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Inhibition of the corrosion of carbon steel in acid solution by the extract of Limonium thouinii (Plumbaginaceae).

Introduction

Carbon steel is used in wide industrial applications. It reacts with corrosive medium to form a stable compound, in which the loss of metal occurs. The compound so formed is called corrosion product and metal surface becomes corroded. Corrosion of metals is a major problem that must be confronted for safety, environment, and economic reasons. It can be minimized by suitable strategies which in turn stifle, retard or completely stop the anodic or cathodic reactions or both [8]. One of the methods used to reduce the rate of material corrosion is addition of inhibitors. Mostly, the heterocyclic compounds containing O, S and N as heteroatom serve as good inhibitors for corrosion [19]. Recently, the use of chemical inhibitors has been limited due to environmental regulations; plant extracts have again become important because they are the environmental and renewable source for a wide range of needed inhibitors. A lot of natural products were previously used as corrosion inhibitors for different metals in various environments [25,10,4,14,12,1,16,22] and their optimum concentrations were reported. The obtained data showed that plant extracts could serve as effective corrosion inhibitors.

The present work is devoted to test the ethyl acetate extract of the aerial parts Limonium thouinii as corrosion inhibitor for C steel in 1M HCl solution. The study is conducted by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy methods.

Materials and Methods

Electrode and solution:

C steel of type API5L Gr B containing 0.025% P, 0.015% S, 1.20% Mn, 0.24% C and remaining Fe was used. Coupons with exposed surface area of 4.36 [cm.sup.2] (1 cm x 0.6 cm x 2 cm) were used for weight loss measurements. For electrochemical studies, the metal was soldered with Cu-wire for electrical connection and was embedded in epoxy resin to expose a geometrical surface area of 0.8 [cms.up.2] to the electrolyte. The electrodes were mechanically polished on wet SiC paper (grade 120-600-1200), degreased with acetone and rinsed with distilled water before experiment [24]. The corrosive medium was 1.0 M HCl prepared from hydrochloric acid 37% analytical grade. Distilled water was used for the preparation of all reagents.

Preparation of the Limonium thouinii extract:

Aerial parts of Limonium thouinii were extracted with 70% MeOH. The MeOH extract was evaporated in vacuo. The condensed solution was diluted with 400 ml of boiling water, and the solution was filtered. The residue was successively treated with ethyl acetate and n-butanol. Then, the solvents were removed to afford n-butanol and ethyl acetate extracts [6,3,7]. The ethyl acetate extracts was then used directly in the experiments.

Weight loss measurements:

The weight loss measurements were performed in beakers containing 100 ml of 1 M HCl with and without addition of different concentrations of L. thouinii at room temperature. The test coupons were mechanically polished with different grades of emery papers and cleaned with acetone and washed with distilled water and finally dried in dry air before every experiment. After weighing accurately, the specimens were suspended in the previous beakers. After 2 days the coupons were taken out, washed, dried and weighed accurately. From the weight loss data, the inhibition efficiency (IE%) was calculated as follows [16].

IE% = [[W.sub.0]-[W.sub.1] / [W.sub.0]] X 100 (1)

Where [W.sub.0] and [W.sub.1] are the weights of the C steel coupons in absence and presence of inhibitor, respectively.

Potentiodynamic polarization measurement:

All electrochemical measurements were carried out using a PC controlled Volta Lab potentiostat (Model PGZ 301) system with Volta master 4 software at room temperature, without and with the addition of a given concentration of L. thouinii extract to 1.0 M HCl solution. Open circuit potential Eocp, was measured for 20 min to allow stabilization of the steady state potential. The potential of potentiodynamic polarization curves was started from -250 mV to -1000 mV at a scan rate of 1mv/s. The inhibition efficiency IE (%) was calculated using the relation [22,19,20]:

IE% = [[I.sub.corr] - [I.sub.corr(i)] / [I.sub.corr]] x 100 (2)

Where [I.sub.corr] and [I.sub.corr(i)] are the corrosion current density without and with the addition of the inhibitor respectively.

