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Mg laminates
Accepted Manuscript A new approach to enhancing interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates Yingcai Pan, Xuan Wu, Zheng Huang, Guoqing Wu, Siqiang Sun, Hengjian Ye, Zongke Zhang PII: DOI: Reference:
S1359-835X(17)30408-6 https://doi.org/10.1016/j.compositesa.2017.11.009 JCOMA 4830
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
27 June 2017 5 November 2017 14 November 2017
Please cite this article as: Pan, Y., Wu, X., Huang, Z., Wu, G., Sun, S., Ye, H., Zhang, Z., A new approach to enhancing interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates, Composites: Part A (2017), doi: https://doi.org/10.1016/j.compositesa.2017.11.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new approach to enhancing interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates Yingcai Pan a, Xuan Wu b, Zheng Huang a, Guoqing Wu a,*,Siqiang Sun a, Hengjian Ye a
, Zongke Zhang a
a
School of Materials Science and Engineering, Beihang University, Beijing 100191,
China b
School of Energy and Power Engineering, Inner Mongolia University of Technology,
Huhhot, 010051, China
Corresponding author at: School of Materials Science and Engineering, Beihang University, Beijing 100191, China. Tel.: +8610 82313240; fax: +8610 82313240. E-mail address: [email protected] (G.-Q. Wu).
Abstract: In the present research, AZ31 magnesium alloy was treated by electrochemical methods in Na2SiO3-KOH-KF and KOH-KF electrolytes, and the morphological features of films formed on magnesium alloy were assessed. Besides, the effect of Mg surface features on interlaminar failure load, failure mode and galvanic corrosion resistance of CFRP/Mg laminates were investigated. The results show that removal of silicate in the Na2SiO3-KOH-KF electrolyte can cause the transition of conversion film from ceramic-like oxide film to pitted oxide film. The pitted oxide film can effectively enhance the peel strength of CFRP/Mg laminates compared with the ceramic-like oxide film, and an average enhancement of 6.5 times was observed. The pitted oxide film on magnesium can provide an excellent protection against the galvanic corrosion in CFRP/Mg laminates as the ceramic-like oxide film. Keywords: A. Laminates; B. Debonding; B. Corrosion; E. Surface treatments
1. Introduction Fiber metal laminates (FMLs), consisting of alternating layers of composite layers and
metal sheets, are widely used as the fuselage skin materials for aeronautical and space engineering due to their low density and high damage tolerance [1]. At present, the most widely used FMLs are a combination of aluminium alloys with aramid or glass fiber reinforced epoxy, labelled as ARALL and GLARE, and their density is about 2.20-2.50 g/cm3 [2]. The need for lightweighting to improve fuel economy is leading to the fact that the magnesium alloys become an attractive option in the FMLs [3-7]. The density of FMLs can be reduced by about 20 % by replacing the 2024 aluminium alloys with AZ31 magnesium alloys [4, 5]. To develop high performance FMLs based on magnesium alloys, the high modulus and strength carbon fibre reinforced polymer (CFRP) is considered as a potential composite layer [5]. When carbon fiber comes in contact with magnesium alloy, it is susceptible to galvanic corrosion [8]. The micro-arc oxidation (MAO) technique was applied to protect against the galvanic corrosion in CFRP/Mg laminates [9]. The influences of MAO parameters (e.g. voltage [10], duty cycle [11] and electrolyte composition [12, 13]) on the bonding strength of different metal have been reported. However, the MAO layer might cause a bad Mode I (peel) interlaminar strength of laminates [14], which will significantly affect the mechanical properties and machinability of FMLs [15, 16]. In this work, the AZ31 magnesium alloy sheets were subjected to electrochemical process in different electrolyte systems, then the carbon fibre reinforced polymer (CFRP)/magnesium laminates were prepared by a hot-press process. The Mode I and II interlaminar strength and the corrosion behavior of CFRP/Mg laminates were evaluated to investigate the effect of Mg surface features generated by electrochemistry on interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates, and to propose an approach for enhancing interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates. 2. Materials and methods 2.1 Materials The FMLs panels consisted of 0.2 mm thick magnesium alloy sheets (AZ31 alloy from Yingkou Yinhe Corp. Liaoning, China) and carbon fiber reinforced polymer
(CFRP) with a nominal thickness of 0.1 mm. The CFRP prepregs, consisting of unidirectional carbon fiber (65 Vol. %, type T300) and epoxy (type 648), were produced by the System Design Institute of Mechanical-Electrical Engineering in Beijing, China. The lay-up scheme of the FMLs was 6/5 (six magnesium alloy layers and five CFRP layers with [0°/0°] stacking sequence), as shown in Figure 1. Prior to manufacture, electrochemical treatment (ET) was applied to magnesium alloy in two different electrolytes. The composition and concentration of the electrolytes are listed in Table 1. The process of ET was carried out by using an inverter pulsed power supply (Type GGMF25/600-A). The magnesium alloy specimens (100 mm × 60 mm × 0.2 mm) were used as anode, and two AISI 316 L panels cathodes were positioned on both sides to ensure a homogeneous film over the entire surface. The distance between both electrodes is 8 cm. During the treatment, the current density used was 100 mA/cm2 with a fixed frequency of 500 Hz and duty ratio of 30 %. After surface preparation, the magnesium alloy sheets and CFRP were placed in a mould with dimensions of 180 mm × 110 mm, and then the specimens were cured in a hot-press machine. During curing, the pressure was kept at 1.5 MPa, and the temperature was increased from room temperature to 180 ℃ at the rate of 2 ℃/min, and held at 180 ℃ for 150 min [17]. Then, the mould was cooled to room temperature in the air. 2.2 Mode I and II interlaminar strength tests The Mode I (peel) interlamintar strength between the CFRP and the magnesium alloys was determined by a roller peel test method utilizing a universal tensile tester according to ASTM D 3167 (Figure 2 a). The specimens were cut down to 15 mm width and 150 mm length, the flexible adherend material was the magnesium alloy. For these tests, a tensile loading rate of 50 mm/min was applied and the peeling force was recorded. The Mode II (shear) interlaminar strength was measured using a double-notch shear (DNS) test method (Figure 2 b) based on available standards (ASTM 3846-08) for pure composites and other available literature [18], as there was no specific test standard for fiber metal laminates. The specimens were cut down to
