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Keywords
Steel, Corrosion prevention
Abstract
Until recently, chlorine used to be an important chemical in bleaching process in paper industry, but as a result of environmental concerns, it is being replaced by chlorine dioxide. However, chlorine dioxide is more corrosive in certain conditions. Plant personnel, therefore need to better understand the reactions taking place in the changed media and search for more resistant materials. It is with this in mind that the present work was undertaken. The paper reports the electrochemical polarisation measurements performed on stainless steels 316L, 317L, 2205 and 254SM0 in chlorine dioxide solutions to observe localised corrosion. The results have been analysed with reference to Pourbaix diagrams, taking into account the various chemical species present in the bleach solutions. Conclusions drawn from electrochemical tests have been compared with those from long-term laboratory and plant tests. Materials options are proposed on the optimal choice of materials for bleach plants, in a context of probable corrosion performance, capital cost and mechanical strength.
Introduction
Chemicals used in the bleaching process are highly corrosive in nature. Additionally, the use of chlorine enhances the pollutants level in nearby streams through discharge of malignant chloro-organics. As a result, chlorine is being replaced increasingly by chlorine dioxide. Changes in the bleach chemicals are likely to influence the corrosivity of the liquid media, which in the worst cases may affect the integrity of the materials of construction of bleach plants. As such, the metallurgy of plants using chlorine dioxide is likely to change. In-plant corrosion tests that were conducted in the past addressed the corrosion behaviour of highly alloyed stainless steels, 6 per cent Mo containing stainless steels, Ni base alloys and Ti, and exhibited a variable degree of corrosion resistance (Nordstrom and Bergquist, 1996; Tuthill and Bardsley, 1990; Tuthill et al., 1992). In a recently conducted in-plant tests, stainless steel alloy 316L, which until recently was considered as the normal material of construction for paper plants, was found to experience excessive attack, as compared to the higher Mo-containing stainless steels, (Singh and Singh, 2001). The tests also showed that solutions containing chlorine dioxide, in certain conditions, could be more corrosive than was chlorine. In consequence, it appeared to be appropriate for the corrosion of stainless...