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Journal of Chemica Acta 2(2013)
Journal of Chemica Acta
journal homepage: www. jchemacta.com
An overview of emerging scenario in the frontiers of eco- friendly corrosion
inhibitors of plant origin for Mild Steel. (Font 15 times new Roman)
R. Rajalakshmi a, A. Prithiba b, and S. Leelavathi c (Font 12 times new Roman)
a
Professor, Department of Chemistry, Avinashilingam Institute for Home Science and Higher Education for Women, Coimbatore, India (Font 8 italic times new
Roman)
b
Assistant Professor, Department of Chemistry, Avinashilingam Institute for Home Science and Higher Education for Women, Coimbatore, India
C
Research Scholar, Department of Chemistry, Avinashilingam Institute for Home Science and Higher Education for Women, Coimbatore, India
ARTICLE INFO
ABSTRACT
Article history:
Received 2 December 2012
Received in revised form 23 December 2012
Accepted 28 December 2012
Available online 31 December 2012
Metals and their alloys are exposed to the action of acid in industrial processes where
acids play important roles such as in cooling water systems, acid pickling, acid
cleaning and acid descaling. Corrosion inhibitors are usually added to the acid
solutions to minimize metal loss and reduce acid consumption. However, some of
these corrosion inhibitors are toxic to the environment and this has prompted the
search for eco-friendly corrosion inhibitors for metals in acid solutions. The
exploration of natural products of plant origin as inexpensive eco-friendly corrosion
inhibitors has become increasingly important in recent years due to the biodegradability, eco-friendliness, cost-effectiveness, less toxicity and easy availability.
In the present review, attention has been focused on the corrosion inhibiting properties
of natural products for mild steel in aggressive media. (Font 8.5 times new
Keywords: (Font 8 italic times new Roman)
Mild steel
Corrosion inhibition
Natural products
Eco- friendly inhibitors
Aggressive media
Roman)
© 2013 Journal of Chemica Acta
1.
Introduction
2. Plant extracts as corrosion inhibitors for MILD STEEL
Corrosion of a metal may be due to chemical or
electrochemical reactions when it comes in contact with
matters present in its environment. Corrosion can be controlled
by suitably modifying the environment which stifles the anodic
or cathodic reactions or both. This is achieved by the use of
inhibitors [1].Corrosion inhibitors are substances which when
added in small concentrations to corrosive media decrease or
prevent the reaction of the metal with the media. Inhibitors are
added to many systems, e.g. cooling systems, refinery units,
acids, pipelines, chemicals, oil and gas production units, boiler
and process waters etc. Though many synthetic compounds are
excellent inhibitors in nature, most of them are found to be
highly toxic to the environment. These toxic effects have led to
the search of alternate resources that can be utilised as
inhibitors. Due to the environmental requirements that are
currently imposed on the development of cleaner inhibitors,
plant extracts, a source of natural, non-toxic, biodegradable
organic compounds, are used as corrosion inhibitors [2].Some
authors have drawn attention to the investigations done in this
area [3,4] This review highlights the utilization of plant
sources in corrosion protection, particularly as corrosion
inhibitors for mild steel in acidic, ethanolic and neutral media
Mild steel, a structural material of choice due to its versatility,
is a very important component of most of the objects and
widely used in most of the chemical industries for fabrication
of various reaction vessels, tanks, pipes etc. Since mild steel is
used under different conditions in chemical and allied
industries, it encounters severe attack from acids, due to their
aggressive nature, resulting in awful degradation .
The addition of inhibitors helps to reduce the corrosive attack
on metallic materials. Several naturally occurring substances
have been tried as corrosion inhibitors for mild steel in acidic
media .The inhibition of the corrosion of mild steel in
hydrochloric acid solution by the seed extract of Karanj
(Pongamia pinnata) [5], Kalmegh (Andrographis paniculata)
leaves [6] and Xylopia ferruginea leaves extract [7] , Leaf
extracts of Occimum viridis (OV) [6] Calyx extract of Hibiscus
sabdariffa [8], pineapple leaves extract [9] were found to be
highly effective in inhibiting corrosion of mild steel in acidic
media. The details of the investigations are tabulated in Table
1.
