As the temperature increased to 350 °C (Fig. ), a distinct spatial separation emerged between 11 B + and Na + . While 23 Na + was observed within the iron oxide regions, B + remained concentrated in the coating. Simultaneously, the NMF map for component 4 showed a more intense distribution of 56 Fe + ions within the borate coating accompanied by a significant decrease in intensity in the areas that were originally iron oxide. Notably, the spatial distribution of Cr + remained consistent under heating, with no signal contribution of these ions in the borate coating. At 412 °C, when sodium largely infiltrated the oxide layers (Fig. ), the NMF maps revealed enhanced signals of Fe + in the borate coating. The distribution of Cr + within the map remained confined to the areas of the chromium oxide layer. Iron present in the borate coating likely originates from the iron oxide layers, implying that portions of these areas were dissolved. Meanwhile, the absence of Cr + signals in the borate coating signifies no or very low level of chromium oxide dissolution at these temperatures.