Ginseng Root: A new Efficient and Effective Eco-Friendly Corrosion Inhibitor for Aluminium Alloy of type AA 1060 in Hydrochloric Acid Solution

Int. J. Electrochem. Sci., 4 (2009) 1277 - 1288

International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.org


Ginseng Root: A new Efficient and Effective Eco-Friendly
Corrosion Inhibitor for Aluminium Alloy of type AA 1060 in
Hydrochloric Acid Solution

I.B. Obot1,*, N.O. Obi-Egbedi2
1 Department of Chemistry, Faculty of Science, University of Uyo, Uyo, Nigeria
2 Department of Chemistry, University of Ibadan, Ibadan, Nigeria
*E-mail: proffoime@yahoo.com

Received: 4 May 2009 / Accepted: 15 September 2009 / Published: 30 September 2009


We report for the first time the inhibitive action of the root of ginseng on aluminium corrosion in HCl
solution using weight loss method at 30-60 oC. Results obtained showed that ginseng root functioned
as an effective and excellent inhibitor in the acid medium. Corrosion rate increased both in the absence
and presence of inhibitor with increase in temperature. Corrosion rate was also found to decrease in the
presence of inhibitor compared to the free acid solution. Inhibition efficiency increases with increase in
concentration of the inhibitor but decreases with increase in temperature reaching a maximum of
93.1% at 30 oC at 50 % v/v concentration of ginseng. Addition of iodide ions to the root extracts of
ginseng enhances the inhibition efficiency considerably and the effect is more pronounced at higher
temperatures. The adsorption of extract components onto the aluminium surface was found to be a
spontaneous process and to follow the Freundlich adsorption isotherm. The free energies, enthalpy and
entropy for the adsorption process as well as the energy of activation, enthalpy of activation and
entropy of activation for the dissolution process were determined and discussed. A mechanism of
physical adsorption of the root components on the surface of the metal is proposed for the inhibition
behaviour.


Keywords: Ginseng, aluminium, corrosion inhibition, adsorption isotherm, thermodynamics


1. INTRODUCTION
Acid solutions are commonly used in the chemical industry to remove the scales from the
metallic surfaces. The addition of inhibitors secures the metal against an acid attack effectively. The
applicability of organic compounds as corrosion inhibitors for metals in acidic media has been
recognized for a long time [1-5]. The existing data show that most organic inhibitors act by adsorption
on the metal surface. The adsorption of inhibitors occurs through heteroatoms such as nitrogen,

---

Int. J. Electrochem. Sci., Vol. 4, 2009
1278

oxygen, phosphorus and sulphur, triple bonds or aromatic rings. These compounds which are adsorbed
on the metallic surface block the active corrosion sites.
Though many synthetic organic compounds showed good anticorrosive activity, most of them
are highly toxic to both human beings and the environment. The safety and environmental issues of
corrosion inhibitors arisen in industries has always been a global concern. These inhibitors may cause
reversible (temporary) or irreversible (permanent) damage to organ system viz, kidney, liver, or disturb
a biochemical process and enzyme system at some site in the body [6]. The toxicity may manifest
either during the synthesis of the compound or during its application.
Recent awareness of the corrosion inhibiting abilities of tannins, alkaloids, organic and amino
acids as well as organic dyes [7-11] 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 friendly and ecologically acceptable, plants products are
inexpensive, readily available and renewable sources of materials. The use of natural products as
corrosion inhibitors have been widely reported by several authors [12-20]. Among plant materials
tested in our laboratory include Dacroydes edulis [21], Pachylobus edulis [22], Vigna unguiculata
[23], Gum Arabic [24-26], and Raphia hookeri [27].
Ginseng refers to species within Panax, a genus of 11 species of slow-growing perennial plants
with fleshy roots. Ginseng; the root and rhizome of Panax ginseng C.A. Meyer (Araliaceae) have been
used as a medicinal herb for more than 2000 years in oriental countries including China, Korea, and
Japan [28]. Panax ginseng has been known to have pharmacological activities over a wide spectrum of
effectiveness such as anti-fatigue and anti-stress and promotion of longevity [29]. The herb has been
shown to contain glycoside, saponin, phenolic acids, alkaloids and lignin [30]. Other components
include large amounts of starch and gum, some resin, a very small amount of volatile oil, and panacon
but the most active ingredient of Panax ginseng is ginsenosides with backbone structure similar to that
of sterol, especially cholesterol. Recently more than 25 dammarane-type tetracyclic triterpernoid
saponins have been isolated from ginseng, the root and rhizome of Panax ginseng [28]. As a
continuation of our current interest on eco-friendly corrosion inhibitors, we report for the first time the
inhibition action of ginseng root on the acid corrosion of aluminium using the weight loss method at
30-60 oC. The effect of iodide addition is also reported.



