Potential of rare-earth compounds as anticorrosion pigment for protection of aerospace AA2198-T851 alloy

Abstract

In this study, the anticorrosion potential of carboxylic compounds; Lanthanum 4-hydroxycinnamate La(4OHCin)₃, Cerium 4-hydroxycinnamate Ce(4OHCin)₃ and Praseodymium 4-hydroxycinnamate Pr(4OHCin)₃ for the protection of Al–Cu–Li alloy was investigated in 3.5% NaCl solution using electrochemical tests (EIS and PDP), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The findings achieved show a very good correlation between electrochemical responses and surface morphologies of the exposed alloy, indicating a modification of the surface by precipitation of the inhibitor species, leading to effective protection against corrosion. At optimum concentration 200 ppm, the trend of inhibition efficiency η (%) increases in the order Ce(4OHCin)₃ 93.35% > Pr(4OHCin)₃ 85.34% > La(4OHCin)₃ 82.25%. XPS complemented the findings by detecting and providing information about the oxidation states of the protective species.

Highlights

• •Rare-earth salts in combination with carboxylic complexes can provide effective protection against corrosion .
• •The adsorption mechanism obeys Langmuir isotherm with physisorption and chemisorption as factors responsible for inhibition.
• •PDP response indicated a cathodic inhibition for Ce(4OHCin)₃ and Pr(4OHCin)₃ but mixed for La(4OHCin)₃.
• •SEM analysis provided pictorial evidence of corrosion and inhibition.
• •XPS data confirms the presence and oxidation states of the protective species.

1. Introduction

Many industries such as the aerospace invested so much into their production and operation processes with the aim of getting higher returns. Currently, the best choice materials for use as structural components in aircrafts are the third generation Al–Cu–Li alloys designated as 2xxx series [1]. Thanks to their high strength-to-weight ratio, great modulus of elasticity, good formability, and corrosion resistance [2]. The Al–Cu–Li alloys are developed to overcome the limitation of low short-transverse fracture toughness, which causes failure during manufacturing process in first- and second-generation alloys, such as AA2024-T3 [3,4]. In a design of aircraft structures where lightweight and damage tolerance are the major priority, Al–Cu–Li alloys receives the best patronage. For example, the Al–Cu–Li alloys have the advantage of 3% weight reduction by addition of 1 wt% Li and this increases the modulus of elasticity by almost 6% [[5], [6], [7]]. Aside of achieving green aviation operation, the other benefits of deploying the lightweight structures in aerospace include: less force and thrust requirement during flight, energy optimisation and reduction in fuel consumption, reduction of global warming and environmental pollution arising from aviation emissions [8]. The enhanced mechanical properties of the aluminium alloys are attributed to the presence of a diversity of alloying elements embedded within the alloy matrix, generally referred to as intermetallic particles (IMP) [9]. For example, formation of strengthening phases and precipitates, such as T1 (Al₂CuLi), δ΄ (Al₃Li), and θ΄ (Al₂Cu) are achieved by the addition of Li within the microstructure. Furthermore, the presence of other alloying elements such as Zr or Cu, and subjecting the alloy to the appropriate heat treatment, may lead to the formation of β (Al₃Zr) and Al₂₀Cu₂Mn₃ precipitates, which affect recrystallization and grain refinement [[10], [11], [12]]. Another interesting thing about aluminium alloys in general, and Al–Cu–Li alloys in particular is that they have great affinity for oxygen, which support a spontaneous formation of 2–3 nm native oxide film that passivate at the surface to provide effective corrosion resistance in ambient environment [1].

However, despite the improved mechanical properties, the complexity of precipitates and subsequent heat treatment of the Al–Cu–Li alloys bring about some changes in the microstructure and electrochemical properties that make them highly susceptible to localised corrosion, characterised by preferential attack at certain grains or grain boundaries [[13], [14], [[15], [16]]. In general, the corrosion susceptibility of aluminium alloys is influenced by differences in potential between the IMP and aluminium matrix, creating a microgalvanic cell which forms the foundation for corrosion [[17], [18], [19], [20]]. For Al–Cu–Li alloys in particular, the T1 precipitates, which forms the major strengthening phase due to their preferential nucleation at dislocations and grain boundaries [21,22], are known to greatly encourage localised corrosion arising from their anodic nature relative to the alloy matrix and Cu depleted zone [23]. Therefore, exposing the Al–Cu–Li alloys to NaCl environment results in dissolving the native oxide film thereby making them extremely vulnerable to localized corrosion attack, such as pitting, intergranular, or exfoliation [3,24]. Most recently, the crucial interest of aerospace industries and academia is channelled towards the study of corrosion behaviour of such important Al–Cu–Li alloy [3,[25], [26], [27], [28], [29], [30]], with a focus to protect the structural integrity and increase their service life. To this end, several corrosion protection approaches are available, among which the deployment of chemicals (corrosion inhibitors) to interact with the alloy surface and drastically impede the rate of corrosion processes is the cheapest and most practical technique [31,32].

The application of corrosion inhibitors is a century-long industrial practice primarily to improve corrosion resistance and longevity of metallic materials [33]. This practice has received significant industrial consideration, and most recently, the heart-burn desire of researchers of corrosion science and nanomaterials fields is to explore how the inhibitors function to enable design of systems and structures that would extend the service life of assets and increase the chances of success in automobile and aircraft industries. The inhibitors are expected to deposit complexes onto the alloy surface or at least reinforce the metal oxide film [[34], [35], [36], [37]]. The known most effective inhibitor to corrosion of alloys is based on chromate materials [38,39]. Thanks for their ability to get adsorbed on the metal surface to form a chromium (III) oxide layer at anodic and cathode sites, while buffering the pH at the metal/electrolyte interface [40]. However, the application of chromium species for the anticorrosion of metals are abolished due to irreversible health damage and environmental concerns [41]. This ban triggered the interest of industries and academia to invest significant resources for the development of new alternatives, which are mostly green benign, and chromate-free inhibitors [42,43]. Among the proposed green organic inhibitors tested are extracts of natural substances [[44], [45], [46]], quaternary amines, and triazoles [47]. Among the viable benign and interesting research inhibitors is the rare earth salts, containing La⁺³, Ce³⁺ and Pr³⁺ species. These species are reported to provide effective protection to carbon steel by reacting with the OH⁻ ions produced at the cathodic sites and forming a complex barrier that inhibit only the cathodic reactions at the surface [[48], [49], [50], [51]].

However, it was discovered that various organic carboxylate compounds (ligands) at sufficient concentrations are able to inhibit metallic corrosion by forming a surface barrier at the anodic sites, thereby inhibiting anodic reaction [52]. It was therefore proposed that these salts mixed together with organic ligands, whose hydrogen-bonding interaction through the carboxylate moieties can form new stable surface complexes that improve corrosion protection by suppressing both anodic and cathodic reactions [[53], [54], [55]]. For example, the recent work by Maria Forsyth and co-workers [52,56] on cerium blended with three substituted carboxylates; including salicylate, anthranilate, and glycolate indicated mixed inhibition behaviour on the corrosion of carbon steel in NaCl solution. Furthermore, the cerium salicylate revealed a synergistic response with greater inhibition efficiency in comparison with the individual components. Similarly, synergistic effect was observed in the work of Blin and co-workers [57] where a range of rare-earth carboxylate compounds exhibited effective corrosion inhibition, behaving as mixed inhibitors on the corrosion mild steel in NaCl environment.

