Why mixing the wrong two metals turns one of them into a sacrifice.
Bolt a steel bracket to an aluminum panel, leave it somewhere damp, and something strange happens: the aluminum around the bolt starts pitting away far faster than an identical aluminum panel with no bolt at all ever would. Nothing about the aluminum changed. What changed is that it's now electrically wired to a different metal, sitting in the same puddle of water — and that's enough to turn ordinary corrosion into something faster, more localized, and much more destructive for one of the two metals involved.