Effects of retrogression and reaging heat treatment on the microstructure,exfoliation corrosion,electrical conductivity,and mechanical properties of AA7050

The effects of different retrogression times and temperatures on the microstructure, electrical conductivity, exfoliation corrosion (EXCO), and hardness of the 7050 aluminium alloy were compared with the T6 and T7451 condition. The EXCO solution was prepared in accordance with ASTM G34‐01 standard and the samples were kept in the solution for 48 hr. Corrosion depths were examined by using scanning electron microscope (SEM) in the cross‐sections microstructure of the samples. The effect of EXCO on the material was investigated using stereo optical microscope images. In microstructure investigations, SEM and energy dispersive X‐ray (EDX) analyses were used to determine the distribution and chemistry of the precipitates at the grain boundaries. The results show that retrogression and reaging (RRA) times and temperatures have significant effects on AA7050 EXCO. Especially after 200°C/30 min RRA process, it has been found that 7050 aluminium alloy has higher mechanical strength than T6 level and high corrosion resistance from T7451 level. According to the results of this study, it has been shown that there is a direct connection between electrical conductivity and EXCO.     

Effects of Severe Cold Rolling on Exfoliation Corrosion Behavior of Al-Zn-Mg-Cu-Cr Alloy

The exfoliation corrosion (EXCO) behavior of Al-Zn-Mg-Cu-Cr alloy after severe cold rolling was investigated by optical microscope, transmission electron microscope, and electrochemical technique. The results show that the EXCO resistance decreased with increasing cold rolling reduction because of the grain boundaries which were decorated with the continuously distributed particles. After the solution treatment, the samples with different reductions retained the fibrous grains, and the elongated grains accelerated the growth of the corrosion cracks according to crack propagation analysis. Furthermore, the increase of deformation enhanced the degree of recrystallization, while the number of corrosion cells increased greatly when in the electrolyte, which tended to reduce the resistance to EXCO.     

Al-Zn-Mg-Cu系铝合金抗剥落腐蚀性能研究

通过观察经剥落腐蚀试验的Al-Zn-Mg-Cu系铝合金试样显微组织,研究了不同成分、热处理及轧制工艺对Al-Zn-Mg-Cu系铝合金的抗剥落腐蚀（EC）性能的影响。结果表明,添加稀土元素Ce的试样抗剥蚀性能低于未添加的合金试样;经过回归再时效处理（RRA）的合金试样抗剥蚀性能优于峰值时效（T6）的合金试样;随着成品冷轧总压下率的增加,合金抗剥蚀性能呈先下降再略上升的趋势,其中10%的成品冷轧总压下率的合金试样抗剥蚀性能最优。     

Effect of Aging Treatments on the Mechanical and Corrosive Behaviors of Spray-Formed 7075 Alloy

Mechanical properties, microstructure, exfoliation corrosion (EXCO), and intergranular corrosion (IGC) behaviors of the spray-formed 7075 aluminum alloy after T6, T73, retrogression (R), and re-aging (RRA) treatment, respectively, were studied by using tensile tester, transmission electron microscope, and scanning electron microscope. The results show that the T6 process can increase the ultimate tensile strength (UTS) up to 760 MPa, while it decreases the elongation, the EXCO, and the IGC resistance of the alloy. The T73 process can improve elongation, the EXCO, and the IGC resistance of the alloy. The corrosion resistance of the alloy can also be improved by R and RRA processes with retrogression times increase. The tiny precipitated phases distributed homogeneously in the matrix can increase the UTS. The close-connected discrete grain boundary phases (GBP) and the narrow precipitate free zones (PFZ) will lower the elongation, the EXCO, and the IGC resistance of the alloy. Contrarily, the discrete GBP and wide PFZ can improve the elongation, the EXCO, and the IGC resistance of the alloy. The EXCO and the IGC behaviors for the spray-formed 7075 alloy after different aging treatments have been established according to the standards of ASTM G34-2001 (2007) and ASTM G110-1992 (2009).     

Effect of RRA heat treatment on corrosion and mechanical properties of AA7075

In this study, a retrogression process (R) has been applied to AA7075 at 180, 240, and 370°C for 15, 30, 90, and 120 min. After the retrogression, reaging treatment (RA) was reapplied with T6 conditions (120°C for 24 hr). The effect of different temperatures and time periods of retrogression and reaging temper on exfoliation corrosion (EXCO) of the AA7075 is examined. The EXCO solution was prepared in accordance with the ASTM G34‐01 standard and the samples were kept in the solution for 48 hr. Susceptibility of the alloy to EXCO was determined by weight loss detection, macro and microstructure studies, scanning electron microscope, transmission electron microscope, and energy dispersive X‐ray analyses. The mechanical properties of aged samples were determined by hardness, tensile tests, and fractography.     

