Thiourea Derivatives as Potent Inhibitors of Aluminum Corrosion

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Thiourea Derivatives as Potent Inhibitors of Aluminum Corrosion: Atomic-Level Insight into Adsorption and Inhibition Mechanisms Nicola Weder, Roger A Alberto, and Ralph Koitz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11750 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 8, 2016

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The Journal of Physical Chemistry

Thiourea Derivatives as Potent Inhibitors of Aluminum Corrosion: Atomic-level Insight into Adsorption and Inhibition Mechanisms Nicola Weder, Roger Alberto and Ralph Koitz* Department of Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland Keywords: Metal corrosion, corrosion inhibition, thiourea, α-alumina, DFT, potentio dynamic measurements ABSTRACT: Suitable corrosion inhibitors are of prime importance in order to prevent degradation of surfaces by oxidizing chemicals. In this work we studied ten symmetrical thiourea derivatives on aluminum and their efficacy in preventing oxidation by hydrochloric acid computationally and experimentally. We carried out DFT calculations of the inhibitors in both tautomer forms adsorbed on an aluminum-terminated α-alumina surface, focusing on the structure and energetics of adsorption, as well as electronic properties. Chemisorption is dominated by electron transfer from the inhibitor S atom towards the surface as well as into the first few layers of the solid. We find that the aggregated amount of transferred charge is an important parameter of the system that correlates with the inhibition efficiency as determined with potentio dynamic measurements. The measurements indicate that the thiourea derivatives are cathodic-type inhibitors, which hinder the reduction of protons and thus indirectly the surface oxidation. This is rationalized with the formation of a positively charged layer on the surface that may repel protons. Our results may serve to further improve corrosion inhibitors on this technologically important surface.

INTRODUCTION Corrosion of metal surfaces is a destructive process, which often leads to irreparable damage to metallic appliances. Hence, corrosion inhibition is a subject of ubiquitous importance in various fields of industry and has attracted the attention of current research. One way to protect metal surfaces from oxidative corrosion is the use of so-called corrosion inhibitors. Small amounts of these compounds are dissolved in corrosive media to protect the metal surface. To date, a large variety of different inhibitors has been studied in several corrosive media and on various metal surfaces. Compounds containing heteroatoms such as sulfur1, nitrogen2,3, phosphorus or oxygen4 are often found to be good inhibitors. Also heterocyclic organic compounds with delocalized π-electrons such as benzotriazole or alkynol derivatives are often considered.5-11 However, thiourea (TU)12,13 and its derivatives have received the greatest attention. Influences of different substituents, the pH value of the corroding medium13 or the concentration of the supporting electrolyte14,15 on the adsorption thermodynamics have been investigated. It has been shown that the high corrosion inhibition efficiency of TU derivatives is due to the sulfur atom, as part of the conjugated system with two amine nitrogens.13-16 Replacement of S by O decreases the inhibition efficiency drastically.4,16 Both, symmetrical15-18 and asymmetrical (di- or mono-substituted)14,19-26 derivatives have been investigated, with the former ones showing significantly better inhibition efficiency. 18 Experimental studies of corrosion inhibition are frequently complemented with computational insight.27 Such calculations are often applied with the aim of understanding the crucial factors that influence the efficiency of a given inhibitor. The majority of these calculations have employed approximate semi-empirical quantum mechanical approaches and only

consider isolated TU compounds in vacuum.10,17,19,22,26 While favorable in terms of computational cost, this neglects that system properties likely change significantly when the inhibitor is adsorbed on its target surface.28 Calculations of the complete adsorbate/substrate system with a fully quantummechanical treatment based on density functional theory (DFT) are much more scarce. They have been carried out, e.g., for triazole derivatives on copper, giving encouraging results that indicate connections between experimentally evaluated inhibition efficiencies and appropriate quantum chemical parameters.29,30 But to the best of our knowledge, TU derivatives have so far not been examined with DFT on any surface. Aluminum is a metal of extraordinary importance for the fabrication of countless objects in everyday use. Its worldwide annual output exceeds 50’000 megatons by now, tendency increasing.31 However, Al surfaces are vulnerable to attack by corrosive liquids such as hydrochloric acid16, and therefore require protection. In addition to the oxidizing power of HCl, chlorine ions form water-soluble aluminum-hydroxy complexes, which accelerate the decomposition of the passivating Al2O3 layer.32 As soon as the acid reaches the bare metal, the oxidation of aluminum and reduction of protons sets in. Due to both, the continuous dissolution of the oxidized aluminum and simultaneous escape of gaseous dihydrogen, the reaction will proceed to completion (as shown for different inhibitors on aluminum in hydrochloric acid with time dependent measurements of H2 evolution by Haleem et al.16). From a practical perspective, HCl is also chosen for its lack of N, O and S heteroatoms, which may compete with the inhibitor and thus distort the measurements. It is thus crucial to identify substances that efficiently bind to passivated aluminum and prevent corrosion of the surface. In this work we examine ten TU derivatives with hydrocarbon

