Emerging surface characterization techniques for carbon steel corrosion: a critical brief review

Abstract

Carbon steel is a preferred construction material in many industrial and domestic applications, including oil and gas pipelines, where corrosion mitigation using film-forming corrosion inhibitor formulations is a widely accepted method. This review identifies surface analytical techniques that are considered suitable for analysis of thin films at metallic substrates, but are yet to be applied to analysis of carbon steel surfaces in corrosive media or treated with corrosion inhibitors. The reviewed methods include time of flight-secondary ion mass spectrometry, X-ray absorption spectroscopy methods, particle-induced X-ray emission, Rutherford backscatter spectroscopy, Auger electron spectroscopy, electron probe microanalysis, near-edge X-ray absorption fine structure spectroscopy, X-ray photoemission electron microscopy, low-energy electron diffraction, small-angle neutron scattering and neutron reflectometry, and conversion electron Moessbauer spectrometry. Advantages and limitations of the analytical methods in thin-film surface investigations are discussed. Technical parameters of nominated analytical methods are provided to assist in the selection of suitable methods for analysis of metallic substrates deposited with surface films. The challenges associated with the applications of the emerging analytical methods in corrosion science are also addressed.

1. Introduction

Extending the service-life and applications of carbon steels in various industrial and domestic applications is a major challenge in the current corrosion science investigations. The general consensus of corrosion inhibition, involving an organic layer that is hydrophobic and thus corrosion-protective to the underlying steel, has been changed with recent evidence showing that hydrophobicity of a surface alone does not ensure the prevention of corrosion, but surface texture also plays a critical role in corrosion of carbon steel. Understanding the effects of the metal properties on the formation of corrosion-protective surface films requires application of analytical methods that provide detailed information on the corrosion processes, lack of which hinders developments in the material science.

There are various characterization techniques for metallic surfaces, such as atomic force microscopy (AFM), scanning tunnelling microscopy, X-ray diffraction (XRD), Kelvin probe atomic force microscopy, X-ray photo electron spectroscopy (XPS), etc., and their applications to carbon steels have been reviewed in literature [1]. It has been highlighted that in elucidating corrosion and corrosion inhibition mechanisms, combination of various surface analytical methods are required to overcome the limitations of the individual methods.

This review presents surface analytical methods that have been less explored for carbon steel corrosion than the above-mentioned techniques. However, their applications in related surface studies strongly suggest their prospective use for mechanistic studies of carbon steel corrosion and its inhibition. It is anticipated that the combined use (including in situ analysis) of the current analytical methods and the emerging techniques discussed in this review, such as XPS-Rutherford backscattering spectroscopy (RBS) or time-of-flight secondary ion mass spectrometry (ToF-SIMS) and XPS, etc., could provide novel insight into the unexplained corrosion processes. There is a scarcity of the literature in this field as the involvement of emerging analytical methods in corrosion science is a relatively new area of research, outcomes of which can be relevant to other applications in material science.

2. Emerging analytical techniques for surface characterization

(a). Time-of-flight secondary ion mass spectrometry

Secondary ion mass spectrometry (SIMS) in its static and dynamic modes can be used to investigate surface properties and uses a primary ion beam of energy 1–30 keV [2]. Static SIMS uses low current density and sputters only a few monolayers of the surface film. Static SIMS is interesting for corrosion research as low current density of the primary ion current density applied in this technique allows less exposure to the corroded samples, whereas dynamic SIMS can be used where one needs to confirm the presence of trace elements. Dynamic SIMS can also be used for quantitative depth profiling as it involves high current density, which can sputter the upper surface. The SIMS technique has numerous advantages, but requires post-analysis data treatment and ultra-high vacuum, which makes this instrument out of reach for small-scale industries [3–5].

ToF-SIMS is a modified SIMS, which allows for capturing the entire mass spectrum with a single pulse of an incident beam with high sensitivity and mass assignment accuracy. ToF-SIMS offers extra advantages for analysis of delicate samples as it causes minimal permanent changes to the sample surface while providing sufficient lateral resolution. Importantly, ToF-SIMS can effectively characterize corrosion inhibitors at a specific surface. Both SIMS and ToF-SIMS are applicable to characterization of thin organic films on metallic or alloyed surfaces [6,7]. The methods provide high degree molecular specificity and can resolve typical organic mixture structures, which makes them important for corrosion inhibitor studies. It should however be mentioned that the SIMS spectra can often be complex and difficult to interpret in terms of structural information of surfaces. Obtaining information about the active chemical species present in the inhibitor molecules as well as measuring the chemical composition of the inhibitor's film on the alloy or metallic substrates can be achieved.

ToF-SIMS has been applied to study corrosion inhibitor films. Swift et al. [8] investigated film formation of ammonium-based compounds (commercial inhibitors) on iron. The presence of quaternary ammonium groups was confirmed through ToF-SIMS along with the aromatic nature of ammonium/amine species defined through its molecular specificity. Intense molecular signals were referred to quaternary ammonium (R₄N⁺) (R = alkyl/aryl), but also represented tertiary amine (R₃N). It was however found difficult to characterize tertiary amine molecules because of the higher sensitivity of ToF-SIMS towards R₄N compared with R₃N. The absence of Fe peaks in the recorded spectra indicated formation of a thick surface film and led to the conclusion that the sample depth was lower than the film thickness, which in turn resulted in no Fe signals. This study shows the limitation of ToF-SIMS and its combined use with NEXAFS (near-edge X-ray absorption fine structure spectroscopy) can be considered (§2g). Furthermore, Fourier transform infrared spectroscopy can provide information about functional groups of the adsorbed compounds. Similarly, ToF-SIMS was used to investigate film formation of dimethylethanolamine-based inhibitors on steel [9]. The observed reduced Fe peaks confirmed the formation of a thick layer of inhibitor molecules on the steel substrate; and the presence of deprotonated molecular ions (88.09 amu) was also confirmed through SIMS spectra. This study established ToF-SIMS as an instrument to determine film thickness. This is particularly useful for corrosion inhibition studies, where film thickness can be one of the parameters that define corrosion-inhibitor performance. Swift et al. [10] demonstrated that ToF-SIMS could identify quaternary amine corrosion inhibitors on iron foil. The ToF-SIMS analysis also indicated the film-formation mechanism and ruled out the possibility of formation of iron–amine complex precipitates that were earlier assumed to be formed in an iron-containing salt. Local accumulation of precipitates on carbon steels is possible as previously shown by Zhang et al. [11]. The authors studied protein film aggregation using in situ AFM and showed that the aggregation was affected by pH. It can be suggested that combination of SIMS and AFM (in situ or ex situ) analysis could provide information about local aggregate formation and shape of aggregates, etc., as parameters that affect the corrosion processes.

