Materials challenges in cyclic carburizing and oxidizing environments for petrochemical applications C. M. Chun * , S. Desai, F. Hershkowitz and T. A. Ramanarayanan Steam cracking is a petrochemical process that cleaves a broad range of hydrocarbon feed molecules into a variety of light olefinic products, including the highly desirable ethylene. Over the course of a cracking operation using a feed mixture of a saturated hydrocarbon and steam around 900–1000 8C in tubular alloy coils located in fired heaters, coke inevitably forms on the inside surfaces of the furnace tubes and must be burned off and/or spalled off periodically using steam or a steam–air mixture. Furnace tube materials are predominantly based on chromia-forming alloys; such alloys can degrade by carburization and oxide–carbide conversion in such a mixed carburizing– oxidizing environment. These hurdles have been largely overcome by using an alumina-forming material that provides superior corrosion and coking resistance. Cracking hydrocarbons at much higher temperatures results in high selectivity to acetylene, which can be converted into many petrochemical products including ethylene. The desired hydropyrolysis reaction from hydrocarbons to acetylene can be realized in a reverse-flow reactor operating above 1500 8C in a scaleable manner. The reactor elements include ceramic components that are placed in the hottest regions of the reactor, and must withstand temperatures in the range of 1500–2000 8C. Moreover, the materials in the hot zone are exposed alternately to a regeneration (heat addition) step that is mildly oxidizing and a pyrolysis (cracking) step that is strongly reducing with a correspondingly high carbon activity. This paper addresses the thermodynamic stability of selected ceramic materials based on alumina, zirconia, and yttria for such an application. Results from laboratory tests involving the exposure of these ceramic materials to simulated process conditions followed by their microstructural characterization are compared with expectations from thermodynamic predictions. 1 Introduction Currently the pyrolysis of hydrocarbon feeds to produce a variety of light olefin products, including the highly desirable ethylene (C 2 H 4 ), is almost exclusively carried out in steam crackers operating around 900–1000 8C [1]. The hydrocarbon feeds with steam diluent flow through tubular coils located in the radiant section of a fired heater. Furnace tube materials, based on chromia-forming austenitic alloys, tend to degrade by carburiza- tion and oxide–carbide conversion in such a mixed carburizing– oxidizing environment having a carbon activity exceeding unity. Coke deposition occurs on the inner walls of the tube and has to be removed periodically. In the present paper the sequential stages of alloy degradation and the role of surface carbide in carburization are discussed; details have been presented earlier [2, 3]. The applicability of an alumina-forming alloy that provides superior corrosion and coking resistance is discussed. Current efforts to use bulk alumina forming alloys as well as alumina- forming overlay materials are also presented. It has long been known that higher temperature hydro- pyrolysis can crack a broad range of hydrocarbon feed molecules, including methane, with high selectivity to acetylene (C 2 H 2 ), which can be further converted into ethylene via a conventional 282 DOI: 10.1002/maco.201307059 Materials and Corrosion 2014, 65, No. 3 C. M. Chun, S. Desai, F. Hershkowitz ExxonMobil Corporate Strategic Research, Annandale, New Jersey 08801 (USA) E-mail: changmin.chun@exxonmobil.com T. A. Ramanarayanan Frick Chemistry Laboratory, Princeton University, Princeton, New Jersey 08544 (USA) ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com www.matcorr.com