Alloys for service in the bleaching plants

The corrosion resistance of heavily alloyed stainless steel grades and nickel base alloys is observed in the chlorine dioxide media. Austenitic stainless steels are the widely corrosion resistant steel grades in the various environments. The tests are conducted in the various service experiences in the Swedish bleach units. The function of this grade in the filter washer parts has generally been very adequate while damages from the localized corrosion have noticed in the pulp pipes in the chlorine dioxide mixer and bleaching tower. Due to their popular resistance to the localized corrosion, nickel base alloys like Hastelloy C276 and C22 are generally selected for service in the conditions that are severe for molybdenum based stainless steels. In the recent studies, although uniform corrosion reduction in case of nickel base alloys is noticed. The advanced steel is widely more resistant to localized corrosion as compare to stainless steels and slightly more resistant to uniform corrosion as compare to standard alloys.  

Preface

In the recent times, it has been common in European and North American bleach units to slightly or fully replace Cl2 with Chlorine dioxide to decrease the magnitude of chlorinated species in the mill effluent. The corrosion prevention of various production materials in the ClO2 conditions has hence attracted the interest of metal engineers. The joint influence of a major redox potential, the availability of chloride ions produced while limiting chlorine dioxide, elevated service temperature and pH value in many conditions varying from 2 to neutral, making bleaching conditions severe to the various traditional manufacturing materials. The stainless steels, nickel base alloys and titanium are recommended alloys.

 Stainless Steels

Inclusion of processing of wash water around 25 years ago made the bleaching plant conditions extremely severe. Major corrosion damages were noticed with stainless steels comprising of 2 to 4 %  molybdenum such as stainless steel 316l that at that time were predominating as washer building materials. In the higher alloyed materials, the austenitic stainless steels offered good service in the field coupon tests. The first commercially available molybdenum steels was 254 SMO that was selected as production material for a major bleach plant installations, beginning in the late 70s. An analysis of experiences from installations in Swedish and Finnish bleach plants was conducted in 1989 by Olsson and Frigren. Several instllations in the stage washers were successful however in few cases pitting corrosion had conducted, usually in the weld areas subjected to the gas phase. Lack of post weld cleaning and welding with a filler metal not overalloyed with regard to molybdenum were referred as feasible reasons for the weld corrosions. The survey also stated grade S31254 to be inadequately resistant for use in the chlorine dioxide mixer to tower piping where aggressive pitting occurred in 6 out of 7 installations.

Further experiences from a wide exposure in North American bleaching units described Molybdenum based stainless steel joined with alloy 625 or higher molybdenum content filler metals to be outstanding materials for the several bleach plant uses. Although, at sample tests in the filter washers, 6Mo steels experienced pitting and crevice attack in the specific media such as when no antichlor was added between tower and vat. In a 14 month exposure of pipe sections in a chlorine dioxide filtrate line, 6Mo stainless steels corroded along the circumferential welds. Residual ClO2 ranged among 20 and 50 ppm and pH 3 to 4.7.

Willis and Johnson stated on samples in the various bleach washers. The conditions included 130 to 600 ppm chlorine ion and chlorine dioxide residual. pH varied from 3 and 3.3 and temperature among 60 – 79oC or 140 to 175oF. In the washer with minimum temperature few steels described crevice attack.

Nickel Base Alloys

Instead stainless steels, nickel base alloys with high magnitudes of chromium and particular of molybdenum offer superior resistance to pitting and crevice attack. Therefore they are the significant materials for installation in the bleaching plants where there is a huge risk of localized attack on stainless steel grades. Additionally for being used for filter washer parts, these materials have offered satisfactory performance in the equipments of mixers and tower top scrapers. Additionally in the various conditions, nickel base alloys for example Hastelloy C276 and Inconel 625 have offered adequate performance. The alloys offer significant resistance to pitting and crevice attack in the filter washers whereas 6Mo steel grades were attacked. However in the recent studies uniform attack on the nickel base alloys is noticed in the chlorine dioxide conditions.

The attack on nickel base alloys was noticed in 1990 when the samples were subjected in a chlorine dioxide pulp bleach washer. Caustic soda was utilized as an antichlor prior the washer and the media had pH value between 6 to 7. The chloride magnitude was 600 ppm and temperature about 70oC and concentration of ClO2 was 20 to 200 ppm as active chlorine. In these experiments, nickel base alloys were corroded by general corrosion at the higher rates whilst the high nickel alloys didn’t not receive any attack however instead from crevice attack.

The lab tests included the conditions of ClO2 and under major control on the base material and welded samples of nickel base alloys and highly alloyed stainless steel grades. In the simulated chlorine dioxide media at free corrosion potential, high alloy austenitic stainless steel types 1.4529 and 1.4565 didn’t get any attack whilst stainless steel grade 904 received crevice and pitting attack whilst Alloy C22, C276 and 625 were attacked by uniform corrosion. In the tests without ClO2, uniform corrosion of nickel base alloys C276 and 625 occurred at above 700 mV and higher.

The weld joints of nickel base alloyed joined with nickel base filler were sensitive to general corrosion however didn’t show the signs of localized attack. The weld joints of stainless steel Din 1.4565 with nickel base filler were corroded by a certain kind of localized attack mainly at the solidification fronts.

The coupon immersion tests were conducted in the lab conditions identical to mixer conditions at the different levels of ClO2 substitution. General environmental factors were pH 1.7 to 3.5, temperature 50oC, chlorine content 1.6 g/l. The attack on nickel base alloys was noticed to be accelerated with increase in concentration of CLO2. For instance nickel base alloy’s functionality in a media where ClO2 slightly chlorine gas. Large shear mixer made from cast alloy Hastelloy C4 functioned well for about 4 years at 25% Chlorine dioxide substitution. After using 50% ClO2, rotor needed to be replaced due to damage, pitting and general corrosion attack at a rate of 2.5mm/y or 100 mpy.

The weight loss measurements in simulated mixer conditions from 0 to 100% chlorine dioxide replacement. High alloy austenitic steels such as alloy 24 attained large corrosive attacks in such media. Hastelloy C276 offered good performance in completely 100% chlorine bleach conditions describes large attacks in ClO2 replaced conditions. The attack of high alloy steel grades was found to reduce with higher concentration of ClO2 whilst it increased for nickel base alloys with increasing ClO2 content.

The outcomes from the lab tests and onsite exposures and performance experiences describe that 6Mo austenitic stainless steels can be successfully used in the various bleach plant conditions. However the drawback is their sensitivity to pitting and crevice attack in the aggressive media. Nickel base alloys with large magnitudes of chromium and molybdenum prevent attack of pitting and crevice corrosion in such plant conditions however may be attacked by transpassive corrosion.