The Electrochemical Studies Of Atmospheric Corrosion Of Typical Metals

This thesis we designed two experiment setups which are "the thin electrolyte film experiment and indoor wet-dry cycle experiment" to simulate the atmospheric corrosion. Electrochemical techniques, including electrochemical impedance spectroscopy (EIS), electrochemical noise (EN), Potentiodynamic polarization and etc., and materials characterization method like SEM/EDX to study the atmospheric corrosion of 2024-T3 aluminum alloy, Q400NQRL and 09CuPCrNi weathering steel in indoor simulating acceleration environment. Besides, based on the three parameters of PSD (Power of Spectral Density) curve and dimensional analysis, two new indexes, S_E & S_G,for electrochemical noise analysis are proposed. Further more, the physical meaning of these two indexes is also discussed.(1) Theory researchâ—†For widely used there electrodes system under thin electrolyte film, a mathematical model is first created, from which, the expression of current, potential and resistance for electrochemical system are deduced and the distribution of current is discussed. It was found that the distribution of current was related to the valueofα^1/2, which is the square root of the ratio of unit solution resistance R₅^* and unit electrode impedance Z^*. The value ofα^1/2 is in a certain rang, in which the smallertheα^1/2 value is, the more uniform of the current distribution is.â—†Based on the three parameters of PSD (Power of Spectral Density) curve, which are the slope of high frequency linear region (k), the critical frequency (f_c) and the low-frequency plateau level (W), and dimensional analysis, two new indexes, S_e & S_G, for electrochemical noise analysis are proposed. And then they are applied to the potential records obtained from stainless steel 1Cr18Ni9Ti and pure Al corrosion systems, which are most susceptible to pitting corrosion in NaCl solution. Meanwhileanother two mathematical methods, statistics and FWT technique, are also carried onthe same data to estimate the physical meaning of those two indexes. The resultssuggest that, the index S_E can be used to indicate the distribution behavior ofelectrochemical noise and the index S_G can be used to depict the slower corrosionprocesses.(1) Applied researchâ—†The EIS and LSV (Linear Scanning Voltammetry) results of 2024-T3 aluminum alloy under different thin electrolyte films illuminate that: the corrosion rate of metal under thin electrolyte films is faster than that in bulk solution, and it decreased with the increase of the thickness of electrolyte film, till to the rate much close to that in bulk solution. The rate determining step (RDS) for the corrosion process of 2024-T3 aluminum alloy differs with the thickness of electrolyte: (i) In the case of 126μm, RDS is the anodic process, i.e. the dissolution of 2024-T3 aluminum alloy.(ii) In the case of 199μm and 399μm thin electrolyte film, RDS is the diffusion of oxygen and the anodic process, and in the case of 399μm, the oxygen diffusion process is the mainly influencing factor.(iii) In bulk solution, RDS is the oxygen diffusion through the electrolyte film and corrosion products layer to the corroding surface.â—†The Electrochemical impedance (EIS) results of 2024-T3 aluminum alloy in different pH environments indicated that: in pH 3.5 system, two independent variables, potential E and the fraction of the area covered with corrosion productsθ, existe; while in pH 4.5 system only one independent variable, potential E, existes. The mathematical analysis is consistent with experiments results.â—†In the EN study of 2024-T3 aluminum alloy corroding in different pH simulating acid rain, the results indicate that the corrosion type of pH 3.5 system can be divided into three stages, i.e. 1) balance region between the pitting process and diffusion process of the cathodic reagents to the active corrosion points; 2) the diffusion of cathodic reagents through the corrosion products is the control process of corrosion; 3) the initiation of new pits in the already formed pits takes up the majority of energy of the corrosion process. In the pH 4.5 and 6.0 systems, the energy is mainly accumulated in the finer crystals (except at very beginning) all through the tests. It is because that pitting is the main corrosion type, although quite limited.â—†For Q400NQRL and 09CuPCrNi two weathering steels corroding under thin electrolyte, the corrosion processes are also strongly effected by the thickness of electrolyte film, and mainly controlled by cathodic reaction. The fastest corrosion rate may appear in 100μm thickness for Q400NQRL weathering steel while in 200μm thickness for 09CuPCrNi weathering steel and the corrosion of both steels are mild under bulk solution. The difference between corrosion processes of two weathering steel are is obvious. At beginning, the corrosion of 09CuPCrNi weathering steel is general, and is strongly affected by thickness, while Q400NQRL is much more localized and is mainly affected by chlorine ion but not the film thickness. These resulted in that the 09CuPCrNi weathering steel is more sensitive to the electrolyte thickness than that of Q400NQRL. At the end of experiments, the corrosion rates of both weathering steels are different, but for same steel, it doesn't change with the electrolyte thickness and attains certain value.â—†In the wet-dry cyclic experiments of 2024-T3 aluminum alloy and two weathering steel (Q400NQRL & 09CuPCrNi), it is found that there a distinctive potential change between sprayer period and drying period, which could be explained by Evens diagram. Meanwhile, the corrosion rate of metal in the wet period is faster than that in the dying period. In the case of pH 3.5, cathodic reaction would change from reduction of proton to oxygen, while it would stay almost the same in the other systems. The SEM images indicates that the 2024-T3 aluminum alloy is mainly suffered from pitting corrosion in simulated acid rain, and like two weathering steels, the resistance of it increased with pH value.â—†In the electrochemical noise study of Q400NQRL & 09CuPCrNi weathering steels, two indexes S_E & S_G, which are proposed by us, are practically used. The analytic results are consistent with wavelet and cathodic potentiodynamic polarization results. It is found that Q400NQRL weather steel is prone to localized corrosion, and 09CuPCrNi weathering steel is sensitive to the pH change of environments. In the acidic environment, the corrosion behavior of these two weathering steel is similar that is serious. However, in the near neutral or neutral environment, 09CuPCrNi steel has better corrosion resistance than Q400NQRL steel does.