Letter 10054

Stainless versus Carbon Steel in Construction 

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 disimilar metal has really piqued my curiousity. 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
Sithe Energies - Everett, Massachusetts, USA

If the surface area of noble metal (stainless) is small compared to the surface area of the base metal (steel), galvanic current is limited by it. Further, the oxide surface of stainless steel is not highly conductive in most environments. Perhaps for these reasons, my own experience has been that--even in the very humid and highly conductive environment of a plating shop--stainless hardware is a solution, not a problem.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

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 the military standard 889, on "Dissimilar Metals". It is available free at http://astimage.daps.dla.mil/quicksearch/, by typing mil-std-889 into the box.

Good luck!

Lee Gearhart
- East Aurora, NY

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