Graphene oxide—Polymethyl methacrylate coatings for Corrosion protection of aerospace aluminium alloy 7075—T651 surfaces

The samples used in this study is Al 7075—T651 with standard composition [6]. It has Aluminium (Al) 87.1–91.4, Zinc (Zn) 5.6, Magnesium (Mg) 2.5, Copper (Cu) 1.5, Iron (Fe) 0.5, Silicon (Si) 0.4, Titanium (Ti) 0.2, Manganese (Mn) 0.3, Chromium (Cr) 0.25 and others 0.15 percentage of weight as composition.

Al 7075—T651 is purchased from the commercial market. It is cut into pieces of equal size and shape for our study. Samples are prepared with and without GO—PMMA coating respectively. PMMA is purchased from sigma Aldrich chemicals. GO is synthesized by modified hummer's method. 0.5 g and 0.25 g of graphite powder and NaNO₃ respectively are mixed with 12 ml of sulfuric acid. It is kept in an ice bath till temperature drops below 5 °C. Then temperature is increased to 35 °C and 1.5 g of KMnO₄ is added to the mixture. It is stirred for 2 h. After the condition of solution is changed to paste, 23 ml of distilled water is added. Then it is heated to 90 °C for 15 min and 80 ml of water is added. Then 3 ml of H₂O₂ is added to the mixture and the final solution in appears in yellow color. The precipitate is washed with HCl and with doubly distilled water five times. It is then dried in a hot air oven at 60 °C for 48 h. 30 mg of obtained GO is mixed with 0.25 g of PMMA in 5 ml acetone to get a high viscous solution for coating. The prepared solution is coated on the Al 7075—T651 alloy and made to dry in an oven for 1 h at 80 °C. Since, the coating is done by pouring drops over the samples, we could not say the thickness accurately. So we can say approximate coating thickness around 0.1 mm.

Corrosion is induced in the prepared samples using 3 M HCl. The prepared 3 M HCl is poured on all the samples for every half an hour, in atmospheric condition. Figure 1 shows the corrosion on uncoated Al 7075—T651 and GO—PMMA coated Al 7075—T651 sample. In this paper, codes are fixed to avoid repetition of words. 0 h, GP 0 h, 24 h, 48 h, 72 h, GP 24 h, GP 48 h, GP 72 h refers to corrosion free, corrosion free GO—PMMA coated, 24, 48, 72 h corroded uncoated sample, 24, 48, 72 h corroded sample with GO—PMMA coating respectively.

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XRD measurements are taken by Rigaku Ultima III X-ray diffractometer in the range of 10 to 90° with an interval of 0.05° using Cu Kα (λ = 1.54 Å). SEM study is done by Hitachi S-3000H scanning electron microscope. The Raman study is carried out in Horiba Jobin Yvon Labram HR Evolution Micro Raman spectrometer using laser of 532 nm wavelength and 5 mW power. The spectra are measured in the range of 50–500 cm⁻¹. Pulsed thermography is carried at a maximum of 6000 Watt/sec. A fixed flash tube and a movable reflector is used. The cable length is 16.5'. The modelling light wattage is 650 with 120 V. The sensor type is InSb. Waveband is 1.5–5.1 μm. The pixel resolution is 320 × 256 and the pitch is 30 μm. Aperture is F/3 with a frame rate of 1 Hz to 380 Hz. Input voltage given is 12 V DC. Power consumption is 30 W/25 W. I–V characteristic study is carried out using keysight B2901A source measuring unit by applying −5 to +5 V on the sample and measure the corresponding current.

The XRD measurements on the surface corrosion are taken from 10 to 90° range with an interval of 0.05°. Most intense peaks are analyzed instead of all the small shoulder peaks. There are several small peaks because of the oxide formation in the samples. Figure 2 shows the XRD measurements for 0, GP 0, 24, GP 24 and 48, GP 48, 72, GP 72. The peaks at 39, 45, 65, 78° corresponds to (111), (200), (220), (311) planes confirms the presence of aluminium in 0, GP 0, 24, GP 24 h samples respectively. Even though the samples 24 and GP 24 are corroded for 24 h by pouring HCl for every half an hour, there is no sign of oxide formation like Al₂O₃, α- Al₂O_3, γ- Al₂O₃ were identified in these samples. The peaks at 45, 65, 78° corresponds to (200), (220), (311) planes confirms the presence of aluminium in 48, GP 48, 72, GP 72 h samples respectively. The peaks at 21, 28, 30° corresponds to (110), (120), (004) planes in 48, GP 48, 72 h samples confirm the Al(OH)₃ formation respectively [21, 22]. The XRD measurements when compared between GO—PMMA coated and uncoated samples, it is observed that the intensity of peaks are high in 24, 48, 72 h samples. So, we conclude that GO—PMMA coating on GP 24, GP 48, GP 72 samples are acting as corrosion resistance, which is observed by the low intensity of peaks in these samples. Several small peaks are identified which are due to the cracks that are formed in the material due to the corrosion. Since aluminium is the major element in this alloy, the probability of Al(OH)₃ formation is high which is observed in the samples.

