The passive films formed on Fe-20Cr and Fe-20Cr-4Mo stainless steels in a pH 8.5 borate buffer solution at 30 °C were investigated using corrected scanning transmission electron microscopy to clarify the influence of molybdenum (Mo) on film structure and composition. Both alloys are considered to form an amorphous-like passive film and the presence of Mo resulted in a modest increase in film thickness. Compositional depth profiling via electron energy loss spectroscopy and X-ray photoelectron spectroscopy revealed chromium (Cr) enrichment within the films due to selective Fe dissolution, while Mo content remained largely uniform. The incorporation of Mo was found to enhance Cr enrichment, implying a synergistic effect on the stability of the passive film and contributing to improved corrosion resistance.
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