Shengxi Li Hawaii Corrosion Laboratory, Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii, USA
Raman spectroscopy and Fourier transform infrared spectroscopy are two useful spectroscopic techniques for corrosion studies especially because they can provide real-time information on various corrosion systems. This chapter discusses the applications of these two techniques in the studies of aqueous corrosion, atmospheric corrosion, corrosion inhibitors, and corrosion protection coatings. Available techniques and instrumentation for each application will be provided as a guideline for related investigations. Various examples from literature are discussed to show the power of these two techniques in corrosion studies. Specifically, these two techniques found a wide application in corrosion protection coating systems, including monitoring the growth of coatings, studying their degradation in corrosive environments, and characterization of the corrosion products.
The author is grateful for the support from College of Engineering, University of Hawaii at Manoa. The author is particularly grateful to Prof. Lloyd Hihara, director of the Hawaii Corrosion Laboratory at the Department of Mechanical Engineering, University of Hawaii at Manoa.
Spectroscopic techniques such as Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR) have been widely used in corrosion research. Both in situ and ex situ experiments using these two vibrational techniques provide valuable information about the corrosion system. Extensive literature search resulted in a considerable number of papers published on corrosion studies using Raman and IR spectroscopy. However, only in situ applications of the two techniques in corrosion studies will be covered here due to their ability to acquire real-time information of the corrosion systems that allows monitoring the corrosion process.
In the following sections, the underlying theory of the two vibrational techniques will be first given. Then, the available techniques and instrumentation for in situ studies will be discussed. Finally, various examples of applications are presented to demonstrate the power of these two techniques for corrosion science and related fields of research. The discussion of literature papers will be divided into four areas: general aqueous corrosion, atmospheric corrosion, corrosion inhibitors, and coatings.
When a monochromatic light is scattered from a molecule, most photons are elastically scattered with the same frequency (energy) as the incident photons. A small fraction of light is scattered at frequencies different from those of the incident photons, namely inelastic scattering. The elastic scattering is named Rayleigh scattering while the inelastic scattering is called Raman scattering, or Raman effect. The Raman effect was discovered in 1928 by V. C. Raman.
The Raman effect arises when incident photons interact with the electric dipole of a molecule. The photons excite the molecule from its ground state to a virtual energy state. The molecule emits a photon when it relaxes and returns to a different energy state. The energy difference between the two states results in the energy of the incident photons being shifted away from the excitation wavelength. If the final energy state of the molecule is higher than the initial state, then the emitted photon will be shifted to a lower frequency, which is designated as Stokes shift. If the final energy state is lower than the initial state, then the emitted photon will be shifted to a higher frequency, and this is designated as an anti-Stokes shift (Figure 20.1). The Stokes shift is what is usually observed in Raman spectroscopy.
The Raman effect is based on molecular deformations in the electric field (E) generated by the laser beam. Assuming that the laser is fluctuating at frequency ν, E can be written as a function of the amplitude, E0, and the time, t:
The dipole moment (μ) of a molecule induced by this light is, therefore:
Here, the proportionality constant α is called the polarizability of the molecule. The polarizability represents the ease with which the electron cloud around a molecule can be deformed.
Similarly, if the molecule is also vibrating or rotating in sinusoidal fashion with frequency νm, the nuclear displacement Δr can be written as:
where r0 is the vibrational amplitude.
When a molecule is at its equilibrium nuclear geometry, the polarizability of the molecule is α0. While at some distance, the instantaneous polarization α is a linear function of the nuclear displacement, Δr:
where the derivative (∂a/∂r) represents the change in polarizability with change in position.
Combining Equations (20.2)–(20.4), we have:
The first term relates to the scattered photon that has the same frequency as the incident photon, and thus explains Rayleigh scattering. The second term relates to the scattered photons with increased frequency (ν − νm) and decreased frequency (ν + νm) as compared to the incident photon, representing Stokes shift and anti-Stokes shift, respectively.
Equation (20.5) also gives a general selection rule that governs Raman scattering. Notice the derivative (∂a/∂r) in the second term. If that derivative equals zero, the entire second term will be zero, and there is no Raman scattering. Therefore, the selection rule for Raman scattering can be given as:
that is, a molecular motion will be Raman-active only if the motion occurs with a change in polarizability. The amount of the polarizability change determines the Raman scattering intensity.
Infrared radiation encompasses a section of the electromagnetic spectrum with wavenumbers () from 12,800 to 10 cm− 1 or wavelengths (λ) from 0.78 to 1000 μm. This infrared portion of the electromagnetic spectrum is usually divided into three subregions: near-IR region (12,800-4000 cm− 1), mid-IR region (4000-200 cm− 1), and far-IR region (200-10 cm− 1). The mid-IR region is the most commonly used region in IR spectroscopy.
