- Email: [email protected]
Eco friendly inhibitor for corrosion inhibition of mild steel in phosphoric acid medium
Electrochimica Acta 49 (2004) 4387–4395
Eco friendly inhibitor for corrosion inhibition of mild steel in phosphoric acid medium G. Gunasekaran∗ , L.R. Chauhan Naval materials Research Laboratory, Shil Badlapur Road, Addl. Ambernath, (E) Dist. Thane 421506 (MH), India Received 7 October 2003; received in revised form 5 April 2004; accepted 12 April 2004 Available online 9 June 2004
Abstract The inhibition effect of Zenthoxylum alatum plant extract on the corrosion of mild steel in 20, 50 and 88% aqueous orthophosphoric acid has been investigated by weight loss and electrochemical impedance spectroscopy (EIS). Plant extract is able to reduce the corrosion of steel more effectively in 88% phosphoric acid than in 20% phosphoric acid. The effect of temperature on the corrosion behaviour of mild steel in 20, 50 and 88% phosphoric acid with addition of plant extract was studied in the temperature range 50–80 ◦ C. Results on corrosion rate and inhibition efficiency have indicated that this extract is effective up to 70 ◦ C in 88% phosphoric acid medium. Surface analysis (XPS and FT-IR) was also carried out to establish the mechanism of corrosion inhibition of mild steel in phosphoric acid medium. © 2004 Elsevier Ltd. All rights reserved. Keywords: Plant extract; Corrosion inhibitors; Adsorption; Weight loss; Electrochemical impedance spectroscopy; ESCA; FT-IR
1. Introduction Phosphoric acid prepared by dihydrate and hemihydrate wet processes generate severe corrosion problems in containers. One of the most practical methods for protection against excessive dissolution of metal by corrosion is use of proper inhibitors [1]. The use of organic compounds containing oxygen, sulphur and nitrogen to reduce corrosion attack on steel has been studied in some details [2–7]. 2-Mercaptobenzimidazole [8], food additives (blue dye no.1) [9], propargyl alcohol [10], polyvinylpyrrolidone and polyethylenimine [11] are used as inhibitors for mild steel in phosphoric acid medium. Extracts of naturally occurring products contain mixtures of compounds having oxygen, sulphur and nitrogen elements and are eco friendly in nature. These mixtures having nitrogen and sulfur as constituent atoms were studied as corrosion inhibitor in HCl medium [12,13]. Ec Hosary et al. [14] studied the corrosion inhibition of aluminium and zinc in 2N HCl using naturally occurring Hibiscus subdariffa (Karkode) extract. Zucchi and co workers [15] studied the dissolution of mild steel in 1N and 2N HCl using plant
extracts of Papaia, Poinciana pulcherrima, Cassia occidentalis and Datura stramonium seeds. Alkaloids such as papaverine, strychnine, quinine and nicotine were also studied as corrosion inhibitors in acid medium [16]. Literature reveal that not much work appears to have been done on the inhibition of mild steel in phosphoric acid solutions at high concentration levels using naturally available plant extracts. Zenthoxylum-alatum, a dicot family plant is generally found at an altitude of around 5000–6000 ft above sea level. In India these plants are mostly found in Himalayan region. However, they have never been exploited for the purpose of corrosion inhibition in phosphoric acid medium. In this paper, the kinetics of the corrosion of mild steel in phosphoric acid have been studied by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy methods. The action of plant extract as inhibitor in phosphoric acid medium over a range of acid concentration and solution temperature has also been examined.
2. Experimental 2.1. Preparation of plant extract
∗
Corresponding author. Tel.: +91-251-2620602; fax: +91-251-2620604. E-mail address: [email protected] (G. Gunasekaran). 0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.04.030
Three hundred fifty grams of dried Zenthoxylum-alatum plant fruits were soaked in 900 ml of reagent grade methanol
4388
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
for 24 h and refluxed for 5 h. The methanolic solution was filtered and concentrated to 500 ml. This extract was used to study the corrosion inhibition properties. To know the mass of plant extract, it was dried at 60 ◦ C ( removal of methanol) under vacuum for 12 h. From the weight of the vacuum dried liquid, it is calculated as each mililiter of plant extract contains 80 mg of plant compounds. The composition of liquid is Carbon (48.24%), Hydrogen (5.56%), Nitrogen (16.5%), Sulfur (1.58%), metal content (8.5%) and rest oxygen.
carried out from 0 to −600 mV potential ranges. Before recording the polarization curves, the WE was maintained at its corrosion potential for 10 min. until a steady state was obtained. The percentage inhibition efficiency (η) was calculated from, Icorr − Icorr (inh) × 100 Icorr where Icorr and Icorr (inh) are the corrosion current density value without and with inhibitor, respectively.
2.2. Preparation of specimens 2.7. Electrochemical impedance spectroscopy (EIS) Mild steel coupons having percent composition of 0.54Mn, 0.05Si, 0.01S, 0.01P, 0.16C and remaining Fe were used. The specimens were metallographically polished according to ASTM A262, degreased with trichloroethylene and washed with distilled water before experiment. 2.3. Electrolyte The solutions used were made of AR grade orthophosphoric acid. Appropriate concentrations of acids were prepared by using triple distilled water. The concentration range of inhibitor (plant extract) employed was varied from 800 to 3200 ppm and the electrolyte used was 200 ml.
EIS experiments were conducted using computer controlled Gamry electrochemical system. EIS 300 software was used for collecting and evaluating the experimental data. Our investigation measured the response of the electrochemical system to ac excitation with a frequency ranging from 50,000 to 0.02 Hz and peak to peak ac amplitude of 10 mV. Various equivalent circuit models were fitted to the impedance data using a non-linear optimization computed programme. 2.8. Surface analysis
Weight loss of rectangular steel specimens of size 1 cm × 5 cm × 2 cm in triplicate immersed in 200 ml of electrolyte with and without the addition of different concentrations of plant extract was determined after 4 h at 30 ◦ C or 1 h in the temperature range 50–80 ◦ C. The percentage inhibition efficiency was calculated from,
The test coupons of the size 1 cm × 1 cm were exposed in 100 ml of 88% phosphoric acid having 3200 ppm plant extract for 2 h at 30 ◦ C and washed with distilled water. After washing, specimens were dried in a critical point drying apparatus (M/S Balzers, Switzerland) and were examined for their structural and topographical features using Electron spectroscopy for chemical analysis (ESCA). For FT-IR studies ( KBr pellet method), the surface of the dried specimens were scratched with a knife and the resultant powder was used.
W1 − W2 × 100 W1
3. Results
where W1 and W2 are weight losses of steel in uninhibited and inhibited solutions.
3.1. Weight loss measurements
2.4. Weight loss method
2.5. Electrochemical measurements A three electrode cell assembly consisting of a mild steel coupon of the size 1 cm×1 cm embedded in specimen holder as working electrode (WE), a large area Platinum mesh of negligible impedance as counter electrode (CE) and a saturated calomel electrode as reference electrode (RE) containing 200 ml of electrolyte was used for electrochemical measurements. The temperature of the electrolyte was maintained at 30 ◦ C. 2.6. Potentiodynamic polarization curves The polarization curves were recorded by using computer controlled Gamry electrochemical system. The potential increased with the speed of 30 mV min−1 . Experiments were
Fig. 1 presents the dependence of corrosion rate and inhibition efficiency IE (%) of mild steel exposed to 20, 50 and 88% phosphoric acid on the concentration (ppm) of the plant extract studied at 30 ◦ C (calculated from gravimetric data). Addition of plant extract increased the corrosion inhibition efficiency. The inhibition estimated to be superior to 95% in 88% phosphoric acid at 3200 ppm concentration. The optimum concentration for maximum efficiency is found to be 3200 ppm of plant extract for 50 and 88% phosphoric acid medium. However in case of 20% phosphoric acid, the optimum concentration is found to be 2400 ppm. To study the effect of temperature on the corrosion inhibition properties of plant extract, mild steel specimens were exposed to 20, 50 and 88% phosphoric acid containing 3200 ppm of plant extract in the temperature range of 50–80 ◦ C (Fig. 2). In case of 20% phosphoric acid, the data
100
60
75
40
50
20
25
-2
Corrosion rate /mg m s
-1
80
0
4389
Inhibition efficiency / %
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
0 0
1000
2000
3000
Fig. 3. Potentiodynamic polarization plots of mild steel immersed in 88% phosphoric acid with and without plant extract (PE).
