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Analysis of effects of process factors on corrosion resistance of adhesive bonded joints for aluminum alloys
Journal of Materials Processing Tech. 276 (2020) 116412
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Analysis of effects of process factors on corrosion resistance of adhesive bonded joints for aluminum alloys
T
Sang Yoon Parka, , Won Jong Choib, Byoung Chul Yoonc ⁎
a
Hyundai Automotive Research & Development Division, Hyundai Motor Group, 772-1, Jangduk-Dong, Hwaseong-Si, Gyeonggi-Do, 445-706, South Korea Department of Materials Engineering, Korea Aerospace University, 200-1, Hwajon-dong, Deokyang-gu, Koyang-city, Gyeonggi-Do, 412-791, South Korea c Muhan Carbon Co. Ltd., Nambu-ro, Samseung-myeon, Boeun-gun, Chungchenongbuk-do, 3750-189, South Korea b
ARTICLE INFO
ABSTRACT
Associate Editor: A Clare
In the adhesive bonding process, the hydrolytic stability or corrosion resistance is of significant importance in achieving acceptable joint durability. The aim of this study is to identify the process factors affecting corrosion durability of adhesive bonded joints for aluminum alloys. For this purpose, experiments were carried out to investigate the influence of varying the surface treatment-primer-adhesive combination. The tests used for the evaluation of corrosion resistance included neutral salt spray and floating roller peel testing. In addition, the residual peel strengths after 300 days of exposure to salt spray were statistically analyzed using analysis of variance (ANOVA) and Tukey test (α=0.05) in order to understand the interaction effects between the process factors. The surface morphology, chemical composition and oxide thickness played a major role in the control of adhesion performance and joint durability. In particular, a hybrid sol-gel coating consisting of TPOZ (zirconium n-propaxine) and γ-GPS (γ-glycidoxypropyltrimethoxysilane) produced a microscopically rough surface with an increased surface energy compared with the standard phosphoric oxide. Furthermore, a non-chromate primer exhibited adequate corrosion protection efficiency when used with a sol-gel coating. The incorporation of zirconium oxide (ZrO2) in the sol-gel coating increased surface densification through pore-filling and planarization, finally resulting in high corrosion durability.
Keywords: Adhesive bonding process Aluminum alloys Surface treatment Non-chromate primer Epoxy adhesive Corrosion resistance
1. Introduction Adhesive bonding of aluminum alloys has been extensively used for joining of parts such as aerospace and automotive components where the combination of uniform load-transfer and production cost effectiveness is important (Yarlagadda and Cheok, 1999). Park et al. (2010) earlier demonstrated that all the process factors and their interactions (i.e. surface treatment-primer-adhesive combination) should be carefully taken into consideration when improving strength and environmental durability of adhesive bonded joints. A critical challenge is to identify a more efficient and sustainable surface treatment technique for a durable adhesive bonded joint, since the adhesive-adherend interface plays an important role in the load transfer between the structural components. The surface treatment of aluminum alloys for adhesive bonding has been reviewed by Critchlow and Brewis (1996), and they have concluded that the ultimate goal of surface treatment is to modify the surface properties of aluminum alloys in order to avoid surface contaminants, realize morphological changes producing a rough and wettable substrate surface, achieve hydrolytic stability, and so
⁎
forth. A mechanically unstable or friable oxide layer that quickly reacts with air and airborne moisture initially improves the joint strength, but it ultimately leads to poor quality bonding. To counter these effects, a bond primer is typically deposited to protect the underlying oxide layer, and to give a strong interfacial bonding when coupled with a structural adhesive. Historically anti-corrosion pigments have been added to primer coating systems in order to achieve a synergistic corrosion inhibition effect. Strontium chromate (SrCrO4) is a typical anti-corrosion pigments, and is effective against filiform corrosion on metallic substrate surfaces. However, the use of chromate is prohibited, or being progressively banned in most industries due to its carcinogenic activity (Bishopp et al., 2001). As the result of a search for alternatives to reduce the emissions of VOC, the water-soluble primer coating has emerged as an environmentally friendly candidate to replace conventional solvent-based primers. Previous research (Stayrook et al., 1999; Blohowiak et al., 2001; Mazza et al., 2004) has reported that comparable durability was obtained by the use of water-soluble primers as with solvent-based primers. Stayrook et al. (1999) were among the earliest to perform a
Corresponding author. E-mail addresses: [email protected] (S.Y. Park), [email protected] (B.C. Yoon).
https://doi.org/10.1016/j.jmatprotec.2019.116412 Received 17 July 2018; Received in revised form 15 September 2019; Accepted 22 September 2019 Available online 24 September 2019 0924-0136/ © 2019 Published by Elsevier B.V.
Journal of Materials Processing Tech. 276 (2020) 116412
S.Y. Park, et al.
Nomenclature and abbreviation Ra Rt τpeel gl AES ANOVA DIM FE-SEM GBS
GBSG Grit-blasting and sol-gel LEFM Linear elastic fracture mechanics NaCl Sodium chloride WORK Owens, Wendt, Rabel, and Kaelble PAA Phosphoric acid anodizing PACs Phosphoric acid containment SDQ Strengths and difficulties questionnaire ToF-SIMS Time-of-flight secondary ion mass spectrometry TPOZ Zirconium n-propaxine γ-GPS γ-glycidoxypropyltrimethoxysilane VOC Volatile organic compounds
average roughness (μm) maximum roughness (μm) apparent peel strength (N/mm) Degree of freedom Auger electron spectroscopy Analysis of variance Diiodomethane Field-emission scanning electron microscope Grit-blasting and silane
comparative study on adhesive bonded joints with three types of bond primers; BR®127 (solvent-based, strontium chromate corrosion inhibitor), BR®6747-1 (water-soluble, strontium chromate) and BR®6757 (water-soluble, no corrosion inhibitor). They found that the PAA followed by water-soluble primers (BR®6747-1 and BR®6757) exhibited a desirable mode of full cohesive failure in wedge joint samples. Davis and Venables (2002) reported that the water-soluble primer (BR®67471) exhibited excellent durability when compared to the standard solvent-based primer (BR®127). The understanding of hydrolytic stability or corrosion resistance on a substrate surface is of significant importance in providing acceptable durability of adhesive bonded joints (Critchlow et al., 2006). Typical environmental fracture modes are schematically illustrated in Fig. 1. Degradation mechanisms are generally associated with moisture diffusion in the adhesive-adherend interface, leading to hydration of aluminum oxide and loss of joint strength. Presence of chloride ions (Cl−) is also shown to accelerate the loss of adhesion due to the propagation of localized corrosion of the substrate surface (Lunder et al., 2002; Hardwick et al., 1986). Nevertheless, a comparative study to establish the impact of process variables on corrosion durability has not yet been reported in the open literature. The aim of this study was to investigate the effect of process factors on the corrosion behavior of adhesive bonded joints for aluminum alloy with 120 °C (250 °F) cure type adhesives. For this purpose, the experiments were concerned with the influence of varying surface treatmentprimer-adhesive combinations. The morphological changes in the substrate surface were monitored by surface roughness measurements and
electron microscope observations. Contact angle measurements and AES analyses were also carried out to analyze the surface wettability, which in turn relates to the surface chemistry and cleaning efficiency with the goal of the improvement of adhesion strength. The tests used for the evaluation of corrosion resistance included neutral salt spray and floating roller peel testing. Finally, the experimental results were statistically analyzed using ANOVA to determine the influence of process factors on the residual peel strength after 300 days of exposure to salt spray. 2. Experimental 2.1. Aluminum substrate The substrate used in this study was an aerospace-grade bare aluminum-copper alloy, 2024-T3 bare (Aerospace material specification code: QQ-A250/4; Alcoa Mill Products Inc., USA). Its composition by weight is 3.8%–4.9% Cu, 1.2%–1.8% Mg, 0.5% Si, 0.5% Fe, 0.3%–0.9% Mn, 0.25% Zn, 0.15% Ti, 0.1% Cr, and balance Al. 2.2. Process factors for adhesive bonding process To assess the effect of process factors on corrosion behaviors of adhesive bonded joints, three process factors; surface treatment (A), primer (B), and adhesive (C) were considered in this study, as given in Table 1. This study was concerned with evaluating the effectiveness of silane-based conversion coatings as environmentally friendly surface treatments for aluminum alloy: (1) γ-GPS coating and (2) hybrid sol-gel coating consisting of TPOZ and γ-GPS. The PACs process was also considered since this is a standard process for on-aircraft bonded repairs. Finally, the PAA process which has been used in the aerospace industry was provided for comparative purposes only. Details of methodology and procedures are summarized in Table 2. The primers used in this study were supplied by Cytec Solvay Group, USA: BR®127 Table 1 Process factors used in this study. Symbol
Process factors
Variables
A B C
Surface treatment Primer Adhesive film
PACs BR®127 a 3 M™ AF163-2 K
GBS BR®6747-1 b Cytec FM®73
GBSG BR®6747-1NC
c
a BR®127 primer is a solvent-based, modified epoxy-phenolic consisting of 10% solids and 15 wt.% strontium chromate as a corrosion inhibiting additive. The emission of VOC (792 g/L) is attributed to the use of solvent-based organic coating. b BR®6747-1 primer is a water-soluble, modified epoxy-phenolic consisting of 20% solids and 15 wt.% strontium chromate as a corrosion inhibiting additive. c BR®6747-1NC primer is a water-soluble and non-chromate modified epoxyphenolic resin consisting of 20% solids.