2. EXPERIMENTAL PART
2.1. Materials
The aluminium sheets of the type AA 1060 and purity 98.8% were obtained from Sky
Aluminium Ltd, Uyo, Nigeria and of the same composition as those reported previously [22-24]. Each
sheet was 0.14 cm in thickness and was mechanically press-cut into coupons of dimension 5cm x 4cm.
These coupons were used as cut without further polishing. They were however degreased in absolute
ethanol, dried in acetone, and stored in moisture-free desiccators prior to use [23]. All reagents used
were BDH analytical grade.

---

Int. J. Electrochem. Sci., Vol. 4, 2009
1279

2.2. Preparation of the root extracts of ginseng
The procedure for the preparation of the root extracts is similar to that reported recently by
Okafor et al. [19]. Ginseng roots were collected from Yaoundé, Cameroon. They were dried in an
N53C-Genlab Laboratory oven at 50 oC, and ground to powder form. Ten gram of the powder was
digested in 1 L of 1 M HCl solution. The resultant solution was kept for 24 h, filtered and stored. From
the stock solution, the root extracts test solutions were prepared at concentrations of 50, 40 and 10
%v/v. The effect of iodide additive was studied by combining 5.0 mM of KI salt with different
concentrations of the root extracts.

2.3. Weight loss method
In the weight loss measurements, aluminium coupons in triplicate were totally immersed in 200
mL of 1 M HCl solutions devoid of and containing various concentrations of the studied inhibitor
(Ginseng root extracts). The metal specimens were withdrawn from the test solutions after 4 h at 30-60
oC and the weight loss determined [31, 32]. The weight loss was taken as the difference in weight of
the specimens before and after immersion determined using LP 120 digital balance with sensitivity of
± 1 mg. The tests were performed in duplicate to guarantee the reliability of the results and the mean
value of the weight loss is reported. Weight loss values obtained allowed calculation of the mean
corrosion rate in mg cm-2 h-1.

The corrosion rate (W) was computed using the expression [26]:

m ? m

W
1
2
=
(1)
At

Where m and m are the weight losses (mg) before and after immersion in the test solutions, A is the
1
2
area of the specimens (cm2) and t is the exposure time (h).
The inhibition efficiency (% I) was computed using the equation [26]:

W
? W

%I
blank
inh
=
1
x 00 (2)
Wblank

where W
and W
are the corrosion rates in the absence and presence of the inhibitor respectively.
blank
inh


3. RESULTS AND DISCUSSION
3.1. Corrosion rates and inhibition efficiency
A general mechanism for the dissolution of Al metal would be similar to that earlier reported
[33]:

---

Int. J. Electrochem. Sci., Vol. 4, 2009
1280

Al(s) + H2O ? AlOHads + H+ + e (3)
AlOHads + 5H2O + H+ ? Al3+ . 6H2O + 2e (4)
Al3+ + H2O ? [AlOH]2+ + H+ (5)
[AlOH]2+ + X- ? [AlOHX]+ (6)

The controlling step in the metal dissolution is the complexation reaction between the hydrated
cation and the anion present Eq. (5). In the presence of chloride ions the reaction will correspond to:


[AlOH]2+ + Cl- ? [AlOHCl]+ (7)

The soluble complex ion formed increases the metal dissolution rate which depends on the
chloride concentration.

Table 1. Calculated values of corrosion rate (mg cm-2 h-1) and inhibition efficiency (%I) for aluminium
corrosion in 1M HCl in the absence and presence of various concentrations of ginseng extracts, 5.0
mM KI and ginseng extracts + 5.0 mM KI at different temperatures using weight loss.

