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Fatigue characteristics of 6061 aluminum alloy subject to 3.5% NaCl environment
International Journal of Fatigue 133 (2020) 105420
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Fatigue characteristics of 6061 aluminum alloy subject to 3.5% NaCl environment
T
Kittisak Chanyathunyaroja, , Sompob Phetchcraia, Ghit Laungsopapunb, Amornsak Rengsomboona ⁎
a
National Metal and Materials Technology Center, National Science and Technology Development Agency, 114 Thailand Science Park, Khlong Luang, Pathum Thani 12120 Thailand b Thailand Institute of Scientific and Technological Research, 35 Moo 3 Khlong Ha, Khlong Luang, Pathum Thani 12120, Thailand
ARTICLE INFO
ABSTRACT
Keywords: 6061 aluminum alloy Pitting corrosion Rotating bending fatigue
The rotating bending fatigue life of extruded 6061 aluminum alloy in a 3.5% NaCl environment was studied. It was found that under this environment solution fatigue life was significantly reduced by over 51 times, the fatigue life being shortened from several days to a few hours. Moreover, fatigue under the 3.5% NaCl solution could remove the endurance limit. The solution promoted the generation of many tiny pits, which were created rapidly on the surface of the specimens. These pits acted as crack nucleation sites leading to fatigue failure. The results indicate that pitting corrosion can markedly decrease the fatigue life.
1. Introduction 6061 aluminum alloy is a popular grade of wrought aluminum alloy used in various applications; such as automobiles, buildings, furniture, pipelines and structures that require strength, weldability and corrosion resistance. In the T6 condition it has a ultimate tensile strength (UTS) and a yield tensile strength (YTS) of 310 MPa and 276 MPa, respectively [1]. Normally, YTS is the factor used to design a static structure or machine. However, the design of parts subjected to dynamic loading must be based on a lower stress level known as the fatigue limit or endurance limit. This limit indicates the resistance of a material to withstand dynamic loads without failure. For most steels the values of stress amplitude at the endurance limit are in the range of 35% to 50% of the tensile strength [2]. The Wohler (stress-life or S-N) diagram, an important tool for fatigue life prediction, of a mild steel shows a characteristic “knee point” (the point where fatigue life abruptly increases from 105 to over 107 cycles), which is called the endurance limit. It is generally accepted that there is no fatigue limit or endurance limit (knee point at 105–107 cycles) in aluminum alloys which have a face-centered cubic (fcc) structure [3–5]. Based on 5 × 108 cycles of completely reversed stress using the R.R. Moore type of machine and specimen, the endurance limit of Al alloys has been defined as the stress amplitude which the specimen can withstand without failure for at least 107 fatigue cycles [2]. The stress amplitude value of 6061 aluminum alloy at this defined endurance limit of 107 fatigue cycles is quoted as
⁎
95 MPa which is 34.55% YTS (30.65% UTS) [6,7]. Metals and alloys are prone to various forms of attack in corrosive environments. For example, the chloride ion, naturally found in the seawater, is a common cause of corrosion damage in steel and aluminum alloys. In aluminum alloys pitting corrosion often develops when chloride is present on metal surfaces as Cl- ions are adsorbed on the natural oxide film in the initiation stage of corrosion [8–10]. In previous studies, the corrosion of 6061 aluminum alloy is generally observed as pitting and intergranular attack [11,12]. The 6061 alloy is susceptible to pitting corrosion in chloride containing solutions [13]. Pitting corrosion can develop when aluminum alloys are in permanent or intermittent contact through aqueous media such as water, rainwater and seawater [14]. Previous studies have reported on the combined effects of corrosion and fatigue in 7xxx aluminum alloys [15–19] showing that the bending fatigue resistance of 7075-T651 specimens was drastically decreased by pre-corrosion in a chloride solution. A decrease in fatigue life by a factor of 10 was observed [20]. The effect of pitting on 7075-T6 significantly reduced the bending fatigue performance to 60% of fatigue strength under no-corrosion conditions [21]. Pre-existing corrosion pits, produced by immersion in seawater, were found to reduce the fatigue life (piezoelectric accelerated fatigue test) of 7075-T6 aluminum alloy by a factor of 10–100 [22]. The fatigue lives of 2xxx aluminum alloys also decreased when tested in NaCl solution [23–25]. In a study of the combined effect of corrosion and fatigue in 6xxx aluminum alloys
Corresponding author. E-mail addresses: [email protected], [email protected] (K. Chanyathunyaroj).
https://doi.org/10.1016/j.ijfatigue.2019.105420 Received 6 September 2019; Received in revised form 15 November 2019; Accepted 11 December 2019 Available online 12 December 2019 0142-1123/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Fatigue 133 (2020) 105420
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Table 1 Chemical composition of 6061 alloy. Alloy
Standard AA6061 Specimen
Element (Weight Percent) Si
Fe
Cu
Mn
Mg
Cr
Al
0.40–0.80 0.70
< 0.7 0.21
0.15–0.40 0.24
< 0.15 0.05
0.8–1.2 1.05
0.04–0.35 0.13
Balance Balance
Fig. 1. (a) Dimensions of tensile specimen conforming to ASTM B557M, (b) Dimensions of rotating bending fatigue specimen conforming to ASTM E466, (c) The schematic of the environmental control module fitted to the fatigue testing machine.
