Introduction

GOLD PLATING is similar to other metal plating in most chemical and electrochemical ways. Gold differs from other metals primarily in that it is much more expensive. Within recent memory, the price of gold metal has gone from $35 per ounce to $850 per ounce and at the time of this writing is characteristically unstable at about $375 per ounce. Thus the cost of a gallon of gold plating solution is quite high.

This price level and the daily variability of its price have required chemists and engineers to severely limit the concentration of gold in the plating solution. Nickel, alkaline copper, and silver are typically plated from solutions that contain 37 g of metal per liter of plating bath. Acid copper is plated from a solution that contains 60 g of metal per liter, and a chromium solution can contain over 240 g of metal per liter. Gold, because of its price and the cost of the dragout losses, is rarely plated from a solution that contains more than 1 troy ounce per gallon (8.2 g/L). Some gold baths used for striking, decorative use, and barrel plating use as little as 0.8 or 0.4 g/L of gold.

These very low metal concentrations, or "starved" solutions, present problems to the gold plater that are quite different from those of other metal plating solutions. With a starved solution, every control parameter in the plating process becomes more critical. Gold concentration, electrolyte concentration, pH, impurity level, and additive level must all be monitored and controlled. Temperature, current density, agitation, and the current efficiency must be accurately known and controlled beyond the degree necessary for copper, nickel, or even silver plating. If any factor changes, even 2 to 3%, the cathode gold deposition efficiency changes. If the efficiency decreases, items being plated under standard conditions will be underplated and the specified thickness will not be attained. Similarly, if the cathode efficiency increases, the plate will be too thick and result in increased cost because of using excess gold.

The engineer and plater of gold must tread the narrow line between not depositing enough gold and giving away too much gold. In addition, those concerned with gold plating must not only keep the chemistry of the process and the peculiarities of electrodeposition in mind, as do other platers, but also be aware of the market price of gold. The plater must be an economist in order to realize when the operating conditions of the solution should be altered or the entire process changed to reflect the changes in the price of gold. Economics also determines the total consumption of gold. In the recent past, when the price of gold vaulted above $500 per troy ounce, many electronics companies replaced some of the total thickness of gold with undercoats of palladium or palladium-nickel alloys. Others abandoned gold completely. Economics is a more important factor in the plating and metallurgy of gold than in the plating of nonprecious metals.

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