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.