INTRODUCTION
In high temperature corrosion, besides oxygen and nitrogen attack, alloys frequently encounter attack by carbon species. This attack can take two forms namely carburization and metal dusting (some times referred to as catastrophic carburization). Carburization generally occurs at carbon activities less than 1 and at temperatures typically greater than 800 ° to 850°C and may result in alloy embrittlement due to formation of internal carbides. Metal Dusting on the other hand, leads to rapid wastage, thinning and disintegration of alloys into graphite, metal particles, and possibly carbides / oxides and occurs in environments with carbon activities significantly greater than 1 and temperatures in range of 430 to 850 ° C. Metal dusting has been encountered in many industries such as in production of hydrogen, syngas, chemical plants for the synthesis of hydrocarbons, methanol, ammonia, pig tails of reformer fum~ices, refineries, petrochemical plants, heat treating and reduction of iron ore plants. Attempts to inhibit metal dusting can take many forms such as using better alloy metallurgy, surface engineering modifications and/or pre-treatments of the alloy surfaces, use of coatings and using gas inhibitors such as addition of sulfide compounds (HzS), which causes other problems of sulfidation attack and catalyst poisoning. This paper presents a case history on tests conducted in a Texas reformer unit, as a pig tail component, where the environment consisted of CO & CO2 with the CO:CO2 ratio between 3 to 4, methane, hydrogen and a small amount ofpropylene at operating temperature range of 1100°F to 1400°F (593°C to 760°C). This is a very severe metal dusting environment. Many nickel base and iron base alloys were tested and a co-relation with "Chromium Equivalent" defined as " % Cr + 3 (% A1 + % Si)" with the metal wastage rates were developed. The nickel base alloy 602CA(UNS N06025) gave the best performance. Also presented are some laboratory data on this alloy in metal dusting and other environments, physical metallurgy characteristics of this alloy along with some other high temperature applications.
Metal dusting, a phenomenon which was first documented in the 1950's has been extensively studied in the last 30 years with various theories as to its mechanistic behavior in both iron base & nickel base alloys. These are well documented in the literature (1-10). Production of syngas, a mixture of CO, H2, C02 and H20 from natural gas via steam reforming technology is a common starting step in production of hydrogen, ammonia, methanol and liquid hydrocarbons which are used in production of many intermediates and consumer end products. To increase production efficiency the trend to use less steam has resulted in lower steam to hydrogen ratios and higher pressures led to higher CO content ( higher CO/CO2 ratio). Both of these result in lower oxygen partial pressures and higher carbon activities. This combination synergistically accelerates the severity and propensity for metal dusting attack to occur. The industry has tried to deal with this problem by designing around the temperature range of metal dusting attack (430 to 850 ° C). This has been achieved by rapidly quenching the syngas to below 300°C in a waste heat boiler from the syngas production temperature range of 800 to 900°C, thus wasting the energy of the high temperature syngas. However in pig tail components, the operating temperature is right around 593°C to 760°C, the worst temperature for metal dusting attack in environments with high carbon activity. At present there are no "alloy metallurgy", whether iron based or nickel based which can resist this type of corrosion over a long period. The next few sections describe the