15 mm width and 150 mm length, respectively. Notches for shear testing samples were cut up to and slightly beyond the interface by low and careful hand cutting, to ensure the length between the two notches is about 10 mm. The interlaminar shear load between the notches was applied by tension on both ends of the DNS specimen, and the process was carried out on a universal test machine at a crosshead displacement rate of 1 mm/min. The machine used is Instron testing machine (Instron 5960) equipped with a 5 kN load cell to measure the load for interlaminar peel and shear fracture, respectively. The peel and shear tests were repeated five times for each material. 2.3 Immersion test Immersion tests on the FMLs were carried out at room temperature according to ASTM-G31-72. Before immersion test, the Mg layer placed at the top and bottom layers were removed in order to evaluate the effect of galvanic corrosion. Then, the all surface of the specimens (15 mm × 10 mm ×1.4 mm) were ground progressively on finer grades of emery paper up to 2000 grit and then weighed. The polished and pre-weighed specimens were exposed to 300 ml of 3.5 wt. % NaCl solution for different intervals of time. The tests were carried out without agitation and disturbing corrosion system. At the end of the experiment, final cleaning of the specimen was done by dipping it in 200 g CrO3, 10g AgNO3, 20 g Ba(NO3)2 and 1000 ml distilled water for 5 min followed by washing with alcohol. The corrosion rate (r) was calculated using (1) where W1 is the weight before immersion (mg), W2 is the weight after immersion (mg), A is the surface area not including the upper and lower surfaces of the specimen (cm2) and T is the immersion time (h). The immersion tests were repeated three times to obtain the reproducible results. 2.4 Material characterization The surface morphology, cross-section microstructure and chemical composition of the films were examined using the scanning electron microscopy (Phenom™ Pro and
S4800-SEM) with energy dispersive analysis of X-rays (EDX). 3. Results and discussion 3.1 Surface characteristics of magnesium alloy Figure 3 shows surface and cross-sectional SEM micrographs of the ET-Si (electrochemical treatment in the Na2SiO3-KOH-KF electrolyte) sample. It can be seen that the ET-Si sample surface exhibits typical features of micro arc oxidation film. A ceramic-like film with a thickness of 5 μm is clearly observed, and some micro-pores with the diameter of 2 μm are observed on the surface of the films. As already discussed in Ref. [19], these cavities/pores are formed by the molten oxide and gas bubbles thrown out of micro-arc discharge channels. Unlike the ET-Si sample, the ET-Si-free (electrochemical treatment in the KOH-KF electrolyte) sample shows the pitted surface films on the magnesium alloy substrate, as shown in Figure 4. Network-like grooves and holes are obviously distributed all over the surface of the magnesium alloy (Figure 4 a3-d3), showing a typical etchpit-like structure. Based on the EDS results (Table 2) and previous research [20], it can be speculated that the remaining formed conversion film mainly consists of oxides of Mg. Therefore, the formation of pitted oxide film can be explained by the tensile stresses in the oxide film, which has a molar volume much smaller than the metal from which it is formed (The Pilling-Bedworth ratio of Mg oxidation is 0.81 [21]). The electrochemical and chemical dissolution of film is also considered as one of the reasons for the pitted surface films. This result indicates that the formation and characteristics of the conversion film process is governed by silicate during electrochemical treatment. The removal of Na2SiO3 in the electrolytic solution causes the transition of conversion films from ceramic-like oxide film to pitted magnesium oxide films on the surface of magnesium alloy. It is interesting to note that the grooves and holes of local regions of magnesium surface is deeper than that of other regions (Figure a1-d1), and the difference in hole depth between two regions increases with the increase of treatment time (Figure a2-d2). As seen from the cross-sectional SEM micrographs (Figure 5), the dimensions of the holes increase with increase in treatment time. The depth of deepest hole
increases from 12.9 μm in case of sample treated for 1 min, to 67.5 μm in the case of sample treated for 4 min. Such a characteristic of the non-uniform surface mainly results from the electrochemical dissolution of the film layer, due to the existence of uneven distribution of elements. It should be noted that although the increased holes depth would provide an increase of surface area and enhance mechanical interlocking between magnesium alloy layer and CFRP layer, such characteristics may enhance crack initiation in the magnesium alloy layer and cause a significant decrease in tensile strength compared to the magnesium alloys with small dimensions of the holes. 3.2 Mode I and II interlaminar strength of the FMLs Figure 6 presents the load–displacement curve from interlaminar peel and shear tests of the laminates, and the mean value of peel and shear interlaminar strength for the ET-Si and ET-Si-free samples is shown in Figure 7. As can be seen from Figure 6 (a), the peel load increases rapidly with the increase of displacement, which corresponds with elastic/plastic deformation of the flexible material (magnesium alloy). When the crack initiation and propagation in interlaminar, the peeling force keeps relatively constant with the increase of displacement, indicating the crack propagation was stable. It can be seen from Figure 6 (b), as the displacement increase, the shear load growth linearly reaches its maximum value and then suddenly drops, indicating that the strain energy has already reached the critical strain energy of the material for interlamianr shear crack propagation. Comparison between previously known data [14] and the results obtained in this study reveals the following facts (Figure 7). No clear differences are found in the shear strength between the ET-Si and the ET-Si-free samples, their values are 37 MPa. However, the ET-Si-free samples show increased peel strength for all treatment times, and an average enhancement of 6.5 times was observed when compared with the ET-Si sample. These results indicate that the ET-Si-free sample displays good overall interlaminar strength of the CFRP/Mg laminates under peel and shear loading when compared with the ET-Si sample. In addition, it can be seen from Fig. 4 the treatment time has a direct effect on the peel strength of the ET-Si-free sample. Increasing the
