Technologies for metallization of parts and structures are widespread in various fields of industry and construction. An additional coating protects the surface from external damage and factors that contribute to the complete destruction of the material. One of these processing methods is chemical nickel plating, whose strong film is characterized by mechanical and corrosion resistance and the ability to withstand temperatures of the order of 400 ° C.
Technology features
Along with a nickel-based chemical coating, there are galvanic and electrolytic processing methods. The features of the technique under consideration should immediately include the precipitation reaction. It is organized under conditions of nickel reduction based on sodium hypophosphite in a saline solution with the addition of water. The industry mainly uses chemical nickel plating technologies with active acidic and alkaline compounds, which just start the deposition processes. The coating thus treated acquires a brilliant metallized appearance, the structure of which is a combined alloy of nickel and phosphorus. A technology made with the presence of the latter substance has lower physicochemical parameters. Acid and alkaline solutions can give different phosphorus content coefficients - the first up to 10%, and the second - about 5-6%.
The physical properties of the coating will depend on the amount of this substance. The specific gravity of phosphorus can be about 7.8 g / cm3, the electrical resistance is 0.60 ohm · mm2 / m, and the melting temperature is from 900 to 1200 °. Using the heat treatment operation at 400 ° C, the hardness of the coating can be increased to 1000 kg / mm2. Along with this, the strength of the coupling of the workpiece with a nickel-phosphorus structure will increase.
As for the fields of application of chemical nickel plating, unlike many alternative methods of protective metallization, it is optimally suited for working with parts and structures of complex shapes. In practice, technology is often applied to coils and internal surfaces of multi-format pipes. The coating is applied evenly and accurately - without gaps and other defects of the protective layer. Regarding the availability of processing for different metals, the restriction applies only to lead, tin, cadmium and zinc. Conversely, nickel phosphorus deposition is recommended for ferrous metals, aluminum and copper parts.
The method of nickel plating on alkaline solutions
Deposition in alkalis provides the coating with high mechanical resistance, which is distinguished by the possibility of easy adjustment and the absence of negative factors such as precipitation of powdered nickel. There are various formulations that are prepared depending on the type of metal being processed and its purpose. Typically, the following chemical nickel plating solution of this type is used:
- Citric acid sodium.
- Sodium hypophosphite.
- Ammonium (chlorinated).
- Nickel.
At a temperature of the order of 80-90 °, the process takes place at a speed of about 9-10 microns / hour, while the deposition is accompanied by the active evolution of hydrogen.
The preparation procedure itself is expressed in the dissolution of each of the above ingredients in a separate order. An exception in this composition of chemical nickel plating is perhaps sodium hypophosphite. It is poured in a volume of about 10-20 g / l by the time all the other components are dissolved, and the temperature is brought to the optimum mode.
Otherwise, special requirements for the preparation of the deposition process in an alkaline solution are not provided. The metal workpiece is cleaned and hung without any special study.
The preparation of the surfaces of steel parts and structures for coating does not have pronounced features. During the process, the solution can be adjusted by adding the same sodium hypophosphite or 25 percent ammonia. In the second case, under the condition of a large volume of the bath, ammonia is introduced from the cylinder in a gaseous state. A rubber tube is immersed to the very bottom of the container and through it a direct feed of the additive is carried out in a continuous mode to the desired consistency.
Nickel plating on acid solutions
Compared to alkaline media, acidic products are characterized by a variety of additives. It is possible to modify the base of hypophosphite and nickel salts with sodium acetate, lactic, succinic and tartaric acid, as well as Trilon B and other organic compounds. Among the large number of formulations used, the following solution for chemical nickelization by acid deposition is most popular:
- Sodium hypophosphite.
- Nickel sulfate.
- Carbon dioxide sodium.
The deposition rate will be the same 9-10 microns / hour, and the pH is adjusted with a solution of caustic soda 2%. The temperature is maintained strictly within 95 °, since its increase can lead to self-discharge of nickel with instant precipitation. Sometimes splashing of the solution from the container is also observed.