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significantly reduce fatigue life as determined via rotating bending fatigue [27,28] and ultrasonic fatigue methods [29]. However, to the authors’ knowledge, there are no published reports on the effects of corrosive environments on aluminum alloy specimens tested by a rotating bending fatigue machine, which performs both torsion and tension actions. Most previous investigations were performed under different actions, such as bending fatigue testing, piezoelectric fatigue testing, tension fatigue, and ultrasonic fatigue testing. This work was undertaken to identify the influence of a corrosion environment on fatigue resistance under rotating bending fatigue test conditions. The rotating bending fatigue test pieces were prepared from commercial extruded 6061 aluminum alloy. During fatigue testing each test piece was subjected to a corrosive environment of 3.5% NaCl solution. 2. Experimental details The fatigue test pieces were prepared from 6061 aluminum alloy extruded rod with 16 mm in diameter. To avoid undesirable variations in chemical compositions, they were prepared from the same batch. Likewise, to avoid any variations in microstructure, macrostructure, and precipitation from heat treatment, they were prepared from asextruded aluminum rod. The chemical composition of the 6061 Aluminum alloy as determined by emission spectrometry is given in Table 1. The rotating bending fatigue specimens with polished surface of controlled roughness, as shown in Fig. 1(b), were prepared by using a CNC machine following ASTM E466 [30]. A Mitutoyo Surftest SV-3000 machine was used to measure the surface roughness. All rotating bending fatigue specimens had Ra values below 1 μm. The minimum diameter was 4 mm, and the radius R was 34 mm (Fig. 1(b)). The stress amplitude for testing was determined as a percentage of the YTS and varied from 35% YTS to 90% YTS (25%-60% UTS). A Yamamoto Giga Quad equipped with the environmental control module as shown in Fig. 1(c) was used for fatigue testing. The testing was divided into 2 conditions; (1) ambient environment and (2) 3.5% NaCl environment, Tests under both conditions were operated at a frequency of 50 Hz. During testing under the potentially corrosive environment, 1 ml/mm of 3.5% NaCl solution was dropped at the center of each specimen. A scanning electron microscope (SEM) equipped with EDS was used to examine the fracture surfaces of specimens after fatigue testing. Also, an Olympus LEXT OLS4100 3D laser measuring microscope was used to study the fracture surfaces and the corroded surfaces. The specimens for macro- and micro-structural examination were prepared by standard metallographic procedures in directions transverse and parallel to the extruded direction. Tucker’s reagent was used as a macro-etchant for 6061 aluminum alloy, following ASTM E340 [31]. Fig. 2 shows the macrographs and optical microstructure image of extruded 6061 aluminum alloy. Fig. 2(a) and (b) show the macrostructures observed in the parallel and transverse sections respectively. The figures show that the grain size inherited from production was variable but within a 5 mm radius from the center of the extruded rod, the grain size was homogeneous. The grain structure viewed in the parallel specimen was elongated in the working direction as shown in Fig. 2(c) and (d). The hardness was determined using a Brinell (2.5 mm ball) for both directions of the extruded rod. The hardness in the cross section (103 HB) was slightly lower than that in the parallel direction (108 HB), as shown in Fig. 3(e). Tensile test pieces following ASTM B557M [32], as shown in Fig. 1(a) were prepared by CNC machining. Gauge length was 75 mm with 12.5 mm diameter. An Instron 8801 (100 kN) machine was used for tensile testing at a strain rate of 1 mm/min.
Fig. 2. Macrographs, optical micrographs and mechanical properties of 6061 aluminum alloy as extruded: (a) parallel direction macrograph; (b) cross section macrograph; (c) parallel direction micrograph; (d) cross section micrograph; (e) Brinell hardness (HB); (f) yield strength, ultimate tensile strength (MPa) and elongation (%).
the fatigue life (tension cycle) of 6061-T651 aluminum alloy was found to be significantly reduced when tested in a 3.5% NaCl solution [26]. Some studies have simulated pitting corrosion by incorporating artificial holes made by mechanical or chemical actions in the surfaces of 6061-T6 aluminum alloy specimens. This artificial pitting could
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Fig. 3. Rotating bending fatigue results: (a) stress amplitude versus the number of cycles; (b) stress amplitude versus fatigue life.
The yield strength was determined based on 0.2% offset. The yield and ultimate tensile strengths of the extruded 6061 aluminum alloy employed in this experiment were 232 MPa and 342 MPa, respectively with elongation of 13.30% as shown in Fig. 2(f). The yield strength in Fig. 2(f) was used to calculate the testing weights. The load applied to the testing machine, W (kg), was calculated according to Eq. (1).
Table 2 Load applied, stress amplitude and % YTS used in fatigue testing. Load (kg)
Stress amplitude (MPa)
% YTS
3.24 3.14 3.04 2.94 2.84 2.74 2.54 2.44 2.24 2.04 1.84 1.74 1.64 1.44 1.34
204 198 192 185 179 173 160 154 141 129 116 110 103 91 84
88 85 83 80 77 75 69 66 61 56 50 47 44 39 36
W=
( d3)/(32gL)
(1)
where σ is the stress amplitude (MPa), d is the gauge diameter (mm), L is the distance from the center of gauge diameter to loading point (mm), and g is the gravitational constant (m/s2). Table 2 showed the applied loading which covered values between 35 and 90% YTS and the stress amplitudes used in this experiment. Previous studies on 6061 aluminum alloy (ambient environment) have reported that the stress amplitude at the endurance limit, based on a minimum fatigue life of 107 cycles, was not below approximately 35% YTS [6,7], and have used a frequency of 50–55 Hz for rotating bending fatigue testing [27,28]. Thus, the testing frequency used in this experiment followed these previous studies.
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Fig. 4. 3D laser measuring image, Macrograph and SEM images of fracture at high-stress amplitude (low number of cycles) in ambient environment.
Fig. 5. 3D laser measuring image, Macrograph and SEM images of fracture at low-stress amplitude (high number of cycles) in ambient environment.
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Fig. 6. 3D laser measuring image, Macrograph and SEM images of fracture at high-stress amplitude (low number of cycles) for exposure with 3.5% NaCl solution.
after 106 cycles took 5–6 h while that for 107 cycles took 2–3 days and that with 108 cycles took 23–24 days. When the stress amplitude was less than 75% YTS, the failure time was over 24 h (1 day). When the 129 and 116 MPa stress amplitudes were employed, the specimens could endure over 566 h (23.5 days) without any rupture. When tested with exposure to 3.5% NaCl solution the fatigue life of the 6061 alloy was reduced by a factor 10 compared to the ambient condition. At high-stress amplitudes (low number of cycles), the impact of the salt solution on the fatigue life was considered low (107 down to 106 cycles). However, at low-stress amplitudes (high number of cycles), its impact was high. The solution decreased the fatigue life from 1.0 × 108 cycles (without any rupture) to 1.9 × 106 cycles at the stress amplitude of 116 MPa (50.00% YTS), a reduction of about 51 times. At high-stress amplitudes, the time to failure was shortened from several hours to a few hours. When the stress amplitude was over 75% YTS, the fatigue life was less than 106 cycles such that no specimens could
3. Results and discussion 3.1. Rotating bending fatigue properties Fig. 3(a) shows the results of rotating bending fatigue testing. The dead weights were used as the loads in the experiment. The endurance limit for aluminum alloys is defined as the stress amplitude that the specimen can withstand for at least 107 fatigue cycles [2]. Under noncorrosive ambient conditions it was found that, when the stress amplitude was less than 75% YTS, the fatigue life of the as-extruded 6061 aluminum alloy was over 107 cycles. As expected, the fatigue life increased with decreasing load, e.g. for a stress amplitude of 56% YTS (129 MPa) the fatigue life approached 108 cycles. The specimens tested with 129 and 116 MPa (50% YTS) loads were not ruptured. Fig. 3(b) shows the observed relationship between stress amplitude and fatigue life in hours. Under the test frequency of 50 Hz, the specimen that failed
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Fig. 7. 3D laser measuring image, Macrograph and SEM images of fracture at low-stress amplitude (high number of cycles) for exposure with 3.5% NaCl solution.