时效处理对新型Al-5.6Zn-1.6Mg-0.15Zr合金显微组织及耐腐蚀性能的影响

通过晶间腐蚀试验(IGC)、剥落腐蚀试验(EXCO)及电化学腐蚀试验,结合光学显微镜(OM)、扫描电镜(SEM)及配套能谱分析仪(EDS)和透射电镜(TEM)等手段,研究Al-5.6Zn-1.6Mg-0.15Zr合金板材在120℃下时效不同时间后微观组织、腐蚀状态及耐腐蚀性能的变化.结果表明:新型Al-5.6Zn-1....     

LY12铝合金剥蚀行为的研究

用金相法，电阻法等研究ＬＹ１２铝合金板材在ＥＸＣＯ溶液中的剥蚀行为，该合金的剥蚀是从点蚀发展到晶间腐蚀，并在内应力的协同作用下产生层状开裂与剥落。腐蚀深度与深度时间的对数呈直线关系，但分成有不同斜度的两个阶段。峰值时效合金的剥蚀敏感性比自然时效合金高。     

ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY DURING CORROSION PROCESS OF 8090 Al-Li ALLOY IN EXCO SOLUTION

1.IntroductionAl-Li alloys, compared to traditional Al alloys, have more excellent properties, such as lower density, greater elastic modulus and higher specific strength[1,2]. In the near future, they would be widely applied to airplane structures. While, exfoliation, a main kind of localized corrosion, lowers their strength, plasticity and fatigue properties, and also decreases their service life[3, 4]. So investigating their exfoliation would be very important to their application.Usua…     

Exfoliation corrosion of 7150 Al alloy with various tempers and its electrochemical impedance spectroscopy in EXCO solution

The exfoliation corrosion susceptibility and electrochemical impedance spectroscopy (EIS) of 7150 Al alloys with T6, T73, and RRA (retrogression at 175 °C for 3 h) tempers in EXCO solution were investigated. The anodic equilibrium precipitate η(MgZn₂) is continuous or closely spaced at the grain boundaries in the 7150‐T6 Al alloy, resulting in its greatest susceptibility to exfoliation corrosion. The grain boundary η precipitates in the RRA and T73 treated 7150 Al alloys are coarsened and show a clear discontinuous nature; they possess similar exfoliation corrosion sensitivity and their exfoliation corrosion resistance is greatly increased. At the beginning of immersion in EXCO solution, the EIS plot of the 7150 Al alloys is composed of a capacitive arc in the high to medium frequency range and an inductive component in the medium to low frequency range. As immersion time is increased, exfoliation corrosion with different corrosion ratings occurs on the surface of the 7150 Al alloy with various tempers, two capacitive arcs appear in the high to medium and medium to low frequency ranges, respectively. The fitted medium to low frequency capacitance C₂ of 7150‐T6 Al alloy, corresponding to the new surface caused by the exfoliation corrosion, is much greater than that of the T73 and RRA treated 7150 Al alloy, which is consistent with the greatest exfoliation corrosion susceptibility of the 7150‐T6 Al alloy.     

含钪Al-Cu-Li-Zr合金的剥蚀性能及电化学阻抗谱

采用剥落腐蚀(Exfoliation corrosion,EXCO)实验和电化学阻抗测试方法,研究时效对新型含钪Al-Cu-Li-Zr合金剥蚀性能的影响。结果表明:合金在EXCO溶液中剥落腐蚀敏感性由高到低的顺序为过时效,峰时效,欠时效;合金在EXCO溶液中浸泡初期,其电化学阻抗谱由一个高频容抗弧和低频感抗弧组成,且随浸泡时间的延长,低频感抗部分逐渐减弱直至消失;一旦发生剥蚀,合金的电化学阻抗谱出现两个部分重叠的容抗弧。依据腐蚀特征和电化学原理设计了等效电路图,对合金腐蚀发展过程的电化学阻抗谱进行了拟合,拟合数据和实验结果一致。     

用时域法EIS评估LY12CZ铝合金的膜下腐蚀

通过对表面带有硫酸阳极化膜和H06－1012H航空环氧底漆的LY12CZ铝合金型材进行剥蚀敏感性加速腐蚀试验（EXCO），采用时域法Laplace转换EIS和截面金相观察来衡量铝合金的膜下腐蚀，结果表明：时域法EIS测出的试样Rd、Cg能够评估膜下腐蚀的损伤程度；不同膜下腐蚀形态对应着不同的Rd、Cg的范围。Rd和Cg随腐蚀时间的变化趋势可以反映基体金属点蚀的萌生、漆膜鼓泡和基体出现剥蚀的历程，大量平行数据的统计分析表明：logRd符合Gumbel分布，logGg符合正态分布；由于logRd服从Gumbel分布，故可以根据小面积试样的logRd值，对大面积试样腐蚀最严重处的状况有所估计，大面积试样在EXCO溶液中浸泡时，其logRd随时间呈指数下降。     

