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Effect of Mg concentration on interfacial strength and corrosion fatigue behavior of thermal-sprayed Al-Mg coating layers
Engineering Failure Analysis 88 (2018) 13–24
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Effect of Mg concentration on interfacial strength and corrosion fatigue behavior of thermal-sprayed Al-Mg coating layers
T
Sarita Morakula, Yuichi Otsukab,*, Yukio Miyashitac, Yoshiharu Mutohb a b c
Graduate School of Material Science, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka-shi, Niigata 940-2188, Japan Department of System Safety, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka-shi, Niigata 940-2188, Japan Department of Mechanical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka-shi, Niigata 940-2188, Japan
AR TI CLE I NF O
AB S T R A CT
Keywords: Al-Mg coating Delamination Crack propagation Corrosion fatigue Fracture mechanics
This study aims at observing the effect of Mg concentration on the interfacial strength and corrosion fatigue behavior of Al-Mg coating layers on structural steel (SS400) substrates. Al-Mg coating has been applied to components of bridges as sacrifice coating layers. Although Al or AlMg coating layers are typically applied to the components located in severely corrosive environment, a mechanism behind the effective protection provided by increasing the concentration of Mg has not yet been clarified. The interfacial strength test using four-point bending revealed that a Al-Mg coating layer of higher Mg concentration showed a higher interfacial strength only before immersion in 3.5-wt% NaCl aq.. After immersion in 3.5-wt% NaCl aq. for 30 days, such the difference in interfacial strength was lost due to rapid dissolution of Mg in the coating layers. As regards the fatigue crack growth behavior, the Al-Mg coating with higher Mg concentration exhibited a lower resistance to vertical crack propagation and interfacial delamination. A fracture mechanics model, which includes both effects of corrosion and delamination/ cracking was proposed. Numerical simulation based on the fracture mechanics model successfully predicted exposure lives of substrates due to fatigue failure of the Al-Mg coating layers. These results could provide a selection policy for Al-Mg coating layers in which a loading level and the severity of corrosive environment were considered.
1. Introduction The components of bridges are usually subjected to cyclic load as well as severely corrosive environment, which lead to a failure of the components. Structural Steel SS400, which is normally used for the components, are coated with sacrificial materials in order to protect it from the corrosive environment. The typical sacrificial-coating materials are pure zinc (Zn) or pure aluminum (Al). The sacrificial coating is deposited by thermal spraying and the pores in the coating are sealed using polymer resin. Although Zn is considered to dissolve easily for effective corrosion protection [1], Al coating layers are preferably used. Further, Al-Mg coating layers has recently been widely applied to the sacrificial coating because of its longer durability compared to that of Al coating layers. However, the effects of different Mg concentration on the durability of Al-Mg coating layers have not yet been clarified. Several researches observed effects of Mg concentration on corrosion behavior of Zn or Al alloys/coating layers [2–6]. They used Zn-Mg-coated steel, Zn-Mg alloys, bulk Mg alloys, Mg-Al alloys or friction-stir-welded (FSWed) Al-Zn Mg alloys, and commonly found that increased Mg concentration did not provide any increased corrosion resistance on their observed materials. Electrochemical tests were also conducted on Mg-foil-coated steel [1, 7] and similar corrosion resistance to those of Al coating layers were observed. Their
*
Corresponding author. E-mail address: [email protected] (Y. Otsuka).
https://doi.org/10.1016/j.engfailanal.2018.02.012 Received 13 November 2017; Received in revised form 30 December 2017; Accepted 19 February 2018 Available online 21 February 2018 1350-6307/ © 2018 Elsevier Ltd. All rights reserved.
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results can be explained by lower corrosion resistance of Mg than that of Al. However, the results are not matched with practitioner's experience that higher concentration of Mg in Al-Mg coating layers can provide “longer” durability in service. Another factor affecting the practical durability of Al-Mg coating layers is an enhanced mechanical property by increasing the concentration of Mg. Fatigue crack growth resistance or fatigue strength of Al-Mg alloys were higher in the case of Al-Mg alloys with higher concentration of Mg [8–10]. The intermetallic phases in Al-Mg alloys or Al-Mg-Zn alloys also exhibited higher strength or corrosion resistance [11, 12]. The above review suggested an idea that the higher concentration of Mg in Al-Mg coating layer can provide higher strength as well as lower corrosion resistance. However, the results can not be directly applied to the case of Al-Mg coating layers, which is different from the cases of bulk alloy because of the existence of interfaces between the coating with steel substrates. In order to suitably select a type of Al-Mg coating layers, effects of interfaces on mechanical or corrosion fatigue properties of Al-Mg coating layers should be revealed. To the best of the authors' knowledge, no comparison of the effects of different concentrations of Mg on these properties of Al-Mg coating layers has been reported. This study aims at observing the effects of Mg concentration on interfacial strength and corrosion fatigue behavior of Al-Mg coating layers on SS400 substrates. First, degradation behavior of interface strength of Al-Mg coating layers after immersion in NaCl aq. was observed. Electrochemical tests were also conducted in order to evaluate the corrosion rates of the different Al-Mg coating layers. Vertical crack propagation behavior and interfacial delamination propagation behavior of the different Al-Mg coating layers by cyclic loading in NaCl aq. were observed. A fracture mechanics approach was applied to determine exposure lives of substrates by fatigue failure of Al-Mg coating layers, by considering both vertical crack and interfacial delamination propagation processes. 2. Experimental procedure 2.1. Specimen fabrication Plate specimens of SS400 with the dimensions of 50 mm × 10 mm × 3 mm were machined. Al-Mg coating layers with different Mg concentration of 2 and 5 wt% (they are labeled as Al-2wt% Mg and Al-5wt% Mg, respectively) were then deposited on the substrate using an arc spray technique. The average thickness of the coating layers was approximately 250 μ m. In order to specify initiation points of delamination, a slit with the dimensions of 3 mm width and 0.3 mm depth was milled. The side surfaces of the slitcontaining specimens were polished using MD-LARGO grinding discs (Struers, Ltd.) with 6- μ m diamond slurry and, subsequently buff-polished using MD-Mol polishing cloths (Struers, Ltd.) with 3- μ m diamond slurry. The thickness of coating was calculated by the average of 10 cross-lines using scanning electron microscopy (SEM) pictures. EDX confirmed that no intermetallic compound was formed in the Al-Mg coating layers (Fig. 1). 2.2. Four-point bending (4PB) test with acoustic emission (AE) measurement Fig. 2 shows the dimensions of the 4PB specimens. The 4PB test was conducted using a fatigue machine (Shimadzu EHF-EUV30K010-0A, servo controller 4830) with a displacement rate of 0.004 mm/s. During the 4PB testing, an acoustic emission (AE) sensor (AE 900M, NF Corporation) was placed on the tension side of the 4PB specimens, in order to detect when delamination initiated. The gain value was set to 30 dB with a 125-kHz high-pass filter and its threshold was set to 0.2 –0.4 V, respectively [13, 14]. Initiation and propagation behavior of the delamination was also observed in situ using a digital microscope (VHX-1000, Keyence). The delamination strength of the Al-Mg coating layers was then determined by a stress when the first AE signal of delamination of the Al-Mg coating layer was detected. Degradation behavior of interfacial strength of Al-Mg coating layers were observed after immersion in 3.5-wt% NaCl aq. [15]. The specimens for four-point bending (4PB) testing were immersed in the solution for different periods, i.e., for 7 or 30 days. The temperature of 3.5-wt% NaCl aq. was maintained at ambient temperature using a bath heater and the solution was exchanged every 3 days during the immersion process. 2.3. Cyclic vertical cracking/delamination test for Al-Mg coating Vertical crack growth behavior through the thickness of Al-Mg coating layers subjected to cyclic loading was observed by using a 4PB specimen without the slit. Delamination behavior at interface of Al-Mg coating under cyclic loading was also observed using slitcontaining specimens. The same servo hydraulic testing machine with a solution chamber (Shimadzu EHF-EUV30K-010-0A, servo controller 4380) was used for these cyclic loading tests. All specimens were tested in air or in 3.5-wt% NaCl aq.. Subsequently, fatigue tests using 4PB specimens without slit were conducted in order to determine fatigue lives of the substrates SS 400. The loading frequency was 5 Hz and stress amplitudes of 75 or 180 MPa with a stress ratio R of 0.1 were applied. The vertical crack length or interfacial delamination length were observed intermittently using a digital microscope (VHX-1000, Keyence Co., Ltd.). 2.4. Potentiodynamic testing and electrochemical impedance spectroscopy (EIS) testing Electrochemical open-circuit potential testing and electrochemical impedance spectroscopy (EIS) were conducted. These tests were performed without mechanical loading in order to investigate the corrosion rates of the Al-2wt% Mg and the Al-5wt% Mg coating layers. An exposed area of both the coating layers was the dimensions of 10 × 10 mm2 and functioned as a working electrode. The side surface of the substrates was sealed by an insulator. The electrolyte solution used in these tests was 3.5-wt% NaCl aq. [16], 14
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(caption on next page)
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Fig. 1. Microstructure of both Al-Mg coating layers on SS 400 substrate, which show no intermetallic compound in the Al-Mg coating layers. SEM pictures showed that thickness of both coating layers were approximately 200 μm (A) Al-2wt% Mg coating layer (B)Al-5wt% Mg coating layer
Fig. 2. Schematic illustrations of four-point bending (4PB) testing (units: mm). An AE sensor was put on the tension side of the specimen and a strain gage was adhered on the compressive side of the specimen, respectively.
