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Abstract
Paint layer was stripped from the 2024 aluminium alloy aircraft skin by either 1000 grit sandpaper or laser with 150 ps pulse width while the laser paint stripping (LPS) process was recorded by a high-speed camera. The surface and cross-section morphologies, chemical compositions and chemical valences of obtained the paint stripping samples were also characterise. The corrosion resistance was determined by the Potentiodynamic Polarization Curve (PPC). On mechanical paint stripping (MPS) samples, a large amount of scratches remained. Surface roughness increased and the oxide film was removed completely. The trace of the laser scan was observable on the surface of LPS samples. Recrystallisation occurred on the LPS surface and eventually formed arrayed micro and sub-micro structures. The oxide film is mainly composed of Al2O3 with a thickness about 2.10 µm. The corrosion current density of mechanical and LPS samples are 3.66 ×10−2 mA·cm−2 and 6.66×10−5 mA·cm−2, respectively. Comparing to MPS which removed all the oxide film and damaged the substrate metal, LPS only damaged the oxide film mildly without damaging metal substrate. The remaining oxide film contributes to a higher corrosion resistance of the LPS sample.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For safety considerations, after a period of working time, the paint of the aircraft skin has to be removed so that cracks etc. safety risk can be detected [1]. The depainting process should not damage the aluminium substrate so that new painting can be applied directly on the paint stripping surface [2,3]. Current depainting methods include MPS and chemical stripping. The MPS has many limitations: low work efficiency, easy damaging of the surface of the workpiece etc [4]. Chemical stripping is lengthy and relates to the use of hazardous chemicals, which are typically high in Volatile Organic Compounds (VOC) and Hazardous Air Pollutants (HAP) [5]. For industrial depainting, LPS seems advantageous compared with chemical or mechanical processes [6]. Comparing to traditional paint stripping approaches, LPS has higher efficiency, more eco-friendly, higher accuracy, wider application ranges, lower operation circumstance requirement [7]. Moreover, it hardly does damage to the substrate [8].
A specific pulse CO2 laser generator was used to depaint the sample by a company in New Jersey, America. The laser evaporates a paint with 5 µm thickness while keep the substrate temperature chilled. The laser has a large spot size which is able to remove the paint layer of a sample with 1 mm thickness and 36 m2 face area in an hour [9]. In the process of LPS, the change and damage of the surface morphology of the aircraft skin will affect the subsequent repainting and corrosion performance. This is of great importance for LPS of aluminium-based aircraft skin. Pantelakis et al. [10] studied the LPS of 2024-T351 aluminum alloy by excimer, CO2, Neodymium-doped Yttrium Aluminium Garnet (Nd:YAG), and Transversely Excited Atmospheric CO2 (TEA-CO2) laser source. They found that the surface roughness increased after paint removal, and the increase in the transverse direction was considerably higher than that in the longitudinal direction. Klingenberg et al. [11] carried out LPS on 2024-T3 aluminium-based aircraft skin by CO2 laser source with pulse width of 1 µs, Nd: YAG laser source with pulse width of 10 ∼ 12 ns, and Nd: YAG laser source with pulse width of 200 ns. They found that after LPS, the roughness of the aluminum alloy substrate becomes larger, the macro temperature of the substrate rises, and the surface is prone to damage. Luo et al. [12] studied the LPS of 2024 aluminum alloy by Continuous Wave (CW) CO2 laser source. It is found that LPS is easy to cause temperature rise of aluminum alloy substrate and damage its surface. It was proved by Zheng [13] that LPS would not damage the aircraft skin substrate. He obtained an excellently paint stripping surface by using a TEA-CO2 laser generator with 200 ns - 3 µs pulse width. Zhu et al. [14] found that under the appropriate energy density, the paint on the skin surface was removed, and the aluminum clad layer was deformed and damaged, while the 2024 aluminum alloy substrate under the aluminum clad layer was not damaged. Unfortunately, they did not mention the specific pulse width information. In calculating the data in this paper, the pulse width is about 500 ns. Jiang et al. [3] carried out LPS on LY12 aluminium-based aircraft skin with a minimum pulse width of 248 µs, and found that there was a strong burning phenomenon occurred. The surface roughness and the micro morphology of the oxide film were same as before LPS, but the color changed. Unfortunately, the thickness change of the oxide film was not analysed.
