Computer Simulation of Internal Corrosion Processes: Internal Nitridation of Nickel-Base Alloys

Ulrich Krupp, Shih Ying Chang, and Hans-Jürgen Christ

Institut für Werkstofftechnik
Fachbereich 11: Maschinentechnik 
Universität Gesamthochschule Siegen
Germany

 

The influence of atmospheric nitrogen on high-temperature corrosion of Ni-base alloys is dealt with in few studies only. Failure of protecting oxide scales by pallation on highly stressed components may lead to a depletion of the scale forming elements (e.g. Al) until the formation of a healing layer becomes impossible. The transition from external to internal Al oxidation permits nitrogen to penetrate into the alloy leading to internal precipitation of Ti , Al and Cr nitrides (Fig. 1).

Figure 1 (click thumbnail for a large version of the image)

Objective of this study is both a detailed experimental analysis of kinetics and thermodynamics of internal nitridation and the development of a computer-based simulation program. In addition to the single-crystalline superalloys CMSX-6, CMSX-2 and SRR99, various model alloys from the system Ni-Cr-Al-Ti has been studied in order to determine the influence of alloying constituents on the rate of the internal nitridation process (e.g. diffusion coefficients and nitrogen solubility). The results of the experimental studies (thermogravimetry, SEM, TEM, XRD etc.) were used as input data for the numerical modelling. Fig. 2 shows schematically the procedure of the computer simulation of internal-precipitation processes combining the finite difference technique with thermodynamic equilibrium calculations using ChemApp.

Figure 2 (click thumbnail for a large version of the image)

Fig. 3 shows an SEM micrograph of the internal-nitridation zone with AlN (black) and TiN precipitates (dark grey) in Ni-20Cr-2Al-2Ti nitrided for 100h at 1000 degrees C in N2 atmosphere and Fig. 4 depicts the corresponding calculated concentration profiles of the reacting species for this internal nitridation reaction (Ni-20Cr-2Al-2Ti, 100h, 1000 degrees C).

Figure 3 (click thumbnail for a large version of the image)

 

Figure 4 (click thumbnail for a large version of the image)

 

Literature:

  • Krupp: Innere Nitrierung von Nickelbasislegierungen, Fortschritt-Berichte VDI, Reihe 5, Nr. 529, VDI Verlag, Düsseldorf 1998
  • Krupp, H.-J. Christ: Innere Nitrierung von Nickelbasis-Legierungen, Proc. Nichtmetalle in Metallen '98, März 23th - 25th 1998, Münster (Ed. D. Hirschfeld), GDMB-Informationsgesellschaft, Clausthal-Zellerfeld 1998, p. 89-98
  • S.Y. Chang, U. Krupp, H.-J. Christ: Selektive Oxidation und innere Nitrierung von einkristallinen Nickelbasis-Superlegierungen, Metall, 6 (1998) p. 355-360
  • U. Krupp, H.-J. Christ: Internal Nitridation of Nickel-Base Alloys Part I: Behavior of binary and ternary alloys of the Ni-Cr-Al-Ti system, Oxidation of Metals, 52(3/4), (1999), p. 277-298
  • U. Krupp, H.-J. Christ: Internal Nitridation of Nickel-Base Alloys Part II: Behavior of quaternary Ni-Cr-Al-Ti alloys and computer-based description, Oxidation of Metals 52(3/4), (1999), p. 299-320
  • U. Krupp, H.-J. Christ: Internal Nitridation of Ni-Base Alloys as a Special Case of an Internal Corrosion Reaction, in: Proc. Eurocorr '98, Working Party 3 and 6: High Temperature and Surface Science, September 28th - October 1st 1998, Utrecht, Netherlands
  • U. Krupp, H.-J. Christ: Selective Oxidation and Internal Nitridation During High-Temperature Exposure of Nickel-Base Superalloys, submitted to: Metallurgical and Materials Transactions A
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  • U. Krupp, H.-J. Christ: Einfluß der Legierungszusammensetzung auf das Oxidations- und innere Nitrierungsverhalten von Nickelbasis-Superlegierungen, submitted to: Werkstoffe und Korrosion?