Research On Technological Process Of Ni-Co-Si₃N₄-Al₂O₃ Nano-Composite Coatings And Corrosion Resistance In Coastal Saline-Soil Simulation Solution

Metallic materials buried in the soil with different physical and chemical properties for a long time,were easy to create an electrochemical environment for metals,which would gradually affect their corrosion or damage.In recent years,how to improve the corrosion resistance of metal surface,reduce the harm of metal corrosion,the most practical and economical protection measure was coating protection.Ni-Co alloy coatings has good mechanical properties and corrosion resistance.With the development of nanotechnology,nano-composite coatings prepared by adding nanoparticles into the alloy can improve their mechanical properties.In this paper,Ni-Co-Si₃N₄-Al₂O₃ nano-composite coating was prepared,and the process parameters and properties of the coatings were analyzed,which can obtain the optimal parameters of the coatings.The corrosion behavior of coatings in different areas of soil simulation solution were investigated by electrochemical corrosion.The main research works and results of this paper were as follows:（1）Ni-Co-Si₃N_4,Ni-Co-Al₂O₃ and Ni-Co-Si₃N₄-Al₂O₃ nano-composite coatings were prepared by electrodeposition method.The morphology,phase structure and composition of the coatings were characterized and analyzed by means of scanning electron microscopy（SEM）,X-ray diffractometer（XRD）and energy dispersive spectrometer（EDS）analysis,and the wettability and corrosion resistance of the coatings were studied by using optical contact angle measuring instrument and electrochemical workstation.The results show that compared with the Ni-Co-Si₃N₄ and Ni-Co-Al₂O₃ coatings,the surface grain of the Ni-Co-Si₃N₄-Al₂O₃ nano-composite coatings is refined,uniform and dense,and the average grain size is the smallest,which is 19.20 nm.The surface static water contact angle of the coatings is 131.4°,and shows the best hydrophobicity.Electrochemical tests show that,under the simulated soil solution conditions,the self-corrosion current density and corrosion rate of the nano-composite coatings are 1.30μA·cm^-2and 0.016 mm·a^-1,respectively,the charge transfer resistance is 214.91 kΩ·cm²,exhibiting the best corrosion resistance.（2）The different process parameters of Ni-Co-Si₃N₄-Al₂O₃ nano-composite coatings were studied by jet electrodeposition.The coating was prepared under different injection voltage,duty cycle and pulse frequency by single factor test.The surface morphology,phase structure,microhardness,hydrophobicity and corrosion resistance of the coatings were characterized.The experimental results show that when the injection voltage was 8 V,the duty cycle was 70%,and the pulse frequency was 4 k Hz,the composite coatings can maintain good comprehensive performance.（3）Process parameters optimization of Ni-Co-Si₃N₄-Al₂O₃ nano-composite coatings.Analysis the interaction of injection voltage,duty cycle and pulse frequency on the microhardness,hydrophobicity and corrosion resistance of the coatings.The fitting function of three factors was optimized by multiple response parameters,and verify the prediction results.Through optimization analysis,it is found that when the injection voltage is 7.72 V,the duty cycle is 71%,and the pulse frequency is 4.16 k Hz,the sample has good comprehensive performance.Predicted by microhardness was 675.52 HV_0.05,contact Angle is 117.47°,the charge transfer resistance at 70.9 kΩ·cm².After verification,the comprehensive performance of the measured results of nano-composite coatings were consistent with the predicted values basically.（4）Corrosion behavior of nano-composite coatings in different saline environments.The corresponding soil simulation solution was configured for the sandy loam tidal flat saline soil（Da feng）with more salt content and the sandy loam meadow coastal saline soil（Dong tai）with less salt content.Electrochemical polarization curves and AC impedance spectroscopy experiments were carried out in simulated soil solutions with different salt contents.According to the experimental results,the formation mechanism of coating corrosion product film was analyzed.The corrosion rate of the coating varies with the soil salinity in different regions.In the study of coating corrosion in Da feng area,the electrochemical impedance spectroscopy test was consistent with the polarization curve,the overall trend first decreases and then increases.With the change of time,the corrosion product film was dense gradually,which plays a protective role on the surface corrosion of the coatings.In the study of status of the coating in Dong tai area,due to the low salt content and the hydrophobicity of the coating surface,the corrosion rate of the coating was relatively slow at the initial stage.With the increase of immersion time,the corrosion rate changes accordingly.