18.2.1 Fiber Bragg gratings

Fiber Bragg gratings (FBGs) are optical filters that allow the transmission of some wavelengths and reflect others; this is achieved by introducing a variation in the refractive index of the core of the fiber periodically along a certain length. FBGs are generally fabricated with the help of a laser by writing a periodic pattern at the core of the fiber for a short length (close to 1 cm). The reflections originated at the core due to refractive index changes in the pattern produce a back reflection at a certain wavelength known as resonant frequency.¹² Resonant wavelength can be obtained by Bragg's law from Equation (18.1):

$\lambda B=2n_{\text{eff}}\Lambda$

where lB is the resonant wavelength, n_eff is the effective refractive index of the fiber core, and Λ is the pitch length of the grating or period of the grating.

Thus, any change in the pitch length or refractive index will induce a shift in the resonant wavelength. Consequently, temperature, strain, cracks, or deformations of the fiber can be monitored by the corresponding resonant wavelength shift.¹³

A strong point of FBGs are their capability of multiplexing in wavelength that enable multiple point or quasi-distributed sensing. FBG sensors have been successfully developed by different companies (www.hbm.com, www.technicasa.com, www.fbgs.com), as represented in Figure 18.1, and are extensively applied in real-life applications.³

FBG sensors are usually embedded into a fiber-reinforced polymer (FRP) composite or welded to steel structures to protect the brittle optic fiber against breakage.¹ Here embedded FBG sensors should be correctly matched with the stiffness of the structural matrix in order to obtain consistent information from civil structures.

18.2.2 Interferometers

Interferometric sensors rely on phase change detections produced by the selected measurand on the light traveling through the optical waveguide. Interferometric sensors can be used in corrosion sensing applications as well. In the case of an optical fiber, the light launched into the fiber is reflected back at two different interfaces forming an interferometric cavity or etalon, as schematically represented in Figure 18.2.

The variation of the transmitted power is originated by interference between the multiple reflections of light between the two reflecting surfaces. Interference can be constructive or destructive when the transmitted beams are in-phase or out-of-phase, respectively. The phase difference (δ) for each reflection is given in Equation (18.2):

$\delta =\frac{2\pi }{\lambda }2n\mathcal{l}cos\theta$

where λ is the wavelength of the light in vacuum, θ is the angle of the light traveling through the etalon, ℓ is the thickness of the etalon, and n is the refractive index of the material that forms the cavity.¹⁴ From Equation (18.1) phase changes in general can be given by changes either in the length, index, or geometry of the etalon. Thus, changes of the cavity parameters, which can be either passive or active, induced by the measurand modify the interference pattern and can serve as the transducer mechanism. The interference pattern can be originated using both coherent and low-coherence techniques depending on the cavity size.

Multiple groups and companies (www.roctest.com and www.fiso.com) have exploited this structure for strain, pressure, displacement, vibration, chemical, and humidity sensing, and some of their commercial devices are represented in Figure 18.2.

18.2.3 Distributed sensing

Distributed measurements exploit the optical fiber as the sensing. Here, an optical pulse is launched into the fiber. Then, the light power that emerges at the launch end as a result of the linear or nonlinear effects in the fiber is time resolved in order to provide information about the state of the fiber. The collected information can be correlated with the spatial position by means of the speed of the light in the fiber as a function of distance and light. Whithin optical fiber distributed sensors, we can include from the traditional optical time domain reflectometry (OTDR) and signal processing techniques that enable to measure optical power attenuations associated with cracks, micro, and macrobends to the more recent as well as more complex nonlinear phenomena based on Raman and Brillouin scattering effects and their corresponding measurement techniques, such as Raman optical time domain reflectometry (ROTDR) or Brillouin optical time domain reflectometry (BOTDR, setup represented schematically in Figure 18.3), which are both temperature and strain dependent.^15–17

