Early studies of the rates of various transformations and reactions in Fe alloys seemed to indicate that certain elements had an influence upon the rate of C diffusion in γ-Fe. In particular, Co exhibited a pronounced accelerating influence. By direct measurements of the rate of C diffusion in Fe-Co alloys (table 50), this conclusion was confirmed and an increase in the diffusion coefficient was established. An addition of 4%Co decreased the activation energy from 32.5kcal to 30.0kcal; corresponding to an almost twofold increase in the diffusion rate at a 1%C concentration. At higher C contents, the accelerating influence of Co was smaller. This property was unusual in view of the fact that both Mn and Ni had practically no influence upon the diffusion of C.

R.Smoluchowski: Physical Review, 1942, 62[11-12], 539–44