The corrosion behavior of cu-al layered composites was investigated by laboratory neutral salt spray corrosion experiments in simulated atmospheric environment. The interface morphology of copper and aluminum was observed by scanning electron microscope (SEM) , and the corrosion products were analyzed by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) , the corrosion mechanism of cu-al composite plate in service was investigated. The results show that the corrosion cell is composed of Cu and Al, Al is anode and cu is cathode, the larger the area ratio of cathode and anode, the larger the corrosion rate. With the continuous corrosion, the Cu side of the Cu-al interface is corroded, and the corrosion near the interface is the most serious. The corrosion products were Al2O3, Al (Oh)3 and Alo (Oh) . The electrochemical results showed that the corrosion rate of cu-Al composite plate increased first, then decreased and then increased. Key words: copper-aluminum Layered Composite Plate; Salt Spray Corrosion; electrochemistry; weight loss; corrosion product copper-aluminum composite plate as a new composite material, it has the advantages of light weight of aluminum, low contact resistance of copper and economy, beautiful appearance, good conductivity and high thermal conductivity, it is widely used in automobile, electronics, electric power, electrical appliances, petroleum, chemical industry, shipbuilding, metallurgy, machinery, aerospace, household appliances, atomic energy and other industrial fields [1-3] . The corrosion types of cu-al composite plates in atmospheric environment for a long period of time are often various, because the galvanic couple is formed between different materials, and in the process of corrosion, they are often affected by the corrosive ions in the medium, therefore, the corrosion mechanism is relatively complex, and the common corrosion types involved include galvanic corrosion, pitting corrosion, crevice corrosion, overall corrosion, chemical corrosion and electrochemical corrosion, etc. , it can protect the film, but there are often local defects on the surface. When there are active ions (Cl, F, Br, I, At) and Ligands (OH-in water) which can destroy the passivation film, local damage of the passivation film can be easily caused, with the extension of service time, copper-aluminum materials suffered a strong damage effect [7-9] . In addition, the interface between copper plate and aluminum plate formed intermetallic compounds due to the diffusion of elements during the preparation process. These intermetallic compounds have certain influence on the corrosion of the composite plate. The results show that there are many intermetallic compounds, such as Cual, CuAl2, CuAl4 and Cu9Al4, which can enhance the adhesion of dissimilar metals but weaken the mechanical properties of the materials. Feng li-chen et Al. [13] think that stress concentration is easy to occur at the edge of intermetallic compound layer, and then crack source is produced, and corrosion is accelerated by corrosive ions. However, it was found that the intermetallic compounds in the copper-aluminum brazed joints were high potential cathodic phase and were not easy to be corroded. Especially when copper and aluminum constitute a couple, the couple corrosion of other corrosion types also have a certain promotion. Zhao Yan et Al. [15] considered that the corrosion resistance of CCL was worse than that of pure copper because of galvanic corrosion. Chen Guohong et Al. [16] studied the corrosion and fracture of copper-aluminum transition line clamp in 220 kV substation, and considered that the corrosion product of weld caused by the bottom of copper-aluminum transition line clamp was due to the electrochemical reaction between Cu and Al, thus, a primary battery with aluminum as negative electrode and copper as positive electrode was formed, which accelerated crevice corrosion of aluminum. The corrosion behavior of aluminum alloy in NaCl solution was observed by combining AFM with SECM, it is found that the initial pitting of aluminum alloy is caused by the preferential corrosion of the anode intermetallic compound in the matrix or the local corrosion of the metal body adjacent to the intermetallic compound, the corrosion deepens. Atmospheric exposure corrosion test is the most accurate and reliable method to study atmospheric corrosion, but it has strong regional character and long experimental period. In this paper, the corrosion behavior of cu-al composite plate with different area ratio of cathode and anode under the action of chloride ion was studied by simulating atmospheric corrosion and conducting neutral salt spray experiment in laboratory to shorten the experimental period, and the effect of corrosion on interfacial bonding was judged, the influence mechanism of interface layer corrosion on bonding failure is obtained, which lays a theoretical foundation for obtaining the service condition constraint standard of composite plate under corrosion condition. 1 The experimental material is made of copper-aluminum laminated composite plate and processed into 20 mm × 20 mm × 6.6 m and 20 mm × 20 mm × 3 mm specimens, as shown in Fig. 1. There are three diffusion layers at the interface of the two plates, which are composed of Alcu, Al2Cu and Al4Cu9. The copper-aluminum interface was polished step by step with 600 to 2000 size sandpaper and polished until the surface was bright and uniform. After cleaning with alcohol, blow-drying and drying in a drying dish for 48 hours, the actual size of the sample was measured by Vernier scale, the original mass of the sample was weighed by an electronic balance with an accuracy of 0.001 MG.

