Accessible Published by De Gruyter January 11, 2019

Organic green corrosion inhibitors (OGCIs): a critical review

Lekan Taofeek Popoola
From the journal Corrosion Reviews

Abstract

Over the decades, corrosion has resulted in loss of lives accorded with damage costs in almost all engineering fields. Thus, it is seen as an environmental threat with catastrophic attributes, which calls for day-to-day research on its final resolution. Recent studies have proven organic green corrosion inhibitors (OGCIs) from plant extracts with biodegradable, environmentally accommodative, relatively cheap, and nonharmful features as the most perfect approach of tackling the problem. This review gives succinct discussion on the mechanisms, classifications, and active functional groups of OGCIs. Measuring ways and factors influencing their efficiency are presented. Also, various plant extracts used as OGCIs in preventing material corrosion in corrosive media coupled with their respective findings, applied characterization techniques, and future challenges are presented. The significance of values obtained from simulating presented mathematical models governing OGCI kinetics, adsorption isotherm, and adsorption thermodynamics is also included. In conclusion, recommendations that will broaden the usage of OGCIs from plant extracts for inhibiting corrosion of materials are presented for prospective researchers in the field of corrosion.

1 Introduction

Corrosion is metal degradation as a result of contact with aqueous corrosive surroundings (air, moisture, or soil; Thompson et al., 2007) through direct chemical or electrochemical reaction to form noble compounds (Uhlig, 1971). As defined by the International Union of Pure and Applied Chemistry, corrosion is an interfacial material (polymer, metal, concrete, wood, and ceramic) reaction (irreversible) with its environment, which results in material consumption or in dissolution into the material of an environmental component (Vadivu et al., 2016). Corrosion is an environmental threat with economic, conservation, and safety impacts in various engineering applications such as building construction, chemical, automobile, mechatronics, metallurgical, and medical (Sharma et al., 2011). Various forms of material corrosion under different environments have been discussed (Popoola et al., 2013). Thus, there is a need to develop novel techniques and methods of tackling this dangerous phenomenon from existing prominent ones, which are protective coatings and linings, cathodic/anodic protection, and corrosion inhibitors. However, the results of numerous research conducted in anticorrosion material applications in previously mentioned engineering fields revealed using corrosion inhibitors as the most effective and simple approach of preventing deleterious degradation of metals and alloys in corrosive media (Basargin et al., 2004); Singh, 2014). Figure 1 depicts the summary of chemical reactions of the corrosion process.

Figure 1: Chemical reactions of the corrosion process (Brylee and Advincula, 2015).

Figure 1:

Chemical reactions of the corrosion process (Brylee and Advincula, 2015).

Corrosion inhibitors minimize or avert corrosion when added in small concentrations to a corrosive medium (Riggs, 1973) by forming monomolecular film-adsorbed surface (Mainier et al., 2003), which obstructs the direct contact between metal and corrosive agents (Ebenso et al., 2012). They have been classified based on sources (as organic or inorganic) and techniques (as synthesized or extracted). Thus, it is required to look for not only applicable corrosion inhibitors but also those that are economically viable and environmentally friendly. However, synthetic organic corrosion inhibitors (SOCIs) and traditional inorganic corrosion inhibitors (TICIs) such as chromates and lead have been known to have restrictive environmental regulations (Raja and Sethuraman, 2008) due to their hazardous effects. Many of the SOCIs are not biodegradable and get accumulated in the environment constituting nuisance to human health or ecological systems (EPA, 1998), the removal of which is complicated and expensive (Bammou et al., 2011). These environmental issues have called for a replacement of these TICIs and SOCIs with natural organic compounds sourced from spices, naturally existing aromatic herbs, and medicinal plants that can hinder the corrosion of materials in corrosive media called organic green corrosion inhibitors (OGCIs), which are inexpensive, harmless, readily obtainable, and environmentally accommodative. Figure 2 presents the various sources of eco-friendly OGCIs.

Figure 2: Sources of eco-friendly OGCIs (Ibrahimi et al., 2017).

Figure 2:

Sources of eco-friendly OGCIs (Ibrahimi et al., 2017).

Historically, the use of OGCIs started in the early 1930s when extracts from plants such as Chelidonium majus (celandine) are first used for sulfuric acid (H2SO4) pickling baths (Sanyal, 1981). Thereafter, researchers around the world found interest in using green anticorrosive agents extracted from several natural plants (Schmitt et al., 2009). Seeds, fruits, leaves, and flowers of natural plants such as Justicia gendarussa plant extract (Satapathy et al., 2009), khillar (El-Etre, 2006), olive leaves (El-Etre, 2007), Phyllanthus amaratus (Okafor et al., 2008), and Murraya koenigii leaves (Quraishi et al., 2010) have been extracted and applied as corrosion inhibitors. Results revealed natural plants extracts to be easily obtainable, biodegradable, and harmless (Ji et al., 2015) with remarkable potential of inhibiting corrosion reaction.

1.1 Mechanisms of OGCIs

The corrosion inhibition efficiency of OGCIs has been linked to the availability of organic compounds with N, O, P, and S atoms (Yildirim and Cetin, 2008), which have shielding effect and corrosion-inhibiting potentials for material attack. Their increasing order of corrosion inhibition efficiency has been stated to be oxygen<nitrogen<sulfur<phosphorus (Neha et al., 2013). OGCIs exhibit their inhibiting action via physisorption or chemisorption onto the metal-solution interface by removing molecules of water on the surface for compact barrier film formation (Finšgar and Jackson, 2014). The occurrence of a coordinate covalent bond by interaction between lone pair and π-electrons available in the molecules of OGCIs with the vacant metal d-orbitals is also experienced (Abdallah, 2002). Nevertheless, compound adsorption on the metal surface is enhanced by p-d bond formation as a result of p-electron overlap to the 3d vacant orbital of Fe atom (Ahamad et al., 2010) due to the availability of N, O, and S atoms and organic structure double bonds (Hong et al., 2008).

Generally, adsorption types can be distinguished by the occurring mechanisms that can be physisorption, chemisorption, interaction between metal and p-electrons, or mixture of the aforementioned (Tian et al., 2012). The chemical structures of OGCIs, inhibitor molecule charge distribution, and metal surface charge determine the process of adsorption. In physisorption, electrostatic attractive force binds ionic charges on OGCI molecules with electric charged metal surface, whereas chemisorption occurs via sharing of free electron pairs or transfer of charge to produce strong chemical bonds between nonionic OGCI molecules and the metal (Papavinasam et al., 2007). However, the chemisorption bond strength is a function of the functional group electron density present on the donor atom and group polarizability. OGCI inhibition efficiency is improved when one of the H atoms attached to the C in the heterocyclic ring is displaced by any of -CHO, -NO, -COOH, or -NH2 substituent group (Ali et al., 2003). Also, cathodic or anodic reactions are being retarded when metal electron density changes at the point of attachment. The consumption of electrons at the cathode occurs while they are furnished at the anode (Kartsonakis et al., 2012).

1.2 Classifications of OGCIs

Corrosion inhibitors from greeners can be either scavengers or interface inhibitors. Scavengers reduce medium corrosivity by scavenging aggressive substances, whereas interface inhibitors inhibit corrosion through film formation at the metal-environment interface. Scavengers work in alkaline and almost neutral solutions by cathodic-oxygen reduction reaction simply written as Eq. (1). The interface inhibitors are classified as either vapor phase or liquid phase. The vapor-phase inhibitors provide temporal atmospheric corrosion protection especially in closed environments by loosely impregnating wrapping paper inside a closed container (Levin et al., 1965). The OGCI transport and metal surface interaction occur during the vapor-phase inhibition process.

(1)O2+2H2O +4e4OH

However, the most prominent are the liquid-phase inhibitors that are further subdivided into cathodic, anodic, or mixed OGCIs based on the reaction-type inhibition, which can be any of the cathodic, anodic, or both electrochemical reactions. In anodic OGCIs, hydroxides, oxides, or salts are produced to enhance passivating film formation, which inhibits anodic metal dissolution reaction. Their mechanism is best explained by an active-passive metal polarization diagram. In cathodic OGCIs, corrosion is controlled by either cathodic poisoning or cathodic precipitation. In the former, sulfides and selenides, which act as cathodic poisons, are adsorbed on the surface of the metal to form protective films that reduce the rate of cathodic reaction through oxygen diffusion minimization on the metal surface. In the latter, insoluble compounds such as carbonates of calcium and magnesium are precipitated on the metal surface to increase alkalinity at cathodic sites. Generally, hydrogen ion reduction to H atoms to form molecules of hydrogen as written in Eqs. (2) and (3) occurs in acidic solution, whereas cathodic reaction occurs via the reduction of oxygen in alkaline solution.

(2)H++eH
(3)2HH2

Lastly, approximately 80% of OGCIs are categorized as mixed inhibitors that protect the metal from corrosion by chemisorption, physisorption, and film formation. Physisorption is facilitated by the electrostatic attraction of negatively charged (anionic) OGCI with positively charged metal surface. Chemisorption process is slower than physisorption such that the inhibition rate and adsorption increase as the temperature increases (Sastri, 2011). Corrosion protection also increases with the increase in polymeric films produced as a result of OGCI molecules adsorbed, which are subjected to reactions on the metal surface. Insoluble adherent films that avert solution access to the metal provide effective inhibition.

1.3 Active functional groups in OGCIs

The OGCI active ingredients consist of phytochemical constituents known to be functional groups with N, O, S, P, or Se heteroatoms via which they are attached onto the metal surface (Helen et al., 2014). Compounds of OGCIs with abundant p-electron and functional electronegative groups with conjugated double or triple bonds have been shown to be most effective (Jiang et al., 2007). The inhibitor molecule efficiency to cover enough surface area is increased due to the attached groups to the parent chain. In lieu of this, the bonding strength of the group on the metal is enhanced by the presence of peculiar repeating units (methyl and phenyl groups) of the parent chain and additional substituent groups. Studies have shown that OGCI molecules with -OH and -OCH3 electron-releasing substituents proved to have better efficiency than the parent molecule with no substituents (Verma et al., 2017). Also, heterocyclic compounds have exhibited higher corrosion inhibition efficiency, as they attach easily on the metal surface via their π- and nonbonding electrons, aromatic rings, and polar functional groups that act as adsorption centers (Ahamad et al., 2010). Table 1 presents some anchoring functional groups present in OGCIs.

Table 1:

Some attaching functional groups in OGCIs (Singh, 1993).

