1-s2.0-S2667266922000159-main

Corrosion Communications 5 (2022) 25–38
Contents lists available at ScienceDirect
Corrosion Communications
journal homepage: www.elsevier.com/locate/corcom
Review
Plant extracts as sustainable and green corrosion inhibitors for protection
of ferrous metals in corrosive media: A mini review
Ali Zakeri
a , b , ∗
, Elnaz Bahmani
b
, Alireza Sabour Rouh Aghdam
b
a
Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada
b
Department of Materials Engineering, Tarbiat Modares University, Tehran P.O. Box: 14115-143, Tehran, Iran
Article history:
Received 22 November 2021
Received in revised form 19 February 2022
Accepted 1 March 2022
Available online 17 April 2022
Keywords:
Corrosion protection
Corrosion inhibitor
Green chemistry
Plant extracts
Phytochemicals
Application of green corrosion inhibitors, which reduce corrosion rates to the appropriate level with low envi-
ronmental impact, is one of the emerging key approaches of controlling corrosion in modern society. From the
standpoint of environmental compatibility, this research eld is undergoing signicant developments. Nowadays,
due to increasing ecological awareness, corrosion inhibitors are subject to stringent restrictions and regulations
enforced by environmental agencies in a number of nations. According to these requirements, these chemicals
must be environmentally acceptable and safe. In light of this, intensive research has been undertaken in recent
years aimed at development of green corrosion inhibitors from plant extracts. Being readily available, inexpensive,
biodegradable, and safe make these substances promising alternatives to the hazardous conventional corrosion
inhibitors. The purpose of this review article is to summarize, in a brief manner, a compilation of recent promi-
nent papers on utilizing plant extracts as sustainable and green corrosion inhibitors. In addition, some discussions
were made on the benets and drawbacks of employing these substances for protection of metals.
© 2022 The Author(s). Published by Elsevier B.V. on behalf of Institute of Metal Research, Chinese Academy of
Sciences.
This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )
1. Introduction
The broad array of structural materials available to advanced and
emerging technologies has expanded the series from which they are cho-
sen, allowing engineers to select the most suited material for each appli-
cation based on physical or mechanical characteristics. Metallic mate-
rials have an essential role in a nation’s development and its long-term
progress in the global economy. There is no application, however, where
the consequence of a metal or alloy’s interaction with its surroundings
can be entirely ignored.
Corrosion is a natural process in which metals and alloys attempt to
return to their more stable thermodynamic state as a result of chemical
attack or reactivity with their surroundings. In other words, metals,
with the exception of platinum and gold, are found in nature in impure
forms, mainly as oxides or suldes, which are stable. In most processing
techniques, energy is consumed to obtain pure metals, causing the pure
metals to be in a higher energy state compared to their ore. As a result,
metals corrosion is the simplest and fastest way to reach their most
stable state. Corrosion can also be triggered by natural or man-made
causes. In general, metals corrosion is described as the natural and
inevitable loss of desired metal characteristics due to interaction with
particular elements present in the environment. Corrosion is proven
∗ Corresponding author.
E-mail address: [email protected] (A. Zakeri) .
to be hazardous to both the environment and human health [1] .
Corrosion-related issues are currently being considered in a number
of situations, ranging from the drinking water pipes to oil and gas
distribution. It should be noted that the term corrosion does not include
predominately physical or mechanical processes such as evaporation,
melting, or mechanical fracture [2] .
Corrosion reactions are frequently electrochemical in nature. The hy-
drogen evolution and the oxygen reduction are the two most prevalent
reactions that keep a corrosion process progressing in acidic media and
neutral/alkaline environments, respectively. Corrosion in metals and al-
loys is caused by a variety of factors; some of the most important en-
vironmental causes are schematically shown in Fig. 1 . Aside from the
factors depicted in this gure, the temperature of the environment has a
signicant eect on corrosion. Furthermore, the presence of some bac-
teria species inside a biolm on steel can speed up and promote the
progress of an already existent corrosion process. It is worth adding that
corrosion chemistry is rapidly evolving, and each chemical process is
scrutinized from the standpoints of economics, environmental impact,
and safety.
