| Zinc-iron alloy coating is a new type anticorrosion coating for mainly iron and steel. It appeared in the early 1970s, and has received considerable attention for its advantages such as excellent anticorrosion property, good paintability and weldability,improved formability, and low cost. In this dissertation, the current research status of the zinc-iron alloy coating has been reviewed. This review mainly focuses on the following five fields of researches including zinc-iron alloy electroplating, the compositions and microstructures of the zinc-iron alloy coating, the corrosive and mechanical properties of the zinc-iron alloy coating, zinc-iron alloy chromate passivating, and the composition, structure and corrosion resistance of the chromate conversion film. The electroplating processes, microstructures, corrosion behaviors and mechanical properties of the zinc-iron alloy coating have been investigated by combining the theoretical and experimental methods. The main results are as follows. 1. A smooth and dense zinc-iron alloy coating containing 0.2-0.9wt.%Fe has been obtained from sulphate baths by using TF bright agent, which is prepared by the author. This bath has the following advantages vs. the alkaline baths: ease of controlling plating bath composition, high cathode current efficiency and low operating cost. Moreover, the zinc-iron alloy electroplating has been ascertained to follow anomalous codeposition mechanism by linear potential sweep method. 2. Zinc-iron alloy coatings with highly basal preferred orientation were successfully obtained from acidic sulphate electrolytes by further optimizing plating parameters. To explore the corrosion behavior of the highly basal preferred orientation zinc-iron alloy coating, the pure zinc coating from acidic sulphate electrolytes and the other zinc-iron alloy coating from alkaline zincate solutions were also prepared for comparison. Chromate passivating for zinc and zinc iron alloy coating is identical in this experiment. The composition, microstructure and corrosion resistance of the pure zinc and zinc-iron coating and their chromate conversion film have been studied using several methods such as X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), potentiodynamic polarization curves and dipping tests. XRD confirmed that the crystalline size of the highly preferred orientation zinc-iron alloy coating is in the nanometer range, its texture coefficient is more than 95 percent and its structure is identified as ηphase. Potentiodynamic polarization measurements and dipping tests showed that after black chromating, the corrosion resistance the highly preferred orientation zinc-iron alloy coating is two times higher than the pure zinc coating with similar thickness, and its alloy composition with optimum corrosion resistance has a composition 0.58wt.%Fe. The corrosion resistance of the zinc-iron alloy coatings with highly preferred orientation coatings from acidic sulphate electrolytes are superior to zinc-iron alloy coatings with similar iron content and thickness from alkaline zincate solutions both after and before chromating. XPS analysis showed that the texture of zinc-iron alloy coatings affects the Cr (VI) and total Cr content of the chromate conversion film on them. Based on the experimental results of the composition, microstructure and corrosion resistance of the zinc-iron alloy coating, it is concluded that the texture and phase parameter (i.e. c /a) of the zinc-iron alloy coating are key factors affecting its corrosion performance. 3. Black chromate conversion films on zinc and zinc iron alloy coatings have been investigated with the latest generation of XPS spectrometer. The results showed that XPS measurements of the above two chromate conversion films are very successful and some valuable XPS data was obtained firstly. The chromate conversion films on the zinc and zinc-iron alloy coatings show a three-layers structure: a surface layer, an interior layer (i.e., bulk film) and an interface layer between the interior layer and the substrate. All constituent elements and their chemical states in the chromate conversion film form gradient distribution through the thickness. During the formation of black chromate conversion film on zinc-iron alloy coatings, Fe was incorporated into the chromate conversion film, and therefore increases Cr (VI) content in the film. Cr (VI) content in the film is the key factor affecting the corrosion resistance of the chromate conversion film and not its thickness. Based on XPS results, a mechanism for the formation of black chromate conversion film on zinc-iron alloy coatings was proposed. 4. According to ion transport characteristic at corrosion interface under metal corrosion potential, a general boundary condition was suggested. The diffusion equation was then solved with the above boundary, and a generalized analytical solution of the diffusion impedance was obtained finally. Hyperbolic tangent and hyperbolic cotangent diffusion impedance was deducted from the generalized analytical solution of the diffusion impedance. low frequency characteristic of hyperbolic tangent and hyperbolic cotangent diffusion impedance was analyzed in detail. In the meantime, Hyperbolic tangent and hyperbolic cotangent diffusion impedance was interpreted in terms of a general transmission line equivalent circuit and circuit parameters were also calculated. 5. EIS (Electrochemical impedance spectroscopy) data for the chromate-passivated andunpassivated zinc-iron alloy coatings immersed in NaCl solutions were measured. Based on the theoretical analysis results of the diffusion impedance in this thesis, in conjunction with SEM, XPS and AFM (Atomic force microscopy) analysis of the chromate-passivated and unpassivated zinc-iron alloy coatings, Impedance models and their equivalent circuits were proposed. All parameters in equivalent circuits were adjusted simultaneously to obtain an optimum fit to measured data. The simulated diagrams for the proposed equivalent circuits are in good agreement with the measured data. 6. In-situ tensile test of the zinc iron coated steel sheet was conducted under SEM. The evolution of cracking and surface microstructure of the zinc-iron coating was in-situ observed. Results reveal that cracks on the surface of the zinc-iron alloy coating are perpendicular to tensile load direction. 7. Using EIS and potentiodynamic polarization curve method, mechanochemical effects (MCEs) of the zinc-iron alloy coatings was firstly confirmed. The results showed that with increasing plastic deformation, the corrosion current and interfacial capacitance of the zinc-iron alloy coating would increase and pass over a maximum. On the contrary, with increasing plastic deformation, the polarization resistance and corrosion potential of the zinc iron coating would decrease and pass over a minimum. In the light of mechanochemistry of solid and electrochemistry theory, the relation between the MCEs characteristics of the zinc iron alloy coating and its different deformation stage was analyzed in detail. Some theoretical formula, which interprets the experimental data qualitatively, was deducted. 8. The influence of zinc-iron alloy coating on mechanical properties of low carbon steel sheet was investigated. The results showed that zinc-iron alloy coatings have little influence on the yield strength ( σs), tensile strength ( σb) and elongation (δ) of low carbon steel sheet, however, decrease its work hardening exponent ( n ) and plastic strain ratio (r ) value. Based on the elastic-plastic theory, the reason why zinc-iron alloy coating decrease n and r of low carbon steel sheet is discussed in detail...
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