Dissimilar Metals Condition

[structural-eng]

We are designing a stack foundation for an industrial power generating building. The foundation and anchor bolts are to be designed for a future 90' tall stack which I'm being told will be painted carbon steel. The owner will be installing a 50' stack for now that they are requiring be stainless steel. We're working for the stack supplier. Our original drawings showed galvanized anchor bolts as we were unaware of the stack being stainless steel. When we learned that the 50' stack would be stainless we mentioned that the anchor bolts may have to be stainless as well and the stack supplier said that they always use galvanized anchor bolts. The supplier also indicated that most of their stainless stacks are much smaller that this one.

My understanding is that the bolts in a dissimilar metal connection should always be more noble would not be the case if we use the galvanized anchor bolts with the stainless steel stack. The anchor bolts are 1.75" diameter and we don't know the base plate size because the stack design isn't complete yet.

Is there a good solution to preventing galvanic corrosion given that the current stack will be stainless but the future stack will be painted carbon steel?

 

[charliealphabravo]

I know F1554 doesn't include stainless steel anchor rods. Aside from the dissimilar metal issue I'd have questions about the compatibility of the stainless rods with desired elongation and ductility characteristics.

 

[TehMightyEngineer]

Your concern is valid in my mind. Galvanized steel can corrode rapidly around stainless steel in a corrosive environment (salt water, chemicals, etc.). For a stack base I suspect the effects are minimized but it's still there and easily prevented. Specify an insulating material between the galvanized steel washers/nuts and the stainless structure. I've seen PTFE sleeves used for pipe flanges that insulate the bolts and washers from the other materials in the flange. I imagine a similar product could be used for your bolts (though obviously you have a much larger size). I'd also detail the anchor bolts such that they can't sit in water and galvanically connect the galvanized metal and the stainless. Should solve all the problems with little cost.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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