the temperature of which was controlled at 25 ± 1 ° C. During the electrochemical potential test, Ag/AgCl was used as a referent electrode and a platinum (Pt) electrode was used as a counter electrode, respectively. A potential measurement range was set from −2.0 V to −1.0 V with the scanning rate of 0.5 mV/s. For the EIS testing, Faraday's first law of electrolysis was used to calculate corrosion rates. Faraday's first law is expressed by Eq. (1) and the equivalent circuit is shown in Fig. 3 [17], respectively.
Q M m = ⎛ ⎞⎛ ⎞ ⎝ F ⎠⎝ z ⎠
(1)
where m is the mass of substance altered at the electrode, Q is total electric current density that passes through the material, F is Faraday's constant (96.485 C mol−1), M is the molar mass of the substance, and z is the ion number. Consequently, the mass transfer rate V mt can be calculated by Eq. (2),
Vmt =
I [s−1 cm−2] nFA
(2)
where n is the number of moles, n = m/M, and t is the total period for which the constant current was applied. V converted to corrosion rate ϕ [mm/cycle] using the following equation,
ϕ=
Vmt × Vunit [mm/cycle] fcycle
where ϕ [mm/cycle] is corrosion rate for fatigue loading, V loading frequency in fatigue tests, respectively.
mt
was finally
(3) unit
= 1 × 1 × 1 [mm3] is unit volume of crack region, fcycle [s−1] is
Fig. 3. Electrochemical impedance spectroscopy (EIS) equivalent circuit. Rt indicates resistance of solution. R1,R2 and C1,C2 are resistance and capacitance of Al-Mg coating layers or SS400 substrates, respectively.
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2.5. Calculations of Δ K and Δ G Stress intensity factor ranges Δ K were calculated by Brown's equation for a 2D surface crack in a finite-width plate. However, Brown's equation does not include an effect of a constrained stress field at a crack tip close to interfaces. Ishida developed an equation for calculating stress intensity factor values for a crack adjacent to interfaces. Thus, by combining Ishida's correction with Brown's equation, the following expression was obtained for the Δ K of cracks in a coating layer loaded under 4PB testing [18, 19]:
a a ⎞ = f1 ⎛ ⎞ f2 ⎛ △σ πa , ⎝ w ⎠ ⎝ tcoat ⎠ a a a 2 a 3 a 4 = 1.12 − 1.4 ⎛ ⎞ + 7.33 ⎛ ⎞ − 13.08 ⎛ ⎞ + 14 ⎛ ⎞ f1 ⎛ ⎞ w w w w w ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ 2 3 a ⎞ a ⎞ a ⎞ a ⎞ ⎛ ⎛ ⎛ ⎛ = 1.0062 − 0.2193 + 0.670 − 0.843 f2 t t t t ⎝ coat ⎠ ⎝ coat ⎠ ⎝ coat ⎠ ⎝ coat ⎠
ΔK
⎜
⎜
⎟
⎟
⎜
⎟
⎜
⎟
⎜
⎟
(4)
where a is the crack length, w is the specimen thickness, and tcoat is the coating thickness. The angle of the slit in Al-Mg coating layers were assumed to be 90°, because the depth of the slit penetrated through the thickness of the coating. The surface waviness and roughness were ignored in the calculation to determine the fracture mechanics parameters because such the variation in shapes did not affect the value of the fracture mechanics parameters. The values of strain energy release rate Δ G, which is a fracture mechanics parameter for interfacial delamination, were calculated using the virtual crack closure technique (VCCT) in finite element method (FEM). The VCCT calculation was performed using Marc/ Mentat 2013 software package. A total of 1306 second elements were considered, and the minimum element size around the delamination tip was approximately 3 μ m. The values of Young's modulus of the Al-Mg coating layer and SS400 were assumed to be 69 and 210 GPa, respectively. At the interface, the coating layer and the substrate were completely bonded at the front region from the tip of the delamination. For the FEM calculation, a simple contact condition was employed to model the contact between the coating layer and the substrate at the interfaces of delamination. Surface friction at the interfaces was neglected in the calculation because values of friction coefficient did not change total value of energy release rate though the ratio of mode I and mode II values of the energy release rate was affected by the friction coefficient [20]. In order to obtain a fatigue life prediction model for the Al-Mg coating layers, Paris' Law was considered. After Δ G was calculated for each crack via FEM, it was found that these values decreased as the delamination propagated. Such the behavior can be shown by using Paris law as shown in Eqs. (5) and (6). The fatigue lives of the Al-Mg coating layers can be calculated by integrating both the equations [19].
(Delamination)
dai = Ci (ΔG )mi dN
(Vertical Cracks)
(5)
dac = Cc (ΔK )mc dN
(6)
3. Results 3.1. AE signal generation during 4PB test of SS 400 substrates with Al-Mg coating layers Fig. 4 shows stress-strain curves of SS400 substrate with/without Al-Mg coating layers and Fast Fourier transform (FFT) of an AE signal, which indicated when interfacial delamination occurred. In the case of SS 400 substrate without coating layers, FFT of an AE signal shown in Fig. 4 (B) exhibited a major frequency range from 250 kHz to 375 kHz probably due to plastic deformation or friction of the specimen. In the case of Al-2wt% Mg coating layer on the SS400 substrate, Fig. 4 (C) showed increased number of AE signals even within elastic region of SS400 substrate. Therefore, main portion of AE signals were from damages in the Al-Mg coating layer. When an initiation of delamination of the Al-Mg coating layer was detected by microscope observation, a typical AE signal whose frequency ranges placed in 400–600 kHz and 750–1000 kHz was also detected (Fig. 4 (D)). The result demonstrated that FFT patterns of AE signals can detect interfacial failure of the Al-Mg coating layers. After immersion in NaCl aq. for 7 days, the Al-2wt% Mg coating layer on the SS400 substrate exhibited a detrimental strength as shown in Fig. 4 (E). The peak intensity of AE signals decreased because of the increased pores by immersion. However, FFT of the AE signal at initiation of delamination still had similar major frequency ranges with those in the case before immersion (Fig. 4 (F)). FFT analyses to detect initiation of delamination are still valid after immersion. 3.2. Effect of Mg concentration on interfacial strength and corrosion potential of Al-Mg coating layers The results of interfacial strength of two types of Al-Mg coating layers are summarized in Fig. 5 (A). After immersion in the NaCl aq. for 30 days, both the coating layers exhibited an identical value of the interfacial strength. Note that the tensile strengths of Al-Mg alloys are 195 MPa for A5052 (2-wt% Mg) and 290 MPa for A5056 (5-wt% Mg), respectively [21]. Therefore, higher interfacial 17
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Fig. 4. 4PB results of the Al-2wt% Mg coating layer (thickness: 200 μm) on the SS 400 substrate to determine interfacial strength of the Al-2wt% Mg coating layer. (A, C, E) Stress-strain curve and detected AE signals. (B, D, F) FFT of typical AE signals to identify failure modes. (A, B) SS 400 substrate without coating. (C, D) the Al-2wt % Mg coating layer on the SS 400 substrate. (E, F) the Al-2wt% Mg coating layer on the SS 400 substrate immersed in NaCl aq. for 7 days.