Because of the presence of acidic environments, metals and alloys in
general exhibit a high susceptibility to corrosion. Metals are vulnerable
to corrosion in acidic solutions because the acid can target the metal’s
https://doi.org/10.1016/j.corcom.2022.03.002
2667-2669/© 2022 The Author(s). Published by Elsevier B.V. on behalf of Institute of Metal Research, Chinese Academy of Sciences. This is an open access article
under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

A. Zakeri, E. Bahmani and A.S.R. Aghdam Corrosion Communications 5 (2022) 25–38
surface via an interfacial reaction and result in its dissolution (releas-
ing ions), which can happen in a variety of industrial operations. Acid
solution has been utilized in industrial applications such as industrial
acid descaling, acid cleaning, acid pickling, and mill scale removal from
metal surfaces, for instance. It is pertinent to mention that corrosion
damages are more prevalent in the processes of obtaining pure metals
from their metallic ores. These techniques use extremely concentrated
acidic solutions, which cause the breakdown of metallic components as
well as forming surface imperfections like scale and rust. Besides, cor-
rosion in the oil and gas industry might be linked to the nature of crude
oil, which promotes corrosion caused by hazardous components such as
naphthenic acid and sulfur.
Corrosion can be regarded as a critical problem that necessitates
prompt attention since it poses a threat to safety, material conserva-
tion, and economic concerns in a variety of engineering applications.
Corrosion, in general, causes serious failure in some structures, such as
rapid corrosion of metallic sections, which can be expensive to restore.
Furthermore, due to system shutdowns, it causes time to be lost during
maintenance. It has been reported that a lack of corrosion prevention led
to increased costs for issues such as repair, maintenance, and rehabil-
itation, as well as the replacement of damaged installations. Corrosion
costs worldwide increased to USD 2.5 trillion in 2013, accounting for
3.4 percent of global GDP. It is projected that implementing available
corrosion control measures might save between 15 and 35 percent of
the cost of corrosion, or between US$375 and $875 billion annually on
a global scale [3] . The indirect corrosion cost for dierent countries
that is reported annually, is displayed in Fig. 2 . Since the world is ex-
periencing technological improvement, this cost is projected to rise in
the upcoming years, if no substantial measures are adopted in corrosion
control and protection.
In the science and engineering of corrosion protection, several
approaches have been used. Material selection, electrochemical
methods, deposition of coatings, and the application of corrosion
inhibitors are some of the popular ones [10–12] . The use of corrosion
inhibitors (CIs) is one of the most economical and convenient of these
approaches to implement. The CIs are substances that, when added
in small concentrations in a corrosive media, can reduce the rate of
metal degradation. For instance, inorganic chemicals, especially chro-
mates and their derivatives, are recognized for their strong inhibitory
properties. Nonetheless, due to their toxicity and harmful inuence
on human life and the ecosystems, environmental regulations have
restricted their employment. On the other hand, natural products, such
as plant extracts, are widely available and cost-eective. As a result,
the twenty-rst century has seen a surge in green chemistry research
and its application in CIs. There has been a massive wave of studies
into corrosion inhibition using extracts from dierent parts of the
plant, as shown in Fig. 3 . In fact, bioactive chemicals contained in plant
extracts have been demonstrated to be just as ecacious as synthetic
inhibitors. Inspired by the large number of research papers published
Fig. 1. Environmental causes of metals corrosion.
on green corrosion inhibitors in recent years [13–20] , we thought it
was important to conduct a short review in order to include up-to-date
information and present future research directions in this eld.
2. Conventional corrosion inhibitors
As previously stated, eective corrosion prevention and control are
required, which can minimize nancial losses while also improving in-
dustrial safety and material longevity. The rst option for preventing
or limiting corrosion phenomena is to choose a suitable material based
on its end design and application. Second, environmental conditions,
including pH, temperature, and content of reactive species to name a
few, should be controlled, and predetermined inhibitor concentrations
should be added if necessary. Separating the component from corro-
sive media by protective coatings [21–24] and compensating for the
electrons that have been lost through cathodic protection [25] are also
among the ways to prevent and control corrosion.
