The Effect of Applied Stress on Environment-Induced Cracking of AluminumAlloy 5052-H 3 in 0 . 5 MNaCl Solution

The environment-induced cracking (EIC) of aluminum alloy 5052-H3 was investigated as a function of applied stress and orientation (Longitudinal rolling direction—Transverse: LT and Transverse—Longitudinal rolling direction: TL) in 0.5 M sodium chloride solution (NaCl) using a constant load method. The applied stress dependence of the three parameters (time to failure; t f , steady-state elongation rate, lss, and transition time at which a linear increase in elongation starts to deviate, tss) obtained from the corrosion elongation curve showed that these relationships were divided into three regions, the stress-dominated region, the EICdominated region, and the corrosion-dominated region. Aluminum alloy 5052-H3 with both orientations showed the same EIC behavior. The value of tss/t f in the EIC-dominated region was almost constant with 0.57± 0.02 independent of applied stress and orientation. The fracture mode was transgranular for 5052-H3 with both orientations in the EIC-dominated region. The relationships between log Iss and log t f for 5052-H3 in the EIC-dominated region became a good straight line with a slope of −2 independent of orientation.


Introduction
The environment-induced cracking (EIC) behavior of metallic alloys in chloride and other corrosive solutions has been extensively investigated using various EIC methods [1][2][3][4][5][6][7], where EIC is composed of stress corrosion cracking (SCC) subjected to anodic reactions such as film formation and dissolution and hydrogen embrittlement (HE) to cathodic reactions such as hydrogen evolution.A number of theories have been developed for cracking mechanisms of aluminum alloys in chloride environments [5][6][7][8][9].Suresh et al. have concluded that EIC behavior in high-strength 7075 aluminum alloy under fatigue loading is mostly governed by three mechanisms, namely, the embrittling effect by the hydrogen products of the electrochemical reactions at the crack tip, the role of microstructure and slip mode and crack closure arising from environmental and microstructural elements [8,9].
It has been reported that the behavior of the metallic alloy can be characterized using a constant load method [10].The method can produce a corrosion elongation curve which can be used to obtain three parameters, namely, time to failure (t f ), steady-state elongation rate (l ss ), and transition time at which a linear increase in elongation starts to deviate (t ss ).These parameters were confirmed to be used in analyzing the failure behavior and to predict time to failure (t f ) [11,12].
The objectives of this research work are (1) to investigate the effect of applied stress on the susceptibility of aluminum alloys to EIC, (2) to evaluate the role of orientation in alloys, and (3) to elucidate a qualitative cracking mechanism for aluminum alloys in the chloride solution.

Experimental
The specimens used were made out of commercial 5052-H3 aluminum alloy.The geometry for EIC experiments was as follows: the gauge length 25.6 mm, its width 5 mm, and the thickness 1 mm.The specimen geometry is shown in Figure 1.The sample orientation used for the experiments  was TL (Transverse-Longitudinal rolling direction) and LT (Longitudinal rolling direction-Transverse) where, in the TL orientation, the former T is the loading direction and the later L is the direction of crack growth whereas, in the LT specimens, the loading direction is in the longitudinal (L) direction and the crack growth direction is the T direction.The chemical composition (wt%) for aluminum alloy 5052-H3 used was shown in Table 1.Prior to the experiments, the specimens were polished to 600 grit emery paper, degreased with acetone in an ultrasonic cleaner, and washed with distilled water.After the pretreatment, the specimens were immediately set into an SCC cell.The SCC tests were conducted in 0.5 M sodium chloride solutions at a test temperature of 353 K.The applied stress range for aluminum alloy 5052-H3 was from 140 MPa to 240 MPa.All experiments were carried out under an open-circuit condition.
A lever-type constant load apparatus (lever ratio 1 : 10) to which three specimens can be separately and simultaneously attached was used with a cooling system on the top to avoid evaporation of the solution during the experiments.The specimens were insulated from rod and grip by surfaceoxidized zirconium tube.A change in elongation of the specimens under the constant load condition was measured by an inductive linear transducer with an accuracy of ±0.01 mm.

Corrosion Elongation
Curve. Figure 2 shows an example of the corrosion elongation curve for 5052-H3 with the LT orientation at a test temperature of 353 K and an applied stress of 240 MPa in 0.5 M sodium chloride solution.From this curve the three parameters were obtained: time to failure (t f ), steady-state elongation rate (l ss ) for the straight part of the corrosion elongation curve, and transition time (t ss ), which is the time when the elongation curve begins to deviate from the linear increase and is indicated as t ss /t f in the paper.

