The effect of sensitization and fatigue loading frequency on corrosion fatigue of AA5083-H131

International Journal of Fatigue 124 (2019) 1–9

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International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue

The effect of sensitization and fatigue loading frequency on corrosion fatigue of AA5083-H131

T

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Rebecca M. Baya, David J. Schrocka, , Allison M. Akmana, Leslie G. Blanda, Ramgopal Thodlab, Jenifer S. (Warner) Lockea a b

Fontana Corrosion Center, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA DNVGL Materials Technology Development, 5777 Frantz Road, Dublin, OH 43017, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: AA5083-H131 Corrosion fatigue Sensitization Loading frequency da/dN

Corrosion fatigue crack growth rates (da/dN) of AA5083-H131 when loaded in 3.5 wt% NaCl increase as the level of sensitization increases and exhibit a strong inverse fatigue loading frequency dependence when heavily sensitized. High Mg 5xxx series Al alloys are known to exhibit decreased resistance to intergranular corrosion and intergranular stress corrosion cracking due to the formation of the active Al3Mg2-β phase on grain boundaries when exposed to slightly elevated temperatures for prolonged periods of time, a phenomenon known as sensitization. High resolution fracture mechanics based experiments conducted at constant ΔK and R-ratio under full immersion in 3.5 wt% NaCl show that da/dN in a heavily sensitized microstructure increases by an order of magnitude for each order of magnitude decrease in fatigue loading frequency, while da/dN for the asreceived and unsensitized microstructure has little to no frequency dependence. Futhermore, for higher frequency loading at 1 Hz, da/dN for the highly sensitized microstructures is one order of magnitude higher than the as-received microstructure; while for the low frequency loading at 0.01 Hz, da/dN for the highly sensitized microstructure is three orders of magnitude higher than the as-received microstructure. The findings of this study suggest that typical higher frequency laboratory testing studies at 10 Hz may severely underestimate fatigue life under service relevant loading conditions.

1. Introduction Strength in 5xxx series Al-Mg alloys is derived from solid solution strengthening and strain hardening. Therefore, the optimal microstructure should have Mg in solid solution and not contain Mg-bearing precipitates. In order to impart sufficient strength, the majority of the commercial alloys used in naval applications today (AA5083, AA5456, and AA5086) contain Mg concentrations greater than the room temperature solubility limit of ∼3 wt% Mg. As a result, there is a thermodynamic driving force for precipitation of the Mg-rich β phase (Al3Mg2). While the kinetics for this precipitation are slow at room temperature, β precipitation does occur along grain boundaries during long term exposure at elevated temperatures (40–∼200 °C), which is possible for the solar exposure experienced in service [1]. This process of β phase formation on grain boundaries at elevated temperatures is known as sensitization. Because β phase is electrochemically more active than the Al matrix, it undergoes rapid dissolution when corrosive environments are present [2]. Therefore, sensitization is undesirable as it drastically decreases corrosion resistance and causes intergranular ⁎

corrosion (IGC) [3–12]. The problem of sensitization leading to IGC is so important that for Al-Mg alloy development and lot release acceptance an ASTM standard test method (ASTM G67 – NAMLT) has been established to quantify alloy susceptibility to IGC by determining the degree of sensitization (DoS) [13]. DoS is a mass loss per unit surface area (mg/cm2) obtained after immersion in reagent grade nitric acid (HNO3) at 30 °C for 24 h. Reviews on 5xxx Al alloy sensitization, grain boundary β precipitation and growth, and IGC/intergranular stress corrosion cracking (IGSCC) behavior have been written by Jones et al., Lim et al., Holroyd et al., and Birbilis et al. [1,4,9,14]. In addition to drastically reducing corrosion resistance, it has been well established that sensitization promotes environment assisted cracking (EAC) [4,6,14–25]. Significant attention has been given to understanding the effects of Al-Mg alloy sensitization on IGSCC [4,6,15,17–21,25]. As a result, a critical level of sensitization above which rapid IGSCC occurs has been determined and it has been found that applying potentials below the β breakdown potential, the electrochemical potential below which the active β phase will no longer experience rapid corrosive dissolution, can eliminate the effect of

Corresponding author. E-mail address: [email protected] (D.J. Schrock).

https://doi.org/10.1016/j.ijfatigue.2019.02.044 Received 28 December 2018; Received in revised form 25 February 2019; Accepted 26 February 2019 Available online 27 February 2019 0142-1123/ © 2019 Elsevier Ltd. All rights reserved.