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Figures 3 and 4 shows the SEM images of 0 h, 24 h, 48 h, 72 h, and GP 0 h, GP 24 h, GP 48 h, GP 72 h samples respectively. The structure of aluminium is very like other metals. It is malleable and ductile due to its polycrystalline structure. After the corrosion process, there are many differences in the phases formed in the base metal with and without GO—PMMA coating. Figures 3(a), (b) shows the 0 h sample which is corrosion free [23, 24]. Figures 3(c), (d) shows the 24 h sample in which the aluminium hydroxide formation is minimum. Various magnification shows the phase formation in detail. Figures 3(e), (f) shows the 48 h in which the formation is even higher than the 24 h sample. It clearly indicates the progress of corrosion on the surface of the sample with respect to time. Figures 3(e), (f) with 500× and 300× magnification shows how high the corrosion is induced in the sample. Figures 3(g), (h) shows the 72 h in which the material gone worse than the previous sample because of the high rate of corrosion induced in the sample due to more exposure time. Comparison of all images in figure 3 shows the phases formed and the progress of the phases formed in the sample with respect to time due to corrosion is clearly shown. Figures 4(a), (b) shows the GP 0 h sample. The coating looks like a bubble-like structure and is uniform in the sample. Figure 4(b), higher magnification of 1000× shows even clearer. Figures 4(c), (d) shows the GP 24 h sample, when compared with 24 h sample shows an improvement in the attempt of making GO—PMMA coating a corrosion resistant one. The phase formation is not much dense as formed in 24 h sample. Figures 4(e), (f) shows the GP 48 h sample in which the oxide formation is much suppressed by the coating on the sample. It is clearly identified with various magnification sizes. Figures 4(g), (h) shows the GP 72 h sample in which phase formation is high due to high exposure time, because of coating the fusion of coating and phase formed is observed highly with various magnification. In overall comparison between figures 3 and 4, it is clearly identified that the attempt to make GO—PMMA coating as a corrosion resistant is achieved as the oxide formation is suppressed in coated samples.

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Figures 5(a), (b) shows the Raman data for 0 h, GP 0 h, 24 h, GP 24 h, 48 h, GP 48 h, 72 h, GP 72 h respectively. The Raman shift for 0 and GP 0 h are identified at 1342, 1593 and 1342, 1582 cm⁻¹ respectively [25, 26]. It confirms the presence of aluminium and GO in 0 h and GP 0 h sample respectively. In 24 h sample, a small peak and a strong peak are identified at 1645 cm⁻¹ and 3365 cm⁻¹ respectively. But in GP 24 h sample, a strong band at 3365 cm⁻¹ is only identified which denotes that it avoids noises at 1645 cm⁻¹ region which act as corrosion resistant medium. In 48 and GP 48 h sample a strong band is identified at 3377 cm⁻¹ regions. GP 48 h avoids the peaks formed at 1366 and 1666 cm⁻¹ in 48 h, indicates it is acting as a corrosion resistant medium. In 72 and GP 72 h sample a strong band is identified at 3437 cm⁻¹ regions. GP 72 h avoids the peaks formed at 1354 and 1642 cm⁻¹ in 72 h, indicates it is acting as a corrosion resistant medium. In this alloy, Aluminium is the major constituent, Zinc is the second major element. Since aluminium is around 90% of material, oxide formations like Al₂O₃, Al(OH)₃ is highly probable. In all the samples, a strong band is identified at 3300–3400 cm⁻¹ region which denotes the formation of Al(OH)₃ in all the samples due to the corrosion induced in the material [27]. It is also observed that when comparing between GO—PMMA coated and uncoated sample, the intensity of peak is more in uncoated sample than GO—PMMA coated sample. It denotes that the GO—PMMA coated layer is acting as a corrosion resistant layer which is the main aim of this work.

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I–V characteristics study gives the behavior of the material within the electric circuit with respect to the voltage applied. The circuit diagram is shown in figure 6(a).

