Startseite Efforts made in enhancing corrosion inhibition potential of organic compounds: recent developments and future direction
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Efforts made in enhancing corrosion inhibition potential of organic compounds: recent developments and future direction

  • Chandrabhan Verma

    Chandrabhan Verma works at the Interdisciplinary Research Center for Advanced Materials, King Fahd University of Petroleum and Minerals, Saudi Arabia. His research is mainly focused on the synthesis and designing of environmental friendly corrosion inhibitors useful for several industrial applications. Dr. Verma is the author of several research and review articles published in various peer-reviewed international journals; currently, he is a book editor for several academic publishers. Dr. Verma has more than 6100 citations to his credit with an H-index of 43 and i-10 index of 106. He received several national and international awards for his academic achievements. He is a member of the American Chemical Society.

     EMAIL logo     und Mumtaz A. Quraishi

    Prof. Mumtaz A. Quraishi is a chair professor at the Interdisciplinary Center for Research in Advanced Materials King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabia. He obtained a Ph.D. in synthetic organic chemistry from Kurukshetra University in 1986 and a D.Sc. in corrosion inhibition of industrial metals and alloys from Aligarh Muslim University Aligarh in 2004. Before joining KFUPM, he was an institute professor at IIT BHU Varanasi, India where he also served as head (chairman) of Department of Chemistry. He has a teaching experience of more than 35 years. Prof. Quraishi received several national and international awards. He is an associate editor of Current Material Science and an editorial board member of more than 30 international journals. Prof. Quraishi is a fellow of the Royal Society of Chemistry UK and a member of the American Chemical Society. He has published more than 400 papers in peer reviewed journals having an H-index of 86 and more than 23000 citations to his credit; his global status is one in terms of H-index in the field of corrosion inhibitors. He is a book co-author of Heterocyclic organic corrosion inhibitors: principles and applications (2020).
Veröffentlicht/Copyright: 24. März 2022
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Abstract

Numerous attempts have been made to enhance the corrosion inhibition (%IE) of organic compounds. Each method has its own advantages and drawbacks. One of the important methods of enhancing %IE of organic compounds is their chemical functionalization which involves addition of polar functional groups, which enhances %IE due to combined effect of enhanced solubility and the number of adsorption centers. A large number of organic compounds derivatized through covalent and noncovalent functionalization are extensively used as corrosion inhibitors. It is well documented that properly functionalized organic compounds show higher %IE than the parent compounds. Other important factors that usually affect corrosion inhibition performance are transportability rate, immersion time, planarity, strength of electrolyte, and synergism. In the present article effect of these factors has been discussed. A proper understanding of these factors will help corrosion scientists and engineers in designing and synthesis (formulation) of effective corrosion inhibitors for industrial scale applications.

Keywords: adsorption centers; organic corrosion inhibitors; planarity and chemical functionalization; synergism; transportability rate

1 Introduction

Organic compounds, especially heterocyclic compounds, serve as the most effective and economic aqueous phase corrosion inhibitors (Quraishi et al. 2020; Verma et al. 2021a). They inhibit corrosion by forming corrosion inhibitive film by their adsorption at the interface of metal and environment (Antonijevic and Petrovic 2008; Dariva and Galio 2014). Based on their percentage inhibition efficiency (%IE), corrosion inhibitors can be classified as poor, (<40%) moderate (41–70%), and excellent (>71%) type, respectively. Corrosion inhibition potential of these compounds depends upon various factors including their electronic structures and nature of metal/electrolyte system. Obviously, organic compounds with polar substituents such as –CN, –COOH, –OH, –OR, –CONH2, >C=S/O, –NO2, –O–, –COOC2H5, –SH, –CHO, –N=N–, >C=N–, and –N=O etc. interact strongly with metal surfaces and form coordination bonding (Goyal et al. 2018; Sanyal 1981). The adsorption of these compounds results into the formation of hydrophobic layer that replace pre-adsorbed electrolyte/water molecules. More so, due to their hydrophobic nature, attack of the aqueous electrolyte on the metallic surface is greatly hindered. It is important to report that adsorption of organic compounds is highly complex phenomenon and it depends upon various factors (Abd El Haleem et al. 2013). One of the major factors is the nature and number of polar substituents (electronic effect). Literature study indicates that organic compounds containing polar functional groups act as better corrosion inhibitors as compared to the compounds without such substituents (Verma et al. 2018). Numerous studies show that electron donating functional groups such as –OH, –NH2, –OCH3, –NHCH3, and–N(CH3)2 etc. produce more pronounced effect as compared to the electron withdrawing functional groups such as –CN, –COOH, and –NO2 etc (Issaadi et al. 2011; Verma et al. 2020a).

