Zinc-Rich Primer

Q. Is there any benefit in using a zinc-rich powder primer under a TGIC powder top coat on aluminum substrates such as sand castings, die castings, and extrusions? If so, I would appreciate any test data quantifying what extension in service life can be expected versus just a TGIC top coat on these aluminum substrates.

Please assume four different scenarios, That these substrates are: 

 

1. just shot blasted before coating,
2. have a non-chromate pretreatment applied before powder coating,
3. are electrocoated before powder coating, or
4. are hard-coat anodized before powder coating.

Thank you. E.H.

 

A. Zinc-rich primers are formulated to be applied only over ferrous substrates. Applying them over anything else would be the equivalent of “putting lipstick on a pig.”

Zinc compounds are useful in improving the corrosion resistance of ferrous substrates because of the galvanic reaction caused by dissimilar metals in a salt-rich atmosphere. They are pretty far from each other on the galvanic scale, so they work well together. Zinc is, however, very close to aluminum galvanically, and therefore the aluminum substrate would obtain no benefit from a zinc-rich primer.

However, using a standard powder epoxy powder primer, or e-coat primer, can improve corrosion resistance when a TGIC polyester topcoat is applied on an aluminum substrate. These are both cheaper than zinc-rich epoxy primers and don’t have the disposal issues associated with coatings containing heavy metals.