5.1.2. Effective Parameters and Challenges of Microcapsule Embedment for Corrosion Inhibition

It is evident that application of self-healing coatings will be the most common and cost effective method of improving the corrosion protection. However, for the excellent fabrication of self-healing coatings several parameters must be considered such as inhibitor material, microcapsule diameter (size), microcapsule core and shell, microcapsules dispersion, presence of catalyst, coating application, coating thickness and coating matrix. Therefore, there is a growing need for investigation of effective parameters in microcapsule formation which is under intense study by various researchers. For example, Nesterova et al. [120] found that an increase in stirring rate, stirrer geometry, correct choice of temperature, and a high stabilizer concentration all can affect the microcapsule size. In capsules with irregular shape mechanical stability will be compromised and capsules will be unacceptable for a coating use. Another interesting and effective parameter which should be considered for obtaining self-healing property is the position of capsules in the coating matrix. Therefore, Kumar et al. [121,122] studied two methods of applying microcapsules in the primer layer: mixing to the primer before applying and sandwiching the microcapsules in the primer during application. Experimental results suggested that the microcapsules should be mixed into the paint formulations at the time of application.

In the other work by Cho et al. [113], two self-healing systems based on siloxane materials have been studied. First system consists of phase-separated polydimethylsiloxane (PDMS) healing agents and microencapsulated catalyst. The limitation of this system is the possible reaction between PDMS healing agent and coating matrix. In order to overcome this drawback, in the second system both catalyst and PDMS healing agent are encapsulated within urea-formaldehyde (UF) microcapsules. Their dual-capsule PDMS healing system showed no evidence of corrosion in the damaged area even after a long time exposure to the corrosive species. Although self-healing or autonomically healing micro cracks is a promising approach for extending the life of coating, still there are significant of unsolved challenges for optimization of the autonomous microcapsule system which is suitable for multiple healing actions.

## **6. Evaluation of Corrosion Inhibitors Using Advanced Characterization Techniques**

Electrochemical methods are most commonly used techniques for the evaluation of the efficiency of corrosion inhibitors. The advantages of electrochemical methods are short measurement time and mechanistic information that they provide which help not only in the design of corrosion protection strategies but also in the design of new inhibitors. Although several electrochemical techniques may be used to study the performance of corrosion inhibitors, potentiodynamic polarization method and electrochemical impedance spectroscopy (EIS) can provide significant useful information, which makes them the most useful method for such study and the number of reports used this method for study of corrosion inhibition performance is limited. As an example, Otmacic Curkovic et al. studied the mechanism of the protective action of three imidazole-based (4-methyl-1-(p-tolyl)-imidazole, 4-methyl-1-(o-tolyl)-imidazole, and 4-methyl-1-phenyl imidazole) corrosion inhibitors on copper in 3% NaCl, using quartz crystal microbalance measurements [123]. This study confirmed that even slight changes in the molecular structure induce a significant effect on the inhibiting properties. Both tolyl-substituted 4-methyl imidazoles rapidly adsorbed onto the copper surface and decreased the copper corrosion rate while the phenyl-substituted 4-methyl imidazole slowly formed a protective 3D layer. On the other hand, the inhibiting effect of o-tolyl-substituted compound did not improve with time, while the inhibiting efficiency of the phenyl-substituted inhibitor increased with immersion time. Figure 12 presents the mass and the corrosion potential changes with respect to immersion period measured by QCM-D in the presence of (a) 4-methyl-1-(o-tolyl)-imidazole and (b) 4-methyl-1-phenyl imidazole. The use of better inhibitor (phenyl based imidazole) shows the increase of the mass of the electrode which is due to the formation of protective layer on the copper surface. However, very little work has been performed on GO/IL systems.

**Figure 12.** Mass and potential change of copper electrode in 3% NaCl with the addition of (**a**) 4-methyl-1-(p-tolyl)-imidazole and (**b**) 4-methyl-1-phenyl imidazole (Reprinted with permission from [123]. Copyright 2009 Elsevier).

On the other hand, the measurement of electrochemical reactions at the interface becomes a matter of particular interest for the prediction of the service life of coating. To achieve such information, new techniques that perform local measurements, such as scanning kelvin probe SKP are increasingly applied [124,125]. This method is a non-destructive, non-contact mode technique based on a vibrating capacitor to measure the surface work function (WF) distribution on the coating surface. Surface work function is an extremely sensitive indicator of the surface condition and can be used to track changes in the surface such as surface contamination and corrosive adhesion of polymers on metal substrates [126].