Furthermore, film formation at austenitic stainless steel has been investigated through ToF-SIMS by Rossi et al. [12]. FeCl₃ solution was used for localized pitting corrosion experiments. The presence of Cr and Mn was confirmed and elucidated the formation of inclusion areas. Furthermore, ToF-SIMS mapping exhibited formation of chromium oxide core along with MnS flakes. It was observed that the distribution of OH⁻ was uniform, and interestingly OH⁻ content in chromium oxide was higher than MnS. It is important to note that information about the cause of inclusions could not been obtained from scanning electron microscopy (SEM). Thus, this study has established ToF-SIMS analysis as one of the important surface characterizing tools for corrosion science, especially for corrosion inhibition and corrosion-protective coating investigations.

Härkönen et al. [13] studied corrosion protection of AISI 52100 steel through Al₂O₃ and Ta₂O₅ films. ToF-SIMS was used for depth profile analysis that successfully detected contaminated interface and Fe, CrO₂, OH⁻ and carbon impurities.

Dall'Agnol et al. [14] used ToF-SIMS to study corrosion of carbon steel with biofilm and observed the presence of NH₄⁺, CH₂N⁺ and other biomolecules. Certain peaks recorded towards higher masses were however left unidentified. This indicated limitations of the SIMS method in analysis of surface films and suggested that one should have prior compositional information about the surface layer.

Recently, Esmaily et al. [15] reported the use of ToF-SIMS for evaluation of atmospheric corrosion of Mg alloy (AM 50). A number of issues, such as non-uniformity in surface profile and hindrance of ion penetration through corrosion products, etc., were resolved by capturing separate ToF-SIMS topographic images for rough and flat surfaces. Topographic images taken at α-Mg grain elucidated the presence of corrosion crust containing chloride ion, which was not identified in other areas, whereas MgOH⁻ and AlO⁻ were located at regions that were not covered with corrosion products. This information is highly valuable for corrosion and computational material scientists who are investigating surface profile and pit formation through the diffusion of ion. As evinced, SIMS can provide useful information on these aspects. This information would also be required for developments of ion-based kinetic models. Depth profile measurements were conducted to get insight about corrosion crust and the analysis illustrated two major outcomes: (i) Na⁺/Mg⁺ ratio is higher at low temperature (−4°C) compared to high temperature (22°C) and (ii) chloride ion intensity is higher at 22°C than at 4°C. This study established ToF-SIMS as an important investigating tool for corrosion process analysis. This study could be further extended to investigate carbon steel and other ferrous and non-ferrous alloys. The effect of surface roughness on the corrosion processes can be further investigated with neutron reflectometry (NR) as described in §2j in this review.

Mosa et al. [16] investigated a Ce-doped organic coating, namely siloxane methacrylate on the basis of the combination of silicon alkoxide as a monomer and organically modified silicon alkoxide as a cross-linker, on mild steel. ToF-SIMS was used for depth profile measurement in the coating. It was observed that the outer undoped layer had a greater thickness than the doped inner layer. After a significant sputtering time, signals for CeO⁺, Si⁺ and Fe⁺ were recorded and proved a thick-film formation.

Swift et al. [8] studied surface activity of corrosion inhibitor with the help of ToF-SIMS and showed that quaternary ammonium and tertiary amine compounds were present in the inhibitor mixture, but no direct bonding was observed between the substrate and inhibitor film. The adsorption process could further be investigated through NEXAFS and NR-based techniques as described in detail later.

Francois Lewis et al. [17] studied plasma fluorocarbon films on stainless steel. ToF-SIMS explained increments in thickness and refractive index, related to post-oxidation of oxide layer during ageing. Díaz et al. [18] investigated the atomic layer of deposited Al₂O₃ and Ta₂O₅ coating on stainless steel, and ToF-SIMS was used to investigate the depth profile of the film. Profiles of C ions and OH⁻ were observed in bulk coating. SIMS results indicated the presence of a large amount of unreacted species of precursors in the oxide layer of Ta₂O₅, and carbon was detected at the interface of stainless steel and Al₂O₃. Bexell et al. [19] investigated silane and organic-coated metals and used ToF-SIMS as a characterizing tool.

Saarimaa et al. [20] studied galvanized steel and composition of the film layer that had a microstructure with hillocks and valleys. The SEM and ToF-SIMS analyses revealed interesting features. SEM involves the application of secondary electrons for image generation. The secondary electron yield was higher in valleys than on hillocks. The opposite observation was observed from ToF-SIMS images where the signal intensities were stronger from hillocks. Mn and Ti distributions were elucidated through SIMS mapping and acquired with positive ions, whereas maps for fluorine and P-O-O were recorded with negative ions. SIMS was sensitive for fluorine, and an elemental depth profile was carried out for each element such as Zr, Mn, Ti, P-O-O, etc.

Even though these studies are not directly related to corrosion of carbon steel, they describe methodologies that are potentially applicable to the analysis of corroded carbon steels. Certain concerns are related to the use of ToF-SIMS for corrosion research. In dynamic SIMS, the number of incident electrons at the substrate surface is larger than the actual number of electrons present at the surface and causes surface damage, which is not a favourable feature for corrosion studies. In static SIMS, the number of incident ions is less than the number of surface atoms. The major advantage of static SIMS is that it can give molecular information. The count rate is however low in the static SIMS and one can only derive surface layer information. Dynamic SIMS on the other hand can provide elemental composition. Dynamic SIMS can also be used to generate three-dimensional image based on information about the elemental distribution.

