1 Introduction

Al is a popular metal used in industrial applications [1,2,3]. This is owing to several factors, including a massively strong argument for mass breaking, corrosion resistance, a low specific gravity 2.72, lightweight, and ease of fabrication. Al in its purest form is a brittle yet malleable metal. When stressed, Al deforms three times more than steel, and the casting procedure has not addressed this issue [4,5,6,7,8]. Al alloys, unlike steel and titanium, are extremely strong while retaining their ductility [9,10,11]. Stress annealing or cold working can significantly improve the strength of this and other Al alloys. Al alloys can be strengthened with several constituents [12,13,14,15]. Whereas in 7xxx series compounds, Zn is the predominant alloying component in amounts ranging from 1 to 8%, resulting in a high-strength, heat-treatable alloy when combined with a reduced proportion of Mg. Other elements, like Cu and Cr, can be used in tiny amounts in aircraft structures and portable devices. Because of its resistance to deterioration and fracture, alloy 7xxx has shown to be more successful in resisting stress corrosion and has been used more frequently at high temperatures [16,17,18,19,20]. Several corrosion protection methods, such as organic or inorganic antirust and metal coatings, are used to build defensive coatings that restrict the rate of corrosion and extend the life of these alloys. Among these helpful coatings are polymers, which have a high potential to protect metals against corrosion [21,22,23]. When using polymers, it is recommended that they have advantages such as the ability to form a strong physical barrier and that they be strengthened with an additional PU coating class, NC. Both PU and NC are based on epoxy PA to ensure good adherence between the substrate and the final coat [24,25,26]. The most important of these coatings, PU, uses a high-strength aliphatic coating with low VOC. As a result, PU coatings are recommended for regularly used coating systems used to protect steel buildings in very corrosive conditions other than seawater [27, 28]. As a result, the purpose of this study was to assess different polymers and their effects on preventing stress corrosion in the Al–Zn 7075 alloy used in the aircraft frame construction [29].

2 Experimental

The alloy used is Al–Zn 7075 with a thickness of about 0.5 mm, with a submerged area of about 1 cm2. PA, NC, NaCl, and PU were used as materials to investigate in this article. The alloy selected is subjected to an XRF analyzer Thermo Scientific Niton XL3T regulating the % of each metal. Chemical analysis of Al 7075; Zn 5.70, Mg 2.40, Cu 1.60, Mn 0.30, Si 0.40, Fe 0.50, Cr 0.20, Ti 0.20, Al 88.55 and 0.15% total as mentioned in Table 1.

Table 1 Elemental analysis of Al 7075 alloy
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The Al–Zn 7075 alloy samples were plotted by applying acetone as a degreaser to remove oil or imprint on the samples, to ensure proper bonding of the applied layer. The electrochemical experiments are carried out using a normal electrochemical cell used a computer-aided Autolab potentiostat/galvanostat device PGSTAT302N is a three-electrode cell at room temperature. The saturated calomel electrode was used as a reference electrode, and the counter electrode was fixed with platinum foil. Coated and uncoated work electrodes were used for the Al alloy samples. The working electrode area of about 1.0 cm2 with and without a coat was exposed to 3.5% NaCl. The OCP was recorded for at least 30 min before each experiment. And the frequency range over AC from 0.1 to 105 Hz with a low-to-low sine wave of ± 10 mV as the excitation signal. Both polarization experiments were performed at a scanning speed of 1.0 mV at 1 and laboratory temperature within the potential range from − 0.8 to 0.1 V. The surface topography of the coated Al in the normal and stress-corrosive condition is performed in saline media after 12 h immersion via a JEOL JAX-840A SEM electron microscope analyzer [30].

