Microstructure evolution and electrochemical corrosion behavior of Al–Zn–Mg aluminum alloy

Ren J., Liu T., Zhang J., Jiang M., Dong Q., Fu C.

Commercial 7050 Al alloys produced by spray forming technique and subsequent heat treatment have been investigated as anodes for Al-air batteries with greatly suppressed self-corrosion. The microstructures of the prepared Al alloy anode have been characterized and compared with those of the cast 7050 Al alloy. The spray formed Al anode possesses finer grains and lower volume fraction and discontinuous distribution of the second phases, leading to a smaller self-corrosion rate. After deformation and heat treatment, the spray formed Al alloy shows a further reduced volume fraction of the second phase and a high fraction of low angle grain boundaries. Therefore, the Al-air battery with the prepared Al anode displays a large specific capacity and a high working voltage. This study demonstrates that commercial Al alloys are likely to be used as anodes for Al-air batteries by the combination of spray forming and proper heat treatment. • Al anodes are prepared by spray forming commercial Al alloys for Al-air batteries. • Spray forming process can eliminate segregation and refine grain size. • The Al-air battery with spray-formed anode displays enhanced discharge performance. • Spray forming can be used to prepare high performance Al anodes at a large scale.

Ma Y., Dong H., Li P., Yang J., Wu B., Hao X., Xia Y., Qi G.

Replacing steel with aluminum alloy is appealing to realize the lightweight of metal structures. Novelis AC 170 PX aluminum alloy and ST06 Z galvanized steel sheets were lap welded by gas tungsten arc welding (GTAW) with ER4047 filler wire and the corrosion behavior of the joint was characterized. The results revealed that the zinc-rich zone had the highest corrosion susceptibility. The difference of localized corrosion between the adjacent area near the rod-shaped and the layered Fe 4 Al 13 phases was explained by galvanic effect. When Zn in the zinc-rich zone was exhausted, the corrosion process of the joint was controlled by the second phases and corrosion products. • Corrosion behavior of aluminum alloy / galvanized steel welded joint was analyzed. • The effect of Fe 4 Al 13 and Al 3 Fe 2 Si 3 phases on corrosion resistance was discussed. • The galvanic corrosion was analyzed from oxide / corrosion products film. • A novel corrosion transformation process of Al/steel dissimilar joint was described.

YUAN D., CHEN S., CHEN K., HUANG L., CHANG J., ZHOU L., DING Y.

The correlations among the corrosion behaviour, grain-boundary microchemistry, and Zn content in Al–Zn–Mg–Cu alloys were studied using stress corrosion cracking (SCC) and intergranular corrosion (IGC) tests, combined with scanning electron microscopy (SEM) and high-angle angular dark field scanning transmission electron microscopy (HAADF-STEM) microstructural examinations. The results showed that the tensile strength enhancement of high Zn-containing Al–Zn–Mg–Cu alloys was mainly attributed to the high density nano-scale matrix precipitates. The SCC plateau velocity for the alloy with 11.0 wt.% Zn was about an order of magnitude greater than that of the alloy with 7.9 wt.% Zn, which was mainly associated with Zn enrichment in grain boundary precipitates and wide precipitates-free zones. The SCC mechanisms of different Zn-containing alloys were discussed based on fracture features, grain-boundary microchemistry, and electrochemical properties.

Zhao Q., Guo C., Niu K., Zhao J., Huang Y., Li X.

The atmospheric corrosion behavior and mechanism of the 7A85 aluminum alloy exposed to the Qingdao industrial-marine atmospheric environment for five years was investigated using the weight-loss method, mechanical property analysis, morphology observations, scanning Kelvin probe force microscopy, and corrosion product analysis. The results showed that the mechanical properties of the exposed 7A85 Al alloy deteriorated considerably, which was mainly induced by pitting corrosion and intergranular corrosion. Al 2 CuMg and Al 7 Cu 2 Fe intermetallic particles were the main precipitates in the 7A85 Al alloy, and the particles not only induced a defective film but also acted as cathode phases in the precipitate–matrix galvanic corrosion couple, which led to pitting corrosion initiation during atmospheric corrosion in Qingdao. Moreover, intergranular corrosion initiated and then propagated owing to micro-galvanic corrosion along the grain boundaries.

Pan Y., Zhang D., Liu H., Zhuang L., Zhang J.

