Understanding stress corrosion cracking behavior of 7085-T7651 aluminum alloy in polluted atmosphere

Mingto WANG, Liwei WANG,*, Kun PANG, Yuxi LIU, Yuxue WANG,Zhongyu CUI

a College of Mechanical and Electrical Engineering, Qingdao University, Qingdao 266071, China

b School of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China

KEYWORDS Aluminum;Atmospheric corrosion;Fractography;Hydrogen embrittlement;Stress corrosion cracking

Abstract The electrochemical and Stress Corrosion Cracking(SCC)behaviors of 7085-T7651 aluminum alloy in different environments are studied by electrochemical and mechanical testing.The research shows that the type,concentration of the corrosive medium and electrolyte state affect the electrochemical and SCC controlling processes of aluminum alloys.The Thin Electrolyte Layer(TEL)state and the addition of HSO3–increase the corrosion rate and SCC susceptibility.The presence of HSO3–in a corrosive environment can significantly accelerate the corrosion rate and mechanical property degradation, and this effect increases with the increase of HSO3– concentration.Compared with the solution environment, the TEL environment will further aggravate corrosion and mechanical property degradation.With the increase of HSO3–concentration,the pH of the corrosive environment exhibits little change,while the SCC degradation is significantly promoted.This is attributed to the HSO3–induced buffer effect and film-assisted stress effect,yielding the overshadowing effect against solution pH.

1.Introduction

As typical 7xxx (Al-Zn-Mg-Cu) aluminum alloys, 7085 alloy has a series of advantages such as high specific strength and toughness, and low density.1–3Therefore, 7085 alloy is widely used in transportation, defense industry, aerospace, and other industrial fields.4–7Due to the oxide film composed of hydrous aluminum oxide, the aluminum alloy has a superior corrosion resistance.This oxide film is stable in a neutral environment but will suffer damage in an acidic or alkaline environment and cause corrosion.In addition, even a neutral environment can cause severe corrosion when aggressive ions such as chloride are present in the environment.8

Many works concerning the corrosion and Stress Corrosion Cracking (SCC) behavior of aluminum alloys in solution and atmospheric environments have been reported.9–13Chanyathunyaroj et al.11proposed that corrosion in 3.5%NaCl solution resulted in a considerably shortened fatigue life of aluminum alloys, and corrosion pits created on the surface served as the crack nucleation sites.The coexistence of pitting corrosion and Intergranular Corrosion (IGC) in aluminum alloys after atmospheric corrosion increases stress concentration and notably reduces the mechanical properties of aluminum alloys.12,13Gao et al.14studied the effect of different solid particles (NaCl, C, and SiO2) on the initial atmospheric corrosion behavior of aluminum alloys and verified that the deposition of NaCl particles significantly accelerated the initial corrosion process,while C particle had no effect.Corrosion of metals in atmospheric environments principally occurs in Thin Electrolyte Layer (TEL) adsorbed on the metal surface.15Compared with the solution environment, TEL will cause more severe corrosion and loss of mechanical properties of the alloy.16For the control of TEL, different simulation approaches have been taken in the literature.16–20Cheng18and Zhou19et al.utilized the method of parallel arrangement of electrodes to form a conductive film with a uniform distribution of known thickness (200–500 μm) on the metal surface and suggested that corrosion was controlled by the oxygen reduction reactions.It is difficult to maintain a uniform specific thickness of TEL on a vertically placed specimen during stretching test.Consequently, the atomization device is designed to represent a constant salt spray environment and generate adsorbed TEL on the metal surface and has been used in previous works.16,20The higher SCC sensitivity of low alloy steel in TEL environment as compared to full-immersion environment has been demonstrated.21,22

2.Experiment

2.1.Materials and corrosive environments

The commercial high-strength aluminum alloy AA7085-T7651(hereafter called 7085 alloy) with the chemical composition listed in Table 1 was used.All the test samples were ground to P1500 using SiC paper, then ultrasonically cleaned with alcohol and dried in cold air.Fig.1 provides an optical micrograph of the 7085 alloy after polishing and etching with Keller’s reagent, as well as the morphology of the second phase and its elemental composition obtained by Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy(EDS).The band structure of the 7085 alloy is not obvious,and the grain size is about 300–400 μm(Fig.1(a)).The second phase of 7085 alloy is primarily composed of Al7Cu2Fe(Fig.1(b)).