Although, rare-earth salts blended with other organic ligands to improve the anticorrosion of carbon steel have been much studied. Currently, very few research works have employed the used of carboxylic acids as ligand. In an attempt to advance understanding in this field of research, Nguyen D. Nam and co-workers [58] investigated some rare earth metals in combination with 4-hydroxycinnamate compounds as novel inhibitors for the safeguarding of steel in saturated CO₂ solution containing NaCl. The correlation of surface examination and electrochemical response suggested that the compounds inhibited the deterioration of the alloy by depositing complexes on the surface which formed a barrier to blocked predominantly the anodic active site. Inhibition efficiency increases with increase inhibitor concentration to reach a peak at 0.63 mM and was ranked in the order Ce(4OHCin)₃ < La(4OHCin)₃ < Pr(4OHCin)₃. However, to the best of the authors’ knowledge, no detailed investigation has focused on the protection of aluminium alloy, having different surface chemistry compared with carbon steel.

Therefore, the prime novelty of this work is to advance the understanding of anticorrosion of these rare earth salts blended with complexes of lanthanoid para-substituted cinnamate ligands in lanthanum 4-hydroxycinnamate (${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$), cerium 4-hydroxycinnamate (${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$), and praseodymium 4-hydroxycinnamate (${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$) on the corrosion of Al–Cu–Li aerospace aluminium alloy in simulated seawater (3.5% NaCl solution) using a combination of analytical techniques, including electrochemical and surface characterisation. The alloy/compounds interaction might provide a promising revolutionary anticorrosion effect on aerospace alloys. In this work, we synthesised and characterised the rare-earth compounds using scanning electron microscopy (SEM) equipped with energy dispersive X-Ray analysis (EDX), X-Ray diffraction, and Fourier transforms infrared spectroscopy. The anticorrosion properties of the compounds were investigated on the corrosion of AA2198-T851 alloy by deploying electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation (PDP) techniques. Furthermore, the anticorrosion mechanism of these compounds on the alloy was evaluated using adsorption isotherm models. SEM combined with X-ray photoelectron spectroscopy (XPS) were deployed to reveal the detailed surface morphological features for the unexposed and exposed substrates and then determined the compositional information of the protective species formed.

2. Experimental

2.1. Chemicals and materials

The reagents used in this work include: trans4-hydroxycinnamic acid, hydrates of lanthanum chloride, cerium chloride, and praseodymium chloride. These reagents were purchased from Sigma Aldrich with purity >99% and were used without further purification. Cerium 4-hydroxycinnamate (${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$), lanthanum 4-hydroxycinnamate (${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$) and praseodymium 4-hydroxycinnamate (${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$) were synthesised and used as corrosion inhibitors in this study. The details of the synthesis and characterization can be found below. Appropriate amounts of ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ were added into 3.5% NaCl solution to obtain final inhibitor concentrations of 10 ppm, 50 ppm, 100 ppm, 200 ppm, and 400 ppm.

The material used in this work is a commercially rolled plate AA2198-T851 aluminium alloy with the chemical composition shown in Table 1.

Table 1.

Chemical composition of AA2198-T851 employed for this project.

Elements	Zr	Li	Ti	Zn	Mg	Cu	Fe	Si	Al
Composition (%wt)	0.12	1.01	0.027	0.01	0.31	3.68	0.08	0.03	94.73

2.2. Rare-earth synthesis procedure

Similar procedure using metathesis reaction in Eq. (1) as reported by previous researcher [59,60] was adopted for the synthesis of all RE(4RCin)₃ ·xH₂O complexes. In this procedure, sodium carboxylate and the corresponding rare earth halide reacted in aqueous media at pH 5 producing microcrystalline powders.

RECl₃·yH₂O + 3 Na(4RCin) RE(4RCinn)₃·xH₂O + 3 NaCl (1)

(RE means La, Ce, Pr, while R means carboxylic OH):

To prepare Na(4OHCin).

• 1.trans 4-Hydroxycinnamic acid (8.23 g, 50.2 mmol) was dissolved in 95% ethanol (50 mL).
• 2.NaOH (2.0 g, 50.2 mmol) in ethanol (95%, 200 mL) was slowly added to the carboxylic acid solution instantly forming a light-yellow precipitate. After stirring for 1 h the solution was filtered, and the precipitate washed with ethanol and dried in a vacuum desiccator for 48 h yielding sodium 4-hydroxycinnamate (Na(4OHCin))

RE 4-hydroxycinnamate hydrates RE(4OHCin)₃ ·xH₂O e.g. La(4OHCin)₃.xH₂O, Ce(4OHCin)₃. H₂O, and Pr(4OHCin)₃. xH₂O

Aqueous solutions (standardised by EDTA titrations) of hydrates LaCl₃·7H₂O, CeCl₃·7H₂O and PrCl₃ · 6H₂O were prepared by dissolving the respective RECl₃ ·xH₂O solid in distilled water and aliquots of these solutions were used for further reactions.

An aqueous solution of Na(4OHCin) (3 equivalents) was slowly added to the RECl₃ ·xH₂O solution LaCl₃·7H₂O (50 mL, 0.018 m, 0.89 mmol), (CeCl₃·7H₂O (10 mL, 0.12 m, 1.20 mmol), PrCl₃ · 7H₂O (100 mL, x m, 0.95 mmol) and a precipitate formed instantly. Upon addition of the sodium salt, the solution was adjusted to pH 5 stirred for 1 h, filtered and the precipitate washed with distilled water and dried in a vacuum desiccator for 48 h.

2.3. Pigment characterisation

2.3.1. X-ray diffraction of pigment

Goniometer X-ray diffraction equipment (Spinner PW3064) was used to find out the nature of the compounds as crystalline or amorphous. The XRD analysis was performed at ambient temperature with an X-ray source of Cu Kα radiation (λ = 1.5406 Å).

2.3.2. Fourier transforms infrared spectroscopy (FTIR)

FTIR (JASCO FTIR-4100, Japan) instrument was employed to identify compounds formed within the crystal pigment.

2.4. Aluminium alloy AA2198-T851 sample preparation

Alkaline etching was carried out by submerging the 3 cm × 2 cm sample in 10% sodium hydroxide solution (100 g NaOH dissolved in 1 L of de-ionised water) for 60 s at 60 °C, and then the samples are rinsed with de-ionised water. This is followed by desmuting in 50% (wt) nitric acid solution (50 mil nitric acid into 50 mil de-ionised water) for 30 s at 25 °C, followed by rinsing with de-ionised water and finally dried in cool air. The specimens were then masked with beeswax and colophony (3:1 wt ratio) to leave an exposed surface area of 1 cm × 1 cm and stored in a desiccator over silica gel.

2.5. Solution preparation

The corrosive environment 3.5 wt% NaCl solution was prepared by dissolving 35.064 g of NaCl in 1 L of de-ionised (15 mΩ) water. Solution of different concentrations (10 ppm, 50 ppm, 100 ppm, 200 ppm and 400 ppm) of inhibitors were prepared by dissolving the appropriate amount (0.01 g, 0.05 g, 0.1 g, 0.2 g, and 0.4 g) of each compound in 1 L of the previously prepared test solution.

2.6. Electrochemical measurements

The potentiometric measurements were carried out with the aid of Schlumberger 1280 ModuLab XM ECS Solartron Analytical Potentiostat. Three-electrode component electrochemical cell was used for impedance spectroscopy (EIS) and potentiodynamic polarisation measurements with Ag/AgCl reference electrode, platinum as counter electrode, and prepared sample as working electrode. In order to satisfy the essential requirement of stationarity, OCP (open circuit potential) was performed for 3600 s before each electrochemical test. The electrochemical measurements of samples in uninhibited and inhibited solution were performed in triplicate to ensure reproducibility. Identical polarisation curves were generated for each set of selected conditions.