Functional groupNameFuctional groupName
-OHHydroxy-NH2Amino
-C-N-C-Amine-SHThiol
-NO2Nitro-C≡C--yne
-CONH2Amide-S=OSulfoxide
-COOHCarboxy-NHImino
-S-Sulfide-N=N-N-Triazole
-C=S-Thio-C-O-C-Epoxy
-P=OPhosphonium-P-Phospho
-Se-Seleno-As-Arsano

Some prominent compounds such as benzoic acid (Akiyama and Nobe, 1970), benzotriazole (Fox et al., 1979), thiourea (Singh, 1993), flavonoids (Bhola et al., 2013), carbohydrates (Umoren and Eduok, 2016), tannins (Nonaka, 1989), and tryptamine (Suleiman et al., 2013) containing these active functional groups whose sources are from natural plants have been applied as corrosion inhibitors for many metals. Flavin mononucleotide from grape pomace extracts has been detected as a good OGCI for hot-rolled steel in acidic medium (Bhola et al., 2013). Its corrosion inhibition potential lies in the presence of heterocyclic isoalloxazine ring anchored to sugar alcohol-ribitol obtained from D(-) pentose sugar (ribose), which consists of a phosphate monosodium salt and three antisymmetric carbons. The bark of Rhizophora racemosa stem investigated to be very rich in tannins has been stated as the most effective OGCI for mild steel. Its basic structure contains residues of garlic acid attached to glucose through glycosidic bonds (Nonaka, 1989) with arrays of hydroxyl and carboxyl groups enhancing molecule adsorption on corroding mild steel surfaces. Chamaerops humilis plant extract, which is also rich in tannins, is effective in inhibiting corrosion of mild steel in 0.5 m H2SO4 with 5% ethanol additive (Benali et al., 2013). Tryptamine, a derivative of the tryptophan, proved effective in inhibiting ARMCO iron corrosion in deaerated 0.5 m H2SO4 within a temperature range of 25°C–55°C. Table 2 presents the various sources of OGCIs with their respective functional groups and inhibitory roles.

Table 2:

Sources of OGCIs, functional groups, and corrosion inhibitory roles.

OGCI sourceFunctional groups and compoundsCorrosion inhibitory rolesReferences
G. biloba leaf extractsFlavonoids and terpenoids; phenol groups and aromatic ringsTerpenoids: quercetin adsorption on mild steel surface based on the interactions of donor-acceptor between O and aromatic ring p-electrons and surface iron vacant d-orbitals

Flavonoids: oxygen-adsorption corrosion inhibited via its oxidation to benzoquinone by O2 resolved in the solution
Chen et al., 2013
Rothmannia longiflora extractMonomethyl fumarate, 4-oxonicotinamide-1-(1-β-D-ribofuranoside), and D-mannitolAkalezi et al., 2015
Petersianthus macrocarpus plantPetersaponin, β-sitosterol, and ellagic acidMolecules are adsorbed on the surface of mild steel surface as a result of hydroxyl group and aromatic ring protonation. Constituent molecules have aromatic rings (π-electrons) with attached electron releasing groups. Also, the increase of the ability of π-electrons to be bonded to vacant d-orbital in FeAkalezi et al., 2015
Extract of Ficus asperifoliaSaponins, alkaloids, tannins, anthraquinones, flavonoids, reducing sugars, n-hexane, ethyl acetate, and butanolThe electron-donating ability was facilitated as a result of rich bond or heteroatoms present in the chemical structures. Thus, the formation of complexes on the material surface to inhibit corrosion was enhancedEbenso et al., 2008
Extracts of D. kaki L.f huskVitamins, p-coumaric acid, gallic acid, catechin, flavonoids, carotenoids, and condensed tanninZhang et al., 2013
Gum arabicArabinogalactan, oligosaccharides, polysaccharides, and glucoproteinsUmoren et al., 2006
Tobacco extractPolyphenols, terpenes, alkaloids, alcohols, carboxylic acids, and nitrogen-containing compoundsCorrosion inhibition on metals by electrochemical activity due to fused benzene ring system with charge dislocation propertyRudresh and Mayanna, 1977
Extract of green wild jute tree (Grewa venusta)Polysaccharides, polyphenols (catechins and flavonoids) vitamins, tannins, minerals, volatile oils, and alkaloidsMixed inhibitor corrosion inhibition actionSuleiman et al., 2013
Anthocleista djalonesisIridoid glucoside (DJN), dibenzo-α-pyrone (djalonensone), ursolic acid, and 3-oxo-∆-4,5-sitosteroneObame et al., 2008
Guar gumPolysaccharides, mainly sugars galactose and mannose1,4-Linked mannose residue linear chain-forming short-side branches, which later formed complexes on the metal surface to inhibit corrosionAbdallah, 2004
Jatropha curcas leaf extractTannins, flavonoids, terpenes, anthraquinone, apigenin, cardiac glycoside, alkaloids, deoxy sugar, saponins, α-D-glucoside, sterols, stigmasterol, and vitexinCorrosion inhibition via the formation of continuous complex metal ions on the metal surface by polar groupsEjikeme et al., 2014; Rani and Selvaraj, 2014
Extracts of banana peelBananadine (3Z,7Z,10Z)-1-oxa-6-azacyclododeca-3,7,10-trieneSangeetha et al., 2012
Aloe vera plant extractMinerals, polysaccharides, vitamins, glycoproteins, and enzymesGupta et al., 2018
Azadirachta indicaAzadirachtin, salannin, meliantriol, and nimbinInhibition effects due to electronic, geometry coupled with binding property bases on the metal surfaceSharma et al., 2015
Locust bean gumGalactomannan-type polysaccharidesJano et al., 2012
Oil palm frondPhenolic constituents (p-hydroxybenzoic acid, syringic acid, vanillic acid, vanillin, p-hydroxybenzaldehyde, p-hydroxyacetophenone, and syringaldehyde)Lignin is cleaved to form aromatic carbonyl compounds (syringaldehyde and vanillin) via alkaline nitrobenzene oxidation to inhibit corrosionYokoi et al., 2001
J. gendarussa plant extractFriedelin, β-sitosterol, o-substituted aromatic amines lupenol, phenolic dimmers, and flavonoidsCorrosion inhibition of metal as a result of mixed-type inhibitor actionsSatapathy et al., 2009
Leaf and flower extracts of Heliconia rostrataAlkaloids, flavonoids, tannins, cellulose, and polycyclic compoundsThe presence of heterocyclic constituents enhanced film formation over the metal surface, thus affording corrosion inhibition
Celery (Apium graveolens) seedsFlavonoids, linoleic acid, D-limonene, sesquiterpene alcohols, coumarins, selinene, sedanolide, and sedanonic anhydrideMegahed et al., 2017
Henna extract (Lawsonia inermis)Lawsone, α-D-glucose, gallic acid, and tannic acidMixed-type corrosion inhibition mechanism with constituents order of inhibition efficiency of tannic acid<α-D-glucose<gallic acid<henna extract<lawsoneOstovari et al., 2009

1.4 Factors influencing OGCI efficiency

OGCI efficiency in inhibiting corrosion is a function of their adsorption characteristics on the metal surface. Factors that have been considered by previous studies affecting OGCI inhibition efficiency depend majorly on their structure, concentration, temperature, and exposure time. An increase in OGCI concentration results in a simultaneous decrease in corrosion rate with an increase in inhibition efficiency, which approaches optimum level at a certain concentration value. This resulted from the formation of additional inhibitor molecules being adsorbed on the surface of the metal, which makes it complex for further corrosive attack to occur by the electrolyte solution. The dissolution of metal increases with corrosion exposure period in the presence of OGCIs. This is linked to previously adsorbed inhibitor molecules from the metal surface resulting from partial desorption. Corrosion rate increases linearly as temperature increases such that an equilibrium exists between adsorption and OGCI molecule desorption at the surface of the metal at a particular temperature. An increase in temperature as a result of a higher desorption rate makes the equilibrium to shift until its reestablishment at various equilibrium constant values. Thus, OGCI inhibitive protectiveness decreases with increasing temperature. As mentioned previously, OGCI structural behavior has a great influence on their efficiencies in corrosive media. The presence of a heteroatom in an OGCI molecule enhances their adsorption onto the metal surface through the formation of an adsorptive bond by Lewis acid-base reaction in which OGCIs and metal act as electron donor and acceptor, respectively. The strength of an adsorption bond is a function of electron density and polarizability of the reaction center. Conclusively, studies have shown surface-active OGCI adsorption to increase with increasing molecular weight and dipole moment.

1.5 Measuring OGCI efficiency

The first step required in measuring the efficiency of OGCIs is the preparation of a metal sample to be examined for corrosion. The selection of metal coupons for checking OGCI efficiency is vital as small changes in metal composition or available impurities during fabrication are reflected in the obtained results (Qian et al., 2013). The metal composition should as much as possible be relevant to metals related to the corrosion problem. Of all the available methodologies for measuring OGCI efficiency in the literature, weight loss measurement (WLM), electrochemical impedance spectroscopy (EIS), linear polarization resistance, and potentiodynamic polarization (PDP) are the prominent ones.

1.5.1 WLM

Many researchers used test solutions prepared from actual field solution for corrosion testing, whereas some used synthetic solutions prepared from analytical-grade reagents in the absence of the former for WLM. Before OGCI efficiency is determined using WLM, metal samples are cleaned by polishing with abrasive paper of different grades and washed thoroughly using solvents (acetone, ethanol, and distilled water) after which they are dried at room temperature. Vernier caliper is used to measure the dimensions of the metal specimen. The prepared metal coupons are then weighed before immersion using high-accuracy digital balance. The thoroughly rinsed corroded metal coupons are reweighed after a specified period of exposure time to check the weight loss. The corrosion rate (Ostovari et al., 2009), surface coverage θ (Al-Zubaidi et al., 2016), and percent corrosion inhibition efficiency η% (Yaro et al., 2013) can then be evaluated using Eqs. (4)–(6), respectively. The influence of OGCI in preventing corrosion attack of the metal coupons is checked by sample WLM in the absence and presence of OGCI. The WLM technique is simple and reliable as it forms the basic fundamental method of measuring OGCI efficiency in many corrosion-monitoring programs.

(4)CR=w1w2At
(5)θ=w1w2w1
(6)η%=w1w2w1×100%

where CR=corrosion rate (g/cm2/h), w1=metal coupon weight loss in the absence of OGCI (g), w2=metal coupon weight loss in the presence of OGCI (g), A=metal coupon surface area (cm2), and t=immersion time (h).

However, there are cases where the inhibition efficiency of OGCI is enhanced as a result of the combination with another OGCI such that the inhibition efficiency is increased by an appreciable value. This is called synergism effect, which can be quantified using Eq. (7) (Murakawa et al., 1967):

(7)S=1θAθB+θAθB1θAB

where θA and θB=respective surface area coverage by compounds A and B when acting separately and θAB=surface area coverage obtained for the mixture of A and B. When S approaches 1, the interaction between A and B is negligible. If S>1, it reveals the existence of synergism, whereas S<1 signifies an opposite effect between A and B (Mobin and Rizvi, 2016).