Fig. 1. Typical environmental fracture modes of adhesive bonded joint: (a) moisture diffusion, and (b) corrosion degradation. 2
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Table 2 Summary of the surface treatment processes applied to aluminum alloys prior to the application of primers. Properties
Process details
Scotch-brite® abrasion and PAC anodizing (PAC)
The aluminum substrate was abraded with a scotch-brite® pad (p/n: SB7447B, 3 M Inc., USA) with an orbital sander. PAC anodizing was then performed in a double vacuum bag assembly. A vacuum pressure was drawn on the inside of the bagging assemblies to infuse phosphoric acid over the repaired area. At this time, a stainless screen mesh was placed between the two breather layers and the area was anodized for 25 min under continuous acid flow (1.55 L/m2/ min). After removing the vacuum bagging assembly, the repair area was rinsed again and air-dried. Conversion coating based on γ-GPS can potentially substitute chromium-containing surface treatments, but they do not ensure sufficient corrosion protection especially for copper-containing aluminum alloys. The aluminum substrate was abraded with a scotch-brite® pad and grit blasted with 50 μm Al2O3 in clean, dry air or in an oil-free nitrogen environment at a pressure of 206–551 kPa. The substrates were then etched in a 1% Z-6040® γ-GPS aqueous solution (Dow Corning Corporation, USA) for 15 min and the γ-GPS deposit layer was dried at 105 °C for 1 h to reach its full-cure state. The GBSG is environmentally friendly and non-toxic process. The properties of coating layer can be tuned easily by modifying the precursor mixture. In particular, sol-gel coatings for aluminum alloys were intensively investigated for corrosion protection, but hardly for surface treatment technique of adhesive bonding process. The aluminum substrates were pre-treated with a scotch-brite® abrasion pad and grit-blasted with 50 μm Al2O3 in clean and dry air or in an oilfree nitrogen environment at a pressure of 206–551 kPa. The substrates were then etched in an AC-130® sol-gel solution (3 M, USA) for 2–3 min and the sol-gel deposit layer was air-dried at room temperature. The aluminum substrates were initially immersed in a sodium chromate-sulfuric acid solution at 68 °C for 10 min prior to anodizing. The aluminum substrates were then anodized in 10 vol% phosphoric acid at 23 °C for 20 min and a DC voltage of 10 V.
Grit-blasting and silane (GBS)
Grit-blasting and sol-gel (GBSG)
Chromic-sulfuric acid etching and phosphoric acid anodizing (PAA) reference only
(VOC level: 792 g/L, chromate: 15 wt.%), BR®6747-1 (zero VOC, chromate: 15 wt.%) and BR®6747-1NC (zero VOC and non-chromate). All primers were applied by a high-volume low-pressure spray gun, and subsequently baked at 120 °C in a convection oven. Finally, the two types of epoxy adhesive films used in this study were 3 M™ ScotchWeld™ AF163-2 K (AF163-2 K) and Cytec Solvay Group FM®73 (FM73), respectively. Both adhesives were manufactured as supported adhesives with either knit or mat carrier, as shown in Fig. 2. The standard cure cycle calls for 60 min hold at 121 °C and vacuum bagging pressure of 75 kPa.
2.3. Surface characterizations A non-contact laser profilometer, Accura 1500 F (Intek IMS, Korea), was used to measure the surface roughness of substrate surfaces subjected to different surface treatments. Two roughness parameters, average roughness (Ra, ISO 4287) and maximum roughness (Rmax, ISO 4288), were used to evaluate surface roughness of the samples. Six measurements were made for each sample and average values and standard deviations were used for statistical analysis. A FE-SEM JEOL JSM-7500 F (JEOL Ltd., Japan) was used to explore the morphological changes in the substrate surface. The surface energies (polar- and dispersive-components) were calculated by the OWRK method (Pantoja et al., 2016). For this purpose, the static contact angle measurements
Fig. 2. Electron micrographs at 20× magnification of (a) AF163-2 K and (b) FM73 adhesives: the two adhesives have non-woven knit- (left) and random mat-type (right) fibrous carriers for resin flow and bond-line thickness control.
Fig. 3. Floating roller peel tests: (a) configuration of the test specimen and fixture (ASTM D3762) and (b) salt spray aging at 35 °C with 5 wt.% salt solution. 3
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Table 4 Chemical compositions of substrate surfaces subjected to different surface treatments. Surface treatments
Elemental concentration, at surface [al%] O 1s
Al 2p
Al 2s
P 2p3
Si 2p
Zr 3d5
PACs Silane Sol-gel PAA
70.6 81.1 70.6 54.5
23.0 4.9 5.7 42.1
0.7 – – 1.6
5.8 – – 1.7
– 6.2 14.9 –
– 5.7 –
Surface film layer thickness [nm] 23 112 203 260
ASTM D3167 standard as shown in Fig. 3(a). The sample dimension was 25 mm in width and 300 mm long with a total joint thickness of 2.35 mm. The apparent peel strength (τpeel) in N/mm was defined by using the following equation: peel
(1)
= P peel /w
where Ppeel is the average peeling load [N], which can be taken from the peel load-displacement curve after the initial peak point, and w is the sample width [mm]. The test samples were exposed in a neutral 5 wt.% salt fog at 35 ± 2 °C in the salt spray chamber as shown in Fig. 3(b). The peel tests were then performed at intervals of 90 days (2160 h), 180 days (4320 h) and 300 days (7200 h) according to the Airbus industry standards (AITM 5-0009). Six measurements were made for each condition and average values and standard deviations were used for statistical analysis. Finally, the residual peel strength values after 300 days of exposure to salt spray were statistically analyzed using ANOVA and Tukey test (α = 0.05). Each fracture surface after being subjected to the floating roller peel test was represented by an array of gray scale values (0–255) corresponding to the differences in failure mode. To obtain a cohesive failure image, a pixel value in the cohesive failure area was set to 0 (black) because it was less than the threshold value, while both adhesive failure and corrosion areas were set to 255 (white), as shown in Fig. 4.
Fig. 4. Image-processing technique used in this study: (a) original fracture image of test specimen; and (b) and (c) binary images showing adhesive failure and corrosion area.
(Sigma 70 Tensiometer, KSV Instrument Ltd., Finland) were carried out using a sessile drop method. The reference liquids used in this study were deionized water (H2O) and 99% DIM (CH2I2). Five drops of each liquid (20 μL) were set down on several points on each sample, and allowed to equilibrate for a few minutes. Finally, AES measurements were performed with a PHI 680 Auger spectroscope (Physical Electronics, Inc., USA) with a field emission gun in order to obtain the depth composition profile on the substrate surface. A range of operating beam conditions was used depending on which spectral regions were being investigated. Areas of interest were identified by secondary electron imaging. For depth profiling, a 1 keV/0.5 mA argon ion beam was used. The sputter rate was 7 nm/min. The quantitative AES results were determined from derivative spectra using elemental sensitivity factors.
3. Results and discussion
2.4. Corrosion resistance testing
3.1. Surface characterizations
2.4.1. Neutral salt-spray testing The corrosion durability of substrate surfaces with epoxy primers was evaluated by a neutral salt-spray test according to the ASTM B117, standard. The experiments were conducted in a salt spray chamber (5 wt.% aqueous NaCl solution at 35 ± 2 °C) for 1512 h. The scribe was a diagonal line cut through the primer coating with a hardened steel tool. The samples were then taken out of the salt spray chamber periodically and visually examined.
Tables 3 and 4 quantitatively describe the effects of surface treatment techniques on surface parameters, which include surface roughness, surface energy and chemical composition. Although the surface roughness showed no significant difference between the two anodizing processes (0.38 μm for PACs and 0.41 μm for PAA), their morphologies were significantly different, as shown in Fig. 5(a) and (d). The classical porous structures reported in the open literatures (Park et al., 2010) were clearly observed in the PAA oxide structure. This microscopic structure is likely to be due to capillary forces drawing the primer into the oxide pores, which in turn increases the mechanical interlocking
2.4.2. Floating roller peel testing The floating roller peel test samples were prepared based on the
Table 3 Surface roughness, surface contact angle, and surface energy of substrate surfaces subjected to different surface treatments (mean value ± standard deviation). Surface treatment
Surface roughness
Degrease only Grit-blasting PAA PACs GBS GBSG
0.32 0.77 0.42 0.47 0.86 0.77
Ra (μm) ± ± ± ± ± ±
a
0.04 0.07 0.03 0.05 0.05 0.04
Contact angle (°) Rt (μm)
b
De-ionized water
99% DIM
3.36 0.77 4.45 5.95 8.62 8.03
0.36 0.07 0.73 0.22 0.95 0.47
67.08 59.78 50.79 55.29 58.72 52.67
37.35 25.77 36.30 39.42 18.43 22.49
± ± ± ± ± ±
a
± ± ± ± ± ±
1.53 6.88 6.12 19.08 2.61 3.43
± ± ± ± ± ±
Surface energy (mJ/m2) 3.25 2.73 5.52 16.20 1.13 1.36
46.82 53.46 55.53 52.19 55.51 57.67
± ± ± ± ± ±
0.28 3.35 4.90 15.11 1.38 2.00
The Ra is the mathematical average of the absolute values of the measured profile height of surface irregularities. The maximum height of the profile (Rt or Rmax) is defined as the peak-to-valley vertical distance along the assessment profiles, and it is sensitive to the high peaks or deep scratches. b
4
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Fig. 5. Typical surface morphologies at a magnification of 2000× and 3D surface images: (a) PACs, (b) GBS, (c) GBSG, and (d) PAA oxide structures.
