[redshift]

That's bimetal corrosion, which I don't understand the chemistry of, but it's a matter of different metals' reactivity with the environment, and states of "charge". I've come across that myself a number of times!

But it also could simply be an area where corrosive material got trapped, and once one metal started corroding, it becomes more agressive against the other. That's about the best explanation can give you without boring you to tears lol.