The same phenomenon happens with modern automobiles and their aluminum suspension parts too. Ever try to replace a wheel bearing hub in an aluminum steering knuckle, if you live in the salt belt? I've had to remove the steering knuckles from the car and chisel out the bearing from behind a few times. This was never a problem on older cars with steel (or is it iron?) components. The dissimilar metals we see now react with the moisture and salt almost as if they've been welded together.