There are two basic types of plating – electrolytic and electroless. Electrolytic plating requires electricity to make the process occur. There is a positive and negative charge. The positive is called the anode and the negative is the cathode. The part to be plated gets the negative charge so it becomes the cathode. The anode is made of conductive metal – such as lead – and becomes the source of the positive charge.
Hard chrome plating is one of the oldest electrolytic plating processes. The chromic acid solution is the medium by which the current transfers. When the plating process occurs, the negative and positive ions transfer in the solution causing a metal (chrome) to reduce onto the base metal of the part being plated. A good analogy would be to look at it as the reverse of EDM. Because the process uses electricity, the plater is constantly fighting against the laws of nature.
The old rule still holds true: electricity travels in straight lines and goes to the closest point. On sharp corners there will be a heavy buildup of plating. In the recesses, the plating will tend to be thinner.
Electroless plating, such as electroless nickel, is just the opposite of electrolytic plating. Additives in the solution take the place of electricity. These additives are known as reducing agents. Since all metals have a charge, when the reducing agents detect the base metal charge in the plating bath, they start to react.
This causes the metal in the bath solution – in this case nickel – to reduce onto the base material of the part being plated. No electrical current is required. Wherever the plating solution touches the base material, the plating will adhere. This gives the plated part a very uniform deposit and the plating thickness can be controlled within .00005-.0001, even on complex shapes.
Within these two families you have several types of deposits. These different types include composite and alloy deposits. As a generality, if you need a perfectly uniform deposit in a complex shape, electroless nickel deposits are best. However, hard chromium is the hardest deposit and has excellent release.