosphite - 25 (pH - 8–8.5). 90 ° С, 8-12 microns / h.
- Nickel chloride - 45, ammonium chloride - 45, sodium citrate - 45, sodium hypophosphite - 20 (pH - 8.5). 90 ° С, 18–20 μm / h.
- Nickel sulfate - 30, ammonium sulfate - 30, sodium hypophosphite - 10 (pH - 8.2-8.5). 85 ° С, 15-18 microns / h.
- Nickel chloride - 45, ammonium chloride - 45, sodium acetate - 45, sodium hypophosphite - 20 (pH - 8-9). 88–90 ° С, 18–20 μm / h.
After the required time has elapsed, the product is washed in water containing a small amount of loose chalk, then dried and polished. The coating obtained in this way is held by steel and iron quite firmly.
The chemical process of nickel plating is based on a reaction in which nickel is reduced from a solution of salts on its basis in the presence of sodium hypophosphite and with the help of other chemical reagents. The applied formulations are divided into alkaline (pH level exceeds 6.5) and acidic (pH value is 4–6.5). The latter are best used for processing ferrous metals, copper, brass, and alkaline ones are intended for nickel plating.
The use of acidic compounds makes it possible to obtain a smoother, more uniform surface on a polished product than with alkaline ones. Acidic solutions have another important feature - the likelihood of their self-discharge when the values are exceeded working temperature less than alkaline. Do-it-yourself nickel plating using alkaline compounds guarantees a stronger and more reliable adhesion of the nickel layer to the metal on which it was applied.
Nickel plating is used in mechanical engineering, instrument making and other industries. Nickel is used to cover parts made of steel and non-ferrous metals to protect them from corrosion, decorative finish, and increase resistance to mechanical wear. Due to its high corrosion resistance in alkali solutions, nickel coatings are used to protect chemical devices from alkaline solutions. In the food industry, nickel can replace tin coatings. Black nickel plating has become widespread in the optical industry
During the electrochemical deposition of nickel on the cathode, two main processes occur: Ni 2+ + 2e - → Ni and 2H + + 2e - → H2.
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. In the fight against pitting, substances are used that reduce the surface tension at the metal-solution interface.
Nickel is easily passivated during anodic dissolution. During the passivation of the anodes in the electrolyte, the concentration of nickel ions decreases and the concentration of hydrogen ions rapidly increases, which leads to a drop in the current efficiency and deterioration of the quality of precipitation. To prevent passivation of the 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.
Be careful! The company "LV-Engineering" does not provide services for the application of galvanic coatings! Our organization carries out the design of electroplating plants, the manufacture of electroplating baths and lines from polypropylene, installation and commissioning works in this direction.
Nickel plating sulfate electrolytes
Sulfuric electrolytes of nickel plating are the most widespread. These electrolytes are stable in operation; if used correctly, they can be used for several years without replacement. The composition of some electrolytes and modes of nickel plating:
| Composition | Electrolyte No. 1 | Electrolyte No. 2 | Electrolyte No. 3 |
| Nickel sulfate | 280-300 | 400-420 | |
| Sodium sulfate | 50-70 | - | - |
| Magnesium sulfate | 30-50 | 50-60 | - |
| Boric acid | 25-30 | 25-40 | 25-40 |
| Sodium chloride | 5-10 | 5-10 | - |
| Sodium fluoride | - | - | 2-3 |
| Temperature, ° C | 15-25 | 30-40 | 50-60 |
| Current density. A / dm 2 | 0,5-0,8 | 2-4 | 5-10 |
| pH | 5,0-5,5 | 3-5 | 2-3 |
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 easily polished deposits are obtained.
Nickel electrolyte is very sensitive even to small changes acidity. To maintain the pH value within the required range, it is necessary to use buffering 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.
For the preparation of nickel plating sulfate electrolytes, it is necessary to dissolve in separate containers in hot water all components. After settling, the solutions are filtered into a working bath. The solutions are stirred, 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 brought up to the required volume with water. In the presence of impurities, it is necessary to study it before using the electrolyte, since nickel electrolytes are extremely sensitive to foreign impurities, both organic and inorganic.
Defects during the operation of the electrolyte of shiny nickel plating and methods of their elimination are given in Table 1.
