【作者】 张伟；

【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， 2010， 博士

【摘要】 有机涂层由于能在金属表面形成一层有机膜阻止腐蚀性介质的渗入而被广泛的应用于海洋环境金属构件的腐蚀防护。但是,不可避免的,有机涂层会在众多复杂的海洋环境腐蚀因素的协同作用下逐渐劣化失效。在这些因素中,水、溶解氧及干湿交替是三个重要的影响因素。有机涂层的吸水过程在涂层的劣化过程中扮演着重要角色,它是涂层屏蔽能力降低,界面腐蚀电化学反应及涂层剥离的前奏。研究发现,氧在涂层/金属界面区的还原是导致涂层剥离的重要因素。然而,当涂层处于干湿交替状态下时,涂层的渗水过程和氧在界面区的还原过程都将被强化。本文的主要研究工作包括：(1)应用电化学阻抗谱技术,结合涂层失效过程的开路电位和表面形貌变化对比研究了全浸泡、4-4h和12-12h干湿循环条件下的有机涂层失效过程。根据阻抗谱响应建立了与各个阶段相对应的等效电路模型,通过解析参数分析了涂层劣化过程涂层性能和涂层下腐蚀反应发生发展的特征。研究发现,根据涂层失效电化学阻抗谱响应特征,全浸泡和干湿交替条件下的有机涂层失效过程均可以分为三个主要阶段：涂层渗水阶段,基底金属腐蚀发生阶段和基底金属腐蚀发展与涂层失效阶段。根据涂层失效过程的阻抗谱结构,等效电路,腐蚀电动势,界面活性区面积百分比和涂层及涂层下的金属腐蚀形貌等多参数多方法分析了涂层的各个子腐蚀过程特征。与单一参数分析相比,这种多参数相关分析从多个不同角度分析涂层失效过程,能在多个平行信息比较印证下获得渗入可靠的研究结果。与浸泡条件下的涂层失效过程相比,不同干湿循环比下的涂层失效过程呈现出较大的差别。4-4h干湿明显加速了涂层的失效过程,主要的加速阶段是界面腐蚀反应开始后的腐蚀反应起始阶段和基底金属腐蚀发展与涂层失效阶段。而12-12h干湿循环却明显的延长了涂层的失效过程。在涂层渗水阶段,4-4h干湿循环与浸泡条件下的涂层渗水时间相当,小于12-12h干湿循环。原因是4-4h干湿循环频率较高,干燥过程涂层失水不完全,又很快进入润湿状态,所以其渗水阶段与浸泡涂层相当。而12-12h干湿循环涂层由于干燥时间较长,涂层干燥较4-4h彻底,所以涂层渗水过程缓慢。基底金属腐蚀发生阶段,4-4h干湿循环与12-12h干湿循环用时相当,远小于浸泡涂层。原因是在干湿循环的干燥阶段,随着液膜厚度逐渐减小,快速的氧还原反应加速了涂层的剥离,使得涂层劣化过程快速进入基体金属腐蚀发展与涂层失效阶段。进入基底金属腐蚀发展与涂层失效阶段后,4-4h干湿循环涂层快速失效,浸泡涂层其次,12-12h干湿循环用时最长。原因是在干湿循环条件下,干燥过程中随着涂层表面水分的蒸发,表面电解液膜层变薄加速了溶解氧向涂层/金属界面区的传输速度和界面区溶解氧的浓度,这直接加速了金属表面阴极还原反应,并导致阴极剥离过程增强和涂层从金属表面的快速剥离。和浸泡涂层相比,干湿循环涂层经历了反复的从湿润到干燥的循环过程,导致涂层很快发生剥离而失去保护作用。这可能就是干湿循环加速涂层失效过程的本质。而对于12-12h干湿循环涂层,虽然在干燥阶段也出现了快速的氧还原反应,但是由于干燥时间长,涂层失水完全,没有了电解质离子在阴阳极区域间的导通,界面腐蚀反应处于停止状态。当涂层表面再次浸泡润湿时,涂层渗水过程重新开始。所以,长时间的干燥阶段使得涂层渗水过程缓慢,再加上干湿循环比较低,导致干燥阶段氧的快速还原加速涂层剥离过程并未明显表现出来。涂层失效时,4-4h干湿循环界面活性面积百分比达到19%,远大于浸泡和12-12h干湿循环,且涂层下金属腐蚀较轻,较均匀。而浸泡和12-12h干湿循环则呈现出严重的局部腐蚀。原因也是因为4-4h干湿循环干燥阶段快速的氧还原反应加速了涂层的剥离,而浸泡和12-12干湿循环则由于缺少快速的氧还原反应,使得涂层下的金属腐蚀反应被局限在一个较小范围内,表面为较严重的局部腐蚀研究结果表明,干湿循环干燥阶段对涂层的影响有两个方面,一是干燥阶段随着水溶液的渗出,涂层微孔收缩,防护性能增强。但另一方面,也加速了溶解氧向界面区的扩散,加速了涂层的剥离。所以干湿循环对涂层劣化过程的影响取决于干燥阶段和浸泡阶段的比值。(2)结合EIS和WBE技术研究结果发现,缺陷涂层阵列电极初始阴极和阳极电流均出现在缺陷区,随着腐蚀过程的发展,阳极电流仍保持在缺陷区,但阴极电流逐渐向涂层完好区扩展,这说明涂层下的剥离反应是由于阴极还原反应的结果,阻抗谱测试结果表明,缺陷涂层阵列电极阻抗响应主要反映了缺陷区电极过程特征,直至涂层完好区也出现剥离和鼓泡的失效现象。而阴极区涂层性能和涂层下金属腐蚀反应信息被“掩盖”。结果证实,阵列电极(WBE)技术能够提供涂层下不同位置的电流分布,分辨出金属表面阴阳极腐蚀反应区域,结合阻抗谱(EIS)技术可以对涂层下任意局部位置进行阻抗谱测试,提供准确的局部涂层劣化过程和涂层下金属腐蚀反应信息。更多还原

