Localized corrosion of stainless steels in chlorine dioxide bleach filtrate solutions

Abstract: Stainless steels are extensively used in pulp bleach plantsbecause of their high corrosion resistance. However, alsostainless steels might suffer from corrosion and in bleachplants this is a problem. Chlorine dioxide (ClO2) is an oxidizing agent, which is used to bleachpulp because of the ability of chlorine dioxide to selectivelyattack the lignin in the pulp, and for environmental reasons.Localized corrosion of stainless steels in chlorine dioxidebleach filtrate in pulp washers is a problem that often limitsthe use of stainless steels in these environments. Thecorrosivity of bleach filtrates depends on a variety ofimportant parameters, including chemical composition,temperature and redox potential. Welds on stainless steels aremore sensitive to localized corrosion, especially if there areweld oxides left on the surface. Since the weld oxides decreasethe corrosion resistance, it is important to be able to detectthese oxides and determine how they affect the properties ofthe stainless steel surface.The aim of the present investigation was to determine thelimiting environmental conditions for the use of stainlesssteels in chlorine dioxide bleach filtrates with respect tocrevice corrosion. The objective was also to establish arelationship between the redox potential and the corrosionpotential of stainless steels as functions of chlorine dioxideconcentration in bleach filtrates. A further goal was toinvestigate whether the scanning Kelvin probe couldbe used todetect detrimental weld oxides on stainless steel.The major experimental techniques included potentiostaticpolarization for testing of the susceptibility of stainlesssteels to crevice corrosion in bleach filtrate, measurements ofredox and corrosion potential, chemical analysis of chlorinedioxide concentration in filtrates using titration andspectroscopy techniques, analysis of surface oxides by emissionspectroscopy, and measurements of Volta potential with thescanning Kelvin probe. Most studies were based on laboratoryexposures, with complementary field exposures to verify some ofthe laboratory results. Austenitic, ferritic-austenitic(duplex), and superaustenitic stainless steels wereinvestigated.The application limits for a number of stainless steels inchlorine dioxide filtrate have been explored with respect totemperature, chloride concentration, and applied potential.Temperature and potential are the two factors that appear tocontribute the most to crevice and pitting corrosion in theinvestigated environments. The results show that the dependenceof redox potential and corrosion potential on chlorine dioxideconcentration approximately follows a logarithmic relationship.The redox potential is almost independent of chlorideconcentration in filtrate in the investigated concentrationrange. The influence of temperature on the redox potential issmall, whereas the influence of temperature on the corrosionpotential is more pronounced. The Kelvin probe study suggeststhat the scanning Kelvin probe technique may be used to detectareas with weld oxides, which are more sensitive to localizedcorrosion under the investigated exposure conditions. The studyfurther shows that the Volta potential decreases as thechromium content increases in the oxide of iron-chromiumalloys.Keywords:stainless steels, bleaching, chlorine dioxide,ClO2, localized corrosion, weld oxides, redox potential,potentiostatic polarization, scanning Kelvin probe