Digester corrosion has become a major topic for the pulp and paper industry in that it brought about the need for high expenditures with repairs, replacement of components and upgrade of materials, as well as losses in connection with unplanned outages. Risks to life and property are also significant, as catastrophic failure of pressurized equipment may result. From the eighties to present day, corrosion problems in many pulp and paper mills around the world have been intensifying, for reasons ranging from materials and design aspects of the digesters themselves, to process modifications introduced ever since. The present work has the main objective to study the corrosive behavior of continuous digesters with modified processes. Actual corrosion cases in two digesters are presented: (i) carbon steel and (ii) stainless steel clad (316L) digester. Electrochemical testing, metallurgical analyses, corrosion testing and field inspections were used to support the discussions and conclusions. In one field case, a carbon steel digester experienced rapid thinning on its top sections with wall loss of 5 mm over 30 months, after a process change. In the second case, also following a process change, a type 316L stainless-clad continuous digester, the only one in such material known in the Kraft pulping, presented an altered electrochemical profile indicating prospective stress corrosion cracking risk. This prompted the execution of 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 demonstrated to be significant.
Continuous digesters were once considered relatively free of corrosion problems. Since their introduction on an industrial scale in the late fifties, and over the following two decades, continuous digester corrosion was not a major concern at the mills, with rare exceptions where particularly corrosive wood species were pulped.