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residues and their inhibition efficiencies using quantum chemical and experimental methods. The calculations are performed on an α-alumina surface, since bare aluminum is always passivated by its oxide.16,33 Experimental results are obtained from potentio dynamic measurements with HCl as corrosive medium. The goal of our work is to provide atomic-scale insight into these as-yet unexplored adsorption systems and to connect quantum chemical results to experimental data. Following this introduction and a description of the employed methods, the paper first highlights the structural properties of the adsorbate-substrate systems, followed by adsorption energetics and electronic effects of adsorption. We then compare DFT results with measured inhibition efficiency and comment on the inhibition mechanism.

COMPUTATIONAL DETAILS

AND

(DMTU), diethylthiourea (DETU), dipropylthiourea (DPTU), dibutylthiourea (DBTU), diallylthiourea (DAllTU), ethylenethiourea (EthTU) and diphenylhioturea (DPhenTU) (all secondary amines), as well as tetramethylthiourea (TMTU) and thiourea (TU). Since all these compounds (except TMTU) may adopt their tautomer forms (Chart 1 (b)), both, the thioland the thion were considered. The equilibrium constant of the tautomerization was calculated according to Gibbs’ law: 𝐾!" = exp −

All calculations are performed with the CP2K code34,35 in the framework of DFT using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional36 in combination with the D3 dispersion correction according to Grimme37, to account for the van der Waals interactions that are otherwise inadequately reproduced by DFT. The Kohn-Sham orbitals are described with Gaussian type orbitals in a double-zeta valence polarized basis set38, in combination with Goedecker-TeterHutter pseudopotentials.39 The electron density is expanded in plane waves up to a cutoff energy of 500 Ry. The adsorption of corrosion inhibitors was modeled on a 4x4x4 slab model (in stoichiometric units) of an aluminum terminated α-alumina(0001) surface, as this is reported to be the most stable surface compared to two other possible oxygen-terminations.38-40 The bottom layer of the slab was fixed at bulk positions and vacuum space of 20 Å was included between adjacent slabs in z-direction. All calculated bulk properties are in good agreement with relevant literature: The lattice constant of α-alumina is found to be 4.790 Å, its surface energy 1.75 J/m2 and the band gap 4.99 eV. 42-44

We chose the following symmetrical N,N’-(thio-)urea derivatives (Chart 1 (a)): diethylturea (DEU), dimethylthiourea

!"

.

!"# !"# 𝐸!"# = 𝐸! − 𝐸!"#$ − 𝐸!"! ,

where 𝐸! is the total energy of the relaxed system and !"# !"# 𝐸!"#$ and 𝐸!"! are total energies of the isolated slab and inhibitor, respectively. Analogously, we define the dispersion contribution to the binding energy (𝐸!"#$ ) as the difference of the D3 energies of the respective systems. The interaction energy 𝐸!"# is taken as the difference between the optimized total energy of the full system (𝐸! ) and that of its constituent parts at the same fixed atomic positions. The contributions of calculated values of both tautomer forms to the final energy value ! (𝐸!"#$% ) were determined by means of the calculated equilibrium constant according to the following equation: 𝐾!" 1 ! 𝐸!"#$% = 𝐸! + 𝐸! 1 + 𝐾!" !!!"# 1 + 𝐾!" !!!"# Electron density difference maps were obtained by subtracting the total electron densities of fixed fragments from that of the adsorbed system according to the following equation: ∆𝜌 = 𝜌!"# − 𝜌!"! − 𝜌!"#$ .

