Chemical nickel plating process

The object of electroless nickel plating is a specific workpiece, and the condition of the workpiece to be plated, including the workpiece material, manufacturing or maintenance methods, workpiece size, and end-use conditions are different; therefore, the pretreatment methods should be different. When determining the correct pretreatment process, it is important to have a good understanding of the workpiece.

Alloy Type　

To ensure adequate bonding and quality of the coating, the substrate material must be identified. Certain materials containing catalytically toxic alloys should be surface adjusted in the pre-treatment to ensure that these alloys are removed before electroless nickel plating is applied. For example lead (leaded steel), sulfur (sulfur-containing steel), excess carbon (high-carbon steel), carbide (carburized steel), etc.. The residue of these substances can cause poor bonding and blistering problems. In addition, pinholes and porosity may occur in the plated layer on surfaces where these substances have not been removed. Another treatment method is to isolate the influence of harmful alloying elements in the base material by pre-plating before plating. In the case of unclear to be plated workpiece material and material analysis is not possible, it is necessary to carry out a pre-test, test qualified before processing work.

　　Manufacturing of workpieces　　　

The surface condition of steel parts due to carburizing, nitriding, quenching and hardening to improve the surface hardness is one of the important ways of change. Usually, chemical nickel plating in the hardness range of HRC58-62 iron parts on the surface of the combination of coating is difficult to qualify. On the one hand, workpieces in the above hardness range have to be subjected to special cleaning methods, i.e. cyclic commutative electrolytic activation in cyanide-containing solutions or other suitable electrolytic cleaning, to dissolve and remove inorganic substances, such as carbides, from the surface. On the other hand, surface stresses generated during plating, such as high tensile stresses on the surface of workpieces used in the aerospace industry, must be de-stressed before and after plating to obtain a competent bond. In the manufacturing process, the surface of the workpiece to remove a large number of communications; the mechanical lubricants and polishing agents, etc. must also be removed before plating.

Workpiece maintenance factors　　

Workpiece repair to remove the surface of the organic coating, rust, or oxidized skin, the use of sandblasting, this workpiece is the most difficult to deal with before chemical plating. Because the surface of these workpieces is not only embedded with residual substances but also corrosion products are firmly attached. In this case, the surface should first be cleaned by mechanical methods to ensure the quality of the subsequent chemical cleaning and activation process. To remove the surface of the workpiece embedded grease and chemical contamination, sometimes pre-baking the workpiece is very effective, although this is not the only good removal method.

Workpiece geometry　　

The geometry of many workpieces prevents the use of certain pre-treatment techniques, as is the case with large-sized containers and pipe fittings with a large internal surface area. Usually cleaning and activation of steel parts should include electrolytic cleaning and activation, in the above cases, mechanical cleaning, chemical cleaning, and activation should be more feasible. For parts with blind holes and complex shapes, the cleaning process needs to be intensified to solve the problem of removing dirt, hydrogen bubble escape, and solution carryover. Consideration should also be given to the method of hanging and placing the workpiece to address these issues.

The problem of plating resistance on the non-plated surface of the workpiece　　

Many workpieces require localized chemical nickel plating, so it must be colored shielding material to protect the non-plated part. Shielding materials available pressure-sensitive tape, paint, special plastic fixtures, and so on. Of course, the market now has a commercial plating barrier coating (called protective paint) for sale, and a little more advanced, can be easily removed after plating, with a special solvent that can be used repeatedly after dissolution.

　　Chemical Cleaning　　

Immersion cleaning is one of the important steps in the pretreatment of chemical plating, its important function is to remove the dirt on the surface of the workpiece, to ensure the cleaning effect, usually using cleaning agents, mechanical agitation, and heating. Alkaline cleaning agents must be heated to 60-80 ℃, to thoroughly clean the dirt, most of the alkaline cleaning takes immersion and strong agitation, and can also be used for spray cleaning. The quality and detergency of commercially available immersion cleaners vary greatly, so it is important to select the cleaner according to the degree of contamination of the workpiece

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　　Electrolytic Cleaning　　

Electrolytic cleaning chemical nickel plating activation treatment before the end of the cleaning method is more applicable to precision parts. DC electrolytic cleaning, that is, electrolytic cleaning of the resistive electrode has the advantage of generating a large amount of hydrogen on the surface of the workpiece to increase the washing effect; the disadvantage is that the workpiece is negatively charged, and thus adsorption of copper, zinc, and other metal ions in the cleaning solution, soaps and some colloidal substances in the formation of loose electrode mud on the workpiece so that it is taken away. Electrolytic cleaning uses periodic commutation of electric injection, when the workpiece for the anode, forces the workpiece surface to positively charge ions and dirt detachment. Moreover, the oxygen generated on the surface of the workpiece effectively washes away the dirt embedded in the workpiece, and the detergent in the cleaning solution wets the dirt and emulsifies the dirt.

　　Water washing　　

The purpose of the washing process between the two pre-treatment processes is to prevent the solution from the previous process from contaminating the solution of the next process and to remove dirt, metal ion contamination, and electrode sludge from the surface of the workpiece to ensure that the bonding force of the plated layer is qualified. It is very important to design a good process flow with a good or bad rinsing process and should avoid the phenomenon of shared rinsing, i.e., going back to the previously used tank for rinsing.