In conclusion, current LPS techniques are based on the pulse width longer than 10 ns. These researches mainly focus on the damage the aluminum alloy substrate, but less on the damage of surface oxide film. During laser processing, the laser with a longer pulse width will cause a larger heat-affected zone on the surface of the material, which may cause damage to the surface [15]. The oxide film on the surface of aluminum alloy is very important for aircraft skin. The shorter pulse width can reduce this damage. In this project, LPS on 2024 aluminium alloy aircraft skin was obtained by pulse width of 150 ps. The damage of the LPS to the oxide film of the aluminum alloy was involved. This helps to contribute the application of the LPS technique on the aluminum alloy aircraft skin.
2. Experimental and sample materials
2.1. Sample materials
Figure 1 shows the scanning electron microscopy (SEM) micrographs of aircraft skin cross-sections. 2024 aluminum alloy was chosen as the substrate material of aircraft skin in this project. The alloy was received in the T351 temper condition. The elemental composition (wt. %) is Cu 4.2, Mg 1.6, Mn 0.8, Si 0.5, Fe 0.3, Zn 0.3, Cr 0.15,Ti 0.1and balance Al. The thickness of each sample is 1 mm. The surface of aluminum alloy was oxidized, the thickness of the oxide film is about 2 µm. STL7470-1 epoxy was then paint on aluminum alloy oxide film by spray painting, the thickness of the paint layer is about 31.5 µm. The main component of STL7470-1 paint is (C3H4O2)n. Due to the large size of the aircraft, the degree of damage to the surface paint of the aircraft skin is inconsistent. It is very difficult to count the damage of the aircraft surface paint of various parts. Therefore, in this study, the research object is simplified to the aircraft skin without damage, and the surface paint is complete.
2.2. Mechanical and LPS
The aircraft skin was depainted by a self-invented LPS apparatus (YLPP-1-150-30, IPG Ltd., USA) in air. The laser’s wavelength is 1064 nm, pulse frequency is 600 kHz, pulse energy is 0.05 mJ, pulse width is 150 ps, and the defocusing amount is 0 mm. The methods for measuring the size of the laser spot include the diameter squared versus fluence method [16], photo paper method, CCD method, etc. The photo paper method is used in this experiment. The laser spot size was measured by the photo paper method. A single pulse laser acted on the photo paper to form 5 holes. Under a 200x optical microscope, the average diameter of the 5 holes was measured as the spot size. The laser spot diameter is about 50 µm. The energy density F could be calculated by Eq. (1):
where Ep, d are pulse energy and laser spot diameter, respectively. To avoid the damage of the oxide film on the surface of the sample, based on the previous test, the energy density of 2.55 J·cm−2 and the LPS times of 4 was chosen. Figure 2 a) shows the schematic of LPS. When more than two pulses repeatedly radiate on same region of material surface, the energy absorption in this region is greater than that of single pulse [17]. Especially the overlapping of adjacent points will have an important influence on the surface topography [17,18]. The overlap ratio U is defined as follows: where l is the distance between two adjacent spots. The overlap ratio UX in the x direction is 60%, and the overlap ratio UY in the Y direction is 94%.Figure 2 b) shows the schematic of MPS. 1000 grit sandpaper was used for the MPS. The 1000 grit sandpaper has very little damage to the surface of the aluminum alloy. With MPS, grind gently in the Y direction until the smooth surface is exposed, and then wiped the surface with alcohol.
2.3. Characterization methods
The micromorphology and composition of the aluminum alloys samples were investigated by scanning electron microscopy (SEM, SU8018, Hitachi Ltd., JPA) with Energy Dispersive Spectrometry (EDS, Oxford X-Man50, Hitachi Ltd., JPA).
The macrostructure and three-dimensional (3D) topography of the samples were examined using a 3D Super Depth Digital Microscope (LY-WN-YH500 CDLY Ltd., CHA).
Surface compositions of aluminum alloys were measured by X-ray photoelectron spectroscopy (Escalab 250xi, Thermo Ltd., USA). The C 1s peak from adventitious carbon at 284.8 eV was used as a reference to correct the charging shifts.
