As kraft pulp mills adopt modified cooking processes, an increasing amount of corrosion of carbon steel digester systems is being encountered. Many mills have had severe corrosion in the flash tanks, in particular, the first (#1) flash tank. The work described in this report was aimed at characterizing the corrosion. Coupons of carbon steel, several stainless steels and titanium were exposed at two mills. At mill A, identical sets of coupons were exposed in the #1 and #2 flash tank. At mill B, three identical sets of coupons were placed in flash tank #1. The results of the exposures showed that both carbon steel and titanium suffered high rates of general corrosion, while the stainless steels suffered varying degrees of localized attack. The ranking of the resistance of corrosion in the flash tank was the same ranking as would be expected in a reducing acid environment. In the light of the coupon results, organic acids is concluded to be the most likely cause of corrosion of the flash tanks.
As mills move towards modified variations of kraft cooking in an effort to reduce the amount of bleaching chemical that is used to produce pulp of market brightness, an increasing amount of equipment corrosion is occurring, especially in the lower part of the kraft continuous digester and the #1 flash tank. The incidence of flash tank corrosion is widespread and has not yet been related to a specific cooking practice. Table I shows the experience of some Canadian mills with different fibre furnishes, and different cooking schemes. As part of Paprican?s on-going effort to determine the limits of existing pulp mill equipment and to determine the optimum materials for new processes, some test work was undertaken at two of these mill-sites. Both mills have experienced flash tank corrosion, one of them at an unexpectedly high rate. Because the environment in the flash tank is a foam, and therefore no continuous electrolyte was present, electrochemical testing was not attempted. This report describes some corrosion testing that was undertaken in flash tanks at the two mills.