Changing the composition with respect to the concentration of its main ingredients is possible only if the content of sodium phosphite in it is about 50 g / l. In this state, nickel phosphite may precipitate. When the solution reaches the above concentration, the solution is drained and replaced with a new one.
When is heat treatment required?
If the workpiece needs to ensure the quality of wear resistance and hardness, a heat treatment operation is performed. An increase in these properties is caused by the fact that, under conditions of increasing temperature, a nickel-phosphorus precipitate precipitates, followed by the formation of a new chemical compound. It helps to increase the hardness in the structure of the coating.
Depending on the temperature regime, microhardness changes with different characteristics. Moreover, the correlation is not at all uniform with respect to an increase or decrease in the heating temperature. When heat treated as part of chemical nickel plating under conditions of 200 and 800 °, for example, the microhardness index will be only 200 kg / mm2. The maximum value of hardness is achieved at temperatures of 400-500 °. In this mode, you can count on providing 1200 kg / mm2.
It should also be borne in mind that not all metals and alloys can in principle accept heat treatment. For example, a ban is imposed on steels and alloys that have already undergone quenching and normalization procedures. Added to this is the fact that heat treatment in air can contribute to the formation of a discoloration color that changes from a golden to a purple hue. Minimizing such factors will help to reduce the temperature to 350 °. The whole process is carried out on the order of 45-60 minutes only with the workpiece free from contamination. External polishing will directly affect the likelihood of obtaining a high-quality result.
Processing Equipment
For the production of this technology does not require highly specialized and industrial units. At home, chemical nickel plating can be arranged in a steel enamel bath or bowl. Sometimes experienced craftsmen use a lining for ordinary metal containers, due to which the surfaces are protected from the action of acids and alkalis.
For tanks with a displacement of up to 50-100 liters, auxiliary enameled tanks resistant to nitric acids can also be used. As for the lining itself, its base is prepared from water-resistant universal glue (for example, “Moment” No. 88) and powdered chromium oxide. Again, in domestic conditions, specialized powder mixtures can be replaced with emery micropowders. To fix and process the applied lining, air drying with a construction hairdryer or heat gun is required.
Professional chemical nickel plating plants do not require special surface protection and are distinguished by the presence of removable covers. The coatings are removed after each treatment session and cleaned separately in nitric acid. The main structural feature of such equipment can be called the presence of baskets and pendants (usually made of carbon steel), which facilitate the manipulation of small parts.
Nickel plating processes of stainless steel and acid resistant metals
The purpose of this operation is to increase the wear resistance and surface hardness of the workpiece, as well as providing corrosion protection. This is the standard procedure for chemical nickel plating for steels that have been alloyed and are prepared for use in aggressive environments. A special place in the coating technique will be the preparation of the part.
For stainless alloys, preliminary refinement in an anode medium with an alkaline solution is used. The workpieces are mounted on suspensions with the connection of internal cathodes. The weighing is carried out in a container with a caustic soda solution of 15%, and the electrolyte temperature in this case is 65-70 °. To form a uniform coating without gaps, electrolytic and chemical nickel plating of stainless alloys should be carried out under conditions of supporting the current density (anode) of up to 10 A / dm2. The duration of the process varies from 5 to 10 minutes, depending on the size of the part. Next, the workpiece is washed in running cold water and decapitated in dilute hydrochloric acid for about 10 seconds at a temperature of 20 °. After this, a typical alkaline deposition procedure is performed.
Nickel plating of non-ferrous metals
Metals that are soft and malleable to chemical processes undergo special training before processing. The surfaces are degreased, and in some cases polished. If the preform had already been nickel-plated before, then the procedure for decapitation in a 25% diluted solution with sulfuric acid should also be carried out within 1 min. Elements based on copper and its alloys are recommended to be processed in contact with electronegative metals like aluminum and iron. Technically, such a combination is provided by a suspension or a tenacious wire of the same substances. As practice shows, sometimes during the reaction one touch of an iron part to a copper surface is enough to achieve the desired deposition effect.
