【作者】 邓博；

【导师】 李劲；

【作者基本信息】 复旦大学 ， 物理电子学， 2010， 博士

【摘要】 不锈钢指含铬量超过12%的铁基合金,其中含镍量较高的奥氏体不锈钢是不锈钢家族中使用量最大的钢种,但是在镍资源节约需求、高性价比追求和高耐蚀性要求的多重推动下,双相不锈钢(即多相铁基合金)在近几十年迅速发展。考虑到工程中不锈钢的失效主要起源于两类重要的局部腐蚀类型(点蚀和晶间腐蚀),因此双相不锈钢在开发过程中对其耐点蚀和晶间腐蚀能力的评价则显得极其重要。双相不锈钢组织中含有奥氏体和铁素体两种晶体结构,并存在大量的相界和晶界,从而导致合金元素在其两相及界面处的分配行为截然不同。特别在300-1300℃生产和使用过程中,经常会有二次相(碳化物、氮化物、s、χ和R等)在双相不锈钢中析出,使其形成复杂的多相结构并导致整体材料的腐蚀性能由最弱相决定。考虑到双相不锈钢组织结构的特殊性,如何澄清合金成分分布、二次相析出规律及分布与腐蚀性能之间的内在联系成为双相不锈钢研究的关键问题,需要从以下三个方向开展工作：首先建立适合于新钢种的先进腐蚀评价技术来甄别各类二次相在多相合金中的作用,其次探索新钢种与环境体系之间的腐蚀规律,最后根据热处理原理提出提高新钢种耐蚀性的合金设计与组织控制原则。基于以上背景,本论文在充分理解单相奥氏体不锈钢点蚀和晶间腐蚀发生机制的基础上,提出了评价双相不锈钢耐局部腐蚀能力的新方法(临界点蚀温度、再钝化温度、微区点蚀控制技术和改进的双环电化学动电位再活化法),表征了氯离子浓度对双相不锈钢耐点蚀能力的影响,研究了合金成分与热处理制度(高温固溶和中温敏化区)对双相不锈钢微观结构演变及其耐局部腐蚀能力的影响,给出了典型双相不锈钢的合金设计与组织控制原则,进而为新型双相不锈钢的设计提供了重要的科学依据,具体的研究内容及创新点如下：(1)由于很少有工作集中研究点蚀形貌、点蚀增长及其稳定性三者直接的潜在关系,本工作采用微工作电极电化学恒电位法细致研究了AISI 304L奥氏体不锈钢表面单一的亚稳态和稳态点蚀生长的动力学过程,研究发现：在点蚀增长的初期,腐蚀电流随时间呈I=at2的关系；一旦进入点蚀稳定增长的扩散控制阶段,腐蚀电流与时间呈I=at0.5的关系。特别地,亚稳态点蚀能否继续增长为稳态点蚀取决于点蚀稳定积能否达到临界值ia*=(D△C*)/(2π/3πF)。最后利用模型示意图定性说明点蚀的整个发生过程,揭示了点蚀的本质。(2)首次采用临界点蚀温度、再钝化温度和微区点蚀评价联合技术研究了合金成分和热处理制度对于系列双相不锈钢(UNS S32101、UNS S31803、UNSS32750、SDSS-Ce和DSS-1、DSS-2、DSS-3)微观结构演变及其耐点蚀能力的影响,研究发现：一方面合金成分的调整可显著影响材料的耐点蚀能力和极化行为；另一方面不同热处理制度(高温1000-1250℃固溶处理、中温鼻尖850℃等温敏化处理和中温450-950℃热循环处理)将改变双相不锈钢中典型合金元素在两相中的分配行为,引起大量二次相(M23C6、Cr2N、s和χ)的析出,导致其点蚀行为的变化,具体指临界点蚀温度和再钝化温度的变化、点蚀萌生位置和发展趋势的转移以及电化学极化行为的不同。(3)采用双环动电位电化学再活法(Double Loop Electrochemical Potentiokinetic Reactivation, DL-EPR)系统研究了AISI 304奥氏体不锈钢在550-850℃温度区间敏化不同时间的晶间腐蚀演变情况,并绘制出温度-时间-敏感性曲线。通过观察材料的微观结构,得到碳化物析出形态、位置以及与晶间腐蚀敏感程度之间的对应关系。最后根据碳化物的形核、长大和贫铬区的演变情况,提出AISI 304奥氏体不锈钢整个晶间腐蚀发生过程的理论模型。(4)基于实验重复性、晶间腐蚀选择性、贫铬区检测的高灵敏性和与晶间腐蚀微观结构一致性的基本原则,首次确定了用于评价UNS S32101和UNS S31803双相不锈钢晶间腐蚀敏感性的DL-EPR优化测试条件,并应用于分析析出相动力学、贫铬区演变以及晶间腐蚀敏感性之间的内在联系中,研究发现：一方面由于合金成分和相结构的不同,UNS S32101和UNS S31803双相不锈钢在DL-EPR优化测试条件的建立上存在很大区别；另一方面UNS S32101和UNS S31803双相不锈钢在鼻尖温度敏化过程中,它们的晶间腐蚀敏感性的变化趋势不尽相同。前者的晶间腐蚀敏感性随着敏化时间的延长而增大,后者的晶间腐蚀敏感性在24h达到最大后开始降低,发生贫铬区的自愈合。深入理解微观结构演变和点蚀、晶间腐蚀敏感性之间的内在联系,对于双相不锈钢的开发和使用具有非常重要的科学和现实意义。更多还原

