Effect Of Cathodic Polarizations On The Failure Behavior Of Epoxy Coating/Low Alloy Steel Systems Under Deep-Sea Hydrostatic Pressures

The combination of organic coating and cathodic protection technology was a more economical and effective anti-corrosion measure for marine engineering equipments.However,the cathodic delamination of the coating would be promoted due to the long-term existence of the cathodic currents,which led to a significant decline of the protective performance of the coating.And the penetration of aggressive media such as water and oxygen into the coating and the coating/metal interface was accelerated in the deep-sea environment,causing metal corrosion to be accelerated.At present,there were still few relatively researches reported the effect of cathodic polarizations on the failure behavior of epoxy coating/low alloy steel systems under deep-sea hydrostatic pressures.Therefore,the failure behavior of epoxy glass flake coating/907A low alloy steel systems by cathodic polarizations(open circuit potential,-850 mV _{vs.Ag/AgCl/seawater},-1100 mV _{vs.Ag/AgCl/seawater})under deep-sea hydrostatic pressures（0.1 MPa,6 MPa,15 MPa）was studied by a simulation test device of the deep-sea environment in this paper,combined with electrochemical impedance spectroscopy（EIS）technique,scanning electron microscopy（SEM）,Fourier transform infrared spectroscopy（FT-IR）and laser Raman spectroscopy.The main results of this paper were as follows:（1）Under the same hydrostatic pressure,with the negative shifting of the cathodic polarization potentials,the degree of coating cathodic delamination increased,and the wet adhesion of the coating decreased,resulting in a gradual decrease for the protective performance of the coating.Because of the long-term existence of cathodic currents,a strong alkaline environment formed at the coating/metal interface and the benzene ring as well as the ether bond of the coating were broken,which resulting in a chemical degradation of the coating.In addition,the corrosion of the metal gradually slowed down to a certain extent with the negative shifting of the cathodic polarization potentials.（2）Under the condition of three polarization potentials,the penetration of aggressive medium into the coating and the coating/metal interface was promoted under high hydrostatic pressures,which accelerated the failure process of the protective performance of the coating.At the same time,the physical structure of the coating was obviously affected by the high hydrostatic pressures.（3）Under the condition of open circuit potential,the formation of the highly conductivity Fe₃O₄ in the corrosion products was promoted and the corrosion of metal was accelerated with the increasing of hydrostatic pressures.Under the condition of-850 mV _vs.Ag/AgCl/seawater and-1100 mV vs.Ag/AgCl/seawater,the increasing content of the weak conductivityα-Fe₂O₃ in the corrosion products might reduce the cathodic currents under the same cathodic polarization potential,which resulting in the protection effect of cathodic polarizations on metal was weakened.（4）Under the condition of three kinds of polarization potentials in different hydrostatic pressure environments,three stages could be classfied for the diffusion of the water into the coating:the rapid diffusion stage of the water into the coating,the diffusion process of the water in this stage conformed to the Fick’s diffusion law;the stage at which the water absorption of the coating reached saturation;the stage where the water further diffused into the coating after the physical structure of the coating was destroyed.（5）With the increasing of hydrostatic pressures,the time of water uptake of the coating that reached saturation and the duration of the saturation state were significantly shortened under the condition of open circuit potential;after applying the cathodic polarization potentials,the time of water uptake of the coating that reached saturation was shortened,while the duration of the saturation state was extended.With the negative shifting of the cathodic polarization potentials,the time of water uptake of the coating that reached saturation was shortened,and the duration of the saturation state was extended in the three hydrostatic pressure environments.This might be related to the increased degree of the cathodic delamination of the coating and the increased consumption of the water by the cathodic reaction under the coating.