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sensitization on IGSCC [15,17,25,26]. While significant strides have been made in quantifying the effect of sensitization on IGC and IGSCC, only limited attention has been given to corrosion fatigue (CF). CF is the material degradation process that results from the simultaneous interaction of corrosion and cyclic deformation. As a result of local crack tip dissolution, da/dN is enhanced and ΔKTH, the threshold below which CF does not occur, is reduced relative to measurements in inert gas or vacuum [27–38]. CF is cycle and/or time dependent [29,39] and depends on both mechanical and environmental driving forces for cracking. The mechanical driving force for cracking is quantified by (1) the stress intensity range, ΔK and (2) the maximum stress intensity, Kmax, or the stress ratio (R = Kmin/ Kmax). The environmental driving force for cracking is quantified by (1) the fatigue loading frequency (f) and the local crack tip electrochemical parameters of (2) electrochemical potential (E) and (3) pH. Most CF studies on Al-Mg alloys are of limited utility because they (1) only examined non-sensitized material in the as-received condition [40–42], (2) tested in vacuum, lab air, or water vapor environments [40,43] that do not sufficiently replicate the corrosive conditions likely encountered by sea-based in-service naval conditions, or (3) quantified the effect of sensitization only under high loading f [22–24]. Previous studies by Holtz et al. [22–24], which utilized high f loading, do provide insight on the effect of sensitization on CF. These studies examined CF of AA5083-H131 loaded in the most susceptible S-L orientation (fatigue loading in the short transverse, S, direction with crack propagation in the longitudinal, L, direction) under full immersion in an aqueous NaCl solution containing a chromate inhibitor. Several important findings came out of the Holtz et al. studies. First, at 10 Hz, it was established that sensitizing AA5083-H131 from the as-received to a highly sensitized condition increases da/dN by approximately an order of magnitude or more for high R (0.85) loading. For low R (0.1) loading, no effect of sensitization was observed. Second, it was established that the ΔKTH was significantly reduced for DoS values above 30 mg/cm2. ΔKTH was high and constant below a DoS of 30 mg/cm2. Third, da/dN increased and ΔKTH decreased as sensitization increased up to a DoS of 42 mg/cm2, after which no continued degradation was measured. In all, the studies by Holtz et al. concluded that 30 mg/cm2 was a critical level of sensitization above which CF properties are appreciably degraded. It was also postulated that the degradation in ΔKTH occurred when Kmax was above the IGSCC threshold (K1SCC, threshold below which IGSCC does not occur, also known as KTH). The terminology KTH is typically used instead of KISCC when comparing values for fracture toughness to values for KISCC [44]. This implies that the onset of CF may be driven by susceptibility to IGSCC. Studies to directly probe this theory were not conducted. While the previous studies by Holtz et al. [22–24] establish that sensitization affects CF, several critical questions remain:

Fig. 1. Optical micrographs of the AA5083-H131 microstructure for a sample sensitized at 175 °C for 240 h.

H131. To accomplish this, da/dN is measured as a function of f and sensitization for a single and constant applied ΔK and R over the f range of 1–0.01 Hz for sensitization levels ranging from 3 to 42 mg/cm2. 2. Experimental procedure 2.1. Material A 3 in. thick plate of AA5083-H131 manufactured by Aleris, Inc. with a nominal composition of Al − 4.7 Mg − 0.67 Mn − 0.1 Cr − 0.054 Zn − 0.027 Ti − 0.026 Cu − 0.27 Fe − 0.24Si (wt%) was utilized in this study. The as-received material has a yield strength of 313 MPa, ultimate tensile strength of 342 MPa, and grain size of 250 μm in the L direction (Longitudinal aka rolling direction), 70 μm in the T direction (Long Transverse direction, direction parallel to the width of a rolled plate), and 25 μm in the S direction (Short Transverse direction, direction parallel to the thickness of a rolled plate). The rolled anisotropic microstructure is shown in Fig. 1. Optical microscopy samples for microstructural characterization were swab etched with Keller’s etch (2 mL HF, 5 mL HNO3, 3 mL HCl, and 190 mL H2O) for 7 min and observed optically.

• Do the findings from the high f studies by Holtz et al. correlate to inservice conditions where low f loading is expected? • Does the ability to apply high f laboratory data to service conditions depend on the level of sensitization? • As f decreases, does the critical level of sensitization below which CF remains un-affected by sensitization change? • Studies by Holtz et al. concluded that CF properties are degraded

2.2. Sensitization All samples investigated in this study were sensitized prior to machining by a furnace hold at 175 °C for 3, 6, 10, 60, 100, or 240 h (h) followed by air cooling. 2.3. Nitric acid mass loss testing (NAMLT)

most severely when Kmax for CF loading is above K1SCC [22–24]. If this is true, can highly sensitized material be rendered immune to CF if Kmax is below K1SCC? Can low levels of sensitization experience CF property degradation similar to that seen for the sensitized material if Kmax is above K1SCC?