between anodic oxide and primer (Critchlow et al., 2006). On the other hand, the PACs oxide layer resulted in the formation of a scalloped surface with a network of shallow pores, as shown in Fig. 5(a). In addition, the pore size and its distribution were dramatically reduced when compared to the standard PAA process. The two silane based surface treatment techniques resulted in microscopically rough surfaces as compared with the oxide layer, as given in Table 3. Their average roughness values (Ra) were 0.86 μm (γ-GPS) and 0.77 μm (TPOZ/γ-GPS), respectively. The pretreatment step (gritblasting, Ra =0.77 μm) could contribute to the increase of surface roughness significantly. In general terms, electron microscopy revealed no significant difference in oxide structures between the two silane based substrates. Visually, the shapes of the resulting oxide structures presented a random distribution of ridges and groove, as shown in
Fig. 5(b) and (c). Furthermore, increasing surface roughness could lead to greater hydrophilicity and low contact angle. The surface energies of the γ-GPS and TPOZ/γ-GPS substrates were 55.51 and 57.67 mJ/m2, respectively. However, the PACs process yielded an inferior wettability (52.19 mJ/m2), indicative of a lower surface quality for adhesive bonding. Fig. 6(a) and (d) shows the AES depth profiles through the anodic oxide layers of the PACs and PAA substrates, respectively. The quantitative chemical compositions in terms of atom percent (al%) are given in Table 4. Note that the carbon level was not included in these figures since the concentration of carbon was below 0.5 al%. A significant variation in oxide layer thickness was apparent between the two anodizing processes; the oxide layer of PACs substrate was much thinner (23 nm), in comparison to the standard PAA process (260 nm). The
Fig. 6. AES depth profiles of substrate surfaces with different surface treatments: (a) PACs, (b) GBS, (c) GBSG, and (d) PAA oxide structures. 5
Journal of Materials Processing Tech. 276 (2020) 116412
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at the AES depth profile of GBS substrates (Fig. 6(b)), the coating thickness of γ-GPS film was found to be 112 nm based on the Si signal. Comparing results with the GBS substrate, it could be seen that the GBSG process yielded a greater oxide layer thickness in the range from 154 nm to 203 nm, and an increase in the Si:Al ratio in the outermost part of the coating layer. Furthermore, the AES depth profile indicated the variation in concentrations of Si and Zr through the thickness of the coating layer; Zr toward metal (max. 2.8 al% at 203 nm depth) and Si on the outermost edge (max. 14.91 al% at 0 nm), respectively. This gradient coating structure of TPOZ/γ-GPS film is schematically shown in Fig. 7 (Osborne et al., 2001); ZrO2 reacts with a metallic surface to produce a covalent chemical bond, while SiO2 offers a reactive organic group for bonding with an adhesive. 3.2. Corrosion resistance tests 3.2.1. Neutral salt-spray tests Images of the corrosion areas of test samples are shown in Fig. 8. To extend the understanding of corrosion damage mechanisms at the micro-scale level, the aged-samples were inspected by electron microscopy at a magnification of 100× as shown in Fig. 9. It can be seen that the PAA substrates did not show any blistering or accumulation of corrosion products around the scribe on the sample after 1512 h exposure. Electron micrographs also supported this result, showing that the scribe line was apparently clean without any corrosion product accumulation, such as precipitates, pits or blisters (see Fig. 9(i)). In addition, no evidence of corrosion was observed either in the scribe line, or underneath the primer with the use of chromate primers (BR127 and BR6747-1). For example, the chromate, water-soluble primer (BR6747-1) exhibited a comparable resistance to the conventional solvent-based, chromate primer (BR127) with PACs and GBS processes. However, the non-chromate, water-soluble primer (BR6747-1NC) showed a limited corrosion resistance during a neutral salt spray test. Electron microscope observations supported this result; the actual
Fig. 7. Schematic representation of a typical sol-gel thin film coating (reproduced from Osborne et al., 2001 with permission from Elsevier).
results in the current study were similar to those reported by Saliba, 1994 on oxide layer dimensions for PAA and PACs substrate surfaces. In addition, the presence of high levels of phosphorus (5.8 al%) in the outermost surface was observed in the PACs oxide structure, as given in Table 4. This result implies that the phosphoric acid did not penetrate into the base of the oxide layer (i.e. barrier layer). This could be due to a number of factors, which include pore narrowing, branching and termination and limitation of the capillary force. AES depth profiles through the organic oxide layers are presented in Fig. 6(b) and (c) for the GBS and GBSG substrates, respectively. Apparently, the presence of Si could be attributed to the application of the γ-GPS (C9H2O5Si) coating agent. Abel et al. (1998), using a ToF-SIMS analysis, demonstrated that the strong Al-O-Si bond was formed between hydrolyzed γ-GPS film and the substrate surface. When looking
Fig. 8. Salt spray exposure panels with different surface treatment/primer combinations: (a) 168 h exposure, and (b) 1512 h exposure. 6
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Fig. 9. Electron micrographs showing corrosion damage in the vicinity of scribe line after 1512 h of exposure to the salt spray: solid and dash arrows represent scribe line and primer coating layer, respectively.
protecting primer layer was severely damaged in the scribe line as shown in Fig. 9(c) and (f). In summary, in the surface treatment study, the GBSG process demonstrated adequate corrosion resistance at a level comparable with non-chromate primer (BR6747-1NC) as shown in Fig. 9(h). On the other hand, the PACs and GBS processes were unable to provide acceptable corrosion protection with a BR6747-1NC primer.
(Mazza et al., 2003, 2004; Blohowiak et al., 2001) on the peel strength with the two adhesive films. Comparison of initial peel strength of un-aged samples demonstrated that a marginal difference could be measured between the two adhesives. These results were probably due to the difference in bondline thickness. Kawashita et al. (2008) have investigated the influence of bond-line thickness on adhesive fracture toughness (GA) using a peel test. They reported that the adhesive fracture toughness of peel joints increased as the bond-line thickness was increased. In addition, the relationship between adhesive fracture toughness and bond-line thickness can be explained by a LEFM model. This was confirmed by the observation that during floating roller peel tests, the energy-dissipating capacity of adhesive bonded joints increased with an increase in bondline thickness. According to our previous research (Park et al., 2018), the nominal bond-line thicknesses of FM73 and AF163-2 K adhesives were 0.30 ± 0.03 and 0.25 ± 0.02 mm, respectively. Typical floating roller peel test load-displacement curves, Fig. 11, clearly show that a thicker adhesive (FM73) results in improved fracture energy. The fracture surfaces and their corresponding failure modes (i.e. % cohesive failure) before and after exposure to salt spray are presented in Fig. 12. Obviously, cohesive failure was the dominant failure mode in all un-aged samples (> 75 areal%). Analysis of failure modes showed that cohesive failure within the adhesive layer prevailed in the PAA samples after 300 days exposure. However, the much thinner PACs oxide layer (nominal thickness, 23 nm) was more susceptible to corrosion attack, showing a high corrosion rate ranging from 45 areal% to 86 areal%. On the other hand, the GBSG sample groups turned out to be more efficient in producing a corrosion resistant surface than the GBS groups. The incorporation of zirconium oxide (ZrO2) into the sol-gel coating can increase in surface densification through pore-filling and
3.2.2. Floating roller peel tests Fig. 10 shows the effect of process factors on residual peel strength according to salt spray exposure time. This figure shows that a significant decrease in peel strength could be observed at 300 days exposure, and the aged samples presented a large scatter in each test result. The oxide layer produced by anodizing in phosphoric acid contains phosphate ion (AlPO4), which is known to inhibit hydration reactions by hindering the penetration of moisture molecules into the anodized film as earlier reported by Johnsen et al. (2004). It subsequently leads to excellent hydrolytic stability or corrosion durability of the PAA oxide layer. On the other hands, the PACs oxide layer was much more susceptible to the effects of moisture and/or corrosion agents than the PAA oxide. This difference in susceptibility to environmental factors may have a significant effect on the long-term properties of adhesive bonded joints. Although the PACs process followed by chromate primers (BR127 and BR6747-1) leads to substantially improved corrosion resistance in comparison to the non-chromate primer (BR6747-1NC), their results were still inferior to other surface treatment techniques. A possible explanation is that the much thinner PACs oxide is more susceptible to hydration or corrosion of the substrate surface as reported by Saliba (1994). Finally, both GBS and GBSG substrates exhibited an intermediate corrosion durability compared to the standard PAA substrate. The results of this study are similar to those reported previously 7
Journal of Materials Processing Tech. 276 (2020) 116412
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Fig. 10. Effect of process factors on residual peel strength according to salt spray exposure time (the error bars represent standard deviation): (a) AF163-2 K and (b) FM73 adhesives.
type carrier adhesive (FM73), as shown in Fig. 2. For example, the GBS substrate with FM73 adhesive has a higher corrosion rate compared to the values obtained by AF163-2 K adhesive. 3.2.3. ANOVA results The results of ANOVA with residual peel strength after 300 days of exposure to salt spray are summarized in Table 5. The effects of process factors on residual peel strength are shown in Fig. 13. In this figure, the results of the PAA sample group are presented for comparative purposes only. It is apparent that the surface treatment was the dominant factor in the determination of residual peel strength with a high statistical term (p = 55.2%). Obviously, both GBSG and PAA sample groups showed a positive effect on the corrosion resistance of peel joints. As mentioned above, the decrease in peel strength was accompanied by an increase in the amount of corrosion damage in the bond-line layer; such degradation was large enough to introduce more local stress concentration than was present in the un-aged samples. Furthermore, premature failure induced by the loss of interfacial bonds could lead to a high error rate; notice that the error value associated with the ANOVA table of residual peel strength was 26.2%. On the other hand, the effects of primer (p = 6.6%) and adhesive (p = 5.5%) were relatively minor. The coupled interaction between surface treatment and primer was found to be 3.7%.
