Imidazoles as highly effective heterocyclic corrosion inhibitors for metals and alloys in aqueous electrolytes: A review (original) (raw)

Imidazoles as highly effective heterocyclic corrosion inhibitors for metals and alloys in aqueous electrolytes: A review

Ankush Mishra a{ }^{\mathrm{a}}, Jeenat Aslam b,∗{ }^{\mathrm{b}, *}, Chandrabhan Verma c,∗{ }^{\mathrm{c}, *}, M.A. Quraishi c{ }^{\mathrm{c}}, Eno E Ebenso d,e,f{ }^{\mathrm{d}, \mathrm{e}, \mathrm{f}}
a{ }^{a} Department of Applied Science, Future University Bareilly, Uttar Pradesh 243503, India
b{ }^{\mathrm{b}} Department of Chemistry, College of Science, Taibah University, Yanbu 30799, Saudi Arabia
c{ }^{c} Center of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum Er Minerals, Dhahran 31261, Saudi Arabia
d{ }^{d} Department of Chemistry, School of Chemical Er Physical Sciences and Material Science, Innovation Er Modelling (MuSIM) Research Focus Area, Faculty of Agriculture, Science and Technology, North-West University, (Mufikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa
e{ }^{e} Nanotechnology and Water Sustainability Research Unit, College of Science, Engineering and Technology, University of South Africa, Johannesburg 1709, South Africa
f{ }^{f} Department of Chemistry, College of Science, Engineering and Technology, University of South Africa, Roodepoort 1710, South Africa

A R T I C L E I N F O

Article History:
Received 24 June 2020
Revised 12 August 2020
Accepted 31 August 2020
Available online xxx

Keywords:
Imidazole based corrosion inhibitors
Langmuir adsorption isotherm
mixed type inhibitors
Metal and alloys

A B S T R A C T

Imidazole represents a special class of heterocyclic compounds that possesses several industrial and biological activities. Imidazole and its derivatives are extensively employed as corrosion inhibitors for different metals in different electrolytes. Most of the imidazole based compounds behaved as mixed and interface type corrosion inhibitors and their adsorption mechanism obeyed the Langmuir adsorption isotherm model. Generally, imidazole and its derivatives interact with metallic surface using electron rich centers. Present article also describes collection of major studies that described the anticorrosive effect of imidazole and its derivatives.
© 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction

1.1. Corrosion: Basic and adverse effect

Most of the known chemical elements (overall 105) are metals and acquire the unique ability to combine with other metal(s) to give more than 40,000 types of metallic alloys [1,2]. The alloys possess unique mechanical, physical and chemical properties depending upon their chemical composition that make them to be used as constructional and building materials in numerous industries including oil and gas industries [3-5]. However, similar to the most of metals, alloys are thermodynamically unstable and their lifespan cut short because their chemical and electrochemical reactions with surrounding environment (corrosion) [6]. Corrosion is a process through which metallic materials undergo degradation and it causes enormous security and profitable fatalities in developed and developing nations predominantly in manufacturing sectors [7,8]. According to a survey conducted by NACE (National Association of Corrosion Engineers) in 2002 it was anticipated that corrosion is one of the most challenging and damaging problems in Japan,

[1]Sweden, Australia, Germany, China, Kuwait, United states and India. In the study it was estimated that because of the corrosion GDP (Gross Domestic Productivity) losses in these countries is around 1−5%1-5 \%. In the same study, in 1998, corrosion results into economic loss of around US 276276\276 276 billion only in United State that constituted around 3.1%3.1 \% of the total U.S. GDP [9-11]. The economic loss of U.S. because of corrosion increased to US 2.22.2\2.2 2.2 trillion in the year 2011. Not only in U.S. corrosion causes enormous safety and economic losses in Asian countries including India. According to an estimation of 1st Global Corrosion Summit (apprehended in New Delhi, India in 2011) corrosion results around US 4545\45 45 billion (Rs. 2 lack crores). As per the most recent analysis of NACE, global cost of corrosion is about US 2.52.5\2.5 2.5 trillion which is equivalent to 3.4%3.4 \% of the global GDP. Recent, annual costs of corrosion in South Africa and India are US $ 9.6 billion (R130-billion) and US 100100\100 100-billion, respectively [9-11]. Recent costs of corrosion in China, Japan and Australia are US 310310\310 310 billion (2127.8 billion RMB; 3.34% GDP), US 9.29.2\9.2 9.2 billion ( 2.5 trillion Yen; 2.0% GDP) and US 3232\32 32 billion, respectively. More so, the cost of corrosion in most of the developed and developing countries is expected to be increased because of the increase in science, technology and industrialization causes an increase in the consumption of the metallic materials for versatile applications [9-11]. Along with monitory losses several accidents are also reported worldwide that

