Study On Corrosion Processes Of Weathering Steels And Zinc/Zn-Fe Alloy By Using Electrochemical Techniques

Corrosion brings about large amounts of wastage of natural materials and causes tragedies. So it is significant to develop exact and in-situ monitoring techniques for material corrosion (corrosion type and intensity), both in theory and in practical application.By using the modern electrochemical methods such as electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) in conjunction with scanning electron microscopy (SEM) technique, this dissertation is devoted to the study of the evolution laws of the corrosion electrochemical behavior and the structure characteristics of the typical materials (such as weathering steels and zinc/ zinc alloys) served in the typical environments, and concentrates on the foundation of the EDP spectroscopy system, which can be used to depict the type and the intensity of corrosion process. The aim of mis dissertation is to provide the basis for future development of a kind of new corrosion monitoring techniques to predict the behavior of typical materials served in typical natural environments. Moreover, it is an important part of the National NSFC project--- electrochemical monitoring techniques of materials corrosion (No. 50499335-02). The main conclusions of this dissertation are drawn as follows:The corrosion processes of weathering steels (Q400NQR1 and 09CuPCrNi), zinc and Zn-Fe alloy in accelerated corrosion solutions (2.0% NaCl, 0.5% NaCl, 4.7% Na₂SO₄ and 1.2% Na₂SO₄, mass fraction hereinafter) were firstly studied by using EIS in conjunction with the SEM technique. According to their evolution features in Nyquist plots and Bode plots of EIS, one can distinguish different corrosion stages of these materials.The whole corrosion processes of weathering steels in 0.5% neutral NaCl solutions can be separated into several segments: (1) metastable pitting region, whose indicator is shrinkage of real part or inductance in complex plots;(2) stable pitting region, whose indicator is disappearance of real part shrinkage or inductance in lowfrequency range and two capacitance arcs with different time constants can be observed in complex plot;(3) pre-general corrosion region, when alloying elements exert important influence on the formation of the dense and packed rust layers that has ability to protection substrate;(4) general corrosion region, when continuous dissolution of rust layer and formation of new rust layer will cause continuous corrosion of substrate.Furthermore, the EIS data were analyzed by using the equivalent circuits supposed to describe corrosion processes of these corrosion ensembles, and subsequently electrode surface fractal dimension variation features during corrosion processes were compared. As a result of comparison, a relationship could be built between corrosion stages and fractal dimension evolution features. So fractal dimension could be used as a parameter to distinguish corrosion segments.Another method, electrochemical noise technique in conjunction with the SEM was also used to study corrosion processes of these materials in NaCl electrolyte. According to open circuit potential (OCP) analysis, there were three segments in electrochemical potential noise spectrum: quick descending segment, slow stabilizing segment and stabilizing segment. Moreover, as for corrosion process of weathering steels in 2.0% neutral NaCl solutions, there was also some periodic fluctuations of OCP during its third segment. None of the so-called typical exponential attenuation curve, which can be used to confirm pit corrosion, could be singled out from fine noise spectrum for all the corrosion ensembles. So in this dissertation Wavelets Transform (WT) technique was mainly used to analyze electrochemical potential noise spectrum, and subsequently time-dependent EDPs were obtained by using this technique. After EDP analysis, a new parameter — standard deviation (SD), was proposed and used to clearly and effectively distinguish different corrosion stages.Prior to study of Zn-Fe alloy corrosion, the decorative and protective Zn-Fe coating was obtained from the light acid chloride solutions, and the corresponding electroplating behavior was investigated using cyclic voltammetry and chronoamperometry methods in conjunction with SEM technique. In the case of highdeposition overpotential, the electroplating of Zn-Fe coating followed 3-D nucleating and subsequent instantaneous grain growth mechanism, while in the case of low deposition overpotential, it followed 3-D nucleating and subsequent progressive grain growth mechanism. Meanwhile, the anomalous co-deposition behavior of Zn-Fe coating can be attributed to that the atomic radius rzn > r?e, which will also result in that the experimental (I/Im)2 was much larger than that calculated from the theoretical model.According to our investigation, Zn-Fe alloy showed special corrosion behavior: at beginning, Zinc atoms at electrode surface tended to dissolve firstly because of their low potential (standard electrode potential is -0.76V) and the content of Fe (standard electrode potential is -0.44V) in electro-deposition layer got a little rise, which in turn exerted some influence to enhance corrosion resistance of the layer;during the following time, the prominent Fe atoms dissolved and the content of Fe in electro-deposition layer descended, which reduced corrosion resistance of the layer. The special dissolution behavior repeated again and again in the corrosion process, till low carbon steel substrate was exposed. Moreover, we believe it is this special dissolution behavior that produce layered phenomena around pits on Zn-Fe alloy surface morphologies and alternating shifts (which have with big amplitudes and different periods) of OCP on electrochemical potential noise spectrum.In order to study the corrosion processes, Ni electroplating process has been firstly investigated to fabricate nickel electrodepsited coatings. The influence of electroplating current density and electroplating time on the features of electrocrystallization EN and the corresponding nickel deposit structure has been studied based on the wavelet technique. With the increase of current density, the growth mechanism of nickel crystallites should change from 2-D to 3-D, the potential turning point was ca. - 1.15 V. Under activation-control, the 2-D growth type of nickel deposit possessed the EN features of the slowly positive potential drift, the corresponding nickel deposit layer was compact and the maximum relative energy accumulated in the region of RP-EDP with smaller scales. While underdiffusion-control, the 3-D growth type of nickel deposit possessed the EN features of the fast positive potential drift and subsequent remarkable negative potential drift, the corresponding nickel deposit layer was dentritic and the maximum relative energy accumulated in the region of RP-EDP with larger scales. The electroplating time affected the nickel deposit structure mainly through its influence on the growth rate of crystallites and the Ni2+ diffusion process around each crystallite. In addition, the corrosion processes of electrodeposited Ni coatings are under for further investigation.