2.6.1. Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) measurements were performed in naturally aerated blank or inhibited 3.5 wt% NaCl test solution as per the standard practice ASTM G106-89 [61]. A sinusoidal signal amplitude of 5 mV was applied at frequency range of 100 kHz to 0.01 Hz after 1 h open circuit potential E_OCP.

2.6.2. Potentiodynamic polarisation

The purpose of conducting the potentiodynamic polarisation (PDP) in this work is to classify the inhibitor compounds as anodic, cathodic, or mixed type. The measurements were conducted as per the standard practice for cyclic potentiodynamic polarisation (ASTM standard practice G5-87) [62]. In this method, the samples are exposed for 1 h open circuit potential (OCP) in de-aerated blank or inhibited 3.5 wt% NaCl test solution. Polarisation measurement commenced at a scan rate of 0.1667 mV/s in the cathodic −250 mV vs E_OCP and anodic +250 mV vs E_OCP directions. The de-aeration was achieved by passing a nitrogen gas (N₂ 99.99%) as oxygen scavenger in the sealed 1 L cell containing the solution for 2 h prior to all the measurements.

2.7. Surface and cross-sectional examination

The surface and cross-sectional morphology of the AA2198-T851 samples were examined prior and after exposure to the test solution without and with inhibitors pigments for 24 h. After exposure, the samples were removed from the test solution, cleaned with distilled water, and dried at room temperature before electron characterisation.

2.7.1. Scanning electron microscopy (SEM)

A FEI Quanta 250 SEM equipped with EDX was used to probe the surface of exposed specimens. The AA2198-T851 samples were submerged for 24 h at 25 °C in 3.5% NaCl with and without the inhibitors. After immersion time, the specimens were removed from the environment, cleaned with distilled water, and allowed to dry in the air before surface investigation at acceleration voltage 20 kV and working distance 10 mm.

2.7.2. Transmission electron microscopy (TEM)

FEI Talos F200A TEM equipped with EDX operating at 1.5 kV was employed to generate the TEM data, including bright field (BF) and high-angle annular dark-field (HAADF) images from the cross–section of the alloy sample which was previously prepared by plasma focused ion beam (FIB) with Xe ion source.

2.8. X-ray photoelectron spectroscopy (XPS)

An incident monochromated X-ray beam from the Al target (15 kV, 10 mA) was focused on the on the alloy substrate (which was previously immersed in the inhibited test solution) using KRATOS Axis Ultra (Kratos Analytical, Manchester, United Kingdom) to identify the presence of inhibitive species and their oxidation states. The electron energy of 20 eV was passed to the sample surface to obtain high resolution of the spectra using step size of 0.02 eV. Scanning was done twice, and the data obtained were analyzed and deconvoluted using Casa XPS software.

3. Results and discussion

3.1. Powder characterisation

Fig. 1 shows the scanning electron microscopy (SEM) images (Fig. 1a–c), and chemical structures of synthesised ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ powders. The dense assembly of the synthesised discrete pigment (powder) particles are characterised with varying diameter, shape, density, colour, and texture. The average size of particles in Ce(4OHCin)₃ and Pr(4OHCin)₃ (Fig. 1b and c) are about 8 μm. The particle sizes in ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ pigment (Fig. 1a) are obviously longer in length and smoother in texture compared to the ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ pigments (Fig. 1b and c). The EDX data presented in (Fig. 1d–f) indicated the existence of the active protective elements, including La, Ce, and Pr. Moreover, Glen and co-workers [59,60] reported that the chemical structures consist of the complexes of lanthanoid para-substituted cinnamate ligands connected to the main components; Lanthanum, Cerium and Praseodymium (Fig. 1g–i).

Fig. 1.

Characterisation of synthesised rare-earth compounds; La(4OHCin)₃, Ce(4OHCin)₃ and Pr(4OHCin)₃ using (a–c) SEM electron images and (d–f) EDX mapping and (g–i) Chemical structure.

3.1.1. X-ray diffraction studies

The structural phases of synthesised ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ powders were determined by Goniometer X-ray diffraction equipment (Spinner PW3064) to find out the nature of the materials as crystalline or amorphous. The XRD analysis was done at ambient temperature with an X-ray source of Cu Kα radiation (λ = 1.5406 Å). It analyses and identifies the unknown crystalline compounds by Brag Brentano method. Fig. 2a shows the XRD patterns of the synthesised inhibitors. The inspection of the profiles indicated that ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ powders are crystalline materials due to presence of high intensity peaks (a.u) at specific angles from each set of lattice planes in the samples. Moreover, the diffraction peaks for ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ are observed at 2θ = 6.993, 8.053, 11.400, 13.390, 13.948, and 16.170 are corresponding to (111), (200), (220), (311), (222), and (400) planes. The obtained reflections corresponding to the cubic crystal system with the lattice parameter of a = b = c = 21.9080 nm. These corresponding planes are associated with the d-spacing values of 12.630, 10.970, 7.756, 6.607, 6.344, and 5.477 Å. The data corresponds to phases obtained for Lanthanum bis(2-N-(2-iminopyridyl) phenolato) perchlorate and are readily assigned to a cubic phase of La(4OHcin)₃ (ICDD file No: 00-050-2267).

Fig. 2.

Characterisation of synthesised rare-earth compounds; La(4OHCin)₃, Ce(4OHCin)₃ and Pr(4OHCin)₃ using (a) XRD pattern (b) FT-IR spectra.

On the other hand, the diffraction peaks for ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ powder are observed at 2θ = 8.239, 13.474, 17.855, 19.099, 21.378, and 24.418 are corresponding to (111), (220), (231), (400), (240), and (150) planes. The obtained reflections corresponding to the cubic crystal system with the lattice parameter of a = b = c = 18.5728 nm. These corresponding planes are associated with the d-spacing values of 10.7230, 6.56648, 4.96379, 4.64320, 4.15300, and 3.64243 Å. The data corresponds to phases obtained for Cerium oxide hydroxide fumarate and are readily assigned to a cubic phase of ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ (ICDD file No: 00-068-0691). Finally, the diffraction peaks for ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ are observed at 2θ = 7.210, 14.480, 18.820, 24.660, and 31.740 are corresponding to (110), (220), (310), (400), and (180) planes. The obtained reflections corresponding to the monoclinic crystal system with the lattice parameter of a = 14.4400, b = 22.9910, and c = 5.6720 nm. These corresponding planes are associated with the d-spacing values of 12.25080, 6.11224, 4.71137, 3.60725, and 2.81691 Å. The data corresponds to phases obtained for Praseodymium tris(4-hydroxy-3,5-dimethoxybenzoate) tetrahydrate and are readily assigned to a cubic phase of ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ (ICDD file No: 00-058-1914). The unnamed peaks in the XRD data could be due to existence of impurities during the synthesis process or during the subsequent characterisation.

3.1.2. Fourier transforms infrared spectroscopy

Fig. 2b presents Fourier transform infrared (FT-IR) spectroscopy results of synthesised ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ powders obtained by high-spectral-resolution over a wide range of wavenumber. Generally, the molecules vibrate, and bonds stretch and bend when they absorb infrared radiation. The spectra for ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ samples indicated that the vibration of the propynyl groups, V(C = C)_propenyl bands complexes are located around ∼1634 cm⁻¹. Furthermore, two bands of the spectra arising from asymmetric V_as(COO⁻) and symmetric V_s(COO⁻) absorptions of the COO⁻ group appear at 1512 and 1403 cm⁻¹, respectively [63]. The bands in the infrared spectra associated with the C–O–C of the methoxy group are located at ∼1243 cm⁻¹ [59]. The peaks in the ranges 850−750 cm⁻¹, precisely at ∼835 cm⁻¹ in this work correspond to aromatic C–H bending [64]. This kind of behaviour perfectly corresponds to the characterisations of a range of rare earth complexes lanthanoid para-substituted cinnamate ligands such as ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ powders are reported to have good anticorrosion properties on mild steel [59].