Plant extract / ppm
Inhibition efficiency/ %
Fig. 1. Corrosion rate and inhibition efficiency plots of mild steel immersed in 20, 50 and 88% phosphoric acid (PA) with and without plant extract at 30 ◦ C. (—) Corrosion rate; (· · · ) inhibition efficiency. 100
∆ - 88 % PA
75
o – 50 % PA - 20 % PA
50 25 0 40
50
60
70
80
90
o
Temperature / C
Fig. 2. Corrosion inhibition efficiency plots of mild steel immersed in 20, 50 and 88% phosphoric acid (PA) with 3200 ppm plant extract in the temperature range of 50–80 ◦ C.
suggest that the inhibition is almost constant irrespective of temperature and close to 20%. In 50% phosphoric acid medium, the inhibition is constant (∼35%) up to 60 ◦ C and decreases further. However, in 88% phosphoric acid, inhibition decreases only after 70 ◦ C. This shows that this plant extract is effective as inhibitor for mild steel in 88% phosphoric acid up to 70 ◦ C. 3.2. Potentiodynamic polarization Typical potentiodynamic polarization curves for the mild steel in 88% phosphoric acid containing different concen-
Fig. 4. Corrosion current vs. plant extract concentration in 20, 50 and 88% phosphoric acid (PA).
trations of plant extract are shown in Fig. 3. The respective kinetic parameters and inhibition efficiency η (%) are given in Table 1. It can be seen that the cathodic reaction (hydrogen evolution) is inhibited and the inhibition increases as the plant extract concentration increases. The addition of plant extract does not change the values of corrosion potential (Ecorr ) and cathodic Tafel slope (bc ). The cathodic current versus potential curves gave rise to parallel Tafel lines indicating that the hydrogen evolution reaction is activation controlled, and the addition of inhibitor studied does not modify the mechanism of the proton discharge reaction [17]. Anodic polarization of mild steel in 88% phosphoric acid (Fig. 3) shows drop in anodic current due to passivation at −280 mV. Decrease in anodic Tafel slope on increasing the plant extract concentration till 2400 ppm indicates that
Table 1 Kinetic parameters derived from potentiodynamic polarization plots of mild steel immersed in 88% phosphoric acid medium containing plant extract at 30 ◦ C Concentration (ppm)
Ecorr (mV/SCE)
Icorr (mA cm−2 )
bc (mV dec−1 )
ba (mV dec−1 )
η (%)
0 800 1600 2400 3200
−378 −380 −376 −387 −390
43.31 10.32 1.55 1.02 0.66
268 277 263 264 268
230 201 106 93 147
– 0.76 0.96 0.98 0.98
4390
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395 15
-Imag / Ohm
100
1600 ppm PE -Imag / Ohm
-Imag /Ohm
75
10 2400 ppm PE
800 ppm PE
5
400 ppm PE
10
800 ppm PE
0 0
10 20 Real / Ohm
50
2400 ppm PE 25
3200 ppm PE
1600 ppm PE 0 ppm PE
0 ppm PE 0
0 0
10
20 Real /Ohm
30
40
addition of plant extract till 2400 ppm decreases the anodic passivation. The increase in anodic Tafel slope at 3200 ppm may be attributed to the reduction of anodic reaction by this plant extract. Fig. 4 shows plot of corrosion current (Icorr ) of mild steel immersed in 20, 50 and 88% phosphoric acid plotted against plant extract concentration. Corrosion current decreases on increasing the plant extract concentration. From the figure it is clear that this plant extract is more effective in 88% phosphoric acid medium than 20 and 50%. Comparison of the above results with weight loss experiments shows almost same trend. But in case of 88% phosphoric acid, the corrosion rate values calculated from Icorr are higher than the corrosion rate values calculated from weight loss method. This trend is only noticed for 88% phosphoric acid medium. This is due to the fact that potentiodynamic polarisation experiment is short duration experiments and their principle is different that of long term experiments like weight loss experiment. On exposure of mild steel in 88% phosphoric acid medium, initially corrosion rate is more and gradually decreases due to the formation and strengthening of black colour layer containing insoluble Iron phosphates and iron oxides. It is therefore not possible to exactly correlate these experiments.
10
2400 ppm PE
5
0
100
200
300
Real / Ohm
Fig. 5. Nyquist plots of mild steel immersed in 20% phosphoric acid with and without plant extract (PE) at 30 ◦ C.
-Imag / Ohm
20
Fig. 7. Nyquist plots of mild steel immersed in 88% phosphoric acid with and without plant extract (PE) at 30 ◦ C.
3.3. Electrochemical impedance spectroscopy The corrosion of mild steel in phosphoric acid solution in the presence of plant extract was investigated by EIS at 30 ◦ C after an exposure period of 20 min. Nyquist plots in inhibited and uninhibited acidic solutions containing various concentrations of plant extract are shown in Figs. 5–7. The impedance diagrams (Nyquist) obtained are depressed semicircles with the center below the real axis. This feature shows contribution from surface roughness, distribution of active sites, adsorption of inhibitors and formation of porous layers as reported by others [14]. Since the corroding surface of mild steel in phosphoric acid medium is inhomogeneous, the system studied here can be characterized by distributed capacitance. The distributed capacitance is presented through constant phase element (CPE, Q). The impedance of CPE (Q) is defined as Z = A (jω)−n , where A is a proportional factor and n has meaning of phase shift. It is seen that at n equal to one, the element is an ideal capacitor and A is equal to C. The capacitance can be calculated from the experimentally determined CPE parameters n and A by the following equation [18,19]. C = ωn−1 /A sin(nπ/2) where ω is angular frequency. It is apparent from these figures that the impedance response for mild steel in 20, 50 and 88% phosphoric acid solutions changes significantly with increasing inhibitor concentration. The structural model of the mild steel–phosphoric acid interface is shown in Fig. 8. This model of the interface nicely fitted the impedance data of mild steel containing various concentrations of plant extract
3200 ppm PE 1600 ppm PE
CPE Ad, nd
800 ppm PE
Rs
0 ppm PE
0 0
5
10 15 Real / Ohm
20
25
Fig. 6. Nyquist plots of mild steel immersed in 50% phosphoric acid with and without plant extract (PE) at 30 ◦ C.
Rct
Fig. 8. Equivalent electrical circuit of the interface of mild steel/phosphoric acid electrolyte.
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
4391
Table 2 Kinetic parameters derived from Nyquist plots of mild steel immersed in 20, 50 and 88% phosphoric acid medium containing plant extract at 30 ◦ C Ad ( s−n cm2 )
Concentration of phosphoric acid (% v/v)
Concentration of plant extract (ppm)
Charge transfer resistance Rct ()
20
0 400 800 1600 2400
7.36 8.62 24.5 30.76 33.4
8333 7109 4882 4616 4187
0.99 0.99 0.83 0.83 0.81
50
0 800 1600 2400 3200
1.25 2 10.2 17.8 20.2
1026 14006 3518 2945 2684
0.68 0.80 0.79 0.79 0.80
88
0 800 1600 2400 3200
7.54 9.16 40.9 65.5 333
53243 5068 20251 20248 4901
0.90 0.90 0.90 0.70 0.51
nd
and the impedance data derived from these investigations are given in Table 2. It is found that as the plant extract concentration increases, the charge transfer resistance (Rt ) values increase as well. The Rt values are comparatively higher in 88% phosphoric acid compare to 20 and 50% phosphoric acid.
The respective polarization resistance (Rp ) versus potential shows that increase of Rp of mild steel in 88% phosphoric acid observed above −280 mV which is due to passivation of mild steel by iron phosphate precipitation.