Fig. 11. Typical load-displacement curves from floating roller peel tests (data is obtained with GBSG/6747-1).
planarization, resulting in high corrosion durability. Finally, the rate of cohesive failure was much better for AF163-2 K adhesive even if FM73 adhesive produced a high initial peel strength. This could be explained by observing that a wide open knit-type carrier (AF163-2 K) results in more contact area to carry the external load in comparison to the mat8
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Fig. 12. Typical fracture surfaces of un-aged and 300 days exposure test samples: (a) AF163-2 K, and (b) FM73 adhesives.
4. Conclusions
Table 5 ANOVA for the residual peel strength results (unit: N/mm). Factors
SDQ
gl
Variance
Test F
p (%)
Surface treatments (A) Primers (B) Adhesives (C) A×B B×C Error Total
1072.94 128.32 107.46 71.07 52.88 509.56 1942.23
2 2 1 2 2 118 127
536.47 64.161 107.46 35.533 26.439 4.318 774.381
124.23 14.86 24.88 8.23 6.12 – –
55.2% 6.6% 5.5% 3.7% 2.7% 26.2% 100.0%
This study has investigated the effect of process factors on corrosion durability of adhesive bonded joints for aluminum alloy with structural epoxy adhesives. Based on the utilized methodologies and experimental results, the following conclusions can be drawn. 1) The corrosion durability of adhesive bonded joints was significantly dependent on the stability of the adhesive-adherend interface. Changes in oxide layer chemistries were expected to modify the interfacial properties and so to affect the joint strength and environmental durability in practice. Furthermore, a hybrid sol-gel coating consisting of TPOZ and γ-GPS produced a microscopically
Fig. 13. Effect of process factors on residual peel strength after 300 days exposure, center line corresponds to the mean value of the observations in all test samples. 9
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rough surface (0.86 μm) with an increase in surface energy (57.67 mJ/m2) when compared with the standard phosphoric oxide. In particular, the enhanced corrosion resistance efficiency was obtained due to the incorporation of zirconium oxide (ZrO2) into the sol-gel coating which can further improve surface stability by increasing surface densification. In contrast, the result of the PACs process was to leave a thin, non-homogenous oxide on the substrate surface with a low level of surface roughness (0.38 μm). 2) The non-chromate primer (BR6747-1NC) exhibited adequate corrosion protection efficiency when used with a TPOZ/γ-GPS based sol-gel coating. Electron micrographs showed that the scribe line was apparently clean without any sign of corrosion product accumulation, such as precipitates, pits or blisters. However, the nonchromate primer applied to PACs and GBS was unable to provide acceptable corrosion protection in the neutral salt spray test. 3) A marginal difference in adhesive fracture toughness could be observed between the two adhesives considered in this study. The thicker FM73 adhesive (nominal bond-line thickness = 0.30 mm) showed a relatively high adhesive fracture toughness in comparison to AF163-2 K (0.25 mm). The influence of bond- line thickness on adhesive fracture toughness can be explained by a LEFM model. In contrast, the rate of cohesive failure after neutral salt spray exposure was much better for AF163-2 K adhesive even though FM73 adhesive produced higher initial peel strength. The wide open knittype carrier in AF163-2 K brings in more contact area to promote wetting with a primer coating. 4) Based on the results of ANOVA, the surface treatment was the main factor influencing the residual peel strength with a high statistical term (p = 55.2%). On the other hands, the effects of primer (p = 6.6%) and adhesive (p = 5.5%) were relatively minor. Consequently, the surface treatments and their coupled interaction with bond primer significantly influence the residual peel strength after 300 days of exposure to salt spray.
of adhesives. Adhesives. ASTM International, West Conshohocken, PA. Bishopp, J.A., Parker, M.J., O’Reilly, T.A., 2001. The use of statistical experimental design procedures in the development and “robust” manufacture of a water-based, corrosion-inhibiting adhesive primer. Int. J. Adhes. Adhes. 21 (6), 473–480. Blohowiak, K.Y., Anderson, R.A., Stephenson, R.R., 2001. Sol–gel Technology for Surface Preparation of Metal Alloys for Adhesive Bonding and Sealing Operations (Report No. AFRL–ML–WP–TR–2003–4169). AFRL Wright-Patterson AFB, Ohio. Critchlow, G.W., Yendall, K.A., Bahrani, D., Quinn, A., Andrews, F., 2006. Strategies for the replacement of chromic acid anodising for the structural bonding of aluminium alloys. Int. J. Adhes. Adhes. 26 (6), 419–453. Critchlow, G.W., Brewis, D.M., 1996. Review of surface pretreatments for aluminium alloys. Int. J. Adhes. Adhes. 16 (4), 255–275. Davis, G.D., Venables, J.D., 2002. In: Pocius, A.V. (Ed.), Adhesion Science and Engineering: Surface, Chemistry and Applications. Elsevier, Amsterdam. Hardwick, D.A., Ahearn, J.S., Desal, A., Venables, J.D., 1986. Environmental durability of phosphoric acid anodized aluminium adhesive joints protected with hydration inhibitors. J. Mater. Sci. 21 (1), 179–187. Kawashita, L.F., Kinloch, A.J., Moore, D.R., Williams, J.G., 2008. The influence of bond line thickness and peel arm thickness on adhesive fracture toughness of rubber toughened epoxy–aluminium alloy laminates. Int. J. Adhes. Adhes. 28 (4-5), 199–210. Lunder, O., Olsen, B., Nisancioglu, K., 2002. Pre-treatment of AA6060 aluminium alloy for adhesive bonding. Int. J. Adhes. Adhes. 22 (2), 143–150. Mazza, J.J., Gaskin, G.B., Piero, W.S.D., Blohowiak, K.Y., 2004. Sol-gel Technology for low-VOC, Nonchromated Adhesive Bonding Applications (Report No. AFRL-ML-WPTR-2004–4063). AFRL Wright-Patterson AFB, Ohio. Mazza, J.J., Avram, J.B., Kuhbander, R.J., 2003. Grti-blast-silane (GBS) Aluminum Surface Preparation for Structural Adhesive Bonding (Report No. WL-TR-94–4111). AFRL Wright-Patterson AFB, Ohio. Osborne, J.H., Blohowiak, K.Y., Taylor, S.R., Hunter, C., Bierwagon, G., Carlson, B., Bernard, D., Donley, M.S., 2001. Testing and evaluation of nonchromated coating systems for aerospace applications. Progr. Org. Coating 41, 217–225. Pantoja, M., Abenojar, J., Martínez, M.A., Velasco, F., 2016. Silane pretreatment of electrogalvanized steels: effect on adhesive properties. Int. J. Adhes. Adhes. 65, 54–62. Park, S.Y., Choi, W.J., Choi, H.S., Kwon, H., Kim, S.H., 2010. Recent trends in surface treatment technologies for airframe adhesive bonding processing: a review (1995–2008). J. Adhes. 86 (2), 192–221. Park, S.Y., Choi, W.J., Choi, C.H., Choi, H.S., 2018. The effect of curing temperature on thermal, physical and mechanical characteristics of two types of adhesives for aerospace structures. J. Adhes. Sci. Technol. 32 (11), 1200–1223. Saliba, S.S., 1994. The Surface Preparation of Aluminum Alloys Using the Phosphoric Acid Containment System for Repair (Report No. WL–TR–94–4112). AFRL WrightPatterson AFB, Ohio. Stayrook, J.J., Bellevou, D.L., Manson, R.T., Martinelli, A.A., Ridpath, S., Arnold, J.R., Gaskin, G.B., 1999. Performance of Solvent and Water Based Primers in Epoxy Adhesive Systems (Accession No. ADA368481). Naval Air Warfare Center Aircraft Division, Maryland. Yarlagadda, P.K.D.V., Cheok, E.C., 1999. Study on the microwave curing of adhesive joints using a temperature-controlled feedback system. J. Mater. Process Technol. 91 (1–3), 128–149.
