Processes for producing acetic acid and determining corrosion therein are described herein. The processes generally include contacting methanol and carbon monoxide in the presence of a liquid reaction medium under carbonylation conditions sufficient to form acetic acid, wherein the liquid reaction medium includes: a carbonylation catalyst selected from rhodium catalysts, iridium catalysts and palladium catalysts; from 1 wt. % to 14 wt. % water; and one or more, in-situ generated derivatives of the one or more additives and combinations thereof; wherein the one or more additives are independently selected from non-benzoyl containing pentavalent phosphine oxides, compound mixtures of at least four phosphine oxides and pentavalent aryl or alkaryl phosphine oxides including one or more benzoyl groups; and recovering acetic acid from the process.