| There is a heavy loss of metal corrosion in the world each year, organic coating is the most simple and effective method to slow the corrosion of steel structure and prevent loss of coat integrity. However, organic coating can be degraded in various corrosion environments and the degradation process of organic coatings are affected by several factors, which are still not clear. The classic electrochemical methods are widely employed in the degradation of organic coatings, but can only get the average electrochemical information of degradation of coating and be hard to obtain the local electrochemical corrosion information. Wire beam electrode technique can get local current distribution, which is an important parameter to research micro-area characteristics of coating corrosion. Therefore wire beam electrode technology is employed in this work, combined with electrochemical impedance spectroscopy technology to investigate the degradation process of organic coating.Wire beam electrode technique is applied to measure current distribution to get the cathodic and anodic distribution characteristics. According to the current distribution obtained, electrochemical impedance spectroscopy of the corresponding local electrode is measured. Through the changes of the current distribution and electrochemical impedance with increasing immersion time, the coating degradation and metal corrosion under the coating are analyzed.This paper mainly contains two parts, the first part: study on degradation of the intact organic coatings coated on wire beam electrode; the second part: study on organic coating coated on wire beam electrode which was polluted by NaCl solution. Two different organic coatings were both immersed in 3.5% NaCl solution. The current map and electrochemical impedance spectroscopy with different immersion time were obtained.According to the data of the current distribution and electrochemical impedance spectroscopy, the following conclusions are obtained: (1) The degradation process of intact coating is divided into three stages: the stage of ooze water not completely penetrated coating/metal interface; the corrosion beginning of metal when corrosive medium seeped into coating/metal interface; the stage of coating failure and the corrosive development of base metal. The impedance spectroscopy of the whole WBE reflects the information of the local coating degradation when local degradation occurs. But when several local coating degradation take place, it will be hard to distinguish each information through the whole impedance spectroscopy.We think that from the coating degradation (impedance from coating to the metal down) to the basal reaction (big corrosion current appears) needs some time. Because usually people think that when the second time constant appearing in the impedance spectroscopy means the corrosive medium has penetrated into coating/metal interface, and corrosion starts to happen, but our results with combined WBE and EIS methods show that obvious corrosion current occurrence will take some time after the second time constant appears in EIS.The current polarity reversal was observed during the coating degradation process. For example, on a single electrode the polarity reversal appears twice. The anodic current is small in the early stage of coating degradation; then the anodic current switchs to cathodic current in the mid-term stage; in the last period current is larger anodic current.(2) In the acceleration experiment of coating gradation, due to the the salt solution on metal surface, metal has started to corrode before coating failed, and EIS contains two capacitance arc. In the whole degradation process, the polluted position has been anodic during the test process.Using WBE and EIS to study the intact coating and the accelerating degradation coating, the above results show that the two techniques are helpful to profoundly understanding the progress of organic coatings aging and steel substrate corrosion. |
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