Downtime corrosion in syngas coolers of entrained slagging gasifiers: Topical report

Materials for syngas coolers of entrained slagging gasifiers are susceptible to corrosion by moist air during the start-up and downtime periods. Modes of attack include general corrosion, pitting, intergranular corrosion, and stress corrosion cracking. Corrosion is accelerated by the presence of HCl in the syngas and deposits of gasifier slag on the surface but will occur for many materials in the absence of either chlorides or deposits. Pitting is the most common form of corrosion and is electrochemical in nature. Sites in which chlorine has been occluded form the anodes while sulfide scales and free carbon in slag deposits form large surface area cathodes that drive the ration at a rapid rate. Chloride salts formed in scales and deposits can absorb moisture from syngas or moist air at temperatures above the dewpoint temperature to initiate corrosion on cooldown from operating temperatures. Plant operations can be modified to mitigate corrosion during downtime with low concentrations of chlorides in scales and deposits but controls may not be fully effective if high concentrations of salts are present. The use of materials that are resistant to chloride enhanced corrosion provides the best approach to control. Ferritic or austenitic stainless steels with 2--6% Mo aremore » effective but may be limited by scaling to use below 400/degree/C. Downtime corrosion controls should be used as standard operating procedures even though they may not be fully effective and pit resistant alloys or coatings should be designed into new plant construction. 21 refs., 45 figs., 22 tabs.« less