QUESTION
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?
Also, what about applying other finishes to the bolts or other pieces to further reduce corrosion, e.g. powder coating, teflon coating, silver plating?
REPLIES
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.
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.
SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=408978
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