Noticeably, apart from using organic compounds, there are numerous other methods of corrosion inhibition. These methods have their own advantages and disadvantages. One of the most effective and traditionally reported methods of corrosion mitigation is the use of inorganic compounds their current employment is restricted because of the toxicity (Raja et al. 2016; Samiee et al. 2019). Thereafter, these toxic alternatives have been replaced by relatedly more ecofriendly alternatives of natural and biological origin. These includes biopolymers, biomolecules, plat extracts etc. However, the commercial extraction and use of these compounds are extremely hindered because of their expensive isolation (Verma et al. 2021b). More so, most of the biomolecules are temperature sensitive and cannot be used as corrosion inhibitors at high temperature. Further, some of the biomolecules, especially biopolymers are not readily soluble in the polar electrolytes that limits their application further (Aleti et al. 2011; Pathan et al. 2020). Lastly, some of these bio-based alternatives, particularly plant extracts, are effective only at high concentration (Alrefaee et al. 2021). Therefore, the use of synthetic organic compounds having a desirable combination of hydrophobicity and hydrophilicity is established as one of the most significant methods of corrosion retardation in aqueous phase (Hu et al. 2010; Verma et al. 2022).

Organic compounds may get adsorbed using physisorption or electrostatic force of attraction which resulted through electrostatic force of attraction between charged metallic surface and charged inhibitor molecules (Bothi Raja and Sethuraman 2009; Elayyachy et al. 2006). This type of adsorption is associated with the standard Gibb’s free energy (ΔG°) value of −20 kJ mol−1 or more positive. Another mode of adsorption is chemisorption, which is due to charge/electrons sharing between metal surfaces and organic molecules (Figure 1) (Rabizadeh and Asl 2019; Saleh et al. 2019). Noticeably, functional groups have major influence of chemisorption. The chemisorption is consistent with the Gibb’s free energy (ΔG°) value of −40 kJ mol−1 or more negative. Literature investigation shows that adsorption of most of the organic molecules follows physiochemisorption i.e. mixed-mode of adsorption for which value of Gibb’s free energy (ΔG°) ranges in between −20 kJ mol−1 and –40 kJ mol−1 (Alrefaee 2021; Gupta et al. 2017; Kumar and Yadav 2021).There are numerous factors that affect adsorption, solubility and corrosion inhibition potential of organic corrosion inhibitors. In the present report, some major factors are described. A significant knowledge of this report will help corrosion scientists and engineers in the synthesis and designing of effective corrosion inhibitors. This can be regarded as an environmentally benign approach as chemical properties and reactivities including their corrosion inhibition potential of an organic molecule can be assessed without its synthesis and experimental trails. This will also save time and avoid unnecessary use of expensive and toxic starting materials, solvents, and catalysts. To enhance solubility and corrosion inhibition potential of organic compounds following structural features are desirable.

Figure 1: Diagrammatic illustrations of chemisorption and physisorption of (a) 1-hexadecylpyridinium chloride and bromide (Saleh et al. 2019) and (b) casein on mild steel surface in acidic solution (Rabizadeh and Asl 2019). Reproduced with kind permission @copyright Elsevier.
Figure 1:

Diagrammatic illustrations of chemisorption and physisorption of (a) 1-hexadecylpyridinium chloride and bromide (Saleh et al. 2019) and (b) casein on mild steel surface in acidic solution (Rabizadeh and Asl 2019). Reproduced with kind permission @copyright Elsevier.

1.1 Chemical functionalization: effect on solubility and adsorption tendency

Chemical functionalization which is also called as derivatization is most frequently used to add hydrophilic or hydrophobic substituents in the chemical structure of parent molecules (Kittlesen et al. 1984; Niemczyk-Soczynska et al. 2020; Vedala et al. 2006). It is important to report that a suitable arrangement of hydrophobic and hydrophilic character is essential for reasonable corrosion inhibition effectiveness (Villamil et al. 1999; Yayoglu et al. 2021). Obviously, addition of polar functional group(s) enhances hydrophilicity and the addition of nonpolar/hydrocarbon chain(s) generally used to enhance hydrophobicity. Literature study reveals that benzene, pyridine, imidazole, quinoline, isoquinoline, indole, triazole, pyrazole etc. based organic compounds are widely used as corrosion inhibitors for versatile metal/electrolyte combinations (Mishra et al. 2020; Quadri et al. 2021; Verma et al. 2020c; Verma and Quraishi 2021). Addition of polar functional group(s), irrespective of electron donating or withdrawing, increases their corrosion inhibition potential. This is attributed due to the increase in the number of electron rich active sites in such small parent molecules. The polar functional groups directly participate in the charge sharing. The frontier molecular electron distribution of benzene and its various derivatives to demonstrate the effect of the substituents is presented in Figure 2. Careful inspection shows that HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), that is responsible for charge donation and acceptance, respectively, are distributed over the entire segments of the molecules including the substituents. This observation suggests that substituents greatly participate in interaction with the metallic surface.

