Study On Corrosion Processes And Repair Technology Of Steel In Splash Zone

The splash zone is the most corrosive area in the marine environment due to higher frequency of alternation of wetting and drying, abundant oxygen supply, high amount of oxygen supply, large amount of salt deposition, high irradiation intensity of ultraviolet, etc. The materials exposed in this area would suffer from serious corrosion and it was very difficult to protect the materials exposed in this region. If appropriate protection methods were applied, it would greatly prolong the service lifetime of the whole offshore steel structures exposed in this area.In the present dissertation, the corrosion processes and mechanisms of steels in splash zone were studied using polarization curve, electrochemical impedance spectroscopy (EIS). The electrochemical results were obtained from the corroded steel samples exposed in splash zone, while, the corrosion morphologies and corrosion products of the steel samples exposed in splash zone were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman.The reason, which the steel samples suffered from serious corrosion in splash zone, was the alternation of wetting and drying. During the drying process, the electrolyte thickness decreased and chloride concentration increased. Oxygen would be much easier to diffuse into the interface of electrolyte/metal, which improved the cathodic reduction processes. Except for this, the rust itself took part in the reduction processes and hence increased the corrosion rate of the steel samples.The composition of the inner layer of the rust formed on the steel samples in splash zone were magnetite(Fe3O4), with tiny flaky lepidocrocite(γ-FeOOH) crystal growing on Fe3O4. The composition of the outer rust layer consisted of scaly sliceγ-FeOOH and small amount of stalactitic goethite(α-FeOOH).γ-FeOOH, which is the typical rust of steel in marine envirnment, took part in the cathodic processes as an oxidant and accelerated the corrosion processes of steel. The metal corroded and reacted withγ-FeOOH and Fe3O4 appeared in wet process, while Fe3O4 was oxidized toγ-FeOOH again during dry process. This consisted a self circulation of electrochemical reduction and chemical oxidization, and Fe3O4 andγ-FeOOH acted as catalysts.Corrosion inhibitors, thickening agents and antioxidants were chosen and series of anti-corrosion grease were produced in the present dissertation. Their anti-corrosion performances were studied using electrochemical methods and salt spray tests. The optimal formula was obtained and one of the anticorrosion greases with best anti-corrosion property was selected. The basic material of anticorrosion tape was selected, and anti-corrosion tape was prepared with the designed dipping machine. With the protection of the prepared grease and tape, the corrosion rate of steels exposed in marine environment was reduced 2~4 orders of magnitude. That is to say, using the anticorrosion grease with the anticorrosion tape could provide effective long term corrosion protection for the steel structure exposed in splash zone.