Potentio dynamic measurements Potentio dynamic measurements evaluate the current density between the measuring and the counter electrode as a function of the applied voltage, yielding the so-called charge-transfercurrent-voltage (CTCV) curve.45 The measurements were carried out in a three-electrode cell, containing a platinum counter electrode, a reference electrode and a measuring (aluminum-) electrode with a surface of 1.021 cm2. For every measurement a new test solution was prepared and the aluminum surface was freshly abraded and polished. The inhibitor was dissolved in HCl(1M) to a concentration of 10-3M. The aluminum electrode was immersed in 150 ml of the test solution. The system reached its open circuit potential (OCP) after 3 to 10 minutes. After that, the actual corrosion measurement was repeated three times. All measurements were taken four times and the average was taken as result. 𝑖!"## =

Chart 1: (a) Chemical structure of examined TU derivatives. (b) Schematic illustration of tautomer conversion.

∆!°

Here, 𝑅 is the ideal gas constant, 𝑇 is the absolute temperature (298 K) and ∆𝐻° is the difference of the total energies of both tautomer conformations. We define the adsorption energy of the inhibitor as,

EXPERIMENTAL

Calculations

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𝑏!!" 𝑏!! 2,3 𝑏!!" + 𝑏!!

d𝑖 d𝜂

!!!!"#

The measurements were evaluated by means of the SternGeary method, which combines the linear polarization method (LPR) and the Tafel approach.46 Accordingly, the corrosion current density 𝑖!"## was defined as a function of the anodic Tafel slope of aluminum (𝑏!!" ), the cathodic slope of hydrogen (𝑏!! ) and the differential of the current density by the applied voltage at the OCP. The tangent points, where the Tafel slopes were laid on the logarithmic CTCV curve were determined by


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a newly developed procedure, which defines the Tafel lines by symmetry arguments in a modified Tafel plot. Thus every measurement was evaluated under well-defined mathematical conditions, which resulted in substantially higher precision compared to the conventional method (the results were equal the ones obtained by conventional evaluation, but the standard deviation of the fourfold measurements could be lowered by 50%). Details on this approach will be published elsewhere. The corrosion current density was converted into the corrosion rate according to 𝑖!"## ∗ 𝑀!" 𝑣= 𝜌!" ∗ 𝑧 ∗ 𝐹

long axis parallel to the surface and the sulfur located directly on an aluminum atom with a distance dS,Al. This Al is pulled out of the plane by 0.46 Å on average over all sulfur-bearing inhibitors, leading to an average dS,Al of 2.34 Å. The individual interatomic distances are summarized in Table 1. See Figure S1 in the Supporting Information for depictions of all adsorbate structures.

Inhibitors

The thion forms of inhibitors with conjugated residues (DAllTU and DPhenTU) exhibit additional secondary interactions between the π-electrons and the surface-aluminum beneath, resulting in a small out-of-plane displacement of these atoms (see Figure 1). This is especially pronounced for DAllTU, where distance and angle between sulfur and the C=C double bonds correspond exactly to the in-plane lattice constant of α-alumina. Thus, the sulfur as well as the πelectrons are optimally located on top of three adjacent surface-aluminum atoms. This is observed to a lesser extent for DPhenTU, where the π-electrons are located slightly to the side of Al. Only EthTU adsorbs in an upright position since the ring structure forces the adsorbed inhibitor to a conformation that brings the amine hydrogen close to a surface-oxygen. The interatomic distance between the amine hydrogen and the surface oxygen of 1.645 Å is in the range of hydrogen bonds, in contrast to the open-chained compounds, where this distance varies between 2.1 and 2.6 Å.

The studied inhibitors were either purchased from commercial sources (TU, DEU, DMTU, DETU, DPTU, DBTU, DPhenTU, EthTU, TMTU) or prepared following adapted literature procedures (DAllTU47).

All thiol species adsorb in an upright position with the sp2hybridized nitrogen located on top of an Al atom, which is also pulled out of plane, however, to a smaller extent (0.42 Å). This leads to an average dN,Al of 1.93 Å.

The units of 𝑣, mm/a, indicate how many millimeters of the metal will be removed by oxidation per year. 𝑀!" and 𝜌!" are the molar mass and the density, respectively, of the metal, 𝑧 represents the charge of the oxidized metal and 𝐹 the Faraday constant (96495.3 C/mol). The inhibition efficiency, 𝑣! − 𝑣 𝐼𝐸% = ∗ 100 𝑣! is relative to a blank value (𝑣! ) measured for a HCl solution (1M) without inhibitor. 𝑣 is the corrosion rate from the (inhibited solution) measurement.

