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Corros Rev 2019; 37(2): 71–
Review
Lekan Taofeek Popoola*
Organic green corrosion inhibitors (OGCIs):
a critical review
https://doi.org/10.1515/corrrev-2018- Received June 28, 2018; accepted December 13, 2018; previously published online January 11, 2019
Abstract: Over the decades, corrosion has resulted in loss
of lives accorded with damage costs in almost all engineer-
ing 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 per-
fect 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 mathemati-
cal models governing OGCI kinetics, adsorption isotherm,
and adsorption thermodynamics is also included. In con-
clusion, recommendations that will broaden the usage of
OGCIs from plant extracts for inhibiting corrosion of mate-
rials are presented for prospective researchers in the field
of corrosion.
Keywords: corrosion; corrosive media; green; inhibitors;
organic.
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 electro-
chemical 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 (irrevers-
ible) 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 differ-
ent 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 coat-
ings and linings, cathodic/anodic protection, and corro-
sion inhibitors. However, the results of numerous research
conducted in anticorrosion material applications in previ-
ously mentioned engineering fields revealed using corro-
sion 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.
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 syn-
thesized 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 restric-
tive environmental regulations (Raja and Sethuraman,
2008) due to their hazardous effects. Many of the SOCIs
are not biodegradable and get accumulated in the environ-
ment constituting nuisance to human health or ecological
systems (EPA, 1998), the removal of which is complicated
*Corresponding author: Lekan Taofeek Popoola, Unit Operation and Material Science Laboratory, Chemical and Petroleum Engineering Department, Afe Babalola University, Ekiti State, Nigeria, e-mail: popoolalekantaofeek@yahoo.com
Angemeldet | chrbfischer@uni-koblenz.de
and expensive (Bammou et al., 2011). These environmen-
tal 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 cor-
rosive media called organic green corrosion inhibitors
(OGCIs), which are inexpensive, harmless, readily obtain-
able, and environmentally accommodative. Figure 2 pre-
sents the various sources of eco-friendly OGCIs.
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 (H 2 SO 4 ) 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, biodegrad-
able, and harmless (Ji et al., 2015) with remarkable poten-
tial 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 inhi-
bition efficiency has been stated to be oxygen < nitro-
gen < sulfur < phosphorus (Neha etal., 2013). OGCIs exhibit
their inhibiting action via physisorption or chemisorption
Oils
Extracts
Plants
Drugs
Amino acids
Surfactants
Biopolymers
Ionic liquids (^) Rare earth
Inorganic green corrosion inhibitor
Organic green corrosion inhibitor
Eco-friendly corrosion inhibitor
Figure 2: Sources of eco-friendly OGCIs (Ibrahimi et al., 2017).
Overall chemical reaction
Neutral or basic conditions w/ oxygen contamination
Carbon dioxide “Sweet” corrosion
Hydrogen sulfide “Sour” corrosion
Oxidation half reaction Reduction half reaction
Fe + 2H +^ → Fe2+^ + H 2
O 2 + 2H 2 O + 4e–^ → 4ΟΗ–
Fe → Fe2+^ + 2e–
Fe2+^ + 2OH–^ → Fe(OH)2,(s)
Fe + CO 2 + H 2 O → FeCO3,(s) + H (^2)
Fe + H 2 S → FeS(s) + Η 2
2H+^ + 2e –^ → H 2
Figure 1: Chemical reactions of the corrosion process (Brylee and Advincula, 2015).
Angemeldet | chrbfischer@uni-koblenz.de
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 electronega-
tive 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 pecu-
liar repeating units (methyl and phenyl groups) of the
parent chain and additional substituent groups. Studies
have shown that OGCI molecules with -OH and -OCH 3
electron-releasing substituents proved to have better
efficiency than the parent molecule with no substitu-
ents (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 func-
tional groups that act as adsorption centers (Ahamad
et al., 2010). Table 1 presents some anchoring functional
groups present in OGCIs.
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 corro-
sion 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 pres-
ence 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 mole-
cule adsorption on corroding mild steel surfaces. Chamae-
rops humilis plant extract, which is also rich in tannins, is
effective in inhibiting corrosion of mild steel in 0.5 m H 2 SO 4
with 5% ethanol additive (Benali et al., 2013). Tryptamine,
a derivative of the tryptophan, proved effective in inhibit-
ing ARMCO iron corrosion in deaerated 0.5 m H 2 SO 4 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.
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 expo-
sure 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 elec-
trolyte 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. Cor-
rosion 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 adsorp-
tion 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 func-
tion 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.
Table 1: Some attaching functional groups in OGCIs (Singh, 1993).
