【作者】 石林；

【导师】 郑志军；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2016， 硕士

【摘要】 奥氏体不锈钢由于具有良好的耐均匀腐蚀性能及成型性能而得到广泛应用,但在含有侵蚀性阴离子(比如Cl-离子)的溶液中却容易发生局部腐蚀(点蚀)。尤其是纳米化和双尺度化后,不锈钢的耐点蚀性能显得更为重要。本文正是在此背景下以304奥氏体不锈钢为研究对象,开展了单一纳米晶和双尺度结构不锈钢的点蚀行为和变化规律。首先,对固溶处理的粗晶不锈钢(晶粒尺寸约为50μm)进行8道次等径角挤压(ECAP)获得晶粒尺寸为100 nm的单一纳米晶结构不锈钢;再对单一纳米晶不锈钢进行750℃-30min的热处理,获得体积比62%、平均晶粒尺寸350nm的细晶部分(即基体部分)和体积比38%、平均晶粒尺寸1.4μm的微米级晶粒部分(即嵌体部分)组成的双尺度结构不锈钢。然后利用静态浸泡法、动电位极化曲线、恒电位极化曲线、电化学阻抗及电化学噪声等方法对三种晶粒尺寸(粗晶、单一纳米晶、双尺度结构)的不锈钢进行点蚀性能研究,重点分析了双尺度结构化和纳米化对奥氏体不锈钢点蚀性能的影响规律。结果发现,单一纳米晶不锈钢和双尺度结构不锈钢的耐点蚀性能均优于粗晶不锈钢,而单一纳米晶不锈钢的耐点蚀性能又优于双尺度结构不锈钢。其原因是双尺度结构和纳米晶结构不锈钢在开始阶段(浸泡10s)可以在表面快速形成一层相对致密的钝化膜,保护性更好,而粗晶不锈钢表面形成的钝化膜相对疏松,因此其保护性较差;另外,纳米晶和双尺度结构不锈钢的点蚀生长发展速率比粗晶不锈钢要小得多。同时,也对三种晶粒尺寸分布的不锈钢进行了单个点蚀孔形貌分析。在相同腐蚀介质中浸泡相同时间,粗晶不锈钢点蚀孔直径和深度均更大,点蚀孔基本是沿纵深立体化发展,最终形成“沟壑”状腐蚀形貌;双尺度和单一纳米晶不锈钢表面出现的点蚀孔更小,没有出现明显的深沟状点蚀现象,只是呈现出二维平面方向发展的特征,最后分别形成“地垄”或“层峦”状的点蚀形貌。分析认为,这与纳米化过程导致不锈钢中Mn S杂质细化有关。电化学阻抗谱结果表明,单一纳米晶不锈钢的阻抗值远远大于双尺度不锈钢和粗晶不锈钢,而双尺度不锈钢又大于粗晶不锈钢。另外,粗晶不锈钢在浸泡76h时阻抗谱出现双容抗弧,这表明浸泡76 h后粗晶不锈钢表面便出现明显点蚀现象,而对于双尺度结构和单一纳米晶结构不锈钢来讲,这一时间分别为120h和160h。以上现象说明,在3.5%Na Cl溶液中,单一纳米晶不锈钢具有更稳定的电化学特性,双尺度结构不锈钢次之,粗晶不锈钢最差。最后,分析了三种不锈钢的电位噪声和电流噪声。结果表明,纳米晶不锈钢钝态期(120小时)和点蚀稳定发展期(70小时)的持续时间最长;粗晶不锈钢以上两种阶段持续时间最短,各为48小时和24小时;双尺度结构不锈钢介于二者之间。这是由于单一纳米晶不锈钢表面钝化膜具有更好的保护能力,粗晶不锈钢表面钝化膜保护能力较差,所以单一纳米晶钝态期较长,而粗晶不锈钢钝态期更短;当不锈钢表面钝化膜遭到破坏以后,由于不同晶粒尺寸的点蚀生长过程不同,单一纳米晶和双尺度结构不锈钢点蚀生长较慢所以稳定态点蚀持续时间也比粗晶不锈钢更长。总体而言,在3.5%Na Cl溶液中,单一纳米晶不锈钢的耐点蚀性能最优,双尺度结构不锈钢次之,粗晶不锈钢最差。这是由三者基体结构依次不同导致表面钝化膜致密性依次变差所致。更多还原

