Is a Diffusion Barrier Needed?
All gold plating is somewhat porous on a microscopic level. This porosity can allow reactive metals such as copper, copper alloys (such as brass or bronze), and silver to corrode under the gold plating. As the base metal corrodes, the base metal oxides can seep up or diffuse through the gold plating and appear on the surface as an unsightly tarnish. This diffusion tarnish can make the gold look dark or dull or seem to wear off sooner than it should, leaving your customers feeling that they didn’t get the quality they expected.
By plating your gold thick enough to effectively seal the base metal, or by adding a less reactive diffusion barrier under the gold, you can avoid this issue. Because of the cost of increasing the gold thickness along with the uncertainty of how thick the gold plating layer actually is, the addition of a diffusion barrier is usually the recommended remedy.
Increasing the gold thickness instead of a diffusion barrier can be tricky. Ensuring that you have a sufficient gold thickness to prevent diffusion involves several considerations:
- Type of gold plating applied (soft pure gold is more porous than hardened gold)
- Base metal reactivity.
- The corrosive nature of the environment.
- Environmental temperature.
- Environmental pressure.
- Environmental humidity.
- How long you expect to avoid diffusion.
It is also important to consider the type of diffusion barrier used. For example, nickel is a common diffusion barrier. However, nickel is often not used for jewelry items since it can cause an allergic reaction in many people. In this case, we would recommend rhodium or palladium. A nickel diffusion barrier can also be problematic for an item exposed to sea air or any other environment where the article may be exposed to salt, salt water, or salt mist. Below is a photo of a gold-plated item with nickel diffusing through the gold plating due to exposure to salt spray. The green crystals are nickel salt crystals that have diffused through the gold plating.
We are often asked how thick the gold should be for the typical gold plating to be able to avoid a diffusion barrier. As explained above, this is a difficult question to answer without more information. However, generally speaking, an item exposed to average room-type conditions should have at least one to two microns (40-80 millionths of an inch, or microinches) of hard gold to reasonably ensure that diffusion isn’t a problem over an extended period of time.