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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):

IVσσ

=nR (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):

)()((t)Rn tI

tVσσ

= (2)

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.

Table1. Chemical composition of studied sample Mo

0.019 Ni

0.022 Cr

0.054 S

0.009 P

0.033 Mn

0.203 Si

1.38 C

0.2 Fe

Balance W

0.01 Co

0.002 Al

0.365 Ti

0.003 As

0.014 Cu 0.06

V 0.001

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 cm2 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.

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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 = 1/2∆t, where ∆t is the sampling interval and fmin =1/N∆t, 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:

)]([)]([)(

tItVtRn ∆

∆=σσ (3)

Where σ[V(∆t)] and σ[I(∆t)] are the standard deviations of voltage and current noise in the same time interval(∆t)[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 ∆t) can be used to calculate noise resistance Rni:

I ... ;I;I ... ;I;I

V ... ;V;V ... ;V;Vmi1ii1m-im-i

mi1ii1m-im-i

+++

+++=

IVRniσ

σ (4)

where V ... ;V;V ... ;V;V mi1ii1m-im-i +++Vσ and I ... ;I;I ... ;I;I mi1ii1m-im-i +++Iσ 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.

t/sec

0

10

20

30

40

50

60

70

80

0 1000 2000 3000 4000 5000

Fig. 2b Fig. 2a R

n/Ωcm

2

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

0.25 % V/V Time(min) 5 15 30 60 140 180 240 300 Rp(Ωcm2) 11.3 18.0 21.6 29.9 46.4 51.8 59.4 62.8 CPE(F) 5.05×10-5 5.61×10-5 6.12×10-5 6.55×10-5 6.70×10-5 7.49×10-5 7.91×10-5 8.19×10-5

n 0.8743 0.8808 0.8907 0.8843 0.8712 0.8853 0.8864 0.8873 0.5 % V/V

Time(min) 5 15 30 60 140 180 240 300 Rp(Ωcm2) 8.57 15.5 24.8 45.9 71.9 144.3 173.3 172.5 CPE(F) 4.89×10-5 5.54×10-5 5.57×10-5 5.05×10-5 4.32×10-5 4.27×10-5 3.77×10-5 3.74×10-5

n 0.8707 0.8714 0.8734 0.8801 0.8596 0.8928 0.8855 0.8837 1% V/V

Time(min) 5 15 30 60 140 180 240 300 Rp(Ωcm2) 10.3 20.3 31.3 58.1 121.1 154.5 186.4 190.3 CPE(F) 4.22×10-5 4.5×10-5 4.63×10-5 4.51×10-5 4.28×10-5 3.70×10-5 3.60×10-5 3.71×10-5

n 0.8507 0.8788 0.8904 0.8950 0.9029 0.8853 0.8852 0.8913

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