Mechanism of low exfoliation corrosion resistance due to slow quenching in high strength aluminium alloy

Pitting corrosion of naturally aged AA 7075 aluminum alloys with bimodal grain size

“…A concurrent decrease in zinc, magnesium content and increase in copper content would make η-phase less anodic. Therefore, the increase in copper content shifts the potential of η-phase towards noble direction, while the presence of zinc and magnesium would decrease the potential of η-phase and make it more active [9,10,40,43,66,76,[81][82][83][84][85][86][87]. In this study, the η-phase in coarse grains has bigger size and higher content of magnesium, zinc and copper compared to that in fine grains.…”

“…As a result, the positive effect of copper on corrosion resistance of η-phase is likely minor and can be overwhelmed by the negative effects of magnesium and zinc. This phenomenon has been proved by Liu's research [66] which showed that more increments of magnesium and zinc, and less increment of copper made the η-phase more active as anode. Therefore, the η-phase in coarse grains shows higher activity, resulting in larger potential difference between them and the surrounding matrix; consequently, they corroded easily during corrosion.…”

“…In the artificial aging condition, with a relatively high temperature (usually higher than 120 o C), the copper can substitutes the zinc sites in η-phase, therefore the copper content will increase with prolonging aging time. This is the main reason why over-aged alloys usually have a higher copper content in η-phase and thus a better corrosion resistance than peak-aged one [9,10,40,43,66,84,[81][82][83][84][85][86][87]. In this work, the natural aging was performed near room temperature (30-35 o C), the copper diffusion should be very slow and restricted.…”

Influence of grain structure on quench sensitivity relative to localized corrosion of high strength aluminum alloy

“…Apart from grain boundary microstructural features, grain boundary microchemical features especially the chemical compositions of GBPs can have significant effect on resistance to exfoliation corrosion [19], and therefore a number of GBPs in the aged samples cooled at 630 C/min and 138 C/min were examined using fine probe EDS measurements in the STEM-HAADF mode. The probe size was 2 nm, so it is likely to attain accurate results.…”

“…Apart from aging, quenching has great effect on the precipitation state at grain boundaries, and therefore can affect resistance to EXCO. For instance, in some typical alloys such as 7075, 7449 and 7055, the resistance to EXCO decreases with the decrease of quench rate after solution heat treatment [17e19,39e41]; and the mechanism has been recently discussed in detail [19]. But when grain structure is changed in the alloy, how this quench rate effect changes is not clear and needs to be studied.…”