Infrared spectroscopy is based on the interaction between IR radiation and molecules that results in the absorption of IR radiation by the molecules. At temperatures above absolute zero, all the atoms in molecules are in continuous vibration with respect to each other. When the frequency of a specific vibration equals the frequency of the incident IR radiation, the molecules absorb the radiation. The absorbed radiation matches the transition energy of the bond or group that vibrates in the molecule.
A molecule can vibrate in many ways, with each way called a vibrational mode. Simple diatomic molecules have only one bond and thus one vibrational mode that is not observed if the molecule is symmetrical. Polyatomic molecules have more bonds and correspondingly more vibrational modes. The total number of fundamental vibrational modes can be determined as follows. A polyatomic molecule with n atoms has 3n total degrees of freedom because each atom has three degrees of freedom. For a nonlinear molecule, 3 degrees of freedom are rotational and 3 are translational. Therefore, there are 3n − 6 degrees of freedom that are fundamental vibrations for nonlinear molecules. For linear molecules, because only 2 degrees are rotational, the total number of fundamental vibrations is 3n − 5. However, not all the vibrations can be observed in IR spectra. For a mode to be observed, changes must occur in the permanent dipole:
where μ is the dipole moment and r is the normal coordinate. In general, the larger the dipole change, the stronger the intensity (I) of the band in an IR spectrum:
A comparison of Raman and IR spectroscopy is shown in Table 20.1. The major difference between the two techniques lies in that Raman spectroscopy determines vibrations that cause a change in the polarizability of a molecule, while IR spectroscopy determines vibrations that cause a change in the dipole moment of a molecule. However, in theory, there are some vibrations that induce changes in both polarizability and dipole moment.
Table 20.1
Comparison of Raman and IR Spectroscopy
Raman | IR |
Originate from scattering of radiation | Originate from absorption of radiation |
Require change in molecular polarizability | Require change in dipole moment |
Water compatible | Water leads to intense absorption |
Record by using a beam of monochromatic radiation | Record by using a beam of radiation having a large number of frequencies |
Optics can be made of quartz or glass | Optics are usually made of salts (e.g., NaCl, KBr, and CsI) |
Homonuclear diatomic molecules are Raman active | Homonuclear diatomic molecules are IR inactive |
There are many variations of Raman spectroscopy that have been developed to enhance the sensitivity (e.g., surface-enhanced Raman), to improve the spatial resolution (Raman microscopy), or to acquire very specific information (resonance Raman). For in situ studies in corrosion science, normal Raman spectroscopy is sufficient in most cases, but sometimes surface enhanced Raman spectroscopy (SERS) is necessary. Therefore, other techniques (e.g., resonance Raman) are not further covered here.
In situ Raman spectroscopic studies of a corrosion system generally do not require specially designed equipment. Cells with or without flow configurations can be made using any material. When an enclosed cell is necessary, a quartz or even glass window can be used for passing the laser beam to the sample.
Surface-enhanced Raman spectroscopy is a technique that offers orders of magnitude increases in Raman scattering by molecules adsorbed on rough metal surfaces or nanostructures. Typical metals used for Raman scattering enhancement are Ag, Au, and Cu. To prepare the roughed surfaces, a conventional approach is to electrochemically rough the substrate through etching. Recently, surfaces are usually prepared by coating metallic nanoparticles on various substrates. There are two primary theories proposed for SERS: the electromagnetic theory and the chemical theory. The electromagnetic theory is based on the enhancement in the electric field provided by localized surface plasmons. The chemical theory proposes the formation of charge-transfer complexes and thus making it only applicable to chemisorbed species on metal surfaces.
There are two primary experimental techniques in IR spectroscopy, transmission and reflection techniques. For in situ studies of corrosion phenomenon, reflectance spectroscopy is the commonly used technique. Among the various types of reflectance techniques, specular reflection, infrared reflection-absorption spectroscopy (IRAS), and attenuated total reflection (ATR) are well suitable for in situ studies.
Because of the intense absorption of IR light by solvent (water), IR spectroscopy cannot be used as readily as Raman spectroscopy for in situ monitoring corrosion phenomenon in aqueous environments. Therefore, specific cells have to be designed with the aim of minimizing radiation losses by solvent absorption. One approach is by depositing very thin layers of electrolyte or small droplets on metal surfaces, which represents the common configuration for atmospheric corrosion studies. Figure 20.2 shows two typical experimental setups for in situ studies of atmospheric corrosion, with the one in Figure 20.2a for specular reflectance study and the one in Figure 20.2b for IRAS. For the IRAS studies, grazing angle incidence is usually used.
For studies in bulk solutions, IR reflectance spectra can still be obtained by reducing the distance between the sample and the IR window (Figure 20.2c). The IR transparent window is pressed against the sample, creating an extremely thin solution layer in between them. One problem with this configuration is the possible depletion of solution species due to electrochemical activities (e.g., corrosion) in the confined environment. To solve this problem, a flow cell can be designed to replenish solution species as reaction proceeds.