When SO2was dissolved in solution, HSO3–would be formed:34

In published work, NaCl and NaHSO3were commonly used to simulate SO2-containing industrial-marine atmospheric environments.31The pure 3.5% NaCl and that with 0.001 mol/L and 0.010 mol/L NaHSO3were used as the corrosive media to simulate the industrial-marine atmosphere with different pollution degrees.The unsteady thin electrolyte layer and solution environment were used to understand the influence of the adsorbed corrosive electrolyte and its difference from the traditionally used immersion test.

2.2.Electrochemical measurements

The electrochemical workstation for electrochemical experiments was Zahner Zennium-E.All the tests were performed at ambient temperature, and each curve was repeated at least three times to ensure data reproducibility.The samples used for potentiodynamic polarization tests were sealed in epoxy resin with an exposed working area of 1 cm2.A threeelectrode cell, with a platinum plate as the counter electrode and a Saturated Calomel Electrode(SCE)as the reference electrode,was used.After an Open Circuit Potential(OCP)test of30 min, polarization curves were measured from –1.05VSCE(where VSCEis the potential of SCE) to the anodic direction with a scan rate of 0.333 mV/s.

Table 1 Chemical composition of 7085 alloy used.

Fig.1 Microstructure and morphology of intermetallic particles of 7085 alloy, as well as elemental mapping results for second phase.

Fig.2 Schematic diagram of sample and device for EIS test in TEL environment.

Fig.2 presents the structural details of the electrode samples used in the EIS experiments, as well as a schematic diagram of the experimental setup which has been used in previous work.15The electrode was composed of two identical electrodes with a size of 8 mm×4 mm×3 mm,placed in parallel with a spacing of 0.5 mm.One of the samples was used as counter electrode and reference electrode, and the other was the working electrode.For the solution environment test, the electrode was directly immersed to conduct the measurement.For the TEL environment test, the atomization device was used instead of a uniformly distributed TEL to generate a salt spray environment and TEL state(Figs.2 and 3(d)).The solution was continuously transmitted to the liquid film deposition device with the atomizer.During the tests, the temperature in the device was controlled at (25 ± 2) °C, and the Relative Humidity (RH) was maintained at 100%, which was used to form an absorbed electrolyte layer on the surface of the aluminum alloy.The EIS tests were performed by applying a sinusoidal perturbation of 30 mV at the OCP in the frequency range from 100 kHz to 0.01 Hz.

2.3.SSRT test

The geometry of the samples and the set-up for the SSRT tests are shown in Fig.3.The length of the gauge segment that was exposed to the corrosive environment was 20 mm, and the remaining surface was coated with silica gel.Prior to the test,the specimens were pre-exposed in the corrosive environment for 12 h, and then the SSRT was performed in the same environment utilizing the WDML-30kN Material Test System with a strain rate of 10–6s-1(0.0018 mm/min) according to ASTM G129.Each set of experiments was repeated three times to check the repeatability.After the SSRT experiment, the ultimate tensile strength,elongation, and energy density were calculated from the stress–strain curves.The energy density was calculated by

where εfis the fracture strain;σ is the stress;ε is the strain.The SCC susceptibility of aluminum alloys was determined by the loss of tensile strength (Iσ) and the loss of elongation (Iδ)according to

where σcand σ0are the tensile strength measured in corrosive media and air, respectively; δcand δ0are the elongations in corrosive media and air, respectively.After removing the corrosion products(rinsing in HNO3at 25 ℃for 5 min),the fracture morphology and side morphology were observed by SEM.

