【作者】 王超；

【导师】 周国治； 钟庆东；

【作者基本信息】 上海大学 ， 钢铁冶金， 2009， 博士

【摘要】 金属腐蚀是金属材料使用过程中在腐蚀性介质的作用下发生不可逆转消耗的现象。多种防护方法中,涂覆有机涂层是金属防腐最好的方法之一。前人对有机涂层失效中的导电机理做了大量研究,研究方法主要基于电化学交流阻抗谱（EIS）,亦建立了较为完善的理论体系。但是,电化学交流阻抗谱存在不易解析的问题,集中表现为:等效电路不唯一、某些元件物理意义不明确。本文作者通过电容-电位测试、光电化学测试、傅立叶变换红外光谱测试（FT-IR）、Mott-Schottky分析、相角分析,结合EIS,研究了不锈钢、碳钢、黄铜裸电极和有机涂层（酚醛、醇酸、环氧涂层）覆盖的电极在电解质（硫酸、氢氧化钠、硫酸钠、氯化钠）溶液中不同腐蚀时间的电化学行为。研究发现,有机涂层在电解质溶液中失效的过程,存在类似于金属表面氧化膜层的半导体电化学行为。不锈钢、碳钢、黄铜裸电极在电解质溶液中氧化膜的载流子密度均随浸泡时间的延长而逐渐增大。由1000Hz数据计算得三种金属在浸泡10分钟时载流子密度数量级分别约为不锈钢10²⁰-10²¹cm^-3,碳钢10²¹-10²⁴cm^-3,黄铜10²¹-10²³cm^-3,故仅考虑载流子密度因素,三种金属的防腐蚀能力从大到小依次为不锈钢、黄铜、碳钢。不锈钢钝化膜腐蚀的主要原因,在氢氧化钠溶液中为富铬层导电能力增强;而在硫酸、硫酸钠溶液中,为富铁层导电能力的增强;在氯化钠溶液中由于氯离子的侵蚀,不锈钢钝化膜表现出深能级供体离子化行为。四种溶液比较而言,硫酸溶液对黄铜的腐蚀最严重,表面氧化膜层的载流子密度在浸泡10分钟便增加到1023cm-3;黄铜在硫酸钠溶液中为溶解-再沉积的脱锌机制,而在氯化钠溶液中为锌优先溶解机制。根据金属/有机涂层在电解质溶液中腐蚀过程的不同电化学行为,可将有机涂层失效过程分为4个阶段:（1）绝缘体阶段。表现为Mott-Schottky曲线平行于电位轴,电容在扫描电位区间保持恒定,相角接近-90o。（ 2 ） MIS（metal-insulator-semiconductor）结构阶段。涂层表面表现为半导体态,涂层内部仍旧为绝缘态。涂层表面半导体层、涂层内部绝缘层以及金属基体构成了一个MIS结构。此阶段Mott-Schottky曲线出现倾斜,载流子密度数量级一般在1015cm-3以下,载流子密度和空间电荷层厚度呈不规律变化,甚至其后续阶段涂层半导体类型也出现转变。（3）偶极极化阶段。由于外界腐蚀性介质的渗透,涂层内部出现可转动的偶极子,涂层总平均偶极距变得不为零,在外加电场下出现偶极极化现象,由于偶极弛豫作用,电容在负电位区随电位升高而增加,正电位区随电位升高而减小,较高频率下（3000Hz、5000Hz）电容的此种变化规律不明显。（4）空间电荷极化阶段。由于腐蚀的深入,涂层整体表现为半导体态,涂层与金属基体构成金属/半导体接触结构,涂层的载流子密度逐渐增加,相角向0o方向趋进。涂层快速失去对金属的保护能力,其自身功函数下降,金属/有机涂层的载流子输运受涂层孔隙率,空间电荷层容抗,金属基底反应动力学的三重控制。涂层载流子密度数量级在10¹⁵cm^-3以下,涂层具有较好的防护能力,基底金属腐蚀未发生;在10¹⁵cm^-3数量级以上,涂层的防护能力急剧下降,金属腐蚀开始发生;数量级在10¹⁵cm^-3时涂层处于防护性能临界状态。由此证明,载流子密度可以作为定性、定量判断涂层防护性能和基底金属腐蚀程度的参量。利用能带模型可以对金属/有机涂层在电解质溶液中发生的半导体电化学现象机理进行描述。根据傅立叶变换红外光谱数据推断,有机涂层发生半导体转变原因之一,为腐蚀使涂层内部发生降解产生自由基,从而使涂层发生一系列半导体导电行为。更多还原