Electrochemical impedance spectroscopy (EIS):

The electrochemical impedance spectroscopy (EIS) measurements were carried out using the same instrument described for potentiodynamic polarization. The open circuit potential, [E.sub.ocp] of every sample was immersed for 30 min over a frequency range of 100 KHz to 0.01 Hz with applied potential signal amplitude of 5 mV. The percentage inhibition efficiency (IE %) was calculated from,

IE% = [[R.sub.2]-[R.sub.1]/[R.sub.2]] x 100 (3)

Where [R.sub.1] and [R.sub.2] are charge transfer resistance of steel in uninhibited and inhibited solutions respectively.

Results and Discussion

Weight loss measurements:

The weight loss method has found broad practical application [13]. A major advantage of this method is its relative simplicity and availability. In addition, the method uses a direct parameter for the quantitative evaluation of corrosion, i.e., the loss in mass of the metal. The data obtained for the corrosion behavior of C steel in 1 M HCl solution with and without the addition of different concentrations of L. thouinii extract, after 2 days of immersion period at room temperature, are presented in Fig 1, as well as table 1. The analysis of these results (Table 1) shows clearly that the weight loss decreases while the inhibition efficiency increases with increasing inhibitor concentration reaching a maximum value of 68.18% at a concentration of 400 ppm. This behavior can be attributed to the increase of the surface covered [theta] (% IE x100), and that due to the adsorption of natural compounds on the surface of the metal [26], as the inhibitor concentration increases. We can conclude that L. thouinii extract is a good corrosion inhibitor for steel in 1.0 M HCl solution.

[FIGURE 1 OMITTED]

Potentiostatic measurement:

From the anodic and cathodic polarization curves (Fig. 2) for L. thouinii in 1M HCl media at different concentrations, various electrochemical parameters were computed and listed (Table 2). From the icorr values, the inhibition efficiency was calculated as in Eq. (2). It was illustrated from the data that the addition of L. thouinii extract decreased the corrosion current density ([i.sub.corr]).

[FIGURE 2 OMITTED]

The decrease may be due to the adsorption of the inhibitor on metal/acid interface [2]. The maximum inhibition was obtained at the concentration of 400 ppm (68.75 % IE). This result was in good agreement with the weight loss measurement which have been discussed above. Moreover, the inspection of Table 2 reveals that the values of anodic and cathodic Tafel constants are markedly changed in the presence of L. thouinii extract. This result reflects the effect of the extract on both anodic and cathodic reactions. Therefore, it could be concluded that L. thouinii extract acts as a mixed inhibitor [11]. Furthermore, the displacement of [E.sub.corr] was <85 mV, hence the inhibitor can be seen as a mixed type inhibitor [18]. This also indicates that the inhibitor merely blocks the reaction sites of the mild steel surface and changes the mechanism of metal dissolution (anodic) and/or hydrogen evolution (cathodic) reaction.

Electrochemical impedance spectroscopy:

The Nyquist plots for steel in 1 M HCl in the absence and presence of different L. thouinii extract concentrations are shown in Fig. 3. The impedance diagram obtained with 1.0 M HCl shows only one capacitive loop. The same trend was also noticed for C steel immersed in 1.0 M HCl containing plant extracts, which indicated that the corrosion of steel is mainly controlled by a charge transfer process [21,5]. On the other hand, it is clear from Nyquist plots that impedance behavior of steel has significantly changed after the addition of inhibitors in acid solutions. The diameter of Nyquist plots increases on increasing the plant extract concentration. This suggested that the formed inhibitive film was strengthened by addition of plant extract [15].

[FIGURE 3 OMITTED]

The electrochemical parameters of charge transfer resistance ([R.sub.ct]) and the double layer capacitance ([C.sub.dl]) are calculated from the Nyquist plots and presented in Table 3. The [R.sub.ct] values increased with the increase of the concentration of inhibitor, which shows protection of steel surface by the inhibitor while the values of Cdl decreased with the increase in the concentration of inhibitor.