© 2013 Journal of Chemica Acta
ISSN: 2314-7083, Jabir Ibn Hayyan Publishing Ltd. All rights reserved
Rajalakshmi et al./J. Chem Acta 2 (2013)
8
00-0000; e-mail: rajivardhan@gmail.com
Table 1: Different types of extraction followed by various
plant extracts and their phytochemicals
S.No
1.
2.
3.
4.
5.
6.
7.
8.
S.No
Name of
the Plant
and Parts
Used
Name of
the Plant
and Parts
Used
Extracts
and
Techniques
Utilised
Phyto
chemicals
responsible
IE (%)
Ref
Karanj(Pongamia
pinnata) Seed
Aqueous Mass loss
Electroche
mical
Karanjin
Pongapin
Kanjone
Pongaglabr
one
98
5
Citrus
paradisi
6
Nypa
fruticans
Wurmb –
Leaves
Kalmegh(A
ndrographi
s
panniculata
) -Leaves
Aqueous Mass loss
Electroche
mical
Xylopia
ferruginea
–Leaves
chloroform
partition
(CP) , nhexane
partition
(HP),
methanol
extracts
(ME)Mass loss,
Electroche
mical,
SEM,
Ocimum
viridis –
Leaves
Acidic (2
M HCl and
1M H2SO4)
–
Gasometry
Hibiscus
sabdariffa –
Calyx
Pine apple
–Leaves
Papaia,
Poinciana
pulcherrim
a, Cassia
occidentalis
and Datura
stramonium
seeds,
Calotropis
procera B,
Azydracta
indica and
Auforpio
turkiale sap
Lawsonia
inermis
9.
10.
Andrograp
holide
ME-82
HP-88
CP-92
7
Alkaloids.
Terpenoids,
carotenoids
and
aromatic
oils
HCl66.89
H2SO4
– 69.1
8
Acidic (2
M HCl and
1M H2SO4)
–
Gasometry
protonated
organic
species in
the extract
HCl90.4
H2SO4
– 93
Ethanol –
Gasometry
ananasate,
1-Ocaffeoylgly
cerol,
1-O-pcoumaroylg
lycerol,
b-sitosterol,
daucosterol
AcidicMass loss,
Electroche
mical
atheroline
alkaloids
98.2
onion
(Allium
cepa)
,Garlic
11.
(Allium
sativum) ,
Bitter gourd
(Momordic
a
charantia)
12.
88%–
96%
Weight
loss,
hydrogen
gas
evolution
DC
methodTafel
extrapolatio
n,
Resistance
polarisation
,"Vicor"computer
software
method;
Mass
loss,dissolv
ed iron
concentrati
on
Cyamopsis
tetragonalo
ba-seed
Acidic
extract
Justica
gendarussa
-Leaves
MethanolWeight loss
Electroche
mical
10
Garcinia
14. mangostan
a - pericarp
of fruit
Proteins
Weight loss
9
13.
72
Extracts
and
Techniques
Utilised
techniques
and surface
analysis
(SEM/EDS
)
11
AcidicMass loss
Phyto
chemicals
responsible
IE (%)
Ref
α-DGlucose ,
tannic acid
vitamins,
pectins,
flavonoids
Nitrogenou
s
compounds
,Tannins
3epikatonic
acid,7-obeta-(2rhamnosylglucosyl)
myricetin,
astragalin,
alkaloids,
lignans,
phenolic
dimmers,
flavonoids ,
Osubstituted
aromatic
amines
oxygenated
prenylated
xanthones,
8hydroxycud
raxanthone
G,
mangosting
one .
14
75.1
15
DC82%,
Mass
loss98%
Dissolv
ed iron
conc88%
19
92
25
92
27
31
Similar studies were carried out by various investigators in
different media – sulphuric acid, sea water. The salient features
of these investigations are listed below.
