Effects Of The Metallic Wire Mesh On Pulsed Eddy Current Detection Of Corrosion Under Insulation

Insulated pipes are widely used in the petrochemical,power and heating industries.This type of pipe is often covered with cladding and is susceptible to Corrosion under Insulation(CUI),which can pose a risk to safety production.Pulsed Eddy Current Testing(PECT),as a branch of eddy current testing,has been successfully used for rapid inspection of CUI.However,controversy exists in existing studies about whether the metallic wire mesh embedded in the insulation affects the testing results.In light of this,this thesis touches on the effect of metallic wire mesh on CUI-PECT,which mainly focuses on the following work:Finite element simulation model regarding the CUI-PECT was developed by simplifying the cladded pipe to a multi-layer plate structure.For the two modes of PECT excitation,the constant current and constant voltage,differences between them were analyzed and finite element simulations of the CUI-PECT were carried out for each of the two excitation modes.It was found that for both excitation modes,the aluminum and stainless-steel metallic mesh had little effect on the eddy currents induced in the sample and the probe induced voltage signal,while the mild steel mesh resulted in a significant increase in the probe footprint and a significant decrease in eddy current intensity.Further,the influence of the electromagnetic and geometrical parameters of the mild steel metallic mesh on the CUI-PECT was analyzed which indicated that magnetic permeability,wire diameter and aperture diameter were the three main factors affecting the probe footprint and eddy current intensity.Subsequently,the effect of a mild steel metallic mesh on the characteristic quantity of the probe’s induced voltage signal was investigated in simulation,using a typical CUI defect such as uniform wall thickness thinning.The results show that the presence of a mild steel mesh increases the probe footprint(up to 47.3%),reduces the amplitude of the induced voltage signal,leads to a reduction in the slope of the late induced voltage signal and reduces the maximum eddy current intensity at the specimen cross-section under constant current excitation conditions.Thus,reducing the ability of the PECT probe to detect CUI.Under constant voltage excitation conditions,the presence of the mild steel metallic mesh also reduces the amplitude of the induced voltage differential signal,while slowing down the variation of the peak differential signal with CUI size.Analyzing the above simulation results,it can be concluded that the effect of the mild steel mesh on the PECT is mainly an increase in the probe footprint(up to 46.3%)and an attenuation of the eddy current intensity,the combined effect of which reduces the probe’s capability to detect the CUI.Finally,experimental verification was carried out on Q235 steel plate specimens with prefabricated wall-thinning defects under the excitation mode of constant current and constant voltage,respectively.The CUI with areas of 120×120 mm,240×240 mm and 400×400 mm and depths of 2,4 and 6 mm,respectively,were tested under constant current excitation,and the results showed that the slope of the late PECT signal in the presence of Q195 mild steel metallic mesh was smaller than that of the PECT signal in the absence of mesh under the same conditions.Under the condition of constant voltage excitation,the CUI with an area of 120 × 120 mm and depths of 2,4,6 and 8 mm were tested,and it was found that the Q195 mild steel metallic mesh would reduce the peak of the differential signal and would slow down the variation of the peak of the differential signal with the depth of the CUI.Both simulation and experimental results show that regardless of the excitation mode of the PECT probe,the mild steel metallic mesh increases the size of the probe footprint and reduces the eddy current intensity in the tested sample,thereby reducing the detection capability of the PECT probe for CUI.