Chemical nickel plating of aluminum and its alloys has its own characteristics. In this case, etching of the preforms in an alkaline solution is organized or clarification to acid based on nitrogen is performed. A double zincate treatment is also used, for which a composition is prepared with zinc oxide (100 g / l) and sodium hydroxide (500 g / l). The temperature regime must be maintained within 20-25 °. The first approach with the immersion of the part lasts 30 seconds, and then the process of etching of the zinc precipitate in nitric acid begins. This is followed by a second, already 10-second dive. At the final stage, the aluminum is washed with cold water and nickel-plated with a nickel-phosphorus solution.
Ceramic nickel plating technology
For materials of this type, the general method of nickel plating of ferrites is used. At the preparation stage, the part is degreased with a solution of soda ash, washed with hot water and etched for 10-15 minutes in an alcohol solution with the addition of hydrochloric acid. Next, the workpiece is again washed with hot water and cleaned with soft abrasives from sludge. Immediately before the chemical nickel plating process begins, the cermet is coated with a layer of palladium chloride. With a brush, a solution with a concentration of 1 g / l is applied to the surface. The procedure is repeated several times and after each pass the workpiece is dried.
For nickel plating use a container with an acidic solution, which contains nickel chloride (30 g / l), sodium hypophosphite (25 g / l) and sodium succinic acid (15 g / l). The temperature of the solution is maintained in the range of 95-98 °, and the recommended hydrogen coefficient is 4.5-4.8. After chemical nickel plating, the cermet is washed in hot water, and then boiled and immersed in a pyrophosphate copper-plated electrolyte. In an active chemical environment, the workpiece is held until a 1-2 micron layer is formed. Different types of ceramics, quartz elements, ticond and thermocond can also be subjected to similar processing. In each case, palladium chloride coating, air drying, immersion in an acid solution and boiling are mandatory.
Nickel plating technology at home
Technically, the operation of nickel plating is possible without special equipment, as already noted. For example, in a garage, it may look like this:
- Appropriate-sized cookware with enameled inner coating is being prepared.
- Pre-prepared dry reagents for the electrolytic solution in an enameled container are mixed with water.
- The resulting mixture is boiled, after which sodium hypophosphite is added to it.
- The workpiece is cleaned and degreased, and then immersed in the solution, but without touching the surfaces of the container - that is, the bottom and walls.
- Features of nickel plating at home are that all the equipment will be made from improvised materials. For the same control of the part, you can provide a special bracket (required from a dielectric material) with a clip, which will need to be left in a stationary position for 2-3 hours.
- For the above time, the composition is left in a boiling state.
- When the nickel plating period passes, the part is removed from the solution. It must be washed under a cold stream of water diluted in slaked lime.
At home, you can nickel steel, brass, aluminum, etc. For all of these metals, an electrolytic solution should be prepared containing sodium hypophosphite, nickel sulfate or chloride, as well as with acid inclusions. By the way, a lead additive can be added to speed up the process.
Conclusion
There are different techniques and approaches for performing nickel plating in active chemical solutions, but the use of sodium hypophosphite is the most advantageous method. This is due to the minimum amount of unwanted precipitation, and a combination of a whole set of technical and physical properties of the coating with a thickness of about 20 microns. Of course, chemical nickel plating is accompanied by certain risks of defect formation. This is especially true for highly sensitive color, but such phenomena can be combated within the framework of a single technological process. For example, experts recommend removing defective areas in a concentrated nitrogen-based acidic medium at temperatures up to 35 ° C. This procedure is performed not only in the event of undesirable flaws, but also for the purpose of regular correction of the applied protective layer.