treatment time from 1 min to 2 min results in a slight increase of peel strength. After 2 min, the sample exhibits a sharp increase in peel strength. The average value of peel strength of the sample treated for 3 min is 1.75 N/mm, which decreases by 60 % compared to the specimens for 1 and 2 min. This result seems to be reasonable since the depth of deepest hole exhibits a sharp increase after 2 min. Further increase in treatment time does not lead to significant increase in peel strength, which is in good agreement with the results of surface micrographs analysis. The cross-sectional and detached Mg surface SEM images of the ET-Si and ET-Si-free samples after peel test are shown in Figure 8 and Figure 9, respectively. It is clear that the ET-Si specimen exhibits interfacial failure between oxide layer and magnesium alloys substrate under Mode I loading conditions, as shown in Figure 8 a. This is because the specimen displays a typical brittle film/ductile substrate system plus a porous MAO film [14]. For the ET-Si-free sample in this study, crack propagation
mainly
occurs
at
the
CFRP/magnesium
interface,
showing
magnesium/epoxy interfacial failure and cohesive failure (Figure 9). It can be speculated that there are two general types of failure modes in the damage, namely the adhesive failure between epoxy and magnesium alloys, as well as the cohesive failure. For the ET-Si-free samples treated for 3 and 4 min, the area corresponding to the fracture of epoxy is greatly increased (Figure 9 c and d), which means that the cohesive failure became dominant for the ET-Si-free samples treated for 3 and 4 min. Hence, the sample exhibits a sharp increase in peel strength after 2 min. The difference on the peel strength and failure mode between the two ET samples mainly results from the different surface texture. Based on the analysis of magnesium surface characteristics, the surface texture generated by micro arc technology can be classified into two types (see Figure 10): the porous two-layered structure (which represents ET-Si surface) and the chaotic structure (which represents ET-Si-free surfaces). The surface texture generated by ET-Si provides a strong adhesive bond between oxide layer and epoxy, the bond strength between the oxide layer and the magnesium substrate is bad due to the typical brittle film/ductile substrate system plus a porous MAO film [14]. As mentioned above, crack propagation mainly occurs at the
MAO film/magnesium substrate interface, as shown in Figure 10 a. The chaotic structure generated by ET-Si-free incorporates the features of the Network-like pitting and the bridge-like or undercut holes (Figure 10 b). In the case of shallow pitting, the adhesive material in the pores is stretched as it is pulled out [22], which results in the adhesive failure between epoxy and magnesium alloys substrate. In the case of bridge-like or undercut holes, a sufficient amount of adhesive penetration into these surface crevices is obtained, thus providing efficient anchoring of the adhesive, which leads to the cohesive failure or the fiber/epoxy interfacial failure under Mode I loading condition and yield higher peel strength, as shown in Figure 10 b. 3.3 Corrosion behavior of the FMLs The appearance of ET-Si-free and control group (CFRP/uncoated Mg laminates, namely the magnesium alloy sheets are subjected to grit blasting) specimens at 15 min, 12 h and 33 h from the beginning of the immersion is shown in Figure 11. The cross-section of control group sample is corroded after only 15 min immersion, while the ET-Si-free sample not shows a significant change from visual inspection. The cross-section of the control group sample is more severely corroded after 12 h immersion. When the immersion time increases to 33 h, the overall collapse of laminates is observed, while the ET-Si-free sample keeps its original shape except visualized minor corrosion damage on the cross-section. The corrosion rate of ET-Si, ET-Si-free and control group specimens after 9 h immersion in 3.5 wt. % NaCl solution is shown in Figure 12. The value of corrosion rate of the ET-Si-free samples after 9 h immersion is 0.040 mg cm-2 h-1, which decreases by 80 % as compared to the CFRP/Mg laminates obtained from grit blasting (0.226 mg cm-2 h-1). It is clear that the oxide film generated by ET-Si-free can provide an excellent protection against the galvanic corrosion in CFRP/Mg laminates. Compared with the control group sample, such a lower corrosion rate of the ET-Si-free sample is mainly attributed to the inhibiting effect of oxide film on the galvanic corrosion. Based on the EDS results (Table 2), it can be speculated that the remaining formed conversion film mainly consisted of MgO phase, its insulating property leading to the fact that the both materials (Mg and carbon fibre) are not in
electrical contact. Thus, galvanic corrosion could not occur in the ET-Si-free sample, and the corrosion behavior of the laminates mainly takes the form of self-corrosion according to literature [23]. However, for the control group (namely, CFRP/Mg laminates obtained from grit blasting), the total corrosion behaviour can be considered to be made up of the galvanic corrosion and the self-corrosion, due to the direct contact between magnesium alloys and carbon fibre. Therefore, the corrosion rate of the ET-Si-free specimen is lower than that of the control group specimen. 4. Conclusions (1) During electrochemical treatment of AZ31 magnesium alloy, removal of silicate in the Na2SiO3-KOH-KF electrolytes causes the transition of conversion film from ceramic-like oxide film to pitted oxide film on the surface of magnesium alloy. (2) The pitted oxide film can effectively enhance the Mode I (peel) interlaminar strength of CFRP/Mg laminates compared with the ceramic-like oxide film, and an average enhancement of 6.5 times was observed. (3) For the CFRP/Mg with ceramic-like oxide film laminates, it shows the oxide film/Mg substrate interfacial failure, while the CFRP/Mg with pitted oxide film laminates mainly shows cohesive failure and adhesive failure. (4) The pitted oxide film on magnesium can provide an excellent protection against the galvanic corrosion in CFRP/Mg laminates as the ceramic-like oxide film, since the isolation between carbon fibre and magnesium substrate can be obtained by this way. Acknowledgments This paper is financially supported by the Science and technology innovation project (No. 009-031-001) and the Aeronautics Fundamental Science Foundation of China (2010ZF51068). References [1] Vermeeren CAJR. An historic overview of the development of fibre metal laminates. Applied Composite Materials 2003; 10: 189-205. [2] Wu GC, Yang JM. The mechanical behavior of GLARE laminates for aircraft structures. Jom 2005; 57: 72-79.