3.2. Electrochemical behaviour of AA2198-T851

Electrochemical impedance spectroscopy spectra of AA2198-T851 substrates exposed to uninhibited and inhibited 3.5% NaCl are presented in the form of Bode plots, and Nyquist plot Fig. 3. The characteristics and shape of the spectra provides information on the level of corrosion protection.

Fig. 3.

EIS spectra plot for the behaviour of AA2198-851 alloy exposed in 3.5% NaCl test solution without and with (a–c) La(4OHCin)₃, (d–f) Ce(4OHCin)₃ and (g–i) Pr(4OHCin)₃.

3.2.1. Effects of inhibitor concentration

The electrochemical response of AA2189-T851 alloy exposed into an uninhibited (blank) and inhibited 3.5% NaCl test solution are described in the form of Bode plots, and Nyquist plot Fig. 3. Lower impedance modulus was recorded for sample exposed to the blank test solution due to direct access of aggressive ions on the alloy surface Fig. 3a. The spectrum in phase angle shows clear two-time constants, with capacitive arc at high and intermediate frequencies accounting for the general surface corrosion while localised corrosion process occurs at low frequency arc [65]. However, introducing different concentrations of carboxylic compounds; ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ into the test solution modified the alloy surface, triggering some considerable rise in impedance modulus (Fig. 3a, d, and Fig. 3g), and the corresponding increase in peak and broadness of phase angle (Fig. 3b, e, and Fig. 3h). The phase angle broadness was an indication of the presence of protective layer absorbed on the substrate surface [66]. To quantitatively assess the anticorrosion ability or the inhibition efficiency, ${\mathrm{\eta }}_{\mathrm{R}}$ (%) was calculated using Eq. (2) from EIS, experiments:

$$\eta \_R(\%)=\left(\frac{R_2-R_1}{{\mathrm{R}}_2}\right)\times 100$$ (2)

Where, R₂ and R₁are the charge transfer resistances in presence and absence of inhibitor, respectively.

Analysis of the data obtained indicated that increasing inhibitor concentrations lead to increased inhibition efficiency η (%), with the maximum solubility of the inhibitors obtained at concentration 400 ppm ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, 200 ppm ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, and 200 ppm ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$. The reason for high performance of Ce(4OHCin)₃ at 200 ppm can be attributed to favourable interaction between the alloy surface and the Ce³⁺ species. It is noteworthy that below the optimum concentration (where peak performance is achieved), an rise in the inhibitor concentration leads to an increase in η (%) due to improved surface coverage and probably, the thickness of the adsorbed species on alloy surface as reported elsewhere [42]. Again, increasing the inhibitor above optimum concentration leads to decline in the η (%). This kind of behaviour has been reported in which inhibitor exhibits a peak performance at an optimum concentration followed by a decline in performance at increased concentration [67]. They attributed this behaviour to parallel and perpendicular adsorption. In the former, the protective species interacted and drives away water molecules from the alloy surface. Whereas the latter was attributed to strong mutual electrostatic repulsion of the protective species resulting in reduction of η (%).

At the optimum concentration, an effective protective film is formed on the surface which acts as a barrier between the corrosive ions and the substrate, drastically reducing the rate of corrosion reactions. This kind of behaviour is consistent with the previous work of M. Forsyth et al. [42]. The inhibitor species glued to the alloy substrate via the functional organic moiety forming a bimetallic complex with the metal. The bimetallic complex works by blocking most corrosion active sites thereby significantly reducing the rate of corrosion reaction. However, some low level of corrosion still occurs as the corrosion is not completely inhibited thereby leading to changes in the local pH. This change in pH at the cathodic sites, leads to the hydrolysis of the carboxylates species forming a precipitate on the alloy surface [51,59]. Beyond the creation of a bimetallic complex precipitate, the orientations of the hydroxyl group from the 4-hydroxycinnamate ligand in the electrolyte is capable of affecting the intermolecular interactions, which in turn affects the inhibition capability [68].

The equivalent electrical circuits (EECs), shown in Fig. 4 are used to fit the experimental data. The Nyquist spectra of the blank sample clearly revealed predominantly two-time constants Fig. 3c, accounting for the kinetically-controlled and a diffusion-controlled regions, respectively [69]. This behaviour was featured in the equivalent electrical circuit (EEC) presented in Fig. 4a by incorporating the Warburg impedance W_s, which is a measure of resistance to mass transfer that accounts for the diffusion of the reactants [69]. The CPE here is used to properly describe the non-ideal capacitive behaviour and electrode surface heterogeneity, arising from alloy microstructure. In the circuit, solution resistance Rs is connected in series with constant phase element for the protective film (CPE_f), which had a parallel connection with film resistance (R_f). These components are connected in series to the double layer capacitance (CEP_dl) of the electrolyte/electrode interface, which had a parallel connection with the charge transfer resistance (R_ct) of adsorbed inhibitor species and Warburg impedance. On the other hand, the Nyquist response of the samples exposed to the inhibited test solution revealed dominantly clear one-time constant, with few low concentrations showing two-time constants (Fig. 3c, f, and Fig. 3i). The spectra show significant increase in impedance modulus, suggesting an enhanced resistance to the charge transfer through the electrode interface and through the adsorbed inhibitor layer [70]. The EEC in Fig. 4b was used for reasonable fitting of the experimental data. The overall goodness of the fitting was confirmed by means of chi-square value < 0.005 and then sum of squares at ≤ 1, and the results are summarised in Table 2.

Fig. 4.

Electric equivalent circuit used to simulate the experimental data of AA2198T851 alloy exposed to 3.5% NaCl test solution (a) without inhibitor – blank (b) in the presence of inhibitor.

Table 2.

Corrosion properties of AA2198-T851 exposed in 3.5% NaCl test solution without and with synthesised rare-earth compounds.

Inhibitor concentration (ppm)	Rs (Ω cm2)	Rf (Ω cm2)	Rct (Ω cm2)	CPEf-T (μF cm−2 sn−1)	n-f	CPEdl-T (μF cm−2 sn−1)	n-dl	Ws-T (Ω cm2)	${\mathbf{\eta }}_{\mathbf{R}}$ (%)	Error value (%)
Blank	8.89	–	3186	–	–	64.39	0.94	6E-4	–	–
La(4OHCin)3
10	9.83	10.16	6592	54.7	0.94	14.5	0.98	–	51.67	2.58
50	9.68	10.17	7537	19.6	0.93	13.3	0.98	–	57.73	2.88
100	9.12	10.21	13,597	8.82	0.96	71.2	0.98	–	76.57	3.82
200	8.83	10.24	17,946	11.8	0.95	12.0	0.97	–	82.25	4.11
400	8.51	8.539	20,812	7.58	0.97	6.97	0.98	–	84.69	4.23
Ce(4OHCin)3
10	8.61	16.24	7219	23.6	0.94	10.4	0.98	–	55.87	3.24
50	8.55	16.26	23,539	7.09	0.92	69.0	0.81	–	86.47	4.32
100	9.14	16.31	33,157	8.20	0.95	2.14	0.98	–	90.39	4.51
200	9.03	16.41	47,876	7.22	0.97	2.33	0.98	–	93.35	4.66
400	8.76	8.529	21,789	7.15	0.94	13.9	0.98	–	85.38	4.26
Pr(4OHCin)3
10	10.88	8.951	9087	46.5	0.98	10.3	0.95	–	64.94	3.24
50	9.89	9.502	10,268	36.0	0.98	15.8	0.94	–	68.97	3.44
100	9.01	8.853	15,722	6.34	0.99	7.82	0.98	–	79.74	3.98
200	8.95	9.791	21,734	4.89	0.99	5.36	0.95	–	85.34	4.26
400	8.63	10.83	21,586	2.45	0.94	5.59	0.97	–	85.24	4.26