strength of the Al-Mg coating layer with higher concentration of Mg only before immersion (Fig. 5 (A)), is similar to the case of tensile strength of bulk Al-Mg alloy. Values of corrosion potential of both Al-Mg coating layers exhibited similar and located more closely to the corrosion potential of Mg ( −1.1 V) [22] than to the one of Al ( −0.7 V) [22], as shown in Fig. 5 (B). The results suggest that Mg contents in the Al-Mg 18
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Fig. 5. Effect of Mg concentration on corrosion behavior of Al-Mg coating layers. (A) Degradation of interfacial strength after immersion in NaCl aq. (B) Corrosion potential by potentiodynamic polarization plot.
coating layer was mainly dissolved. In order to quantify the corrosion resistance of the Al-Mg coating layers, EIS testing was conducted and the corrosion rate was calculated using Eq. (2). Higher concentration of Mg in Al-Mg coating layers provided higher corrosion rate, as shown in Table 1. In order to include the effect of corrosion in crack/delamination growth model shown in Eqs. (5) and (6), corrosion rate was converted in the unit of mm/cycle. Changes in surface morphology of both Al-Mg coating layers were also observed by SEM (Fig. 6). Splats made by thermal-spraying process were observed before immersion, and dissolution of both the coating layers and precipitation of oxides were observed during the immersion. EDX results shown in Table 2 revealed that chemical composition of both the coating layers became similar after the 30 days immersion, probably due to rapid dissolution of Mg contents. Table 1 EIS test results and corrosion rates of two types of Al-Mg coating layers. Type
R1
C1
[Ω] Al-2 wt% Mg Al-5 wt% Mg
R2
[F] 5
2.99 × 10 6.30 × 104
C2
[Ω] −12
1.00 × 10 3.98 × 10−3
[F] 5
2.99 × 10 2.80 × 102
19
Corrosion rate ϕ
Rt [Ω] −11
5.15 × 10 1.00 × 10−12
1.00 × 10 13.07
[mm/cycles] −5
1.04 × 10−11 5.91 × 10−8
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Fig. 6. Changes in surface morphology of both Al-Mg coating layers after immersion in NaCl. aq. (A) Al-2wt% Mg coating layer (B)Al-5wt% Mg coating layer.
Table 2 Changes in element concentration of Al-Mg coating layers after immersion in NaCl aq. Type
Al-2 wt% Mg
Al-5 wt% Mg
Immersion
Element concentration (atom %)
Periods
Fe
O
Al
Mg
None 7 days 30 days None 7 days 30 days
0.01 1.30 1.21 0.04 4.75 4.53
3.86 7.15 48.07 0.92 33.10 52.09
94.40 91.34 50.53 94.45 60.98 42.22
1.73 0.20 0.19 4.59 1.17 1.16
3.3. Effects of Mg concentration on interfacial delamination and vertical crack growth in Al-Mg coating layers subjected to cyclic load Fig. 7 (A) shows the relationships between the cyclic delamination growth rate and the energy release rate Δ G, which followed Paris's law [19]. Though delamination growth rapidly decreased with increase its length and became stable, initial stage of delamination within the length of 1 mm was used for calculation of Δ G. Both NaCl aq. environment and higher concentration of Mg in AlMg coating layers led to higher growth rate of delamination. Fig. 7 (B) shows the relationships between the crack growth rates and the stress intensity factor range ΔK. All curves in Fig. 7 (B) were kinked because crack tips approached to the interface between the Al-Mg coating layers with the substrates, which was considered in Eq. (4). The result also demonstrated that both NaCl aq. environment and higher concentration of Mg in Al-Mg coating layers provided higher crack growth rate in the Al-Mg coating layers.
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Fig. 7. Effects on Mg concentration and NaCl aq. environment on interfacial delamination and vertical crack propagation behavior arranged by Paris Law. (A) Relationships between interfacial delamination growth rate vs energy release rate Δ G. (B) Relationships between crack growth rate vs stress intensity factor range Δ K.
3.4. Fracture mechanics approach to estimate exposure lives of the SS400 substrate in corrosion fatigue Fig. 8 (A) shows corrosion fatigue behavior of SS400 substrates with two types of Al-Mg coating layers. The fatigue life of the substrate with the Al-5wt% Mg coating layer was shorter than that of the substrate with the Al-2wt% Mg coating layer. Fig. 8 (B, C) demonstrated that fatigue failure of SS400 substrates was occurred after exposure to the corrosive environment by vertical cracking and subsequent connection between two interface delaminations in the Al-Mg coating layers. In order to estimate exposure lives of the substrate, Eqs. (5) and (6) were numerically integrated on the assumptions that a penetrating length of vertical crack was 250 μ m and connecting length of delamination was 200 μ m, respectively. Fig. 9 shows simulated growth plots of vertical crack and interfacial delamination in Al-Mg coating layers. Especially in the case of lower stress amplitude, a higher concentration of Mg in Al-Mg coating layers led to shorter exposure lives of the substrate. Comparisons of simulated exposure lives with the experimentally observed ones were also shown in Fig. 8 (A), which demonstrated that the simulated result provided conservative prediction of exposure lives of substrate. Such the shorter exposure lives of the SS400 substrate could accelerate crack initiation in the substrate by corrosion fatigue. Therefore, shorter fatigue lives of the SS400 substrate were followed by the shorter lives of the Al-Mg coating layer. The result in Fig. 8 suggests that higher concentration of Mg in Al-Mg coating layers is not beneficial in the case of highly-loaded conditions due to earlier exposure of the substrates to corrosive environment. Under practical conditions of σa = 80 MPa [23], there is no significant difference in the fatigue lives of the substrates. This fact can explain the reason why the effectiveness of different Mg concentration in Al-Mg coating layers is difficult to be observed in service environment, which has low stress amplitudes as well as 21
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Fig. 8. Effect of Mg concentration on corrosion fatigue behavior of SS400 substrates with Al-Mg coating layers in NaCl aq. (A)S-N curves. The ∘ and △ symbols show penetration lives of vertical crack and connecting lives of delamination (= exposure lives of substrates) in Al-Mg coating layers, respectively. Solid and dash lines suggest simulated results of failure lives of Al-Mg coating layers using Eqs. (5) and (6). (B, C) Crack paths in the Al-Mg coating layers. Red arrows in the figures indicate directions of propagation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Substrate
M
Substrate
M
Fig. 9. Simulated crack growth curves in Al-Mg coating layers immersed in NaCl aq. calculated by integration of Eqs. (5) and (6).
