阴极保护作用下模拟剥离涂层底部Q345钢的腐蚀行为

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	阴极保护作用下模拟剥离涂层底部Q345钢的腐蚀行为
	吴志斌
学位类型	硕士
导师	孙超 ; 李京
	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	材料加工工程
关键词	阴极保护缝隙腐蚀电位电流扰动 Cathodic Protection Crevice Corrosion Potential Current Agitation
摘要	"阴极保护和有机涂层组成的联合防护系统已被广泛应用于解决自然环境中金属构件腐蚀问题。然而，涂层内不可避免地存在针孔和裂纹等形式的漏点，在阴极保护下，这些漏点处的涂层易发生阴极剥离形成缝隙。已有研究表明缝内金属表面可遭受均匀腐蚀、点蚀、应力腐蚀开裂和微生物腐蚀破坏，造成结构件断裂，引发安全事故。因此，研究阴极保护能否有效抑制缝内金属腐蚀具有重要的实际意义。目前，对阴极保护作用下低电导率环境中缝内金属腐蚀行为的认识尚未统一；对缝内电流分布的研究较少；缝内电位变化机制不明确；关于阴极保护有效性的已有认识集中在静态环境中，而对动态环境中的研究很少。在此背景下，本文采用分段钢电极技术，测量了阴极保护下模拟剥离涂层底部Q345钢表面电位和电流的连续变化以及实验结束时缝内溶液的平均pH和电导率，同时借助动电位扫描技术，研究了静态低电导率环境中，阴极保护下模拟剥离涂层底部Q345钢的腐蚀行为，静态环境中缝内电流分布和缝内电位随时间变化机制，动态环境中阴极保护抑制缝内金属腐蚀有效性，得到了如下结论： 1、静态低电导率环境（电导率 0.07 ms/cm）中，阴极保护电流进入缝内范围有限，这和理论计算结果一致。缝内一直存在电偶电流和局部腐蚀电池，这表明阴极保护不能有效抑制缝内钢片腐蚀。此外，阴极极化过程中，缝底钢表面测得的阴极电流为局部腐蚀电池诱发的电偶电流，并非已有工作所认为的阴极保护电流。 2、在静态溶液中，溶液电导率、阴极保护电位、以及溶液氧浓度对阴极极化过程中缝内电流分布有明显影响。溶液电导率越高，阴极保护电位越负，溶液氧浓度越浓，钢片表面阴极电流越大。然而，上述因素对稳态时的电流密度分布影响较小。 3、阴极保护下，高电导率环境中模拟剥离涂层底部钢电位随极化时间不断负移的机制为电荷传递极化，溶液欧姆电阻极化和化学极化共同作用。低电导率环境中，缝内电位变化机制和钢片所处的位置以及极化时间有关。缝口附近钢片为电荷传递极化，溶液欧姆电阻极化和化学极化共同作用；缝深处钢片起始时受化学极化和钢表面状态共同影响，随着极化时间的延长，也会发生电荷传递极化和溶液欧姆电阻极化；缝底部的钢片电位变化主要为化学极化和钢表面状态变化所致。 4、扰动作用能够迫使本体溶液中的氧进入缝内，降低缝内Q345钢的阴极极化程度。这导致相比静态高电导率环境，动态环境中需要更高水平的阴极保护，才能有效抑制缝内钢片腐蚀。在低电导率环境中，扰动作用还能够在缝内诱发明显的氧浓差腐蚀电池，加剧缝底部Q345钢的腐蚀，此时，很难通过阴极保护抑制缝隙腐蚀。另外，缝隙宽度和漏点尺寸对扰动改变缝内极化水平的影响明显。"
其他摘要	"A synergistic system of cathodic protection (CP) and organic coating has been widely applied in protecting structural metal from corrosion in natural environment. However, coating inevitably develops holidays, such as pinholes, cracks and so on. Crevice forms when the coating in the vicinity of holiday is cathodically disbonded from metal substrate under the influence of CP. The existing results have confirmed that general corrosion, pitting corrosion, stress corrosion cracking, or microbiological influenced corrosion can occur on metal surface in crevice, and leads to the rupture of structural metal which causes safety accidents. Thus, it is of practical significance to study the effectiveness of CP inhibiting corrosion in crevice. To date, in the presence of CP, unanimous understanding on the corrosion behavior in crevice in stagnant solution of low conductivity has not been obtained; researches on current in crevice are not as many as those on potential; the mechanism of potential shifting has not been clarified; the effectiveness of CP inhibiting corrosion in crevice in agitated solution is also unknown. In this thesis, on the basis of segmented steel electrodes, the continuous variations of potential and current in crevice were recorded, and the average pH and conductivity of crevice solution at the end of experiment were also measured. Meanwhile, the potentiodynamic scanning measurements were employed. Then, the corrosion behavior of Q345 steel beneath disbonded coating in stagnant solution of low conductivity was studied. Moreover, the current distribution in crevice and the mechanism of potential evolving with time in stagnant solution were discussed. The effectiveness of CP inhibiting corrosion in crevice in agitated solution was also studied. The conclusions are summarized as follows: 1. In stagnant solution of low conductivity (0.07 ms/cm), CP current flows into a finite region in crevice, in line with the theoretical distribution of current. The coupling current and local corrosion cells are always located on steel surface in crevice, indicating that the CP ineffectively protects steel in crevice from corrosion. Furthermore, cathodic current measured at the rear of crevice, which was considered to be CP current in previous studies, is the coupling current from local corrosion cells. 2. In stagnant solution, the effects of solution conductivity, CP potential and dissolved oxygen concentration on transient distributions of current in crevice are significant. The higher the solution conductivity, more negative CP potential, higher dissolved oxygen concentration, the more cathodic current in crevice. However, the above factors have insignificant effects on steady distributions of current in crevice. 3. In high conductivity solution, the negative shift of potential in crevice is in relation with the combined effect of charge transfer polarization, ohmic resistance of electrolyte and the change of solution chemistry on steel surface. In low conductivity solution, the shifting mechanism of potential in crevice varies with distance from crevice mouth and polarizing time. Shift of potential near the crevice mouth is caused by the factors similar with those in high conductivity solution. The initial variation of potential in deep region of crevice is attributed to crevice solution chemistry and steel surface state. With time increasing, CP current can arrive there, and potential in deep region can be also influenced by the charge transfer polarization and ohmic resistance of electrolyte. Variation of potential at the end of crevice is always controlled by the crevice solution chemistry and steel surface state. 4. Agitation forces the oxygen in bulk solution to enter the crevice, and decreases cathodic polarization level of Q345 steels in crevice. Thus, in agitated solution of high conductivity, more negative CP potential is necessary to inhibit corrosion in crevice effectively, compared with that in stagnant solution. In low conductivity solution, agitation results in the oxygen concentration cell, which leads to the severe corrosion in crevice. This result indicates that it is unfeasible to effectively protect steel in crevice by CP. Furthermore, crevice thickness and holiday size have significant influence on the phenomenon of agitation weakening the polarization level in crevice."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64538
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	吴志斌. 阴极保护作用下模拟剥离涂层底部Q345钢的腐蚀行为[D]. 北京. 中国科学院金属研究所,2012.