System / Concentrations Corrosion rate (mg cm-2 h-1) Inhibition efficiency (%I)
30 oC 40 oC 50 oC 60 oC 30 oC 40 oC 50 oC 60 oC
Blank 7.35 29.60 31.80 33.90 - - - -
5.0 mM KI 3.01 17.00 20.10 20.40 59.0 42.6 36.8 39.8
10% v/v 0.87 11.10 13.90 20.00 88.2 62.5 56.3 41.0
40% v/v 0.68 6.10 10.60 14.00 90.7 79.4 66.7 58.7
50% v/v 0.51 4.90 9.30 12.50 93.1 83.4 70.8 63.1
10% v/v + KI 0.60 10.00 12.30 18.10 91.8 66.2 61.3 46.6
40% v/v + KI 0.36 4.90 9.40 11.90 95.1 83.4 70.4 64.9
50% v/v + KI 0.30 3.40 7.90 10.90 96.0 88.5 75.2 67.8

The values of percentage inhibition efficiency and corrosion rate obtained from weight loss
method at different concentrations of the ginseng extracts and ginseng extracts in combination with KI
at 30 - 60 oC are summarized in Table 1, and the variation of inhibition efficiency with concentrations
of ginseng extracts and ginseng extracts in combination with KI are shown in Fig.1. Inspection of the
data in the table reveals that the addition of ginseng extract decreases the corrosion rate of aluminium.
This result indicates the inhibitive effect of the added extract on aluminium corrosion in the acidic
solution. From Fig. 1, it is observed that the inhibition efficiency increases as the concentration of
added extract is increased. The observed inhibition action of the ginseng extract could be attributed to
the adsorption of its components on aluminium surface. The formed layer, of the adsorbed molecules,
isolates the metal surface from the aggressive medium leading to decreasing the corrosion rate. The
chemical components of the extract of ginseng roots was identified and determined as reported in
literature [28-30]. As earlier stated in the introductory remarks, ginseng root is a complex mixture of
glycoside, saponin, phenolic acids, alkaloids and lignin. Other components include large amounts of
starch and gum, some resin, a very small amount of volatile oil, panacon, dammarane-type tetracyclic

---

Int. J. Electrochem. Sci., Vol. 4, 2009
1281

triterpernoid saponins and ginsenosides (or panaxosides), which implies that it contains heteroatom
such as oxygen and nitrogen in its molecules. The corrosion inhibition of aluminium may be attributed
to adsorption of ginseng extract components through these atoms, which are regarded as centers of
adsorption onto the metals surface. Owing to the complex chemical composition of the extract, it is
quite difficult to assign the inhibitive effect to a particular constituent. Further investigation using
surface analytical techniques will enable the characterization of the active materials in the adsorbed
layer and assist in identifying the most active ingredients.

(a)
100
90
80
I
)
70

(
%
c
y
60
n
f
f
i
c
i
e
50
30oC

e
n
40
40oC
i
t
i
o
i
b
h
50oC
30
I
n
60oC
20
10
0
10
40
50
Conc.( %v/v)

(b)
120
100
I
)

(
%
80
c
y
n
f
f
i
c
i
e
60

e
n
30oC
i
t
i
o
40
i
b
h
40oC
I
n
20
50oC
60oC
0
10
40
50
Conc. (%v/v)

Figure 1. Plots of inhibition efficiency against concentration for (a) Ginseng extracts and (b) Ginseng
extracts + KI mixtures at 30-60 oC .

---

Int. J. Electrochem. Sci., Vol. 4, 2009
1282

0
-0.1
-0.2
-0.3
-0.4
?
-0.5
l
n
30oC
-0.6
40oC
-0.7
y = 0.145x - 0.587
50oC
y = 0.216x - 1.064
-0.8
R² = 0.876
R² = 0.869
60oC
-0.9
y = 0.027x - 0.154
y = 0.114x - 0.671
R² = 0.999
R² = 0.928
-1
2.3
3.6
3.9
lnC

Figure 2. Freundlich adsorption isotherm for aluminium corrosion in 1M HCl at 30 -60oC


Table 2. Adsorption parameters for the adsorption of ginseng extracts in 1M HCl on aluminium at
different temperatures.

