Electroplating is a widely used chemical process in engineering. It is sometimes treated as a finishing process intended to condition a part to meet specific functional requirements. Take, for instance, a steel electrical connector used in automobile assembly. Without electroplating, the connector can easily corrode when the vehicle is exposed to moisture and contaminants. However, the part can survive in these corrosive environments for longer when electroplated, depending on the plating material used. The technology is also widely used in the beauty industry. Through electroplating, earrings and necklaces are made to appear gold, hence more attractive.
Electroplating effectiveness does not only revolve around the selection of the right electroplating material, but also on how the process parameters are managed. This discussion examines various parameters of electroplating, including current density, plating time, bath chemistry, temperature, pH, agitation, anode-to-cathode arrangement, surface preparation, and current efficiency. These parameters must be well monitored to achieve specified coating thickness, adhesion, corrosion resistance, and hardness. Further, by understanding these parameters, manufacturers can achieve electrical performance, appearance, and dimensional condition with acceptable process variation.
What Is Electroplating?
Electroplating is a manufacturing process in which a thin layer of metal atoms is deposited onto another material through electrolysis. The metal added is known as the deposition metal, and the underlying material or workpiece is known as the substrate material.
The main components of an electroplating setup include the cathode (the object to be plated), the anode (a metal supplying ions for plating), and the electrolyte solution containing dissolved ions, which collectively form an electrolytic cell governed by precise electrical parameters [1]. DC power supply provides the electrical energy required for deposition. Electroplating also includes a process control system for controlling process parameters.
How Does the Electroplating Process Work?
The electroplating process is sequential, starting with parts inspection and ending with final inspection after deposition. In this discussion, we will be demonstrating the deposition of nickel onto a steel component.
1. Part Inspection
Inspection of the substrate (in our case, steel) before plating. Key areas to observe include base metal composition, surface roughness, machining marks, and burrs. Other areas to note include porosity, weld defects, and dimensional accuracy. The substrate should be checked for any existing oxide, as it can affect the resulting deposited layer. These defects must be corrected before etching. The electroplating process does not hide these defects. They will still be noticed later on the final part.
2. Surface Preparation
Preparation of the substrate is essential before deposition, as a high degree of cleanliness is critical to ensure proper coating adhesion and prevent defects [3]. Even though steel can appear clean to the naked eye, it can still carry grease, oils, fingerprints, oxides, dust, and residues from previous processing. These contaminants can inhibit the bonding of the nickel layer on the steel substrate. The preparation includes degreasing the steel component in an alkaline solution to remove oils, rinsing to remove dust, and acid activation to remove oxides. In some cases, nickel strike is applied. This is a thin nickel layer applied before main nickel is deposited. Nickel strike improves substrate adhesion and provides an effective starting surface.
3. Main Electroplating
The electroplating process involves electron transfer. An anode and cathode are created to create a potential difference. In our case, the nickel plate is connected to the positive terminal, creating an anode. On the other side, the steel component is connected to the negative terminal, creating a cathode. Both the anode and cathode are immersed in an electrolyte containing nickel. Once the DC power is switched on, an electric current drives nickel ions towards the steel component. Once deposited, the nickel ions become nickel metal, forming a layer. At the anode, nickel metal loses electrons to become an ion, as described by the reaction;
Ni → Ni²⁺ + 2e⁻
At the cathode, nickel ions around the steel gain electrons to form nickel metal, as described by the reaction;
Ni²⁺ + 2e⁻ → Ni
4. Rinsing and Post-treatment
In every stage of the electroplating process, the component is rinsed. After this electrolysis process, rinsing helps wash the electrolyte off the surface. Rinsing helps to prevent cross-contamination between baths. After rinsing, the component undergoes post-treatment, which may include sealing, passivation, anti-tannish treatment, lubrication, and drying.
What Are the Most Important Electroplating Process Parameters?