Electroche
mical
(lawsone,
gallic acid,
16
The inhibition of the corrosion of mild steel in 1 M HCl and 1
M H2SO4 by Spirulina platensis has been studied by weight
Rajalakshmi et al./J. Chem Acta 2 (2013)
9
loss method, potentiodynamic polarization method,
electrochemical impedance spectroscopy measurements and
SEM analysis [12]. The inhibition efficiency increased with
increasing concentration of the inhibitor in both HCl and
H2SO4 media. From the results of weight loss studies at various
temperatures, the mode of adsorption is confirmed to be
physisorption. Further the adsorption has been found to follow
Temkin isotherm.
Matheswaran and Ramasamy, [13] tested the inhibitive nature
of Adhatoda vasica (AV) on the corrosion of mild steel in 1 N
HCl, 1N H2SO4 and 1 N H3PO4 acid solutions by weight loss
and polarization methods. From the results it was noticed that
the corrosion rate decreased with increase in concentration of
inhibitor in the entire investigated acid medium. Maximum
inhibition efficiency was obtained for 1 N HCl (1 h) = 77.77%
(0.1% of AV). In this investigation AV acted as good inhibitor
in HCl than H2SO4 and H3PO4. It was suggested that the
presence of alkaloids is responsible for the inhibitive nature of
AV Electrochemical studies reveal that the inhibitive action is
of mixed type .The inhibition was essentially based on the
coverage of the metal surface by the inhibitor molecules, thus
preventing contact of the corroding species with acid.
of extract in acid medium retard corrosion degradation of the
deposited mild steel and thereby reduce corrosion rate.
Pandian, Bothi Raja and Mathur Gopalakrishnan
Sethuraman[20,21] tested the acid extract of Solanum
tuberosum and alkaloid extract of Rauvolfia serpentina as
corrosion inhibitor for mild steel in 1 M HCl and H2SO4 media
using
weight
loss,
Potentiodynamic
polarization,
Electrochemical Impedance Spectroscopy (EIS) and SEM
techniques. The studies reveal that the plant extracts act as
good inhibitors in both acid media. The inhibitive effect may
be attributed to the adsorption of the inhibitor on the surface of
MS and was found to follow Temkin adsorption isotherm.
Tafel polarization method indicated that the plant extract
behaves as mixed inhibitor. SEM studies provide the
confirmatory evidence for the protection of MS by the green
inhibitor. The inhibition efficiency of the inhibitors may be due
to the adsorption of alkaloids and other phyto chemical
constituents present in the plant extracts. The authors proposed
the following mode of mechanism (Fig 2).
The inhibitive action of leaves (LV), seeds (SD) and a
combination of leaves and seeds (LVSD) extracts of
Phyllanthus amarus on mild steel corrosion in HCl and H2SO4
solutions ( fig 1)was evaluated using weight loss and
gasometric techniques [17]. The results indicate that the
extracts functioned as a good inhibitor in both environments
and inhibition efficiency increased with extracts concentration.
Temperature studies revealed an increase in inhibition
efficiency with rise in temperature and activation energies
decreased in the presence of the extract. A mechanism of
chemical adsorption of the plants components on the surface of
the metal is proposed for the inhibition behaviour. The
adsorption characteristics of the inhibitor were approximated
by Temkin isotherm.
Figure 2 : a) Competitive and b) Cooperative adsorption
of green inhibitor in acid solutions [20,21]
The leaves extract of Murraya koenigii has been evaluated
as an inhibitor for the corrosion of mild steel in hydrochloric
acid and sulphuric acid by Quraishi et al [22] using weight
loss, electrochemical impedance spectroscopy (EIS), linear
polarization and potentiodynamic polarization techniques.
Inhibition was found to increase with increasing concentration
of the leaves extract. The inhibition was assumed to occur via
adsorption of the inhibitor molecules on the metal surface. The
adsorption of the extract on the mild steel surface was found to
obey Langmuir adsorption isotherm.
