Local Electrochemical Study Of The Effect Of Residual Stress On The Corrosion Sensitivity Of Carbon Steel

Due to its excellent properties on machinery, strength and anti-corrosion, carbon steel has been widely used in a variety of fields such as building, defense, transportation, petroleum and chemical industry as well as ocean development, etc. In the process of various machining, residual stress can be inevitably produced inside the parts of metals. Therefore, residual stress in the surface layer of metals or machine parts will certainly play an important role in fatigue properties of the metals or parts. In history, many disastrous accidents were mostly caused by the residual stress in the metals or parts. On the basis of "mechanical～chemical interaction", it is significant to explore the effects of residual stress on the corrosion susceptibility of carbon steel.In this thesis, the bending carbon steel, in which macroscopic residual stress existed, was used as specimens. On one hand, the residual stress in the surface of bending carbon steel was measured by X-ray diffraction. On the other hand, conventional corrosion-electrochemistry tests and local electrochemical measurements carried out by means of scanning electrochemical microscopy (SECM) were combined to study respectively the corrosion characteristics of carbon steel and the effect of residual stress on corrosion sensitivity of the carbon steel.The main work is as follows:1. Measurements of residual stress in carbon steelThe residual stress on the surface of the bending carbon steel specimen was measured on X-ray diffractometer based on the principle of X-ray diffraction. Seven positions on the surface of specimen, where concentrated residual stress existed, were selected to perform the measurements of residual stress.2. Study of macroscopic corrosion-electrochemistry characteristics of carbon steelCorrosion-electrochemistry characteristics such as corrosion potential and corrosion current density were measured and deduced by means of corrosion potential-time curves, polarization curves and electrochemical impedance spectroscopy (EIS), etc. The results indicated that some factors such as composition and concentration of corrosion media and immersion time played an important role in the corrosion-electrochemistry characteristics as mentioned above.3. Local electrochemical measurements by SECM(1) Determination of experimental parameters of SECMCyclic voltammetric experiments were performed to detrmine the the appropriate potentials applied to the Pt tip in Fe(CN)64-or I" ion-containing KCl solution, where Fe(CN)64-or I-ion served as as redox mediator. That is, it was decided to set a potential value on the Pt tip where a diffusion limited current is attained. Similarly, by using oxygen as redox mediator, the appropriate potential applied to the Pt tip was also obtained in KCl solution by the cyclic voltammetric experiments.(2) Effects of compressive residual stress on corrosion sensitivity of carbon steel in neutral chloride solutionThe bending carbon steel sample was used as the working specimen in neutral KCl solution. In the sample generation-tip collection mode of SECM measurements, three-dimensional images of probe current were obtained from regions of carbon steel with different kinds of residual stress to both investigate the localized electrochemical activities for the initiation and propagation of pitting corrosion at a microscopic level and establish the relationship between the non-uniformity of surface electrochemistry and the heterogeneity induced by residual stress on microstructure of material. At the same time, the effects of residual stress on the corrosion sensitivity of carbon steel were verified on the basis of varing test methods such as area scan and line scan and test conditions including concentration of solution and substrate polarization, and so(3) Effects of compressive residual stress on heterogeneous electron transfer rateBy using I-as redox mediator in the feedback mode of SECM measurements, the approach curve is employed to study the effects of compressive residual stress on heterogeneous electron transfer rate.(4) Concentration distribution of corrosion products of carbon steel in the neutral chloride medium The cyclic voltammetric experiment was performed on the SECM-tip in0.1M KCl solution in which Fe(CN)64" ions with varing concentration were added as redox mediator. The diffusion coefficient of Fe2+ions was calculated by means of the limited difiusion current iT,∞. The concentration of Fe2+ions arising from the dissolution of carbon steel specimen immersed in0.1M KCl solution at open-circuit potential can be achieved by approach curve of the SECM-tip.(5) Oxygen as redox mediator to detect the corrosion of carbon steelApart from the Fe(CN)64" or I-ion which can be added in the solution as redox mediator as mentioned above, the dissolved oxygen in solution was also served as redox mediator to detect the corrosion situation of carbon steel substrate.