Figure 2: Optimized, HOMO and LUMO frontier molecular orbital pictures of benzene, methyl benzene, phenol, methoxy benzene, thiophene, aniline, cyanobenzene, nitrobenzene, and benzoic acid (reproduction permission not required).
Figure 2:

Optimized, HOMO and LUMO frontier molecular orbital pictures of benzene, methyl benzene, phenol, methoxy benzene, thiophene, aniline, cyanobenzene, nitrobenzene, and benzoic acid (reproduction permission not required).

However, it is also clear that in nitrobenzene and carboxyl benzene (benzoic acid) contribution of contribution of HOMO i.e. electron donation is highly reduced. This is might be attributed to the very strong electron withdrawing effect of –NO2 and –COOH substituents. Therefore, in small and medium size of organic molecules to be used as corrosion inhibitors, addition of highly electron withdrawing substituents such as –CN, –COOH, and –NO2, that can decrease HOMO and/or LUMO contribution, should be avoided. Nevertheless, in the complex molecules containing three or more aromatic rings, macromolecules, oligomers and polymers addition of electron donating and withdrawing substituents increase their corrosion inhibition potential. Generally, such complex molecules experience solubility problems in the aqueous polar electrolytes therefore presence of both (donating and withdrawing) type of polar substituents enhances their solubility as well as corrosion inhibition potential (Ata et al. 2014; Naqvi et al. 2020).

1.1.1 Covalent functionalization

Covalent functionalization is most frequently used to add substituents in the molecular structure of organic corrosion inhibitors. The covalent functionalization can be achieved either through one step multicomponent reactions (MCRs) or by multistep reactions (MSRs) (Khan and Nishina 2021; Subrahmanyam et al. 2009). Obviously, for covalent functionalization, the use of MCRs would be preferred over the MSRs because of their association with various advantages including being environmentally sustainable. MCRs, especially in association with microwave (MW) and ultrasound (US) irradiations have developed as one of the greenest protocols for the synthesis of organic compounds having different industrial and biological applications (Cravotto and Cintas 2007; de la Hoz and Loupy 2013; Zakeri et al. 2014). Apparently, the MCRs are connected with several benefits such as ease of handling, high atom economy i.e. high synthetic yield, high selectivity, shorter reaction time, lower number of works-up and purification steps and many more. Organic compounds derived from MCRs and MSRs are widely used as corrosion inhibitors for different metal/electrolyte combination.

Singh et al. extensively reported the corrosion inhibition potential of organic corrosion inhibitors having electron donating –CH3,–OCH3, and –N(CH3)2 functional groups (Singh et al. 2016a, b, c). The organic compounds substituted by these electron donating groups exhibit relatively higher corrosion inhibition potential as compared to the nonsubstituented parent compound i.e. their inhibition efficiencies follow the order: –N(CH3)2>–OCH3> –CH3> –H. This order of corrosion inhibition effectiveness can be explained on the basis of electron donating ability of these functional groups. Electrochemical investigations suggest that these compounds become effective by adsorbing on the metallic surface. This type of adsorption results in the decrease in corrosion current density and corresponding increase in the charge transfer resistance. Generally, presence of these compounds increases the diameter of Nyquist curves and in most of the cases this increase is consistent with the concentration of organic corrosion inhibitors. For an example, inhibition effect of four thiopyrimidine derivatives (TPs) having –H, –CH3, –OCH3, and –N(CH3)2 follows the order: –H (TP-1) < –CH3 (TP-2) < –OCH3 (TP-3) < –N(CH3)2(TP-4) (Singh et al. 2016b). Potentiodynamic polarization and Nyquist curves for mild steel corrosion in 1 M HCl are shown in Figures 3 and 4.

Figure 3: Potentiodynamic polarization curves for mild steel corrosion in 1 M HCl with and without TPs (Singh et al. 2016b). Reproduced with kind permission @copyright Elsevier.
Figure 3:

Potentiodynamic polarization curves for mild steel corrosion in 1 M HCl with and without TPs (Singh et al. 2016b). Reproduced with kind permission @copyright Elsevier.

Figure 4: Nyquist curves for mild steel corrosion in 1 M HCl with and without TPs (Singh et al. 2016b). Reproduced with kind permission @copyright Elsevier.
Figure 4:

Nyquist curves for mild steel corrosion in 1 M HCl with and without TPs (Singh et al. 2016b). Reproduced with kind permission @copyright Elsevier.

Authors observed that these compounds acquire flat or horizontal orientation on the metallic surface. Thereby they cover large metallic surface area and provide excellent surface protection. DFT based analyses show that these electron donating substituents are greatly participate in the charge sharing. For example, while investigating the corrosion inhibition potential of authors observed that naphthyridine derivatives (Ns) acquire flat orientation and substituents greatly participated in the charge sharing (Figure 5) (Singh et al. 2016a).

