Corrosion resistance of AISI 1018 carbon steel in NaCl solution by plasma-chemical formation of a barrier layer

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Corrosion Science 50 (2008) 1422–1432 www.elsevier.com/locate/corsci

Corrosion resistance of AISI 1018 carbon steel in NaCl solution by plasma-chemical formation of a barrier layer F. Depenyou Jr. a,b, A. Doubla a, S. Laminsi a, D. Moussa b, J.L. Brisset b,*, J.-M. Le Breton c a Department of Inorganic Chemistry, Laboratory of Mineral Chemistry, University of Yaounde´ I, Cameroon Electrochemistry Laboratory (LEICA), UFR Sciences, University of Rouen, 76821 Mont Saint-Aignan Cedex, France c Groupe de Physique des Mate´riaux UMR 6634, UFR Sciences, University of Rouen, 76801 St. Etienne du Rouvray, France b

Received 23 November 2007; accepted 22 December 2007 Available online 2 February 2008

Abstract The oxidizing OH and NO radicals generated by a gliding electric discharge in humid air induce passive film formation at the surface of AISI 1018 steel discs, as showed by changes in corrosion electric parameters (Ecor, Jcor) in aqueous (0.5 M) NaCl solution with the exposure time to the plasma. The protecting treatment and wettability are largely improved by rotating the sample during exposure. X-rays analysis evidences the apparition of a-Fe phase at the surface and Mo¨ssbauer spectra enlights a slight contribution of lepidocrocite FeOOH phase. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: A. Mild steel; B. Gliding electric discharge; B. Polarization; B. X-ray diffraction; C. Passive film

1. Introduction The non-thermal quenched plasma generated at atmospheric pressure by a gliding electric discharge (i.e., the ‘‘gliding arc” or ‘‘glidarc”) in humid air is a technique operated close to ambient temperature which has been successfully applied to the degradation of liquid solutes [1–3] for pollutant abatement, decontamination of micro-organisms [4] and oxidation of stainless steels surfaces [5]. The anticorrosive properties of the stainless steel materials with a chromium content higher than 12% are assigned to the spontaneous formation on the surface of a passive thin film layer, mainly composed of chromium and iron oxides/ hydroxides. Other metallic materials such as copper alloys were considered [6] but mild steel has not yet been exposed to the *

Corresponding author. Present address: University of Rouen, UFR Sciences, LMDF (UPRES-2123), 55 rue St Germain, F-27000 Evreux, France. Tel.: +33 681 945 719; fax: +33 232 291 566. E-mail addresses: [email protected], [email protected] (J.L. Brisset). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2007.12.011

gliding discharge but changes in surface properties may reasonably be expected [7,8]. The AISI 1018 carbon steel is widely used in industry despite its low corrosion resistance in various aggressive environments. We report in this paper on the ability of the glidarc technique to enhance the anticorrosion properties of this material. The plasma treatments represent nowadays an important technique for modifying and improving the chemical and surface properties of solid materials, such as increasing the corrosion resistance, etching, and cleaning [7–9]. A large part of these treatments is achieved under reduced pressure and at ambient temperature, while high temperature treatments are usually performed at atmospheric pressure. The gliding discharge (or ‘‘glidarc”) is one of the few plasma technique that allows working at atmospheric pressure and quasi-ambient temperature: this makes it attractive to the industrial world. It was first proposed by Lesueur et al. [10] for the treatment of gases and is still under development for surface treatments, solute pollutants abatement and inactivation of bacteria [4]. The glidarc is actually a quenched plasma which involves the