【Abstract】 Corrosion of metals is a kind of irreversible phenomena of consumption under action of aggressive medium in use of metals. Covering surface of metals with organic coating was one of the most excellent anti-corrosion methods. Lots of researches about conduction mechanism of coatings during their degradation were carried out by predecessors. The leading research technology was electrochemical impedance spectroscopy （EIS） and relatively perfect theory system had been established. But the complex analysis baffled the development of it. The most important problems were non-unique equivalent circuits and indefinite physical meanings of certain circuit components. Capacitance-potential testing, photo-electrochemistry testing, Fourier transform infrared spectroscopy （FT-IR） and Mott-Schottky analysis, phase angle analysis combined with EIS were utilized to study respective semiconductor behaviors of naked stainless steel （SS）, carbon steel （CS）, yellow brass （YB） electrodes and coated electrodes （phenolic coating, alkyd coating and epoxy coating were involved in this paper） immersed in electrolytes （sulfuric acid solution, sodium hydroxide solution, sodium sulfate solution, sodium chloride solution） in this paper. It is discovered that the electrochemical behaviors of coated electrodes during its degradation in electrolytes is similar to the electrochemical behaviors of semiconductor oxide film generated on surface of above mentioned metals in electrolytes.Carrier density of oxide films on surface of naked SS, CS and YB electrodes in electrolytes rises with growing immersion time. The carrier density of oxide films on surface of SS, CS and YB immersed for 10 minutes calculated from the data at 1000Hz respectively is 10²⁰cm^-3-10²¹cm^-3, 10²¹cm^-3-10²⁴cm^-3, 10²¹cm^-3-10²³cm^-3, so just as far as carrier density is concerned, descending sort about anti-corrosion property of the 3 kinds of metal in electrolytes is SS, YB, CS. The main cause of SS’s corrosion in sodium hydroxide solution is the rising conductibility of chromium-rich layer, while it is due to the ascending conductance of iron-rich layer in sulfuric acid and sodium sulfate solutions. Owing to aggression of chloride ions, deep level donors in passive film of SS are ionized. The corrosion action of sulfuric acid solution to YB is more intensive than the other 3 solutions. The dezincification mechanism of YB is resolution-redeposition theory in sodium sulfate solution and is preferential solution of zinc theory in sodium chloride solution.According to the different electrochemical behaviors of metal/organic coating in electrolytes during its degradation, the corrosion process of coatings can be divided into 4 stages. （1） Insulator stage. In this stage, the Mott-Schottky curves parallel to the potential axis, also the capacitance values keep constant in the scanning potential region and the phase angles closes to -90o. （2） MIS structure （metal-insulator-semiconductor） stage. Surface of organic coating is under semiconductor state but inner layer of coating is still under insulating state, so the semiconductor layer, insulating layer and the metal substrate form a MIS structure. In this stage, Mott-Schottky curves of electrodes start to incline and the carrier density’s order of magnitude is generally lower than 1015cm-3. Carrier concentration and thickness of space charge layer irregularly vary with growing immersion time, even, the semiconductor type also transform sometimes in its subsequent stage. （3） Dipole polarization stage. Owing to permeation of outer corrosive medium, rotatable dipoles appear in the coating what make the overall mean of dipole moment being nonzero. Under the action of applied electric field, dipole polarization occurs in the inner of coating. Because of dipole relaxation, the capacitance increases in the negative potential region and the capacitance decreases in the positive region with rising applied potential. Under relative higher frequency （3000Hz, 5000Hz）, these phenomenon are obscure. （4） Space charge polarization stage. In this stage, overall coating changes into semiconductor. The semiconductor and metal substrate make up of a metal/semiconductor contact structure. Carrier density of coating gradually increases with rising immersion time and the phase angle tends towards zero degree. Protective performance of organic coating to substrates is weakened quickly and work function of coating is reduced by corrosion in the stage. The carrier’s transportation of the electrode was controlled by pore resistance of the coating, space charge layer and kinetics of corrosion reaction of metal substrate.When the order of magnitude of carrier density is below 10¹⁵cm^-3, the organic coating has excellent anti-corrosion performance; and above 10¹⁵cm^-3, organic coating’s anticorrosive ability falls drastically, meantime, substrate’s corrosion initiating. 10¹⁵cm^-3 is a critical value. Carrier density can be utilized as a parameter to qualitatively or quantitatively evaluate organic coating’s protection performance and substrate’s corrosion level.Mechanism of semiconductor electrochemical behaviors of metal/organic coating in electrolytes during its degradation can be demonstrated by energy band model. One of the reasons for above-mentioned semiconductor behaviors is organic coating’s degradation which results in generation of free radicals according to datas from Fourier transform infrared spectroscopy （FT-IR）.更多还原