Unit
- Electrochemical cell basics: An electroplating cell consists of an anode, cathode, electrolyte solution, and an external power source. The workpiece serves as the cathode and the metal to be deposited is supplied by the anode or dissolved salts in the bath.
- Faraday’s laws of electrolysis: The mass of metal deposited is directly proportional to the quantity of electricity passed through the cell and the equivalent weight of the metal. One Faraday (96485 coulombs) deposits one gram-equivalent of metal.
- Current density and distribution: Current density, expressed in amperes per square decimeter (A/dm²), determines the rate and quality of deposition. Non-uniform current distribution leads to variations in coating thickness across the part geometry.
- Overpotential and polarization: The difference between the applied potential and the equilibrium potential drives the deposition reaction. Activation overpotential, concentration overpotential, and resistance overpotential each affect the efficiency and morphology of the deposit.
- Faradaic efficiency: Not all current contributes to metal deposition; side reactions such as hydrogen evolution consume a portion. Current efficiency varies by metal type, bath chemistry, and operating conditions.
- Throwing power: The ability of a plating bath to deposit uniform thickness on irregularly shaped parts. High throwing power baths are essential for complex geometries such as threaded fasteners or internal bores.
- Electroplating versus electroless plating: Electroplating requires an external current source and deposits metal only on conductive surfaces. Electroless plating uses a chemical reducing agent in solution and can coat non-conductive substrates after appropriate activation.
- Typical deposition rates: For common metals like copper, nickel, and chromium, deposition rates range from 0.1 to 1.0 micrometers per minute depending on current density. Higher rates risk poor adhesion, roughness, and increased porosity.
- Bath composition and additives: Electroplating baths contain metal salts, complexing agents, buffers, and brighteners. Additives such as levelers, grain refiners, and wetting agents control deposit properties like hardness, brightness, and internal stress. Regular analysis of metal ion concentration, pH, and additive levels is required to maintain optimal plating performance.
- Anode types and behavior: Anodes can be soluble or insoluble. Soluble anodes, such as copper or nickel, dissolve to replenish metal ions in the bath. Insoluble anodes, like platinum-coated titanium, are used for baths where metal ions are supplied by salts. Anode area and geometry influence current distribution and dissolution uniformity.
- Pulse and reverse pulse plating: Modulating the current waveform between on-time and off-time periods allows improved deposit properties. Pulse plating can reduce porosity, enhance adhesion, and refine grain structure by controlling diffusion layer thickness. Reverse pulse periodically removes loosely adsorbed impurities, leading to smoother deposits. Typical pulse frequencies range from 10 to 1000 Hz, with duty cycles of 10-50% depending on the application.
Process Flow
- Step 1 - Part Preparation: Clean and degrease the workpiece to remove organic contaminants. Rinse thoroughly with deionized water.
- Step 2 - Activation: Immerse the part in an acid or alkaline activator to remove oxides and create a chemically active surface for adhesion.
- Step 3 - Electroplating: Submerge the prepared part in the electrolyte bath and apply a controlled current between anode and cathode for a specified time.
- Step 4 - Rinsing and Drying: Remove the plated part, rinse to remove dragout solution, and dry using hot air or centrifugation to prevent water stains.
Quick Revision
- Electroplating relies on an electrochemical cell where the workpiece is the cathode.
- Faraday’s law governs the mass of metal deposited per unit charge passed.
- Current density distribution affects coating thickness uniformity across part geometry.
- Throwing power determines the ability to plate complex shapes evenly.
- Electroless plating differs by using chemical reduction instead of external current.