For all three configurations in Figure 20.2, the IR windows are in direct contact with liquid electrolyte or high-humid environment. Therefore, non-water-soluble IR window materials have to be used, such as zinc selenide (ZnSe) and calcium fluoride (CaF2).
When an IR radiation enters from optically denser ATR crystals (refractive index n1) to the optically rarer samples (refractive index n2) at an angle exceeding the critical angle (θc), total internal reflection will occur inside the crystal. The critical angle is defined as:
This internal reflectance creates an evanescent wave that extends beyond the surface of the crystal into the sample that is in contact with the crystal. The evanescent wave protrudes only a few microns (0.5-5 μm) beyond the crystal surface and into the sample.
Some common ATR crystals are ZnSe, thallium bromide-thallium iodide (KRS-5), germanium (Ge), or silicon (Si). Prisms (Figure 20.3a) and hemispheres (Figure 20.3b) made of ATR crystals, for example, ZnSe, can be used for ATR-FTIR studies. In these configurations, IR beam is reflected once inside the crystal. Multiple internal reflection can be achieved by using the configuration shown in Figure 20.3c. In all three configurations, samples have to be in close contact with the crystals or can be plated on the crystals as thin films.
In situ Raman spectroscopy has several major advantages for corrosion studies especially in aqueous environments. First, water does not interfere with Raman acquisition in aqueous environments. Second, the laser beam does not perturb the corrosion system under the protection of water, that is, without altering the corrosion species. This enables the study of those unstable compounds such as green rust, which would have transformed to other phases in the ambient. Third, the high surface roughness caused by corrosion does not hinder Raman acquisition, but instead enhances Raman scattering.
The corrosion of metallic materials in aqueous environments is one of the most prevalent forms of corrosion and has been extensively studied over the past few decades. In situ Raman spectroscopy is well suited for monitoring the corrosion process in metal/liquid interface. A great number of in situ Raman spectroscopic studies have been conducted to characterize the anodic corrosion films or monitor the formation of corrosion products on metal surfaces in corrosive environments.
The passive oxide films on metals enable the self-protection of metals against corrosion in aqueous environments. A series of work has focused on in situ Raman spectroscopic identification of oxide films grown on various metal surfaces, including Fe, Ni, Co, Ag, Ti, Pb, and stainless steel. Most of the oxide films studied were either formed by anodic polarization1–15 or by exposing the metals to hydrothermal environments.16–20
At anodic potentials, oxide films grow on metal surfaces; while at cathodic potentials, the oxide films dissolve. During this process, the formation, transformation (if any), and dissolution of the oxide films could be followed using in situ Raman spectroscopy. For example, according to in situ Raman data, the oxide film on Ni was found to be Ni2O3 (477 and 555 cm− 1) in 0.05 M NaOH solution2 and NiO (500 and 555 cm− 1) in concentrated (1-10 M) H2SO4 solution.3 On Cu in 0.1 M NaOH, Cu2O was detected for potentials from 50 to about 500 mV during anodic sweeps, but was converted to Cu(OH)2 at more anodic potentials (> 637 mV), as evidenced by the appearance of the band at ~ 488 cm− 1.6,7 The oxide film on Ti in acidic sulfuric solution was found to be anatase-type TiO2 (145, 400, 515, and 640 cm− 1), which changed from amorphous to crystalline as the potential exceeded a particular value.8 The protective oxide films also grow on Fe polarized in alkaline solutions and were determined to consist mainly of magnetite (Fe3O4) (550 and 670 cm− 1) in the inner layer.11–13,15 An outer layer consisting of goethite (α-FeOOH) or lepidocrocite (γ-FeOOH) was also detected. The in situ capabilities of Raman spectroscopy were also extended to monitoring the surface films on iron and steel in high-temperature environments,16–19 and various phases of iron oxides and oxyhydroxides were detected depending on the temperature.