3.4. The potential of zero charge of mild steel in phosphoric acid
A reflection FT-IR spectrum of plant extract and the iron surface treated with 3200 ppm plant extract in 88% phosphoric acid medium is displayed in Fig. 10. A broad bands in the range of 3000–3500 cm−1 , can be assigned to the presence of a superficial adsorbed water, stretching mode of an O–H and/or N–H (from plant extract) [21,22]. The band at 1300–1800 cm−1 is related to plant extract. The stretching mode of the C–H, C–C and C–S are indicated as bands at 2930, 1451 and 1388 cm−1 . Presence of C=O and C=C are indicated by their stretching modes at 1720 and 1600 cm−1 [23]. The band at 1720 cm−1 clearly indicates the presence of Iron–plant extract complex [24]. It is also observed that
200
30
160
25 20
120 15 80 10 40
5
0 -390
Polarisation resistance/ Ω
Capacitance / µF cm2
For a given solution the adsorbability of inhibitors will depend on the potential of metal on the potential of zero charge scale (pzc). The different charge of metals under conditions of their corrosion might be considered as one of the reasons for the selective action of the inhibitors. It is therefore necessary to calculate the pzc of mild steel in phosphoric acid to explain the inhibiting nature of plant extract. Ma et al. [20] calculated pzc of copper electrode in 0.5 mol dm−3 H2 SO4 solution from double layer capacitance by measuring a series of impedance spectra in a wide potential range between −512 and −20 mV versus SCE. We have measured a series of impedance spectra for the mild steel electrode in 88% phosphoric acid at various potential ranges close to Ecorr . The calculated capacitance and polarization resistance are shown in Fig. 9. The pzc of mild steel in 88% phosphoric acid is estimated to be close to −340 (±10) mV versus SCE from the dependence of double layer capacitance on potential shown in Fig. 9 which is less negative than the corrosion potential (−378 mV). This means that the mild steel surface in phosphoric acid is negatively charged at the corrosion potential. The plant extract contains salts of alkaloids, which are positively charged in nature. Considering the electrostatic attraction, electrostatic adsorption of plant extract is favoured. The reason that plant extract gives the highest inhibition efficiency is related to the specific adsorption of plant extract at the mild steel/solution interface.
3.5. Surface analysis of steels by FT-IR
0 -340
-290
-240
Potential / mV vs SCE
Fig. 9. Double layer capacitance of mild steel as a function of potential in 88% phosphoric acid solution.
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
3500
3000
2500
FP
-C-O
-C-S-
FP -C=C-
FPE -C=O-
4000
B
-C=C-
-C-H -O-CH3
-O-H -N-H
Transmittance / %
FP
IO
IO
4392
2000
A
1500
1000
500
Wave number / cm-1
Fig. 10. Infrared spectra of samples: (A) plant extract, (B) mild steel immersed in 88% phosphoric acid containing 3200 ppm of plant extract. The absorption bands belonging to iron phosphate FP, iron–plant extract complex FPE and iron oxides IO are indicated.
the 1620 cm−1 band is due to the contribution of Iron phosphate. The bands at 1265 (P=O) and 1030 cm−1 (P–O–Fe) are due to the presence of iron phosphate [25,26]. The band at 450–700 cm−1 probably originates mainly from ␥-Fe2 O3 (670 cm−1 [27]). Band at 793 cm−1 indicates the presence
O1s
B
A 404
Fe 2p Intensity / cps
Intensity / cps
Intensity / cps
N 1s
of iron oxide. The ferric phosphate bands are relatively more intensive than the plant extract bands. This is an indication that the inhibitive film consists mainly ferric phosphate and small amount of iron–plant extract complex and iron oxides.
B
A 400
396
542
Binding energy / eV
B
A 536
532
726
528
718
710 702
Binding energy / eV
Binding energy / eV
Fig. 11. Electron spectroscopy for chemical analysis (ESCA) spectra of Fe 2p3/2 , O 1s and N 1s for the mild steel immersed in 50 (A) and 88% (B) phosphoric acid containing 3200 ppm plant extract.
S 2p
B
A 136
C1s Intensity / cps
Intensity / cps
Intensity / cps
P 2p
B
A 132
Binding energy / eV
164
B
A 162
160
Binding energy / eV
292
288
284
Binding energy / eV
Fig. 12. Electron spectroscopy for chemical analysis (ESCA) spectra of P 2p3/2 , S 2p3/2 and C 1s for the mild steel immersed in 50 (A) and 88% (B) phosphoric acid containing 3200 ppm plant extract.
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
4. Discussions The composition and the structure of the films formed on iron remains subjects of continued interest [31]. X-ray diffraction studies on the oxides of iron revealed the presence of ␥-Fe2 O3 in solutions at all pH levels irrespective of the nature of the iron substrate [32]. At the interface of iron and acid electrolyte, the dissolution of iron can be written as following, Fe + H2 O ⇔ FeOHads + H+ + e FeOHads → FeOH+ + e FeOH+ ⇔ Fe2+ + OH− At medium and high concentrations of phosphoric acid, precipitation of iron–phosphate occurs at interface [33]. 6H3 PO4 + 3Fe → 3Fe(H2 PO4 )2 + 3H2 3Fe(H2 PO4 )2 → Fe3 (PO4 )2 + 4H3 PO4
-1
However, this precipitation can be weakly observed when the mild steel is treated with phosphoric acid solutions with low concentration. The formation of insoluble phosphate depends on the metal ions present in solution at interface, concentration of metal ion in the solution and the reactivity of metal surface. Volklan et al. [34] observed precipitation of corrosion inhibiting iron phosphate (vivianite, Fe3 (PO4 )2 8H2 O) on mild steel incubated in phosphate-buffered cultures of aerobic, biofilm forming Pseudomonos putida mt2 and Rhodococcus sp. Strain C125. Breur et al. [35] explained that as soon 4
-2
The electron spectroscopy for chemical analysis (ESCA) measurements was performed to obtain information on the surface coverage of the inhibitor after the different electrochemical experiments. Fig. 11 shows ESCA of Fe 2p3/2 , O 1s and N 1s for the mild steel specimen after immersion for 1 h in 50 and 88% phosphoric acid solution containing 3200 ppm plant extract. The Fe 2p3/2 spectrum has large peak at about 705–712 eV. Using a non linear least squares algorithm with a Shirley base line and a Gaussian–Lorentzian combination, the Fe 2p3/2 spectrum was deconvoluted into two components in case of 50% phosphoric acid medium. The peaks are at around 711.3 and at 707 eV. The latter peak is attributable to metallic iron which is reported to appear at 706.9 ± 0.10 [28]. The former peak is attributable to ferric compounds such as the iron oxides, iron phosphates and iron–plant extract complex mentioned previously in the explanation of FT-IR spectrum. For example, the binding energies of ␥-Fe2 O3 is reported to be 711.0 ± 0.15 [28]. The peak intensity for metallic iron was larger than the ferric compounds in the spectrum for the mild steel after immersion in 50% phosphoric acid with plant extract. This indicates that the formation of iron oxides and or iron phosphate is enhanced by the addition of plant extract. Only one peak for ferric compounds at 711 eV is noticed in case of mild steel immersed in 88% phosphoric acid containing plant extract. It shows addition of plant extract increases the formation of ferric compounds such as ferric phosphates and iron–plant extract complex mentioned in the explanation of FT-IR spectrum. In the O 1s spectrum a broad peak was observed at about 530–534 eV. The O 1s spectra could be deconvoluted into two components. The O 1s spectra show the strong presence of H2 O/O–H (532.1 eV) and C=O/S=O groups (534 eV). The peak that originates from plant extract is included in these two peaks because in general the O 1s peaks of organic compounds have been reported to appear at this energy position [29]. In the N 1s spectrum, a peak appeared at 399.9 eV. This peak probably comprised the nitrogen peak in the cyclic ring. The peaks of the amino group of cyclic ureido compounds and its anologues have been reported to appear at 399.5 and 399.6 eV, respectively [30]. The peak of the amino group is appeared at somewhat higher position than the above values because the coordination to the steel surfaces caused the peak shift towards the higher binding energy. Fig. 12 shows C 1s, S 2p3/2 and P 2p3/2 for the mild steel specimen after immersion for 1 h in 50 and 88% phosphoric acid solution containing 3200 ppm of plant extract. The respective C 1s spectra could be deconvoluted into three components. It shows that the peaks at 285, 287 and 288.6 eV are mainly due to the C–C, C–O and C=O bonds from plant extract. This result also supports the presence of adsorbed plant extract on the mild steel surface. The peaks at 161.8 and 133.0 eV are due to S 2P3/2 and P 2P3/2 electrons. The
presence of P 2P3/2 electrons indicates the presence of iron phosphate on the mild steel surface. The above results show that the inhibitive film formed on the mild steel surface contains iron plant extract complex, iron phosphate and iron oxides.
Ln (Corrosion rate) / mg m s
3.6. Surface analysis of steels by ESCA
4393
3
2 28
29
30 10000/T / K
31
32
-1
Fig. 13. Variations of Ln (corrosion rate) with 1/T in 20, 50 and 88% phosphoric acid electrolyte.