Exfoliation corrosion susceptibility of 8090 Al-Li alloy examined by electrochemical impedance spectroscopy

The exfoliation corrosion susceptibility and electrochemical impedance spectroscopy(EIS) of rolled and peak-aged 8090 AI-Li alloys in EXCO solution were studied, and the EIS after exfoliation was simulated. Once exfoliation occurs, two capacitive arcs appear in the EIS at high-mediate frequency and mediate-low frequency respectively. The exfoliation-attacked alloy surface consists of two parts, an original flat alloy surface and a new interface exposed to EXCO solution due to the exfoliation. The capacitance corresponding to the new exfoliation inter-face increases approximately linearly with time at early exfoliation stage, due to the enlargement of the new inter-face.Then it maintains stable, due to the corrosion product covering on the new inter-face. The exfoliation susceptibility can be judged through the average slope of the capacitance vs time curve of the early exfoliation stage. This average slope of the rolled 8090 alloy is much higher than that of the peak-aged 8090 alloy, accordingly the rolled 8090 alloy is more susceptible to exfoliation than the peak-aged 8090 alloy.     

0.1 mol/L NaCl溶液中不同剥蚀程度LY12CZ合金的EIS特征

通过EXCO溶液浸泡腐蚀实验制备了不同剥蚀程度的LY12CZ合金试样.研究了不同剥蚀程度的LY12CZ合金在0.1 mol/L NaCl溶液中的电化学阻抗谱(EIS)特征.结果表明不同剥蚀程度的IY12CZ合金在0.1 mol/L NaCl溶液中,EIS呈现不同特征;根据电化学阻抗谱特征可区分铝合金的剥蚀状态.研究结果为铝合金构件剥蚀的现场监检测提供了必要的依据.     

ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY DURING CORROSION PROCESS OF 8090 A1-Li ALLOY IN EXCO SOLUTION

The corrosion behavior and electrochemical impedance spectroscopy (EIS) features of 8090 AI-Li alloys in EXCO solution were investigated, and the EIS was simulated using an equivalent circuit. At the beginning of immersion in EXCO solution, the EIS is comprised by a depressed capacitive arc at high-mediate frequency and an inductivearc at low frequency, and the inductive component decreases and disappears with immersion time. Once exfoliation or severe pitting corrosion is produced, two capacitive arcs appear in the EIS. These two capacitive arcs are originated from the two parts of the corroded aUoy surface, the original flat alloy surface and the new inter-face exposed to the aggressive EXCO solution due to the exfoliation or pitting corrosion.Some corrosion development features of 8090 AI-Li alloys in EXCO solution can be obtained through simulated EIS information.     

Exfoliation and intergranular corrosion resistance of the 2198 Al–Cu–Li alloy with different thermomechanical treatments

In this study, the resistance to exfoliation and intergranular corrosion (IGC) of the 2198 Al–Cu–Li alloy submitted to different thermomechanical treatments (T3, T8, and T851) was investigated. The tests were carried out following the standard practices, ASTM G34-18 and ASTM G110-15, respectively. All the tested alloys showed susceptibility to exfoliation and some alloys showed susceptibility to IGC, but the artificially aged alloys presented a higher tendency to exfoliation. The extensive hydrogen evolution reaction (HER) was observed on the surfaces of artificially aged alloys when immersed in the EXCO solution. The HER resulted in an increase in solution pH with the time of immersion. Also, the weight losses related to the artificially aged alloys were higher than that of the naturally aged ones. The T8 treatment was the only condition that resulted in susceptibility to both, intergranular and transgranular corrosion, whereas the T851 treatment did not show IGC susceptibility, only transgranular corrosion. Finally, the 2198-T3 condition showed the highest corrosion resistance among the thermomechanical treatments tested. The results of the 2198 alloy subjected to various treatments were compared with that of the 2024-T3 alloy. This last alloy showed higher resistance to exfoliation and IGC as compared with the 2198 alloy.     

