Corrosive environments in modern kraft batch cooking

Corrosive environments have been studied in three modern Finnish kraft batch digesters during their normal operation. The potentials of a carbon steel, an austenitic stainless steel and a duplex stainless steel have been measured for a long period of time using an on-line digester process and corrosion monitoring system. Corrosion potentials of the steels varied extensively depending on the stages of the batch process and on the cooking processes. The lowest potentials were in stable stage of cooking, where temperature was over 150°C and hot white liquor has been added. The differences of corrosion potentials in different cooking processes has been analysed with the operating process parameters such as temperature and composition of green, impregnation, hot black, hot white, split white and displacement liquors. Linear polarisation resistances of the studied steels showed good correlation of the weight losses of the steels. The highest resistances were monitored during filling and impregnation stages and the lowest in stable stage of cooking. The corrosion of the studied steels depends notably also on the operation sequences in batch digesting.