The neutral salt spray corrosion test was carried out on DG-170211 salt spray corrosion machine according to GB 6458-86. The samples were suspended by cotton rope in a salt fog corrosion chamber at 35 °C for 24 h, 144 h and 240 h, respectively, and the concentration of 5% (mass fraction) NACL solution was determined. After salt spray test, the corrosion products and deposited salt were scraped off the surface, cleaned and dried by alcohol, then weighed for 48 hours, and finally calculated and tested. The experiment was carried out in the VSP300 electrochemical working station. Saturated Calomel was used as the reference electrode, platinum wire was used as the opposite electrode, the working electrode, namely the copper-aluminum composite plate, was coated with hot melt glue except the measured surface, which was the interface of copper and aluminum, the polarization curves were scanned from-1 to 1 v, the scanning speed was 10 mV/s, and the electrochemical impedance frequency was from 105 to 10-2 Hz, the surface and corrosion products of copper-aluminum composite plate were analyzed by S-3400 scanning electron microscope (SEM) and 132-1D EDX energy spectrum (EDS) . The corrosion products were analyzed by XRD-7000 X-ray diffraction (XRD) . The scanning angle was 10 ° ~ 90 ° , the scanning speed is 8 °/Min. The corrosion weight loss is calculated by formula, where W 1 is the post-corrosion weight, G, W 0 is the pre-corrosion weight, G, s is the area of the sample exposed to salt spray, m 2. Results and analysis 2.1 in order to study the effect of the area ratio of cathode to anode (the ratio of the area of Cathode to the area of anode to aluminum) on the corrosion resistance of cu-al composite plate, the corrosion resistance of cu-al composite plate was measured by electrochemical method, the polarization curves and electrochemical impedance spectra of cu-al composite plates with area ratios of 0.16 and 0.18 were obtained by immersion in NACL solution. Figure 2 shows the measured polarization curve. The self-corrosion current density and self-corrosion potential calculated by Tafel extrapolation method are shown in Table 1. The higher the self-corrosion current density is, the faster the corrosion rate is, the worse the corrosion resistance is. From Table 1, it can be seen that the corrosion current density of cu-al composite plate with an area ratio of 0.16 is 19.2 ΜA cm-2, and that of cu-al composite plate with an area ratio of 0.18 is 36.8 ΜA cm-2, the corrosion current density of pure aluminum is 1.852 μa cm-2. The corrosion resistance of aluminum is reduced by the couple of cu-al composite plate. The self-corrosion current density increases with the increase of the area ratio of cathode and anode

The NYQUIST diagram is composed of a capacitive arc. The Phase Angle Diagram in the bode diagram shows that the copper-aluminum composite plate is a time constant, and the pure aluminum is two time constants, the equivalent circuit diagrams as shown in figures 4A and B are fitted by the software Zsimpwin. The fitted data are shown in Table 2. The RS is the solution resistance, the capacitance is replaced by the constant phase angle element, and the CPEDL represents the double layer capacitance on the surface of the working electrode, rB represents the resistance of corrosion product layer and RCT represents the charge transfer resistance of working electrode surface. The smaller the charge transfer resistance is, the worse the corrosion resistance is. From the fitted data in Table 2, it can be seen that the Rct of cu-al composite plate with an area ratio of 0.16 is 1421ω CM2, and the Rct of cu-al composite plate with an area ratio of 0.18 is 219ω CM2, indicating that with the increase of the area ratio of cathode to anode, the charge transfer resistance of the working electrode decreases, the corrosion resistance of the material decreases, and the charge transfer resistance of pure aluminum is the largest. It can be seen from the impedance curve in Bode Diagram that the impedance value of pure aluminum is the highest, and the impedance value decreases with increasing the area ratio of cathode and anode, which also indicates that the corrosion resistance of cu-al composite plate decreases.

Figure 5 shows the polarization curves of cu-al composite plates after salt spray corrosion in different time. The corresponding fitting data are shown in Table 3, the corrosion current density increases first, then decreases and finally increases, because before the corrosion begins, the surface of the material is covered with a passivation film to protect the substrate from the corrosion solution, and the corrosion resistance of the material is better. When the corrosion develops to 24h, the oxide film on the surface of the material is destroyed by corrosion, and the contact area between the exposed matrix and the corrosion solution increases, which makes the corrosion resistance of the material worse. When the corrosion process reaches 144H, a new corrosion product is formed on the surface of the Matrix, which prevents the corrosion solution from further corroding the Matrix and improves the corrosion resistance. When the corrosion time reaches 240h, the corrosion product is loose and porous, which makes the Matrix contact with the solution again, and the corrosion resistance of the sample decreases again.