The SIMS technique could be an important technique for corrosion of carbon steel as it allows for detection of diffusion of chemical species in grain boundaries of alloys, metals or at the interface between the inhibitor films and substrate. At the same time, this technique helps to acquire information about scale formation above the substrate. Inhomogeneity can clearly be seen in SIMS images. Furthermore, SIMS can characterize almost all elements. In certain corrosion studies, the presence of hydrogen detection is very important, specifically for hydrogen embrittlement, and is possible with SIMS. Similarly, failure analysis from grain boundaries due to formation of hard phases can also be attained with SIMS. Chemical profile detection can be obtained through three-dimensional SIMS images with good lateral resolution.

(b). X-ray absorption spectroscopy

In-depth understanding of surface properties and surface composition is mandatory for corrosion investigations of carbon steel. Two major techniques are essential: (i) X-ray absorption near-edge structure (XANES) and (ii) extended X-ray absorption fine structure (EXAFS). EXAFS works in the range of 1 keV above the K spectrum because of the scattering of electron by atoms. EXAFS can be used to examine corrosion products, including complex corrosion product formations, and can determine their structure and bond strength [21]. In XANES, beyond the absorption edge, multiple scattering of electrons (photo electrons) gives information about the clusters, whereas in EXAFS, interference of photoelectron waves and backscattered photoelectron waves (from neighbouring atoms) provides structural information. EXAFS can give local structural information as elastically scattered electrons participate in the interference with short mean free path [22,23]. Owing to the high energy of the X-ray beam, in situ studies are always difficult to conduct and this is a major disadvantage of X-ray absorption techniques. EXAFS results can be correlated with XRD data to obtain information about the coordination number.

Malinovschi et al. [24] used EXAFS with hW1B beam lines in Beijing synchrotron radiation with Si double-crystal monochromator at room temperature. EXAFS results showed that the coordination of Fe decreased due to the presence of Fe₃O₄. This study established the importance of EXAFS as a tool for carbon steel corrosion characterization. Characterization of specific corrosion phases, especially conducted in situ, is important for corrosion scientists. De Marco et al. [25] showed that CO₂ corrosion of carbon steel leads to the formation of various corrosion phases, such as Fe₂(OH)₂CO₃, Fe₂O₂(CO₃), Fe₆(OH)₁₂(CO₃) and Fe₆(OH)₁₂(CO₃).2H₂O, with porous nature and electrolyte ingression further increasing corrosion of carbon steel.

Heald et al. [26] investigated pertechnetate [Tc (VII)] and perrhenate [Re (VII)] in corroded steel in a study related to nuclear fuel and nuclear waste materials. EXAFS was used to study oxidation states of T_c and R_e, as interactions of Tc (VII) and Re (VII) with carbon steel dependent on their oxidation states.

Chen et al. [27] studied microbiologically influenced corrosion and used X-ray absorption technique for in-depth understanding of surface film. XAS could also be useful to investigate the coordination of atoms in different layers, for example, an inhibitor layer or a protective coating. The SAX analysis could help to understand the corrosion mechanism of carbon steel through corrosion product examination.

Kerkar et al. [28] proceeded with in situ investigation of corrosion of Fe, and Cr alloyed Fe. These materials are beyond the scope of this review, but it is important to note that this study established the use of EXAFS and XANES for in situ applications that are often found difficult to conduct in practice. After exposing iron to Cr-containing solution, significant disordering was noticed in the surrounding iron. At the same time, amorphous Cr(OH)₃ formation was observed, which could be the key factor responsible for the enhanced corrosion resistance.

Monnier et al. [29] used in situ XAS to investigate the reduction and re-oxidation of atmospheric corrosion of iron. The study was performed in transmission mode at iron K edge with a Si (220) monochromator. The obtained XANES data were used to analyse the oxidation state of iron during reduction, whereas EXAFS data were used to study the short range order of phase. Re-oxidation led to the formation of maghemite, which was considered as a stable phase, whereas the formed lepidocrocite and ferrihydrite phases were reactive in nature. Properties of Fe(II) hydroxide under anoxic medium are still unclear and require extensive studies.

The published literature reveals the importance of EXAFS and XANES for corrosion studies of carbon steel, either through study of passive or corrosive films formed on carbon steels; or through in situ monitoring of a corrosion process. It should be noted that these techniques have not been extensively used in corrosion investigations, leaving a large scarcity of information in this research field.

(c). Rutherford backscattering spectroscopy

RBS is a very powerful technique for corrosion analysis as it allows investigating the thickness of the corrosion product film, of protective or non-protective nature, or inhibitor film [30,31]. RBS could be an alternative to ellipsometry, which is an optical-based method used for film-thickness measurements. In addition, RBS spectra are known as energy spectra that can be used for compositional and depth profile measurements. Depth profiles can be obtained through energy loss of the ion beam. The obtained RBS spectrum is dependent on the experimental set-up and specific experimental parameters. At present, modern RBS instruments can perform precise measurements for beam energy, spectrometer gain, etc., and chances of errors are significantly reduced compared to earlier instrumentations. Thickness measurements of a passive layer above the substrate in the presence of corrosion inhibitor molecules have become possible in an in situ manner through RBS. This measurement cannot be conducted effectively with optical methods that can only be used for less than 50 nm oxide films.

Sakai et al. [32] studied oxidation of iron at 573 K in oxygen partial pressure range of 10⁻¹–10⁵ Pa and measured the oxide layer thickness with the help of RBS. This study showed formation of hematite and magnetite layers on the Fe, and also revealed information about the phase formation and their coverage on the substrate under various pressure conditions. It was also observed that in the case of a thin oxide layer, the measured thickness value always contains an error (less than 100 nm). The results of thickness measurements by RBS are valid when the oxide–substrate interface is clearly seen in the spectrum. Despite this study dealing with Fe oxidation, which is out of scope of this study, the work suggests a possible experimental set-up for carbon steel investigations in order to get a deeper insight into the passive layer formation in the presence of corrosion inhibitors.