Mathematically, a function of CPE can be calculated from Eq. (3):

$${\mathrm{Z}}_{\mathrm{C}\mathrm{P}\mathrm{E}}=\frac{1}{{\mathrm{Y}}^0{\left(\mathrm{j}\mathrm{\omega }\right)}^{\mathrm{n}}}$$ (3)

The function Z is the impedance of the CPE,Y₀ represents the constant of the function CPE [S(s·rad⁻¹)ⁿ], j² is an imaginary number equals to −1, ω is angle frequency (rad·s⁻¹), n is the deviation parameter defined by (−1 ≤ n ≤ 1) [[71], [72], [73], [74]]. By convention, when n = 0, the CPE presumes a pure resistance, while, n = 1 means a pure capacitance, and n = −1 indicates a pure inductance [75]. Furthermore, the Eq. (4) can be employed to determine the value of double layer capacitance (C_dl):

$${\mathrm{C}}_{\mathrm{d}\mathrm{l}}={\mathrm{Y}}_0{\left({\mathrm{j}\mathrm{\omega }}_{\mathrm{m}}^{″}\right)}^{\mathrm{n}-1}$$ (4)

where ${\mathrm{\omega }}_{\mathrm{m}}^{″}$ denotes the maximum angular frequency [73]. At concentration of 200 ppm where peak performances are recorded, the trend of inhibition efficiencies η (%) (shown in Table 2 and Fig. 5) increase in the order ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ > ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ > ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$.

Fig. 5.

Variation of inhibition efficiency η (%) against concentration for AA2198-T851 alloy in 3.5% NaCl test solution in the presence of La(4OHCin)₃, Ce(4OHCin)₃ and Pr(4OHCin)₃.

3.2.2. Adsorption isotherms

The process at a constant temperature and pH where certain substances are retained or released from an aqueous phase to a solid phase can be represented by a vital curve, refers to as adsorption isotherm [76]. Since the adsorption mechanism of inhibitor species is simply defined by means of adsorption isotherms [77], it is therefore crucial to evaluate the distribution and determine the maximum adsorption power at equilibrium. Adsorption equilibrium is the vital information required to aid understanding of an adsorption phenomenon [[78], [79], [80]]. The correlation coefficient (R²) has recently been the most employed tool for evaluating thermodynamic parameters and obtaining the best fit on adsorption models. These models examine the adsorption system, compute the distribution of adsorbed species, and confirm or otherwise the consistency of theoretical assumptions [80]. In the present work, the following popular models were tested: Langmuir (Eq. (5)), Temkin (Eq. (6)), and Freundlich (Eq. (7)).

$$K_{ads}C=\frac{\theta }{1-\theta }$$ (5)

$$K_{ads}C=e^{f\theta }$$ (6)

$$K_{ads}=\theta$$ (7)

Where, $K_{ads}$ represents the adsorption equilibrium constant normally obtained as the ‘inverse of intercept’ from Table 3, which denotes the strength between adsorbate and adsorbent. C is the inhibitor concentration; fis the interaction constant (0.010 mg/g), and θ is the fractional surface coverage, which is linked to the inhibitor efficiency as shown in the following Eq. (8).

$$\theta =\frac{\%IE}{100}$$ (8)

$$K_{ads}=\frac{1}{\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{c}\mathrm{e}\mathrm{p}\mathrm{t}}$$ (9)

Table 3.

Surface coverage of inhibitive species on the AA2198-T851 substrate exposed in 3.5% NaCl test solution in the presence of various concentrations of La(4OHCin)₃, Ce(4OHCin)₃ and Pr(4OHCin)₃.

Inhibitor concentration (ppm)	η (%)	Surface coverage (θ)	C/θ	exp (θ)
3.5% NaCl Blank	–	–	–	–
La(4OHCin)3
10	51.67	0.51	0.02	1.67
50	57.73	0.57	0.09	1.73
100	76.57	0.76	0.13	2.15
200	82.25	0.82	0.24	2.17
400	84.69	0.84	0.47	2.33
Ce(4OHCin)3
10	55.87	0.55	0.01	1.54
50	86.47	0.86	0.05	2.37
100	90.39	0.90	0.11	2.46
200	93.35	0.93	0.21	2.54
400	85.38	0.85	0.46	2.32
Pr(4OHCin)3
10	64.94	0.64	0.01	1.91
50	68.97	0.68	0.07	1.99
100	79.74	0.79	0.12	2.21
200	85.34	0.85	0.23	2.34
400	85.24	0.85	0.46	2.34

Fig. 6 and Table 4 show that the experimental data was best fitted using Langmuir's adsorption model. This model predicts and provides information about the interaction between adsorbed species occurring at localized sites [81].

Fig. 6.

Langmuir adsorption isotherm plot for the adsorption of La(4OHCin)₃, Ce(4OHCin)₃ and Pr(4OHCin)₃ molecules on AA2198-T851 substrate in 3.5% NaCl test solution.

Table 4.

Thermodynamic parameters for the adsorption of La(4OHCin)₃, Ce(4OHCin)₃ and Pr(4OHCin)₃.

Inhibitor	R2	Kads = 1/intercept	ΔG0ads (kJ mol−1)	Isotherm
La(4OHCin)3	0.9974	5.376E4	−36.94	Langmuir
Ce(4OHCin)3	0.9993	5.001E5	−42.46
Pr(4OHCin)3	0.9993	1.136E5	−38.79

The obtained correlation coefficients (${\mathrm{R}}^2$) were close to the unity, which suggested that protective species from ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ spontaneously adsorbed on the metal substrate. Large values of Kads (obtained from Eq. (9)) also suggested greater adsorption and hence better inhibition efficiency. The adsorption equilibrium constant Kads is related to the standard Gibb's free energy of adsorption ${\Delta \mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^0$, via the Eq. (10) below:

$${\Delta \mathrm{G}}_{ads}^0=-RTIn\left(55.5K_{ads}\right)$$ (10)

where 55.5 is the molar concentration of water (mol/L), R is the universal gas constant (8.314 J mol⁻¹K⁻¹), and T is the temperature in Kelvin (298 K) [77]. Calculated ${\Delta \mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^0$ for ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ on AA2198-T851 showed negative values, thereby confirming that the process of adsorption of the protective species was spontaneous.

By convention, the value of ${\Delta \mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^0$ up to −20 kJ mol⁻¹ is an indication of the electrostatic interaction of the charged molecule and the charged surface of the metal (physisorption). Whereas ${\Delta \mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^0$ more negative than −40 kJ confirms that inhibitor species are adsorbed strongly on the metal surface through coordinate type bond (chemisorption) [77,82,83].

