Repairs corrosion build up around oil drain plug

I just got my troy bilt tb 30 rear engine mower, i got it for a real good price. it had some corrosion around the oil plug and white corrosion spots around the engine and muffler.
I cleaned it up and having it look brand new. It started right up and drove great and cut just fine. however, after running it, i could see the corrosion starting to build back up.
what do you think is causing this to happen?

Picture so we can see what you are seeing.