Optical sensor system for corrosion monitoring

Abstract

Corrosion is one of the main degradation phenomena to act on metallic structures, resulting in great economic losses. As a result, different techniques to evaluate and monitor this parameter have become the object of study in recent years. There is a great tendency towards the use of optical fiber sensors for monitoring different parameters, thanks to their advantages over traditional electronic sensors, for example, possible use in aggressive and/or explosive environments, immunity to electromagnetic fields, electrical isolation, low weight and size, longer useful life, among others. Optical accelerometers have also gained prominence, integrating the characteristics of optical sensors, and allowing a greater possibility of applications. The application of these sensors is already widespread for monitoring temperature, stress, strain, refractive index, acceleration, and relative humidity, among others. However, despite the wide possibilities of use in engineering structures, there is still a need for further studies on its application in other areas, such as monitoring corrosion in metallic structures, in particular, in pipelines for the transport of flammables. The variation of the natural frequency of a material is related to the variation of its thickness and stiffness, parameters related to corrosion. This elationship establishes the principle governing the measurements of this work. In this way, it is proposed in this dissertation the development and implementation of an optical sensor based on Bragg gratings for continuous monitoring of the loss of thickness in metallic elements subjected to controlled corrosion, in a generalized and uniform way in a carbon steel plate 1020, as well as the development of an optical fiber accelerometer based on Bragg gratings together with pairs of optical fibers based on the same principle for monitoring the corrosion of a pipeline section of the same material, buried and subjected to controlled corrosion, through monitoring the variation of its natural frequency. The results obtained were promising and indicated the detection of corrosion through fiber Bragg gratings on the metallic plate, with a coefficient of determination superior to 67% to all nine sensors, however, in the monitoring of the pipe section, only the optical fiber accelerometer was able to identify corrosion properly by identifying the variation of the natural frequency, with a coefficient of determination equivalent to 74.43% throughout the monitoring.