Functional group Name Fuctional group Name
-OH Hydroxy -NH 2 Amino -C-N-C- Amine -SH Thiol -NO 2 Nitro -C≡C- -yne -CONH 2 Amide -S=O Sulfoxide -COOH Carboxy -NH Imino -S- Sulfide -N=N-N- Triazole -C=S- Thio -C-O-C- Epoxy -P=O Phosphonium -P- Phospho -Se- Seleno -As- Arsano
Angemeldet | chrbfischer@uni-koblenz.de
Table 2:
Sources of OGCIs, functional groups, and corrosion inhibitory roles.
OGCI source
Functional groups and compounds
Corrosion inhibitory roles
References
G. biloba
leaf extracts
Flavonoids and terpenoids; phenol groups and aromaticrings
Terpenoids: quercetin adsorption on mild steel surface based on theinteractions of donor-acceptor between
O
and aromatic ring
p
-electrons and
surface iron vacant
d
-orbitals
Flavonoids: oxygen-adsorption corrosion inhibited via its oxidation tobenzoquinone by O
resolved in the solution 2
Chen et al., 2013
Rothmannia longiflora extract
Monomethyl fumarate, 4-oxonicotinamide-1-(1-
β-D-
ribofuranoside), and D-mannitol
–^
Akalezi et al., 2015
Petersianthusmacrocarpus
plant
Petersaponin,
β
-sitosterol, and ellagic acid
Molecules are adsorbed on the surface of mild steel surface as a result ofhydroxyl group and aromatic ring protonation. Constituent molecules havearomatic 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 Fe
Akalezi et al., 2015
Extract of
Ficus
asperifolia
Saponins, alkaloids, tannins, anthraquinones,flavonoids, reducing sugars,
n
-hexane, ethyl acetate, and
butanol
The electron-donating ability was facilitated as a result of rich bond orheteroatoms present in the chemical structures. Thus, the formation ofcomplexes on the material surface to inhibit corrosion was enhanced
Ebenso et al., 2008
Extracts of
D. kaki
L.f
husk
Vitamins,
p
-coumaric acid, gallic acid, catechin,
flavonoids, carotenoids, and condensed tannin
–^
Zhang et al., 2013
Gum arabic
Arabinogalactan, oligosaccharides, polysaccharides, andglucoproteins
–^
Umoren et al., 2006
Tobacco extract
Polyphenols, terpenes, alkaloids, alcohols, carboxylicacids, and nitrogen-containing compounds
Corrosion inhibition on metals by electrochemical activity due to fusedbenzene ring system with charge dislocation property
Rudresh andMayanna, 1977
Extract of green wildjute tree (
Grewa
venusta
Polysaccharides, polyphenols (catechins and flavonoids)vitamins, tannins, minerals, volatile oils, and alkaloids
Mixed inhibitor corrosion inhibition action
Suleiman et al., 2013
Anthocleista djalonesis
Iridoid glucoside (DJN), dibenzo-
α-pyrone
(djalonensone), ursolic acid, and 3-oxo-∆-4,5-sitosterone
–^
Obame et al., 2008
Guar gum
Polysaccharides, mainly sugars galactose and mannose
1,4-Linked mannose residue linear chain-forming short-side branches, whichlater formed complexes on the metal surface to inhibit corrosion
Abdallah, 2004
Jatropha curcas
leaf
extract
Tannins, flavonoids, terpenes, anthraquinone, apigenin,cardiac glycoside, alkaloids, deoxy sugar, saponins,
α
-D-
glucoside, sterols, stigmasterol, and vitexin
Corrosion inhibition via the formation of continuous complex metal ions on themetal surface by polar groups
Ejikeme et al.,2014; Rani andSelvaraj, 2014
Extracts of banana peel
Bananadine (3Z,7Z,10Z)-1-oxa-6-azacyclododeca-3,7,10-triene
–^
Sangeetha et al., 2012
Aloe vera
plant extract
Minerals, polysaccharides, vitamins, glycoproteins, andenzymes
–^
Gupta et al., 2018
Azadirachta indica
Azadirachtin, salannin, meliantriol, and nimbin
Inhibition effects due to electronic, geometry coupled with binding propertybases on the metal surface
Sharma et al., 2015
Locust bean gum
Galactomannan-type polysaccharides
–^
Jano et al., 2012
Oil palm frond
Phenolic constituents (
p -hydroxybenzoic acid, syringic
acid, vanillic acid, vanillin,
p
-hydroxybenzaldehyde,
p -hydroxyacetophenone, and syringaldehyde)
Lignin is cleaved to form aromatic carbonyl compounds (syringaldehyde andvanillin) via alkaline nitrobenzene oxidation to inhibit corrosion
Yokoi et al., 2001
Angemeldet | chrbfischer@uni-koblenz.de
where CR = corrosion rate (g/cm^2 /h), w 1 = metal coupon
weight loss in the absence of OGCI (g), w 2 = metal coupon
weight loss in the presence of OGCI (g), A = metal coupon
surface area (cm^2 ), and t = immersion time (h).