3 Results and Discussion

3.1 Open Circuit Potential Measurements

The gap in free corrosion potentials in 3.5% NaCl with time for uncoated and PA, NC, and/or PU-coated Al7075 is present in Fig. 1a. The stable OCP values in both the untreated and treated Al7075 were obtained after 30 min of immersion. For the coated sample a positive variation of the OCP was observed in respect of the untreated specimen in the corrosive medium. The steady OCP for uncoated and NC, PA, and/or PU-coated are − 790 mV, − 740, − 680, and − 577 mV, respectively. This conduct designates that; the coated PU is the highest positive potential shift. So it is the effective polymer for inhibition of the 7075- alloy relative to PA and NC. Under stress, it was observed that the possible potential of steady-state for the untreated alloy was shifted to more negative values. The OCP for 100,150 and 175 g m−2 are − 720, − 742 and − 865 mV as seen in Fig. 1b.

Fig. 1
figure 1

The recorded OCP throughout immersion in 3.5% NaCl solution for a corrosion and b stress corrosion of uncoated Al 7075 alloy at the ambient temperature

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3.2 Measurements of Potentiodynamic Polarization PD

Figure 2a displays the measurements of PD curves of untreated and treated Al–Zn 7075 alloy in 3.5% NaCl at lab temperature, with a scanning rate of around 1 mV s−1 varying from − 0.8 to 0.1 mV from the OCP values. All curves tend to show the same general characteristics. The cathodic region shows a restrictive current density connected with the diffusion-controlled oxygen reduction reaction. Table 2 lists the parameters of PD such as icorr current density, Ecorr corrosion potential, Rp anodic βa, and cathodic βc Tafel slopes. The Ecorr and icorr principles were gotten by extrapolating the linear portion of both Tafel curves and the inhibition efficiencies were calculated [31]. The inhibition efficiencies for NC, PA, and PU are measured in the following order 65, 92, and 96%, respectively. The uncoated Al alloys Icorr was 1.19 × 10−5A cm−2. The Icorr was moved to 8.47 × 10–6, 2.06 × 10–6, and 4.38 × 10–8 A cm−2 for NC, PA, and PU coating. Crosslink capability of PU results from containing two or more groups of isocyanates. The PU coating demonstrated the greatest anti-corrosion capabilities compared to PA and NC coating, as indicated by its highest and lowest values for Rp and Icorr, respectively. PA and NC matrix has low coating resistance and easy design of conductive. However, at a consistent time, filling the micropores of the polymeric matrix influence the barrier effect which gives the high protective features of the coating. Figure 2b shows uncoated Al alloy behavior under 100,150 and 175 gm cm−2 stresses. The stress corrosion potentials were − 0.983, − 1.16, and − 1.04 V, respectively. This stress 175 g m−2 was then loaded to the coated alloy as publicized in Fig. 2c. The potential for uncoated, NC, PA and, PU-coated samples were − 1.041, − 1.039, − 1.173, and − 1.12 V, respectively. The parameters of corrosion results are listed in Table 2. It is found that the current density shifted to less value 6.24 × 10–7 A cm−2 and the resistance increased for PU coating. It indicated that the most protective coating is PU because it forms a shield between the corrosive species and the metallic superficial [32,33,34,35].

Fig. 2
figure 2

The recorded potentiodynamic polarization curves of a corrosion, b stress corrosion of uncoated and c stress corrosion of 175 g cm−2 of uncoated and coated Al 7075 alloy in 3.5% NaCl, at ambient temperature and the scan rate of about 1 mV s−1

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Table 2 Electrochemical parameters and inhibition efficiency of corrosion and stress corrosion for coated and uncoated Al7075 alloy in 3.5% NaCl
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3.3 Electrochemical Impedance Measurements (EIS)