1. a{ }^{a} Corresponding authors.
   E-mail addresses: drjeenataslam@outlook.com (J. Aslam), chandraverma.rs. apc@tibhu.ac.in (C. Verma). ↩︎

Fig. 1. Change in corrosion ratewith Hammett constantfor different series of corrosion inhibitors with different substituents[46, 47]. Observation showed that in the presence of electron donor substituents corrosion rate is lower than that of electron withdrawing substituents.
are happened because of the corrosion failure. In view of the enormous economic and safety losses, several attempts have been made to minimize the cost of corrosion. The cost of corrosion can be minimize up to 15−35%15-35 \% using previously established methods of corrosion mitigation [12].

1.2. Driving force for corrosion

Most of the metals (except noble metals: silver, gold and platinum) are thermodynamically unstable and readily undergo chemical or electrochemical reaction with the component of environment and corrode to form unwanted corrosion products such as rusts and scales [13-15]. Obviously, pure metals are associated with high internal energy (U) that acquires during industrial cleaning of the metallic ores in metallurgy. Therefore, pure metals have ability to react spontaneously with constituents and form corrosion products. A metal species associated with high internal energy (U) and more
negative value of ΔG∘\Delta G^{\circ} is connected with higher corrosion rate as compared to its counterpart and vice versa [16]. Nevertheless, rate of corrosive dissolution is greatly depends upon the nature and aggressiveness of the surrounding environment [9,17]. Humidity, presence of salts, temperature, surface condition, availability of oxygen, microbial and non-microbial pollutants etc. are some major parameters that greatly affect the corrosion rate [18-20]. Apart from that, corrosion products (scales and rusts) accumulated over the metallic surface plays an important role while protecting and de-protecting nature. Generally, nature of corrosion products is measured by Pilling-Bedworth ratio that is defined the same as Md/nmD\mathrm{Md} / \mathrm{nmD} [21,22]. In Pilling-Bedworth equation, dd and mm are the density and molecular mass of the metal under investigation, respectively, MM and DD represent the molecular weight and density of corrosion product (scales and rusts), and nn represents the number of metallic atoms in the molecular formula of corrosion products e.g. in Al2O3,Fe3O4\mathrm{Al}_{2} \mathrm{O}_{3}, \mathrm{Fe}_{3} \mathrm{O}_{4} and Fe2O3,n\mathrm{Fe}_{2} \mathrm{O}_{3}, n is equal to 2,3 and 2 , respectively [21,22]. There are two
Please cite this article as: A. Mishra et al., Imidazoles as highly effective heterocyclic corrosion inhibitors for metals and alloys in aqueous electrolytes: A review, Journal of the Taiwan Institute of Chemical Engineers (2020), https://doi.org/10.1016/j.jtice.2020.08.054

possibilities depending upon the value of Pilling-Bedworth ratio namely, Md/nmD<1M d / n m D<1 and Md/nmD>1M d / n m D>1. In later case, corrosion products are expected to be corrosive or non-protective because of the presence of pores and cracks whereas in former case, corrosion product is expected to be protective or non-corrosive as they (corrosion product) do not contain any cracks or pores [21,22].

1.3. Corrosion inhibition: Heterocycles as inhibitors

In view of huge safely, fatality and economic losses, several methods such as alloying and de-alloying, annealing, dehumidification, galvanization, passivation, and coating etc. are developed and consumed depending upon the nature of electrode (metal) and electrolyte [23-28]. Corrosion inhibitors can be broadly categorized as inorganic and organic type. Inorganic inhibitors may be further classified as anodic and cathodic type depending upon their major influence on the either types of reactions. Some of the major inorganic corrosion inhibitors are chromates, phosphates, nitrates; molybdate etc. on the other hands, organic inhibitors may be classified as physical, chemical or mixed (physiochemical) types. Chemical medicines (drugs), carbohydrates, amino acids, polymers, oligomers, macromolecules, heterocyclic compounds etc. represent the common representatives of organic corrosion inhibitors. Among the various methods of corrosion monitoring, use of synthetic organic compounds is one of the most profitable, ease and effective methods [29-32].

