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Galvanic Reaction: Galvanised Flanges with Stainless Bolts? [South Africa] 

June 22, 2008

Hi, we have an HVAC project in Cape Town, South Africa. We are installing Diam.200mm hot-dip galvanised mild-steel piping using steel flanges.
We have been requested to install 316 Stainless steel bolts and nuts.
I question this, as I think there could be galvanic reaction, leading to premature corrosion. Would appreciate any response.

Craig Boswell
HVAC Contractor - Cape Town, South Africa

Hi, Craig. Yes, there is a galvanic corrosion issue there, but it's always hard to predict how serious a matter it is. Why not just use galvanized bolts and nuts?

Regards,

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Hi Ted,
Many thanks for your prompt response, this has assisted us in avoiding the problem altogether; we will be using galvanised bolts and nuts.
Thanks for a great service!

Craig Boswell
HVAC Contractor - Cape Town, South Africa

"All metals are electrolytically active to a greater or lesser degree and if sufficiently different in their electrolytic potential will create a battery in the presence of a conductive electrolyte such as seawater or even tap water." -Machine Design, Robert L. Norton.

Here is a list of metals from least noble to most noble:
Magnesium, Zinc, Aluminum, Cadminum, Steel, Cast Iron, Stainless steel, Lead, Tin, Nickel, Brass, Copper, Bronze, Monel, Silver, Titanium, Graphite, Gold, Platinum.

Combinations close to each other are ok, combinations further apart are bad. In your case, the stainless steel nuts and bolts will corrode the pipe faster than the mild steel. I am guessing it will not happen for at least 15 years.

Bunhom Ngem
Automotive - Troy, MI, USA

There are a couple of free internet references I'd recommend. First is at www.npl.co.uk/upload/pdf/bimetallic_20071105114556.pdf, a brochure on "Bimetallic Corrosion" published by the National Physical Laboratory of the UK. Second is Mil-Std-889 on Dissimilar Metals, which you can find by typing "ASSIST Quicksearch" into Google, to get to the US Military Standards page, and then "889" into the number box.

Lee Gearhart metallurgist East Aurora, NY