Figure 5: Frontier molecular orbital pictures (left side) and Monte Carlo (right) based orientation of N-1 to N-3 on mild steel surface (Singh et al. 2016a). Reproduced with kind permission @copyright American Chemical Society.
Figure 5:

Frontier molecular orbital pictures (left side) and Monte Carlo (right) based orientation of N-1 to N-3 on mild steel surface (Singh et al. 2016a). Reproduced with kind permission @copyright American Chemical Society.

Compounds containing electron withdrawing substituents such as –NO2, –COOH, and–CN have also been used as corrosion inhibitors for different metal/electrolyte systems. Our research team extensively reported that –NO2 group extremely decreases the HOMO and/or LUMO contribution and forces inhibitor molecules to gain vertical orientation (Figure 6) (Verma et al. 2015; Verma et al. 2016a, b, c). Based on these discussions it can be concluded that though inhibition potential of both electron donating and withdrawing substituents are reported extensively however electron donating functional groups exert more prominent effect that electron withdrawing substituents.

Figure 6: Effect of substituents on HOMO/LUMO electron distribution and orientation of organic corrosion inhibitors on metallic surface. (a) HOMO and LUMO of –NO2 and –OH substituted 2-amino-4-arylquinoline-3-carbonitriles (AACs) (Verma et al. 2015). (b) HOMO and LUMO of –NO2 and –OH substituted 2, 4-diamino-5-(phenylthio)-5H-chromeno [2, 3-b] pyridine-3-carbonitriles (DHPCs) (Verma et al. 2016c). (c) HOMO and LUMO of –NO2 and –OH substituted 5-arylpyrimido-[4, 5-b] quinoline-diones (APQDs) (Verma et al. 2016a). (d) MC simulation based orientations of –NO2 and –OH substituted pyrimidine-fused heterocycles (CARBs) (Verma et al. 2016b) (reproduction permission not required).
Figure 6:

Effect of substituents on HOMO/LUMO electron distribution and orientation of organic corrosion inhibitors on metallic surface. (a) HOMO and LUMO of –NO2 and –OH substituted 2-amino-4-arylquinoline-3-carbonitriles (AACs) (Verma et al. 2015). (b) HOMO and LUMO of –NO2 and –OH substituted 2, 4-diamino-5-(phenylthio)-5H-chromeno [2, 3-b] pyridine-3-carbonitriles (DHPCs) (Verma et al. 2016c). (c) HOMO and LUMO of –NO2 and –OH substituted 5-arylpyrimido-[4, 5-b] quinoline-diones (APQDs) (Verma et al. 2016a). (d) MC simulation based orientations of –NO2 and –OH substituted pyrimidine-fused heterocycles (CARBs) (Verma et al. 2016b) (reproduction permission not required).

1.1.2 Noncovalent functionalization

Organic compounds, especially macromolecules and polymers, functionalized with metal and metal oxides i.e. in their composite form are widely employed as corrosion inhibitors for different electrolyte/metal systems (Chen et al. 2017; Cui et al. 2018; Luo et al. 2021). The composites are mainly used in coatings however some of the composites have also been used as anticorrosive materials in solution phase (Shetty and Shetty 2017; Solomon and Umoren 2015). Literature study reveals that corrosion inhibition potential of organic compounds, macromolecules, oligomers, and polymers greatly enhanced in the presence of metal/metal oxide. In anticorrosive coatings, the metals and metal oxides fill and repair the rapture surface coating structures and thereby avoid the penetration of corrosive species such aqueous electrolyte, salt solution, corrosive gases, and moisture (Verma et al. 2020b). Therefore, presence of the metals and metal oxides in such coating formulations enhances their durability along with the anticorrosive effectiveness.

Our research group widely reported the corrosion inhibition potential of epoxy resins (ERs) in solution as well as coating formulations. The ERs act as effective anticorrosive materials, especially in the coatings condition. Their anticorrosive applications in solution phase are limited because of their limited solubility in the aqueous electrolytes. Nevertheless, recently Dagdag et al. (2020) investigated the corrosion inhibition potential of two epoxy resins namely, tetra glycidyl of ethylene dianiline (TGEDA), and hexaglycidyl Tris (p-ethylene dianiline) phosphoric triamide (HGEDPAT) cured with methylene dianiline (MDA), designated as TGEDA-MDA and HGEDPAT-MDA, respectively. The corrosion inhibition potential of TGEDA-MDA (ER1) and HGEDPAT-MDA (ER2) also evaluated in the presence of 5% zinc i.e. TGEDA-MDA-5%Zn (ER3) and HGEDPAT-MDA-5%Zn (ER4). Preparation of ER1-4 based coatings is presented Figure 7.

Figure 7: Preparation of ER1-4 based anticorrosive coating (Dagdag et al. 2020). Reproduced with kind permission @copyright Elsevier.
Figure 7:

Preparation of ER1-4 based anticorrosive coating (Dagdag et al. 2020). Reproduced with kind permission @copyright Elsevier.