RESULTS AND DISCUSSION Adsorption Structures The geometry optimization of the isolated surface causes the outermost aluminum layer to move 0.49 Å towards the slab center, nearly on the same level as the adjacent oxygen layer, in agreement with earlier studies.40,42,48 The first interlayer spacing decreases by 83.90 % relative to its value in the bulk. Table 1: Geometric data of adsorbed inhibitors on αalumina: S-Al distance, dS,Al, for thion and N-Al distance, dN,Al, for thiol tautomers, and vertical displacement of the surface Al in contact with the inhibitor (Δz) Compound

Thion tautomer

Thiol tautomer

dS,Al, Å

ΔzAl, Å

dN,Al, Å

ΔzAl, Å

DEU

1.831

0.434

1.900

0.420

TMTU

2.301

0.466

−

−

TU

2.324

0.481

1.926

0.420

DPhenTU

2.377

0.381

1.953

0.331

EthTU

2.317

0.458

1.904

0.415

DBTU

2.346

0.507

1.962

0.513

DMTU

2.338

0.483

1.918

0.435

DETU

2.332

0.481

1.916

0.447

DAllTU

2.386

0.437

1.943

0.394

DPTU

2.332

0.483

1.920

0.430

The relaxed structures of the adsorbed inhibitors are similar for all compounds: The thion tautomer is positioned with its

Figure 1: Adsorbed inhibitors on α-alumina. Left: DETU, center: DAllTU with visible secondary interactions, right: DETU in the thiol form. Atom colors: Brown, Al; red, O; blue, N; black, C; yellow, S; white, H.

Adsorption Energetics The calculated adsorption energies, 𝐸!"# , and corresponding dispersion contributions, 𝐸!"#$ , of the thion and thiol forms, the equilibrium constants 𝐾!" of the tautomerization as well as the experimentally evaluated corrosion rate and inhibition efficiency, are summarized in Table 2. 𝐸!"# tends to increase with molecular size, from -236.6 kJ mol-1 for DMTU (MW = 104.17 g mol-1) to -315.5 kJ mol-1 for DBTU (MW = 188.33 g mol-1). For alkyl derivatives, Eads increases linearly (R2>0.99) with the number of side chain carbon atoms, with each -CH2- unit contributing -13.1 kJ mol-1 and an extrapolated Eads of -209.9 kJ mol-1 for 0 C atoms in the side chain. Similarly, Edisp increases linearly with the side chain length with a slope of -11.1 kJ mol-1 C-1, accounting for virtually all of the binding strength increase. Notably, Edisp of DEU is half that of DETU, although their Eads values are the same. The stronger electrostatic interactions between Al an O are compensated by stronger dispersion forces between Al and S, leading to equal Eads of the urea and thiourea compounds.



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Table 2: Calculated parameters for inhibitors on the alumina surface: Equilibrium constant of the tautomerization (Keq), adsorption (Eads) and dispersion energies (Edisp) on α-alumina, integrated electron density difference (IDD) as well as the experimentally evaluated corrosion rate (v) and inhibition efficiency (IE%). Values written in italics refer to the thiol forms. Bold values include both, the thion and thiol form, weighted with Keq. Compound