The LPS process were observed using high-speed camera (FASTCAM SA4, Photron Ltd., JPA). The sample frequency was 3600 fps. As shown in Fig. 2(b)), in order to clearly observe the whole process of LPS, an auxiliary laser lighter with 808 nm wavelength is added beside.
2.4. Methodology of measuring potentiodynamic polarization curve
The potentiodynamic polarization curve was measured on a CS310 electro-chemical workshop in a three-electrode system which includes a working electrode (testing sample), a reference electrode (saturated calomel electrode, SCE) and a supportive electrode (Pt electrode). The corrosion reagent is a 3.5% NaCl solution. The potentiodynamic polarization curve was scanned by a speed of 1 mv s−1, a voltage of −0.6 ∼ 0.3 V (Open Current Potential, OCP), a scanning time of 15 min.
3. Results and analysis
3.1. Surface macro and 3D topography analysis
The paint stripping effect can be clearly visualized by the change of surface macro-topography before and after paint stripping, and it can also reflect the damage to the aircraft skin surface. Surface macro-topography of aircraft skin before and after paint stripping is shown in Fig. 3(a)), b) and c). Both LPS and MPS were effective in removing the paint on the surface of the aircraft skin, revealing the metallic luster of aluminum alloy.
In Fig. 3(b)), after MPS, there are mechanical scratches on the paint stripping part and the non paint stripping part of the aircraft skin surface, and the paint cannot be removed according to the designed dividing line. Figure 3(b)) compared with Fig. 3(a)), there are also a lot of scratches on the paint surface of the part without paint stripping. In Fig. 3(c)), after LPS, the paint is removed according to the designed dividing line, and the laser scanning traces are faintly visible on the paint stripping part. It means that the excess laser energy has already interacted with the substrate, thus yielding laser scanning traces [19]. In Fig. 3 c), many grayscale markings are also observed, which are produced by the laser acting on the oxide film, which is consistent with the research result of Gedvilas et al [20]. Figure 3(c)) compared with Fig. 3(a)), the macro morphology of the paint surface of the unpainted part is the same. Compared with Fig. 3 b) and c), LPS is more uniform. The laser moves regularly under the control of the galvanometer, and the movement track and energy control are consistent, so the paint stripping is more uniform. During MPS, the movement of sandpaper and the force of friction can not keep the same, so the paint stripping is very uneven, and the paint stripping can not be carried out according to the design boundary.
Surface 3D topography of aircraft skin sample before and after paint stripping is shown in Fig. 4(a)), b) and c). In Fig. 4(a)), the paint surface is relatively smooth before paint stripping. In Fig. 4(b)), after MPS, a large number of scratches increase the unevenness and damage the surface of the aluminum alloy. In Fig. 4(c)), the unevenness of the surface after LPS is consistent with the before paint stripping. Surface profile curve of aircraft skin is shown in Fig. 4(d)) and e). The undulation degree of surface profile curve in X and Y directions after MPS is much greater than before paint stripping, while the undulation degree of surface profile after LPS is slightly less than before paint stripping. Along the X direction, the roughness Ra is 0.47 µm before paint stripping, 2.96 µm after MPS and 0.34 µm after LPS. Along the Y direction, the roughness Ra is 0.52 µm before paint stripping, 3.10 µm after MPS and 0.47 µm after LPS. The roughness after MPS is greater than that before, and increases by 529.79% along X direction and 496.15% along Y direction. The roughness after LPS is lower than that before, and decreases by 27.66% along X direction and 19.23% along Y direction. When MPS is used, the movement track and force of sandpaper are different, which makes the surface scratch, the undulation of surface profile become larger and the roughness increase. When LPS is used, the movement track and energy of the light spot are consistent, so that the paint stripping effect is also consistent, and the undulation of surface profile and the roughness are small.
