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topic 13029

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.

Thanks,

Steve Wade
- 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.

Gurvin Singh
Coatec India
supporting advertiser
Mohali, Punjab, India
coatec india


thumbs up signThanks 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.

Regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



Salt spray test failure of CED coating

May 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.

premlata p
- india
  ^- Privately contact this inquirer -^


May 2017

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.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



August 26, 2019

Q. For SG500 casting we need to protect the machined surface from corrosion. Minimum salt spray life to be achieved is 600 Hrs.
What is recommended surface protection CED or Powder coating?

Arun Solunke
- Pune, Maharashtra, India


August 2019

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

CED plus powder coating is an ideal finish. CED alone can be a final finish, but it's generally not as durable as powder coating. It's advantage over powder coating is that it can 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 would be best. Good luck!

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha

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