Keq

Eads, kJ/mol

Edisp, kJ/mol

IDD, e-

v, mm/a

IE%

DEU

6.11E-10

-261.1

-315.4

-261.1

-65.2

-61.0

-65.2

0.022

121.5

2.8%

TMTU

−

-287.4

−

-287.4

-87.6

−

-87.6

0.131

68.1

45.5%

TU

5.78E+11

-374.3

-243.1

-243.1

-59.6

-39.6

-39.6

0.070

55.4

55.7%

DPhenTU

3.41E+3

-331.4

-342.9

-342.9

-142.3

-134.8

-134.8

0.099

52.8

57.8%

EthTU

9.30E-5

-244.5

-267.4

-244.5

-54.5

-54.3

-54.5

0.094

47.4

62.1%

DBTU

1.09E-2

-315.2

-345.3

-315.5

-151.2

-132.7

-151.0

0.124

24.8

80.1%

DMTU

3.65E-2

-235.1

-277.8

-236.6

-84.3

-62.4

-83.5

0.124

22.0

82.4%

DETU

4.78E-3

-260.3

-301.8

-260.5

-103.3

-84.4

-103.2

0.127

20.4

83.7%

DAllTU

1.85E-3

-380.2

-333.5

-380.1

-125.5

-98.4

-125.4

0.135

14.4

88.5%

DPTU

1.14E-2

-285.3

-325.4

-285.7

-123.3

-109.5

-123.1

0.127

13.0

89.6%

The thiol forms of alkyl derivatives exhibit smaller (i.e. more positive) Edisp than the corresponding thion tautomers, since the S atom is turned away from the surface. In spite of the lower van der Waals contribution, the Eads are up to 20% stronger, likely due to electrostatic interactions between N and Al. The significantly higher Eads of DAllTU and DPhenTU are explained by the strong side-on interactions between the πelectrons and the surface, as shown in Figure 1. In the thiol tautomers these secondary interactions are reduced, due to the upright adsorption geometries. Hence, in contrast to the alkyl derivatives, the adsorption energies of these thiol forms are equal to or even lower than the ones of the corresponding thion. The equilibria of the tautomerization reactions are all clearly on the thion-side, except for TU and DPhenTU. The significantly lower 𝐾!" of urea derivatives compared to thiourea analogues is in line with earlier studies.49 Contrary to what is expected intuitively, the measured inhibition efficiency is not strongly correlated with adsorption energy. This surprising observation is supported by two conclusions drawn from the results in Table 2: First, strong van der Waals forces between saturated hydrocarbon residues and the surface, which increase the adsorption energy, do not have a direct influence on the inhibition efficiency. Secondly, the presence of sulfur in the molecular structure of the inhibitor is crucial for a good inhibition but has no influence on the adsorption energy, as the direct comparison between DEU and DETU shows. Thus both, the adsorption as well as the dispersion energies are insufficient predictors of inhibition efficiency.

Electronic Interactions Evidently, the sulfur-aluminum interaction is the decisive factor for good inhibition: The large and energetically higher valence shells of the sulfur are more easily displaced towards the surface than for oxygen. This is seen in electron density difference plots, which show the displacement of electrons mostly from the amine nitrogen atoms towards the interatomic space between aluminum and sulfur (top left panel in Figure 2). The aforementioned π-electron-aluminum interactions are also visible, since a smaller accumulation of negative charge is

located between the conjugated residues and the surface (top right panel). In contrast, no such process is observed for alkyl derivatives. Consequently, this parameter of transferred electrons into the interspace between the inhibitor and the surface excludes the induced dipole interactions and only concentrates on the sulfur-aluminum- as well as the π-electron-surface interactions. The total amount of transferred charge is obtained by integration of the electron density difference up to a cutoff (0.01 e-/Bohr3) and restricted to the volume element between these particular atoms (referred to as “IDD” from here on, for “integrated density difference”, which corresponds to the amount of electrons within the red “bubbles” in Figure 2). The IDD varies between the different compounds and is lowest by far for the oxygen-containing compound. The IDD values of all inhibitors are given in Table 2.

Figure 2: Electron density difference plots of thion (upper row) as well as thiol tautomers (lower row) of adsorbed DETU (left column) and DAllTU (right column). Red indicates positive and blue negative differences (electron accumulation and depletion, respectively). Cutoff values: ± 0.01 e-/Bohr3 (dark), ±0.005 e-/Bohr3 (semi-transparent).

Clearly, the length of the open-chained alkyl residues has no influence on the IDD. Also noticeably and in contrast to the adsorption energy, the side interactions of compounds with conjugated residues have no increasing effect on this parameter: DAllTU, including as many C atoms as DPTU, does not exhibit higher IDD in spite of the additional secondary interactions. An equal amount of transferred charge is distributed


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between all three interaction points rather than concentrated on the sulfur as in case of DPTU. The less optimally positioned πelectrons of DPhenTU even decrease the IDD; in this case the residues withdraw electrons from the sulfur. EthTU shows also lower IDD than the open-chained alkyl derivatives due to its upright adsorption geometry. All these observations are clearly reflected in the experimental results. When we correlate the IDD with the experimentally evaluated corrosion rate, the tendency of decreasing corrosion rate with increasing IDD is clearly discernible. This applies especially to (thio)urea derivatives with secondary amine structures, which are shown as black dots in Figure 3. The grey dots represent TU and TMTU. Probably due to different pKB values of non-secondary amines, the observed tendency does not apply to these compounds, since our model does not account for protonation reactions. This effect does not affect our conclusions, but merits further exploration in future work.

Figure 3: Measured corrosion rate of aluminum as a function of the calculated IDD. Black dots indicate (thio)urea species with secondary amine structures, linear fit drawn to guide the eye. Grey dots indicate non-secondary amines that are excluded from the correlation.

