两种特定腐蚀环境下铜腐蚀行为的研究

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	两种特定腐蚀环境下铜腐蚀行为的研究
	王长罡
学位类型	博士
导师	董俊华
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	腐蚀科学与防护
关键词	Cu 腐蚀高放废物地质处置换热器 Cu Corrosion High Level Radioactive Waste Disposal Heat Exchanger
摘要	" 铜具有良好的导电性、导热性、耐磨性、耐蚀性以及必要的强度，因此被广泛应用于建筑、造船、电力、石油、化工等工业领域。铜具有贵金属性质，因为其平衡电位高于析氢电位而低于吸氧电位，所以在自然环境中只有氧还原才能支持铜的电化学腐蚀。耐腐蚀性能强是铜作为结构材料的较大优势，然而在金属构件服役的某些特殊腐蚀环境中，铜经常因为腐蚀而失效，从而造成严重的环境污染和巨大的经济损失。因此，针对特定腐蚀环境中铜的腐蚀行为进行机理上的分析和防护对策上的研究对于避免因铜腐蚀失效而造成的环境污染和经济损失具有重要的意义。本文主要针对两种特定腐蚀环境，系统研究了铜的腐蚀行为与腐蚀机理。为避免铜构件在相应环境中腐蚀失效、延长铜构件的服役寿命提供了基础数据和理论支持。第一种腐蚀环境为模拟高放废物地质处置环境。装有高放废物的铜处置罐被深埋地下数百米，其周围填充膨润土后掩埋。在漫长的地质处置过程中，铜处置罐周围的环境由初期的有氧状态逐渐转变为无氧状态，带有HCO3-、Cl- 和SO42-的地下水将浸入处置环境并与铜处置罐接触。在初期的有氧状态联合侵蚀性离子的作用下，铜处置罐将发生复杂的腐蚀行为。 Cu处置罐表面的Cu2O膜形成于处置施工前的大气环境，它的存在可以大大降低Cu的腐蚀速率，延长处置罐的服役寿命。然而在处置初期的有氧条件下，当地下水浸入处置环境并与Cu处置罐接触后，Cu2O保护膜将受到地下水化学环境的影响。pH值和Cl-是影响Cu2O保护膜稳定性的主要因素。利用循环伏安测试、扫描电子显微镜、电化学阻抗谱、MS曲线测试等方法和手段法研究了在开放有氧条件下，Cu2O膜的制备方法，以及在硼酸缓冲条件下，pH值和Cl-浓度对Cu2O膜稳定性的影响。结果表明，选择0.1mol/LNa(OH)溶液为体系，电位为-0.0588V（vsSCE）恒电位0.5小时制备的Cu2O表面膜最为致密且纯度高。低的pH值和（或）高Cl-浓度时Cu2O表面膜不稳定。低pH值促进Cu2O部分溶解，Cl-以间隙离子形式进入Cu2O膜。高Cl-浓度时，电极表面膜为Cu2O/CuCl的双层结构。在有氧的HCO3-缓冲条件下，当水环境中含有SO42-和Cl-中的一种或两种离子时，Cu存在发生点蚀的可能性。根据对北山地区地下水离子含量的总结看，该地区地下水的成分满足Cu发生点蚀的条件。因此，在地质处置环境中Cu处置罐将面临点蚀的威胁。利用循环极化曲线测试法和扫描电子显微镜等试验手段研究了在开放有氧条件下，HCO3- /Cl-混合体系、HCO3- / SO42-混合体系以及[HCO3-]=0.08mol/L、Cl-/SO42-混合体系中Cu的点蚀行为。在Cl-/HCO3-体系中，Cu的点蚀可分为活性溶解型点蚀和钝化膜破裂型点蚀。在点蚀敏感区域图的活性溶解型点蚀区。Cu点蚀的几率随[HCO3-]/[Cl-]的改变呈现先增多后减少的变化趋势。高浓度HCO3-时，出现钝化膜破裂型点蚀区。Cl-和HCO3-起协同作用时，Cu才发生点蚀，二者缺一不可。活性溶解型点蚀区域中，Cl-提高点蚀敏感性。随HCO3-浓度升高，点蚀的敏感性先升高后降低，HCO3-对点蚀的作用出现浓度极值。蚀坑的自修复能力和自催化效应均对阴离子较为敏感。钝化膜破裂型点蚀区域中，Cl-促进点蚀,HCO3-抑制点蚀。二者对蚀坑的自催化效应和自修复能力影响较大，对点蚀的诱发能力无明显影响。在SO42-/HCO3-体系中，Cu的点蚀可分为活性溶解型点蚀和钝化膜破裂型点蚀。在点蚀敏感区域图中，Cu由较低HCO3-浓度时的活性溶解型点蚀转变为较高HCO3-浓度时的钝化膜破裂型点蚀。SO42-促进Cu的点蚀。随着HCO3-浓度的升高，点蚀敏感性先增大后减小，HCO3-对点蚀的作用出现浓度极值。蚀坑的自修复能力受阴离子影响较大。在钝化膜破裂型点蚀中，SO42-促进Cu的点蚀。HCO3-抑制Cu的点蚀。蚀坑的点蚀诱发能力受阴离子影响较大。在[HCO3-]=0.08mol/L,Cl-/SO42-体系中，SO42-和Cl-促进Cu的阳极溶解。Cl-降低Cu电极的腐蚀电位，增强其电化学活性。在点蚀敏感区域图中，Cu发生点蚀的临界Cl-浓度为0.02mol/L。当Cl-为低浓度时，SO42-对点蚀敏感性无影响。当Cl-为中浓度时，SO42-抑制点蚀。当Cl-为高浓度时，SO42-先提高后降低点蚀敏感性。无论SO42-浓度的高低，Cl-都能促进点蚀。蚀坑的自催化效应受阴离子影响较大。根据对数据的综合分析得出结论：为减小铜处置罐的均匀腐蚀速率，延长其服役寿命，可采取提高缓冲材料pH值的方法；而为了使铜处置罐免遭点蚀穿孔而发生核泄漏，可采取尽量降低处置库环境中Cl-浓度的手段。第二种腐蚀环境为某换热器内部环境。换热器的壳体为304不锈钢筒体结构，内部由T2紫铜冷凝管和304不锈钢折流板构成。304不锈钢折流板上开有直径稍大于铜管外径的折流孔，这些折流孔的作用是在换热器筒体内固定T2紫铜管。换热器工作时，水在筒体内循环，氟利昂在T2紫铜管内循环。换热器储存保养前，因设备调试用水未排尽，造成筒体底部有残留水存在。筒体内部空气湿度为水汽平衡湿度。铜管腐蚀穿孔的位置处于铜管与折流孔孔壁之间形成的缝隙处以及与自来水水线接触的铜管部位，腐蚀时间为2年。通过扫描电子显微镜分析，金相分析，体式显微镜观察，发现在铜管与不锈钢折流孔构成的所有缝隙部位都发生了严重的腐蚀，而且在少数位置发生了穿孔。润湿实验表明，T2紫铜管外表面与折流孔孔壁之间形成的缝隙足够小，以至于可以对铜管外表面上结露的薄水膜产生虹吸作用，形成连接铜管表面与折流孔孔壁的液体。因此在铜管外表面与缝隙部位铜管表面间存在供氧差异，铜管外表面为富氧区，而缝隙部位铜管表面为贫氧区。电位监测结果表明：表面带有氧化皮的外部铜管的电极电位高于缝隙部位裸铜管的电极电位，二者之间形成电偶电池，折流孔部位的铜管表面为阳极区，外部铜管表面为阴极区。氧浓差电池和电偶电池的联合作用是导致铜管折流孔部位发生严重局部腐蚀的原因。通过扫描电子显微镜分析发现在铜管与水线接触的部位都发生了严重的腐蚀，而且在少数位置发生了穿孔。潮湿的环境使水线上部的铜管表面形成薄液膜，水线部位的铜管与液面形成润湿薄液膜，二者相互连接，使铜管形成连通的电解液环境。水线上部铜管与水线下部铜管间形成供氧差异，水线上部铜管为富氧区，水线下部铜管为贫氧区。通过对腐蚀产物元素分析发现：腐蚀体系中含有HCO3-和SO42-，这两种离子提高了T2紫铜管的点蚀敏感性，促使水线部位铜管发生点蚀。氧浓差电池与点蚀敏感性离子的联合作用是导致水线部位铜管发生严重局部腐蚀的原因。