Applied Stress
Dependence of Three Parameters (t f , l ss , and t ss /t f ). Figure 3 depicts the logarithm of time to failure (t f ) versus applied stress (σ) for 5052-H3 with the TL and LT orientations in a 0.5 M sodium chloride solution at a test temperature of 353 K.The relationships fell into three regions shown by Arabic numerals 1-3 in the figures, whereas the applied stress dependence of t f was identical independent of the orientation.Figure 4 shows the relationship between the logarithm (l ss ) and applied stress (σ).The relationships were also divided into three regions with the same applied stress ranges as those in Figure 3.The value of l ss in region 3 became significantly small compared to those in region 2, although this is not clear in Figure 3. Figure 5 shows the relationship between t ss /t f and σ.The value of t ss /t f was divided into three regions with the same applied stress ranges as those in Figures 3 and 4. The value of t ss /t f in region 2 kept constant with 0.57 ± 0.02 independent of applied stress and orientation.However, that of t ss /t f in region 3 was 0.8 to 0.9 irrespective of orientation, while in region 1 it became smaller with a value of 0.3 to 0.4.
Thus, the applied stress dependences of three parameters were divided into three regions (regions 1 to 3).In region 1 t f decreased rapidly with increasing applied stress with the rapid increase in l ss .In region 2 log t f increased and log l ss decreased linearly with decreasing applied stress and in region 3 log t f was not clear for deviation from the liner relationship in region 2, but log l ss showed clearly deviation.This behavior was almost the same as those for the solution annealed and sensitized austenitic stainless steels [13][14][15][16] and pure titanium [17].Therefore, it was decided that region 1 was the stress-dominated, region 2 the EICdominated region, and region 3 the corrosion-dominated region, respectively.

Fracture Appearance.
The fracture appearances of 5052-H3 with both orientations were investigated by using the scanning electron microscopy.Figure 6 shows the fracture appearance of 5052-H3 with the LT orientation in the EICdominated region (region 2).It was found that the fracture mode was predominantly transgranular (TG) with a dimple pattern, which was observed over the whole applied stresses in the EIC-dominated region.

4.1.
A Parameter for Predicting Time to Failure. Figure 7 shows the relationship between log t f and log l ss for 5052-H3 with both orientations, which was obtained by using those in Figures 3 and 4. The log t f versus log l ss curve in the EICdominated region for 5052-H3 became a straight line with a slope of m = −2 independent of orientation.Thus, we got the empirical equation as expressed in the following: where C 1 is a constant.From (1), it was found that l ss becomes a useful parameter for predicting t f independent of applied stress as well as for austenitic stainless steels in hydrochloric acid and sulphuric acid solutions [8], because l ss can be obtained at a time within 10-20% of t f from the corrosion elongation curve.

A Qualitative Proposal of EIC Mechanism.
In general, the fracture mode of aluminum alloys is recognized to be intergranular [18].In 5xxx series alloys, a precipitate of Mg 2 Al 3 is considered to exist along a grain boundary and to become anodic compared to grain itself [18], which may suggest that the EIC of 5052-H3 would be an intergranular stress corrosion cracking (IG-SCC).However, the present fracture mode was transgranular.This means that the precipitate would not affect the EIC behavior under the present experimental condition.
As for the EIC of the solution annealed and sensitized austenitic stainless steels in boiling saturated magnesium chloride solutions [13][14][15][16], when the EIC was TGSCC, the value of t ss /t f was 0.57 ± 0.02 and the slope of the linear relationship between log t f and log l ss was −2.On the other hand, in the case of HE, the value of t ss /t f was 0.8 ± 0.02 and the slope of the linear relationship was −1, which were similar to those for HE of pure titanium [17].Therefore, the present results obtained suggest that the EIC of 5052-H3 is TGSCC and would be explained by adopting the TGSCC mechanism of the austenitic stainless steels.The TGSCC was based on a cyclic film rupture-formation event.A crack is initiated at slip steps and a film formed at the crack tip inhibits dislocation movement, which is enhanced by metal dissolution.As a result, a dislocation pileup takes place and  an additional local stress in the vicinity of crack tip (σ app ) is generated in addition to a local stress caused by applied stress (σ app ), where a net local stress (σ tip ) = σ app + σ add .When σ tip reaches a critical film fracture stress (σ cri ), film fracture occurs.Such a film rupture-formation event is repeated inducing crack propagation until t ss and as a result the steady state elongation rate can be obtained.

Conclusions
The following conclusions can be drawn from this work.
(i) The applied stress dependences of the three parameters (t f , l ss , and t ss /t f ) obtained from the corrosion elongation curve showed that these relationships were divided into three regions, the stress-dominated region, the SCC-or HE-dominated region, and the corrosion-dominated region.(ii) Aluminum alloy 5052-H3 showed identical behavior in both orientations at all applied stress range.
(iii) The fracture mode for aluminum alloy 5052-H3 was transgranular in both orientations.
(iv) The relationships between log t f and log l ss for the aluminum alloy used became a good straight line.The slope of the line was −2.

Figure 3 :
Figure 3: The logarithm of time to failure (t f ) versus applied stress (σ) for A5052-H3 in a 0.5 M sodium chloride solution.