Study On The Deterioration Processes Of Organic Coating/low Alloy Steel Systems In Simulated Deep-sea Environment

With the excessive consumption of land resources, more and more efforts are being devoted to exploit marine resources in the world. Organic coating is one of the most important methods to protect ocean engineering structures in deep-sea environment. The pressure of seawater could accelerate the deterioration process of organic coating, and it may lead to the damage of ships, oil platforms and submersible boats. Therefore, the research of corrosion behaviors of metal and the deterioration mechanism of organic coating is being paid attention to. This dissertation based on the experiment of simulated deep-sea environment. Corrosion electrochemical behaviors and morphology of low alloy steels under different pressure of seawater were investigated by various electrochemical methods and physical testing technologies. The deterioration processes of intact coatings and the coatings with artificial defect and water transport behaviors in coatings under different pressure were investigated. The evolutions of impedance of coating with immersion time were measured by electrochemical impedance spectroscopy (EIS), and electrical equivalent circuits (EEC) corresponding to the different immersion stages were established. The EIS data of coating/metal systems were fitted to suitable EECs. The deterioration processes of coatings under atmospheric pressure of seawater were studied using the wire beam electrode (WBE) method and electrochemical impedance spectroscopy (EIS). The main results and progresses of this work are outlined as following:(1) The corrosion electrochemical behaviors and morphology of low alloy steels under different pressure of seawater were investigated. The results showed that with the increase of hydrostatic pressure of seawater, the corrosion resistance of X steel and Y steel were deteriorated, which were attributable to the increase of the anodic reaction rates. And the morphology of two steel changed significantly and differently. Under high pressure of seawater, some shallow-dish shape localized corrosion appeared on the surface of X steel, whereas on the surface of Y steel some tunnel localized corrosion appeared.The water transport behavior in the organic coating immersed under 2.5MPa pressure of seawater was investigated. The results suggested that the water transport behavior followed the Fick diffusion laws during the initial period of immersion. Contrast to the water transport behavior under atmospheric pressure, the time that the water uptake of the coating kept a saturation state has been shorten. The"turning point"appeared in the lnC_c t^0.5 curve when the pressure critical value of coating has been reached. Subsequently, the coating capacitance sharply increased and the coating was complete failure.The EIS response characteristic of the deterioration process of organic coating under 2.5MPa pressure of seawater was studied. The results demonstrated that the impedance of coating with artificial defects at 2.5 MPa pressure decreased faster than that under atmospheric pressure, and with the increase of immersion time, the capacitance of the samples increased, and the resistance decreased at 2.5 MPa pressure, indicating that high sea pressure greatly accelerated the degradation of organic coating. Moreover, the effect of accumulation of corrosion products at defect location of coating to the corrosion process of coating with artificial defect under different pressure was investigated.The deterioration processes of organic coating under alternating pressure of seawater were investigated. The results showed that the deterioration process of organic coating has been accelerated under alternating pressure of seawater, and the reason is that the permeation and reversed-permeation of seawater into the coating accelerated the delamination of coating from the substrate metal.(2) The current distribution results of the WBE showed that the substrate metal were corroded seriously because the high anode current appeared in the interface in a long time. And there were the phenomena of polarity transition of current on the coating/steel interface. During the entire coating deterioration process, the EIS characteristics were dominated by the substrate corrosion process of the most serious defect, and the coatings and underlying corrosion process were"averaged"out. Through the combination of the WBE and EIS, the local deterioration of the coating and corrosion process of the substrate metal could be exactly detected.