Prior to CF testing, ASTM G67 NAMLT testing [45] was conducted to determine the level of sensitization for AA5083-H131 samples utilized in this study. NAMLT specimens of dimension 2.54 cm × 2.54 cm × 0.635 cm were sectioned from the plate such that one face with dimension 2.54 cm × 2.54 cm contained the rolled surface. These samples were sensitized identically to the CF testing samples and used as a measure for the DoS of the CF samples. NAMLT testing of the CF samples themselves is not possible as it is a destructive test. In accordance to ASTM G67 [45], all samples were weighed, the

The goal of this work is to bridge the gap in the fundamental understanding of the effect of Al-Mg alloy sensitization on IGSCC, which is well understood, and CF, where critical and systematic studies are lacking. Specifically, the objective of this study is to understand the effect of f on da/dN as a function of level of sensitization in AA50832

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quantified for samples with DoS values 7, 10, 16, 18, 33, and 42 mg/ cm2 using test methods defined in ASTM E1820 [48]. Experiments were run under displacement control with a displacement rate of 4.6 × 10−9 m/s (nominal K-rate of 0.57 MPa√m/h) and were conducted in the same closed stainless steel cell described above using a servo-electric Interactive Instruments load frame. Specimens, grips, and portions of the pull rods were fully immersed in 3.5 wt% NaCl solution and electrically isolated from the sample grips and load frame with alumina (Al2O3) coated pins and an epoxy or platers tape coating on the interiors of the grips to ensure that an electric couple between the specimen and grips was not created. An 80%N2-20%O2 air mixture was bubbled through the closed cell and the open circuit potential (OCP) was recorded with a potentiostat during the test.

specific surface areas measured, and then immersed in 5% NaOH at 80 °C for 60 s to desmut, rinsed with water, and immersed in reagent grade HNO3 for 30 s. Samples were then immersed three at a time in a beaker of HNO3 lined with glass beads for maximum surface area exposure and exposed for 24 h at 30 °C. Following the exposure to HNO3, samples were cleaned per ASTM G67 guidelines and dried overnight, after which their final weight was measured and DoS reported. NAMLT testing was conducted twice to ensure repeatability. The average DoS is reported in this work. 2.4. CF crack growth testing For CF testing, compact tension (CT) specimens with a width (W) of 2.54 cm and thickness (B) of 1.27 cm were machined in the short transverse (S) – longitudinal (L) orientation (S-L, loading in the through plate thickness direction (S) and crack propagation in the rolling direction (L)). Specimens were pre-cracked on a servo-hydraulic mechanical testing frame at Westmoreland Mechanical Testing and Research Inc. to a crack length of 0.899 cm based on surface measured crack lengths. Precracking was performed according to ASTM E647 [46] at a f of 25 Hz, with Kmax < 7.7 MPa√m and an R of 0.16. CF experiments were performed under load control in a closed stainless steel cell using a servo-electric Interactive Instruments load frame. CF specimens, grips, and portions of the pull rods were fully immersed in 3.5 wt% NaCl solution. CF specimens were electrically isolated from the clevis style grips and load frame using alumina (Al2O3) coated clevis pins and an epoxy or platers tape coating on the CT sample to ensure that a galvanic couple between the specimen and grips/load frame was not created. An 80%N2-20%O2 air mixture was bubbled through the closed cell and the open circuit potential (OCP) was recorded with a potentiostat during testing. Constant ΔK experiments with a ΔK of 4.4 MPa√m, R of 0.43, and varied f were conducted using a sinusoidal waveform. The f was varied over a range of 1–0.01 Hz. For a majority of experiments, f was initially high and progressively decreased. Each f segment was maintained for at least 1.27 mm (0.05 in) of linear crack growth. After each f segment, f was manually changed to the next f after ensuring at least 1.27 mm on constant da/dN. The fatigue crack growth rate was determined for each individual f in all experiments by plotting crack length (a) as a function of fatigue loading cycles (N) and determining the slope. Steady linear growth was determined by an R2 value of at least 0.960 (a majority of R2 values were above 0.99). Crack length was monitored using the direct current potential difference (DCPD) method [47] with a setup having an average minimum detectable crack length extension of 25–30 μm. The applied current was 4 amps. For each CF CT test sample, the starting notch length and the final crack length at the end of testing (af) were measured optically or through scanning electron microscopy (SEM). The actual af was measured and compared to the af calculated via Johnson’s equation at the end of testing. For any experiments containing a greater than 5% error in actual versus Johnson’s equation af, post-test correction was conducted to ensure the actual ΔK applied was within –0.4 and +0.6 MPa√m of that intended (intended ΔK is 4.4 MPa√m). Post-test analyses were performed by calculating the actual applied ΔK using the applied maximum and minimum load and corrected crack length as opposed to the Johnson’s equation crack length value. The Johnson’s equation crack length value at any given crack length was corrected to an actual crack length by assuming a linear accumulation in % error from the visually confirmed notch length to the visually confirmed actual af, when viewable. Any da/dN associated with a ΔK less than 4 MPa√m and greater than 5 MPa√m was deemed unacceptable and was not reported.

