Results are presented of a metallurgical evaluation of diffusion coated, micro-alloyed HP-Nb (HP-Nb MA) ethylene furnace tubes after 27 months in service in a furnace. The diffusion coating system applied to the ID surfaces of the tubes by a modified pack cementation process was a bi-diffused aluminum-chromium-silicon system. Results indicate that the coating was thermally stable and successful at eliminating carburization at temperatures exceeding the maximum design temperature for the furnace. Tube sections excised from a coil section exhibiting flame impingement damage confirmed the coating prevented carbon ingress at tube metal temperatures of at least 2050℉(1121°C). Earlier published work verified the ability of the coating system in reducing coke formation during the first 14 months of service. Details of additional research to determine detailed phase characterization and the mechanisms of protective action are discussed.
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