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Zinc passivated and galvanized metal corrosion

Can anyone expand on why, when bolting two galvanized flanges together, using zinc passivated bolts, the bolts start to corrode?

The customer suggests stainless steel bolts, but this gives a potential greater the 0.3V, greater then the recommended value for outdoor use - as per this application.

Is it down to the quality of the zinc passivating process?

Neville Tubb

There is at least one problem here, and possibly two. The sure thing is that the zinc plating will be much thinner than the galvanizing, so after a while the protection would give out. The other factor is that I have also seen zinc plating seem to corrode preferentially to galvanizing; I say 'seem to' because I did no testing of corrosion products and it is possible that the galvanization was corroding right along with the zinc plating but less obviously. The right answer is probably to galvanize the hardware.

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

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