Assessment of Corrosion Damage in Steel Samples Using Electro-Magnetic Acoustic Measurements

Abstract

Ultrasonic thickness resonance and pulse propagation speed can be effectively used to find the thickness of thin-walled structures present in the skin of aircraft, fluid handling equipment such as pipes and in fluid storage tanks. Electromagnetic Acoustic Transducers (EMATs) yield several advantages over more traditional piezoelectric based transducers, namely that they can be used in a non-contact fashion allowing measurement regardless of surface condition or temperature. The main disadvantage of EMATs is their low efficiency. Therefore, resonance techniques, in which most of the energy is concentrated near structural resonance frequencies, are preferable to boost efficiency of electro-magnetic acoustic measurements. Thickness resonance is a technique commonly used to measure small changes in structural thickness, by integration of a ringdown curve in a wide time-domain window at different excitation frequencies. The Pulse Propagation Speed technique relies on the fact that ultrasonic waves travel faster in ticker plates in comparison to thinner plates at the same frequency. These techniques were applied to 316L stainless steel thin plates. Samples consisted of different thickness rolled material, machined samples, and samples artificially corroded with both uniform corrosion as well as pitting corrosion. These samples were used to determine the effect of thickness reduction and corrosion on the resonant peaks and lamb wave velocity. EMAT liftoff distance was also studied to quantify its effect on the amplitude, spread, and frequency of resonant frequencies and signal transition. Detectability of corrosion using EMATs was explored and quantified. Recommendations for use of EMATs for assessing corrosion damage were presented.