Similar to monitoring the oxide film formation using in situ Raman spectroscopy, the general aqueous corrosion has been widely studied using this approach in order to identify the corrosion products formed during the corrosion process. Most of the work has been focused on iron and steel (e.g., galvanized steel, carbon steel, and stainless steel). Earlier work has characterized the corrosion products formed on galvanized steel during exposure to corrosive solutions.21,22 Zinc hydroxycarbonates and hydroxychlorides were successfully identified as the two primary corrosion products. For the corrosion of iron and steel in corrosive media, especially chlorinated solutions, nonstable green rust has been detected as the corrosion products generated during pitting.23–30 Two major Raman bands at ~ 433 and 507 cm− 1 for the green rust were assigned to the Fe2 +OH and Fe3 +OH stretching modes, respectively. Depending on the corrosive media, the green rust can have different ions integrated in it, such as Cl−, CO32 −, SO42 −, and HCOO−. Other types of rust, such as Fe3O4 and γ-FeOOH, were also detected together with green rust. Siderite (FeCO3) is another corrosion product that forms on steel in concentrated carbonate/bicarbonate solutions, simulating groundwater.31,32
Other corrosion phenomena of metals that have been investigated using in situ Raman spectroscopy include molybdenum33 and tin34 in NaCl and KOH solutions, zirconium-niobium alloy35 and Ni-based alloy 60036 under hydrothermal conditions, and Pt-Ni alloys in HCl solution.37
Atmospheric corrosion proceeds under thin electrolyte layers or small droplets that form by condensation or absorption of water by airborne sea salt particles. The small amount of electrolyte facilitates the use of Raman spectroscopy to monitor the corrosion process in situ. Corrosion of zinc caused by predeposited NaCl particles with a density of ~ 0.4 μg/cm2 serves as an example.38 Zinc chloride (ZnCl2) with a Raman band at 290 cm− 1 was detected on the Zn surface 2 h after the sample was exposed to high humidity (84%). After 6 h exposure, when ZnCl2 in the surface layer reached saturation, simonkollite (ZnCl2(Zn[OH]2)4) with major Raman bands at 255 and 390 cm− 1 was detected. The corrosion process under a single droplet was also studied for Zn39 and carbon steel.40–42 The Raman spectra from Zn under a seawater droplet showed the formation of ZnO, a sulfate-containing and a carbonate-containing compound.39 The Raman spectra from carbon steel under single NaCl or Na2SO4 droplets indicated green rust and γ-FeOOH as the initial rust formation during sea salt droplets-induced marine atmospheric corrosion.40–42 The spatial distribution of the corrosion reaction products was also revealed by in situ Raman spectroscopy, as shown in Figure 20.4.41 Ferrous chloride (FeCl2) and also possibly its chloride ions (245 and 286 cm− 1) were detected inside the corrosion initiation site or the corrosion pit, as the corrosion reaction products (Figure 20.4a). Green rust (GR1(Cl−)) (425 and 501 cm− 1) was identified as the corrosion product formed in the vicinity immediately outside of the pit, while γ-FeOOH (246, 377, 525, and 645 cm− 1) was detected in regions outside of the GR region (Figure 20.4b and d). In the transitional region from GR to γ-FeOOH, Fe3O4 (666 cm− 1) also formed together with GR and γ-FeOOH (Figure 20.4c).
Corrosion inhibitors are chemical compounds that are added to the surroundings (liquids or gases) of metallic materials to decrease their corrosion. Inhibitors work by the formation of a coating on metals, which prevents access of corrosive substance to the metal surfaces. The inhibitor molecules either attach to the metal or react with the surface to form a thin, adherent layer. In both cases, in situ Raman spectroscopy has been employed to study the adsorption mechanism and the film formation mechanism.
Benzotriazole (BTAH) has been used as an effective corrosion inhibitor for Cu and its alloys since the 1950s,43 and was also extended to other metals. The high corrosion inhibition efficiency of BTAH is attributed to the formation of a compact, polymerlike metal-BTAH complex on the metal surface.44–46 The structure and composition of this complex can be revealed by in situ SERS, which provides information on even only a few monolayers of adsorbed species. Most of the in situ SERS studies on BTAH as corrosion inhibitors have been focused on Cu and Ag due to their strong Raman-enhancing effect. The inhibition effect of BTAH on Cu has been studied in various environments, such as halide solutions,44,46–48 sulfuric acid media,49–52 organic solution,53 and ionic liquids.54
Earlier studies showed that, in near neutral KCl solution, the complex formed by the reaction between Cu and BTAH was [Cu(I)BTA] according to in situ Raman data. In KCl/acid solution, the Raman spectra obtained at less negative potentials resembled that of [Cu(I)BTA] complex but, at more positive potentials the spectra were similar to that of [Cu(I)-ClBTAH]4.44 A recent study reported that, in neutral KCl solution, the initially formed complex was [CuI(BTA)]n and might have transformed to [CuI(BTAH)]4 at more negative potentials. This was evidenced by the redshifts of the in-plane trigonal breathing mode from 1041 cm− 1 at − 0.5 V to 1021 cm − 1 at − 1.1 V and the increase in intensity of the NH in-plane bending mode (1144 cm− 1) at more negative potentials (Figure 20.5).46 Therefore, it is probably necessary to take into account the effect of the presence of the [CuI(BTAH)]4 or [Cu(I)-ClBTAH]4 complexes in the corrosion inhibition of Cu in mild acidic KCl solution containing BTAH. In relatively strong acidic environment (e.g., H2SO4), [Cu(I)BTA]n complex was found as the film formed on the surface of Cu and its alloys.
