Galvanic Reaction between zinc plated bolts and S.S. washers
A discussion started in 2004 but continuing through 20192004
Q. 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
A. 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.
Pine Beach, New Jersey
July 17, 2008
A. 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.
zinc coatings - London, England
April 19, 2012
Q. Much of the topics related to the Zinc/SS are about clamps and sea locations.
We have a project where we are placing 3" L 1/4-20 countersunk zinc plated steel screws through a post and attaching with stainless steel washers, lock washers and nuts. Would there be a concern for the dissimilar metals? This is an outdoor product nowhere near an ocean.
Manufacturer - Reading, Pennsylvania, USA
January 17, 2013
Q. I am simply a homeowner attempting to do a job correctly and prevent future issues.
We recently purchased a basketball system which utilizes a large and very heavy main pole that will attach to a concrete pier. Most all hardware that shipped with the system is stainless steel with the exception of the 4 "J" bolts that are sunken into the concrete pier. The J bolts aren't quite as dull as I would have expected them to be if galvanized and aren't quite as shiny as I would have expected for zinc coated. Anyway, I'm assuming they are simply zinc coated as I really do not feel they are galvanized.
Since the main pole is heavy and I do not wish to remove it later to replace any hardware, I was thinking of using stainless steel nuts and washers underneath the main pole's mounting plate. The nuts are used for adjusting the level and the large washers simply offer more surface area support. Since the J bolts are zinc coated, will it be acceptable to mount the stainless steel nuts and washers on the J bolts underneath the pole? Or would it be recommended that I stick with only zinc coated hardware? My main concern is having to remove the pole in later years just to replace badly rusted washers or nuts. But, on the other-hand, the use of stainless steel hardware may cause the J bolts to deteriorate at a much faster rate causing more serious issues ... am I thinking correctly?
We are located in South Carolina and not near the coast, but since this hardware will be very near ground level, it will be wet at times. Thanks so much for your help!
- Lugoff, South Carolina, USA
A. Hi Tony. Galvanic corrosion issues are relative. What would be unacceptable on an airliner might be a trivial concern on a basketball backboard. What you haven't mentioned is what the pole is made of, and that may affect the answer...
Zinc is cathodic to stainless steel and will corrode to try to "protect" the stainless. But stainless can also be rather 'passive', i.e., not encouraging that electrical flow. In general, if you have a tiny area of zinc coating and a large area of stainless steel, the zinc will be consumed fairly quickly. In other words, galvanized bolts are not ideal for connecting stainless steel structures. But if you have a large area of zinc and a small area of stainless, it's probably not a problem. Stainless hardware can usually be used to connect zinc coated structures in non critical applications.
Ted Mooney, P.E.
Pine Beach, New Jersey
Can I use Stainless steel washer for A193 GrB7 boltsJune 17, 2017
Q. I've got a requirement for stainless steel washers for carbon steel bolts and nuts. Am afraid to use this as there may be corrosion if I use dissimilar metals. Please advise.Abdul Hakeem
Buyer - Dubai, United Arab Emirates
June 25, 2017
A. For ASTM A193 [link by ed. to spec at TechStreet] Grade B7 Bolts/Studs suitable washer is F436 although you can use SS304 washers.Vishal Agarwal
INDANA STEEL PVT.LTD - kolkata,West Bengal,India
February 4, 2019
- 1/4" stainless flat head screws
- 1/4" stainless floating nutplate bonded to painted aluminum plate
- countersunk holes in a painted aluminum plate
- all installed on an indoor piece of machinery located in central Florida with regular exposure to outdoor humidity, but no direct exposure to rain
1. Galvanic corrosion concerns given the small surface areas?
2. Galvanic corrosion concerns if I switch to a black zinc plated screw, again given the small surfaces areas?
- Orlando, Florida, USA
A. Hi Pam. I don't think stainless screws are a problem for painted aluminum structures except in truly bad conditions like offshore oil rigs. The black zinc screws are probably okay indoors too but are not as safe an application. Just personal opinion; you need to check if your machinery is subject to any building codes which dictate such stuff.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"
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