03656 ALLOY 602CA(UNS N06025) SOLVES PIG TAIL CORROSION PROBLEMS IN REFINERIES

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Product Number: 51300-03656-SG

ISBN: 03656 2003 CP

Author: D.C. Agarwal, Lee Stewart & Mike McAllister

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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 (H2S), 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 (% Al + % 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. Keywords: Metal dusting, pig tails, refinery, nickel alloys, alloy 602CA, UNS N06025, high temperature corrosion, carburization, chromia-alumina scale, syngas, ammonia synthesis, methanol production