Last year I decided to do a vinegar soak on my water heater since I could see a lot of scale in the bottom of the tank.
I think I used too much vinegar though - didn't dilute it much, and let it sit overnight.
Imagine my surprise the next day when my anode rod was about half gone (was a new one that year)!
I flushed all the vinegar out (the tank was nice and clean!) and thought all was well.
I thought I flushed everything good enough with copious amounts of water, but should have neutralized the vinegar also - now my external water heater area where the vinegar ran out/over it is a little rusty. I'm chalking that up to the vinegar reacting with the metal & promoting corrosion.
Oh well, live and learn!
CC