Letter 31153

Galvanic Reaction between zinc plated bolts and S.S. washers

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We are currently doing a Liquefied Natural Gas Subcontract on shore. Part of our scope is providing Cold supports for Gas lines. These are usually heavy density foam around a pipe clamped by saddles onto a beam or base. Original specs requires Bolts with Bright Zinc plated finishing to 25 microns with stainless steel washers. We would like to inquire if there will be galvanic reaction when this two different metals come in contact. The bolts will have to be torqued so any corrosion in the future would affect the design principle. Also, is there any remedy for this should we have the least option of changing the materials. Otherwise, please advise so.

Ver Chavez
Insulation - Kuala Belait, Brunei

Although 25 microns is a very heavy zinc electroplating, the useful life of zinc plated hardware in outdoor exposure is rather limited even without the more noble stainless steel washers accelerating the corrosion. I don't know what other design constraints you have, and there are always plenty, but galvanized everything would sound more durable to me.

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

 

I agree with Ted that the best route to go is with galvanised fasteners and washers, then the electrical potentials of all components will 'balance-out' which means that nothing is going anodic to the rest. This means no rapid dissolution of the zinc coatings.

Remember, when using zincs with both carbon and stainless steels you must observe cathodic/anode ratios. For example, a small galvanised bolt-head sitting on a bare metal beam would cause a huge negative anode/cathode ratio, hence the zinc would dissolve to protect the bare steel but would do so at an alarming rate.
Near the sea in salty coastal atmospheres (and with sulphur salts from refinery stacks) the zinc layer would probably be gone in a short space of time, sometimes even in few months.

If you have hundreds of galvanised bolts and stainless washers already in place, a 'quick fix' is to take a brush and coat them with tar or bitumen.
Note: not the modified versions with fancy solvents (for fast flash-off times) but the good 'ole fashioned' sticky stuff. You would be surprised at the results.

Rick Simpson
zinc coatings - London, England

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