Acoustic Emission Monitoring Of Small Hole Corrosion

Metal materials are widely used in the areas of production and lifebecause of their low price and good mechanical properties.However,the corrosion resistanceto aqueous environment of it is very low.Especially in the presence of saline medium,the probability of corrosion increases greatly,and most of them are localized corrosion.How to do a good job of corrosion monitoring,especially of pitting corrosion monitoring,is a great significance for the formulation of anti-corrosion measures.In this paper,in view of the pitting process of metal,the widely used Q235 steel and 316L austenitic stainless steel as the research object,using acoustic emission monitoring technology for in situ monitoring of pitting corrosion behavior under natural conditions.In addition,on this basis,developed the signal processing software including tail cutting,shape preserving interpolation,wavelet denoising,feature extraction,signal parameter re-definition.Then,the parameters of the processed signal were analyzed,and a good clustering was obtained.Finally,based on the SSIM index,a kind of corrosion type identification model,which is based on the whole acoustic emission process during metal corroding,was established on the MATLAB platform.Through the above research methods,the results obtained are as follows:1)The OCP has no obvious correlation between its change and the acoustic emission signalwhen Q235 carbon steel specimen in the medium containing Cl-.The concentration of NaCl had pronounced influence on OCP.With the increasing of NaCl concentration,OCP moves negatively with wider range and stabilized in a rather negative potential.The AE result accorded the relative narrow variation range of OCP.However,with the increase of NaCl concentration,AE events and OCP behaved differently.OCP only presented general corrosion information of the interface,while AE sensor can record small damage in occluded pits under metallic surface.AE signals can be classified into three types by waveform parameters clustering after AE waveform processing,including pre-treatment,shape preserving interpolation and denoising.The result indicated that this is a highly efficient method for classifying AE signals and preparing waveform for data analysis.2)The law of the correlation between acoustic emission signals and the pitting corrosionwas researched.The pitting corrosion of 316L austenitic stainless steel in 3.5%NaCl solution with different pH value has different incubation periods,and the occurrence of pitting corrosion is random.The electrochemical information mainly reflects the electrochemical reaction on the macroscopic surface of the metal,but in the process of pitting corrosion,the corrosion is mainly in the pit because of occlusion effect.However,acoustic emission can record the actual changes in the material,reflecting the pitting has been growing,or stop.When the 316L is subjected to static loading,the probability of pitting corrosion increases with the increase of stress.After the waveform processing,the parameters of the acoustic emission signal of the 316L austenitic stainless steelpitting corrosion after treatment are analyzed,and three kinds of signals are obtained.There are two kinds of resonance signals among the propagation stage of 316Laustenitic stainless steel pitting corrosion,and the resonance signals arehigh duration low counts and high duration high counts.In addition,the low duration and low count signal is distributed in the whole process of corrosion,and the waveform is dominated by a single impact signal.3)Through the acoustic emission signal further parsing,the method of using acoustic emission technology to identify corrosion category was established.The pitting corrosion process of different metal materials has great similarity.Pitting corrosion and crevice corrosion have the similar acoustic emission process.In addition,there are obvious differences between the corrosion signals and noise signals.The comparison results showed that the corrosion type identification method based on SSIM can be used to divide not only the uniform corrosion and localized corrosion,but also corrosion and noise effectively,while it is not very effective to distinguish the different types of localized corrosion.