Defect Mapping of Steel Substrate under Fire Protection Layer using EM NDE Methods Ibukun Dapo ADEWALE 1 , Hong ZHANG 1 , Gui Yun TIAN 1,2 , Tom HOPE 3 1 School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne, U.K. 2 School of Automation Engineering, University of Electronic Science and Technology of China, Chengdu, China 3 International Paints Ltd, Gateshead, U.K. Abstract: Accurate and reliable defect detection and characterization of structures and critical components is becoming of increasing importance. Electromagnetic (EM) methods like eddy current (EC), electromagnetic acoustic transducer (EMAT), alternating current field measurement (ACFM) and microwave imaging have lend credence to this search for a reliable defect detection and characterization probe. However, a probe that would exhibit high sensitivity and spatial resolution in detecting corrosion of substrate under fire protection layer (FPL) with relatively high liftoff is desirable. This paper therefore presents a comparative investigation of the potentials of eddy current displacement sensor and microwave imaging in detecting corrosion in cladded steel samples of varying clad thickness. The possibilities and potentials of these EM NDE techniques for defect-under FPL detection are brought to light in the results presented. Keywords: EC; defect-under fire protection layer; steel substrate; liftoff 1 Introduction To prolong the useful operation life of in-service key components in the petrochemical and nuclear power plants, coating techniques are widely adopted for such structures like pipelines, tubes amongst others to prevent wall thinning or pitting. Again, fire protection layers (FPL) are often used in chemical plants to protect assets like chemical and fuel storage tank cleans and factory cleans. These fireproof coatings help maintain the structural integrity of such assets as the substrate beneath the coating retains its shape and stability in the event of fire or high temperature. Over the years, extensive work has been done to monitor the thickness of such coatings[1], however, the integrity of the substrate under FPL needed to be monitored overtime to ensure that it is in a fit-for-purpose state. It is therefore of utmost importance to identify a non-destructive testing and evaluation technique for effective defect mapping of local wall thinning and pitting. To evaluate the integrity of the substrate under FPL, electromagnetic non- destructive testing (NDT) techniques are preferred and are hitherto being adopted in in-service inspections [2]. Microwave imaging is a credible NDE technique for mapping steel thickness under thick coating in the range of 10 to 40mm offering high resolution as a result of its relative short wavelength[3]. However, the open-ended wave guide used in microwave imaging operates in the Giga hertz frequency band, thus, its sensitivity and spatial resolution is lowered at lower frequencies[3]. It is therefore crucial to point out that a portable low frequency EM energy NDE probe that would not compromise sensitivity and resolution is desirable. Hence, eddy current sensors which are non-contact, low cost and operating at a relatively low frequency [4, 5] is employed to comparatively investigate the structural integrity of steel substrate under FPL of varying thickness. The rest of the paper is organised thus; section 2 gives the theory and principles of eddy current method and report the experimental investigation while section 3 focuses on near field microwave imaging for NDE applications thereafter salient and emergent issues from the investigation is discussed and summarized in the conclusion. 2 EC Method Eddy current technique is widely used in NDT&E. It works on electromagnetic induction principles consisting mainly of an excitation coil and a sensing coil or a magnetic field sensor. When a time varying current in the excitation coil creates an alternating magnetic field commonly called the primary magnetic field in literature, eddy currents are induced in the conductive sample. The eddy currents simultaneously generate a secondary magnetic field, which resists the variation of primary magnetic field, the net field is measured and analyzed to evaluate the integrity of various target samples [6]. There are a number of factors that affects the interaction between the primary magnetic field and the secondary magnetic field amongst which are probe-sample gap called lift-off(l), excitation frequency(f), the electromagnetic properties of the sample(permeability,μ, and conductivity, ), probe geometry() and type of excitation ( ) [4, 7]. The penetration depth of these eddies is governed by the skin depth effect (), which may be expressed mathematically as . 1    f  (1) ____________________________________ 978-1-4673-6020-3/13/$31.00 ©2013 IEEE