Summary for zinc plating brazed steel Discussion
Zinc plating on brazed steel is presented as a corrosion fix, not a galvanic risk. Mick Brown asked about an auto AC hard line that had rust-pitted under insulation, then was blasted and rebuilt with bronze brazing; he wanted it zinc dichromate plated and worried the bronze braze on steel might create galvanic corrosion if condensation occurred. The reply says that once the tube is zinc plated, neither the brazing nor the steel is exposed, so galvanic corrosion should not occur. Instead, the zinc acts sacrificially and protects the tube if moisture is present. The answerer also notes that the plater may recommend copper plating first, since copper plating of brazed steel is commonplace. The overall advice is that zinc plating and chromating the repaired line should be acceptable for this lightly exposed automotive part.
Q. I have an auto AC hard line that had insulation which retained condensation and caused rust pitting. It’s steel. I brought it to a welder asking if they could blast it and build material via welding to repair it. They fixed it by blasting it bare and brazing bronze material to build it back up where it had pitted. It was holding pressure before, and i’m not worried about the mechanical integrity of the line.
I wanted to have it zinc dichromate plated/finished but the braze on part of the steel line with chromated zinc over it sounds like a galvanic problem in the wings. The line may be subject to light condensation from time to time but is otherwise out of the weather. Thoughts?
Mick Brown
Engineer (but this is for a car restoration) – Midland, Texas
April 17, 2025
A. Hi Mick.
If I am understanding the situation correctly, no, the zinc plating and chromating will not cause a potential galvanic corrosion problem, it will solve one.
Once the tube is zinc plated neither the brazing nor the steel will be exposed, so no galvanic corrosion will occur; and the zinc will preferentially corrode to protect the tube if any corrosive forces are at work.
It should not be a problem to zinc plate and chromate the tube, although the plater may suggest copper plating it first. Copper plating of brazed steel is a commonplace.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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