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.     

2B96K铝锂合金剥蚀过程的电化学阻抗研究

研究了不同时效状态下的2896K铝锂合金在EXCO溶液中剥蚀的电化学阻抗特征.结果表明,腐蚀初期,合金的电化学阻抗谱由一个容抗弧及一个感抗弧组成,一旦发生剥蚀,即变成两个容抗弧.对于欠时效合金,这两个容抗弧出现的时间较长,表明其剥蚀发展较慢.对电化学阻抗谱进行了拟合分析,得到了剥蚀的相关信息,如剥蚀的发展及形貌等.     

Research on intercrystalline corrosion,exfoliation corrosion,and stress corrosion cracking of Al–Zn–Mg–Sc–Zr alloy

The intercrystalline corrosion, exfoliation corrosion (EXCO), and stress corrosion cracking (SCC) of Al–Zn–Mg–Sc–Zr alloy were investigated by means of constant temperature immersion corrosion method, optical microscopy, transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS). The results show that intercrystalline corrosion, and EXCO susceptibility of Al–Zn–Mg–Sc–Zr alloy decrease gradually with increasing of aging time. Corrosion susceptibility order from low to high is as follows: OA > PA > UA > NA. The SCC susceptibility index of PA temper is more than OA temper at the same strain rate. According to TEM observation, with aging time prolonging, a part of η′ phases transform to η equilibrium phases, which become coarse gradually. The distribution discontinuity of the grain boundary precipitates increases. In addition, for Al–Zn–Mg–Sc–Zr alloy without EXCO, the EIS is comprised by a capacitive impedance arc at high frequency and an inductive impedance arc at low frequency. Once EXCO occurs, the EIS is composed of two capacitive impedance arcs at high frequency and at low frequency, respectively.     

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…     

时效状态对新型Al-Cu-Li系合金剥落腐蚀性能的影响

研究了一种新型Al-Cu-Li系合金在T6、T8两种时效状态的剥落腐蚀行为,根据电化学阻抗谱,结合显微组织形貌分析了其腐蚀机理.研究表明,两种时效状态的合金几乎同时发生点蚀,T8态合金剥落腐蚀要比经T6态合金发生晚.着重讨论了合金中T1相对腐蚀的影响,实验表明,时效前的预变形(6%)使T1相析出细小、均匀,有利于提高该合金的抗剥落腐蚀性能.     

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.     

Evaluation of pitting corrosion of Al-Mg-Mn-Sc-Zr alloy in EXCO solution by EIS

In order to evaluate the degree and severity of Al-Mg-Mn-Sc-Zr alloy in exfoliation corrosion（EXCO） solution quickly and nondestructively, electrochemical impedance spectroscopy（EIS） technique was employed. The Al- Mg-Mn-Sc-Zr alloy suffers pitting corrosion in the EXCO solution. During pit incubation, the Nyquist diagram is composed of a depressed capacitive arc at high-mediate frequency and an inductive arc at low frequency. The inductive arc fades with immersion time, and the beginning of pitting corrosion and the appearance of two capacitive arcs have simultaneity. During pit propagation, the Nyquist diagram is composed of two overlapped capacitive arcs. As time goes on, two time constants are more clearly distinguished. The high frequency and low frequency capacitive arc are aroused hy passive surface and new interface, respectively. An equivalent circuit is designed to fit EIS, and the experimental results and the fitted results have good correspondence. The degree and severity of pitting corrosion can be obtained by the features of EIS and comparing the fitted values of parameters at different times.     