Figure 1: Performance evaluation of leaves, seeds extract
and their mixture in different acidic media
Popoola and Fayomi [18] reported the corrosion inhibition
efficiency of Nicotiana tobacum (tobacco leaves) on zinc
electrodeposited mild steel in 2 M HCl using weight loss and
gasometric methods. Results obtained indicate that the addition
An evaluation of the effective performance of extract of
Ervatamia coronaria leaves on the corrosion inhibition of mild
steel in 1M HCl and 0.5M H2SO4 and Staminate Flower
extract of Cocos Nucifera as corrosion inhibitor for mild steel
in HCl medium at ambient temperature has been made by
Rajalakshmi et al [23,24]. Conventional weight loss and
electrochemical measurements were used for the evaluation.
Electrochemical studies reveal that both the tested extracts
acted as mixed type inhibitors. Ervatamia coronaria leaves
extracts yielded a maximum of 98% in acid medium.
Staminate flowers of Cocos nucifera yielded a maximum of
97.3% inhibition of mild steel corrosion in 1M HCl.
Rajalakshmi et al./J. Chem Acta 2 (2013)
10
In a similar study , Vijayalakshmi et al reported the
inhibitive effect of petiole extract Cocos nucifera [30] palmyra
palm (Borassus flabellifer Linn.) shell [32], coconut (Cocos
nucifera) shell extract [33] for the corrosion of mild steel in
0.5 M H2SO4 and 1 M HCl using mass loss, polarization and
electrochemical impedance techniques. The results obtained
indicate that the extracts behave as good inhibitor for the
corrosion of mild steel in 0.5 M H2SO4 and 1 M HCl. The
adsorption of the inhibitor on mild steel surface was found to
be spontaneous, endothermic and consistent with the
assumptions of Langmuir adsorption isotherm. The
electrochemical measurements reveal that the extracts behaved
like mixed type inhibitor. The maximum inhibition efficiency
for Palmyra palm in 0.5 M H2SO4 and 1 M HCl was found to
be 97.65% and 98.11% for a period of 24 h with 3%v/v
concentration of the inhibitor respectively.
Plant extract Ricinus communis was analysed for its
inhibitory effects towards mild steel in 100 ppm sodium
chloride by means of weight loss, electrochemical polarization
and impedance measurements [26]. It was found from weight
loss measurements that the inhibition efficiency was about
84% in 300 ppm of the plant extract. Polarization
measurements indicated that the plant extract generally act as
cathodic inhibitors on the fourth day. Electrochemical
impedance results also showed that the natural plant extract
increases the corrosion resistance of mild steel.
The inhibitive effect of cocoa (Theobroma Cacao) and
kolanut (Cola Acuminata) extracts on the corrosion of mild
steel in seawater at room temperature has been evaluated by
Umoru et al [28] .The study was carried out using the
gravimetric technique. The results showed kola and cocoa
leaves extracts as potential inhibitors of mild steel corrosion in
seawater and marine environment. The highest inhibition
efficiency was obtained when the concentration of the
inhibitors were increased up to optimum. According to the
authors , a sign of synergism was observed when 4% of each
of the extracts were used to inhibit corrosion in the seawater.
The efficacy of Acacia senegal seeds in mitigating the
corrosion of mild steel in HCl solution was tested by utilizing
weight loss and thermometric techniques in 0.1N – 5N HCl
[29] .It was reported that Acacia senegal alcoholic extract
acted as a good corrosion inhibitor in hydrochloric acid
solution (Fig 3) and the adsorption of the extract provides a
good protection against mild steel corrosion.
100
IE (%)
80
60
0.1N HCl
40
1N HCl
20
0
Seed
Bark
Root
Gum
Figure 3: IE obtained for different parts of Acacia senegal in
acidic media
Corrosion inhibition of mild steel in 1 M H 2SO4 by leaves
and stem extracts of Sida acuta was studied using chemical
(weight loss and hydrogen evolution) and spectroscopic (AAS,
FTIR and UV-V) techniques at 30–60˚C [34]. It was found that
the leaves and stem extracts of S.acuta inhibited the acid
induced corrosion of mild steel. The inhibition efficiency
increased with increase in concentration of the extracts but
decrease with rise in temperature. The adsorption of the extract
was found to follow Freundlich adsorption isotherm.