1.5.2 PDP

PDP is another means of measuring OGCI efficiency, corrosion rate, and corrosion mechanism protection through electrochemical-based measurements. In most cases, the basic laboratory setup involves using three electrodes in the electrochemical cell, which are working, counter, and reference electrodes for the measurement immersed in the test solution of known volume and concentration. Platinum electrode (Shah et al., 2017) and graphite rod (Al-Zubaidi et al., 2018) are mostly used as the counter electrode, whereas saturated calomel electrode (Akalezi et al., 2015) and Ag/AgCl aqueous electrode are used as the reference electrode. The working electrode is the metal substrate under examination. The voltage (V) of the system is measured and controlled by the reference electrode, whereas the current (I) is measured by the counter electrode. As the electrochemical reactions progress, open circuit potential (Eocp) of the metal fluctuates. At equilibrium, a stable value is then measured after which a PDP scan is performed. After this, a Tafel plot is obtained by applying a potential from a value below the initially measured Eocp to a higher potential (between −0.25 and +0.25 V). The corrosion current (icorr) and corrosion potential (Ecorr) are then measured from the plots. Figure 3 represents the typical polarization curves for Q235A steel corrosion in 1 m HCl in the absence and presence of varying concentrations of persimmon husk extracts as OGCI. Corrosion rate is measured using Eq. (8) (Al-Sabagh et al., 2012), whereas η% is calculated by measuring icorr in the presence and absence of OGCIs using Eq. (9) (Verma et al., 2015):

Figure 3: Polarization curves for Q235A steel corrosion in 1 m HCl in the absence and presence of varying concentrations of persimmon husk extracts as OGCI (Zhang et al., 2013).

Figure 3:

Polarization curves for Q235A steel corrosion in 1 m HCl in the absence and presence of varying concentrations of persimmon husk extracts as OGCI (Zhang et al., 2013).

(8)CR=icorr×κ×EWρ×A
(9)η%=icorroicorr1icorro×100%

where κ=conversion factor, EW=equivalent weight (g), ρ=density (g/cm3), A=sample area (cm2), and icorro and icorr1=icorr density values in the absence and presence of OGCI molecules, respectively.

1.5.3 EIS

EIS is an essential method of monitoring in situ electrochemical changes with critical understanding of physical processes occurring at the metal-electrolyte interface (Mourya et al., 2014) such that information related to electrode kinetics, surface properties, and mechanistic can be taken from impedance diagrams (Lorenz and Manfield, 1981). Just like PDP, the experiment is conducted in a three-electrode electrochemical cell with small potential upsetting between 5 and 50 mV of AC voltage over frequency variation between 100 kHz and 10 mHz (Ramanavicius et al., 2010). The EIS parameters are obtained using experimental EIS spectral (Nyquist plot) obtained with the aid of suitable circuits from values of frequencies that correspond to real (Z′) and imaginary (Z″) impedance values. A typical Nyquist plot for examining mild steel in 1 m H2SO4 at 30°C by means of a new Schiff base extract with different concentrations as OGCI is shown in Figure 4. The adopted equivalent circuit comprises Rs (electrolyte solution resistance), in series with a parallel arrangement of constant phase element (CPE) and Rct (charge transfer resistance; Roy et al., 2014) modeled in a system of metal substrate, adsorbed inhibitors, and electrolyte solution.

Figure 4: Mild steel Nyquist plot in 1 m H2SO4 at 30°C for varying OGCI concentrations (Al-Amiery et al., 2014).

Figure 4:

Mild steel Nyquist plot in 1 m H2SO4 at 30°C for varying OGCI concentrations (Al-Amiery et al., 2014).

However, one study has used polarization resistance (Rp) obtained as real impedance difference at reduced and higher frequencies to replace the usual Rct (Gupta et al., 2018). Rp is noticed to include Rct, accumulation resistance (Ra) resulting from species accumulated at the metal-electrolyte interface, diffusion layer resistance (Rd), and inhibitor film resistance (Rf) on the metal surface. Anode-cathode charge transfer causes metal oxidation, which is usually obstructed by the presence of solvent molecules in aqueous acid solution. The resistance by the electrolyte solution is called the solution resistance (Rs). Rct represents the protective film capacity of adsorbed organic molecules on the metal surface to impede charge transfer to the metal-solution interface. Impedance parameters that include Rp, n, Cdl, and η% can then be obtained from a Nyquist plot by the equivalent circuit.

For a better explanation of a frequency-independent phase shift existing between an applied alternating potential and its current response, a CPE represented mathematically as Eq. (10) is used instead of capacitance (C; Satapathy et al., 2009):

(10)ZCPE=1A(jω)n

where ZCPE=CPE impedance, A=CPE constant, ω=angular frequency, j=imaginary number (i.e. i2=−1), and n=phase shift exponent that is a measure of surface irregularity/inhomogeneity. The significance of n is that a lower surface roughness is obtained at a higher n and vice versa. Also, n determines the nature of CPE and states what A in Eq. (10) represents as briefly summarized in Table 3. Electrical double-layer capacitance values can be calculated using any of Eqs. (11)–(13), whereas percent inhibition efficiency ηE(%) in the presence and absence of OGCIs is determined by Eq. (14) (Shah et al., 2017):

Table 3:

Significance of n values on the CPE nature.

nCPE nature (A)SignificanceReferences
0ResistanceA metal-solution interface operating as a resistorBai, 2015
1CapacitancePlane and homogeneous electrode surface with the metal-solution interface behaving as a capacitor with a regular surfaceLin et al., 2015
−1InductanceNonplane and heterogeneous electrode surface with the metal-solution interface behaving as an inductor with an irregular surfaceDeyab et al., 2007
1/2Warburg impedanceA metal-solution interface acting as both capacitor and inductorYurt et al., 2006
(11)Cdl=(ARct1n)1n
(12)Cdl=A(ωmax)n1
(13)Cdl=12πωmaxRct
(14)ηE(%)=Rct(i)Rct(o)Rct(i)×100%

where ωmax=maximum frequency of impedance imaginary quantity (rad/s) and Rct(i) and Rct(o)=Rct in the presence and absence of OGCI various concentrations, respectively.

In general, Table 4 summarizes the significance/implication of changes in trends and variations in the values of parameters associated with the techniques of measuring OGCI efficiencies as observed in previous studies.

Table 4:

Summary of the significance of parameter variations obtained from methods of measuring OGCI efficiencies as observed in previous studies.

ObservationSignificance/implicationReferences
WLM
 Inhibition efficiency increases as OGCI concentration increasesThe adsorption of enough molecules of OGCI on the metal surface at higher concentration causes higher surface coverageAkalezi et al., 2015
 The inhibition efficiency of OGCI “A” in combination with a small concentration of OGCI “B” is higher than the summation of inhibition efficiencies of OGCI “A” aloneThe synergism parameter is greater than 1, which suggests better corrosion protection metallic specimens by OGCIs “A+B” than using OGCI “A” aloneMobin and Rizvi, 2016
 The inhibition performance of OGCI molecules decreased with increasing solution temperatures1. This resulted from an increased mobility of OGCI molecules, which decreased the existing interaction between the metal surface and OGCI molecules

2. Rapid etching, molecular rearrangement, and/or fragmentation and desorption of adsorbed OGCI molecules at higher temperature might decrease inhibition efficiency
Bai, 2015
PDP
 Change of Ecorr values to more negative values in different OGCI concentrations coupled with a decrease in cathodic and anodic current density with increasing OGCI concentrationsAdsorption of molecules of OGCI on the sample surface, forming a protective metal surface. Cathodic polarization occurred. Anodic polarization occurs when anode potential shifts to positive direction. Ecorr>85 mV implies anodic or cathodic OGCI, whereas Ecorr displacement of <85 mV means mixed-type OGCIAhamad et al., 2010; Khadom et al., 2010
 Increase in OGCI concentrations causing reduction of icorr density (icorr) with increase in inhibition efficiency (IE)OGCI is effective in protecting the metal in acidic medium solutionKartsonakis et al., 2012
 Cathodic Tafel slope (βc) and anodic Tafel slope (βa) changed due to the addition of OGCIOGCI influences anodic and cathodic reactionsBilgic and Caliskan, 2001
 Anodic and cathodic branches of Tafel plot shifted to lower values for all examined concentrations of OGCI addedOrganic constituents of OGCI inhibited both hydrogen evolution (cathodic reaction) and metal dissolution (anodic reaction), suggesting that OGCI acted as mixed typeObot et al., 2009
EIS
 Significant increase in Rp value as a result of adding inhibitorCharge transfer reaction retarded by inhibitors and corrosion occurring on the metal surface with the formation of a protective filmSolmaz et al., 2008
 Reduction of Cdl values due to the presence of inhibitor molecules1. This resulted from local dielectric constant decrease and/or electrical double-layer thickness increase

2. Also, surface metal inhibition resulted from adsorption mechanism by the replacement of water molecules

3. Increase in surface coverage by OGCIs causing an increase in inhibition efficiency
Yousef et al., 2015
 Imperfect semicircle obtained in Nyquist plots when the concentration of OGCI increases in the solutionThis is attributed to metal surface imperfections and roughness called the dispersing effectTang et al., 2009
 Phase angle values in Bode plot for inhibited metallic specimens higher than uninhibited specimenSurface becomes appreciably smooth due to protective film formation by OGCIs over the metal surfaceVerma et al., 2015
 Increase in Nyquist plot diameter with increasing OGCI concentrationThis indicates inhibitive film strengthening with decrease in corrosion rateDe Souza and Spinelli, 2009
 Nyquist plot containing a depressed semicircle for used solid electrodeThis is linked to metal electrode inhomogeneities and surface roughnessRaja and Sethuraman, 2008
 Significant increase in Rct in the presence of OGCI moleculesAdsorption of OGCI molecules on the metal surface. High corrosion protection efficiencyAmin and Ibrahim, 2011
 Decrease in Nyquist plot diameter with increasing solution temperatureCorrosion inhibition rate decreases with an increase in solution temperatureAl-Amiery et al., 2014
 Decrease in values of Rct and IE as a result of an increase in temperatureAdsorbed OGCI molecules on the metal surface subjected to desorption with continuity in an increase of solution temperatureFouda et al., 2006
 Values of slope and phase angle deviating from the ideal capacitive behavior of the electric double layer (slope=1 and phase angle=−90°) in the Bode impedance and phase angle plots for inhibited and uninhibited metallic specimensThis resulted from metal surface inhomogeneitySingh et al., 2016

2 Previous studies on using OGCIs

Table 5 summarizes the literature consulted for different OGCI sources used for testing various kinds of metallic materials in different corrosive media, extraction methodology, employed characterization of OGCIs, findings, and prospective future challenges.

Table 5:

Summary of literature on sources of previously used OGCIs.

