Influence of cooking conditions on continuous digester corrosion in a Brazilian pulp mill

Digester corrosion is costly because of repairs, replacement of components and upgrade of materials, and losses in connection with unplanned outages. Risks to life and property also are significant, especially if catastrophic failure of pressurized equipment occurs. From the 1980s to present, corrosion problems in many pulp and paper mills worldwide have been intensifying, some resulting from materials and design aspects of the digesters themselves or later process modifications. The main focus of this study is the corrosive behavior of continuous digesters with modified processes. We discuss actual corrosion cases in two continuous digesters at a pulp mill in Brazil and the protection measures adopted. Results of electrochemical testing, metallurgical analyses, corrosivity testing, and field inspections are discussed. In one case, a carbon steel digester experienced rapid thinning on its top sections, with wall loss of 5 mm over 30 months following a process change. In the second case, also following a process change, a unique, type 316 stainless-clad continuous digester was found to have an altered electrochemical profile, indicating prospective corrosion hazards. This prompted studies and protective measures that are unique for stainless cooking vessels. The risks to carbon or austenitic stainless steel digesters running on modern cooking processes are shown to be significant. The effectiveness of the protective technologies, and their compatibility, was proved, and the need to consider these technologies upon cooking retrofit projects was established.