However, there are cases where the inhibition effi-
ciency 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 syn-
ergism effect, which can be quantified using Eq. (7)
(Murakawa et al., 1967):
A B A B AB
S
where θA and θB = respective surface area coverage by com-
pounds 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 negligi-
ble. 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, cor-
rosion 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 elec-
trode, whereas the current ( I ) is measured by the counter
electrode. As the electrochemical reactions progress, open
circuit potential ( E ocp) of the metal fluctuates. At equilib-
rium, 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 meas-
ured E ocp to a higher potential (between −0.25 and +0.25 V).
The corrosion current ( i corr) and corrosion potential ( E corr)
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 concentra-
tions of persimmon husk extracts as OGCI. Corrosion rate
is measured using Eq. (8) (Al-Sabagh et al., 2012), whereas
η% is calculated by measuring i corr in the presence and
absence of OGCIs using Eq. (9) (Verma et al., 2015):
CR i^ corr^ EW
A
× ×
×
1 corr corr corr
o o
i i
i
= × (9)
where κ = conversion factor, EW = equivalent weight (g),
ρ = density (g/cm^3 ), A = sample area (cm^2 ), and i corr^ o and
1
i c orr = i c rordensity values in the absence and presence of
OGCI molecules, respectively.
1.5.3 EIS
EIS is an essential method of monitoring in situ electro-
chemical changes with critical understanding of physi-
cal processes occurring at the metal-electrolyte interface
(Mourya et al., 2014) such that information related to
electrode kinetics, surface properties, and mechanis-
tic can be taken from impedance diagrams (Lorenz and
Manfield, 1981). Just like PDP, the experiment is con-
ducted 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
–0.
10 –
10 –
10 –
10 –
10 –
10 –
10 –
–0. E (volts)
I (Amps/cm
2 )
–0.
Blank10 mg/l 50 mg/l100 mg/l 200 mg/l 500 mg/l1000 mg/l
–0.
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).
Angemeldet | chrbfischer@uni-koblenz.de
imaginary ( Z ″) impedance values. A typical Nyquist plot
for examining mild steel in 1 m H 2 SO 4 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 R s (electrolyte solution resistance), in series
with a parallel arrangement of constant phase element
(CPE) and R ct (charge transfer resistance; Roy et al., 2014)
modeled in a system of metal substrate, adsorbed inhibi-
tors, and electrolyte solution.
However, one study has used polarization resistance
( R p) obtained as real impedance difference at reduced and
higher frequencies to replace the usual R ct (Gupta et al.,
2018). R p is noticed to include R ct , accumulation resist-
ance ( R a) resulting from species accumulated at the metal-
electrolyte interface, diffusion layer resistance ( R d), and
inhibitor film resistance ( R f ) 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 electro-
lyte solution is called the solution resistance ( R s). R ct rep-
resents 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 R p, n , C dl, 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 poten-
tial and its current response, a CPE represented math-
ematically as Eq. (10) is used instead of capacitance ( C ;
Satapathy et al., 2009):
CPE
Z ( j ) n
A
where Z CPE = CPE impedance, A = CPE constant, ω = angular
frequency, j = imaginary number (i.e. i^2 = −1), and n = phase
shift exponent that is a measure of surface irregular-
ity/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. Elec-
trical 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):
1 1
dl (^ ct )
C = AR − n^ n (11)
1
dl (^ max)
C = A ω n − (12)
dl max ct
C
π R
ct( i) ct(o) E ct(i)
R R
R
= × (14)
where ωmax = maximum frequency of impedance imagi-
nary quantity (rad/s) and R ct(i) and R ct(o) = R ct in the pres-
ence and absence of OGCI various concentrations,
respectively.
In general, Table 4 summarizes the significance/impli-
cation of changes in trends and variations in the values of
parameters associated with the techniques of measuring
OGCI efficiencies as observed in previous studies.
0 200 400 Zreal (ohm cm 2 )
–Zimag (ohm cm
2 )
600 800
0.00 m M 0.05 mM 0.10 mM 0.15 mM 0.20 mM 0.25 mM
1000
0
50
100
150
200
250
300
350
Figure 4: Mild steel Nyquist plot in 1 m H 2 SO 4 at 30°C for varying OGCI concentrations (Al-Amiery et al., 2014).
Table 3: Significance of n values on the CPE nature.
n CPE nature (A) Significance References
0 Resistance A metal-solution interface operating as a resistor Bai, 2015 1 Capacitance Plane and homogeneous electrode surface with the metal-solution interface behaving as a capacitor with a regular surface
Lin et al., 2015
−1 Inductance Nonplane and heterogeneous electrode surface with the metal-solution interface behaving as an inductor with an irregular surface
Deyab et al., 2007
1/2 Warburg impedance A metal-solution interface acting as both capacitor and inductor Yurt et al., 2006
Angemeldet | chrbfischer@uni-koblenz.de
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 method-
ology, employed characterization of OGCIs, findings, and
prospective future challenges.