F. Depenyou Jr. et al. / Corrosion Science 50 (2008) 1422–1432

interesting chemical properties of activated gaseous matter without noticeable thermal effect. Briefly, two diverging conductors are connected to a High Voltage source such as a transformer. An arc forming at the electrode smallest gap is actually a thermal plasma. A gas flow directed along the axis of the reactor gently pushes the arc feet along the conductors towards the electrodes tips up to the arc is short-circuited by a new one and breaks in a plasma plume. Then the process resumes. Therefore, the length of the electric channel increases and the gas temperature falls, so that the resulting plasma plume is actually a quenched plasma, the macroscopic temperature of which is close to ambient. The solid target placed in front of the electrodes is thus submitted to the flux of the impinging activated species formed in the discharge. The glidarc device is an easy to operate source of chemically reactive species the nature of which depends on those of the gas and the electrodes. Interesting chemical properties of gliding discharges in humid air are mainly due to the presence of the highly reactive hydroxyl OH and nitric oxide NO radicals in the plasma plume [11,12]. The radical OH is known as a strong oxidizing agent (Standard potential E°OH/H2O = 2.85 V/ NHE) while NO is more usually referred to as an acidic one, since it induces the formation of transient nitrous acid and stable nitric acid in the treated solution or at the target surface. The aim of this study is to enhance the corrosion resistance of AISI 1018 carbon steel by generating a passive film on the surface using a treatment of non-thermal gliding arc plasma of humid air. The modifications of the corrosion resistance characteristics of the steel due to the treatment are investigated by electrochemical tests. The Tafel’s technique is performed in this work in order to compare and analyse the corrosion potential and the corrosion current density of treated samples in corroding 0.5 M NaCl solution. The influence of the rotation of the steel surface during plasma treatment is evaluated. The properties of the protective barrier layer

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formed during plasma-surface interaction are investigated by contact angle measurements, electrochemical analysis in NaOH 0.025 mol L1, X-rays diffractometry and Mo¨ssbauer spectroscopy. 2. Experimental setup A scheme of the experimental setup is given in Fig. 1. This cheap atmospheric plasma reactor uses compressed humid air saturated by passing through a bubbling water flask. The gas flow is directed along the axis of the reactor, i.e., between two diverging electrodes connected to a 220 V/10 kV Aupem-Sefli high voltage transformer. The ignited electric arc at the neck (shortest electrode gap = 2.5 mm wide) is pushed downwards to the electrode lower tips, and its length increases. The arc voltage increases until it reaches the breakdown voltage of the starting gap; then a new arc forms at the electrode gap, short-circuiting the first one that gives rise to a plasma plume. The gas flow rate can be adjusted from 0 to 25 L min1 by means of a Show Rate Brooks flow meter. All the treatments were made at a distance d/ = 2.5 cm from the tips of the electrodes to the steel surface and at the gas flow rate 16.25 L min1. The selected AISI 1018 carbon steel samples provided by Goodfellow contain about 0.18% by weight of carbon and 0.6–0.9% by weight of manganese. The carbon steel discs (15 mm diameter and 5 mm thick) were polished using various grinding papers (400, 800, and 1200) and finished with a Mecaprex self-adhesive polishing disc using 3 lm diamond paste. They were cleaned with acetone, rinsed with distilled water and dried with non-abrasive filter paper. The mirror-like surface obtained was then exposed to the incident plasma gas as shown in Fig. 1. Contact angle to water was measured after the plasma treatment, using a classic contact angle apparatus, when samples are cooled to room temperature.

Generator (H.T )

Humid air

starting Flow meter

Thermal zone Bubbling water flask

Tempered zone out equilibrrium d

Rupture

Glass wall Air inlet M Steel sample

Motor

Fig. 1. Scheme of the experimental setup, transformer HV = 10 kV, electrode gap d = 2.5 cm. (a) Immobile sample holder and (b) rotating sample holder.

F. Depenyou Jr. et al. / Corrosion Science 50 (2008) 1422–1432

The electrochemical analyses were performed using a Potentiostat/Galvanostat Model 273 (Princeton Applied Research) coupled to an IBM PC. The electrochemical cell is fitted with a platinum auxiliary electrode, the working electrode involving the plasma treated sample, and a SCE reference electrode. The electrolyte solution, (i.e., 0.5 M NaCl; pH = 6.4; T = 20 °C), is purged before each run for nearly 30 min with nitrogen. The potential sweep rate is fixed at 1 mV s1 for all runs. The characteristic curve current intensity (A) vs. applied potential (V) is plotted straightaway.