2.6. SEM SEM was performed on an FEI Quanta 200 SEM with an accelerating voltage of 20 keV and a spot size of 5. This technique was used to measure af and characterize the fracture surface. The true nature of the posttest fracture surfaces were typically obscured by corrosion product. In order to remove this corrosion product without changing the character of the underlying fracture surface, samples were sonicated at room temperature in reagent grade HNO3 for 10 min, rinsed with running water, and dried with compressed air. A similar technique for removing corrosion product from aluminum test specimens using short duration exposure in reagent grade HNO3 is detailed in ASTM G01-03 [49]. Samples were cleaned by sonication in ethanol for 2 min immediately before imaging. 3. Results 3.1. NAMLT The effect of sensitization time at 175 °C on the DoS as measured by the ASTM G67 NAMLT is shown in Fig. 2. The DoS levels examined in this study range from 3 mg/cm2, for the as-received condition, to 42 mg/cm2 after a 240 h sensitization at 175 °C, as shown in Fig. 2. Three of the six DoS levels examined have an average DoS below 25 mg/cm2 (3, 7, and 18 mg/cm2) and the remaining three are above 25 mg/cm2 (33, 37, and 42 mg/cm2). For the purposes of this study, DoS values below 25 mg/cm2 are referred to as “low” sensitization, the DoS value of 33 mg/cm2 is referred to as “intermediate” sensitization, and DoS values at and around 35 mg/cm2 are referred to as “high”

50 AA5083-H131 ASTM G67

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Sensitization Time (h) 2.5. KTH testing Fig. 2. ASTM G67 NAMLT DoS values for AA5083-H131 as a function of time held at 175 °C.

The threshold below which cracking does not occur, KTH, was 3

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10-1

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AA5083-H131 (S-L) ΔK = 4.4 MPa√m R = 0.43 3.5 wt% NaCl, OCP

ƒ = 0.1 Hz

da/dN (mm/cyc)

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Fig. 3. Fatigue crack growth kinetics as a function of ASTM G67 NAMLT DoS for AA5083-H131 loaded in the S-L orientation under a ΔK of 4.4 MPa√m and R of 0.43 while being fully immersed in 3.5% NaCl under freely corroding conditions. The data in each plot were measured at a specific f. (a) corresponds to 1 Hz, (b) to 0.1 Hz, and (c) to 0.01 Hz. Data points represent the average DoS calculated from multiple ASTM G67 NAMLT tests at a single sensitization time at 175 °C. Error bars indicate the highest and lowest ASTM G67 NAMLT DoS value measured for the specific sensitization time.

clusters of behavior become apparent with clustering according to “low”, “intermediate”, and “high” sensitization. For 0.01 Hz loading, da/dN is accelerated by ∼3 orders of magnitude when comparing the “low” sensitization with a DoS of 42 mg/cm2. It is important to note that the lowest da/dN measured for “low” sensitization levels was that measured for the DoS of 7 mg/cm2 where da/dN was 1 × 10−4 mm/ cyc, which is ∼3 orders of magnitude lower than that measured for DoS of 42 mg/cm2. In general, each order of magnitude decrease in f produced about an order of magnitude increase in da/dN when comparing “high” versus “low” sensitization levels.

sensitization. According to ASTM G67 [45], DoS values between 1 and 15 mg/cm2 are considered IGC resistant and DoS values at and above 25 mg/cm2 are considered IGC susceptible. 3.2. Effect of sensitization on da/dN at a singular f Fig. 3 establishes that da/dN increases as DoS increases at 1 Hz (Fig. 3a), 0.1 Hz (Fig. 3b), and 0.01 Hz (Fig. 3c) loading. At 1 Hz, da/dN is accelerated by approximately an order of magnitude when comparing da/dN between the as-received condition (3 mg/cm2) and DoS levels of 33 mg/cm2 and greater. Additionally, there appear to be two clusters of data. DoS at and below 18 mg/cm2 are low and near 1 × 10−4 mm/cyc, while DoS at and above 33 mg/cm2 are higher and near 7 × 10−4 mm/ cyc. Table 1 shows the specific da/dN measured for the as-received, 33 mg/cm2, and 42 mg/cm2 conditions. At 0.1 Hz loading, the acceleration of “high” DoS da/dN over “low” DoS levels is approximately 1.5 orders of magnitude, as shown in Table 1. Additionally, at 0.1 Hz, three

3.3. Effect of sensitization on f-dependence of da/dN for a singular sensitization level The effect of sensitization on the f-dependence of da/dN varies depending on sensitization level, as shown in Fig. 4. For “low” sensitization levels, da/dN does not exhibit a f dependence (i.e. for the entire f range examined, da/dN is within typical experimental scatter [46]). 4

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Table 1 Fatigue crack growth kinetics for DoS levels of 3 mg/cm2 (“low” sensitization), 33 mg/cm2 (“intermediate” sensitization), and 42 mg/cm2 (“high” sensitization) at fatigue loading f of 1, 0.1, and 0.01 Hz. Where more than one test was conducted, more than one da/dN is shown. Loading frequency

3 mg/cm2 (AR), Low sensitization (mm/cyc)

33 mg/cm2, Intermediate sensitization (mm/cyc)

42 mg/cm2, High sensitization (mm/cyc)

1 Hz 0.1 Hz 0.01 Hz

6.3 × 10−5 1.6 × 10−4 3.0 × 10−4

4.5 × 10−4, 9.6 × 10−4 1.3 × 10−3 1.3 × 10−3

5.0 × 10−4 3.9 × 10−3, 6.2 × 10−3 9.2 × 10−2

Fig. 4. Fatigue crack growth kinetics as a function of fatigue loading f for AA5083-H131 sensitized to various ASTM G67 NAMLT levels, loaded in the S-L orientation under constant ΔK of 4.4 MPa√m and R of 0.43 while fully immersed in 3.5% NaCl under freely corroding conditions.

