Zinc nickel
With zinc nickel electroplating, nickel is added to the solution used in the plating process. Typically, the solution contains 12 to 15% nickel. Though not as common as standard zinc electro-plating, zinc nickel offers superior corrosion resistance. The major disadvantage of any electroplating process is that it can cause hydrogen embrittlement in fasteners with a tensile strength greater than 320 HV.
Usually, the corrosion resistance is enhanced by a post plating process called chromate passivation. This applies a thin coating on the zinc surface. To comply with RoHS and ELV directives, the solution used in this process must be hexavalent chrome free and the most common solutions used today use trivalent chrome.
As standard, all fasteners that are case hardened and all fasteners with a property class of 8.8 or higher are heat treated after electroplating to help reduce the risk of hydrogen embrittlement. This process is called de-embrittlement, but it does not guarantee that the risk of hydrogen embrittlement is removed completely. If any risk of hydrogen embrittlement is unacceptable than a non-electroplated finish should be considered.
| Chromate treatment | Thickness | White rust | Red rust | Appearance |
|---|---|---|---|---|
| Clear | 8 microns | 120 hours | 720 hours | Transparent with slight bluish tinge |
| Black | 8 microns | 240 hours | 720 hours | Black |
| Heavy | 8 microns | 240 hours | 720 hours | Iridescent yellow |
| Black with top coat | 8 microns | 240 hours | 1200 hours | Black |
Performance results are approximate only and are based on laboratory salt spray tests.