Sensing systems can be classifi ed as passive or active; passive systems being those where the sensing system induces a change in the coating that then requires an external sensor to detect, active systems being when the sensing system outputs a signal. This distinction is not rigid as there are some systems, such as fl uorescent molecules or quantum dots, that do output a signal but require detectors to record that signal. Nevertheless it is useful and so in this chapter responses such as colour change will be classifi ed as passive and fl orescent systems as active. In the section sensing for parameters that may be related to corrosion events or contribute to corrosive events (temperature, strain, etc.) are reviewed in addition to more direct corrosion sensing. Further, this section will discuss molecular scale sensing (addition of fl uoresceins, etc.) and nano-scale sensing (surface enhanced Raman spectroscopy (SERS)/quantum dots, etc.).
2.4.1 Passive molecular-scale sensing
Colorimetric sensing
Colour change may be used to detect corrosion [76] , temperature [77] or strain [78] . Zhang and Frankel [76] placed colour changing indicator (phenolphthalein) in clear epoxy paint on an aluminium substrate in order to detect the onset of corrosion. The cathodic reactions (oxygen reduction to form hydroxyl ions) caused a local pH change and thus a change in indicator colour. More recent work has focused on the use of fl uorescent molecules (see Fluorescence section on page 44). Seeboth et al. [77] induced thermochromic changes in a hydrogel. They placed 2,6 diphenyl-4(2,4,6 triphenylpyridinio) phenolate (DTTP) in a polyvinyl achohol–borax surfactant gel which promoted a colour change from colourless at 10 °C to deep purple at 80 °C. DTTP undergoes proton transfer between its phenol and phenolate form with the percentage of phenolate increasing with temperature and promoting the colour change. Since the pioneering work of Seeboth et al. [77] many researchers have developed diverse systems both to add colour change functionality to industrial materials (roofi ng, windows) and for sensing purposes, with the aim of producing cheap, stable and biodegradable systems. For example in a recent study Li et al. [79]
placed two non-ionic surfactants, triblock copolymer poly(ethylene oxide)- poly(propylene oxide)-poly (ethylene oxide) (EPE) and 4-octylphenol
polyethoxylene (TX-100) in the hydrogel Agarose. At low temperatures the surfactants stay in solution but at high temperatures they come out of solution, changing the colour and optical properties of the coating. Fudouzi et al. [78] developed a system where the strain in an aluminium plate could be detected by the colour change in a fi lm applied to the plate. A silicon elastomer fi lm was used with embedded sub-microm polystyrene collodial particles. When the plate and fi lm were placed under strain the separation of the polystyrene particles increased, changing the colour of the fi lm.
Other types of sensing
Nanto et al. [80] demonstrated how gaseous (toluene, xylene, diethyether, chloroform and acetone) absorption can be measured by coating a quartz resonator microbalance with copolymerized propylene-butyl. Park et al.
[81] constructed pH-sensitive graft co-polymers that change their phase and thus transmittance as a function of pH. They grafted basic monomers of varying p K a (pyridine, imidazole and tertiary amine groups) to co-polymers (based on 2-hydroxyethyl apartamide). At a pH < p K a the polymer is transparent, at a pH close to the p K a the co-polymer precipitates and the transmittance falls dramatically, while at pH signifi cantly greater than p K a the co-polymer dissolves again and the polymer is again transparent. Such systems could be used as part of an optically based pH sensor or for drug or active agent delivery.
2.4.2 Active molecular sensing
Fluorescence
There have been a number of recent reviews on fl uorescent materials including one by Chi et al. on mechanofl uorochromic materials [82] , Shirota [83] and Bhattacharya and Samanta [84] on photochomic materials and Hu and Liu on systems responsive to chemical signals [85] . In this chapter we will only summarise major trends and refer the reader to these reviews for more details. The principle that many of such materials rely on is the rearrangement of chromophores dispersed in the polymer matrix as a result or an applied stress. The chromophores will in general have a longer energy in the aggregated form than in the monomer form [86] . For example early work of Crenshaw and Weder [87] dispersed small volumes of oligo( p - phenylene vinylene) synthetic chromophores in a polymer matrix (linear low density polyethylene, LLDPE). As originally mixed the dye was aggregated and the LLDPE matrix was randomly orientated. Application of a mechanical force caused the macromolecular chains of the LLDPE to align, promoting the breakup of the aggregated dye into isolated monomers.
The aggregated chemophores (orange-red) have markedly different emission properties from the isolated monomer (green) and thus the colour of the composite changes. More recently Pucci et al. [88] applied the same principle to polypropylene containing the food grade dye bis(benzoxazolyl) stilbene (BBS). The same breakup of the aggragated dye to its monomer form occurred, changing the colour to blue. Sagara and Kato [89] reported on the use of liquid crystals for mechanically induced luminescence. They consider a pyrene derivative and an anthracene derivative, both having long alkaline chains attached to luminescent cores. As shown in Fig. 2.6 they can change their colour when the structure changes. As formed they have a metastable cubic form displaying a yellow emission; however, mechanical shearing promotes a transformation to a stable columnar form and causes the pyrene derivative to have blue-green emissions and the anthracene to have light blue emissions.
2.4.3 Corrosion-sensing coatings
Johnson and Agarwala [90] investigated the sensory capability of epoxy coatings containing fl uorescein coating aluminium surfaces. In particular they assessed whether fl uoescein would fl uoresce under UV light in response to temperature, pH and reduction and oxidation conditions. Fluorescence was observed when exposed to corrosive conditions; however, under certain pH conditions fl uorescence was permanent.
