A Micro-Macro Electrochemistry Comparative Study On The Corrosion Process Of Electroplated Ni And Ni-P Composite Coating

Due to the good electrocatalytic activity for hydrogen evolution, special paramagnetic characteristics, excellent microhardness and corrosion-resistance of Nickel-phosphorus alloy coating, it has been widely used in many fields, especially in the corrosion protection. Conventional and classical electrochemical techniques such as steady-state polarization curves and electrochemical impedance spectroscopy (EIS) have been used to provide important information about the global properties of the metal/electrolyte solution interface. However, these techniques lack spatial resolution and do not offer much detailed information about the localized dynamics of corrosion processes, and they are not used for the investigation of corrosion processes in nanometers and micrometers scales with spatial resolution. The Scanning Electrochemical Microscopy (SECM) can direct identify of chemical species and has electrochemical sensitivity to characterize the topography and redox activities of the metal/electrolyte interface with spatial resolution using a scanning ultramicroelectrode (UME). SECM with high spatial resolution has become a very significant technique for investigating the local corrosion process.In the current research, Ni and Ni-P composite coatings with 10.7 and 25.7 at.% P. content were fabricated by electrodeposition. In this thesis, three coatings were called as pure-Ni, Ni-LP and Ni-HP respectively. Microstructure, surface morphology and composition of electroplated Ni and Ni-P coatings were studied using SEM, XRD and AFM. Macro-electrochemical technology, such as EIS and the potentiondynamic polarization measurements, combined with SECM, were used to evaluate the corrosion behavior of Ni-P coating in NaCl solution with different Cl-concentration and pH value. The uniform and compact Pure-Ni and Ni-P deposition were performed on brass substrates by electrodeposition, and the Ni-HP composite coating has a better corrosion resistance. The corrosion resistances of three coatings decrease with Cl- concentration increasing, and increase with P content and pH value in composite coating increasing. The Nyquist plots of EIS for three coatings have two capacitive arc time constants. With the P content and pH value increasing, the sum of charge transfer resistance (Rct) and film resistance (Rfilm) of coatings increases. With Cl" concentration increasing, the sum of Rct and Rfilm decreases. The results indicated that SECM can monitor Ni and Ni-P composite coating failure process and the effect of Cl- concentration and pH value. Ni-LP and Ni-HP composite coatings maintain good stability in low Cl- concentration in the solution for a short immersion time. When soaking to 6 hours, the typical active site and corrosion products appeared on Ni-LP alloy coating, but corrosion products and the active area did not appeared on Ni-HP alloy coating until soaking in NaCl solution to 24 hours. Low concentration of Cl- has a stronger activating effect on pure-Ni coating, while the higher Cl-concentration accelerates localized corrosion formation for pure-Ni coating. Localized corrosion appeared in 0.1 mol/1 NaCl solution and general corrosion appeared in 0.3 mol/1 NaCl solution for Ni-LP composite coating.The micro-electrochemical results show that the in pH 2.0 and pH 7.0 solution, the corrosion resistance of the coating increases gradually with the increase of P content in the coating. In a solution of pH 11.0, the corrosion resistance of the coating decrease with the increase of P content in the coating. From surface mapping in NaCl solution with different pH, in pH 2.0 and pH 11.0 solution, coatings are given priority to with localized corrosion, and in the pH 7.0 solution, more prone to uniform corrosion. The result of SECM micro zone corrosion and the corrosion of the electrode surface of the average information of EIS and polarization curve results are not completely consistent, which also shows that the corrosion reaction in micro scale heterogeneity, spatial distribution phenomenon.According to the characteristics of the SECM approach curve and surface mapping, COMSOL multiphysics simulation software was applied to build 2D and 3D model. Based on boundary conditions and reaction model, the feedback effect of approach curve and active point size were quantified. The simulation results show that the positive and negative feedback effect is strong dependent on the gap between tip and substrate. The smaller gap, the feedback effect is more apparent. 3D simulation results indicate that the spatial scale of active point for Ni-LP coating failure process is micron level, which is less than 10 microns. SECM can effectively in-situ monitor the corrosion behavior of Ni metal coating and Ni-P composite coating, such as the formation of the active points, the generation and accumulation of corrosion products, which can complement each other with macroscopic electrochemical results.