Conclusions:

From the overall experimental results the following conclusions can be deduced:

* Limonium. thouini extract acts as an inhibitor for the corrosion of API 5L Gr B steel in 1.0 M HCl solution.

* Potentiodynamic polarization measurements demonstrate that Limonium. thouinii extract acts as mixed-type inhibitor.

* EIS measurements show that charge transfer resistance ([R.sub.ct]) increases and double layer capacitance ([C.sub.dl]) decreases in the presence of plant extract indicating the adsorption of natural compounds at the surface API 5L Gr B steel.

References

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(1) M. Benahmed, (2) M. Lafhal, (1) N. Djeddi N, (3) H. Laouer, (2) S. Akkal

(1) Laboratoire des molecules actives et applications, Universite Tebessa, Route de Constantine 12000 Tebessa, Algerie. (2) Laboratoire de Phytochimie et Analyses physicochimiques et Biologiques, Departement de Chimie, Faculte de Sciences exactes, Universite Mentouri Constantine, Route d'Ain el Bey, 25000 Constantine, Algerie. (3) Departement de biologie, Universite de Setif.

M. Benahmed, M. Lafhal, N. Djeddi N, H. Laouer, S. Akkal: Inhibition of the corrosion of carbon steel in acid solution by the extract of Limonium thouinii (Plumbaginaceae)

Corresponding author

Benahmed Merzoug, Laboratoire des molecules actives et applications, Universite Tebessa, Route de Constantine 12000 Tebessa, Algerie. E-mail: riad43200@yahoo.fr
Table 1: Gravimetric results of steel corrosion
in 1.0 M HCl (2 days immersion) without and with
various concentrations of L. thouini extract at
room temperature.

Conc (ppm)   W. loss (g/[cm.sup.2])   % I E

0                    0.005             --
100                  0.0027           45.45
200                  0.0020           59.09
300                  0.0016           68.18
400                  0.0013           72.72
500                  0.0019           62.00

Table 2: Potentiodynamic polarizations parameters
of API 5L Gr B in 1.0 M HCl for various
concentrations of plant extracts.

[conc.] ppm   -[E.sub.corr]    [i.sub.corr]      [[beta].sub.a]
                  (mV)        (mA.[cm.sup.-2])        (mV)

0                 534.8            0.1929             84.4
100               522.3            0.1374             67.5
200               534.3            0.0866             69.4
300               503.2            0.0642             82.0
400               492.0            0.0603             92.2
500               467.6            0.0895             55.0

[conc.] ppm   -[[beta].sub.c]          CR           EI%
                    (mV)         (mm.[y.sup.-1])

0                  186.0              2.256         --
100                181.2              1.606        28.82
200                142.9              1.013        55.09
300                213.0              0.751        66.71
400                235.0              0.705        68.75
500                182.3              1.047        53.59

Table 3: Electrochemical impedance spectroscopy
parameters of API 5L Gr B in 1.0 M HCl for
various concentrations of L. thouini extracts.

[conc.]      [C.sub.dl]           [R.sub.s]
ppm       [mu]F*[cm.sup.-2]   [omega].[cm*sup.2]

0               104.3               2.519
100             78.85               3.827
200             77.47               16.34
300             60.90               16.29
400             33.88               18.35

[conc.]       [R.sub.tc]        EI%
ppm       [omega]*[cm.sup.2]

0                318            --
100             410.8          22.43
200             452.0          29.64
300             827.8          61.58
400              1060          70.21
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Title Annotation:Original Article
Author:Benahmed, M.; Lafhal, M.; Djeddi N., N.; Laouer, H.; Akkal, S.
Publication:Advances in Environmental Biology
Article Type:Report
Geographic Code:6ALGE
Date:Dec 1, 2012
Words:2614
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