References Abel, M.L., Watts, J.F., Digby, R.P., 1998. The adsorption of alkoxysilanes on oxidised aluminium substrates. Int. J. Adhes. Adhes. 18 (3), 179–192. ASTM Standard B117-16, 2016. Standard Practice for Operating Salt Spray (Fog) Apparatus. ASTM International, West Conshohocken, PA. ASTM Standard D3167-10, 2017. Standard test method for floating roller peel resistance
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Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Analysis of effects of process factors on corrosion resistance of adhesive bonded joints for aluminum alloys
T
Sang Yoon Parka, , Won Jong Choib, Byoung Chul Yoonc ⁎
a
Hyundai Automotive Research & Development Division, Hyundai Motor Group, 772-1, Jangduk-Dong, Hwaseong-Si, Gyeonggi-Do, 445-706, South Korea Department of Materials Engineering, Korea Aerospace University, 200-1, Hwajon-dong, Deokyang-gu, Koyang-city, Gyeonggi-Do, 412-791, South Korea c Muhan Carbon Co. Ltd., Nambu-ro, Samseung-myeon, Boeun-gun, Chungchenongbuk-do, 3750-189, South Korea b
ARTICLE INFO
ABSTRACT
Associate Editor: A Clare
In the adhesive bonding process, the hydrolytic stability or corrosion resistance is of significant importance in achieving acceptable joint durability. The aim of this study is to identify the process factors affecting corrosion durability of adhesive bonded joints for aluminum alloys. For this purpose, experiments were carried out to investigate the influence of varying the surface treatment-primer-adhesive combination. The tests used for the evaluation of corrosion resistance included neutral salt spray and floating roller peel testing. In addition, the residual peel strengths after 300 days of exposure to salt spray were statistically analyzed using analysis of variance (ANOVA) and Tukey test (α=0.05) in order to understand the interaction effects between the process factors. The surface morphology, chemical composition and oxide thickness played a major role in the control of adhesion performance and joint durability. In particular, a hybrid sol-gel coating consisting of TPOZ (zirconium n-propaxine) and γ-GPS (γ-glycidoxypropyltrimethoxysilane) produced a microscopically rough surface with an increased surface energy compared with the standard phosphoric oxide. Furthermore, a non-chromate primer exhibited adequate corrosion protection efficiency when used with a sol-gel coating. The incorporation of zirconium oxide (ZrO2) in the sol-gel coating increased surface densification through pore-filling and planarization, finally resulting in high corrosion durability.
Keywords: Adhesive bonding process Aluminum alloys Surface treatment Non-chromate primer Epoxy adhesive Corrosion resistance
1. Introduction Adhesive bonding of aluminum alloys has been extensively used for joining of parts such as aerospace and automotive components where the combination of uniform load-transfer and production cost effectiveness is important (Yarlagadda and Cheok, 1999). Park et al. (2010) earlier demonstrated that all the process factors and their interactions (i.e. surface treatment-primer-adhesive combination) should be carefully taken into consideration when improving strength and environmental durability of adhesive bonded joints. A critical challenge is to identify a more efficient and sustainable surface treatment technique for a durable adhesive bonded joint, since the adhesive-adherend interface plays an important role in the load transfer between the structural components. The surface treatment of aluminum alloys for adhesive bonding has been reviewed by Critchlow and Brewis (1996), and they have concluded that the ultimate goal of surface treatment is to modify the surface properties of aluminum alloys in order to avoid surface contaminants, realize morphological changes producing a rough and wettable substrate surface, achieve hydrolytic stability, and so
⁎
forth. A mechanically unstable or friable oxide layer that quickly reacts with air and airborne moisture initially improves the joint strength, but it ultimately leads to poor quality bonding. To counter these effects, a bond primer is typically deposited to protect the underlying oxide layer, and to give a strong interfacial bonding when coupled with a structural adhesive. Historically anti-corrosion pigments have been added to primer coating systems in order to achieve a synergistic corrosion inhibition effect. Strontium chromate (SrCrO4) is a typical anti-corrosion pigments, and is effective against filiform corrosion on metallic substrate surfaces. However, the use of chromate is prohibited, or being progressively banned in most industries due to its carcinogenic activity (Bishopp et al., 2001). As the result of a search for alternatives to reduce the emissions of VOC, the water-soluble primer coating has emerged as an environmentally friendly candidate to replace conventional solvent-based primers. Previous research (Stayrook et al., 1999; Blohowiak et al., 2001; Mazza et al., 2004) has reported that comparable durability was obtained by the use of water-soluble primers as with solvent-based primers. Stayrook et al. (1999) were among the earliest to perform a
Corresponding author. E-mail addresses: [email protected] (S.Y. Park), [email protected] (B.C. Yoon).
https://doi.org/10.1016/j.jmatprotec.2019.116412 Received 17 July 2018; Received in revised form 15 September 2019; Accepted 22 September 2019 Available online 24 September 2019 0924-0136/ © 2019 Published by Elsevier B.V.
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Nomenclature and abbreviation Ra Rt τpeel gl AES ANOVA DIM FE-SEM GBS
GBSG Grit-blasting and sol-gel LEFM Linear elastic fracture mechanics NaCl Sodium chloride WORK Owens, Wendt, Rabel, and Kaelble PAA Phosphoric acid anodizing PACs Phosphoric acid containment SDQ Strengths and difficulties questionnaire ToF-SIMS Time-of-flight secondary ion mass spectrometry TPOZ Zirconium n-propaxine γ-GPS γ-glycidoxypropyltrimethoxysilane VOC Volatile organic compounds
average roughness (μm) maximum roughness (μm) apparent peel strength (N/mm) Degree of freedom Auger electron spectroscopy Analysis of variance Diiodomethane Field-emission scanning electron microscope Grit-blasting and silane
comparative study on adhesive bonded joints with three types of bond primers; BR®127 (solvent-based, strontium chromate corrosion inhibitor), BR®6747-1 (water-soluble, strontium chromate) and BR®6757 (water-soluble, no corrosion inhibitor). They found that the PAA followed by water-soluble primers (BR®6747-1 and BR®6757) exhibited a desirable mode of full cohesive failure in wedge joint samples. Davis and Venables (2002) reported that the water-soluble primer (BR®67471) exhibited excellent durability when compared to the standard solvent-based primer (BR®127). The understanding of hydrolytic stability or corrosion resistance on a substrate surface is of significant importance in providing acceptable durability of adhesive bonded joints (Critchlow et al., 2006). Typical environmental fracture modes are schematically illustrated in Fig. 1. Degradation mechanisms are generally associated with moisture diffusion in the adhesive-adherend interface, leading to hydration of aluminum oxide and loss of joint strength. Presence of chloride ions (Cl−) is also shown to accelerate the loss of adhesion due to the propagation of localized corrosion of the substrate surface (Lunder et al., 2002; Hardwick et al., 1986). Nevertheless, a comparative study to establish the impact of process variables on corrosion durability has not yet been reported in the open literature. The aim of this study was to investigate the effect of process factors on the corrosion behavior of adhesive bonded joints for aluminum alloy with 120 °C (250 °F) cure type adhesives. For this purpose, the experiments were concerned with the influence of varying surface treatmentprimer-adhesive combinations. The morphological changes in the substrate surface were monitored by surface roughness measurements and
electron microscope observations. Contact angle measurements and AES analyses were also carried out to analyze the surface wettability, which in turn relates to the surface chemistry and cleaning efficiency with the goal of the improvement of adhesion strength. The tests used for the evaluation of corrosion resistance included neutral salt spray and floating roller peel testing. Finally, the experimental results were statistically analyzed using ANOVA to determine the influence of process factors on the residual peel strength after 300 days of exposure to salt spray. 2. Experimental 2.1. Aluminum substrate The substrate used in this study was an aerospace-grade bare aluminum-copper alloy, 2024-T3 bare (Aerospace material specification code: QQ-A250/4; Alcoa Mill Products Inc., USA). Its composition by weight is 3.8%–4.9% Cu, 1.2%–1.8% Mg, 0.5% Si, 0.5% Fe, 0.3%–0.9% Mn, 0.25% Zn, 0.15% Ti, 0.1% Cr, and balance Al. 2.2. Process factors for adhesive bonding process To assess the effect of process factors on corrosion behaviors of adhesive bonded joints, three process factors; surface treatment (A), primer (B), and adhesive (C) were considered in this study, as given in Table 1. This study was concerned with evaluating the effectiveness of silane-based conversion coatings as environmentally friendly surface treatments for aluminum alloy: (1) γ-GPS coating and (2) hybrid sol-gel coating consisting of TPOZ and γ-GPS. The PACs process was also considered since this is a standard process for on-aircraft bonded repairs. Finally, the PAA process which has been used in the aerospace industry was provided for comparative purposes only. Details of methodology and procedures are summarized in Table 2. The primers used in this study were supplied by Cytec Solvay Group, USA: BR®127 Table 1 Process factors used in this study. Symbol
Process factors
Variables
A B C
Surface treatment Primer Adhesive film
PACs BR®127 a 3 M™ AF163-2 K
GBS BR®6747-1 b Cytec FM®73
GBSG BR®6747-1NC
c
a BR®127 primer is a solvent-based, modified epoxy-phenolic consisting of 10% solids and 15 wt.% strontium chromate as a corrosion inhibiting additive. The emission of VOC (792 g/L) is attributed to the use of solvent-based organic coating. b BR®6747-1 primer is a water-soluble, modified epoxy-phenolic consisting of 20% solids and 15 wt.% strontium chromate as a corrosion inhibiting additive. c BR®6747-1NC primer is a water-soluble and non-chromate modified epoxyphenolic resin consisting of 20% solids.
