In Plant Test for Corrosion Investigations in Digester of a Paper Mill

In the Pulp and Paper mill Industry, Digester house is the main section and corrosion is a big problem in this section. The presence of sulfate, sulfide, and chloride in liquor and Silicone in steel produce the severe corrosion problem in digester. An In plant test of six month duration was done in the bottom part of a batch digester, made of mild steel of volume 85m3. The wood chips are filled in the digester and cooking liquor is then charged keeping liquor to wood ratio as 1:2.8. Volume of cooking liquor inside the digester was kept between 37 to 42 m3.The cooking liquor consists of mainly sodium hydroxide (NaOH) (92 gpl as NaOH) and sodium sulfide (Na2S) (22.4 gpl as NaOH) having pH ~13.2. Maximum temperature attained during the pulping process is 1680C and it operates at a pressure of 6.2 Kg/Cm2.The results showed that mild steel (MS) experiences highest corrosion rate while Duplex stainless steels experienced least corrosion rate. It is concluded that the bottom part of the digester is affected due to erosion as well as localized assisted corrosion and the bottom part can be constructed of duplex stainless steel without severe risk of corrosion