Powder Coating vs. Electrocoating

Question:

I am responding to the article in the January 2001 issue regarding the comparison between powder coat and electrocoat performance. I work for a large custom coater who applies both powder coating and electrocoating. Our electrocoat is a cathodic design and applies an epoxy coating. Concerning the corrosion resistance, we send out samples for salt spray testing (ASTM B117) monthly for both powder coat and electrocoat. The results of the testing always indicate that the electrocoat outperforms the powder coat. Most of our automotive specifications stipulate that powder coat should be tested for 500 hr and electrocoat tested for 1,000 hr. The excellent chip and impact resistance allows for metal-to-metal contact (bulk packaging) of finished product. I agree that a powder coat finish allows for more options to meet customer’s product requirements, but for corrosion protection and durability electrocoat is far superior. J.E.

Answer:

You have dealt a crushing blow to my ego. Like all consultants, I depend on my knowledge and experience to present an image of infallibility. But, you proved that only the Pope is infallible, and consultants shouldn’t take strong stands on technologies that they have limited experience with. I was so sure that there was something in your process that was skewing the test results and your experience. However, after we talked, I realized that the electrocoat technology and coatings have improved and that my experience was dated. I’ll recap our discussion here so that all the readers can benefit from this information.

We discussed your salt spray testing method with the expectation that some advantage was inadvertently given to the electrocoat test samples. However, the facts proved otherwise. You use standard untreated ACT test panels (audit panels) for all your testing to ensure that the substrate is the same. We discussed that your salt spray testing is performed on X-scribed panels to a rating of six or higher.

Both pretreatment systems are remarkably similar, including the DI rinses. I thought I had you here because normally the pretreatment systems that service an electrocoat line have more rinses and must use DI water. We both agree that corrosion resistance is most influenced by pretreatment, so I thought that this must have been skewing your test results. However, you described your powder pretreatment system; it has DI as the final rinse (although less of them) and has the same chemicals in the working stages. Just the same, you said you would take test panels from each process and switch them before coating to see if the pretreatment system made a difference in the salt spray results. The jury is still out on this point, but I don’t see an obvious advantage for electrocoat here.

We talked about the higher cross-linked density of powder coatings and how electrocoat doesn’t have this advantage. But, your in-house testing didn’t reveal any difference in functional or corrosion properties that would prove this to be an advantage for powder coatings. Furthermore, you are using an “improved” electrocoating that has a higher solids content than previous formulations. I even thought that you might be comparing an epoxy electrocoat to a polyester powder coating. Everyone knows that epoxies have better corrosion and chemical resistance than polyesters. But, you stated that electrocoat still outperforms powder coating in corrosion tests at your facility even when you test epoxy powder formulations. No special advantage for electrocoat here.

We discussed that electrocoat systems traditionally required more process control effort and tighter process monitoring. This may have meant that since the powder coating system’s pretreatment system is more forgiving, then you may have let it run on the edge of its process control tolerances while the electrocoat pretreatment system must be held to a tighter standard. Everyone knows that a well-maintained pretreatment system will provide better corrosion performance than one run out of tolerance. But you said that you run both systems, and always have, to the same tight standards. No special advantage for electrocoat here.

We both agreed that powder coatings come in many formulations and offer many choices to meet customer requirements. This is especially true for appearance properties where there are many choices of color and gloss that favor powder coatings over electrocoatings. We also discussed that you use powder coatings for customers who have outdoor exposure requirements, since the epoxy electrocoat will chalk quickly when exposed to UV light. These issues make powder coatings more suitable for anyone who needs good functional performance and superior appearance properties in an outdoor environment.

After our talk, I honestly felt that there was no inadvertent advantage given to the electrocoat test samples processed in your facility. Furthermore, your testing methods are fair and do not influence the test results. Therefore, the only explanation is that electrocoat formulations have been vastly improved, leading to improved performance properties and that only the best consultants admit they are fallible.