My utility trailer has a sheet metal floor, but it has rusted through in the 25 years I've had it, so I'm gonna cut it out. I got a good deal on a sheet of 1/4" diamond plate aluminum, so that will be my new floor.
I'm a little concerned about galvanic corrosion due to the plate resting on the steel understructure, and my plan on using steel bolts to secure the plate (flat-heads countersunk in the plate and screwed into holes tapped in the frame).
Are my concerns warranted, and if so, what can I do to mitigate any problems ?
Yes you will have galvanic corrosion (if the trailer gets wet :-). One solution is to use a sealant between the two materials.
With a similar problem - stainless bolts in aluminum fittings, in a marine environment, I once used 3M 5200 Adhesive Sealant which is a high-performance polyurethane adhesive sealant that stays flexible and waterproof, yet resists weathering and salt water.
Several years after installation I had reason to disassemble part of the structure and found no signs of corrosion. Cheers, John B.
The bed will get wet, right? So, fasten it with blind aluminum fasteners (aluminum rivets would be ideal) to a substructure that bolts to the steel. The wet surface will be aluminum vs. aluminum (no galvanic corrosion) and the brackets that bolt to the steel can be ... sacrificial. Use of nonconducting washers may help, too.
Wherever there is steel to aluminium contact you will get galvanic corrosion over time. If you can find out the valency of the steel, mild steel bolts and aluminium you can get the closest valency match which will reduce/slow the corrosion. SS bolts are far worse than mild steel bolts for causing corrosion as their valency is much greater and they react with the steel and the aluminium.
If you use steel washers over the aluminium you are just enlarging the area of the corrosion.
The corrosion will occur whether wet or dry, the moisture in the air will assist the corrosion but does not double the rate.
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This is an extremely common problem in marine applications and the techniques for minimising the corrosion are well established.
Although stainless fastenings are worse galvanically, at least they dont rust and expand, disrupting the protective oxide layer of the aluminium. The problem then becomes protecting the aluminium from the steel (stainless or not). The heads of the fastenings are relatively small area and are unlikely to cause a significant problem if the correct sealent is used to exclude air and moisture from the interface. The main problem is the large area of contact between the sheet and the frame. If any moisture is trapped here, you *WILL* get poltice corrosion.
You need a non-moisture absorbing insulating layer between the aluminium and the steel frame + sealent between the insulator and the aluminium. You should also use sealent between the insulator and the steel. If you are through bolting, use fibre washers under the nuts to protect the steel from the stainless.
For all dissimilar metal fastenings exposed to moisture, especially in aluminium, use Duralac sealent to exclude moisture and inhibit electrolytic action.
Apply to all mating surfaces, screw threads etc.
It is my personal experiance that a stainless fastener in an aluminium boat mast (frequently wet with salt spray) will typically cause no noticable corrosion over a ten year period if properly assembled with Duralac. Its use is pretty much industry standard.
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