Al-Li合金在EXCO溶液中腐蚀的电化学阻抗研究

研究了T6态峰时效AA2090及AA8090A1—Li合金在EXCO溶液中腐蚀的电化学阻抗持征．结果表明，剥蚀发生之前，AA8090合金电化学阻抗谱由一个压缩的高频容抗弧和一个低频感抗弧组成，且低频感抗成分随浸泡时间延长而减弱并消失；一旦发生剥蚀，其电化学阻抗谱即由一个高频和一个低额两个容抗弧组成．而AA2090合金在较短浸泡时间内由于较大面积蚀孔的产生，其电化学阻抗谱上即出现一个高频及一个低频两个容抗弧．发生剥蚀后，两种合金具有相同的电化学阻抗持征．     

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。     

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.     

Investigation of exfoliation corrosion of rolled AA8090 Al-Li alloy using electrochemical impedance spectroscopy

1　INTRODUCTIONAl Lialloys ,comparedtotraditionalAlalloys ,havemoreexcellentproperties,suchaslowerdensi ty ,greaterelasticmodulusandhigherspecificstrength[1,2 ] .Inthenearfuture ,theywillbewidelyappliedtoairplanestructures.While ,exfoliation ,amainkindoflocalizedcorrosion ,lowerstheirstrength ,plasticityandfatigueproperties ,andalsodecreasestheirservicelife[35] .Soinvestigatingtheirexfoliationwillbeveryimportanttotheirapplication .　　　　　　Usually ,theexfoliationratingsareobservedbyna…     

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.     

T6态2090 Al-Li合金在EXCO溶液中的剥蚀行为和剥蚀发展过程中的电化学阻抗谱

采用电化学阻抗谱（EIS）和光学显微镜原位观察等方法研究了T6态峰时效的2090 Al-Li合金在EXCO溶液中的剥蚀行为.在浸泡过程中局部腐蚀形态演变经历了4个阶段：点蚀、晶间腐蚀、剥蚀和剥蚀后期.在点蚀阶段形成的以杂质相为局部阴极生成的大蚀孔成为剥蚀引发的中心,剥蚀鼓泡沿大蚀孔边缘生长,直到把表层以下完全穿透.点蚀引发阶段的EIS由一个高频容抗弧和一个中低频感抗弧构成,在点蚀发展阶段感抗弧消失.晶间腐蚀阶段的EIS的容抗部分中很难分辨出腐蚀区域对应的时间常数,剥蚀发展阶段和剥蚀后期的EIS由两个容抗弧构成.     

铝合金剥蚀过程的电化学阻抗谱分析

研究了轧制态及峰时效8090铝—锂合金、拉应力作用下及无应力作用时双级过时效7075铝合金的剥蚀发展过程，测量并拟合了合金发生剥蚀后的电化学阻抗谱。结果表明：发生剥蚀后，合金电化学阻抗谱由高—中频和中—低频容抗弧组成；可从电化学阻抗谱的拟合参数(变化趋势)得到剥蚀的相关信息。如剥蚀发展速度、剥蚀形貌、剥蚀表层是否脱落等．     

Exfoliation corrosion of aluminum alloy AA7075 examined by electrochemical impedance spectroscopy

A typical aluminum alloy, AA7075, was immersed in the EXCO solution, and its corrosion properties during different immersion time were measured repetitively using electrochemical impedance spectroscopy technique (EIS). The EIS data a were simulated using equivalent circuit with ZView program. The results show that once the exfoliation occurs, the low frequency inductive loop in the Nyquist plot associated with the relaxation phenomenon of reaction intermediates disappears, and the Nyquist plane is mainly composed of two capacitive arcs in the high frequency range and low frequency range respectively. The former originates from the original corroded surface, while the latter from the newly formed interface by exfoliation corrosion (EXCO). With the increased immersion time, the high frequency capacitance arc decreases gradually, while the low frequency capacitance arc increases gradually. From the beginning of immersion up to 9 hours, charge transfer resistance gradually decreases, illustrating the acceleration of the corrosion rate, whereas the proton concentration decreases steeply, indicating the cathodic process is pre‐dominant. Then the corrosion rate decreases gradually corresponding to the exhausting of proton ions. The results also show that the exfoliation corrosion is developed from pitting corrosion through intergranular corrosion to general corrosion at the end.     