【Abstract】 Stainless steels are iron-based alloys that contain a minimum of approximately 12 wt.% Cr and austenite stainless steels with higher content of Ni constitute the largest stainless steel family. With the requirements of resource saving, high quality and high corrosion resistance, duplex stainless steels (DSSs) have been developed in the past decades. Considering that the failure of stainless steels are mainly induced by pitting and intergranular corrosion, it is very important to evaluate the pitting and intergranular corrosion resistance of DSS during its manufacture process.Because there are two phases with different crystal structure in DSSs, the distributation behavior of alloy elements in ferrite, austenite and interface are quite different. In addition, DSSs are prone to form some unwanted secondary phases (carbides, nitrides, s,χand R) during exposure to elevated temperatures between 300 and 1300℃and their corrosion resistance are determined by the weakest phase. Thus, it is extremely important to clarify the underling relationship between alloy element distribution, secondary phase precipitation and the associated corrosion resistance. Based on the complexity in DSSs, the relative research work have been done from three viewpoints:Firtsly, the advanced techniques for corrosion evaluation should be established for detecting the role of secondary precipitates in multiphase alloys; Secondly, the relationship between new develpoed alloys and environment condition should be expored; At last, the rule for the chemical composition design and microstruture control should be put forward.Through understanding mechanism of pitting and intergranular corrosion of austenite stainless steel, we put forward new methods for evaluating the localized corrosion resistance of DSSs. And then we check the effect of chloride concentration on corrosion resistance of DSSs, investigate the effect of the chemical composition and heat treatment on microstructure evolution and the localized corrosion resistance and finally obtain the rule for the chemical composition design and microstruture control in typical DSSs, which is very useful for the development of DSSs. The detailed research contents and highlight of innovations are as follows:(1) Because there are seldom research focused on the relationship between pitting morphology, growth and stability, this work is to investigate the potentiostatic metastable and stable pitting behaviors of AISI 304L stainless steel. The results demontrate that the current-time follows the prevailing relationship I=at2 during initial growth and then their relationship changes as I=at0.5 during stable growth. The transition from metastable pitting to stable pitting is determined by the pitting stability products, e.g., ia*=(DΔC*)/(2π/3nF). At last, a model is put forward for understanding the whole pitting process.(2) The effect of chemical composition and heat treatment on microstructure evolution and pitting corrosion resistance of DSSs has been investigated by the integrated technique of critical pitting temperature, repassivation temperature and microscopic pitting control. The results show that the chemical composition can greatly affect the pitting corrosion resistance and polarization behavior of DSSs.In addition, different heat treatment will result in the alloy elements redistribution and secondary precipitates, which will change the value of critical pitting and repassivation temperature, the position of pitting initiation and propagation and polarization behavior of the alloy.(3) Intergranular corrosion (IGC) behavior of AISI 304 austenite stainless steel has been evaluated in relation to the influence of aging temperature and time. For this evaluation, double loop electrochemical potentiokinetic reactivation (DL-EPR) is performed to produce time-temperature-sensitization (TTS) diagram for AISI 304 stainless steel. The relationship between carbide morphology, precipitation position and the associated IGC has been determined by microstructure analysis. At last, we put forward a model for IGC evoluation of AISI 304 stainless steel based on the change of Cr concentration at the depleted zone.(4) The optimal test conditions using DL-EPR method for evaluating IGC susceptibility of UNS S32101 and UNS S31803 DSSs have been firstly defined on the basis of test response reproducibility, etching selectivity, high sensitivity to detect the dechromized zone and aggrement with IGC microstructure criteria. The results demonsrate that the optimal test condition of UNS S32101 is quite different from that of UNS S31803 due to the difference of chemical composition and microstructure. In addition, the IGC susceptibility of UNS S32101 DSS increases with increasing the aging time. However, the IGC susceptibility of UNS S31803 DSS approaches the highest value after aging up to 24 h and then decreases with aging time.更多还原