Fig. 4. Surface 3D topography of aircraft skin, a) as received, b) MPS, c) LPS, d) surface profile curve in X-direction and e) surface profile curve in Y-direction
3.2. Surface microstructure and elemental composition analysis
In 3.1, it was observed that there were different paint stripping traces and damages on the surface of aircraft skin after MPS and LPS respectively. In order to further judge the influence of these traces and damages on the surface of aircraft skin, SEM and EDS were used for surface microscopic observation and element analysis. The microstructure of the surface morphology of the aircraft skin paint stripping by MPS is shown in Fig. 5. In Fig. 5 a), b) and c), the friction lines are relatively obvious and the surface is relatively even. In Fig. 5 d), stripped structure with width of 7.17 µm can be observed, spot 1 and spot 2 are located at the top and bottom of the scratch, respectively. In spot 2, the Al and O wt. % are 91.08 and 3.68, respectively. The part of spot 2 is aluminum alloy substrate. In spot 1, the Al and O wt. % are 79.61 and 16.32, respectively. Due to the residual oxide film in spot 1, the O wt. % in this part is 4.43 times that of aluminum alloy substrate. Uneven MPS leaves a residual oxide film on the surface of the aircraft skin.
The elemental mappings of aircraft skin after MPS is shown in Fig. 6. In Fig. 6 d) and f), the distribution of the oxygen is generally homogeneous over the entire surface but also very close to the distribution of the magnesium. This is due to the oxygen affinity of the magnesium is higher. However, as the percentage of Mg is relatively low, after the paint stripping process, the surface oxides are generally composed by MgOx, (AlMg)Ox and AlOx [21]. After the MPS, both of the paint and the oxide film were removed. After MPS, the aluminum alloy substrate reacts with oxygen in the air, and the residual oxide film make the O wt. % on the surface of aircraft skin to be 3.68%.
The surface microstructure morphology of the aircraft skin after the LPS is shown in Fig. 7. The pulse width is 150 ps in this experiment. The temperature of the laser spot can be calculated by [22,23]:
At this time, the surface temperature of the aircraft skin sample is up to 3000 °C while the melting temperature of Al2O3 is 2045 °C [22,24]. In this process, the paint layer and aluminum alloy oxide film will be removed by thermal vibration and thermal ablation under the action of laser, of which thermal vibration peeling is the main one [25,26]. This means the oxide film on the sample surface experienced a re-melting in the LPS process. The laser beam has simultaneously affected both paint stripping and the re-melting [19]. During the re-melting process, the aluminum alloy expanded and evaporated, which created a counterforce to the substrate so that some of the molten oxides splashed and recrystallized on the surface. The melted oxides flows and moves instantaneously, leading to the formation of a corrugated paint stripping substrate surface [27]. The micro-structure coated sub-long wave structure was formed on the surface when the ultra-short pulse laser was applied to the aluminum alloy [28]. In Fig. 7 a), there are some periodic striped LPS traces. The micro-pores, micro-floccules and micro-bulge are shown in Fig. 7 b), c) and d). The apophysis of the ridge from the micro-wave geometry was produced by the fluid expansion when the material melt and flow. When the sample was opposed to a high energy density (larger than 0.4 J·cm−2), explosion happened due to the reaction between laser pulse and the material and hence produce the random-distributed micro-structure [29]. The size of the white spot convex structure was around 326 nm and oxide film micro-pores were observed under the spot.
The chemical composition of the spot 1 and spot 2 was measured and shown in Fig. 7. In spot 1, the Al and O wt. % are 52.93 and 41.47, respectively. Combined with its microscopic morphology, it is the aluminum alloy oxide film. In spot 2, the Al and O wt. % are 84.5 and 10.37, the Al wt. % is lower than the substrate and higher than the oxide film, and the O wt. % is higher than the substrate and lower than the oxide film. It is concluded that point 2 is the recrystallized grain of oxide film. During the remelting and crystallization of the oxide film, part of the oxygen combines with the C and H elements in the paint to produce CO, CO2, and H2O gas overflowing the surface, resulting in a 31% reduction of oxygen content after remelting and crystallization. The oxygen loss process was shown in Fig. 8 [24,25].
The elemental mappings of aircraft skin after LPS is shown in Fig. 9. Compared with MPS, the uniformity of aluminum and oxygen on the surface of the aircraft skin after LPS is better, and the content of oxygen and magnesium is higher. This means the loss of magnesium is larger than aluminum after the LPS. This is due to the melting temperature and the evaporating temperature of the pure magnesium is lower than that of aluminum. Also, the ionization energy of Al3+ is 5139 kJ mol−1 and that of Mg2+ is 2188.4 kJ mol−1. This means the Mg2+ is easier to evaporate and for a plasma [30]. The oxygen composition of after LPS surface is larger than that of the MPS surface, which also indicates that the oxide film remained after the LPS.
