1.
INTRODUCTION:
Aluminum alloys are a material frequently used for its
light weight, high conductivity, easy fabrication and resistant properties
against corrosion1,2. It is covered with thin film of aluminum oxide
and resist to corrosion in many aggressive environments3. However,
acidic solution causes corrosion of aluminum alloys. The use of aluminum alloys as foods
container widely available in food industries but acidity of some foods such as
sauce and soft drink cause pitting corrosion of aluminum alloys surface. The
leaching of aluminum from food container consider as hazardous to human health5,6,7.
In order to prevent the corrosion of aluminum alloys in acidic medium organic
and inorganic compounds are used as inhibitors. The biological toxicity and the
cost limited the use of such inhibitors8,9. So the introduced of
green corrosion inhibitors are more important and preferred.
The
researchers over the previous dozen years reported some natural products to be
effective corrosion inhibitor in acidic media such as Newbouldia laevis Leaves
Extract10, Bassia muricata extract11, Almond fruits
extract12, Ananas sativum leaves extract13, Green Coffee
Bean Extract14, Borassus flabellifer dust extract15,
Ficus benjamina gum16. The inhibiting mechanism of natural products
is related to organic constituents of these products whose electronic
structures resemble to synthesis organic corrosion inhibitors.
The
high abundance of milk thistle drew our attention to testing it as an
inhibiting substance for corrosion of AA7051 aluminum alloy in 0.01M HCl,
especially since, to our knowledge, it was not previously tested. This plant
grown in any type of soil and distributed in Europe, North Africa and Middle
East under different names Mary's milk thistle, Mediterranean milk thistle,
variegated thistle and the scientific name is Silybum marianum17,18.
The milk thistle containing Flavonolignans such as silybin, isosilybin,
silydianin, silychristin as well as fatty acids, including linoleic acid mainly
extracted from seeds19. However, the stem and leaves remain unused
while it is consist more than 98% of plant weight. The present work concern on
aqueous extract of milk thistle leaves as it is abundant, low cost and
eco-friendly corrosion inhibitor tested in acidic media by potentiostatic
polarization. The electrochemical behavior of the inhibitor discussed based on
the effect of temperature, inhibitor concentration, adsorption isotherm and
kinetic data.
2.
MATERIALS AND METHODS:
2.1.
Raw materials and extraction:
The
milk thistle leaves were collected from Bismayah, Baghdad, Iraq. The leaves
were crushed, grinding to a fine powder by food processer, and then sieved.
Next, 3.0g of the powder with 100ml of water was refluxed in soxhlet for 7
hours. After that, the water was evaporated. Finally, dried at room temperature
and kept for further use.
2.2.
Inhibitor and electrolyte:
The
corrosive media was prepared by dilution of concentrated hydrochloric acid
(A.R. grade from Fluka) to 0.01M by distilled water. A different amount of the
milk thistle extract was added to an acid solution in five volumetric flasks,
0.1, 0.2, 0.3, 0.4 and 0.5g/L with agitation to obtain good solubility.
2.3.
Electrochemical experiments:
The
electrochemical measurements were performed in one letter Pyrex corrosion
cell with three electrodes reference (saturated calomel electrode), working
(aluminum sheet) and auxiliary (high purity platinum metal) connected to
potentiostate (Germany, MLab-2000) with computer controlling. The working
electrode with 1cm2 exposed surface area was immersed in 0.01M HCl
solution and the open-circuit potential (OCP) was measured for15 minutes. Based
on the OCP value, the potential was scanned from -200 to +200mV. The MLabSci
electrochemical software was used to analyze the polarization data by Tafel
extrapolation method and the value of corrosion current density; corrosion potential,
weight loss and penetration loss were obtained. The efficiencies of different
concentrations of ML extract were calculated using the following equation0:
Where
and are the current density of corrosion in the free and
inhibited acidic solution, respectively21.
2.4.