The adsorption of the inhibitor includes a transfer of negative charge from the inhibitor towards the interspace between the adsorbate and the surface and also into the first few layers of the Al2O3-lattice. For the example of DETU, integration of the whole electron density difference (positive and negative) from the lowermost atom of the adsorbate upwards in positive z-direction yields -0.17 electrons. The integration over the whole slab volume amounts to +0.13, leaving 0.04 electrons in the interspace between adsorbate and surface.

Figure 4: Electron density difference plots of DETU, DAllTU and DPhenTU (from left to right). Cutoff values: ± 0.0001 e-/Bohr3. Colors as in Fig. 2.

The details of the electron density redistribution are shown in Figure 4, which depicts the density difference of several αalumina/adsorbate systems with a cutoff value of ±0.0001 e/Bohr3. The described situation is the same for all studied

inhibitors. Notably, the surface becomes partially negatively charged, covered by a positively charged monolayer of adsorbed inhibitor. In direction to the interior of the solid, the polarization alternates with the frequency of the z-lattice constant and is damped after about two to three oscillations. These electronic events are also discernible in the density of states (PDOS) projected on the molecule, the sulfur atom and the slab how is shown in Figure 5 on the example of αalumina/DAllTU. The plots are similar for all examined compounds. The top panel shows the thion form of the inhibitor located 10 Å above the surface, i.e. in an unbound configuration, while the lower one shows the adsorbed state.

Figure 5: Density of states of the DAllTU/α-‐Al2O3 system of the thion tautomer projected on the inhibitor molecule (green), the sulfur atom (blue) and the alumina slab (red). The upper panel shows the inhibitor located 10 Å above the surface, whereas it is adsorbed in the lower panel. The energies are given in eV relative to the HOMO of the adsorbed molecule.

Before the interaction sets in, the highest molecular orbitals (MO) of the inhibitor lie at the position of the valence band of the solid state, except for the highest occupied molecular orbital (HOMO), which lies 0.5 eV above it. The HOMO and HOMO-1 are mostly located on the sulfur, as is visible in both, the PDOS and the depiction of the MOs in Figure 6. When interacting with the surface, the molecular states broaden to energy bands. However, since the HOMO is not interacting with the surface, it retains its molecular character and remains located on the sulfur. Noticeably, HOMO-1 and HOMO-2 localize predominantly on the α-alumina surface upon adsorption. The lowest molecular state, which interacts with the surface, lies about 7 eV below the HOMO. The LUMO of the inhibitor lies in the energy gap of α-alumina, hence, it does not interact and remains located on the adsorbate. The HOMO of the isolated thiol tautomer lies 1 eV lower than the one of the thion form so that it can interact with the valence band of the solid state. The HOMO of the adsorbed system is mostly located on the alumina lattice and shows no molecular character, in contrast to the aforementioned thion form. But analogously, the LUMO also lies in the energy gap of the solid; consequently it does not interact and remains located on the adsorbate. See Figures S2-S4 in the Supporting Information for depictions of the PDOS and energy states of the thiol species.



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Figure 6: Molecular orbital plots of the DAllTU/α-Al2O3 system from the HOMO-2 to the LUMO of the isolated (upper row) and adsorbed state (lower row).

Inhibition Mechanism Experimentally obtained CTCV curves from our potentio dynamic measurements are shown in Figure 7 (a).

the inhibitor does not affect the oxidation of aluminum at all. We thus conclude that all examined (thio)urea derivatives are cathodic-type inhibitors. Due to the asymmetric deformation of the CTCV curve, the OCP becomes dependent on the inhibitor and its concentration, as shown in Figure 7 (b). The decreasing OCP value with increasing concentration is also an indication of cathodic inhibition. We can rationalize this effect with the observations in Figure 4: The partially positively charged monolayer of adsorbed inhibitor might exert a repellent effect on the also positively charged protons. This hinders the charge transfer process from the electrode towards the surrounding medium by preventing the oxidizing protons from passing the protective layer. And since aluminum is only oxidized as fast as the reductive counter reaction proceeds, its corrosion rate is decreased. The inhibition process functions without direct influence on the oxidation process.

Adsorbate Stacking

Figure 7: (a) Measured CTCV-curves of an HCl (1M) solution (dashed line) and DBTU (10-3M) in HCl (straight line) and (b) Open Circuit Potential (OCP) of EthTU, DBTU and DETU as a function of concentration in HCl (1M).