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lower corrosive conditions. 4. Discussion In order to explain the effects of the Mg concentration on the durability of the Al-Mg coating layers, the following possible mechanisms were considered. 1. Intermetallic compound phases, which increase the corrosion resistance, form in the Al-Mg coating matrix in response to the increased Mg concentration; 2. The mechanical properties of the Al-Mg coating layers are enhanced by the increased Mg concentration; 3. The corrosion resistance of the Al-Mg coating layers are deteriorated by the increased Mg concentration; 4. The fatigue properties of the Al-Mg coating layers are altered by the increased Mg concentration. At first, we discuss the effect of different Mg concentration on microstructure and mechanical property. SEM pictures shown in Fig. 1 clearly exhibited that no intermetallic compound phases were formed at the adjacent area of the interfaces. Higher concentration of Mg in Al-Mg coating layers provided a higher interfacial strength of Al-Mg coating layers (Fig. 5 (A)). However, such the enhancement effect will be deteriorated when we consider severe corrosive environment (Al-Mg coating layers are for sacrifice coating in corrosive environment.). The energy dispersive X-ray spectroscopy (EDX) analysis results shown in Table 2 exhibited that there was no significant difference in the remained concentration of Mg in both the coating layers. The dissolution of Mg contents can decrease interfacial strength of Al-Mg coating layers (Fig. 5 (A)) and then the enhancement effect of the higher Mg concentration is limited by the severity of corrosion. On the other hand, the deteriorating effect of corrosion resistance of Al-Mg coating layers by higher concentration of Mg (Fig. 5 (B)) should always be considered. Higher concentration of Mg induced a lower corrosion resistance and lower vertical crack/ interfacial delamination growth resistances, as shown in Table 1 and Fig. 7, respectively. Such the decreased resistances eventually led to shorter fatigue lives of substrates (Fig. 8). When considering a higher stress amplitude as well as severe corrosive environment, such the difference in exposure lives of the substrates can not be ignored. Finally, a selection policy of Al-Mg coating layers considering practical loading conditions is discussed. Under practical loading conditions of a lower stress amplitude without corrosive environment, the Al-5wt% Mg coating layer has a higher interfacial strength than the one of Al-2wt% Mg coating layer. However, consideration of corrosive environment is essential when an Al-Mg coating layer is employed as a sacrificial coating layer. Unplanned instances of higher loading, such as those experienced during earthquakes, storms, or heavy loading, can also be considered, although the typical design stress is lower at σa = 80 MPa. If higher stress amplitudes as well as the corrosive environment are considered, Al-Mg coating layers with lower Mg concentration are preferable, because of their higher fatigue resistances. The above discussion facilitates the selection of Al-Mg coating layers with different Mg concentration by considering the extent of the loading conditions and the severity of corrosion. This study has investigated specimens with a fixed coating thickness. Note that it is also important to observe the effects of different coating thicknesses and substrates on the examined properties and behaviors. Such investigations are topics for further study. 5. Conclusion The effect of different Mg concentration (2 and 5 wt%) of Al-Mg coating layers on the interracial strength and corrosion fatigue behavior of the Al-Mg coating layers were observed. The obtained results can be summarized as the follow: 1. Higher concentration of Mg in Al-Mg coating layers induced higher interfacial strength of the Al-Mg coating layer only for short immersion periods (7 days). After longer immersion in NaCl aq., such the enhancement effect of higher Mg concentration on interfacial strength of the Al-Mg coating layer is lost due to rapid dissolution of Mg content. No intermetallic phase in the Al-Mg coating layers was formed by this thermal spray process. 2. Higher concentration of Mg in Al-Mg coating layers reduced corrosion resistance of the Al-Mg coating layers. The Al-5wt% Mg coating layer showed a higher corrosion rate than that of the Al-2wt% Mg coating layer. 3. Higher concentration of Mg in Al-Mg coating layers decreased vertical crack growth resistance and interfacial delamination growth resistance of the Al-Mg coating layers. Both the crack/delamination propagation behavior of the Al-Mg coating layer were accelerated in NaCl aq., and causes earlier exposure of the substrate, resulting in shorter fatigue lives of the substrate. 4. Simulation based on fracture mechanics model could predict exposure lives of substrate. Especially in the cases of higher stress amplitude, higher concentration of Mg in the Al-Mg coating layers shortened exposure lives of the substrates, which resulted in shorter fatigue lives of the substrates. 5. Higher concentration of Mg in Al-Mg coating layer is more suitable in the case of low-severity corrosive environment and low stress levels. However, when fluctuations in the loading conditions and corrosive environment are considered, an Al-Mg coating layer with lower Mg concentration is recommended. This study provides a selection policy for Al-Mg coating layers by considering the loading conditions and corrosive environment. For this selection, observation of the corrosion fatigue properties of various types of Al-Mg coating layers are important. The effect of the coating thickness is another important factor to be examined in future work, along with the effects of different substrate types. 23
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Acknowledgments The Al-Mg coated specimens were provided by the Thermal Spraying for Corrosion Protection Cooperative and NIIGATA Metallicon Co., Ltd. The authors appreciate the partial support provided by the Suga Weathering Technology Foundation (2013–2014). References [1] X. Li, W. Liang, X. Zhao, Y. Zhang, X. Fu, F. Liu, Bonding of Mg and Al with Mg-Al eutectic alloy and its application in aluminum coating on magnesium, J. Alloys Compd. 471 (1-2) (2009) 408–411. [2] N. Hosking, M. Strom, P. Shipway, C. Rudd, Corrosion resistance of zinc-magnesium coated steel, Corros. Sci. 49 (9) (2007) 3669–3695. [3] P. Volovitch, T. Vu, C. Allely, A.A. Aal, K. Ogle, Understanding corrosion via corrosion product characterization: II. Role of alloying elements in improving the corrosion resistance of Zn-Al-Mg coatings on steel, Corros. Sci. 53 (8) (2011) 2437–2445. [4] A. Pardo, M. Merino, M. Mohedano, P. Casajus, A. Coy, R. Arrabal, Corrosion behaviour of Mg/Al alloys with composite coatings, Surf. Coat. Technol. 203 (9) (2009) 1252–1263. [5] Y. Deng, R. Ye, G. Xu, J. Yang, Q. Pan, B. Peng, X. Cao, Y. Duan, Y. Wang, L. Lu, Z. Yin, Corrosion behaviour and mechanism of new aerospace Al-Zn-Mg alloy friction stir welded joints and the effects of secondary Al3ScxZr1x nanoparticles, Surf. Coat. Technol. 90 (2015) 359–374. [6] W. Liu, M.-C. Li, Q. Luo, H.-Q. Fan, J.-Y. Zhang, H.