[unclebilly]

This is galvanic corrosion. The steel parts are ‘eating’ the aluminum parts. The moisture (or rain) was the electrolyte and you have ion transfer from the aluminum to the steel.

It’s pretty common and a lot faster in marine environments especially with salt water which is a better ionic conductor that moisture with trace amounts of salt to create an electrolyte.

But then ask Higgins, he’s the group metallurgist...