Thats pretty interesting, you made me take a look at my charging contacts and they're still completely new looking.
I wonder why you experienced that and I didn't - I don't take particularly good care of my AirPods. The only thing I do occasionally is hit them with a paperclip to remove any earwax buildup because they get pretty gross after a few weeks of use.
I mean I'll go for runs in the rain, run in super humid weather, sleep with them in sometimes, and a host of other probably not good ideas and they just keep on working perfect after a year of this 'abuse'.
I would guess however that they weren't corroded in the traditional sense of the word, more likely just covered in a gross film?
[Edited to answer your question to which I cannot reply because it is marked as dead:] The corrosion to which I was referring is clearly oxidation to my eye. It has a green color and requires scraping. Maybe our environments and usage are too different for comparison. I live in Central Florida, where it is quite hot and humid. According to my receipts, I bought these AirPods on April 5, 2017. I have been running with them almost every day since then.