Aluminum plate + stainless steel bolts + exterior condition = how much corrosion in one year? 1

[DCStructures]

I am designing a base for a sculpture. The artist would prefer that the base be stainless steel, the sculpture be aluminum, the connecting plate (between the sculpture and the base) be aluminum, and the bolts (between the connecting plate and the base be stainless steel. This is a temporary installation, expected to be up for one year. It will be located outside, but not in a coastal environment. How much corrosion should I expect in one year? Is the expected amount of corrosion enough to break the bolts or any of the other pieces in one year? Thanks!

 

[tolchijb]

Per table 2-7 of AISC 14th edition, the corrosion of the base metal (aluminum) is marginally increase by the fastener (Austenitic Stainless Steel: Type 302/304, 303, 305). Note that Martensitic Stainless Steel (Type 410) is not recommended. In my opinion, as long as you are using Austenitic stainless steel, corrosion over 1 year would be a minimal concern. Personally, I would use a piece of TPO membrane to separate the aluminum connecting plate from the stainless steel base to isolate the dissimilar metals. That way the only galvanic reaction would be between the SS bolts and the aluminum plate.

 

[DCStructures]

Thanks, tolchijb! What about applying other finishes to the bolts or other pieces to further reduce corrosion, e.g. powder coating, teflon coating, silver plating, ???

 

[tolchijb]

It is our office standard with aluminum to isolate dissimilar base metals using TPO or mastic and use stainless steel fasteners. I will caution you that galvanized steel base metal and stainless steel fasteners can actually cause the zinc coating of the galvanized steel to be more rapidly consumed. However, since you are talking aluminum and stainless, I would be perfectly comfortable with the procedure mentioned above, especially if it is only for 1 year.

As far as using other finishes on the bolts, I generally caution against coating or plating bolts since the threads would need to be over-tapped (tapped again) after plating/coating, which reduces the thread stripping strength. If you do not have net tension on the bolts, it may not be a big deal, but over-tapping really reduces the positive effects you are looking for from the coating/plating.

 

[SAIL3]

I would not expect any problem, especially when considering that it is only for 1yr....if it was a 10 or 15yr span, then maybe.....

 

[Ron]

Not an issue! I agree that 400 series fasteners should not be used. 304 or 316 should be fine. Have designed over 600 aluminum structures, most in Florida and many have been in place over 30 years.

 

[ColinPearson]

You might find something like what we would call an "iso kit" for bolting stainless piping flanges with carbon stud bolts. The kits consist of a plastic tube where BOLT_Outside_Diam < TUBE_Outside_Diam < Bolt_Hole_Inside_Diam so there is a "sheath" that prevents contact between the flange and the bolt. There are also plastic washers that go between the flange and the underside of the nuts. If you're relying on thread engagement, then this obviously wouldn't work, but if the thing is to be held down by nuts on a base plate then it may.

 

[DCStructures]

Very helpful. Thank you all!