High-speed camera can clearly observe the ablation process in LPS. High-speed photography of aircraft skin with LPS is shown in Fig. 10. A large of smoke is produced by laser ablation of aircraft skin paint. In Fig. 10 f), g) and h), when the laser acts on the aluminum alloy plate, the laser spot is smaller. In Fig. 10 a), b), c) and d), when it acts on aircraft skin, the laser spot is larger. In addition to ablation phenomenon, there is vibration phenomenon during LPS [7,31]. Part of the paint is peeled off the aircraft skin by vibration and falls near the skin. In Fig. 10 f), g) and h), the closer it is to the aircraft skin, the more paint is dropped, and the more severe the ablation phenomenon produced when the laser is applied to these paints, making the light spot larger. In the LPS process, the ablation effect is strong and the process time is very short. The air near the ablation interface cannot meet the needs of oxidation during paint stripping. The oxygen in the oxide film near the ablation interface is in a high-energy state under the action of high temperature. The oxygen reacts with the paint to reduce the oxygen content in the oxide film.
There are ablation effect, thermal vibration effect and plasma shock effect in the process of LPS [25,32]. In the initial stage of LPS, the ablation effect is the main removal mechanism. The laser irradiates the surface of the paint layer, the paint layer absorbs laser energy, and the temperature rises above the melting point or boiling point, resulting in physical and chemical changes such as decomposition, combustion or gasification [33].
It can be seen in Fig. 11 that there is ablation in the whole process of LPS. The schematic diagram of ablation process is shown in Fig. 11(a)). The penetration depth of the laser in the paint layer b could be calculated by Eq. (5) [22]:
where $\alpha $ is the thermal diffusivity. When the $\tau $ is 150 ps, the penetration depth of the laser is shallow, and the thickness of single LPS is small.With the progress of LPS process, the temperature increases due to the action of laser on the paint layer A temperature gradient is formed inside the paint layer, which causes an uneven distribution of thermal stress and produces thermoelastic vibration effects, resulting in different displacement of each part of the paint [2,7]. The displacement $\delta $ could be calculated by Eq. (6) [33]:
where $\beta $ is the linear expansion coefficient of the material, A is the effective absorption coefficient of the material, and I is the laser energy density, respectively.During thermoelastic vibration, the greater the laser energy density, the greater the displacement, the paint is easy to separate from the original surface. In Fig. 10 e), f), g) and h), part of the paint on the test panel is splashed out of the test panel under the action of thermoelastic vibration, and ablation occurs when the laser is applied.
The melting and vaporization caused by laser ablation produces a large number of nanometer and micrometer aerosol particles, which generate plasma when the laser is applied [34]. Plasma shocked the paint layer. The process of plasma shock is accompanied by plasma flash and shock wave generated by rapid expansion of absorbed laser energy [35].
Maximum shock force during LPS ${{\rm P}_{max}}$ could be calculated by Eq. (7) [36]:
where $\varphi $ is the internal energy distribution coefficient, Z is the impedance of the shock wave propagating in the paint, respectively.The spot observed in Fig. 10 is a composite spot of plasma flash and laser. In Fig. 10 a) to d), as the vapor generated by laser ablation becomes more and more, the plasma effect becomes stronger and stronger, and the plasma flash spot becomes larger and larger, so the composite spot becomes larger and larger. In Fig. 10 e) to h), the farther away from the test plate, the less paint stripped by thermoelastic vibration, the weaker the ablation effect, the less steam generated, and the smaller the plasma flash spot, so the composite spot is smaller and smaller.