Electrochemical analyses show that inhibition effectiveness of studied formulations followed the order: TGEDA-MDA < HGEDPAT-MDA < TGEDA-MDA-5%Zn < HGEDPAT-MDA-5%Zn. Potentiodynamic polarization study suggests that presence of the TGEDA-MDA and HGEDPAT-MDA causes significant reduction in the values of corrosion current densities and this decrease was more prominent in the presence of TGEDA-MDA-5%Zn and HGEDPAT-MDA-5%Zn. Both anodic and cathodic Tafel curves were greatly affected in the presence of these formulations (Figure 8). Similarly, corrosion inhibition potential of composites of other polymers such as aniline, chitosan, cellulose, polypyrrole, gum arabic, etc. are widely investigated as corrosion. In these composites, metals and metal oxides incorporate in the polymer matrixes without any covalent bonding. Beside the chemical functionalization, corrosion inhibition potential of organic compounds can also be controlled using experimental conditions such as transportability rate, immersion time, electrolyte and temperature etc.

Figure 8: Potentiodynamic polarization curves for copper corrosion in 3% NaCl solution with and without ERs (Dagdag et al. 2020). Reproduced with kind permission @copyright Elsevier.
Figure 8:

Potentiodynamic polarization curves for copper corrosion in 3% NaCl solution with and without ERs (Dagdag et al. 2020). Reproduced with kind permission @copyright Elsevier.

1.2 Transportability rate

During adsorption process, corrosion inhibitors are transported from solution/electrolyte to the metal surface (Berdimurodov et al. 2020). The rate through which a corrosion inhibitor transported on the metallic surface can be termed as transportability rate (TR). Therefore, adsorption rate (AR) is directly proportional to transportability rate i.e. Adsorption rate (AR) ∝ transportability rate (TR). Therefore, for an effective adsorption and corrosion protection, transportability rate should be high. By increasing the transportability rate of corrosion inhibitors, one can improve their corrosion inhibition potential. The transportability rate depends upon numerous factors including temperature, solvent and coordination ability of the corrosion inhibitors. These factors are briefly described below:

1.2.1 Effect of temperature

Temperature has exceptionally noticeable effect on the corrosion inhibition potential of organic corrosion inhibitors. In most of the cases, rise in the temperature causes subsequent increase in corrosion rate i.e. decrease in the inhibition potential (Yin et al. 2009; Zarrouk et al. 2010). This might be attributed to the following facts:

  1. Rise temperature causes increase in the kinetic energy (KE or EK) of inhibitor molecules and the fast-moving molecules are less vulnerable to adsorb on the metal surface.

  2. Rise in the temperature increase metallic surface energy that adversely affects its attraction with the inhibitor molecules.

  3. On increasing temperature, inhibitor molecules may get precipitated due to increase in the intermolecular force of attraction between them.

  4. On increasing temperature, inhibitor molecules may undergo electrolyte catalyzed fragmentation or breaking that can adversely affect the inhibition potential. However, it is also possible that fragments of the molecules enhance inhibition potential of each other through synergism.

  5. On increasing temperature, inhibitor molecules may undergo electrolyte catalyzed rearrangement that can affect their inhibition potential positively or negatively.

  6. At elevated temperature, chemical bonds involved in the coordination with metallic surface may weekend due to increase in the vibrational energy (Evib).

However, this is not always true that rise in temperature causes reduction in the corrosion inhibition potential. Sometime rise in temperature exerts opposite effect as a number of cases reported where increase in the corrosion inhibition potential of organic compounds is derived on elevating the temperature (Ebenso 2003; Touhami et al. 2000). On assuming that bonding between metal surface and inhibitor molecule involves chemical bonding, inhibition efficiency of the molecule is expected to increase in increasing the temperature. On the other hand, if bonding between them is ionic or electrostatic, rise in temperature is expected to decrease the inhibition potential (Figure 9). Therefore, temperature may have positive or negative effect on transportability rate and inhibition potential of organic compounds depending upon their chemical structures and metal/electrolyte system.

Figure 9: Effect of temperature on physisorption and chemisorption of organic corrosion inhibitors.
Figure 9:

Effect of temperature on physisorption and chemisorption of organic corrosion inhibitors.

1.2.2 Nature of electrolyte and corrosion inhibitor

Transportability rate of a corrosion inhibitor apparently depends upon its nature. Obviously, an organic compound having greater number of polar functional groups that can act as adsorption centers would be more potent to adsorb on the metallic surface as compared to the compounds having no or lower number of polar functional group. The compound containing greater number of adsorption centers is expected to behave as stronger ligand as compared to the compound having lesser number of adsorption centers. Therefore, affinity of an organic corrosion inhibitor with the metallic surface therefore its transportability rate can be enhanced by adding polar functional groups as discussed above in the covalent functionalization of corrosion inhibitors. Generally, functional groups are susceptible for solvolysis/hydrolysis in the specific type of electrolyte. Therefore, selection of electrolyte should be carried out in such a way so that functional groups should not by hydrolyzed, fragmented or rearranged.