Fig. 1. Typical environmental fracture modes of adhesive bonded joint: (a) moisture diffusion, and (b) corrosion degradation. 2
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Table 2 Summary of the surface treatment processes applied to aluminum alloys prior to the application of primers. Properties
Process details
Scotch-brite® abrasion and PAC anodizing (PAC)
The aluminum substrate was abraded with a scotch-brite® pad (p/n: SB7447B, 3 M Inc., USA) with an orbital sander. PAC anodizing was then performed in a double vacuum bag assembly. A vacuum pressure was drawn on the inside of the bagging assemblies to infuse phosphoric acid over the repaired area. At this time, a stainless screen mesh was placed between the two breather layers and the area was anodized for 25 min under continuous acid flow (1.55 L/m2/ min). After removing the vacuum bagging assembly, the repair area was rinsed again and air-dried. Conversion coating based on γ-GPS can potentially substitute chromium-containing surface treatments, but they do not ensure sufficient corrosion protection especially for copper-containing aluminum alloys. The aluminum substrate was abraded with a scotch-brite® pad and grit blasted with 50 μm Al2O3 in clean, dry air or in an oil-free nitrogen environment at a pressure of 206–551 kPa. The substrates were then etched in a 1% Z-6040® γ-GPS aqueous solution (Dow Corning Corporation, USA) for 15 min and the γ-GPS deposit layer was dried at 105 °C for 1 h to reach its full-cure state. The GBSG is environmentally friendly and non-toxic process. The properties of coating layer can be tuned easily by modifying the precursor mixture. In particular, sol-gel coatings for aluminum alloys were intensively investigated for corrosion protection, but hardly for surface treatment technique of adhesive bonding process. The aluminum substrates were pre-treated with a scotch-brite® abrasion pad and grit-blasted with 50 μm Al2O3 in clean and dry air or in an oilfree nitrogen environment at a pressure of 206–551 kPa. The substrates were then etched in an AC-130® sol-gel solution (3 M, USA) for 2–3 min and the sol-gel deposit layer was air-dried at room temperature. The aluminum substrates were initially immersed in a sodium chromate-sulfuric acid solution at 68 °C for 10 min prior to anodizing. The aluminum substrates were then anodized in 10 vol% phosphoric acid at 23 °C for 20 min and a DC voltage of 10 V.
Grit-blasting and silane (GBS)
Grit-blasting and sol-gel (GBSG)
Chromic-sulfuric acid etching and phosphoric acid anodizing (PAA) reference only
(VOC level: 792 g/L, chromate: 15 wt.%), BR®6747-1 (zero VOC, chromate: 15 wt.%) and BR®6747-1NC (zero VOC and non-chromate). All primers were applied by a high-volume low-pressure spray gun, and subsequently baked at 120 °C in a convection oven. Finally, the two types of epoxy adhesive films used in this study were 3 M™ ScotchWeld™ AF163-2 K (AF163-2 K) and Cytec Solvay Group FM®73 (FM73), respectively. Both adhesives were manufactured as supported adhesives with either knit or mat carrier, as shown in Fig. 2. The standard cure cycle calls for 60 min hold at 121 °C and vacuum bagging pressure of 75 kPa.
2.3. Surface characterizations A non-contact laser profilometer, Accura 1500 F (Intek IMS, Korea), was used to measure the surface roughness of substrate surfaces subjected to different surface treatments. Two roughness parameters, average roughness (Ra, ISO 4287) and maximum roughness (Rmax, ISO 4288), were used to evaluate surface roughness of the samples. Six measurements were made for each sample and average values and standard deviations were used for statistical analysis. A FE-SEM JEOL JSM-7500 F (JEOL Ltd., Japan) was used to explore the morphological changes in the substrate surface. The surface energies (polar- and dispersive-components) were calculated by the OWRK method (Pantoja et al., 2016). For this purpose, the static contact angle measurements
Fig. 2. Electron micrographs at 20× magnification of (a) AF163-2 K and (b) FM73 adhesives: the two adhesives have non-woven knit- (left) and random mat-type (right) fibrous carriers for resin flow and bond-line thickness control.
Fig. 3. Floating roller peel tests: (a) configuration of the test specimen and fixture (ASTM D3762) and (b) salt spray aging at 35 °C with 5 wt.% salt solution. 3
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Table 4 Chemical compositions of substrate surfaces subjected to different surface treatments. Surface treatments
Elemental concentration, at surface [al%] O 1s
Al 2p
Al 2s
P 2p3
Si 2p
Zr 3d5
PACs Silane Sol-gel PAA
70.6 81.1 70.6 54.5
23.0 4.9 5.7 42.1
0.7 – – 1.6
5.8 – – 1.7
– 6.2 14.9 –
– 5.7 –
Surface film layer thickness [nm] 23 112 203 260
ASTM D3167 standard as shown in Fig. 3(a). The sample dimension was 25 mm in width and 300 mm long with a total joint thickness of 2.35 mm. The apparent peel strength (τpeel) in N/mm was defined by using the following equation: peel
(1)
= P peel /w
where Ppeel is the average peeling load [N], which can be taken from the peel load-displacement curve after the initial peak point, and w is the sample width [mm]. The test samples were exposed in a neutral 5 wt.% salt fog at 35 ± 2 °C in the salt spray chamber as shown in Fig. 3(b). The peel tests were then performed at intervals of 90 days (2160 h), 180 days (4320 h) and 300 days (7200 h) according to the Airbus industry standards (AITM 5-0009). Six measurements were made for each condition and average values and standard deviations were used for statistical analysis. Finally, the residual peel strength values after 300 days of exposure to salt spray were statistically analyzed using ANOVA and Tukey test (α = 0.05). Each fracture surface after being subjected to the floating roller peel test was represented by an array of gray scale values (0–255) corresponding to the differences in failure mode. To obtain a cohesive failure image, a pixel value in the cohesive failure area was set to 0 (black) because it was less than the threshold value, while both adhesive failure and corrosion areas were set to 255 (white), as shown in Fig. 4.
Fig. 4. Image-processing technique used in this study: (a) original fracture image of test specimen; and (b) and (c) binary images showing adhesive failure and corrosion area.
(Sigma 70 Tensiometer, KSV Instrument Ltd., Finland) were carried out using a sessile drop method. The reference liquids used in this study were deionized water (H2O) and 99% DIM (CH2I2). Five drops of each liquid (20 μL) were set down on several points on each sample, and allowed to equilibrate for a few minutes. Finally, AES measurements were performed with a PHI 680 Auger spectroscope (Physical Electronics, Inc., USA) with a field emission gun in order to obtain the depth composition profile on the substrate surface. A range of operating beam conditions was used depending on which spectral regions were being investigated. Areas of interest were identified by secondary electron imaging. For depth profiling, a 1 keV/0.5 mA argon ion beam was used. The sputter rate was 7 nm/min. The quantitative AES results were determined from derivative spectra using elemental sensitivity factors.
3. Results and discussion
2.4. Corrosion resistance testing
3.1. Surface characterizations
2.4.1. Neutral salt-spray testing The corrosion durability of substrate surfaces with epoxy primers was evaluated by a neutral salt-spray test according to the ASTM B117, standard. The experiments were conducted in a salt spray chamber (5 wt.% aqueous NaCl solution at 35 ± 2 °C) for 1512 h. The scribe was a diagonal line cut through the primer coating with a hardened steel tool. The samples were then taken out of the salt spray chamber periodically and visually examined.
Tables 3 and 4 quantitatively describe the effects of surface treatment techniques on surface parameters, which include surface roughness, surface energy and chemical composition. Although the surface roughness showed no significant difference between the two anodizing processes (0.38 μm for PACs and 0.41 μm for PAA), their morphologies were significantly different, as shown in Fig. 5(a) and (d). The classical porous structures reported in the open literatures (Park et al., 2010) were clearly observed in the PAA oxide structure. This microscopic structure is likely to be due to capillary forces drawing the primer into the oxide pores, which in turn increases the mechanical interlocking
2.4.2. Floating roller peel testing The floating roller peel test samples were prepared based on the
Table 3 Surface roughness, surface contact angle, and surface energy of substrate surfaces subjected to different surface treatments (mean value ± standard deviation). Surface treatment
Surface roughness
Degrease only Grit-blasting PAA PACs GBS GBSG
0.32 0.77 0.42 0.47 0.86 0.77
Ra (μm) ± ± ± ± ± ±
a
0.04 0.07 0.03 0.05 0.05 0.04
Contact angle (°) Rt (μm)
b
De-ionized water
99% DIM
3.36 0.77 4.45 5.95 8.62 8.03
0.36 0.07 0.73 0.22 0.95 0.47
67.08 59.78 50.79 55.29 58.72 52.67
37.35 25.77 36.30 39.42 18.43 22.49
± ± ± ± ± ±
a
± ± ± ± ± ±
1.53 6.88 6.12 19.08 2.61 3.43
± ± ± ± ± ±
Surface energy (mJ/m2) 3.25 2.73 5.52 16.20 1.13 1.36
46.82 53.46 55.53 52.19 55.51 57.67
± ± ± ± ± ±
0.28 3.35 4.90 15.11 1.38 2.00
The Ra is the mathematical average of the absolute values of the measured profile height of surface irregularities. The maximum height of the profile (Rt or Rmax) is defined as the peak-to-valley vertical distance along the assessment profiles, and it is sensitive to the high peaks or deep scratches. b
4
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Fig. 5. Typical surface morphologies at a magnification of 2000× and 3D surface images: (a) PACs, (b) GBS, (c) GBSG, and (d) PAA oxide structures.