Table 1. Defects during the operation of nickel plating sulfate electrolytes and ways to eliminate them
| Defect | The cause of the defect | Remedy |
| Nickel does not precipitate. Abundant evolution of hydrogen | Low pH | Correct pH with 3% sodium hydroxide solution |
| Partial nickel plating | Poor degreasing of parts | Improve preparation |
| Incorrect position of the anodes | Distribute the anodes evenly | |
| Parts mutually shield each other | Change the arrangement of parts in the bath | |
| The coating has grey colour | Presence of copper salts in the electrolyte | Remove copper from electrolyte |
| Brittle, cracking coating | Treat electrolyte with activated carbon and work with current | |
| The presence of iron impurities | Remove iron from electrolyte | |
| Low pH | Correct pH | |
| Pitting formation | Electrolyte contamination organic compounds | Work out electrolyte |
| Low pH assignment | Correct pH | |
| Weak mixing | Increase stirring | |
| The appearance of black or brown streaks on the coating | The presence of zinc impurities | Purify electrolyte from zinc |
| Formation of dendrites on the edges of parts | High density current | Reduce current density |
| Excessive nickel plating process | Introduce an intermediate copper sublayer or reduce the electrolysis time | |
| Anodes are covered with a brown or black film | High anode current density | Increase the surface of the anodes |
| Low concentration of sodium chloride | Add 2-3 g / l sodium chloride |
For nickel plating, hot-rolled anodes are used, as well as non-passivating anodes. Anodes in the form of plates (cards) are also used, which are loaded into sheathed titanium baskets. Card anodes promote uniform nickel dissolution. To avoid contamination of the electrolyte with anode sludge, nickel anodes should be enclosed in cloth covers, which are pre-treated with a 2-10% hydrochloric acid solution.
The ratio of the anode surface to the cathodic surface during electrolysis is 2: 1.
Nickel plating small parts 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 the mismatch of the surface 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 at which the deposition of nickel stops altogether. A disadvantage when working in bells and drums is also a large entrainment of electrolyte with parts from the baths. Specific loss rates in this case are from 220 to 370 ml / m 2.
Brilliant Nickel Plating Electrolytes
For protective and decorative finishing of parts, shiny and mirror-like nickel coatings are widely used, obtained directly from electrolytes with gloss-forming additives. 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
Phthalimide 0.02-0.04 g / l
pH = 4-4.8
Temperature = 50-60 ° С
Current density = 3-8 A / dm 2
To obtain shiny nickel coatings, electrolytes with other brightening additives are also used: chloramine B, propargyl alcohol, benzosulfamide, etc.
When applying a shiny coating, it is necessary to intensively stir the electrolyte with compressed air, preferably in combination with the rocking of 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 with stirring sulfate and nickel chloride, boric acid. The electrolyte brought up to the working volume with water is subjected to chemical and selective cleaning. To remove copper and zinc, the electrolyte is acidified with sulfuric acid to pH 2-3, cathodes of a large area made of corrugated steel are hung and the electrolyte is worked through for a day at a temperature of 50-60 ° C, stirring with compressed air. Current density 0.1-0.3 A / dm 2. 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 stirred with compressed air... The solution is settled for 7-12 hours, then filtered into a working bath.
Brightening agents are introduced into the purified electrolyte: saccharin and 1,4-butynediol directly, phthalimide - after dissolving in a small amount of electrolyte heated to 70-80 ° С. Bring the pH to the required value and start work. Consumption of brighteners when adjusting the electrolyte is: saccharin 0.01-0.012 g / (A.h); 1,4-butnediol (35% solution) 0.7-0.8 ml / (A.h); phthalimide 0.003-0.005 g / (A.h).
Defects during the operation of the electrolyte of shiny nickel plating and methods of their elimination are given in Table 2.
Table 2. Defects during the operation of the electrolyte of shiny nickel plating and ways to eliminate them
| Defect | The cause of the defect | Remedy |
|
Insufficient gloss of the coating |
Low concentration of brighteners | Introduce brighteners |
| The specified current density and pH are not maintained | Adjust current density and pH | |
|
Dark color coverings and / or dark spots |
The electrolyte contains impurities of heavy metals | Perform selective cleaning of electrolyte at low current density |
| Pitting | The presence of iron impurities in the electrolyte | Purify electrolyte and introduce antipitting additive |
| Insufficient mixing | Increase air mixing | |
| Low electrolyte temperature | Raise electrolyte temperature | |
| Brittle sediments | Electrolyte contamination with organic compounds | Purify electrolyte with activated carbon |
| Decreased content of 1,4-butynediol | Add 1,4-butyndiol supplement |
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Titanium nitride deposition
We apply titanium nitride (TiN) by vacuum deposition on products with dimensions up to 2500x2500x2500 mm.