OGCI sourceExtraction methodologyMaterial tested; solution usedOGCI characterization; laboratory analysisFindingsFuture challengesReferences
Camellia sinensis (green tea)Dried and ground leaves subjected to reflux in 70% acetone for 4 hMild steel in 1 m HClSEM, EIS, WLM, FTIR, EDX1. 79% inhibition efficiency achieved in 200 ppm solution

2. Inhibition effect increases with an increase in solution concentration and temperature
Adsorption kinetics and isotherms studies were not examinedNofrizal, 2012
R. longiflora extractExtractionMild steel in 1 m HCl and 0.5 m H2SO4PDP, EISIncrease in corrosion inhibition efficiency as extract concentration and temperature increase1. Extraction methodology was not presented

2. Extract was not characterized for functional groups inhibiting corrosion
Akalezi et al., 2015
A. djalonesis leaf extract20 g dried leaves under reflux for 3 h in 1 m HCl and 0.5 m H2SO4Mild steel in 1 m HCl and 0.5 m H2SO4EIS, PDP, DFT-based QCC1. Corrosion inhibition via mixed-type inhibition mechanism

2. Djalonenoside (DJN) and its hydrolysis product DJN-hyd were extracts enhancing corrosion inhibition in the medium
Corrosion of other metals besides mild steel was not investigatedOgukwe et al., 2012
Theobroma cacao peel polar extractBoiling dried pods under reflux for 4 h in 1.0 m HClMild steel in 1 m HClWLM, EIS, PDP1. Increase in corrosion inhibition efficiency as OGCI concentration increases but decreased with temperature

2. Langmuir isotherm was obeyed
More metallic materials should be testedYetri et al., 2014
o,m,p-Decanoyl thiourea derivativesMixed substitution and addition reaction using decanoyl chloride, ammonium thiocyanate, and 2-aminopyridine in acetone solution for 10 minMild steel in 0.1 m H2SO4FTIR, 1H and 13C NMR1. Compound D3 of the derivatives possessed the highest efficiency

2. Compound corrosion inhibition efficiencies affected by N atom at o-, m-, and p-positions affects the pyridine chemical structure
1. Only mild steel was considered

2. Limited laboratory analysis

3. Although chemical structures were presented, the structural morphology of the synthesized inhibitors need be investigated
Kamal et al., 2014
Extracts of D. kaki L.f huskHusk powder heated under reflux with water or alcohol for 4 hQ235A steel in 1 m HClPDP, GM1. Extracts behaved like a mixed-type inhibitor

2. Extracts exhibited antibacterial activity against microbial influenced corrosion (MIC) of oil field microorganism
1. There is a need to fully explore the corrosion inhibitory feature of extracts from this biomass in other corrosion types besides microbial influenced corrosion

2. SEM analysis was not carried out to ascertain that the corrosion type inhibited on the metal surface by the extracts was exactly MIC
Zhang et al., 2013
Schiff bases8 h refluxing of 3-amino-2-methylquinazolin-4(3H)-one with 4-hydroxybenzaldehyde and N,N-dimethyl-4-aminobenzaldehyde in acetic acidMild steel in 1.0 m HClSEM, NMR, DFT1. p-position substituent enhanced inhibition efficiency

2. Inhibition efficiency relies on OGCI nitrogen amount and their molecular weight and concentration
1. m-position substituent on OGCI molecule affected inhibition efficiency negatively

2. Corrosion type prevented was not specified

3. Only mild steel was used to test the inhibitor efficiency
Jamil et al., 2018
G. biloba leaf extractsPowder was heated with water or alcohol for 4 h after drying under 60°CQ235A steel in 1 m HClPDP1. Extracts exhibited mixed-type inhibitor behavior

2. Extract revealed antibacterial activity against oil field microorganism
1. More laboratory analysis is required to corroborate the findings

2. Although corrosion mechanism and antibacterial activity of the extracts were presented, the adsorption and kinetic studies of the process are needed in future studies
Chen et al., 2013
Musa sapientumMild steel in 0.5 m H2SO4ECM1. Decrease in corrosion rate as inhibitor concentration increases

2. Physisorption was experienced
1. Only acidic medium was examined

2. Reaction kinetics was not studied

3. Limited laboratory analysis
Salami et al., 2012
Isoxazolidine derivatives of aldehydeNitrone cycloaddition reactionMild steel in 1 m HCl, 0.5 m H2SO4, CO2-saturated 0.5 m NaClGM, LPR, TEM, EIS, ST1. Inhibitor molecules primarily acted as anodic inhibitors

2. Adsorption of inhibitors on the metal surface was due to physisorption and chemisorption

3. Surface tension revealed the formation of film on the metal surface by inhibitor molecules

4. Inhibitor molecules fitted well using Temkin isotherm in both acids

5. Langmuir isotherm performed excellently in CO2-saturated saline media
1. Solution media were not tested in the absence of synthesized inhibitors for comparison purposes

2. The kinetics of the process was not studied
Mazumder and Jafar, 2016
Hydroxyethyl-imidazoline derivatives based on coffee oilCarbon steel in CO2-saturated emulsion at 50°CEIS1. Inhibitor decreased corrosion rate by >99.9%

2. Unprotected sites were linked to electrostatic repulsion forces between the negative charges and insufficient added concentration to form protective film
1. Unprotected sites were formed on the metal surface

2. Reference was made to a previous study for synthetic methodology used

3. Only EIS was used to check inhibitor efficiency
Porcayo-Calderon et al., 2015
Sida acuta (Malvaceae) leavesImmersion of pounded leaves into HCl at 15, 30, and 45 g/l of 0.7, 1.2, and 2.2 m HClMild steel in 0.7, 1.2, and 2.2 m HClWLM1. 71.16% maximum inhibition efficiency attained at 15 g/l of 0.7 m HCl with corrosion rate reduction from 1.0485 to 0.3006 mg/cm2/h

2. Phytochemical analysis revealed the presence of alkanoids, tannins, saponins, phytates, flavonoids, and phenol

3. Out of the examined isotherms, experimental data fitted well into Langmuir
1. SEM image revealed the degradation of mild steel surface in an uninhibited 0.7 m HCl to be localized

2. Only WLM as a means of checking inhibitor efficiency was used

3. Only acidic medium

4. Only mild steel
Ndukwe and Anyakwo, 2017
P. macrocarpus plantBoiling dried leaves under reflux for 3 h in 1.0 m HCl and 0.5 m H2SO4Mild steel in 1 m HCl and 0.5 m H2SO4GM, PDP, EIS1. EIS data revealed organic matter extract influence on corrosion inhibitory effect on mild steel

2. Inhibition efficiency increased with an increase in concentration and temperature up to 50°C

3. Lower activation energy in the presence of corrosion inhibitor resulted from the adsorption chemisorptive nature
1. The influence of the inhibitor in alkaline medium was not investigated

2. Only mild steel was examined

3. The kinetics of the process was not studied

4. The efficiency of the inhibitor used was not compared to those of previous inhibitors used by the researchers
Akalezi et al., 2015
Hibiscus rosa-sinensis leaf extractMild steel in 1 m HClWLM, EIS1. Inhibition efficiency increased as temperature and solution concentration increased

2. OGCI behaved as mixed type

3. Spontaneous reaction

4. Data agreed well with Langmuir, Flory-Huggins, and Freundlich adsorption isotherms
1. Limited laboratory analysis to affirm the inhibitor efficiency

2. Only mild steel in only acidic medium was investigated
Desai, 2015
D-glucose derivativesMulticomponent reactionsMild steel in 1 m HClSEM, EDX, AFM, EISThe presence of -OH and -OCH3 groups exhibited higher inhibition efficiencyEads values did not exhibit any regular trend for aqueous and protonated inhibitor moleculesVerma et al., 2017
Silica extract from rice husk ash+Na2OSilica extract was prepared by mixing 80 ml of 2.5 m NaOH with rice husk ash produced by calcination at 600°C for 6 h. 0.2 m NaOH and distilled water were then added to form the inhibitor99.9% Cu, Al alloy (AA6061), carbon steel (SAE1045) in 0.5 m HClXRF, XRD1. Each metal alloy influenced the optimal SiO2:Na2O ratio determination sodium silicate formulation

2. Used silicate-based inhibitor has a potential of inhibiting corrosion in tested samples under examined acidic medium
1. Limited laboratory analysis for more confirmation of inhibitor efficiency

2. Only acidic medium solution was tested

3. More metallic samples should be examined
Mohamad et al., 2013
Gum arabicMild steel and Al in H2SO4WLM, TT1. Inhibition efficiency increases with an increase in the concentration of the inhibitor

2. Inhibitor obeyed Temkin adsorption isotherm for tested samples

3. Mild steel corrosion was chemical adsorption, whereas Al corrosion was physical adsorption

4. Inhibitor acted better on Al than mild steel with adsorption being spontaneous
1. The methodology of inhibitor extraction was not adequately presented

2. The kinetics of the adsorption process was not presented

3. The reaction mechanism of the inhibitor adsorption process on mild steel and Al process was not available

4. Limited laboratory analysis to support inhibitor efficiency on samples
Umoren, 2008
Coconut coir dust extractHydrogen evolution extraction methodAl corrosion in 1 m HClWLM, HEM1. As temperature and concentration increased, inhibition efficiency increased

2. Langmuir isotherm was obeyed
1. Only Al was considered. It would be better if inhibitor efficiency is tested in other metals

2. Also, only HCl as acidic medium was tested. Both acidic and alkaline solutions should be checked
Umoren et al., 2006
Jatropha stemJatropha fine powder obtained by sun drying and grinding soaked in ethanol for 24 h. Evaporation of filtrate to remove excess alcoholMild steel in seawaterSEM, WLM, FTIR1. Coupons without inhibitor corroded more in seawater than those with inhibitor

2. Presence of active corrosion inhibitors Jatropha extracts revealed by FTIR

3. Maximum inhibition efficiency of 81.7% at 0.90 g/l inhibition concentration
1. Adsorption isotherm and thermodynamics were not studied

2. Few laboratory analysis for more affirmation of inhibitor efficiency

3. Inhibitor influence and efficiency in acidic and alkaline media were not investigated
Olawale et al., 2016
Tobacco extractExtraction by weighing aqueous solutions, boiling of water, and weighing residue1008/1010 cold-rolled steel and 3105 H24 Al Q-panels in 1–3% NaCl solutionZRA, PDP, WLM1. Tobacco extracts proved to be excellent inhibitors for the corrosion of Al and steel in alkaline solution

2. Extract also worked in acidic solution and could prevent corrosion during descaling processes

3. Inhibition effect greater than chromates within a solution concentration range as low as 100 ppm
1. Thermodynamics, kinetics, and adsorption isotherm equilibrium of inhibitor effect were not investigated