Eng et al. [33] investigated the presence of uranium in corrosion layers of carbon steel (AISI 1010). RBS was used with ⁴He²⁺ (2 MeV) beam with 170° angle and 3 mm beam diameter. RBS results showed the incorporation of uranium in the corrosive layer. Uranium was distributed in either lepidocrocite- or hydroxyl-rich phases. Other methods, such as XAS-based techniques, could be employed to further investigate the coordination of uranium in lepidocrocite- and hydroxyl-rich phases.

Besides containing various positive features, RBS also has certain drawbacks as it requires an expensive accelerator. RBS is a non-destructive technique suitable for chemical depth profile measurements, but is very difficult to obtain chemical information from the sample. RBS can give precise information about the ratio of the elements present in the film, thickness and crystallinity of film. It is a quantitative method with no standard requirements, but requires a smooth and thin sample.

Murayama et al. [34] investigated the presence of H₂O₂ for intergranular corrosion of stainless steel and observed large O/Fe ratio in the case of Fe₂O₃ compared to Fe₃O₄, possibly due to the presence of water, Fe(OH)₂ or Fe(OH)₃. It should be noted that X-ray photoemission electron microscopy (XPEEM) can also provide significant information in this type of study as described later in this work (§2h).

Ningkang et al. [35] used carbon steel substrates to study the effect of Stellite and tantalum in NaCl solution and RBS was used (with 168°–2 MeV He⁺ ion) for depth profiling.

Scholes et al. [36] used cerium (III) di-butyl-phosphate for Al alloy 2024-T3 corrosion inhibition and the inhibitor film was analysed through RBS. The presence of Ce was confirmed on the surface of Al alloy. The reason for selecting RBS for this inhibitor film analysis was to get a good signal-to-noise ratio offered by RBS for heavy elements such as Ce, which was present in the inhibitor film. Depth profile measurement depicted the formation of a 17 nm thick oxide film. Since RBS is sensitive to small concentrations of heavy element detection, it is always useful for corrosion studies. In some of the RBS spectra, Ce was absent, which could be the result of Ce diffusion in one monolayer only. This argument was further supported through Raman and X-ray photoelectron spectroscopic techniques, and the correlative information contributed to the proposed inhibitor adsorption mechanism where the inhibitor adsorbs on anodic sites. RBS spectra were also recorded for Al₂CuMg samples, which exhibited the formation of a thick oxide layer with Al and Cu present along with a small amount of Ce. This study was conducted on Al alloys, but RBS could provide compositional depth profiling for carbon steel with multilayer inhibitor films. These films can also be analysed by XAS as described earlier.

RBS was also used for Mg corrosion studies by Cain et al. [37], who examined the alloy surface and showed that the freshly prepared Mg alloy surface contained a layer of MgO/Mg(OH)₂ with minute presence of Si, whereas Cr and Fe were detected along with oxide/hydroxide. The presence of Fe affects the tolerance limit of the corrosion rate. Increments of Fe concentration over the tolerance concentration limit boost the corrosion rate. Therefore, determination of these elements even in trace amounts is very important for corrosion scientists. This study established the importance and application of RBS in corrosion science.

Alishahi et al. [38] studied DC magnetron sputtered TaN coatings that are considered as anti-corrosive coatings on 316 stainless steel and Si (100). RBS spectra provided information about stoichiometry of the different phases in the surface film (TaN). The analysis allowed for determining a single and mixed phases, formations of which depended on [N₂] values. The presence of Ta₅N₆ and Ta₃N₅ was suggested for a film with [N₂] more than 5% that shows N/Ta more than 1. XRD was used as a complimentary technique to RBS and combination of the data could determine exactly the phases formed on the surface. The surface crystallography could further be investigated using low-energy electron diffraction (LEED), which is discussed in detail in §2i.

The limited number of studies describing the use of the RBS technique for corrosion of carbon steel is likely to be related to the need for a sophisticated accelerator. This technique, however, can provide insights into corrosion mechanisms either by depth profiling or film thickness measurements.

(d). Auger electron spectroscopy

Auger electron spectroscopy (AES) provides information about elemental and chemical states of a surface, mostly up to 5 nm. Usually, depth profile investigations are done with the simultaneous application of ion milling or sputtering. Chemical identification is performed through measurements of the kinetic energy of Auger electrons and Auger intensity peaks. Peak shape and peak position help to investigate chemical state of the surface. This technique is widely applicable in corrosion research. Pan et al. [39] investigated carbon steel polarization in molybdite solution with AES and found that surface roughness was an important factor in film failure (film breakage). In AES depth profile, new Mo compounds were observed at the pitted area, where new metal surface was exposed.

Carbon steel was also exposed to industrial atmosphere in order to see its atmospheric corrosion behaviour, and AES was used to get information about the initial corrosion product. It was observed that Cl⁻ and sulfate ions adsorbed quickly on the carbon steel surface and could destroy the oxide film through ion dissolution [40]. Ito et al. [41] carried out an experiment to develop a suppression method through Ni and Ni-ferrite layer formation on carbon steel. Since Ni and Ni-ferrite film layer were involved in the experiment, AES was used to obtain important information about the depth profile. AES results showed that the inner layer was enriched with Ni, whereas Ni_1−x²⁺Fe_x²⁺Fe₂³⁺O₄ was observed in the outer layer.

AES was also used to study mild steel inhibition with triazoles and confirmed the adsorption of DPTT (5-dec-9 enyl-4-phenyl-4H-[1,2,4] triazole-3-thiol) on mild steel through sulfur and nitrogen atoms. The AES helped to figure out the mechanism of corrosion inhibition [42]. EL-Nabey et al. [43] studied the effect of 1-phenylthiosemicarbazide as corrosion inhibitor for mild steel and AES was used to examine the steel surface. Mild steel after exposure in a deoxygenated solution of 0.1 M H₂SO₄ and 10% methyl alcohol exhibited O, S, C and Fe peaks. Depth profile measurements were carried out, which revealed formation about iron oxide as a corrosion product. As the sputtering time increased, O peak intensity was decreased. This confirmed the formation of an oxide on the surface.

