Surface Finsh: Electroplating Explained

Electroplating is currently widely used in the electronics and electrical industry. Components such as circuit boards, connectors, and other small metal parts all require an electroplated finish. The strong growth of the telecommunications sector and the continuously rising demand for electronic devices have significantly boosted the application of electroplating. This article will discuss what the electroplating process is, and plating types based on the plated metal.

What is Electroplating？

Electroplating is a process that uses electrolysis to deposit a layer of metal or alloy onto the surface of a workpiece, resulting in a metal coating that is uniform, dense, and possesses good adhesion. During electroplating, the workpiece to be plated is connected to the negative terminal of the power supply, serving as the cathode. It is submerged in an electrolyte solution containing the metal ions to be deposited. The anode is typically a plate or rod of the metal being deposited. However, some electroplating processes also use insoluble anodes like graphite, stainless steel, lead, or lead-antimony alloys.

Electroplating can be divided by its method of application into rack plating, barrel plating, continuous plating, and brush plating. The appropriate method is selected based on the size and production volume of the parts being plated.

Rack plating is the most common method. The parts are suspended on a highly conductive rack and immersed in the plating solution as the cathode. This method is suitable for workpieces of general or large dimensions, such as bicycle handlebars and car bumpers.

Barrel plating is another common method, suitable for small-sized parts produced in large volumes. During plating, the parts are placed in a polygonal barrel and rely on their own weight to make contact with the cathode inside the barrel.

Basic Principle of Electroplating

The workpiece to be plated is placed in a plating tank filled with the electrolyte solution and connected to the negative terminal (cathode) of a DC power source. The plating metal (or an inert material like graphite) is also placed in the tank and connected to the positive terminal (anode).

When power is applied, the metal ions in the plating solution migrate toward the cathode, gain electrons, are reduced to metal atoms, and subsequently deposit onto the surface of the workpiece, forming the coating. Since electroplating primarily relies on electrochemical principles, three essential conditions are required: an electrode potential difference, a plating solution (electrolyte), and a power source.

Taking copper plating as an example: The workpiece acts as the cathode in a plating solution whose main component is copper sulfate, with metallic copper as the anode. When the DC power is connected, the current passes through the electrodes and the electrolyte. The anions and cations in the plating solution undergo electromigration, meaning that under the influence of the electric field, anions move toward the anode and cations move toward the cathode. Copper ions (Cu2⁺) are reduced and deposited onto the cathode, while the metallic copper at the anode is oxidized.

The chemical reactions are as follows:

• Cathode (Workpiece) Reaction (Reduction): Cu²⁺ + 2e⁻ → Cu
• Anode Reaction (Oxidation): Cu – 2e⁻ → Cu²⁺

The final coating must be complete, uniform, dense, meet specific thickness requirements, be firmly bonded to the base metal, and possess certain physical and chemical properties to provide effective protection.

Factors Affecting Electroplating Quality

Many factors influence the quality of the electroplated layer. Key ones include plating solution composition, cathode current density, temperature, and surface pretreatment.

Plating Solution Composition

The composition of the plating solution mainly includes the following parts:

Main Salt (Primary Salt): The metal salt that contains the ions that will deposit as the coating metal on the cathode. Higher concentrations of the main salt, all else being equal, make metal deposition easier but decrease cathode polarization, leading to coarser crystal grains in the coating. However, excessively high concentrations reduce the solution’s throwing power (dispersing ability) and stability, and increase production and waste treatment costs. Conversely, very low concentrations provide better throwing and covering power and better cathode polarization, but result in poor conductivity, lower permissible current density, lower current efficiency, and a slow deposition rate. Therefore, the main salt concentration must be within an appropriate range, which varies based on the specific application requirements.

Additional Salts (Conductive Salts): Salts of alkali metals or alkaline earth metals (including ammonium salts) added primarily to increase the solution’s conductivity. These salts can also improve the plating solution’s deep plating ability, throwing power, and covering power, leading to a finer and denser coating quality. Examples include sodium sulfate and magnesium sulfate in nickel plating baths. Excessive additional salts, however, can reduce the solubility of the main salt and cause the solution to become cloudy.

