Electroplating and Artificial Jewellery

Overview & Fundamental Concept

Detailed AISSEE Class 9 General Science guide for Electroplating and Artificial Jewellery (Topic 17). Contains pointwise electrochemistry principles, jewellery plating schematics, electrode half-reactions, solved models, and 10 practice drills.

• Electroplating: An electrolytic process in which a thin, cohesive layer of a superior, non-corrosive metal is deposited onto the surface of a base metal by passing a Direct Current (DC) through an electrolyte solution.
• Fundamental Setup Rules:
  • 1. Anode (+ve Electrode): Made of a clean sheet of the pure metal to be deposited (e.g. pure Gold, Silver, Copper, or Chromium). It connects to the positive (+) terminal of the DC battery.
  • 2. Cathode (−ve Electrode): The article/jewellery piece to be coated (e.g. brass bangle, iron spoon, copper ring). It connects to the negative (−) terminal of the DC battery.
  • 3. Electrolyte Solution: A water-soluble chemical salt of the anode metal containing its mobile positive cations (e.g. $CuSO_4$ for copper plating; $AgNO_3$ or potassium argentocyanide for silver plating).
• Electrochemical Mechanism (Redox Reactions):
  • At the Anode (Oxidation): Pure metal atoms lose electrons and enter the electrolyte as positive cations:
  $$M \text{ (solid)} \longrightarrow M^{n+} \text{ (aqueous)} + n\,e^-$$
  • In the Solution: Metal cations ($M^{n+}$) migrate electrostatically toward the negative cathode.
  • At the Cathode (Reduction): Metal cations gain electrons and deposit as a solid, mirror-smooth metallic coating on the jewellery:
  $$M^{n+} \text{ (aqueous)} + n\,e^- \longrightarrow M \text{ (solid film)}$$
• Constant Electrolyte Concentration: Because the rate of metal dissolution at the anode equals the rate of metal deposition at the cathode, the concentration of the electrolyte bath remains absolutely constant throughout the operation.

• Artificial / fashion jewellery is manufactured from inexpensive base metals and alloys such as Brass (Copper + Zinc), Copper, Nickel, or Steel, and electroplated with gold, silver, or rhodium for four primary reasons:
• 1. Aesthetic Elegance at Low Cost: Real gold and silver are extremely expensive precious metals. Electroplating deposits a microscopic film (often only a few micrometres thick) of 24-karat gold or pure silver on brass jewellery, providing the exact radiant shine, color, and finish of solid gold at less than 1% of the cost.
• 2. Resistance to Corrosion & Tarnish: Base metals like copper and brass oxidize rapidly when exposed to atmospheric moisture, sweat, and air, turning dull green or black (verdigris / copper carbonate). Gold and rhodium are noble, unreactive metals that do not corrode, tarnish, or oxidize.
• 3. Prevention of Skin Irritation & Allergies: Many individuals suffer from contact dermatitis (skin rashes and green skin discoloration) caused by nickel and copper salts reacting with acidic body perspiration. An impermeable electroplated layer of gold or silver creates a hypoallergenic barrier.
• 4. Increased Surface Hardness & Durability: Precious metals like Rhodium (a platinum-group metal) are electroplated over silver or white gold to provide superior scratch resistance and prevent silver from tarnishing black from sulfur compounds in air.

• The single most important factor determining the adhesion and smoothness of electroplating is the cleanliness of the substrate surface. Even a microscopic speck of oil or oxide will prevent the plating metal from bonding, causing the film to blister or peel off.
• Step-by-Step Industrial Preparation Protocol:
  • 1. Mechanical Polishing & Buffing: The jewellery article is smoothed with abrasive polishing wheels to remove casting marks, scratches, and burrs.
  • 2. Alkaline Degreasing (Cleaning): The article is washed in a hot alkaline detergent bath (containing sodium hydroxide and sodium carbonate) to dissolve greases, machine oils, and fingerprints.
  • 3. Water Rinsing: Rinsed thoroughly in deionized water to remove residual cleaner.
  • 4. Acid Pickling (Acid Dip): Dipped into a dilute acid bath (usually 5–10% sulphuric acid or hydrochloric acid) for a few seconds. The acid dissolves surface tarnish, rust, and invisible metal oxide films, exposing pure metallic crystalline lattice.
  • 5. Electroplating Bath: Immediately transferred into the electroplating tank while wet, without touching with bare fingers.
  • 6. Post-Plating Rinsing & Drying: Washed in warm water and dried with warm air or sawdust to prevent water spotting.

• The visual quality, adherence, and uniformity of an electroplated deposit depend on controlling four physical variables:
• 1. Current Density ($I / A$):
  • The amount of electric current passing per unit surface area of the cathode.
  • Too High Current Density: Metal deposits too rapidly, resulting in a dark, powdery, rough, or burnt deposit.
  • Too Low Current Density: Deposition is agonizingly slow and thin.
  • Optimal Current: Produces a fine-grained, smooth, reflective crystal layer.
• 2. Direct Current Voltage: Typically low voltage (between 1 to 6 Volts DC) is maintained to avoid vigorous hydrogen gas evolution at the cathode that causes pinholes and pitting in the plated film.
• 3. Electrolyte Concentration & Agitation:
  • The concentration of metal salts must remain within specified limits.
  • Mechanical or air agitation of the bath ensures a fresh supply of metal ions continuously surrounds the article, preventing local depletion.
• 4. Temperature: Warm baths (around 40–60°C) increase ionic mobility and solubility, producing brighter deposits.
• Cyanide Baths in Precious Metal Plating: In commercial gold and silver plating, potassium gold cyanide or potassium silver cyanide complexes are used. Cyanide ions hold the metal ions in complex form, releasing them very slowly. This slow, steady release produces an exceptionally smooth, mirror-bright coating.