For a long time now, international competitivity and more recently environmental regulations have been leading to more aggressive process conditions for Kraft wood cooking. As a consequence, carbon steel digesters have been most often replaced by stainless steels (SS) of the 300 series. Nevertheless, such grades suffer from Corrosion Thinning (CT) more or less assisted by hot-wall boiling effects and sometimes from Stress Corrosion Cracking (SCC) especially when the process environment involves high sulphidity and high chloride level.

Laboratory investigations were conducted on several austenitic and duplex SS using an experimental device simulating hot plate boiling conditions which appeared to be responsible for corrosion processes existing during filling and blowing batch digesters. Duplex SS were found to be much more resistant than 304L and 316L grades regarding to CT and SCC. The results permitted to state conservative limiting environmental conditions for each material expressed as chloride, sulphidity and temperature values. The comparison of the corrosion behaviour of the various SS tested showed that among the main alloying elements, chromium is the most efficient while molybdenum and nickel are not corrosion rate determining.

According to these results, more than 70 batch or continuous digesters made of duplex stainless steels, either solid or clad, have been erected in Europe, South America, Far East and recently in USA. A survey taking into account 8 of the oldest batch digesters of a Kraft pulp mill which operates an aggressive Kraft process showed an average corrosion rate of 0.05 mm/year which corresponds to a minimum service life of 15 years without any maintenance operation. In the same conditions, carbon steel digesters have to be regularly repaired by SS weld overlaying, and the maintenance cost comparison clearly demonstrated that the use of plain or clad duplex SS leads to important cost savings.