Complexing Agents: Substances that can complex with the metal ions of the main salt. Solutions without these ions are called single-salt baths, which generally have poor stability, coarse crystal grains, and lower coating quality. Adding complexing agents increases cathode polarization, resulting in a finer crystal structure and also promoting anode dissolution. If the amount of complexing agent exceeds the requirement for complexing the main salt metal ions, it forms free complexing ions. Too high a concentration of free complexing ions can reduce the cathode current efficiency, slow down the deposition rate, or even prevent plating entirely.

Buffering Agents: Substances, usually a weak acid/salt or weak base/salt combination, used to maintain the plating solution’s acidity or alkalinity within a specific pH range necessary for normal operation. They help to minimize fluctuations in the bath’s pH.

Additives: Small amounts of organic substances are added to the plating bath to improve the solution’s performance and the coating’s quality. They are mainly categorized by their function:

• Brighteners: Increase the luster/gloss of the coating. Primary brighteners often contain sulfonyl groups or unsaturated carbon bonds (e.g., saccharin). Secondary brighteners are typically aldehydes, ketones, alkynes, cyanides, or heterocyclics with unsaturated bonds.
• Levelers: Significantly improve the smoothness of the coating. They cause the coating to be thicker in microscopic valleys of the workpiece than on microscopic peaks.
• Wetting Agents (Anti-Pitting Agents): Lower the interfacial tension between the electrode and the solution, allowing the solution to spread easily over the electrode surface. They also promote the detachment of gas bubbles from the electrode surface, inhibiting the formation of pinholes in the coating.
• Stress Reducers: Decrease the internal stress in the coating, thus improving its toughness.
• Grain Refiners: Promote a finer and denser crystal structure in the coating.

Cathode Current Density

The optimal cathode current density depends on factors like plating solution composition, main salt concentration, pH, temperature, and agitation.

Too low a current density reduces cathode polarization, leading to coarse crystal growth or no plating at all.

Increasing the current density (within limits) increases cathode polarization, resulting in a finer coating structure.

Excessively high current density causes crystals to grow rapidly along the lines of force into the solution, leading to nodules (lumps) or tree-like crystals (dendrites). Corners and edges may even burn (scorch). High current density also leads to intense hydrogen evolution at the cathode surface, increasing the pH, which can result in metal basic salts being entrapped in the coating, making it black. Furthermore, it can cause anode passivation, leading to a depletion of metal ions in the solution and potentially yielding a loose, spongy coating. Every plating bath has an ideal operating current density range.

Temperature

Temperature is a crucial factor. Increasing the temperature accelerates particle diffusion and ion dehydration, increases the activity of ions and the cathode surface, and lowers both diffusion and electrochemical polarization. Consequently, a higher temperature reduces cathode polarization, leading to coarser crystal grains. In production, increasing the bath temperature is often done to increase salt solubility and the solution’s conductivity. Plating temperature must be carefully controlled to stay within the optimal range.

Surface Pretreatment

Before electroplating, the workpiece must undergo surface pretreatment to remove burrs, inclusions, residues, grease, scale (oxide film), and passivation films. Proper pretreatment exposes a clean, active base metal surface, which is essential for obtaining a continuous, dense, and well-bonded coating. Inadequate pretreatment is a major cause of defects such as blistering, peeling, poor adhesion, burrs, and staining.

Electroplating Process

The electroplating process generally includes three stages: surface pretreatment before plating, electroplating, and post-plating treatment.

Surface Pretreatment Before Plating

This stage aims to achieve a clean, active base metal surface, preparing it for a high-quality coating. Pretreatment mainly involves:

Grinding or Polishing: To meet surface roughness requirements.

Degreasing: Chemical or electrochemical methods to remove oil and grease.

Rust/Scale Removal: Mechanical, acid pickling, or electrochemical methods.

