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Corros Rev 2019; 37(2): 71–102
Review
Lekan Taofeek Popoola*
Organic green corrosion inhibitors (OGCIs):
acritical review
https://doi.org/10.1515/corrrev-2018-0058
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
Heruntergeladen am | 29.06.19 00:07
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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

  • (^2) ref 2 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