Study On AC/DC Stray Current Corrosion Behavior Of L245 Pipeline Steel Under Stripping Coating

Pipeline is one of the main transportation methods for oil and gas,and buried pipelines usually use protective coatings for corrosion control.When there are defects or damages in the protective coating,external factors can easily cause the coating to peel off and create a crevice corrosion environment,causing severe corrosion of the pipeline steel under the peeled coating.Stray current will flow into and out of the pipeline from the point of coating damage,exacerbating corrosion in this environment.This article uses electrochemical testing,morphology observation,and product analysis methods to study the effects of gap depth,AC/DC stray current interference,and coupling interference on the corrosion behavior of L245 pipeline steel under peeled coating using a self-made experimental device.The results indicate that:(1)When there is no stray current interference,the self corrosion potential and corrosion rate of L245 pipeline steel under peeling coating generally decrease with the increase of gap depth.However,the corrosion rate at 90 mm is higher than that at other locations,where there is a Warburg impedance diffusion arc with significant dispersion effect,and there are obvious cracks in the corrosion product layer.Its composition is mainly loose reddish brown Fe2O3,with crevice corrosion characteristics.Therefore,it is determined that this point is a corrosion sensitive point under the peeled coating.The diffusion effect of the impedance arc at 120 mm and 150 mm is not significant,indicating that the limit of oxygen diffusion has been reached.(2)Based on the identified sensitive points,the direct current stray current corrosion behavior of L245 pipeline steel under peeling coating was studied.Under the interference of DC stray current,the corrosion potential of L245 steel electrode significantly shifts forward,and the corrosion rate is positively correlated with the corrosion current.According to the order of interference current density from high to low,it is 67.5,35.4,and 9.1 times higher than that under no interference,respectively.150 A/m2 DC interference can damage the passivation layer on the surface of the steel electrode,exacerbate corrosion,and the center of the capacitive arc in the low-frequency region of the impedance spectrum deviates from the real axis of the impedance spectrum,indicating the existence of dispersion effect.The interference of DC stray current mainly leads to pitting corrosion of steel electrodes,and the severity of corrosion increases with the increase of current density,forming a phenomenon of pitting pits gathering in local areas,forming larger local corrosion.Under the interference of DC stray current,the surface roughness of the steel electrode increases,with a maximum corrosion depth of 0.551 mm.(3)There is a significant difference in corrosion behavior between AC interference and DC interference.AC interference not only promotes corrosion,but also has a certain corrosion inhibition effect at higher current densities.The electrode reaction anode process is mainly controlled by activation,but the degree of activation reaction is relatively weak.There is a critical current density near 90 A/m2,above which corrosion products accumulate faster on the surface of the steel electrode,making it easier to form a dense oxide film to inhibit the development of corrosion reactions.In the impedance spectrum,the diameter of the low-frequency capacitive arc decreases with the increase of current density,indicating that an increase in AC current density will reduce the corrosion resistance of the steel electrode.Morphological analysis shows that the corrosion of steel electrodes is mainly pitting corrosion,and the longitudinal development of corrosion pits is more severe at critical current density.(4)Under the interference of AC and DC coupling,the self corrosion potential of L245 steel electrode undergoes a significant positive shift,resulting in more severe corrosion.Under all coupling conditions,the corrosion rate increased significantly in the initial stage of the experiment,but under strong AC and weak DC coupling conditions,the corrosion rate decreased compared to other conditions.Under coupling interference,the maximum corrosion rate is 5 times that of DC interference and 37 times that of AC interference.The oscillation effect of communication causes damage to the corrosion product layer,making it difficult to form a dense protective film on the surface of the steel electrode to suppress corrosion development.Its pitting tendency is more obvious and more inclined towards etching.The maximum corrosion pit depth on the surface of the steel electrode is 1.137 mm,which is twice and nine times that under separate interference of DC and AC.This indicates that AC/DC coupling interference can lead to increased corrosion of L245 pipeline steel under peeling coating,and attention should be paid to the daily protection of aircraft apron oil pipelines using this type of material.