BobMarieLitt wrote:
Looks good! Hope those bolts are stainless steel! Regular bolts and aluminum react in time. You likely knew that. Good luck on your build. Good job of cutting the aluminum!
I did a lot of research on this galvanic corrosion subject, and the bottom line is that I wound up using regular-steel bolts. Almost all of them are grade 5, but I used grade 8 on the axle and the tongue attachments. No real difference in potential corrosion, but at least they are stronger.
Here was my total thought process in choosing steel bolts:
First of all, Stainless is actually further than steel from aluminum on the Galvanic chart, meaning SS is more cathodic than regular steel and theoretically the aluminum should corrode more, not less, when in contact with the SS. I realize there is a lot of anecdotal talk out there, even on here, about how SS bolts work better with aluminum, but I went with the science on this one.
Since the bolts are steel and the base metal is aluminum, this is way better than the opposite for this corrosion. I found numerous charts that rated galvanic corrosion on a scale of 1-4, one being OK, 4 being really bad. I found one chart showing steel bolts in aluminum base being a "1", another couple it being a "2". Mixed message her, though even "2" isn't that bad.
SS bolts are a lot more expensive.
I'm taking numerous precautions using the steel bolts. First, I coated all my washers with multiple coats of paint, providing a small layer of resistance between the steel washers and the aluminum frame. The bolts themselves generally don't even touch the aluminum as all the holes are larger than the bolt diameter. Of course, there will be some thread contact with the aluminum in the holes when the bolt is off center, and there was no easy way to control this.
After the trailer floor is complete (basically now, almost), I will flip the whole thing over and very thoroughly undercoat the trailer, covering all bolts/nuts/washers with multiple layers of product, so as to keep all the bolts/nuts/washers dry. Of course, there will be some leakage in the long run. Galvanic corrosion does not occur if everything is kept dry. We also live in the dry west, which should help slightly in the long run.
The front of the a-frame tongue is bolted together with two pieces of triangular plate using these same bolts (see pic on my first post below). I am not coating these upper bolts as I want them to be my "indicators" of any galvanic corrosion issues. These bolts are in plain view, so this will alert me to issues in the actual frame. I am thoroughly coating the lower bolts, as they are harder to see and would get the worst of the road spray. I'll keep my eye on those as well.
Overall, it was a tough call choosing these fasteners, and even choosing aluminum for the frame, but I'm happy with my choices.