18.2.4 Optical intensity modulations

Light intensity modulations based on the direct interaction of the measurand are a conventional method for the fabrication of OFSs. These sensors are based on the contour conditions of the total internal reflection phenomenon that occurs at the interface between two mediums, the first with high refractive index (guiding medium) and the latter with lower refractive index (reflecting medium). In the boundary of these mediums the light is totally reflected, but some part will enter through the low refractive index medium, which is called evanescent field. The main problem of these devices is that the light entering in the low refractive index medium will decay exponentially as a function of the penetration depth, making it paramount to come as close as possible to the guiding part in order to obtain good sensitivity. In the case of optical fibers the interaction of the evanescent field and the measurand is related to the opto-geometrical parameters of the fiber. Different configurations have been proposed in order to enhance evanescent field interaction by means of the modification of the optical fiber geometry, such as polished fibers, cladding removed fibers, chemically etched fibers, stretched fibers, microstructured fibers or D-shape fibers. Then, the detection will just resort on direct or indirect interactions of the evanescent field with the selected measurand. An example of evanescent field based OFS for pH sensing is shown in Figure 18.4.

18.2.5 Surface plasmon resonances

Surface plasmon resonance (SPR) refers to the excitation of surface plasmons (SPs), which are electromagnetic waves coupled on the surface between a metal and a dielectric medium (or air) propagated along the interface of the metal and dielectric material (or air) as it is schematically represented in Figure 18.5.

SPR devices are fabricated typically by adding a thin-metal layer (normally gold or silver) close to the optical fiber core. Consequently the evanescent field excites the SPs at the fiber core-metal layer interface. The correct coupling and SP generation strongly depend upon light wavelength, fiber parameters, fiber geometry, and metal layer properties.^18–20

Many optical sensing systems have been developed using the SPR phenomenon thanks to the high sensitivity of the resonant wavelength to changes of refractive index of both the metal layer and the sensing medium as well as thanks to the thickness of the metal layer and overlayer. Thus, the proper selection of sensitive thin-films enables researchers to easily obtain SPR-based devices for a wide range of applications including corrosion detection.^21,22

18.3.1 Direct measurements of corrosion

Strain and deformation of a structure at an early stage can give valuable information about its mechanical properties and particularly the corrosion of the same. Optical fibers fully integrated into structures are able to monitor directly the initiation and progress of corrosion-induced mechanical degradation. The sensors are commonly embedded into a structure to form a novel self-strain and temperature monitoring system. The embedded sensor, due to its extremely small physical size, can provide the information at a high accuracy and resolution without influencing the dimension and mechanical properties of the structure. A common application consists of temperature and strain detection in reinforced concrete structures where cracks can be developed and propagated as a result of corrosion originating strain, deformation, or displacements. A few works have studied FBGs strain sensors to monitor the corrosion rate in civil engineering structures at single or multiple points.^23–27 In Ref. [26] it is presented and experimentally demonstrated the utilization of a new fiber glass-reinforced polymer (FGRP) optical fiber Bragg gratings (OFBG), which is merged within the stay cable (see Figure 18.6) in bridges for corrosion detection. This work shows the importance of the optical fiber embodiment and armor in order to withstand the harsh working conditions and obtain large durability of the sensor.

Strain and deformation sensing in reinforced concrete structures has been also addressed by means of optical fiber interferometers.^28–32 In particular, the location of the sensor in Ref. [29] enables the detection of tensile strain perpendicularly to the plane of the reinforced steel bars. This configuration enables to monitor the strain or delamination associated to the longitudinal cracks along the reinforcing bars resulting from the radial expansion of the corroding rebars, particularly in those places where visual inspection is not possible. In Ref. [28] the strain response of an OFS based on white light interferometry (WLI) and FBGs is compared showing the feasibility of obtaining low-cost and stable sensors for corrosion detection. Additionally, WLI corrosion sensors (WLI-CS) were embedded in concrete and subjected to corrosion tests showing good agreements with other sensing technologies.