Up till now, employment of the corrosion inhibitors has been a
widely-known method to control, avoid, or limit metal corrosion [26] .
Indeed, the utilization of corrosion inhibitors is one of the most ideal and
cost-eective strategies for reducing corrosion rates since these chemi-
cals can be used in batch and/or continuous procedures with minimal
concentration and strong adaptability [ 27 , 28 ]. In general, a CI is a chem-
ical that, when introduced to an environment (e.g., process uid) in a
low quantity, can eectively slow down the rate of corrosion of exposed
metal to its environment by modifying metal dissolution and excess acid
consumption (in cleaning and pickling process of metals) [29] . The pro-
cess of corrosion inhibition starts with the adsorption of CIs onto the
surface of the metal, which forms a protective layer and interacts with
anodic/cathodic reaction sites, lowering oxidation-reduction reactions.
That being said, their inhibitory mechanism is not always clear to iden-
tify and investigate. Based on the electrode process types, anodic, ca-
thodic, or mixed inhibitory eects are possible. It’s also worth noting
that some CIs are highly particular to a specic material, as well as the
environment, and may not function in other conditions.
Traditional inorganic and organic corrosion inhibitors are two
widely-used groups of CIs, mainly due to their simple processability and
application, as well as their excellent eciency at low concentrations
[30] . Inorganic CIs are preferred in near-neutral environments, whereas
organic CIs are desired in acidic environments [31] . An eective organic
inhibitor will have polar functional groups in its structure that contain
heteroatoms like O, S, or N atoms together with 𝜋electrons, as well as a
hydrophobic moiety that repels corrosive aqueous species o from the
material surface. Fatty amides, pyridines, polymers, and imidazolines
are among the organic chemicals used for corrosion inhibition purposes
[32] . The molecules of these CIs can adsorb on the substrate surface via
various mechanisms, including [33] :
(i) hydrolysis of organic compounds
(ii) donor-acceptor exchanges occurring between the 𝜋-electrons of
CI molecules and the vacant d-orbital of iron substrate
(iii) interaction of unshared electron pairs of the heteroatoms with
the vacant d-orbital of iron substrate
(iv) interaction of vacant orbital of the heteroatoms with the d-orbital
electrons of iron substrate
Furthermore, inorganic CIs can work at elevated temperatures for
longer durations and are more aordable than organic ones; never-
theless, inorganic CIs cannot function in acid solutions stronger than
17% HCl (hydrochloric acid). Commercially available chemicals of this
group include chromates, dichromates, phosphates, and arsenates [34] .
It should be noted that the mentioned substances are nowadays con-
sidered as toxic and hazardous compounds which do not simply break
down or lose their potency upon disposal; hence, threatening both hu-
mans and environmental health. This can lead to the accumulation of
harmful substances, which has a larger risk of causing long-term prob-
lems such as cancer [1] . Moreover, the synthesis processes of conven-
26

A. Zakeri, E. Bahmani and A.S.R. Aghdam Corrosion Communications 5 (2022) 25–38
tional CIs involve using expensive and toxic reagents, solvents, and cat-
alysts; consequently, releasing large quantities of undesirable chemicals
into the environment, which has a negative impact on soil and marine
life [ 35 , 36 ].
As public understanding of the health and environmental conse-
quences of industrial products comprising toxins has grown, more re-
strictive regulations on their usage have been enacted in the United
States and Europe [37] . With this in mind, the widespread use of conven-
tional CIs is being banned because of their toxicity and pollution eect
on the environment. This has stimulated great interest in the develop-
ment of green corrosion inhibitors (GCIs) among researchers [ 1 , 30 ]. It is
worth noting that some of the rst patented substances as CIs for the cor-
rosion mitigation of iron in acidic media were of natural products, like
our and yeast, together with by-products of food industries [ 38 , 39 ].
Moreover, an increasing number of patents have been issued in the last
two decades, dealing with the application of plant extracts as eective
GCIs [40–44] .
3. Green corrosion inhibitors
The principle of "green chemistry" refers to eorts toward establish-
ing a comprehensive approach to chemical risk management. This con-
Fig. 2. Indirect corrosion cost for dierent countries (data collected from Refs. [4–9] ).