2. Inhibition effect in other media was not investigated
Davis et al., 2001
Citrus aurantiifolia leavesDried and ground leaves under reflux for 3 h in 1 m H2SO4Mild steel in 1 m HClSEM, WLM1. Corrosion inhibition increases with an increase in solution concentration with 97% efficiency

2. Experimental data conformed to Langmuir isotherm
Only mild steel in acidic medium was investigatedSaratha et al., 2009
Cashew wasteSun dried and pulverized fruits soaked in 250 ml ethanol for 24 hMild steel in 1 m HCl and 0.1 m H2SO4WLM, SEM, FTIR1. Inhibitor efficiency increased with an increase in inhibitor concentration with optimum 80.5%

2. Cashew waste was seen as a valuable corrosion inhibitor
1. Adsorption kinetics, isotherms, and thermodynamics were not studied for in-depth investigation

2. Only mild steel in acidic medium was investigated
Olawale et al., 2015
Locust bean gumCarbon steel 39, 44, and B500 in H2SO4PDP, EISInhibition effect on steel 39 in acidic medium in the presence of NaCl was revealedAlthough different carbon steel samples were tested, there was a shallow investigation on the extracted corrosion inhibitor on the examined samplesJano et al., 2012
Extract of banana peel+ZnCarbon steel in distilled waterAFM, WLM, GM, FTIRZn addition decreased inhibition efficiency. It later increases after increasing Zn concentrationNo mathematical model was presented as a predictive tool for the future corrosion of the sample testedSangeetha et al., 2012
Fig leaf extractExtraction by reflux in Soxhlet extractor for 5 h in alcoholMild steel in 2 m HClWLM, EIS1. Mixed-type inhibitor formed 2. Data support Langmuir isotherm 3. 87% inhibition efficiency with 200 ppm inhibitor solutionOnly mild steel in acidic medium was examinedTaleb and Mohamed, 2011
Rice straw extractReflux of 10 g rice straw in 100 ml of 1 m HCl for 1 hMild steel in 1 m HClEIS, PDP1. Physisorption of rice straw extract on mild steel electrode surface

2. Both anodic and cathodic reactions were inhibited, whereas OGCI exhibited mixed type

3. Adsorption data obeyed Langmuir isotherm
1. Few characterization of inhibitor extracte

2. Only mild steel in HCl alone
Mahross et al., 2016
Dialum guineense and Euphorbia hirta leaf extractsSome grams of dried leaves subjected to reflux for some hours 0.5 m H2SO4Al alloy (AA8011) in HClGM1. Both extracts behaved as good inhibitors

2. Inhibition efficiency improved with concentration

3. Experimental data conformed with Langmuir adsorption isotherm
Limited laboratory analysis for more investigation of inhibitor efficienciesAnozie et al., 2011
Extract of F. asperifoliaPulverized stem was obtained after oven drying at 40°C. Powdered stem bark was then soaked in 80% ethanol for 72 hMild steel in 5 m HClWLM, SEM, FTIR, AAS1. Inhibition efficiency increased with an increase in extract concentration but decreased with temperature

2. Inhibition of corrosion resulting from mixed plant constituent adsorption on the metal surface by spontaneous reaction

3. Extracts contain tannins, alkaloids, anthraquinones, flavonoids, saponins, and reducing sugars

4. Kinetic studies revealed that data agreed with first-order reaction
1. Although detailed kinetics, isotherm, and thermodynamic studies of adsorption process were presented, limited laboratory analysis for inhibitor characterization was done

2. Only mild steel in acidic medium was examined
Fadare et al., 2016
Seeds of Areca catechuAfter drying and grinding, seeds were subjected to reflux for 8 h in ethanol solutionMild steel in 0.5 m HClWLM, PDP, EIS, SEM, FTIR1. Mixed-type inhibitor revealed

2. 96.97% corrosion inhibition achieved at 500 ppm inhibitor concentration

3. Experimental data agreed with Temkin isotherm
1. Only mild steel was examined

2. Higher inhibitor concentration required to achieved better corrosion inhibition
Kumar et al., 2011
Elephant grass extract (Pennisetum purpureum)10 g dried and pulverized leaves soaked in 100 ml ethanol for 48 h. Filtrates further subjected to evaporation to obtain ethanol-free sampleMild steel in 1 m HClAAS, SEM, FTIR1. Steel dissolution rate sensitive to extract concentration in acidic solution. Mass loss and corrosion rates decreased with an increase in extract concentration

2. Inhibition influenced by absorption via the presence of extract functional groups
Although detailed work was done, the inhibition effectiveness of elephant grass extract was only tested using mild steel in HCl as acidic mediumAlaneme et al., 2016
3. Langmuir adsorption isotherms and activation energies revealed physical adsorption

4. SEM images of corroded substrates showed primary corrosion mechanism to be by pitting

5. 95% Inhibition efficiencies at room temperature achievable

6. Corrosion inhibition increased with an increase in extract concentration but decreased with increasing temperature
Guar gum1. Pods dried in sunlight and separated manually from seeds

2. Seeds heated under reflux with water or alcohol for 6 h
Carbon steel in 1 m H2SO4+NaClWLM, EIS, PDP1. Increase in resistance of pitting corrosion was exhibited

2. Guar gum acted as a mixed-type inhibitor whose efficiency increases with an increase in concentration

3. All data supported Langmuir adsorption isotherm
Abdallah, 2004
Oil palm frondNitrobenzene oxidation method for lignin depolymerizationMild steel in 1 m HClWLM, PDP, EIS, SEM, XRD1. Inhibition efficiency increased with increased concentration of lignin depolymerized products

2. Mixed-type inhibitors revealed

3. Experimental data well fitted with Langmuir adsorption isotherm

4. Adsorption was dominated by physisorption

5. SEM revealed reduction of surface roughness in the presence of an inhibitor
1. Only Langmuir isotherm was used. For comparative purposes, other existing isotherms should be used

2. Oil palm frond extracts have been shown to have potential of corrosion inhibition in alkaline medium. Thus, various types of metallic materials should checked in alkaline medium
Shah et al., 2017
Celery seeds (A. graveolens)Ground and powdered seed boiled in distilled H2O for 2 h. Filtrate evaporated to dryness and residue used high concentrated stock solutionCarbon steel in 1 m HClWLM, PDP1. Optimum inhibition efficiency obtained at 500 ppm inhibitor concentration

2. Spontaneous adsorption process that conforms to Temkin isotherm

3. Percent inhibition efficiency decreased with increased temperature

4. Inhibition efficiency increased as celery doses increased
1. Only WLM and PDP were used to check inhibitor efficiency

2. Only HCl was used for carbon steel alone to check OGCI efficiency

3. Active functional groups present in A. graveolens seeds enhancing corrosion inhibition were not deeply investigated
Megahed et al., 2017
Eichhornia crassipes (water hyacinth) leaves and roots4 g dried and ground leaves and roots soaked in 1000 ml of 5 m HClMild steel in HClDFT, GT1. Root and leaf extracts performed excellently well as effective OGCIs

2. Physisorption of extract organic constituents on corroding mild steel surface
1. Insufficient laboratory analysis

2. Equilibrium isotherms and kinetics were not investigated
Ulaeto et al., 2012
G. venusta plant extractG. venusta cut into pieces, dried for 3 days, and ground into powder. Product was refluxed for some hours using ethanolMild steel in 0.5 m H2SO4SEM1. Corrosion rate was reduced when OGCI concentration was increased above 2% (v/v) with time

2. Increase in temperature massively increased corrosion rate

3. Plant extract exhibited effective corrosion inhibition potential for mild steel in acidic medium

4. At 8% (v/v) optimum concentration of plant extract in acid solution, 86.47% highest efficiency was obtained
1. Only SEM was used to authenticate inhibitor efficiency

2. Thermodynamics of adsorption process was not studied
Suleiman et al., 2013
Crude glycerol from residue of biodiesel produced from a plant seedTransesterification processSteel in 0.5 m HCl at 25°CWLM, SEM, PDP1. Corrosion inhibition increased with inhibitor concentration

2. Maximum inhibition efficiency of 98%) was achieved after 70 h of residence time with 1% inhibitor concentration
1. Plant source of oil used for biodiesel production from which glycerol was obtained was not mentioned

2. Inhibition efficiency remained unchanged after residence time
Al-Zubaidi et al., 2018

  1. AAS, Atomic absorption spectroscopy; AFM, atomic force microscopy; DFT; density functional theory; ECM, electrochemical measurements; EDX, energy-dispersive X-ray spectroscopy; FTIR, Fourier transform infrared; GM, gravimetric method; GT, gasometric technique; HEM, hydrogen evolution method; LPR, linear polarization resistance; NMR, nuclear magnetic resonance; QCC, quantum chemical computation; SEM, scanning electron microscopy; ST, surface tension; TEM, transmission electron microscopy; TT, thermometric techniques; XRD, X-ray diffraction; XRF, X-ray fluorescence; ZRA, zero-resistance ammeter.

2.1 Industrial applications of OGCIs

Industrial applications of corrosion inhibitors from greeners can be found in petroleum production, steel pipeline-making industry, refrigeration industry, automobile, paint industry, acid-producing companies, and so on. Table 6 summarizes the industrial applications of OGCI with active functional groups responsible for each application.

Table 6:

Industrial applications of OGCIs.

Industrial applicationActive functional groups; complexes; ingredientsInhibitor source from greener%IEHow it works; how to solve the problemSide effectsReferences
Petroleum productionPyrocatechol

4-Methylpyrocatechol

4-n-Butylpyrocatechol

4-n-Hexylpyrocatechol
−14

84

93

96
Petroleum industries are characterized by wet corrosion of materials as a result of aqueous phase existence, which may contain H2S, CO2, and Cl. The injection of these film-forming long-chain nitrogenous inhibitors anchors to the metal surface via existing polar group. The nonpolar tail extends out vertically such that the physisorption of hydrocarbons on them increases the thickness of the film coupled with hydrophobic barrier effectiveness to prevent corrosionEmulsification occurs, which leads to foaming as a result of inhibitors being interfacial in natureUhlig and Revie, 1985
Steel pipeline internal corrosionGalactose and mannose

Iridoid glucoside and dibenzo-α-pyrone

Flavonoids and terpenoids
Guar gum

Leaf extract of A. djalonesisG. biloba leaf extracts
86

97

98
Flow-induced corrosion and erosion-corrosion are influenced by the high flow rates of multiphase fluids in steel pipelines. At low flow rates, corrosion pitting occurs due to sediment formation at the bottom. The inhibitors being mixed type prevent corrosion by physical adsorption, chemisorption, and film formation. Also, the pigging of steel pipelines is employed to avoid internal corrosionDue to mixed reaction, unwanted products and intermediates may be formed in the course, causing the formation of unwanted sedimentsAbdallah, 2004; Obame et al., 2008
AutomobilesPhosphates and silicates