2.4.Surface morphology observation

The morphology and composition of the corrosion products were observed by SEM and EDS after immersion in the solution and TEL environment with or without 0.010 mol/L HSO3–for 48 h.Subsequently, the corrosion products were removed by the method in Section 2.3 and the surface corrosion damage was detected.

3.Results and discussion

3.1.Potentiodynamic polarization analysis

Fig.3 Dimensions of sample, sample preparation for SSRT tests and schematic diagram of device for SSRT test in solution and TEL environment.

Fig.4 Polarization curves of 7085 alloy in 3.5% NaCl solution with different concentrations of NaHSO3.

Fig.4 exhibits the polarization curves of 7085 alloy in chloride solution with different concentrations of NaHSO3.No passivation zone is observed in the anodic region of 7085 alloy,implying that pitting occurs at the OCP.35When a small amount of NaHSO3(0.001 mol/L)is added,the anodic process does not change significantly, while the cathodic process is slightly promoted.As the concentration reaches 0.010 mol/L,the corrosion potential is shifted positively,which is attributed to the acceleration of the cathodic reactions.

In the polarization curves,the corrosion potential can forecast the thermodynamic information of corrosion,and the corrosion current density is an important parameter to evaluate the corrosion resistance.The cathodic current density of 7085 alloy is not altered significantly with the potential in the strong-polarization domain, showing the characteristics of the diffusion-controlled phenomenon.Therefore, the pseudo-diffusion current density is used as corrosion current density (icorr).32The corrosion potential (Ecorr) and icorrare listed in Table 2.The addition of HSO3–accelerates the corrosion of 7085 alloy, especially the promotion of the cathodic process.With the increase of HSO3–concentration, icorrgradually increases, indicating that the corrosion process is accelerated (Table 2).

On the one hand,HSO3–in 3.5%NaCl solution acidifies the electrolyte and promotes the cathodic reaction of the aluminum alloy, making the oxide film unstable.On the other hand, due to the formation of island-shaped aluminum hydroxy sulfate in the corrosion product, the propagation of pitting corrosion is slowed down and the anodic process is inhibited.36

3.2.Electrochemical impedance spectroscopy analysis

The EIS results of 7085 alloy exposed to the solution and TEL environment for 1 h are shown in Fig.5,where Zrand Zimare the real and imaginary parts of the impedance, Z is the impedance, θ is the phase.The Nyquist plot is primarily composed of capacitive loops in the solution environment (Fig.5(a)),while the inductive loop is observed in the TEL environment(Fig.5(b)).The capacitance loops at the high and middle frequency are mainly contributed by the oxide film and the charge transfer process.15The inductive loop implies the initiation of pitting corrosion, which may be caused by noble impurities or reactive precipitates.37,38Three capacitive loops and an inductive loop are distinguished in the Nyquist plots in the TEL environment (Fig.5(c)), while two capacitive arcs can be distinguished in the immersion environment (Fig.5(d)).

Table 2 Fitting results of Ecorr and icorr of 7085 alloy in different solutions.

The Constant Phase Element(CPE)is used to replace pure capacitors to address the non-ideal behavior in the system.39Fig.6 shows the equivalent electrical circuit and the fitting results, where Rsis electrolyte resistance, Roxand CPEoxrepresent the resistance and capacitance of the oxide layer,respectively, CPEdland Rctrepresent the double layer capacitance and charge transfer resistance, respectively, RLand L are inductive parameters, indicating the surface film failure and the degradation of the corrosion resistance, CPEfand Rfare the properties of the thick corrosion product layer, Rpis the polarization resistance.37,40The circuit shown in Fig.6(a) is used to fit the EIS data in the solution environment.For the TEL environment, the corrosion product layer is formed,and additional CPEfand Rfare used (Fig.6(b)).All fitting parameters are listed in Table 3.The polarization resistance,Rp, is a crucial parameter for evaluating the corrosion resistance of alloy, the reciprocal of which is proportional to the corrosion rate,41which is calculated by