2.1 Industrial applications of OGCIs
Industrial applications of corrosion inhibitors from green-
ers 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.
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 follow-
ing assumptions are made: (1) anodic i corr density is used
for Fe 2 +^ ion boundary condition at anode, (2) anodic i corr
density accounts for Fe 2 +^ ion generation via electrochemi-
cal reactions on the metal surface as the source term, (3)
zero concentration of Fe 2 +^ ion is applied at cathode due to
scale formation, (4) Fe^2 +^ ion concentration in the shielded
solution is the same as bulk solution in chemical equilib-
rium, and (5) H+^ ( C H +)and CO 2 ( C CO 2 )surface concentra-
tions enhance the rate of corrosion via exchange current
density. Thus, the anodic electrochemical reaction is
given as Eq. (15) (Popoola et al., 2013):
Fe → Fe 2 +^ + 2e− (15)
The anodic i corr density is calculated using Eq. (16) (Tafel’s
law):
a (^) rev,Fe 2 Fe^2
Fe^2 0,Fe^2
b
i i
φ φ (^) +
−
where i Fe 2 + = iron oxidation current density (A/m^2 ),
i 0,Fe 2 + = iron oxidation exchange current density (A/m^2 ),
φrev = reversible potential of iron oxidation (V), φa = anodic
potential (V), and b = Tafel slope of oxidation (V).
The iron oxidation exchange current density ( i 0,Fe 2 +)
in Eq. (16) is determined from Eq. (17) (Nordsveen et al.,
1 2
1 1 H CO 0,Fe 0,ref H ,ref CO ,ref
e
a a H
C C R T T
i i
C C
− ∆ (^) −
^
^
where i 0,ref = reference exchange current density (A/m^2 ),
C H + = surface concentration of hydrogen ion (mol/l),
C H + ,ref=^ reference^ hydrogen^ ion^ concentration^ (mol/l),
CO 2
C = surface concentration of CO 2 (mol/l),
CO ,ref 2
C = refer-
ence CO 2 concentration (mol/l), ∆ H = change in enthalpy
(kJ/mol), R = gas constant (J/mol K), T = solution tempera-
ture (/K), and T ref = reference temperature (/K).
The mass flux of Fe^2 +^ at anode ( J Fe 2 + )is determined by
Eq. (18) (Gavrilov et al., 2007):
2 2 2
Fe Fe Fe
i
J
n F
where J Fe 2 + = mass flux of Fe^2 +^ at anode (mol/m 2 s),
i Fe 2 + = current density of iron oxidation (A/m^2 ), F = Fara-
day’s constant (C/mol), and n Fe 2 + = number of moles of Fe^2 +
(mol).
3.1.2 Cathodic modeling
The derivation of equations governing the kinetics of cor-
rosion at the cathode is based on the assumption that
oxygen and water reduction in the system is negligible
Observation Significance/implication References
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 specimens
This resulted from metal surface inhomogeneity Singh et al., 2016
Table 4 (continued)
Angemeldet | chrbfischer@uni-koblenz.de
Table 5:
Summary of literature on sources of previously used OGCIs.
OGCI source
Extraction methodology
Material tested;solution used
OGCI characterization;laboratory analysis
Findings
Future challenges
References
Camelliasinensis (green tea)
Dried and ground leavessubjected to reflux in 70%acetone for 4 h
Mild steel in 1 m HCl SEM, EIS, WLM, FTIR,
EDX
79% inhibition efficiency achieved in200 ppm solution
Inhibition effect increases with an increasein solution concentration and temperature
Adsorption kinetics and isothermsstudies were not examined
Nofrizal, 2012
R. longiflora extract
Extraction
Mild steel in 1 m HCland 0.5 m H
SO 2
4
PDP, EIS
Increase in corrosion inhibition efficiencyas extract concentration and temperatureincrease
Extraction methodology was notpresented
Extract was not characterized forfunctional groups inhibiting corrosion
Akaleziet al., 2015
A. djalonesis leaf extract
20 g dried leaves underreflux for 3 h in 1 m HCl and0.5 m H
SO 2
4
Mild steel in 1 m HCland 0.5 m H
SO 2
4
EIS, PDP, DFT-basedQCC
Corrosion inhibition via mixed-typeinhibition mechanism
Djalonenoside (DJN) and its hydrolysisproduct DJN-hyd were extracts enhancingcorrosion inhibition in the medium
Corrosion of other metals besides mildsteel was not investigated
Ogukweet al., 2012
Theobromacacao
peel
polar extract
Boiling dried pods underreflux for 4 h in 1.0 m HCl
Mild steel in 1 m HCl WLM, EIS, PDP
Increase in corrosion inhibition efficiencyas OGCI concentration increases butdecreased with temperature
Langmuir isotherm was obeyed
More metallic materials should betested
Yetri et al., 2014
o ,
m
, p
Decanoylthioureaderivatives
Mixed substitution andaddition reaction usingdecanoyl chloride,ammonium thiocyanate,and 2-aminopyridine inacetone solution for 10 min
Mild steel in 0.1 mH^2
SO
4
FTIR,
1 H and
(^13)
C NMR
Compound D3 of the derivatives possessedthe highest efficiency
Compound corrosion inhibition efficienciesaffected by N atom at
o
-,^
m
-, and
p -positions affects the pyridine chemicalstructure
Only mild steel was considered
Limited laboratory analysis
Although chemical structures werepresented, the structural morphologyof the synthesized inhibitors need beinvestigated
Kamalet al., 2014
Extracts of
D.