[3] Hu FP, Zhang Y, Zeng QW, Jia N, Qin J, Zhang X, Wang SZ, He JM, Xie WD. Research on potassium permanganate-phosphate treatment of magnesium alloy surface and fiber/magnesium alloy composite laminate. Materials and Corrosion 2016; 67: 1128-1134. [4] Pärnänen T, Alderliesten RC, Rans C, Brander T, Saarela O. Applicability of AZ31B–H24 magnesium in Fibre Metal Laminates–An experimental impact research. Composites: Part A 2012; 43: 1578-1586. [5] Cortés P, Cantwell WJ. The fracture properties of a fibre–metal laminate based on magnesium alloy. Composites: Part B 2006; 37: 163-170. [6] Múgica JI, Aretxabaleta L, Ulacia I, Aurrekoetxea J. Impact characterization of thermoformable fibre metal laminates of 2024–T3 aluminium and AZ31B–H24 magnesium based on self-reinforced polypropylene. Composites: Part A 2014; 61: 67-75. [7] Sadighi M, Pärnänen T, Alderliesten RC, Sayeaftabi M, Benedictus R. Experimental and numerical investigation of metal type and thickness effects on the impact resistance of fiber metal laminates. Applied Composite Materials 2012; 19: 545-559. [8] Pan YC, Wu GQ, Cheng X, Zhang ZK, Li MY, Ji SD, Huang Z. Galvanic corrosion behaviour of carbon fibre reinforced polymer/magnesium alloys coupling. Corrosion Science 2015; 98: 672-677. [9] Pan YC, Wu GQ, Huang Z, Wu X, Liu YH, Ye HJ. Corrosion behaviour of carbon fibre reinforced polymer/magnesium alloy hybrid laminates. Corrosion Science 2017; 115: 152-158. [10] Wang CC, Wang F, Han Y. The structure, bond strength and patite-inducing ability of micro-arc oxidized tantalum and their response to annealing. Applied Surface Science 2016; 36: 190-198. [11] Tang Y, Zhao X, Jiang K, Chen J, Zuo Y. The influences of duty cycle on the bonding strength of AZ31B magnesium alloy by microarc oxidation treatment. Surface and Coatings Technology 2010; 205: 1789-1792. [12] Yuan XH, Tan F, Xu H, Zhang SJ, Qu FZ, Liu J. Effects of different electrolytes
for micro-arc oxidation on the bond strength between titanium and porcelain. Journal of Prosthodontic Research 2017; 61 (3): 297-304. [13] Gao H, Zhang M, Yang X, Huang P, Xu K. Effect of Na2SiO3 solution concentration
of
micro-arc
oxidation
process
on
lap-shear
strength
of
adhesive-bonded magnesium alloys. Applied Surface Science 2014; 314: 447-452. [14] Pan YC, Wu GQ, Huang ZH, Li MY, Ji SD, Zhang ZK. Effects of surface pre-treatments on Mode I and Mode II interlaminar strength of CFRP/Mg laminates. Surface & Coatings Technology 2017; 319: 309-317. [15] Lawcock G, Ye L, Mai YW, Sun CT. The effect of adhesive bonding between aluminum and composite prepreg on the mechanical properties of carbon fiber reinforced metal laminates. Composites Science and Technology 1997; 57: 35-45. [16] Sinke J. Manufacturing of GLARE parts and structures. Applied Composite Materials 2003; 10: 293-305. [17] Pan YC, Wu GQ, Cheng X, Zhang ZK, Li MY, Ji SD, Huang Z. Mode I and Mode II interlaminar fracture toughness of CFRP/magnesium alloys hybrid Laminates. Composite Interfaces 2016; 25: 453-465. [18] Naghipour P, Schulze K, Hausmann J, Bartsch M. Numerical and experimental investigation on lap shear fracture of Al/CFRP laminates. Composites Science and Technology 2012, 72: 1718-1724. [19] Sankara Narayanan TSN, Park IS, Lee MH. Strategies to improve the corrosion resistance of microarc oxidation (MAO) coated magnesium alloys for degradable implants: Prospects and challenges. Progress in Materials Science 2014; 60: 1-71. [20] Ryu HS, Hong S. Effects of KF, NaOH, and KOH Electrolytes on Properties of Microarc-Oxidized Coatings on AZ91D Magnesium Alloy. Journal of The Electrochemical Society 2009; 156: C298-C303. [21] Song LW, Song YW, Shan DY, Zhu GY, Han EH. Product/metal ratio (PMR), A novel criterion for the evaluation of electrolytes on micro-arc oxidation (MAO) of Mg and its alloys. Science China Technological Sciences 2011; 54: 2795-2801. [22] Gent AN, Lin CW. Model studies of the effect of surface roughness mechanical interlocking on Adhesion. The Journal of Adhesion 1990; 32: 113-125.
[23] Jia JX, Atrens A, Song G, Muster TH. Simulation of galvanic corrosion of magnesium coupled to a steel fastener in NaCl solution. Materials and Corrosion 2005; 56: 468-474.
Table and Figure captions Table 1. The composition and concentration of two electrolytes, and the treatment time in each of electrolytes. Table 2. Relative element contents (in wt. %) of conversion film. Figure 1. Schematic illustration of the stacking sequence of the FMLs. Figure 2. Schematic diagram and experimental setup of (a) floating roller peel test, and (b) double-notch shear test. Figure 3. The surface and cross-sectional SEM images of the ET-Si sample. Figure 4. Surface morphology of magnesium alloys sheets after ET-Si-free. Figure 5. The cross-sectional SEM images of the ET-Si-free sample. Figure 6. The load–displacement curve from interlaminar peel (a) and shear (b) tests of the FMLs Figure 7. The mean values of Mode I and II interlaminar strength for the FMLs obtained from ET-Si and ET-Si-free. Figure 8. The cross-sectional and detached Mg surface SEM images of the ET-Si specimen after peel test. Figure 9. The cross-sectional and detached Mg surface SEM images of the ET-Si-free specimen after peel test. Figure 10. Schematic depiction of sections and fracture surfaces for samples with (a) ET-Si and (b) ET-Si-free under peel loading condition. Figure 11. The photos of two FMLs during the immersion test in 3.5 wt. % NaCl solution. Figure 12. The corrosion rates of the control group and the samples with ET-Si and ET-Si-free.