In this work, the calculated standard Gibbs free energy of adsorption values for the ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ are −36.94 kJ mol⁻¹, −42.46 kJ mol⁻¹, and −38.79 kJ mol⁻¹, respectively. The values of ${\Delta \mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^0$ for ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ are more negative than −20 kJ mol⁻¹ but less than −40 kJ mol⁻¹ which suggested that the adsorption mechanism involved both physisorption and chemisorption as combine factors responsible for the inhibition process. Then again, the ${\Delta \mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^0$ for ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ is −42.46 kJ mol⁻¹, which is more negative than −40 kJ mol⁻¹ suggesting chemisorption as the sole factor responsible for inhibition. This clearly indicated that inhibitor molecules adsorbed firmly on the substrate surface. Chemisorption mechanism involved transfer of electrons from inhibitor molecules to the partially filled orbital from atoms of aluminium alloy at the surface [83].

3.2.3. Variation of open circuit potential

Establishing a quasi or a steady state condition is an in situ requirement before the commencement of subsequent electrochemical measurements [67]. However, the dynamic nature of the corrosion process makes it challenging to achieve perfect stability. Although, reliable electrochemical data could still be acquired provided this quasi condition is attained. To fulfil this important requirement, the working electrode, AA2198-T851 alloy was immersed in 3.5% NaCl solution in the absence (blank) and presence of 200 ppm ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ for approximately 3600 s to measure the variation of potential E vs Ag/AgCl as a function of time (seconds) as presented in Fig. 7a. The result suggested that the time allowed for OCP was sufficient to at least imitate steady conditions. Furthermore, introduction of 200 ppm ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ in the test solution triggered a significant shift in the corrosion potential compared to the blank. The final potential shifts recorded over the test duration are −608.1 mV vs Ag/AgCl ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, −967.2 mV vs Ag/AgCl ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, and −788.6 mV vs Ag/AgCl ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ relative to the blank −608.1 mV vs Ag/AgCl.

Fig. 7.

(a) Variations of open circuit potential (OCP) and (b) potentiodynamic polarisation of AA2198-T851 exposed in 3.5% NaCl test solution without and with 200 ppm rare-earth compounds.

3.2.4. Potentiodynamic polarisation

Fig. 7b shows the potentiodynamic polarisation curves for AA2198-T851 alloy taken immediately at the final open circuit potential in the test solution without and with carboxylic compounds; 200 ppm ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$. The extrapolated values of Tafel polarisation parameters, including corrosion potential (E_corr), anodic and cathodic Tafel slopes (βa and βc), and corrosion current density (I_corr), are summarised in Table 5. As stated above, the introduction of 200 ppm inhibitors in the test solution resulted in some changes in both anodic and cathodic slopes, accompanied by a significant reduction in the corrosion rate of the alloy. For example, corrosion current density (I_corr) has reduced from 7.214 μA/cm² for the blank sample to 1.15, 0.462, and 1.076 μA/cm² for ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, respectively. The inhibition efficiency ${\mathrm{\eta }}_{\mathrm{I}}$ (%) was obtained from the Eq. (11) below:

$${\mathrm{\eta }}_{\mathrm{I}}(\%)=\frac{{\mathrm{I}}_{\mathrm{u}}-{\mathrm{I}}_{\mathrm{i}}}{{\mathrm{I}}_{\mathrm{u}}}\times 100$$ (11)

where $I_u$ and $I_i$ are the values of current density for the alloy exposed in uninhibited and inhibited test solutions, respectively. At the same time, the corrosion potential E_corr for the inhibited samples shifted towards the more negative values against the blank, from −608.1 mV vs Ag/AgCl reference electrode to −642.2, −967.2, and −788.6 mV vs Ag/AgCl, respectively. These decline of the corrosion rates and shift in potential suggested that the inhibitor species are adsorbed on the alloy surface, resulting in predominantly suppressing the cathodic activities [84]. Classification of an inhibitor as anodic, cathodic, or mixed type requires a corrosion potential E_corr displacement of up to 85 mV and the values corrosion current density |I_corr| for alloy exposed in solution containing inhibitor must be lower relative to that of blank solution [85,86]. Inspecting from the Table 5, all the samples exposed to the test solution containing 200 ppm recorded lower current density. However, the difference in potential displacement of −359.1 mV and −180.5 mV vs Ag/AgCl, which are way greater than −85 mV, were recorded for samples exposed in the presence of ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, respectively suggesting that both be classified as cathodic inhibitors. On the other hand, sample exposed to the test solution in the presence of 200 ppm ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ revealed difference of displacement −34.1 mV vs Ag/AgCl, which is lower than the required −85 mV and therefore be classified as a mixed inhibitor in the test environment.

Table 5.

Tafel parameters extrapolated from the potentiodynamic polarisation curves for AA2198-T851 alloy exposed to uninhibited and inhibited 3.5% NaCl test solution.

Inhibitor	Ecorr (mV) vs Ag/AgCl	Einh - Euninh (mV)	βa (mV/dec)	-βb (mV/dec)	Icorr (μA/cm2)	${\mathbf{\eta }}_{\mathbf{I}}$ (%)	Inhibitor type
Blank	−608.10	–	13.60	388.90	7.21	–	–
La(4OHCin)3	−642.20	−34.10	45.60	163.90	1.15	84.05	Mixed
Ce(4OHCin)3	−967.20	−359.10	90.10	84.33	0.46	93.59	Cathodic
Pr(4OHCin)3	−788.60	−180.50	199.40	220.40	1.07	85.08	Cathodic

3.3. Electron microscopy (SEM/TEM) and EDX analysis of substrate

Fig. 8a–d shows the electron images of AA2198-T851 alloy surface and cross-sectional morphology prior to exposure to the test solution, obtained by SEM and TEM, respectively. Note that during TEM sample preparation using FIB, a platinum (Pt) layer was deposited to protect the alloy surface Fig. 8c and d. The surface and cross-sectional views indicated the presence of constituent intermetallic particles (IMP) (Fig. 8a–d). The EDX analysis was conducted to ascertain the presence and content of certain elements on the alloy. For easy understanding, EDX features showing brighter regions on the maps or peaks on the spectra indicate the presence of higher content of certain element, whereas the darker regions on the map or no peaks on the spectra indicate absence or lesser content of that element. The chemical composition of some selected particles was determined by point EDX and presented in Fig. 9a and b and Table 6. This result agreed with our previous work [87], where we indicated that the particles represent the major intermetallic composition in Al–Cu–Li alloys, consisting mainly of Al, Cu, Fe, and C, thereby allowing us to assigned them as Al₇Cu₂Fe. The particles Al₇Cu₂Fe and Al₂Cu are reported to support micro-galvanic interaction and preferential dealloying of active element (such as Fe) upon exposure to corrosive environment since they are cathodic with respect to the alloy matrix [[88], [89], [90]].

Fig. 8.

AA2198-T851 alloy prior to immersion test showing the constituent intermetallic particles (IMP); (a and b) SEM of alloy surface (c and d) TEM of cross section with bright field (BF) and high angular annular dark field (HAADF) images.

Fig. 9.

Point EDX at the selected IMP particles taken from spectrum 1 and 2 on; (a) alloy surface and (b) the cross-sectional area in Fig. 8.

Table 6.

Point EDX response detailing elemental compositions (wt.%) along with phase assignment obtained from spectrum 1 and 2 on intermetallic particles remnants in Fig. 8.

