Chemical nickel plating - features, technology and recommendations

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

Technologies for metallization of parts and structures are widespread in various areas 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 such treatment is electroless nickel plating, which has a durable film that is mechanically and corrosion resistant and can withstand temperatures of around 400°C.

Technology Features

Along with nickel-based chemical plating, there are electroplating and electroplating methods. The precipitation reaction should immediately be attributed to the features of the technique under consideration. It is organized under the conditions of nickel reduction on the basis of sodium hypophosphite in a saline solution with the addition of water. In industry, chemical nickel plating technologies are predominantly used with the connectionactive acidic and alkaline compounds, which just start the precipitation processes. The coating treated in this way acquires a shiny metallized appearance, the structure of which is a combined alloy of nickel and phosphorus. The technology, made with the presence of the last substance in the composition, has lower physical and chemical indicators. Acid and alkaline solutions can give different coefficients of phosphorus content - the first up to 10%, and the second - about 5-6%.

The physical qualities of the coating will also 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 point is from 900 to 1200°. By means of a heat treatment operation at 400°, the hardness of the deposited coating can be increased up to 1000 kg/mm2. At the same time, the adhesion strength of the workpiece with a nickel-phosphorus structure will also increase.

As far as chemical nickel plating is concerned, unlike many alternative protective plating methods, it is best suited for working with parts and structures of complex shape. In practice, the 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 in 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 andcopper parts.

Nickel plating method in alkaline solutions

Alkali precipitation provides the coating with high mechanical resistance, which is characterized by the possibility of easy adjustment and the absence of negative factors such as precipitation of nickel powder. There are different recipes that are prepared depending on the type of metal being processed and its purpose. The composition of this type of chemical nickel plating solution is usually used as follows:

• Citric acid sodium.
• Sodium hypophosphite.
• Ammonium (chlorinated).
• Nickel.

At a temperature of about 80-90°, the process takes place at a speed of about 9-10 microns/hour, while the deposition is accompanied by active evolution of hydrogen.

The procedure for preparing the recipe itself is expressed in the dissolution of each of the above ingredients in a separate order. The only exception in this composition of chemical nickel plating will be sodium hypophosphite. It is poured in a volume of about 10-20 g / l by the time when all other components are dissolved, and the temperature is brought to the optimal mode.

Otherwise, there are no special requirements for the preparation of the deposition process in an alkaline solution. The metal blank is cleaned and hung without special treatment.

Preparation of surfaces of steel parts and structures for coating has no pronounced features. During the process, you can adjust the solution by adding the same sodium hypophosphite or25% ammonia. In the second case, under the condition of a large bath volume, ammonia is introduced from a cylinder in a gaseous state. A rubber tube is immersed to the very bottom of the container and the additive is directly fed through it in a continuous mode to the desired consistency.

Nickel plating with acid solutions

Compared to alkaline media, acid media are characterized by a variety of additives. The base of hypophosphite and nickel s alts can be modified with sodium acetate, lactic, succinic and tartaric acids, as well as Trilon B and other organic compounds. Among the large number of formulations used, the most popular solution for chemical nickel plating by acid deposition:

• Sodium hypophosphite.
• Nickel sulphate.
• Sodium carbonate.

The deposition rate will be the same 9-10 microns/hour, and the pH value is corrected with 2% sodium hydroxide solution. 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.

It is possible to change the parameters of the composition relative to the concentration of its main ingredients only if the content of sodium phosphite in it is about 50 g/l. In this state, nickel phosphite precipitation is possible. When the parameters of the solution reach the above concentration, the solution is drained and replaced with a new one.

When thermalprocessing?

If the workpiece needs to ensure the quality of wear resistance and hardness, a heat treatment operation is performed. The increase in these properties is due to the fact that under conditions of an increase in the temperature regime, nickel-phosphorus precipitation occurs, followed by the formation of a new chemical compound. It contributes to the increase in hardness in the structure of the coating.

Depending on the temperature regime, there is a change in microhardness with different characteristics. Moreover, the correlation is not at all uniform with respect to an increase or decrease in the heating temperature. During heat treatment in chemical nickel plating at 200 and 800°, for example, the microhardness index will be only 200 kg/mm2. The maximum value of hardness is reached 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 are, in principle, acceptable heat treatment. For example, the ban is imposed on steels and alloys that have already undergone hardening and normalization procedures. Added to this is the fact that heat treatment in air can contribute to the formation of a tint color that changes from golden to purple. Lowering the temperature to 350 ° will help minimize such factors. The whole process is performed in the order of 45-60 minutes only with the workpiece cleaned of contaminants. External polishing will directly affect the likelihood of obtaining a quality result.

