Effects Of Aging Conditions And Surface Abrasion On Localized Corrosion Behavior Of 7xxx Series Aluminum Alloys

Effects of aging conditions and surface abrasion on localized corrosion behavior of 7xxx series aluminum alloys were studied systematically. The major experimental work includes:a study of changes of precipitates in the grain and at the grain boundaries and solid solution resulting from single and double-step aging, retrogression and reaging, and non-isothermal aging in 7055 Al alloy using scanning transmission electron microscope and three-dimensional atom probe tomography; effects of compositions of solid solution and precipitates, and typical microstructure on the pitting corrosion damage accumulation and stress-corrosion cracking behavior of 7055 and 7N01 Al alloys in different aging conditions using ASTM B117 salt-spray exposure and slow strain rate tensile tests; microstructure evolution at the near-surface of 7055 and 7N01 Al alloys processed by surface abrasion with grinding paper using focus ion beam and scanning transmission electron microscope; effects of abrasion-induced altered surface layer on localized corrosion behavior of 7055 and 7N01 Al alloys in different aging conditions studied by an in situ observation system during polarization.3D-APT analyses show that Zn, Mg and Cu contents in the η′/η precipitates of 7055 Al alloy increased according to: peak-aged T6 temper < H170 temper < T77 temper < over-aged T73 temper. The overall matrix solid solutes of 7055 Al alloy decreased according to: H170 temper > T6 temper > T77 temper > T73 temper. The matrix Zn, Mg and Cu contents decreased separately according to the above order. TEM-EDS analyses show that the Cu content in the η precipitates at the grain boundary in the T6 samples of 7055 Al alloy was lower than that in the double-step aged, RRA and non-isothermal aged samples. The Cu content in the η′/η precipitates increased with over-aging and increasing the aging temperature.ASTM B117 salt-spray exposure tests show that the susceptibility to pitting corrosion damage of 7055 alloy was found to decrease according to: over-aged samples > under-aged samples > peak-aged sample. The matrix composition of 7055 alloy changed with aging because of precipitation. Thus, the galvanic relationship between Al7Cu2 Fe intermetallic particles and the surrounding matrix correspondingly changed, which affected the susceptibility to pitting corrosion attack. Despite the more active breakdown potential of 7N01 Al alloy, this alloy exhibited less severe pitting corrosion damage than 7055 Al alloy. Furthermore, aging had only a small influence on pitting corrosion damage for 7N01 Al alloy. This was attributed to the weaker galvanic coupling between Al3 Fe intermetallic particles and the surrounding matrix.SSRT results and fracture morphologies show that 7055 Al alloy in the T6 temper exhibited higher susceptibility to SCC, while the over-aged samples exhibited lower susceptibility to SCC. High strength and low susceptibility to SCC were obtained through RRA and non-isothermal aging processes. The tensile strength of the peak-aged samples processed by direct cooling aging in Na Cl solution were close to that of the T6 sample, and the elongation were close to that of the T73 sample. The Cu content in the precipitates at the grain boundary affe cts the susceptibility to SCC. Lower SCC susceptibility of samples processed by direct cooling aging was attributed to high Cu content in the grain-boundary precipitates.An altered surface layer with thickness of several hundred nanometers forms on an Al-Zn-Mg-Cu alloy during surface preparation by abrasion with grinding paper, which contains gradient microstructure. The surface layer contained equiaxial subgrains with size of 50 nm, elongated subgrains along with the final abrasion direction, and high-density dislocations. η′/η precipitates resulting from aging in the surface layer were cut and broken by dislocations and dissolved. The segregation of solute elements along dislocations and subgrain boundaries was observed. The precipitation of Al2 Cu phase at the extreme surface, newly-formed sub-boundaries in the surface layer and the original subgrain/grain boundaries in the underlying bulk substrate. The growth and coarsening of this phase at room temperature was found.Polarization measurements accompanied by in situ observation system show that three potentials at which the current increased rapidly were found for peak-aged 7055-T6 samples. The first breakdown potential corresponded to the transient dissolution of an active surface layer formed during surface preparation by abrading with grinding paper. The second potential at which the current increased rapidly was associated with crevice corrosion and was not a true breakdown potential. The second breakdown potential was associated with pitting of the underlying bulk alloy. Similar behavior was found for over-aged 7N01-T73 sample, which exhibited two breakdown potentials. However, no surface layer attack and a single breakdown potential were observed for 7055-T73 and 7N01-T6 samples. The segregation of Zn atoms at the dislocations and sub-boundaries in the abrasion-induced surface layer made the layer susceptible to corrosion; on the other hand, the Zn and Cu contents in matrix solid solution of the underlying bulk alloy affected the matrix breakdown potential; both of the factors contributed to the difference in breakdown potential between the surface layer and underlying bulk substrate, and thus were responsible for surface layer attack. Furthermore, the 7055-T77 sample abraded to 320 grit exhibited two breakdown potentials, whereas the 400 grit abraded sample exhibited only one breakdown potential. Surface layer attack occurred for both samples, but was absent for the 1200 grit abraded sample, which exhibited one breakdown potential. Abrasion-induced surface layers containing various microstructural features were observed on all surfaces. Rougher surfaces exhibited thicker surface layers with more Zn-enriched sub-boundaries and fewer η′ precipitates. These changes in surface microstructure with different surface roughness resulted in differences in susceptibility of the surface layer, and in the activity gap between the surface layer and underlying bulk matrix, which altered the breakdown behavior of 7055 alloy.