The resistance and capacitance values are shown in Figs.6(c)and(d).Brug’s method is used to calculate the pure capacitance from CPE.42Rox, Rct, and Rpin the solution environment are significantly higher than those in the corresponding TEL environment, which indicates that the oxide film in the TEL environment is damaged, resulting in the initiation of the pitting corrosion43and the increase of the corrosion rate.The polarization resistance decreases with the increase of HSO3–concentration, suggesting a consistent phenomenon with the polarization curves.Meanwhile, the polarization resistance in the TEL environment is much lower than that in the solution environment, which indicates that changes in the environment can affect the corrosion resistance of aluminum alloys more than additional aggressive ions.However,the oxide film resistance and charge transfer resistance do not always decrease with the increase of HSO3–concentration.This phenomenon may be due to the different effects of different amounts of HSO3–on the oxide film and on the initiation and propagation of pitting.

3.3.Corrosion morphology observation

Fig.5 Nyquist and Bode diagrams of 7085 alloy in 3.5% NaCl solution environment and TEL environment.

Fig.6 Equivalent circuits used to fit EIS of 7085 alloy in solution and TEL environments,as well as fitting parameters of resistance and capacitance.

Fig.7 illustrates the corrosion morphologies before and after removal of corrosion products from 7085 alloy after immersion for 48 h.Some aggregated clusters and dispersed tiny corrosion products are observed in the solution environment(Figs.7(a) and (c)).After removal of the corrosion products,only slight traces of corrosion are detected (Figs.7(d) and(f)).Corrosion product rings are found in the TEL environment (Figs.7(b) and (g)), which are related to the interaction between electrolytes of different pH.44After removing the corrosion products,the surface of the sample is mainly dominated by single corrosion pits (Figs.7(e) and (h)).Fig.8 shows the detailed corrosion product characteristics after immersion in 3.5% NaCl solution with or without 0.010 mol/L HSO3–for 48 h.The corrosion products on the aluminum alloy surface are full of cracks and loose in the solution environment without HSO3–(Fig.8(a)).In the HSO3–-containing solution, compact corrosion product islands appear on the 7085 alloy surface(Fig.8(b)).The chemical composition of the corrosionproducts is listed in Table 4.The corrosion products are mainly composed of O and Al, which can be ascribed to Al(OH)3, Al2O3, and AlOOH.12,24,45With the addition of NaHSO3,a trace amount of S is detected due to the formation of insoluble aluminum sulfate.19

Table 3 Fitted electrochemical parameters for EIS of 7085 alloy in solution and TEL environments.

Fig.7 Corrosion morphologies of 7085 before and after removal of corrosion products in 3.5%NaCl solution,3.5%NaCl TEL,3.5%NaCl + 0.010 mol/L NaHSO3 solution and 3.5% NaCl + 0.010 mol/L NaHSO3 TEL.

Fig.8 Corrosion product characteristics after immersion in 3.5%NaCl solution and 3.5% NaCl+0.010 mol/L NaHSO3 solution for 48 h.

