1
CONTINUOUS NOISE RESISTANCE CALCULATION TO EVALUATE THE
INHIBITION EFFECT OF POMEGRANATE HULL EXTRACT AGAINST STEEL
CORROSION IN 1M HCl
D.SEIFZADEH, H. ASHASSI-SORKHABI
Electrochemistry Research Laboratory, Physical Chemistry Department, Chemistry Faculty, University of
Tabriz, Tabriz-Iran
ABSTRACT: Among the several methods of corrosion control, the use of corrosion inhibitors is very popular.
Though many synthetic compounds have been shown good anticorrosive activity, most of them are highly toxic
to both human beings and environment. In resent years, several researchers have been focused on the use of
plant extracts as green inhibitors for metals corrosion in different media.
In the present work, the effect of pomegranate hull extract as green inhibitor against steel corrosion in
1M HCl solution was investigated using electrochemical noise (EN) and electrochemical impedance
spectroscopy (EIS) methods. Continuous noise resistance calculation (CNRC) technique was used to detect the
continuous changes of corrosion rate due to inhibitor adsorption on the steel surface and its results were
compared with EIS results. The results showed that the pomegranate hull extract acts as effective inhibitor even
in low concentration. Results obtained from both electrochemical methods showed that the inhibition
efficiencies increase as the inhibitor concentration increase. Also the effect of immersion time on inhibition
efficiency was investigated. Results obtained by two different methods were in good agreement.
Keywords: Corrosion, Inhibition, Electrochemical noise, EIS
1. INTRODUCTION
Pure metals and alloys react chemically/electrochemically with corrosive medium to
form a stable compound, in which the loss of metal occurs. Among the several methods of
corrosion control and prevention, the use of corrosion inhibitors is very popular. Though
many synthetic compounds have been shown good anticorrosive activity, most of them are
highly toxic to both human beings and environment. These toxic effects have led to the use of
natural products as anticorrosion agents which are eco-friendly and harmless (1). In the resent
years several authors have been focused on the plant extract as cheap and non-toxic corrosion
inhibitors (2-5).
There are a number of different electrochemical methods such as linear polarization
(LP), electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) which
use for evaluation of corrosion inhibitor effects. EN is one of the new corrosion analysis
methods that describe the low level spontaneous fluctuations of potential and current that
occurs during electrochemical processes in steady state condition. Measurements of
electrochemical noise involves the monitoring of fluctuations of current and potential using a
three-electrode cell configuration consist of two identical working electrode and reference
electrode(6,7).
First stage in calculation of corrosion rate using EN method is the statistical
calculation of noise resistance parameter (Rn) that can be comparable with Polarization
resistance (Rp) that obtain from LP and EIS methods respectively. In many applications
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(though not all) the noise resistance determined as the standard deviation of potential noise
divided by the standard deviation of current noise as follow(8):
Rn 
V
I
(1)
Since in many cases such as unstable electrochemical system, noise resistance is a
time related parameter, Tan et al defined continuous noise resistance as time related
parameter (9):
V (t )
(2)
R n (t) 
I (t )
Tan et al (10) have been used the continuous noise resistance calculation (CNRC)
technique to study the continuous formation of inhibitor film on steel surface in CO2
corrosion of pipeline.
In the present study the effect of pomegranate hull extract as green inhibitor on mild
steel corrosion in hydrochloric acid was investigated using CNRC technique and the obtained
results were compared with the EIS results.
2. EXPERIMENTAL METHODS
2.1. Materials
The chemical composition (wt. %) of the steel specimen (determined by
SPECTROLAB quantometer) is given in Table 1. The samples were mounted in polyester in
such a way that only 1cm2 of samples was in contact with corrosive media. The steel
specimens were polished with emery papers no. 400–1200 grade. They were degreased with
acetone, washed with double-distilled water, and finally dried at room temperature before
immersion in the acid solution. The acid solutions (1 M HCl) were made from analytical
grade 37% HCl using double-distilled water.
The powder of the crushed pomegranate hull was purchased from a local market in
Tabriz. 25 gram of powder is boiled in water for 30 min. After filtration, the water was
evaporated to obtain 7 gram extract.
C
0.2
V
0.001
Si
1.38
Cu
0.06
Table1. Chemical composition of studied sample
Mn
P
S
Cr
0.203
0.033
0.009
0.054
As
Ti
Al
Co
0.014
0.003
0.365
0.002
Ni
0.022
W
0.01
Mo
0.019
Fe
Balance
2.2. Methods
Electrochemical noise (EN): Electrochemical noise data were recorded using an AUTOLAB
Potentiostat-Galvanostat (PGSTAT30) and GPES (General Purpose Electrochemical
Software) version 4.9 005Beta software. A three electrodes cell configuration (Fig. 1)
consists of two identical steel electrodes as dual working electrodes (each other with 1 cm 2
surface area) and a saturated Ag/AgCl electrode as a reference electrode was used.
Electrochemical current noise was measured between the two working electrodes
(coupled with zero resistance ammeters) and simultaneously, the potential noise of coupled
electrodes was measured with respect to reference electrode.
