| Aluminum alloys are commonly used light metal materials, widely applied in automobile, aerospace, machinery manufacturing, construction industry, electronics and other fields, due to its high strength-to-weight ratio and specific modulus, high electrical and thermal conductivity, excellent ductility and workability. However, the application of Al alloys is limited due to their poor corrosion and wear resistance. The electrode potential of aluminum is more negative, and the materials easily become corroded in many working environment, leading the component to failure, causing unavoidable economic loss.In this study, electroless nickel(EN) plating is adopted to modify the surface of LY12. Scanning electronic microscopy(SEM), X-ray diffraction spectrometer(XRD), and X-ray energy dispersive spectrometry(EDX), were used to characterize the surface morphologies, microstructure and compositions. According to the results,the proper pre-treatment and optimal EN bath were decided. Influences of various main parameters, such as plating time, temperature and pH on plating rate(R) and efficiency of EN were investigated. The value of R was calculated by the gravimetrical method. Different organic coatings were brushed to the electroless deposits, aiming to improve the corrosion resistance of composite coatings. The adhesion of the composite coatings was observed by scanning electronic microscopy, and the bonding strength was estimated by scribe method. The electrochemical behavior of composite coatings was assessed by potentiodynamic polarization and neutral salt spray test. After neutral salt spray test, the morphology of the samples was compared before and after corrosion, the type of corrosion destruction was observed. The distribution of elements was analysed by EDX for the inducement of corrosion. The chemical structure of the composite coatings before and after corrosion was characterized by FTIR, and the possible corrosion mechanism of the composite coatings was also discussed.The results were concluded with the following aspects:(1)Compared to alkaline activation, uniform, compact, low porosity deposits was gained by zincate treatment. The properties of the deposits was modified by decreasing the concentration of(CH3COO)2Pb. When the concentration of(CH3COO)2Pb was 0.16 mg/L, the deposits had the optimal corrosion resistance. The bath parameters, including temperature, pH value and plating time had the important influence on plating rate. The results showed that the deposition rate decreases as the plating time extends, but increases rapidly with increasing the bath temperature and solution pH value. According to the results, when the plating time, temperature and pH value was 2 hours, 85?, 5.0 respectively, the plating bath gained good stability and excellent plating rate.(2)Epoxy resin, polyimide resin, and spray of polyurea elastomer(SPUA) were coated on the deposits to fabricate three different anti-corrosion composite coatings, the thickness of the coatings is 120?m, 82.4?m, 120?m respectively. According the potentiodynamic polarization test, the corrosion property of the composite coatings are higher than Ni-P coatings, and the coatings were all bonded well. Corrosion evolution of different coatings was demonstrated by neutral salt spray test. It could concluded that the aluminum samples coated with polyimide resin and SPUAmaintained complete more than 1000 hours in the neutral salt spray test, had the best corrosion resistance, however, Epoxy resin had the relatively poor corrosion resistance, it peeled off in 200 hours.(3)It's found that, for Ni-P deposits coated with polyimide resin, during the neutral salt spray test, the polyimide resin film was permeated by corrosive medium. Some bubbles was observed in the defect of the deposits. The film was degraded, and then the coating had peeled. Pitting corrosion also occurred on the substrate by corrosive medium. According to the analysis of EDX, it was inferred that chlorine was the main reason caused the pitting corrosion. |
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