Figure 3a demonstrates the semi-circular for uncoated sample indicates the corrosion process was regulated by the charge transfer resistance. However, in the attendance of polymeric paint, the increased semicircular is regarded as an indicator of the Rp between the metal and the external Helmholtz plane. The Nyquist plot of PU coating was nearly semicircle arcs. We found that the charge transfer resistances of the samples, as determined by the intersection of the low-frequency end of each semicircle arc with the real axis, were 3 × 106 Ω cm2. However, the EIS semicircles rely on numerous considerations such as; coupling effect, current density distribution, electrode surface uniformity, surface electrode roughness, and the physical and/or chemical nature of conductor substances [32]. Plots of Bode Fig. 3b, c break up into three sections. The first one represents the solution resistance Rs at HF. The second part, in the medium frequency region, represents a capacitive behavior but not an ideal capacitor. In the last part at LF, the electrode resistance increases, and |Z| values do not depend on the frequency for uncoated and PU coating. The uncoated Al alloy is assigned to the Al2O3 layer resistance. The results display the untreated and treated Al alloy phase angle diagram as exhibited in Fig. 4a, b. The wide peak of high value 80° observed on the phase bode plot at PU coating is indicative of high corrosion protection at higher frequencies. For stress corrosion, the same tools were performed. Figure 4c shows the Nyquist plots for the untreated sample in NaCl. It is noticed that the impedance declines with increasing the stress from 100 to 175, and the diffusion appears in 175 stresses and dispersion suggests that, the start of metastable pitting [36,37,38].

Fig. 3
figure 3

Effect of coat on the a Bode plots of electrochemical impedance spectra of Al 7075 alloy in 3.5% NaCl solution. b, c Nyquist plots of electrochemical impedance spectra of Al 7075 alloy in 3.5% NaCl solution at ambient temperature

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Fig. 4
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Effect of coat on the a, b phase of electrochemical impedance spectra of Al 7075 alloy in 3.5% NaCl solution. c Bode plots of electrochemical impedance spectra of stress corrosion of Al 7075 alloy in 3.5% NaCl solution at ambient temperature

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Furthermore, due to microscopic surface irregularity and the existence of a spongy weathering creation film. Figure 5, the phase plot illustrates that the phase angle decreases from the blank sample to the 175-stress sample from 75 to 180. The plots of coated alloy as exhibited in Figs. 5, 6 represent that the Nyquist and bode plots the impedance increase in order NC, PA then PU. And Fig. 7 shows the phase value of PU-coated equal 60 and it is higher than other polymers but still lower than without stress. The organic coatings thus acted as efficient natural walls to avoid the attacked ions from infiltrating the coat into the electrolyte to cause alloy corrosion [39].

Fig. 5
figure 5

Effect of coat on the Nyquist plots of electrochemical impedance spectra of stress corrosion of Al 7075 alloy in 3.5% NaCl solution at ambient temperature

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Fig. 6
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Effect of coat performance on the a, b phase of electrochemical impedance spectra of stress corrosion of Al 7075 alloy in 3.5% NaCl solution. c Bode plots of stress corrosion of Al 7075 alloy in 3.5% NaCl solution at 175 g cm−2 at ambient temperature

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Fig. 7
figure 7

Effect of coat performance on the Nyquist plots of electrochemical impedance spectra of stress corrosion of Al 7075 alloy in 3.5% NaCl solution at 175 g cm−2 at ambient temperature

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R1 and R2 in upper case: are the outer and inner layer resistance of Q1 and Q2 in μF cm−2, respectively. ω is the angular frequency, J = About − 1: the imaginary component n1 and n2 is the non-ideal constituent of the outer and inner layer capacities, respectively.

The diffusion process may be initiated by the diffusion of Al3+ cations from the metallic superficial to the wholesale environment under stress, which is created by the anodic reaction. EIS spectrum was fitted using a two-time measurements electrical equivalent circuit as displayed in Fig. 8a. Rs corresponds to the solution resistance; R1 is the film resistance, C1 is the capacitance of the coating film, R2 is the Rp charge transfer, C2 is the double-layer capacitance. The barrier properties of the coating can be evaluated through the pore resistance values R1 obtained from modeling electrochemical impedance diagrams. The resistance of pores indicates the ease with which the electrolyte penetrates inside the coating see Table 3. The n component is compensate for the non-ideality of the roughness of the surface, the adsorption, and the incomplete homogeneity [40].