Table 1 The composition and concentration of two electrolytes, and the treatment time in each of electrolytes. Group
Solution composition
Time
ET-Si group
5.68 g/L Na2SiO3+7.28 g/L KOH+2.9 g/L KF
2 min
ET-Si-free group
7.28 g/L KOH+2.9 g/L KF
1/2/3/4 min
Table 2 Relative element contents (in wt. %) of conversion film. Element content (wt. %) Sample Mg
O
F
Al
Si
Na
ET-Si-free (1 min)
75.8
18.4
4.3
1.5
-
-
ET-Si-free (2 min)
77.1
18.4
3.5
1.0
-
-
ET-Si-free (3 min)
74.3
21.0
3.4
1.3
-
-
ET-Si-free (4 min)
74.3
21.6
3.0
1.2
-
-
ET-Si
45.9
45.0
-
1.3
6.7
1.2
S1359-835X(17)30408-6 https://doi.org/10.1016/j.compositesa.2017.11.009 JCOMA 4830
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
27 June 2017 5 November 2017 14 November 2017
Please cite this article as: Pan, Y., Wu, X., Huang, Z., Wu, G., Sun, S., Ye, H., Zhang, Z., A new approach to enhancing interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates, Composites: Part A (2017), doi: https://doi.org/10.1016/j.compositesa.2017.11.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new approach to enhancing interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates Yingcai Pan a, Xuan Wu b, Zheng Huang a, Guoqing Wu a,*,Siqiang Sun a, Hengjian Ye a
, Zongke Zhang a
a
School of Materials Science and Engineering, Beihang University, Beijing 100191,
China b
School of Energy and Power Engineering, Inner Mongolia University of Technology,
Huhhot, 010051, China
Corresponding author at: School of Materials Science and Engineering, Beihang University, Beijing 100191, China. Tel.: +8610 82313240; fax: +8610 82313240. E-mail address: [email protected] (G.-Q. Wu).
Abstract: In the present research, AZ31 magnesium alloy was treated by electrochemical methods in Na2SiO3-KOH-KF and KOH-KF electrolytes, and the morphological features of films formed on magnesium alloy were assessed. Besides, the effect of Mg surface features on interlaminar failure load, failure mode and galvanic corrosion resistance of CFRP/Mg laminates were investigated. The results show that removal of silicate in the Na2SiO3-KOH-KF electrolyte can cause the transition of conversion film from ceramic-like oxide film to pitted oxide film. The pitted oxide film can effectively enhance the peel strength of CFRP/Mg laminates compared with the ceramic-like oxide film, and an average enhancement of 6.5 times was observed. The pitted oxide film on magnesium can provide an excellent protection against the galvanic corrosion in CFRP/Mg laminates as the ceramic-like oxide film. Keywords: A. Laminates; B. Debonding; B. Corrosion; E. Surface treatments
1. Introduction Fiber metal laminates (FMLs), consisting of alternating layers of composite layers and
metal sheets, are widely used as the fuselage skin materials for aeronautical and space engineering due to their low density and high damage tolerance [1]. At present, the most widely used FMLs are a combination of aluminium alloys with aramid or glass fiber reinforced epoxy, labelled as ARALL and GLARE, and their density is about 2.20-2.50 g/cm3 [2]. The need for lightweighting to improve fuel economy is leading to the fact that the magnesium alloys become an attractive option in the FMLs [3-7]. The density of FMLs can be reduced by about 20 % by replacing the 2024 aluminium alloys with AZ31 magnesium alloys [4, 5]. To develop high performance FMLs based on magnesium alloys, the high modulus and strength carbon fibre reinforced polymer (CFRP) is considered as a potential composite layer [5]. When carbon fiber comes in contact with magnesium alloy, it is susceptible to galvanic corrosion [8]. The micro-arc oxidation (MAO) technique was applied to protect against the galvanic corrosion in CFRP/Mg laminates [9]. The influences of MAO parameters (e.g. voltage [10], duty cycle [11] and electrolyte composition [12, 13]) on the bonding strength of different metal have been reported. However, the MAO layer might cause a bad Mode I (peel) interlaminar strength of laminates [14], which will significantly affect the mechanical properties and machinability of FMLs [15, 16]. In this work, the AZ31 magnesium alloy sheets were subjected to electrochemical process in different electrolyte systems, then the carbon fibre reinforced polymer (CFRP)/magnesium laminates were prepared by a hot-press process. The Mode I and II interlaminar strength and the corrosion behavior of CFRP/Mg laminates were evaluated to investigate the effect of Mg surface features generated by electrochemistry on interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates, and to propose an approach for enhancing interlaminar strength and galvanic corrosion resistance of CFRP/Mg laminates. 2. Materials and methods 2.1 Materials The FMLs panels consisted of 0.2 mm thick magnesium alloy sheets (AZ31 alloy from Yingkou Yinhe Corp. Liaoning, China) and carbon fiber reinforced polymer
(CFRP) with a nominal thickness of 0.1 mm. The CFRP prepregs, consisting of unidirectional carbon fiber (65 Vol. %, type T300) and epoxy (type 648), were produced by the System Design Institute of Mechanical-Electrical Engineering in Beijing, China. The lay-up scheme of the FMLs was 6/5 (six magnesium alloy layers and five CFRP layers with [0°/0°] stacking sequence), as shown in Figure 1. Prior to manufacture, electrochemical treatment (ET) was applied to magnesium alloy in two different electrolytes. The composition and concentration of the electrolytes are listed in Table 1. The process of ET was carried out by using an inverter pulsed power supply (Type GGMF25/600-A). The magnesium alloy specimens (100 mm × 60 mm × 0.2 mm) were used as anode, and two AISI 316 L panels cathodes were positioned on both sides to ensure a homogeneous film over the entire surface. The distance between both electrodes is 8 cm. During the treatment, the current density used was 100 mA/cm2 with a fixed frequency of 500 Hz and duty ratio of 30 %. After surface preparation, the magnesium alloy sheets and CFRP were placed in a mould with dimensions of 180 mm × 110 mm, and then the specimens were cured in a hot-press machine. During curing, the pressure was kept at 1.5 MPa, and the temperature was increased from room temperature to 180 ℃ at the rate of 2 ℃/min, and held at 180 ℃ for 150 min [17]. Then, the mould was cooled to room temperature in the air. 2.2 Mode I and II interlaminar strength tests The Mode I (peel) interlamintar strength between the CFRP and the magnesium alloys was determined by a roller peel test method utilizing a universal tensile tester according to ASTM D 3167 (Figure 2 a). The specimens were cut down to 15 mm width and 150 mm length, the flexible adherend material was the magnesium alloy. For these tests, a tensile loading rate of 50 mm/min was applied and the peeling force was recorded. The Mode II (shear) interlaminar strength was measured using a double-notch shear (DNS) test method (Figure 2 b) based on available standards (ASTM 3846-08) for pure composites and other available literature [18], as there was no specific test standard for fiber metal laminates. The specimens were cut down to
