Corrosion Detection: A Fibre Optic Approach

Abstract

Corrosion is a multi-billion dollar problem faced by industry. High acquisition costs associated with modern military and civilian aircraft coupled with tighter budgets has resulted in the need for greater utilisation of existing aircraft eets. With advancing aircraft age there is increased possibility that protective coatings will break down or be damaged, resulting in exposure of the base material to the environment and an increased possibility of corrosion. Corrosion is most difficult to detect in inaccessible metallic structures within aircraft. Monitoring these areas requires a sensor capable of spatially resolved detection of corrosion (distributed measurements), so that the location of the detected corrosion can be determined. Optical fibre based sensors are inherently suited to distributed sensing and are typically in the order of only a few hundred microns in diameter making them very lightweight and suitable for embedding in otherwise inaccessible corrosion-prone areas. This thesis describes the development of an optical fibre based corrosion sensing element. Transition of exposed-core microstructured optical fibres from soft glass to silica is shown to provide a platform for optical fibre sensors requiring long term and/or harsh environmental applications while providing real time analysis anywhere along the fibres length. The portion of light guided outside of the glass core, often described as the `evanescent field,' is affected by the refractive index and absorption characteristics of the surrounding medium. Functionalising this core with chemosensors sensitive to corrosion by-products, turns the light guiding fibre into a corrosion sensing element, with which insitu kinetic measurements of accelerated corrosion in simulated aluminium aircraft joints is demonstrated. This provides a fibre optic approach for detection of corrosion inside the hidden part of structures and opens up new opportunities for distributed optical fibre chemical sensing with a capacity for long-term application in harsh environments.