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Now it is possible to perform works on decorative application of brass and bronze
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N - Nickel plating
- Codes of applied coatings: N, N.b., Chem.N.tv, Chem.N, N.m.ch.
- Processed steels: any, including aluminum and titanium alloys
- Product dimensions: up to 1000x1000x1000 mm. Weight up to 3 tons.
- Coating products of any complexity
- Quality control department, quality certificate, work under the state defense order
general information
Nickel plating is a process of electroplating or chemical deposition of nickel from 1 micron to 100 microns thick.
Nickel coatings have high corrosion resistance, high hardness and good decorative properties.
Melting point of nickel: 1445 ° C
Microhardness of nickel coatings: up to 500 HV (chem. 800 HV)
The areas of application of nickel-plated parts depend on whether the nickel plating is used as a finishing or the nickel plating acts as a sub-layer (substrate) for other electroplating coatings.
Nickel plating can be applied to almost all metals.
Main areas of application for electroplating and chemical nickel plating:
Using nickel as a stand-alone coating
- For decorative purposes.
Nickel coatings have a good high-gloss sheen and practically do not tarnish in air. The coatings are well suited for use in atmospheric conditions due to their high corrosion resistance. Nickel is often plated decorative items, fences, equipment and tools. - For technical purposes.
For corrosion protection of electrical contacts or machinery operating in humid environments, as well as a coating for soldering. The black nickel plating process has become widespread in the optical industry.
- As a replacement for chrome plating.
In some cases, it is possible to replace chrome coatings with nickel ones, due to the technological difficulties of applying chromium to products with complex surface geometry. If the coating properties and application modes are selected correctly, the difference in the resource of coated products can be almost imperceptible (nodes and parts for various purposes, including for the food industry)
Nickel use in combination with other electroplating
- When applying multi-layer protective and decorative coatings.
Typically in combination with copper and chromium (copper plating, nickel plating, chrome plating) and other metals as an intermediate layer to increase the gloss of the chrome plating, as well as to protect against corrosion and prevent the diffusion of copper through the pores of the chromium to the surface, which can result in a short time for the appearance of red spots on the chrome finish.
Examples of parts with nickel plating
Nickel plating technology
During the electrochemical deposition of nickel on the cathode, two main processes take place: 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.
In the fight against pitting, substances are used that reduce the surface tension at the metal-solution interface.
Nickel is easily passivated during anodic dissolution. During the passivation of the anodes in the electrolyte, the concentration of nickel ions decreases and the concentration of hydrogen ions rapidly increases, which leads to a drop in the current efficiency and deterioration of the quality of precipitation. To prevent passivation of the 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 plating sulfate electrolytes are most widely used. These electrolytes are stable in operation; if used correctly, they can be used for several years without replacement. The composition of some electrolytes and modes of nickel plating:
| Composition | Electrolyte No. 1 | Electrolyte No. 2 | Electrolyte No. 3 |
| Nickel sulfate | 280-300 | 400-420 | |
| Sodium sulfate | 50-70 | - | - |
| Magnesium sulfate | 30-50 | 50-60 | - |
| Boric acid | 25-30 | 25-40 | 25-40 |
| Sodium chloride | 5-10 | 5-10 | - |
| Sodium fluoride | - | - | 2-3 |
| Temperature, ° C | 15-25 | 30-40 | 50-60 |
| Current density. A / dm 2 | 0,5-0,8 | 2-4 | 5-10 |
| pH | 5,0-5,5 | 3-5 | 2-3 |
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 easily polished deposits are obtained.
Nickel electrolyte is very sensitive to even small changes in acidity. To maintain the pH value within the required range, it is necessary to use buffering 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.
For the preparation of nickel plating 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 stirred, 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 brought up to the required volume with water.
In the presence of impurities, it is necessary to study it before using the electrolyte, since nickel electrolytes are extremely sensitive to foreign impurities, both organic and inorganic.
Defects during the operation of the electrolyte of shiny nickel plating and methods of their elimination are given in Table 1.
