electrofinish

Electrofinishing is an electrochemical process that deposits a thin layer of metal or alloy onto a substrate surface by passing electric current through an electrolyte bath. The workpiece acts as the cathode, receiving electrons; metal ions in solution migrate toward it and reduce to solid metal, building up a protective or decorative coating. Common electrofinishes include chrome plating, nickel plating, zinc plating, and copper plating, each chosen for corrosion resistance, wear resistance, or appearance.

The process works by immersing the workpiece in a solution containing dissolved metal salts and a rectified direct current source (typically 6 to 12 volts for most work). Current density, measured in amperes per square decimeter, controls coating thickness and deposit quality. Higher current speeds deposition but risks burning (brittleness, pitting, or burned spots), while lower current yields smoother but slower buildup. Coating thickness typically ranges from 5 micrometers for decorative finishes to 25 micrometers or more for functional protection.

Preparation is critical. The workpiece must be cleaned of oils, scale, and oxides through alkaline cleaning, acid pickling, or both. Without proper cleaning, the coating will not adhere. Surface geometry matters too: high-current-density areas plate thicker than recesses, so punch-through designs or masked areas require deliberate electrical engineering to achieve uniform thickness.

Variants and trade-offs

Pulse plating, in which current is switched on and off at controlled intervals, produces denser deposits and can coat complex shapes more uniformly. Selective electrofinishing masks certain areas to plate only the required regions, saving material and time. Barrel plating handles hundreds of small parts simultaneously in a rotating cathode assembly, making it economical for fasteners and small stampings but sacrificing uniformity. Hard chrome plating uses higher current density and specialized electrolytes to deposit extremely hard, wear-resistant coatings for hydraulic cylinders and machined components.

Common faults include nodular deposits (rough, bumpy surface from hydrogen evolution), pitting (small holes from gas entrapment), and adhesion failure if the substrate was not properly activated. Chromium plating, especially bright nickel followed by chrome, dominates decorative work but requires hexavalent chromium handling under strict environmental controls. Zinc plating is cheaper and environmentally friendlier, favored for fasteners and structural steel in corrosive environments. Barrel electrofinishing of mass-produced parts demands precise bath composition, temperature (typically 18 to 27 degrees Celsius for most processes), and current monitoring to maintain tolerance.