An investigation on anti-coking behavior of gas phase aluminide coatings applied on a high performance micro alloyed (HP-MA) steel

Abstract

In this study, aluminide coatings were applied on a high performance micro alloyed (HP-MA) steel through a gas phase aluminizing process in order to investigate anti-coking performance of the coated steels. In this regard, powder mixtures with compositions of 10% Al-5% NH4Cl-85% Al2O3 and 30% Al-15% NH4Cl- 55% Al2O3 were used for the coating process, hereafter named Al-10 and Al-30 coatings, respectively. The anti-coking performance of the specimens was evaluated using a simulated cracking setup. In this setup, specimens were placed in a tube furnace, exposed to Ethane (C2H6) and Argon gases as the primary feedstocks, using optimized parameters. Results of the coking test showed that the bare steel was largely covered with catalytic coke; while enhanced performance was observed for the gas phase aluminide coated steels. Formation of catalytic coke on the bare steel was due to the reaction between feedstocks and active substrate elements such as Fe and Ni. In fact, the released carbon during the cracking process diffuses into the bare steel, and removes metal crystals from the surface by its outward growing, which eventually led to the formation of catalytic coke. Besides, it was observed that the Al-30 coating had superior performance than the Al-10 coating. In fact, for the Al-10 coating, the amount of catalytic coke was reduced as compared to the bare steel, while the Al-30 coating was mainly covered with spherical carbon deposits, released from chemical reactions in the gas phase. In this regard, the anti-coking ratios of the coatings were 63 and 80%, for Al-10 and Al-30 coatings, respectively. This was mainly due to the higher content of Al and Cr, in the Al-30 coating.