15 mm width and 150 mm length, respectively. Notches for shear testing samples were cut up to and slightly beyond the interface by low and careful hand cutting, to ensure the length between the two notches is about 10 mm. The interlaminar shear load between the notches was applied by tension on both ends of the DNS specimen, and the process was carried out on a universal test machine at a crosshead displacement rate of 1 mm/min. The machine used is Instron testing machine (Instron 5960) equipped with a 5 kN load cell to measure the load for interlaminar peel and shear fracture, respectively. The peel and shear tests were repeated five times for each material. 2.3 Immersion test Immersion tests on the FMLs were carried out at room temperature according to ASTM-G31-72. Before immersion test, the Mg layer placed at the top and bottom layers were removed in order to evaluate the effect of galvanic corrosion. Then, the all surface of the specimens (15 mm × 10 mm ×1.4 mm) were ground progressively on finer grades of emery paper up to 2000 grit and then weighed. The polished and pre-weighed specimens were exposed to 300 ml of 3.5 wt. % NaCl solution for different intervals of time. The tests were carried out without agitation and disturbing corrosion system. At the end of the experiment, final cleaning of the specimen was done by dipping it in 200 g CrO3, 10g AgNO3, 20 g Ba(NO3)2 and 1000 ml distilled water for 5 min followed by washing with alcohol. The corrosion rate (r) was calculated using (1) where W1 is the weight before immersion (mg), W2 is the weight after immersion (mg), A is the surface area not including the upper and lower surfaces of the specimen (cm2) and T is the immersion time (h). The immersion tests were repeated three times to obtain the reproducible results. 2.4 Material characterization The surface morphology, cross-section microstructure and chemical composition of the films were examined using the scanning electron microscopy (Phenom™ Pro and
S4800-SEM) with energy dispersive analysis of X-rays (EDX). 3. Results and discussion 3.1 Surface characteristics of magnesium alloy Figure 3 shows surface and cross-sectional SEM micrographs of the ET-Si (electrochemical treatment in the Na2SiO3-KOH-KF electrolyte) sample. It can be seen that the ET-Si sample surface exhibits typical features of micro arc oxidation film. A ceramic-like film with a thickness of 5 μm is clearly observed, and some micro-pores with the diameter of 2 μm are observed on the surface of the films. As already discussed in Ref. [19], these cavities/pores are formed by the molten oxide and gas bubbles thrown out of micro-arc discharge channels. Unlike the ET-Si sample, the ET-Si-free (electrochemical treatment in the KOH-KF electrolyte) sample shows the pitted surface films on the magnesium alloy substrate, as shown in Figure 4. Network-like grooves and holes are obviously distributed all over the surface of the magnesium alloy (Figure 4 a3-d3), showing a typical etchpit-like structure. Based on the EDS results (Table 2) and previous research [20], it can be speculated that the remaining formed conversion film mainly consists of oxides of Mg. Therefore, the formation of pitted oxide film can be explained by the tensile stresses in the oxide film, which has a molar volume much smaller than the metal from which it is formed (The Pilling-Bedworth ratio of Mg oxidation is 0.81 [21]). The electrochemical and chemical dissolution of film is also considered as one of the reasons for the pitted surface films. This result indicates that the formation and characteristics of the conversion film process is governed by silicate during electrochemical treatment. The removal of Na2SiO3 in the electrolytic solution causes the transition of conversion films from ceramic-like oxide film to pitted magnesium oxide films on the surface of magnesium alloy. It is interesting to note that the grooves and holes of local regions of magnesium surface is deeper than that of other regions (Figure a1-d1), and the difference in hole depth between two regions increases with the increase of treatment time (Figure a2-d2). As seen from the cross-sectional SEM micrographs (Figure 5), the dimensions of the holes increase with increase in treatment time. The depth of deepest hole
increases from 12.9 μm in case of sample treated for 1 min, to 67.5 μm in the case of sample treated for 4 min. Such a characteristic of the non-uniform surface mainly results from the electrochemical dissolution of the film layer, due to the existence of uneven distribution of elements. It should be noted that although the increased holes depth would provide an increase of surface area and enhance mechanical interlocking between magnesium alloy layer and CFRP layer, such characteristics may enhance crack initiation in the magnesium alloy layer and cause a significant decrease in tensile strength compared to the magnesium alloys with small dimensions of the holes. 3.2 Mode I and II interlaminar strength of the FMLs Figure 6 presents the load–displacement curve from interlaminar peel and shear tests of the laminates, and the mean value of peel and shear interlaminar strength for the ET-Si and ET-Si-free samples is shown in Figure 7. As can be seen from Figure 6 (a), the peel load increases rapidly with the increase of displacement, which corresponds with elastic/plastic deformation of the flexible material (magnesium alloy). When the crack initiation and propagation in interlaminar, the peeling force keeps relatively constant with the increase of displacement, indicating the crack propagation was stable. It can be seen from Figure 6 (b), as the displacement increase, the shear load growth linearly reaches its maximum value and then suddenly drops, indicating that the strain energy has already reached the critical strain energy of the material for interlamianr shear crack propagation. Comparison between previously known data [14] and the results obtained in this study reveals the following facts (Figure 7). No clear differences are found in the shear strength between the ET-Si and the ET-Si-free samples, their values are 37 MPa. However, the ET-Si-free samples show increased peel strength for all treatment times, and an average enhancement of 6.5 times was observed when compared with the ET-Si sample. These results indicate that the ET-Si-free sample displays good overall interlaminar strength of the CFRP/Mg laminates under peel and shear loading when compared with the ET-Si sample. In addition, it can be seen from Fig. 4 the treatment time has a direct effect on the peel strength of the ET-Si-free sample. Increasing the