4394
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
Table 3 Calculated values of activation energy (Ea ) for the corrosion of mild steel in phosphoric acid with and without 3200 ppm plant extract Phosphoric acid concentration (%)
Activation energy (Ea )a without plant extract (k cal mol−1 )
Activation energy (Ea )a with 3200 ppm of plant extract (k cal mol−1 )
20 50 88
12 14 14
12 18 30
a
Error ± 1.5.
as bacteria start to grow and produce extra polymeric substance (EPS), the EPS interact with dissolving iron to form an organo-metal complex (FeEPS). This layer reacts with the phosphate ions to form a layer of FeHPO4 /FeH2 PO4 . This reaction takes place in series with the formation of FeEPS, since it is mediated or catalysed for the formation of iron phosphates. Surface analysis of mild steel specimens immersed in 88% phosphoric acid showed that the surface contains iron phosphate, iron–plant extract complex and iron oxide. From the corrosion rate of mild steel in 88% phosphoric acid that it is clear that insoluble iron phosphate precipitation is not taking place. Based on the electrochemical and surface analysis information, a hypothesis can be derived for the mechanism of inhibitive layer formation by plant extract. Initially, when no layers are present dissolution of iron takes place. The plant extract (PE) reacts with dissolving iron to form an organo-metal complex such as Fe–plant extract (Fe–PE) and forms a layer. Fe2+ + PE → [Fe–PE]
4
-2
Ln (Corrosion rate) / mg m s
-1
This layer reacts with phosphate ions to form a layer of FeHPO4 /FeH2 PO4. This reaction takes place in series with the formation of Fe–PE, since it is mediated or catalysed by this compound, as is observed by the increased formation rate of iron phosphates. After a certain period, the formation of iron phosphate results in a dense layer and formation of Fe–PE will contribute less. This was reflected by FT-IR analysis of mild steel immersed in 88% phosphoric acid containing 3200 ppm of plant extract.
3
2
The change of the corrosion process rate in presence and absence of plant extract containing 20, 50 and 88% phosphoric acid was plotted against temperature for calculation of activation energy (Figs. 13 and 14) and the calculated activation energy [4] are given in Table 3. The effect of chemically stable surface active inhibitors increase the energy of activation and to the diminution of the surface available for corrosion [36]. The value of activation energy (Ea ) in solutions containing phosphoric acid with plant extract (3200 ppm) was greater than that of without plant extract and also increase in concentration of phosphoric acid containing 3200 ppm of plant extract increases activation energy. The hindrance to dissolution is due to the formation of the Fe–PE and iron phosphate layer. ESCA and FT-IR results showed that on increasing concentration of phosphoric acid in presence of 3200 ppm of plant extract increases the concentration of iron-phosphates and iron oxides on the surface along with Fe–plant extract organometallic complex. Due to the above, on increasing concentration of phosphoric acid, the mild steel surface available for corrosion diminishes resulting in the increase in activation energy.
5. Conclusions 1. The inhibition efficiency of mild steel in 20, 50 and 88% phosphoric acid medium increases on increasing the concentration of plant extract. These plant extracts act as inhibitor in 88% phosphoric acid medium up to 70 ◦ C. The performance of this extract as corrosion inhibitor is better in 88% phosphoric acid than in 20 and 50%. 2. AC impedance plots of mild steel in 20, 50 and 88% phosphoric acid medium shows that polarisation resistance increases with increase of plant extract concentration. 3. XPS and FT-IR show that the inhibitor layer contains compounds present in plant extract, iron oxide and iron phosphate. 4. Results indicated that formation of iron phosphate was catalysed by the formation of Fe–PE organo metallic complex.
1 28
29
30 10000/ T / K
31
32
-1
Fig. 14. Variations of Ln (corrosion rate ) with 1/T in 20, 50 and 88% phosphoric acid electrolyte containing 3200 ppm plant extract.
Acknowledgements The authors are grateful to Dr. P.C. Deb, Director and Dr. J. Rangarajan, Head, Corrosion Protection and
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
Electrochemistry, NMRL for their valuable guidance and Mrs. Lalitha Chandrasekar and Mr. Vishal Dalvi for FT-IR. References [1] J.M. Sykes, Br. Corros. J. 25 (1990) 175. [2] M. Elachauri, M.S. Hajji, S. Kertit, E.M. Salem, R. Coudert, Corros. Sci. 37 (1995) 381. [3] B. Mernari, H. Elattari, M. Traisnel, F. Bentiss, M. Lagrenee, Corrosion 40 (1998) 391. [4] L. Wang, Corros. Sci. 43 (2001) 2281. [5] M.E. Azhar, M. Mernari, M. Traisnel, F. Bentiss, M. Lagrenee, Corros. Sci. 43 (2001) 2229. [6] M.A. Quraishi, M.A.W. Khan, M. Ajmal, S. Muralidharan, S.V. Iyer, J. Appl. Electrochem. 26 (1996) 443. [7] F. Bentiss, M. Traisnel, M. Lagrenee, Corros. Sci. 42 (2000) 127. [8] L. Wang, Corros. Sci. 43 (2001) 2281. [9] A.C. Makrides, Corrosion 29 (1973) 180. [10] M.S. Morad, Mat. Chem. Phys. 60 (1999) 188. [11] Y. Jianguo, W. Lin, V. Otieno-Alego, D.P. Schweinsberg, Corros. Sci. 37 (1995) 975. [12] K. Srivasthava, P. Srivasthava, Corros. Prev. Contr. 27 (1980) 5. [13] R.M. Saleh, A.A. Ismail, A.A. El Hosary, Corros. Prev. Contr. 31 (1984) 21. [14] A.A. Ec Hosary, R.M. Saleh, A.M. Shams El Din, Corros. Sci. 12 (1972) 897. [15] F. Zucchi, I.H. Omar, Surface Tech. 24 (1985) 391. [16] B.C. Jain, J.N. Gour, J. Electrochem. Soc. India 27 (1978) 165. [17] L. Elkadi, B. Mernari, M. Traisnel, F. Bentiss, M. Lagrenee, Corros. Sci. 42 (2000) 703.
4395
[18] S.F. Merttens, C. Xhoffer, B.C. Cooman, E. Temmerman, Corrosion 53 (1993) 381. [19] E.P.M. van Westing, G.M. Ferrari, J.H.W. de Wit, Corros. Sci. 34 (1993) 1511. [20] H. Ma, S. Chen, B. Yin, S. Zhao, X. Liu, Corros. Sci. 45 (2003) 867. [21] R.M. Cornell, U. Schwertmann, The Iron-Oxides-Structure, Properties, Reactions, Occurences and Uses, VCH, Weinheim, 1996, p. 573. [22] M. Mizushima, T. Shimauchi, in: Sekigaisenkyuushuu to Raman Kouka, Kyoritsu Shuppan, 1958 (Chapter 8). [23] C.J. Alice, C.P. Prabhakaran, Ind. J. Chem. 29A (1990) 491. [24] C.A. Barrero, L.M. Ocampo, C.E. Arroyave, Corros. Sci. 43 (2001) 1003. [25] J. Gust, Corros. NACE 47 (1991) 453. [26] S. Nasrazadani, Corros. Sci. 39 (1997) 1845. [27] R. Jasinski, A. Lob, J. Electrochem. Soc. 135 (3) (1988) 551. [28] N.S. McIntyre, D.G. Zetaruk, Anal. Chem. 49 (1977) 1521. [29] D. Briggs, M.P. Seah, Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Wiley, Sussex, 1983. [30] J. Peeling, F.E. Hruska, N.S. McIntyle, Can. J. Chem. 56 (1978) 1555. [31] G. Gunasekaran, N. Palaniswamy, B.V. Appa Rao, V.S. Muralidharan, Proc. Indian Acad. Sci. (Chem. Sci.) 108 (1996) 399. [32] C.L. Foley, J. Kruger, C.J. Bechtoldt, J. Electrochem. Soc. 114 (1967) 994. [33] E. Almeida, D. Pereira, M.O. Figueiredo, V.M.M. Lobo, M. Morcillo, Corros. Sci. 39 (1997) 1561. [34] H.A. Volkland, H. Harms, B. Muller, G. Repphun, O. Wanner, A.J.B. Zehnder, Appl. Environ. Micribiol. 66 (2000) 4389. [35] H.J.A. Breur, J.H.W. de Wit, J. van Turnhout, G.M. Ferrari, Electrochim. Acta 47 (2002) 2289. [36] L.I. Antropov, Corros. Sci. 7 (1967) 607.