Because of their task specific, high efficiency, ease of synthesis and application and cost-effectivity, heterocycles are being used as most frequently during different industrial cleaning processes including oil-well acidification, acid cleaning, descaling and acid pickling processes [29-31]. Literature revision established that most of the successful and financially viable inhibitors are heterocycles that are connected with functional groups including −OH,−NH2,−OCH3-\mathrm{OH},-\mathrm{NH}_{2},-\mathrm{OCH}_{3}, −NHMe,−NMe2,−CN,−CONH2,−COOC2H5,−O−,−NO2-\mathrm{NHMe},-\mathrm{NMe}_{2},-\mathrm{CN},-\mathrm{CONH}_{2},-\mathrm{COOC}_{2} \mathrm{H}_{5},-\mathrm{O}-,-\mathrm{NO}_{2}, etc. and extensive conjugation in the form of non-bonding and π\pi-electrons of the hetero- and homo-atoms such as >C=O,>C=N−,>C=S,−C≡N>\mathrm{C}=\mathrm{O},>\mathrm{C}=\mathrm{N}-,>\mathrm{C}=\mathrm{S},-\mathrm{C} \equiv \mathrm{N}, −N=O,−N=S,>C=C<,−N=N−-\mathrm{N}=\mathrm{O},-\mathrm{N}=\mathrm{S},>\mathrm{C}=\mathrm{C}<,-\mathrm{N}=\mathrm{N}-, and −C≡C−-\mathrm{C} \equiv \mathrm{C}- etc. that act as adsorption (electron rich) center during interaction with the metallic surface [29-31]. The adsorption (of heterocycles) lying on metallic outer surface form a shielding surface film which segregate the metal and shelter from corrosion [30,31,33]. There are two mechanisms, namely chemisorption and physisorption are reported for heterocycles adsorption over the metallic surface [34,35]. Most of the heterocycles adsorb by means of their chemisorption mechanism which is initially originated and proceed through electrostatic or physisorption mechanism. Adsorption ability of the heterocycles depends upon numerous factors including charges on the metallic surface, temperature, aggressiveness of the electrolyte and immersion time [36-38]. Obviously, adsorption tendency of heterocycles decreases on increasing the electrolyte temperature which is attribute to the increase in kinetic energy that adversely affect the intermolecular attraction between adsorbate and adsorbent as well as high temperature catalyzed fragmentation and /or rearrangement [39,40]. Protection efficiency of the heterocycles also decreases with increasing the electrolyte aggressiveness and immersion time [39-41]. Generally, the nature and magnitude of adsorption is assigned by the standard Gibb’s free energy (ΔG∘)\left(\Delta G^{\circ}\right) as a negative sign of ΔG∘\Delta G^{\circ} indicates the spontaneous nature of metal-heterocycles interactions and converse is true for positive sign of ΔG∘\Delta G^{\circ} [42,43]. More so, the magnitude of ΔG∘\Delta G^{\circ} gives information about the physiochemical nature of metal-heterocycles adsorption. The ΔG∘\Delta G^{\circ} value of −20 kJ mol−1-20 \mathrm{~kJ} \mathrm{~mol}^{-1} or more positive indicate the physisorption mechanism of heterocycles adsorption and ΔG∘\Delta G^{\circ} value of −40 kJ mol−1-40 \mathrm{~kJ} \mathrm{~mol}^{-1} more negative confirms the chemisorption mechanism [42,43]. Whereas, ΔG∘\Delta G^{\circ} values in between -20 to −40 kJ mol−1-40 \mathrm{~kJ} \mathrm{~mol}^{-1} is consistent with the physiochemisorption mechanism. There are numerous factors associated with heterocycles that affect
their adsorption on the metallic surface and one of such factors is planarity of heterocyclic ring. A planer molecule having parallel or horizontal alignment will wrap bigger outside region and should behaves the same as better adsorbate as compared to the molecule having perpendicular or vertical orientation [44,45]. Protectiveness of the heterocyclic inhibitors is also fundamentally depends their electronic factors (nature of substituents) as electron releasing substituents (ERSs) enhance the effectiveness of the adsorption by increasing the electron density over the active sites and electron withdrawing substituents (EWSs) can decrease the effectiveness of metal-heterocycles interaction by decreasing the electron density over the active sites. The electronic effect on protection efficiency of heterocycles can be best explained using Hammett equation presented below [11,46-48]:
log⁡KBKH=ρσ\log \frac{K_{B}}{K_{H}}=\rho \sigma
log⁡1−η%=1−η%=H=ρσ\log \frac{1-\eta \%=}{1-\eta \%=H}=\rho \sigma
log⁡η%=η%=H=log⁡CcHCcg=ρσ−log⁡θgθH\log \frac{\eta \%=}{\eta \%=H}=\log \frac{C_{c H}}{C_{c g}}=\rho \sigma-\log \frac{\theta_{g}}{\theta_{H}}
In above Hammett equations, all physical symbols have their typical meaning as described in our previous reports. One of the most significant indices in above equations is Hammett substituent constant (σ)(\sigma) that reflect the total electron density over the active sites. Generally, ERSs such as −OH,−NH2,−OMe,−NHMe,−NMe2-\mathrm{OH},-\mathrm{NH}_{2},-\mathrm{OMe},-\mathrm{NHMe},-\mathrm{NMe}_{2}, etc. are associated with negative sign of σ\sigma and EWSs such as −CN,−COOH,−NO2,−-\mathrm{CN},-\mathrm{COOH},-\mathrm{NO}_{2},- COOC2H5\mathrm{COOC}_{2} \mathrm{H}_{5} etc. are connected with positive sign of σ\sigma [11]. Consequently, ERSs increase electron density over the active sites and enhance the inhibition effectiveness in their incidence and contrary is true for EWSs. Nevertheless, exception is also present in a number of cases. In these cases, addition of EWSs causes significant increase in the molecular size that overcomes the substituent effect. Fig. 1 represents the effect of some common substituents on anticorrosive effect of inhibitors.

