| Austenitic stainless steel with excellent performance is widely used,but it is prone to suffer from pitting corrosion in the environment containing corrosive anions,which can cause great security risks.Electrochemical methods used in pitting corrosion inspection of stainless steel are not nondestructive and highly effective.Therefore,it is of great significance to study the nondestructive testing of pitting corrosion in the process of using austenitic stainless steel.In order to know the pitting corrosion of 304 stainless steel in different levels,the nonlinear ultrasonic surface wave detection technique is offered to characterize and evaluate it.? The influence of the reliability of the surface wave wedge/detector assembly parts and the parameters of the detection system on the stability of the experimental system are studied by using CSK-IA standard test block,and based on it,the choice rules of the size parameters of the surface wave wedge and the system parameters are studied.The results show that the conversion efficiency of surface wave is higher and the reliability is better when the wedge angle ? is 65°,the length l is 25 mm,and the inclination angle ? of the bottom end is 17°.In the case of the experimental detection system,the more cycles of the pulse excitation,the better.When the output voltage level increases from 30 to 90,there is a linear relationship between the amplitude of the harmonic A2 and the square of the amplitude of the fundamental wave A12,to the experiment,under the condition of ensuring the safety of the sensor,the larger voltage output level should be used;when the gain increases from 24 dB to 34 dB,there is a linear relationship between the amplitude of the harmonic A2 and the square of the amplitude of the fundamental wave A12,as it increases to 40 dB,the discreteness of the linear correlation between the two becomes larger;the nonlinear results of the testing system are affected by the surface wave propagation distance and the surface roughness of the sample.? Nonlinear ultrasonic surface wave detection was carried out on 304 stainless steel samples soaked in Fecl3 solution,with mass fraction of 6%,10% and 14% respectively,for6 h,12h and 18 h respectively and combined with electrochemical impedance spectroscopy(EIS),the feasibility of nondestructive characterization and evaluation of pitting corrosion of304 stainless steel is studied by nonlinear ultrasonic technique,the damage of the samples is evaluated by confocal laser scanning microscopy technique and the weight loss rate.The following results are obtained:1.Microscopic technique and weight loss results show that,with the increase of immersion time and the mass fraction of the solution,the discontinuous interface area,the diameter of pitting hole extends from the initial 600?m to about 1500?m,the pitting depth increases from 450?m to 950?m,and the weight loss rate increases at the same time,which leads to the discontinuity of the material and the increase of the interfacial area,and the pitting corrosion of the sample is more serious.2.With the increase of the immersion time,the mass fraction of the solution and propagation distance,the normalized nonlinear coefficient ?'/?0' increases.3.The results of Pearson correlation coefficient and regression analysis show that there is a good linear correlation between the normalized nonlinear coefficient ?'/?0' and the equivalent circuit parameters Cc,Cd,Rt.Therefore,it is feasible to characterize and evaluate the pitting corrosion of 304 stainless steel by using the nonlinear surface acoustic wave technique.4.The results show that,the change of the nonlinear coefficient in the process of pitting corrosion of 304 stainless steel is attributed to the interaction between finite amplitude ultrasonic and the discontinuous interface caused by the pitting corrosion of 304 stainless steel.,which lead to the nonlinear effect between stress and strain is caused by the interface,and it tends to increase with the increase of the discontinuous interface area. |
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