Temperature (oC) Kads n R2
(kJ/mol)
30 0.86 0.027 0.999 -9.73
40 0.56 0.145 0.876 -8.92
50 0.51 0.114 0.928 -8.98
60 0.35 0.216 0.869 -8.17


3.2. Adsorption isotherm and thermodynamic studies
The efficacy of an organic compound as a successful inhibitor is mainly dependent on its
ability to get adsorbed on the metal surface, which consists of the replacement of water molecules at
the corroding interface [26]. The adsorption of the inhibitor is influenced by the nature and the charge
of the metal, the chemical nature of the inhibitor, distribution of the charge in the molecule, and the
type of electrolyte [34]. Basic information dealing with the interaction between the inhibitor molecule
and metal surface can be provided by adsorption isotherm. The degree of surface coverage values for
different concentrations of ginseng extracts from the weight loss measurements obtained from (? = %
I/100), assuming a direct relationship between surface coverage and inhibition efficiency has been
adapted to determine the adsorption characteristics of ginseng extracts in 1 M HCl solution. To
ascertain the nature of adsorption, the surface coverage values for ginseng extracts for 30 – 60 oC were
fitted into different adsorption isotherm models and correlation coefficients (R2) were used to
determine best fit which was obtained with the Freundlich adsorption isotherm. The observed changes

---

Int. J. Electrochem. Sci., Vol. 4, 2009
1283

in ? are shown in Fig. 2 as a function of concentration of ginseng extracts in 1M HCl at 30 - 60 oC. The
linear plots obtained ( R2 > 0.86) suggest that the experimental data fit the Freundlich adsorption
isotherm which is given by [34]:


n
? = KC (8)

Where ? < n< 1, or


ln ? = ln K + n ln C (9)

C is the concentration of ginseng extracts and K the equilibrium constant for adsorption is
related to the thermodynamic parameters for adsorption shown in Table 2 according to the following
equations [35]:

? ? ? o ?
1
G

=
K
?
ads
exp
? (10)
5 .
5 5
?
RT
?

o
o
o
?G
=
H
?
? T?S
(11)
ads
ads
ads


0
y = 0.460x - 10.10
-2
R² = 0.874
Ginseng
l
)
o
-4
Ginseng + KI
y = 0.355x - 10.07
/
m
J
R² = 0.809
-6

(
k
s
d
a
G
-8
?
-10
-12
303
313
323
333
T (K)

Figure 3. Plot of free energy of adsorption versus temperature

The negative values of
o
?G
suggest that the adsorption of ginseng extracts onto aluminium
ads
surface is a spontaneous process and the adsorbed layer is stable. Usually the adsorption free energy
involved in a physisorption process is less than -25 kJ/mol [33]. The plot of
o
?G
versus T was used to
ads
determine the enthalpy
o
?H
, and the entropy
o
?S
for the adsorption process using the basic
ads
ads

---

Int. J. Electrochem. Sci., Vol. 4, 2009
1284

thermodynamic equation (Eq. 11) (Fig. 3). The values of
o
?H
obtained were -10.10 kJ/mol and -
ads
10.07 kJ/mol for the ginseng extracts and ginseng extracts + KI respectively. Whereas the values of
o
?S
were -0.46 J/mol/K and -0.36 J/mol/K for the ginseng extracts and ginseng extracts + KI
ads
respectively. The negative values of
o
?H
and
o
?S
obtained show that the adsorption is exothermic
ads
ads
with an ordered phenomenon. Similar report has been documented [35].

0
Blank
-0.5
50 v/v%
-1
40 v/v%
10 v/v %
-1.5

W
g
o
L
-2
-2.5
-3
-3.5
3
3.09
3.19
3.3
1/T (K-1 ) x 10-3

Figure 4. Arrhenius plot for aluminium corrosion in 1M HCl in the absence and presence of various
concentrations of ginseng extracts

0
Blank
-1
50 v/v%
40 v/v%
-2
/
T
10 v/v %
-3
g

W
L
o
-4
-5
-6
3
3.09
3.19
3.3
1/T (K -1) x10-3

Figure 5. Transition State plot for aluminium corrosion in 1M HCl in the absence and presence of
various concentrations of ginseng extracts

---

Int. J. Electrochem. Sci., Vol. 4, 2009
1285

Table 3. Activation parameters of aluminium dissolution in 1M HCl in the absence and presence of
different concentrations of ginseng extracts.