3. Results and discussion 3.1. Rotation of the sample 3.1.1. Effects of surface rotation on the profile of incident flowing plasma gas The interaction between a reactive incident gas and a solid surface depends on the ability of the actives species to reach the surface and react at it. We have modified the previously used apparatus [5] by mounting the sample holder on the rotation axis of an electric motor, and protecting the plasma flame from the external atmosphere with an isolating glass cylinder. The rotation speed of the motor is monitored by a variator. Fig. 2 illustrates the plasma flows obtained for immobile (a) and rotating (b) sample holder. The rotation modifies the impinging flow of the plasma gas at the surface. Three regions are observed on the picture for the discharge, i.e., a central reddish region constituted of arc discharge filaments, a blue region surrounding the reddish filaments, and a black region located outside and at the end of the blue regions. These different domains may correspond to various compositions and properties of the plasma. For non rotating samples, the incident plasma gas is deviated from the metal surface, according to the behaviour of a gas flow falling perpendicular to a plane solid surface. The black region is placed directly above the sample

and its area is much larger than that of the blue region. Oppositely, the volume of the plasma is more important above the rotating sample, and the blue region is larger and closer to the surface. The rotation of the sample holder creates a vortex above the surface centred on the treated sample. More numerous active species are thus forced to go into contact with the surface before leaving the reactor. Finally, the contact between the plasma and the treated surface is more effective in case of a rotating target. 3.2. Tafel’s method The interaction of the plasma gas with the carbon steel surface induced surface modification which was followed by electrochemical methods: the Tafel’s curves of the treated samples were recorded from 800 mV to 0.00 mV/SCE at a sweeping rate of 1 mV s1 in sodium chloride NaCl (0.5 mol L1, pH: 6.4) electrolyte. The corrosion potential Ecorr is directly determined on the figure, and the corrosion current density Jcorr is determined by Tafel’s extrapolation method (Fig. 3).

-1 -2

Log(J(A/cm2))

1424

-3 -4

y = 18.725x + 7.6297 R2 = 0.9943

-5 -6

non treated D1 D2 Linéaire (D1) Linéaire (D2)

y = -2.4305x - 6.6832 R2 = 0.9933

-7 -1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

E(V)/SCE Fig. 3. Tafel curve of non-treated AISI 1018 carbon steel in 0.5 mol L1 NaCl solution (pH¼ J6.4), showing Tafel lines for the calculation of corrosion current density. Scan rate: 1 mV s1.

Fig. 2. Plasma gas profile for the treatment of immobile (a) and rotating (b) steel sample. The tones of pictures are treated to display the regions of the gliding discharge.

F. Depenyou Jr. et al. / Corrosion Science 50 (2008) 1422–1432 -400 -450

Ecor (mV)/SCE

3.2.1. Tafel curve of the AISI 1018 carbon steel in 0.5 mol L1 NaCl solution Fig. 3 shows the Tafel’s curves of the non-plasma-treated AISI 1018 carbon steel with the anodic and cathodic linear Tafel’s lines. The corrosion potential calculated from the recorded data is 680 mV/SCE, with a corrosion current density evaluated close to 9.3 ± 1.0 lA cm2. The potentiodynamic polarization diagram shows that carbon steel has no active/passive transition when immersed in a sodium chloride solution, which agrees with the literature [13]. The anodic branch of the curves shows that the carbon steel undergoes a generalized corrosion.

1425

-500 -550

3100 rpm 0 rpm

-600

2400 rpm

-650 -700

t (min) -750

3.2.3. Influence of exposure to discharge on the corrosion parameters Ecor, Jcor in 0.5 M NaCl The influence of the exposure time to discharge on the corrosion potential Ecor and corrosion current density Jcor has been studied (Figs. 5 and 6) in 0.5 M NaCl solution since this medium may account for sea water. The Jcor values globally decrease with treatment duration, but the cor-

0

5

10

15

20

25

30

35

40

45

50

55

60

Fig. 5. Corrosion potential (mV/SCE) of AISI 1018 treated samples in 0.5 M NaCl, pH = 6.4 as function treatment duration. Rotation speed of sample under arc discharge: (a) 0 rpm, (b) 2400 rpm, and (c) 3100 rpm.