endure over 5 h when exposed to 3.5% NaCl solution. At low-stress amplitudes, the time was shortened from several days to several hours. At stress amplitudes below 75% YTS, the fatigue life was less than 107 such that no specimens could endure over 28 h when exposed to 3.5% NaCl solution. The presence of 3.5% NaCl solution significantly reduced the 107 fatigue cycles endurance limit of the 6061 alloy. Moreover, when the minimum stress amplitude (84 MPa) was applied, the testing time was quite short, e.g., only 13 h The fatigue failure under exposure to 3.5% NaCl solution was more severe than was expected. In general, due to good corrosion resistance of the 6061 aluminum alloy, exposure for 24 h to 3.5% NaCl solution is not likely to cause significant corrosion attack [33]. However, the experimental results indicate that in 6061 exposed to 3.5% NaCl solution fatigue failure can take place in a relatively short period of time even at low stress amplitudes. Therefore, fatigue behavior of 6061 must be carefully considered when the alloy is be used in environments exposed
to salt water. Moreover, an understanding of the mechanisms by which fatigue failure of 6061 can be accelerated by exposure to chloride ions is crucial in mitigation and prevention of problems during service. 3.2. Fracture under rotating bending fatigue Figs. 4 and 5 show SEM and 3D laser measurement images of fractures that occurred under high-stress and low stress amplitudes (low and higher number of cycles). Typical fatigue failure consists of 3 stages: stage I is crack initiation, stage II is crack propagation and stage III is final fracture [34]. The fracture surfaces given in Figs. 4 and 5 show typical fatigue fracture characteristics. Fig. 4(c) and Fig. 5(c) show only one crack initiation site on each specimen. Fig. 4(d) and Fig. 5(d) show the fatigue striations which appear in stage II of the fatigue failure. The fatigue striations that have been produced under high-stress amplitude are wider than those produced at lower stress
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Fig. 8. Fracture of specimens: (a) ambient environment; (b) with 3.5% NaCl solution, the red line is center of the specimens.
millimeters. These fracture positions have larger diameters and are capable of bearing higher stress levels than the center. Hence the displaced positioning of these fractures has most likely resulted from corrosion damage due to contact between the specimen surface and the NaCl solution in the presence of oxygen. Fig. 9 shows optical micrographs of parallel and cross sections taken from fractured specimens. The fracture of the specimens tested without 3.5% NaCl solution (Fig. 9(a)) shows round and smooth surface. Whereas, the fracture of specimens tested with 3.5% NaCl solution (Fig. 9(b) and (c)) show some pits at the surface near fracture position. It is considered that these pits have been produced by corrosion and that such pitting corrosion facilitates crack nucleation leading to reduced resistance to fatigue failure.
amplitude. The ratio of stage II and stage III area in Fig. 4(b) was approximately 1:1. The specimen shown in Fig. 5 was subjected to a lower stress amplitude than the specimen shown in Fig. 4 and hence the area of stage II is larger than that of stage III, as indicated in Fig. 5(b). The final fracture areas of each specimen, as shown in Fig. 4(f) and Fig. 5(f), both exhibit dimpled surfaces that are characteristic of ductile failure via micro-void nucleation and coalescence. Figs. 6 and 7 show SEM and 3D laser measurement images of fractures produced at high-stress amplitude and low-stress amplitude under exposure to 3.5% NaCl solution. The fracture surfaces given in Figs. 6 and 7 also show typical features of fatigue fracture, but several crack initiation sites are observed on the surfaces. The crack initiation sites are described as major and minor. The major crack initiation sites are shown in Fig. 6(d) and Fig. 7(d) while the minor sites are shown in Fig. 6(b), (c) and (f) and Fig. 7(b), (c) and (f). The presence of minor crack initiation sites indicates the effect of 3.5% NaCl solution on multiplication of crack nucleation sites. Stages II and III of the fracture surfaces of specimens tested under 3.5% NaCl solution are similar to those of the specimens tested without the solution. The fracture surfaces of the specimens tested without the solution (Fig. 4(b) and Fig. 5(b)) are smoother than those of the specimens tested with 3.5% NaCl solution (Fig. 6(e) and Fig. 7(e)). The roughness around the major crack initiation sites are illustrated by the 3D laser measurement images (Fig. 4(a), 5(a), 6(a) and 7(a)). These images confirm that the specimens tested without the solution (Fig. 4(a) and Fig. 5(a)) have fracture surfaces around the major crack initiation sites that are smoother than those of the specimens tested with 3.5% NaCl solution (Fig. 6(a) and Fig. 7(a)). Fig. 8 shows the fractured specimens tested with and without 3.5% NaCl solution. The red line refers to the center of specimens. The fracture of the specimens tested without 3.5% NaCl solution (Fig. 8(a)) frequently occurs at the specimen center which has minimum crosssectional area and hence can withstand less loading than other positions. In contrast, some fracture positions of the specimens tested with 3.5% NaCl solution (Fig. 8(b)) are displaced from the center by a few
3.3. The effect of 3.5% NaCl solution on the surface of the 6061 aluminum alloy To better understand the corrosion behavior of 6061alloy in 3.5% NaCl solution, polarization tests were conducted using an Ag/AgCl standard reference electrode. As shown in Fig. 10(a) the polarization curve for the as-extruded surface has a current peak at approximately at −0.65 V (Ecorr = -0.65 V), whereas the polarization curve of the asmachined surface has a peak at approximately −0.83 V (Ecorr = −0.83 V) The polarization curves indicate that the machined surface of 6061 was more vulnerable to corrosion than the as-extruded surface. The form of corrosion that occurred during the polarization tests was identified as pitting corrosion. The nature of corrosion was also examined by testing a fatigue specimen on the fatigue machine under no applied load with 3.5% NaCl solution dropped at the center. After 1, 4 and 24 h, the specimen was removed from the machine and its surface was examined. Fig. 10 shows a 2D photograph of this specimen taken by using a 3D laser measurement microscope. Fig. 10(b) shows some scratches, remaining from polishing, on the surface of specimen. After 1 h (at the same area), there some small pits appeared on the surface of specimen as shown in Fig. 10(c). When the time was
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Fig. 10. (a) Polarization curves for as-extruded and as-machined surfaces. Also shown are 2D images (from 3D laser measurement microscope) for a fatigue specimen under no applied loading (zero stress amplitude) tested with 3.5% NaCl solution at different times: (b) 0 h; (c) 1 h; (d) 4 h; (e) 24 h.
indicated in Fig. 11). The area A in Fig. 11 is a free from pits: for this area no Fe, Cu, Cr and Mn was detected by EDS. For the B area, which contains pitting, EDS revealed the presence of Fe, Cu, Cr, and Mn alloying elements, indicating that pitting corrosion had occurred near the intermetallic compounds present in the microstructure of 6061alloy. Park et Al [35] suggested that Al3Fe intermetallic in 6061 Aluminium alloy serves as a local cathode and creates cavities in the host metal. Acosta and Veleva [36] investigated the corrosion of 6061-T6 aluminum alloy in diluted substitute ocean water and also found that Fe-rich intermetallic particles of Al-Fe-Si act as cathodes. Liang et Al [37] reported that alloying elements in the 6061 aluminum alloy affected corrosion rate. In agreement with other work [35,36] Zander et Al [38] concluded that pitting penetration followed the distribution of the Fe-rich intermetallic particles. The addition of Cu increases strength of 6061 but decreases corrosion resistance. The naturally formed oxide
Fig. 9. Optical micrographs of parallel direction and cross section from fractured specimens: (a) ambient environment; (b) and (c) with 3.5% NaCl solution.
