The Research Of The Cathodic Protection Effect On Broken Organic Coating

Organic coating is one of the most effective methods to prevent metal corrosion because of its economy and convenient. But in fact, coating protection used as the only prevention is insufficient, often,we employ both coating and cathodic protection to obtain more effective result. The cathodic polarization has great impact on the coating's lifetime when cathodic protection is used. What's more, the potential of cathodic protection will influence the coating's corrosion. At present, the admissive potential criteria is -850mv(vs. Cu/CuSO4) but whether it is suitable for the breaked coating is still unclear.In this thesis,we discussed the effect of different cathodic polarization potentials and breaked area on organic coating's lifetime.(1) Firstly, we observed the electrodes' corrosion morphologies and self-corrision potentials and applied EIS to study the influence of different potentials on the cathodic protection with 1% breaked area.The results indicated that the synergistic effect of cathodic protection and organic coating played a role in the range from-1030mv (vs. SCE)to -950mv(vs. SCE),that is-1100mv (vs. Cu/CuSO4) to-1020mv (vs. Cu/CuSO4), when the breaked area is very small. As a result of the electrostatic screen of the coating's defect and the great IR drop, the traditional potential criteria -850mv(vs. Cu/CuSO4) will be out of act for this breaked coatings. In order to obtain the effect of cathodic protection, we should change the potential to a more negative one.(2) Then, EIS was employed to study the effect of different breaked area ratio (4% and 7%) on cathodic protection under-1030mv (vs. SCE). The results showed that the stripping ratio of metal/coating interface increased while the breaked area ratio got bigger. Although the bigger breaked area ratio would decrease the electrostatic screen and IR drop and gave better protection, yet this would cause more severe coating delamination and tha coating would be out of protection in a shorter time.(3) We researched the delamination behaviors of the the coating specimen with the 4% damaged area ratio under different pH alkaline conditions.The results showed that:the degradation behaviors under alkaline conditions were different from the natural soak and cathodic polarization conditions, the bare metal of the coating specimen under alkaline conditions would generate a passivation film. Also the stability of the passivation film had some connections with the pH value, the higher of the pH value,the better of the passivation effects and more stable of the passivation film. When the passivation film dissolved,the corrosion reaction occurred at the breaked area and other locations of the electrode surface.The main reason is that the OH- generaged from the cathodic polarization reacted with the coating, which destroyed the adhesion between the coating and the metal.