Evaluating stress corrosion cracking behaviour of high strength AA7075-T651 aluminium alloy

1. Introduction

High strength heat-treatable aluminium alloys are generally used in marine components due to their high strength-to-weight ratio, excellent machinability and good formability. Aluminium alloys in general are passive and corrosion resistant in aqueous environments except for pitting corrosion due to the fact that they become chemically reactive in a chloride environment. AA7075-T651 high strength aluminium alloy (Al-Zn-Cu-Mg) has good resistance to general corrosion due to the formation of an adherent oxide film, but the major drawback of AA7075-T651 alloys in service is that they are susceptible to stress corrosion cracking (SCC) in chloride environments which can lead to system failure in the form of catastrophic fracture. In AA7075 there are soluble alloying elements such as zinc, copper and magnesium precipitate that grow along the grain boundaries. This phenomenon plays an important role in initiating the SCC mechanism. However, the fine magnesium and zinc precipitates within the grains improve the strength of the aluminium alloy and they are anodic with respect to the matrix elements. MgZn₂, which is the typical precipitate of the Al-Zn-Mg alloy, is potentially in an active phase with respect to the matrix [1], [2]. Anodic dissolution of grain boundary precipitates (GBP), which initiates the SCC phenomena, leads to many premature service failures in marine and aircraft structures owing to the conjoint effect of tensile stress and corrosive environment in the intergranular mode [3].

In aluminium alloys, SCC predominantly propagates in two different modes corresponding to region I and region II [4]. In particular, region I is formed in the crack tips or in the regions of metal lost due to pits formed by anodic dissolution. In the case of region II, SCC propagation is not influenced by tensile load. During this period, the material corrodes irrespective of time and degradation occurs at the material surface; the degradation is controlled by a physiochemical mechanism [5], [6]. Region II ends at the critical stress above which crack propagation results in mechanical fracture, which is not a result of the corrosion environment [7]. Moreover, the AA7075-T651 alloy contains insoluble intermetallic particles (Al₂Cu) or single element (Cu), which leads to a normal electrochemical reaction between the precipitates and matrix material and causes severe localised pitting in aggressive corrosion environments [8], [9]. Figure 1 shows the trend of the crack growth rate against the stress intensity factor.

Figure 1:

Schematic diagram of general trend of crack growth rate against stress intensity factor [4].

For high strength aluminium alloy, if it is exposed to a NaCl or a seawater environment, the entire system would be conducive to SCC. In addition, the study of threshold stress intensity including the effect of a corrosive environment at the crack tip is essential for any mechanical engineering design factors which are traditionally carried in large specimens like the double-end cantilever beam (DCB) specimen and compact tension (CT) specimen [10]. As the manufacturing cost of such specimens is too expensive, new types of circumferential notch tensile (CNT) specimens kindle more interest among researchers [11], [12], [13]. Also, it is advantageous to lessen the time required for identifying the corrosion morphology of various regions such as notch, SCC and ligament [14]. However, it is evident that research on SCC in high strength aluminium alloys using CNT specimens is scant. The present study is aimed to determine the threshold stress for SCC of AA7075-T651 alloy in a 3.5 wt.% NaCl solution using CNT specimens.

2. Experimental procedure

In this study, a high strength heat-treatable AA7075-T651 aluminium alloy was used as the base material. The chemical composition and mechanical properties of AA7075-T651 alloy (in received condition) are presented in Tables 1 and 2, respectively. The SCC test was carried out using CNT specimen (shown in Figure 2A and B). The CNT specimens were prepared as per the ASTM E8 M 04 standard with 60° notch with a 0.5-mm depth machined at the middle of the specimen for SCC study.

The SCC test was conducted in a 3.5 wt.% NaCl environment at various axial stress conditions expressed as percentages of the AA7075-T651 yield strength (YS). The stress was varied in decreasing order (100%, 80%, 60% and 40% of YS). The axial stress was applied to the CNT specimen. For each stress condition, three specimens were tested and the average value was used for the analysis. The corrosive environment was maintained with the help of a 3.5 wt.% NaCl environment flowing onto the specimen as shown in Figure 2. The tensile load is applied through the compression ring supported with the spring. The applied load was checked at 12-h intervals and adjusted as needed to the initial loading condition [15], hence a constant load was maintained in a NaCl environment. Figure 3 shows the constant load SCC test setup used in this study.

Figure 2:

Stress corrosion crack test specimen details.

(A) Specimen design. (B) Photograph of the specimen.

Figure 3:

Axial type constant load SCC test setup.