kaki
L.f husk
Husk powder heated underreflux with water or alcoholfor 4 h
Q235A steel in 1 mHCl
PDP, GM
Extracts behaved like a mixed-type inhibitor
Extracts exhibited antibacterial activityagainst microbial influenced corrosion(MIC) of oil field microorganism
There is a need to fully explorethe corrosion inhibitory feature ofextracts from this biomass in othercorrosion types besides microbialinfluenced corrosion
SEM analysis was not carried outto ascertain that the corrosion typeinhibited on the metal surface by theextracts was exactly MIC
Zhanget al., 2013
Schiff bases
8 h refluxing of 3-amino-2-methylquinazolin-4(
H )-one with 4-hydroxybenzaldehydeand
N
, N
-dimethyl-4-
aminobenzaldehyde inacetic acid
Mild steel in 1.0m HCl
SEM, NMR, DFT
p
-position substituent enhanced inhibitionefficiency
Inhibition efficiency relies on OGCI nitrogenamount and their molecular weight andconcentration
m
-position substituent on OGCI molecule affected inhibitionefficiency negatively
Corrosion type prevented was notspecified
Only mild steel was used to test theinhibitor efficiency
Jamil et al., 2018
Angemeldet | chrbfischer@uni-koblenz.de
OGCI source
Extraction methodology
Material tested;solution used
OGCI characterization;laboratory analysis
Findings
Future challenges
References
P.macrocarpus plant
Boiling dried leaves underreflux for 3 h in 1.0 m HCland 0.5 m H
SO 2
4
Mild steel in 1 m HCland 0.5 m H
SO 2
4
GM, PDP, EIS
EIS data revealed organic matter extractinfluence on corrosion inhibitory effect onmild steel
Inhibition efficiency increased with anincrease in concentration and temperatureup to 50
°C
Lower activation energy in the presenceof corrosion inhibitor resulted from theadsorption chemisorptive nature
The influence of the inhibitorin alkaline medium was notinvestigated
Only mild steel was examined
The kinetics of the process was notstudied
The efficiency of the inhibitorused was not compared to thoseof previous inhibitors used by theresearchers
Akaleziet al., 2015
Hibiscus rosa-sinensis
leaf
extract
–^
Mild steel in 1 m HCl WLM, EIS
Inhibition efficiency increased astemperature and solution concentrationincreased
OGCI behaved as mixed type
Spontaneous reaction
Data agreed well with Langmuir, Flory-Huggins, and Freundlich adsorptionisotherms
Limited laboratory analysis to affirmthe inhibitor efficiency
Only mild steel in only acidic mediumwas investigated
Desai, 2015
D-glucosederivatives
Multicomponent reactions
Mild steel in 1 m HCl SEM, EDX, AFM, EIS
The presence of -OH and -OCH
groups 3
exhibited higher inhibition efficiency
E ads
values did not exhibit any regular trend for aqueous and protonatedinhibitor molecules
Vermaet al., 2017
Silica extractfrom rice huskash
Na
O 2
Silica extract was preparedby mixing 80 ml of 2.5 mNaOH with rice husk ashproduced by calcination at 600
°C for 6 h. 0.2 m NaOH and distilled water werethen added to form theinhibitor
99.9% Cu, Al alloy(AA6061), carbonsteel (SAE1045) in0.5 m HCl
XRF, XRD
Each metal alloy influenced the optimalSiO
:Na 2
O ratio determination sodium 2
silicate formulation
Used silicate-based inhibitor has apotential of inhibiting corrosion in testedsamples under examined acidic medium
Limited laboratory analysis for moreconfirmation of inhibitor efficiency
Only acidic medium solution wastested
More metallic samples should beexamined
Mohamadet al., 2013
Gum arabic
–^
Mild steel and Al inH^2
SO
4
WLM, TT
Inhibition efficiency increases with anincrease in the concentration of theinhibitor
Inhibitor obeyed Temkin adsorptionisotherm for tested samples
Mild steel corrosion was chemicaladsorption, whereas Al corrosion wasphysical adsorption
Inhibitor acted better on Al than mild steelwith adsorption being spontaneous
The methodology of inhibitorextraction was not adequatelypresented
The kinetics of the adsorption processwas not presented
The reaction mechanism of theinhibitor adsorption process on mildsteel and Al process was not available
Limited laboratory analysis to supportinhibitor efficiency on samples
Umoren, 2008
Table 5
(continued)
Angemeldet | chrbfischer@uni-koblenz.de
OGCI source
Extraction methodology
Material tested;solution used
OGCI characterization;laboratory analysis
Findings
Future challenges
References
Coconut coirdust extract
Hydrogen evolutionextraction method
Al corrosion in 1 mHCl
WLM, HEM
As temperature and concentrationincreased, inhibition efficiency increased
Langmuir isotherm was obeyed