Fig. 5. SCC KTH as a function of ASTM G67 DoS level. The solid horizontal line shows Kmax of 7.7 MPa√m used in CF testing.

product during testing, despite use of the nitric cleaning procedures described in Section 2.6. All fracture surfaces in the current work are comparable to those shown by Holtz et al. [23], which were reported to be intergranular in nature.

While “intermediate” sensitization levels show increased da/dN over the “low” sensitization levels, f-independent behavior is observed causing da/dN to be consistently ∼1 order of magnitude higher than the “low” sensitization levels. It is important to note that for both the “low” and “intermediate” sensitization levels, there may be an increasing da/dN with decreasing f trend at the higher loading f, but this is very speculative due to the scatter in measured da/dN for the few repeat data sets generated in this study. More testing to generate statistics around the scatter in da/dN at each f would be required to definitively state that anything other than a f-independent trend appears. For “high” DoS values (above 33 mg/cm2), there exists an inverse f dependence for da/dN. As shown in Table 1, comparing da/dN for 42 mg/cm2, da/dN is accelerated by ∼1 order of magnitude and over 2 orders of magnitude (225×) for 0.1 and 0.01 Hz, respectively, when compared against 1 Hz.

4. Discussion The results of this study confirm the hypothesis that performing CF testing at high f can underestimate the effect of sensitization on da/dN. Specifically, da/dN measured under high f loading, greater or equal to 1 Hz, poorly estimates low f da/dN for high levels of sensitization, particularly those above 33 mg/cm2. It is important to note that high f testing is likely sufficient to estimate “low” DoS (up to 18 mg/cm2) da/ dN and possible “intermediate” level da/dN. 4.1. Effect of sensitization at a singular f

3.4. KTH testing

The results shown in Fig. 3 establish that sensitization negatively impacts CF crack growth. At high f, sensitization accelerates da/dN by approximately an order of magnitude for heavily sensitized AA5083H131 over the as-received microstructure. This is similar to the findings of Holtz et al. [22,23] where 10 Hz loading was conducted. For low f (0.01 Hz), sensitization accelerates da/dN by approximately three orders of magnitude for heavily sensitized AA5083-H131 over the as-received microstructure. This implies that the formation of β phase along grain boundaries has a deleterious effect on CF da/dN. For all f tested, the data generated at and below a DoS of 18 mg/cm2 show similar da/dN across three sensitization levels suggesting that there is a critical amount of beta phase below which CF properties are unaffected by sensitization. This is consistent with the findings of Holtz et al. who found no effect of sensitization on CF properties below a DoS of 30 mg/cm2 [22,23]. It is important to note that Holtz et al. additionally shows that da/dN is accelerated over vaccum in aqueous

Results of KTH testing, shown in Fig. 5, establish a decreasing KTH with increasing DoS trend. This is consistent with the literature [18,19,23]. KTH decreases from ∼24–21 MPa√m for “low” sensitization levels to ∼17 MPa√m for 33 mg/cm2 and to ∼8 MPa√m for 42 mg/ cm2. 3.5. Fractography Figs. 6, 7, and 8 show fracture surfaces for sensitization levels of 7, 33, and 42 mg/cm2 at 1 Hz (Fig. 6), 0.1 Hz (Fig. 7), and 0.01 Hz (Fig. 8). In general, all fracture surfaces appear similar for all sensitization levels and f examined, indicating that the fracture mode is not dependent on the amount of β phase on the grain boundaries. The character of the fracture surfaces is difficult to discern due to the buildup of corrosion 5

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Fig. 6. SEM micrographs of CF fracture surfaces tested at 1 Hz for DoS of (a) 7 mg/cm2, (b) 33 mg/cm2, and (c) 42 mg/cm2. Fatigue crack propagation was horizontal to the plane of page.

that are sensitization level dependent. Even though an effect of sensitization exists with “intermediate” sensitization, da/dN is accelerated by approximately an order of magnitude over that for “low” sensitization da/dN and the “low” and “intermediate” sensitization da/dN is findependent. This suggests that high f testing can estimate low f performance. Yet for the “high” sensitization levels, da/dN is inversely dependent on f with da/dN increasing by at least an order of magnitude for every order of magnitude decrease in loading f. This implies that a critical level of sensitization, and possibly a degree of grain boundary β phase coverage, exists where the mechanism driving CF changes.