Similar in situ SERS results were observed on Ag in either halide media or acetonitrile solution with BTAH.45,55 In addition, in situ SERS studies were also extended to Fe, Ni, and Co by proper electrochemical roughing procedures.46,56
Other inhibitors that have been studied using in situ SERS include triethylstibine for Fe and Ni,57 propargyl alcohol for Fe,58 2-mercaptobenzothiazole (MBT) for Cu,59 phytic acid (IP6) for Ag,60 volatile corrosion inhibitors for carbon steel,61 salicylate for Cu,62 and benzyldimethylphenylammonium chloride (BDMPAC) for low carbon steel,63 methimazole (MMI) for Cu,64 2-amino-5-mercapto-1,3,4-thiadiazole (AMT) for Ag65 and Co,66 3-amino-5-mercapto-1,2,4-triazole (AMTA) for Fe,67 and 2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole (4-APTD) for Cu.68
Coatings including both organic and inorganic coatings are an effective approach to achieve corrosion protections for metals. In situ Raman spectroscopy is a powerful tool for the study of corrosion-protection coatings. First, the formation of coatings on metallic materials could be monitored using in situ Raman spectroscopy. Second, the changes and degradation of coatings especially organic coatings upon exposure to corrosive environments can be studied using in situ Raman spectroscopy. Last, the corrosion products underneath the coatings can be characterized through in situ Raman spectroscopy provided that the coatings are thin and transparent.
Conversion coatings refer to the coatings produced by a chemical or electrochemical treatment of the metallic surface using coating solutions. During the conversion process, part of the metal surface is converted into a protective surface layer. Examples of conversion coatings include chromate conversion coating (CCC), phosphate conversion coating (PCC), and black oxide coating.
In situ Raman spectroscopy was successfully used to study CCC on aluminium alloys. Given that the Raman scattering from water is quite week, Raman spectroscopy could be used to monitor the formation process of CCC in aqueous solution.69 A band at approximately 858 cm− 1 appeared following the addition of Alodine solution (or K2CrO4, K2Cr2O7) to fresh Cr(OH)3 precipitate, indicating the formation of CCC. By monitoring the change of the band intensity at 858 cm− 1, the growth rates of the CCC as well as their dependence on coating bath composition were studied.70 It was found that the redox mediation action by Fe(CN)6− 3/− 4 is the mechanism for the acceleration of CCC formation.
In situ Raman spectroscopy was also used to monitor the release of chromate (CrO42 −) from CCC and its migration to neighboring regions of exposed alloy to protect them.71–73 The chromate species redeposited as a film on fresh alloy surface, making the initially untreated alloy less active toward corrosion. The redeposition process occurred more rapidly in or near pits, showing the self-healing properties of CCC films.74 Besides pitting corrosion, filiform corrosion was also effectively prevented by CCC films.75
PCC was also studied using in situ Raman spectroscopy. The dissolution of phosphate layers on zinc-coated steel in dilute NaOH solution was monitored by measuring the decay of the 996 cm− 1 (PO43 −) band intensity, from which the rate of phosphate loss could be obtained.76
Raman spectroscopy is especially suitable for studying polymers thanks to its high sensitivity towards the structure and conformation of the polymer backbone. Therefore, in situ Raman spectroscopy could be used to follow the growth of polymeric coatings on metals and their degradation in corrosive environments. In addition, this technique enables monitoring the corrosion product formation under the coatings provided that the coatings are very thin and transparent. The thin coating also reduces the time frame for corrosion to occur to match with that of Raman acquisition. Another requirement is the absence of fluorescence caused by the epoxy coating. Finally, it is better if the Raman bands of the coating do not interfere with those from the corrosion products.77
The earliest applications of in situ Raman spectroscopy in the study of polymer coatings were discussed in a series of papers on the corrosion of painted galvanized steel.77–80 After being immersed in NaCl solutions for a certain time, the epoxy-polyamide (15-20 μm) coated galvanized steel developed two clearly different regions, with and without blisters. Two types of blisters were observed, black and large blisters and white and small blisters.77 Raman spectra of the corrosion products inside the blisters could be obtained directly through the thin paint layer. The Raman spectra from the nonblistered regions corresponded to epoxy-polyamide resin coating with characteristic bands at 570 and 637 cm− 1. The Raman spectra from inside the black blisters showed the presence of zinc hydroxychloride with sharp bands at 260 and 390 cm− 1, while those from the white blisters indicated the existence of amorphous zinc oxide with a prominent band at 569 cm− 1.78–80 It was believed that the black blisters correspond to the cathodic zones and the white blisters are anodic. This study was also extended to the case of zinc-coated steel samples with chromate or phosphate conversion layers.79
Recently, there has been an increasing interest in the use of electronically conducting polymers (ECPs) for corrosion resistant coatings. Some of the ECPs that have been investigated for corrosion protection include polyaniline (PANI), polypyrrole (PPy), and polythiophene (PT).