Only Al was considered. It would bebetter if inhibitor efficiency is testedin other metals
Also, only HCl as acidic mediumwas tested. Both acidic and alkalinesolutions should be checked
Umorenet al., 2006
Jatropha
stem
Jatropha
fine powder
obtained by sun drying andgrinding soaked in ethanolfor 24 h. Evaporation offiltrate to remove excessalcohol
Mild steel inseawater
SEM, WLM, FTIR
Coupons without inhibitor corroded more inseawater than those with inhibitor
Presence of active corrosion inhibitors^ Jatropha
extracts revealed by FTIR
Maximum inhibition efficiency of 81.7% at0.90 g/l inhibition concentration
Adsorption isotherm andthermodynamics were not studied
Few laboratory analysis for moreaffirmation of inhibitor efficiency
Inhibitor influence and efficiency inacidic and alkaline media were notinvestigated
Olawaleet al., 2016
Tobaccoextract
Extraction by weighingaqueous solutions, boilingof water, and weighingresidue
1008/1010cold-rolled steeland 3105 H24 AlQ-panels in 1–3%NaCl solution
ZRA, PDP, WLM
Tobacco extracts proved to be excellentinhibitors for the corrosion of Al and steel inalkaline solution
Extract also worked in acidic solution andcould prevent corrosion during descalingprocesses
Inhibition effect greater than chromateswithin a solution concentration range aslow as 100 ppm
Thermodynamics, kinetics, andadsorption isotherm equilibrium ofinhibitor effect were not investigated
Inhibition effect in other media wasnot investigated
Davis et al., 2001
Citrusaurantiifolia leaves
Dried and ground leavesunder reflux for 3 h in 1 mH^2
SO
4
Mild steel in 1 m HCl SEM, WLM
Corrosion inhibition increases with anincrease in solution concentration with97% efficiency
Experimental data conformed to Langmuirisotherm
Only mild steel in acidic medium wasinvestigated
Sarathaet al., 2009
Cashew waste
Sun dried and pulverizedfruits soaked in 250 mlethanol for 24 h
Mild steel in 1 m HCland 0.1 m H
SO 2
4
WLM, SEM, FTIR
Inhibitor efficiency increased with anincrease in inhibitor concentration withoptimum 80.5%
Cashew waste was seen as a valuablecorrosion inhibitor
Adsorption kinetics, isotherms, andthermodynamics were not studied forin-depth investigation
Only mild steel in acidic medium wasinvestigated
Olawaleet al., 2015
Locust beangum
–^
Carbon steel 39, 44,and B500 in H
SO 2
4
PDP, EIS
Inhibition effect on steel 39 in acidic mediumin the presence of NaCl was revealed
Although different carbon steel sampleswere tested, there was a shallowinvestigation on the extracted corrosioninhibitor on the examined samples
Jano et al., 2012
Extract ofbananapeel
+^
Zn
–^
Carbon steel indistilled water
AFM, WLM, GM, FTIR
Zn addition decreased inhibition efficiency.It later increases after increasing Znconcentration
No mathematical model was presentedas a predictive tool for the futurecorrosion of the sample tested
Sangeethaet al., 2012
Table 5
(continued)
Angemeldet | chrbfischer@uni-koblenz.de
OGCI source
Extraction methodology
Material tested;solution used
OGCI characterization;laboratory analysis
Findings
Future challenges
References
Langmuir adsorption isotherms andactivation energies revealed physicaladsorption
SEM images of corroded substrates showedprimary corrosion mechanism to be bypitting
95% Inhibition efficiencies at roomtemperature achievable
Corrosion inhibition increased with anincrease in extract concentration butdecreased with increasing temperature
Guar gum
Pods dried in sunlightand separated manuallyfrom seeds
Seeds heated underreflux with water oralcohol for 6 h
Carbon steel in 1 mH^2
SO
NaCl
WLM, EIS, PDP
Increase in resistance of pitting corrosionwas exhibited
Guar gum acted as a mixed-type inhibitorwhose efficiency increases with an increasein concentration
All data supported Langmuir adsorptionisotherm
–^
Abdallah, 2004
Oil palm frond
Nitrobenzene oxidationmethod for lignindepolymerization
Mild steel in 1 m HCl WLM, PDP, EIS, SEM,
XRD
Inhibition efficiency increased withincreased concentration of lignindepolymerized products
Mixed-type inhibitors revealed
Experimental data well fitted with Langmuiradsorption isotherm
Adsorption was dominated byphysisorption
SEM revealed reduction of surfaceroughness in the presence of an inhibitor
Only Langmuir isotherm was used.For comparative purposes, otherexisting isotherms should be used
Oil palm frond extracts have beenshown to have potential of corrosioninhibition in alkaline medium. Thus,various types of metallic materialsshould checked in alkaline medium
Shah et al., 2017
Celeryseeds (
A.
