The process of depositing a thin layer of gold over another metal for added beauty and durability has been used commercially since the late 1800s. Besides the glamour of having gold detailing or the appearance of solid gold on a piece, gold is plated for industrial purposes and is important for use in circuit boards. There are two main electroplating methods, tank and brush. Both involve use of electric current, electrodes (anode and cathode) and an electrolyte solution or preparation containing gold.
The object or areas to be plated have to be absolutely clean for plating to occur properly. In order to remove both organic and inorganic materials as well as grit and soil, a combination of different treatments are used, including acid cleaners, alkaline cleaners, abrasives and solvents.
Depending on the type of metal to be plated, treatment may be needed to deposit an intermediate plating metal or smooth the surface layer for gold deposition. For instance, in plating gold onto a copper alloy, nickel is plated first, then gold. Sometimes other finishes, such as chrome, need to be removed with a chemical stripping agent.
In order to get an electrolyte, the metal has to be in a state where it can disassociate and form ions. Gold is a stable metal and it takes harsh chemicals to accomplish this. Usually gold is complexed with cyanide, called cyanaurate, although techniques using sulphites and thiosulfites exist. There are many proprietary formulae for these solutions. In tank electroplating, cyanaurate is dissolved in an acidic bath that receives the electrodes. In brush electroplating, an applicator with a stainless steel core puts on the cyanaurate as a gel. Electric current passes from the steel applicator to the metal object being plated as the gel goes on.
The pH of the electroplating solutions for tank electroplating have to be adjusted to prevent formation of hydrogen cyanide, a lethal gas, at pH values above eight. However, below pH three, cyanaurate precipitates out of solution. Both inorganic and organic acids have been used to adjust pH in the workable range, including phosphoric acid, sulphuric acid and citric acid.
Brighteners are metal salts of transitional metals like cobalt, nickel and iron. They give improved wear resistance and brighter colours to the gold deposit. Some organic compounds are added to improve the density of the gold plating. Some of these organic additives are polyethyleneimine, pyridine sulphonic acid, quinoline sulphonic acid, picoline sulphonic acid and substituted pyridine compounds. Buffering agents such as a citrate/oxalate buffer can be added to help keep pH in the proper range. Wetting agents may be added as well.