This is from a Malaysian 33 kit. Its not a serious issue unless you go rolling in a swamp and then put in your basement for the next few , the reality is that if abuse and neglect steel you will get an issue no matter what platform. When I go to the range, rain or shine I shoot. (You will commonly see rain drops on everything in my range pics that I post.) I use a towel to wipe down then do a full strip and clean. Never had a problem.
If buying one do a full inspection. Look for wear around ejection port, mag well bottom, underside of bolt. There will be signs if used and abused. That malaysian bolt even though it has pitting will keep chuggingvrounds down range for years as is.