Research On Electrochemical Corrosion Behaviors And Parameters Of Rusted Carbon Steel In Marine Environment

Sea is the human survival and the development in future, and also blue economyand marine resources exploitation is one of the most important development directionfor China in the future. The corrosion of carbon steel which was widely used in themarine industrial development process will directly affect the safety of all kinds ofengineering equipment such as ships, offshore oil platform, bulidings and so on, so itneeds to make deepgoing research on the corrosion behaviors and laws of carbonsteel. Rust/metal structure is one of the multiphase and multiple interface complexsystems. The corrosion under rust is the uppermost and longest form of carbon steel’scorrosion evolution process. But the corrosion behaviors and laws are toocomplicated to obtain precisely analysis results due to the influence of the rust layer,especially, when attention focused on the corrosion rate of carbon steel under rust bytraditional electrochemical method. However, the corrosion rate is the most importantcorrosion electrochemical parameter to examine materials’ corrosion behavior. Solooking for an accurate and efficient corrosion velocity measurement method forcarbon steel has siginificant meaning to corrosion research field.In this paper, the corrosion morphologies of the Q235carbon steel withindifferent immersing period under two different experiment conditions, meaning staticseawater and simulated flowing seawater, were carefully observed firstly throughtaking pictures. The result shows that in both systems, the corrosion products formedon the surface of the carbon steel stratified apparently after a spell of immersion. Theouter rust layer was yellow and relatively loose, while the inner layer was black andcompact, the thickness of which gradually thickened with the extension of theimmersing time, resulting in an increasing proportion of the inner black corrosionproducts of the whole corrosion products. After that, weight-loss method and severalelectrochemical methods (such as electrochemical impedance spectroscopy, linearpolarization resistant method, polarization curve) were carried out to study the corrosion behavior of the carbon steel in long-term immersing experiment under twoconditions. The result has been discovered that in both static and flowing systems,the short-term corrosion rate measured by electrochemical methods was consistentwith that measured by weight-loss method (8weeks in still seawatersystems and2weeks in flowing system); with the extension of immersing time, the weight-lossmethod measured corrosion rate of both systems decreased and gradually stabilized,while the corrosion current density detected by electrochemical methods showed anincreasing trend, and the longer the time extends, the more conspicuously thedeviation between them becomes, which might be explained by the reduction currentpeak appearing at the potential about-950mV in the cathodic polarization curve.The constituent analysis and the physical chemical property study of rust layersshow that the rust layer on carbon steel surface has a great effect on its corrosionbehavior, especially the measurement of the corrosion rate. For piled corrosionproducts on the surface of carbon steel, the outer rust layer is mainly consisted ofγ-FeOOH, and a mass of reductive β-FeOOH and magnetic Fe3O4appeared in theblack inner products besides γ-FeOOH and α-FeOOH. After a long-term immersionin seawater, the Ecorr of carbon steel is in the reduction reaction potential range ofβ-FeOOH, so plenty of β-FeOOH accumulated in the rust layer after long-time soakwill act as depolarizer and participate in cathodic reduction reaction, making adeviation of the corrosion rates measured by electrochemical methods from thatdetected by weight-loss method, and the longer the immersion time is, the moreβ-FeOOH formed, which leads to the proportion of the rust reduction increasing inthe cathodic reaction, coming along with the larger deviation between the twomethods.Potentiostatic polarization experiments prove that the cathodic reaction of rustlayers during the electrochemical tests can be eliminated by additional pretreatmentway of cathodic current. In order to efficiently calibrate this deviation and achievethe fast and accurate measurement of carbon steel corrosion rate by electrochemicalmethods, cathodic polarization pretreatment were carried out on long-term immersedcarbon steel electrode in both still and flowing seawater condition before electrochemical measurement tested. The results indicated that, after-25μA/cm2and-35μA/cm2pre-polarization for static and flowing seawater system respectively, thepolarization resistant Rp of carbon steel increased obviously than that ofno-polarization, and then the corrosion current density obtained by electrochemicalmethod showed an analogous variation tendency and similar value with weight-lossmeasurement results (the error is minished to less than10%), which means thepre-polarization treatment has good correction effect on calibration of the deviationbetween electrochemical measurement and weight-loss method.Meanwhile, the longer immersed term, the pre-polarization time got longer,demonstrates that rust reduction current also increased gradually with theimmersion-term prolonged, and the variation law of pre-polarization time showed asimilar pattern with the proportion of rust layer reduction current in the total apparentcorrosion current. This means this pre-polarization treatment can obtain a goodcalibration roles as we have did by deaerated method. Moreover, XRD analysis wasapplied on the carbon steel electrode of immersed in still seawater for48weeks withand without pre-polarization, the results indicate that the outer rust layer have littlevariation no matter pre-treat or not; but the active component β-FeOOH in inner rustlayer almostly disappeared completely after pre-polarization, and the deviationbetween two measurement results decreased greatly at the same time. This fatherlyproved the reduction of β-FeOOH during the electrochemical test procedure is theprimary cause for the deviation of the corrosion rate between electrochemicalmeasurement and weight-loss method.