12

0 rpm 2400 rpm 3100 rpm

10

Jcor (µA/cm2)

3.2.2. Effects of the glidarc plasma treatment and the surface rotation speed on the electrochemical properties of carbon steel The AISI 1018 carbon steel samples were treated by plasma for 30 min at different rotation speeds, i.e., 0, 2400, and 3100 rotations per minute (rpm) of the sample holder. The relevant Tafel curves are displayed in Fig. 4. This figure shows that the plasma treatment induces an increasing corrosion potential and a reducing current density of the anodic branch of mild steel. The corrosion potentials Ecor of samples treated on rotating holder are higher than those of samples treated on immobile sample holder. Ecor also increases when increasing the rotation speed of the samples during plasma treatment.

8 6 4 2 0 0

5

10

15

20

25

30

35

40

45

50

55

60

t (min) Fig. 6. Corrosion current density (lA/cm2) of AISI 1018 treated samples in 0.5 M NaCl, pH = 6.4 as function treatment duration. Rotation speed of sample under arc discharge: (a) 0 rpm, (b) 2400 rpm, and (c) 3100 rpm.

-2

Log(J(A/cm2))

-3

-4

-5

-6

-7

E(V/SCE) -8 -0.8

-0.6

non treated

-0.4

N= 0 rpm

N= 2400 rpm

Fig. 4. Voltammograms of AISI 1018 steels treated under plasma for 30 min. 0.5 M NaCl solution (pH= arc discharge: (b) 0 rpm, (c) 2400 rpm, (d) 3100 rpm, and (a) non-treated.

-0.2

N= 3100 rpm

6.4). Rotation speeds ( rpm) of sample under

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rosion potential Ecor does not clearly increases for immobile sample treated by plasma. However, we observe a clear decrease of the Jcor values and an increase of the Ecor values when rotating the sample under the gliding discharge. Fig. 5 shows that the corrosion potential increases with treatment duration to a first maximum after 5 or 10 min, then slightly decrease and then increases to a maximum after 30 min of treatment. The decrease of Ecor after the first maximum coincides with an increase of Jcor for the same treatment duration. This observation was already made by different authors [11,14] and is consistent with the behaviour of an oxide film growing on a metal surface exposed to corroding gas. The behaviour of Ecor and Jcor as functions of the exposure time to the discharge suggests the formation of a protective film, which first causes an increase in the corrosion potential and a decrease in the corrosion current. The film uniformly and progressively covers the surface of the sample exposed to plasma. After 5 or 10 min of treatment, the oxide/hydroxide begins to crack, due to differences in the crystal lattice parameters between the metal and the (hydr)oxide, or due to temperature increase [13]. It is assumed that cracks on the film expose the metal to the corroding solution, which leads to a slight decrease of corrosion potential and increase of corrosion current. For longer exposure times to the discharge, Ecor increases to a maximum value, and Jcor falls to a minimum: this suggests an optimisation of the covering of the surface, or a physical–chemical transformation of the oxide/hydroxide layer. Table 1 sums up the improvement of anticorrosion parameters, such as the corrosion current density and the corrosion potential by plasma treatment as functions of the exposure time to the discharge for different rotation speed of the sample. The protection represents the difference in percentage between the corrosion current of untreated sample and that of the plasma treated sample   J corrt¼0  J corrt¼x Protectiont¼x ¼ 100  J corrt¼0

The increase of protection percentage upon rotation of the treated surface implies an improved contact between the plasma and the surface. The rotation of the sample creates a vortex above its surface, which increases the contact between the active species of the plasma and the sample surface. The reactive plasma gas is not deviated before it has reached the surface, as it is the case for an immobile sample. Moreover, an enhanced homogenized treatment of the sample results. A rotating sample exposes all the regions of its surface to the incident flux of active species composing the plasma gas, improving the quality of the barrier film compared to that of an immobile sample under plasma. 3.2.4. Effect of pH of the solution on the polarization curves of treated samples The effect of pH on the electrochemical behaviour of steels samples treated for 30 min was evaluated at neutral and basic pH, using sodium chloride (NaCl 0.5 mol L1) and nitrate solutions (NaNO3 102 mol L1). Figs. 7–9 show the relevant polarization curves. In 0.5 mol L1 NaCl solution (Fig. 7), the corrosion potential of plasma treated mild steel does not change with pH in the considered acidity range, but corrosion current density decreases as the pH increase. At pH = 11.8, the voltammogram presents a large passive domain of about 318 mV width (from 500 mV to 182 mV/SCE). The apparition of this large domain is due to the presence on the metal surface of an oxide/hydroxide layer that is more stable at basic pH (Fig. 7). Similar experiments were performed in another electrolyte and a non-complexing medium was selected, e.g., 0.01 mol L1 NaNO3, pH = 6.5 solution (Fig. 8). The corrosion potential Ecor of plasma treated sample increases and the corrosion current density decreases as pH increases. The curves also show a larger domain of passivity close to 880 mV (i.e., from 284 mV/SCE to +628 mV/ SCE at pH 11.9). The Ecor of the 30 min plasma treated mild steel shifts toward anodic potentials, by 82 mV at pH = 6.5 and 154 mV at pH = 11.8. At basic pH, the width of the passive region increases from 300 mV (for the untreated sample) to 500 mV for the plasma treated sample. This agrees with the occurrence of a passive layer