Fig. 3. Number of publications in the past two decades reporting the use of plant
extracts as green corrosion inhibitors (data collected from Scopus database).
cept is based on the ideas of sustainability, reducing environmental con-
sequences, and preserving natural resources for the following centuries
[45] . The requirements for a chemical to be approved as a green corro-
sion inhibitor have been explicitly set out by legislative bodies such as
the Paris Commission (PARCOM) and the Registration, Evaluation, Au-
thorization and Restriction of Chemicals (REACH), which are: (i) non-
bio-accumulative, (ii) bio-degradable, and (iii) zero or very minimal ma-
rine toxicity level [16] . PARCOM provides an environmental evaluation
to nd out how long a chemical can remain in the natural environment
before it can be accepted for biodegradation, with a maximum of 60%
degrading after 28 d. Moreover, the level of chemical deposition in the
body is considered in the bioaccumulation assessment. The partition co-
ecient –a test of the chemical distribution between water mixture
and octanol expressed as log ( Po / w ) –is used to investigate it. To be
accepted, this value must be lower than 3 for each chemical [46] .
In general, GCIs include various biodegradable and natural materi-
als such as plant extracts, natural honey, herbs, oil, and drugs. A num-
ber of synthetic GCIs have also been reported in the literature, like sur-
factants and ionic liquids, which are out of the scope of this review.
Fig. 4 presents some of the natural materials used as GCIs in corrosion
studies.
Fig. 4. Natural materials used as green corrosion inhibitors.
27

A. Zakeri, E. Bahmani and A.S.R. Aghdam Corrosion Communications 5 (2022) 25–38
3.1. Plant extracts as GCIs
3.1.1. Preparation of extracts
According to the existing literature, various plant components’ ex-
tracts, including fruit, leaves, bark, peel, ower, root, seed, and even
whole plant extracts, are commonly utilized as GCIs. It should be noted
that phytochemical type and content dier based on the plant compo-
nent selection for extraction. It has been reported that phytochemical
contents of Sida acuta varied by plant part, with avonoids, saponins,
alkaloids, tannins, organic acid, and anthraquinones found in the leaves
extract and alkaloids, tannins, and anthraquinones exclusively con-
tained in the stem extract [47] . Leaves extracts, out of a variety of
extracts, were reported to exhibit the best overall protective perfor-
mance at low concentrations. This is mainly because phytochemicals are
primarily produced in leaves, where they are synthesized in the pres-
ence of sunlight, water, and CO
2
[ 10 , 48 ]. Some of the most common
phytochemicals that have a corrosion-inhibiting eect are avonoids,
glycosides, alkaloids, saponins, phytosterol, tannins, anthraquinones,
phenolic compounds, triterpenes, and phlobatannins. Most such phy-
tochemicals have polar functional groups like amide ( —CONH
2
), hy-
droxyl ( —OH), ester ( —COOC
2
H
5
), carboxylic acid ( —COOH), and
amino ( —NH
2
) that assist in their absorption [49–53] .
Following the selection of plant parts, the drying process is usually
accompanied by the sieving and grinding procedures to turn the plant
extract into powder. The traditional drying operation is usually carried
out at room temperature, in the shade or in the sun, and takes a lengthy
period. Following the drying process, several methods for separating
and extracting the desired extract from plants can be applied. In gen-
eral, the extraction principle is based on heating, followed by cooling,
and isolating active compounds (phytochemicals) [ 1 , 10 , 54 ]. It is worth
adding that the amounts of phytochemicals present in the extract are
also aected by a number of factors, including the plant’s age, vege-
tative cycle, geographic region, and the impact of weather conditions
[55] . Besides, there are extract powders that are commercially available
and can be used in the preparation of inhibitor stock solutions. Some of
the commonly used extraction processes along with their advantages
are presented in Table 1 . Besides, a typical experimental procedure for
yielding the Chamomile extract in powder form is shown schematically
in Fig. 5 .