其他摘要	Because of the characters of good electrical conductivity, thermal conductivity, abrasion resistance, corrosion resistance and the necessary strength, copper is widely used in various industrial environments. As a precious metal, the equilibrium potential of copper is above the hydrogen evolution potential and its electrochemical corrosion could only be supported by oxygen reduction in the natural environment. Therefore, corrosion-resistant is a greater advantage of copper when it was used as structural material. However, due to different forms of corrosion when they are exposed in some certain harsh corrosion environments, copper could failure which will lead to serious environmental pollution and huge economic loss. Then，taking into account the environmental and economic factors, it is significant to study the mechanism of corrosion and protection of copper. In this work, corrosion behavior and mechanism of copper exposed in two special corrosion environments are studied systematically, which will provide basic data and theory support for avoiding the failure and prolonging service life of copper exposed in certain environments. The first kind of corrosion environment is simulated high-level radioactive disposal environment. The bentonite coated copper tank containing high-level waste would be buried deep underground for hundreds of meters. During the process of geological disposal, environment around the copper tank will change from the initial aerobic state to the anaerobic condition. Meanwhile, groundwater contained HCO3-, Cl- and SO42- will be immersed into the disposal environment and contact the copper bank. Under the synergic influence of initial oxygen and aggressive ions, copper tank will be suffered from complicated corrosion behaviors. Cu2O film formed on surface of the copper tank in atmospheric environment could significantly reduce the copper’s corrosion rate and extend its service life. However, in the early aerobic conditions, the copper will be affected by the groundwater chemistry once the groundwater contact the tank. And pH and Cl- are the main factors which will influence the stability of the Cu2O film. By means of cyclic voltammetry, scanning electron microscopy, electrochemical impedance spectroscopy and MS curve test, the preparation method of Cu2O film and the influence of pH and [Cl-] on the stability of Cu2O film were studied in aerobic boric acid buffer solutions. It was found that the most pure and compact Cu2O film could be obtained by imposing the potential of -0.0588V (vs SCE) on copper electrode for 0.5 hour in aerobic 0.1mol/L NaOH solution. In the solution of low pH value and (or) high concentration of Cl-, Cu2O film was instability. When pH value was Low, Cu2O film partly dissolved and Cl- penetrated into the Cu2O film as the form of interstitial ions. When Cl- concentration was high, the electrode surface had a kind of two-layer structure of Cu2O/CuCl. In aerobic HCO3- buffer solution, pitting corrosion of copper could occur if the buffer solution containing either SO42- or Cl-, or both of the two ions. Then according to the components of groundwater in Beishan, the copper disposal tank will be