How solute atoms control aqueous corrosion of Al-alloys

Abstract

Aluminum alloys play an important role in circular metallurgy due to their good recyclability and 95% energy gain when made from scrap. Their low density and high strength translate linearly to lower greenhouse gas emissions in transportation, and their excellent corrosion resistance enhances product longevity. The durability of Al alloys stems from the dense barrier oxide film strongly bonded to the surface, preventing further degradation. However, despite decades of research, the individual elemental reactions and their influence on the nanoscale characteristics of the oxide film during corrosion in multicomponent Al alloys remain unresolved questions. Here, we build up a direct correlation between the near-atomistic picture of the corrosion oxide film and the solute reactivity in the aqueous corrosion of a high-strength Al-Zn-Mg-Cu alloy. We reveal the formation of nanocrystalline Al oxide and highlight the solute partitioning between the oxide and the matrix and segregation to the internal interface. The sharp decrease in partitioning content of Mg in the peak-aged alloy emphasizes the impact of heat treatment on the oxide stability and corrosion kinetics. Through H isotopic labelling with deuterium, we provide direct evidence that the oxide acts as a trap for this element, pointing at the essential role of the Al oxide might act as a kinetic barrier in preventing H embrittlement. Our findings advance the mechanistic understanding of further improving the stability of Al oxide, guiding the design of corrosion-resistant alloys for potential applications.

Fig. 1. Corrosion properties and element-specific dissolution rates of an AlZnMgCu alloy in 0.01 M KCl.

a The potential applied for the ICP-MS measurement. Inset is a schematic illustration of the ICP-MS. b Online ICP-MS dissolution profiles of current densities. c–f Online ICP-MS dissolution profiles of dissolved metal ions as a function of time. c As solution heat treated (475 °C, 24 h). d, Under-aged (120 °C, 2 h). e Peak-aged (120 °C, 24 h). f Over-aged (120 °C, 24 h + 180 °C, 6 h). Cu is shown on the right axis label.

Fig. 2. APT analysis on the distribution of individual atoms and composition of the as quenched Al-2.69 Zn-2.87Mg-0.95Cu alloy (at. %) after immersion in 0.1 M KCl in D₂O for 3 h.

a Atom maps of Al, O, Mg, Zn, Cu, and D. b One-dimensional composition analysis across the oxide. c One-dimensional compositional analysis across the grain boundary. D is shown on the right axis label. The shaded bands of the traces represent the standard deviations in each bin of the composition profile. GB grain boundary.

Fig. 3. APT analysis of the peak-aged (120 °C, 24 h) Al-2.69 Zn-2.87Mg-0.95Cu alloy (at. %) after immersion in 0.1 M KCl in D₂O for 3 h.

a Distribution of O and Al along with iso-surfaces of 10 at.% Zn highlighting the precipitates. b Atom maps of Al, O, Mg, D, Zn, and Cu. c One-dimensional composition analysis across the oxide. d, e Precipitate composition and matrix composition with average Zn to Mg ratios analyzed in three neighboring zones, with zones 1–3 marked in pink, purple, and red rectangles in Fig. 3a respectively. These three regions are: zone 1 directly below the oxide where all the former (Mg,Zn)-rich precipitates were dissolved, a transition zone 2 below this region, and zone 3 about 50 nm below the oxide. The reference is referred to the uncorroded sample reported in previous work. The shaded bands of the traces represent the standard deviations in each bin of the composition profile.

Fig. 4. STEM analysis of a peak-aged Al-Zn-Mg-Cu alloy after exposure to 0.1 M KCl for 3 h.

a Low magnification HAADF image showing the cross-section from the corroded surface (covered by Pt) to oxide and Al matrix. b STEM-EELS analysis of the oxide and Al matrix. The two EELS spectra are normalized by their maximum counts for a better comparison. c FFT patterns obtained from different regions near the oxide.

Fig. 5. Potential-pH diagrams for the Al-2.69 Zn-2.87Mg-0.95Cu alloy (at. %) in 0.1 M KCl at 25 °C and 1 atm.

a Potential-pH diagram showing equilibrium phase regimes of oxides, aqueous solution (Aqs.), and alloy matrix (FCC). b–d Potential-pH diagrams showing the equilibrium dissolution characteristics of matrix phase: b Decrease of Al matrix content (moles) as a function of potential and pH. c, d Equilibrium compositions of Zn and Cu of dealloyed solid matrix. Reversible potentials for H evolution are highlighted as gray dashed lines.