-S. Lu, K.-C. Chou, X.-L. Wang, Q. Li, Influence of alloyed magnesium on the microstructure and long-term corrosion behavior of hot-dip Al-Zn-Si coating in NaCl solution, Corros. Sci. 104 (Supplement C) (2016) 217–226. [7] S. Li, B. Gao, G. Tu, L. Hu, S. Sun, G. Zhu, S. Yin, Effects of magnesium on the microstructure and corrosion resistance of Zn-55Al-1.6Si coating, Constr. Build. Mater. 71 (2014) 124–131. [8] G.M.D. Almaraz, J.L.A. Ambriz, E.C. Calderon, Fatigue endurance and crack propagation under rotating bending fatigue tests on aluminum alloy {AISI} 6063T5 with controlled corrosion attack, Eng. Fract. Mech. 93 (2012) 119–131. [9] J.T. Burns, S. Kim, R.P. Gangloff, Effect of corrosion severity on fatigue evolution in Al-Zn-Mg-Cu, Corros. Sci. 52 (2) (2010) 498–508. [10] K. Jones, D.W. Hoeppner, Prior corrosion and fatigue of 2024-T3 aluminum alloy, Corros. Sci. 48 (10) (2006) 3109–3122. [11] H. Bu, M. Yandouzi, C. Lu, D. MacDonald, B. Jodoin, Cold spray blended Al + Mg17Al12 coating for corrosion protection of {AZ91D} magnesium alloy, Surf. Coat. Technol. 207 (2012) 155–162. [12] D. Pradhan, M. Manna, M. Dutta, Al-Mg-Mn alloy coating on steel with superior corrosion behavior, Surf. Coat. Technol. 258 (2014) 405–414. [13] T. Laonapakul, Y. Otsuka, A.R. Nimkerdphol, Y. Mutoh, Acoustic emission and fatigue damage induced in plasma-sprayed hydroxyapatite coating layers, J. Mech. Behav. Biomed. Mater. 8 (2012) 123–133. [14] T. Laonapakul, A.R. Nimkerdphol, Y. Otsuka, Y. Mutoh, Failure behavior of plasma-sprayed {HAp} coating on commercially pure titanium substrate in simulated body fluid (SBF) under bending load, J. Mech. Behav. Biomed. Mater. 15 (2012) 153–166. [15] M. Es-saheb, E.-S.M. Sherif, A. El-Zatahry, M.M.E. Rayes, K.A. Khalil, Corrosion passivation in aerated 3.5% NaCl solutions of brass by nanofiber coatings of plyvinyl chrolide and plostyrene, Int. J. Electrochem. Sci. 7 (2012) 10442–10455. [16] D. Ramesh, R.P. Swamy, T.K. Chandrashekar, Corrosion behaviour of Al6061-Frit particulate metal matrix composites in sodium chloride solution, J. Miner. Mater. Charact. Eng. 1 (1) (2013) 15–19. [17] B. Bhattacharyya, Electrochemical Micromachining for Nanofabrication, MEMS and Nanotechnology, William Andrew publications, 2015. [18] J.R. Rice, Thermodynamics of the quasi-static growth of griffith cracks, J. Mech. Phys. Solids 26 (1978) 163–186. [19] T. Anderson, Fracture Mechanics Fundamentals and Application, CRC press, 2005. [20] Y. Otsuka, D. Kojima, Y. Mutoh, Prediction of cyclic delamination lives of plasma-sprayed hydroxyapatite coating on Ti6Al4V substrates with considering wear and dissolutions, J. Mech. Behav. Biomed. Mater. 64 (2016) 113–124. [21] the Japan Institute of light metals, Aluminum Handbook, Microstructure and Features of Aluminum, the Japan Institute of light metals, 1991. [22] A. Epstein, J. Smallfield, H. Guan, M. Fahlman, Corrosion protection of aluminum and aluminum alloys by polyanilines: a potentiodynamic and photoelectron spectroscopy study, Synth. Met. 102 (1-3) (1999) 1374–1376. [23] S. Kawabata, T. Kitamura, Y. Mizokami, N. Yanadori, Masahiro Sakano, Stress measurement and fatigue life evaluation on transverse rib of non composite orthotropic steel deck in suspension bridge, J. Struct. Eng. 61A (2015) 1095–1103.
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Effect of Mg concentration on interfacial strength and corrosion fatigue behavior of thermal-sprayed Al-Mg coating layers
T
Sarita Morakula, Yuichi Otsukab,*, Yukio Miyashitac, Yoshiharu Mutohb a b c
Graduate School of Material Science, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka-shi, Niigata 940-2188, Japan Department of System Safety, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka-shi, Niigata 940-2188, Japan Department of Mechanical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka-shi, Niigata 940-2188, Japan
AR TI CLE I NF O
AB S T R A CT
Keywords: Al-Mg coating Delamination Crack propagation Corrosion fatigue Fracture mechanics
This study aims at observing the effect of Mg concentration on the interfacial strength and corrosion fatigue behavior of Al-Mg coating layers on structural steel (SS400) substrates. Al-Mg coating has been applied to components of bridges as sacrifice coating layers. Although Al or AlMg coating layers are typically applied to the components located in severely corrosive environment, a mechanism behind the effective protection provided by increasing the concentration of Mg has not yet been clarified. The interfacial strength test using four-point bending revealed that a Al-Mg coating layer of higher Mg concentration showed a higher interfacial strength only before immersion in 3.5-wt% NaCl aq.. After immersion in 3.5-wt% NaCl aq. for 30 days, such the difference in interfacial strength was lost due to rapid dissolution of Mg in the coating layers. As regards the fatigue crack growth behavior, the Al-Mg coating with higher Mg concentration exhibited a lower resistance to vertical crack propagation and interfacial delamination. A fracture mechanics model, which includes both effects of corrosion and delamination/ cracking was proposed. Numerical simulation based on the fracture mechanics model successfully predicted exposure lives of substrates due to fatigue failure of the Al-Mg coating layers. These results could provide a selection policy for Al-Mg coating layers in which a loading level and the severity of corrosive environment were considered.
1. Introduction The components of bridges are usually subjected to cyclic load as well as severely corrosive environment, which lead to a failure of the components. Structural Steel SS400, which is normally used for the components, are coated with sacrificial materials in order to protect it from the corrosive environment. The typical sacrificial-coating materials are pure zinc (Zn) or pure aluminum (Al). The sacrificial coating is deposited by thermal spraying and the pores in the coating are sealed using polymer resin. Although Zn is considered to dissolve easily for effective corrosion protection [1], Al coating layers are preferably used. Further, Al-Mg coating layers has recently been widely applied to the sacrificial coating because of its longer durability compared to that of Al coating layers. However, the effects of different Mg concentration on the durability of Al-Mg coating layers have not yet been clarified. Several researches observed effects of Mg concentration on corrosion behavior of Zn or Al alloys/coating layers [2–6]. They used Zn-Mg-coated steel, Zn-Mg alloys, bulk Mg alloys, Mg-Al alloys or friction-stir-welded (FSWed) Al-Zn Mg alloys, and commonly found that increased Mg concentration did not provide any increased corrosion resistance on their observed materials. Electrochemical tests were also conducted on Mg-foil-coated steel [1, 7] and similar corrosion resistance to those of Al coating layers were observed. Their
*
Corresponding author. E-mail address: [email protected] (Y. Otsuka).
https://doi.org/10.1016/j.engfailanal.2018.02.012 Received 13 November 2017; Received in revised form 30 December 2017; Accepted 19 February 2018 Available online 21 February 2018 1350-6307/ © 2018 Elsevier Ltd. All rights reserved.