High-performance aluminum alloys are desired for applications that require lightweight materials, but it is challenging to overcome the tradeoff between their strength and intergranular corrosion resistance. To overcome this tradeoff, we have prepared novel Al–Mg–Zn (-Cu) alloys with highly Zn content combined with Cu addition. The alloys with a (Zn + Cu)/Mg ratio below 1.5 are different from traditional 2000, 5000 and 7000 series aluminium alloys. The results show that the number density and lattice spacing of intragranular precipitates increase, while the precipitate sizes decreases with the increase of (Zn + Cu)/Mg ratio, which is the reason for the increase of the alloy strength. Meanwhile, the intergranular corrosion resistance of the alloys is also enhanced by decreasing the potential between grain boundary precipitates and the Al matrix, especially in alloys with high (Zn + Cu)/Mg ratios. These results provide a new method for obtaining high-strength and corrosion-resistant aluminum alloys. • The novel Al-Mg-Zn(-Cu) alloys with (Zn + Cu)/Mg ratio below 1.5 are different from traditional 2000, 5000 and 7000 alloys. • The precipitation hardening of T-Mg 32 (AlX) 49 phase is the main reason to improve the alloy strength. • The IGC resistance of the higher (Zn + Cu)/Mg ratio alloy are mainly depended on the continuity of GBPs.

Xu X., Liu D., Zhang X., Liu C., Liu D.

In this work, ultrasonic surface rolling process (USRP) was utilized to produce a gradient structured layer on 7B50-T7751 aluminum alloy, and the mechanical properties and corrosion fatigue behavior of treated samples were studied. These results reveal that underwent USRP, a 425 μm thick gradient structure and a 700 μm deep compressive residual stress field are created, aluminum grain size become fine(∼ 67 nm), and the corrosion rate of treated surface reduces by 60.08% owing to the combined effect of compressive residual stress and surface nanocrystallization. The corrosion fatigue strength is enhanced to 117% of that of 7B50 Al alloys by means of USRP due to the introduced compressive residual stress, which is considered as the major favorable factor in suppressing the initiation and early propagation of corrosion fatigue cracks. Besides, the gradient structure is an important factor in providing a significant synergistic contribution to the improvement of corrosion fatigue performance.

Xu X., Liu D., Zhang X., Liu C., Liu D., Zhang W.

The surface integrity and corrosion fatigue (CF) performance of the 7B50-T7751 aluminum alloy treated by one (UR1), three (UR3), and six passes (UR6) of ultrasonic surface rolling process (USRP) were investigated in this paper. The results revealed that a modified surface layer with refined microstructure and increased microhardness was formed after the USRP, and a compressive residual stress (CRS) was induced. The CF life of all USRP-treated samples was improved, especially the life of the UR1 sample with an optimal surface integrity. The synergistic effect of surface layer microstructure refinement and CRS effectively have inhibited the occurrence and development of pitting and intergranular corrosion, prevented the initiation of CF cracks at the surface, thus led to the movement of the fatigue crack nucleation site toward the subsurface. Overall, the CRS has played a major critical role. The severe surface damage in the modified layer of the UR6 sample resulted in a partial relaxation of the CRS, in favor of secondary fatigue crack initiation, therefore when using the USRP treatment to improve the CF life of the 7B50 aluminum alloy, the effect of surface integrity should be fully considered.

Abreu C.M., Cristóbal M.J., Figueroa R., Nóvoa X.R., Pena G.

This work reports the corrosion behavior of the AA7075 aluminum alloy under two different tempers (T6 and T73) after nitrogen implantation in 0.5 mol·L−1 NaCl solution. A combination of Electrochemical Impedance Spectroscopy and surface microstructure characterization was used to analyze the effects of nitrogen implantation on corrosion resistance. Nitrogen implantation, at 2 × 1017 N+ ions·cm−2 and 50 keV energy, induces the formation of AlN layer improving the corrosion resistance of aluminum alloys as evidenced by initial corrosion tests and a less corroded area. The evolution of the impedance indicates that the corrosion improvement vanishes over time. Implanted and unimplanted alloys show similar electrochemical parameters after 20 h immersion. However, microscopic examination shows less corrosion damage for implanted alloys after 72 h of immersion. These findings suggest that the implantation only delays the onset of corrosion probably due to degradation of AlN layer in aqueous medium. The corrosion resistance at long periods only depends on the aging heat treatment applied to the AA7075 aluminum alloy and the effect of implantation is negligible.

Liu L.L., Pan Q.L., Wang X.D., Xiong S.W.

The relationship among microstructures, mechanical properties and corrosion behaviors of spray formed 7055 aluminium alloy has been investigated upon peak-aging (T6), double-aging (DA) and retrogression and re-aging (RRA). The research is estimated by hardness test, tensile test, immersion test, detailed microstructure observation and potentiodynamic polarization under different aging treatments. Results demonstrate that the corrosion resistance rank of aging treatments is as follow: DA > RRA > T6. The continuous grains boundary precipitations (GBPs) and coarse Al7Cu2Fe particles increase the corrosion susceptibility. The appearance of the pit cavity attributes to coarse grains and the considerable potential difference between the matrix and GBPs or precipitate-free zones (PFZs) during intergranular corrosion (IGC) process. The wedging stress from hydrogen cracks and the accumulation of the corrosion products help induce the exfoliation corrosion (EXCO). Anodic dissolution and hydrogen embrittlement cause stress corrosion cracking (SCC) of the alloy under different aging treatments. The strengthening mechanism attributes to the distribution of matrix phases (MPs), η′ phase and Al3Zr particles.

