The visualization of the chemical propagation fronts in space and in real time in these in situ FIB-SIMS measurements also supports direct measurement of the mass transfer rates. The diffusion coefficient of active elements in solid-state materials have been measured more commonly using radiotracer methods combined with a grinder sectioning , differential calorimetry , in situ TEM heating , and calculations . Here, using in situ FIB-SIMS mapping, we can directly track the motion of sodium ions in the oxide layer to extract a diffusion coefficient of sodium as a function of temperature. Figure provides an overview image of the specimen generated by integrating the 23 Na + intensity from 200-315 °C, illustrating the spread of sodium from the borate coating to the oxide layers, with cyan boxes marking the areas used for the measurement of the transformed segment length, L.