3
Fig. 1. Electrochemical noise measurements setup
EN measurements were started immediately after samples immersion in aggressive
solution and 1 point per second (∆t = 1 Sec) recorded during the measurements. The
frequency domain corresponding to the sampling conditions was evaluated to be between fmax
= 1/2Δt, where Δt is the sampling interval and fmin =1/NΔt, where N is the total number of
data points. All experiments were carried out in open circuit potential condition and in 25 ºC.
Electrochemical Impedance Spectroscopy (EIS): EIS measurements were also carried out
using AUTOLAB Potentiostat-Galvanostat (PGSTAT30) and the FRA version 4.9. 005Beta
software. A sinusoidal voltage signal of 5 mV was applied over a frequency range of 10 KHz
– 10 mHz. All EIS experiments were performed in open circuit potential using a three–
electrode cell configuration consist of a saturated Ag/AgCl electrode as reference, platinum
sheet as counter electrode and mild steel sample as working electrode. All experiments were
performed under atmospheric conditions and in 25 ºC.
3. STATISTICAL ANALYSIS
3.1. CNRC technique
The noise resistance (Rn) is given by the ration of the standard deviation of voltage
and the standard deviation of current noise:
Rn (t ) 
 [V (t )]
 [ I (t )]
(3)
Where σ[V(Δt)] and σ[I(Δt)] are the standard deviations of voltage and current noise in the
same time interval(Δt)[10].
The CENRC technique uses a small and moving time window for time related noise
resistance estimation. This analyzing method explained as follow [11]:
Suppose that a series of voltage-time records, {V1; V2; V3; ...Vi-1; Vi, Vi+1; ...Vk}, and
corresponding current time records, {I1; I2; I3; ... Ii-1; Ii, Ii+1; ... Ik} are experimentally
recorded. For each datum value of voltage and current noise ,Vi, Ii, a small data series of
2m+1 neighbor points (in time window Δt) can be used to calculate noise resistance Rni:
Rni 
V {V i - m; Vi - m  1; ... Vi; Vi  1; ... Vi  m}
I {Ii - m; Ii - m  1; ... Ii; Ii  1; ... Ii  m}
(4)
where V {V i - m; Vi - m  1; ... Vi; Vi  1; ... Vi  m} and I {Ii - m; Ii - m  1; ... Ii; Ii  1; ... Ii  m} are the
standard deviation of voltage and current noise data (2m+1 neighbor of i). In this work m
value was 20.
So that we obtain a series of Rn values ({Rn,m …Rn,i-1; Rn,i; Rn,i+1; …Rn,k-m})
corresponding each voltage and current value which respectively recorded in different time
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during the experiment. Thus this technique can be used to calculate corrosion rate of systems
where polarization resistance are subject to rapid change (such as during inhibitor film
formation or destruction).
3.2. DC trend removal
The EN is regarded as consist of random fluctuations about some mean value. For the
case of voltage noise, that mean value is the corrosion potential. It is observes that the
corrosion potential tends to drift during the measurements and it has been shown that this
drift can greatly influence the results obtained from an analysis of the EN. The phenomena
are referred to as DC trend and the process of removing it is called trend removal (9). There
are some mathematical methods for trend removal of random data. The two most prominent
methods of trend removal that have been applied to EN are linear trend removal (LTR) and
moving average removal (MAR). In this experimental work we used the MAR method that
previously describes by Tan t al (10). We wrote a suitable computer program in Matlab 6.5
software for trend removal process based on MAR method. P value (10) during this process
was 10.
4. RESULTS AND DISCUSSION
Continuous noise measurements were used to study the inhibition effect of
pomegranate hull extract on the steel corrosion. Fig. 2a shows the raw voltage and raw
current noise records of mild steel in 1 M HCl during the first 4096 sec of immersion.
Noise data were analyzed in time domain by calculating of continuous noise
resistance parameters. Continuous noise resistance was calculated using Matlab 6.5 software
by writing a suitable program as described in section 3.1. Fig. 2b shows the changes of
calculated noise resistance with time for mild steel in 1 M HC solution.
80
Fig. 2a
Fig. 2b
70
60
Rn/Ωcm2
50
40
30
20
10
0
0
1000
2000
3000
4000
5000
t/sec
Fig. 2. Raw potential and current noise of mild steel in 1M HCl (a) and corresponding continuous noise
resistance (b)
As clear from Fig. 2b, the noise resistance of mild steel in 1 M HCl is constant during
the immersion time and has a low values indicating the high corrosion rate of metal. It should
be mentioned that the values of continuous noise resistances were calculated using the trend
removed noise data (not showed). MAR trend removal method was used in this study. The
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value of p parameter (10) during the process was 10. Trend removal was also performed
using the Matlab 6.5 software.
Fig.s 3 show the raw potential and raw current noise of mild steel in 1 M HCl solution
in the presence of 0.25 %, 0.5 % and 1 % V/V pomegranate hull extract during the first 8192
sec of immersion. The maximum measurements time (8192 sec) was selected because of
GPES software limitation.