treatment time from 1 min to 2 min results in a slight increase of peel strength. After 2 min, the sample exhibits a sharp increase in peel strength. The average value of peel strength of the sample treated for 3 min is 1.75 N/mm, which decreases by 60 % compared to the specimens for 1 and 2 min. This result seems to be reasonable since the depth of deepest hole exhibits a sharp increase after 2 min. Further increase in treatment time does not lead to significant increase in peel strength, which is in good agreement with the results of surface micrographs analysis. The cross-sectional and detached Mg surface SEM images of the ET-Si and ET-Si-free samples after peel test are shown in Figure 8 and Figure 9, respectively. It is clear that the ET-Si specimen exhibits interfacial failure between oxide layer and magnesium alloys substrate under Mode I loading conditions, as shown in Figure 8 a. This is because the specimen displays a typical brittle film/ductile substrate system plus a porous MAO film [14]. For the ET-Si-free sample in this study, crack propagation
mainly
occurs
at
the
CFRP/magnesium
interface,
showing
magnesium/epoxy interfacial failure and cohesive failure (Figure 9). It can be speculated that there are two general types of failure modes in the damage, namely the adhesive failure between epoxy and magnesium alloys, as well as the cohesive failure. For the ET-Si-free samples treated for 3 and 4 min, the area corresponding to the fracture of epoxy is greatly increased (Figure 9 c and d), which means that the cohesive failure became dominant for the ET-Si-free samples treated for 3 and 4 min. Hence, the sample exhibits a sharp increase in peel strength after 2 min. The difference on the peel strength and failure mode between the two ET samples mainly results from the different surface texture. Based on the analysis of magnesium surface characteristics, the surface texture generated by micro arc technology can be classified into two types (see Figure 10): the porous two-layered structure (which represents ET-Si surface) and the chaotic structure (which represents ET-Si-free surfaces). The surface texture generated by ET-Si provides a strong adhesive bond between oxide layer and epoxy, the bond strength between the oxide layer and the magnesium substrate is bad due to the typical brittle film/ductile substrate system plus a porous MAO film [14]. As mentioned above, crack propagation mainly occurs at the
MAO film/magnesium substrate interface, as shown in Figure 10 a. The chaotic structure generated by ET-Si-free incorporates the features of the Network-like pitting and the bridge-like or undercut holes (Figure 10 b). In the case of shallow pitting, the adhesive material in the pores is stretched as it is pulled out [22], which results in the adhesive failure between epoxy and magnesium alloys substrate. In the case of bridge-like or undercut holes, a sufficient amount of adhesive penetration into these surface crevices is obtained, thus providing efficient anchoring of the adhesive, which leads to the cohesive failure or the fiber/epoxy interfacial failure under Mode I loading condition and yield higher peel strength, as shown in Figure 10 b. 3.3 Corrosion behavior of the FMLs The appearance of ET-Si-free and control group (CFRP/uncoated Mg laminates, namely the magnesium alloy sheets are subjected to grit blasting) specimens at 15 min, 12 h and 33 h from the beginning of the immersion is shown in Figure 11. The cross-section of control group sample is corroded after only 15 min immersion, while the ET-Si-free sample not shows a significant change from visual inspection. The cross-section of the control group sample is more severely corroded after 12 h immersion. When the immersion time increases to 33 h, the overall collapse of laminates is observed, while the ET-Si-free sample keeps its original shape except visualized minor corrosion damage on the cross-section. The corrosion rate of ET-Si, ET-Si-free and control group specimens after 9 h immersion in 3.5 wt. % NaCl solution is shown in Figure 12. The value of corrosion rate of the ET-Si-free samples after 9 h immersion is 0.040 mg cm-2 h-1, which decreases by 80 % as compared to the CFRP/Mg laminates obtained from grit blasting (0.226 mg cm-2 h-1). It is clear that the oxide film generated by ET-Si-free can provide an excellent protection against the galvanic corrosion in CFRP/Mg laminates. Compared with the control group sample, such a lower corrosion rate of the ET-Si-free sample is mainly attributed to the inhibiting effect of oxide film on the galvanic corrosion. Based on the EDS results (Table 2), it can be speculated that the remaining formed conversion film mainly consisted of MgO phase, its insulating property leading to the fact that the both materials (Mg and carbon fibre) are not in
electrical contact. Thus, galvanic corrosion could not occur in the ET-Si-free sample, and the corrosion behavior of the laminates mainly takes the form of self-corrosion according to literature [23]. However, for the control group (namely, CFRP/Mg laminates obtained from grit blasting), the total corrosion behaviour can be considered to be made up of the galvanic corrosion and the self-corrosion, due to the direct contact between magnesium alloys and carbon fibre. Therefore, the corrosion rate of the ET-Si-free specimen is lower than that of the control group specimen. 4. Conclusions (1) During electrochemical treatment of AZ31 magnesium alloy, removal of silicate in the Na2SiO3-KOH-KF electrolytes causes the transition of conversion film from ceramic-like oxide film to pitted oxide film on the surface of magnesium alloy. (2) The pitted oxide film can effectively enhance the Mode I (peel) interlaminar strength of CFRP/Mg laminates compared with the ceramic-like oxide film, and an average enhancement of 6.5 times was observed. (3) For the CFRP/Mg with ceramic-like oxide film laminates, it shows the oxide film/Mg substrate interfacial failure, while the CFRP/Mg with pitted oxide film laminates mainly shows cohesive failure and adhesive failure. (4) The pitted oxide film on magnesium can provide an excellent protection against the galvanic corrosion in CFRP/Mg laminates as the ceramic-like oxide film, since the isolation between carbon fibre and magnesium substrate can be obtained by this way. Acknowledgments This paper is financially supported by the Science and technology innovation project (No. 009-031-001) and the Aeronautics Fundamental Science Foundation of China (2010ZF51068). References [1] Vermeeren CAJR. An historic overview of the development of fibre metal laminates. Applied Composite Materials 2003; 10: 189-205. [2] Wu GC, Yang JM. The mechanical behavior of GLARE laminates for aircraft structures. Jom 2005; 57: 72-79.
