An in-depth investigation of the native oxide formed on Al-Zn-Mg-Cu alloys by XPS, XRR and ToF-SIMS

The native oxide formed on polished surfaces of two commercial Al-Zn-Mg-Cu alloys of the 7xxx series with high (7.3 wt%) and moderate (6.2 wt%) Zn content was studied to elucidate the surface chemistry and potential influences on corrosion initiation. XPS, ToF-SIMS and XRR were used to analyze surface chemistry, oxidation states of major alloying elements and oxide thickness. Both matrix and Cu containing coarse intermetallic particles were considered. The thicknesses of the hydroxylated matrix oxides were 5.2 ± 0.6nm. A metallic Cu enrichment of ∼10at.% was found at the oxide/metal interface for Cu contents of 1.7 and 2.1 wt%, while no Cu incorporation into the matrix oxide layer was observed. Surface concentrations of Zn and Mg were correlated with the bulk alloy composition. Besides minor Mg enrichment in the oxide, we report an unprecedented amount of Zn near the oxide/metal interface that exceeds the bulk concentration by a factor of ∼2. Significant Zn oxidation was found within the oxide layer, where the metallic/oxidized Zn ratio increases for higher bulk Zn concentrations, potentially altering the surface reactivity during corrosion. Additionally, the oxide on a Cu containing coarse intermetallic particle is thinner than on the matrix with similar segregation mechanisms.