JNTU B.Tech IV ELECTROMETALLURGY AND CORROSION ENGINEERING Previous Question Paper (October/November - 2020 - R16)

Electroplating of copper is an electrolytic process used to deposit a layer of copper onto a conductive substrate to improve its appearance, conductivity, or corrosion resistance. The process involves an electrolytic cell consisting of an anode, a cathode, and an electrolyte solution.

The substrate to be plated acts as the cathode, while a pure copper plate serves as the anode. Both are immersed in an acidic copper sulfate solution. When a direct current is applied, copper atoms at the anode undergo oxidation, releasing electrons and entering the solution as copper ions:

These copper ions migrate through the electrolyte toward the cathode, where they undergo reduction by gaining electrons to form a uniform metallic copper deposit on the surface of the substrate:

The concentration of copper ions in the electrolyte remains constant because the rate of dissolution at the anode matches the rate of deposition at the cathode, provided the current density is maintained within optimal limits to ensure a smooth, adherent, and bright coating.

Electrolytes used in copper plating are primarily categorized into two types: acidic baths and alkaline (cyanide) baths. Acidic baths, typically based on copper sulfate and sulfuric acid, are widely used for high-speed, thick deposits due to their high current efficiency. Alkaline cyanide baths are employed when plating on base metals like steel or zinc, as they provide better adhesion and prevent displacement plating (immersion plating) that would otherwise occur in acidic solutions.

The commercial preparation of a standard acid copper electrolyte involves the following steps:

1. Dissolve high-purity copper sulfate crystals (CuSO₄·5H₂O) in deionized or distilled water to reach the desired concentration, typically 150–200 g/L.
2. Slowly add concentrated sulfuric acid (H₂SO₄) to the solution while stirring constantly. The acid increases the conductivity of the bath and prevents the hydrolysis of copper salts.
3. Add specific organic additives, such as brighteners, levelers, or wetting agents, in small quantities to control the grain size and surface finish of the deposit.
4. Filter the solution to remove any suspended impurities that could cause roughness or pitting in the plated layer.
5. Analyze the bath for pH and copper ion concentration before commencing the plating operation.