3.4.Stress corrosion cracking behavior

3.4.1.Stress–strain curves

Fig.9 shows typical stress–strain curves and mechanical property loss of 7085 alloy in 3.5%NaCl with different concentrations of HSO3–.The ultimate tensile strength, elongation, and energy density are calculated and listed in Table 5.Obviously,SCC occurs in corrosive environments as indicated by the loss in ductility and strength.The latter is very obvious in the TEL environment with 0.010 mol/L HSO3–(Fig.9(b)).Moreover,the degradation of the 7085 alloy becomes more severe when the HSO3–concentration is increased,yielding the fast fracture,the low ultimate tensile strength, elongation, and energy density (Table 5).After adding 0.001 mol/L HSO3–to the solution environment, the ductility and strength of the 7085 alloy reduce slightly, while the ductility drops significantly when the HSO3–concentration reaches 0.010 mol/L (Fig.9(a)).At the concentration of 0.001 mol/L HSO3–, the elongation of 7085 alloy in solution is greatly higher than that in the TEL environment.As the HSO3–concentration rises to 0.010 mol/L,the elongation of the alloy in the solution and the TEL environment tends to be the same, and the strength in the TEL environment was much lower than that in the solution environment(Table 5).The 7085 alloy exhibits sufficient sensitivity in ductility (Fig.9(c)), while the apparent loss in strength occurs only when the HSO3–concentration reaches 0.010 mol/L(Fig.9(d)).The TEL environment exhibits higher SCC sensitivity than the solution under the same ion type and concentration.In addition,the dramatic difference in ductility loss between solution and TEL environments in HSO3–-free and low concentrations of HSO3–(0.001 mol/L) can be confirmed.However, when the HSO3–concentration reaches 0.010 mol/L, the difference between the ductility loss in solution and the TEL environment is small.This implies that the detrimental effect of environmental changes(from solution to TEL)on the ductility of 7085 alloy decreases at high concentrations of HSO3–.The index of strength loss increases with the increase of HSO3–concentration (Fig.9(d)), which may be attributed to the reduction in the effective thickness of the alloy due to corrosion attack.46

In conclusion,both the additional aggressive ion HSO3–and TEL environments will exacerbate SCC compared to the 3.5%NaCl solution environment, which results in the worst mechanical properties of 7085 alloy in the TEL environment with 0.010 mol/L HSO3–,indicating that polluted marine atmospheric environment will gravely threaten the service safety of 7085 alloy.

3.4.2.Fractographic analysis

Table 6 exhibits the fracture surface and side morphologies of 7085 alloy after SSRT tests in different environments.After fractured in air, the samples show the ductile characteristics,and there are obvious dimples and aggregated micro-dimples on the fracture surface (②in Table 6).In the solution (⑤in Table 6)and TEL(⑧in Table 6)environments of 3.5%NaCl,a large number of similar smooth brittle surfaces appear on the fracture surface,which indicates that Cl–would induce embrittlement.With the addition of HSO3–, the fracture mode of the sample changes.In the solution environment, the fracture morphology is dominated by the brittle region, showingquasi-cleavage and intergranular mixed morphology (?in Table 6).In the TEL environment,7085 alloy exhibits obvious intergranular fracture morphologies (?in Table 6).In both air and Cl–-containing environments, a small number of side cracks appear in the fractured specimens (③, ⑥and ⑨in Table 6).With the addition of HSO3–in the solution environment, the side cracks of the fractured specimen disappear (?in Table 6).In the TEL environment containing HSO3–,a large number of deep pits appear on the side surface,and cracks are nucleated at the edges of the pits,which drastically reduces the SCC resistance of the aluminum alloy(?in Table 6).Among them,in the solution environment(⑥and ?in Table 6),only slight corrosion traces appear on the side surface, while in the TEL (⑨and ?in Table 6) environment, obvious corrosion pits are observed, which is consistent with the surface morphology after removing the corrosion products in Fig.7.

Table 4 Chemical composition of corrosion products in Fig.7.

Table 5 Ultimate tensile strength, elongation, and energy density of 7085 alloy in solution and TEL environments.

3.4.3.SCC mechanism

Fig.10 shows the schematic diagram of the corrosion process of 7085 alloy in four environments, where Hadsand Habsrepresent adsorption hydrogen and absorption hydrogen, respectively.The Anodic Dissolution (AD) produces corrosion defects, and the Al3+hydrolyzes near the corrosion defects to produce hydrogen and acidify the solution (②in Fig.10(c)).Part of the produced hydrogen is trapped by the matrix and the other part is recombined into hydrogen gas and release.47Even though the cathodic reaction in the solution environment is principally oxygen reduction, the corrosionfront is locally acidified due to the hydrolysis of Al3+and the formation of occluded cells.In the TEL environment,the electrolyte on the alloy surface can transport oxygen more easily than the solution, resulting in an enhanced cathodic oxygen reduction reaction and more generated OH–.It is difficult for hydroxide ions to diffuse in the limited space, leading to alkalization of the surface electrolyte layer15and promoting surface corrosion.48The surface electrolyte is alkalized in a Cl–containing TEL environment (pH is about 9, roughly measured with pH testing strips).The deeper corrosion attack in the occluded cell leads to more trapped hydrogen, thereby enhancing the Hydrogen Embrittlement (HE) mechanism.49

