"Immersion Deposit"

Immersion/replacement plating is seen in science class when you put a galvanized nail into a solution of copper sulphate and it acquires a thin coating of copper on it.

The reason is that copper is a noble metal and zinc an active metal, meaning that copper exerts more pull on electrons than zinc does. So the copper in the solution which touches the zinc-covered nail "steals" electrons from it. As the electrons move from the zinc to the copper, the zinc is no longer an electrically neutral atom but becomes a positively charged ion and dissolves into the solution; at the same time the positively charged copper ions in solution becomes neutral copper metal atoms.

Immersion/replacement plating has some disadvantages. Firstly, adhesion is often very poor because as the copper (in this example) is trying to deposit, the zinc (in this example) which it is depositing upon is dissolving underneath it. Secondly, the thickness is very limited because as soon as no more zinc is exposed, no more copper will deposit.

Immersion plating is not limited to copper on zinc but can occur when depositing any more noble metal on a less noble metal. Sometimes it is unwanted and problematical due to the poor adhesion. For example, steel cannot be usefully electroplated directly from a copper sulfate plating solution because the inadvertent immersion deposit prevents good adhesion.