However, as indicated by NMF analysis (Fig. ), iron cations predominantly fulfil this role, causing selective dissolution of iron oxide over chromium oxide. Similar observations were reported previously , attributable to the smaller ionic radius of Fe 3+ compared to Cr 3+ ions with the harder Fe 3+ ion interacting more favorably with hard bases (BO 2 -in this case). The detection of the secondary ion fragments of iron borate compounds in the mass spectra (Fig. ) supports this interpretation. Further examination of the mass spectra from the two oxide layers (Fig. ) revealed an increase in the intensity of a peak assigned to Li + , suggesting the incorporation of lithium within the oxide layers at high temperatures. Lithium may originate from trace contaminants in the sodium borate, which is reported to have a purity of 99.5 wt.%. Examination of the mass spectrum exported from the coating at low m/Q region (Fig. ) suggests the presence of Li + at below 223 °C, albeit at low intensity. As the temperature increased above 223 °C, the Li + peak disappeared in the coating region, while it gradually appeared in the oxide layers (Fig. ). The 7 Li + intensity in the oxide layers gradually increased with temperature and became detectable at much higher intensity at above 500 °C (Fig. ), suggesting a temperature-dependent lithium ion yield. These observations indicate there lithium also diffused from the coating to the oxide scale. NMF analysis, performed on the TOF-SIMS dataset cropped at 580 o C (Fig. ), depicts the spatial distribution of lithium localized within the oxide layers. Lithium may diffuse to and react with the oxide layers in a similar fashion to sodium. The detection of lithium at trace levels in this study highlights the substantial potential of TOF-SIMS analysis for investigating the impact of impurities in solid-state reactions.