The fractured surface of the specimen was cleaned with orthophosphoric acid and analysed using scanning electron microscopy (SEM). As an extension of this study, the electrochemical polarisation behaviour of AA7075-T651 alloy was also evaluated. The 10×10×10 mm samples were prepared from the rolled plate of AA7075-T651 alloy for testing in the 3.5 wt.% NaCl environment. The electrochemical measurements were taken using an electrochemical system. In this test, the anodic polarisation curve was derived by exposing 10×10 mm surface area of high strength aluminium alloy to a 3.5 wt.% NaCl solution. In this condition, the applied current densities were maintained between 10 and 20 mA/cm² and the corresponding potentials were recorded for a period of 1 h. It is inferred from the results that the alloy shows a passivation region. A light optical microscope was employed to observe the microstructure of the polished parent metal using the standard metallographic method and etched using Keller’s reagent. To observe the preferential corrosion, the specimen was exposed to weight 2% of oxalic acid and examined using SEM to find the mode of fracture. The microstructure of AA7075-T651 specimen was analysed in a field emission transmission electron microscope.

3. Results

3.1 Characteristics of AA7075-T651 aluminium alloy

The optical micrograph of the AA7075-T651 alloy shown in Figure 4 reveals elongated equiaxed grain boundaries and the presence of precipitates oriented along the rolling direction.

Figure 4:

Optical micrograph of AA7075-T651.

Figure 5 shows the X-ray diffraction (XRD) results of the high strength aluminium alloy. The XRD pattern for as-received base metal is shown in Figure 5. The XRD pattern was confirmed by using JCPDS database; from these five peaks were identified and they were formed at 38°, 43°, 45°, 65° and 78° in 2θ angles. Each peak was matched based on its d-spacing from the JCPDS database. Three major peaks were identified as Al with higher intensity and two peaks corresponded to MgZn₂ and Al₇Cu₂Fe phase with lower intensity values. The microhardness of AA7075-T651 aluminium alloy was found to be 170 HV.

Figure 5:

XRD pattern of bulk AA7075-T651.

The fracture surface of both notched and unnotched specimens are shown in Figure 6A–D. A low magnification image of unnotched specimen consists of irregular peaks and valleys (Figure 6A) with some flat facets. A higher magnification of unnotched specimen reveals a stepped fracture with a small number of ductile dimples and cavities (Figure 6B). A low magnification image of notched specimen (Figure 6C) shows nearly flat surface compared to unnotched fractured surface; the ductile dimples were less in the notched specimen and some of the secondary phase particles were observed, which act as crack initiators during tensile load (Figure 6D).

Figure 6:

Fracture surfaces of tensile tested AA7075 in air environment.

(A, B) Fractured surface of unnotched specimen. (C, D) Fractured surface of notched specimen.

The finer microstructural features of the AA7075-T651 alloy specimens are shown in the transmission electron microscopy images (Figure 7). The AA7075-T651 alloy shows a continuous network of coarse GBP (Figure 7A). In addition, the grain matrix precipitates remain finer in the AA7075-T651 alloy raising the strength, while the matrix precipitates become finer (Figure 7B).

Figure 7:

Microstructure evolution of the precipitate morphology of AA7075-T651.

(A) Finer matrix precipitates. (B) Grain boundary precipitates.

3.2 Stress corrosion cracking behaviour

Figure 8 shows the relationship between applied load and time to failure. It is observed that the survival of each specimen is indirectly proportional to the applied stress. At an axial stress of 242 MPa, the specimen did not fracture up to 900 h and hence the test was stopped at 1000 h (refer Figure 8). This load is considered as the threshold stress. The corrosion elongation curves provide information about the SCC mechanisms involved in the high strength superstructural marine aluminium AA7075-T651 alloy. The corrosion elongation curve tested at 291 MPa (0.6×YS parent alloy) tensile stress in a 3.5 wt.% NaCl solution at room temperature is shown in Figure 9 and corrosion elongation parameters such as i_ss, t_ss and t_f were derived from this corrosion elongation curve. The slope of the curve in the secondary region before the time to transition (t_ss) from secondary to tertiary region represents the rate of steady state elongation (i_ss) and t_f represents the time to final failure. In the steady state (i_ss) period, the material involves corrosion reaction; however, it may prolong until crack initiates. This stage is called time to transition state (t_ss) and the material is severely affected by applied stress which leads to final fracture.

Figure 8:

Relationship between applied stress and time to failure.

Figure 9:

Corrosion elongation curve in a 3.5 wt.% NaCl solution and its parameters.

The fractured surface of SCC specimen is shown in Figure 10A–D. Before examining the fractured surface of the specimen using SEM, a mixture of chromium trioxide and orthophosphoric acid was used to clean the surface. A low magnification SCC specimen (Figure 10A) shows notched, SCC and mechanical failure regions. The interface between notch and SCC region reveals initiation of multiple cracks prior to mechanical failure. A higher magnification SEM image of SCC region is shown in Figure 10C. Intergranular fracture (along the grain boundaries) dominated the SCC region, whereas both intergranular and transgranular fracture mode were observed in the mechanical failure region (Figure 10D).

