Study On Cathodic Oxygen Reduction Reaction At Steel In Seawater

It is well known that the corrosion processes of steel in sea water are accompanied by cathodic depolarization of oxygen. Lots of the intermediate active substance like peroxidized ion (O₂^-) are formed at the interface between steel and solution during the oxygen reduction, which can decrease the adhesive force between the coating and the steel, leading to the delamination and destroy of polymeric coatings. The reduce of the adhesive force definitely accelerates the rupture of the steel which causes a great deal of poisonous heavy metal ion to dissolve into the seawater, polluting seriously marine entironment as well as endangering the heath of human being. The corrosion of steel brings not only tremendous economic loss, but also becomes a unneglectable safety trouble. In view of all of these damages and hidden dangers, it's very important to investigate the oxygen reduction reaction on the steel in seawater to understand fully of the corrosion mechanism of the steel. In this thesis, we have investigated qualitatively the oxygen reduction reaction and the electrochemical behaviors of the steel using cyclic voltammetry, studied the influence of different treatment ways on the oxygen reduction under different potential by the electrochemical impedance spectroscopy to confirm the electron transfer resistance and different time constants of the reactions; explored the dynamic behaviors of the oxygen reduction on the steel in seawater using rotating ring/disk electrode and rotation cylinder electrode.Our results from analysis of metallographic structure of the steel show that the X60steel used in the experiment is hot rolled steel that is composed by white nubby ferrite and black pealite. A3 and 20 steel, on the other hand, have a typical annealed structure of white ferrite and black pealite, respectively. From the open circuit potential vs time plots we found that both X60steel and A3steel exhibit a characteristics in potential negative shift-positive shifr-negative shift with an arrival of a stable state in the end. This is because of the creation and hydrolyzation of the iron oxide and the formation of an oxide film. Two reduced peaks in the cathodic part in the cyclic voltammograms of oxygen reduction have been observed, which can be ascribed to the reduction of the iron oxide and oxygen. The appearance of two oxide peaks in anodic part, however, can be considered to be attributed to the oxidation of the iron. The occurrence of the Fe(â…¡) catalyses the reduction of the oxygen and the entire transition progresses of steel in oxygen reduction are as follow Three time constants appear in the bode plot of the oxygen reduction which correspond to frequent logarithm 0, 1, 2, respectively. The peak corresponding to time constant 0 denotes the reduction of the oxide while those corresponding to time constants 1 and 2 denote the reduction of oxygen and the evolution of hydrogen. The same reaction has a similar phase angle. We also found using the rotating electrode system that the oxygen reduction reaction on X60 and A3 are characterized as diffusion control and coexistence of reduction of two and four electrons, which the four-electron reduction reaction dominates and the number of reaction electron are 3 and 3.3, respectively.