Corrosion behaviour of mechanically polished AA7075‐T6 aluminium alloy

Abstract

In the present study, the effects of mechanical polishing on the microstructure and corrosion behaviour of AA7075 aluminium alloy are investigated. It was found that a nano-grained, near-surface deformed layer, up to 400 nm thickness, is developed due to significant surface shear stress during mechanically polishing. Within the near-surface deformed layer, the alloying elements have been redistributed and the microstructure of the alloy is modified; in particular, the normal MgZn 2 particles for T6 are absent. However, segregation bands, approximately 10-nm thick, containing mainly zinc, are found at the grain boundaries within the near-surface deformed layer. The presence of such segregation bands promoted localised corrosion along the grain boundaries within the near-surface deformed layer due to microgalvanic action. During anodic polarisation of mechanically polished alloy in sodium chloride solution, two breakdown potentials were observed at −750 mV and −700 mV, respectively. The first breakdown potential is associated with an increased electrochemical activity of the near-surface deformed layer, and the second breakdown potential is associated with typical pitting of the bulk alloy.

Microstructure Evolution in the Near-Surface Region During Homogenization of a Twin-Roll Cast AlFeMnSi Alloy

“…[4,6,[8][9][10][11][12][13][14][15][16][17][18] The near-surface deformed layers are characterized by an ultrafine-grained microstructure, consisting of a large population of grain boundaries which represents high stored energy. At elevated temperatures, the ultrafine grains in the near-surface deformed layer are susceptible to grain growth.…”

Near-Surface Deformed Layers on Rolled Aluminum Alloys

“…The near-surface deformed layer has a significant influence on the electrochemical and corrosion behavior, the joining and welding behavior, optical properties, and potentially mechanical properties of the alloys such as formability and bendability. [5,[9][10][11][12][13][14][15][16][17][18][19] Understanding the evolution of the near-surface deformed layer through conventional fabrication processing is, therefore, both of technological importance and of significant scientific interest.…”

“…[1][2][3][4] Although conventional fabrication and finishing processes, for example rolling, do not strain the bulk material to the same extent as ECAE, it has been shown that the surface and near-surface regions of alloy sheet are subjected to enhanced shear deformation. As a consequence, a heavily deformed near-surface layer, up to a few microns in thickness, comprising ultrafine grains with oxide-decorated boundaries and fine dispersoids, [5][6][7][8][9][10][11][12][13] is developed. The near-surface deformed layer has a significant influence on the electrochemical and corrosion behavior, the joining and welding behavior, optical properties, and potentially mechanical properties of the alloys such as formability and bendability.…”

Effect of Post Weld Heat Treatments on Mechanical and Stress Corrosion Cracking Behaviour of Aa7075 Friction Stir Welds

“…Liu [12] reported that that overaging and dissolving of the metastable precipitates lead to the decrease in the hardness in the WZ. But the presence of the fine equiaxed grains and the resolution of the dissolved precipitates partially remedy the loss of hardness.…”