Effect Of Chloride Ion On Corrosion Behavior Of Metals Under Epoxy Coating

Epoxy powder coating is the main anti-corrosion primer and topcoat for oil pipelines.In the process of coating or pipeline construction,defects such as scratches and pinholes may occur.As the service time extends,the coatings can be degraded,which seriously affects its protective effect on metal.Chloride ion is an important factor affecting the performance of the coating and the corrosion of the metal under the coating.It can penetrate the surface of the pipeline steel through the epoxy powder coating to affect the corrosion behavior of the metal and cause the degradation of the coating performance.However,it remains unclear about its influence mechanism and law.Therefore,for salt-rich soil and seawater environment,it is of great significance to study the effect of Cl~-on the metal corrosion under epoxy coating in different conditions.In this paper,the effects of chloride ion on the water absorption failure of coatings and the corrosion behavior of metals under coatings has been studied by electrochemical impedance spectroscopy technology,combined with weighing method,self-corrosion potential method and corrosion morphology observation method.The study finds that with the increase of Cl~-concentration,the total water absorption of the coating system will decrease,but when reaching saturation,the rate of water-absorption will become faster.By analyzing the impedance spectrum,the failure process of the complete coating can be divided into three main stages:coating water absorption stage,interface reaction initiation and coating failure stage.With the increase of Cl~-concentration,the coating failure rate is significantly accelerated.Combined with metallographic analysis,it can be seen that the corrosion area of the metal under the coating increases and the degree of corrosion becomes more serious with the high Cl~-concentration.The corrosion potential gradually decreases during the failure process,but the self-corrosion potential of the coating system is higher than the uncoated metal in the solution during the entire experiment.In the case of scratches on the coating,the WBE technique is used to measure the distribution of the corrosion current density.It finds that the location of the damage always shows an anodic current,and the cathodic reaction takes place adjacent to the damaged location.The cathode area surrounds the defect and migrates to the far end over time.With the increase of Cl~-concentration,the corrosion reaction speeds up,the rate of cathode migration also increases,and the speed of coating stripping does as well.Combined with electrochemical impedance spectroscopy,uniform laws are obtained.In the case of pinhole defects on the coating,the anodic area of the corrosion cell first appears at the pinhole defects,and the cathode gradually develops around it.In the distilled water environment,corrosion has stagnated due to the rust layer covering.With the increase of Cl~-concentration in the solution,the corrosion stagnation is eliminated.Both the corrosion rate and the cathode migration speed are accelerated.In the case of pinhole defects,the anode can not remain in the defect position all the time.It will move to the area around the damaged position while the cathodic current may appear in the damaged position.