【Abstract】 Organic coatings are widely used to control the corrosion of steel structure, both to maintain appearance and to prevent loss of structural integrity. Thus, organic coatings should isolate the metal from the corrosive environments. However, organic coatings can be degraded under various service conditions, because they comprise polymeric material, leading to reduction of its anti-corrosive effectiveness. The degradation process of organic coatings is complex since it is affected by several factors. Under weathering exposure, wet-dry cycles, H2O and O2 are three critical factors for coatings degradation.The evolution of the water absorption process in an organic coatings immersed in a solution is a very important phenomenon because it precedes phenomena such as paint/metal interface loss of adhesion, electrochemical reactions and the reduction of the resistance to the passage of ions through the coating itself. Kinds of important effects of water penetration have been reported and the parameter of water up-take is often used for predicting anti-corrosive performances of organic coatings. It has been proven that the loss of adhesion between the organic coatings and the iron sample is induced by cathodic delamination which is caused by reaction products, like radicals from the paint/metal interface redox. The synergistic effect of oxygen reduction and water penetration was strengthened by wet-dry cycles during the coatings degradation and interface metal corrosion processes.The corresponding experimental results are listed as follows:(1) Under immersed and wet-dry cyclic conditions, the deterioration processes of the organic coating on carbon steel surface have been comparatively studied using electrochemical techniques. The wet-dry cycles are carried out by exposure to 4 h immersion and 4h drying (4-4h cycles) and 12h immersion and 12h drying (12-12h cycles), respectively. The immersion condition is carry out in a 3.5% NaCl solution and drying at 298K and 50% RH. According to the EIS responses under above three states, the equivalent circuit models are established, and then the changes of coating performance and the underlying metal corrosion processes are evaluated by analyzing the fitting parameters.According to the EIS characteristics, the entire deterioration processes under above three mentioned conditions can be divided into three main stages, consisting of the medium penetration into coatings, corrosion initiation and corrosion extension underlying coatings. The deterioration processes are assessed depending on the EIS characteristics, the equivalent circuits, open circuit potential, the morphology pictures of the coatings surface and the corresponding metal substrate analysis. Comparing with single parameter analysis, The multi-parameter correlation analysis is able to provide rich correlated information from different analysis aspects on the same sub-process, giving reliable evaluation in details on the coatings performance and deterioration mechanism.Comparing with the immersed, the 4-4h wet-dry cycles greatly accelerate the entire deterioration process; especially during the corrosion initiation and the corrosion extension periods, leading the paint system lose its anti-corrosive performance in a short period. However, the 12-12h wet-dry cycles decelerate the entire deterioration process, prolonging the anticorrosive ability.During the medium penetration stage, the 4-4h wet-dry cycles and the immersed coatings experienced the same degrading period, shorter than the 12-12h wet-dry cycles. For the 4-4h wet-dry cycles, the cyclic ratio is so higher that the absorbed water during the wetting period do not lost completely when it comes into the drying process, and then the coatings enter the next immersion cycles again. So, the coatings wetting situation is the same like the immersed. But for the 12-12h wet-dry cycles, the penetrated water can be exudates completely for the long drying process.However, comparing with the immersed, both the 4-4h and the 12-12h wet-dry cycles accelerate the corrosion initiation period for the faster oxygen transportation and reduction at the polymer/metal interface during drying stages. The oxygen reduction at the coating/metal interface has to be regarded as the main detrimental reaction which gives rise to the destruction of the interface and, therefore, to the extension of the delaminated zone. The oxygen transportation is much faster in air pore of the coatings during drying process than the immersed state, and the high oxygen content in the wetting coating accelerate the cathodic reduction rate resulting in a rapid coating delamination from the metal substrate.When it comes into the corrosion extension stages, for the 4-4h wet-dry cycles, the quickly oxygen reduction at the coating/metal interface during drying stage accelerate coating delaminating process, making the coatings lost its anti-corrosive ability quickly. For the immersed state, without the fast oxygen transportation process, the delamination speed is lower, and the anodic reactions can only take place in the limited blisters, making heavy local substrate corrosion. However, although the 12-12h wet-dry cycles also experience the fast oxygen reduction processes during the drying stages, the longer dying period made the water desorbed completely and then the underlying metal corrosion stagnated without the connection of the electrolyte solution between the anodes and canodes. So, as the results show, comparing with the immersed, the deterioration process are decelerated.The current distribution results of the WBE show that the high anode current and cathodic current are found at the defect when the coating was immersed in the 3.5% sodium chloride solution, and then the new cathodic lacations are found around the defect, meaning that the coating delamination is accelerated by the oxygen reduction process in the interface.During the entire coating deterioration process, the EIS diagrams are dominated by the substrate corrosion process of the defect, the coatings and the underlying electrochemical process are "averaged" out. However, through detecting the current distribution information on the metal surface, the local electrochemical process occurring in the coatings and the underlying can be monitored.The responses of EIS and WBE are consistent with the progress of the degradation of organic coatings. Through the combination of the WBE and EIS, the local coating deterioration and substrate metal corrosion processes can be detected. The results show that the WBE and EIS techniques are available to study the progress of breakdown of organic coatings on a steel substrate.更多还原