1.2.3 Solubility of corrosion inhibitors

In aqueous phase corrosion inhibition, solubility of the corrosion inhibitors is one of the primary requirements. Most of the inorganic and small organic molecules containing polar functional groups are soluble in aqueous electrolytes. However, highly complex molecules containing three or more aromatic rings, macromolecules, oligomers, and polymers experience solubility problems in such electrolytes. Therefore, they easily get precipitated instead of being adsorbed on the metallic surface. In order to make them soluble in such medium and get transported on the metal surfaces the following attempts should be made:

  1. Functionalization and addition of highly polar functional groups including –COOH and –SO3H.

  2. These molecules can be fragmented into relatively smaller species i.e. polymers can be broken in oligomers through solvolysis/hydrolysis.

  3. Stock solution of these compounds can be prepared in organic cosolvents such as acetone or ethanol in which the compounds are fully dissolved. In this process, a specific amount (100 mg) of organic corrosion inhibitor is dissolve in a very small amount (≈1–3 ml) of cosolvent followed by the addition of the electrolyte (Figure 10).

Figure 10: Illustrative diagram for the preparation of stock solution using an organic cosolvent (acetone).
Figure 10:

Illustrative diagram for the preparation of stock solution using an organic cosolvent (acetone).

Noticeably, precipitated organic corrosion inhibitors cannot be transported to the metal surface. The fully dissolved organic corrosion inhibitors are easily transported to the metal surface through electrolyte. Therefore, for a better transportability rate and effective corrosion mitigation, organic corrosion inhibitors should be fully dissolved in the electrolytes.

1.3 Immersion time

Immersion time of a metal specimen in any electrolyte plays a crucial role while determining the corrosion inhibition potential of organic compounds (Al-Moubaraki et al. 2015; Asadi et al. 2015; Benali et al. 2013). Theoretically, corrosion inhibition effectiveness of the inhibitor should increase on increasing the immersion time as on increasing immersion time inhibitor molecules experience relatively more time to adsorb on the metal surface. However, in most of the cases, organic corrosion inhibitors adsorb instantaneously through chemisorption and form uniform monolayer. It is important to mention that all active sites cover by the inhibitor molecules. Therefore, it can be concluded that during first stage of metal-inhibitor interaction, organic corrosion inhibitors adsorb on the metal surface following through chemisorption mechanism and form compact monolayer. On increasing the immersion time, second, third, fourth etc. layers of corrosion inhibitors are formed via the intermolecular force of attractions between pre-adsorbed inhibitor molecules and inhibitor molecules dispersed in the electrolyte. Apparently, first inhibitor layer is stabilized by chemisorption while remaining layers are formed by intermolecular force of attraction. However, sometime corrosion inhibition potential of organic inhibitors decreases on increasing the immersion time. This is might be degradation, decomposition or rearrangement of the inhibitor molecules after long exposure to the electrolyte. The effect of immersion time can be summarized as follows:

  1. Generally, inhibition potential of organic inhibitors increases on increasing immersion time due to increased time for the adsorption.

  2. Inhibitor molecules instantaneously adsorb through chemisorption and form compact and stable monolayer.

  3. Each active site contains one inhibitor molecule.

  4. Second, third, fourth etc. layers of inhibitor molecules formed by intermolecular force of attraction between pre-adsorbed inhibitor molecules and the molecules dispersed in the solution/electrolyte (Figure 11).

  5. Sometime immersion time has adverse effect and inhibition potential of corrosion inhibitors decreases with time due to their decomposition or rearrangement on long exposure.

Figure 11: Monolayer and multilayer adsorption of corrosion inhibitor molecules on a metal surface after exposing in an electrolyte.
Figure 11:

Monolayer and multilayer adsorption of corrosion inhibitor molecules on a metal surface after exposing in an electrolyte.

1.4 Planarity

Planarity is another very significant parameter that determines corrosion inhibition potential of organic compounds (Chakravarthy et al. 2014; Döner et al. 2013; Nataraja et al. 2011). Noticeably, organic compounds having planar orientation envelop greater metal surface and act as superior corrosion inhibitors as compared to the organic compounds with vertical orientation. The orientation of corrosion inhibitors in aqueous electrolytes mainly depends upon two the following factors:

1.4.1 Concentration

Concentration is one of the most significant parameters which determine the orientation of organic corrosion inhibitors on the metallic surface. Obviously, at lower concentration, intermolecular force of attraction between metal surface and inhibitor molecules dominates and they acquire flat orientation. However, on increasing their concentration beyond a certain limit i.e. beyond optimum concentration, where all active sites are covered by inhibitor molecules, intermolecular force of repulsion among adsorbed inhibitor molecules dominates that forced adsorbed to gain vertical orientation. This is why corrosion inhibition potential of most of the organic corrosion inhibitors increases on increasing their concentration however after certain concentration (called as optimum concentration) there is no appreciable increase in the corrosion inhibition performance of the inhibitor is observed (Jiang et al. 2005; Ketrane et al. 2009; Varvara et al. 2008). For an example, various possible orientations of at its lower, intermediate, and higher concentrations are illustrated in Figure 12.

