Corrosion of BLRB Floor Tubes in Reduced Kraft Smelts: Experimental and Theoretical Studies, 1996 Engineering Conference Proceedings

Martti Mäkipää and Sari Mäkinen

VIT Manufacturing Technology

Rainer Backman

Åbo Akademi University

Marko Hämäläinen

Helsinki University of Technology

Carbon steel and chromium nickel steels are known to be severely corroded in reduced kraft smelts containing polysulfide species. To clarify this type of corrosion, a systematic course of study was cho-sen including laboratory corrosion testing and theoretical calcula-tions. Mainly the effects of sodium polysulfide species are consid-ered. A model was developed for estimation of the melting behaviour of salt mixtures similar to the inorganic salts in the smelt bed of recov-ery boilers. In normal conditions in the lower part of the furnace no high concentrations of polysulfide are expected. In extremely reduc-ing conditions small amounts of a melt rich in polysulfide can form at temperatures below 500 °C. A model suitable to analyse thermo-dynamic stability of various corrosion products of steels and alloys was also developed. For the corrosion tests a method where a relatively small amount of sodium polysulfide melt was present was chosen. Corrosion kinet-ics of carbon steel and chromium-nickel alloy steels containing about 20 % chromium and 8 to 40 % nickel were determined at tempera-ture interval from 300 to 450 °C. The least resistant material at the lower temperature range (300-400 “C) is carbon steel. The corrosion rate of Type 304L stainless steel reach the corrosion rate of carbon steel at temperature interval of 400-450 “C. The most resistant mate-rial in the all experimental conditions was a chromium-nickel-iron alloy containing about 20 % chromium and 40 % nickel. On the basis of the combined results of the model calculations and corrosion tests it was, however, concluded that the corrosion of the floor tubes because of the presence of sodium polysulfide melt is not very probable without the contribution of additional factors stabilis-ing polysulfide melts.