Do you mean the rate of increase (or decrease) of corrosion when using corten steel instead of mild steel?
Are you considering using corten steel for boatbuilding?
The rate of corrosion of steel will depend on various things such as how exposed it is (eg if it is kept perfectly dry, or it is exposed to the weather), or exposed to salt air (or salt water).
Also if it is properly blasted and painted or not.
I asked Google this question "corten steel advantage vs mild steel" and it came up with many links.