Table 6 Fracture surface and side morphologies of 7085 alloy in different environments.

With addition of HSO3–in the 3.5%NaCl solution,the corrosion environment is acidified,enhancing the hydrogen evolution reaction.Meanwhile, the generated insoluble aluminum hydroxy sulfate (Fig.10(d)) inhibits the pitting.50In addition,with the oxidation of oxygen under the catalysis of transition metal (Fe, Mn), HSO3–will be transformed into SO42–(Reactions (7) & (8)), which destroys the oxide film on the surface and accelerates corrosion.24,51

The synergistic effect of HSO3–and Cl–enhances the conductivity of TEL films,promotes anodic dissolution,and accelerates the pitting corrosion of aluminum alloys.With the enhancement of pitting corrosion,the generation and propagation of SCC cracks are promoted.The local release,entry,and embrittlement of hydrogen are promoted,and the SCC susceptibility of the 7085 alloy is improved.The HE and AD mechanisms are strengthened by the hydrogen evolution reaction and corrosion intensification in the industrial-marine atmospheric environment, resulting in higher SCC susceptibility.

Fig.10 Schematic diagram of corrosion process of 7085 aluminum alloy in solution and TEL environment without and with NaHSO3.

3.5.Controllingfactorof HSO–3-inducedSCCresistance degradation of 7085-T7651alloy inpresence of Cl–

An interesting behavior from Fig.9 is that there is little difference in bulk solution pH between the chloride solution containing 0.001 mol/L and 0.010 mol/L HSO3–(4.70 and 4.04,respectively), while the SCC behavior of 7085 alloy at these two concentrations is quite different.To further understand the effect of acidity and HSO3–in the alloy surface electrolyte on the SCC of 7085 alloy, HCl was used to adjust the pH of 3.5%NaCl solution to 2,4,and 5,respectively,and in situ tensile experiments were carried out.The stress–strain curves and the relationship between solution pH and SCC sensitivity are shown in Fig.11.The strength of the 7085 alloy is not affected by the pH from 7 to 4 of the 3.5%NaCl solution,and the elongation slightly decreases.However, when the solution is acidified to pH=2,both strength and ductility are degraded obviously,and the samples are fractured near the yield point.This is consistent with the work of Tsai et al.,52who showed that when the pH of the solution was between 4 and 10,the ductility hardly changed,and when the pH was reduced to be lower than 4, the SCC sensitivity was highly dependent on pH.Sedriks53and Brown54et al.also pointed out that the SCC behavior of Al-Zn-Mg-Cu alloys was not sensitive to solution pH,and they believed that the pH of the crack tip was around 3.5 regardless of the solution pH.Therefore, acidification of corrosive environment caused by HSO3–is not the main factor causing severe SCC at high concentration (0.010 mol/L).

Fig.11 Stress–strain curves of 7085 in 3.5% NaCl solution at different pH, as well as dependence of ductility loss on pH.

Fig.12 Variations of solution pH as a function of exposure time in 3.5% NaCl solution with initial pH of 4.70 and 4.04.

