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Chinese Journal of Aeronautics

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Chinese Journal of Aeronautics 24 (2011) 681-686

Corrosion Properties of Light-weight and High-strength 2195 Al-Li Alloy

XU Yuea,*, WANG Xiaojinga, YAN Zhaotongb, LI Jiaxuea aSchool of Chemistry and Environment, Beihang University, Beijing 100191, China

bInstitute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China

Received 29 November 2010; revised 10 December 2010; accepted 17 December 2010

Abstract

The intergranular corrosion and exfoliation corrosion of 2195 Al-Li alloy treated by multi-step heating-rate controlled aging (MSRC) are studied. The corrosion features of 2195 Al-Li alloys which are respectively treated by high-temperature nucleation MSRC (H-M) and low-temperature nucleation MSRC (L-M) are contrasted. And the corrosion mechanism of 2195 Al-Li alloy is also discussed from the viewpoint of microstructure (types, distribution, etc.) of the strengthening phase. The results show that 2195 Al-Li alloy after H-M is more susceptible to intergranular corrosion and exfoliation corrosion than that of alloy after L-M. The degree of intergranular corrosion increases with the increase of predeformation amount and the surface parallel to the rolling direction is more prone to exfoliation corrosion. The main reason of intergranular corrosion and exfoliation corrosion is the for-mation of corrosion galvanic couples among T1 phase, θ′ phase and grain boundary precipitate-free zones (PFZ).

Keywords: 2195 Al-Li alloy; corrosion resistance alloys; multi-step heating-rate controlled aging; intergranular corrosion; exfo-liation corrosion; corrosion mechanism

1. Introduction1

2195 Al-Li alloy is widely known as a kind of ideal light-weight high-strength structural material in aero-space field owing to high specific strength, good duc-tility and toughness [1]. And 2195 Al-Li alloy has been made into cryogenic propellant tank of space shuttle by National Aeronautics and Space Administration (NASA). For the future, it may have new application in the field of aeronautics. But it is highly susceptible to intergranular corrosion and exfoliation corrosion un-dergoing severe corrosive circumstance and moisture atmosphere [2]. So it is important to investigate corro-

*Corresponding author. Tel.: +86-10-82339930. E-mail address: xuyue@buaa.edu.cn Foundation item: National Basic Research Program of China

(2010CB934700) 1000-9361 © 2011 Elsevier Ltd.doi: 10.1016/S1000-9361(11)60080-0

sion properties of 2195 Al-Li alloy in order to ensure its secure and effective use.

At present, the researches [3-6] about corrosive be-havior of Al-Li alloy focus on the 2090 alloy, 8090 alloy, 1420 alloy, etc. While the reports about 2195 Al-Li alloy are very few. It is well known that the properties of alloys are closely related to microstruc-ture of alloys. Thus, the mechanical property and cor-rosion resistance of Al-Li alloy can be improved by regulating the structure of alloy through varying aging treatment. Al-Li alloys have three kinds of aging treatments. They are single-stage aging, split aging and multi-step heating-rate controlled aging (MSRC) re-spectively. The effects of single-stage aging and split aging on the mechanical property and corrosion resis-tance of alloy have been substantially studied [7-10]. Recently, the influences of MSRC on the performances of alloys have caused wide public concern. It was re-ported in Ref. [11] that the mechanical property of 2195 Al-Li alloy could be significantly improved Open access under CC BY-NC-ND license.

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through adopting MSRC treatment [11], while a system-atic research work on the corrosion properties of 2195 Al-Li alloy by such treatment has not been reported in literature currently. So on that basis, this paper 1) re-searches intergranular corrosion and exfoliation corro-sion which are the main corrosion forms of 2195 Al-Li alloy; 2) explores the relationship between the corro-sion degree of 2195 Al-Li alloy after MSRC and the microstructure (types, distribution, etc.) of the strength-ening phase; 3) understands the influence of predefor-mation amount on the corrosion properties of 2195 Al-Li alloy and presents the explanation from the precipitation mechanism of the strengthening phase; 4) discusses preliminarily corrosion mechanism of 2195 Al-Li alloy by metallographic microstructure and electrochemical impedance spectroscopy (EIS). These provide references for the application of 2195 Al-Li alloy as practical engineering materials.