chloride solutions regardless of level of sensitization [22,23]. In contrast, prior studies by Crane et al. showed SCC degradation occurred at DoS as low as ∼0 mg/cm2 [18,19]. This may suggest that SCC is negatively impacted by sensitization at lower DoS levels than CF. The grain boundary coverage of β phase was not quantified in this study or the prior Crane et al. study [15,18,19,50]. As such, it is possible that the grain boundary β coverage was not equivalent for the same levels DoS levels between prior work in the literature and this work. Results for loading f at and below 0.06 Hz are inconclusive on the existence of an upper limit to the effect of sensitization as DoS of 37 and 42 mg/cm2 have similar da/dN for all f at and above 0.06 Hz, but at 0.01 Hz a separation between 37 and 42 mg/cm2 exists. Hotlz et al. [22,23] reported that no further degradation in CF properties occurred at DoS greater than 42 mg/cm2 . No sensitization levels above a DoS of ∼42 mg/cm2 were studied here. An upper limit in the effect of sensitization would be expected should all Mg supersaturated in solid solution precipitate out into β phase or grain boundary saturation of β phase occur, i.e. complete coverage as suggested by Holtz et al. [23]. Further testing is needed at 0.01 Hz to conclusively determine if a separation occurs between these two “high” sensitization levels. It is possible that da/dN is artificially low for the 0.01 Hz 37 mg/cm2 data point as corrosion product induced crack closure was observed in some testing segments of “high” sensitization levels when the loading f was very low. While the a vs N data for the 0.01 Hz 37 mg/cm2 data point does not appear to be influenced by crack closure, it was not conducted past the usual segment length or using a crack mouth opening displacement (CMOD) to definitively rule out any small influence of crack closure.

4.3. Possible mechanism(s) for cracking For 5xxx Al-Mg alloys, definitive evidence establishing the mechanism driving the decreased resistance to IGSCC and CF is lacking. For IGSCC, recent experimental evidence supports a mechanism of coupled grain boundary β dissolution followed by crack solution acidification and subsequent Hydrogen Environment Assisted Cracking (HEAC) of the α-Al ligament between the discrete grain boundary β particles [4,15–17,26]. Another proposed mechanism depends solely on the role of β phase dissolution followed by crack solution acidification [4,25,51] and cannot be ruled out. For CF, little attention has been paid to determining the driving mechanism as it has been assumed to be the same for IGSCC and CF. For the “low” and “intermediate” sensitization levels where da/dN is f independent, behavior is consistent with that predicted by HEAC. In fact, HEAC as the mechanism driving SCC in sensitized AA5083-H131 was suggested by Crane et al. [18,19]. Crane et al. proposed that above a critical sensitization level sufficient grain boundary β phase coverage existed to produce a hydrolytic crack solution acidification upon crack tip β phase dissolution. This crack tip acidification then promoted the production and uptake of embrittling H, which can allow cracking of the α-Al ligament between discrete β phase particles by HEAC. A f-

4.2. Effect of sensitization on f-dependence of da/dN for a singular sensitization level The data in Fig. 4 clearly establishes two different frequency trends

Fig. 7. SEM micrographs of CF fracture surfaces tested at 0.1 Hz for DoS of (a) 7 mg/cm2, (b) 33 mg/cm2, and (c) 42 mg/cm2. Fatigue crack propogation was horizontal to the plane of page. 6

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Fig. 8. SEM micrographs of CF fracture surfaces tested at 0.01 Hz for DoS of a) 7 mg/cm2, b) 33 mg/cm2, and c) 42 mg/cm2. Fatigue crack propagation was horizontal to the plane of page.

10-1

above which SCC is possible, KTH, is below Kmax in a fatigue loading cycle. During any portion of the fatigue loading cycle where K is above KTH, SCC can occur and increase da/dN over that expected for CF alone [54,55]. The superposition principle governs this interplay between SCC and CF [54,55]. Eq. (1) is a mathematical expression of the superposition principle [54,56]. The parameter ф accounts for the environmental acceleration of da/dN over rates in an inert environment and is ΔK dependent. The second term accounts for contributions from SCC and is inversely proportional to f and directly proportional to the average crack growth rate for SCC (dā/dt) for the applied K during the fatigue cycle.

AA5083-H131 (S-L) ΔK = 4.4 MPa√m R = 0.43 3.5 wt% NaCl, OCP

10-2

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AR (DoS 3) 1hr (DoS 7) 10 hr (DoS 18) 60 hr (DoS 33) 100 hr (DoS 37) 240 hr (DoS 42) Crane (DoS 42)