FTIR:
Fourier
transform-infrared spectra were measured by (Shimadzu 8400S, Japan)
spectrophotometer using the potassium chloride Pellet method. The samples were
mixed in a 1:10 weight ratio (sample to potassium chloride) and grinding. Then
pressed by Pelletizer to form the pellet and finally, scanning in the range
400–4000 cm-1.
3.
RESULT AND DISCUTION:
3.1.
Characterization of ML extract:
Presence
of characteristics groups in the aqueous extract of milk thistle was
investigated by FTIR spectroscopy. Part A in Fig. 1 shows the FTIR spectra for
a crude extract of milk thistle, the very strong broad band from 3463.92 to
3375.20cm2 are refer to O–H stretching and intense band at 1631.67cm2 for C=O group22,23. The part B
from fig.1 represent milk thistle extract after adsorption on aluminum alloy
surface, the O–H and C=O related bands are shifted as well as their intensity
reduced. The FTIR data supported by the chemical structure of main component of
milk thistle (fig.2) indicate the hydroxyl and carbonyl groups participated in
adsorption.
The
open circuit potential (OCP) increases slightly when the temperature rises to
303K or 313K, but at a temperature of 323K and 333K the OCP increases
noticeably and that shows the rate of corrosion of AA7051 alloy reaches the
maximum at 333K. The addition of milk thistle extract reduces the corrosion
potential as well as the corrosion current due to adsorption of extract
component on the surface of AA7051 alloy and that decreases the weight loss
(W.L) and prevent the penetration of the acid to the surface hence the penetration
loss (P.L) reduces from 1.26mm/year to 0.172mm/year at 333K. As the amount of
the inhibitors increases the inhibition efficiency increases and attained the
maximum at 0.4g with 86.41% inhibition efficiency. Polarization resistance (RP)
of inhibited solution more than free acid solution and that implies the ability
of milk thistle extract to inhibited the corrosion of AA7051 alloy at 0.01M
hydrochloric acid solution.
3.3.
Adsorption studies:
To
interpretations the adsorption of milk thistle extract on AA7051 alloy surface
and to calculate the thermodynamics adsorption parameters, the surface coverage
Ɵ and inhibitor concentrations were substituted in Langmuir, Freundlich, and Temkin isotherms and
the best fit with Langmuir isotherm represented by following equation7:
Where
is adsorption equilibrium constant obtained from the plot of against. As shown in Fig. 3a the linear correlation (> 0.98) assume that
the adsorption of milk thistle on the AA7051 alloy agreed with this isotherm.
According to this model the extract components adsorb on the aluminum surface,
presumably in monolayer thickness and that blocks the aluminum dissolution and 
discharge. Hence, the addition of extract to acid solution not
considerably altered the corrosion potential, although the rate of corrosion
dramatically decreases8.
The thermodynamic parameters of adsorption were calculated by using the
following equations9:
Where
T (K) is the absolute temperature, R is the gas constant (8.314 J.), 55.5 is concentration of water in the molar unit, is the free energy of adsorption, is the standard adsorption enthalpy calculated from the slope of
logversus 1000/T (Fig.3b) and from the intercept standard entropy of adsorption calculated. The thermodynamic data
represented in table.2 refer to spontaneous process of chemical adsorption on
aluminum surface and the positive values of and indicate endothermic process which interpreted the increases of
inhibition efficiency with the temperature and the entropy of adsorption
increases.
Table 2: Thermodynamic values of ML on AA7051 alloy in 0.01
M HCl.
|
T(K)
|
(kJ.mol-1)
|
(kJ. mol-1)
|
(J. mol-1. K-1)
|
|
293
|
-11.861
|
52.96
|
220.91
|
|
303
|
-14.169
|
|
313
|
-16.002
|
|
323
|
-17.612
|
|
333
|
-21.259
|
Figure 3. (a) Langmuir adsorption model and (b) Straight
line of log K versus 1000/T for adsorption thermodynamic of ML on AA7051 alloy
in 0.01M HCl.
