Topic: Pre-dip Before Nickel

Whether or not you are plating over copper, brass or a ferrous substrate with nickel, you should prepare the surface by removing oxidation prior to nickel plating. This can be done by using a pre-dip usually a mild acid with enough activity to remove the flash rust and mild oxidation.

On process systems that involve a great deal of time during passes from the preparation cycle to the plating bath, predips are very important, and they can make the difference between success and failure in a plating operation.

Pre-dip solutions should have the following properties:

> They must be amenable to drag-in to the nickel solution, so the acid used needs to be non-contaminating to the nickel bath. Because pre-dips are either rinsed with a single rinse, or not rinsed at all, it is important that drag-in does not contaminate the nickel bath.

> Pre-dip acids should also be clean and uninhibited. Good grades of mineral acids fit the bill well.

> They should be cheap and replaced often.

> They should be effective at room temperature.

> The last station before the plating tank can be used to collect errant material and prevent it from entering the plating tank. That is the reason the pre-dips should be replaced often, or purified to remain clean.

Sample Pre-Dips For Steel Or Iron Parts:

(Direct plating of nickel over steel)

To remove flash rust that will form on the surface of steel during the pre-plate process, a pre-dip should be used. A simple one would be 2 percent sulfuric acid.

If the pretreatment is excessive, or there is a great deal of flash rust on the surface, 1-2 percent of hydrochloric acid should be included.

Woods Nickel or modified Woods Nickel can be used as a pre-dip, with current. The electrified Woods Nickel will remove flash rust as a chloride and deposit a thin coating of an active nickel on the surface. This will increase adhesion by increasing the integrity of the deposit, and it will prevent drag-in of iron to the nickel bath.

Sample Pre-Dips For Nickel Over Copper Alloys

Whether plating over a copper substrate or over a copper deposit, you should use an acid pre-dip, prior to nickel plating.

In this case, you are removing any oxides that may have formed over the copper surface, or you can condition the surface by neutralizing the surface and leaving a slightly acidic film, if the copper was plated from an alkaline plating bath. Sulfuric acid at 1-2 percent is usually enough to activate and remove any slight surface oxidation prior to the nickel plating sequence. Chlorides are more soluble than sulfates, so a small addition of either sodium chloride or hydrochloric acid will help to remove the oxidation.

Because you are dissolving copper as copper, the oxide and the copper will exist in the ionic form in the bath. It will have to be replaced often, to prevent the drag-in of copper into the nickel bath. Also, the copper in the pre-dip solution may immersion-deposit onto the surface of your parts, if the concentration is allowed to get too high.