3.3. XPS surface analysis of aircraft skin surfaces
EDS elemental and EDS elemental mapping of aircraft skin surface are different with MPS and LPS. Thus, it is necessary to study the chemical valence of aircraft skin surface after paint stripping. XPS was used to analyze the binding energy of constituent elements. XPS survey scan of aircraft skin after paint stripping, as well as high-resolution scans of Al 2s and Al 2p are shown in Fig. 12. Figure 12 i and iv show full XPS spectrum from the surface of aircraft skin with after paint stripping. As shown in the figures, six XPS peaks, which were assigned to Mg 1s, Cu 2p, O 1s, C 1s, Al 2s and Al 2p from the side of lower binding energy, were observed on the as-annealed surface of aircraft skin after paint stripping.
Figure 12 ii shows XPS spectrum of Al 2p core levels from the surface of aircraft skin after MPS. As shown in the figure, the XPS doublet peaks at 71.8 eV and 72.24 eV, which were assigned to Al metal 2p3/2 and Al metal 2p1/2, were observed [37], and the XPS single peak at 74.1 eV, which was assigned to Al2O3, were observed [38]. Figure 12 iii shows XPS spectrum of Al 2s core levels, the XPS single peak at 118 eV, which was assigned to Al metal, was observed [39]. The high-resolution XPS spectrum of Al 2p and Al 2s confirmed that the chemical form of aluminum of aircraft skin after MPS presented two components of Al metal (76.39%) and Al2O3(23.61%).
Figure 12 v shows XPS spectrum of Al 2p core levels from the surface of aircraft skin after LPS. As shown in the figure, the XPS single peaks at 74.5 eV, which was assigned to Al2O3, was observed [40]. Figure 12 vi shows XPS spectrum of Al 2s core levels, two XPS single peaks at 118.1 eV and 121.1 eV, which were assigned to Al metal and Al2O3, were observed [39,41]. The high-resolution XPS spectrum of Al 2p and Al 2s confirmed that the chemical form of aluminum of aircraft skin after LPS presented two components of Al2O3 (83.54%) and Al metal (16.46%). Compared with MPS, the Al metal content on the surface of aircraft skin after LPS is reduced by 78.45%, while the content of Al2O3 is increased by 71.73%.
3.4. Elemental mapping analysis of aircraft skin cross-sections
3.1, 3.2, and 3.3 all observed and analyzed the effect of different paint stripping methods on the surface of aircraft skin. It can also be seen directly from the cross-section that different paint stripping methods have damaged the surface of aircraft skin. Therefore, it is very necessary to observe the cross-section of aircraft skin before and after paint stripping. In the cross-sectional elemental mapping, the thickness of the oxide film can be measured by comprehensively analyzing the distribution of aluminum and oxygen. The damage degree of oxide film was judged by the thickness change of oxide film before and after paint stripping.
The elemental mapping on the cross-section before paint stripping is shown in Fig. 13 a). Where the aluminum substrate, the oxide film and paint were included. The difference between weight percent of Al and O were large in the aluminum substrate region, the oxide film, and the paint. Comparing the elemental mapping in the three regions, highest atom percent of aluminum was found in the aluminum substrate, and lowest atom percent of aluminum was found in the paint. In the oxide film region the oxygen content was maximum. In the aluminum substrate region the oxygen content was minimum. At the line L1, mappings of Al at% show a sudden increase while that of the oxygen shows a sudden increase, particularly Al. This indicates that line L1 is the interface of the paint and the oxide film. At the line L2, mappings of Al at% show a sudden increase while that of the oxygen shows a sudden decrease. This indicates that line L2 is the interface of the aluminum alloy substrate and the oxide film. Hence the thickness of the oxide film at the aircraft skin before paint stripping is 2.42 µm.
Fig. 13. EDS elemental mapping in subsurface cross-sections of aircraft skin, a) before paint stripping, b) mechanical paint stripping and c) laser paint stripping.
The elemental mapping on the cross-section after MPS is shown in Fig. 13 b). No painted region can be observed at the cross-section. At the line L3, mappings of Al at% show a sudden increase while that of the oxygen shows a sudden increase. This indicates that line L3 is the interface of the vacuum and the aluminum alloy substrate. There is no oxide film in the MPS sample. The oxide film was removed during the MPS process.