Figure 12: Orientations of pyridine at low, intermediate, and high concentrations.
Figure 12:

Orientations of pyridine at low, intermediate, and high concentrations.

1.4.2 Nature of substituents

Nature of substituents greatly affects the orientation and therefore corrosion inhibition potential of organic corrosion inhibitors. The effect of substituents on the orientation of corrosion inhibitors is extensively described in our previous reports (Verma et al. 2016b, 2022). The orientation of corrosion inhibitors on metallic surface can be assessed using molecular dynamics simulations (MDS) and Monte Carlo simulations (MCS) techniques. In general, organic corrosion inhibitors containing electron donating substituents such as –OH, –NH2, –OMe etc. acquire flat or horizontal orientation. On the other hand, electron withdrawing substituents such as –COOH, –NO2 and –CN etc. force organic inhibitors to gain vertical orientation. Orientations of some series of corrosion inhibitors with electron donating and withdrawing substituents are presented in Figure 13.

Figure 13: Orientation of (a) 5-arylpyrimido-[4, 5-b] quinoline-diones (APQDs) with electron donating –OH and electron withdrawing –NO2 substituents (Verma et al. 2016a) and (b) substituted pyrimidine-fused heterocycles (CARBs) with electron donating –CH3 and electron withdrawing –NO2 substituents (Verma et al. 2016b) (reproduction permission not required).
Figure 13:

Orientation of (a) 5-arylpyrimido-[4, 5-b] quinoline-diones (APQDs) with electron donating –OH and electron withdrawing –NO2 substituents (Verma et al. 2016a) and (b) substituted pyrimidine-fused heterocycles (CARBs) with electron donating –CH3 and electron withdrawing –NO2 substituents (Verma et al. 2016b) (reproduction permission not required).

1.5 Electrolyte strength

The electrolyte strength in another factor that affects the inhibition potential of organic corrosion compounds (Dalmoro et al. 2019; Das and Pradhan 2019; Agarwal and Landolt 1998). Obviously, different electrolytes of numerous concentrations i.e. acidic, basic, and neutral solutions are widely for different metals and alloys depending upon the nature of surface impurities (Table 1). Generally, corrosion inhibition potential of organic compounds decreases on increasing the electrolyte concentration. Noticeably, lower concentrations of electrolytes are extensively used for educational purposes and highly concentrated electrolytes are widely used for industrial cleaning process.

Table 1:

Some common acidic, basic, and neutral electrolytes.

S. no. Acidic electrolytes Basic electrolytes Neutral electrolytes
1 HCl NH4OH NaCl
2 H2SO4 NaOH KCl
3 HNO3 KOH
4 H3PO4 Ammonia solution
5 H3NSO3.

1.6 Synergism: synergistic effect

Recently, a lot of works have been reported in which presence of small amount of second constituent increases the corrosion inhibition potential of organic compounds up to an appreciable level. This phenomenon is known as synergistic effect or synergism (Umoren and Solomon 2017). Synergism may be caused (Umoren and Solomon 2017):

  1. Metal cations (mixture of organic corrosion inhibitor and metal cations).

  2. Inorganic salts/compounds (mixture of organic and inorganic compounds).

  3. Organic compounds (mixture of organic compounds).

  4. Polymers (mixture of organic compound and polymers).

Literature investigation shows that synergism using metal cations, inorganic salts, organic compounds, and polymers is widely investigation. A detailed description on synergism can be found elsewhere (Umoren and Solomon 2017).

2 Summary and outlook

Organic compounds having electron dense centers, called as adsorption centers, are most frequently used as corrosion inhibitors because of their various beneficial properties including their high %IE, cost-effectivity, and ease of synthesis. However, all tested organic compounds do not fulfill the requirements of a corrosion inhibitor. Some of them show reasonably low %IE at relatively high concentration. More so, some of them are not properly soluble in the testing electrolytes. Therefore, before formulation/synthesis of an effective corrosion inhibitor a proper understanding of electronic structure of the compound to be used as corrosion inhibitor is highly essential. The understanding of electronic structure can be extracted from the recent advancements or attempts made in the formulation of effective corrosion inhibitors. There are numerous attempts have been made in order to increase the %IE of organic corrosion inhibitors. One of the most common attempts is enhancing the number of adsorption sites through chemical functionalization and addition of polar functional group(s) by covalent and/or noncovalent functionalization. The chemical functionalization is also useful for enhancing the solubility of the organic compounds, especially macromolecules and polymers. Another attempt that has been made to enhance the %IE of organic corrosion inhibitors is enhancing the transportability rate (rate of inhibitor transport from electrolyte to metal surface). Obviously, it depends upon numerous factors including testing temperature, nature of electrolyte, nature of corrosion inhibitors and their solubility. Generally, increase in the temperature decreases the transportability rate however exact influence of temperature varies depending upon the chemisorption or physisorption mode. Planarity is another factor that affects the %IE of organic inhibitors. A more planar compound should act as better corrosion inhibitor than less planar compound. In solution phase, orientation of corrosion inhibitors depends upon numerous factors including nature of substituents and inhibitor concentration. Apart from this, the %IE of organic compounds can be increased by adding small amount of second constituent i.e. synergism.

