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


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)



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