The straight line presents the response of a solution with inhibitor (for the example of DBTU, very similar for all TU derivatives), which exhibits some remarkable differences compared to the blank measurement. The inhibited CTCV curve shows a clear decrease of the slope in the cathodic region (where the reduction of protons is predominant at the measuring electrode) while the slope at large anodic overpotential does not deviate from the blank measurement. This flattening of the cathodic curve arises from a decreased reaction rate of the reductive partial reaction. On the other hand,

The correlation between the inhibitor concentration and the inhibition efficiency at low concentrations has already been shown in several studies.17,21,23 It is known that thiourea derivatives adsorb according to the Langmuir isotherm. However, at high concentrations the relation between inhibitor concentration and inhibition efficiency is not linear any more. The dilution series of DETU on a logarithmic scale with the corresponding inhibition efficiency is shown in Figure 8. The inhibition efficiency saturates beyond a certain inhibitor concentration. Since at high concentrations the surface is completely covered by inhibitor molecules, we can straightforwardly rationalize this observation. Moreover, high inhibitor concentrations may also lead to “stacking effects”, i.e. the formation of multiple inhibitor layers on the substrate. A second layer of inhibitor placed on the first monolayer competes for the available electrons, and thus reduces the charge transfer to the surface. In the case of DETU our calculations show that the interaction energy between the inhibitor and the surface is reduced by 58 kJ mol-1 upon addition of a second DETU molecule atop the first (Table S1 in the Supporting Information). This effect is also reflected in the electron density difference plot, which shows the difference of the densities of the second DETU and the first DETU adsorbed on the surface from the whole (doubly adsorbed) system (right picture in Figure 8). Electron density is redistributed to the space between the inhibitor molecules. Electrons are withdrawn from between


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the surface and bottom DETU molecule. This loss of electron density from the sulfur-aluminum interaction during the adsorption process due to the second inhibitor is the most probable reason for the observed saturation and decrease of inhibition efficiency at high concentrations.

may facilitate the future design of efficient corrosion inhibitors for aluminum-based surfaces. Supporting Information The complete selection of adsorption geometries of both tautomer forms, the PDOS and depictions of molecular states of the thiol form of DAllTU as well as the table of energies of the doubly adsorbed α-alumina/DETU system are available free of charge as Supporting Information on the ACS Publications website.

Corresponding Author * Ralph Koitz, [email protected]

ACKNOWLEDGMENT The authors acknowledge generous computing resources from the University of Zurich. RK is grateful for funding from the Swiss National Science Foundation (grant number 20021_140441). Figure 8: Left: Inhibition efficiency of DETU vs. inhibitor concentration. Right: Electron density difference of the doubly adsorbed system. See text for details.

CONCLUSIONS In this work, we investigated the corrosion inhibition efficiency of different symmetrical N,N’-thiourea derivatives on aluminum in hydrochloric acid. Quantum chemical parameters were determined by means of DFT calculations, complemented by experimental data from potentio dynamic measurements. Adsorption energies were discussed for both tautomers. For most of the examined compounds, the equilibrium strongly favors the thion side. PDOS analysis revealed the interaction of the molecular HOMO-1 with the surface orbitals. It was seen that the corrosion rate does not correlate with the calculated adsorption energies of the inhibitors on the αalumina surface, contrary to intuition. An increase of the size of the inhibitor molecule increases the magnitude of dispersive interactions with the surface. These, however, do not improve inhibition efficiency. The inhibition itself is an electrontransferring process, from the inhibitor towards the surface. The parameter that quantifies this charge transfer is the integrated electron density difference. The experimentally evaluated corrosion rate correlates well with the IDD between inhibitor and substrate. The mechanism of the inhibition process was elucidated by a detailed analysis of the measured CTCV curves and supported by quantum chemical considerations. The corrosion rate measurements indicate cathodic-type inhibition by all compounds: The adsorbed inhibitor obstructs the reduction of hydrogen, but has no direct influence on the oxidation of aluminum. Quantum chemical calculations showed that at full coverage the surface is shielded towards the corrosive medium by a partially positively charged layer. This exerts a repellent effect on the also positively charged protons from the acidic solution, which will not get near the surface anymore. Consequently the charge transfer from the aluminum towards the protons is hindered. Further studies should consider the protonation equilibrium of the TU derivatives at low pH. This study assumed similar pKB values for all inhibitors, which is likely the case for pure hydrocarbon residues and secondary amines. However, the degree of protonation seems to differ for non-secondary amines, such as TU and TMTU. This merits further investigation. Our combination of theoretical and experimental results

