define chemical plating

Electroless plating, also known as chemical or auto-catalytic plating, is a non-galvanic type of plating method that involves several simultaneous reactions in an aqueous solution, which occur without the use of external electrical power. The reaction is accomplished when hydrogen is released by a reducing agent, normally sodium hypophosphite, and oxidized thus producing a negative charge on the surface of the part. The most common electroless plating method is electroless nickel plating.
Electroless nickel plating, or "EN", is a technique used to deposit a layer of nickel- phosporous alloy on a solid workpiece, such as metals or plastic. The alloys with different percentage of phosphorus, ranging form 2-5 (low phosphorus) to up to 11-14 (high phosphorus) are possible. The metallurgical properties of alloys depend on the percentage of phosphorus.
Electroless nickel plating is an auto-catalytic reaction used to deposit a coating of nickel on a substrate. Unlike electroplating, it is not necessary to pass an electric current through the solution to form a deposit. This plating technique is to prevent corrosion and wear. EN techniques can also be used to manufacture composite coatings by suspending powder in the bath.
Electroless nickel plating has several advantages versus electroplating. Free from flux-density and power supply issues, it provides an even deposit regardless of workpiece geometry, and with the proper pre-plate catalyst, can deposit on non-conductive surfaces.
Pretreatment of parts for EN plating
Before performing electroless nickel plating, the material to be plated must be cleaned by a series of cleaning chemicals such as bases and acids, this process is called the pre-treatment process. Failure to remove unwanted "soils" from the part's surface would result in poor plating. Each pre-treatment chemical must be followed by water rinsing (normally two to three times) to remove the chemical that adheres to the surface. Degreasing removes oils from surface; acid cleaning removes scaling. Activation is done with a weak acid etch, or nickel strike, or, in the case of non-metallic substrate, a proprietary solution. After the plating process, plated materials must be finished with an anti-oxidation or anti-tarnish chemical (trisodium phosphate, chromate etc) and pure water rinsing to prevent unwanted stains. The rinsing materials must then be completely dried off or sometimes baked off to obtain the full hardness of the plating film.
Advantages of EN
- Does not use electrical power.
- Even coating on parts surface can be achieved.
- No sophisticated jigs or racks are required.
- There is flexibility in plating volume and thickness.
- The process can avoid plate recesses and blind holes with stable thickness.
- Chemical replenishment can be monitored automatically.
- Complex filtration method is not required
Disadvantages of EN
- Lifespan of chemicals is limited.
- Waste treatment cost is high due to the speedy chemical renewal.
Applications
Electroless nickel plating produces a non-toxic coating; as a result, this method is commonly used in medical devices. Other applications include optical surfaces for diamond turning, door knobs, kitchen utensils, bathroom fixtures, electrical/mechanical tools and office equipment. It is also commonly used as a plating in electronics printed circuit board manufacturing, typically with an overlay of gold to prevent corrosion. This process is known by the acronym ENIG, which stands for "Electroless Nickel, Immersion Gold".
I hope this can help your understanding. =)

chemical plating = plating with chemicals