High protection percentage is more rapidly reached on rotating samples. A decrease in Jcor by more than 88% is obtained for 30 min exposures. That proves the efficiency of glidarc treatment.

Table 1 Corrosion potential and current density of AISI 1018 carbon steel in 0.5 mol L1 NaCl solution, pH = 6.4 after gliding arc plasma treatment for different exposure times and various sample rotation speeds Sample rotation speed (rpm)

Treatment duration (min)

0

0

Ecor(mV/SCE) Jcor (lA cm2) Protection (%)

680 9.3 0

2400

Ecor(mV/SCE) Jcor (lA cm2) Protection (%)

680 9.3 0

3100

Ecor(mV/SCE) Jcor (lA cm2) Protection (%)

680 9.3 0

2

5

10

652 4.1 55.5

632 7.2 22.9

552 2.9 69.4

532 2.9 69.3

554 2.1 77.3

614 5.7 38.9

542 2.3 75.9

536 1.9 80.1

15

20

30

40

60

600 8.5 9.1

632 6.3 32.2

698 1.7 82.1

666 3.9 58.4

514 1.5 83.4

526 3.4 63.9

518 1.0 88.8

 508 1.2 87.6

496 0.7 92.1

554 4.2 55.1

508 2.6 71.6

474 0.6 93.5

 472 0.6 93.8

464 0.5 94.3

F. Depenyou Jr. et al. / Corrosion Science 50 (2008) 1422–1432

1427

-1

NaCl 0.5 mol L-1 -2

Log(J(A/cm2))

-3

b -4

c

PH6.4; t=0 PH6.4; t=30

-5

PH7.6; t=30

d

PH11.8 ; t=30

-6

a

-7

E(V)/SCE

-8 -0.8

-0.6

-0.4

-0.2

0.0

1

Fig. 7. Polarization curves in 0.5 mol L NaCl of AISI 1018 steels treated under plasma for 30 min. Rotation speed: 2400 rpm. Electrolyte: 0.5 M NaCl (a) non-treated, pH = 6.4, (b) pH = 6.4, (c) pH = 7.6, and (d) pH = 11.8.

3E-05

(NaCl 0,5 mol.L-1 + NaOH 0,006 mol.L-1). pH = 11,8

pH 11,8 t= 0 pH11,8 t=30

2E-05

t = 30 min

t = 0 min

2E-05

242 mV

1E-05

5E-06

365 mV

t -0.70

u

-0.65

-0.60

-0.55

-0.50

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

0E+00 -0.10

-0.15 J (A/cm2)

90 mV E(V)/ECS

-5E-06

-1E-05

Fig. 8. Voltammograms in 0.01 mol L1 NaNO3 of AISI 1018 steels plasma treated for 30 min. Rotation speed: 2400 rpm. Electrolyte: NaNO30.01 mol L1. (a) untreated, pH = 6.5, (b) plasma treated for 30 min, pH = 6.5, (c) Untreated, pH = 11.9, and (d) plasma treated for 30 min, pH = 11.9.