[3] Hu FP, Zhang Y, Zeng QW, Jia N, Qin J, Zhang X, Wang SZ, He JM, Xie WD. Research on potassium permanganate-phosphate treatment of magnesium alloy surface and fiber/magnesium alloy composite laminate. Materials and Corrosion 2016; 67: 1128-1134. [4] Pärnänen T, Alderliesten RC, Rans C, Brander T, Saarela O. Applicability of AZ31B–H24 magnesium in Fibre Metal Laminates–An experimental impact research. Composites: Part A 2012; 43: 1578-1586. [5] Cortés P, Cantwell WJ. The fracture properties of a fibre–metal laminate based on magnesium alloy. Composites: Part B 2006; 37: 163-170. [6] Múgica JI, Aretxabaleta L, Ulacia I, Aurrekoetxea J. Impact characterization of thermoformable fibre metal laminates of 2024–T3 aluminium and AZ31B–H24 magnesium based on self-reinforced polypropylene. Composites: Part A 2014; 61: 67-75. [7] Sadighi M, Pärnänen T, Alderliesten RC, Sayeaftabi M, Benedictus R. Experimental and numerical investigation of metal type and thickness effects on the impact resistance of fiber metal laminates. Applied Composite Materials 2012; 19: 545-559. [8] Pan YC, Wu GQ, Cheng X, Zhang ZK, Li MY, Ji SD, Huang Z. Galvanic corrosion behaviour of carbon fibre reinforced polymer/magnesium alloys coupling. Corrosion Science 2015; 98: 672-677. [9] Pan YC, Wu GQ, Huang Z, Wu X, Liu YH, Ye HJ. Corrosion behaviour of carbon fibre reinforced polymer/magnesium alloy hybrid laminates. Corrosion Science 2017; 115: 152-158. [10] Wang CC, Wang F, Han Y. The structure, bond strength and patite-inducing ability of micro-arc oxidized tantalum and their response to annealing. Applied Surface Science 2016; 36: 190-198. [11] Tang Y, Zhao X, Jiang K, Chen J, Zuo Y. The influences of duty cycle on the bonding strength of AZ31B magnesium alloy by microarc oxidation treatment. Surface and Coatings Technology 2010; 205: 1789-1792. [12] Yuan XH, Tan F, Xu H, Zhang SJ, Qu FZ, Liu J. Effects of different electrolytes
for micro-arc oxidation on the bond strength between titanium and porcelain. Journal of Prosthodontic Research 2017; 61 (3): 297-304. [13] Gao H, Zhang M, Yang X, Huang P, Xu K. Effect of Na2SiO3 solution concentration
of
micro-arc
oxidation
process
on
lap-shear
strength
of
adhesive-bonded magnesium alloys. Applied Surface Science 2014; 314: 447-452. [14] Pan YC, Wu GQ, Huang ZH, Li MY, Ji SD, Zhang ZK. Effects of surface pre-treatments on Mode I and Mode II interlaminar strength of CFRP/Mg laminates. Surface & Coatings Technology 2017; 319: 309-317. [15] Lawcock G, Ye L, Mai YW, Sun CT. The effect of adhesive bonding between aluminum and composite prepreg on the mechanical properties of carbon fiber reinforced metal laminates. Composites Science and Technology 1997; 57: 35-45. [16] Sinke J. Manufacturing of GLARE parts and structures. Applied Composite Materials 2003; 10: 293-305. [17] Pan YC, Wu GQ, Cheng X, Zhang ZK, Li MY, Ji SD, Huang Z. Mode I and Mode II interlaminar fracture toughness of CFRP/magnesium alloys hybrid Laminates. Composite Interfaces 2016; 25: 453-465. [18] Naghipour P, Schulze K, Hausmann J, Bartsch M. Numerical and experimental investigation on lap shear fracture of Al/CFRP laminates. Composites Science and Technology 2012, 72: 1718-1724. [19] Sankara Narayanan TSN, Park IS, Lee MH. Strategies to improve the corrosion resistance of microarc oxidation (MAO) coated magnesium alloys for degradable implants: Prospects and challenges. Progress in Materials Science 2014; 60: 1-71. [20] Ryu HS, Hong S. Effects of KF, NaOH, and KOH Electrolytes on Properties of Microarc-Oxidized Coatings on AZ91D Magnesium Alloy. Journal of The Electrochemical Society 2009; 156: C298-C303. [21] Song LW, Song YW, Shan DY, Zhu GY, Han EH. Product/metal ratio (PMR), A novel criterion for the evaluation of electrolytes on micro-arc oxidation (MAO) of Mg and its alloys. Science China Technological Sciences 2011; 54: 2795-2801. [22] Gent AN, Lin CW. Model studies of the effect of surface roughness mechanical interlocking on Adhesion. The Journal of Adhesion 1990; 32: 113-125.
[23] Jia JX, Atrens A, Song G, Muster TH. Simulation of galvanic corrosion of magnesium coupled to a steel fastener in NaCl solution. Materials and Corrosion 2005; 56: 468-474.
Table and Figure captions Table 1. The composition and concentration of two electrolytes, and the treatment time in each of electrolytes. Table 2. Relative element contents (in wt. %) of conversion film. Figure 1. Schematic illustration of the stacking sequence of the FMLs. Figure 2. Schematic diagram and experimental setup of (a) floating roller peel test, and (b) double-notch shear test. Figure 3. The surface and cross-sectional SEM images of the ET-Si sample. Figure 4. Surface morphology of magnesium alloys sheets after ET-Si-free. Figure 5. The cross-sectional SEM images of the ET-Si-free sample. Figure 6. The load–displacement curve from interlaminar peel (a) and shear (b) tests of the FMLs Figure 7. The mean values of Mode I and II interlaminar strength for the FMLs obtained from ET-Si and ET-Si-free. Figure 8. The cross-sectional and detached Mg surface SEM images of the ET-Si specimen after peel test. Figure 9. The cross-sectional and detached Mg surface SEM images of the ET-Si-free specimen after peel test. Figure 10. Schematic depiction of sections and fracture surfaces for samples with (a) ET-Si and (b) ET-Si-free under peel loading condition. Figure 11. The photos of two FMLs during the immersion test in 3.5 wt. % NaCl solution. Figure 12. The corrosion rates of the control group and the samples with ET-Si and ET-Si-free.
Table 1 The composition and concentration of two electrolytes, and the treatment time in each of electrolytes. Group
Solution composition
Time
ET-Si group
5.68 g/L Na2SiO3+7.28 g/L KOH+2.9 g/L KF
2 min
ET-Si-free group
7.28 g/L KOH+2.9 g/L KF
1/2/3/4 min
Table 2 Relative element contents (in wt. %) of conversion film. Element content (wt. %) Sample Mg
O
F
Al
Si
Na
ET-Si-free (1 min)
75.8
18.4
4.3
1.5
-
-
ET-Si-free (2 min)
77.1
18.4
3.5
1.0
-
-
ET-Si-free (3 min)
74.3
21.0
3.4
1.3
-
-
ET-Si-free (4 min)
74.3
21.6
3.0
1.2
-
-
ET-Si
45.9
45.0
-
1.3
6.7
1.2