Effect Of AC On Corrosion Behavior Of Buried Metal Pipeline Under Stripped Coating Defects

Pipelines,high-voltage transmission lines and AC electrified railways are connected in parallel due to the space constraints,which results in the forming of a public corridor.The alternating current(AC)stray current has an increasingly serious impact on buried metal pipelines.As the first barrier to protect the pipeline,the external anti-corrosion coating can isolate the pipeline from the corrosive medium.However,the coating can be mechanically damaged during the transportation,installation and use of the pipeline.The soil electrolyte will penetrate into gap between the coating and the pipeline,causing the serious corrosion in the stripped area.The existence of AC interference is bound to accelerate the further corrosion of the pipeline in the stripped area.Therefore,it is of great significance to study the interference of alternating current on the corrosion behavior of metals under the stripped coating defects.In this paper,the effects of AC current density,frequency and waveform on the corrosion behavior of X80 steel under stripped coating defects were studied by electrochemical method,wire beam electrode(WBE)technology and surface technology.The results show that:(1)Under the interference of different AC current densities,the open circuit potential of X80 steel undergoes negative deviation,and the degree of negative deviation increases with the increase of AC current density.Under the action of low AC current density(0~100 A/m~2),the corrosion current density has little difference.While under the action of high AC current density(200~300 A/m~2),the corrosion current density increases and it is higher than that of low AC current density.The cathode area of the wire beam electrode mainly distributes outside the gap and at the edge of the gap,while the anode area mainly distributes inside the gap.Under low AC density,only a few pitting pits appears on the surface of the wire beam electrode.While under high AC density,the pitting pits are distributed densely and connect into pieces on the surface of X80 steel.(2)Under the interference of different AC frequencies,the open circuit potential of X80steel increases first and then decreases with the increase of AC frequency,and it has a positive offset under 200 Hz.Under the action of low AC frequency(50~100 Hz),the corrosion current density decreases with the increase of frequency.Under the action of high AC frequency(200~300 Hz),the corrosion current density increases slightly.The local corrosion of the wire beam electrode appears under the alternating current of 50 Hz.In other cases,the anode current is evenly distributed on the surface of the wire beam electrode.The X80 steel is severely corroded under the interference of AC frequency of 50 Hz.And the pitting corrosion is obvious,the number and the diameter of the pits are large,and the distribution is dense.With the increasing of frequency of alternating current,the corrosion is weakened.(3)Under the interference of different AC waveforms,the negative deviation of the open circuit potential of X80 steel varies from large to small is:square wave,triangle wave and sine wave.The corrosion current density under different waveforms of AC is also different.The corrosion current density under sine wave interference is larger than that of square wave and triangle wave,and the corrosion current density under the latter two waveforms is not much different.The local corrosion of the wire beam electrode under square wave and triangular wave interference is serious,and the local corrosion is relatively light under the action of sine wave.On the whole,the diameters of pits under square wave and triangle wave are smaller and are not connected into pieces.While the pits under sine wave have a tendency to connect into pieces and have larger diameters.