I am a metallurgist, but no means an expert in corrosion, but I'll try my best here.
When aluminum and steel are in contact, aluminum becomes the anode, and is the metal that \"corrodes\" (disappears!). The problem with using aluminum in steel is that the corrosion rate is related to the area ratio between the two. So with a large area of steel, and a small area of aluminum, the aluminum will corrode quickly.
What anodizing does, as Bruce mentions, is to increase the thickness of the oxidized layer on the aluminum. Aluminum is a metal which \"passivates\" which means it forms an oxide layer that actually protects it (steel does not passivate because the volume change in forming iron oxide is such that it breaks the passive layer and that's why we get rust \"bubbles\" when rust forms on cars). Most people associate anodizing with fancy colours (oh, for the hey day of purple billet of the 90's, eh?). But anodizing is actually a protective layer. However, the problem is that if you scar the anodizing when installing it, you now have exposed bare aluminum with a small surface area, and it will corrode quickly.
One thing to bear in mind is that you still need an electrolyte to connect the two. Here in dry, dry Calgary it's not so bad. With Vancougar's nice humid sea air with salt in it, the electrolyte is puuuuurrrrfect. Plus your fenders seams are a perfect place to trap moisture.
So what's the conclusion? You know what, for fenders, aluminum bolts aren't going to matter. It's not like your fenders are going to fall off or anything, so go nuts. And at least if the bolts shear off when you ever remove your fender, they are easy to drill out.
They look pimp, but aren't the best choice for everyone. I'll stick to stainless, personally.
Rock on.
-Keith
