On nitrogen diffusion during the internal nitridation of Ni-based alloys

The utility of Wagner's theory of internal precipitation in evaluating the nitrogen permea tion through nickel has been discussed. Nitridation experiments on Ni-Cr alloys at 1398 K under 6000 bar of N-2 pressure give evidence of ''up-hill'' diffusion of nitrogen in the matrix of the reaction zone. This is attributed to the attractive Cr-N interaction in the FCC Ni-Cr solid solution. Such a behaviour is contrary to the boundary conditions in Wagner's analysis. It was shown that during nitriding at extremely high effective nitrogen fugacity (like under flowing NH3+H-2 -gas mixtures) the ''up-hill'' diffusion of nitrogen through the matrix of the precipitation zone does not occur and the boundary conditions for Wagner's treatment can be fulfilled. In order to obtain useful permeability data the external nitrogen activity in the gas phase at the gas/metal interface needs to be specified.