increased to 4 h (Fig. 10(d)), the number and size of the pits increased. After 24 h the number of pits has further increased with the pit size becoming much larger than in the 4-hour example. Fig. 11 shows a SEM image of pitting corrosion caused by 3.5% NaCl solution dropped on the surface of the un-loaded 6061 fatigue specimen tested for 24 h. Table 3 shows chemical compositions determined by EDS of a pitting area (as
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leading to migration of Cl- ions toward this position in order to balance the positive charge on the Al3+ ions thus resulting in AlCl3 formation inside each pit [39]. Hydrolysis of AlCl3 then leads to formation of HCl and Al3+ with the HCl at the root of the pit causing acidification up to a pH value < 3 which further accelerates pit propagation. The Al3+ ions concentrated inside the pit will diffuse towards the pit opening and form Al(OH)3 corrosion product via the reduction reaction at the lateral surfaces near the top of the pit. For electron transfer pitting corrosion takes place near the intermetallic compounds. Thus, the distribution and size of pitting corrosion depend on the distribution and size of the intermetallic compounds in the microstructure. It has been recognized that the presence of pits, nucleated on second phase particles or precipitates, can significantly reduce the fatigue life under tension fatigue conditions [26]. The pits provide crack nucleation sites. After crack nucleation, subsequent fatigue cracking will depend on the loading conditions. Thus, when the crack initiation occurs rapidly, the fatigue of the aluminum alloy under the rotating bending fatigue is accelerated. Fig. 12 shows fracture paths in the 179 MPa stress amplitude specimen with exposure to 3.5% NaCl solution. Fig. 12 (a) and (b) show macrographs of fracture paths in different areas of the same specimen. The white arrow in Fig. 12(b) indicates pitting corrosion which can act as minor crack nucleation. Fig. 12(c) shows the major fracture path follows pitting corrosion damage. These observations strongly confirm that the pitting corrosion facilitates crack nucleation of the fracture path.
Fig. 11. SEM image of no load (zero stress amplitude) specimen after testing for 24 h with 3.5% NaCl solution.
Table 3 Chemical compositions of the pitting area from Fig. 11(b) measured by EDS (weight %). Positiion
A B
4. Conclusions Fatigue characteristics, under rotating bend testing of 6061 aluminum alloy exposed to 3.5% NaCl environment can be summarized as follows:
Element (Weight Percent) Mg
Al
Si
Cr
Mn
Fe
Cu
0.33 –
98.69 65.47
0.97 17.02
– 1.62
– 1.19
– 12.69
– 2.01
1. At high stress amplitude, greater than 75% yield strength, the fatigue life of extruded 6061 aluminum alloy is less than 107 cycles. Exposed to the 3.5% NaCl environment the fatigue life is decreased to around 106 cycles and the fatigue test duration is shortened from one day to just a few hours. 2. At low stress amplitude, lower than 75% yield strength, the fatigue life is almost 108 cycles. Exposure to the 3.5% NaCl environment decreased the fatigue life from 1.0 × 108 cycles to 1.9 × 106 cycles, a significant reduction of 51x. The fatigue test duration is also decreased from several days to several hours. 3. The 3.5%NaCl solution causes corrosion pits on the surface of 6061 aluminum alloys specimens which can act as crack nucleation sites. During the rotating bending fatigue, such pitting corrosion rapidly facilitates the nucleation of small cracks which are considered to be responsible for premature fatigue failure.
film on aluminum alloy becomes unstable in an environment containing chloride ions since the Cl- is adsorbed into the oxide film causing film breakdown with formation of micro-cracks a few nanometers in size [9,14,36]. The oxide film formed near the intermetallic compounds is thinner than in other areas. Hence preferential corrosion will tend to occur where this film is damaged by chloride attack. leading to formation of pits. The pitting corrosion propagates according to the following electrochemical reactions [14,39]: Oxidation at the anode (inside a pit): Al → Al3+ + 3e
Further study should be focused on effective protection of aluminum alloy subject to NaCl or other corrosive environments. Practical surface treatments should be investigated in terms of efficiency and effectiveness.
Al3+ + H2O → Al(OH)3 + 3H+ (Hydrolysis of AlCl3) Reduction at the cathode (outside the pit cavity): 2H
+
+ 2e− → H2
O2 + 2H2O + 4e− → 4OH−
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.
Previous studies have suggested that for electron transfer the pits would appear near the intermetallic compounds. The formation of Al3+ ions from the oxidation reaction takes place at the bottom of a pit
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Fig. 12. Macrograph and 2D images (from 3D laser measurement microscope) of fracture path from 179 MPa stress amplitude specimen with 3.5% NaCl solution: (a) and (b) macrograph of fracture path (from same specimen); (c) minor fracture; (d) major fracture.
Acknowledgements
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Aluminium-Copper-Lithium alloy 2050 in chloride solution. Mater Des 2015. https://doi.org/10.1016/j.matdes.2015.08.003. Ishihara S, Saka S, Nan ZY, Goshima T, Sunada S. Prediction of corrosion fatigue lives of aluminium alloy on the basis of corrosion pit growth law. Fatigue Fract Eng Mater Struct 2006. https://doi.org/10.1111/j.1460-2695.2006.01018.x. Sriraman MR, Pidaparti RM. Life prediction of aircraft aluminum subjected to pitting corrosion under fatigue conditions. J Aircr 2009. https://doi.org/10.2514/1. 40481. Mutombo K, Du Toit M. Corrosion fatigue behaviour of aluminium alloy 6061–T651 welded using fully automatic gas metal arc welding and ER5183 filler alloy. Int J Fatigue 2011. https://doi.org/10.1016/j.ijfatigue.2011.06.012. Dominguez Almaraz GM, Mercado Lemus VH, Villalón López JJ. Effect of proximity and dimension of two artificial pitting holes on the fatigue endurance of aluminum alloy 6061–T6 under rotating bending fatigue tests. Suppl Proc Mater Fabr Prop Charact Model. 2011. https://doi.org/10.1002/9781118062142.ch78. Takahashi Y, Shikama T, Yoshihara S, Aiura T, Noguchi H. Study on dominant mechanism of high-cycle fatigue life in 6061–T6 aluminum alloy through microanalyses of microstructurally small cracks. Acta Mater 2012. https://doi.org/10. 1016/j.actamat.2012.01.023. Takahashi Y, Yoshitake H, Nakamichi R, Wada T, Takuma M, Shikama T, et al. Fatigue limit investigation of 6061–T6 aluminum alloy in giga-cycle regime. Mater Sci Eng A 2014. https://doi.org/10.1016/j.msea.2014.07.039. MATERIALS. ASFTA. ASTM E466 − 15: Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials. ASTM 2015. http://doi.org/10.1520/E0466-15.
