Detection of Inner Corrosion of Steel Construction Using Magnetic Resistance Sensor and Magnetic Spectroscopy Analysis

The detection of the inner corrosion of steel construction, specifically backside corrosion of a steel plate, is highly demanded to maintain safety. Eddy-current testing (ECT) is widely used as a nondestructive testing and an evaluation method for detecting surface and subsurface flaws. However, the ECT is usually applied to non-ferromagnetic materials, but has difficulties when applied to ferromagnetic materials with high permeability, since the skin depth is smaller, and the signal-to-noise ratio (SNR) decreases due to magnetic flux fluctuation. In this paper, we developed a magnetic probe using an anisotropic magnetic resistance (AMR) sensor with the sensitivity of 1 nT/root Hz that can detect inner corrosion at extremely low frequency. We also developed an analysis method using magnetic spectroscopy to delete the magnetic flux fluctuation. To improve the SNR of the detected signal, a small cancellation coil around the AMR sensor canceled the applied magnetic field directly coupled to the sensor. The frequency of the applied magnetic field ranged from 1 Hz to 1 kHz. The sensor output at each frequency was lock-in detected. The weak ac magnetic field was applied to the initial permeability region of the magnetization curve. The obtained magnetic vector signal consisting of a real part and an imaginary part at each frequency was plotted as magnetic spectroscopy, and the magnetic component of the eddy current and the magnetic component of the magnetization of the steel were separated. By using the magnetic component of the eddy current, the thickness of an iron steel plate thinner than 16 mm could be measured. Moreover, the shape of the back-side corrosion was determined by scanning with the magnetic probe.