Rapid corrosion thinning occurred in the wash zone of a hardwood continuous digester between the 1999 and 2000 annual inspections. The major process change during that time was the introduction of anthraquinone (AQ) pulping. AQ pulping was temporarily halted until a laboratory investigation was completed to determine the corrosivity of digester liquors sampled during anthraquinone (AQ) pulping and during pulping without AQ addition. Potentiostatic polarization at 315°F
(157°C) demonstrated that active corrosion rates of A516-Grade 70 and A285-Grade C carbon steels in the liquor sample taken during AQ pulping were significantly lower than in the liquor sampled during non-AQ pulping. It was concluded that the rapid corrosion thinning was not directly the result of AQ addition but may have been due to secondary factors such as increased temperature or decreased hydroxide residual, as well as high temperature acid cleaning.
Keywords: corrosion testing, pulp and paper.