Corrosion of superheater steel materials under alkali salt deposits Part 1: The effect of salt deposit composition and temperature

This paper is the first in a series of two reporting on results from an extensive laboratory-scale corrosion study where tailor-made well-characterized synthetic alkali salt deposits were used for corrosion testing of several steel materials used in or aimed for recovery boiler superheater tubing.The corrosion testing was done in temperatures ranging from 450 to 600 °C. The synthetic alkali salt deposits, containing sodium, potassium sulfates and chlorides, were composed in such a way that their first melting temperature, T0, and the amount of melt formed at this temperature, varied for each salt mixture.The results showed on one hand that an increased amount of melt in the salt deposit increased the corrosion of the steel material markedly. The results showed, however also, that corrosion could take place at temperatures clearly below any melting of the salt deposits if the composition was suitable. This took place with salts that contained chlorine. Already a very low amount of chlorine in the salt caused corrosion at temperatures typical for superheaters in the recovery boiler. These effects are qualitatively well-known from earlier but it was surprising that already a very small amount of chlorine caused significant increase in corrosion.To stress the importance of the deposit layer on the corrosion we introduce two new terms: (1) sub-T0 corrosion, indicating corrosion taking place below any melting of the deposit and (2) super-T0 corrosion, indicating corrosion taking place when the deposit contains melt.