Study On Electrochemical Mechanism And Growth Kinetics Of Pitting Corrosion On X100 Pipeline Steel Under Cathodic Protection Potential Fluctuations

Buried oil and gas pipelines are one of the key infrastructures of national energy transportation.The combination of coating and cathodic protection is the main method to prevent external corrosion of pipelines.Currently,buried oil and gas pipelines are generally used impressed current cathodic protection with“internal feedback”loop mode.However,during the service of pipeline,the cathodic protection potential fluctuations would occur when the soil conductivity changes because of the soil properties of physics,chemistry,electrochemistry,and biology transformed.The pitting corrosion would still initiate even under cathodic protection loaded on the pipeline steel matrix.In this paper,square wave polarization?SWP?technology,scanning electron microscope?SEM?,pitting quantitative statistics,theoretical calculations of circuit model and orthogonal tests are employed to study the pitting corrosion behaviors on X100pipeline steel in an anaerobic acid soil under cathodic protection potential fluctuations.The achievements of this research would describe the electrochemical mechanism and the growth kinetics of pitting corrosion under non-steady electrochemical environments.The following main results have been achieved:?1?The equivalent circuit model of the electrochemical system is constructed.According to the methods of transient electrochemical and the theories of electronic information,the mathematical functions of polarization current density in the time domain and potential in the frequency domain at the double-charge layer of electrode/solution interface under the cathodic protection potential fluctuation are derived.And the potential in the frequency domain functions are also predicted the trend of pitting density in quantitative or semi-quantitative.The results show that,the polarization current density formula in the time domain describes the quantitative relationship among pulse,attenuation and steady of the current density with periodic square wave potential,and also expounds the positive relationship between the current density and square wave fluctuation amplitude.The frequency domain potential equations at the double-charge layer of electrode/solution interface predicts the pitting density trend under different cathodic protection potential fluctuation parameters.?2?An electrochemical three-electrode system is constructed for X100 pipeline steel in a simulated acid soil environment.Square wave polarization signals with different fluctuation parameters are loaded on the pipeline steel to quantitative statistics the pitting of electrode surface.The results present that the pitting corrosion will occur on the surface of pipeline steel due to the cathodic protection potential fluctuations.The pits would initiate on the steel matrix rather than the non-metallic inclusions.In the conditions of single variable,the statistic density of pits increases sharply from about 450 pits·mm^-2 to about 45000 pits·mm^-2 with increasing of fluctuation frequency?f?.In the same,when the fluctuation range?E?and the total loading time?t_t?increased,the statistics density of pitting would arise linearly.In addition,the pitting density would increase from about1100 pits·mm^-2 to over 6500 pits·mm^-2,and then decrease to 803 pits·mm^-2 while the fluctuation duty cycle???increase,where the highest density is at?=50%.Furthermore,the statistics pitting density of electrochemical corrosion experiments under different cathodic protection potential fluctuation parameters can be used to prove the correctness of the derived theoretical functions.?3?The effect of the cathodic protection potential fluctuation parameters on the pitting density is obtained through orthogonal test.From the results of orthogonal experiments,the order of the effect of potential fluctuation parameters on pitting density is:t_t>?>f>E.Simultaneously,when f is 0.5 Hz,E is-0.95?-0.7 V,?is 50%,and t_t is3 d,the pitting density is the largest,which means the local corrosion behavior of X100pipeline steel in an anaerobic acid soil under cathodic protection potential fluctuations is the most seriously.