Evolution of surface characteristics of two industrial 7xxx aluminium alloys exposed to humidity at moderate temperature

Abstract

This study analyses the evolution of surface characteristics of two industrial high‐strength 7xxx aluminium alloys with a focus on alloy composition and environmental parameters. Based on storage and transport conditions of as‐machined products, the effect of humidity—as liquid and vapour phase—on the natural oxide layer has been studied. The evolution of the natural oxide layer has been analysed by scanning electron microscopy and X‐ray photoelectron spectroscopy. The growth behaviour of the surface layer is dominated by environmental conditions, while microgalvanic activity depends mainly on the alloys' chemical composition and differs significantly for tested alloys. Scanning transmission electron microscopy images demonstrated that the long‐term exposure at moderate temperatures affects the microstructure near the surface, which differs for the analysed alloy compositions. An anomalous precipitation of zinc‐rich particles at the surface and along the precipitate‐free zone is observed for the alloy with higher Zn/Mg ratio and lower Cu content.

Revisiting stress-corrosion cracking and hydrogen embrittlement in 7xxx-Al alloys at the near-atomic-scale

“…A two-layer structure appears, with first a ~5 nm zone enriched in Mg and O, followed by a region with increased Zn and lower in O and Mg. O/H increase again further ahead, but Mg and Zn do not show increased levels. The formation of an Mg-rich oxide film in aluminium alloys was suggested by Green et al 38 and reported experimentally 39 , 40 . The oxide film is also found to be hydrated, which is in line with previous observations 3 , 39 , 41 .…”

“…The formation of an Mg-rich oxide film in aluminium alloys was suggested by Green et al 38 and reported experimentally 39 , 40 . The oxide film is also found to be hydrated, which is in line with previous observations 3 , 39 , 41 . Levels of Cl of up to 14 at.%, not included in the profiles for clarity, are also incorporated in the oxide film, as expected 4 .…”

“…Similar APT observations of substantial compositional changes and associated dissolutions in oxidised and stressed regions in superalloys had previously been reported 51 , 65 . In Al alloys, if corrosion is occurring at a high enough temperature, Schwarzenböck et al, and Alani and Swann 39 , 58 show Zn-rich precipitates forming in this solute-rich subsurface region, following oxidation in high humidity environments at 70 °C and 80 °C respectively. These observations are consistent with Mg diffusion to the oxide, precipitate dissolution and re-precipitation, similarly to what they observed in the matrix.…”

“…2g ). Precipitate dissolution and solute transport to an oxidising surface, possibly accelerated by diffusion along defects, occurs readily in the absence of stress 39 , 58 . This was referred to as the “reservoir effect” by Young and Gleeson 59 and has been observed in other systems like Cr-containing Fe-based alloys 60 for instance.…”

A New Method of Edge Crack Suppression in the Rolling Process of Magnesium Alloy Sheet and Study on the Method of Cathodic Protection

“…In the process of rolling, the embrittlement would result in cracks in the middle of the magnesium alloy that affects the quality of the magnesium alloys [31][32][33]. erefore, it is necessary to dehumidify in the rolling environment [34,35].…”