Fig. 6 shows the electrochemical impedance spectra of cu-al composite plate after different salt spray corrosion time. As can be seen from Fig. 6A, the impedance spectra are all composed of a capacitive arc, and the radius of the capacitive arc is increased with the corrosion time, it shows that the corrosion resistance of the material increases first, then decreases and then increases with the increase of salt spray corrosion time. Figure 6B shows the phase angle diagram as a time constant. The data for the equivalent circuit Diagram (figure 4) is shown in Table 4 with the ZSIMPWIN software. It can be seen from table 4 that Rct of cu-al composite plate without salt spray corrosion is 1.0 × 104ω/cm2, Rct decreases to 1.9 × 103ω/cm2 after 24 hours of salt spray corrosion, and rises to 7.6 × 103ω/cm2 after 144h of salt spray corrosion, rCT was again reduced to 3.0 × 103 Ω CM 2. It can be seen that the charge transfer resistance decreases first, then increases and finally decreases with the increase of the corrosion time, and the impedance value in the baud chart shows the trend of decreasing first, then increasing and finally decreasing, the results are in good agreement with those obtained from polarization curves.

2.2 The corrosion microcosmic appearance and the corrosion product composition analysis of the copper-aluminum interface corrosion microcosmic appearance after the salt spray corrosion of different time is as shown in Fig. 7. It can be seen from the figure that the corrosion is concentrated on the AL side, and the Cu side has little change, and the corrosion is most serious at the interface. At 24h of corrosion (Fig. 7B) , a long and narrow corrosion pit with uniform width and nearly 500μm appears on the aluminum side, and the Matrix is honeycomb-like at the corrosion pit, while the white bulk corrosion products are distributed along the copper-aluminum interface on the aluminum side When the corrosion time is 144h, the area of the corrosion pit becomes larger, and a new crazing corrosion product can be seen in the corrosion pit, the exposed aluminum substrate has a larger area. Fig. 8 is the composition analysis of the corrosion product. Fig. 8A is the XRD result of the corrosion product. The results show that the composition of the corrosion product is Al2O3, Alo (Oh) and Al (OH)3. It can be seen from the energy spectrum analysis of Fig. 8B that in addition to Al and O, there are some Cl in the corrosion products in the corrosion pit, which shows that Cl-is the main factor causing the corrosion of cu-Al composite plate.

2.3 corrosion kinetics Fig. 9 shows the kinetics curves of continuous corrosion of cu-al composite plates in 5% NACL salt spray for different time. The curve equation fitted by origin software is shown in Fig. 2, where R2 = 0.9962, it shows that the fitting effect is good. The fitted equation shows power exponent form: W = ABN, where the size of n represents the effect of corrosion products on corrosion. When n < 1, the corrosion products can inhibit the corrosion process, the results show that the corrosion products can promote the corrosion process. The corrosion kinetics equation of cu-al composite plate in salt spray corrosion is 1.55359, which indicates that the corrosion products can promote the corrosion of cu-al composite plate.

2.4 The salt spray corrosion mechanism of Cu-Al composite plate Fig. 10 is the schematic diagram of salt spray corrosion of Cu-Al composite plate. When Cu and Al are combined, the potential difference between the two metals forms a couple, and the potential of Cu is higher than that of Al., therefore, aluminum is the anode and Cu is the cathode in the corrosion process, so the corrosion of Al is serious, and the corrosion of Cu side is almost non-corrosion. The chemical equation involved in the corrosion process are:

At the beginning of corrosion, the corrosion rate is slow because the metal surface is covered with a dense passivation film to protect the substrate from corrosion solution, the corrosion rate is accelerated because of the contact between the substrate and solution, the pitting on the aluminum substrate and the new corrosion products, and the corrosion of Al near the interface between copper and aluminum is serious, the small pitting corrosion pits on the aluminum substrate gradually expand, the corrosion pits near the copper-aluminum interface connect to form larger and deeper corrosion pits, the corrosion products increase, in addition to the block-like corrosion products, there are also cracks-like corrosion products, which hinder the contact between the substrate and Cl-, the corrosion rate slows down and the reaction equations involved are:

Therefore, the corrosion process is a cyclic process, until the aluminum matrix is gradually stripped away. Conclusion (1) the corrosion of Cu-Al composite plate is accelerated by the electric couple in the process of corrosion. After the corrosion of Cu-Al composite plate by salt spray, the corrosion mainly occurs on the side of Al and almost no change on the side of Cu, the corrosion resistance of the material becomes worse. (2) because the corrosion of cu-al composite plate is a cyclic process, including the dissolution of Matrix, the formation and detachment of corrosion products; The results of electrochemical detection showed that the corrosion resistance of cu-al composite plate increased first, then decreased and then increased. (3) the weight loss curve of cu-al composite plate accords with power exponential equation w = 1.948 × 10-5t3.17156. The corrosion products can promote the corrosion process. The corrosion products were Al2O3, Alo (Oh) and Al (OH)3, Cl-, which caused pitting corrosion of the material, and the corrosion of aluminum matrix was most serious near the interface, and then reacts with Cl-to Break Off repeatedly. 

Source: Chinese Journal of Corrosion and protection