Table 1 shows the values of σ\sigma for different substituents. It is important to mention that substituent effect is not significant due to appreciable steric effect at ortho-position. Apart from electron

Table 1
Values of Hammett substituent constant for different electron withdrawing and releasing substituents [11,46,47][11,46,47].

Table 3
Synthetic schemes for some common imidazole derivatives used as corrosion inhibitors.

Fig. 2. HOMO and LUMO frontier molecular orbital picture of (a) AAC-1 ( −NO2-\mathrm{NO}_{2} ), (b) AAC-2 (-H) and © AAC-3 (-OH). Electron withdrawing −NO2-\mathrm{NO}_{2} decreases HOMO contribution whereas electron donating -OH increases HOMO contribution[133].

Molecular dynamic (MD) and Monte Carlo (MC) simulations are two other useful computational simulation techniques that provide information about the orientation of corrosion inhibitors on the metallic surface.

Measurement of the orientation of corrosion inhibitors on the metallic surface provide vital information about the relative effectivity of different corrosion inhibitors as a compound with flat or horizontal orientation will be better corrosion inhibitor that of the compound with vertical orientation. The effect of substituents on adsorption behavior of organic compounds can also be determined using these techniques. Generally, in the presence of electron withdrawing substituents inhibitor molecules acquire vertical orientation and in the presence of electron donating substituents they gain flat orientation. While studying the anticorrosive properties of 5-Arylpyr-imido- [4,5−b][4,5-b] quinoline-diones (APQDs) for mild steel in acidic medium we observed similar finding (Fig. 4).

2. Imidazole based corrosion inhibitors: A literature survey

Imidazole is an organic compound (C3H4 N2)\left(\mathrm{C}_{3} \mathrm{H}_{4} \mathrm{~N}_{2}\right) containing two nitrogen atoms in the five membered heterocyclic rings at position 1 and 3. Imidazole is an important constituent of several biologically and industrially useful heterocyclic compounds. Imidazole possesses excellent biological activities including analgesic, anxiolytic, antimycobacterial, anti-diabetic, anti-protozoa, anti-cancer, anti-tubercular, anti-HIV, antibacterial, antifungal and anti-inflammatory
activities [134-136]. This ring is also building blocks of histamine and histidine. A German chemist A. R. Hantzsch coined the term imidazole in 1887 [137]. Imidazole is highly water miscible and it exists in two equivalent tautomeric forms. Because of the 6π6 \pi electron system, imidazole is classified as aromatic compound having dipole moment of 3.67 D [138]. Because of its high solubility, highly polar nature (due to resonance; Fig. 5) and highly nucleophilic behavior, imidazole can interacts strongly with metallic surface and behaves as efficient corrosion inhibitor in aqueous electrolytes.

