An acid electroplating solution is a mixture of water, acid, and the metal to be plated. To this may be added a number of organic constituents that serve to regulate and distribute the delivery of metal to the surface being plated.
A basic electroplating “cell� consists of a tank full of the above 'electrolyte' with baskets of nuggets or arrays of bars of the metal to be plated arranged along two opposite sides. These are referred to as the “anodes� and are connected to the positive terminal of a DC current source called a “rectifier.� A rectifier is built to supply a large amount of current (1,000 amps is not unusual) but with a very low voltage (maybe only 3 volts). Situated between these anode banks is the material or part that is to be plated. It is variously referred to as the cathode or the work-piece and is connected to the negative terminal of the rectifier.
In the simplest terms, metal deposition occurs when an electrical potential (voltage) is established between the anodes and the cathode (the connected rectifier is turned on). The resulting electrical field causes movement of metal ions to the cathode (the work-piece) where the ionic charge is neutralized as they plate out of solution. In a properly maintained bath, sufficient metal erodes into solution from the anodes to exactly make up for the deposited material, maintaining a constant concentration of dissolved metal.
As in all electrolytic solutions, there is a tendency of electrical charges to build up on the nearest high spot, thereby creating a higher electrical potential. This area of increased potential attracts more metal ions than the surrounding areas which in turn makes the high spot even higher. Inhibiting and controlling this nonlinear behavior is where the organic grain refiners and brighteners come in to play.
Without the use of special additives, the electroplating process is uncontrolled and will result in a rough surface deposit. To control the process, chemicals referred to as wetters and grain refiners are used. The result is an even, consistent appearance finish.
The wetter serves to create a controlled plating zone at the surface of the work piece making it easier for the metal ions to deposit. The molecule has a “head� and a “tail�. One end is hydrophilic (attracted to water) and the other end is hydrophobic (repelled by water). The wetter assembles itself between the plating solution and the work piece to create a consistent environment for metal ion deposition.
A grain refiner helps to further control the process. Grain refiners are attracted to the “high points� on the plated surface. Left untreated, the high points would tend to attract more metal deposition compared to lower points resulting in a rough surface. A grain refiner inhibits further deposition. As these points are replaced by other higher points the grain refiner will drift away and reposition itself.
Because wetters and grain refiners do not co-deposit with the plating they do not affect the purity of the plating. In order to obtain a bright, or reflective, finish a “brightener� is used. Brighteners will co-deposit with the plating and their presence results in a smaller grain structure. The smaller grain size is smoother and thus more reflective.