OTDR techniques enable distributed sensing in order to monitor optical fiber bents associated to corrosion in structures and can be a cost-effective solution.³³ Cutting-edge detection techniques based on BOTDR enable distributed sensing of strain in steel-reinforced concrete in long-distance applications and with high spatial resolution.^34–37 A novel arrangement based on Brillouin scattering, known as fiber optic coil winding, is described in Ref. [36]. Here, the authors exploit the distributed sensing ability of the technique to directly measure the expansion strain caused by corrosion. The sensing element is quite simple and consists of winding the optical fiber around the steel rebar, as in Figure 18.7. Thus, the fiber optic will be stretched when the steel rebar expands due to corrosion and will be monitored using the BOTDR analyzer.

In Ref. [35], the authors describe a novel method in order to overcome some of the challenges of long-distance embedded optical fiber sensor (LEOFS), such as the fragility of the optical fiber or interference with construction techniques. This method employ air-blowing in order to lay the optical fiber into the preinstalled tubes and uses the vacuum grouting technique to fix the optical fiber tightly inside the tubes. Given that this technique enables LEOFS to be installed into the tubes separately, it has no interference in the construction and the sensors can be easily replaced and repaired if necessary.

Moving out from reinforced concrete structures, the degradation of civil and military airframes is a main concern because they are operating in many cases far beyond their original expected lifetime. Here, corrosion is also an important factor that can lead to material loss and cracks, which can be costly to support in terms of reparations and human lives. The role of OFS and the accurate interpretation of their signals can be crucial in order to proceed with the correct maintenance.⁷ For example, in Ref. [38] it is shown the utilization of a LPG sensor to measure corrosion in aging structures as well as to avoid the temperature dependence of FGB sensors with the same characteristics, while in Ref. [39] the detection of the delamination of metals using a FGB sensor is simulated. A different application consists of corrosion monitoring in marine environments.³¹ Here, the authors employ a metal-coated optical fiber that acts as a corrosion fuse that interrupts the optical fiber transmission when it is corroded, as is schematically represented in Figure 18.8.

Apart from the mentioned continuous in situ corrosion monitoring structures, also described can be other devices, handhold devices, intended for discrete study and characterization of corrosion as a function of the color⁴⁰ or roughness^41–43 of the samples. Table 18.1 summarizes the characteristics and main features of the sensors described in this section.

18.3.2 Corrosivity direct measurements using metallic sacrificial layers

The sensors proposed here do not detect the corrosion of the material in a direct way. Instead, a layer made of a material similar to that which degradation is going to be monitored coats the chosen optical fiber architecture. In other words, the corrosion of the structure of interest is estimated by monitoring the corrosion of a similar material deposited onto an optical fiber. The sensors are placed near the structure that needs to be preserved. Most commonly, degradation is detected on metals using this strategy. Some examples are aluminum, steel, copper, and nickel and are of special importance in construction, civil engineering, and aeronautics. Nevertheless, some of these structures can be also applied to detect the degradation of other materials such as paintings or protective coatings.

Different optical fiber topologies have been used as corrosion detectors, from typical transmission configurations to FBGs or sensors based on SPRs. Obviously, the dynamical range of these sensors strongly depends on the thickness of the deposited coating. For this reason, this parameter has a crucial importance, and it has to be chosen according to the concrete application of the sensor.

One of the simplest sensing architectures applied to this kind of sensor is based on a reflection setup. The sensing device consists of an optical fiber tip perpendicularly cleaved coated by a layer of the material to be monitored. If the reflectivity of this material is high enough, most of the light sent through the fiber will be reflected and the reflected optical power will present its maximum value. As long as the coating is suffering from degradation, its thickness and the value of the reflected optical power will decrease. This way, when the coating has been completely removed from the optical fiber tip, the reflected optical power will reach its minimum value.

Some examples of this configuration can be found in Refs. [44–47] where the authors use this simple configuration to develop OTDR multipoint sensing systems devoted to the detection of aluminum and steel degradation, respectively. As an example, in Figure 18.9 the response of an array of optical fiber corrosion sensors is shown. A 200 nm-thick iron coating was sputtered onto each sensor. The sensing devices were placed at variable distances from seawater, which explains the differences in the output signal of each sensor.

