1. Concept
Electroplating is a process that uses electrodeposition to coat the surface of a workpiece with one or multiple layers of metal, providing an attractive appearance or specific functional requirements. During electroplating, the coating metal or an insoluble material serves as the anode, while the metal product to be plated acts as the cathode. The metal ions of the coating are reduced on the workpiece surface to form the plating layer. To avoid interference from other cations and ensure a uniform and牢固 (firm) coating, a solution containing the metal cations of the coating is used as the electrolyte to maintain a constant concentration of these ions.
The purpose of electroplating is to apply a metal coating to a substrate, altering its surface properties or dimensions. It enhances corrosion resistance (using corrosion-resistant metals), increases hardness, prevents wear, improves conductivity, lubricity, heat resistance, and enhances aesthetic appeal.
2. Principle
In an electroplating tank containing the electrolyte, the prepared workpiece serves as the cathode, and the coating metal acts as the anode. Both are connected to the negative and positive terminals of a DC power supply, respectively. The electrolyte consists of an aqueous solution containing compounds of the coating metal, conductive salts, buffers, pH regulators, and additives. When current is applied, metal ions in the electrolyte move to the cathode under the potential difference and form the coating. The anode metal dissolves into the electrolyte as ions to maintain the concentration of the coating metal ions. In some cases, such as chromium plating, insoluble anodes made of lead or lead-antimony alloys are used, which only facilitate electron transfer and current conduction. The chromium ion concentration in the electrolyte is maintained by periodically adding chromium compounds.
Factors such as anode material quality, electrolyte composition, temperature, current density, duration of current application, agitation intensity, precipitated impurities, and power supply waveform can affect the quality of the coating and must be controlled appropriately.
3. Function
Electroplating is a technology that uses electrolysis to deposit a firmly adherent metal coating on mechanical products, with properties different from those of the substrate material. Electroplated layers are more uniform than hot-dip layers and are generally thinner, ranging from a few micrometers to tens of micrometers. Electroplating can provide decorative, protective, and functional surface layers on mechanical products and can also repair worn or incorrectly processed workpieces.
Coatings are typically single metals or alloys, such as titanium, zinc, cadmium, gold, brass, or bronze. They can also be dispersion layers (e.g., nickel-silicon carbide, nickel-fluorinated graphite) or composite layers (e.g., copper-nickel-chromium on steel, silver-indium on steel). Substrate materials除了 (besides) iron-based materials like cast iron, steel, and stainless steel include non-ferrous metals and plastics such as ABS, polypropylene, polysulfone, and phenolic plastics. However, plastic substrates must undergo special activation and sensitization treatments before electroplating.
4. Performance
1. Key Performance Metrics
The primary performance metrics of electroplated workpieces include adhesion to the substrate, corrosion resistance, heat resistance, and mechanical strength.
Adhesion: The adhesion strength depends on the material itself (e.g., SUS304 steel plate, thickness 0.35 mm, size 32*40 cm) and its chemical and physical properties. Different materials exhibit significant variations in adhesion to metal coatings.
Corrosion Resistance: The corrosion resistance of electroplated workpieces varies greatly depending on the coating combination and thickness. Electroplated workpieces exhibit higher corrosion resistance than metal workpieces with the same coating because their corrosion mechanisms differ. First, electroplated workpieces corrode via an anode protection mechanism. In mild cases, the coating may fully corrode and dissolve, leading to complete脱落 (detachment). Therefore, double-layer or multi-layer nickel systems are used for workpieces requiring high corrosion resistance. Second, metal coatings cannot form corrosion cells. Even if corrosion occurs, it spreads laterally rather than deepening.
Heat Resistance: Heat resistance primarily depends on the inherent heat resistance of the metal coating (e.g., nickel, chromium) and its adhesion to the substrate.
Mechanical Strength: The mechanical strength of electroplated workpieces is closely related to the material (e.g., nickel, chromium). Generally, rigidity improves after electroplating.