Table 1. Defects during the operation of nickel plating sulfate electrolytes and ways to eliminate them
| Defect | The cause of the defect | Remedy |
| Nickel does not precipitate. Abundant evolution of hydrogen | Low pH | Correct pH with 3% sodium hydroxide solution |
| Partial nickel plating | Poor degreasing of parts | Improve preparation |
| Incorrect position of the anodes | Distribute the anodes evenly | |
| Parts mutually shield each other | Change the arrangement of parts in the bath | |
| The coating is gray | Presence of copper salts in the electrolyte | Remove copper from electrolyte |
| Brittle, cracking coating | Treat electrolyte with activated carbon and work with current | |
| The presence of iron impurities | Remove iron from electrolyte | |
| Low pH | Correct pH | |
| Pitting formation | Electrolyte contamination with organic compounds | Work out electrolyte |
| Low pH assignment | Correct pH | |
| Weak mixing | Increase stirring | |
| The appearance of black or brown streaks on the coating | The presence of zinc impurities | Purify electrolyte from zinc |
| Formation of dendrites on the edges of parts | High current density | Reduce current density |
| Excessive nickel plating process | Introduce an intermediate copper sublayer or reduce the electrolysis time | |
| Anodes are covered with a brown or black film | High anode current density | Increase the surface of the anodes |
| Low concentration of sodium chloride | Add 2-3 g / l sodium chloride |
For nickel plating, hot-rolled anodes are used, as well as non-passivating anodes. Anodes in the form of plates (cards) are also used, which are loaded into sheathed titanium baskets. Card anodes promote uniform nickel dissolution. To avoid contamination of the electrolyte with anode sludge, nickel anodes should be enclosed in cloth covers, which are pre-treated with a 2-10% hydrochloric acid solution.
The ratio of the anode 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 the mismatch of the surface 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 at which the deposition of nickel stops altogether. A disadvantage when working in bells and drums is also a large entrainment of electrolyte with parts from the baths. Specific loss rates in this case are from 220 to 370 ml / m 2.
For protective and decorative finishing of parts, shiny and mirror-like nickel coatings are widely used, obtained directly from electrolytes with gloss-forming additives. 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
Phthalimide 0.02-0.04 g / l
pH = 4-4.8
Temperature = 50-60 ° С
Current density = 3-8 A / dm 2
To obtain shiny nickel coatings, electrolytes with other brightening additives are also used: chloramine B, propargyl alcohol, benzosulfamide, etc.
When applying a shiny coating, it is necessary to intensively stir the electrolyte with compressed air, preferably in combination with the rocking of 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 with stirring sulfate and nickel chloride, boric acid. The electrolyte brought up to the working volume with water is subjected to chemical and selective cleaning.
To remove copper and zinc, the electrolyte is acidified with sulfuric acid to pH 2-3, cathodes of a large area made of corrugated steel are hung and the electrolyte is worked through for a day at a temperature of 50-60 ° C, stirring with compressed air. Current density 0.1-0.3 A / dm 2. 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 stirred with compressed air. The solution is settled for 7-12 hours, then filtered into a working bath.
Brightening agents are introduced into the purified electrolyte: saccharin and 1,4-butynediol directly, phthalimide - after dissolving in a small amount of electrolyte heated to 70-80 ° С. Bring the pH to the required value and start work. Consumption of brighteners when adjusting the electrolyte is: saccharin 0.01-0.012 g / (A.h); 1,4-butnediol (35% solution) 0.7-0.8 ml / (A.h); phthalimide 0.003-0.005 g / (A.h).
Defects during the operation of the electrolyte of shiny nickel plating and methods of their elimination are given in Table 2.
Table 2. Defects during the operation of the electrolyte of shiny nickel plating and ways to eliminate them
| Defect | The cause of the defect | Remedy |
|
Insufficient gloss of the coating |
Low concentration of brighteners | Introduce brighteners |
| The specified current density and pH are not maintained | Adjust current density and pH | |
|
Dark coating color and / or dark spots |
The electrolyte contains impurities of heavy metals | Perform selective cleaning of electrolyte at low current density |
| Pitting | The presence of iron impurities in the electrolyte | Purify electrolyte and introduce antipitting additive |
| Insufficient mixing | Increase air mixing | |
| Low electrolyte temperature | Raise electrolyte temperature | |
| Brittle sediments | Electrolyte contamination with organic compounds | Purify electrolyte with activated carbon |
| Decreased content of 1,4-butynediol | Add 1,4-butyndiol supplement |
Multilayer nickel plating is used to improve the corrosion resistance of nickel plating over single-layer plating.
This is achieved by sequential deposition of nickel layers from several electrolytes with different physicochemical properties of the coating. Multilayer nickel coatings include bi-nickel, tri-nickel, sil-nickel.
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 matt and shiny nickel coatings.