3.3.
Kinetic Of Corrosion:
Corrosion
kinetics of the AA7051 alloy in presence and absence of various concentrations
of milk thistle extract were examined at range of temperature 293–333K, using
polarization measurements. Fig.5 exhibit polarization plots for AA7051 alloy in
0.01M HCl, in presence as well as in absence of different amounts of milk
thistle extract at 293–333K. The relative kinetic parameters are listed in
Table 3. The finding exhibit that the rates of corrosion for AA7051 alloy, in a
non-inhibited and inhibited acidic solutions rose linearly with temperature due
to an increase in passive film dissolution. Nevertheless, inspection of table.
1 exhibit increases in inhibition efficiency with increases of temperature and
that evidently confirm the effectiveness of milk thistle extract as corrosion
inhibitor in the range of temperature of this investigation. The energy of
activation for the corrosion process calculated by using the following natural
logarithm form of Arrhenius equation0:
Where
are the current density of corrosion, A the Arrhenius
pre-exponential factor and the apparent activation energy of the corrosion reaction. The
apparent activation energy of the non-inhibited and inhibited corrosion
reaction was determined from the slope of the linear plot of ln
versus 1/T (Fig. 4a). The values obtained listed in Table 3 and exhibit the of aluminum
alloy in acidic solutions with milk thistle extract are less than the acidic
solutions without milk thistle extract. The decline in the apparent activation
energy values for acidic solutions containing milk thistle extract compared
with pure acidic solutions, in addition to the noticeable increases in
efficiency of inhibition with increases of temperature assume that a few of
milk thistle components would be chemisorbed on the aluminum surface31.
The activation parameters were calculated by Eyring transition state equation32,33:
Where
k is Boltzmann constant, R gas constant (8.314 J.), h Planck constant, the activation entropy
and the activation enthalpy. From the plot of ln(/T) versus 1/T in Fig. 4b, the and values were calculated from the slope (-/R) and an intercept (ln(R/h) + ()) respectively, and the observed
data recorded in Table 3. The values carry positive signs and that refer
to gentle endothermic dissolution reaction of aluminum under the condition of
this investigation. The large negative values of activation entropy in the
presence and absence of milk thistle extract exhibited that the activated
complex in the rate-determining step denotes association instead of
dissociation and the disorder of activated complex reduced1.
Table 3: Activation parameters of AA7051 alloy in 0.01M HCl at
different concentration of inhibitor.
|
Inhibitor conc. (g.L-1)
|
Ea
(kJ. mol-1)
|
(kJ. mol-1)
|
(J.mol-1K-1.)
|
|
0.0
|
32.911
|
30.317
|
-114.694
|
|
0.1
|
9.585
|
6.991
|
-195.147
|
|
0.2
|
9.572
|
9.988
|
-188.217
|
|
0.3
|
9.913
|
7.319
|
-200.177
|
|
0.4
|
18.946
|
16.352
|
-172.113
|
|
0.5
|
17.246
|
14.652
|
-178.713
|
4.
CONCLUSION:
1. The aqueous extract of milk
thistle act as good corrosion inhibitor for AA7051 in 0.01M hydrochloric acid
solution.
2. The mechanism of inhibition shows
mixed type inhibitor with the ability to inhibit the acid corrosion at high
temperature.
3. Adsorption of inhibitor components
form a monolayer on the surface of AA7051 and prevent the surface from acid
degradation.
4. The corrosion rate in free acid
solution increases noticeably with temperature. In inhibited solution, On the
other hand, the corrosion inhibition efficiency increases with temperature.
5.
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=0.98) and the slight change in corrosion potential
refer to mixed-type inhibitor attach to the surface by chemisorption. FTIR
spectra for crude extract supported by literatures shows the molecular
structure of the main component of milk thistle extract and exhibit the O-H
group as a major group participate in adsorption.
