Accuracy of tracer diffusion measurements in iron-chromium system using tracer analysis

A mathematical model has been developed and used to examine the accuracy of the ‘thin film’ approximation commonly used to calculate tracer diffusion coefficients in binary diffusion couples. A range of diffusion conditions relevant to the iron-chromium system has been used to explore the effects of tracer layer thickness, solubility, and annealing time. An equation has been developed which quantifies the error involved in the thin-film approximation for a wide range of diffusion conditions. The equation demonstrates that the accuracy of tracer diffusion coefficients obtained under less than optimum diffusion conditions can be substantially improved. Measured tracer and interdiffusion coefficients in the literature are not consistent with thermodynamic considerations and an attempt is made to explain this in terms of the accuracy of the thin-film approximation used in tracer diffusion measurements. It is concluded that a satisfactory explanation does not exist and that further diffusion studies in the ironchromium system are required.