【Abstract】 Austenitic stainless steel(SS) has been used widely rafor its excellent general corrosion resistance and formability, but it is easily corroded in the solution containing aggressive anions(such as Cl-). The issue of pitting corrosion resistance of SS is more important especially for SS with the nanocrystalline and bimodal structure. In this paper, the behaviors and evolution rule of the pitting corrosion of SS with the bimodal and nanocrystalline structure was systematically studied and compared with that of theas-received SS with the coarse grain.Firstly, the mono-nanocrystalline SS with about 100 nm grain size was obtained by ECAP of SS with the coarse grain(50μm) for 8 passes. Then, the as-ECAPed SS with nanocrystalline microstructure was annealed at 750℃ for 30 min, which resulted in the bimodal microstructure including the ultrafine grains with a volume fraction of 62 % and an verange grain size of 350 nm and the coarser grains with a volume fraction of 38% and an average size of 1.4μm.Secondly, the pitting corrosion behavior of three SS with different grain size(the coarse grained structure the bimodal structure; the nanocrystalline structure) was systematically investigated by static immersion, potentiodynamic polarization curves, potentiostatic polarization curves, electrochemical impedance spectroscopy(EIS) and electrochemical noise(EN). Additionally, the influence of the nanocrystalline and bimodal structure on the pitting corrosion of SS was also analyzed emphatically. The results showed that the pitting corrosion resistance of the bimodal structure and nanocrystalline SS was better than that of the coarse grained SS, and the nanocrystalline SS was superior in the pitting corrosion resistance to the bimodal structure SS. The reason is that passive films formed on the bimodal structure and nanocrystalline SS are more compact than that formed on the coarse grained SS. In addition, the growth rate of pitting corrosion for the bimodal structure and nanocrystalline was much slower than that of the coarse grained SS,contribuating to the better pitting corrosion resistance of the formers.Meanwhile, the pitting morphologies were also studied. The diameter of pitting hole in the coarse grained SS was the biggest in the same immersion time and same corrosive medium, the pitting hole both longitudinally and horizontally in three-dimensional direction and ultimately the “gully” like morphology is formed on the SS. The pitting holes in the bimodal structured and nanocrystalline SS were smaller, and finally the “ridging” “layering” morphology was formed on both SS, respectively.EIS test showed that the impedance values of SS increased in the decreasing order of the grain size in the coarse, bimodal structured and nanocrystalline SS. The coarse grained SS came into pitting after immersion for 76 h in 3.5% Na Cl, however, this time is 120 h and 160 h for the bimodal structured and nanocrystalline SS, respectively. The results suggested that the nanocrystalline SS had more stable electrochemical properties, the bimodal structured SS is in the second position and the coarse grained SS was in the worst position.Finally, EN of three types of SSs were analysized including potentio EN and current density EN. The results showed that the nanocrystalline SS had the longest duration of passive period(120h) and stable pitting period(70h), and the corresponding value in terms of two preiods mentioned above respectively was 48 h and 12 h, which is smaller than that of NC SS. The corresponding values for the bimodal stracttured SS are in betweent that of the nanocrystalline SS and that of the coarse grained SS. Passive film formed on the surface of nanocrystalline SS obtained higher protection ability. When the passive film of SS was compromised, the growth rate of pitting corrosion of the bimodal structured and nanocrystalline SS was much slower than that of coarse grain after the passive film was destroyed.In a word, the mono-nanocrystalline SS exhibts better pitting corrosion resistance, the bimodal structured SS is in the second position, the coarse grained SS is in the last position. The causes why these phenomena occurs results from a fact that the compactness of the passive film on three SSs decreases in the increasing order of the grain size in three SSs.更多还原