Spectra	Al	Cu	C	Fe	Phase assignment
1	69.23	17.25	6.06	6.15	Al–Cu–Fe
2	68.11	17.11	8.29	6.49

Fig. 10 represents the electron images of AA2198-T851 surface morphology after 24 h immersion in uninhibited and inhibited 3.5% NaCl solution. The surface micrographs of the substrate exposed in uninhibited test solution was severely corroded (Fig. 10a), revealing the details when scanned at a low magnification (10 μm) (Fig. 10b). The major corroded area was also scanned at much lower magnification (2 μm) to reveal the morphology, which consists of feature liken to corrosion products formed due to attack by the aggressive ion during the immersion test (Fig. 10c). The EDX response for the sample immersed in blank test solution in Fig. 11, Fig. 12 confirmed severely corroded surface and the corrosion product with composition 39.7 wt% Al, 53.8 wt% O, 3.6 wt% Cu, 2.1 wt% C, and 0.8 wt % Cl, consists mainly of Al and O, likely forming aluminium oxide/hydroxide Al₂O₃/Al(OH)₃ [91,92], while the maps and peaks revealing Cu and C are the alloying element, and adventitious carbon, respectively. The aggressive Cl‾ ion is from the test solution which is responsible for the corrosion attack.

Fig. 10.

SEM micrograph showing different magnifications of AA2198-T851 substrates after 24 h immersion in 3.5% NaCl test solution (a–c) without inhibitor – blank (d–f) in the presence of 200 ppm La(4OHCin)₃, (g–i) Ce(4OHCin)₃ and (j–l) Pr(4OHCin)₃.

Fig. 11.

SEM micrograph showing EDX mapping of AA2198-T851 substrates after 24 h immersion in 3.5% NaCl test solution without inhibitor – blank.

Fig. 12.

SEM micrograph showing AA2198-T851 alloy after 24 h immersion in blank 3.5% NaCl test solution with X-ray spectrum of the mapped area.

On the other hand, reasonably protected surfaces were observed on substrates immersed in the inhibited test solution for 24 h (Fig. 10d, g, and Fig. 10j), which indicated that the protective species La, Ce, and Pr interacted with the alloy surface to form the precipitates on top of the background native oxide film. High magnification (2 μm) images clearly revealed the randomly deposited precipitates (Fig. 10f, i, and l). Interestingly, the alloy sample immersed in test solution with lanthanum species exhibits images with cracking and peeling films of aluminium oxides/hydroxides, along with powdery precipitate (Fig. 10d–f). Generally, these protected surfaces observed by microscopy apparently indicated a very good correlation with the electrochemical impedance response obtained during the immersion tests, which shows increasing charge transfer resistance (R_ct). Furthermore, the EDX mapping (Fig. 13a–f, Fig. 14a–f, and Fig. 15a–f) and spectra taken from the precipitates confirmed the presence of protective species (La, Ce, and Pr, respectively) with different concentrations at the locations of the elements’ main peaks, i.e., Kα and Kβ (Fig. 16). The chemical composition (wt.%) of the species extracted are presented in Table 7.

Fig. 13.

SEM micrograph showing AA2198-T851 substrates after 24 h immersion in 3.5% NaCl test solution with 200 ppm La(4OHCin)₃ with EDX mapping; Al, O, Cu, C, and La.

Fig. 14.

SEM micrograph showing AA2198-T851 substrates after 24 h immersion in 3.5% NaCl test solution with 200 ppm Ce(4OHCin)₃ with EDX mapping; Al, O, Cu, C, and Ce.

Fig. 15.

SEM micrograph showing AA2198-T851 substrates after 24 h immersion in 3.5% NaCl test solution with 200 ppm Pr(4OHCin)₃ with EDX mapping; Al, O, Cu, C, and Pr.

Fig. 16.

Electron images of AA2198-T851 alloy after 24 h immersion in the inhibited test solution showing EDX points with different contents of inhibitive species at different locations.

Table 7.

EDX analysis results showing elemental composition (wt.%) from the AA2198-T851 alloy surface after immersion in uninhibited and inhibited 3.5% NaCl test solution.

Sample immersed in the presence of:	EDX spectrum	Elemental composition wt.%
		Al	O	Cu	C	Cl	La	Ce	Pr
Blank	Mapped area	46.6	44.1	2.4	4.2		–	–	–
La(4OHCin)3	1	23.6	32.9	1.3	15.1	1.3	25.4	–	–
	2	34.4	34.9	0.8	20.3	0.9	8.7	–	–
	3	85.3	3.9	3.7	7.1	–	0.2	–	–
Ce(4OHCin)3	1	13.6	31.3	0.8	18.4	–	–	34.6	–
	2	30.6	20.5	1.0	32.2	–	–	15.0	–
	3	52.0	9.6	1.5	30.9	–	–	5.8	–
Pr(4OHCin)3	1	18.3	27.3	0.5	31.5	0.3	–	–	22.0
	2	9.1	30.6	1.1	31.3	0.9	–	–	26.0
	3	80.3	3.1	2.9	13.2	0.1	–	–	0.4

The formation of these precipitates is attributed to the hydrolysis reactions between rare earth cations (La³⁺, Ce³⁺, and Pr³⁺) and OH⁻ anions. This is because, in naturally aerated test solution, oxygen reduction being the major cathodic reaction preferentially takes place at intermetallic inclusions (known to be cathodically active sites relative to the aluminium matrix) [[93], [94], [95]]. This oxygen reduction reaction resulted in the formation of OH‾ ions, leading to increasing the local pH at the active sites [[96], [97], [98], [99]]. Due to local chemistry, the OH‾ ions react with rare earth protective species La³⁺, Ce³⁺, and Pr³⁺ to produce the precipitates (in form of La₂O₃/La(OH)₃ Ce₂O₃/Ce(OH)₃ Pr₂O₃/Pr(OH)₃) that preferentially adsorb/deposit at the cathodic active sites [96,100]. The strong adsorption of these precipitates effectively suppresses not only the cathodic reaction but also the overall electrochemical activities at active sites. This perfectly agreed with the improved inhibition efficiency observed during EIS measurement.

Our previous work along with further literature search indicated that the intermetallic particles present in aluminium alloys are the most preferential sites for deposition of inhibitor species precipitation; especially the rare-earth compounds [87,101]. Previous work indicated a strong cerium precipitation was observed on intermetallic particles than the alloy matrix after immersing 2024-T3 alloy in 0.05 M NaCl solution containing 5 g/L Ce(NO₃)₃·6H₂O for 12 h [102]. Furthermore, N Birbilis and co-workers [103] highlighted that inhibitor species interact with intermetallic particles and inhibit the localised corrosion by stabilising the anodic passivity and effectively suppressing the oxygen reduction on each phase. Although, the precipitation of inhibitive species at the alloy surface drastically reduce the rate of corrosion attack, yet, it shall be noted that the efficiency cannot be 100%, as extremely slow-rate anodic and cathodic processes still continue to take place via microdefects in the film [104]. To determine the chemical state of the inhibitive species after immersion test, another powerful surface characterisation technique [105], known as X-ray Photoelectron Spectroscopy (XPS) was employed.

3.3.1. X-ray photoelectron spectroscopy results

In this work, XPS technique was employed to provide insight about the oxidation state of the protective species formed during the 24 h immersion of AA2198-T851 alloy in 3.5% NaCl with 200 ppm of ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$. A Shirley-type background subtraction using a mixture of 70:30 Gaussian–Lorentzian functions of adjustable width and intensity was used to fit the curves obtained.

Fig. 17 shows the survey spectra acquired from the alloy surface after 24 h immersion in the inhibited test solution. Generally, the core level photoelectron spectra detected the presence of aluminium, oxygen (O 1s) and adventitious contaminates such as carbon species (C 1s), along with the respective inhibitive species (La 3d, Ce 3d, and Pr 3d) on the alloy surfaces. This is an indication of the predominance of aluminium oxide. Separate analysis of the detected species in each sample was conducted using a narrow scan to reveal the high-resolution spectra. It has been reported that carbon and oxygen could have been originated from surface contamination during exposure to air, as the samples were measured without any in situ cleaning in ultra-high vacuum (UHV) [[106], [107], [108]].