Zhang and Frankel [76] used fl uorescing pH indicators 7-hydroxycoumarin and coumarin to detect pH changes in a clear acrylic paint over an aluminium alloy substrate in NaCl solutions. In particular they determined the critical pit size that can be detected by the fl uorescing indicator and found that it
2.6 The change of luminescent colour and molecular assemblies for the pyrene derivative 1a (a) and the anthracene derivative 1b (b).
These emission images were taken under UV irradiation (365 nm), and both compounds are sandwiched between quartz substrates.
Reproduced with permission from [89] . Copyright 2009 Nature Publishing Group.
Mechanical shearing Mechanical
shearing
(a)
(b)
was 2 μm for 7–hydroxycoumarin. Liu and Wheat [91] used 7-amino-4- methylcoumarin (Coumarin 120) as a fl uorescence indicator. The advantage of Coumarin 120 was that its high fl uorescence intensity allows it to be used in ‘standard’ non-clear paints. Liu and Wheat used epoxy-polyamide as primer and top coat. Coumarin 120 fl uorescence decreases at high or low pH, enabling Li and Wheat to demonstrate that loss of fl uorescence could be correlated with pit initiation on coated Al.2024.
Augustyniak et al. [8] investigated the application of spiro[1 H -isoindole- 1,9 ′ -[9 H ]xanthen]-3(2 H )-one, 3 ′ ,6 ′ -bis(diethylamino)-2-[(1-methylethylidene) amino] (‘FD1’) in epoxy coatings over steel. FD1 fl uoresces on forming a complexes with ferric ions. Augustyniak et al. demonstrated that FDI (at 0.5 wt%) fl uoresced when the epoxy impregnated coating was placed in a 0.002 M FeCl 3 solution for 24 hours. They associated this fl uorescence with the FD1 chelating with Fe 3 + (produced by substrate corrosion) in solution and producing a fl uorescent complex.
2.4.4 Nano-scale sensors and sensor networks
The placement of nano-scale sensors in fi lms offers the possibility of accurate real-time measurement and if combined with a sensor network it can provide information on the exact point of activity as well as a knowledge of the spread of the activity.
Nano-scale sensors
SERS is often cited as a very prospective sensing system as very small amounts of material can be detected. In this context it has been demonstrated that Ag nanoparticles (5 nm) can be deposited onto the surface of silica spheres [92] which can, in principle, be used as a sensor. The presence of Ag nanoparticles provide a SERS response for the interaction of inhibitors with corrodents even when only a small quantity of materials absorbed onto the surface [93] . This mechanism provides the possibility of using SERS to probe reactions within the coating, such as the release of inhibitors and/or indicators.
Quantum dots are fl uorescent semiconductor nanoparticles. Trinchi et al.
[94] examined the use of CdSe/ZnS quantum dots as a sensor to signal how much chromate inhibitor was left in a coating after a specifi ed period of exposure. CdSe/ZnS quantum dots degrade in low pH or corrosive environments. Trinchi et al. were able to design the quantum dots so that when the quantum dot were in a paint primer they decrease their fl uorescence at a rate that was proportional to the rate of chromate inhibitor leached out of the primer. Thus they were able to use the fl uorescence signal at 647 nm to determine the amount of chromate in the chromate inhibited
primer. In Fig. 2.7 the fl uorescent signal given off from a chromate and quantum dot containing primer when exposed to cyclic humidity and salt accelerated test (GM9540) is presented. It is apparent that the fl uorescence decreased from day 1 and was extinguished by day 25. Parallel measurements of the chromate level within the primer should a similar depletion of chromate with exposure time.
The separation of the anode and the cathode during a corrosion process provide a voltage, which, in principle, can be used to drive a mechanical device. In the case of aluminium the voltage is around 1.7 volts [95] . This approach was discussed in a little detail by Kendig and Mansfeld [95] for aluminium corrosion applications. While the incorporation of nanoparticles into coatings may present unique opportunities for sensing or improving the mechanical properties of coatings, their high surface energy means that they are reactive, often leading to increased health issues but also unknown infl uences on the long-term viability of products [96] .
Distributed sensors and transduction
Distributed sensor systems for a range of sensing applications have been envisioned for many years. These types of system are of most value for applications where the damage is likely to result in a signifi cant reduction in the structural integrity but there is little chance of visual inspection or where access is limited or impossible for inspection using external instrumentation. These conditions generally apply for aerospace
2.7 Decay of CdSe/ZnS quantum dot fl uorescence from within a primed aluminium sample in response to exposure in a salt spray chamber (from 1 to 25 days).
60 50 40 30 Intensity (a.u.) 20
Average 0 day Average 1 day Average 10 days Average 25 days
Wavelenght (nm) 10
0
500 550 600 650 700 750
applications particularly in the military where airframes may experienc loads close to the designed specifi cations. Corrosion and mechanical damage are the normal targets for these applications.
A range of corrosion sensing systems has been proposed. This includes direct sensing of pH changes, corrosive agents such as chloride, or corrosion products such as Al or Cu ions. Inferential sensing involves sensors that react to the local environment and the level of corrosion is inferred from the sensor output. In this latter case the sensor may be made of similar material to the structure that is to be protected and located in corrosion prone areas. Typical sensors for this type of application include a range of materials that rely on attack of the sensor via for example a galvanic couple.
There is also a strong link to self-healing systems with distributed sensors since sensing damage is the fi rst stage in the process of repair or healing.
The healing responses can be built on the identifi cation of the location and degree of damage.