The elemental mapping on the cross-section after LPS sample is shown in Fig. 13 c). At the line L1´, mappings of Al at. % show a sudden increase while that of the oxygen shows a sudden increase. This indicates that line L1´ is the interface of the vacuum and the oxide film. At the line L2´, mappings of Al at. % show a sudden increase while that of the oxygen shows a sudden decrease. This indicates that line L2´ is the interface of the aluminum alloy substrate and the oxide film. Hence the thickness of the oxide film at the aircraft skin after LPS is 2.10 µm. Compared with before LPS, the thickness of the oxide film is reduced 0.32µm.
3.5. Electro-chemical analysis
The corrosion resistance of aircraft skin will decrease after the oxide film is damaged. The integrity of the oxide film can be analysed from the corrosion resistance. Thus, it is necessary to test the corrosion resistance of aircraft skin after paint stripping. Aluminum alloy has a high tendency of localized corrosion, and the electrochemical reaction dominates the corrosion process [42]. It is very suitable to analyze the corrosion behavior of aircraft skin by electrochemical test.
Aircraft skin were depainted by different paint stripping methods and the PPC were plotted and compared as shown in Fig. 14. The polarization parameters were fitted by Tafel function as shown in Table 1. The corrosion current density of aircraft skin is 6.66×10−5 mA·cm−2 after LPS and 3.66×10−2 mA·cm−2 after MPS. The rate of corrosion corresponds to the thickness of the alloy corroded per year. The rate of corrosion is linear in the current density. In the aspect of the corrosion dynamics, the larger the corrosion current density, larger the rate of corrosion would be and hence the worse corrosion resistance would be [43]. The corrosion current density of aircraft skin after LPS was in agreement with existing studies [44,45], while the corrosion current density of aircraft skin after MPS is two orders of magnitude lower than that of the existing research. The corrosion resistance of aircraft skin after LPS has no obvious change, while the corrosion resistance of aircraft skin decreases greatly after MPS.
Table 1. Corrosion characteristic parameters for aircraft skin after paint stripping in unpurged and quiescent 3.5wt% NaCl solution
The thickness of the oxide film is 2.10 µm after LPS as shown in Fig. 13, which is nearly undamaged. After LPS, the oxide film on the aluminum alloy surface is less damaged and the oxide film integrity is better, making its corrosion performance consistent with existing research. The oxide film did not exist anymore after the MPS. Comparing to the MPS sample, the surface corrosion current density of the LPS aircraft skin sample was decreased by 90.83%.
4. Conclusions
Both mechanical and LPS on the 2024 aluminium alloys aircraft skin was compared on four main aspects: the surface morphology, microstructure, chemical composition and corrosion resistance.
- 1. 1) Due to the low consistence between the energy input and the motion trace, the paint removal is not uniform enough. Also, the large quantity of scratches indicate that the substrate material was damaged a lot. On the other hand, by using a laser with pulse width of 150 ps, the energy input and the motion trace possesses a better consistence, hence the substrate is hardly damaged.
- 2. 2) As incineration occurs in the process of LPS, carbon and hydrogen contained in the paint reduced part of the alumina film and hence the oxygen content decreased in the sample surface. Under the effect of the counter-acting force to the thermal expansion and evaporation, arrayed micro and sub-micro structures were formed. After LPS, the surface of 2024 aluminium alloys aircraft skin is covered with Al2O3.
- 2. 3) The thickness of the oxide film on LPS samples was 13.22% thinner comparing to non-paint stripping samples. However, the integrity of the oxide film was still maintained. Therefore, the corrosion resistance is 99.82% higher comparing to MPS samples with hardly alumina film remained. The corrosion voltage was increased significantly and the corrosion current density is only 4.33% of the MPS sample.
Funding
National Key Research and Development Program of China (2017YFB1303720201); Sichuan Science and Technology Programs (2019JDRC0130, 2020JDRC0047, 2020ZDZX0002).
Acknowledgements
Thanks to Qiang Peng of Sichuan Xiye New Materials Limited Liability Company for his help in the experiment!
Author contributions
Hui Chen and Feisen Wang designed the experiments; Feisen Wang, Qian Wang, Haiqi Huang, Yinfen Cheng and Sifei Ai performed the experiments; Hui Chen, Lihua Wang, Chuang Cai, Haiqi Huang and Feisen Wang contributed to the interpretation of the results; Feisen Wang prepared the original draft; all authors edited the draft and contributed to the final manuscript.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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