Zinc chromate, or ZnCrO4, has been used for decades in the aeronautics and automotive industry as a cover or coating on aluminum or iron. While originally developed by the Ford Motor Company in the early part of the 1920s zinc chromate plating was also used very early in early airplane manufacturing and production.
The main use of zinc chromate plating historically, as well as today, is to provide top corrosion resistance without adding weight or thickness to the part. This was and is particularly important in the aerospace and automotive industries where weight and durability of the part is critical. The process also is effective in retaining some degree of the natural electrical conductivity, but this can vary based on several factors.
It is also important to note the application of zinc chromate plating, or chromate conversion, creates an aluminum surface which can be painted and glued. Untreated aluminum is highly resistant to both, not to mention also more likely to corrode.
While the benefits of zinc chromate plating have been summarized above, it is important to note the use of this plating process is essential in protecting aluminum from white rust.
White rust, which may be known as white corrosion, white oxidation or wet storage stain, is very common in most areas. It occurs when aluminum is exposed to moisture without time for the natural corrosion resistance of the metal to develop. Although a visual issue, the presence of white rust doesn’t necessarily impact the structural soundness of the part.
Adding zinc chromate plating to the aluminum will prevent the development of white rust, regardless of the storage conditions. This will allow stacking and packing of the aluminum parts and pieces in areas otherwise unsuitable for this type of storage.
With the chemical composition of zinc chromate plating there are toxicity issues with the traditional applications, especially those containing hexavalent chromium. New technology and new alternatives and formulations are being developed and used which greatly reduce this concern by using non-hexavalent chromium processes.
Different companies offering zinc chromate plating or chromate conversion can provide different options in colors and options for the process. This can make part identification easier or clearly identify parts as being of a specific brand. It can also be done purely for decorative purposes, but it will still help to increase the resistance of the part to any type of white rust or white oxidation.