Literature survey showed that several articles dealing with anticorrosion effect of imidazole and its derivatives [139-141]. Anticorrosion of imidazole derivatives with and without different substituents is reported by our research team elsewhere [11]. Present review article describes recent advancement (2010-2020) in the anticorrosion effect of imidazole and its derivatives on metals in different electrolytes. Effect of -Me and -SH substituents on the anticorrosion effect of benzimidazole for MS/1 M HCl system were reported by several authors using different techniques [77-79]. Results showed that inhibition power of different derivatives followed the order: −SH>−Me>−H-\mathrm{SH}>-\mathrm{Me}>-\mathrm{H} (non-substituted). Experimental studies also support the outcomes of the studies. All substituted benzimidazole acted as mixed and interface type corrosion inhibitors. Their nature of adsorption was best explained by Langmuir adsorption isotherm model. Ghanbari and coworkers [142] corrosion inhibition effect of three imidazole derivatives for mild steel/1 M H3PO4 system using electrochemical techniques. EIS and DC polarization results showed that inhibition effectiveness of the imidazole derivatives obeyed the order: 2ABI ==

[1]

1. Please cite this article as: A. Mishra et al., Imidazoles as highly effective heterocyclic corrosion inhibitors for metals and alloys in aqueous electrolytes: A review. Journal of the Taiwan Institute of Chemical Engineers (2020), https://doi.org/10.1016/j.jtice.2020.08.034 ↩︎

Fig. 3. HOMO and LUMO frontier molecular orbital picture of (a) APQD-1 ( −NO2-\mathrm{NO}_{2} ), (b) APQD-2 (-H),© APQD-3 (-OH) and (d) APQD-2 (2-OH). Electron withdrawing −NO2-\mathrm{NO}_{2} decreases HOMO contribution whereas electron donating -OH increases HOMO contribution[93].
BI>2MBI\mathrm{BI}>2 \mathrm{MBI}. EIS studies showed all studied compounds behaved as interface type corrosion inhibitors and 2ABI, BI and 2MBI showed highest inhibition efficiencies of 69.5%,62.1%69.5 \%, 62.1 \% and 51.3%51.3 \%, respectively. DC polarization study showed that2ABI, BI and 2MBI behaved as mixed type corrosion inhibitors. The maximum inhibition efficiencies of the tested imidazole derivatives were observed at ×10−4M\times 10^{-4} \mathrm{M} concentration. Benaliet al. [143] demonstrated the inhibition effect of 2-mercapto-1-methylimidazole (MMI) (another -SH substituted imidazole) for mild steel corrosion in 5%5 \% HCl solution using chemical and electrochemical methods. The MMI behaved as mixed type corrosion inhibitor and it showed 91.18%91.18 \% efficiency at 5×10−3M5 \times 10^{-3} \mathrm{M} concentration. Formation of single semicircle in the Nyquist curves suggested that mild steel corrosion in 5%HCl5 \% \mathrm{HCl} solution involves single charge transfer mechanism. Polarization results showed that MMI acted as cathodic type corrosion inhibitor and its adsorption mechanism obeyed the Langmuir isotherm model. Inhibition effect of different benzimidazole and its derivatives having different other substituents are also investigated extensively. Table 4 represents the chemical