[31] ASTM International. ASTM E340 - 15, standard practice for macroetching metals and alloys. ASTM Int 2015:1–11. https://doi.org/10.1520/E0340-15.2. [32] Int ASTM. B557–15 standard test methods for tension testing wrought and cast aluminum- and magnesium-alloy products. Annu B ASTM Stand 2016. https://doi. org/10.1520/B0557-15. [33] Davis JR. Corrosion of aluminum and aluminum alloys. ASM Int 1999. https://doi. org/10.1361/caaa1999p001. [34] ASM International. Metals handbook volume 11 - failure analysis and prevention. ASM Handb 2002. https://doi.org/10.1016/S0026-0576(03)90166-8. [35] Park JO, Paik CH, Huang YH, Alkire RC. Influence of Fe-rich intermetallic inclusions on pit initiation on aluminum alloys in aerated NaCl. J Electrochem Soc 1999. https://doi.org/10.1149/1.1391637. [36] Acosta G, Veleva L. Mapping initial stages of localized corrosion of AA6061-T6 in diluted substitute ocean water by LEIS and SKP. Rev Metal 2018. https://doi.org/ 10.3989/revmetalm.134. [37] Liang WJ, Rometsch PA, Cao LF, Birbilis N. General aspects related to the corrosion of 6xxx series aluminium alloys: Exploring the influence of Mg/Si ratio and Cu. Corros Sci 2013. https://doi.org/10.1016/j.corsci.2013.06.035. [38] Zander D, Schnatterer C, Altenbach C, Chaineux V. Microstructural impact on intergranular corrosion and the mechanical properties of industrial drawn 6056 aluminum wires. Mater Des 2015. https://doi.org/10.1016/j.matdes.2015.05.079. [39] Alsamuraee A, Ameen H, Al-Rubaiey S. Evaluation of the pitting corrosion for aluminum alloys 7020 in 3.5% NaCl solution with range of temperature (100–500) °C. Am. J Sci Ind Res 2011. https://doi.org/10.5251/ajsir.2011.2.2.283.296.
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Fatigue characteristics of 6061 aluminum alloy subject to 3.5% NaCl environment
T
Kittisak Chanyathunyaroja, , Sompob Phetchcraia, Ghit Laungsopapunb, Amornsak Rengsomboona ⁎
a
National Metal and Materials Technology Center, National Science and Technology Development Agency, 114 Thailand Science Park, Khlong Luang, Pathum Thani 12120 Thailand b Thailand Institute of Scientific and Technological Research, 35 Moo 3 Khlong Ha, Khlong Luang, Pathum Thani 12120, Thailand
ARTICLE INFO
ABSTRACT
Keywords: 6061 aluminum alloy Pitting corrosion Rotating bending fatigue
The rotating bending fatigue life of extruded 6061 aluminum alloy in a 3.5% NaCl environment was studied. It was found that under this environment solution fatigue life was significantly reduced by over 51 times, the fatigue life being shortened from several days to a few hours. Moreover, fatigue under the 3.5% NaCl solution could remove the endurance limit. The solution promoted the generation of many tiny pits, which were created rapidly on the surface of the specimens. These pits acted as crack nucleation sites leading to fatigue failure. The results indicate that pitting corrosion can markedly decrease the fatigue life.
1. Introduction 6061 aluminum alloy is a popular grade of wrought aluminum alloy used in various applications; such as automobiles, buildings, furniture, pipelines and structures that require strength, weldability and corrosion resistance. In the T6 condition it has a ultimate tensile strength (UTS) and a yield tensile strength (YTS) of 310 MPa and 276 MPa, respectively [1]. Normally, YTS is the factor used to design a static structure or machine. However, the design of parts subjected to dynamic loading must be based on a lower stress level known as the fatigue limit or endurance limit. This limit indicates the resistance of a material to withstand dynamic loads without failure. For most steels the values of stress amplitude at the endurance limit are in the range of 35% to 50% of the tensile strength [2]. The Wohler (stress-life or S-N) diagram, an important tool for fatigue life prediction, of a mild steel shows a characteristic “knee point” (the point where fatigue life abruptly increases from 105 to over 107 cycles), which is called the endurance limit. It is generally accepted that there is no fatigue limit or endurance limit (knee point at 105–107 cycles) in aluminum alloys which have a face-centered cubic (fcc) structure [3–5]. Based on 5 × 108 cycles of completely reversed stress using the R.R. Moore type of machine and specimen, the endurance limit of Al alloys has been defined as the stress amplitude which the specimen can withstand without failure for at least 107 fatigue cycles [2]. The stress amplitude value of 6061 aluminum alloy at this defined endurance limit of 107 fatigue cycles is quoted as
⁎
95 MPa which is 34.55% YTS (30.65% UTS) [6,7]. Metals and alloys are prone to various forms of attack in corrosive environments. For example, the chloride ion, naturally found in the seawater, is a common cause of corrosion damage in steel and aluminum alloys. In aluminum alloys pitting corrosion often develops when chloride is present on metal surfaces as Cl- ions are adsorbed on the natural oxide film in the initiation stage of corrosion [8–10]. In previous studies, the corrosion of 6061 aluminum alloy is generally observed as pitting and intergranular attack [11,12]. The 6061 alloy is susceptible to pitting corrosion in chloride containing solutions [13]. Pitting corrosion can develop when aluminum alloys are in permanent or intermittent contact through aqueous media such as water, rainwater and seawater [14]. Previous studies have reported on the combined effects of corrosion and fatigue in 7xxx aluminum alloys [15–19] showing that the bending fatigue resistance of 7075-T651 specimens was drastically decreased by pre-corrosion in a chloride solution. A decrease in fatigue life by a factor of 10 was observed [20]. The effect of pitting on 7075-T6 significantly reduced the bending fatigue performance to 60% of fatigue strength under no-corrosion conditions [21]. Pre-existing corrosion pits, produced by immersion in seawater, were found to reduce the fatigue life (piezoelectric accelerated fatigue test) of 7075-T6 aluminum alloy by a factor of 10–100 [22]. The fatigue lives of 2xxx aluminum alloys also decreased when tested in NaCl solution [23–25]. In a study of the combined effect of corrosion and fatigue in 6xxx aluminum alloys
Corresponding author. E-mail addresses: [email protected], [email protected] (K. Chanyathunyaroj).
https://doi.org/10.1016/j.ijfatigue.2019.105420 Received 6 September 2019; Received in revised form 15 November 2019; Accepted 11 December 2019 Available online 12 December 2019 0142-1123/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Chemical composition of 6061 alloy. Alloy
Standard AA6061 Specimen
Element (Weight Percent) Si
Fe
Cu
Mn
Mg
Cr
Al
0.40–0.80 0.70
< 0.7 0.21
0.15–0.40 0.24
< 0.15 0.05
0.8–1.2 1.05
0.04–0.35 0.13
Balance Balance
Fig. 1. (a) Dimensions of tensile specimen conforming to ASTM B557M, (b) Dimensions of rotating bending fatigue specimen conforming to ASTM E466, (c) The schematic of the environmental control module fitted to the fatigue testing machine.