The inhibition effect of Parthenium hystophrous L extract
on the corrosion of mild steel in 1N sulphuric acid has been
studied at different concentrations as well as different
temperatures of inhibitor by mass loss studies, adsorption
isotherms, Tafel polarization and electrochemical impedance
spectroscopy (EIS) between 308 and 338K [35]. The corrosion
rate increased with increase in temperature and decrease with
increase in concentration of inhibitor compared to blank. The
adsorption of this extract has been found to obey Temkin and
Langmuir adsorption isotherm. Polarization results reveal that
the extract is a mixed type inhibitor. The results demonstrated
that the maximum inhibition efficiency at room temperature is
95.12%
Ethanol extract of Phyllanthus amarus was analysed for its
corrosion inhibitive effect on mild steel in H2SO4 using
gravimetric, thermometric and gasometric methods [36]. The
results indicate that ethanol extract of Phyllanthus amarus
leaves act as excellent inhibitors for mild steel in H2SO4. The
adsorption characteristic of the inhibitor is consistent with the
assumptions of Langmuir adsorption isotherm. From the
results and findings of the study, a physical adsorption
mechanism was proposed for the adsorption of ethanol extract
of Phyllanthus amarus on mild steel surface. The inhibition
potentials of ethanol extract of Phyllanthus amarus leaves are
enhanced by its phytochemical constituents.
The inhibitive effect of the ethanol extract of the
mixture of leaves, fruits and twigs of Muntingia calabura
(EEMC) on mild steel corrosion in 1M HCl and 1M H2SO4
solutions was studied by using weight loss method at room
temperature [37]. The results demonstrate that EEMC
suppressed the corrosion reaction in both acid media and
inhibition efficiency increased with EEMC concentration with
slightly higher values obtained in 1 N H2SO4. The adsorption
of this EEMC in both acids on the mild steel surface obeyed
the Langmuir adsorption isotherm. Frumkin isotherm is best
applicable in inhibitor with H2SO4. The addition of halide ions
to the EEMC enhanced the inhibition efficiency due to
synergistic effect.
Extract of Bauhinia purpurea leaves was investigated as a
corrosion inhibitor of mild steel in 1 N H2SO4 using
conventional weight loss, electrochemical polarization,
electrochemical impedance spectroscopy, and scanning
electron microscopic studies [38]. The weight loss results
showed that the extract of Bauhinia purpurea leaves acted as
excellent corrosion inhibitor. Electrochemical polarization data
revealed the mixed mode of inhibition. Scanning electron
microscope studies provided the confirmatory evidence of an
improved surface condition, due to the adsorption of the
inhibitor, for the corrosion protection.
Eddy et al [39] determined the inhibitive and adsorption
properties of ethanol extract of Lasianthera africana for
inhibition of corrosion of mild steel in H2SO4 using
gravimetric, thermometric, gasometric, and infrared (IR)
methods. The extract was found to be a good inhibitor of
corrosion of mild steel in H2SO4. Inhibitive properties of the
Rajalakshmi et al./J. Chem Acta 2 (2013)
11
extract were attributed to enhancement in adsorption of the
inhibitor on mild-steel surface. Adsorption of the inhibitor was
found to be consistent with assumptions of Langmuir and
Temkin adsorption isotherms. Synergistic study revealed that,
of the investigated halides, only KCl may enhance adsorption
of the inhibitor, whereas KBr and KI antagonized its
adsorption.
The inhibitor effect of the naturally occurring biological
molecule caffeic acid on the corrosion of mild steel in 0.1 M
H2SO4 was investigated by weight loss, potentiodynamic
polarization, electrochemical impedance and Raman
spectroscopy [40]. The different techniques confirmed the
adsorption of caffeic acid onto the mild steel surface and
consequently the inhibition of the corrosion process.
