Anodic or Cathodic E-Coat?

Q. I’m only familiar with cathodic E-coat. However, I recently heard about a company using an anodic E-coat on castings, and that it cures at a much lower temp. If the anodic E-coat takes less energy to cure, why isn’t it more widely used? F.F.

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A. There is more to it than the cure temperature. Here is a little of what the Electrocoating Guidebook says about anodic electrocoats. “Anodic electrocoating involves the use of negatively charged paint particles that are deposited onto positively charged metal substrates. The polymer species are acid functional and amine solubilized. On the positive side, some anodic electrocoats, like acrylics, offer economical finishes with excellent color and gloss control.

“However, on the negative side, the anodic deposition process leads to some dissolution of metal ions from the parts being coated. The ions become trapped in the depositing paint film and, due to their ability to interact with moisture, limit the corrosion performance of these films. Other undesirable effects can include film staining and discoloration due to the presence of iron in the anodic films.”

Even with its drawbacks, the anodic epoxy E-coat film still offers respectable corrosion resistance when compared with other coating technologies. This is especially true for dense parts like castings, engine blocks and for temperature-sensitive substrates, because it can be cured at less than 200°F.
Now, here is basically what the guidebook says about cathodic E-coat. “Cathodic deposition, where positively charged paint particles are attracted to a negatively charged part, involves much less iron incorporation into the depositing film and consequently offers substantially improved corrosion resistance. Additionally, the polymer species are amine functional and acid solubilized, with the alkaline nature of the polymer leading to better inherent corrosion resistance than can be obtained with acid functional species.”

When high coating performance is required, cathodic E-coat systems are generally specified. Market penetration into the appliance and automotive industries over the last 30 years bears evidence to the attractiveness of these coatings.