Fig. 8
figure 8

Effect of coat performance on the phase of electrochemical impedance spectra of corrosion and stress corrosion of Al 7075 alloy in 3.5% NaCl solution at 175 g cm−2 at ambient temperature

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Table 3 Electrochemical impedance parameters and inhibition efficiency of corrosion for coated and uncoated Al 7075 alloy in 3.5% NaCl
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The equivalent circuit was used to fit the (a) corrosion behavior and (b) stress corrosion behavior of uncoated and coated Al7075 alloy immersed in 3.5% NaCl solution. Rs represents solution resistance used in the test, CPE is constant phase angle element, Rct and W is charge transfer resistance, and Warburg impedance, respectively. The corresponding circuits Fig. 8a, b were used to match the untreated and treated Al7075 alloy’s stress corrosion behavior plunged into a 3.5% NaCl solution. Rs characterize the solution resistance, CPE is the constant phase angle part, Rct and W are the resistance to transfer charges, and Warburg impedance, respectively. In general, the constant phase element CPE uses instead of the pure double-layer capacitor to give a more accurate fit in the equivalent circuit. The correct value of double-layer capacitance Cdl was determined at the frequency fmax, at which the imaginary element of the impedance [41]. It is clear that, is the solution resistance Rs values increased in presence of polymer coating. it refers to a decrease in the conductivity of the solution. The Rct values increased indicating the formation of protective. Table 4 demonstrates the higher resistance value for the covering is up to 2.69 × 103 KΩ cm2 with 99% efficiency.

Table 4 Electrochemical impedance parameters and inhibition efficiency of stress corrosion at 175 for coated and uncoated Al7075 alloy in 3.5% NaCl
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It can be strong-minded that the treatment of uncoated samples in corrosive media, therefore, increases the capacity of the Al7075 alloys surface layer and, in the circumstance of PU, the thinness of the transfer of charge decreases by 99% concerning its performance. However, in the attendance of polymeric paint, the increased semicircular is regarded as an indicator of the Rp between the metal and the external Helmholtz plane. This event undistinguished the shielding layer on the superficial of the Al7075 alloy and therefore reduces the dissolution cycle. Both EIS results and Tafel results confirmed the order: PU>PA>NC> uncoated [42].

3.4 Microstructure Characterization

Results exposed in Fig. 9a reveal that the polished sample of Al7075 electrode and the corroded sample in saline media is mentioned in Fig. 9b as well as the stress corroded sample at 175 g cm−2. It is identified that, underwent energetic dissolution, as designated by the superficial coarseness in the untreated specimens with the polymer as mentioned in Fig. 9c. It is beyond that, with no insignia of pitting attack, and the formed rust has existed via general corrosion [37]. Furthermore, the roughness of the Al surface is seen to decrease in the polymer-coated sample, which represents prevention or guideline of the influence of corrosive media on the superficial of Al outstanding to the creation of protective deposits as seen in Fig. 9d for NC, Fig. 9e for PA and finally Fig. 9f for PU coat. The PU reveals the highest performance for corrosion resistance in saline solutions in both corrosion and stress corrosion [43].

Fig. 9
figure 9

SEM images of the Al7075 a polished sample, b uncoated sample, c uncoated sample under stress conditions, d NC-coated sample, e PA coated sample, and f PU-coated sample under stress conditions in 3.5% NaCl solution

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4 Conclusions

This work describes the efficiency of sequences of coating materials of PU, PA, and NC in protection against corrosion. The Rp increased, while corrosion rate and corrosion current are diminished by PU coating, confirming that PU coating acts as an operative shield against deterioration of Al7075, suggesting the excessive possibility for application in industry. The investigational plots indicated that the crash tip-up tension rate and crack rapidity-based stress corrosion are restrained at the functional stress rate. In addition, the crack speed depended on the crack tip tension rate and the functional stress rate and was not predisposed in this study by the arrangement of the assessed Al sheet. It is also defined the consequences data of coated samples, that the rate of crack tip tension regulated the limiting rate of cracking from the stress corrosion of coated electrodes. The PU is recommended for application in coating the Al alloys to be protected in corrosive media since it is proved that gives a perfect resistant coat under stressed conditions.