2. Factors Affecting Adhesion Between Metal and Substrate
Steel Plate: To ensure strong adhesion between the coating and the steel substrate, factors such as the steel plate’s dimensions, model, material uniformity, stability, surface hardness, internal stress (205 N/mm²), tensile strength (520 N/mm²), and the use of steel with good electroplating properties must be considered.
Brushed Steel Plate: The brushed pattern on the steel plate must be consistent in depth. Currently, imported 2000-grit sandpaper is used.斜丝 (Oblique patterns) or乱丝 (uneven patterns) can adversely affect adhesion between the steel plate and the electroplated nickel layer.
Pretreatment: Pretreatment processes such as brushing (mechanical roughening), degreasing, and activation significantly impact adhesion.
Coating Thickness: Adhesion is related to the coating structure for two reasons: First, coatings of different thicknesses exhibit varying stress levels. Second, differences in coating structure and the thermal expansion coefficients between the coating and the steel plate, as well as the coating’s toughness, affect adhesion.
5. System Components
1. Electroplating Equipment
The primary functional advantage of electroplating is its ability to create a全新 (brand-new) and highly glossy metal layer on the surface of metal and non-metal parts, significantly enhancing the visual appeal of the original component. Compared to directly using metal-formed parts, electroplating is much more cost-effective. For plating specific areas on the same component, special waxes and coatings can be applied to areas not requiring plating. Chromium plating is widely used for surface treatment in the transportation and furniture industries.
2. Main Materials
There are over 30 types of metals used in electroplating, with the most common being zinc, cadmium, copper, nickel, chromium, silver, tin, gold, iron, cobalt, lead, antimony, platinum, titanium, and rhodium. In addition to single-metal coatings, there are many alloy coatings, such as copper-tin, copper-zinc, copper-nickel, nickel-iron, lead-tin, zinc-tin, zinc-iron, zinc-nickel, copper-cadmium, zinc-cadmium, tin-iron, tin-cobalt, and tungsten-iron.
For plastics, ABS is most commonly used because it can withstand the high temperatures of electroplating (60°C or 140°F) and exhibits strong adhesion between plated and non-plated areas. Most metals can be electroplated, but they vary in purity and plating efficiency. The most common include tin, chromium, nickel, silver, gold, and rhodium (a type of platinum that is extremely expensive, maintains high brightness for long periods, and resists most chemicals and acids. It is often used for products requiring极高 (extremely high) surface gloss, such as trophies and medals). Nickel should not be used for products contacting the skin due to its irritant and toxic properties.
3. Coating Types
Based on the coating material, electroplating can be categorized as chromium plating, copper plating, cadmium plating, tin plating, zinc plating, etc.
6. Process Flow
1. Basic Steps
(Finishing → Polishing) → Racking → Degreasing → Rinsing → (Electropolishing or Chemical Polishing) → Acid Pickling Activation → (Pre-plating) → Electroplating → Rinsing → (Post-treatment) → Rinsing → Drying → Unracking → Inspection and Packaging.
2. Functions of Each Step
Pretreatment: All steps before plating are称为 (called) pretreatment. Their purpose is to prepare the workpiece surface by removing oils, rust, oxide layers, etc., to provide a suitable surface for plating. Pretreatment primarily affects appearance and adhesion. According to statistics, 60% of electroplating defects are caused by poor pretreatment, making it a critical part of the process. In countries with advanced electroplating technology, pretreatment is highly valued and can account for half or more of the entire process, resulting in high-quality coatings and significantly reduced defect rates.
De-smutting: The colloidal palladium adsorbed on the part surface from the catalytic solution is not initially catalytically active because it is surrounded by divalent tin ions. De-smutting dissolves these tin ions, exposing the palladium and activating it.
Bright Nickel Plating: Gives the part a mirror-like bright appearance, with a white-yellow hue. When exposed to corrosive media, the bright nickel layer, which contains sulfur and has a lower potential than semi-bright nickel, corrodes preferentially as an anodic layer. This changes the corrosion direction from vertical to horizontal, preventing deep corrosion and significantly slowing the corrosion rate.