Another topology widely applied in corrosion sensing is the absorption configuration. The cladding of a portion of optical fiber is replaced by a high-reflecting coating of the same material to be monitored. This way, a significant attenuation of the higher order modes is produced in this coated zone. For this reason, the transmitted optical power will be significantly lower. As the coating corrodes, the cladding around the core is removed, the absorption losses decrease, and the transmitted optical power increases.⁴⁸

Sensors used to detect corrosion on aluminum,^49,51 copper,⁵² nickel,^53,54 or steel^55,56 are present in the bibliography. These examples apply different deposition methods to create the sensitive coatings, such as sputtering, thermal evaporation, or electrodeposition, depending on the material to be deposited.

In Ref. [49] the authors develop and characterize transmission sensors for detecting aluminum corrosion. Aluminum coatings were deposited via thermal evaporation onto the optical fiber core. For a coating thickness of 2 μm, the response of the output optical power when the sensitive zone is immersed in nitric acid (1 N, 10-1 N, and 10-2 N) is shown in Figure 18.10. It can be clearly appreciated how the transmitted power rises as far as the aluminum layer corrodes. This rise is faster if the concentration of nitric acid is higher.

SPR OFSs have been also used to detect the corrosion of metals. Abdelmalek studied the optical properties of gold-aluminum films, using a sensor based on this phenomenon.⁵⁷ When the sensitive region is immersed in water, the SPR peak shifts slightly to higher angles of incidence due to the difference between the refractive index of water and air. After 3.5 h, the corrosion of aluminum produces an important shift of the peak and a change in its shape, as can be seen in Figure 18.11. A deeper study of this phenomenon where a combination of SPR and FBGs is applied to the fabrication of corrosion sensors can be found in Ref. [58].

Some other authors have taken advantage of the sensitivity of FBGs to strain by developing optical fiber corrosion sensors.^59,60 In 1996, Green et al. proposed corrosion sensors based on prestrained short-period gratings and LPGs.⁶¹

Decades later, Hu et al. developed an FBG sensor for steel corrosion monitoring.⁶² In particular, a steel sensing film of 12-15 μm thick is electroplated on the surface of the FBG. When this film is corroded, it suffers a longitudinal change that causes the strain of the FBG. Consequently, the optical output of the FBG will experience a wavelength shift that can be detected with a spectrometer. As FBG are also sensitive to temperature, an FBG without steel coating is used as reference. In order to induce accelerated corrosion of the film, sensors are immersed into NaCl solutions (0.5 M). In Figure 18.12, the shift of the FBG peak during an 18-day experiment is shown.

One main peak can be observed in the spectrum of the sensor before being introduced into the NaCl solution. This peak shifts to a higher wavelength 4 days later. Eight days after the immersion, multiple peaks are observed, and, finally, after 18 days in the cabinet, they merge into one major peak. The authors conclude that the multiple peaks are related to nonuniform strain and, consequently, can be used as a critical signal of the intense corrosion of the steel.

There are other examples of OFSs for indirect corrosion monitoring, but those presented here are the most representative. A summary with the main features of the approaches reviewed in this section is given in Table 18.2.

18.3.3 Measurement of corrosion products and precursors

The detection of corrosion precursors and by-products derived from progressive deterioration can be perfectly monitored when a proper coating is deposited onto an optical fiber. The results obtained can help the engineer to make a final decision about the need for preventive maintenance or premature replacement of the structure. In Figure 18.13, a scheme about the chemical elements of analysis (precursors and by-products) is shown using embedded optical fibers in order to help ensure the safety of the structure.

