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topic 10054

Stainless plus Carbon Steel in Construction -- galvanic problems?


Q. I've been a few different projects as either a Construction Supervisor, Resident Engineer or Project Engineer and almost always during the life of a project the question comes up, Is it detrimental to have stainless steel in contact with carbon steel for the service life of the contact which could be 20 years or more.

I've been told that stainless steel anchor bolts will cause premature or accelerated corrosion when used on A36 base plates. I've seen vendors provide skid mounted equipment with 316 piping isolated with a rubber pad from the carbon steel supports.

I decided to get on the net to see if I could get my question answered when I stumbled on this page.

The latest use I've seen of this dissimilar metal has really piqued my curiosity. The project I'm on now has a proposal to use a stainless steel, I haven't determined the type or grade yet but it shines like a mirror, as a shim plate between an A36 steel pier and an A36 equipment frame.

Is there a problem here for the longevity of this connection? Is there a rule of thumb to go by to help decide in which situations this would be acceptable, i.e., stainless with chrome in this range, 17 to 19%, is ok with A36 or A 500 grB? Or is it more simple than that.

Michael Brown
- Everett, Massachusetts, USA


 

A. Hello Michael!

Yes, dissimilar metals in contact can accelerate corrosion of the more active metal in the pair. Think of it as forming a battery, which will lead you to the other critical component: the electrolyte. Having stainless in contact with A36 carbon steel is fine, until some moisture is also in contact with the joint. So, if you can guarantee that the joint will always be dry, then there is no problem. If it can get wet, the carbon steel in the vicinity of the joint will corrode faster than the surrounding carbon steel.

A nice introduction to the topic is Mil-STD-889 [link is to free spec at Defense Logistics Agency, dla.mil], on "Dissimilar Metals". It is available free at astimage.daps.dla.mil/quicksearch/, by typing mil-std-889 into the box.

Good luck!

lee gearhart
Lee Gearhart
metallurgist


----
Ed. note 5/23/13: Make that "quicksearch.dla.mil/" nowadays



 

A. As Lee notes and you already know, stainless and carbon steel are not galvanically compatible; there is something like a 0.3 to 0.5 volt potential difference. Still, if the surface area of the noble metal (stainless) shims is small compared to the surface area of the base metal (steel), galvanic current is limited by that. Further, the oxided surface of stainless steel is not highly conductive in most environments.

Perhaps for these reasons, my personal experience has been that--even in the very humid and highly conductive environment of a plating shop--stainless hardware has often been a solution, while I have no experience with it being an actual problem. But opinions are not a substitute for construction codes, so at the least you must make sure that there is no prohibition against using these shims for your application. Good luck.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



Stainless bolts screwed into painted tapped steel plate

June 29, 2015

Q. Hello

We are trying to source a rust-proof bolt that will be attached through a 1/2" steel plate, into a threaded 1/2" steel plate (type of steel unknown at the moment). The plates have been painted (paint type unknown at the moment).

Currently the Engineer has just spec'd a SS bolt to be used. It will need to have the capacity of a Grade 5 bolt.

I take it from perusing some of the other posts on here that b/c of the steel plate, rust may potentially have an effect on the SS. Currently we are considering 18-8 SS but could potentially use a different grade.

How would 316SS compare to a treated by some means bolt?

3/8" bolt, 1-1/2" length, Grade 5 or better.

Thank you,
J

Jonathan Skibicki
Construction Professional - Winnipeg, Manitoba, Canada



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