suffered from pitting corrosion in the geological disposal environment. By means of cyclic polarization curve test method and scanning electron microscopy, pitting behavior of copper has been studied in the aerobic solution of HCO3-/Cl- mixed system, HCO3-/SO42- mixed system, and [HCO3-]=0.08mol/L, Cl-/SO42- mixed system. The pitting corrosion behaviors of Cu could be divided into the type of active dissolve and the type of film rupture in Cl-/HCO3- mixed solution. The probability of Cu pitting firstly increased and then decreased with the increase of [HCO3-]/[Cl-]. The area of film rupture type pitting appeared in the solution with high [HCO3-]. The pitting of Cu only occurred in the solution with both of HCO3- and Cl-. In the area of active dissolve type pitting, Cl- could improve pitting. With the increase of [HCO3-], HCO3- firstly increased and the decreased the Cu pitting sensitivity. The self-repair ability and self-catalyze effect of Cu pitting were all sensitivity to Cl- and HCO3-. In the area of film rupture type pitting, Cl- could improve pitting and HCO3- could inhibit pitting. The self-repair ability and self-catalyze effect of Cu pitting were all sensitivity to Cl- and HCO3-, however the pitting-induced ability of Cu was not sensitivity to Cl- and HCO3-. The pitting corrosion behavior of Cu could be divided into the type of active dissolve and the type of film rupture in SO42-/HCO3- mixed solution. The pitting of Cu changed from active dissolve-type to film rupture-type with the increase of [HCO3-]. SO42- could improve pitting. With the increase of [HCO3-], HCO3- firstly increased and the decreased the Cu pitting sensitivity. The self-repair ability of Cu pitting was sensitivity to SO42- and HCO3-. In the area of film rupture-type pitting, SO42- could improve pitting and HCO3- could inhibit pitting. The pitting-induced ability of Cu was sensitivity to SO42- and HCO3-. In the mixed solution of [HCO3-]=0.08mol/L ,SO42-/Cl- , SO42- and Cl- could promote the anodic dissolution of copper electrode, Cl- could reduce the corrosion potential of Cu to enhance its electrochemical activity. In the area picture of pitting sensitivity of Cu, the pitting critical concentration of Cl- was 0.02mol/L.When [Cl-] was low, pitting susceptibility of Cu was not significantly effected by SO42-; when [Cl-] was in the center, SO42- played a significant inhibitory effect on Cu pitting; when [Cl-] was high, SO42- played a reducing role in the first and then a increasing role in pitting susceptibility of Cu. Regardless of the concentration of SO42-, Cl- could promote the pitting of Cu. In this system, the self-catalytic effect of Cu pitting was sensitivity to SO42- and Cl-. Comprehensive analysis showed that: In order to reduce the corrosion rate of copper disposal tank and extend its service life, the method of improving the pH value of buffer material could be taken; in order to avoid pitting corrosion perforation of copper disposal tank, the method of minimizing [Cl-] of the repository environment could be taken. The other kind of corrosion environment is a heat