Fatty acids, phosphonates, and sulfonates
Rice husk extract

D. kaki L.f husk extracts

Oil palm fond
92

95
The inhibitors dissolve in antifreeze to prevent internal corrosion caused by coolants, aeration, temperature, flow, and so on. External corrosion is controlled by mixing additives such as grease, wax resin, metalloorganic, and asphaltic compounds that enhance film formation on the metal surfaceFoaming due to emulsification occursZhang et al., 2013
Paint industryCalcium plumbate, lead azelate, and lead suboxideThe displacement of water by polar compounds in inhibitors occurs after which they arrange themselves with hydrophobic ends facing the environment. The augmentation of coating bonding on the metal surface occurs afterIntermediate pigments may be formed
Water transmission industryPhosphates, amine, volatiles (cyclohexylamine and morphine)Tobacco extract78.3Inhibitors anchor to the metal using their polar group, which increases film thickness and hydrophobic barrier effectiveness for corrosion inhibitionThe interaction between the organic inhibitor and water makes the water unsuitable for domestic usage in most casesAbdel-Gaber et al., 2011
Refrigerating industryBenzotriazole

p-Hydroxybenzoic acid and vanillic acid
A. djalonesisOil palm frond88

67.8
Galvanic corrosion evolves due to the increase in dissolved mineral salt content as evaporation proceeds with the presence of several dissimilar metals and nonmetals. Inhibitors control corrosion by film formation that inhibits anodic metal dissolution reaction and cathodic poisoningMatsuda and Uhlig, 1964; Obame et al., 2008
Building constructionPhosphate ionWhen mixed with cement, the durability of reinforced concrete structures is improvedYohai et al., 2013
BoilerAmmonia, alkanol, cyclohexylamine, and morpholineCorrosion attack of pipes prevented by solubilization of limescaleSanyal, 1981

3 Mathematical modeling of OGCI influence on metals

3.1 Kinetics of corrosion modeling

3.1.1 Anodic modeling

To model the kinetics of corrosion at the anode, the following assumptions are made: (1) anodic icorr density is used for Fe2+ ion boundary condition at anode, (2) anodic icorr density accounts for Fe2+ ion generation via electrochemical reactions on the metal surface as the source term, (3) zero concentration of Fe2+ ion is applied at cathode due to scale formation, (4) Fe2+ ion concentration in the shielded solution is the same as bulk solution in chemical equilibrium, and (5) H+(CH+) and CO2(CCO2) surface concentrations enhance the rate of corrosion via exchange current density. Thus, the anodic electrochemical reaction is given as Eq. (15) (Popoola et al., 2013):

(15)FeFe2++2e

The anodic icorr density is calculated using Eq. (16) (Tafel’s law):

(16)iFe2+=i0,Fe2+10ϕaϕrev,Fe2+bFe2+

where iFe2+=iron oxidation current density (A/m2), i0,Fe2+=iron oxidation exchange current density (A/m2), ϕrev=reversible potential of iron oxidation (V), ϕa=anodic potential (V), and b=Tafel slope of oxidation (V).

The iron oxidation exchange current density (i0,Fe2+) in Eq. (16) is determined from Eq. (17) (Nordsveen et al., 2003):

(17)i0,Fe2+=i0,ref(CH+CH+,ref)a1(CCO2CCO2,ref)a2eΔHR(1T1Tref)

where i0,ref=reference exchange current density (A/m2), CH+=surface concentration of hydrogen ion (mol/l), CH+,ref=reference hydrogen ion concentration (mol/l), CCO2=surface concentration of CO2 (mol/l), CCO2,ref=reference CO2 concentration (mol/l), ∆H=change in enthalpy (kJ/mol), R=gas constant (J/mol K), T=solution temperature (/K), and Tref=reference temperature (/K).

The mass flux of Fe2+ at anode (JFe2+) is determined by Eq. (18) (Gavrilov et al., 2007):

(18)JFe2+=iFe2+nFe2+F

where JFe2+=mass flux of Fe2+ at anode (mol/m2 s), iFe2+=current density of iron oxidation (A/m2), F=Faraday’s constant (C/mol), and nFe2+=number of moles of Fe2+ (mol).

3.1.2 Cathodic modeling

The derivation of equations governing the kinetics of corrosion at the cathode is based on the assumption that oxygen and water reduction in the system is negligible such that the two cathodic reactions are stated as Eqs. (19) and (20) (Nesic et al., 1996):

(19)2H++2eH2
(20)2H2CO3+2eH2+2HCO3

A general form used in the calculation of H+ reduction partial cathodic icorr densities and H2CO3 reduction is stated as Eq. (21) (Nordsveen et al., 2003):

(21)ic=i010ϕcϕrevbηScale

where ic=current density of any cathodic reaction (A/m2), i0=cathodic reaction exchange current density (A/m2), ϕc=cathodic potential (V), ϕrev=cathodic reaction reversible potential (V), b=cathodic Tafel slope (V), and ηscale=scale factor at cathode.

The exchange current densities of H+ and H2CO3 reduction at cathode are determined using Eq. (17). The electric field in the solution is governed by Poisson’s equation stated as

(22)2ϕ=Fεi=1nzici

where ε=dielectric constant and ϕ=potential (V).

For electroneutrality condition in the solution, Eq. (22) reduces to

(23)i=1nzici=0

Thus, Eq. (23) reduces to

(24)2ϕ=0

3.1.3 Electrochemical modeling

Assuming that corrosion rate is governed only by electrochemical reaction, the total anodic reaction current density is used in determining the corrosion rate of CO2 stated as (Nešić et al., 2009)

(25)CR=iaMw,FeρFenF

where CR=corrosion rate (mm/y), ia=anodic current density (A/m2), Mw,Fe=atomic mass of iron (kg/mol), ρFe=density of iron (kg/m3), n=number of moles of electrons involved in iron oxidation (2 mol/mol), and F=Faraday’s constant.

The current density for iron dissolution is obtained by Eq. (26) stated as (Anderko and Young, 1999)

(26)ia,Fe=io,Fe×10[αFeF(EcorrErev,Fe)RT]

The Tafel slope of iron oxidation bFe as defined as

(27)bFe=RTαFeF

where R=ideal gas constant (J/mol K), T=temperature (K), F=Faraday’s constant, and αFe=iron dissolution constant.

Thus, Eq. (26) reduces to

(28)ia,Fe=io,Fe×10[(EcorrErev,Fe)bFe]

where ia,Fe=current density for iron dissolution (A/m2), io,Fe=exchange current density of iron oxidation (A/m2), Ecorr=corrosion potential (V), Erev,Fe=reversible potential of iron oxidation (V), and bFe=Tafel slope of iron oxidation (V).

The current density of any cathodic reaction is calculated as (Craig, 1995)

(29)1ic=1ict+1ilim

where ic=cathodic reaction current density (A/m2), ict=charge transfer current density component (A/m2), and ilim=limiting current density component (A/m2).

The charge transfer current density of cathodic reactions (ict) is determined by (Chokshi et al., 2005)

(30)ict=io10ηbc

where io=exchange current density of cathodic reactions (A/m2), η=EErev is the overpotential (V), E=potential (V), Erev=reversible potential (V), and bc=cathodic Tafel slope (V/decade).

The limiting current is determined from the mass transfer limitation for the case of H+ reduction. Thus,

(31)ilim(H+)d=kmF[H+]b

where ilim(H+)d=diffusion limiting current density (A/m2), km=mass transfer coefficient of corrosive species (m/s), [H+]b=bulk hydrogen ion concentration (mol/m3), and F=Faraday’s constant (96,490 C/equiv).

Suppose that there is a restriction of carbonic acid reduction due to CO2 hydration reaction rate being very slow, the limiting current density (ilim(H2CO3)r) is calculated as (Vetter, 1961)

(32)ilim(H2CO3)r=F[CO2]b(DH2CO3Khydkhydf)0.5

where [CO2]b=bulk concentration of dissolved CO2 (mol/m3), DH2CO3=diffusion coefficient of H2CO3 in water (m2/s), Khyd=equilibrium constant for CO2 hydration reaction, and khydf=forward reaction rate constant for CO2 hydration reaction (/s).

A theoretical flow multiplier f for Eq. (32), which takes into account the flow effect on the chemical reaction limiting current, is calculated by (Nešić et al., 2009)

(33)f=1+e2δm/δr1e2δm/δr

where δm=mass transfer thickness (m) and δr=reaction layer thickness (r) whose values are determined by Eqs. (34) and (35), respectively:

(34)δm=DH2CO3km,H2CO3

and

(35)δr=DH2CO3Khydkhydf

3.2 Rate modeling of corrosion-type inhibition using OGCIs

3.2.1 Pitting corrosion

The risk of pitting corrosion can be increased under stagnant conditions in which corrosive microenvironments are established on the surface. The accumulation of stagnant electrolyte at the bottom of pipes, tubes, and tanks can be prevented by both drying and ventilation. The build-up of local highly corrosive conditions can also be prevented through agitation (Roberge, 2000). The pitting corrosion rate, defined as Fe2+ ion mass flux leaving the metal surface, can be determined using Eq. (36) based on the following assumptions: (1) pitting corrosion results in the removal of Fe2+ ion from the metal surface by diffusion and electromigration and (2) Fe2+ ion distribution in the solution is governed by Fick’s second law. Thus, Fe2+ flux can be solved using Nernst-Planck equation (Papavinasam et al., 2005):

(36)JFe2+=DFe2+CFe2+FzFe2+TDFe2+CFe2+ϕ

where J=mass flux (mol/m2 s), D=diffusivity (m2/s), C=concentration (mol/l), F=Faraday’s constant (C/mol), z=valence (mol/mol), ℜ=ideal gas constant (J/mol K), T=absolute temperature (K), and ϕ=electric potential (V).

The pitting corrosion rate is determined after computing the distributions of Fe2+ ion and electrical field in the solution such that change in concentration with time (Ct) is determined as (Hoeppner et al., 1981)

(37)Ct=(J)+R

where R=source term of chemical reactions in the solution.

3.2.2 Stress corrosion cracking (SCC)

SCC in metallic materials has been discussed previously (Popoola et al., 2013). The outstanding models for predicting SCC rate are the active path dissolution and film rupture model and hydrogen-assisted cracking model. Active path dissolution occurs as a result of accelerated corrosion within a narrow path with higher corrosion susceptibility in comparison to the overall material or structure. In contrast, hydrogen-assisted cracking involves the entrapment of H atoms onto the metal crystal structure and the subsequent local cracking resulting from the local pressure build-up (Mohanty et al., 2012).

3.2.2.1 Active path dissolution and film rupture model

For this, the crack growth rate or crack velocity (a˙) is given as (Ford and Andresen, 1988)

(38)a˙=dadt=A(ε˙ct)n

where A and n=constants related to the material and environmental composition at the crack tip and ε˙ct=crack tip strain.