Processing equipment

For productionThis technology does not require highly specialized and industrial units. At home, chemical nickel plating can be arranged in an enameled steel bath or dish. Sometimes experienced craftsmen use a lining for ordinary metal containers, thanks to which the surfaces are protected from the action of acids and alkalis.

With reference to capacities 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 waterproof 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 building hair dryer or heat gun is required.

Professional chemical nickel plating installations do not require special surface protection and are distinguished by the presence of removable covers. Coatings are removed after each treatment session and cleaned separately in nitric acid. The main design feature of such equipment can be called the presence of baskets and suspensions (usually made of carbon steels), which facilitate the manipulation of small parts.

Nickel plating processes for stainless steel and acid-resistant metals

The purpose of this operation is to increase the wear resistance and hardness of the workpiece surface, as well as to provide anti-corrosion protection. This is the standardchemical nickel plating procedure for steels that have been alloyed and are being prepared for use in aggressive environments. Part preparation will have a special place in the coating technique.

For stainless alloys, preliminary refinement in an anode environment in an alkaline solution is used. The workpieces are mounted on hangers with the connection of internal cathodes. The weighing is carried out in a container with a 15% caustic soda solution, and the electrolyte temperature 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 maintaining a current density (anodic) 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 °. This is followed by a typical alkaline precipitation procedure.

Non-ferrous nickel plating

Metals that are soft and malleable to chemical processes also undergo special preparation before processing. Surfaces are degreased and, in some cases, polished. If the workpiece has already been subjected to nickel plating before, then the pickling procedure in a 25% dilute solution with sulfuric acid should also be carried out within 1 minute. Elements based on copper and its alloys are recommended to be processed in contact with electronegative metals such as aluminum and iron. Technically, such a combination is provided by a suspension or tenacious wire.from the same substances. As practice shows, sometimes in the reaction process, one touch of the iron part to the copper surface is enough to achieve the desired deposition effect.

Chemical nickel plating of aluminum and its alloys also has its own characteristics. In this case, pickling of workpieces in an alkaline solution is organized, or clarification is performed to a nitrogen-based acid. A double zincate treatment is also used, for which a composition is prepared with zinc oxide (100 g / l) and caustic soda (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 the zinc precipitate in nitric acid begins. This is followed by a second, already 10-second dive. At the final stage, aluminum is washed with cold water and nickel-plated with nickel-phosphorus solution.

Cermet Nickel Plating Technology

For materials of this type, the general method of ferrite nickel plating is used. At the stage of preparation, 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 start of the chemical nickel plating process, the cermet is covered with a layer of palladium chloride. A solution with a concentration of 1 g / l is applied to the surface with a brush. The procedure is repeated several times and the workpiece is dried after each pass.

For nickel plating use a container with an acidic solution containing 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 ceramic-metal part 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 layer of 1-2 microns is formed. Different types of ceramics, quartz elements, ticond and thermocond can also be subjected to similar processing. In each case, the procedures for plating with palladium chloride, air drying, immersing in an acid solution and boiling will be mandatory.

Nickel plating technology at home

Technically, nickel plating operations can be organized without special equipment, as already noted. For example, in a garage environment, it might look like this:

• Cooking the right size pot with an enamelled interior.
• 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 allequipment will be made from improvised materials. For the same control of the part, you can provide a special bracket (necessarily from a dielectric material) with a clamp, 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 technological period of nickel plating passes, the part is removed from the solution. It must be rinsed under cold running water diluted in slaked lime.

At home, you can nickel plate steel, brass, aluminum, etc. For all of the listed metals, an electrolytic solution containing sodium hypophosphite, nickel sulfate or chloride, as well as acid inclusions, should be prepared. By the way, to speed up the process, you can add a lead additive.

Conclusion

There are different techniques and approaches to 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 of metal is accompanied by certain risks of defect formation. This is especially true for highly sensitive non-ferrous metal, but such phenomena can also be combated within the framework of a single technological process. For example, experts recommend removing defective areas in a concentrated, acidic nitrogen-based environment withtemperatures up to 35°C. This procedure is performed not only in case of unwanted flaws, but also for the purpose of regular correction of the applied protective layer.