How does Electroless Plating work?

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When people in the trade speak of "electroless plating" they are not referring to "immersion plating", such as you see when an iron nail is placed in copper sulphate and it spontaneously becomes copper plated.

Rather, they are referring to autocatalytic plating. Autocatalytic nickel plating processes contain dissolved nickel plus a reducing agent like sodium borohydride or sodium hypophosphite. When a catalytic metal surface like steel is immersed in the plating solution, that metal surface catalyzes the reduction of nickel ions to nickel metal by the reducing agent. One of the surfaces that comprises a catalyst is nickel itself (hence the name autocatalytic). Thus, even though the steel surface becomes fully covered with nickel metal, the reaction does not stop because the newly deposited nickel metal also serves as a catalyst for the continuing reaction. The plating bath also contains stabilizers to slow down the deposition reaction and (hopefully) prevent the nickel values from spontaneously plating out.

Ted Mooney, P.E.  finishing.com Brick, New Jersey