Influences of Excitation Current Frequency and Amplitude on Corrosion Evaluation Based on Analytical Model for Surface Magnetic Field

Abstract

The influences of excitation current on the evaluation of the corrosion with a cylindrical shape in a semi-infinite plate are presented in this paper. First, axi-symmetric model of the cylindrical coil above the corroded plate is established and an analytical solution to the surface magnetic field is derived by means of the truncated region eigenfunction expansion method (TREE). Then, the effect of the excitation current frequency and amplitude on the surface magnetic field is investigated and quantitatively evaluated. It is found that the radial magnetic induction intensity increases as the excitation current frequency increases, whereas the axial magnetic induction intensity decreases. The excitation current amplitude is proportional to the radial and axial magnetic induction intensities and does not affect the phase of magnetic induction intensity. As a result, the influence node for detecting the radius and the hollow depth for detecting the hidden depth are obviously dependent on the excitation current frequency in terms of the butterfly-shaped graph. The calculation results are in good agreement with the numerical simulation, showing that the developed model can be used to predict the optimal excitation current frequency. This work indicates that an appropriate excitation current frequency, such as f = 500 Hz, can improve the characteristic parameter resolution for detecting the corrosion radius. For the detection of hidden depth, the lower excitation current frequency, such as f = 100 Hz, can amplify the defect phenomenon and improve the detection accuracy. It is not only helpful to develop the analytical model for the surface magnetic field, but also to optimise the parameters of the test device.