Influence of partially molten alkali deposits on superheater corrosion in black liquor recovery boilers

Corrosion tests with steel materials of interest for the boiler manufs. were done in lab. conditions.  The material samples were covered by synthetic salt mixts. contg. Na2SO4, K2SO4, NaCl, KCl, and K2CO3 with compn. typical for deposits in kraft recovery boilers.  Six different salt mixt. were used with varying first m.p., amt. of melt formed, and compn. of the first melt.  Expts. were done in isothermal oxidizing conditions in the range 440-​590°.  Exposure times varied between 6 and 168 h.  The samples were studied by optical microscopy and SEM.  The thickness of the corrosion product layer was detd. by optical microscopy and the chem. compn. of different product layer were detd. by energy dispersive x-​ray anal. (EDS)​.  Chem. phases were identified by x-​ray diffraction.  The melting behavior of the salt samples were detd. by thermal anal. (TG​/DTA)​.  Conventional chem. anal. was used for detn. of bulk salt compn. after the expt.  The results show both the temp. dependency and the effect of salt compn. of the corrosion.  Differences between different alloys could be seen and the corrosion mechanism could be explained in some of cases.  Based on the results, practical corrosion criteria can be drawn for predicting of corrosion in full-​scale operation.