2. Experimental

2195 Al-Li alloy hot rolled plate with a thickness of 2.2 mm provided by Southwest Aluminium (Group) Co., Ltd. was cut into the corresponding size speci-mens according to the test requirement. The chemical composition of the specimen is given in Table 1.

Table 1 Chemical composition of 2195 Al-Li alloy

wt%

Element Li Cu Mg Ag Zr Fe

Content 1.00 4.02 0.40 0.41 0.11 0.16

Element Si Ti Na Al H

Content 0.03 0.068 0.000 3 Allowance 0.17 cm3 /100 g

In the process of heat treatment, the solid solution

and the aging process are the two major factors to de-termine the performance of Al-Li alloy. In this re-search, heat treatment techniques were as follows. The solid-solution treatment of specimens was carried out at 510 °C for 40 min in the air tube furnace and the transfer time before quenching should be less than 2 s. Then the pre-stretching cold deformation treatment was operated at the SANS-1000 mechanical testing ma-chine by motion rate of 0.5 mm/min.

Two kinds of MSRC were applied in this paper. And technique parameters of aging are shown in Table 2. One was the low-temperature nucleation MSRC (L-M) that controlled the nucleation stage of precipitates at 127 °C which made specimens with better mechanical properties at room temperature; the other was the high-temperature nucleation MSRC (H-M) that con-trolled the nucleation stage of precipitates at 180 °C which made specimens with better mechanical proper-ties at low temperature. The temperature distribution in furnace was not more than 2.0 °C; the temperature de-viation was not more than 0.22 °C; the effectively con-trolled heating rate was 0.60 °C/h.

Table 2 Technique parameters of aging

Aging technique Stage 1 Stage 2 Stage 3

L-M 127 °C, 5 h 133 °C, 5 h 143 °C, 20 h

H-M 180 °C, 3 h 187 °C, 5 h 193 °C, 20 h

Note: heating rate was 0.60 °C/h.

The effects of the above two aging treatments on the corrosion properties of 2195 Al-Li alloy were mainly researched by the corrosion tests. In intergranular cor-rosion test, the corrosive media were the aqueous solu-tion involving 1 mol/L NaCl+10 mL 30%H2O2/L (the intergrannlar corrosion (IGC) solution), and the speci-mens were immersed in the IGC solution for 6 h. Then the corroded specimens were cut off by 5 mm at the end of the vertical rolling direction. The cross-section was grinded and polished according to the preparation method of metallographic specimen, then was observed by metallographic microscope (100-500 times magni- fication). If there exists grain boundary network, it was intergranular corrosion and the maximum corrosion depth was measured.

The exfoliation corrosion test was performed ac-cording to exfoliation corrosion (EXCO) test. The tem-perature of EXCO (4.0 mol/L NaCl+0.5 mol/L KNO3+ 0.1 mol/L HNO3, pH=0.4) solution was maintained at (25.0 ± 0.5) °C. The specimens were immersed in EXCO solution continuously with the ratio of exposed surface to the solution volume maintained at 1 cm2/25 mL and were used as working electrode. Three electrode systems with Pt being the auxiliary electrode and saturated calomel electrode being the reference electrode were used. EIS measurement was conducted in a CHI660C electrochemical workstation at open- circuit potential under the since-wave current with a frequency from 20 kHz to 0.01 Hz. The signal voltage is 10 mV. And the overall corrosion morphology of the specimen was taken by digital camera.

3. Results and Discussion

3.1. Intergranular corrosion

The specimen surface after removing the corrosion products is observed by naked eyes. It is found that the surface of specimen with 3% predeformation shows various sizes of corrosion pits while the specimen sur-faces with 6% and 8% predeformation have linear etching vessel distributing along the rolling direction.

The intergranular corrosion morphologies of cross section of specimens after L-M with different prede-formation are analyzed by the metallographic micro-scope and shown in Fig. 1. And the depth of inter-granular corrosion is listed in Table 3. As shown in Fig. 1 and Table 3, when predeformation amount is 3%, the specimen after L-M appears slight intergranu-lar corrosion and the corrosion depth is the least. While the specimens with 6% and 8% predeformation have obvious intergranular corrosion and the corrosion lon-

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gitudinally develops along the rolling direction under the surfaces of alloy materials.

Fig. 1 Intergranular corrosion morphologies of specimens

after L-M with different predeformation.