Mild steel immersed in a 0.1 M H₂SO₄ solution that contained 0.5 mM 1-phenylthiosemicarbazide inhibitor elucidated the presence of O, C and Fe, and exhibited oxide formation on the surface of mild steel. Interestingly, it was observed that the intensity of the O peak was less in the case of steel exposed in an inhibitor-containing solution than an inhibitor-free solution.

Macrocyclic compounds, such as tetraphenyl-dithia-octaaza-cyclotetra-decahexaene (PTAT), tetraphenyl-dithia-hexaaza-cyclobidecane-hexaene (PTAB) and tetraphenyl-dioxa-hexaaza-cyclobidecane-hexaene (POAB) were used as corrosion inhibitors for mild steel. The highest efficiency was observed for PTAT, whereas POAB showed lowest inhibition efficiency. AES spectra confirmed adsorption of PTAT on mild steel through N and S atoms, whereas N, S and I were evidenced in PTAT inhibitor in the presence of KI in HCl solutions (1 M, 3 M and 5 M) and with variable inhibitor concentrations [44]. 4-Phenyl semicarbazide hydrochloride (PSC) was used as corrosion inhibitor for aluminium bronze with 4 wt% of NaCl (pH-1.8–2.0, at 60°C) as the corrosion medium. AES mapping was done at the sample surfaces and it was evident that chloride ions were present in a small area. This observation was related to corrosion protection of aluminium bronze as CuI-PSC formed a protective film and prevented chloride ions to adsorb on Al bronze surface [45]. Another approach was to select an orthophosphate solution as corrosion inhibition solution for iron and AES was used to acquire the depth profile of the film. In inhibitor solutions with no Ca, orthophosphate was not present in significant amounts in the film, whereas in the presence of Ca in the solution the formed inhibitor film exhibited significant amounts of orthophosphate [46].

2-Mercapto benzothiazole and 2-mercapto benzimiddazole were used as the Cu corrosion inhibitor and AES showed the formation of thick Cu₂O film. AES helped to reveal the film formation mechanism. It was suggested that under acidic conditions, the Cu₂O surface layer was dissolved and later a Cu inhibitor complex precipitated. It was however found that if the thickness of the inhibitor film is small enough with a thin Cu₂O layer, Cu ion from Cu₂O starts to precipitate. This study elucidated the importance of AES in corrosion inhibitor mechanism investigations [47].

A similar study was done by Shaban et al. [48] regarding carbon steel corrosion in the presence of N-phosphono-methyl-glycine under neutral conditions. AES was used to measure the depth profile and surface composition along with the sputtering through Ar beam.

Zhang et al. [49] studied exposure of steel in borate solution and explained the reaction kinetics of H₂O₂ with carbon steel. The authors used AES along with Raman spectroscopy to confirm the presence of Fe₃O₄ inner layer.

Leas & Hondros [50] investigated stress corrosion cracking in mild steel. AES technique was used to measure grain boundary segregation, which is also relevant to corrosion of carbon steel.

This review confirms in general the applicability of AES for carbon steel corrosion (phase formation, grain boundary corrosion-related studies) and corrosion prevention (e.g. corrosion inhibitor films) studies. To date, only a few investigations have been conducted for carbon steel, and future applications could include for example studies on grain boundary segregation of various elements in carbon steel corrosion along with the surface texture using the AES.

(e). Particle-induced X-ray emission

Particle-induced X-ray emission (PIXE) is another ion beam technique, which is based on the principle of exposing samples to ion beams (proton) in the MeV energy range. PIXE can detect most elements even if they are present at the ppm level and is therefore extensively used for elements with Z in the range of 11–92. PIXE can also be used for mechanistic studies, such as inspecting diffusion of elements into multilayer films through depth profiling. PIXE is a non-destructive technique, which makes it suitable for corrosion inhibitor and anti-corrosive film investigations. Owing to its non-destructive nature, micro-PIXE techniques are nowadays being widely used for the analysis of ancient artefacts as it is capable to perform elemental mapping in homogeneous and heterogeneous materials, etc. [51]. The use of PIXE in corrosion science was discussed in 1981 by Chaudhri et al. [52] and was used to detect corrosion in tooth paste tubes which had high amount of Ti, Fe, Ga and Zn.

PIXE was used to define the performance of volatile corrosion inhibitors (VCIs) in anti-corrosive polymer films. PIXE revealed the presence of Mo in all anti-corrosive polymer films. High concentration of Mo in the film established VCIs as Mo-based corrosion inhibitors where MoO₄²⁻ was often employed as an eco-friendly and anionic inhibitor. PIXE was very useful in this study as earlier studies were not able to define the concentration of Mo in VCI. This concentration measurement was much needed in order to establish an understanding of the effect of Mo content in the polymer film. High Mo content was detected through FTIR, whereas low concentrations of Mo remained unidentified. PIXE made this possible and detected a minute presence of Mo [53]. Matsuyama et al. [54] used PIXE to get elemental information about the corroded layer on carbon steel. PIXE was conducted with a proton beam energy of 2.4 MeV with 50 pA beam current. PIXE and RBS techniques were combined in the surface evaluation where the region of interest was selected by PIXE and then examined by RBS for depth profiling.

Furman et al. [55] used PIXE as an investigating tool to examine chromate-containing paint on AA 2024-T3 (Al alloy) which were treated in neutral salt spray (NSS). PIXE maps revealed the Cr depletion zone (8 µm width) and elucidated that within a specific area, the primer was Cr depleted. It was noticed that long time exposure of primer in NSS elucidated large Cr depletion zones. This analysis concluded that depletion of Cr was non-uniform and deletion depth was much higher after 5 days of exposure (30–40 µm) of primer in NSS than after 1 day.

It is important to note that PIXE is a non-destructive technique to surface films at metallic substrates and is thus highly suitable for corrosion and corrosion inhibition studies in particular.

