Electroplating is a surface treatment process that deposits a dense, uniform, and well-adhered metal or alloy coating on the surface of CNC metal workpieces based on the principle of electrolytic deposition. With the support of the coating, it can achieve core functions such as corrosion resistance, wear resistance, decorative appearance, conductive shielding, and welding adaptability for parts, making it one of the most widely used surface treatment methods in the field of precision mechanical processing.
Electroplating is suitable for most commonly used CNC metal substrates, including 45# steel, Q235 carbon steel, stainless steel, brass, aluminum alloy, etc.; insulating materials such as engineering plastics and acrylic need to be pre-treated with electroless plating before electroplating, while purely insulating and non-conductive substrates cannot be electroplated.
CNC machined workpieces shall be free of burrs, sharp edges and corners, with no obvious tool marks, deep pits, oil stains, oxide scales, sand holes or air holes on the surface; blind holes, deep holes and narrow slot structures shall be provided with process avoidance in advance to avoid internal plating leakage in subsequent processes.
It is mainly suitable for CNC structural parts, fasteners, chassis brackets and other accessories made of carbon steel and iron. The unilateral coating thickness is 5～25μm, which has the characteristics of sacrificial anode corrosion resistance, low process cost, and medium wear resistance and conductivity. The conventional surface colors are blue-white zinc, colored zinc, black zinc and military green zinc, and the salt spray protection performance is respectively: blue-white zinc 24～48h, colored zinc 72～120h, black zinc 48～96h. It is suitable for ordinary industrial corrosion-resistant parts, but not for high-precision decorative parts.
It is divided into three types: bright nickel plating, semi-bright nickel plating and electroless nickel plating. Bright nickel plating has a thickness of 10～50μm, with a mirror-like bright and delicate surface, strong acid and alkali resistance and oxidation resistance, and is mostly used for precision decorative parts such as electronic connectors and instrument casings; semi-bright nickel plating has a thickness of 15～60μm, with a matte and non-reflective industrial texture, suitable for medical accessories and internal structural parts of industrial control equipment. Electroless nickel plating does not require power supply and is deposited by chemical reaction. The thickness is 5～50μm with excellent uniformity, which can completely cover deep holes and complex cavities. After heat treatment, the hardness can reach more than HV 800, with excellent wear resistance, suitable for precision parts with complex structures, hydraulic parts and mold accessories.
It is divided into two main application types: decorative chrome and hard chrome. Decorative chrome adopts a structure of copper + nickel underlayer with surface chrome, the surface thickness is only 0.3～1μm, with a mirror-like high-gloss texture, corrosion resistance and non-fading, and is mostly used for bathroom accessories, automotive exterior CNC decorative parts. Industrial hard chrome coating has a thickness of 20～150μm, hardness up to HV 900-1150, with ultra-high wear resistance and high temperature resistance. It can be used for wear-resistant parts such as shafts, piston rods and mold cores. It can also repair workpieces with small processing dimensions by thick plating, and then achieve the tolerance standard through precision grinding.
The coating thickness is 10～30μm, with excellent conductivity, good ductility and easy polishing. It is mainly used as the underlayer for electroplating to improve the adhesion of nickel and chrome coatings; it is also suitable for the processing of electromagnetic shielding parts, conductive copper bars and mold heat dissipation coatings.
It is mostly used for CNC precision parts such as aerospace and high-end radio frequency. The gold plating thickness is 0.1～2μm, with strong oxidation resistance and low resistance, which can protect circuit contacts from ablation; the silver plating thickness is 1～5μm, with top-level conductivity in the industry, which needs sealing treatment to prevent sulfide discoloration, suitable for connector terminals, high-frequency cavity parts.
Mainly including zinc-nickel plating, nickel-cobalt plating and tin-nickel plating. Compared with ordinary zinc plating, the salt spray protection ability can reach more than 500h, which is mostly used for high-end automotive precision components and industrial parts with high corrosion resistance requirements.
Aluminum Alloy CNC Parts: A dense oxide film is easy to form on the surface, and direct electroplating is prone to blistering and peeling. Solution: Adopt the process of alkaline etching and ash removal → zinc dipping replacement → pre-plating thin nickel → final plating forming to completely break the oxide film and improve coating adhesion.
Stainless Steel CNC Parts: It has its own passivation film, leading to easy peeling and poor adhesion of the coating. Solution: First perform pickling activation treatment, then pre-plate strike nickel, and then carry out conventional nickel plating and chrome plating processes to enhance the bonding stability between the coating and the substrate.
Brass/Bronze CNC Parts: The material is easy to oxidize and discolor, and easy to contaminate the plating solution. Solution: Directly pre-plate nickel after light degreasing and activation of the workpiece, and strictly control the immersion time of the chemical solution to avoid material oxidation and plating solution contamination.
Complex Structure Parts: Deep holes, blind holes, narrow slots and other structures are prone to internal plating leakage and excessive coating accumulation at corners. Solution: Add process holes and increase structural fillets in the design stage, customize special hangers, and match auxiliary anodes to reasonably adjust the current density to ensure uniform coating deposition.
