| Magnesium containing RE alloy with the excellent characteristics of light weight, high strength, shielding performance, excellent impact shock absorption and heat resistance, et al, has been extensively studied and applied. However, its poor corrosion resistance limits the large-scale application in the engineering field, so how to improve the corrosion resistance of magnesium alloy is very important and has drawn great attention at home and abroad. In this paper, inductively coupled plasma direct-reading spectrometer, optical microscopy, scanning electron microscopy with energy spectrum, X-ray diffraction, mechanical testing machine, electrochemical test systems and other analytical tools were adopted to research on the changing rules of Y on the microstructures, mechanical properties, corrosion rates, corrosion morphologies, the open-circuit potentials, polarization curves, et al for the Mg-Y alloys in as-cast, homogenization state, squeezed state and aged state.The grain size of as-cast Mg-Y alloy reduced to about180μm from1100μm with increasing Y content. After the homogenization treatment, the second phases of Mg-(0.25,2.5,5and8)Y alloys dissolved into substrate substantially, but there were a large number of Mg24Y5phase of Mg-15Y remained on the grain boundaries. The grain size significantly refined with the increase in the content of the Y elements. The microstructures of Mg-0.2Y and Mg-2.5Y were consisted of the single a-Mg phases, and the microstructures of Mg-5Y, Mg-8Y and Mg-15Y had the a-Mg phases and the bone-shaped eutecticsβ-Mg24Y5phases along the grain boundaries. With increasing Y content, the tensile strength and elongation of Mg-Y alloy with different states increased and decreased respectively.The optimum heat treatment processes were determine through series of experiments:the treatment homogenized at525℃of Mg-0.25Y Mg-2.5Y, Mg-5Y, Mg-8Y, Mg-15Y were2h,2h,4h,8h and28h respectively, the treatment homogenized at535℃of Mg-0.25Y Mg-2.5Y, Mg-5Y, Mg-8Y, Mg-15Y were2h,2h,2h,6h and24h respectively, the peaking treatment aged at250℃of Mg-0.25Y Mg-2.5Y, Mg-5Y, Mg-8Y, Mg-15Y were4h,6h,10h,12h and16h. The treatment processes could act as the basis for drafting the homogenization and aging treatment process. According to the potentiodynamic polarization curves, the pitting point potentials were positive than the self-corrosion potentials of Mg-(0.25,2.5.5.8and15)Y of different states in3.5%NaCl solution, which stated that the alloys could generate localized corrosion spontaneously. With the increase of the Y content, the corrosion potential and open-circuit potentials of different states alloys moved positively first, then moved positively, and corrosion potential and open-circuit potentials of Mg-2.5Y alloy had the highest values.’The polarization curves of different alloys in the different states had the similar shape, and the anode current rise faster than the cathodic current. The anode and cathode branches of polarization curves were asymmetric. The anode branch slopes were greater than that of the cathode branch, indicating that the cathodic process played a more important role in the corrosion reaction.The impedance spectroscopies of Mg-(0.25,2.5,5,8and15) Y alloy in different states were consisted of a high-frequency capacitance and a low-frequency capacitance, with the increase of the Y content, the radius of high-frequency capacitive arc increased apparently, and the radius reached the maximum value when the Y content was up to2.5%. but the radius of high-frequency capacitive arc decreased when Y content continued to increase above2.5%, which indicated that the corrosion resistance of the alloy increased first increased and then decreased with increasing of Y content. According to the theory of electrochemical reaction process and impedance analysis, the equivalent circuit of impedance spectrum curve of Mg-(0.25,2.5,5.8and15) Y alloys in different states was consisted of a solution resistance Rs, a corrosion film resistance Rf, a The charge transfer resistance Rt, an inductor L and a constant phase angle element CPE. The analysis of impedance spectroscopy analysis showed that the corrosion resistance decreased progressively as following: Rt (Mg-0.25y)<Rt (Mg-8Y)<Rt (Mg-15Y)<RT (Mg-5Y)<Rt (Mg-0.25Y).The corrosion processes of Mg-(0.25,2.5,5.8and15)Y alloys in different states could be divided into three stages: the initial stage of corrosion, corrosion interim and the late stage of corrosion. According to weight loss, the corrosion rate of Mg-(0.25,2.5,5,8and15)Y alloys in different states decreased progressively as following: Mg-0.25Y<Mg-8Y <Mg-15Y<Mg-5Y<Mg-0.25Y, and the results kept in line with the changes of the polarization curves and electrochemical impedance spectroscopy.According to the corrosion behavior and corrosion morphology of Mg-(0.25,2.5,5,8and15)Y alloys in different states in3.5%NaCl solution, the corrosion kinetics model, the corrosion mechanism model and pitting formation model were established. The potential difference of α-Mg and β-Mg24Y5phases produced the galvanic corrosion in the solution, and the β-Mg24Y5phases were the origins of pitting corrosion. The corrosion mechanism model of Mg-(0.25,2.5)Y alloys and Mg-(8and15)Y alloy were the uniform corrosion, pitting and localized corrosion. After the homogenization treatment, the corrosion mechanism model of Mg-5Y became to the uniform corrosion from the localized corrosion in other state.Mg-Y alloy withstand the three-dimensional stress during the extrusion process, and the extrusion was accompanied by the occurrence of dynamic recrystallization and the grain refinement. After extrusion, the grains rearranged on the preferred orientation of the direction of extrusion, and the grains arranged in any order perpendicular to the squeeze direction. The corrosion resistance of the longitudinal section was better than that of the transverse section, and the corrosion resistance of the alloys improved with increasing of the extrusion ratio.The main corrosion products of Mg-(0.25,2.5,5,8and15)Y alloys were Mg (OH)2and small amounts of Mg2(OH)3Cl·4H20in the corrosion process.The corrosion resistance of Mg-(0.25,2.5,5.8and15)Y alloys increased progressively as following:cast<heat treatment state<aged state<squeezed state. |
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