Corresponding author: Chandrabhan Verma, Interdisciplinary Research Center for Advanced Materials, King Fahd University of Petroleum and Minerals, Dhahran31261, Saudi Arabia, E-mail:

Funding source: King Fahd University of Petroleum and Minerals

About the authors

Chandrabhan Verma

Chandrabhan Verma works at the Interdisciplinary Research Center for Advanced Materials, King Fahd University of Petroleum and Minerals, Saudi Arabia. His research is mainly focused on the synthesis and designing of environmental friendly corrosion inhibitors useful for several industrial applications. Dr. Verma is the author of several research and review articles published in various peer-reviewed international journals; currently, he is a book editor for several academic publishers. Dr. Verma has more than 6100 citations to his credit with an H-index of 43 and i-10 index of 106. He received several national and international awards for his academic achievements. He is a member of the American Chemical Society.

Mumtaz A. Quraishi

Prof. Mumtaz A. Quraishi is a chair professor at the Interdisciplinary Center for Research in Advanced Materials King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabia. He obtained a Ph.D. in synthetic organic chemistry from Kurukshetra University in 1986 and a D.Sc. in corrosion inhibition of industrial metals and alloys from Aligarh Muslim University Aligarh in 2004. Before joining KFUPM, he was an institute professor at IIT BHU Varanasi, India where he also served as head (chairman) of Department of Chemistry. He has a teaching experience of more than 35 years. Prof. Quraishi received several national and international awards. He is an associate editor of Current Material Science and an editorial board member of more than 30 international journals. Prof. Quraishi is a fellow of the Royal Society of Chemistry UK and a member of the American Chemical Society. He has published more than 400 papers in peer reviewed journals having an H-index of 86 and more than 23000 citations to his credit; his global status is one in terms of H-index in the field of corrosion inhibitors. He is a book co-author of Heterocyclic organic corrosion inhibitors: principles and applications (2020).

  1. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: CV thankfully acknowledges the Interdisciplinary Research Center for Advanced Materials, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia, for providing financial support under the Postdoctoral Fellowship scheme.

  3. Conflicts of interest: The authors declare that they have no conflicts of interest regarding this article.

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Received: 2021-12-15
Accepted: 2022-02-09
Published Online: 2022-03-24
Published in Print: 2022-06-27

© 2022 Walter de Gruyter GmbH, Berlin/Boston

Artikel in diesem Heft

  1. Frontmatter
  2. Reviews
  3. Review of passivity and electrochemical properties of nanostructured stainless steels obtained by surface mechanical attrition treatment (SMAT): trend and progress
  4. Galvanic corrosion based on wire beam electrode technique: progress and prospects
  5. Efforts made in enhancing corrosion inhibition potential of organic compounds: recent developments and future direction
  6. Original Articles
  7. The corrosion behavior of HVOF TiAlNb coating in molten Zn-0.2 wt.% Al
  8. Pitting initiation on 304 stainless steel in a chloride-contaminated pore solution under alternating temperature conditions
  9. Immersion corrosion behavior and electrochemical performance of laser cladded Ni60–CeO2 coatings in 5% NaCl solution
  10. Effect of imidazoline derivatives on the corrosion inhibition of Q235 steel in HCl medium: experimental and theoretical investigation
https://doi.org/10.1515/corrrev-2021-0101
Schlagwörter für diesen Artikel
adsorption centers; organic corrosion inhibitors; planarity and chemical functionalization; synergism; transportability rate

Artikel in diesem Heft

  1. Frontmatter
  2. Reviews
  3. Review of passivity and electrochemical properties of nanostructured stainless steels obtained by surface mechanical attrition treatment (SMAT): trend and progress
  4. Galvanic corrosion based on wire beam electrode technique: progress and prospects
  5. Efforts made in enhancing corrosion inhibition potential of organic compounds: recent developments and future direction
  6. Original Articles
  7. The corrosion behavior of HVOF TiAlNb coating in molten Zn-0.2 wt.% Al
  8. Pitting initiation on 304 stainless steel in a chloride-contaminated pore solution under alternating temperature conditions
  9. Immersion corrosion behavior and electrochemical performance of laser cladded Ni60–CeO2 coatings in 5% NaCl solution
  10. Effect of imidazoline derivatives on the corrosion inhibition of Q235 steel in HCl medium: experimental and theoretical investigation
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