Inhibitive and adsorption properties of ethanol extract of
Colocasia esculenta for the corrosion of mild steel in H2SO4
were investigated using weight loss, hydrogen evolution and
IR methods of monitoring corrosion [41]. The results obtained
indicated that ethanol extract of C. esculenta was a good
inhibitor for the corrosion of mild steel in H 2SO4 and its
inhibitive action was attributed to its phytochemical
constituents which aided its adsorption on the surface of mild
steel. Calculated values of activation energy and inhibition
efficiency at 303 and 333 K revealed that the mechanism of
adsorption of ethanol extract of C. esculenta on mild steel
surface is physical adsorption. The adsorption of the inhibitor
on mild steel surface was found to be spontaneous,
endothermic and consistent with the assumptions of Langmuir
adsorption isotherm.
The inhibitive action of ethanol extracts from leaves (LV),
bark (BK) and roots (RT) of Nauclea latifolia on mild steel
corrosion in H2SO4 solutions at 30 to 60ºC was studied using
weight loss and gasometric techniques [42]. The extracts were
found to inhibit the corrosion of mild steel in H 2SO4 solutions
and the inhibition efficiencies of the extracts follow the trend:
RT > LV > BK. The Inhibition efficiency increased with the
extracts concentration but decreased with temperature rise.
Physical adsorption of the phytochemical components of the
plant on the metal surface was proposed as the mechanism of
inhibition. The adsorption characteristics of the inhibitor were
approximated by the thermodynamic-kinetic model of ElAwady et al.
The inhibitive and adsorptive characteristics of ethanol
extract of Gnetum africana for the corrosion of mild steel in
H2SO4 solutions have been studied using weight loss,
gasometric, thermometric, and IR methods of monitoring
corrosion [43]. Ethanol extract of Gnetum africana is a good
adsorption inhibitor for the corrosion of mild steel in H 2SO4.
The inhibitive property of the extract was attributed to the
presence of alkaloid, saponin, tannin, terpene, anthraquinone,
cardiac glycoside, and alkaloid in the extract. The adsorption
of the inhibitor on mild steel surface was exothermic,
spontaneous and was consistent with the mechanism of
physical adsorption. In addition, Langmuir and Temkin
adsorption isotherms best described the adsorption
characteristics of the inhibitor. Efforts to improve the
adsorption of the inhibitor through synergistic combinations
with halides indicated that only KCl may enhance the
efficiency of the inhibitor.
Okafor et al [44] investigated the inhibitive effect of ethanol
extract of Garcinia kola (EXG) for the corrosion of mild steel
in H2SO4 solutions using the hydrogen evolution technique at
30-60°C. The results obtained indicate that EXG inhibits the
corrosion of mild steel in acidic medium and that the inhibition
efficiency increased with an increase in the concentration of
ethanol extract and decreasing temperature. The inhibition
efficiency increased on addition of potassium iodide to EXG,
indicating synergism. The experimental data obeyed the
Langmuir adsorption isotherm as well as the El-Awady et al.
thermodynamic-kinetic model. The activation energy of
inhibition of 6.8508 KJ/mol for the corrosion process
suggested that the EXG molecules are physically adsorbed on
the metal surface.
The corrosion inhibitive effect of the extract of black
pepper on mild steel (MS) in 1 H2SO4 media was evaluated
using conventional weight loss studies, electrochemical studies
viz., Tafel polarization, AC impedance and scanning electron
microscope (SEM) studies [45]. Results of weight loss study
reveal that black pepper extract acts as a good inhibitor even at
high temperatures also. The inhibition was through adsorption
which is found to follow Temkin adsorption isotherm. Tafel
polarization method revealed the mixed mode inhibition of
black pepper extract. Analysis of impedance data has been
made with equivalent circuit with constant phase angle element
for calculation of double layer capacitance value. SEM studies
provide the confirmatory evidence for the protection of MS by
the green inhibitor.