The use of PANI for steel corrosion protection was first reported in the early 1980s.81,82 Subsequently, a substantial amount of research has been devoted to the use of PANI for corrosion protection of various metals and alloys. The conventional approach to obtain the protective PANI layers was by electrochemical polymerization. In situ Raman spectroscopy has been extensively used to study the formation mechanisms of the PANI films,83,84 normally on platinum electrodes. Even though not directly related to corrosion study, these studies laid the foundation for studying the protective PANI coating on metals using in situ Raman spectroscopy.
In situ Raman spectroscopy combined with cyclic voltammetry was employed to investigate the interaction between the passive layer on iron and PANI coating in sulfate medium.85 The oxidation state of the PANI film on iron, which changed after voltammetric cycling, could be followed. After one redox cycle (0 to − 1.0 V), the PANI film deprotonation was verified by the depletion of the band at 1330 cm− 1 (CN+ stretching). Also, the depletion of benzenoid units was evident due to the decrease of the 1620 cm− 1 band (CC ring stretching) and the depletion of 1185 cm− 1 band (quinine ring CH bending).85 After further cycling, the Raman spectra evolved toward a less oxidized form of PANI with a predominance of benzenoid rings (1185 and 1620 cm− 1). The reduced PANI on iron is no longer able to provide enough electrons to maintain iron in its passive state, and the passive layers break down. They also analyzed the PANI layer potentiostatically polymerized in phosphoric/metanilic solution as compared to the PANI layer grown in inorganic acid (HCl) by collecting Raman spectra at different polarization potentials (e.g., from 150 to − 700 mV/SSE).86 They found that the film prepared in metanilic acid had a more oxidized and less protonated state than that grown in inorganic acid solution, as evidenced by the strong Raman bands at 1330 cm− 1 (CN+·) and 1500 cm− 1 (C = N+·) observed from the PANI film formed in metanilic acid. Therefore, the reduction of the PANI film is more difficult in the presence of metanilic acid, resulting in higher protective properties.
Another strategy for protecting metals using PANI is by forming epoxy and acrylic blends. The acrylic blend formed by PANI, poly(methyl methacrylate) (PMMA), and camphorsulfonic acid (CSA) is an example,87–90 which had a low percolation threshold, a high conductivity, and enhanced mechanical properties as compared to PANI. Raman spectra of the PANI-PMMA-CSA blend indicated characteristic features of “secondary doped” PANI,91 with a significant increase of the band at 1333 cm− 1 corresponding to the C = N+· mode. The ratio of the band at 1333 cm− 1 and that at 1599 cm− 1 corresponding to the aromatic CC mode is equal to ~ 1, indicating that the film is in the emeraldine state. However, it diminished to ~ 0.7 after the coating (on iron) was immersed in H2SO4 solution for 20 days, suggesting the reduction of the polymeric coating.87 This ratio varies for different metal substrates, which indicates that the extent of the redox reaction in the film depends on the metals that have different reducing power (Figure 20.6).88 Raman data also showed the presence of a second layer formed by counter ions released from PANI and various metal cations.
Another ECP that has been widely used for corrosion protection coatings is PPy. The formation or the electropolymerization of the PPy films on various metals could be followed by using in situ Raman spectroscopy.92–96 Raman bands characteristic of the oxidized form of PPy at 927, 1086, 1238, 1368, and 1605 cm− 1 were observed during the growth of PPy film, and those typical of the reduced form of PPy at 988, 1038, 1260, 1313, and 1564 cm− 1 were detected during corrosion in corrosive medium.94
Take the thin PPy film on iron electrode and immersed in a 0.1 M K2SO4 solution (pH = 4) as an example.97 Raman spectra were recorded during immersion and at different OCPs to monitor the redox states of the PPy and composite films. The Raman data revealed that the PPy film was in the oxidized form in the original state (Eoc = 0 V) and was protective to the iron substrate. Then, the OCP dropped to − 0.4 V during protection and finally to − 1.0 V after corrosion. During this process, the Raman bands at 1606 cm− 1 (inter-ring C = C) and 1385 cm− 1 (ring C = C) corresponding to the oxidized PPy both shifted towards lower wavenumbers, ~ 1570 and 1315 cm− 1, respectively, for the reduced form of PPy (Figure 20.7a). The bands for the defect modes at 929, 1410, and 1240 cm− 1 also decreased. The Raman bands which increased significantly during the OCP measurement are at 985 and 1050 cm− 1, which correspond to the reduced form of PPy. As compared to the PPy film alone, the PPy-PDAN composite film showed better ability to protect iron because the Raman bands that are typical of the oxidized form of PPy (929, 1240, 1410, and 1610 cm− 1) decreased more slowly upon reduction (Figure 20.7b).97
FTIR has been widely used for corrosion studies, both in situ and ex situ. An extensive treatment of both in situ and ex situ application of FTIR in corrosion studies is found in Ref. [98] In this chapter, only in situ application of FTIR for corrosion studies will be covered.