3.1.2. Inhibition mechanism and efficiency of phytochemicals
Oftentimes, the electronic structures of the phytochemicals present
in the plant extracts are similar to those of organic CIs; therefore, it
is commonly recognized that these substances are capable of protecting
the ferrous metals in corrosive media [33] . To study the corrosion inhibi-
tion mechanism and performance of a specic plant extract, it is impor-
tant to determine the chemical compounds present in the plant extract.
Identication of the phytochemical substances such as volatile and non-
volatile compounds as well as heavy metals, fatty acids, and amino acids
can be carried out by chemical analysis techniques. Several commonly
used methods for determining phytochemical compounds are Fourier-
transform infrared spectroscopy (FTIR) [57] , liquid chromatography-
mass spectrometry (LC-MS) [58] , and gas chromatography-mass spec-
troscopy (GC-MS) [59] .
Gravimetric investigation together with surface and electrochemi-
cal analyses are some of the approaches used to study corrosion inhibi-
tion behavior. The most frequent method for determining the eciency
of GCIs is weight loss measurement, and an inhibitory mechanism can
be suggested based on the results. In this method, the eciency of an
inhibitor is evaluated on the account of substrate weight loss, which
is recorded before and after immersion in the electrolyte [60] . In the
meantime, modern electrochemical techniques such as electrochemi-
cal impedance spectroscopy (EIS), potentiodynamic polarization (PDP),
and electrochemical noise (EN) analysis have also been successfully im-
plemented. These ecient tests have provided certain results that are
comparable to those obtained using the traditional gravimetric method
[ 1 , 61 ]. These methods are briey described in this section.
3.1.2.1. Weight loss measurement (WLM). Prior to performing WLM,
metal specimens are prepared by polishing with several grades of abra-
sive paper, then thoroughly rinsed with solvents (ethanol, acetone,
and/or distilled water), and allowed to dry at ambient temperature. The
cleaned metal coupons are then weighed using a sensitive electronic bal-
ance before being immersed. To check for weight loss, the thoroughly
cleaned corroded specimens are reweighed after a dened duration of
exposure time. In addition to the synthesized solutions made from an-
alytical ingredients, many studies employed test solutions made from
original eld solution [62] . Specimen WLM in the absence and presence
of GCI is used to examine the eect of GCI in reducing corrosion rate on
metal coupons. The WLM methodology is straightforward and credible,
and it is used in several corrosion-monitoring systems as the primary
method of determining GCI eciency [63] .
3.1.2.2. Potentiodynamic polarization (PDP). PDP is another
electrochemical-based method for determining GCI performance,
corrosion rate, and corrosion prevention mechanism. In most studies,
the basic laboratory setup is employing three electrodes in the electro-
chemical cell for the measurement: counter (Pt or graphite), working
(metal substrate), and reference (calomel or Ag/AgCl) electrodes
immersed in a specied volume and concentration test solution [64] .
The reference electrode measures and controls the system’s voltage ( V ),
while the counter electrode measures and controls the current ( I ). The
open circuit potential ( E
ocp
) of the metal changes as the electrochemi-
cal reactions take place. After reaching equilibrium, a steady value is
measured, and then the PDP scan is conducted. Afterward, a Tafel plot
is established by providing a potential that ranges from 0.25 V below
the E
ocp
to 0.25 V higher potential value. The plots are then used to
calculate the corrosion potential ( E
corr
) and corrosion current density
( i
corr
). Moreover, dierent concentrations of GCIs and experimental
temperatures can be used to examine dierent impacts of GCIs on
corrosion inhibition performance [61] .
Table 1
Some extraction processes for plant extracts (phytochemicals) and their benets.