ABBREVIATIONS CTCV, charge-transfer-current-voltage; DAllTU, N,N’diallylthiourea; DBTU, N,N’-dibutylthiourea; DETU, N,N’diethylthiourea; DEU, N,N’-diethylurea; DMTU, N,N’dimethylthiourea; DPhenTU, N,N’-diphenylthiourea; DPTU, N,N’-dipropylthiourea; EthTU, Ethylenethiourea; IDD, integrated electron density difference; IE, inhibition efficiency; OCP, open circuit potential; TMTU, Tetramethylthiourea TU, thiourea;

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Chart 1: (a) Chemical structure of examined TU deriva-tives. (b) Schematic illustration of tautomer conversion. 67x54mm (600 x 600 DPI)


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Figure 1: Adsorbed inhibitors on α-alumina. Left: DETU, center: DAllTU with visible secondary interactions, right: DETU in the thiol form. Atom colors: Brown, Al; red, O; blue, N; black, C; yellow, S; white, H. 84x20mm (300 x 300 DPI)



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Figure 2: Electron density difference plots of thion (upper row) as well as thiol tautomers (lower row) of adsorbed DETU (left column) and DAllTU (right column). Red indicates positive and blue negative differences (electron accumulation and depletion, respectively). Cutoff values: ± 0.01 e-/Bohr3 (dark), ±0.005 e/Bohr3 (semi-transparent). 84x45mm (300 x 300 DPI)


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Figure 4: Electron density difference plots of DETU, DAllTU and DPhenTU (from left to right). Cutoff values: ± 0.0001 e-/Bohr3. Colors as in Fig. 2. 84x23mm (300 x 300 DPI)



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Figure 5: Density of states of the DAllTU/α-Al2O3 system of the thion tautomer projected on the inhibitor molecule (green), the sulfur atom (blue) and the alumina slab (red). The upper row shows the inhibitor located 10 Å above the surface, whereas it is adsorbed in the lower row. The energies are given in eV relative to the HOMO of the adsorbed molecule. 84x54mm (300 x 300 DPI)


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Figure 6: Molecular orbital plots of the DAllTU/α-Al2O3 system from the HOMO-2 to the LUMO of the isolated (upper row) and adsorbed state (lower row). 177x50mm (300 x 300 DPI)



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Figure 8: Left: Inhibition efficiency of DETU vs. inhibitor concentration. Right: Electron density difference of the doubly adsorbed system. See text for details. 84x41mm (300 x 300 DPI)


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Figure S1: Optimized structures of adsorbed inhibitors in the thion (each upper row) and thiol form (each bottom row) on α-alumina. Atom colors: Brown, Al; red, O; blue, N; black, C; yellow, S; white, H. 177x174mm (300 x 300 DPI)



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Figure S2: Molecular orbital plots for the DAllTU/α-alumina system from the HOMO-2 to the LUMO of the isolated (each upper row) and adsorbed state (each lower row) of the thion form (upper two rows) and thiol form (bottom two rows). 177x93mm (300 x 300 DPI)


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Figure S3: Density of states of the DAllTU/α-alumina system projected on sulfur (blue), the inhibitor molecule (green) and the surface (red) of the thion form (left column) and the thiol form (right column) in the isolated (top row) and the adsorbed state (bottom row). The energies are given in eV relative to the respective HOMO of the adsorbed state. 177x60mm (300 x 300 DPI)



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Figure S4: Orbital energies in eV of (from left to right): The thion conformer in the isolated and adsorbed state on α-alumina and the thiol conformer in the isolated and adsorbed state. The red line indicates the HOMO of the isolated thion form, which lies ~1.0 eV higher than the HOMO of the isolated thiol conformer (blue line). This allows the latter one to interact with the alumina surface, while the former one retains its molecular character in the adsorbed state. 84x139mm (300 x 300 DPI)


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Figure 3: Measured corrosion rate of aluminum as a function of the calculated IDD. Black dots indicate (thio)urea species with secondary amine structures, linear fit drawn to guide the eye. Grey dots indicate non-secondary amines that are excluded from the correlation. 52x32mm (600 x 600 DPI)



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Figure 7: (a) Measured CTCV-curves of an HCl (1M) solution (dashed line) and DBTU (10-3M) in HCl (straight line) and (b) Open Circuit Potential (OCP) of EthTU, DBTU and DETU as a function of concentration in HCl (1M). 99x118mm (600 x 600 DPI)


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Graphic for Table of Contents 82x22mm (300 x 300 DPI)


Thiourea Derivatives as Potent Inhibitors of Aluminum Corrosion

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