Activation: Typically, a brief immersion in a weak acid to ensure the surface is chemically active.

Start Electroplating

The method of electroplating varies depending on the part’s shape, size, and batch volume. Rack plating is the most common for parts of ordinary or larger dimensions. Parts are suspended on a highly conductive rack, immersed in the plating solution as the cathode, with anodes positioned appropriately on both sides.

Good contact between the rack and the cathode bar is critical. Poor contact can cause contact resistance, especially in high-current hard chrome plating or decorative plating with cathode movement agitation, leading to intermittent power interruptions, resulting in poor coating adhesion and reduced thickness uniformity, thus lowering corrosion resistance.

For smaller parts or large production volumes, barrel plating is used. The parts are placed in a polygonal barrel and contact the internal cathode by their own gravity as the barrel rotates, facilitating uniform deposition. Barrel plating saves labor, has high production efficiency, requires less equipment maintenance, takes up less floor space, and produces uniform coatings. However, it is not suitable for overly large or very light workpieces. Also, barrel plating tanks operate at a higher voltage, leading to faster solution temperature rise and greater solution drag-out.

Brush plating is used for local plating or repair. Continuous plating is used for wire and strip materials in high-volume production.

Post-Plating Treatment

This stage includes passivation, hydrogen embrittlement relief (de-hydrogenation), and surface polishing.

Passivation is performed to improve the coating’s corrosion resistance, increase its luster, and enhance its anti-tarnishing properties.

Hydrogen Embrittlement Relief is typically a heat treatment at a specific temperature for several hours to prevent hydrogen absorbed during plating from causing brittleness in the part.

Surface Polishing is a finishing process applied to the coating to reduce surface roughness, achieve a mirror-like decorative finish, and further improve corrosion resistance.

Types of Electroplating

A common way to classify electroplating is by the plating metal or alloy applied to the substrate material.

Zinc Plating (Zn)

Zinc plating is primarily used as a protective coating for steel. For steel materials, zinc is an anodic coating, providing both electrochemical (sacrificial) and mechanical protection, resulting in good corrosion resistance. The protective ability is related to the coating thickness and porosity: thicker coatings and lower porosity yield better corrosion resistance. Zinc coating thickness typically ranges from 6 to 20μm, with thicknesses over 25μm required for parts in severe conditions. Passivation of the zinc layer increases its protective capacity by 5 to 8 times.

Zinc plating solutions are categorized as alkaline (e.g., cyanide, zincate, pyrophosphate), neutral (e.g., chloride, sulfate bright), and acid (e.g., sulfate, ammonium chloride). Due to its low cost, good corrosion resistance, aesthetics, and long storage life, zinc plating is widely used in the light industry, instrumentation, machinery, agriculture, and defense. However, because zinc is harmful to the human body, it is not suitable for the food industry.

Read More: Properties Comparing | Zinc Plating vs Nickel Plating

Copper Plating (Cu)

Copper plating is used on base metals such as zinc and iron. The copper coating on these metals is cathodic. If the copper layer has defects, damage, or pores, the base metal acts as the anode in a corrosive medium, accelerating its corrosion, often faster than if it were unplated. Therefore, single-layer copper plating is rarely used for protective-decorative purposes. It is often used as an underlayer (intermediate coating) to improve the adhesion between subsequent coatings and the base metal. Thick copper underlayers combined with thin nickel layers can reduce the porosity of the nickel layer and conserve nickel consumption. Copper layers can also protect localized areas from carburizing or nitriding, as the diffusion and penetration of carbon and nitrogen into copper is difficult.

Various copper plating baths exist, including cyanide, sulfate, pyrophosphate, citrate, nitrilotriacetic acid, and fluoborate.

Chromium Plating (Cr)

Chromium is a silver-white metal with a slight bluish tinge. It exhibits excellent chemical stability in the atmosphere due to strong passivation, forming a thin, dense oxide film. Chromium is stable in alkalis, nitric acid, sulfuric acid, sulfides, and many organic acids, but it dissolves in hydrohalic acids and hot, concentrated sulfuric acid.