Roughening: Creates micro-roughness on the part surface, increasing the contact area between the plating layer and the plastic substrate and enhancing adhesion.
Sandblasting: Removes rust, weld slag, carbon deposits, old paint, and other dry contaminants from the part surface; removes sand and oxide scale from castings, forgings, or heat-treated parts; eliminates burrs and directional grinding marks; reduces surface roughness to improve adhesion of paints and other coatings; and creates a diffuse反射 (reflective) matte finish.
Finishing: Removes macroscopic defects such as burrs, rust, scratches, weld seams, weld bumps, sand holes, and oxide scale to improve flatness and plating quality.
Polishing: Further reduces surface roughness to achieve a bright appearance. Methods include mechanical, chemical, and electrochemical polishing.
Degreasing: Removes oils from the workpiece surface using organic solvents, chemical agents, electrochemical methods, wiping, or tumbling.
Acid Pickling: Removes rust and oxide layers using chemical or electrochemical methods.
Electroplating: This is the core step where the desired coating is applied to the workpiece surface. The quality of this step directly affects the coating’s properties. Key factors include:
Main Salt System: Each plating type has multiple main salt systems and corresponding additive systems. For example, zinc plating includes cyanide zinc plating, zincate zinc plating, chloride zinc plating (potassium salt zinc plating), ammonia salt zinc plating, and sulfate zinc plating. Each system has its advantages and disadvantages. For instance, cyanide zinc plating offers good dispersion and depth capability, fine crystal structure, strong adhesion, excellent corrosion resistance, wide process range, and stable operation with low sensitivity to impurities. However, it is highly toxic and severely pollutes the environment. Chloride zinc plating uses a single-salt solution without complexing agents, making wastewater easy to treat. It provides better brightness and leveling than other systems, high current efficiency, and fast deposition. It is suitable for materials with low hydrogen overpotential, such as high-carbon steel, castings, and forgings. However, the weakly acidic chloride ions can corrode equipment, and this solution is not suitable for deep-hole or tubular parts requiring auxiliary anodes.
Additives: Additives include brighteners, stabilizers, softeners, wetting agents, and low-area agents. Brighteners are further divided into primary brighteners, carrier brighteners, and auxiliary brighteners. For the same main salt system, additives from different manufacturers can produce significant differences in coating quality. Generally, additives from Europe, the US, and Japan are the best, followed by those from Taiwan, while mainland Chinese additives are relatively inferior. The combination of the main salt and specific additives determines the overall performance of the plating solution. High-quality additives can compensate for certain deficiencies of the main salt. For example, excellent chloride zinc plating additives combined with a chloride main salt can achieve better depth capability than many cyanide zinc plating solutions.
Electroplating Equipment:
Racks: Square racks are used with square tanks, and round racks with round tanks. Round tanks and racks provide more uniform current distribution. Square racks require additional current-dispersing devices, such as iron wire meshes, or shorter anode plates on both sides, using elliptical anode configurations.
Agitation Devices: Promote solution flow, ensure uniform distribution, and prevent bubbles from adhering to the workpiece surface.
Power Supply: Must provide stable DC current with low ripple coefficient.
Post-treatment: Treatments after plating enhance various properties of the coating, such as corrosion resistance, anti-tarnishing ability, and solderability.
Dehydration Treatment: Adding dehydrating agents to water, e.g., after bright nickel plating.
Passivation Treatment: Improves corrosion resistance, e.g., for zinc-plated parts.
Anti-tarnishing Treatment: Adding anti-tarnishing agents to water, e.g., for silver, tin, or imitation gold-plated parts.
Solderability Improvement: e.g., for tin-plated parts.
The quality of post-treatment directly affects the functional performance of the coating.
7. Application in Cosmetic Packaging
In the cosmetic packaging industry, electroplated coatings cannot directly contact the contents. Therefore, electroplating is primarily used for external components, such as various packaging shells, lipstick cases, bottle cap exteriors, and cosmetic tool components.
