MIXED OXIDANT CORROSION IN NONEQUILIBRIUM GASIFIER ENVIRONMENTS

The major use of high-temperature steel alloys in gasifiers is in heat exchangers for cooling hot syngas, consisting mainly of CO and H-2 wi th lesser amounts of H2O and CO2 and minor quantities of H2S and HCl. Metal temperatures range from 250 to 600 degrees C, gas temperatures f rom 250 to 1200 degrees C. Because of rapid cooling the composition of the gas does not change with temperature. Therefore the gas is not in equilibrium at the metal surface. Calculations show that such gases h ave lower oxygen and sulfur pressures than equilibrated gases at the s ame temperature. This makes the results of previous laboratory studies less appropriate for predicting mixed oxidant corrosion in gasifiers. For this reason the present study was carried out using nonequilibriu m gas mixtures, similar to gases, produced in entrained-slagging gasif iers. Most corrosion experiments were carried out at 540 degrees C, as this is a common temperature for superheaters and hot-gas cleanup sys tems. Iron-base model alloys containing 35% Ni, 20% Cr, and various mi nor alloying additions were studied. Three corrosion regimes were iden tified over the range of conditions studied, depending on the sulfur-t o-oxygen pressure ratio of the gas and the alloy composition. At high P-S2/P-O2 ratios a somewhat protective FeCr2S4 scale formed on all all oys. Below this layer internal oxidation and sulfidation occurred at a slow rate. At lower P-S2/P-O2 ratios nonprotective Fe(Ni, Cr)S extern al scales formed. These allow rapid internal oxidation of the chromium in the alloy, resulting in high corrosion rates. Under the same condi tions very low corrosion rates are obtained when silicon is added to t he alloy, because the presence of SiO2 precipitates makes the internal -oxidation layer protective. Thus, the same corrosion model is operati ve in all three corrosion regimes: external sulfidation of iron and ni ckel, together with internal oxidation of chromium and other strong-ox ide formers.