exchanger internal environment. The shell of heat exchanger was made of stainless steel 304. Inside the shell there are T2 copper tube and 304 stainless steel baffle plate. On the baffle plate, there are holes with slightly larger diameter than the T2 copper tube outer diameter, which are used to fixe the copper tube. When the heat exchanger was working, water cycled in the heat exchanger, and Freon cycled in copper tube. Before the storage of heat exchangers, residual water used for equipment debugging was still in bottom of the exchanger. Then the relative humidity in the shell was the water vapor equilibrium humidity. There were two kinds of perforation position on T2 copper tube. The first one was in the gaps between the T2 copper tube and baffle hole, the other one was along the waterline. The corrosion time was 2 years. By SEM, OM, stereo microscope observation, it was found that serious corrosion happened on the surface of copper in the gaps constituted by copper tube and stainless steel baffle holes, and perforation occurred in a few locations. Wetting experiments showed that the gap formed between the T2 copper tube and the baffle hole was small enough that it could produce siphon liquid film, which could connect the copper tube surface and baffle hole. Therefore there was a difference of oxygen supply between the copper tube outer surface and the copper tube in the gap, the outer surface of tube becomed oxygen-rich zone and the tube in the gap oxygen-poor zone. Potential monitoring results show that the potential of the external surface of copper tube with anoxide is higher than that of copper in the gap site leading to a galvanic cell formed between them. The surface of copper in the gap site is anode region and the external copper tube surface is cathode region. The differences of oxygen supply combined the effect of the galvanic cell lead to the severe localized corrosion of copper tube in the gap site. By scanning electron microscopy observation, it was found that severe corrosion happened at the contact part of copper tube and waterline, and perforation occurred in a few locations. Due to the humid environment, thin liquid film formed on the surface of copper tube above the waterline, and wetting thin film formed on the special part of copper tube along the waterline, both film was connected to each other, therefore a connected electrolyte environment formed. There was difference of oxygen supply between the parts of copper tube above and below the waterline, the part above the waterline was oxygen-rich zone and the part below the waterline was oxygen-poor zone. Result of element analysis of the corrosion products showed that there were HCO3-and SO42- in the corrosion environment, which could improve the pitting sensitivity of T2 copper tube. Oxygen concentration cell combined with effect of aggressive ions lead to severe localized corrosion on the waterline parts of copper tube."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64505
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	王长罡. 两种特定腐蚀环境下铜腐蚀行为的研究[D]. 北京. 中国科学院金属研究所,2012.