Q. Our plating shop is having problems with plating gold components that have been nickel plated. We find that we get inconsistent results with the gold adhering to the nickel plate. This causes peeling and blistering of the gold plate. Most of the problems arise with parts that have been nickel plated at a different location and shipped to us. What is the secret to getting a good gold plate on a nickel plated part? L.A.
A. Your problem is a fairly common issue when plating gold over nickel. In fact this is a problem when plating any type of metal over a nickel plated surface or a nickel alloy. Nickel forms oxides when exposed to the atmosphere. These oxides make the surface passive. The nickel surface essentially becomes chemically inactive and will not bond with other materials, in this case, gold. This process takes place whenever the surface is exposed to the air. Surprisingly, passivation can even take place if the nickel plated parts are not directly exposed to air but instead are held in a rinse tank for long periods of time prior to gold plating.
To get consistent results, the nickel surface must always be activated prior to gold plating. The parts should be precleaned in a mild alkaline soak cleaner followed by an activation step. There are a number of different activation procedures available. Cathodic activation in dilute sulfuric acid, proprietary activators or a Woods nickel strike are all possibilities.
It is critical that as soon as you complete the activation step you go directly to the gold plating step. Failure to do so can result in passivation of the surface all over again.