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significantly reduce fatigue life as determined via rotating bending fatigue [27,28] and ultrasonic fatigue methods [29]. However, to the authors’ knowledge, there are no published reports on the effects of corrosive environments on aluminum alloy specimens tested by a rotating bending fatigue machine, which performs both torsion and tension actions. Most previous investigations were performed under different actions, such as bending fatigue testing, piezoelectric fatigue testing, tension fatigue, and ultrasonic fatigue testing. This work was undertaken to identify the influence of a corrosion environment on fatigue resistance under rotating bending fatigue test conditions. The rotating bending fatigue test pieces were prepared from commercial extruded 6061 aluminum alloy. During fatigue testing each test piece was subjected to a corrosive environment of 3.5% NaCl solution. 2. Experimental details The fatigue test pieces were prepared from 6061 aluminum alloy extruded rod with 16 mm in diameter. To avoid undesirable variations in chemical compositions, they were prepared from the same batch. Likewise, to avoid any variations in microstructure, macrostructure, and precipitation from heat treatment, they were prepared from asextruded aluminum rod. The chemical composition of the 6061 Aluminum alloy as determined by emission spectrometry is given in Table 1. The rotating bending fatigue specimens with polished surface of controlled roughness, as shown in Fig. 1(b), were prepared by using a CNC machine following ASTM E466 [30]. A Mitutoyo Surftest SV-3000 machine was used to measure the surface roughness. All rotating bending fatigue specimens had Ra values below 1 μm. The minimum diameter was 4 mm, and the radius R was 34 mm (Fig. 1(b)). The stress amplitude for testing was determined as a percentage of the YTS and varied from 35% YTS to 90% YTS (25%-60% UTS). A Yamamoto Giga Quad equipped with the environmental control module as shown in Fig. 1(c) was used for fatigue testing. The testing was divided into 2 conditions; (1) ambient environment and (2) 3.5% NaCl environment, Tests under both conditions were operated at a frequency of 50 Hz. During testing under the potentially corrosive environment, 1 ml/mm of 3.5% NaCl solution was dropped at the center of each specimen. A scanning electron microscope (SEM) equipped with EDS was used to examine the fracture surfaces of specimens after fatigue testing. Also, an Olympus LEXT OLS4100 3D laser measuring microscope was used to study the fracture surfaces and the corroded surfaces. The specimens for macro- and micro-structural examination were prepared by standard metallographic procedures in directions transverse and parallel to the extruded direction. Tucker’s reagent was used as a macro-etchant for 6061 aluminum alloy, following ASTM E340 [31]. Fig. 2 shows the macrographs and optical microstructure image of extruded 6061 aluminum alloy. Fig. 2(a) and (b) show the macrostructures observed in the parallel and transverse sections respectively. The figures show that the grain size inherited from production was variable but within a 5 mm radius from the center of the extruded rod, the grain size was homogeneous. The grain structure viewed in the parallel specimen was elongated in the working direction as shown in Fig. 2(c) and (d). The hardness was determined using a Brinell (2.5 mm ball) for both directions of the extruded rod. The hardness in the cross section (103 HB) was slightly lower than that in the parallel direction (108 HB), as shown in Fig. 3(e). Tensile test pieces following ASTM B557M [32], as shown in Fig. 1(a) were prepared by CNC machining. Gauge length was 75 mm with 12.5 mm diameter. An Instron 8801 (100 kN) machine was used for tensile testing at a strain rate of 1 mm/min.
Fig. 2. Macrographs, optical micrographs and mechanical properties of 6061 aluminum alloy as extruded: (a) parallel direction macrograph; (b) cross section macrograph; (c) parallel direction micrograph; (d) cross section micrograph; (e) Brinell hardness (HB); (f) yield strength, ultimate tensile strength (MPa) and elongation (%).
the fatigue life (tension cycle) of 6061-T651 aluminum alloy was found to be significantly reduced when tested in a 3.5% NaCl solution [26]. Some studies have simulated pitting corrosion by incorporating artificial holes made by mechanical or chemical actions in the surfaces of 6061-T6 aluminum alloy specimens. This artificial pitting could
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Fig. 3. Rotating bending fatigue results: (a) stress amplitude versus the number of cycles; (b) stress amplitude versus fatigue life.
The yield strength was determined based on 0.2% offset. The yield and ultimate tensile strengths of the extruded 6061 aluminum alloy employed in this experiment were 232 MPa and 342 MPa, respectively with elongation of 13.30% as shown in Fig. 2(f). The yield strength in Fig. 2(f) was used to calculate the testing weights. The load applied to the testing machine, W (kg), was calculated according to Eq. (1).
Table 2 Load applied, stress amplitude and % YTS used in fatigue testing. Load (kg)
Stress amplitude (MPa)
% YTS
3.24 3.14 3.04 2.94 2.84 2.74 2.54 2.44 2.24 2.04 1.84 1.74 1.64 1.44 1.34
204 198 192 185 179 173 160 154 141 129 116 110 103 91 84
88 85 83 80 77 75 69 66 61 56 50 47 44 39 36
W=
( d3)/(32gL)
(1)
where σ is the stress amplitude (MPa), d is the gauge diameter (mm), L is the distance from the center of gauge diameter to loading point (mm), and g is the gravitational constant (m/s2). Table 2 showed the applied loading which covered values between 35 and 90% YTS and the stress amplitudes used in this experiment. Previous studies on 6061 aluminum alloy (ambient environment) have reported that the stress amplitude at the endurance limit, based on a minimum fatigue life of 107 cycles, was not below approximately 35% YTS [6,7], and have used a frequency of 50–55 Hz for rotating bending fatigue testing [27,28]. Thus, the testing frequency used in this experiment followed these previous studies.
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Fig. 4. 3D laser measuring image, Macrograph and SEM images of fracture at high-stress amplitude (low number of cycles) in ambient environment.
Fig. 5. 3D laser measuring image, Macrograph and SEM images of fracture at low-stress amplitude (high number of cycles) in ambient environment.