Fig. 3a
Fig. 3b
Fig. 3c
Fig. 3. Raw potential and raw current noise of mild steel in 1 M HCl solution in the presence of 0.25 %(a), 0.5
%(b) and 1 %(c) pomegranate hull extract during the first 8192 sec of immersion.
Corresponding noise resistances values are calculated by CNRC method and
schematically are shown in Fig.s 4. As it clear from the Figures, noise resistance values of
mild steel in 1 M HCl solution has significantly increased in the presence of pomegranate
hull extract. The values of noise resistance at certain time increased as the extract
concentration increased. This may be related with more inhibitor adsorption at high
concentrations. Since the values of noise resistance is inversely related to corrosion rate, it
can be concluded that the pomegranate hull extract acts as effective green inhibitor for mild
steel in 1 M HCl and the inhibition effect strengthens when the extract concentration
increases. Also, the noise resistance values of mild steel in the presence of extract at certain
concentration increase as immersion time increase. This fact may be related with continuous
adsorption of inhibitor molecules on the mild steel surface (11).
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Fig. 4. Changes of noise resistance with immersion time for mild steel in I M HCl in the presence of extract at
different concentrations, 0.25%(a), 0.5%(b) and 1%(c).
Pomegranate hull contains some organic compounds such as Punicalagin (12),
Punicalin (12) and Granatin B (14). Chemical structure of these compounds is shown in Fig.
5.
Fig. 5. Chemical structures of Punicalagin, Punicalin and Granatin B.
Inspection of the chemical structures of these compounds reveals that these
compounds can adsorb on the metal surface via the lone pairs of electrons present on their
oxygen atoms and different aromatic rings. The adsorption of such compounds on the metal
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surface make a barrier for charge and mass transfer leading to decrease the interaction of the
metal with the corrosive environment. As a result, the corrosion rate of the metal is decreased
[5].
EIS experiments have been performed in this study to confirm the EN results. Fig.s 6
shows the Nyquist plots of mild steel in 1 M HCl in the absence and presence of inhibitor at
different concentrations and in different immersion times.
Fig. 6. Nyquist plots of mild steel in 1 M HCl in the absence and presence of inhibitor at different
concentrations, 0.25%(a), 0.5%(b) and 1 %(c) at different immersion times.
Nyquist plots of this system consist of one depressed semicircle thus an equivalent
circuit consists of polarization resistance (Rp), double layer constant phase element (CPE),
and solution resistance (Rs) were used to calculate impedance parameters (14). Nyquists plots
were fitted to equivalent circuit using the Zview software. Calculated EIS parameters are
collected in Table 2. As it clear from this table, the polarization resistance of mild steel
increases as inhibitor concentration increases at certain time and also the polarization
resistance increases with time in the presence of inhibitor in certain concentration. There is a
tail in the Nyquist plots especially at the initial immersion times (5 and 15 min). This may be
related with the continuous increasing of polarization resistance (due to inhibitor adsorption)
during the impedance measurements at low frequencies.
To compare the results obtained by different electrochemical methods, plots of
corrosion resistance (noise resistance and polarization resistance) versus immersion time
obtained by two methods are shown in Figs 7.
Fig. 7. Comparsion of EIS and EN results for mild steel in 1 M HCl in the presence of inhibitor at
different concentrations, 0.25% (a), 0.5 % (b) and 1 % (c)
As it clear from the Figs, the agreement between the EN and EIS results in the
presence of 0.25 % concentration of extract is excellent. Also agreement for 0.5%
concentration is good. However in the case of highest extract concentration the agreement is
relatively poor.
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Table 2. EIS parameters obtained for mild steel in 1 M HCl at the presence of extract at different
concentrations in different immersion times
Time(min)
Rp(Ωcm2)
CPE(F)
n
5
11.3
5.05×10-5
0.8743
15
18.0
5.61×10-5
0.8808
30
21.6
6.12×10-5
0.8907
Time(min)
Rp(Ωcm2)
CPE(F)
n
5
8.57
4.89×10-5
0.8707
15
15.5
5.54×10-5
0.8714
30
24.8
5.57×10-5
0.8734
Time(min)
Rp(Ωcm2)
CPE(F)
n
5
10.3
4.22×10-5
0.8507
15
20.3
4.5×10-5
0.8788
30
31.3
4.63×10-5
0.8904
0.25 % V/V
60
29.9
6.55×10-5
0.8843
0.5 % V/V
60
45.9
5.05×10-5
0.8801
1% V/V
60
58.1
4.51×10-5
0.8950
140
46.4
6.70×10-5
0.8712
180
51.8
7.49×10-5
0.8853
240
59.4
7.91×10-5
0.8864
300
62.8
8.19×10-5
0.8873
140
71.9
4.32×10-5
0.8596
180
144.3
4.27×10-5
0.8928
240
173.3
3.77×10-5
0.8855
300
172.5
3.74×10-5
0.8837
140
121.1
4.28×10-5
0.9029
180
154.5
3.70×10-5
0.8853
240
186.4
3.60×10-5
0.8852
300
190.3
3.71×10-5
0.8913
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