graveolens
Ground and powdered seedboiled in distilled H
O for 2
2 h. Filtrate evaporated todryness and residue usedhigh concentrated stocksolution
Carbon steel in 1 mHCl
WLM, PDP
Optimum inhibition efficiency obtained at500 ppm inhibitor concentration
Spontaneous adsorption process thatconforms to Temkin isotherm
Percent inhibition efficiency decreased withincreased temperature
Inhibition efficiency increased as celerydoses increased
Only WLM and PDP were used tocheck inhibitor efficiency
Only HCl was used for carbon steelalone to check OGCI efficiency
Active functional groups presentin
A. graveolens
seeds enhancing
corrosion inhibition were not deeplyinvestigated
Megahedet al., 2017
Table 5
(continued)
Angemeldet | chrbfischer@uni-koblenz.de
OGCI source
Extraction methodology
Material tested;solution used
OGCI characterization;laboratory analysis
Findings
Future challenges
References
Eichhorniacrassipes (waterhyacinth)leaves androots
4 g dried and groundleaves and roots soaked in1000 ml of 5 m HCl
Mild steel in HCl
DFT, GT
Root and leaf extracts performed excellentlywell as effective OGCIs
Physisorption of extract organicconstituents on corroding mild steel surface
Insufficient laboratory analysis
Equilibrium isotherms and kineticswere not investigated
Ulaetoet al., 2012
G. venusta plant extract
G. venusta
cut into
pieces, dried for 3 days,and ground into powder.Product was refluxed forsome hours using ethanol
Mild steel in 0.5 mH^2
SO
4
SEM
Corrosion rate was reduced when OGCIconcentration was increased above 2%(v/v) with time
Increase in temperature massivelyincreased corrosion rate
Plant extract exhibited effective corrosioninhibition potential for mild steel in acidicmedium
At 8% (v/v) optimum concentration of plantextract in acid solution, 86.47% highestefficiency was obtained
Only SEM was used to authenticateinhibitor efficiency
Thermodynamics of adsorptionprocess was not studied
Suleimanet al., 2013
Crude glycerolfrom residueof biodieselproducedfrom a plantseed
Transesterification process
Steel in 0.5 m HClat 25
°C
WLM, SEM, PDP
Corrosion inhibition increased withinhibitor concentration
Maximum inhibition efficiency of 98%) wasachieved after 70 h of residence time with1% inhibitor concentration
Plant source of oil used for biodieselproduction from which glycerol wasobtained was not mentioned
Inhibition efficiency remainedunchanged after residence time
Al-Zubaidiet al., 2018
Table 5 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-rayfluorescence; ZRA, zero-resistance ammeter.
(continued)
Angemeldet | chrbfischer@uni-koblenz.de
such that the two cathodic reactions are stated as Eqs. (19)
and (20) (Nesic et al., 1996):
2H 2e H 2
2H CO 2 3 2e H 2 2HCO 3
+ −^ → + − (20)
A general form used in the calculation of H+^ reduc-
tion partial cathodic i corr densities and H 2 CO 3 reduction is
stated as Eq. (21) (Nordsveen et al., 2003):
φ φ
−^ −
c rev
c 0 10 Scale
i i b (21)
where i c = current density of any cathodic reaction (A/m^2 ),
i 0 = cathodic reaction exchange current density (A/m^2 ),
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 H 2 CO 3 reduc-
tion at cathode are determined using Eq. (17). The electric
field in the solution is governed by Poisson’s equation
stated as
2 i i 1
n
i
F
φ z c
∇ = − (^) ∑ (22)
where ε = dielectric constant and φ = potential (V).