It is difficult to probe corrosion phenomenon in bulk solutions using in situ FTIR due to intense absorption of IR radiation by water. However, by reducing the distance between the sample surface and the IR window, the corrosion process on the metal surface could be followed. The electrochemically controlled oxidation and reduction of lead in sulfuric acid was successfully monitored using in situ IRAS.99 The growth and reduction of lead sulfate (PbSO4) was followed by monitoring the band at 631 cm− 1, the integrated area of which showed a linear relationship with the consumed charge in the potential region where PbSO4 forms. At more anodic potentials, the formation of lead dioxide (PbO2) was observed as evidenced by an increased absorption band at 5200 cm− 1. Similar experiments were conducted to characterize the corrosion products formed on Ni electrodes in aqueous solutions containing pseudohalide ion series (OCN−, SCN−, and SeCN−).100 Ni corroded in all three solutions with the formation of Ni(II) cyanate complex, Ni(II) thiocyanate complex, and Ni(II) selenocyanate complex depending on the ion species in the solution. In the Ni/OCN− system, the in situ data obtained using subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) also showed the formation of CO2 (2343 cm− 1), which was believed to be the ultimate product of electrolysis of the cyanate ion.
Other in situ FTIR studies of corrosion-related phenomena include anion adsorption on colloidal chromium (III) oxide hydroxide film simulating passive stainless steel surface,101 characterization of the anodic film on iron in neutral phosphate solution (FeIIIPO4),102 and monitoring of the transport of corrosive species through the mortar layer.103
Given that atmospheric corrosion occurs under extremely thin electrolyte layers, there is less absorption of IR radiation by water. Therefore, a great amount of work has been conducted to study atmospheric corrosion using in situ IRAS techniques. In situ IRAS has been developed to study the corrosion of metals (e.g., Cu, Zn, Al, and Mg) exposed to humid air104 or humid air mixed with corrosive gases, such as SO2,104–110 NO2,107,109–111 O3,109 and SO3.112 The corrosive environments were generated by passing air with humidity and certain levels of corrosive gases through the sample chamber. The interaction of metals with corrosive environments and the formation of corrosion products could be followed using in situ IRAS. For example, the initial corrosion products on Cu exposed to humid air (90% RH) were identified as copper(I) oxide while that formed in humid air containing SO2 (0.23 ppm) was sulfite.104 Similarly, the corrosion products formed on Ni,105 Zn,105 and Al112 exposed to SO2-containing humid air were also characterized using in situ IRAS. As complementary to in situ IRAS, quartz crystal microbalance (QCM)106,109,111 and atomic force microscopy (AFM)108,109 were the other two in situ techniques used for the study of humid air-induced atmospheric corrosion, to obtain the mass changes and morphological information, respectively.
The influence of organic constituents such as acetic acid, acetaldehyde, and formic acid on the atmospheric corrosion of metals, especially in indoor environments, was also studied using in situ IRAS.113–117 Zinc acetate was detected as the major corrosion products in humid environments with sub-ppm concentration of acetic acid and acetaldehyde,113,114 while zinc formate formed on zinc exposed to environments containing formic acid.115
Atmospheric corrosion of metals is greatly enhanced by NaCl deposition, which can originate from either airborne sea salt particles or deicing salts. NaCl particle- induced atmospheric corrosion has been studied using in situ IRAS for Zn,118,119 Al,120 Fe,121 Cu,122,123 and Mg.124 For Zn with predeposited NaCl particles and then exposed to high humidity (> 90%), a surface film containing ZnO, Zn5(OH)8Cl2·H2O, and zinc Zn5(OH)6(CO3)2 was detected by in situ IRAS.118 The rate of the corrosion process could also be evaluated from the rate of surface film formation obtained from the IRAS measurements. In situ IRAS also detected carbonate ions (1390 cm− 1) in the thin solution films in the secondary spreading region close to the original NaCl droplet.119 Similar secondary spreading effects were observed on Cu and studied using in situ IRAS.122,123 Another important application of in situ IRAS in atmospheric corrosion studies is the investigation of filiform corrosion of Al120 and Fe.121 The movement of the active filament head on coated Al surface could be followed with in situ FTIR microspectroscopy using the characteristic IR band at ~ 2500 cm− 1 from Al(H2O)63 +, the corrosion product in the filament.120 Notice that the coating on Al must be thin and transparent in the spectral region of interest.