10-6

1 da¯ ⎛ da ⎞ = ϕ ⎛ da ⎞ + ⎛ ⎞ f ⎝ dt ⎠SCC ⎝ dN ⎠CF ⎝ dN ⎠inert

Fig. 5 shows the results of KTH testing and establishes that KTH decreases with increasing DoS. Additionally, the KTH for a DoS of 42 mg/ cm2 is ∼8 MPa√m. The Kmax during all CF testing in this study was 7.7 MPa√m. Therefore, it is likely that SCC is superimposing on CF for the “high” sensitization levels when K is at or near Kmax. The observed behavior of KTH vs DoS also suggests that at low values of DoS, the Kmax was significantly below KTH; and as such, there is no contribution of static SCC crack growth on CF da/Dn. No KTH test was conducted on a DoS 37 mg/cm2 sample. In order to investigate the role of SCC in CF da/dN for the “high” sensitization levels, fatigue crack growth kinetics were plotted in terms of da/dt with units of mm/s instead of da/dN in units of mm/cyc to facilitate co-plotting of data with SCC data from Crane et al. [18,19]. Fig. 9 shows the fatigue crack growth kinetics in units of mm/s as a function of f. da/dN for all DoS values except 42 mg/cm2 show a decreasing da/dt with decreasing f trend. For a DoS of 42 mg/cm2, da/dt increases with decreasing f and approaches that measured by Crane et al. [18,19] for SCC testing with a similar DoS. It is important to note that SCC da/dt from Crane et al. [18,19] was measured via slow rising displacement tests, which would mimic the loading portion of a fatigue loading cycle, particularly at low f. This lends support to the theory that the inverse f dependence is due to a superposition between SCC and CF for the highest sensitization levels when Kmax is approaching KTH. Further supporting evidence is found in Fig. 10, which shows da/dN and a corresponding linear fit as a function of f for only the DoS of 42 mg/cm2. It can be seen that the slope is ∼−1, which is consistent with the superposition principle. Another possible reason for the inverse f dependence is a crack length effect. For most tests within this work, the tests were conducted starting at high f (1 Hz) and systematically lowering f. As such, the high f data was typically collected at shorter crack lengths than the high f data. Longer cracks are predicted to have more acidic electrolytes [57–66], which could cause an increase in da/dN from either increased H production and uptake per HEAC or increased crack tip dissolution for crack advance. Note, there is no f dependence for DoS of 33 mg/cm2

-7

10

0.0001 1E-4

0.001

0.01 0.1 Frequency (Hz)

1

(1)

10

Fig. 9. Crack growth kinetics plotted in units of mm/s as a function of fatigue loading f. The open star represents SCC data by Crane et al. [18,19] with a DoS of 42 mg/cm2.

independent behavior below a critical f is predicted by the HEAC mechanism. Per the HEAC mechanism, da/dN increases with decreasing f when f is above this critical f because lower f provides more time per loading cycle for H to diffuse to the fatigue process zone (FPZ) damage sites. Therefore, as f decreases, more H diffuses to the FPZ in a single cycle to increase crack propagation rates. At and below this critical f, f is low enough to provide sufficient time for the critical maximum amount of H needed for HEAC damage to diffuse to FPZ damage sites achieving the maximum environmental contribution to CF [52,53]. This is supported by the fact that the f at which da/dN becomes f-independent is predicted by H diffusion modeling [52,53]. For the “low” sensitization level, a possible decreasing da/dN with increasing f trend is observed above 0.3 Hz implying that 0.3 Hz may be this critical f for AA5083-H131, but more testing is required to definitively determine an inverse f trend above 0.3 Hz for the “low” sensitization levels. This is partially supported by the fact that the da/dN values measured by Holtz et al. [22,23] are about 5-10x lower that the plateau values measured in this study. It is not unreasonable to conclude that da/dN increases as f decreases from 10 Hz and reaches a plateau at or above 0.3 Hz. For the “high” sensitization levels, the f dependence is not consistent with lower sensitization levels or HEAC; and there are several possible reasons for this. The first possible mechanism that may be driving the inverse f dependence of the “high” sensitization levels is a superposition of SCC onto CF. This has been proposed previously for AA5083-H131 by Holtz et al. [22,23] and can occur when the threshold 7

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0

10

• For DoS levels of 18 mg/cm

AA5083-H131 (S-L) ΔK = 4.4 MPa√m R = 0.43 3.5 wt% NaCl, OCP

•

da/dN (mm/cyc)

10-1

10-2

•

10-3

•

10-4 240 hr (DoS 42) Linear Fit 10-5 0.001

0.01

• 0.1

1

10

•

Frequency (Hz) Fig. 10. Fatigue crack growth kinetics as a function of fatigue loading f for AA5083-H131 sensitized to a DoS of 42 mg/cm2 and loaded in the S-L orientation under constant ΔK of 4.4 MPa√m and R of 0.43 while fully immersed in 3.5% NaCl under freely corroding conditions.

and below. Therefore, it does not seem likely that a crack length effect is driving the inverse f dependence. Increased crack tip acidity to drive increased da/dN may also be due to increasing β phase dissolution as a larger portion of the crack path is covered with β phase. For the more sensitized microstructures, there is more grain boundary β phase present along the fracture path to dissolve. As suggested by Crane et al. [19], this increased grain boundary β will drive hydrolytic crack solution acidification. It is possible that the “high” sensitization levels have sufficient grain boundary β phase to facilitate severe hydrolytic crack solution acidification. Should this occur, lower f would provide more time for crack path anodic dissolution to drive crack advance. This increased anodic dissolution can do one of two things. (1) Increased crack tip dissolution will facilitate increased H production and uptake per HEAC. (2) Per the anodic dissolution mechanism, crack advance will occur due to active dissolution of fresh metal at the crack tip. As such, the inverse f dependence may solely be due to crack solution acidification from increased coverage of β phase and subsequent crack tip anodic dissolution driving CF. The evidence in Figs. 9 and 10 lends strong evidence in support of superposition of SCC on CF being the mechanism driving the inverse f dependence and the large effect of sensitization (accelerations in da/dN of over an order of magnitude). However, the other two possible theories cannot be ruled out. As such, further studies are warranted to definitively determine the underlying mechanism. CF studies conducted on “low” and “intermediate” levels of sensitization where the Kmax is approaching KTH should be conducted to determine if an inverse f dependence appears. Additionally, CF testing on “high” sensitization levels where Kmax is far below KTH should be conducted to determine if an order of magnitude acceleration in da/dN persists for all f. Unfortunately these studies were not conducted here, but are being conducted on AA5456-H116, which will be the subject of further publications.

