Study On Corrosion Behavior Of Steel Under Disbonded Coating Area Of Submarine Pipeline With Cathodic Protection

At present,cathodic protection combined with anticorrosive coating is usually used to protect the outer wall of pipelines from corrosion.The coating can isolate the corrosive medium such as seawater from contacting the metallic substrate,but it is prone to mechanical damage,crazing and blistering defects in the process of transportation,installation and service,resulting in the metallic substrate exposed in seawater.As an auxiliary means of the coating system,cathodic protection can protect the exposed metal.The reaction between the alkaline substance produced by cathodic protection current and the coating composition will cause the coating to lose adhesion and peel off.The cathodic shielding produced by the high insulation coating makes it difficult for the cathodic current to reach the disbonded coating area and the metal corrosion occurred after contacting the corrosive medium.In this paper,the effect of conductivity and coating disbondment height on the cathodic shielding and the corrosion behavior of the metal under disbonded coating area of submarine pipeline under different conditions are studied by using the self-made simulated coating disbondment experimental device,the wire beam electrode technology and the morphology observation technology.In the second chapter,the effects of typical conductivity of marine and soil environment and different crevice gap size on cathodic shielding are compared.It is found that the cathodic shielding effect in marine environment is weaker than that in soil environment,and the cathodic protection current can enter the gap more quickly to protect the steel.The narrower the gap produced by disbondment,the more obvious the cathodic shielding effect is.But the content of dissolved oxygen in the gap decreases and the corrosion is inhibited.The narrower gap does not mean that corrosion in the gap is more serious.The severity of corrosion is also closely related to the content of dissolved oxygen in the gap.Therefore,in the same cathodic protection potential but different conductivity environment,the crevice gap width is different to make the corrosion rate in the gap reach the maximum.Under the obvious cathodic shielding effect,the cathodic current cannot penetrate into the depth of the gap.But,the excessive OH~-generated by the cathodic protection current can gradually diffuse into the gap,gradually forming a high pH environment in the gap,which is conducive to the inhibition of steel corrosion.In the third chapter,the corrosion behavior of steel under disbonded coating area with cathodic protection failure has been studied by setting two experimental conditions:no cathodic protection is applied and cathodic protection is disconnected after applying cathodic protection for one day.When the cathodic protection fails before the coating is damaged and disbonded,the entire corrosion process in the damaged and disbonded area is mainly affected by the macro-oxygen concentration cell effect.The most serious corrosion location of the steel occurs in about 2 mm long area from the crevice mouth,and the corrosion rate can reach0.54 mm/a.When the cathodic protection fails after the coating is damaged and disbonded,due to the cathodic protection current,the excess OH~-generated in the holiday diffuses into the crevice,causing the pH in the crevice to rise.After the cathodic protection fails,pH outside the crevice quickly drops,while pH in the crevice is kept stable,so the steel in the crevice is protected.The corrosion tendency of the steel outside the creviece is higher than that of the steel in the crevice.The location of the most serious corrosion is transferred from the inside of the crevice to the outside of the crevice.The corrosion rate can reach 0.35 mm/a.In the fourth chapter,the corrosion behavior of the pipeline under disbonded coating area in the tidal zone is studied,and the effect of different crevice orientations is compared.The rising and falling tide affects the effectiveness of cathodic protection.The tide drop process is a process in which cathodic protection gradually fails,and the tide rise process is a process in which cathodic protection gradually restores.When the mouth of crevice faces up,seawater solution is always kept inside.When the cathodic protection fails at low tide,the corrosion is the most serious,and the corrosion current gradually increases with the tidal cycle.In the middle of the disbondment area,non-uniform pitting occurs.Therefore,the corrosion rate calculated by average anode current density may be conservative.When the mouth of the crevice faces down,the seawater solution cannot be retained in the crevice.When the water level is lower than the mouth of the crevice,there is still a water column to separate the crevice from the environment outside.The environment in the crevice is relatively dry and oxygen-depleted,so the corrosion is slight.Therefore,when the cathodic protection is insufficient or even fails,the steel under disbonded coating area must be in direct contact with the corrosive medium to cause corrosion in the tidal zone.