In situ FTIR is a powerful tool for the investigation of the mechanism of corrosion inhibitors. In combination with electrochemical techniques, SNIFTIRS was used to study the formation of BTAH on Cu(100) surface.125 The Cu(I)BTA complex was detected on Cu surface at potentials >−0.3 V as evidenced by the negative-going bands at 1119 and 1155 cm− 1. The bands assigned to BTAH in the solution were observed as the positive-going bands at 1014, 1217, 1248, 1268, and 1308 cm− 1. The effects of anions (Cl− and SO42 −) in the supporting electrolyte on the formation and decomposition of the Cu(I)BTA film on Cu were also investigated using in situ FTIR.126 It was found that the Cu(I)BTA complex was formed more easily in Cl−-containing solution than in sulfate or bisulfate solutions. Other corrosion inhibitors that have been studied using in situ FTIR include BDMPAC, glutamic acid, triethylenetetramine (TETA), sodium tartrate, and sodium benzoate.127
In situ FTIR studies of corrosion protection coating systems have been limited to polymeric coatings. Generally, two techniques can be used: FTIR-reflection spectroscopy and ATR-FTIR. While FTIR-reflection spectroscopy was employed to monitor the growth of organic coatings on metal surfaces, ATR was used mostly to study the transport of water and some ionic species through the organic coatings.
Electrochemical cell with CaF2 windows was designed to study the formation of polymer films on metal surfaces.128,129 For the poly(thiophene-3-methanol) (PTOH) film on platinum electrodes, in situ FTIR spectra indicated that the perchlorate anions entered the PTOH film during the oxidation process as evidenced by the bands in the region of 1000-1500 cm− 1. When the film was reduced, the anions were expelled from the film because the band for the perchlorate anions decreased.128 In situ FTIR was also used to study the growth of poly(o-phenylenediamine) (PPD) film on stainless steel, and the results suggested a ladder-type structure with alternating pyrazine and phenazine rings.129 Similarly, in situ FTIR could be used to detect the solution species, that is, CO2, during the formation of a semi-passivating layer on Cu.62 The CO2 was believed to be from the decarboxilation of the salicylate anion.
The adhesion between the polymeric coating and the metal substrate plays an important role in the protective properties of the coatings. Water that transports through the coating to the metal/coating interfaces is a major cause of degradation and delamination of the coating. In addition, water also promotes corrosion of the metal substrate.
ATR-FTIR has been extensively used to study the transport of water and/or ionic species through polymer films to metal/polymer interfaces.130–139 FTIR in the multiple internal reflection mode (FTIR-MIR) was used to obtain information on the water layer at the organic coating/substrate interface.130–132 The effects of water evident in the 3000-3650 and 1625-1645 cm− 1 regions were observed after the coating was exposed to water for a certain time. These features increased in intensity with time while those for the coating decreased. The fact that the intensity change of 3400 cm− 1 is proportionally related to the total amount of water in the coating enables a quantitative analysis of the water layer. By comparing the intensity changes of the water OH stretching band as a function of time, the rates at which water entered the coating/substrate interfacial region for three coating systems can be determined and compared.132 Normal ATR-FTIR using ZnSe crystal as substrate was also employed to study the transport kinetics of both water and inhibitor anion (HPO42 −).133,134
In a series of papers, ATR-FTIR with the Kretschmann configuration was applied for in situ studies of the transport of water and ionic species (thiocyanate ions) through a polymer film to metal/polymer interfaces, such as aluminium/polymer interface and conversion coated-zinc/polymer interface. The fast increase of the stretching and bending vibrations of water at 3450 and 1650 cm− 1 suggested a fast sorption of water in the coating and the aluminium/coating interface (Figure 20.8).135 Another band at 950 cm− 1 due to the vibrations of Al-O groups indicated the formation of corrosion products—aluminium oxide or hydroxide at the aluminium surface. In the thicyanate solution, a band at 2075 cm− 1 due to the C = N vibration of the thicyanate ion was detected after prolonged exposure. As complementary to ATR-FTIR, which provided information about a specific region, electrochemical impedance spectroscopy (EIS) was employed to take into account the whole system.137,138 The combination of ATR-FTIR and EIS was also extended to investigate the hidden interface between a conversion-coated zinc surface and a polymer coating upon exposure to an electrolyte.139
This chapter demonstrated the potential of Raman and IR spectroscopy for in situ studies of corrosion-related phenomena. Both techniques provide valuable real-time information on various corrosion systems. However, literature survey showed that more in situ studies were conducted using Raman spectroscopy than IR spectroscopy because of less interference from water for Raman acquisition. In addition, special sample preparation and experimental setup are required for in situ FTIR studies, especially in aqueous solutions. Therefore, challenges exist for IR spectroscopy to be applicable to more corrosion-related in situ studies.
Among the several applications discussed in this chapter, in situ study of corrosion protection coating systems is a prevalent subject, especially in recent years. The studies have been focused on all the important aspects in coating systems, which include in situ monitoring the growth of coatings, their degradation in corrosive environments, and characterization of the corrosion products underneath the coating if coating delamination occurs.