Table 3 Intergranular corrosion depths of specimens after L-M with different predeformation

Depth of intergranular corrosion/mm Predeformation amount Average depth Maximum depth

3% 0.012 0.021 6% 0.050 0.061 8% 0.060 0.079

Note: the error of the value is ±0.000 5 mm.

The intergranular corrosion morphologies of cross- section of specimens after H-M with different prede-formation are presented in Fig. 2. And Table 4 displays the depth of intergranular corrosion. It is found from Fig. 2 and Table 4 that the depth of intergranular corro-sion increases and the susceptibility to intergranular corrosion of specimens elevates with the increase of predformation amount. The result shows that the inter-granular corrosion characteristics of specimens respec-tively adopting H-M and L-M are similar.

Fig. 2 Intergranular corrosion morphologies of specimens after H-M with different predeformation.

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Table 4 Intergranular corrosion depths of specimens after H-M with different predeformation

Depth of intergranular corrosion/mm Predeformation amount Average depth Maximum depth

3% 0.021 0.038

6% 0.045 0.052

8% 0.080 0.089

Note: the error of the value is ±0.000 5 mm.

Comparing the intergranular corrosion of the speci-mens treated by different aging with the same prede-formation, it is not difficult to find that the specimen after H-M has slightly greater intergranular corrosion depth than the one after L-M under the same prede-formation. The reason is that alloy specimens after L-M have less amount of T1 phase and the driving force of corrosion provided by the corrosion galvanic couples is less due to comparable open circuit potential of the internal θ′ phase and grain boundary precipi-tate-free zones [11-13]. Thus, the degree of intergranular corrosion is relatively slighter. However, as for the alloy specimens after H-M, because the density of T1 phase at the grain boundary is higher, the driving force provided by the corrosion galvanic couples consisting of T1 phase and θ′ phase/grain boundary precipi-tate-free zones increases. Meanwhile, the anodic dis-solution of T1 phase leads to the increase of the sus-ceptibility to intergranular corrosion. Through the comparison of the above corrosion mechanisms, it can be found that the susceptibility to intergranular corro-sion of specimens after H-M is higher than that of specimens after L-M, which is consistent with the con-clusion based on the intergranular corrosion mor-phologies observed by the metallographic microscope.

3.2. Exfoliation corrosion

Exfoliation corrosion which does critical harm to Al-Li alloys is a main kind of localized corrosion, and is also one of the main corrosion forms of aircraft components. The strength, ductility and fatigue prop-erties of materials would greatly decrease due to the presence of exfoliation corrosion [14-17].

The EIS plots of the specimens after L-M with dif-ferent immersion time are shown in Fig. 3, where Z′ and Z″ are the real part and imaginary part of electro-chemical impedance spectra, respetively. After im-mersing for 10 min in EXCO solution, the EIS plot of 2195 Al-Li alloy specimen consists of a capacitive impedance arc at high frequency and an inductive im-pedance arc at low frequency, which illustrates that there exists no exfoliation corrosion. With the increase of immersion time, the inductive impedance arc gradu-ally reduces. When exfoliation corrosion occurs, the EIS plot consists of a capacitive impedance arc at high frequency and a capacitive impedance arc at low fre-quency.

Fig. 3 EIS plots of specimens after L-M with different immersion time.

The EIS plots of specimens after H-M are given in Fig. 4. It can be seen that the EIS plot of the specimen is composed of a compressed capacitive impedance arc at high frequency and an inductive impedance arc at low frequency after immersing for 10 min in EXCO solution. The inductive impedance arc would decline with immersion time. However, there is only one ca-pacitive impedance arc on the EIS plot after appearing exfoliation corrosion. Fig. 4 also shows that the radius of capacitance impedance arc decreases rapidly at first, then there is a trend to increase with immersion time. This phenomenon is due to the fact that the surfaces of specimens are covered with protective oxide films whose protection weakens with immersion time, and the pH of EXCO solution decreases after a period of immersion. The EIS study shows the rapid decrease of the radius of capacitance impedance arc is in response to the quick reduction of reaction resistance.

Fig. 4 EIS plots of specimens after H-M with different immersion time.