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results can be explained by lower corrosion resistance of Mg than that of Al. However, the results are not matched with practitioner's experience that higher concentration of Mg in Al-Mg coating layers can provide “longer” durability in service. Another factor affecting the practical durability of Al-Mg coating layers is an enhanced mechanical property by increasing the concentration of Mg. Fatigue crack growth resistance or fatigue strength of Al-Mg alloys were higher in the case of Al-Mg alloys with higher concentration of Mg [8–10]. The intermetallic phases in Al-Mg alloys or Al-Mg-Zn alloys also exhibited higher strength or corrosion resistance [11, 12]. The above review suggested an idea that the higher concentration of Mg in Al-Mg coating layer can provide higher strength as well as lower corrosion resistance. However, the results can not be directly applied to the case of Al-Mg coating layers, which is different from the cases of bulk alloy because of the existence of interfaces between the coating with steel substrates. In order to suitably select a type of Al-Mg coating layers, effects of interfaces on mechanical or corrosion fatigue properties of Al-Mg coating layers should be revealed. To the best of the authors' knowledge, no comparison of the effects of different concentrations of Mg on these properties of Al-Mg coating layers has been reported. This study aims at observing the effects of Mg concentration on interfacial strength and corrosion fatigue behavior of Al-Mg coating layers on SS400 substrates. First, degradation behavior of interface strength of Al-Mg coating layers after immersion in NaCl aq. was observed. Electrochemical tests were also conducted in order to evaluate the corrosion rates of the different Al-Mg coating layers. Vertical crack propagation behavior and interfacial delamination propagation behavior of the different Al-Mg coating layers by cyclic loading in NaCl aq. were observed. A fracture mechanics approach was applied to determine exposure lives of substrates by fatigue failure of Al-Mg coating layers, by considering both vertical crack and interfacial delamination propagation processes. 2. Experimental procedure 2.1. Specimen fabrication Plate specimens of SS400 with the dimensions of 50 mm × 10 mm × 3 mm were machined. Al-Mg coating layers with different Mg concentration of 2 and 5 wt% (they are labeled as Al-2wt% Mg and Al-5wt% Mg, respectively) were then deposited on the substrate using an arc spray technique. The average thickness of the coating layers was approximately 250 μ m. In order to specify initiation points of delamination, a slit with the dimensions of 3 mm width and 0.3 mm depth was milled. The side surfaces of the slitcontaining specimens were polished using MD-LARGO grinding discs (Struers, Ltd.) with 6- μ m diamond slurry and, subsequently buff-polished using MD-Mol polishing cloths (Struers, Ltd.) with 3- μ m diamond slurry. The thickness of coating was calculated by the average of 10 cross-lines using scanning electron microscopy (SEM) pictures. EDX confirmed that no intermetallic compound was formed in the Al-Mg coating layers (Fig. 1). 2.2. Four-point bending (4PB) test with acoustic emission (AE) measurement Fig. 2 shows the dimensions of the 4PB specimens. The 4PB test was conducted using a fatigue machine (Shimadzu EHF-EUV30K010-0A, servo controller 4830) with a displacement rate of 0.004 mm/s. During the 4PB testing, an acoustic emission (AE) sensor (AE 900M, NF Corporation) was placed on the tension side of the 4PB specimens, in order to detect when delamination initiated. The gain value was set to 30 dB with a 125-kHz high-pass filter and its threshold was set to 0.2 –0.4 V, respectively [13, 14]. Initiation and propagation behavior of the delamination was also observed in situ using a digital microscope (VHX-1000, Keyence). The delamination strength of the Al-Mg coating layers was then determined by a stress when the first AE signal of delamination of the Al-Mg coating layer was detected. Degradation behavior of interfacial strength of Al-Mg coating layers were observed after immersion in 3.5-wt% NaCl aq. [15]. The specimens for four-point bending (4PB) testing were immersed in the solution for different periods, i.e., for 7 or 30 days. The temperature of 3.5-wt% NaCl aq. was maintained at ambient temperature using a bath heater and the solution was exchanged every 3 days during the immersion process. 2.3. Cyclic vertical cracking/delamination test for Al-Mg coating Vertical crack growth behavior through the thickness of Al-Mg coating layers subjected to cyclic loading was observed by using a 4PB specimen without the slit. Delamination behavior at interface of Al-Mg coating under cyclic loading was also observed using slitcontaining specimens. The same servo hydraulic testing machine with a solution chamber (Shimadzu EHF-EUV30K-010-0A, servo controller 4380) was used for these cyclic loading tests. All specimens were tested in air or in 3.5-wt% NaCl aq.. Subsequently, fatigue tests using 4PB specimens without slit were conducted in order to determine fatigue lives of the substrates SS 400. The loading frequency was 5 Hz and stress amplitudes of 75 or 180 MPa with a stress ratio R of 0.1 were applied. The vertical crack length or interfacial delamination length were observed intermittently using a digital microscope (VHX-1000, Keyence Co., Ltd.). 2.4. Potentiodynamic testing and electrochemical impedance spectroscopy (EIS) testing Electrochemical open-circuit potential testing and electrochemical impedance spectroscopy (EIS) were conducted. These tests were performed without mechanical loading in order to investigate the corrosion rates of the Al-2wt% Mg and the Al-5wt% Mg coating layers. An exposed area of both the coating layers was the dimensions of 10 × 10 mm2 and functioned as a working electrode. The side surface of the substrates was sealed by an insulator. The electrolyte solution used in these tests was 3.5-wt% NaCl aq. [16], 14
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(caption on next page)
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Fig. 1. Microstructure of both Al-Mg coating layers on SS 400 substrate, which show no intermetallic compound in the Al-Mg coating layers. SEM pictures showed that thickness of both coating layers were approximately 200 μm (A) Al-2wt% Mg coating layer (B)Al-5wt% Mg coating layer
Fig. 2. Schematic illustrations of four-point bending (4PB) testing (units: mm). An AE sensor was put on the tension side of the specimen and a strain gage was adhered on the compressive side of the specimen, respectively.
the temperature of which was controlled at 25 ± 1 ° C. During the electrochemical potential test, Ag/AgCl was used as a referent electrode and a platinum (Pt) electrode was used as a counter electrode, respectively. A potential measurement range was set from −2.0 V to −1.0 V with the scanning rate of 0.5 mV/s. For the EIS testing, Faraday's first law of electrolysis was used to calculate corrosion rates. Faraday's first law is expressed by Eq. (1) and the equivalent circuit is shown in Fig. 3 [17], respectively.
Q M m = ⎛ ⎞⎛ ⎞ ⎝ F ⎠⎝ z ⎠
(1)
where m is the mass of substance altered at the electrode, Q is total electric current density that passes through the material, F is Faraday's constant (96.485 C mol−1), M is the molar mass of the substance, and z is the ion number. Consequently, the mass transfer rate V mt can be calculated by Eq. (2),
Vmt =
I [s−1 cm−2] nFA
(2)
where n is the number of moles, n = m/M, and t is the total period for which the constant current was applied. V converted to corrosion rate ϕ [mm/cycle] using the following equation,
ϕ=
Vmt × Vunit [mm/cycle] fcycle
where ϕ [mm/cycle] is corrosion rate for fatigue loading, V loading frequency in fatigue tests, respectively.
mt
was finally
(3) unit
= 1 × 1 × 1 [mm3] is unit volume of crack region, fcycle [s−1] is
Fig. 3. Electrochemical impedance spectroscopy (EIS) equivalent circuit. Rt indicates resistance of solution. R1,R2 and C1,C2 are resistance and capacitance of Al-Mg coating layers or SS400 substrates, respectively.