To evaluate the degradation caused by HSO3–,the SCC sensitivity in the HSO3–-free and HSO3–-containing solutions with a similar pH is compared and shown in Fig.11(b).A simple approximation was used to calculate the role of HSO3–.The loss of ductility in an HSO3–-containing environment minus the loss of ductility in HSO3–-free environment with the same pH yields the loss of elongation due to aggressive ions.The SCC sensitivity is assumed to show a linear decrease within the pH range from 4 to 7.The results shown in Fig.11(b)reveal that the sensitivity caused by the HSO3–is 52.7% in the solution containing 0.010 mol/L HSO3–, contributing to 82.0% of the total ductility degradation.The actual value and the contribution percentage decrease to 7.3% and 43.0%, respectively, when the HSO3–concentration is 0.001 mol/L.Several reasons may be responsible for the HSO3–triggered SCC degradation besides the pH effect.Firstly, the HSO3–belongs to a weak acid radical, of which the ionization effect can continuously produce the H+and supply the hydrogen evolution reaction.Therefore,the interfacial area is difficult to alkalize and the hydrogen can be produced effectively.In the TEL environment with 0.010 mol/L HSO3–, the pH of the electrolyte on the surface of the sample hardly changes after the experiment.However, in the TEL environment of 0.001 mol/L, the pH of the experimental surface rises by about 2 after the experiment, which is roughly measured with pH testing strips.Fig.12 shows the variations of the solution pH as a function of time in the solutions with an initial pH of 4.70 and 4.04.At initial pH of 4.70, the pH in the pure chloride environment continues to rise during the immersion test, while that in the solution containing 0.001 mol/L HSO3–shows an overall upward with lower increase rate and some fluctuations (Fig.12(a)).As the HSO3–concentration is increased to 0.010 mol/L, the initial pH exhibits little change during the test (Fig.12(b)).These experiments confirm that HSO3–in the environment can inhibit the alkalinization of the corrosive medium, and the inhibition effect is more obvious with the increase of its concentration.This special effect is similar to the effect of NH4+on the corrosion of magnesium alloy in the chloride environment,in which the hydrolysis can provide sufficient H+to support the corrosion of Mg and its dominant role is also demonstrated.55–57The higher the HSO3–concentration is,the more the H+is produced.Secondly, the presence of HSO3–facilitates the formation of the corrosion product layer.It has been reported that the corrosion product Al2SO4(OH)4?7H2O is more compact than the Al2O3/Al(OH)3layer.36Therefore, its high adhesion could produce more enhanced effect on the dislocation emission and motion near the surface,58resulting in the more pronounced film-induced stress that contributes to the brittle fracture.Moreover, the thin film layer formed in the solution containing 0.001 mol/L HSO3–may be not sufficient to trigger the cracking of the ductile substrate because of the low interfacial strength.

To sum up, when the HSO3–concentration is low, the decrease of the solution pH and the HSO3–induced effects almost equally contribute to the SCC degradation.When the HSO3–concentration is high enough, however, the continuous supply of H+caused by HSO3–ionization and the sulfate corrosion product film-induced stress play the dominant role in the ductility deterioration of 7085 alloy.

4.Conclusions

(1) In 3.5% NaCl-based corrosion environments, the addition of HSO3–and the change of the corrosive environment (from solution to TEL) aggravate the corrosion of 7085 alloy.The corrosion rate increases with the increase of HSO3–concentration.

(2) In different 3.5% NaCl-based corrosion environments,SCC will occur in 7085 aluminum alloy, resulting in the decline of mechanical properties and the appearance of brittle areas in the fracture.SCC sensitivity is increased by increasing HSO3–concentration and changing test conditions from solution to TEL.

(3) The decrease of the solution pH and the HSO3–induced effects almost equally contribute to the SCC degradation when the HSO3–concentration is low.However,the continuous supply of H+caused by HSO3–ionization and the sulfate corrosion product film-induced stress play the dominant role in the ductility deterioration of 7085 alloy in environment containing high concentration of HSO3–.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the financial support of the Ministry of Industry and Information Technology Project, China (No.MJ-2017-J-99), the National Science Foundation of China(No.51701102), and the National Science and Technology Resources Investigation Program of China (No.2019FY101400).