Figure 10:

SEM fractographs of AA7075-T651 in a 3.5 wt.% NaCl solution.

(A) Overall fractured surface. (B) Interface region. (C) SCC region. (D) Mechanical failure region.

3.3 Electrochemical behaviour

Determining the corrosion potential in aluminium alloys is very difficult as the alloy can be quickly passivated through the formation of a protective thin film, which can make it difficult to measure the current densities. The Tafel polarisation test was carried out to determine the corrosion rate of the aluminium alloy. Figure 11 shows a typical polarisation curve obtained in a solution of 3.5 wt.% NaCl, where i_corr is the corrosion current density derived by extrapolating the anodic and cathodic Tafel lines at E_corr in the absence of an inhibitor. This Tafel polarisation technique provides direct measurement of the corrosion current, which can be directly proportional to the corrosion rate.

Figure 11:

Tafel polarisation curves for high strength aluminium alloy in a 3.5 wt.% NaCl solution.

Figure 11 shows the total anodic and cathodic polarisation curve corresponding to hydrogen evolution and metal dissolution superimposed as a continuous curve. With this method, it is possible to measure low corrosion rate and it can be used for monitoring the corrosion rate of a system. The corrosion potential (E_corr) and current density (i_corr) and corrosion rate values of the AA7075-T651 alloy are presented in Table 3. Figure 12A shows a low magnification macrograph of potentiodynamic polarisation tested specimen of AA7075-T651 alloy in a 3.5 wt.% NaCl solution. SEM secondary electron imaging of the corroded surface at higher magnification reveals that localised pitting is initiated by anodic dissolution on the surface (Figure 12B).

Table 3:

Potentiodynamic polarisation test summary.

Specimen	Ecorr mV (vs. SCE)	icorr (mA/cm2)	Corrosion rate (mm/year)
AA7075-T651	−715.18	0.1099	1.372

Figure 12:

The corroded surface of the potentiodynamic polarisation test specimen of AA7075-T651 in a 3.5 wt.% NaCl solution.

(A) Low magnification. (B) High magnification.

4 Discussion

SCC in AA7075-T651 alloy is caused by a 3.5 wt.% NaCl solution and a constant tensile load. This simulated sea water environment contains a large amounts of chloride ions and these ions are very reactive with aluminium matrix. The chloride ions initiate the electrochemical pitting with aluminium matrix. In turn, pits initiate the anodic dissolution in the surface. The SCC of high strength aluminium alloys generally occurs in the dissolved anodic mode due to the aggressive influence of chloride ions on Zn containing second phase GBP [16], [17], [18], [19], [20]. The combined action of electrochemical attack and continuous tensile load ruptures the protective anodic film. The crack size and further the crack growth rate are increased by preferential attack of grain boundary zinc-rich precipitates. Figure 10C shows the intergranular SCC morphology of high strength aluminium alloy.

Recently, two SCC mechanisms in both zinc- and copper-rich-based aluminium alloys have been reported. One is chemical model in which constant load film ruptures and another is an anodic dissolution mechanism [21], [22]. According to the first mechanism, the oxide films are broken down in the aluminium zinc matrix surface because the contact surfaces are highly active in the 3.5 wt.% NaCl environment. The oxide film rupture leads to formation of a slip step that can influence the continuous crack growth of aluminium alloy. The key factor of SCC susceptibility attributed to a continuous distribution of GBP. This leads to corrosion crack initiation as it provides significant differences of potential between GBPs and the matrix element [23].

The results of a horizontal-type SCC constant load test of AA7075-T651 alloy in a 3.5 wt.% NaCl solution indicate that high strength aluminium alloy is susceptible to SCC in the anodic dissolution mode. Figure 12B shows a higher magnification micrograph of the degraded surface of the AA7075-T651 aluminium alloy sample after the potentiodynamic polarisation test in a 3.5 wt.% NaCl solution. It is observed from the figure that pits and corrosion products are formed on the exposed surface. It is evident that the effect of potentiodynamic anodic polarisation leads to formation of pitting. A similar trend has been reported in the literature [24], [25], [26]. Further, a micrograph revealed that SCC initiated from localised pitting corrosion in the base material.

5 Conclusions

The SCC behaviour of high strength aluminium AA7075-T651 alloy in a 3.5 wt.% NaCl solution was studied using a CNT specimen under constant load in a horizontal SCC testing machine. The important conclusions of this study are as follows:

1. The SCC is intergranular in nature under various tensile stress conditions, and the SCC mechanism is found to be localised anodic dissolution.

2. The threshold stress of high strength marine grade AA7075-T651 alloy in a 3.5 wt.% NaCl environment at room temperature is found to be 242 MPa.

3. The localised pitting corrosion is associated with GBP. This preferential attack is initiated near the second phase intermetallic GBP (MgZn₂) which are anodic to the AA7075-T651 aluminium alloy matrix.