To achieve high corrosion resistance, the first nickel layer (matt or semi-gloss), which is at least 1/2 - 2/3 of the total thickness of the coating, deposited from a standard electrolyte, practically does not contain sulfur. A second layer of nickel is deposited from a shiny nickel plating electrolyte; the sulfur contained in organic brighteners is part of the nickel coating, while the electrode potential of the second shiny layer is shifted by 60-80 mV towards electronegative values with respect to the first layer. Thus, the shiny nickel layer becomes the anode in the galvanic pair and protects the first layer from corrosion.
Three-layer nickel plating has the highest corrosion resistance. With this method, after the first layer of nickel is deposited from the same electrolyte as in two-layer nickel plating, the middle layer of nickel is deposited from the electrolyte, which includes a special sulfur-containing additive that ensures the inclusion of a large amount of sulfur (0.15-0.20%) in composition of the intermediate nickel layer. Then a third top layer of electrolyte is applied to obtain glossy coatings. In this case, 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 deposited from a shiny nickel plating electrolyte. The parts are then transferred to a second electrolyte where the silica-nickel is deposited. This electrolyte contains a non-conductive highly dispersed kaolin powder in an amount of 0.3-2.0 g / l. Temperature 50-60 ° C, current density 3-4 A / dm 2. The process is carried out without continuous filtration. To ensure a uniform distribution of kaolin particles throughout the electrolyte volume, intensive air mixing is used. The silica-nickel layer increases the wear resistance of the coating and has a high corrosion resistance.
Sil-nickel is used as the last layer before chromium in a protective and decorative coating. Due to the high dispersion of inert particles thin layer silt-nickel (1-2 microns) does not change the decorative appearance of the shiny nickel-plated surface, and with subsequent chromium plating it allows 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 reducer. Sodium hypophosphite is used as a reducing agent.
Chemical nickel plating is used to cover parts of any configuration with nickel. Chemically reduced nickel has high corrosion resistance, high hardness and wear resistance, which can be significantly increased during heat treatment (after 10-15 minutes of heating at a temperature of 400 ° C, the hardness of chemically deposited nickel rises to 8000 MPa). At the same time, the adhesion strength also increases. Nickel coatings restored with hypophosphite contain up to 15% phosphorus. The reduction of nickel with 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 gppophosphite occurs. The degree of useful use of gppophosphite is assumed to be about 40%.
The reduction of nickel from its salts with hypophosphite spontaneously puffs only on metals of the iron group, which catalyze this process. For the coating of other catalytically inactive metals (for example, copper, brass), contact of these metals in solution with aluminum or other metals more electronegative than nickel is required. 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, nickel plating does not form even when using the contacting and activation method.
Chemical deposition of nickel is possible from both alkaline and acidic solutions. Alkaline solutions are characterized by high stability and easy adjustment. The composition of the solution and the mode of nickel plating:
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 water, 25-% - 70-100 ml / l
pH = 8-9
Temperature = 80-90 ° С
The coatings obtained in acidic solutions are characterized by lower porosity than from alkaline solutions (at a thickness above 12 μm, the coatings are practically non-porous). Of acidic solutions of chemical nickel plating, the following composition (g / l) and nickel plating mode are recommended:
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 ° С
Deposition rate = 10-15 μm / h
Chemical nickel plating is carried out in glass, porcelain or enamelled iron baths. Carbon steel is used as the material for the suspensions.
V recent times a nickel-boron alloy is chemically applied using boron-containing compounds as a reductant - sodium borohydride and dimethyl borate, which have a higher reducibility than hypophosphite.
The resulting nickel-boron alloy coatings have high wear resistance and hardness.
To estimate the cost of work, please send a request by e-mail[email protected]
It is advisable to attach a drawing or sketch of the products to the request, as well as indicate the number of parts.
The price section shows the cost of nickel plating of products
The most widespread are chemical coatings with nickel, copper, silver, palladium, cobalt, and less often with tin, chromium and other metals.
Chemical nickel plating. The recovery 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, the restoration 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 recovery).
The released hydrogen also reduces phosphite to phosphorus; therefore, the nickel coating contains 6–8% phosphorus, which largely determines its specific properties (Table 24).
24. Properties of chemical and electroplating nickel plating
Despite the fact that nickel, besieged chemically, has significant corrosion resistance, it cannot be used for corrosion protection in the environment of nitric and sulfuric acids. After heat treatment, such nickel has a hardness of HV 1000-1025.
Basically, the technological process of nickel plating is as follows. Parts made of steel, copper and its alloys are prepared in the same way as for electroplated coatings.
