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Research of the Relationship between Microstructure Evolution and Corrosion Behavior of Typical Stainless Steel

Author: GongJia
Tutor: LiJin
School: Fudan University
Course: Physical Electronics
Keywords: Ferritic stainless steel Austenitic stainless steel Duplex stainless steel High-temperature oxidation Intergranular corrosion Selective etching Isotopic tracer Secondary Ion Mass Spectrometry Electrochemical potentiodynamic reactivation Scanning Electrochemical Microscopy Diffusion mechanism Secondary precipitated phase
CLC: TG142.71
Type: PhD thesis
Year: 2011
Downloads: 431
Quote: 0
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Abstract


This thesis work is divided into two main fronts: high temperature oxidation of ferritic stainless steel, as well as localized corrosion of austenitic and duplex stainless steels. Ferritic stainless steel having excellent high temperature corrosion and stress corrosion resistance, low thermal expansion coefficient, and excellent in cost performance, and therefore widely used in high temperature environments. In the high temperature environment, the ferritic stainless steel can form a dense oxide film on the surface and to increase its high temperature oxidation resistance. For use in high-temperature water vapor environment, however, will change the mechanism of the formation of the oxide film and accelerate the oxidation rate, and eventually lead to rupture of the protective oxide film and failure, therefore necessary depth understanding of the mechanism of oxidation in vapor environment. Provide theoretical support to the practical application and development of new steel. Oxide film is mainly composed of metallic elements to specific research and innovations are as follows: in the country for the first time successfully established water isotope tracer techniques, combined with secondary ion mass spectrometry (SIMS) depth research ferrite oxidation mechanisms in water vapor environment: external diffusion oxidized to form, but there are still a small amount of water molecules diffuse into the involved in the oxidation; further add elements such as Nb and Ti oxidation, concluded: Nb elements alone add 430SS of the oxidation rate increases, however, the composite add Nb and Ti is able to effectively reduce the rate of oxidation. Austenitic and duplex stainless steel in practical application process which is often due to improper heat treatment and welding, etc. lead to secondary phase precipitation, such as: carbides, nitrides, and σ, χ and R, etc., and cause corrosion easier occur, eventually leading to failure of the material. Therefore, it is very necessary to study the alloy composition and heat treatment (high temperature solution and isothermal aging sensitized) control principle of duplex stainless steel microstructure evolution and its resistance to localized corrosion ability, draw the design and organization of a typical duplex stainless steel alloy provide the scientific basis for the design of the new duplex stainless steel. The duplex stainless steel is a more complex system, which contains a two-phase of ferrite and austenite. Therefore, the first starting from a simple system to study the understanding of single-phase austenitic stainless steel corrosion mechanism. Specific research and innovations are as follows: using bicyclic the electrochemical potentiodynamic back sweep method (DL-EPR) study to understand single-phase austenitic AISI 301B and 301S different time processing temperature range of 500-900 ℃ aging intergranular corrosion evolution situation, and to map out the temperature - time - sensitivity curve. By observe the microstructure of the material, to give the carbide precipitation form, location and the intergranular corrosion of the correspondence relationship between the degree of sensitivity. Finally, according to the evolution of the carbide nucleation, grew up, and chromium-depleted zone proposed measures AISI 301B and 301S austenitic stainless steel to intergranular corrosion during the theoretical model and the corresponding prevention intergranular corrosion. Studies carbide precipitation surrounding the formation of the depleted Cr is the root cause of intergranular corrosion occurred; 301B carbon content lower than the 301S, its resistance to intergranular corrosion also better; 301B and 301S stainless steel nose temperature of 800 ℃, carbide precipitation dominates at low temperatures, high temperature Cr dominant element in the re-distribution of the grain boundaries (healing); Finally the three preventive intergranular corrosion measures, and reduce the carbon content of the system, Add beneficial alloy elements such as: Nb, Ti, etc., and control parts and proportions of carbide precipitates. Understanding of single-phase austenite intergranular corrosion based on a complex system of duplex stainless steel research. Specific research and innovation as follows: DL-EPR method to optimize establish DL-EPR electrochemical method for duplex stainless steel; application DL-EPR analysis precipitated phase dynamics, the evolution of the chromium-depleted zone for the first time as well as selective the intrinsic link between the corrosion sensitivity; and, for the first time using the micro-electrochemical (SECM) scans further study localized electrochemical characteristics of duplex stainless steel microstructure. Studies the optimization of the DL-EPR conditions: solution medium for 2 mol / L of H2SO4 lmol / L HCl, scan rate of 1.66mV / s, the solution temperature of 30 ° C; selective corrosion occurs the root cause of duplex stainless steel phase precipitation, corrosion susceptibility increases with the aging time, until 24 hours after the \The K0 of body 0.009cm s non-active region of ferrite and The Ko 0.056 cm s-1.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Steel > Special performance of steels > Stainless steel, acid-resistant steel
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