Corrosive environments have been studied in a modem Finnish kraft batch digester during its normal operation. The potentials of a carbon steel, an austenitic stainless steel and a duplex stainless steel have been measured using an on-line digester process and corrosion monitoring system for a long period of time. Corrosion potentials of the steels varied extensively depending on the stages of the batch process. The lowest potentials were in stable stage of cooking, where temperature was over 150°C and hot white liquor has been added. Linear polarization resistances of the studied steels showed notable, where the highest resistances were monitored during filling and impregnation stages and the lowest in stable stage of cooking.