Study On The Detection Method Of Steel Plate Corrosion By Electromagnetic Acoustic C-Scan Imaging

The steel plate has comprehensive mechanical properties such as high strength,high hardness,and strong pressure resistance,which is widely used in large ships,bridges,power stations,high-pressure boiler vessels and other fields.However,during the long-term service of steel plates,metal damage or metamorphic corrosion is very common due to chemical,physical or electrochemical effects.Electromagnetic acoustic testing technology has the advantages of no coupling agent and non-contact,and has become a new technology for steel plate corrosion detection.Firstly,this paper investigates the causes of corrosion of steel plate and the classification of corrosion defects based on morphology.By analyzing the excitation,reception,propagation and scattering rules of electromagnetic acoustic in the detection of corrosion defects of steel plate,the corrosion detection scheme of steel plate based on EMAT thickness probe is determined.A simulation model of EMAT excitation and reception based on COMSOL finite element software is established.The experimental results show that the thickness variation greater than 2mm × 8mm can be directly measured by pulse reflection method.Further,the orthogonal test method was used to study the local corrosion detection rule.It was found that the bottom echo height method is more suitable for the detection of small-scale local corrosion defects,and the bottom area of the defect is more obvious for the attenuation of the primary echo signal.Secondly,on the basis of theoretical research and simulation analysis,an experimental study on the EMAT system was carried out.The signal-to-noise ratio of the reflection echo of four different shapes of excitation coils,the amplitude-frequency characteristics of the coils and the effective area of the coils are compared and analyzed.Aiming at the low efficiency of electromagnetic acoustic transduction,L-matching network is designed,the impedance matching optimization of the circuit is studied by experiment,and better reflected echo signal is obtained.According to the thickness change caused by different degrees of corrosion,the experiment shows that the square coil has higher resolution for the elongated local corrosion,and the spiral coil has a good detection effect for pitting corrosion defects.Finally,an EMAT system for steel plate corrosion was built,combined with a motion control platform to achieve C-scan imaging,and a testing software based on LabVIEW was designed.A C-scan imaging algorithm based on the RFS is designed to image the corrosion defects of the processed sample plate.An improved algorithm based on the limiting filter algorithm is designed to post process the corrosion image of the sample plate.The final results show that the EMAT C-scan system can detect the corrosion defects with a depth width product of 1mm × 3mm.