INTRODUCTION
Stress corrosion cracking susceptibility (SCC) of the austenitic-ferritic stainless steels, i.e. duplex steels EN1.4462 (UNS S31803), EN1.4362 (UNS S32304) and EN1.4410 (UNS S32750) and their welds, which replicated weld structures in real digesters, have been examined in simulated kraft cooking environments. The effect of macro and micro stress state, phase ratio (ferrite content), liquor temperature and sulphidity on SCC has been clarified on the base steel grades and their welds. Guidelines to prevent SCC in digesters are presented.
The cooking environments in pulp mills are continually changing. The high sulphidity increases, in addition of hannfial smell, also corrosion risk of digesters and related equipment like alkaline liquor tanks, accumulators, piping and heat exchangers. Thus, within the development of modem digesting processes the standard requirements of the construction materials, especially against corrosion, have increased resulting the usage of stainless steels. In many cases austenitic structures are made of cladded carbon steel plate, but also solid austenitic structures are used. The latest digesters and tanks are, however, manufactured from austenitic-ferritic stainless steels or so called duplex stainless steels.
The corrosion resistance and especially the stress corrosion cracking (SCC) resistance of stainless steels depend on several factors in an alkaline environment, e.g. the chemical composition of stainless steels, temperature and chemical composition of the liquors. The corrosion resistance of duplex stainless steels in alkaline solutions without dissolved oxygen is better than that of austenitic stainless steels (Nordstr6m 1994, Clarke & Stead 1999, Alfonson & Olsson 1999). The knowledge from SCC resistance of stainless steels in alkaline and sulphur rich environments is limited compared to that of stainless steels in alkaline environments. Generally, the SCC resistance of duplex stainless steels in an alkaline sodium sulphide solution is proved to be better than that of austenitic stainless steels (Honda et al. 1991, Rondelli et al. 1997). Based on the laboratory test results mentioned above, the use of duplex stainless steels in the modem digester houses is justified.
How well do the simulated laboratory measurements fit with the field measurements of the modem cooking processes? In the laboratory tests with the cooking liquors taken from a mill (Wensley 1998) revealed corrosion rates orders of magnitude higher than the rates measured at the field (Leinonen 2000). During the three year field tests in a Swedish mill the corrosion rates in heavy black liquor of evaporators at 120-140°C were about 0.1 mm/year for the austenitic stainless steel Type AISI 316 but for duplex stainless steels only 0.01-0.02 mm/year (Troselius 2004). It is reported, that a duplex pipe in a calorimeter was susceptible to SCC because of overheating and increased stresses (Alfonsson & Olsson 1999). In modified cooking processes small amounts of polysulphides and thiosulphate were observed to be always present in solution. These anions were observed to be detrimental for corrosion of stainless steels (Rämö et al. 2001).
On the other hand, the most critical corroded structures of the digesters do not correlate with simplified and standardised laboratory corrosion and SCC experiments, which are in the most cases performed with small base metal specimens. To obtain the corrosion resistance comparable to the base metal is a particularly demanding job in welded duplex structures in spite of over-alloyed filler metal and good welding practice (Moskal et al. 1999). During weld solidification alloying elements like molybdenum will be irregularly distr
