Behavior of heat exchanger tube materials in simulated coal gasification atmospheres with different H2O-content

Corrosion behavior of three Fe-base heat-exchanger-tube alloys was investigated in CO-CO2-H-2-H2S-HCl-containing gas mixtures of different (0-17%) H2O content which simulated the heat-recovery-boiler environment of a dry-fed, entrained slagging-type coal gasifier. Sample temperatures during the 500 hour-long tests were kept at 300, 450 and 600 degrees C in the low-pressure (LP; 1atm) tests and at 450 degrees C in the high-pressure (HP; 31 arm) tests. Surface recession of the 2.25Cr1Mo steel increased while those of the 12Cr steel and 20Cr32Ni alloy decreased with increasing H2O-content of the gas in the LP-600 degrees C tests. Surface recession of the alloys decreased with temperature regardless of the H2O-content of the gas mixture. Increasing the total gas pressure at 450 degrees C caused changes in the surface recession of all alloys. However, the effect of H2O-content on corrosion at high pressure was less significant for the 20Cr32Ni alloy. The effect of the steam content of the gas on alloy corrosion was observed to be related to alloy composition, total gas pressure and the test temperature which affected the formation of corrosion products. Increased dissolution of gaseous oxidants in the alloy at high pressures and/or temperatures was concluded to result in thicker subscales in low-Cr alloys due to the lack of formation of protective oxide layers which would act as barriers to the ingress of corrosive species. For low-Cr alloys, corrosion products containing chlorine were observed during the low temperature tests. For these alloys, carbide corrosion products were found in the subscales as predicted by the thermodynamic properties of the gas mixtures.