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Quantitative Detection of Shallow Corrosion Damage by Targeted Use of the Dispersive Behavior of Guided Wave Modes



Pipelines in chemical and petrochemical processing plants are fixed at various locations through attachments and supports either on ground or on pipe bridges. Those locations are prone to accumulate moisture which may result in corrosive substances directly leading to corrosion and affecting the outer pipe wall. Those corroded locations are often difficult to be accessed for visual inspection and possibly also other means of inspection too. The same applies for gas and water supply lines, polls for roofs or signpost dug into the ground. Monitoring of corrosion along those tubes and polls is a must with respect to operational and general safety as well as for environmental reasons mainly imposed by public authorities. Targeted use of dispersive ultrasound wave modes offers the possibility to detect corrosion-related wall thickness reductions along the path of the ultrasonic wave considering ultrasonic transmission measurements. The effect measured is based on the fact that both phaseand group velocity of the ultrasonic wave are a dispersive mode that varies with wall thickness. Through analysis of phase position and / or travelling time of specific modes, information resulting from the wall thickness reduction along the acoustic wave path can be accessed. This is made possible through Electromagnetic Acoustic Transducers (EMATs) which allow a pure mode excitation of a guided ultrasonic wave to be generated such that this excitation as well as receiving configuration allows an exact evaluation of the phase position to be obtained. Furthermore, a targeted impression of a desired trace wavelength is possible. Due to the EMAT principle, generation and recording of an ultrasonic signal is done without direct contact with the specimen. Fraunhofer IZFP’s newly developed EMATs not only allow the variation of ultrasonic modes, but also a change in the trace wavelength. Hence, by analysis of the phase in different specific modes and locations on the component to be inspected, important information for the quantitative error determination can be found. Particularly, this approach can be used to detect shallow wall thickness changes, which generate no measurable reflection signals.

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