This paper is the final installment of EPRI?s project on corrosion in singes coolers of integrated coal gasification combined cycle plant. In previous papers we explored the effect of chloride species on mixed oxidant corrosion of stainless steels (Corrosion 97, Paper 134) and iron aluminizes (Corrosion 98, Paper 185). It was found that the amount of chloride species in the gas and in deposits largely determined the rate of corrosion. Armed with this understanding the effect of a few common process variables is explored in this paper. It was found that corrosion rates decline at higher temperatures, when chloride-enhanced mixed oxidant corrosion occurs at lower temperatures, but corrosion rates increase with temperature for pure sulfidation/oxidation, as maybe expected. System pressure was generally of secondary importance, but again higher corrosion rates were generally experienced at elevated pressure, when chloride-enhanced corrosion was predominant, especially at lower temperatures. A simulated temperature excursion below the syngas dew point had similar effects, and also caused chloride-enhanced corrosion in 310 stainless steel, which usually does not show this type of corrosion, except when exposed to aqueous corrosion during downtime. A pleasant surprise was the almost complete lack of corrosion when H2S and HC1 levels were reduced to those expected downstream from gasifiers using in-bed desulphurization. It would appear that 12Cr steels may be adequate here. This is being explored further.
During the last 18 years EPRI and its partners have studied the effects of mixed oxidant corrosion (sulfidation, oxidation, chlorination) mainly to select the most appropriate materials for syngas coolers, hot gas filtration systems and other key components of gasification systems. This effort included both laboratory corrosion studies, field exposures and analyses of failed components after service. From this work the major causes of accelerated corrosion were determined. They have been discussed in detail in recent publications(1?2 ?3>4)and will be summarized in the technical background section. In this paper we will discuss the effects of process variables which generally do not affect the basic corrosion mechanisms, but may have a significant effect on the corrosion rate.