【作者】 梁利花；

【导师】 王佳；

【作者基本信息】 中国海洋大学 ， 海洋化学工程与技术， 2009， 博士

【摘要】 大气腐蚀是金属材料在大气环境下发生的一种电化学腐蚀过程。金属材料的大气腐蚀机制主要是材料受到大气中所含的水份、氧气和腐蚀性介质的联合作用引起的电化学破坏。当金属表面存在吸湿性较强的固体沉积物时,它能够从大气中吸附水份,促进金属表面薄液膜的形成,诱发和加剧电化学腐蚀过程。微液滴现象是在大气腐蚀初期出现的一种实验现象。在合适的相对湿度下,大气中的水汽会在金属表面凝聚吸附形成水膜而诱发大气腐蚀。沉积的无机盐颗粒会增强水汽的凝聚和吸附过程从而使其更容易发生潮解形成液滴,当潮解形成的液滴与金属形成的组合体系具有腐蚀性时,那么,在潮解液滴的周围将会有大量微米直径的微液滴形成并向四周扩展。但是如果潮解液滴对金属材料不具有腐蚀性,微液滴将不会形成。由此可见,微液滴现象与大气腐蚀密切相关。本论文采用电化学技术结合显微镜在线观察方法,对微液滴的形成特征、形成动力、形成途径等进行了研究,探讨了微液滴现象与大气腐蚀发展行为的相关性,并建立了微液滴的形成机理。研究发现自然环境条件下,合适的环境相对湿度、氧气环境和腐蚀性的组合体系是微液滴形成的必备条件。只有当环境相对湿度高于同温度下主液滴饱和溶液的相对湿度值时,微液滴才能形成;并且微液滴只在有氧环境下和腐蚀性组合体系中才能形成。采用电化学极化方法对微液滴的形成动力进行了探讨。发现主液滴边缘的三相界面区性质对微液滴形成能够产生重要影响。通过改变界面区的电化学界面状态,发现即使在较低的环境相对湿度下,无氧环境下以及非腐蚀性组合体系中,只要通过电化学极化使金属表面的剩余电荷达到一定程度,微液滴现象也能出现。电化学界面状态是微液滴形成的决定性条件,即形成动力。显色实验表明微液滴是由主液滴蒸发出的水气通过气相环境再在金属表面吸附凝聚形成。三相界面区接触角实验反映了电化学极化会导致液/固界面能的降低。根据以上实验结果,微液滴的形成机理可以描述如下:电化学极化不仅降低了液/固界面能,使三相界面微区的水分子打破平衡状态而从主液滴逸出,导致微区环境相对湿度增大达到过饱和状态;同时,电化学极化产生电流,由于Peltier效应的影响,使阴极区温度降低。三相界面微区局部水气过饱和以及阴极区温度降低,这都将促使水汽在阴极区发生凝聚形成众多小液滴,即微液滴形成。最后,显微镜连续在线观察实验结果表明,微液滴能够促进金属表面薄液膜的形成,促使金属表面不同部位的液膜扩展和交联,形成大面积连续液膜,从而促进腐蚀过程的发展。更多还原

【Abstract】 The atmospheric corrosion of metals is usually electrochemical corrosion process, and which is the interaction result of water vapour, oxygen and corrosive media contained in the atmosphere. Air contaminant such as hygroscopic particles deposited on metal surface could absorb water molecules and promote formation of thin electrolyte layers. The formation of thin electrolyte layer on metal surface could accelerate the development of atmospheric corrosion.The micro-droplets formation on metal surfaces, an important phenomenon closely related to atmospheric corrosion, can be described as follows. Under suitably relative humidity, salt particles deposited on metals surface can absorb water molecules to form electrolyte droplet (primary-droplet) and when this primary-droplet can corrode the metal substrate, then, there will be a large amount of tiny water droplets with 1～10μm in diameter formed (micro-droplets) around primary-droplet, which have a closely relationship to the atmospheric corrosion initiation.In this paper, the behavior characteristics of micro-droplets were studied in the climate chamber, and the role of micro-droplets in the atmospheric corrosion process were also investigated using microscope observation method and electrochemical polarization techniques.It was found out that micro-droplets could only form in a corrosive combination of metal and electrolyte droplet, namely, the metal substrate could be corroded for the primary-droplet, and appeared easier at higher relative humidities (RH), while lower RH and oxygen-free atmosphere were helpless for the micro-droplets formation.Formation power of micro-droplets was systematically studied by electrochemical polarization method. The results show that electrochemical polarization of the three-phase boundary was the critical condition for micro-droplets formation. Cathodic polarization accelerated micro-droplets formation and anodic polarization reduced the formation of micro-droplets, independent of metal type, electrolyte species, composition of air, and environmental humidity. That is to say that the electrochemical interface state was the key condition for micro-droplets formation on metal surfaces.Phenolphthalein test proved that the micro-droplets came from the adsorption and condensation of the water vapor around the primary-droplet through the gas phase, instead of through surface spread from the primary-droplet. Contact angle test reflected electrochemical polarization could lead to the decrease of liquid-metal interfacial energy.Based on the above-mentioned results, negatively charged three-phase boundary could caused a decrease in local surface tension of liquid/metal interface and cooling of the metal surface near to the three-phase boundary, which led to the local liquid evaporating and condensing again on the nearby metal surface were regarded as the mechanism of micro-droplets formation.In addition, the in situ observations indicated that micro-droplets could intensify metal surface wetting capacity and promote formation and spreading of thin electrolyte layer. Meanwhile, micro-droplets could accelerate thin electrolyte layers distributed on various locations merging together to form continuous thin film. The formation of thin electrolyte layer on metal surface means that atmospheric corrosion steps in the rapid development stage.更多还原