Corrosion Behaviors Of A Carbon Steel/Copper-nickel Alloy Couple Under Organic Coating With Artificial Defect In Static And Flowing Seawater

Dissimilar metals were often overlapped in marine engineering equipment,which could easily cause galvanic corrosion of the metals in harsh marine environments.Organic coating was one of the most commonly used methods to delay metal corrosion.Organic coating was affected by various environmental factors in the marine environment,which led to its performance degradation or complete failure.Therefore,the organic coating/dissimilar metal couple system was taken as the research object in this paper,and the immersion experiments of the carbon steel/copper-nickel alloy couple under organic coating with artificial defect in static and flowing seawater were carried out.Corrosion behaviors of the carbon steel/copper-nickel alloy couple under organic coating with artificial defect were studied by current density distributions of the wire beam electrode(WBE),evolution of the electrochemical impedance spectroscopy(EIS),cathodic delamination of the coating,surface morphology of the electrodes and current density changes of the single electrodes.The main results were as follows:(1)When the sample of the coating with artificial defect at the carbon steel area was immersed in static seawater,the carbon steel electrode at the defect was mainly acted as an anode,and the corrosion reactions of the electrode were accelerated,while other electrodes of the WBE were mainly acted as a cathode to be protected to a certain extent in the whole test period.When the sample was immersed in flowing seawater for the first 186 h,the anodic reaction of the electrode at the defect was accelerated;after immersed for 186 h,the carbon steel electrodes at the defect and under the coating were mainly acted as anodes,while the copper-nickel alloy electrodes were mainly acted as cathodes.(2)When the sample of the coating with artificial defect at the carbon steel area was immersed in static seawater,the degree of cathodic delamination of the coating at the carbon steel area was obviously higher than that at the copper-nickel alloy area,which was mainly related to the electrochemical activity of the metal and the location of the coating defect on the WBE.The analysis results of the sample in flowing seawater were the opposite.During most of the immersion time,the degree of cathodic delamination of the coating at the carbon steel area in flowing seawater was significantly lower than that in static seawater;the degree of cathodic delamination of the coating at the copper-nickel alloy area in flowing seawater was obviously higher than that in static seawater.(3)When the sample of the coating with artificial defect at the copper-nickel alloy area was immersed in static seawater for the first 186 h,the anodic reaction of the copper-nickel alloy electrode at the defect was accelerated;after immersed for 186 h,part of the carbon steel electrodes under the coating were acted as anodes,and the corrosion reactions of these electrodes were accelerated.When the sample was immersed in flowing seawater,the carbon steel electrodes under the coating were mainly acted as anodes during most of the time of immersion,and the corrosion reactions of these electrodes were accelerated,while the copper-nickel alloy electrodes at the defect and under the coating were mainly acted as anodes,and the reason was that the high dissolved oxygen content in flowing seawater promoted the cathodic reduction reaction.(4)When the sample of the coating with artificial defect at the copper-nickel alloy area was immersed in static seawater,the degree of cathodic delamination of the coating at the carbon steel area was obviously higher than that at the copper-nickel alloy area,which was contrary to the analysis results in flowing seawater.During most of the immersion time,the degree of cathodic delamination of the coating at the carbon steel area and the copper-nickel alloy area in flowing seawater was significantly higher than that in static seawater,respectively,which was related to the high dissolved oxygen content in flowing seawater promoting the cathodic reduction reaction.(5)In static and flowing seawater,there were electrochemical inhomogeneities of carbon steel and copper-nickel alloy under organic coating with artificial defect during galvanic corrosion.(6)In static and flowing seawater,polarity reversals of some carbon steel electrodes under the organic coating with artificial defect were observed during the corrosion process.The reason for the polarity reversal from cathodic to anodic was that the anode dissolution reaction occurred,while the polarity reversal from anodic to cathodic was mainly ascribed to the involvement of Fe OOH in cathodic reaction.