The following table summarizes key parameters of the electroplating process.
| Paramètres | Typical engineering role | What happens if poorly controlled? |
|---|---|---|
| Current density | Controls deposition rate and deposit characteristics | Burning, poor coverage, excessive thickness |
| Plating time | Determines deposited thickness | Under- or over-plating |
| Bath temperature | Controls reaction kinetics and conductivity | Slow deposition, roughness, chemistry instability |
| pH | Influences deposition chemistry and deposit quality | Poor efficiency, rough or brittle deposits |
| Metal-ion concentration | Supplies depositing metal | Thickness variation, unstable deposition |
| Agitation | Improves mass transport | Poor uniformity and local depletion |
| Anode/cathode spacing | Influences current distribution | Edge build-up and thin areas |
| Current efficiency | Determines how much electrical charge produces metal deposition | Thickness prediction errors |
| Surface preparation | Determines adhesion and cleanliness | Peeling, blistering, poor coverage |
| Filtration | Removes particles and contaminants | Pits, nodules and roughness |
| Bath contamination | Affects deposit chemistry | Brittleness, discoloration or poor adhesion |
| Part orientation | Influences solution flow and current distribution | Uneven coating |
Current Density
The electroplating process is founded on Faraday’s law that;
m= (M × I × t × η) / (n × F)
Où m is the mass of nickel deposited, M is the molar mass of nickel ≈ 58.69 g/mol, I is the current in amperes, and t is the plating time in seconds. This provides a quantitative framework for calculating the mass of metal deposited during electrolysis [2].
The plating process does not depend only on the amount supplied. Designers and engineers consider current density (J). This is the amount of current supplied per unit area. Current density is given by;
J = I / A
Où A is the plated area.
Even though increasing current density increases deposition rate, excessive current density must be avoided. It can lead to burning, rough deposits, excessive hydrogen evolution, poor appearance, and increased internal stress.
Current density is often expressed as A/dm² (ASD), A/cm², or A/ft².
Plating Time
From Faraday’s law, plating time affects the quantity of metal that can theoretically be deposited directly. This means that by keeping other factors like current density, electrolyte concentration, and substrate components constant, increasing plating time can increase the amount of deposited material (in our case, nickel deposition thickness).
Suppose the process has been validated so that a particular current density produces approximately a known deposition rate. The following table shows that plating thickness increases with plating time.
However, the deposition rate should not be evaluated solely by plating time. All other factors as described in Faraday’s law must be obeyed. A major current-control challenge is the uneven distribution of current around the part. Sharp or protruding mold features tend to receive higher local current density than recessed areas. This results in high-current regions leading to thicker coatings and lower-current regions leading to thinner coatings.
Electrolyte Temperature and pH
Temperature is a key activation factor affecting all chemical processes in electroplating. It controls bath stability, additive behavior, reaction kinetics, and electrical conductivity. Take, for instance, MacDermid Alpha. This process operates at a preferred average electroplating temperature of 55°C, with a range of 50 to 60°C. This temperature range is lower than that of the Atotech electroless nickel process, which operates at 74–76°C.
Temperature relations are not a constraint for continuous operation. It is directly correlated to pH values. For example, in our case, an operating temperature of 55°C would result in maximum deposition at a pH of 3.5- 4.5. The temperature differences between different operating technologies show that engineers must define a specific operating environment. Temperature must be highly controlled to avoid deviation from the electroplating chemistry.
Effects of pH
pH affects the chemical equilibrium of the electrolyte. It affects current efficiency, metal-ion availability, hydrogen evolution, additive behavior, deposit morphology, and deposit stress. The average preferred pH level specified for the MacDermid Alpha electrolytic nickel system is 4.0, with a range of 3.5-4.5. pH must not exceed or fall below the recommended levels. Higher pH levels can lead to less ductile deposits. On the other hand, lower pH levels can reduce plating efficiency.
How Can Electroplating Be Controlled in Production?
Electroplating process control involves managing key parameters such as bath (electrolyte) chemistry, pH, current, and plating time. Bath control involves controlling metal concentration. This monitoring ensures that sufficient ionic transfer occurs from the anode to the cathode. Controlling pH focuses on additive concentration and reaction by-products. A control system measures the pH of the reactants and the final products to ensure proper balance. The electrical control records current, voltage, and current density. The current density must fall within the recommended ranges. All these measures are computer-programmed, and red signals and sirens in the industrial setup identify any deviation.
Références
[1] ASTM International. (2024). Standards for Electroplating: Turning the Screw. ASTM News & Technical Insights. https://www.astm.org/news/standards-electroplating-turning-screw-mj24
[2] National Institute of Standards and Technology (NIST). (1997). Advanced Technology Program motor vehicle manufacturing. NIST Technical Report Archive. https://nvlpubs.nist.gov/nistpubs/Legacy/IR/nistir6079.pdf
[3] ASM International. (n.d.). Electroplated Coatings. ASM Digital Library Handbooks. https://dl.asminternational.org/handbooks/edited-volume/46/chapter-abstract/552799/Electroplated-Coatings?redirectedFrom=PDF
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