Eco friendly inhibitor for corrosion inhibition of mild steel in phosphoric acid medium G. Gunasekaran∗ , L.R. Chauhan Naval materials Research Laboratory, Shil Badlapur Road, Addl. Ambernath, (E) Dist. Thane 421506 (MH), India Received 7 October 2003; received in revised form 5 April 2004; accepted 12 April 2004 Available online 9 June 2004
Abstract The inhibition effect of Zenthoxylum alatum plant extract on the corrosion of mild steel in 20, 50 and 88% aqueous orthophosphoric acid has been investigated by weight loss and electrochemical impedance spectroscopy (EIS). Plant extract is able to reduce the corrosion of steel more effectively in 88% phosphoric acid than in 20% phosphoric acid. The effect of temperature on the corrosion behaviour of mild steel in 20, 50 and 88% phosphoric acid with addition of plant extract was studied in the temperature range 50–80 ◦ C. Results on corrosion rate and inhibition efficiency have indicated that this extract is effective up to 70 ◦ C in 88% phosphoric acid medium. Surface analysis (XPS and FT-IR) was also carried out to establish the mechanism of corrosion inhibition of mild steel in phosphoric acid medium. © 2004 Elsevier Ltd. All rights reserved. Keywords: Plant extract; Corrosion inhibitors; Adsorption; Weight loss; Electrochemical impedance spectroscopy; ESCA; FT-IR
1. Introduction Phosphoric acid prepared by dihydrate and hemihydrate wet processes generate severe corrosion problems in containers. One of the most practical methods for protection against excessive dissolution of metal by corrosion is use of proper inhibitors [1]. The use of organic compounds containing oxygen, sulphur and nitrogen to reduce corrosion attack on steel has been studied in some details [2–7]. 2-Mercaptobenzimidazole [8], food additives (blue dye no.1) [9], propargyl alcohol [10], polyvinylpyrrolidone and polyethylenimine [11] are used as inhibitors for mild steel in phosphoric acid medium. Extracts of naturally occurring products contain mixtures of compounds having oxygen, sulphur and nitrogen elements and are eco friendly in nature. These mixtures having nitrogen and sulfur as constituent atoms were studied as corrosion inhibitor in HCl medium [12,13]. Ec Hosary et al. [14] studied the corrosion inhibition of aluminium and zinc in 2N HCl using naturally occurring Hibiscus subdariffa (Karkode) extract. Zucchi and co workers [15] studied the dissolution of mild steel in 1N and 2N HCl using plant
extracts of Papaia, Poinciana pulcherrima, Cassia occidentalis and Datura stramonium seeds. Alkaloids such as papaverine, strychnine, quinine and nicotine were also studied as corrosion inhibitors in acid medium [16]. Literature reveal that not much work appears to have been done on the inhibition of mild steel in phosphoric acid solutions at high concentration levels using naturally available plant extracts. Zenthoxylum-alatum, a dicot family plant is generally found at an altitude of around 5000–6000 ft above sea level. In India these plants are mostly found in Himalayan region. However, they have never been exploited for the purpose of corrosion inhibition in phosphoric acid medium. In this paper, the kinetics of the corrosion of mild steel in phosphoric acid have been studied by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy methods. The action of plant extract as inhibitor in phosphoric acid medium over a range of acid concentration and solution temperature has also been examined.
2. Experimental 2.1. Preparation of plant extract
∗
Corresponding author. Tel.: +91-251-2620602; fax: +91-251-2620604. E-mail address: [email protected] (G. Gunasekaran). 0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.04.030
Three hundred fifty grams of dried Zenthoxylum-alatum plant fruits were soaked in 900 ml of reagent grade methanol
4388
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
for 24 h and refluxed for 5 h. The methanolic solution was filtered and concentrated to 500 ml. This extract was used to study the corrosion inhibition properties. To know the mass of plant extract, it was dried at 60 ◦ C ( removal of methanol) under vacuum for 12 h. From the weight of the vacuum dried liquid, it is calculated as each mililiter of plant extract contains 80 mg of plant compounds. The composition of liquid is Carbon (48.24%), Hydrogen (5.56%), Nitrogen (16.5%), Sulfur (1.58%), metal content (8.5%) and rest oxygen.
carried out from 0 to −600 mV potential ranges. Before recording the polarization curves, the WE was maintained at its corrosion potential for 10 min. until a steady state was obtained. The percentage inhibition efficiency (η) was calculated from, Icorr − Icorr (inh) × 100 Icorr where Icorr and Icorr (inh) are the corrosion current density value without and with inhibitor, respectively.
2.2. Preparation of specimens 2.7. Electrochemical impedance spectroscopy (EIS) Mild steel coupons having percent composition of 0.54Mn, 0.05Si, 0.01S, 0.01P, 0.16C and remaining Fe were used. The specimens were metallographically polished according to ASTM A262, degreased with trichloroethylene and washed with distilled water before experiment. 2.3. Electrolyte The solutions used were made of AR grade orthophosphoric acid. Appropriate concentrations of acids were prepared by using triple distilled water. The concentration range of inhibitor (plant extract) employed was varied from 800 to 3200 ppm and the electrolyte used was 200 ml.
EIS experiments were conducted using computer controlled Gamry electrochemical system. EIS 300 software was used for collecting and evaluating the experimental data. Our investigation measured the response of the electrochemical system to ac excitation with a frequency ranging from 50,000 to 0.02 Hz and peak to peak ac amplitude of 10 mV. Various equivalent circuit models were fitted to the impedance data using a non-linear optimization computed programme. 2.8. Surface analysis
Weight loss of rectangular steel specimens of size 1 cm × 5 cm × 2 cm in triplicate immersed in 200 ml of electrolyte with and without the addition of different concentrations of plant extract was determined after 4 h at 30 ◦ C or 1 h in the temperature range 50–80 ◦ C. The percentage inhibition efficiency was calculated from,
The test coupons of the size 1 cm × 1 cm were exposed in 100 ml of 88% phosphoric acid having 3200 ppm plant extract for 2 h at 30 ◦ C and washed with distilled water. After washing, specimens were dried in a critical point drying apparatus (M/S Balzers, Switzerland) and were examined for their structural and topographical features using Electron spectroscopy for chemical analysis (ESCA). For FT-IR studies ( KBr pellet method), the surface of the dried specimens were scratched with a knife and the resultant powder was used.
W1 − W2 × 100 W1
3. Results
where W1 and W2 are weight losses of steel in uninhibited and inhibited solutions.
3.1. Weight loss measurements
2.4. Weight loss method
2.5. Electrochemical measurements A three electrode cell assembly consisting of a mild steel coupon of the size 1 cm×1 cm embedded in specimen holder as working electrode (WE), a large area Platinum mesh of negligible impedance as counter electrode (CE) and a saturated calomel electrode as reference electrode (RE) containing 200 ml of electrolyte was used for electrochemical measurements. The temperature of the electrolyte was maintained at 30 ◦ C. 2.6. Potentiodynamic polarization curves The polarization curves were recorded by using computer controlled Gamry electrochemical system. The potential increased with the speed of 30 mV min−1 . Experiments were
Fig. 1 presents the dependence of corrosion rate and inhibition efficiency IE (%) of mild steel exposed to 20, 50 and 88% phosphoric acid on the concentration (ppm) of the plant extract studied at 30 ◦ C (calculated from gravimetric data). Addition of plant extract increased the corrosion inhibition efficiency. The inhibition estimated to be superior to 95% in 88% phosphoric acid at 3200 ppm concentration. The optimum concentration for maximum efficiency is found to be 3200 ppm of plant extract for 50 and 88% phosphoric acid medium. However in case of 20% phosphoric acid, the optimum concentration is found to be 2400 ppm. To study the effect of temperature on the corrosion inhibition properties of plant extract, mild steel specimens were exposed to 20, 50 and 88% phosphoric acid containing 3200 ppm of plant extract in the temperature range of 50–80 ◦ C (Fig. 2). In case of 20% phosphoric acid, the data
100
60
75
40
50
20
25
-2
Corrosion rate /mg m s
-1
80
0
4389
Inhibition efficiency / %
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
0 0
1000
2000
3000
Fig. 3. Potentiodynamic polarization plots of mild steel immersed in 88% phosphoric acid with and without plant extract (PE).
