Hard X-ray Photoelectron Spectroscopy (HAXPES) characterisation of electrochemical passivation oxide layers on Al–Cr–Fe complex metallic alloys (CMAs)

• HAXPES characterisation of electrochemical passivation layers for Al–Cr–Fe CMAs
• Oxide and hydroxide signals identified for Al 2s/Cr 2p photoelectron core levels
• Shallow and deep core levels analysed to achieve diverse surface sensitivity
• Cr 18 % at. concentration threshold necessary for stable passivation at pH 1 H2SO4
• X-ray energy variation for cations and hydroxide/oxide distribution identification

A Hard X-ray Photoelectron Spectroscopy (HAXPES) characterisation of the passivation layers formed by electrochemical polarisation of Al–Cr–Fe complex metallic alloys is presented. By employing X-ray excitation energies from 2.3 to 10.0 keV, the depth distributions of Al- and Cr-oxide and hydroxide species in the (Al,Cr)-containing passive layers could be determined. Simultaneous analyses of the shallow Al 2s and deep Al 1s core level lines (respectively, more bulk- and surface-sensitive) provided complementary information to effectively determine the depth-resolved contributions of hydroxide and oxide species within the passivation layer. A Cr threshold concentration of 18 (at.%) was found for effective passivation at pH 1.