Chromium plating offers excellent corrosion resistance and has poor wettability, exhibiting hydrophobic and oleophobic properties. It also boasts high hardness, good wear resistance, and good heat resistance. Its appearance and hardness remain essentially unchanged when heated in air up to 500°C; oxidation begins above 500°C, and softening begins above 700°C. Chromium also has high reflectivity, second only to silver.

Based on application, chromium coatings are divided into protective-decorative chromium plating (thin, prevents rust and beautifies the appearance) and functional chromium plating (generally thicker, used to improve hardness, wear resistance, corrosion resistance, and high-temperature resistance of mechanical parts). Functional chromium is further classified as hard chrome, milky chrome, and porous chrome.

The composition of the chromium plating solution is relatively simple: the main component is chromic anhydride (CrO3​), not a salt of the plating metal itself, along with small amounts of catalysts like sulfate, fluoride, or fluorosilicate. Lead alloys are generally used as anodes. While chromic anhydride must be continuously replenished, hexavalent chromium (Cr(VI)) is highly toxic, so trivalent chromium (Cr(III)) is increasingly being used as an alternative.

Nickel Plating (Ni)

Nickel has a yellowish-tinged silver-white metallic luster and is ferromagnetic. Nickel has strong passivation capability, forming a thin passivation film in the air, resulting in high chemical stability and a long-lasting, unchanged luster on the surface. At room temperature, nickel exhibits good corrosion resistance to the atmosphere, water, alkalis, salts, and organic acids. It dissolves slowly in dilute hydrochloric and sulfuric acids but more quickly in dilute nitric acid. It becomes passive in fuming nitric acid and does not react with strong alkalis.

Nickel’s electrode potential is more positive than iron’s, making it a cathodic coating for iron. Therefore, it provides good protection to a steel substrate only when the coating is complete and flawless. Since nickel coatings are generally somewhat porous, they are often used in multi-layer systems with other metals to enhance corrosion resistance, with nickel serving as an underlayer or intermediate layer to reduce porosity (e.g., Cu/Ni/Cr or Ni/Cr combinations).

Nickel coatings are divided into protective-decorative and functional types.

Protective-decorative nickel plating is used for corrosion protection of low-carbon steel, zinc die-castings, and some aluminum and copper alloys. A bright decorative effect is achieved either by polishing semi-bright nickel or by directly plating bright nickel. As nickel easily tarnishes in the atmosphere, a thin layer of chromium is often plated over the bright nickel layer to improve corrosion resistance and aesthetics.

Functional nickel plating is mainly used to repair worn, corroded, or oversized parts. These coatings are typically plated thicker than needed and then mechanically machined back to the specified dimensions.

The main salts used in nickel plating are nickel sulfate and nickel chloride.

Alloy Electroplating

Alloy electroplating is a method of improving coating performance, capable of producing hundreds of coatings with diverse properties. This technique is highly effective in addressing issues related to decoration, corrosion resistance, wear resistance, magnetism, solderability, and conductivity.

Alloys produced by electroplating have unique characteristics compared to those produced by thermal fusion:

1. Alloys composed of high-melting-point and low-melting-point metals can be obtained.
2. Alloys not present on the thermal fusion phase diagram can be achieved.
3. Amorphous alloys that are very dense and have excellent properties can be produced.
4. Alloys can be formed that are difficult to deposit as single metals from aqueous solutions.
5. Under certain controlled conditions, metals with more negative potentials can be preferentially deposited.

Alloy coatings are classified by the element with the highest concentration (e.g., copper-based, silver-based, zinc-based, nickel-based alloys). Copper-tin alloy is a widely used alloy coating due to its low porosity, good corrosion resistance, easy polishing, and ability to be directly chrome-plated. Cyanide copper-tin alloy plating is often used because the composition and color of the coating are easy to control, and the solution has good throwing power. However, a major disadvantage is the use of large amounts of highly toxic cyanide, which requires strict safety measures.

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