Salt Spray Comparison, CED vs. CED Plus Powder Coating
Q. A customer asked: What is the expected salt spray resistance using cathodic Electrocoat on top of Tricationic low Zinc phosphate? What is the increase, if any of the salt spray resistance if a Powdercoat finish is applied to the above? If anyone has data I would appreciate it.
- Romulus, Michigan
A. Cathodic Electrocoating can give 1000 hours of salt spray resistance in accordance to test standard ASTM ASTM B117 [link by ed. to spec at TechStreet] . The choice of pre treatment is more of 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 B 117. However, if a test is carried out without the inscribed cross, the thicker the film the better it may perform.
Mohali, Punjab, India
Thanks Gurvin. Yet more evidence for stopping this madness of designing finishes for salt spray hours :-)
As you say, the unquestionably superior finish of powder coating on top of CED coating rates the same in salt spray hours as just the CED coating. The salt spray test should be used as an early warning QA tool to guard against the finishing process going out of control; it should never be used to compare one finish against another.
Ted Mooney, P.E.
Pine Beach, New Jersey
Salt spray test failure of CED coatingMay 5, 2017
Q. What effects the salt spray test hours on CED coating. Customer has given us 1000 hrs salt spray test with no red rust but we are not achieving the same; only until 600 hrs.
Please help in this matter.
^- Privately contact this inquirer -^
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, 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 may need to be made. Good luck.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"
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