Electroless Deposition (Autocatalytic Plating)
By this process the deposited metal is reduced from its ionic state in solution by a chemical reducing agent. The reducing agent supplies the electrons for the following reaction:
Me2+ + 2e = Me0
This reaction takes place only on a catalytic surface. Therefore, once deposition is initiated, the metal deposited must itself be catalytic for the deposition to continue. Electroless plating uses a redox reaction to deposit metal on an object without the passage of an electric current. Because it allows a constant metal ion concentration to bathe all parts of the object, it deposits metal evenly along edges, inside holes, and over irregularly shaped objects which are difficult to plate evenly with electroplating. Electroless plating is also used to deposit a conductive surface on a nonconductive object to allow it to be electroplated. Not all metals can be plated autocatalytically. The reducing agents are usually more expensive electron sources as compared with the electric current.
Example:
Electroless nickel plating (EN) is an auto-catalytic chemical technique used to deposit a layer of nickel-phosphorus or nickel-boron alloy on a solid part, such as metal or plastic. The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPO2H2·H2O) which reacts with the metal ions to deposit metal. The alloys with different percentage of phosphorus, ranging from 2-5 (low phosphorus) to up to 11-14 (high phosphorus) are possible. The metallurgical properties of alloys depend on the percentage of phosphorus.