The numerous mechanisms of corrosion and cracking in kraft batch and continuous digesters are reviewed. For carbon steel these include stress corrosion cracking, general corrosion thinning, pitting corrosion, and preferential attack of welds and weld buildup, and erosion-corrosion. For austenitic stainless steels these include general corrosion thinning, pitting corrosion, intergranular attack and stress corrosion cracking. Corrosion protection measures include anodic protection, application of stainless steel weld overlay, and thermal spray coating. Construction of new digesters from duplex stainless steels offers the best opportunity for long corrosion-free and maintenance-free service.
Batch and continuous kraft digesters constructed from either carbon steel or clad with stainless steel are often plagued with corrosion problems that at worst have resulted in catastrophic failures (Pine Hill, Alabama, continuous digester in 1980; Panama City, Florida, batch digester in 1994). Failure prevention now necessitates expensive annual inspection and maintenance of these vessels. The TAPPI Digester Corrosion Task Group serves as a forum for the discussion of corrosion problems affecting batch and continuous digesters. It is hoped that by sharing information on inspection results and on the success (or failure) of repair and protection techniques, future failures can be avoided. This paper describes the various kinds of corrosion that can cause the deterioration of both carbon steels and stainless steels in batch and continuous digesters, and the various types of protective measures that can be taken to prevent corrosion.
CORROSION OF CARBON STEEL BATCH DIGESTERS
Batch digesters have traditionally been constructed from carbon steel even though carbon steel has is not particularly resistant to corrosion in alkaline liquors. For many years the poor corrosion resistance of carbon steel was taken into account in new batch digester construction by specifying a generous corrosion allowance (e.g., 25 ram) that at least postponed the repair and protection problem for some years.
The thinning of batch digesters typically occurs during the liquor filling and initial heating stages. Liquor splashing on the hot digester wall is particularly aggressive and a characteristic "inverted horseshoe" pattern can often be seen on the digester wall'. Liquor may splash on the wall either directly from the liquor filling nozzle or indirectly after hitting the chip pile. Corrosion rates as high as 1 to 2.5 mm/year are commonly observed 2'". The appearance of corrosion in batch digesters can range from perfectly smooth and uniform, to pitted, to rough and nonuniform. Corrosion can also take the form of vertical grooves aligned under thermocouple nozzles. In a conventional batch cook white and black liquors are added to a batch digester at the beginning of the cook, often concurrently with chip filling. The volume ratio of white-to-black liquor appears be important, with batch digester liquors with higher proportions of white liquor being potentially more corrosive. Certainly the initial liquor is the strongest in concentration of hydroxide and sulfide. Corrosion potential measurements u show that upon filling the digester becomes active with a very low corrosion potential such as -250 millivolts versus a molybdenum reference electrode (mV vs Mo). During the heating stage (that can last for half the cook) the corrosion potential gradually rises to passive values (to values above -50 mV vs Mo) by the time the cooking temperature (usually 170°C) has been reached. There may be a return to active conditions before the end of the cook as the liquor becomes depleted. Co