(f). Electron probe microanalysis

Electron probe microanalysis (EPMA) is a micro-beam instrument used as a non-destructive technique. The EPMA fundamentals are similar to SEM, which can be used for quantitative elemental analysis at a very small spot size (1–2 µm) along with imaging. EPMA and SEM are similar in many aspects, except for the working distance. The parameters make EPMA useful as a quantitative elemental analyser. EPMA however cannot detect light elements, such as H, Li, He, etc. Therefore, hydrous products (likely to be corrosion products) cannot be analysed through EPMA. This technique also gives information about the distribution of elements. Li et al. [56] used EPMA for rust analysis of carbon steel, and was successful in mapping O, Cl, S, Mg, Si and Fe.

Sodium n-diethyldithiocarbamate was used as a corrosion inhibitor for mild steel in HCl (0.5 mol l⁻¹). It was identified as a mixed inhibitor and EPMA confirmed the uniform distribution of S on the mild steel surface, and elucidated the uniform film formation [57]. Yamamoto et al. [58] investigated the mechanism of sodium-3-n-octylmercapto-propionate on iron corrosion with the application of 0.5 M NaCl (aerated) as a corrosive medium, and EPMA was used to investigate the local corrosion of iron surfaces. EPMA confirmed localized corrosion in the presence of chloride ions. This study established the importance of EPMA in corrosion science.

EPMA can also be used for investigations of anti-corrosive polymer coatings. Aramaki et al. [59] used 1,2-bis(triethoxy-silyl)ethane polymer containing sodium silicate and cerium nitrate on the Zn electrode. A self-healing nature was observed in the coating, when it was exposed to NaCl solution with a scratch. The self-healing nature was confirmed through EPMA as the pitting corrosion rate decreased due to the formation of Zn(OH)₂, ZnSi₂O₅ and Ce³⁺Si₂O₅²⁻. Bilyi et al. [60] investigated the effect of polyurethane primer with Zn/iron phosphate and wollastonite on anticorrosion properties. EPMA was used to examine corrosion products that contained Zn, Si, P and Ca. It was concluded that calcium silicate and Zn/iron phosphate were two probable compounds in corrosion products. The presence of Si and P established a concept of interaction of Zn/iron phosphate and calcium silicate which was useful in mechanistic inhibition studies. The analysis demonstrated the importance of a certain concentration of wollastonite and phosphate to polyurethane that increases the inhibition performance.

Metal matrix composites (MMCs) corrosion inhibitors were also investigated through EPMA [61]. AA-6061 T6 (20% Al₂O₃ particulates) and AA-2014 T6 (10% Al₂O₃ particulates) were immersed in 0.1 M NaCl solution (25°C). EPMA showed that after immersion in corrosion media for 5 days and one month, the AA-6061 T6 samples had a few pits, whereas 6061 T6 MMC showed small pits around Al₂O₃ particles. AA 2014 had extensive micro-pitting and few intergranular cracks. EPMA confirmed the presence of W and Mo on specimens exposed for one month in the inhibitor solution. EPMS investigation can be complemented with XPEEM analysis (see §2h in this review).

(g). Near-edge X-ray absorption fine structure spectroscopy

As described in this review, adsorption of inhibitor molecules on carbon steel and other metallic substrates is highly desirable for improved corrosion prevention. Adsorption processes of complex molecules and surfactants, the key components of inhibitor formulations, can be studied through NEXAFS. Importantly, NEXAFS allows studying orientation of the adsorbate bond on the metallic substrate, and specific peaks in the NEXAFS spectra can elucidate the π-bonding. Stohr et al. [62] studied adsorption of CO on Pd nanoparticles. The authors observed that the adsorption process involved termination of (111) on Pd nanoparticles and that π-plane bonded with the surface through threefold rotational symmetry.

A similar approach can be adopted for studying adsorption of corrosion-inhibitor molecules on carbon steel. For example, the type of π-bonding of a corrosion inhibitor molecule, namely 1-dodecylpyridinium chloride, with carbon steel has been suggested from Fourier transform infrared spectroscopy study [63]. The application of NEXAFS can further help in this type of study and provide supplementary information about bonding of inhibitor molecules with a substrate. This is supported by the work of Seal et al. [64], who used NEXAFS spectroscopy to investigate inhibitor adsorption on carbon steel. Adsorption kinetics of imidazoline inhibitor and two nitrogen peaks (N_I in five-membered benzene ring, and N_II in pendant chain) were studied. The use of variable angles of incidence showed no significant differences in the peak intensities of N_I, indicating that the benzene ring was oriented flat at the surface; whereas the observed intensity changes of N_II in C–N functional group showed orientation in between 30° and 35°. This study proved NEXAFS as a valuable tool for inhibitor adsorption studies on carbon steel.

There are a few other potential applications of NEXAFS on inhibited carbon steels. NEXAFS can assist in investigating the effect of grain orientation (texture) of carbon steel in the adsorption process of the inhibitor molecules. It is known that the adsorption of molecules strongly depends on the surface energy. Song et al. [65] observed that the γ-phase (austenitic phase) had higher surface energy than the α-phase (ferrite). It was further noticed that surface energy decreased due to adsorption of hydrogen. Detailed information on this surface phenomenon can be obtained from a recent review article by Dwivedi et al. [1].

It is envisaged that understanding the effect of texture (orientation of grains) of carbon steel on adsorption of inhibitor molecules with a specific bond plane would bring new mechanistic insights and advance the developments of highly efficient corrosion inhibitors. Furthermore, understanding the relationship between the effect of texture and the adsorption process can be relevant to other fields. Dorkhan et al. [66] studied biofilm formation and proved the competency of NEXAFS for phase formation detection. NEXAFS has also been used for corrosion studies on ancient archaeological samples. Michelin et al. [67] successfully used NEXAFS for analysis of 450 year old iron nails and established the composition within the thin surface film.

Despite the limited number of studies using NEXAFS to investigate corrosion of carbon steel, the few examples presented here show potential for this method as a tool for both phase identification and corrosion inhibition mechanistic investigations of carbon steels.

