Comparison of internal nitridation reactions in ammonia and in nitrogen

Journal Article · Sun Oct 01 04:00:00 EDT 1995 · Oxidation of Metals

DOI:https://doi.org/10.1007/BF01058247· OSTI ID:207129

Tjokro, K; Young, D J ^[1]

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1. Univ. of New South Wales, Sydney (Australia)

Nine commercial high-temperature alloys were reacted with NH{sub 3} and with N{sub 2}-5% H{sub 2} at temperatures of 1000-1200{degrees}C. In all cases extensive internal nitridation developed according to parabolic kinetics. Reaction with N{sub 2} produced precipitates of Cr{sub 2}N plus, at 1000{degrees}C, some external CrN, in accordance with thermodynamic prediction. However, NH{sub 3} produced an external scale of CrN and a near-surface zone of internal CrN as well as a deeper zone of Cr{sub 2}N at 1100 and 1200{degrees}C. The CrN phase is metastable, and results from catalytic dissociation of NH{sub 3}, which produces a high effective nitrogen activity. This high activity also leads to faster internal-precipitation reactions, whereas reaction rates in nitrogen are in reasonable agreement with Wagner`s model of rate control by inwardly diffusing nitrogen at its equilibrium solubility. Precipitate morphologies are complex, reflecting the importance of the energy barrier to homogeneous nucleation.

• Sponsoring Organization: USDOE
• OSTI ID: 207129
• Journal Information: Oxidation of Metals, Journal Name: Oxidation of Metals Journal Issue: 3-4 Vol. 44; ISSN OXMEAF; ISSN 0030-770X
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
AMMONIA
COMPARATIVE EVALUATIONS
HEAT RESISTING ALLOYS
NITRIDATION
NITROGEN
SCALING
TEMPERATURE RANGE 1000-4000 K