Corrosion Characteristic In Cl^- And HCO₃^- Solutions Based On Electrochemical And Acoustic Emission

With the implementation of industrialization and supply-side reform,the consumption capacity of oil and gas in China has been rapidly improved.The policy of low-carbon energy will effectively promote the construction of natural gas pipeline network.New domestic oil and gas pipelines will mainly be natural gas pipelines.The use of large-diameter,high-pressure,high-grade pipelines has become a necessary means to increase pipeline throughput,reduce material costs and solve energy demand.Therefore,a variety of high-strength pipeline steels emerge as the times require.X90pipeline steel is expected to be widely used in the future due to its high impact toughness,high uniform elongation,low yield-strength ratio,and excellent welding performance.However,there are few reports on soil corrosion characteristics of high-strength pipeline steel.Therefore,the corrosion characteristics of X90 and higher grade pipeline steel under different p H,Cl^-concentration,and HCO₃^-concentration are studied by the combined electrochemical and acoustic emission test method,which provides effective theoretical support for field popularization and application of X90 and higher grade pipeline steel.The main conclusions are as follows:Through the research of the Tafel dynamic polarization curve and electrochemical impedance spectroscopy（EIS）,it is found that in acidic simulated soil solution at p H=5.5and neutral simulated soil solution at p H=7,the corrosion rate of steel sheets increases with the increase of Cl^-concentration,and increases first,then decreases and then increases with the increase of HCO₃^-concentration.Under an acidic environment,the promotion effect of HCO₃^-on corrosion is obvious.In alkaline soil solution with p H=9,the corrosion rate of the steel sheet first decreases,then increases,and then decreases with the increase of Cl^-concentration,in which the corrosion rate of 0.03mol/L Na Cl and 0.05mol/L Na Cl solution is not significantly different,but decreases first,then increases and then decreases with the increase of HCO₃^-concentration.The presence of HCO₃^-in an alkaline environment has little influence on the corrosion rate;It is also found that the corrosion rate decreases with the increase of p H,and according to the importance of influencing the corrosion rate,the corrosion factors can be arranged as follows:p H>HCO₃^->Cl^-.The 24h electrochemical polarization corrosion process was detected by acoustic emission（AE）.It can be seen that the ringing number of corrosion AE monitoring decreases exponentially with the increase of p H.The acoustic emission signal generated in an acidic environment is the most intense,which can be divided into metal dissolution,corrosion product accumulation,and stable corrosion stage.There are two strong and weak signals generated in each stage,of which the signal is mainly metal dissolution signal and corrosion product accumulation,passivation film rupture signal.The peak frequency is concentrated between 100k Hz and 150k Hz.The number of signals generated in a neutral environment is less than that in an acidic environment,which can be divided into metal dissolution signal,passivation film formation signal,corrosion product accumulation signal,and passivation film breakdown signal.The peak amplitude of the passivation film breakdown signal is larger than 7×10^-3;the signals generated in an alkaline environment are the least and all are metal dissolution signals.The signal is denoised by wavelet transform and compared with the wavelet basis used for denoising.Wavelet basis coif4 is selected to carry out wavelet denoising,decomposition,and reconstruction of the signal.The processed signal eliminates high-frequency noise interference and makes the signal more simple and intuitive.The processed signal eliminates high-frequency noise and the signal is more single and intuitive.Wavelet packet analysis shows that the energy is concentrated in the peak frequency range,which can be used as the basis for signal type judgment and quickly track and determine signal components’time and frequency domain position.