Fig. 4. Orientation of (a) APQD-1 ( −NO2-\mathrm{NO}_{2} ), (b) APQD-2 (-H),© APQD-3 (-OH) and (d) APQD-2 (2-OH) on mild steel surface. In the presence of electron withdrawing −NO2-\mathrm{NO}_{2} (APQD-1) inhibitor acquires vertical orientation and in the presence of one or two -OH substituents inhibitors (APQD-3 and APQD-4) acquire the flat orientation [93].
structures and abbreviations of imidazole and its derivatives that are applied as corrosion inhibitors for different metals in various electrolytes. Careful observation of the informations presented in Table 4 showed that imidazole and its derivatives behaved as efficient corrosion inhibitors. It is also cleared that most of the imidazole based corrosion inhibitors are evaluated as for MS (mild steel)/ CS (carbon steel) HCl system therefore inhibition effect of imidazole based compounds should also be tested for other metals and alloys in different electrolytes. Mostly, in substituted imidazole compounds imidazole ring interact with the metallic surface as it is highly electron rich and act as adsorption centers. Presence of substituents plays an important role in delocalization of the electron densities. Mostly, electron donating substituents enhances protection efficiency and converse is true for electron withdrawing substituents.

Some of the authors also reported the anticorrosive effect of imidazole based compounds is also investigated in NaCl media. Otmacic and Stupnisek-Lisac [173] reported the anticorrosive effect of nontoxic imidazole derivatives for copper corrosion in NaCl solution using electrochemical and weight loss methods. Results showed that the tested imidazole derivatives behaved as reasonably good corrosion inhibitors for copper in neutral solution. Polarization study revealed that tested imidazole based compounds behaved as mixed-type corrosion inhibitors. In other study [174] corrosion inhibition property of imidazole based compounds is investigated for iron in 3.5%NaCl3.5 \% \mathrm{NaCl} solution using theoretical and experimental methods. Weight loss study showed that tested

Fig. 5. Resonating structures of imidazole.

Table 4
Chemical structure of some common imidazole derivatives evaluated as corrosion inhibitors for different metals in various electrolytes.

(continued)

Please cite this article as: A. Mishra et al., Imidazoles as highly effective heterocyclic corrosion inhibitors for metals and alloys in aqueous electrolytes: A review. Journal of the Taiwan Institute of Chemical Engineers (2020), https://doi.org/10.1016/j.jtice.2020.08.034

Table 4 (Continued)

Table 4 (Continued)

compounds inhibit iron corrosion by adsorbing on the metallic surface and their adsorption also obeyed the Langmuir isotherm model. Polarization studies showed that investigated imidazole based compounds acted as anodic type corrosion inhibitors. Quantum chemical calculations provide good correlation to the experimental results and showed that tested imidazole derivatives interact with metallic surface using donor-acceptor interactions. The corrosion inhibition effect of imidazole based compounds is also reported in other reports [175-178]

3. Summary and outlooks

Present review article features the collection of recent advancement in the imidazole based compounds as corrosion inhibitors for metals in different electrolytes. Because of their high dipole moment, imidazole and its derivatives are effectively evaluated as corrosion inhibitor especially for mild steel and carbon steel in acidic media. Therefore, the implementation of these compounds for other metals and electrolytes should be tested. Most of the imidazole based compounds behaved as mixed and interface type corrosion inhibitors and their adsorption mechanism obeyed the Langmuir adsorption isotherm model. Inhibition effects of imidazole based organic compounds are tested using different chemical, surface, electrochemical and computational techniques. Imidazole derivatives possess excellent industrial and biological activities along with the anticorrosion effect. Although, a plenty of literature are available on the anticorrosive effect of imidazole based organic compounds however present review article describes only few major collections that are published recently. Mostly, in substituted imidazole compounds imidazole ring interact with the metallic surface as it is highly electron rich and act as adsorption centers. Inhibition effects of imidazole derivatives are mostly evaluated using experimental methods. Therefore, their inhibition effect measurement using computation techniques that are green and cost-effective should be explored. The imidazole derivatives are mostly used as aqueous phase corrosion inhibitors therefore their anticorrosive effect in coatings should also be evaluated extensively as they have strong ability to interact with metallic substrate. Present article also describes different aspects of corrosion such as corrosion and its adverse effect, mechanism of corrosion with and without organic inhibitors, mechanism of corrosion inhibitors and green corrosion inhibitors. Therefore, outcomes of this study will help in understanding the mechanism of corrosion and corrosion inhibition. It was observed that most of the effective imidazole based organic compounds are those that contain electron donating substituents in their molecular structures. This understanding will help in designing of effective imidazole and other heterocyclic corrosion inhibitors.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared toinfluence the work reported in this paper.

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