Iron aluminizes, both in the form of diffusion coatings on low alloy steels, as well as in alloy or weld overlay form have generally shown excellent corrosion resistance in laboratory tests simulating gasifier and other reducing environments in energy conversion systems. However, the performance of aluminized coatings in gasifiers has generally been unacceptable. Coatings, containing up to 30!LoA1 near the surface, were quite often totally destroyed in 2000-5000 hrs exposure. Here we report on laboratory corrosion tests to elucidate this discrepancy. Both high pressure (41 atrn) and atmospheric pressure tests were carried out. The test confirmed the excellent corrosion resistance of iron aluminide in reducing gases containing HZS, even when significant quantities of HC1 are present. However, iron aluminizes were found to be extremely susceptible to aqueous corrosion during downtime. Moreover once the protective scale was destroyed, it did not reform easily. Concentration of chloride species at the metal surface during downtime is considered the root cause of the scale degradation mechanism.
Mixed oxidant corrosion in gasifiers is a complex process, especially at low temperatures (300-600C) where relatively small amounts of chlorine species in the environment can significantly influence both corrosion morphology as well as corrosion rate. Aqueous corrosion during shutdowns (downtime corrosion, D. T.C.) can further change the corrosion processes and resulting metal wastage rates. In previous paperstl?2JJ the corrosion of Fe-Ni-Cr alloys was reported. Here it was found that the presence of HC1 in a reducing gas, containing H# as the main corrosive species, influences the relative rates at which Ni, Fe and Cr diffuse to the alloy surface to react with the sulfidizing gas to form outward growing sulfide rich scales. In gases with a low HC1/H20 ratio, the relative diffusion rates of Ni, Fe and Cr are such that a somewhat protective FeCr2Sl subscale will form under a non protective outward growing Fe, Ni, S scale. This was labeled Type A corrosion. When the HC1 content of the gas is increased, the outward diffusion rates of Fe and Ni are increased relative to those of Cr. This prevents the formation of a Fecrzsi subscale and results in internal oxidation of Cr to Cr2O~ in the Fe and Ni depleted alloy surface. Depending on the chromium content of the alloy, this can lead to very high corrosion rates, when the density of the Cr2O~ precipitates results in a porous inward growing Cr2O~ rich scale, which is non protective. This was labeled Type B corrosion. It occurs in alloys with a relatively low Cr content (18-22Yo) and may also occur in higher chromium alloys in the presence of chloride rich deposits. When the amount of oxide formers in the alloy is very high, the formation of a very dense oxide precipitation zone can lead to very low corrosion rates. Here the dense precipitation zone apparently blocks further outward diffusion of Fe and Ni. This situation is especially prevalent in alloys containing chromium as well as smaller amounts of other oxide formers such as Si or Al. This was labeled Type C corrosion. (Figure 1)
Corrosion during shutdowns becomes significant in the presence of deposits containing chloride salts such as Fe Cl~ and NaCl, when the humidity is high enough to cause the formation of a chloride rich liquid in the deposit. This can accelerate the overall corrosion rate in two ways. Firstly, the chloride rich liquid tends to concentrate at the scale/ metal interface, This increases the available chloride species during future service and leads to non protective Type B scale formation and a high corrosion rate. Secondly, the chloride rich liquid can cause aque