between anodic oxide and primer (Critchlow et al., 2006). On the other hand, the PACs oxide layer resulted in the formation of a scalloped surface with a network of shallow pores, as shown in Fig. 5(a). In addition, the pore size and its distribution were dramatically reduced when compared to the standard PAA process. The two silane based surface treatment techniques resulted in microscopically rough surfaces as compared with the oxide layer, as given in Table 3. Their average roughness values (Ra) were 0.86 μm (γ-GPS) and 0.77 μm (TPOZ/γ-GPS), respectively. The pretreatment step (gritblasting, Ra =0.77 μm) could contribute to the increase of surface roughness significantly. In general terms, electron microscopy revealed no significant difference in oxide structures between the two silane based substrates. Visually, the shapes of the resulting oxide structures presented a random distribution of ridges and groove, as shown in
Fig. 5(b) and (c). Furthermore, increasing surface roughness could lead to greater hydrophilicity and low contact angle. The surface energies of the γ-GPS and TPOZ/γ-GPS substrates were 55.51 and 57.67 mJ/m2, respectively. However, the PACs process yielded an inferior wettability (52.19 mJ/m2), indicative of a lower surface quality for adhesive bonding. Fig. 6(a) and (d) shows the AES depth profiles through the anodic oxide layers of the PACs and PAA substrates, respectively. The quantitative chemical compositions in terms of atom percent (al%) are given in Table 4. Note that the carbon level was not included in these figures since the concentration of carbon was below 0.5 al%. A significant variation in oxide layer thickness was apparent between the two anodizing processes; the oxide layer of PACs substrate was much thinner (23 nm), in comparison to the standard PAA process (260 nm). The
Fig. 6. AES depth profiles of substrate surfaces with different surface treatments: (a) PACs, (b) GBS, (c) GBSG, and (d) PAA oxide structures. 5
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at the AES depth profile of GBS substrates (Fig. 6(b)), the coating thickness of γ-GPS film was found to be 112 nm based on the Si signal. Comparing results with the GBS substrate, it could be seen that the GBSG process yielded a greater oxide layer thickness in the range from 154 nm to 203 nm, and an increase in the Si:Al ratio in the outermost part of the coating layer. Furthermore, the AES depth profile indicated the variation in concentrations of Si and Zr through the thickness of the coating layer; Zr toward metal (max. 2.8 al% at 203 nm depth) and Si on the outermost edge (max. 14.91 al% at 0 nm), respectively. This gradient coating structure of TPOZ/γ-GPS film is schematically shown in Fig. 7 (Osborne et al., 2001); ZrO2 reacts with a metallic surface to produce a covalent chemical bond, while SiO2 offers a reactive organic group for bonding with an adhesive. 3.2. Corrosion resistance tests 3.2.1. Neutral salt-spray tests Images of the corrosion areas of test samples are shown in Fig. 8. To extend the understanding of corrosion damage mechanisms at the micro-scale level, the aged-samples were inspected by electron microscopy at a magnification of 100× as shown in Fig. 9. It can be seen that the PAA substrates did not show any blistering or accumulation of corrosion products around the scribe on the sample after 1512 h exposure. Electron micrographs also supported this result, showing that the scribe line was apparently clean without any corrosion product accumulation, such as precipitates, pits or blisters (see Fig. 9(i)). In addition, no evidence of corrosion was observed either in the scribe line, or underneath the primer with the use of chromate primers (BR127 and BR6747-1). For example, the chromate, water-soluble primer (BR6747-1) exhibited a comparable resistance to the conventional solvent-based, chromate primer (BR127) with PACs and GBS processes. However, the non-chromate, water-soluble primer (BR6747-1NC) showed a limited corrosion resistance during a neutral salt spray test. Electron microscope observations supported this result; the actual
Fig. 7. Schematic representation of a typical sol-gel thin film coating (reproduced from Osborne et al., 2001 with permission from Elsevier).
results in the current study were similar to those reported by Saliba, 1994 on oxide layer dimensions for PAA and PACs substrate surfaces. In addition, the presence of high levels of phosphorus (5.8 al%) in the outermost surface was observed in the PACs oxide structure, as given in Table 4. This result implies that the phosphoric acid did not penetrate into the base of the oxide layer (i.e. barrier layer). This could be due to a number of factors, which include pore narrowing, branching and termination and limitation of the capillary force. AES depth profiles through the organic oxide layers are presented in Fig. 6(b) and (c) for the GBS and GBSG substrates, respectively. Apparently, the presence of Si could be attributed to the application of the γ-GPS (C9H2O5Si) coating agent. Abel et al. (1998), using a ToF-SIMS analysis, demonstrated that the strong Al-O-Si bond was formed between hydrolyzed γ-GPS film and the substrate surface. When looking
Fig. 8. Salt spray exposure panels with different surface treatment/primer combinations: (a) 168 h exposure, and (b) 1512 h exposure. 6
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Fig. 9. Electron micrographs showing corrosion damage in the vicinity of scribe line after 1512 h of exposure to the salt spray: solid and dash arrows represent scribe line and primer coating layer, respectively.
protecting primer layer was severely damaged in the scribe line as shown in Fig. 9(c) and (f). In summary, in the surface treatment study, the GBSG process demonstrated adequate corrosion resistance at a level comparable with non-chromate primer (BR6747-1NC) as shown in Fig. 9(h). On the other hand, the PACs and GBS processes were unable to provide acceptable corrosion protection with a BR6747-1NC primer.
(Mazza et al., 2003, 2004; Blohowiak et al., 2001) on the peel strength with the two adhesive films. Comparison of initial peel strength of un-aged samples demonstrated that a marginal difference could be measured between the two adhesives. These results were probably due to the difference in bondline thickness. Kawashita et al. (2008) have investigated the influence of bond-line thickness on adhesive fracture toughness (GA) using a peel test. They reported that the adhesive fracture toughness of peel joints increased as the bond-line thickness was increased. In addition, the relationship between adhesive fracture toughness and bond-line thickness can be explained by a LEFM model. This was confirmed by the observation that during floating roller peel tests, the energy-dissipating capacity of adhesive bonded joints increased with an increase in bondline thickness. According to our previous research (Park et al., 2018), the nominal bond-line thicknesses of FM73 and AF163-2 K adhesives were 0.30 ± 0.03 and 0.25 ± 0.02 mm, respectively. Typical floating roller peel test load-displacement curves, Fig. 11, clearly show that a thicker adhesive (FM73) results in improved fracture energy. The fracture surfaces and their corresponding failure modes (i.e. % cohesive failure) before and after exposure to salt spray are presented in Fig. 12. Obviously, cohesive failure was the dominant failure mode in all un-aged samples (> 75 areal%). Analysis of failure modes showed that cohesive failure within the adhesive layer prevailed in the PAA samples after 300 days exposure. However, the much thinner PACs oxide layer (nominal thickness, 23 nm) was more susceptible to corrosion attack, showing a high corrosion rate ranging from 45 areal% to 86 areal%. On the other hand, the GBSG sample groups turned out to be more efficient in producing a corrosion resistant surface than the GBS groups. The incorporation of zirconium oxide (ZrO2) into the sol-gel coating can increase in surface densification through pore-filling and
3.2.2. Floating roller peel tests Fig. 10 shows the effect of process factors on residual peel strength according to salt spray exposure time. This figure shows that a significant decrease in peel strength could be observed at 300 days exposure, and the aged samples presented a large scatter in each test result. The oxide layer produced by anodizing in phosphoric acid contains phosphate ion (AlPO4), which is known to inhibit hydration reactions by hindering the penetration of moisture molecules into the anodized film as earlier reported by Johnsen et al. (2004). It subsequently leads to excellent hydrolytic stability or corrosion durability of the PAA oxide layer. On the other hands, the PACs oxide layer was much more susceptible to the effects of moisture and/or corrosion agents than the PAA oxide. This difference in susceptibility to environmental factors may have a significant effect on the long-term properties of adhesive bonded joints. Although the PACs process followed by chromate primers (BR127 and BR6747-1) leads to substantially improved corrosion resistance in comparison to the non-chromate primer (BR6747-1NC), their results were still inferior to other surface treatment techniques. A possible explanation is that the much thinner PACs oxide is more susceptible to hydration or corrosion of the substrate surface as reported by Saliba (1994). Finally, both GBS and GBSG substrates exhibited an intermediate corrosion durability compared to the standard PAA substrate. The results of this study are similar to those reported previously 7
Journal of Materials Processing Tech. 276 (2020) 116412
S.Y. Park, et al.
