Meanwhile, a sharp decrease in the intensity of Na + occurred above 340 °C (Fig. ), aligning with the sodium diffusion seen in the NMF maps. The mass spectra also revealed the presence of iron borate, by the detection of 100 FeHBO2 + , as well as other secondary ion fragments reflecting the structure of borate glasses, such as 31 H4BO + , 119 B5O4 + and 138 H3B5O5 + , above 400 °C (Fig. ). In the iron oxide layer, the mass spectra showed the detection of 79 NaFe + at ~200 °C, and the intensity of this peak increased with temperature, indicating the gradual formation of NaFeO2 phases before the sodium borate melted (Fig. ). Similarly, in the chromium-rich layer, the mass spectra included 75 NaCr + above 342 °C, suggesting the formation of NaCrO2 phases. These observations align with prior studies and show the favorable formation of NaFeO2 and NaCrO2 phases at relatively low temperatures.