Effects Of Cathodic Polarization On Corrosion Behaviors Of Organic Coating/Carbon Steel Systems In Static And Flowing Seawater

The marine environment was a very complex corrosive environment,and various metal materials were easily corroded in the environment.The combined use of organic coating and cathodic protection technology is currently the most economical and effective measure for corrosion control of marine equipments such as ships and oil platforms.However,the cathodic delamination of organic coating on the metal surface would be promoted due to the long-term existence of the cathodic polarization current,which resulted in the reduction of the service life of the coating.In this paper,wire beam electrode（WBE）and electrochemical impedance spectroscopy（EIS）techniques,combined with infrared spectroscopy（FT-IR）,scanning electron microscopy（SEM）and other structural characterization techniques were used to study the behaviors of artificially damaged organic coating/carbon steel systems under cathodic polarization in static and flowing seawater.The main results of this paper were as follows:（1）The failure process of organic coating in the two seawater environments could be divided into three phases:corrosive particles such as water and oxygen penetrated into the coating;corrosive particles reached the metal substrate,electrochemical reactions occurred at the coating/metal interface and the cathodic delamination of the coating was initiated;the cathodic delamination area was enlarged,and extensive bubbling of the coating occurred until the coating failed.（2）Under the condition of open circuit potential,the electrodes in the damaged area of the coating acted as the anode in the two seawater environments,and the electrodes under the coating acted as the cathode,which was protected to a certain extent.Under the condition of the potential,the cathodic delamination rate of organic coating in flowing seawater was always greater than that in static seawater,which was attributed to the accelerated diffusion of corrosive particles into the coating.In flowing seawater,the dissolved oxygen consumed by the cathodic reaction at coating/metal interface was replenished in time,which increased the electrochemical reaction rate at the coating/metal interface,and the cathodic delamination of organic coating was promoted.（3）In static and flowing seawater environments,the failure process of organic coating was accelerated due to the application of cathodic polarization,and the cathodic delamination rate of organic coating increased with the negative shift of polarization potential.The reason was that the cathodic reactions at the coating/metal interface were accelerated at a relatively negative polarization potential.This led to a continuous increase in the alkalinity of the coating/metal interface,which in turn increased the driving force for cathodic disbondment of the coating.（4）Under the conditions of-850 mV vs.SCE and-1100 mV vs.SCE,in the first 72 h of the immersion experiments,there was no significant difference in the cathodic delamination rate of the coating in the two seawater environments.After immersed for 72 h,the cathodic delamination rate of the coating in flowing seawater was lower than that in static seawater.The reason was that the anodic dissolution reaction of the metal substrate in flowing seawater was enhanced after the coating delaminated.Under the condition of constant polarization potential,the effect of cathodic polarization on the metal substrate was weakened.Therefore,the promotion effect of cathodic polarization on the cathodic reaction would decrease,resulting in a decrease in the cathodic delamination rate of the coating.（5）The cathodic polarization had a better protection affect on the electrodes at the damaged area of the coating and the electrodes under the coating in static seawater,and the protective effect of cathodic polarization on the electrodes at the damaged area of the coating and the electrodes under the coating in flowing seawater decreased.The reason was that the diffusion rates of corrosive particles such as oxygen were increased in flowing seawater,and the shear stress generated by flowing seawater made it difficult for corrosion products to accumulate on the metal surface,resulting in aggravated corrosion of metal substrate.（6）In the two seawater environments,the ester bonds in the molecular chain of the coating resin were easily hydrolyzed by the strong alkaline environment of the coating/metal interface.This led to the scission of the molecular chain and caused the degradation of organic coating.With the increase of the cathodic polarization potential,the alkalinity at the coating/metal interface continued to increase due to the continuous enhancement of the cathodic reduction reaction on metal surface.Therefore,the degradation degree of organic coating would increase accordingly.