Plant extract / ppm
Inhibition efficiency/ %
Fig. 1. Corrosion rate and inhibition efficiency plots of mild steel immersed in 20, 50 and 88% phosphoric acid (PA) with and without plant extract at 30 ◦ C. (—) Corrosion rate; (· · · ) inhibition efficiency. 100
∆ - 88 % PA
75
o – 50 % PA - 20 % PA
50 25 0 40
50
60
70
80
90
o
Temperature / C
Fig. 2. Corrosion inhibition efficiency plots of mild steel immersed in 20, 50 and 88% phosphoric acid (PA) with 3200 ppm plant extract in the temperature range of 50–80 ◦ C.
suggest that the inhibition is almost constant irrespective of temperature and close to 20%. In 50% phosphoric acid medium, the inhibition is constant (∼35%) up to 60 ◦ C and decreases further. However, in 88% phosphoric acid, inhibition decreases only after 70 ◦ C. This shows that this plant extract is effective as inhibitor for mild steel in 88% phosphoric acid up to 70 ◦ C. 3.2. Potentiodynamic polarization Typical potentiodynamic polarization curves for the mild steel in 88% phosphoric acid containing different concen-
Fig. 4. Corrosion current vs. plant extract concentration in 20, 50 and 88% phosphoric acid (PA).
trations of plant extract are shown in Fig. 3. The respective kinetic parameters and inhibition efficiency η (%) are given in Table 1. It can be seen that the cathodic reaction (hydrogen evolution) is inhibited and the inhibition increases as the plant extract concentration increases. The addition of plant extract does not change the values of corrosion potential (Ecorr ) and cathodic Tafel slope (bc ). The cathodic current versus potential curves gave rise to parallel Tafel lines indicating that the hydrogen evolution reaction is activation controlled, and the addition of inhibitor studied does not modify the mechanism of the proton discharge reaction [17]. Anodic polarization of mild steel in 88% phosphoric acid (Fig. 3) shows drop in anodic current due to passivation at −280 mV. Decrease in anodic Tafel slope on increasing the plant extract concentration till 2400 ppm indicates that
Table 1 Kinetic parameters derived from potentiodynamic polarization plots of mild steel immersed in 88% phosphoric acid medium containing plant extract at 30 ◦ C Concentration (ppm)
Ecorr (mV/SCE)
Icorr (mA cm−2 )
bc (mV dec−1 )
ba (mV dec−1 )
η (%)
0 800 1600 2400 3200
−378 −380 −376 −387 −390
43.31 10.32 1.55 1.02 0.66
268 277 263 264 268
230 201 106 93 147
– 0.76 0.96 0.98 0.98
4390
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395 15
-Imag / Ohm
100
1600 ppm PE -Imag / Ohm
-Imag /Ohm
75
10 2400 ppm PE
800 ppm PE
5
400 ppm PE
10
800 ppm PE
0 0
10 20 Real / Ohm
50
2400 ppm PE 25
3200 ppm PE
1600 ppm PE 0 ppm PE
0 ppm PE 0
0 0
10
20 Real /Ohm
30
40
addition of plant extract till 2400 ppm decreases the anodic passivation. The increase in anodic Tafel slope at 3200 ppm may be attributed to the reduction of anodic reaction by this plant extract. Fig. 4 shows plot of corrosion current (Icorr ) of mild steel immersed in 20, 50 and 88% phosphoric acid plotted against plant extract concentration. Corrosion current decreases on increasing the plant extract concentration. From the figure it is clear that this plant extract is more effective in 88% phosphoric acid medium than 20 and 50%. Comparison of the above results with weight loss experiments shows almost same trend. But in case of 88% phosphoric acid, the corrosion rate values calculated from Icorr are higher than the corrosion rate values calculated from weight loss method. This trend is only noticed for 88% phosphoric acid medium. This is due to the fact that potentiodynamic polarisation experiment is short duration experiments and their principle is different that of long term experiments like weight loss experiment. On exposure of mild steel in 88% phosphoric acid medium, initially corrosion rate is more and gradually decreases due to the formation and strengthening of black colour layer containing insoluble Iron phosphates and iron oxides. It is therefore not possible to exactly correlate these experiments.
10
2400 ppm PE
5
0
100
200
300
Real / Ohm
Fig. 5. Nyquist plots of mild steel immersed in 20% phosphoric acid with and without plant extract (PE) at 30 ◦ C.
-Imag / Ohm
20
Fig. 7. Nyquist plots of mild steel immersed in 88% phosphoric acid with and without plant extract (PE) at 30 ◦ C.
3.3. Electrochemical impedance spectroscopy The corrosion of mild steel in phosphoric acid solution in the presence of plant extract was investigated by EIS at 30 ◦ C after an exposure period of 20 min. Nyquist plots in inhibited and uninhibited acidic solutions containing various concentrations of plant extract are shown in Figs. 5–7. The impedance diagrams (Nyquist) obtained are depressed semicircles with the center below the real axis. This feature shows contribution from surface roughness, distribution of active sites, adsorption of inhibitors and formation of porous layers as reported by others [14]. Since the corroding surface of mild steel in phosphoric acid medium is inhomogeneous, the system studied here can be characterized by distributed capacitance. The distributed capacitance is presented through constant phase element (CPE, Q). The impedance of CPE (Q) is defined as Z = A (jω)−n , where A is a proportional factor and n has meaning of phase shift. It is seen that at n equal to one, the element is an ideal capacitor and A is equal to C. The capacitance can be calculated from the experimentally determined CPE parameters n and A by the following equation [18,19]. C = ωn−1 /A sin(nπ/2) where ω is angular frequency. It is apparent from these figures that the impedance response for mild steel in 20, 50 and 88% phosphoric acid solutions changes significantly with increasing inhibitor concentration. The structural model of the mild steel–phosphoric acid interface is shown in Fig. 8. This model of the interface nicely fitted the impedance data of mild steel containing various concentrations of plant extract
3200 ppm PE 1600 ppm PE
CPE Ad, nd
800 ppm PE
Rs
0 ppm PE
0 0
5
10 15 Real / Ohm
20
25
Fig. 6. Nyquist plots of mild steel immersed in 50% phosphoric acid with and without plant extract (PE) at 30 ◦ C.
Rct
Fig. 8. Equivalent electrical circuit of the interface of mild steel/phosphoric acid electrolyte.
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
4391
Table 2 Kinetic parameters derived from Nyquist plots of mild steel immersed in 20, 50 and 88% phosphoric acid medium containing plant extract at 30 ◦ C Ad ( s−n cm2 )
Concentration of phosphoric acid (% v/v)
Concentration of plant extract (ppm)
Charge transfer resistance Rct ()
20
0 400 800 1600 2400
7.36 8.62 24.5 30.76 33.4
8333 7109 4882 4616 4187
0.99 0.99 0.83 0.83 0.81
50
0 800 1600 2400 3200
1.25 2 10.2 17.8 20.2
1026 14006 3518 2945 2684
0.68 0.80 0.79 0.79 0.80
88
0 800 1600 2400 3200
7.54 9.16 40.9 65.5 333
53243 5068 20251 20248 4901
0.90 0.90 0.90 0.70 0.51
nd
and the impedance data derived from these investigations are given in Table 2. It is found that as the plant extract concentration increases, the charge transfer resistance (Rt ) values increase as well. The Rt values are comparatively higher in 88% phosphoric acid compare to 20 and 50% phosphoric acid.
The respective polarization resistance (Rp ) versus potential shows that increase of Rp of mild steel in 88% phosphoric acid observed above −280 mV which is due to passivation of mild steel by iron phosphate precipitation.