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2.5. Calculations of Δ K and Δ G Stress intensity factor ranges Δ K were calculated by Brown's equation for a 2D surface crack in a finite-width plate. However, Brown's equation does not include an effect of a constrained stress field at a crack tip close to interfaces. Ishida developed an equation for calculating stress intensity factor values for a crack adjacent to interfaces. Thus, by combining Ishida's correction with Brown's equation, the following expression was obtained for the Δ K of cracks in a coating layer loaded under 4PB testing [18, 19]:
a a ⎞ = f1 ⎛ ⎞ f2 ⎛ △σ πa , ⎝ w ⎠ ⎝ tcoat ⎠ a a a 2 a 3 a 4 = 1.12 − 1.4 ⎛ ⎞ + 7.33 ⎛ ⎞ − 13.08 ⎛ ⎞ + 14 ⎛ ⎞ f1 ⎛ ⎞ w w w w w ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ 2 3 a ⎞ a ⎞ a ⎞ a ⎞ ⎛ ⎛ ⎛ ⎛ = 1.0062 − 0.2193 + 0.670 − 0.843 f2 t t t t ⎝ coat ⎠ ⎝ coat ⎠ ⎝ coat ⎠ ⎝ coat ⎠
ΔK
⎜
⎜
⎟
⎟
⎜
⎟
⎜
⎟
⎜
⎟
(4)
where a is the crack length, w is the specimen thickness, and tcoat is the coating thickness. The angle of the slit in Al-Mg coating layers were assumed to be 90°, because the depth of the slit penetrated through the thickness of the coating. The surface waviness and roughness were ignored in the calculation to determine the fracture mechanics parameters because such the variation in shapes did not affect the value of the fracture mechanics parameters. The values of strain energy release rate Δ G, which is a fracture mechanics parameter for interfacial delamination, were calculated using the virtual crack closure technique (VCCT) in finite element method (FEM). The VCCT calculation was performed using Marc/ Mentat 2013 software package. A total of 1306 second elements were considered, and the minimum element size around the delamination tip was approximately 3 μ m. The values of Young's modulus of the Al-Mg coating layer and SS400 were assumed to be 69 and 210 GPa, respectively. At the interface, the coating layer and the substrate were completely bonded at the front region from the tip of the delamination. For the FEM calculation, a simple contact condition was employed to model the contact between the coating layer and the substrate at the interfaces of delamination. Surface friction at the interfaces was neglected in the calculation because values of friction coefficient did not change total value of energy release rate though the ratio of mode I and mode II values of the energy release rate was affected by the friction coefficient [20]. In order to obtain a fatigue life prediction model for the Al-Mg coating layers, Paris' Law was considered. After Δ G was calculated for each crack via FEM, it was found that these values decreased as the delamination propagated. Such the behavior can be shown by using Paris law as shown in Eqs. (5) and (6). The fatigue lives of the Al-Mg coating layers can be calculated by integrating both the equations [19].
(Delamination)
dai = Ci (ΔG )mi dN
(Vertical Cracks)
(5)
dac = Cc (ΔK )mc dN
(6)
3. Results 3.1. AE signal generation during 4PB test of SS 400 substrates with Al-Mg coating layers Fig. 4 shows stress-strain curves of SS400 substrate with/without Al-Mg coating layers and Fast Fourier transform (FFT) of an AE signal, which indicated when interfacial delamination occurred. In the case of SS 400 substrate without coating layers, FFT of an AE signal shown in Fig. 4 (B) exhibited a major frequency range from 250 kHz to 375 kHz probably due to plastic deformation or friction of the specimen. In the case of Al-2wt% Mg coating layer on the SS400 substrate, Fig. 4 (C) showed increased number of AE signals even within elastic region of SS400 substrate. Therefore, main portion of AE signals were from damages in the Al-Mg coating layer. When an initiation of delamination of the Al-Mg coating layer was detected by microscope observation, a typical AE signal whose frequency ranges placed in 400–600 kHz and 750–1000 kHz was also detected (Fig. 4 (D)). The result demonstrated that FFT patterns of AE signals can detect interfacial failure of the Al-Mg coating layers. After immersion in NaCl aq. for 7 days, the Al-2wt% Mg coating layer on the SS400 substrate exhibited a detrimental strength as shown in Fig. 4 (E). The peak intensity of AE signals decreased because of the increased pores by immersion. However, FFT of the AE signal at initiation of delamination still had similar major frequency ranges with those in the case before immersion (Fig. 4 (F)). FFT analyses to detect initiation of delamination are still valid after immersion. 3.2. Effect of Mg concentration on interfacial strength and corrosion potential of Al-Mg coating layers The results of interfacial strength of two types of Al-Mg coating layers are summarized in Fig. 5 (A). After immersion in the NaCl aq. for 30 days, both the coating layers exhibited an identical value of the interfacial strength. Note that the tensile strengths of Al-Mg alloys are 195 MPa for A5052 (2-wt% Mg) and 290 MPa for A5056 (5-wt% Mg), respectively [21]. Therefore, higher interfacial 17
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Fig. 4. 4PB results of the Al-2wt% Mg coating layer (thickness: 200 μm) on the SS 400 substrate to determine interfacial strength of the Al-2wt% Mg coating layer. (A, C, E) Stress-strain curve and detected AE signals. (B, D, F) FFT of typical AE signals to identify failure modes. (A, B) SS 400 substrate without coating. (C, D) the Al-2wt % Mg coating layer on the SS 400 substrate. (E, F) the Al-2wt% Mg coating layer on the SS 400 substrate immersed in NaCl aq. for 7 days.
strength of the Al-Mg coating layer with higher concentration of Mg only before immersion (Fig. 5 (A)), is similar to the case of tensile strength of bulk Al-Mg alloy. Values of corrosion potential of both Al-Mg coating layers exhibited similar and located more closely to the corrosion potential of Mg ( −1.1 V) [22] than to the one of Al ( −0.7 V) [22], as shown in Fig. 5 (B). The results suggest that Mg contents in the Al-Mg 18
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Fig. 5. Effect of Mg concentration on corrosion behavior of Al-Mg coating layers. (A) Degradation of interfacial strength after immersion in NaCl aq. (B) Corrosion potential by potentiodynamic polarization plot.
coating layer was mainly dissolved. In order to quantify the corrosion resistance of the Al-Mg coating layers, EIS testing was conducted and the corrosion rate was calculated using Eq. (2). Higher concentration of Mg in Al-Mg coating layers provided higher corrosion rate, as shown in Table 1. In order to include the effect of corrosion in crack/delamination growth model shown in Eqs. (5) and (6), corrosion rate was converted in the unit of mm/cycle. Changes in surface morphology of both Al-Mg coating layers were also observed by SEM (Fig. 6). Splats made by thermal-spraying process were observed before immersion, and dissolution of both the coating layers and precipitation of oxides were observed during the immersion. EDX results shown in Table 2 revealed that chemical composition of both the coating layers became similar after the 30 days immersion, probably due to rapid dissolution of Mg contents. Table 1 EIS test results and corrosion rates of two types of Al-Mg coating layers. Type
R1
C1
[Ω] Al-2 wt% Mg Al-5 wt% Mg
R2
[F] 5
2.99 × 10 6.30 × 104
C2
[Ω] −12
1.00 × 10 3.98 × 10−3
[F] 5
2.99 × 10 2.80 × 102
19
Corrosion rate ϕ
Rt [Ω] −11
5.15 × 10 1.00 × 10−12
1.00 × 10 13.07
[mm/cycles] −5
1.04 × 10−11 5.91 × 10−8
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Fig. 6. Changes in surface morphology of both Al-Mg coating layers after immersion in NaCl. aq. (A) Al-2wt% Mg coating layer (B)Al-5wt% Mg coating layer.
Table 2 Changes in element concentration of Al-Mg coating layers after immersion in NaCl aq. Type
Al-2 wt% Mg
Al-5 wt% Mg
Immersion
Element concentration (atom %)
Periods
Fe
O
Al
Mg
None 7 days 30 days None 7 days 30 days
0.01 1.30 1.21 0.04 4.75 4.53
3.86 7.15 48.07 0.92 33.10 52.09
94.40 91.34 50.53 94.45 60.98 42.22
1.73 0.20 0.19 4.59 1.17 1.16
3.3. Effects of Mg concentration on interfacial delamination and vertical crack growth in Al-Mg coating layers subjected to cyclic load Fig. 7 (A) shows the relationships between the cyclic delamination growth rate and the energy release rate Δ G, which followed Paris's law [19]. Though delamination growth rapidly decreased with increase its length and became stable, initial stage of delamination within the length of 1 mm was used for calculation of Δ G. Both NaCl aq. environment and higher concentration of Mg in AlMg coating layers led to higher growth rate of delamination. Fig. 7 (B) shows the relationships between the crack growth rates and the stress intensity factor range ΔK. All curves in Fig. 7 (B) were kinked because crack tips approached to the interface between the Al-Mg coating layers with the substrates, which was considered in Eq. (4). The result also demonstrated that both NaCl aq. environment and higher concentration of Mg in Al-Mg coating layers provided higher crack growth rate in the Al-Mg coating layers.