2 and below, da/dN is f independent implying that typical high f, on the order of 10 or 1 Hz, laboratory testing can accurately predict in-service loading conditions that may be a lower f. For high DoS levels at and above 37 mg/cm2, an inverse f dependence exists causing an order of magnitude increase in da/dN for every order of magnitude decrease in f. This suggests that utilizing high f laboratory testing to understand in-service behavior may severely underestimate CF damage. For the intermediate DoS level of 33 mg/cm2, da/dN was f independent, but did exhibit an order of magnitude acceleration in da/dN over the as-received microstructure. For high f loading (1 Hz), sensitization increases da/dN an order of magnitude, which is consistent with prior literature reports for AA5083-H131. For low f loading, sensitization can increase da/dN by over three orders of magnitude when comparing the as-received microstructure with a heavily sensitized one. Strong evidence exists suggesting that the inverse f dependence is due to the superposition of SCC on CF. Specifically, when sensitization causes KTH to fall below CF Kmax, SCC can occur during a loading cycle to further increase CF over that expected for CF alone. This implies that keeping loading conditions below those causing SCC can minimize, but not eliminate, the effect of sensitization on CF.

Acknowledgments The research conducted at OSU was funded by the Office of Naval Research Young Investigator Award (ONR YIP) managed by Dr. Airan Perez under contract number N00014-16-1-2756. Support for the graduate student Rebecca Bay was provided by the DoD Technical Corrosion Collaboration managed by the US Air Force Academy (USAFA) under contract number FA7000-12-2-0015. Research conducted at DNV-GL was funded under the Office of Naval Research managed by William Nickerson through grant number N00014-11-10892. SEM was supported in part by The Ohio State University Institute for Materials Research. The support technicians at DNV-GL, particularly Frankie Guglielmi, were instrumental in this research and are greatly acknowledged. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Office of Naval Research. The opinions, findings, views, conclusions or recommendations contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of USAFA or the US government. The US government is authorized to reproduce and distribute reprints for governmental purposes not withstanding any copyright notation therein. Distribution Statement A: distribution unlimited. References [1] Birbilis DN, Zhang MR, Knight DS, Holtz DR, Goswami DR, Davies PC. A survey of sensitisation in 5xxx series aluminium alloys. Corrosion, doi:10.5006/1787. [2] Lyndon JA, Gupta RK, Gibson MA, Birbilis N. Electrochemical behaviour of the βphase intermetallic (Mg2Al3) as a function of pH as relevant to corrosion of aluminium–magnesium alloys. Corros Sci 2013;70:290–3. [3] Dix EHJ, Anderson WA, Shumaker MB. Influence of service temperature on the resistance of wrought aluminum-magnesium alloys to corrosion. Corrosion 1959;15:19–26. [4] Jones RH, Baer DR, Danielson MJ, Vetrano JS. Role of Mg in the stress corrosion cracking of an Al-Mg alloy. Metall Mat Trans A 2001;32:1699–711. [5] Lim MLC, Scully JR, Kelly RG. Intergranular corrosion penetration in an Al-Mg alloy as a function of electrochemical and metallurgical conditions. Corrosion 2013;69:35–47. [6] F.S. Bovard, Sensitization and environmental cracking of 5xxx aluminum marine sheet and plate alloys. In: Shifler DA, Tsuru T, Nitishan PM, Ito S, editors. Corrosion in marine and saltwater environments II. Pennington, New Jersey: The Electrochemical Society; 2005. p. 232–43. [7] Bumiller E. Intergranular corrosion in AA5xxx aluminum aloys with discontinuous

5. Conclusions AA5083-H131 was sensitized to DoS levels between 3 and 42 mg/ cm2 at a temperature of 175 °C and underwent CF testing in 3.5 wt% NaCl solution under freely corroding conditions conducted at a constant ΔK of 4.4 MPa√m and R of 0.43 while varying the fatigue loading f between 1 and 0.1 Hz to observe the effects of sensitization and f on CF behavior. The following conclusions can be drawn from this work: 8

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[8]

[9]

[10]

[11] [12]

[13]

[14] [15] [16] [17] [18] [19] [20] [21] [22]

[23]

[24]

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[35]

[36]

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