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Fig. 6. 3D laser measuring image, Macrograph and SEM images of fracture at high-stress amplitude (low number of cycles) for exposure with 3.5% NaCl solution.
after 106 cycles took 5–6 h while that for 107 cycles took 2–3 days and that with 108 cycles took 23–24 days. When the stress amplitude was less than 75% YTS, the failure time was over 24 h (1 day). When the 129 and 116 MPa stress amplitudes were employed, the specimens could endure over 566 h (23.5 days) without any rupture. When tested with exposure to 3.5% NaCl solution the fatigue life of the 6061 alloy was reduced by a factor 10 compared to the ambient condition. At high-stress amplitudes (low number of cycles), the impact of the salt solution on the fatigue life was considered low (107 down to 106 cycles). However, at low-stress amplitudes (high number of cycles), its impact was high. The solution decreased the fatigue life from 1.0 × 108 cycles (without any rupture) to 1.9 × 106 cycles at the stress amplitude of 116 MPa (50.00% YTS), a reduction of about 51 times. At high-stress amplitudes, the time to failure was shortened from several hours to a few hours. When the stress amplitude was over 75% YTS, the fatigue life was less than 106 cycles such that no specimens could
3. Results and discussion 3.1. Rotating bending fatigue properties Fig. 3(a) shows the results of rotating bending fatigue testing. The dead weights were used as the loads in the experiment. The endurance limit for aluminum alloys is defined as the stress amplitude that the specimen can withstand for at least 107 fatigue cycles [2]. Under noncorrosive ambient conditions it was found that, when the stress amplitude was less than 75% YTS, the fatigue life of the as-extruded 6061 aluminum alloy was over 107 cycles. As expected, the fatigue life increased with decreasing load, e.g. for a stress amplitude of 56% YTS (129 MPa) the fatigue life approached 108 cycles. The specimens tested with 129 and 116 MPa (50% YTS) loads were not ruptured. Fig. 3(b) shows the observed relationship between stress amplitude and fatigue life in hours. Under the test frequency of 50 Hz, the specimen that failed
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Fig. 7. 3D laser measuring image, Macrograph and SEM images of fracture at low-stress amplitude (high number of cycles) for exposure with 3.5% NaCl solution.
endure over 5 h when exposed to 3.5% NaCl solution. At low-stress amplitudes, the time was shortened from several days to several hours. At stress amplitudes below 75% YTS, the fatigue life was less than 107 such that no specimens could endure over 28 h when exposed to 3.5% NaCl solution. The presence of 3.5% NaCl solution significantly reduced the 107 fatigue cycles endurance limit of the 6061 alloy. Moreover, when the minimum stress amplitude (84 MPa) was applied, the testing time was quite short, e.g., only 13 h The fatigue failure under exposure to 3.5% NaCl solution was more severe than was expected. In general, due to good corrosion resistance of the 6061 aluminum alloy, exposure for 24 h to 3.5% NaCl solution is not likely to cause significant corrosion attack [33]. However, the experimental results indicate that in 6061 exposed to 3.5% NaCl solution fatigue failure can take place in a relatively short period of time even at low stress amplitudes. Therefore, fatigue behavior of 6061 must be carefully considered when the alloy is be used in environments exposed
to salt water. Moreover, an understanding of the mechanisms by which fatigue failure of 6061 can be accelerated by exposure to chloride ions is crucial in mitigation and prevention of problems during service. 3.2. Fracture under rotating bending fatigue Figs. 4 and 5 show SEM and 3D laser measurement images of fractures that occurred under high-stress and low stress amplitudes (low and higher number of cycles). Typical fatigue failure consists of 3 stages: stage I is crack initiation, stage II is crack propagation and stage III is final fracture [34]. The fracture surfaces given in Figs. 4 and 5 show typical fatigue fracture characteristics. Fig. 4(c) and Fig. 5(c) show only one crack initiation site on each specimen. Fig. 4(d) and Fig. 5(d) show the fatigue striations which appear in stage II of the fatigue failure. The fatigue striations that have been produced under high-stress amplitude are wider than those produced at lower stress
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Fig. 8. Fracture of specimens: (a) ambient environment; (b) with 3.5% NaCl solution, the red line is center of the specimens.
millimeters. These fracture positions have larger diameters and are capable of bearing higher stress levels than the center. Hence the displaced positioning of these fractures has most likely resulted from corrosion damage due to contact between the specimen surface and the NaCl solution in the presence of oxygen. Fig. 9 shows optical micrographs of parallel and cross sections taken from fractured specimens. The fracture of the specimens tested without 3.5% NaCl solution (Fig. 9(a)) shows round and smooth surface. Whereas, the fracture of specimens tested with 3.5% NaCl solution (Fig. 9(b) and (c)) show some pits at the surface near fracture position. It is considered that these pits have been produced by corrosion and that such pitting corrosion facilitates crack nucleation leading to reduced resistance to fatigue failure.
amplitude. The ratio of stage II and stage III area in Fig. 4(b) was approximately 1:1. The specimen shown in Fig. 5 was subjected to a lower stress amplitude than the specimen shown in Fig. 4 and hence the area of stage II is larger than that of stage III, as indicated in Fig. 5(b). The final fracture areas of each specimen, as shown in Fig. 4(f) and Fig. 5(f), both exhibit dimpled surfaces that are characteristic of ductile failure via micro-void nucleation and coalescence. Figs. 6 and 7 show SEM and 3D laser measurement images of fractures produced at high-stress amplitude and low-stress amplitude under exposure to 3.5% NaCl solution. The fracture surfaces given in Figs. 6 and 7 also show typical features of fatigue fracture, but several crack initiation sites are observed on the surfaces. The crack initiation sites are described as major and minor. The major crack initiation sites are shown in Fig. 6(d) and Fig. 7(d) while the minor sites are shown in Fig. 6(b), (c) and (f) and Fig. 7(b), (c) and (f). The presence of minor crack initiation sites indicates the effect of 3.5% NaCl solution on multiplication of crack nucleation sites. Stages II and III of the fracture surfaces of specimens tested under 3.5% NaCl solution are similar to those of the specimens tested without the solution. The fracture surfaces of the specimens tested without the solution (Fig. 4(b) and Fig. 5(b)) are smoother than those of the specimens tested with 3.5% NaCl solution (Fig. 6(e) and Fig. 7(e)). The roughness around the major crack initiation sites are illustrated by the 3D laser measurement images (Fig. 4(a), 5(a), 6(a) and 7(a)). These images confirm that the specimens tested without the solution (Fig. 4(a) and Fig. 5(a)) have fracture surfaces around the major crack initiation sites that are smoother than those of the specimens tested with 3.5% NaCl solution (Fig. 6(a) and Fig. 7(a)). Fig. 8 shows the fractured specimens tested with and without 3.5% NaCl solution. The red line refers to the center of specimens. The fracture of the specimens tested without 3.5% NaCl solution (Fig. 8(a)) frequently occurs at the specimen center which has minimum crosssectional area and hence can withstand less loading than other positions. In contrast, some fracture positions of the specimens tested with 3.5% NaCl solution (Fig. 8(b)) are displaced from the center by a few
3.3. The effect of 3.5% NaCl solution on the surface of the 6061 aluminum alloy To better understand the corrosion behavior of 6061alloy in 3.5% NaCl solution, polarization tests were conducted using an Ag/AgCl standard reference electrode. As shown in Fig. 10(a) the polarization curve for the as-extruded surface has a current peak at approximately at −0.65 V (Ecorr = -0.65 V), whereas the polarization curve of the asmachined surface has a peak at approximately −0.83 V (Ecorr = −0.83 V) The polarization curves indicate that the machined surface of 6061 was more vulnerable to corrosion than the as-extruded surface. The form of corrosion that occurred during the polarization tests was identified as pitting corrosion. The nature of corrosion was also examined by testing a fatigue specimen on the fatigue machine under no applied load with 3.5% NaCl solution dropped at the center. After 1, 4 and 24 h, the specimen was removed from the machine and its surface was examined. Fig. 10 shows a 2D photograph of this specimen taken by using a 3D laser measurement microscope. Fig. 10(b) shows some scratches, remaining from polishing, on the surface of specimen. After 1 h (at the same area), there some small pits appeared on the surface of specimen as shown in Fig. 10(c). When the time was
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Fig. 10. (a) Polarization curves for as-extruded and as-machined surfaces. Also shown are 2D images (from 3D laser measurement microscope) for a fatigue specimen under no applied loading (zero stress amplitude) tested with 3.5% NaCl solution at different times: (b) 0 h; (c) 1 h; (d) 4 h; (e) 24 h.
