ABSTRACT

This paper describes the cracking failure of a UNS S30403 (304L) stainless steel 72% heavy black liquor storage tank at a Canadian bleached kraft pulp mill. The tank was installed in 1995 and operated reliably for 20 years when leaks were found in the upper part of the cylindrical shell. Samples of the cracked area were removed for laboratory examination. Investigation showed that the cracking initiated on the inside surface of the tank in the “tidal” zone where the liquor level fluctuates during normal operation of the mill. The laboratory investigation showed that the cracking was chloride stress corrosion cracking. This failure is consistent with findings at other mills in that high chloride was found in the crack tips even though the bulk chloride content of the liquor is much lower (of the order of 0.28 % on a dry solids basis which equates to 0.39% in the “liquid” 72% black liquor). Analysis of the organic residue on the corroded surface indicated that the organic species in the black liquor were oxidized in the tidal zone, and formed organic acids, which combined with the concentration effect allowed stress corrosion cracking to occur.

INTRODUCTION

This paper describes the experience of the kraft pulp mill that is located at the Reversing Falls site in Saint John New Brunswick, Canada. The mill uses the usual kraft recovery cycle, which involves the combustion of concentrated black liquor to produce steam and regenerate cooking chemicals. The black liquor that is to be fired in the recovery boiler is stored in a 304L stainless steel storage tank. The tank was installed in 1995 and operated reliably until the spring of 2015 when leaks were found in the upper part of the cylindrical shell. When the insulation in this area of the tank was removed an extensive network of cracking was found. Further investigation of the same area was done during the August 2015 mill-wide maintenance shutdown. During this shutdown a large area (approximately 4 m × 1.5 m) of the corroded shell plate was removed to allow insertion of a flush patch of 304L. Samples of the area that was removed were taken away for laboratory examination. Figures 1 and 2 show the area that was affected by the cracking in the upper part of the tank. The affected area is in the “tidal” zone of the tank where the liquor level fluctuates during normal operation of the mill. As far as is understood, the rest of the tank, both above and below the tidal zone is unaffected. It was noted that the outside of the tank, even around the areas that were severely cracked, was not corroded, whereas the inside surface of the cracked area of the tank was severely corroded. The purpose of this investigation was to identify the cause of the corrosion and to identify candidate alloys for the replacement vessel.

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