Fig. 17.

XPS survey spectra of AA2198-T851 alloy surface showing the presence of protective species after 24 h immersion in 3.5% NaCl solution with 200 ppm La(4OHCin)₃, Ce(4OHCin)₃, and Pr(4OHCin)₃.

Fig. 18 shows the high-resolution spectra of the C 1s, O 1s, and complex multi-peaked $\mathrm{L}\mathrm{a}3\mathrm{d}$, $\mathrm{C}\mathrm{e}3\mathrm{d}$, and $\mathrm{Pr}3\mathrm{d}$ corresponding to the binding energies of the different spin-orbital ${3\mathrm{d}}_{3/2}$ and ${3\mathrm{d}}_{5/2}$ core holes. In all cases, the binding energies in the spectra were corrected with reference to peak with the lowest binding energy on the adventitious C 1s, which is at 284.8 eV. The deconvolution spectra of the C 1s core level for all samples (Fig. 18a, d, and g) indicated that the peaks can be divided into three main sub-peaks at the binding energies approximately 284.8 eV, 285.3 eV, and 288.8 eV corresponding to carbonate C–C or C–H, C–OH, and C–O, respectively. Note that the precision of values presented is ±0.2 eV to nearest tenth. Similarly, the deconvolution of the O 1s core level spectra for all samples (Fig. 18b, e, and Fig. 18h) can be fitted with two main sub-peaks at approximately 530.5 eV and 531.9 eV for La(4OHCin)₃; 530.7 eV and 532.0 eV for Ce(4OHCin)₃; and 530.3 eV and 531.7 eV for Pr(4OHCin)₃. The peaks at 530.5 eV, 530.7 eV, and 530.3 eV are usually associated with adsorbed oxygen, forming La₂O₃; Ce₂O₃; and Pr₂O₃, whereas the peaks at 531.9 eV, 532.0 eV, and 531.7 eV are usually attributed to hydroxide components or carbonate species in form of OH⁻ forming La(OH)₃; Ce(OH)₃; and Pr(OH)₃ [[109], [110], [111], [112], [113]].

Fig. 18.

High resolution XPS deconvolution spectra of AA2198-T851 alloy after 24 h immersion in 3.5% NaCl solution with 200 ppm La(4OHCin)₃ showing (a–c): C 1s, O 1s, and La 3d; Ce(4OHCin)₃ (d–f): C 1s, O 1s, and Ce 3d; Pr(4OHCin)₃ (g–i): C 1s, O 1s, and Pr 3d.

The deconvolution each of the complex multi-peaked spin-orbit ${3\mathrm{d}}_{3/2}$ and ${3\mathrm{d}}_{5/2}$ core holes of the $\mathrm{L}\mathrm{a}3\mathrm{d}$, $\mathrm{C}\mathrm{e}3\mathrm{d}$, and $\mathrm{Pr}3\mathrm{d}$ spectra are dominated by doublet peaks (Fig. 18c and f, ang Fig. 18i). The complex photoelectron La 3d (Fig. 18c) shows strong satellite peaks whose intensity and energy separation relative to the main peak are highly sensitive to the ligands [114]. The two main peaks 3d_5/2 and 3d_3/2 signals are separated by splitting energy of ∼16.7 eV but their relative positions to each satellite peak is 3.2 eV. The two components of peak 3d_5/2 with binding energies 835.5 and 838.7 eV indicated the characteristic of La³⁺ contributing to La₂O₃ species [111]. Whereas components of peak 3d_3/2 with binding energies 852.2 eV and 855.5 eV are contributing to La(OH)₃ species [115]. The Ce 3d spectrum (Fig. 18f) indicated two main peaks located around 885.5 eV and 904.1 eV, which are attributed to the Ce³⁺ final state (Ce(III)3d⁹4f²O2p⁵), whereas the satellite peaks at 881.5 eV and 900 eV can be attributed to Ce⁴⁺ final state (Ce(IV)3d⁹4f⁰O2p⁶) with similar chemical environment of CeO₂ [116]. The spin-orbit splitting of 18.6 eV agreed perfectly with the work of Eric Beche and co-workers [117]. The presence of another satellite peak located around 916 eV suggested that Ce³⁺ was partially oxidised to Ce⁴⁺ during the immersion session [118,119]. The Pr 3d spectrum (Fig. 18i) shows two asymmetric peaks with binding energies located around 933.3 eV and 953.8 eV corresponding to the Pr 3d_5/2 and Pr 3d_3/2 core-levels, respectively [120]. The typical spin-orbit-splitting energy of ∼20.5 eV shows a good agreement with several works reported in the literature [121,122]. The deconvolution of 3d_5/2 peak indicated a satellite peak located around 928.8 eV, which could be assigned to the Pr³⁺ oxidations state, but and the signals at the main peaks 933.3 eV and 953.8 eV are attributed to Pr⁴⁺ [[123], [124], [125]]. The larger area under the peak corresponding to Pr⁴⁺ indicated that Pr⁴⁺ is the major oxidation state of Pr, despite coexisting with Pr³⁺ on the sample.

4. Conclusions

• 1.Under the prescribed test conditions, the rare earth compounds effectively inhibit the corrosion of AA2198-T851 substrate in 3.5% NaCl test solution by adsorption process, whose efficiency ranks in the order ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ > ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ > ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ at 200 ppm concentrations.
• 2.The adsorption mechanism obeys Langmuir isotherm with calculated Gibbs free energy of ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ (−36.94 kJ mol⁻¹), and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ (−38.79 kJ mol⁻¹), indicating physisorption and chemisorption as combine factors responsible for inhibition whereas ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ (−42.46 kJ mol⁻¹) indicated chemisorption process as the sole factor responsible for inhibition.
• 3.At 200 ppm concentration, potentiodynamic polarisation results indicated that ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ provided strong cathodic inhibition for AA2198-T851 alloy in the test solution. Whereas ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ behaved as a mixed inhibitor.
• 4.SEM analysis of alloy substrate immersed in the inhibited test solution revealed a relatively unaffected morphology suggesting sufficient protection. Whereas the substrate exposed to uninhibited test solution showed clear signs of severe corrosion.
• 5.The XPS results indicated the presence of inhibitive species on the alloy surface, which oxidised from one state to the other due to interaction with oxygen, water, and adventitious carbon.
• 6.The benign green inhibitors ${\mathrm{L}\mathrm{a}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, ${\mathrm{C}\mathrm{e}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$, and ${\mathrm{Pr}\left(4\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{i}\mathrm{n}\right)}_3$ may potentially provide industries with new options for the corrosion protection of aerospace Al–Cu–Li alloys in place of the hazardous chromate materials.

Author contribution statement

Shedrack Musa Gad: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper. Zelong Jin: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper. Seydgholamreza Emad: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper. Javier Espinoza Vergara: Performed the experiments; Analyzed and interpreted the data; Wrote the paper. Danjuma Saleh Yawas, Ishaya Musa Dagwa, Ibrahim Momoh-Bello Omiogbemi: Analyzed and interpreted the data; Wrote the paper.

Funding statement

Dr Shedrack Gad was supported by Petroleum Technology Development Fund [PTDF/ED/OSS/PHD/POF/1281/17].

Data availability statement

The authors do not have permission to share data.

Declaration of interest's statement

The authors declare no competing interests.