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 ° С, deposition rate 18 μm / h (at 90 ° С and loading density 1 dm 2 / l), pH = 4.1 ÷ 4.3.
Parts during the nickel plating process must be shaken. It is allowed to replace thiourea with maleic anhydride in the amount of 1.5 - 2 g / l.
To initiate the deposition of nickel 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 plastic wrap, as well as in silicate glass containers.
With high-speed deposition and with a high loading density of parts of a simple profile, 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 ° С, deposition rate 18 μm / h (at 90 ° С and loading density 3 dm 2 / l), pH = 5.6 ÷ 5.7.
After chemical nickel plating, the parts are washed in a trap, then in running cold and hot water, dried at 90 ± 10 ° С for 5 - 10 min and thermally treated at 210 ± 10 ° С for 2 hours (in order to remove internal stresses and increase adhesion strength to the base). Further, depending on the operating conditions, the parts are varnished, treated with a hydrophobic liquid (GKZh, etc.) or fed to the assembly without processing.
The main reasons for poor quality plating in 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 places on parts complex configuration due to the formation of gas bubbles and uneven washing of parts with a 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) a decrease in the acidity of the solution (cracking, brittle coating);
5) an increase in the acidity of the solution (the coating is rough and rough).
The pH value is adjusted by adding 10% acetic acid or sodium hydroxide solution.
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 the parts, depending on the conditions of their operation, is indicated in table. 25.
25. Coating thickness values depending on operating conditions
So, at pH = 5.5, the sediments contain 7.5% phosphorus, and at pH = 3.5, 14.6%. An increase in the hardness of the coating to 1100-1200 kgf / mm 2 at 200-300 ° C is caused by the precipitation 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. In this case, the modulus of elasticity is 19000 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 h. Coefficient of friction of the coating (with a load> 10 kgf) after its application is the same as and shiny chrome. Specific wear of the nickel coating at 100 ° C is 2.10 -3 mm 3 / m.
Stirring the acidic solution increases the brilliance of the precipitation and the rate of precipitation. If the deposition process is interrupted for a few minutes, then the parts can be loaded into the bath without additional activation. For 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 speed is a function of the ratio Ni 2+: H 2 PO - 2. For a normal acidic bath, it should fluctuate between 0.25 - 0.60 (for acetate buffered 0.3-0.4).
In the presence of ammonium salts, the deposition rate decreases. In newly prepared solutions, the deposition rate is high at first, and then decreases with aging. So, 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 a coating is determined by the quality of the surface preparation of the substrate to be polished. In alkaline baths, the 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 tinge. The spread over the coating thickness of 30 µm, even on parts of complex configuration, is, for example, no more than 1–2 µm. When the bath is operated at a constant pH, 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 / cm 2), but worsens if the temperature of the solution 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 composition of the solution is the formation of a white foam due to excessive hydrogen evolution in the entire volume of the bath. Then a very fine black suspension of Ni-P appears, which accelerates the decomposition reaction of the solution.
The reasons for the premature decomposition of the solution may be: too rapid introduction of alkali and hypophosphite (add a dilute aqueous solution with vigorous stirring); local overheating; too high a content of hypophosphite (you need to lower the pH and temperature); introduction of palladium into a solution with parts activated in PdCl 2, incorrect ratio of the total area of parts to the volume of the solution.
The level of the solution in the bath must be kept constant, since lowering it due to evaporation leads to the concentration of the solution. In the process of covering parts, the heaters (steam, heat and electric heating, etc.) should not be turned off.
In contrast to hydrozine, sodium hypophosphite has an important advantage, since the sediment contains 8-10 times less gases. The addition of sodium thiosulfate helps to reduce the porosity of nickel. So, with a thickness of 20 microns, it decreases from 10 to 2 pores / cm 2. When choosing a material for a bath, it should be borne in mind 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 have to be removed from the walls of the bath from time to time. Bathtubs with a volume of 20 liters are made of pyrex, and the larger one is made of polished ceramics. Inner surface steel tanks covered with vitreous 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 to be 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 rod-type 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 ° С, from parts made of corrosion-resistant steels in a 25% solution of HNO 3, and from brass and copper ones - by anodic dissolution in H 2 SO 4.
In order to improve the stability of the composition of the solution, foreign companies recommend adding chromium salts. The porosity of the coatings obtained in a solution containing 10 g / l of K 3 Fe (CN) 6 and 20 g / l of NaCl is determined within 10 minutes. Pores are completely absent at coating thickness => 100 µm.