indicated in Fig. 11). The area A in Fig. 11 is a free from pits: for this area no Fe, Cu, Cr and Mn was detected by EDS. For the B area, which contains pitting, EDS revealed the presence of Fe, Cu, Cr, and Mn alloying elements, indicating that pitting corrosion had occurred near the intermetallic compounds present in the microstructure of 6061alloy. Park et Al [35] suggested that Al3Fe intermetallic in 6061 Aluminium alloy serves as a local cathode and creates cavities in the host metal. Acosta and Veleva [36] investigated the corrosion of 6061-T6 aluminum alloy in diluted substitute ocean water and also found that Fe-rich intermetallic particles of Al-Fe-Si act as cathodes. Liang et Al [37] reported that alloying elements in the 6061 aluminum alloy affected corrosion rate. In agreement with other work [35,36] Zander et Al [38] concluded that pitting penetration followed the distribution of the Fe-rich intermetallic particles. The addition of Cu increases strength of 6061 but decreases corrosion resistance. The naturally formed oxide
Fig. 9. Optical micrographs of parallel direction and cross section from fractured specimens: (a) ambient environment; (b) and (c) with 3.5% NaCl solution.
increased to 4 h (Fig. 10(d)), the number and size of the pits increased. After 24 h the number of pits has further increased with the pit size becoming much larger than in the 4-hour example. Fig. 11 shows a SEM image of pitting corrosion caused by 3.5% NaCl solution dropped on the surface of the un-loaded 6061 fatigue specimen tested for 24 h. Table 3 shows chemical compositions determined by EDS of a pitting area (as
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leading to migration of Cl- ions toward this position in order to balance the positive charge on the Al3+ ions thus resulting in AlCl3 formation inside each pit [39]. Hydrolysis of AlCl3 then leads to formation of HCl and Al3+ with the HCl at the root of the pit causing acidification up to a pH value < 3 which further accelerates pit propagation. The Al3+ ions concentrated inside the pit will diffuse towards the pit opening and form Al(OH)3 corrosion product via the reduction reaction at the lateral surfaces near the top of the pit. For electron transfer pitting corrosion takes place near the intermetallic compounds. Thus, the distribution and size of pitting corrosion depend on the distribution and size of the intermetallic compounds in the microstructure. It has been recognized that the presence of pits, nucleated on second phase particles or precipitates, can significantly reduce the fatigue life under tension fatigue conditions [26]. The pits provide crack nucleation sites. After crack nucleation, subsequent fatigue cracking will depend on the loading conditions. Thus, when the crack initiation occurs rapidly, the fatigue of the aluminum alloy under the rotating bending fatigue is accelerated. Fig. 12 shows fracture paths in the 179 MPa stress amplitude specimen with exposure to 3.5% NaCl solution. Fig. 12 (a) and (b) show macrographs of fracture paths in different areas of the same specimen. The white arrow in Fig. 12(b) indicates pitting corrosion which can act as minor crack nucleation. Fig. 12(c) shows the major fracture path follows pitting corrosion damage. These observations strongly confirm that the pitting corrosion facilitates crack nucleation of the fracture path.
Fig. 11. SEM image of no load (zero stress amplitude) specimen after testing for 24 h with 3.5% NaCl solution.
Table 3 Chemical compositions of the pitting area from Fig. 11(b) measured by EDS (weight %). Positiion
A B
4. Conclusions Fatigue characteristics, under rotating bend testing of 6061 aluminum alloy exposed to 3.5% NaCl environment can be summarized as follows:
Element (Weight Percent) Mg
Al
Si
Cr
Mn
Fe
Cu
0.33 –
98.69 65.47
0.97 17.02
– 1.62
– 1.19
– 12.69
– 2.01
1. At high stress amplitude, greater than 75% yield strength, the fatigue life of extruded 6061 aluminum alloy is less than 107 cycles. Exposed to the 3.5% NaCl environment the fatigue life is decreased to around 106 cycles and the fatigue test duration is shortened from one day to just a few hours. 2. At low stress amplitude, lower than 75% yield strength, the fatigue life is almost 108 cycles. Exposure to the 3.5% NaCl environment decreased the fatigue life from 1.0 × 108 cycles to 1.9 × 106 cycles, a significant reduction of 51x. The fatigue test duration is also decreased from several days to several hours. 3. The 3.5%NaCl solution causes corrosion pits on the surface of 6061 aluminum alloys specimens which can act as crack nucleation sites. During the rotating bending fatigue, such pitting corrosion rapidly facilitates the nucleation of small cracks which are considered to be responsible for premature fatigue failure.
film on aluminum alloy becomes unstable in an environment containing chloride ions since the Cl- is adsorbed into the oxide film causing film breakdown with formation of micro-cracks a few nanometers in size [9,14,36]. The oxide film formed near the intermetallic compounds is thinner than in other areas. Hence preferential corrosion will tend to occur where this film is damaged by chloride attack. leading to formation of pits. The pitting corrosion propagates according to the following electrochemical reactions [14,39]: Oxidation at the anode (inside a pit): Al → Al3+ + 3e
Further study should be focused on effective protection of aluminum alloy subject to NaCl or other corrosive environments. Practical surface treatments should be investigated in terms of efficiency and effectiveness.
Al3+ + H2O → Al(OH)3 + 3H+ (Hydrolysis of AlCl3) Reduction at the cathode (outside the pit cavity): 2H
+
+ 2e− → H2
O2 + 2H2O + 4e− → 4OH−
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.
Previous studies have suggested that for electron transfer the pits would appear near the intermetallic compounds. The formation of Al3+ ions from the oxidation reaction takes place at the bottom of a pit
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Fig. 12. Macrograph and 2D images (from 3D laser measurement microscope) of fracture path from 179 MPa stress amplitude specimen with 3.5% NaCl solution: (a) and (b) macrograph of fracture path (from same specimen); (c) minor fracture; (d) major fracture.
Acknowledgements
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