Study Of X70Steel Corrosion Behavior And Cathodic Protection Effect Under Disbonded Coatings

In this paper, experimental apparatus required in this study was designed, based on thestudy of the relative experimental apparatus and the experimental methods used at home andabroad. The corrosion behavior and cathodic protection effect of X70steel under disbondedcoatings was studied by the methods of open circuit potential, EIS, polarization curves.Finally, cathodic polarization potential distribution in corrosion system under theexperimental state was calculated and simulated by mathematical method.In this paper, the corrosive environment of the metal substrate under disbonded coatingswas simulated well with the experimental apparatus required in this study, in which variouselectrochemical measuring methods performed conveniently.EIS of17days was gotten. With the EIS, it is found that as to the experimentalapparatus required in this study, there is sign of crevice corrosion on the working electrodesmore than9cm away from damaged point during the experimental period, and the incubationtime was about6days. As to the working electrodes more than9cm away from damagedpoint, with the increase of depth, polarization resistance on the working electrode reduced.However, as to the working electrodes less than6cm away from damaged point, there is nosign of crevice corrosion. In addition, with the increase of depth, polarization resistance onthe working electrode increased because of the oxygen concentration.Polarization curves were gotten when the experimental apparatus was placed stably for17days. With the Polarization curves, corrosion current density of each working electrodewas gotten, the information by which is consistent with that of EIS. In addition, with thepolarization curves and relevant standards, effective cathodic protection range of eachworking electrode was gotten, and as to the working electrodes in the area of crevicecorrosion, with the increase of depth, the minimum protection potential and maximumprotection potential both become more positive. The distribution changes of the potential on the working electrode surfaces were gottenduring the period of energizing, voltage regulating, power failure, energizing after powerfailure on energized point by open circuit potential test and surface observing. Followingconclusions can be gotten. Firstly, during the period of energizing, the potential of eachworking electrode would be stable in a more negative value in about1h, and with the increaseof depth, the potential becomes more positive because of shielding effect on the CP of gapenvironment. However, with the increase of depth, the minimum protection potentialbecomes more positive, so shielding effect on the CP of gap environment is not so serious.Secondly, during the period of voltage regulating, the potential of each working electrodewould become more negative soon. However, with the increase of depth, the potential wouldchange in less degree, which leads to the aggrandizement of potential difference between theworking electrodes at the head and tail of the crevice. Thirdly, during the period of powerfailure, the potential of each working electrode would become more positive soon, and withthe increase of depth, the potential would change in less speed, which caused the current fromthe inside to outside. At the same time, as to the working electrodes in the area of crevicecorrosion, there would be chemical reaction that Fe changed into Fe2+. Lastly, during theperiod of energizing after power failure, each working electrode would become morenegative again. However, the degree of polarization reduced in a certain time resulting fromoxygen consumption and reduction of the pH value with corrosion.Finally, cathodic polarization potential distribution in corrosion system under theexperimental state was calculated and simulated by the method of the empirical formula andBEASY software. The cathodic polarization potential distribution from the method of theempirical formula is consistent with that of experiment, while that from the method ofBEASY software is not. What’s more, the minimum polarization current of0.39mA wasgotten by the method of BEASY software.