Corrosion and Corrosion Prevention of Digesters Operating with Modern Cooking Processes, 2005 Engineering, Pulping & Environmental Conference

Digesters used in Kraft pulping were originally believed to be free of corrosion problems. However, approximately 25 years ago a catastrophic accident gave a signal that corrosion was indeed a problem and more attention must be paid to the inspection and protection of digesters. The major concern at that time was caustic stress corrosion cracking (SCC) of carbon steel, which was frequently found in the impregnation zones.

Pulping techniques are evolving rapidly with increasing demands for efficiency. Modern cooking methods can use higher temperatures, modified alkali profiles and pulp to lower lignin levels. In addition, little or no alkali may be present in the lower wash zone. At the same time, mill closure and recycling are increasing chemical dead-load and, consequently, corrosion of digesters. Corrosion in the form of rapid thinning, pitting and stress corrosion cracking is nowadays taking place in various parts of digesters decreasing the availability of cooking lines.

When changes are made in the cooking process, corrosion monitoring is often used to determine corrosion risks and to evaluate protection needs. In addition, anodic corrosion protection has successfully been used to minimize all kinds of corrosion in continuous digesters worldwide. This paper presents experiences of how implementation of modern cooking processes has affected corrosion of these critical pulping vessels. In addition, results of corrosion monitoring and anodic corrosion protection of continuous digesters from different parts of the world are presented.