Systems/Concentrations (g/l) E
o
a (kJ/mol)
o
?H (kJ/mol) ?S (J/mol/K)
Blank 3.80 1.95 -58.20
10% v/v 8.00 6.08 -57.17
40% v/v 8.00 6.06 -60.23
50% v/v 8.50 6.58 -60.52


3.3. Effect of Temperature
Temperature study was carried out to get more information about the performance of ginseng
extract and the nature of adsorption and thereafter to evaluate the activation processes. For this
purpose, weight-loss measurements are determined in the range of temperature 30-60 oC, in the
absence and presence of inhibitor at various concentrations during 4 h of immersion. The
corresponding data are shown in Table 1. It is clearly seen from the table that increase in temperature
leads to an increase in the corrosion rate with or without inhibitor
To calculate activation thermodynamic parameters of the corrosion reaction such as the
activation energy E , the enthalpy
o
?H
and the entropy
o
?S
of activation, Arrhenius Eq. (12) and
a
ads
ads
its alternative formulation called transition state Eq. (13) were employed:

? E

logW
a
=
+ log A (12)
.
2 303RT
? RT ?
? ? o ?
? ? ? o ?
S
H
=
W
?
? ex ??
p
?
? exp??
?
?
? Nh ?
? R ?
? RT ?

(13)

where W is the corrosion rate, E is the apparent activation energy, R is the molar gas constant, T is
a
the absolute temperature, A is the frequency factor, h is the Planck’s constant and N is the Avogadro’s
number.
The activation energy E is calculated from the slope of the plots of Log W versus 1/T (Fig.4).
a
Plots of Log (W/T) as a function of 1/T (Fig. 5) give a straight line with a slope of (–?Ho/ 2.303R) and
an intercept of (logR/Nh + ?So/2.303R) from which the values of
o
?H
o
and ?S were calculated, and
listed in Table 3. Inspection of Table 3 reveals that the presence of ginseng root extracts increases the
values of E as compared to the blank (without ginseng extracts) indicating physical adsorption of the
a
extracts on the metal surface [21-23]. The positive values of
o
H
?
reflect the endothermic nature of the
o
aluminium dissolution process in the 1 M HCl. The values of ?S in the presence and absence of the
extracts are large and negative implying that the activation complex in the rate determining step
represents association rather than dissociation. This is an indication that a decrease in disorder takes
place on going from reactants to the activated complex [36].

---

Int. J. Electrochem. Sci., Vol. 4, 2009
1286

3.4. Synergism considerations
The upgrading of inhibition efficiency of organic compounds in the presence of some anions,
particularly halide ions, have been reported by some authors and was ascribed to a synergistic effect
[37-39]. It is thought that the anions are able to improve adsorption of the organic cations in solution
by forming intermediate bridges between the metal surface and the positive end of the organic
inhibitor. Corrosion inhibition synergism thus results from increased surface coverage arising from
ion-pair interactions between the organic cations and the anions.
The synergistic parameters were calculated using the relationship initially given by Aramaki
and Hackerman and reported elsewhere [40]:

1 ? I1+2
=
S
(14)
1
'
1 ? I1+2

where
= (I1 + I2); I1 = inhibition efficiency of the iodide; I2 = inhibition efficiency of ginseng
exteacts; I`= measured inhibition efficiency for the ginseng extracts in combination with iodide ions.
S1 approaches 1 when no interaction between the inhibitor compounds exists, while S1 > 1 points to a
synergistic effect. In the case S1 < 1, the antagonistic interaction prevails, which may be attributed to
competitive adsorption. Values of S1 for different concentrations of ginseng extracts in combination
with iodide ion are given in Table 4. S1 values given in Table 4 are more than unity, thereby
suggesting that the enhanced inhibition efficiency caused by the addition of iodide ions to ginseng
extract is only due to synergistic effect and the effect is more pronounced at higher temperatures
(Table 1). Thus, it can be suggested that iodide ion (I-) is initially adsorbed on the metal surface,
ginseng components in the form of cation are then adsorbed by the coulombic attraction on the metal
surface, where the iodide ions are already chemisorbed and thus suppresses the self-corrosion rate by
the stabilization of the adsorbed anion and by the increase in surface coverage [33].

Table 4. Synergism parameters (S1) for the different concentrations of ginseng extracts.

Concentrations of Ginseng extracts ( % v/v)
S1
10
1.61
40
1.53
50
1.50



4. CONCLUSIONS
The following conclusions can be drawn from this study:

1. Extracts from the root of ginseng acts as an effective and efficient inhibitor for aluminium
corrosion in 1 M HCl.

---