Optical Sensors for Corrosion Monitoring
Corrosion can be defined in a simple way as the reaction of a material with its environment which leads to the consumption of the material or its adulteration by a component from the environment . In this post we will briefly explain the existing methods to monitor corrosion by means of optical fiber sensors.
Corrosion is one of the main concerns of structural health monitoring (SHM), which refers to the implementation of technological solutions to control damages in aerospace, civil and mechanical engineering infrastructure . The reasons are that corrosion possesses high repairing costs and, if left unattended, in extreme cases, it can cause the collapse of structures. Therefore, it is crucial to detect corrosion at an early stage.
Optical fiber sensors are a suitable technology for monitoring the health of constructions, including corrosion, as they provide nondestructive, real time and low cost techniques to check its appearance. Among the optical fiber sensors, different solutions are employed for corrosion monitoring such as FBGs, interferometers, surface plasmon resonances (SPR), distributed sensing or optical intensity modulations.
Depending on how they detect the presence of corrosion in the structure, optical fiber sensors for corrosion monitoring can be classified in 4 different categories :
1. Direct measurement of corrosion effects
These sensors detect the strain, deformation and displacements within the structure that are caused by corrosion. In order to do this, optical fiber sensors are embedded in the structure of the building. Their small size enables high accuracy and resolution while avoiding a negative influence in the mechanical properties of the structure.
On the other hand, sensors have to be strategically placed so the strain caused by corrosion is transmitted towards them . Additionally, they have to be protected so they are only affected by the effects of the corrosion that is being monitored and not by other environmental conditions.
These type of sensors are commonly used to monitor corrosion in concrete structures as well as in civil and military airframes, where corrosion is a concern as they are working in some cases beyond their expected lifetime. Among the optical sensors in this category, FBGs, white light interferometry or optical time domain interferometry (OTDR), the latter being a particular technique within optical distributed sensing, are employed. Apart from the embedded optical fiber sensors, there are also fiber-optic hand-held devices, which measure corrosion as a function of the roughness or the color of the sample.
2. Measurements by correlation
In this case the corrosion of the structure of interest is not directly measured, but estimated by monitoring the corrosion of a layer of an analogous material deposited onto an optical fiber. Obviously, this sensor has to be placed next to the structure whose corrosion is being studied to experience the very same conditions .
This approach is commonly employed in metals such as steel, aluminum, copper or nickel in domains like construction, civil engineering or aeronautics. However, it is also used in protective coatings or paintings. The critical parameter for these sensors is the thickness of the coating, which has to be selected bearing in mind the expected life of the structure under study.
The simplest setup is based on reflection, where the reflected power is monitored and decreases as the coating (which has a high reflectivity) degrades. Other configurations, based on absorption, OTDR, SPR or FBG are also used.
3. Detection of corrosion precursors or products
Regarding precursors of corrosion, optical fiber sensors can be used to detect ions that induce a higher corrosion rate, where waterborne chlorides are the most relevant ones. These sensors overcome the disadvantages of traditional chemical analysis, which is destructive, expensive and does not provide real time data .
An interesting option for monitoring the presence of chlorides is placing a fluorescence-based sensor on the tip of an optical fiber, as in . In the absence of the chloride, fluorescence is detected. When the chloride is present, it acts as quencher, reducing the fluorescence and therefore enabling corrosion detection. However, further research in this setup is required.
Other approach consists in monitoring the presence of corrosion products, such as ions Ca2+, Cu2+, Mg2+, Al3+. In this case, the most common setup is based on measuring the light transmitted through an optical fiber with a coating that has an affinity to corrosion by-products.
4. Measurements of environmental parameters
Water plays an important role in corrosion, transporting agents, such as the previously mentioned chlorides, and being the medium in which some chemical reactions that contribute to corrosion take place . Therefore, humidity sensors based on different topologies (LPGs, FBGs, interferometers among others) are also employed in corrosion monitoring.
On the other hand, the pH of a medium can also be linked to corrosion. In particular, it is considered that a pH below 9 can damage steel structures, so monitoring pH can be quite useful to prevent corrosion . Optical sensors for this purpose are commonly based on fluorescence or colorimetry.
As a conclusion, optical fiber sensing devices are an attractive solution for corrosion monitoring, as they can equal or even surpass conventional sensors by providing remote and distributed measurements and offering simple integration and installation as well as high durability and reliability.
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