Quote Originally Posted by jrevens View Post
?...Any structural guys out there with a good working knowledge of this issue? I'd like to hear if I'm off-base about this. BTW, I did use micro on my build, and I'm certainly not saying anyone shouldn't based on these thoughts, but I'm curious about the whole thing.
Yes John, I'll put my hand up as an aircraft structures design guy with 40+ years as one ... and you're right. The use of micro balloons has not a lot to do with corrosion protection. It's a bulking and filling measure. As Lowell suggests, If you want some strength, add chopped fibre. If you want more strength, use epoxy and directional tapes or fabric. S/S fasteners contacting the sides of the hole when the rivet expands will most likely have a very thin layer of adhesive between fastener and hole sides anyway, which is a good thing.

Using S/S rivets in aluminium has been the practice in home built aircraft for many years. As Jim said, the two materials are reasonably close on the galvanic corrosion chart, but having said that, I've wet installed all my rivets using an epoxy primer. Just a personal preference. For corrosion to start, you need as a minimum, moisture and a potential difference between the two metals to start pushing ions around. In this case, the adhesive acts as the barrier between the two, similar to the use of a fibreglass ply between a carbon skin and an aluminium alloy rib, spar, frame or longeron. So I'm not surprised that there haven't been corrosion issues in the steel spar doublers to aluminium tube joints. But as a further precaution for my build, I've also heavily coated this area with primer just to make sure I don't get moisture ingress. Again, a personal preference. .....