时效制度对含钪超高强铝合金晶间腐蚀和剥落腐蚀的影响

通过晶间腐蚀、剥落腐蚀和电化学腐蚀试验,结合扫描电镜和透射电镜等分析手段,研究了含钪Al-Mg-Zn-Cu-Zr合金在不同时效状态下的的晶间腐蚀和剥落腐蚀行为。结果表明,合金抗晶间腐蚀和剥落腐蚀能力随着时效时间的延长而提高,在4.0mol/L NaCl+0.1mol/L HNO3+0.4mol/L KNO3(EXCO)溶液中测试的极化曲线也表现出相同的趋势。透射电镜观察表明,晶界析出相和晶界无沉淀析出带(PFZ)是影响合金腐蚀性能的主要因素。随着时效时间的延长,非平衡相η′和S′相逐渐向平衡相η和S相转变,晶界析出相粗化并呈链状分布,PFZ变宽。晶界粗大平衡相的不均匀分布和PFZ阻断了腐蚀的阳极通道,使合金的腐蚀敏感性降低。     

ELECTROCHEMICAL STUDY ON THE LOCALIZED CORROSION OF TWO DIFFERENT TEMPER LY12 ALLOYS

The corrosion behavior of two different aged LY12 alloys in EXCO （234g/L NaCl, 50g/L KNO3 0.1M HNO3） and MIL-H-600 （35g/L NaCl, 5g/L （NH4）2SO4） solutions were studied using electrochemical impedance spectroscopy （EIS） in conjunction with SEM technique respectively, and the pH variation of the above accelerated corrosion solution during materials corroding was also measured to characterize the corrosion processes. At the beginning of corrosion, EIS plots exhibited two capacitive arcs in the high frequency range （Cd） and mediate frequency range （ Cf） originating from the original corroded surface and the newly formed interface respectively, and a low frequency inductive loop due to the relaxation phenomenon of reaction intermediates. In the EXCO,solution, the EIS results showed that once the exfoliation corrosion occurred, the inductive loop disappeared. The results also showed that the exfoliation corrosion susceptibility of state T6 is much higher than that of state T3, but the intergranular corrosion susceptibility has a reverse order. Based on the relationship of EIS results with SEM and metalloscopy morphologies of the corroding substrates and physic-chemical meaning of EIS parameters, an index （Cf/Cd） has been proposed to characterize the extent of the localized corrosion, which variation tallies with the real corrosion extent correctly. Meanwhile, good agreement was found between the experimental results and the electrochemical model proposed and, good coherence between the electrochemical features and the distribution type of constituent particles of corroding material was observed.     

拉应力对7075铝合金的剥蚀及其电化学阻抗谱的影响

研究了拉应力对双级过时效(121℃/35 h+160/20 h ) 7075铝合金剥蚀的影响及合金在EXCO溶液中的电化学阻抗谱.研究表明，拉应力可显著地促进铝合金剥蚀的发生及发展.浸泡初期，合金电化学阻抗谱由一个高-中频容抗弧及一个中-低频感抗弧组成.合金一旦发生剥蚀，电化学阻抗谱上即出现一个高-中频容抗弧及一个中-低频容抗弧.     

Electrochemical impedance spectroscopy analysis on aluminum alloys in EXCO solution

Electrochemical impedance spectroscopy (EIS) of Al‐Zn‐Mg‐Cu alloy in EXCO solution has been investigated. The results show that the impedance spectroscopy of the investigated electrode consists of two capacitive loops in the high and middle frequency domain respectively and an inductive loop in the low frequency domain at the first 30 h of immersion; the inductive arc disappears between 30 h and 60 h while it takes on at the low frequency domain at the last 36 h of the immersion time. A model based on the corrosion mechanism of the aluminum alloy and corroding morphology was proposed and applied to analyze the EIS results.     

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.