Fig. 10. Effect of process factors on residual peel strength according to salt spray exposure time (the error bars represent standard deviation): (a) AF163-2 K and (b) FM73 adhesives.
type carrier adhesive (FM73), as shown in Fig. 2. For example, the GBS substrate with FM73 adhesive has a higher corrosion rate compared to the values obtained by AF163-2 K adhesive. 3.2.3. ANOVA results The results of ANOVA with residual peel strength after 300 days of exposure to salt spray are summarized in Table 5. The effects of process factors on residual peel strength are shown in Fig. 13. In this figure, the results of the PAA sample group are presented for comparative purposes only. It is apparent that the surface treatment was the dominant factor in the determination of residual peel strength with a high statistical term (p = 55.2%). Obviously, both GBSG and PAA sample groups showed a positive effect on the corrosion resistance of peel joints. As mentioned above, the decrease in peel strength was accompanied by an increase in the amount of corrosion damage in the bond-line layer; such degradation was large enough to introduce more local stress concentration than was present in the un-aged samples. Furthermore, premature failure induced by the loss of interfacial bonds could lead to a high error rate; notice that the error value associated with the ANOVA table of residual peel strength was 26.2%. On the other hand, the effects of primer (p = 6.6%) and adhesive (p = 5.5%) were relatively minor. The coupled interaction between surface treatment and primer was found to be 3.7%.
Fig. 11. Typical load-displacement curves from floating roller peel tests (data is obtained with GBSG/6747-1).
planarization, resulting in high corrosion durability. Finally, the rate of cohesive failure was much better for AF163-2 K adhesive even if FM73 adhesive produced a high initial peel strength. This could be explained by observing that a wide open knit-type carrier (AF163-2 K) results in more contact area to carry the external load in comparison to the mat8
Journal of Materials Processing Tech. 276 (2020) 116412
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Fig. 12. Typical fracture surfaces of un-aged and 300 days exposure test samples: (a) AF163-2 K, and (b) FM73 adhesives.
4. Conclusions
Table 5 ANOVA for the residual peel strength results (unit: N/mm). Factors
SDQ
gl
Variance
Test F
p (%)
Surface treatments (A) Primers (B) Adhesives (C) A×B B×C Error Total
1072.94 128.32 107.46 71.07 52.88 509.56 1942.23
2 2 1 2 2 118 127
536.47 64.161 107.46 35.533 26.439 4.318 774.381
124.23 14.86 24.88 8.23 6.12 – –
55.2% 6.6% 5.5% 3.7% 2.7% 26.2% 100.0%
This study has investigated the effect of process factors on corrosion durability of adhesive bonded joints for aluminum alloy with structural epoxy adhesives. Based on the utilized methodologies and experimental results, the following conclusions can be drawn. 1) The corrosion durability of adhesive bonded joints was significantly dependent on the stability of the adhesive-adherend interface. Changes in oxide layer chemistries were expected to modify the interfacial properties and so to affect the joint strength and environmental durability in practice. Furthermore, a hybrid sol-gel coating consisting of TPOZ and γ-GPS produced a microscopically
Fig. 13. Effect of process factors on residual peel strength after 300 days exposure, center line corresponds to the mean value of the observations in all test samples. 9
Journal of Materials Processing Tech. 276 (2020) 116412
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rough surface (0.86 μm) with an increase in surface energy (57.67 mJ/m2) when compared with the standard phosphoric oxide. In particular, the enhanced corrosion resistance efficiency was obtained due to the incorporation of zirconium oxide (ZrO2) into the sol-gel coating which can further improve surface stability by increasing surface densification. In contrast, the result of the PACs process was to leave a thin, non-homogenous oxide on the substrate surface with a low level of surface roughness (0.38 μm). 2) The non-chromate primer (BR6747-1NC) exhibited adequate corrosion protection efficiency when used with a TPOZ/γ-GPS based sol-gel coating. Electron micrographs showed that the scribe line was apparently clean without any sign of corrosion product accumulation, such as precipitates, pits or blisters. However, the nonchromate primer applied to PACs and GBS was unable to provide acceptable corrosion protection in the neutral salt spray test. 3) A marginal difference in adhesive fracture toughness could be observed between the two adhesives considered in this study. The thicker FM73 adhesive (nominal bond-line thickness = 0.30 mm) showed a relatively high adhesive fracture toughness in comparison to AF163-2 K (0.25 mm). The influence of bond- line thickness on adhesive fracture toughness can be explained by a LEFM model. In contrast, the rate of cohesive failure after neutral salt spray exposure was much better for AF163-2 K adhesive even though FM73 adhesive produced higher initial peel strength. The wide open knittype carrier in AF163-2 K brings in more contact area to promote wetting with a primer coating. 4) Based on the results of ANOVA, the surface treatment was the main factor influencing the residual peel strength with a high statistical term (p = 55.2%). On the other hands, the effects of primer (p = 6.6%) and adhesive (p = 5.5%) were relatively minor. Consequently, the surface treatments and their coupled interaction with bond primer significantly influence the residual peel strength after 300 days of exposure to salt spray.
of adhesives. Adhesives. ASTM International, West Conshohocken, PA. Bishopp, J.A., Parker, M.J., O’Reilly, T.A., 2001. The use of statistical experimental design procedures in the development and “robust” manufacture of a water-based, corrosion-inhibiting adhesive primer. Int. J. Adhes. Adhes. 21 (6), 473–480. Blohowiak, K.Y., Anderson, R.A., Stephenson, R.R., 2001. Sol–gel Technology for Surface Preparation of Metal Alloys for Adhesive Bonding and Sealing Operations (Report No. AFRL–ML–WP–TR–2003–4169). AFRL Wright-Patterson AFB, Ohio. Critchlow, G.W., Yendall, K.A., Bahrani, D., Quinn, A., Andrews, F., 2006. Strategies for the replacement of chromic acid anodising for the structural bonding of aluminium alloys. Int. J. Adhes. Adhes. 26 (6), 419–453. Critchlow, G.W., Brewis, D.M., 1996. Review of surface pretreatments for aluminium alloys. Int. J. Adhes. Adhes. 16 (4), 255–275. Davis, G.D., Venables, J.D., 2002. In: Pocius, A.V. (Ed.), Adhesion Science and Engineering: Surface, Chemistry and Applications. Elsevier, Amsterdam. Hardwick, D.A., Ahearn, J.S., Desal, A., Venables, J.D., 1986. Environmental durability of phosphoric acid anodized aluminium adhesive joints protected with hydration inhibitors. J. Mater. Sci. 21 (1), 179–187. Kawashita, L.F., Kinloch, A.J., Moore, D.R., Williams, J.G., 2008. The influence of bond line thickness and peel arm thickness on adhesive fracture toughness of rubber toughened epoxy–aluminium alloy laminates. Int. J. Adhes. Adhes. 28 (4-5), 199–210. Lunder, O., Olsen, B., Nisancioglu, K., 2002. Pre-treatment of AA6060 aluminium alloy for adhesive bonding. Int. J. Adhes. Adhes. 22 (2), 143–150. Mazza, J.J., Gaskin, G.B., Piero, W.S.D., Blohowiak, K.Y., 2004. Sol-gel Technology for low-VOC, Nonchromated Adhesive Bonding Applications (Report No. AFRL-ML-WPTR-2004–4063). AFRL Wright-Patterson AFB, Ohio. Mazza, J.J., Avram, J.B., Kuhbander, R.J., 2003. Grti-blast-silane (GBS) Aluminum Surface Preparation for Structural Adhesive Bonding (Report No. WL-TR-94–4111). AFRL Wright-Patterson AFB, Ohio. Osborne, J.H., Blohowiak, K.Y., Taylor, S.R., Hunter, C., Bierwagon, G., Carlson, B., Bernard, D., Donley, M.S., 2001. Testing and evaluation of nonchromated coating systems for aerospace applications. Progr. Org. Coating 41, 217–225. Pantoja, M., Abenojar, J., Martínez, M.A., Velasco, F., 2016. Silane pretreatment of electrogalvanized steels: effect on adhesive properties. Int. J. Adhes. Adhes. 65, 54–62. Park, S.Y., Choi, W.J., Choi, H.S., Kwon, H., Kim, S.H., 2010. Recent trends in surface treatment technologies for airframe adhesive bonding processing: a review (1995–2008). J. Adhes. 86 (2), 192–221. Park, S.Y., Choi, W.J., Choi, C.H., Choi, H.S., 2018. The effect of curing temperature on thermal, physical and mechanical characteristics of two types of adhesives for aerospace structures. J. Adhes. Sci. Technol. 32 (11), 1200–1223. Saliba, S.S., 1994. The Surface Preparation of Aluminum Alloys Using the Phosphoric Acid Containment System for Repair (Report No. WL–TR–94–4112). AFRL WrightPatterson AFB, Ohio. Stayrook, J.J., Bellevou, D.L., Manson, R.T., Martinelli, A.A., Ridpath, S., Arnold, J.R., Gaskin, G.B., 1999. Performance of Solvent and Water Based Primers in Epoxy Adhesive Systems (Accession No. ADA368481). Naval Air Warfare Center Aircraft Division, Maryland. Yarlagadda, P.K.D.V., Cheok, E.C., 1999. Study on the microwave curing of adhesive joints using a temperature-controlled feedback system. J. Mater. Process Technol. 91 (1–3), 128–149.
References Abel, M.L., Watts, J.F., Digby, R.P., 1998. The adsorption of alkoxysilanes on oxidised aluminium substrates. Int. J. Adhes. Adhes. 18 (3), 179–192. ASTM Standard B117-16, 2016. Standard Practice for Operating Salt Spray (Fog) Apparatus. ASTM International, West Conshohocken, PA. ASTM Standard D3167-10, 2017. Standard test method for floating roller peel resistance
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