Copper Plating on Zinc

Copper Coating Solution

A copper strike applied in a copper cyanide solution is normally the first plating step for all die castings electroplated with nickel and chromium. The thickness of the strike should be at least 1.0 µm for castings that will be plated with nickel after copper striking. A thickness of 3 to 4 µm is recommended for die castings that will be subsequently electroplated with bright, leveling copper in copper sulfate-sulfuric acid solutions. The copper strike is a critical step in plating zinc die castings that is sometimes inadequately controlled.

Solutions containing 20 to 45 g/l of copper cyanide, 10 to 20 g/l of free sodium cyanide, and 15 to 75 g/l of sodium carbonate are customary for strike solutions. A solution containing 20 to 27 g/l of copper cyanide and 10 to 15 g/l of free sodium cyanide is relatively popular. A few formulations include the molecular equivalents of potassium cyanide in place of sodium cyanide. Other contain 15 to 25 g/l sodium hydroxide or 30 to 45 g/l of sodium potassium tartrate in addition to the major constituents. Agent such as sodium hydrosulfite that reduce hexavalent chromium ions are sometimes added to prevent the reduction in efficiency caused by only a few parts per million of Cr(VI). Periodic additions of such as reducing agent may be required, especially in solutions containing tartrate ions. Cathode current densities range from 2.3 to 5.5 A/dm2 and solution temperatures from 50 to 57oC should be avoided, or there will be danger of blister formation.

The average cathode current density dust be balanced with the free sodium cyanide and the temperature of the solution, or burning might occur at edges and other high current density areas. With an average cathode current density of 2.3 A/dm2 the cathode current efficiency varies from 30 to 60% for strike plating, depending on the free sodium cyanide concentration. Although a low concentration (10-15 g/l) favor a high efficiency, throwing power is reduced, so that recesses receive thin deposits that are unsatisfactory for protecting the zinc from chemical attack during nickel plating in acid solution. Ultrasonic agitation increases the cathode efficiency, the covering power in recessed areas, and the density of the copper deposit.

High purity copper anodes are recommended for copper cyanide strike solutions. Solutions should be continuously filtered to avoid the inclusion of small particles that nucleate nodules during subsequent plating operations. Anode bagging is sometimes adopted for ensuring smooth copper strike deposits. In this connection, the control of the free cyanide concentration is important. A relatively high free cyanide concentration is also to maintain satisfactory anode dissolution in solution operated at 50 to 57oC.