Corrosion resistance of CED? Salt spray hours compared to CED Plus Powder Coating

A. Cathodic Electrocoating can give 1000 hours of salt spray resistance in accordance to test standardASTM B117. The choice of pre treatment is more a function of the base metal. However, it needs to be compatible with CED system. The second part of the question is tricky! CED coating normally has a DFT of 20 microns or .8 MIL. A final coat of powder coating on it will not improve upon the salt spray test resistance under the test standard ASTM B117. However, if a test is carried out without the inscribed cross, the thicker the film the better it may perform.

Thanks Gurvin. Still more evidence for stopping this madness of designing finishes to meet salt spray hours :-)

As you say, the absolutely unquestionably far superior finish of powder coating on top of CED coating rates the same in salt spray hours as just the CED coating, demonstrating hw truly ridiculous this approach is :-(

Hot dip galvanizing, which is the most durable finish yet developed for many applications(50-75-100 years with no maintenance is not unheard of) performs poorly in salt spray testing.

The salt spray test should be used as an early warning QA tool to guard against a finishing process going out of control; it should never be used to try to compare one finish against another.

Regards,

A. CED system (Cathodic Electro deposition) requires a huge set up cost and the running cost is also high. E-coat is only suitable when you have a very large volume of production.

A. Hi Pradeep. Although Manish's answer may not be as quantitative as you'd like, it is the answer :-) E-coat, as far as I've ever seen, always involves automation, ultrafiltration, high-voltage electrification, and a series of steps which makes it expensive and impractical unless you have the production volume to justify it. But it's often a good finish by itself, and a great finish under powder coating.

Luck & Regards,

A. Hi Premlata. What is the substrate? ... a good grade of low carbon cold finished steel with no pitting or scratching? Are you using a top quality tri-cationic zinc phosphate pretreatment system from a respected vendor?

600 hours isn't bad at all :-) ... it's just not good enough for your customer. The point of saying it "isn't bad" is that 600 hours leads me towards thinking it's not an issue of your process being out of control, but of your process not being quite robust enough; processing changes probably need to be made. Good luck.

Regards,

A. Hi Arun. The recommended surface protection depends on the application and the environment.

Designing to salt spray hours is like teaching to the test; but if you're going to design to salt spray hours, the best approach is to simply test both, and you'll have a proven answer in less than a month.

CED plus powder coating is the ideal finish. CED alone can be a final finish, depending on the environment, but it's not as durable as powder coating. It's advantage over powder coating is that it can usually be pinhole-free, and offer consistent coating thickness.

In either case a phosphate pretreatment should be done; if the parts are actually exposed to salt or other corrosives, a zinc phosphate under powder coating would usually be best. Good luck!

Regards,