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Figure 6(b) clearly indicates the shift in the curve between 24 h, 48 h, 72 h samples. Figure 6(c) shows the variation in the curve for GP 24 h, GP 48 h, GP 72 h samples. The variation is because of the oxide formation which is formed on the surface. When time prolongs it starts to be worn out which results in a shift in current because of the pitting formed that is confirmed in SEM. When time prolongs it starts to be worn out which results in a shift in current [28]. It is observed from 24 h to 72 h samples. In figure 6(c), because of GO—PMMA coating on the samples, the oxide formation is not much observed on the samples as like previous samples. There is no much difference in profile but with variation in current scale. It shows passivation behavior of GO—PMMA coating, which resists oxide formation in the samples. Thus, we come to conclude that more compact filler matrix GO—PMMA coating used in our work reduced much oxide formation to act as corrosion resistant layer.

In pulsed thermography, the heat pulse from the flash lamb reaches the material and penetrates through it. There are two modes in thermography, namely transmittance and reflectance mode. We have used transmittance mode of thermography for our study. The diffusion of heat through the material is recorded. If there is no defect, the temperature decreases uniformly. Instead if there is any defect, there will be changes in the temperature variation pattern accordingly. We have calculated temperature difference between defect area (corroded area) and non-defect area (uncorroded area), material loss and running contrast to show the variation in the temperature decay pattern between the accelerated corrosion in both GO—PMMA coated and uncoated sample. The thermal conductivity of the Al 7075—T651, k is 130 W m⁻¹ k⁻¹ , specific heat capacity, C is 960 J kg⁻¹ k⁻¹ , and density ρ is 2810 kg m⁻³ . The heat pulse given is 6 KJ for 2 ms for every sample. The dimensionless Fourier number for the heat transfer is ${F}_{0}=\,\propto \tau /{L}^{2}$ [29]

$$\begin{eqnarray*}&&{{\boldsymbol{F}}}_{{\bf{0}}}=4.82\times {10}^{-5}\times 0.002/\left(6\times {10}^{-3}\right)={\bf{0.000016}}\end{eqnarray*}$$

The diffusivity ( α ) is the ability of a given material to transmit a temperature variation.

$\propto \,=\,k/\rho \,C$ where C is the specific heat and it is calculated as [30–32]

$$\begin{eqnarray*}&&\propto \,=\,130/\left(2810\times 960\right)={\bf{4}}.{\bf{82}}\times {\bf{1}}{{\bf{0}}}^{-{\bf{5}}}\,{{\bf{m}}}^{{\bf{2}}}\,{{\bf{s}}}^{-{\bf{1}}}\end{eqnarray*}$$

The temperature variation pattern is shown in figure 7

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From figure 7(a), it is observed that the after the initial high heat pulse, the temperature continuous to follow a steady state. The temperature reaches a steady state at 31.2, 27.9, 27.4, 27.3 °C in 0 h, 24 h, 48 h, 72 h respectively. Since 0 h is uncorroded sample, it transmits high heat but in case of corroded samples, the oxide formation reduces the temperature transmitted through the material. The more the oxide formation, the less the temperature is. From figure 7(b), it is observed that after initial high heat pulse, the temperature reaches a steady state at 30.6, 28.9, 27.9, 27.3 °C in GP 0 h, GP 24 h, GP 48 h, GP 72 h respectively. On comparing figures 7(a) and (b), we observed that because of more oxide formation very less temperature difference is identified after steady state in without coating samples but in GO—PMMA coated sample, the temperature difference is identified higher than that the uncoated samples which supports our work on using GO—PMMA coating as corrosion resistant layer. Here, because of less oxide formation, the temperature variation is identified higher. The temperature difference, running contrast, material loss are calculated by the following equations [29].

$$\begin{eqnarray*}&&{\rm{\Delta }}T={T}_{d}-{T}_{nd}\end{eqnarray*}$$

$$\begin{eqnarray*}&&{C}_{run}={\rm{\Delta }}T/{T}_{nd}\end{eqnarray*}$$

$$\begin{eqnarray*}&&\delta ={C}_{run}/1+\,{C}_{run}\end{eqnarray*}$$

${\rm{\Delta }}T$ indicates the temperature difference, ${T}_{nd}$ denotes temperature in non-defect area (corrosion free area), ${T}_{d}$ denotes temperature in defect area (corroded area), ${C}_{run}$ indicates running contrast, $\delta$ indicates material loss.

In figure 7(c), the material loss for 24 h, 48 h, 72 h varies from −0.12 to −0.14 but the material loss is less in GO—PMMA coated samples, GP 24 h, GP 48 h, GP 72 h, which varies from −0.06 to −0.12 in figure 7(d). Material loss is a serious factor in corrosion. The more the material loss is, the quicker the material will go worse. So, by using the GO—PMMA coating on the samples the material loss is reduced due to more compact filler-matrix interface. Thus the presence of passivation layer clearly indicates it is acting as a corrosion resistant layer.