The pure oxidation dissolution Faradic equation is stated as (Hall, 2009)

(39)a˙=dadt=MzρFita

where M=atomic weight, z=oxidation number, ρ=density (kg/m3), F=Faraday’s constant, and ita=anodic current density at time t.

The anodic current density at time t is given as (Eason and Nelson, 1994)

(40)ita=ioa(tot)n

where ioa=base metal dissolution rate parameter and to=repassivation time scaling parameter (s or min or h).

Substituting Eq. (40) into (39) gives

(41)a˙=dadt=MzρFioa(tot)n

If a=MzρFioa,Eq. (41) reduces to

(42)a˙=dadt=a(tot)n

Integrating Eq. (42) over time from totf and averaging the SCC growth rate over tfto, the average SCC growth rate is expressed as

(43)a˙¯=1tftoa˙dt=a1n(totf)n

where tf=time per oxide fracture event (s or min or h) defined as

(44)tf=εfε˙ct

where εf=oxide film rupture strain rate and ε˙ct=crack tip strain rate.

Assuming that ε˙ct and tf are constant and independent of time, Eq. (44) can be substituted in Eq. (43) to give the average SCC growth rate as

(45)a˙¯=a1n(toεf)n(ε˙ct)n
3.2.2.2 Hydrogen embrittlement

The related SCC model is based on the assumption that crack advance occurred due to the hydrogen-assisted creep fracture of hydrogen embrittled grain boundaries. Thus, the SCC growth rate is expressed as (Mohanty et al., 2012)

(46)a˙=rct=rc(εfε˙cfz)=rcε˙cfzεf

where rc=radius of the fracture zone in front of the crack tip (mm), ε˙cfz=strain rate in the creep fracture zone, and εf=critical fracture strain.

The critical fracture strain (εf) can be stated as (Symons, 2001)

(47)εf=εfo(CoCgb)12

where Cgb=grain boundary hydrogen concentration, εfo=fracture strain at a reference grain boundary hydrogen concentration, and Co=reference grain boundary hydrogen concentration.

Substituting Eq. (47) into Eq. (46) gives

(48)a˙=rcε˙cfzεfo(CgbCo)12

3.2.3 H2S (sour) corrosion

H2S corrosion of mild steel proceeds predominantly through a solid-state reaction as a result of a dense, very thin, and protective nonstoichiometric iron sulfide film formation called mackinawite (Sun and Nesic, 2006); Popoola et al., 2013). Assuming that H2S corrosion is controlled by mass transfer resulting from the presence of mackinawite layers and the liquid boundary layer, H2S flux through the mass transfer boundary layer is expressed as (Nešić et al., 2009)

(49)JH2S=km(H2S)(cH2Sco(H2S))

The flux of H2S through the porous outer mackinawite layer is calculated as

(50)JH2S=DH2Sεψδos(co(H2S)ci(H2S))

The flux of H2S through the inner mackinawite film is determined using

(51)JH2S=AH2SIn(ci(H2S)cs(H2S))

From Eq. (49),

(52)co(H2S)=cH2SJH2Skm(H2S)

From Eq. (50),

(53)co(H2S)=ci(H2S)+JH2SδosDH2Sεψ

At steady state, the fluxes through different layers are equal to each other. Equating Eqs. (52) and (53) and making ci(H2S) the subject, we have

(54)ci(H2S)=cH2SJH2S[1km(H2S)+δosDH2Sεψ]

The corrosion rate caused by H2S in metallic materials is obtained by substituting Eq. (54) into Eq. (51). Thus,

(55)CRH2S=AH2SIncb,H2SJH2S[1km(H2S)+δosDH2Sεψ]cs,H2S

where CRH2S=corrosion rate caused by H2S (mol/m2 s), AH2S=constant for solid-state diffusion, cb,H2S=concentration of H2S in bulk solution (mol/m3), cs,H2S=concentration of H2S at the steel surface (mol/m3), JH2S=flux of H2S at different mackinawite layers (mol/m2 s), km(H2S)=mass transfer coefficient of H2S in the liquid boundary layer (m/s), δOS=outer scale thickness (m), DH2S=diffusion coefficient of H2S in water (m2/s), ε=outer mackinawite scale porosity, and ψ=outer mackinawite scale tortuosity.

3.3 OGCI adsorption isotherms

Adsorption isotherms play A key role in giving detailed information about the existing interaction between molecules of OGCIs and the metal surface (Herrag et al., 2010) to prevent the dissolution reaction of such metal in the corrosive medium. Influencing factors on the adsorption process using OGCIs include the (1) structure of OGCI compounds, (2) types of corrosive media under examination, (3) nature of surface-charged metals, (4) electronic characteristics of the metal surface, and (5) charge distribution in the molecules of OGCIs (Vracar and Drazic, 2002); Khaled, 2010). The values obtained from the simulation of existing isotherm models relating surface coverage (θ) and OGCI concentration together describe the most suitable adsorption isotherm for the process. The prominent OGCI adsorption isotherms have been stated to be Langmuir, Temkin, Frumkin, Freudlich, Virial Parson, and Bockris-Swinkels isotherms (Gopal et al., 2011), which are summarized in Table 7 with their verification plots and significance of obtained values. The most suitable adsorption isotherm that best describes the adsorption nature of OGCI on the examined metal surface will give a correlation coefficient (R2) value that is very close to unity or equal to 1.

Table 7:

OGCI adsorption isotherm models and significance of the values obtained.

IsothermModelPlotSignificance of valuesReferences
Temkinθ=(1f)InKCOGCI Concθ vs. log COGCI ConcIf f=0 (no interaction), f=+ve (attraction), and f=−ve (repulsion) between OGCI molecules and the metal surfaceTemkin and Pyzhev, 1940
Virial Parsonθe2fθ=Kads.COGCI Concθ vs. log(θCOGCI conc)1. If f=0 (no interaction), f=+ve (attraction), and f=−ve (repulsion) between OGCI molecules and the metal surface

2. If a larger value is obtained for Kads (5×10−3–20×10−3/m), it is an indication of a strong adsorption capacity of OGCI attributed to abundant p-electron in conjugated double or triple bonds between OGCI and vacant d-orbital of the metal specimen
Smialowska and Wieczorek, 1971
Langmuirθ(1θ)=KadsCOGCI concθ(1θ) vs. log COGCI ConcA smaller Kads implies a weak adsorption capacity of OGCILangmuir, 1947
Freudlichlogθ=logKads+nlogCOGCI Conclog θ vs. log COGCI Conc1. n>1 implies a favorable adsorption of OGCI molecules on the metal surface.

2. A larger Kads implies a strong adsorption capacity of OGCI
Piccin et al., 2011
Bockris-Swinkelsθ(1θ)n[θ+n(1θ)]n1nn=COGCI ConceKads55.4θ(1θ) vs. log COGCI Conc1. n<1 implies an unfavorable adsorption of OGCI molecules on the metal surface.

2. A smaller Kads implies weak adsorption capacity of OGCI
Bockris and Swinkels, 1964
Frumkin[θ(1θ)]efθ=KadsCOGCI Concθ vs. log COGCI ConcIf f=0 (no interaction), f=+ve (attraction), and f=−ve (repulsion) between OGCI molecules and the metal surfaceTrasatti, 1974

  1. θ, Surface coverage; COGCI Conc, concentration of bulk OGCI (mm); f, OGCI interaction parameter; K, constant; Kads, adsorption equilibrium constant (mol−1 dm3 or m−1); n, number of H2O molecules replaced per OGCI molecule.

3.4 OGCI adsorption thermodynamics

The consideration of thermodynamic studies in OGCI adsorption reveals the significance of Gibbs free energy of adsorption (ΔGadso), enthalpy of OGCI adsorption (ΔHadso), entropy of OGCI adsorption (ΔSadso), and apparent activation energy (Ea) of the process. The value of adsorption equilibrium constant (Kads) obtained from the best-fitted isotherm tabulated in Table 8 is used to calculate ΔGadso in Eq. (56) (Aljourani et al., 2009). The adsorption heat ΔHadso can be calculated using van’t Hoff equation in Eq. (57) (Zhao and Mu, 1999). The entropy of OGCI adsorption ΔSadso can be calculated using Eq. (58) (Solmaz et al., 2008). Also, the enthalpy (∆Ha) and entropy (∆Sa) of activation for the corrosion process can be calculated from the results obtained from temperature studies via Eq. (59) such that a plot of logCRT against 1T gives a slope of (ΔH2.303R) and intercept of (log(Rnh)+ΔS2.303R), which enhance the computation of ∆Ha and ∆Sa (Alaneme et al., 2016). However, many corrosion studies have shown that corrosion rate increases as temperature increases, as justified by the Arrhenius equation stated as Eq. (60) (Deyab et al., 2007). Table 8 summarizes the significance of values of thermodynamic parameters on the adsorption of OGCIs on metals.

Table 8:

Significance of values of the thermodynamic parameters on the adsorption of OGCIs on metals.

Thermodynamic parameterSignificance of values on the adsorption processReferences
ΔGadsoIf a negative value is obtained, it implies that the adsorption process is spontaneous with the formation of a stable protective OGCI layerKeleş et al., 2008
ΔGadso≤−20 kJ/mol indicates an electrostatic interaction existence between charged molecules of OGCI and charged metal. This process is called physisorptionMusa et al., 2011
ΔGadso≤−40 kJ/mol implies that there are electrons sharing or transfer from OGCI molecules to the examined metal surface to enhance the formation of a coordinate type of bond. This process is called chemisorptionBentiss et al., 2009
For −20 kJ/mol≤ΔGadso≤−40 kJ/mol, the adsorption process is a mixture between physical and chemical adsorptionNoor and Al-Moubaraki, 2008
ΔHadsoPositive ΔHadso implies that the adsorption process is endothermic in nature; negative ΔHadso suggests an exothermic adsorption exhibition for tested OGCIOlasehinde et al., 2013
ΔSadsoPositive ΔSadso indicates that the adsorption process of OGCI on the corroded metal surface under investigation in a corrosive medium is supported by an increase in entropy and vice versaLazarova et al., 2009
EaEa>20 kJ/mol suggests the inhibition process to be a controlled surface reactionHerrag et al., 2010
An increase in Ea in the presence of OGCI indicates OGCI adsorption on the examined metal surface by increasing the energy barrier for the corrosion process without changing the mechanism of dissolution. Also, physical adsorption (electrostatic) has occurred at the initial stage of the processKhadom et al., 2010; Boudalia et al., 2013
A decrease in Ea at higher OGCI efficiency exhibits shift in net corrosion reaction from the uncovered metal surface to the adsorbed sitesTang et al., 2003
SaAn increase in ∆Sa in the presence of OGCI suggests an increase in the degree of disorderliness resulting from the conversion of reactants to activated complexes. Such exhibition could also be attributed to the reduction in the release of H+ on the metal surface, making the system to shift from a more organized into a more random order, thereby increasing the entropy of activationKhaled and Amin, 2009
If ∆Sa is positive, the adsorption process is enhanced by an increase in entropy, which acts as a driving force for the adsorption of OGCI on the metal surfaceYurt et al., 2006
Negative ∆Sa suggests the occurrence of the degree of disorderliness reduction taking place on moving from reactants to the activated statesTang et al., 2006
HaPositive ∆Ha implies that the adsorption process of OGCI on the metal surface is endothermic, whereas negative ∆Ha means exothermic reactionZaafarany et al., 2010
An increase in ∆Ha in the presence of OGCI suggests the presence of an energy barrier for reaction due to the adsorption of OGCISingh et al., 2012
(56)ΔGadso=RTIn(55.5Kads)
(57)InKads=(ΔHadsoRT)+constant
(58)ΔGadso=ΔHadsoTΔSadso
(59)logCRT=log(RNh)+ΔSa2.303RΔHa2.303RT
(60)logCR=logAEa2.303RT

where CR=corrosion rate (mm/year), N=Avogadro’s number (6.02×1023/mol), h=Plank’s constant (6.63×10−34 m2 kg/s), R=gas constant (8314 J/mol K), T=absolute temperature (K), Ea=activation energy (kJ/mol), and A=pre-exponential factor.

