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Abstract

At temperatures < 1000 degrees C vapour deposited silica coatings prevent carburization of Incoloy 800H in gas mixtures with a-c = 0.8 and a Po-2 (4 x 10**-23 atm) sufficiently high to maintain the stability of silica. The objective of this investigation was to examine whether, applied to ethylene cracker tubes, this protection would continue in severe carburizing environments with a lower oxygen potential than that in thermodynamic equilibrium with silica. Silica coatings, between 4 and 9 microns thick, were applied to Incoloy 800H by a plasma assisted vapour deposition process. Coated and control specimens were exposed, for defined periods up to 1000h, to 1.22**v/oCH-4 + 98.78**v/o H-2 at 825 degrees C and to 4.39 v-/o CH-4 + 95.61 v-/o H-2 at 1000 degrees C. Carbon activity was 0.8; residual oxidizing contaminants were observed, principally water vapour, which resulted in a Po-2 < 10**-25 bar. The vapour deposited silica coating completely prevented carburization of Incoloy 800H at 825 degrees C; at 1000 degrees C it was ineffective, because the Po-2 was insufficient to prevent the reduction of silica to volatile silicon monoxide in the high carbon activity gaseous environment.

Additional information

Authors: LANG E JRC PETTEN ESTAB. (THE NETHERLANDS) BENNETT M J AERE, HARWELL (UK) , JRC PETTEN ESTAB. (THE NETHERLANDS);AERE, HARWELL (UK)
Bibliographic Reference: COLLOQUE INTERNATIONAL DE CORROSION A HAUTE TEMPERATURE, UNIVERSITE DE PROVENCE, MARSEILLE (FRANCE), JULY 7-11, 1986 WRITE TO CEC LUXEMBOURG, DG XIII/A2, POB 1907 MENTIONING PAPER E 32768 ORA
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