3.4. The potential of zero charge of mild steel in phosphoric acid
A reflection FT-IR spectrum of plant extract and the iron surface treated with 3200 ppm plant extract in 88% phosphoric acid medium is displayed in Fig. 10. A broad bands in the range of 3000–3500 cm−1 , can be assigned to the presence of a superficial adsorbed water, stretching mode of an O–H and/or N–H (from plant extract) [21,22]. The band at 1300–1800 cm−1 is related to plant extract. The stretching mode of the C–H, C–C and C–S are indicated as bands at 2930, 1451 and 1388 cm−1 . Presence of C=O and C=C are indicated by their stretching modes at 1720 and 1600 cm−1 [23]. The band at 1720 cm−1 clearly indicates the presence of Iron–plant extract complex [24]. It is also observed that
200
30
160
25 20
120 15 80 10 40
5
0 -390
Polarisation resistance/ Ω
Capacitance / µF cm2
For a given solution the adsorbability of inhibitors will depend on the potential of metal on the potential of zero charge scale (pzc). The different charge of metals under conditions of their corrosion might be considered as one of the reasons for the selective action of the inhibitors. It is therefore necessary to calculate the pzc of mild steel in phosphoric acid to explain the inhibiting nature of plant extract. Ma et al. [20] calculated pzc of copper electrode in 0.5 mol dm−3 H2 SO4 solution from double layer capacitance by measuring a series of impedance spectra in a wide potential range between −512 and −20 mV versus SCE. We have measured a series of impedance spectra for the mild steel electrode in 88% phosphoric acid at various potential ranges close to Ecorr . The calculated capacitance and polarization resistance are shown in Fig. 9. The pzc of mild steel in 88% phosphoric acid is estimated to be close to −340 (±10) mV versus SCE from the dependence of double layer capacitance on potential shown in Fig. 9 which is less negative than the corrosion potential (−378 mV). This means that the mild steel surface in phosphoric acid is negatively charged at the corrosion potential. The plant extract contains salts of alkaloids, which are positively charged in nature. Considering the electrostatic attraction, electrostatic adsorption of plant extract is favoured. The reason that plant extract gives the highest inhibition efficiency is related to the specific adsorption of plant extract at the mild steel/solution interface.
3.5. Surface analysis of steels by FT-IR
0 -340
-290
-240
Potential / mV vs SCE
Fig. 9. Double layer capacitance of mild steel as a function of potential in 88% phosphoric acid solution.
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
3500
3000
2500
FP
-C-O
-C-S-
FP -C=C-
FPE -C=O-
4000
B
-C=C-
-C-H -O-CH3
-O-H -N-H
Transmittance / %
FP
IO
IO
4392
2000
A
1500
1000
500
Wave number / cm-1
Fig. 10. Infrared spectra of samples: (A) plant extract, (B) mild steel immersed in 88% phosphoric acid containing 3200 ppm of plant extract. The absorption bands belonging to iron phosphate FP, iron–plant extract complex FPE and iron oxides IO are indicated.
the 1620 cm−1 band is due to the contribution of Iron phosphate. The bands at 1265 (P=O) and 1030 cm−1 (P–O–Fe) are due to the presence of iron phosphate [25,26]. The band at 450–700 cm−1 probably originates mainly from ␥-Fe2 O3 (670 cm−1 [27]). Band at 793 cm−1 indicates the presence
O1s
B
A 404
Fe 2p Intensity / cps
Intensity / cps
Intensity / cps
N 1s
of iron oxide. The ferric phosphate bands are relatively more intensive than the plant extract bands. This is an indication that the inhibitive film consists mainly ferric phosphate and small amount of iron–plant extract complex and iron oxides.
B
A 400
396
542
Binding energy / eV
B
A 536
532
726
528
718
710 702
Binding energy / eV
Binding energy / eV
Fig. 11. Electron spectroscopy for chemical analysis (ESCA) spectra of Fe 2p3/2 , O 1s and N 1s for the mild steel immersed in 50 (A) and 88% (B) phosphoric acid containing 3200 ppm plant extract.
S 2p
B
A 136
C1s Intensity / cps
Intensity / cps
Intensity / cps
P 2p
B
A 132
Binding energy / eV
164
B
A 162
160
Binding energy / eV
292
288
284
Binding energy / eV
Fig. 12. Electron spectroscopy for chemical analysis (ESCA) spectra of P 2p3/2 , S 2p3/2 and C 1s for the mild steel immersed in 50 (A) and 88% (B) phosphoric acid containing 3200 ppm plant extract.
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
4. Discussions The composition and the structure of the films formed on iron remains subjects of continued interest [31]. X-ray diffraction studies on the oxides of iron revealed the presence of ␥-Fe2 O3 in solutions at all pH levels irrespective of the nature of the iron substrate [32]. At the interface of iron and acid electrolyte, the dissolution of iron can be written as following, Fe + H2 O ⇔ FeOHads + H+ + e FeOHads → FeOH+ + e FeOH+ ⇔ Fe2+ + OH− At medium and high concentrations of phosphoric acid, precipitation of iron–phosphate occurs at interface [33]. 6H3 PO4 + 3Fe → 3Fe(H2 PO4 )2 + 3H2 3Fe(H2 PO4 )2 → Fe3 (PO4 )2 + 4H3 PO4
-1
However, this precipitation can be weakly observed when the mild steel is treated with phosphoric acid solutions with low concentration. The formation of insoluble phosphate depends on the metal ions present in solution at interface, concentration of metal ion in the solution and the reactivity of metal surface. Volklan et al. [34] observed precipitation of corrosion inhibiting iron phosphate (vivianite, Fe3 (PO4 )2 8H2 O) on mild steel incubated in phosphate-buffered cultures of aerobic, biofilm forming Pseudomonos putida mt2 and Rhodococcus sp. Strain C125. Breur et al. [35] explained that as soon 4
-2
The electron spectroscopy for chemical analysis (ESCA) measurements was performed to obtain information on the surface coverage of the inhibitor after the different electrochemical experiments. Fig. 11 shows ESCA of Fe 2p3/2 , O 1s and N 1s for the mild steel specimen after immersion for 1 h in 50 and 88% phosphoric acid solution containing 3200 ppm plant extract. The Fe 2p3/2 spectrum has large peak at about 705–712 eV. Using a non linear least squares algorithm with a Shirley base line and a Gaussian–Lorentzian combination, the Fe 2p3/2 spectrum was deconvoluted into two components in case of 50% phosphoric acid medium. The peaks are at around 711.3 and at 707 eV. The latter peak is attributable to metallic iron which is reported to appear at 706.9 ± 0.10 [28]. The former peak is attributable to ferric compounds such as the iron oxides, iron phosphates and iron–plant extract complex mentioned previously in the explanation of FT-IR spectrum. For example, the binding energies of ␥-Fe2 O3 is reported to be 711.0 ± 0.15 [28]. The peak intensity for metallic iron was larger than the ferric compounds in the spectrum for the mild steel after immersion in 50% phosphoric acid with plant extract. This indicates that the formation of iron oxides and or iron phosphate is enhanced by the addition of plant extract. Only one peak for ferric compounds at 711 eV is noticed in case of mild steel immersed in 88% phosphoric acid containing plant extract. It shows addition of plant extract increases the formation of ferric compounds such as ferric phosphates and iron–plant extract complex mentioned in the explanation of FT-IR spectrum. In the O 1s spectrum a broad peak was observed at about 530–534 eV. The O 1s spectra could be deconvoluted into two components. The O 1s spectra show the strong presence of H2 O/O–H (532.1 eV) and C=O/S=O groups (534 eV). The peak that originates from plant extract is included in these two peaks because in general the O 1s peaks of organic compounds have been reported to appear at this energy position [29]. In the N 1s spectrum, a peak appeared at 399.9 eV. This peak probably comprised the nitrogen peak in the cyclic ring. The peaks of the amino group of cyclic ureido compounds and its anologues have been reported to appear at 399.5 and 399.6 eV, respectively [30]. The peak of the amino group is appeared at somewhat higher position than the above values because the coordination to the steel surfaces caused the peak shift towards the higher binding energy. Fig. 12 shows C 1s, S 2p3/2 and P 2p3/2 for the mild steel specimen after immersion for 1 h in 50 and 88% phosphoric acid solution containing 3200 ppm of plant extract. The respective C 1s spectra could be deconvoluted into three components. It shows that the peaks at 285, 287 and 288.6 eV are mainly due to the C–C, C–O and C=O bonds from plant extract. This result also supports the presence of adsorbed plant extract on the mild steel surface. The peaks at 161.8 and 133.0 eV are due to S 2P3/2 and P 2P3/2 electrons. The
presence of P 2P3/2 electrons indicates the presence of iron phosphate on the mild steel surface. The above results show that the inhibitive film formed on the mild steel surface contains iron plant extract complex, iron phosphate and iron oxides.
Ln (Corrosion rate) / mg m s
3.6. Surface analysis of steels by ESCA
4393
3
2 28
29
30 10000/T / K
31
32
-1
Fig. 13. Variations of Ln (corrosion rate) with 1/T in 20, 50 and 88% phosphoric acid electrolyte.