Long R.S., Boettcher E., Crawford D.

Aluminum use is growing in automotive closures and body in white applications to improve vehicle performance and fuel economy. The auto industry is looking for higher-strength aluminum materials needed for strength-driven safety–critical parts. Through cooperation with industrial partners and support from the Department of Energy (DOE), multiple experimental 7xxx alloys were developed for automotive applications. The objective is to enable complex shapes to be formed at temperatures below 225°C. A demonstration part has been developed that is representative of the forming challenges within a current hot-stamped door ring component. This part tooling has been built and installed into a press line which includes blank heating and robotic transfer. Forming trials of these alloys are currently underway and the formability, strength and corrosion performance of these materials are being evaluated.

Zuo J., Hou L., Shi J., Cui H., Zhuang L., Zhang J.

An improved low-temperature thermo-mechanical treatment (LTMT based on DYA: dynamic aging) aiming to discover the action of deformation-induced precipitation to microstructures and corrosion resistance was proposed that included a conventional T6 peak aging (120 °C/24 h) and a subsequent dynamic aging. Its influence on mechanical properties, electrochemical behavior and intergranular/exfoliation corrosion resistance of 7055 Al alloy were investigated by tensile testing, polarization curve, intergranular/exfoliation corrosion testing combined with optical microscope and transmission electron microscope observation. It shows that after the conventional T6 peak aging, the matrix precipitates were fine and dispersive while the grain boundary precipitates were coarse and continuously distributed. With the introduction of dynamic aging, the grain boundary precipitates become discontinuously distributed gradually without much coarsening of matrix precipitates. As a result, the LTMT process can improve intergranular/exfoliation corrosion resistances without sacrificing strength that could be attributed to the discontinuous grain boundary precipitates and fine dispersed matrix precipitates, respectively. The improved corrosion resistance was also confirmed by polarization curves and conductivity test. It indicates that the present LTMT process may be a good alternative to produce heat-treatable Al alloy sheets with good strength and corrosion resistance.

Liu J., Wang D., Gao L., Zhang D.

The synergistic inhibition effect of rare earth cerium nitrate and sodium dodecylbenzenesulfonate (DBS) on corrosion of AA5052 aluminium alloy in 3 wt.% NaCl solution was investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curve, scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FT-IR). The results show that the single cerium nitrate or DBS has a limited inhibition effect against corrosion of AA5052 alloy. The combination cerium ions with DBS produced strong synergistic effect on corrosion inhibition for AA5052 alloy and rendered a negaitve shift of the corrosion potential. The formation of the complex of Al(DBS)3 and Ce(DBS)3 stabilized the passive film of Al2O3 and CeO2, retarding both the cathodic and anodic processes of AA5052 alloy corrosion reaction significantly.

Henriksson F., Johansen K.

Weight reduction and material substitution are increasing trends in the automotive industry. In this project, the task of substituting the steel body side in the monocoque of a large SUV towards an ...

Wang D., Yang D., Zhang D., Li K., Gao L., Lin T.

Two quinoline derivatives, 8-aminoquinoline (8-AQ) and 8-nitroquinoline (8-NQ), have been used as inhibitors to examine their corrosion protection effect on AA5052 aluminium alloy in 3% NaCl solution. The weight-loss and electrochemical measurement have indicated that 8-AQ and 8-NQ play as anodic inhibitor to retard the anodic electrochemical process. SEM/EDS analysis clearly shows that 8-AQ and 8-NQ form a protective film on the AA5052 alloy surface. Density functional theory (DFT) calculation confirmed the formation of strong hybridization between the p-orbital of reactive sites in the inhibitor molecules and the sp-orbital of the Al atom. 8-aminoquinoline and 8-nitroquinoline may be useful as effective corrosion inhibitors for aluminium alloys.

Yang X.B., Chen J.H., Liu J.Z., Qin F., Xie J., Wu C.L.

An Al–7.60Zn–2.55Mg (wt.%) alloy with a characteristically high Mg-to-Zn atomic ratio has been investigated for its strength and hardening precipitates in comparison with that of 7150 alloy. It is shown that this alloy yields a rather high strength upon thermal ageing. Interesting is that this high-strength alloy is hardened by the coherent polyhedral T-phase [(AlZn)49Mg32] particles and their early-stage precipitates, which is very different from that of other high-strength AA7xxx AlZnMg(Cu) alloys hardened by the disc-like precipitates of the η-type phases.

Dai Z., Xiao Z., Zhao D., Xiao S., Huang Y., Dai Q.