Exfoliation corrosion of T6- and T8-aged AlxCuyLiz alloy

The exfoliation corrosion behavior of a novel Al-Li alloy treated by T6- and T8-peak ageing was studied by electrochemical impedance spectroscopy(EIS) technique. The surface morphology of corroded samples was examined by scanning electron microscope(SEM). The microstructure of un-corroded samples was observed by transmission electron microscope(TEM). At early stage of immersion in EXCO, EIS plots of the two differently processed samples are composed of a capacitive arc in the high frequency range and an inductive loop in the low frequency range. Inductive loop disappears with the increasing of immersion time and two capacitive arcs appear. T6-treated alloy has higher exfoliation susceptibility than T8-treated one, suggested by different exfoliation starting time, which is 23 h and 27 h respectively. T1 phase and equilibrium precipitate at the grain boundary of T6-treated alloy are larger in amount and size than those of T8-treated alloy. This is the main reason for the higher exfoliation susceptibility of T6-treated alloy.     

Influence of various aging treatments on microstructure,strength and corrosion behaviour of high Zn content Al–Zn–Mg–Cu alloy

In this paper, the influence of T6, T74 and RRA aging treatments on microstructure, strength and corrosion behaviour of high Zn content Al–Zn–Mg–Cu alloy was investigated by tensile properties tests, inter-granular corrosion (IGC) tests, exfoliation corrosion (EXCO) tests, polarisation tests, metallographic microscope and transmission electron microscopy (TEM) analysis. The results show that the T74 and RRA temper can increase the size and the distribution discontinuity of the grain boundaries precipitates (GBPs), thus leading to improvement of the corrosion resistance. However, with the coarser matrix precipitates (MPs) relative to T6 treatment, RRA and T74 temper both have a decrease in strength. Besides, all the performances (including mechanical properties and corrosion properties) of the RRA treatment show an intermediate level relative to T6 and T74. Therefore, we can select the appropriate heat treatment process according to the different performance requirements in the industrial production.     

The effect of applied tensile stress on the exfoliation corrosion of 7075‐T6 alloy

The effect of the applied tensile stress on the exfoliation corrosion of 7075‐T6 alloy in EXCO solution has been studied. Electrochemistry Impedance Spectroscopy (EIS), Scanning Electron Microscope (SEM) and optical microscope techniques were used to characterize the initiation and propagation of the exfoliation corrosion (EC) under different applied tensile stress levels. Two equivalent circuit models were developed to fit the EIS data of the initiation and propagation stages of EC according to their corresponding physicochemical properties respectively. The correlation between the characteristics of EIS evolution and the localized corrosion types was established, and a new criteria for detecting the initiation of EC of aluminum alloys was brought forward. The results also showed that the tensile stress accelerates EC by increasing the number of the active sites during its initiation stage.     

Exfoliation corrosion and electrochemical impedance spectroscopy of an Al‐Li alloy in EXCO solution

The exfoliation corrosion and electrochemical impedance spectroscopy (EIS) of an Al‐2.8%Cu‐1.5%Li‐0.3%Mg‐0.3%Zn‐0.3%Mn‐0.15%Zr alloy with various aging states in EXCO solution were investigated. The equilibrium precipitates at grain boundaries are anodic to the alloy base at their adjacent periphery. With prolonging aging time, the amount and the size of the equilibrium precipitates at grain boundaries are increased, resulting in an enhanced susceptibility to exfoliation corrosion. At the beginning of immersion in EXCO solution, the EIS plot of the alloys is composed of a capacitive arc in the high frequency range and an inductive loop in the low frequency range. As immersion time is increased, two capacitive arcs appear in the high‐mediate and mediate‐low frequency ranges respectively and the appearance time of two capacitive arcs could be an indication of the speed of localized corrosion development in EXCO solution. The longer appearance time of two capacitive arcs of the under‐aged alloy indicates its slower localized corrosion development.     

时效处理对铝锂合金电子束焊接头耐蚀性能的影响

对铝锂合金电子束焊接头进行焊后热处理,研究了时效处理前后接头各区域的晶间腐蚀、剥蚀和电化学腐蚀行为。结果表明,接头经过时效处理后,焊缝晶界析出的T1(Al_2CuLi)相数量增加,并且形成了明显的晶界无沉淀带(Precipitate Free Zone,PFZ)。焊态下接头未出现晶间腐蚀,热影响区和母材区均出现了孔蚀;焊后时效处理增大了接头的晶间腐蚀倾向,热影响区同时发生了孔蚀和晶间腐蚀,母材区出现了严重的晶间腐蚀。焊态下焊缝和热影响区均具有优异的抗剥蚀能力,母材区对剥蚀的敏感性较高;焊后时效处理可提高接头母材区的抗剥蚀能力,但会增大热影响区的剥蚀敏感性。电化学腐蚀测试表明,与时效后的接头焊缝相比,焊态下焊缝的自腐蚀电位较高,腐蚀电流密度小,具有相对较好的耐蚀性。