4394
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
Table 3 Calculated values of activation energy (Ea ) for the corrosion of mild steel in phosphoric acid with and without 3200 ppm plant extract Phosphoric acid concentration (%)
Activation energy (Ea )a without plant extract (k cal mol−1 )
Activation energy (Ea )a with 3200 ppm of plant extract (k cal mol−1 )
20 50 88
12 14 14
12 18 30
a
Error ± 1.5.
as bacteria start to grow and produce extra polymeric substance (EPS), the EPS interact with dissolving iron to form an organo-metal complex (FeEPS). This layer reacts with the phosphate ions to form a layer of FeHPO4 /FeH2 PO4 . This reaction takes place in series with the formation of FeEPS, since it is mediated or catalysed for the formation of iron phosphates. Surface analysis of mild steel specimens immersed in 88% phosphoric acid showed that the surface contains iron phosphate, iron–plant extract complex and iron oxide. From the corrosion rate of mild steel in 88% phosphoric acid that it is clear that insoluble iron phosphate precipitation is not taking place. Based on the electrochemical and surface analysis information, a hypothesis can be derived for the mechanism of inhibitive layer formation by plant extract. Initially, when no layers are present dissolution of iron takes place. The plant extract (PE) reacts with dissolving iron to form an organo-metal complex such as Fe–plant extract (Fe–PE) and forms a layer. Fe2+ + PE → [Fe–PE]
4
-2
Ln (Corrosion rate) / mg m s
-1
This layer reacts with phosphate ions to form a layer of FeHPO4 /FeH2 PO4. This reaction takes place in series with the formation of Fe–PE, since it is mediated or catalysed by this compound, as is observed by the increased formation rate of iron phosphates. After a certain period, the formation of iron phosphate results in a dense layer and formation of Fe–PE will contribute less. This was reflected by FT-IR analysis of mild steel immersed in 88% phosphoric acid containing 3200 ppm of plant extract.
3
2
The change of the corrosion process rate in presence and absence of plant extract containing 20, 50 and 88% phosphoric acid was plotted against temperature for calculation of activation energy (Figs. 13 and 14) and the calculated activation energy [4] are given in Table 3. The effect of chemically stable surface active inhibitors increase the energy of activation and to the diminution of the surface available for corrosion [36]. The value of activation energy (Ea ) in solutions containing phosphoric acid with plant extract (3200 ppm) was greater than that of without plant extract and also increase in concentration of phosphoric acid containing 3200 ppm of plant extract increases activation energy. The hindrance to dissolution is due to the formation of the Fe–PE and iron phosphate layer. ESCA and FT-IR results showed that on increasing concentration of phosphoric acid in presence of 3200 ppm of plant extract increases the concentration of iron-phosphates and iron oxides on the surface along with Fe–plant extract organometallic complex. Due to the above, on increasing concentration of phosphoric acid, the mild steel surface available for corrosion diminishes resulting in the increase in activation energy.
5. Conclusions 1. The inhibition efficiency of mild steel in 20, 50 and 88% phosphoric acid medium increases on increasing the concentration of plant extract. These plant extracts act as inhibitor in 88% phosphoric acid medium up to 70 ◦ C. The performance of this extract as corrosion inhibitor is better in 88% phosphoric acid than in 20 and 50%. 2. AC impedance plots of mild steel in 20, 50 and 88% phosphoric acid medium shows that polarisation resistance increases with increase of plant extract concentration. 3. XPS and FT-IR show that the inhibitor layer contains compounds present in plant extract, iron oxide and iron phosphate. 4. Results indicated that formation of iron phosphate was catalysed by the formation of Fe–PE organo metallic complex.
1 28
29
30 10000/ T / K
31
32
-1
Fig. 14. Variations of Ln (corrosion rate ) with 1/T in 20, 50 and 88% phosphoric acid electrolyte containing 3200 ppm plant extract.
Acknowledgements The authors are grateful to Dr. P.C. Deb, Director and Dr. J. Rangarajan, Head, Corrosion Protection and
G. Gunasekaran, L.R. Chauhan / Electrochimica Acta 49 (2004) 4387–4395
Electrochemistry, NMRL for their valuable guidance and Mrs. Lalitha Chandrasekar and Mr. Vishal Dalvi for FT-IR. References [1] J.M. Sykes, Br. Corros. J. 25 (1990) 175. [2] M. Elachauri, M.S. Hajji, S. Kertit, E.M. Salem, R. Coudert, Corros. Sci. 37 (1995) 381. [3] B. Mernari, H. Elattari, M. Traisnel, F. Bentiss, M. Lagrenee, Corrosion 40 (1998) 391. [4] L. Wang, Corros. Sci. 43 (2001) 2281. [5] M.E. Azhar, M. Mernari, M. Traisnel, F. Bentiss, M. Lagrenee, Corros. Sci. 43 (2001) 2229. [6] M.A. Quraishi, M.A.W. Khan, M. Ajmal, S. Muralidharan, S.V. Iyer, J. Appl. Electrochem. 26 (1996) 443. [7] F. Bentiss, M. Traisnel, M. Lagrenee, Corros. Sci. 42 (2000) 127. [8] L. Wang, Corros. Sci. 43 (2001) 2281. [9] A.C. Makrides, Corrosion 29 (1973) 180. [10] M.S. Morad, Mat. Chem. Phys. 60 (1999) 188. [11] Y. Jianguo, W. Lin, V. Otieno-Alego, D.P. Schweinsberg, Corros. Sci. 37 (1995) 975. [12] K. Srivasthava, P. Srivasthava, Corros. Prev. Contr. 27 (1980) 5. [13] R.M. Saleh, A.A. Ismail, A.A. El Hosary, Corros. Prev. Contr. 31 (1984) 21. [14] A.A. Ec Hosary, R.M. Saleh, A.M. Shams El Din, Corros. Sci. 12 (1972) 897. [15] F. Zucchi, I.H. Omar, Surface Tech. 24 (1985) 391. [16] B.C. Jain, J.N. Gour, J. Electrochem. Soc. India 27 (1978) 165. [17] L. Elkadi, B. Mernari, M. Traisnel, F. Bentiss, M. Lagrenee, Corros. Sci. 42 (2000) 703.
4395
[18] S.F. Merttens, C. Xhoffer, B.C. Cooman, E. Temmerman, Corrosion 53 (1993) 381. [19] E.P.M. van Westing, G.M. Ferrari, J.H.W. de Wit, Corros. Sci. 34 (1993) 1511. [20] H. Ma, S. Chen, B. Yin, S. Zhao, X. Liu, Corros. Sci. 45 (2003) 867. [21] R.M. Cornell, U. Schwertmann, The Iron-Oxides-Structure, Properties, Reactions, Occurences and Uses, VCH, Weinheim, 1996, p. 573. [22] M. Mizushima, T. Shimauchi, in: Sekigaisenkyuushuu to Raman Kouka, Kyoritsu Shuppan, 1958 (Chapter 8). [23] C.J. Alice, C.P. Prabhakaran, Ind. J. Chem. 29A (1990) 491. [24] C.A. Barrero, L.M. Ocampo, C.E. Arroyave, Corros. Sci. 43 (2001) 1003. [25] J. Gust, Corros. NACE 47 (1991) 453. [26] S. Nasrazadani, Corros. Sci. 39 (1997) 1845. [27] R. Jasinski, A. Lob, J. Electrochem. Soc. 135 (3) (1988) 551. [28] N.S. McIntyre, D.G. Zetaruk, Anal. Chem. 49 (1977) 1521. [29] D. Briggs, M.P. Seah, Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Wiley, Sussex, 1983. [30] J. Peeling, F.E. Hruska, N.S. McIntyle, Can. J. Chem. 56 (1978) 1555. [31] G. Gunasekaran, N. Palaniswamy, B.V. Appa Rao, V.S. Muralidharan, Proc. Indian Acad. Sci. (Chem. Sci.) 108 (1996) 399. [32] C.L. Foley, J. Kruger, C.J. Bechtoldt, J. Electrochem. Soc. 114 (1967) 994. [33] E. Almeida, D. Pereira, M.O. Figueiredo, V.M.M. Lobo, M. Morcillo, Corros. Sci. 39 (1997) 1561. [34] H.A. Volkland, H. Harms, B. Muller, G. Repphun, O. Wanner, A.J.B. Zehnder, Appl. Environ. Micribiol. 66 (2000) 4389. [35] H.J.A. Breur, J.H.W. de Wit, J. van Turnhout, G.M. Ferrari, Electrochim. Acta 47 (2002) 2289. [36] L.I. Antropov, Corros. Sci. 7 (1967) 607.