4 Conclusions and recommendations

  1. Many of the greeners considered for the extraction of OGCIs are edible and very useful for human need in many areas such as medicinal, pharmaceutical, and food consumption, thus making them to be very competitive in terms of functionality. There is a need for future research works to focus more on wastes from greeners constituting environmental nuisance causing harmful effects on both human and aquatic natures. Recently, Al-Zubaidi et al. (2018) used crude glycerol (by-product of biodiesel production) with a concentration range from 0.1% to 1.0% (w/w) as a potential OGCI for steel corrosion in a corrosive medium containing 0.5 m HCl at a constant room temperature of 25°C.

  2. It has been established that many silicates also possess corrosion inhibitory attributes due to their capability to block corrosion active sites on metals in acidic medium. Researchers should focus on synthesizing composite OGCIs from greener extracts and silicates (such as rice husk waste) to improve their efficiency. This phenomenon is referred to as a synergism effect, which can be quantified by applying Eq. (7). This will also enable researchers to widen their scope and knowledge of using naturally endowed greeners.

  3. None of the examined studies presented chain reaction mechanisms and reaction pathways showing the presence of intermediates in the course of corrosion reaction inhibitory effects of used OGCIs and examined metals. This is necessary to know how these intermediates contribute or affect the inhibition via the adsorption of OGCIs on metals. It is an established fact that when extracts from greeners react, intermediates are formed in the course.

  4. Also, there is a complexity of adequate separation techniques to be employed in obtaining tested acidic or alkaline solution (as the case may be) in pure form and the spent OGCIs. The reusability of spent OGCIs, and decrease in their efficiency over time, is a great challenge for prospective researchers in the field of corrosion engineering.

  5. Cost-effective modern techniques that will also maximize OGCI extraction in pure form from their sources are required. A detailed financial implication from pilot scale to full industrial plant is needed for the general public to see this as a key means of internal revenue generation. Previous studies focused majorly on refluxing greeners in HCl, ethanol, and H2SO4 for a period of time. In support of these, existing optimization tools such as response surface methodology and central composite design of design experts coupled with predictive tools such as artificial neural network-based Monte Carlo simulation and sum of square errors will be of help (Popoola and Susu, 2014).

  6. The gap between research and industrial application of these OGCIs has not been bridged. From investigation, many industries are still using corrosion inhibitors already in existence, which have been proven to be expensive and environmental unfriendly although researchers are working assiduously to generate cheap, environment friendly, and readily available OGCIs from greeners. Researchers’ efforts should be appreciated in this regard.

  7. In the examined functional groups presented in Table 2, flavonoid has been observed to be a peculiar active corrosion inhibition constituent present in almost 85% of the greeners presented. Flavonoid is a good candidate to explain the corrosion inhibitory effects observed in greeners. Its detailed chemistry must be studied to enhance further works on the contribution of flavonoid in OGCIs on the corrosion inhibition of metals.

  8. Phytochemical analysis provides information on active ingredients present in plant extracts acting as corrosion inhibitors of OGCIs. Therefore, it is most likely that a mixture of constituents is acting as corrosion inhibitors (Raja and Sethuraman, 2008). Few studies are engaged in using this analysis.

  9. Additional advanced characterization techniques coupled with fundamental studies are required to further differentiate OGCI mechanism and investigate the relationship binding their structure with experienced corrosion inhibition. This knowledge will help tailor the OGCI structure in obtaining the necessary corrosion inhibitory properties (Taghavikish et al., 2017).

  10. Although synthetic inhibitors have been shown to be expensive and toxic with restrictive environmental regulations in many countries, they have high corrosion inhibitory effectiveness. However, OGCIs from plant extracts with biodegradability, nontoxic, and environment friendly potential exhibit low corrosion inhibition efficiency as presented in many studies (Finšgar and Jackson, 2014). Due to this point, further research works are required on using additives such as water wetting agents, iron control agents, viscoelastic surfactants, nonemulsifiers, antisludge agents, and stabilizers to green corrosion inhibitors. They do not have corrosion inhibition potential but can enhance the corrosion inhibition performance of OGCIs and significantly reduce the corrosion rate.

  11. Although many studies suggested Langmuir as the best predictive adsorption isotherm that conforms to experimental data, further studies are required for the kinetics of the corrosion inhibition of materials by OGCIs via the previously presented modeling equations. Also, more studies are needed in the aspect of developing mathematical models with reduced assumptions involving kinetics and mechanistic studies in predicting the corrosion inhibitory effects of OGCIs from plant extracts. Computer software that will enhance quick prediction and other applications pertaining to corrosion inhibition exhibits of these OGCIs can be developed.

  12. It has been established that organic compound structures of OGCIs play a vital role on how they effectively inhibit the corrosion of metal. This means that changing the organic compound chemical structure will directly sectionize corrosion inhibition. This calls for advanced research works in the development of quantitative models bridging chemical structure to properties using existing machine learning or statistical approaches. Although quantitative structure-activity relationships and quantitative structure-property relationships modeling have been presented (Winkler and Burden, 2000); Le et al., 2012; Fujita and Winkler, 2016), there is a need for more machine learning modeling methods and computational models that are applicable in studying the corrosion inhibitory properties of OGCI organic compounds. Also, improvements in robotics and machine learning will pave ways to tremendous increase in the efficiencies and dependency of methods for designing OGCIs within a short period.

  13. The corrosion inhibitory effects of OGCIs on mild steel have been the major consideration in numerous previous studies because of its relatively low price with acceptable material properties for many domestic and industrial applications (Singh et al., 2016). However, its low corrosion resistance in acidic environments is a major challenge (Alaneme et al., 2016). There is a need to work assiduously on other metallic components such as copper, alloys, aluminum, and stainless steel. All these work in unison and play specific roles in material selection for domestic/industrial purposes. They also corrode when subjected to certain environmental conditions. They are also the major components of automobiles, whose major concern is corrosion.

  14. Building collapse has been linked majorly to the weakening of iron steel rods (used in concrete beams) as a result of corrosion over a period of time. Thorough research is required on the use of OGCIs from greeners that have strong affinity for concrete cements and constituents (pastes) to increase the life span of reinforced concrete structures damaged as a result of high alkalinity and tackle rusting of these iron rods used in building construction.

  15. Some corrosion inhibitors extracted from Ginkgo biloba leaf (Chen et al., 2013) and Diospyros kaki L.f husk (Zhang et al., 2013) that have exhibited potential for corrosion inhibition for microbial-induced corrosion type need to be further investigated for other forms of corrosion to widen the scope of corrosion control in the oil and gas industries. More research should be carried out on other corrosive environments such as CO2, H2S, and NaCl with great impact on metals used in the oil and gas industries. Nevertheless, the identification of corrosion type inhibited by OGCIs based on the solution examined for the materials is necessary.

  16. Acid solutions have found industrial application in mill scale removal from metal surfaces, acid descaling, acid pickling, industrial acid cleaning, and oil well acidizing. An important stimulation technique for enhancing oil production is petroleum oil well acidification. However, studies of metal corrosion using organic acid solutions are rare compared to similar studies with mineral acids as corrosive media (Sekine et al., 1987). Among acid solutions, HCl and H2SO4 are the most widely used because of their high corrosive nature to most metals and alloys even at low concentrations (Bekkouch et al., 1999), whereas HNO3 and H3PO4 are explored in isolated cases (El-Din, 2009). It is very important to put these mineral acids and organic acids into consideration during the corrosion inhibition of metals using OGCIs as they are among the highly ranked important industrial chemicals.

  17. Previous researchers have been using days and months to critically figure out the adsorption corrosion process of examined materials, media (either acidic or alkaline), and inhibitors from greeners to truly determine the percent inhibition efficiency of inhibitors. Although very few researchers have been working on methodologies and technologies to reduce the corrosion examination period as presented in Table 9, there is still a need for thorough research works by many researchers on novel methodologies that can examine hundreds or thousands of materials within a few minutes with optimum accuracy of inhibition efficiency determination. The most common methodologies that have been employed and adopted are EIS, WLM, and PDP.

Table 9:

Employed methodologies to reduce corrosion examination time.

MaterialsMethodologiesNo. of inhibitorsObservation periodObservation(s)References
Al alloy (AA2024)Direct current polarization509 hResults obtained correlated perfectly with those with 10 days of extended testing periodChambers et al., 2005
Al3+Fluorometric detection141–7 daysExcellent results obtained with high accuracy within a limited periodChambers and Taylor, 2007
Al3+Direct current polarization, cyclic voltammetry, and fluorometric detection1003–5 daysBetter results with high accuracy within a short periodTaylor and Chambers, 2008
Fe and ZnScanning vibrating electrode technique4≈2 hAccurate determination of percent corrosion inhibition efficienciesKallip et al., 2010
Carbon steelHigh-throughput testing rig88≈1 dayBest ever methodology that can handle many inhibitors within a short period on a single plate with negative and positive controlsWhite et al., 2012
Mild steelHigh-throughput EIS12≈3 hAn electrochemical platform with spatially addressable feature interfaced to a commercial EIS instrument was developedHe et al., 2008
Mild steelRobust computerized optical image processing method25≈2 hA linear relationship binding image apparent grayscale value with a corrosive pitting depth in the specimens was revealedVillamizar et al., 2006

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Received: 2018-06-28
Accepted: 2018-12-13
Published Online: 2019-01-11
Published in Print: 2019-03-26

©2019 Walter de Gruyter GmbH, Berlin/Boston