Zinc plated steel screw, assembled with brass nut, fails Salt Spray test
May 4, 2012Q. Dear All,
We are facing a problem of Steel Screw failing in salt spray test for 72 hrs. The screw is 10 micron zinc plated with yellow passivation.
The screw is assembled with brass nut and Salt spray test is being done in assembled condition. We question whether the brass (nut) is giving any effect during testing and cause for the failure? Kindly provide your suggestion. Thank you.
Regards
Rajakumar
- Chennai, Tamilnadu, India
May 4, 2012
A. Hi, Rajakumar.
Although we could guess, this strikes me as a somewhat foolish thing to conjecture about. Let's not squander this opportunity for a teachable moment for you, me, and all of the readers :-)
Please put some screws in the test chamber without nuts assembled and tell us what you find. Thanks!
Regards,
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 Ted Mooney, P.E. RET finishing.com Brick, New Jersey |
May 10, 2012
Q. Dear Mr. Ted Mooney,
Thank your very much for the reply.
If steel screws are tested alone, then it is passing in Salt spray test for 72 hrs.
But as per our organization standard, this is to be tested in assembled condition only and it is failing in this condition.
Please provide your suggestion.
Thanks,
Regards,
- Chennai, Tamilnadu, India
A. Hi, Rajakumar. I'm working from sketchy details and a limited understanding of the situation, but zinc plating and chromate conversion coating the brass nut should solve the problem. If you don't want to do that, then simply lacquering the brass nut should reduce the damage its doing to the plating. Good luck.
Regards,
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Ted Mooney, P.E. RET finishing.com Brick, New Jersey |
May 22, 2012
Q. Dear Mr. Ted Mooney,
Thank your very much for the reply.
Please note that Only Steel screws are failing in Salt spray test and not the brass nut. Do you recommend to do zinc plating and chromate conversion coating for the Brass nut?
Please clarify.
Regards,
- Chennai, Tamilnadu, India
A. Hi, Rajakumar.
You have apparently determined that the nut and bolt will independently survive the salt spray test, but the assembly will not, and you require that the assembly will do so.
In this case, the problem then appears to be galvanic incompatibility, with the exposed brass of the nut accelerating the corrosion of the screw. In that case, the galvanic incompatibility must be addressed, and one way to do that is to zinc plate and chromate conversion coat the brass nut. So, yes, that is what I suggest. Please let us know how it works out.
Regards,
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Ted Mooney, P.E. RET finishing.com Brick, New Jersey |