Coating Blistering and Peeling: The core cause is incomplete degreasing of the workpiece surface, with residual oil stains or oxide films; aluminum alloy and stainless steel workpieces do not complete the priming and activation treatment in accordance with the specifications, resulting in insufficient adhesion between the coating and the substrate. Solution: Strengthen the pre-degreasing and pickling processes to ensure no residual oil stains or oxide films on the workpiece surface; strictly implement the substrate-specific priming process, aluminum alloy needs to complete zinc dipping replacement before pre-plating thin nickel, and stainless steel needs to be pickled and activated before pre-plating strike nickel to fundamentally improve coating adhesion.
Surface Blooming and Color Difference: The main causes are incomplete water washing process, residual chemical solution from the previous process on the workpiece surface; the plating solution is contaminated by impurities and its composition is unstable; the unreasonable design of the hanger leads to uneven current distribution. Solution: Increase the overflow water washing process, adopt a step-by-step rinsing mode to completely remove the residual chemical solution on the workpiece surface; regularly filter the plating solution, remove impurities and supplement the plating solution components to ensure the stability of the plating solution; optimize the hanger structure and adjust the suspension angle of the workpiece to ensure uniform current distribution on the workpiece surface.
Pinholes and Pitting Defects: The causes include solid impurities in the plating solution, hydrogen evolution bubbles generated during electrolysis that are not discharged in time, and insufficient wetting agent in the plating solution. Solution: Regularly filter and purify the plating solution to remove solid impurities; add an appropriate amount of wetting agent to the plating solution to reduce the generation of hydrogen evolution bubbles; strengthen the stirring of the plating solution to promote the discharge of bubbles and avoid bubbles adhering to the workpiece surface to form defects.
Blackening and Burning at Corners: Due to the sharp corner structure of the workpiece, current is easy to gather, resulting in excessive current density and too fast and thick coating deposition. Solution: Perform fillet treatment on the sharp corners of the workpiece to ensure the fillet R≥0.3mm to avoid current gathering; appropriately reduce the electroplating current density, or add auxiliary cathodes at the sharp corners to disperse the current and achieve uniform coating deposition.
No Coating Inside Deep Holes: Due to the special structure of deep holes and blind holes, the current cannot effectively reach the inside of the holes, and the plating solution circulation is not smooth. Solution: Prioritize changing blind holes to through holes in the design stage, and add process circulation holes for deep holes (depth > 3 times the hole diameter); customize special hangers and adjust the hanger position to ensure that the current can reach the inside of the holes; strengthen the plating solution circulation, such as air injection circulation, to ensure the timely update of the plating solution inside the holes.
Unqualified Salt Spray Test: It is mainly because the coating thickness does not meet the design requirements, or the passivation and sealing processes of zinc-plated workpieces are not in place. Solution: Strictly control the coating thickness and thicken the coating according to the design requirements; extend the passivation treatment time and optimize the sealing process to ensure that the passivation film is complete and dense, improve the corrosion resistance of the coating, and meet the salt spray test standards.
Dimensional Tolerance Reservation: For ordinary electroplating, reserve 0.02～0.05mm unilaterally; for hard chrome thick plating, reserve 0.05～0.15mm unilaterally; for electroless nickel plating, reserve 0.03～0.08mm unilaterally to ensure that the size after electroplating meets the tolerance requirements.
Structural Fillet Requirements: Eliminate sharp corner design, the fillet shall meet R≥0.3mm to avoid coating accumulation, blackening and burning at the sharp corners and ensure uniform coating appearance.
Hole Structure Design: Prioritize changing blind holes to through holes. When the hole depth is greater than 3 times the hole diameter, process exhaust holes shall be added to effectively avoid internal plating leakage.
Surface Roughness Standard: The roughness of the CNC machined surface shall be controlled at Ra1.6～Ra3.2. Excessively high roughness cannot cover tool marks, and excessively smooth surface will reduce coating adhesion.
Processing Requirements for Threaded Parts: The hole diameter of precision threads tends to shrink after electroplating, so the thread tolerance needs to be enlarged in advance, or secondary tapping correction shall be carried out after electroplating to ensure smooth thread fit.
| Process Type | Applicable Substrates | Conductivity | Wear Resistance | Corrosion Resistance | Available Colors | Dimensional Change |
|---|---|---|---|---|---|---|
| Electroplating | Metals such as steel/aluminum/copper/stainless steel | Conductive | High | High | Few styles (mainly silver/black/gold) | Coating thickens, need to reserve tolerance in advance |
| Anodizing | Aluminum alloy only | Insulating | Medium-high | Medium | Rich and diverse colors | Slight thickening, minimal impact on size |
| Sandblasting | Suitable for all metal substrates | Conductive | General | General | Mainly natural color, can be colored later | Almost no dimensional change |
As a core surface treatment process for CNC precision machining, electroplating has a wide range of applicable substrates and comprehensive functions, which can meet multiple needs such as corrosion resistance, wear resistance, conductivity and appearance. In practical application, the type of coating should be reasonably selected according to the workpiece substrate type, structural characteristics, service conditions and salt spray tolerance requirements; at the same time, strictly follow the CNC machining electroplating design specifications, control the pre-treatment and the whole electroplating process, effectively avoid various common defects, and ensure the service performance and service life of precision parts.