(h). X-ray photoemission electron microscopy

Submicrometre and nanometre imaging is possible through XPEEM. The details about operating procedures and principles are discussed by Heun et al. [68]. New-generation XPEEM is represented by an instrument that is equipped with bandpass energy filter and is capable of providing high lateral resolution at around 0.5 µm with an X-ray source [69,70]. Kang et al. [71] studied outward diffusion of Cr and Cr₂O₃, which occurred due to the carbon precipitation in 304 stainless steel. The size of precipitates usually lies in the range of micrometres, which makes XPEEM potentially useful for acquiring images and spectroscopic details from corroded surfaces, including carbon steel. XPEEM can also be used to investigate the work function influenced by grain size and crystalline orientation of metals [72,73]. The work function was investigated on 316 stainless steel by Barret et al. [74], specifically its chemical and electronic properties. XPEEM was used to identify Cr-enriched and Ni-depleted regions along with the ratio of these two elements. This study was also important for metallurgists and corrosion scientists as it established the relation of the work function with grain orientation, and involved the application of EBSD (electron backscatter diffraction) to identify grain orientation. Since the work function is affected by surface electron affinity and can have an effect on electrochemical properties of the steel, XPEEM could be used in combination with EBSD to study the dependency of grain size and grain orientation (metallurgical texture) on the work function in carbon steel. New insights into the dependency of electrochemical properties of the metal on the work function along with grain orientation and other grain properties could be useful to design new alloys for various high- and low-temperature applications.

Genuzio et al. [75] also used XPEEM to study transformation of Fe₃O₄ and Fe₂O₃ thin films grown on Ag(111) and Pt(111) substrates. Apart from XPEEM, this study uses other surface characterization methods, such as LEED (see §2i in this review). Importantly, the study also uses a combination of methods for a thorough surface analysis, which can be applicable to corrosion science investigations. For example, it was interesting to observe that XPEEM can assist in distinguishing between Fe₃O₄ and γ-Fe₂O₃, compounds that otherwise have the same LEED pattern. The XPEEM however cannot identify γ-/α-Fe₂O₃ in a mixed layer, and only shows Fe³⁺ ions. The compositional contrast between γ-Fe₂O₃ and α-Fe₂O₃ can be analysed with LEEM (low-energy electron microscopy). Similar applications, combining XPS and LEEM, could be useful for mechanistic studies in corrosion science, specifically for studying phase transformations that are known to alter the corrosion potentials (electrode potentials differ for various phases).

We have shown that XPEEM has been used in corrosion-related disciplines, such as surface science, nanoscience and also in mineralogy [76], and has relevance to corrosion studies of carbon steel, including the option of real-time monitoring of the corrosion processes.

(i). Low-energy electron diffraction

LEED can provide information about crystallographic orientation of assemblies of organic molecules on metallic substrates [77]. Crystalline molecular assemblies deposited on substrate can be identified with LEED, which is relevant to the analysis of corrosion products. The crystallographic data from a LEED pattern refer to the symmetry and lattice parameters, and can be used to determine porosity (packing fraction) of the surface films.

The LEED technique has been recently applied by Genuzio et al. [75] and showed that the transformation of iron phases, such as magnetite and hematite, depends on the nature of the substrate (Pt(111) and Ag(111)). The study also demonstrated the benefits of combination of LEED with other methods, including XPEEM (§2h in this review).

Cooil et al. [78] established LEED as a valuable method for the surface analysis of Al-Mg-Si-Li alloys at variable temperatures (room temperature to melting). It was observed that migration of Li occurs at higher temperatures compared with Si and Mg, and that migration takes places across the surface or at grain boundaries depending on the alloying element. It is known that migration of alloying elements from bulk to surface and vice versa can affect corrosion resistance of metals, including carbon steel. Therefore, a similar study performed on carbon steel could provide information about thermal diffusion of alloying elements and assist in developments of materials for high-temperature applications in corrosive media. High-temperature processing such as normalizing, annealing, etc., can lead to grain boundary sliding and grain coarsening and thus comparing high-temperature corrosion performance of carbon steels with high-angle grain boundary and carbon steel with low-angle grain boundary could be of interest to corrosion scientists.

McBride et al. [79] investigated corrosion of Pd(111) single crystal immersed in 0.05 M H₂SO₄ and 0.5 mM NaI solution. The LEED pattern after anodic dissolution suggested layer by layer dissolution and pit formation. This study can be relevant to carbon steel applications as it has been shown that different textured carbon steel offer different level of corrosion resistance. LEED analysis could supply useful information about local nano-/microscopic crystallography of different sites at the carbon steel surfaces. Furthermore, LEED could be used to study a specifically oriented single phase such as ferritic steel as a substrate for various inhibitor adsorption studies and define the effect of solution elements, such as Na, Cl, HCO₃⁻, etc., on adsorption sites with particular crystallographic orientation. A similar study was conducted by Markovic et al. [80], who used LEED to investigate the effect of chloride on deposition of Cu on Pt(111).

These studies show apparent relevance of the application of LEED to corrosion studies of carbon steels. Other studies have shown that LEED can provide information about multiphase formation, such as that by Agnoli et al. [81] at the TiOx/Pt(111) interface. Soares et al. [82] combined XPS and LEED to study the graphene/Fe/Ni(111) system. Graphene coating is being used as a corrosion-protective layer, and it would be interesting to analyse graphene coating on carbon steel. LEED can also be useful to study the effect of orientation of specific phases on the adherence of graphene to carbon steel with regards to its crystallographic orientation and the related surface coverage.

(j). Small-angle neutron scattering and neutron reflectometry

This section refers to neutron-based techniques and their application in corrosion, with emphasis on corrosion inhibition studies. Examination of corrosion-inhibitor adsorption on carbon steel through both small-angle neutron scattering (SANS) and NR represent an interesting prospect as carbon steel phases, such as ferrite and pearlite. These phases offer different adsorption sites for the inhibitor molecules as observed by Oblonsky et al. [83]. SANS can characterize structural and morphological surface features in the range of 1–300 nm. Taghavikish et al. [84] in their pioneering study applied SANS to study corrosion protection by polymeric ionic liquid (PIL) nanoparticle emulsion of carbon steel. Neutron scattering showed the formation of cylindrical ionic micelles in the PIL spherical nanoparticles that participated in the corrosion inhibition action. This study establishes SANS as a suitable method for investigating corrosion-inhibitor films of carbon steels.