18.3.3.1 Detection of corrosion precursors

The ingress of harmful ions such as carbon dioxide, nitrate, sulfur dioxide, or chloride has a deteriorating impact on the performance of engineering materials because the ions are capable of inducing a higher corrosion rate, compromising the integrity of the structure.⁶ However, not only these ions can accelerate the corrosion process, but also the presence of microorganisms, such as sulfate-reducing bacteria (SRB), which can have an important role in localized corrosion.⁶⁹ Of all these, the waterborne chlorides are one of the most influential factors in the rapid development of the corrosion. Their presence can be due to the use of deicing salts on roadways or bridges to lower the melting point of the ice, during which process some of these salts react with the H₂O that transports the chloride into cracks near metal.^70,71 Several studies have reported that the rate of corrosion strongly depends on the amount of chlorides present within the structure. Once chloride ion penetration occurs, active corrosion is initiated with a sequence of events such as cracking and spalling.⁷² The process is slow and complex as a function of the involved materials and environmental variables. An interesting aspect is that the corrosion process is accelerated in marine places in comparison with other environments such as inland structures due to the higher presence of chloride ions in coastal areas.

In order to detect chloride ion penetration in concretes, the usual methods are based on a chemical analysis in an external laboratory. However, this analysis shows inherent limitations, such as being destructive and expensive, and do not provide continuous information in real time because it is necessary to collect the sample and a further analysis to know the chloride content. To overcome these limitations, the use of optical fiber as a nondestructive technique allows us to obtain the information in situ with high accuracy through remote measurements and to control the state of the structure.³ Once optical fibers are embedded in a concrete structure, most of the sensing schemes to detect corrosion are based on a spectroscopic analysis of the surface of the material in which light signals are reflected from both corroded and uncorroded areas. A relevant study is reported in Refs. [11,10], where the use of a twin-fiber technique (separate transmit and receive fibers) can determine the presence of corrosion via color modulation of the broadband input signal. However, this spectroscopic technique is based on a local inspection of the corrosion area due to a color shift, but it is not possible to estimate the chloride ion concentration present in the final structure.

An interesting approach is proposed in Ref. [73], where an absorbance-based sensor is developed to monitor the chemical interaction of silver nitrate-fluorescein-chloride mixture, which results in a red coloration. The end of an optical fiber is coated by a solid net that contains the mixture of fluorescein and silver nitrate. If the chloride ions penetrate this net, a colorimetric change is immediately observed. The range of chloride detection is from 0 to 40 mM, and higher values of absorbance correspond to the higher values of chloride concentration. However, all the measurements are made in a short period of time after the addition of chloride, a critical parameter in the development of the sensor. The absorption peak is located in the 500 nm region and can be perfectly measured with blue or green laser diodes. The signal remains stable for the first 10 minutes, but a higher period of time results in a total precipitation. In addition, once the chemical interaction has occurred, the process is completely irreversible.

According to the previous results, it is necessary to implement an optical sensor more attractive in terms of reversibility and time dependence in order to detect the chloride concentration. The use of a fluorescence-based sensor combined with optical fiber is one of the most promising alternatives.^74,75 In these cases, the sensors are based on fluorescence quenching, by which the excited reagent is deenergized by nonradioactive collision with the molecules of the analyte (halide ions). The collisional quenching is a reversible process whereby the emitted fluorescence intensity decreases linearly when the quencher concentration is increased. This process can be described by the Stern-Volmer equation:

$\frac{I_{\text{F}}^{\text{o}}}{I_{\text{F}}}=1+K_{\text{SV}}\left[Q\right]$

I_F^o, fluorescence intensity in the absence of quencher; I_F, fluorescence intensity in the presence of quencher; [Q], quencher concentration; K_SV, diffusion dependent Stern-Volmer constant.

The extinction mechanism is composed of a contact between the excited state of the fluorescent specie (dye) and the quencher (chloride), as can be observed in Figure 18.14.