Berberine extracted from Coptis chinensis was tested for its
inhibition efficiency on corrosion of mild steel in 1 M H 2SO4
using weight loss experiment, electrochemical techniques and
scanning electronic microscope (SEM) with energy disperse
spectrometer (EDS) [46]. The weight loss results showed that
berberine is an excellent corrosion inhibitor for mild steel
immersed in 1 M H2SO4. Potentiodynamic curves suggested
that berberine suppressed both cathodic and anodic processes
for its concentrations higher than 1.0 × 10−4 M and mainly
cathodic reaction was suppressed for lower concentrations. A
good fit to Flory–Huggins isotherm was obtained between
surface coverage degree and inhibitor concentration.
Okafor and Ebenso [47] evaluated the effect of different
parts of Carica papaya leaves (LV), seeds (SD), heart wood
(HW) and bark (BK)) as eco-friendly and non-toxic mild-steel
corrosion inhibitors in H2SO4 at 30 to 60°C. Gravimetric and
gasometric techniques were used to characterise the
mechanism of inhibition. The LV, SD, HW and BK extracts
were found to inhibit mild steel corrosion in H2SO4. The
inhibition efficiencies of the plant's part extracts follow the
trend: LV>SD>HW>BK. Inhibition efficiency increased with
extracts concentration but decreased with temperature.
Physical adsorption of the phytochemical components of the
plant on the metal surface is proposed as the mechanism of
inhibition. The experimental data fitted well into the Langmuir
and Temkin
adsorption isotherms.
The potential of ethanol extract of Ocimum gratissimum as
a green inhibitor for the corrosion of mild steel in H 2SO4 was
assessed by Eddy et al [48] using gasometric, gravimetric, and
thermometric methods of monitoring corrosion. The inhibition
efficiency of the inhibitor was found to decrease with increase
in the period of contact and with increasing temperature, but
increased with increase in the concentration of the inhibitor.
The authors proposed a physical adsorption mechanism for the
adsorption of the inhibitor on the surface of mild steel. The
Rajalakshmi et al./J. Chem Acta 2 (2013)
12
adsorption was found to be exothermic, spontaneous, and was
best described by Langmuir adsorption isotherm.
The corrosion inhibitive effects of Pachylobus edulis
exudate gum for mild steel in 2M H2SO4 and influence of
potassium halide additives on the inhibition efficiency were
investigated using hydrogen evolution and thermometric
methods at 30–60°C [49]. Inhibition efficiency was determined
by comparing the corrosion rates in the absence and presence
of additives. The trend of inhibition efficiency with
temperature was used to propose the mechanism of inhibition.
It was found that the exudate gum acted as an inhibitor for
acid-induced corrosion of mild steel. Inhibition efficiency (%I)
of the exudate gum increased with an increase in concentration
of the exudate gum and synergistically increased on addition of
potassium halides but decreased with increase in temperature.
Inhibitor adsorption characteristics were approximated by
Temkin adsorption isotherm at all the concentrations and
temperatures studied. The phenomenon of physical adsorption
was proposed from the activation parameters obtained.
Umoren et al [50] tested the effectiveness of Raphia hookeri
(RH) exudate gum as a corrosion inhibitor for mild steel in
H2SO4 using weight loss and hydrogen evolution techniques at
30– 60 °C. Results obtained revealed that RH act as corrosion
inhibitor for mild steel in sulfuric acid medium. The corrosion
rates in all concentrations studied increased with rise in
temperature. The inhibition efficiency was observed to
increase with increase in RH concentration but decreased with
rise in temperature, which is suggestive of physical adsorption
mechanism.The adsorption of the exudate gum onto the steel
surface was found to follow the Langmuir adsorption isotherm.
Aisha Turkustani and Sanaa T. Arab [51] evaluated the
corrosion inhibition of mild steel in 1.0 M H 2SO4 containing
10% ethyl alcohol (EtOH) using Zizyphus Spina-Christi (ZSC)
extracts (aqueous extract and alcoholic extract) by chemical
techniques ( hydrogen evolution(HE) and mass loss(ML)) and
electrochemical
techniques(electrochemical
impedance
spectroscopy(EIS) and potentiodynamic polarization(PDP)).