The SANS technique could further be applied to mechanistic studies of carbon steel corrosion. Mechanistic studies, involving characterization of phases formed on carbon steels in oil and gas pipelines are possible applications. Ingham et al. [85] recently used small-angle X-ray scattering (SAXS) techniques to investigate formation of an iron carbonate layer in CO₂-saturated brine solution, which represents oil and gas operation conditions. Further analysis with SANS could reveal additional morphological and structural features of the surface film particles that are smaller than 300 nm and cannot be analysed with SAXS techniques. The possible mechanistic corrosion studies with SANS could further include examination of grain boundary segregation, as de-alloying has been investigated in corrosion media earlier through neutron-based techniques by Pugh et al., who studied de-alloying of Cu_0.75Pt_0.25 alloy in H₂SO₄ to produce nanoporous platinum substrate [86]. SANS is also a sensitive technique for detecting coarsening of intermetallic phase precipitates and nanoscopic voids in a surface film as demonstrated by Rother et al. [87] for T347 stainless steel and alumina forming austenitic stainless steel.

Similar to the applications of SANS in corrosion studies, the NR technique can be applied for analysis of corrosion products (phase formations) and corrosion-inhibitor films on metallic substrates. In neutron-based techniques, such as NR, an up spin and down spin neutron generates clear contrast in the reflectometry profile for iron oxide phases (e.g. Fe₂O₃ and Fe₃O₄) that represent corrosion products typically formed on carbon steel surfaces. Furthermore, Wood et al. [88] showed that the NR technique is very sensitive to even small levels of corrosion, which is important for corrosion inhibition studies. The authors used polarized NR to investigate corrosion inhibitors such as sodium dodecyl sulfate (negatively charged molecule) and dodecyl trimethyl ammonium bromide (DTAB, positively charged molecule) to prevent Ni corrosion. The results showed advanced performance of the DTAB corrosion inhibitor in acidic medium with minute change in surface roughness.

Barkhudarov et al. [89] used NR to investigate super-hydrophobic films for Al corrosion prevention and proved its importance for super-hydrophobic film investigations. A super-hydrophobic surface is very important for carbon steel corrosion inhibition. We encourage the use of NR techniques for super-hydrophobic film formed on carbon steels, as super-hydrophoic surfaces usually depict hierarchical structures. Morphological characteristics and characterizing hierarchical super-hydrophobic film through NR could provide information about the film thickness and surface roughness along with the reflectometry profile contrast.

The reviewed literature clearly suggests the potential for the neutron-based techniques, SANS and NR, for characterization of carbon steel surfaces in corrosive media and/or treated with corrosion inhibitors. The obtained data can be further correlated with AFM results (for surface roughness) and ellipsometry results for film thickness measurements.

(k). Additional surface investigating technologies

Conversion electron Moessbauer spectroscopy (CEMS) is also a non-destructive technique to investigate corrosion products. Fujinami et al. [90] studied corrosion products of carbon steel with the help of CEMS. Mackinawite was observed as a corrosion product in the initial stage of the corrosion process and was transformed into greigite (Fe₃S₄) at a later stage of corrosion.

Microcomputed X-ray tomography is a technique used in corrosion science for rust formation monitoring in reinforcement, and it has now been established as a non-destructive technique of live corrosion monitoring [91]. This technique was also used to observe pit formation and helped in the formation of pitting mechanism development in austenitic steel. High resolution (0.8 µm³ voxel size) revealed that pitting corrosion shows intergranular corrosion due to low pH and high molarity of metal cations in the pit cavities [92].

3. Comparison between the emerging techniques

technique	advantages	disadvantages	applications
AES	fast technique	semi-quantitative, limited chemical information	elemental analysis of surface
SIMS	very high sensitivity	destructive technique, sometimes unreliable results	compositional profile of surface
XAS	any form of sample (powder, film, etc.) can be analysed, the analysis is element specific	possible damage to samples, especially in case of biological corrosionEXAFS cannot distinguish between atoms with very close atomic numbers such as N and Ocoordination number analysis is done by Debye–Waller factor involving curve fitting, a possible source of error	XANES-oxidation state analysisEXAFS-radial distribution of atoms around particular atom.
RBS	depth profile analysis, rapid analysis, direct and simple conversion	high accelerator required	depth profile measurements

4. Specific features of instruments

properties	AES	XPS	ToF-SIMS
lateral resolution	10 nm	30 µm	100 nm
depth resolution	0.2 nm	0.2 nm	1–5 nm
depth information	5 nm	5 nm	1 nm
detectability	0.1 wt%	0.1 monolayer	109 atoms cm−2
type of information	chemical composition map and depth profile	chemical bonding, mapping and depth profile	molecular and elemental surface composition

5. Summary

The emerging surface analytical methods suitable for investigating carbon steels are critically reviewed with respect to their applicability to the analysis of films formed on carbon steel surfaces, including layers of organic corrosion-inhibitor molecules and phase formations (iron oxides/hydroxides). It is shown, for example, that AES and XPS are suitable for thin surface compositional evaluations, but not preferred for thick scale products or film layers, which can be analysed with Raman spectroscopy along with other spectroscopic and microscopic analysis techniques. Some of the emerging techniques discussed in this review, such as PIXE, RBS, etc., have not been extensively used specifically for carbon steel, and relevant corrosion studies are presented. To provide the reader with a general overview of the emerging methods, a table showing advantages and disadvantages, along with technical parameters of the techniques is presented. Given the limitations of the current and emerging analytical techniques for corrosion studies, it is inevitable to combine these methods for investigations of the mechanisms of corrosion and corrosion inhibition. It is known that the initial stages of corrosion play a crucial role in the formation of surface films, properties of which define the level of corrosion protection to the steel. This highlights the need for use of the advanced analytical methods in in situ mode. We believe that application of these emerging techniques for in situ corrosion studies remains a great challenge for surface scientists, and significant efforts in the development of methodologies for corrosion science are required.

Data accessibility

No datasets were used for this review article.

Competing interests

We declare we have no competing interests.

Funding

No external funding has been received for this article. Financial support from Curtin University for D.D. (Curtin International Postgraduate Research Scholarship) is greatly appreciated.