Method Advantage Note
Solvent extraction Improved energy eciency, high production output,
fast and easy operation
The most commonly used technique among others. Operation factors
aecting the properties of extract include type of solvents,
solvent-to-solid ratio, extraction time and temperature
Microwave-assisted extraction Enhanced reaction eciency, reduced reaction
durations, and minimized active component damage
Irradiation with microwaves has a synergistic impact of both breaking
and heating, while other methods do not have such capability
Enzyme-assisted extraction Suitable method for releasing bounded substances,
increased overall yield
Allowing for the utilization of practically the entire plant matrix
Ultrasound-assisted extraction Higher extraction yield, reaching almost 99.99% in
some studies. Without posing any damaging eects
to the structure of phytochemicals
Ultrasonic energy-assisted extraction aids the leaching of organic and
inorganic components. Ultrasound energy causes severe bubble
collapse, assisting diusion into the plant matrix, which in turn
enhances eciency
28

A. Zakeri, E. Bahmani and A.S.R. Aghdam Corrosion Communications 5 (2022) 25–38
Fig. 5. Sequence of experimental steps for the preparation of Chamomile extract together with its major constituents and corresponding molecular structures
(Reproduced from Ref. [56] with permission from Elsevier).
3.1.2.3. Electrochemical impedance spectroscopy (EIS). EIS is a powerful
approach for tracking in-situ electrochemical progressions with a funda-
mental insight into physical phenomena acting at the metal-electrolyte
interface, by which it provides valuable information on surface charac-
teristics and electrode kinetics via impedance diagrams [65] . The anal-
ysis is carried out in a three-electrode cell, similar to PDP, with slight
potential changes between 5 and 50 mV of AC voltage throughout a fre-
quency range of 10
5
-10
− 2 Hz. EIS is typically used to determine the
values of resistance and current ow, both when a GCI is present in
the solution and when it is not. Typically, the reported result of the EIS
analysis is a Nyquist plot, with the real part of the impedance ( Z ′ ) on
the X -axis and the imaginary part ( Z ″ ) on the Y -axis [66] .
The nature of adsorption with an organic molecule on a substrate
surface is largely governed by features such as the actual state of the
molecule, the corrosive media’s pH, the medium’s temperature, the an-
ions existing in the medium, and the charge on the metal surface. The
dierence (denoted by d ) of the E
corr
and the potential of zero charge
( Ei
q = 0
) could be used to estimate the charge on a metal surface. The
metal surface is predicted to gain positive charges if d = E
corr
- Ei
q = 0
is negative. On the other hand, the metal surface should be negatively
charged if d = E
corr
- Ei
q = 0
is positive. An organic molecule can be neu-
tral or protonated based on the pH of a system. In a system with neg-
ative anions, a positively charged surface will undoubtedly draw the
anions to itself. If inhibitor compounds present in the medium as proto-
nated species, they will electrostatically adsorb on the surface, a process
known as physisorption. A large number of studies on the plant-based
GCIs has reported the physisorption mechanism as the adsorption mode
[67–69] . In this situation, an electrostatic force keeps the GCI molecules
on the metal surfaces, and when the system temperature rises, the ad-
sorption bond weakens, resulting in a drop in inhibitory eciency [47] .
In a corrosive medium, electrons sharing or donation from neu-
tral inhibitor molecules to a metal substrate’s unoccupied orbital, es-
tablishes a covalent or co-ordinate type of bond known, and the ad-
sorption mechanism is known as the chemisorption mode. This form
of adsorption benets from a rise in temperature, as the inhibition ef-
ciency rises with the rise in system temperature. Plant-based GCIs
have also been found to exhibit chemisorption process [70] . It is per-
tinent to add that both mechanisms can take place together on the
same substrate surface [71–73] . In addition, inuence of the corro-
sive solution on the adsorption mechanism must be addressed. Ac-
cording to Oguzie et al. [74] , whenever specic corrosive media are
introduced, dierent adsorption mechanisms are established. In this
study, researchers stated that Hibiscus sabdariffa extract demonstrated
a physisorption mechanism in 2 mol/L HCl and chemisorption in
1 mol/L H
2
SO
4
.
Further, analyzing the standard enthalpy of adsorption ( ΔH
0
ads
) is
a proper methodology to determine the primary adsorption mechanism
on a metal surface. Chemical adsorption is indicated by a ΔH
0
ads
value
greater than zero in an endothermic system, while physical adsorption
is represented by a ΔH
0
ads
value smaller than zero. Additionally, in an
exothermic system, a ΔH
0
ads
value of less than 40 kJ/mol indicates ph-
ysisorption, while a ΔH
0
ads
value of more than 100 kJ/mol indicates
29