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Fig. 7. Effects on Mg concentration and NaCl aq. environment on interfacial delamination and vertical crack propagation behavior arranged by Paris Law. (A) Relationships between interfacial delamination growth rate vs energy release rate Δ G. (B) Relationships between crack growth rate vs stress intensity factor range Δ K.
3.4. Fracture mechanics approach to estimate exposure lives of the SS400 substrate in corrosion fatigue Fig. 8 (A) shows corrosion fatigue behavior of SS400 substrates with two types of Al-Mg coating layers. The fatigue life of the substrate with the Al-5wt% Mg coating layer was shorter than that of the substrate with the Al-2wt% Mg coating layer. Fig. 8 (B, C) demonstrated that fatigue failure of SS400 substrates was occurred after exposure to the corrosive environment by vertical cracking and subsequent connection between two interface delaminations in the Al-Mg coating layers. In order to estimate exposure lives of the substrate, Eqs. (5) and (6) were numerically integrated on the assumptions that a penetrating length of vertical crack was 250 μ m and connecting length of delamination was 200 μ m, respectively. Fig. 9 shows simulated growth plots of vertical crack and interfacial delamination in Al-Mg coating layers. Especially in the case of lower stress amplitude, a higher concentration of Mg in Al-Mg coating layers led to shorter exposure lives of the substrate. Comparisons of simulated exposure lives with the experimentally observed ones were also shown in Fig. 8 (A), which demonstrated that the simulated result provided conservative prediction of exposure lives of substrate. Such the shorter exposure lives of the SS400 substrate could accelerate crack initiation in the substrate by corrosion fatigue. Therefore, shorter fatigue lives of the SS400 substrate were followed by the shorter lives of the Al-Mg coating layer. The result in Fig. 8 suggests that higher concentration of Mg in Al-Mg coating layers is not beneficial in the case of highly-loaded conditions due to earlier exposure of the substrates to corrosive environment. Under practical conditions of σa = 80 MPa [23], there is no significant difference in the fatigue lives of the substrates. This fact can explain the reason why the effectiveness of different Mg concentration in Al-Mg coating layers is difficult to be observed in service environment, which has low stress amplitudes as well as 21
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Fig. 8. Effect of Mg concentration on corrosion fatigue behavior of SS400 substrates with Al-Mg coating layers in NaCl aq. (A)S-N curves. The ∘ and △ symbols show penetration lives of vertical crack and connecting lives of delamination (= exposure lives of substrates) in Al-Mg coating layers, respectively. Solid and dash lines suggest simulated results of failure lives of Al-Mg coating layers using Eqs. (5) and (6). (B, C) Crack paths in the Al-Mg coating layers. Red arrows in the figures indicate directions of propagation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Substrate
M
Substrate
M
Fig. 9. Simulated crack growth curves in Al-Mg coating layers immersed in NaCl aq. calculated by integration of Eqs. (5) and (6).
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lower corrosive conditions. 4. Discussion In order to explain the effects of the Mg concentration on the durability of the Al-Mg coating layers, the following possible mechanisms were considered. 1. Intermetallic compound phases, which increase the corrosion resistance, form in the Al-Mg coating matrix in response to the increased Mg concentration; 2. The mechanical properties of the Al-Mg coating layers are enhanced by the increased Mg concentration; 3. The corrosion resistance of the Al-Mg coating layers are deteriorated by the increased Mg concentration; 4. The fatigue properties of the Al-Mg coating layers are altered by the increased Mg concentration. At first, we discuss the effect of different Mg concentration on microstructure and mechanical property. SEM pictures shown in Fig. 1 clearly exhibited that no intermetallic compound phases were formed at the adjacent area of the interfaces. Higher concentration of Mg in Al-Mg coating layers provided a higher interfacial strength of Al-Mg coating layers (Fig. 5 (A)). However, such the enhancement effect will be deteriorated when we consider severe corrosive environment (Al-Mg coating layers are for sacrifice coating in corrosive environment.). The energy dispersive X-ray spectroscopy (EDX) analysis results shown in Table 2 exhibited that there was no significant difference in the remained concentration of Mg in both the coating layers. The dissolution of Mg contents can decrease interfacial strength of Al-Mg coating layers (Fig. 5 (A)) and then the enhancement effect of the higher Mg concentration is limited by the severity of corrosion. On the other hand, the deteriorating effect of corrosion resistance of Al-Mg coating layers by higher concentration of Mg (Fig. 5 (B)) should always be considered. Higher concentration of Mg induced a lower corrosion resistance and lower vertical crack/ interfacial delamination growth resistances, as shown in Table 1 and Fig. 7, respectively. Such the decreased resistances eventually led to shorter fatigue lives of substrates (Fig. 8). When considering a higher stress amplitude as well as severe corrosive environment, such the difference in exposure lives of the substrates can not be ignored. Finally, a selection policy of Al-Mg coating layers considering practical loading conditions is discussed. Under practical loading conditions of a lower stress amplitude without corrosive environment, the Al-5wt% Mg coating layer has a higher interfacial strength than the one of Al-2wt% Mg coating layer. However, consideration of corrosive environment is essential when an Al-Mg coating layer is employed as a sacrificial coating layer. Unplanned instances of higher loading, such as those experienced during earthquakes, storms, or heavy loading, can also be considered, although the typical design stress is lower at σa = 80 MPa. If higher stress amplitudes as well as the corrosive environment are considered, Al-Mg coating layers with lower Mg concentration are preferable, because of their higher fatigue resistances. The above discussion facilitates the selection of Al-Mg coating layers with different Mg concentration by considering the extent of the loading conditions and the severity of corrosion. This study has investigated specimens with a fixed coating thickness. Note that it is also important to observe the effects of different coating thicknesses and substrates on the examined properties and behaviors. Such investigations are topics for further study. 5. Conclusion The effect of different Mg concentration (2 and 5 wt%) of Al-Mg coating layers on the interracial strength and corrosion fatigue behavior of the Al-Mg coating layers were observed. The obtained results can be summarized as the follow: 1. Higher concentration of Mg in Al-Mg coating layers induced higher interfacial strength of the Al-Mg coating layer only for short immersion periods (7 days). After longer immersion in NaCl aq., such the enhancement effect of higher Mg concentration on interfacial strength of the Al-Mg coating layer is lost due to rapid dissolution of Mg content. No intermetallic phase in the Al-Mg coating layers was formed by this thermal spray process. 2. Higher concentration of Mg in Al-Mg coating layers reduced corrosion resistance of the Al-Mg coating layers. The Al-5wt% Mg coating layer showed a higher corrosion rate than that of the Al-2wt% Mg coating layer. 3. Higher concentration of Mg in Al-Mg coating layers decreased vertical crack growth resistance and interfacial delamination growth resistance of the Al-Mg coating layers. Both the crack/delamination propagation behavior of the Al-Mg coating layer were accelerated in NaCl aq., and causes earlier exposure of the substrate, resulting in shorter fatigue lives of the substrate. 4. Simulation based on fracture mechanics model could predict exposure lives of substrate. Especially in the cases of higher stress amplitude, higher concentration of Mg in the Al-Mg coating layers shortened exposure lives of the substrates, which resulted in shorter fatigue lives of the substrates. 5. Higher concentration of Mg in Al-Mg coating layer is more suitable in the case of low-severity corrosive environment and low stress levels. However, when fluctuations in the loading conditions and corrosive environment are considered, an Al-Mg coating layer with lower Mg concentration is recommended. This study provides a selection policy for Al-Mg coating layers by considering the loading conditions and corrosive environment. For this selection, observation of the corrosion fatigue properties of various types of Al-Mg coating layers are important. The effect of the coating thickness is another important factor to be examined in future work, along with the effects of different substrate types. 23
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