Multi-scale study on the electrochemical behavior and corrosion mechanism of 5083 aluminum alloy with different microstructures in a NaCl environment.

Yi Z., Xu G., Tang L., Zhao Z., Tang Y., Li H., Wang R., Peng X.

Compared with conventional isothermal creep aging, non‐isothermal creep aging does not involve a prolonged holding stage, but only has heating and cooling stages. During the non‐isothermal creep aging process, the precipitates nucleate and grow up in the early part of heating stage, resulting in fluctuations in creep rate and an increase in strength. The coarsening of precipitates in the later part of the heating stage, can lead to significantly increase of creep rate. Upon approaching the peak temperature, the dissolution of the precipitates occurs in conjunction with a partial coarsening of the remaining precipitates, causing a reduction in strength. However, the secondary precipitation during the cooling stage facilities a significant strength enhancement in a relatively shorter period. In contrast to the isothermal creep aging, the targeted non‐isothermal creep aging treatment gives an increase in ultimate strength while improves the stress corrosion cracking resistance. Moreover, the time required for non‐isothermal creep aging to obtain ultimate strength is only 13.9 % of that of the ICA treatment. A large amount of creep strain can be generated during the NICA process, which is equivalent to 630 % of that of creep age forming treatment.This article is protected by copyright. All rights reserved.

Wang Y., Su R., Jin M., Li G.

The effect of the maximum temperature of non-isothermal aging on the properties of 7075 aluminum alloy was investigated by hardness, friction and wear, as well as intergranular and electrochemical corrosion tests. The results show that when the maximum temperature is 190 °C, the hardness of the alloy is the highest when cooled to 100 °C, which is 183.5 HV. The wear rate is minimal at 4.5×10-3 mm3‧m-1, the mechanical properties exceed that of T6 tempering. The TEM images show that in the high-temperature stage, as the temperature increases, the large-size precipitated phases increase, resulting in a decrease in the mechanical properties of the alloy, and in the cooling stage, "secondary precipitation" as well as the phenomenon of back dissolution occurs. At the same time, the grain boundary precipitation phases of Tmax190 sample were intermittently distributed and staggered, forming the appropriate width of the precipitation-free zone (PFZ). The depth of intergranular corrosion is only 20.1 μm, icorr and Vcorr are 0.61×10-3 mA/cm2 and 0.0198 mm/a, respectively, and the corrosion resistance exceeds that of T73 tempering, while the time is greatly shortened to improve efficiency.

Hao K., Xia W., Li Q., Yan H., Chen J., Su B.

Microalloying is an effective method to improve the properties of Al–Mg alloys. The microstructure, mechanical properties and corrosion behaviours of Al–9.2Mg–0.8Mn– xCu ( x = 0–1.2 wt-%) alloys are studied to promote the application of Al–Mg alloys. The addition of Cu increases the strength of the alloy, where the ultimate tensile strength of 0.6 wt-% Cu alloy is improved by 43 MPa. Moreover, the addition of Cu significantly affects the corrosion behaviours of alloys. For as-sensitised alloys, compared to the matrix, the 0.1 wt-% Cu alloy shows a 28.3% reduction in pitting corrosion mass loss, which is attributed to the Cu element can form a stable passivation film, and the 0.3 wt-% Cu alloy shows a 15% reduction in intergranular corrosion mass loss, which is attributed to the addition of Cu can restrict the β phase precipitation at grain boundaries. The study shows that Cu-alloying can improve the comprehensive properties of Al–9.2Mg–0.8Mn alloys.

Zhao H., Ye L., Cheng Q., Kang Y., Zhang W.

The effect of variable-rate non-isothermal aging (VR-NIA) on the properties of 7055 aluminum alloy was investigated by hardness, electrical conductivity, tensile tests and immersion corrosion, stress corrosion and electrochemical corrosion experiments. A four-stage VR-NIA process is proposed and applied to 7055 alloy, and a good combination of mechanical properties and corrosion resistance can be obtained in a relatively short time. TEM and HRTEM images show that GPⅠ zones, GPⅡ zones and η' phases are formed during the heating process of 100→180 °C, which enhances the mechanical properties; in the process of 180→210→180 °C, GPⅠ zones, part of the small-sized GPⅡ zones and η' phases are redissolved into the matrix, while part of η' phases grows. Meanwhile, the grain boundary precipitates (GBPs) change from continuous to discontinuous, forming the proper width of the precipitate free zone (PFZ), which greatly improves the electrical conductivity and corrosion resistance. In the cooling process of 180→100 °C, "secondary precipitation" of the GPⅠ zones occurs, and the η' phases grow up, with a small part transforming into the η phases, but no remarkable coarsening occurs. During the process of VR-NIA, the content of Zn, Mg, and Cu elements in GBPs is increasing continuously.