First experiments with fluorescence-based sensors to detect chloride concentration were reported in Refs. [73,74], where the use of two different quinoline derivatives dyes (ABQ and MEQ) is presented. The results indicate that ABQ displays higher fluorescence efficiency than MEQ. The next steps were a further immobilization of both dyes in a microporous silica coating using the sol-gel technique, and then the coating of a declad optical fiber. The desired configuration for in situ chloride detection was an evanescent wave excited coating. However, once the sol-gel coating was applied onto the optical fiber, no fluorescence signal was detected. The main reason could be because smaller analyte molecules (chloride) can penetrate the microenvironment of the sol-gel matrix; as a result, no interaction between anlayte and dye was observed. Different sol-gel formulations at different pH values were investigated, but none was successfully proved as an evanescent-wave sensor.

An interesting approach using an optical fiber combined with luminescence measurements for free chloride ion detection in concrete is presented in Ref. [75]. In this case, the use of a new fluorophore, known as lucigenin, is of great interest because it has a higher quenching sensitivity than other products and its Stern-Volmer's constant (K_SV) is the highest for chlorine indicators. Here, a sol-gel method was also chosen to encapsulate the dye into a polymeric matrix at the end of the optode. The term optode refers to detectors that are transducers between the measured quantity and optical fibers. The optical design of the sensor is shown in Figure 18.15, where the four elements of the fluorescence system are present (an excitation source, indicator for the analyte detection, wavelength filters, and the detector). In addition, multimode optical fibers are used to connect the emission module to the optical probe (optode). Two experimental tests, hot maritime climate and cold continental climate, were performed in a controlled environment to determine the chloride content. The results can detect free chloride concentration between 30 and 350 mM with a good reproducibility and a good stability.

Other optical fiber based techniques have also been reported^76,78 for chloride detection, which do not require expensive optical sources or detection systems as was previously thought in the fluorescence-based sensors. Of these, the use of long period grating pair (LPGP)⁷⁹ is of great interest because small quantities of chloride ions in NaCl solutions can be detected (10 ppm). In addition, the results reveal that through monitoring the induced wavelength shift, the LPGP could reach a better precision (over six times) than single LPG when changes in the refractive index of the surrounding medium were monitored. This high sensitivity makes possible the use of LPGP in early stage monitoring for corrosion of materials in both the marine environment and for salt sprays used for deicing road surfaces in cold climates.

18.3.4 Relative humidity monitoring for corrosion control

Presence of water is one of the most frequent causes of infrastructure deterioration. Water acts as a transporter for aggressive agents, such as chloride or sulfate ions. In addition, water is a reaction medium in some destructive chemical processes. In general, water plays a key role in the processes of corrosion. For this reason, it is of great importance to implement sensing tools to allow the early detection of the humidity in order to avoid the structures damage and the presence of water, which will remediate the situation before the damage to the structures is produced.

A huge number of optical fiber humidity sensors can be found in the bibliography.^89,90 Several optical fiber topologies have been applied for the fabrication of these devices, such LPGs, FBGs, sensors based on evanescent field, sensors based on lossy mode resonances (LMR), and interferometers.^91–99 An important part of these devices have been specifically designed for early corrosion detection.^100–106 Some examples are presented in this section.

Mendoza et al. developed a distributed optical system for detecting humidity and pH changes in aircraft joints.^102,103 This system consists of optical fiber pH and humidity sensors based on the evanescent field. The fabrication of the humidity sensors involved the doping of the optical fiber permeable polymer cladding with a solvatochromic reagent. The color of this indicator strongly depends on the presence of humidity in the ambient. When the humidity increase, the dye absorbs at a different wavelength, and, consequently, the fiber loss will depend on the humidity percentage. This way, a difference of more than 1 dB (at a wavelength of 1300 nm) can be appreciated between the attenuation due to the evanescent field when the sensor is placed in a wet medium and the same magnitude when it is in a dry medium.