It can also be seen from Fig. 3 and Fig. 4 that there is a significant difference between the radii of capaci-tive impedance arc on the EIS plots of the specimens treated by different aging processes under the same immersion time (e.g. 10 min). The radius of capacitive impedance arc of the specimen after L-M is obviously greater than that of the specimen after H-M. This indi-cates that the reaction resistance of the specimen after

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L-M is larger than that of the specimen after H-M in EXCO solution. In other words, the susceptibility to exfoliation corrosion of specimens after H-M is higher than that of specimens after L-M in EXCO solution. The reason may be that the main strengthening phase [11-13] is θ′ phase within 2195 Al-Li alloy after L-M while it is T1 phase within 2195 Al-Li alloy after H-M. And the two phases are both dominated in the strengthening phases and distribute densely.

As the size of the θ′ phase is far less than that of the T1 phase, when corrosion galvanic couples consisting of grain boundary precipitate-free zones and θ′ phase or T1 phase lead to the happening of corrosion, the influence of massive T1 phase on the corrosion is much greater than that of the tiny θ′ phase. For in-stance, the corrosion is caused by corrosion galvanic couple consisting of θ′ phase and grain boundary pre-cipitate-free zones, which makes the dissolved amount of anode (grain boundary precipitate-free zones) much smaller than that of anode when θ′ phase is replaced by T1 phase under the same conditions. Thus, the de-fects left in grain boundary are less after corrosion and the amount of corrosion solution seeping into alloy is little. The corrosion solution has few chance to expose to the new θ′ phase, which decreases the possibility of further corrosion and weakens the damage to speci-mens.

Generally, the different distributions and sizes of precipitation phase of 2195 Al-Li alloy adopting dif-ferent aging processes result in the variation of the susceptibility to exfoliation corrosion of alloy; while the difference of reaction resistance is reflected in the capacitive impedance arc on the EIS plots.

In order to further research the exfoliation corrosion of 2195 Al-Li alloy, the surface images of specimens with different immersion time in EXCO solution were taken by digital camera. Fig. 5 and Fig. 6 respectively show exfoliation corrosion morphologies of specimens after L-M and H-M under different immersion time in EXCO solution. As shown, the corrosion degree of surface perpendicular to the rolling direction has no obvious change with immersion time, and the degree is small. While the corrosion degree of surface parallel to the rolling direction has a obvious tendency to increase as immersion time prolongs. When the specimen is immersed in EXCO solution for 12 h, bubbling corro-sion appears on the alloy surface in small degree. And when immersion time is up to 48 h or 72 h, “peeling” phenomenon occurs on the alloy, and some corrosion products peel and fall away. In conclusion, the exfolia-tion corrosion of 2195 Al-Li alloy occurs more easily on the surface parallel to the rolling direction than the surface perpendicular to the rolling direction. Com-paring Fig. 5 and Fig. 6, it can also be seen that the 2195 Al-Li alloy after H-M is more susceptible to ex-foliation corrosion than alloy after L-M.

Fig. 5 Exfoliation corrosion morphologies of specimens

after L-M with different immersion time.

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Fig. 6 Exfoliation corrosion morphologies of specimens after H-M with different immersion time.

4. Conclusions

(1) The degree of intergranular corrosion increases with the increase of predeformation amount. Under the same predeformation, the 2195 Al-Li alloy after H-M is more prone to the occurrence of intergranular corro-sion.

(2) At early stage of immersion in EXCO, EIS plots of specimens after H-M or L-M are composed of a capacitive impedance arc at high frequency and an inductive impedance arc at low frequency. When exfo-liation corrosion occurs, EIS plots consist of a capaci-tive impedance arc at high frequency and a capacitive impedance arc at low frequency. The radius of capaci-tive impedance arc of the specimen after H-M is smaller than that of the specimen after L-M. This indi-cates that the electrochemical reaction resistance of 2195 Al-Li alloy after H-M is lower and the alloy after H-M is more susceptible to exfoliation corrosion.

(3) The main reason of intergranular corrosion and exfoliation corrosion is the formation of corrosion gal-vanic couples among T1 phase, θ′ phase and grain boundary precipitate-free zones respectively. The ex-foliation corrosion of 2195 Al-Li alloy occurs more easily on the surface parallel to the rolling direction than the surface perpendicular to the rolling direction, which develops along the grain distributing zones.

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Biography:

XU Yue Born in 1961, he received Ph.D. degree from Harbin Institute of Technology in 2002. Now, he is an asso-ciate professor in School of Chemistry and Environment, Beihang University. His main research interest is surface modification of alloys. E-mail: xuyue@buaa.edu.cn