Among many studies, Choudhury et al. [127] recently presented the sol-gel derived hybrid coatings containing three different compositions of methacrylate-phosphosilicate on mild steel substrates where SKP microscopy was used to evaluate the adhesion and corrosion protection properties. Following equation can be used to correlate the absolute WF measured using SKP (*VKP*) to the corrosion potential (*E*corr) [128]:

$$V\_{KP} = E\_{\text{corr}} + \text{const.}\tag{5}$$

Phosphorus containing methacrylate hybrids were synthesized from 2-(methacryloyloxy)ethyl phosphate (EGMP) and 3-[(methacryloyloxy)-propyl] trimethoxysilane (MEMO) via dual-cure process involving sol-gel reaction and addition polymerization. Similar experimental procedures were used to synthesize hybrids at other composition namely M:E–3:7 [129]. Figure 13 illustrates the SKP maps of the gold-coated aluminum (reference), bare metal substrate (MS), MEMO, EGMP, M:E–1:1 and M:E–3:7 coated samples. The average WF values of the samples shift positively towards noble potential in the order of bare MS < EGMP < MEMO < M:E–1:1 < M:E–3:7.

The deviation in WF values can be correlated to the interfacial interaction between the coating and the substrate. The SKP measurements showed the presence of strong interfacial interaction, which is attributed to the interaction of phosphate group with the metallic substrate [130].

An understanding of the correlation between the structure and observed corrosion inhibition properties, such as mechanism of adsorption, is essential for designing corrosion inhibitors with enhanced properties. Adsorption of an inhibitor on a metal surface depends on various parameters such as the nature and the surface charge of the metal, the inhibitor's chemical structure etc. One of the most common scattering techniques used for corrosion and corrosion inhibition study is surface-enhanced Raman scattering [131]. However, this technique studies the film that forms on the metal and is a surface analysis technique.

**Figure 13.** SKP maps of pristine gold on aluminum reference, bare MS, MEMO, M:E–1:1 and M:E–3:7 coated samples, respectively. (Reprinted with permission from [127]. Copyright 2010 Elsevier).

In the field of organic corrosion inhibitors, more attention is paid to the mechanism of adsorption and also to the relationship between inhibitor structures and their adsorption properties. Neutron scattering techniques including small angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) are valuable techniques to study the structure of corrosion inhibitors in different forms including gels and nanoparticles. SANS and USANS are ideal and can be used to provide information relating to the crosslink porosity of structure, which is directly related to the mechanical properties of inhibitors. A literature survey of neutron scattering demonstrates that this technique has been employed to study the microstructure of a range of different types of polymers, predominantly from synthetic polymers. Furthermore, polymers studied by this method are classified into polymer blends, block copolymers and polymer gels [132].

Shibayama et al. [133] studied the structure of tetra-arm polyethylene glycol (PEG) gels by SANS. It has been investigated that there is no inhomogeneities appeared even by swelling. However, a steep upturn was observed in SANS curves, indicating the presence of PEG chain clusters or defects

where these inhomogeneities disappear in swelled sample. Furthermore, Bhatia and co-workers designed a unique form of chemically cross-linked PEG gels to minimize defects in the network [134]. SANS was utilized to investigate the network structures of gels in two different solvents: D2O and d-DMF. SANS results show the resulting network structure is dependent on PEG length, transitioning from a more homogeneous network structure at high molecular weight PEG to a two phase structure at the lowest molecular weight PEG. It has been shown that with qualitative analysis and model fitting of SANS data, the highest molecular weight tetra-functional PEG hydrogels have a remarkably homogeneous network structure with low junction functionality. However, there are still some small indications of inhomogeneity for the lowest molecular weight networks even in d-DMF, suggesting a higher level of defect formation during cross-linking for these systems. Despite the extensive research on structural study of materials using SANS and USANS, to date there has not been any reports of using a combination of SANS and USANS to study the effect of corrosion inhibitors' structure on their efficiency. Recently, using combined USANS/SANS tools, Taghavikish et al. [74] investigated the hierarchical gel network structure and their relation to the observed bulk properties for polymeric ionic liquid nanoparticle emulsion based corrosion inhibitor in anticorrosion coatings.
