Electrochemical And In-situ SECM Study On The Corrosion Characteristics Of Aluminum In Neutral Media

Among numerous of metallic materials, aluminum and its alloys, due to their advantages of small density, high mechanical strength as well as excellent electrical and thermal conductivity, have been widely applied in many areas such as aviation, aerospace, defense, transportation, building materials and high-voltage transmission etc. With the development of the aluminum materials applying in the ocean development projects, the amounts of aluminum using in the marine environment are generally increasing. Due to the characteristics of high salinity and containing high concentration of aggressive Cl-ion, the sea can lead to serious corrosion hazards to the metals. As common structural materials, therefore, the corrosion and protection of aluminum and its alloys in the marine environment have been a hot research spot of corrosion science and engineering technology at home and abroad in recent years.Traditional electrochemical experiment techniques (such as polarization curves and electrochemical impedance spectroscopy) can provide the global and average corrosion electrochemistry information related to the metal surface. Since the traditional electrochemical methods can not meet the requirements of real-time and in-situ study for localized corrosion, however, the electrochemical scanning probe technology emerged in recent years to overcome the shortages of the traditional electrochemical methods as mentioned above. As one of the electrochemical scanning probe techniques, scanning electrochemical microscopy (SECM) is a novel technique in electrochemistry with high spatial resolution, which can in-situ study the initiation and propagation mechanisms of localized corrosion in the micro-, submicro-or nano-meter scale, explore the growth and breakdown mechanisms of the passivation film on metal surface, as well as characterize the micro-electrochemical activity of metal surface, etc.In this thesis, the pure aluminum and industrial aluminum were used as the working electrodes. The NaCl and Na2SO4solutions, which constitute the main ingredients in marine environment, were employed as the corrosion medium. Corrosion characteristics such as corrosion potential, corrosion current density and corrosion rate of aluminum electrodes were measured and deduced by means of corrosion potential-time curve, polarization curves and electrochemical impedance spectroscopy, etc. On the basis of the results provided by the traditional electrochemical methods as mentioned above, the concentration profiles of the corrosion products, the electrochemical activity, the characteristics and mechanism of localized corrosion of aluminum in neutral sodium chloride solutions (with KI or K4Fe(CN)6as redox mediator) have been investigated by SECM methods of approach curve, area scan imaging and in combination with cyclic voltammetry (CV).The main work is as follows:1. Study of the characteristics of corrosion electrochemistry of Al in the simulated marine corrosion environmentTests of pure and industrial Al materials with Ecorr～t, EIS and Tafel curves in different concentrations of NaCl and Na2SO4solutions are carried out in order to discuss the effects of various factors, such as texture of metal material, corrosive medium composition, concentration, immersion time and the characteristics of aggressive ions etc., on the characteristics of corrosion electrochemistry including material corrosion potential, the corrosion current of the anode and cathode process as well as the corrosion rate etc. According to the experimental EIS results, a reasonable electrochemical equivalent circuit to analyse the measured EIS results is proposed, and a good corresponding relationship has been established between the fitted and the measured data. Meanwhile, by using of the charge transfer resistance (Rct) which is one of the kinetics parameters of the corrosion electrochemistry process, the material-chemistry synergy mechanisms which govern the corrosion rate and corrosion mechanism have been deduced further.2. In situ SECM study of characteristics of Al corrosion in NaCl solution(1) Determination of SECM experimental parametersFirstly, the appropriate potential applied to the scanning Pt probe is determined. To do this, the CV curve of the probe is measured in0.1M NaCl solution with5mM K4Fe(CN)6or KI (redox mediator). After then, the polarization potential applied on the probe during the measurement of the approach curve is determined in terms of the potential range corresponding to the limited diffusion current plateau. Secondly, the appropriate potentials applied on the Al substrate electrode were derived on the basis of the potential ranges corresponding to the active dissolution and passivation processes in the potentiodynamic polarization curves of the Al substrate electrode.(2) SECM study of the surface corrosion process and mechanism of pure Al in0.1M NaCl solution by the current feedback and generation/collection modesWith the current feedback mode of SECM, the surface electrochemical activity of the substrate electrode is explored by using of approach curves which show the variations of Pt probe current with probe-substrate distance.By means of the area scan test on the pure Al electrode with the feedback mode at open circuit potential, the localized surface electrochemical activity of pure Al electrode is obtained. With changing the immersion time of Al in NaCl solution, the corresponding SECM area scan images are obtained and subsequently the corrosion type and mechanism of Al are analyzed. According to the differences of pure Al reaction at different potentials in0.1M NaCl+5mM K4Fe(CN)6solution, SECM area scan tests are proceeded with different polarization potentials applied on the substrate by combination of feedback mode with substrate generation/probe collection mode, to analyze the surface corrosion process and mechanism of substrate under different polarized states (activation, passivation or pitting corrosion).(3) SECM study of the effects of texture of materials on the differences in surface electrochemical activity and topological morphology of Al corrosion processSECM area scan measurements are carried out on pure and industrial Al in0.1M NaCl solution (containing5mM KI redox mediator) at open circuit potential. The topological morphology and corrosion characteristics on substrate surface are characterized by the corresponding tip current in feedback mode. Consequently, the surface electrochemical activity differences in the two type of Al materials are compared, to effectively clarify the effects of texture of metal materials on the corrosion behaviors. Using the area scan images of pure and industrial Al immersed in neutral corrosive media for different times, the localized corrosion characteristics of Al in NaCl solution have been intuitively demonstrated, which as a result provides theoretical and experimental basis for further understanding of the initial and developing process of the localized corrosion of metal surfaces.