Research on magnetic inversion in alternating current magnetic flux leakage testing

Defects bring significant security risks to pipelines, which are essential carriers in oil and gas transportation. However, when alternating current magnetic flux leakage (AC-MFL) testing is applied to pipeline defect detection, the morphological reversal of the detection signal under specific parameters has adverse effects on defect identification and quantification. This paper identifies and explains the phenomenon of detection signal morphological reversal in AC-MFL testing, which is defined as magnetic inversion (MI). Based on the AC-MFL testing mechanism, the causes of MI are analysed. The influence of the excitation current frequency, excitation current intensity and defect depth on MI is numerically analysed using the finite element method. The excitation current frequency and the defect depth are important factors causing MI, but the excitation current density is not. MI only occurs when deep defects are detected under low-frequency excitation. The boundary conditions of AC-MFL testing are further clarified.