Improvement of oxidation resistance of Ni-base alloys by the halogen effect

SCHU 729/18

Bild Forschungsprojekt

Hochtemperaturofen in Betrieb

Period: 2007-02-01 to 2011-01-31
Funder: German Research Foundation (DFG)
Project Manager: Dr. Hans-Eberhard Zschau
Research Group: High Temperature Materials

The Ni-base superalloys with Al-contents of less than 10 wt.-% are widely used in high temperature technology due to their beneficial mechanical properties. In contrast to this their oxidation behaviour may be particular insufficient at temperatures above 1000°C. Oxidation of these Ni-base alloys does not form a pure continuous Al2O3   protective scale on the surface, but rather a complex layer structure. This structure is characterized by internal oxidation. However the formation of a dense continuous alumina scale without significant internal oxidation would theoretically be possible, if a "critical" Al-concentration Nc is realized. This critical value Nc was calculated from Wagner's oxidation theory for Ni-base alloys to be less than 10 wt.-% Al.

In this project a new concept for the formation of a protective alumina scale on the surface is presented. The focus of this work is to change the alumina formation from a discontinuous internal to a continuous external oxide scale. The method is based on the "artificial" increase of the Al-amount near the surface by using the halogen effect. The critical Al-concentration for alumina formation can be increased by increasing the Al - diffusion coefficient D through an increased Al transport rate to the alloy surface via the halogen effect:

f505_formel   kp = parabolic rate constant
  D  = Diffusion coefficient of Al in the Ni-base alloy


An enhanced transport of Al from the alloy to the oxide/metal interface via the gas phase can be obtained by a selective formation and transport of Al-halogenides. Detailed thermodynamic calculations show the existence of a region dominated by Al-fluorides in the gas phase. Based on these thermodynamic results samples of the Ni-base alloy IN 738 were implanted with F-ions. After oxidation at 1050°C a thin continuous external alumina scale was formed on the surface, whereas the untreated alloy showed mixed oxide scale with significant internal oxidation. The results can offer a new way for oxidation protection of Ni-base superalloys at temperatures of 1000°C - 1200°C.

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