The effect of acid concentration on the corrosion rate of mild
steel was found to increase with acid concentration (0.25-1.5)
M. Electrochemical impedance spectroscopy results showed
that the corrosion and corrosion inhibition of steel occurred
mainly by charge transfer. The electrochemical results of
polarization also showed that the extracts of ZSC plant act as
mixed type inhibitors, they retarded both cathodic and anodic
reaction. The experimental results from chemical and
electrochemical studies followed Langmuir isotherm model.
Eduok et al [52] investigated the synergistic action caused
by iodide ions on the corrosion inhibition of mild steel in 1 M
H2SO4 by leaves and stem extracts of Sida acuta using weight
loss and hydrogen evolution methods at 30 – 60°C. It was
found that the leaves and stem extracts of Sida acuta inhibited
the acid induced corrosion of mild steel. Addition of iodide
ions enhances the inhibition efficiency to a considerable
extent. The inhibition efficiency increased with increase in the
iodide ion concentration but decreased with rise in
temperature. Adsorption of the extracts and in combination of
iodide ion was found to obey Freundlich adsorption isotherm
at all investigated temperatures. Inhibition mechanism was
deduced from the temperature dependence of the inhibition
efficiency as well as from assessment of kinetic and activation
parameters that govern the processes. The synergism parameter
(S1) for the different concentrations of iodide ions from the
two techniques employed was found to be greater than unity
indicating that the enhanced inhibition efficiency of the
extracts caused by the addition of iodide ions was due to
synergism.
Various authors have tried the inhibition effect of different
plant extracts namely Zenthoxylum alatum [53] ,Acalypha
indica L.[54], Allamanda blanchetii [55], Uncaria gambir[56]
, Musa species peel [57], Vernonia amygdalina[58] for
ethanolic and ethyl acetate medium and the salient features are
tabulated in Table 2
Table 2: Investigated Plant extracts and their extraction
techniques
S.No
Name of
the
Plant/Par
ts Used
1. Zenthoxy
lum
alatum
Fruits
2. Acalypha
indica L
Extract/
Techniques
Utilised
Methanol /
Weight loss ,
Electrochem
ical, ESCA
Alcohol /
Weight loss ,
Electrochem
ical
53
3. Allaman
da
blanchett
i
Aqueous
/Weight loss
,
Electrochem
ical,SEM
Ethyl
acetate/
Weight loss,
Electrochem
ical , SEMEDX
Ethanol
/hydrogen
evolution
and
thermometri
c methods
Ethanol
/Weight loss
, hydrogen
evolution
and
thermometri
c methods
55
4. Uncaria
gambir
5. Musa sp
peel
6. Vernonia
amygdali
na
Phyto
chemicals
responsible
Ref
54
Catechin
56
57
tannin,
saponnins,
flavanoid
and
anthraquino
ne
58
3. Conclusion:
The mammoth increase in industrial activities has led
to the use of mineral acids in acid cleaning/pickling and
descaling. Mild steel-a structural material of choice-encounters
severe attack of these acids resulting in aweful degradation.
Since it is impossible to eliminate corrosion, controlling, rather
than preventing it, is the only remedy. Application of the acid
corrosion inhibitors is one such corrosion control measures.
Recent awareness of the corrosion inhibiting abilities of
tannins, alkaloids, organic and amino acids has resulted in
sustained interest on the corrosion inhibiting properties of
natural products of plant origin. Such investigation is of much
importance because in addition to being environmentally
Rajalakshmi et al./J. Chem Acta 2 (2013)
13
friendly and ecologically acceptable, plant products are
inexpensive, readily available and renewable sources of
materials.
Acknowledgements
The authors wish to acknowledge Avinashilingam Institute for
Home Science and Higher Education for Women, Coimbatore,
India, for providing the necessary facilities to carry out the
studies. One of the authors S.Leelavathi, wish to thank DST
CURIE.
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