Environmental concern about chlorinated organic compounds generated from traditional bleach plants has led to a development of new environmentally friendly bleaching processes, Recently, the development of ozone bleaching has progressed from pilot operations to production scale installations. Ozone is currently used as a bleaching agent of pulp as a replacement for chlorine based bleaching chemicals. Yet, there are few articles which are dealing with ozone and its effect to corrosion resistance of stainless steels or any other construction materials used in bleaching equipment.
In this paper corrosion behavior of austenitic stainless steels, grades AISI 316, AISI 317LNM, UNS S3 1254 and UNS 32654 PM, were studied in simulated ozone bleaching environments. The laboratory tests showed that in ozone environments without chlorides the corrosion resistance of AISI 316 was superior to that of the high-alloyed stainless steels, due to the relatively low amount of alloying elements. The sequence was reversed in ozone environments containing chlorides. In the presence of chlorides AISI 316 was susceptible to localized corrosion whereas the high-alloyed UNS S3 1254 and UNS S 32654PM were resistant to localized corrosion in all chloride concentrations examined.
Environmental regulations, market forces and public concern are forcing the pulp and paper industry to reduce and ultimately eliminate the chlorinated organic compounds from the waste water stream. To achieve this, new environmentally friendly bleaching processes have been developed. Ozone as bleaching chemical has a great potential for reducing and replacing chlorine based chemicals in acidic bleaching stages. Ozone can be used both in elementary chlorine free (ECF) bleaching processes, where it replaces gaseous chlorine, and in totally chlorine free (TCF) bleaching where ozone replaces chlorine dioxide[1-3].
Ozone is a very strong oxidizing agent and also an effective lignin solvent. Another advantage of ozone is that ozone produces effluents that can be recycled. But as a strong oxidizing agent ozone may also react with bleaching equipment and in some cases cause unexpected corrosion failures. According to the literature data[1-3] the corrosion resistance of traditional austenitic AIS13L and 316L type stainless steels is good in ozone containing solutions (pH ˜3) and ozone gas. Their resistance is superior compared to highly alloyed 6%Mo type austenitic and duplex stainless steels.Due to the good corrosion resistance AISI 304L and 316L type stainless steels are commonly used as a construction material for ozone generators.
However, there have been suspicions that the traditional AISI 304L and 316L type stainless steels may suffer from localized corrosion in ozone containing solutions if critical amounts of chlorides are present in the pulp[4,5] . The aim of this paper was to study the effects of ozone and chlorides to the corrosion resistance of stainless steels in simulated ozone bleaching conditions.
Corrosion behavior of four austenitic stainless steels, gradesAISI316, AISI 317LNM, UNS S3 1254 and UNS S32654 PM, were studied in simulated ozone bleaching environments. The AISI 316, AISI 317LNM and UNS S31254 specimens were taken from commercially produced stainless steel sheets.