In Ref. [106], the authors fabricate an OFS based on a LPG. First of all, they inscribed the gratings by focused beam of CO₂ laser onto single mode fibers with core and cladding diameters of 8.2 and 125 μm, respectively. Once the LPG was ready, they deposited a nanocoating made of polyallylamine hydrochloride (PAH), polyacrylic acid (PAA) poly-sodium 4-stirenesulfonate (PSS), and alumina (Al₂O₃) by using the LbL technique. According to the authors, the purpose of the PAH/PAA nanofilm is to increase the sensitivity of the sensor, while the Al₂O₃/PSS nanofilm increases the water absorption. The response of the coated LPG when it is subjected to relative humidity variations from 20% to 90% is shown in Figure 18.16a. A double effect can be distinguished. On one hand, the resonance shifts to the left, showing a change of 3.6 nm. On the other hand, an intensity variation of 6.978 dBm is appreciated. Both responses are more clearly shown in Figure 18.16b.

These data suppose a wavelength sensitivity of 0.051 nm/%RH and an intensity sensitivity of 0.099 dBm/%RH. In addition, this optical fiber humidity sensor showed a small dependence to temperature changes, what make it suitable for implementation of concrete structures health monitoring.

Other optical fiber humidity sensors designed for corrosion control are summarized in Table 18.4.

18.3.5 Optical pH sensors for corrosion control

Among other physical and chemical conditions, such as moisture (see Section 18.3.4) or chloride ions concentration (see Section 18.3.3), the pH of a medium is a parameter that is altered when corrosion occurs. Therefore it is very important to have information about the stability of the pH of the medium in which corrosion may occur as long as it can provide an early warning signal prior to the massive degradation of the metallic structure, and it may be possible to take predictive maintenance actions. It has been demonstrated that acidic pH conditions below pH 9 can damage the steel structures, and therefore long-term monitoring of pH values in the range of 9-13 with a resolution of about 0.5 pH units can be very useful for early detection of potential corrosion condition.⁸

The most common approaches to optical pH sensing are based on a primary transducting material which transforms the pH of the analyzed medium into a useable optical signal that can be collected using an optical fiber. Basically it is possible to differentiate two main approaches regarding the optical phenomena underneath the chemical-optical transduction: fluorescence-based sensors and colorimetric sensors.

This last group involves probably the most straightforward method to get optical information from the pH of a medium because there are many pH indicators traditionally used in acid-base titration. Nevertheless, other fluorescence-based approaches for optical fiber pH sensing for corrosion detection can be found. In a basic OFS configuration, the colorimetric or the fluorescent dyes are immobilized into a proton permeable membrane and the changes of their UV-VIS spectra or fluorescence emission can be monitorized using an optical fiber or an optical fiber bundle. Some authors have reported different immobilization media for creating the sensitive areas such as polyurethanes,^109–111 hydrogels,^112,113 and celluloses.^114–116 But one of the most used immobilization media are the sol-gel matrices ^117–120 because they are fully inorganic matrices, mechanically robust, optically transparent, and permeable to H⁺ and OH⁻ ions.¹¹⁷

All those matrix materials can be used to place the pH indicators or fluorophores within the light path of a measuring optical system, and therefore any optical change in the indicators can be registered. This region where the indicators interact with the external medium and at the same time with the light conducted by the optical fiber is often called sensitive region, or sensitive coating (or chamber), depending on the particular application. The configuration of this sensitive region is not always the same, and it strongly depends on the nature of the indicator (colorimetric or fluorescent). When fluorescent indicators are used it is very important to maximize the collection of the emitted fluorescent signal and at the same time minimize coupling of the excitation light as it can mask the fluorescence signal at the detector. This is shown in Figure 18.17a and B, where the excitation light interacts with the fluorescent material, and passes through it, reflecting back only in a small proportion. The emitted fluorescent light is emitted in all directions, and some of this emission will be collected by the optical fiber and further analyzed. Figure 18.17C shows an alternative transmission setup arranged in a U-bend typically used with colorimetric indicators. In such configuration white light can be used as excitation for the sensor, and the transmitted light is guided towards the detector (spectrometer). Changes in the UV-VIS absorption spectrum will give information about the pH of the colorimetric indicator immobilized into the sample chamber. Other simpler approaches can be used replacing the white light source and the spectrometer by LEDs and simple photodetectors ¹²¹ to achieve information only at the sensitive bands of the colorimetric indicators.