For electroneutrality condition in the solution,
Eq. (22) reduces to
i i 1
n
i
z c
=
∑ = (23)
Thus, Eq. (23) reduces to
3.1.3 Electrochemical modeling
Assuming that corrosion rate is governed only by elec-
trochemical reaction, the total anodic reaction current
density is used in determining the corrosion rate of CO 2
stated as (Nešić et al., 2009)
a w,Fe Fe
i M
CR
ρ nF
where CR = corrosion rate (mm/y), a i = anodic current
density (A/m^2 ), M w,Fe = atomic mass of iron (kg/mol),
Industrialapplication ρFe = density of iron (kg/m^3 ), n = number of moles of
Active functional groups;complexes; ingredients
Inhibitor source fromgreener
IE
How it works; how to solve the problem
Side effects
References
Refrigeratingindustry
Benzotriazole p -Hydroxybenzoic acid andvanillic acid
A. djalonesis Oil palm frond
Galvanic corrosion evolves due to theincrease in dissolved mineral salt contentas evaporation proceeds with the presenceof several dissimilar metals and nonmetals.Inhibitors control corrosion by film formationthat inhibits anodic metal dissolution reactionand cathodic poisoning
–^
Matsuda andUhlig, 1964;Obame et al., 2008
Buildingconstruction
Phosphate ion
–^
–^
When mixed with cement, the durability ofreinforced concrete structures is improved
–^
Yohai et al., 2013
Boiler
Ammonia, alkanol,cyclohexylamine, andmorpholine
–^
–^
Corrosion attack of pipes prevented bysolubilization of limescale
–^
Sanyal, 1981
Table 6
(continued)
Angemeldet | chrbfischer@uni-koblenz.de
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)
Fe (^ corr rev ,Fe)
a,Fe o,Fe 10
F E E
i i RT
α −
= × ^ (26)
The Tafel slope of iron oxidation b Fe as defined as
Fe Fe
RT
b
α F
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
corr rev ,Fe Fe
( )
a,Fe o,Fe 10
E E b
i i
−
= × ^ (28)
where i a,Fe = current density for iron dissolution (A/m^2 ),
i o,Fe = exchange current density of iron oxidation (A/m^2 ),
E corr = corrosion potential (V), E rev,Fe = reversible potential
of iron oxidation (V), and b Fe = Tafel slope of iron oxida-
tion (V).
The current density of any cathodic reaction is calcu-
lated as (Craig, 1995)
c ct lim
i i i
where i c = cathodic reaction current density (A/m^2 ),
i ct = charge transfer current density component (A/m^2 ),
and i lim = limiting current density component (A/m^2 ).
The charge transfer current density of cathodic reac-
tions ( i ct) is determined by (Chokshi et al., 2005)
c
ct o 10
i i b
−^ η
where i o = exchange current density of cathodic reactions
(A/m^2 ), η = E − E rev is the overpotential (V), E = potential (V),
E rev = reversible potential (V), and b c = cathodic Tafel slope
(V/decade).
The limiting current is determined from the mass
transfer limitation for the case of H+^ reduction. Thus,
lim(H ) m^ [H ]b
id k F
where i lim(H d + )= diffusion limiting current density (A/m^2 ),
k m = mass transfer coefficient of corrosive species (m/s),
[H+]b = bulk hydrogen ion concentration (mol/m^3 ), and
F = Faraday’s constant (96,490 C/equiv).
Suppose that there is a restriction of carbonic acid
reduction due to CO 2 hydration reaction rate being very
slow, the limiting current density
lim(H CO ) 2 3
( i r )is calculated
as (Vetter, 1961)
2 3 2 3
lim(H CO ) [CO ] 2 b^ (^ H CO hyd hyd)
i r^ = F ⋅ ⋅ D K kf
where [CO 2 ]b = bulk concentration of dissolved CO 2
(mol/m 3 ),
H CO 2 3
D = diffusion coefficient of H 2 CO 3 in water
(m 2 /s), K hyd = equilibrium constant for CO 2 hydration reac-
tion, and k hyd f = forward reaction rate constant for CO 2
hydration reaction (/s).
A theoretical flow multiplier f for Eq. (32), which takes
into account the flow effect on the chemical reaction limit-
ing current, is calculated by (Nešić et al., 2009)
m r m r
2 / 2 /
e
f
e
δ δ δ δ
− −
where δm = mass transfer thickness ( m ) and δr =reaction
layer thickness ( r ) whose values are determined by
Eqs. (34) and (35), respectively:
2 3 2 3
H CO m m,H CO
D
k
and
H CO 2 3 hyd r hyd
f
D K
k
3.2 Rate modeling of corrosion-type inhibition using OGCIs
3.2.1 Pitting corrosion
The risk of pitting corrosion can be increased under stag-
nant conditions in which corrosive microenvironments
are established on the surface. The accumulation of stag-
nant 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 Fe 2 +^ 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 Fe 2 +^ ion from the metal surface by diffusion
and electromigration and (2) Fe^2 +^ ion distribution in the
Angemeldet | chrbfischer@uni-koblenz.de
