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 lab.-​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 carried out at the temps. ranging from 450 to 600°.  The synthetic alkali salt deposits, contg. sodium, potassium sulfates and chlorides, were composed in such a way that their first melting temp., T₀, and the amt. of melt formed at this temp., varied for each salt mixt.  The results showed on one hand that an increased amt. of the melt in the salt deposit increased the steel corrosion significantly.  Also, corrosion could take place at the temps. clearly below any melting of the salt deposits if the compn. was suitable.  This took place with the salts that contained chlorine.  Even a small amt. of chlorine in the salt caused corrosion at the temps. typical for superheaters in the recovery boiler. To stress the importance of the deposit layer on the corrosion, two new terms are introduced: (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.