One of the most robust optical approaches for civil engineering applications is a reflection setup using a multiple fiber optode.¹²² As it is shown in Figure 18.18, those fiber bundles use a complex structure of peripheral optical fibers to maximize the amount of illumination light (excitation), and a central optical fiber to collect the reflected optical spectrum.

Habel and coworkers have developed colorimetric dye-based pH optodes and have successfully measured pH in situ a steel-reinforced concrete structure for more than 6 years installed in cooling towers of a power plant.⁸ They have used multi-wavelength measurements that give ratiometric information about the acid-basic state of the pH indicator, which is more robust to optical power fluctuations or even signals to the detector. The colorimetric or fluorescent indicator is immobilized into a disc-shaped matrix (polymeric, sol-gel, etc.), where optical properties are altered by the pH of the external medium. When fluorescent indicators are used the peripheral optical fibers excite the fluorescence in the sensitive disc and only a small portion of this excitation optical power is collected by the central interrogation fiber. As the fluorescence is emitted omnidirectionally, this signal can be collected and analyzed at the detector. When colorimetric indicators are used, the optode ends with a mirror that focuses back the transmitted light into the central sensing fiber, keeping the information of the optical absorption of the dyed disc. Such optical fiber bundles have become a very robust choice for pH optical sensing⁸ and some of them are commercially available.¹²³ It has been observed than the most serious attacks concerning stability of pH-sensitive membrane occurred in the first few months.⁸ In Figure 18.19 a real application of pH colorimetric optodes is shown. The optical pH sensors are immobilized close to the steel structure prior to the concrete pouring during the construction of a cooling tower of a power plant. The color (pH) of the sensitive elements could be monitored remotely for more than 6 years.

Other researchers have proposed alternative colorimetric pH-sensitive dyed matrices. For example, Carmona and coworkers have proposed a colorimetric approach for monitoring the acidity in historic monuments and prevent the stone degradation.¹¹⁹ In their work they propose a sensitive coating using Chlorophenol Red integrated into an inorganic silica matrix fabricated using the sol-gel technique. Figure 18.20 shows the chemical change in the pH colorimetric indicator, and the change in the absorption spectrum of the sensitive coating as pH is varied in a Kesternich chamber.

Alternatively use of fluorescence-based optical pH sensors is commonplace because fluorescent labeling is a widely used tool in biology and medicine to study tissues of even intracellular mechanisms. Consequently there is very wide knowledge of the fluorophores and the techniques for immobilizing them into substrates, and it is possible to find in the literature a lot of fluorescent-based optical fiber pH sensors^124,125 for applications such as ammonia sensing,^126,127 CO₂ monitoring,^128–131 or oxygen detection.^132,133 Nevertheless the implementation of fluorescent OFSs in civil engineering has fewer applications than do other scientific disciplines. The main drawbacks of fluorescence-based pH sensors in civil or industrial engineering environments are the higher degradation rates in fluorescence dyes compared to colorimetric ones in harsh environments and the high attenuation of the optical fiber at the typical excitation wavelengths that strongly limits the distance of the optode from the excitation light source.

Nevertheless, some authors have proposed alternative approaches for metal corrosion detection and characterization using the variation of fluorescence to detect the local changes in pH. Some researchers have used fluorescence microscopy to study and characterize the corrosion mechanisms in the surface or interfaces of metals.^134–136 Alternatively Walt and coworkers present an interesting fiber optic sensor tip that can easily get images from a surface.¹³⁷ In this case an optical fiber bundle is placed very close to the observed surface (see Figure 18.21). Each optical fiber collects the emitted light of a small portion of surface, acting as a single pixel of an image, so it is possible to reconstruct the fluorescence of the emitted surface with some spatial resolution, having an image. Using this approach they have been capable of visualizing corrosion sites and measuring local chemical concentrations in a real-time corrosion monitorization process. Figure 18.21 (bottom) shows the apparition of a corroded region denoted by a dark region where the pH locally decreases and this results in a fluorescence quenching.