Passive oxide film characterisation on Al-Cr-Fe and Al-Cu-Fe-Cr complex metallic alloys in neutral to alkaline electrolytes by photo- and electrochemical methods

The formation, chemical composition and especially electrochemical reactivity of the passive oxide formed on the Al64.2Cr27.2Fe8.1 (at. %) and Al66.9Cu11.6Fe11.6Cr10.6 (at. %) complex metallic alloys (CMAs) were investigated in borate buffer electrolyte of pH8.4 and 0.1 M Na2SO4 electrolyte of pH 12. Open Circuit Potential evolution, potentiodynamic and potentiostatic anodic polarisation as well as electrochemical impedance measurements revealed high stability of the investigated materials in the alkaline domain relevant to cathodic areas. The high stability of these phases is attributed to the presence of a passive oxyhydroxide layer consisting of thin chromium oxyhydroxide as outermost layer and an inner aluminium oxyhydroxide layer close to the metal/oxide interface as found by XPS. The semiconducting properties characterised by means of photoelectrochemistry reveal further that the bandgap of the passive oxide on the CMAs corresponds to a less hydrated situation (closer to alumina) compared to the one grown on pure Al, thus supporting the higher stability of the CMAs phases.