(NaNO3 0,01 mol.L-1 + NaOH 0,006 mol.L-1). pH = 11,9

t = 0 min

pH 11,9 t=0 pH 11,9 t=30

7.0E -06

5.0E -06

t = 30 min 300 mV

3.0E -06

500 mV 1.0E -06

x -0.6

y -0.4

-0.2

0.0

154 mV E(V)/ECS

0.2

0.4

0.6

0.8 -1.0E-06

J(A/cm2)

-0.8

-3.0E-06

-5.0E-06

Fig. 9. Polarization curves of AISI 1810 steel exposed to the discharge for 30 min. Rotation sped:2400 rpm. Electrolyte 0.01 mol L1 NaNO3 (pH:11.9).

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stable ascribed to FeO and other Fe(II) compounds at the surface. Hydrolysis in alkaline medium yields Fe(OH)2 or other Fe(II) oxides or hydroxides. At basic pH, the nitrate ions NO 3 may be reduced by iron and iron oxide/hydroxide to form a stable oxide/hydroxide compound. The following reaction scheme was proposed to explain the denitrification in basic nitrate solutions by iron [15]:  8FeðOHÞ2 # þNO 3 þ 6H2 O ! NH3 þ FeðOHÞ3 # þOH

The protective iron (II) oxide/hydroxide layer formed under plasma tends to stabilize in the presence of nitrate ions by formation of iron (III) hydroxide. Other reactions were proposed to explain passivating of carbon steel by basic NaNO3 solution in the presence of nitrite [16–18].

80

AISI 1018

 Fe þ NO 3 þ H2 O ! FeðOHÞ2 þ NO2 FeO þ H2 O ! FeðOHÞ2  3FeðOHÞ2 þ NO 3 ! Fe3 O4 þ NO2 þ 3H2 O  Fe3 O4 þ 2H2 O þ NO 2 ! 3FeOOH þ NO þ OH  2Fe3 O4 þ 2NO 2 þ H2 O ! 3Fe2 O3 þ 2NO þ 2OH

3.3. Contact angle to water Contact angles were measured on plasma treated samples and plotted as a function of the exposure time to the discharge. Fig. 10 shows that the contact angle decreases and tends to a steady value for increasing exposure times: this feature suggests that polar species form on the surface and are responsible for the increase in wettability. For exposure times longer than 10 min, the contact angle is almost constant, which suggests that the surface is totally covered by the hydrophilic passive layer. Increasing treatment duration would lead to the transformation of compounds on the surface [14].

70

angle (°)

60

3.4. Linear cathodic polarization

50 40 30 20 10 0 0

5

10

15

20

25

30

35

40

45

50

55

60

t (min)

AISI 1018

Fig. 10. Contact angle (°) to water of AISI 1018 treated samples as function of the exposure time to the discharge.

-1.3

-1.2

-1.1

-1.0

-0.9

Polarization curves of plasma treated samples were recorded in 0.025 mol L1 NaOH solution (pH = 12.6; scan rate 1 mV min1) for various exposure times to the discharge (Fig. 11). The height of the cathodic polarization wave increases with the exposure time as the reduction wall attributed to water reduction progressively shifts from 1.1 V/SCE to about 1.25 V/SCE. Theses features agree with an increasing coating of the sample surface by oxide/hydroxide moieties with the exposure time to the discharge. For 30 min treatments, the cathodic polarization curves show a large reduction band, from 0.8 V/SCE to 1.0 V/

-0.8

-0.7

-0.6

-0.5 -.40 0.0E+00

E(V)/SCE

J(A/cm2)

-5.0E-06

-1.0E-05

t = 0 min t = 30 min t= 40 min t = 60 min

-1.5E-05

Fig. 11. Cathodic polarization curves in 0.025 mol L1 NaOH solution, pH = 12.6, of AISI 1018 steels treated under plasma. Surface rotation speed under plasma: 2400 rpm. Scan rate 1 mV min1. Exposure times to discharge: (a) t = 0 min, (b) t = 30 min, (c) t = 40 min, and (d) t = 60 min.

F. Depenyou Jr. et al. / Corrosion Science 50 (2008) 1422–1432

SCE, centred close to 0.9 V/SCE. Many iron oxide/ hydroxides are reduced in this potential range, e.g., Fe2O3/Fe(0), Fe3O4/Fe(0), Fe(OH)2/Fe(0) or FeOOH/ Fe(OH)2 as showed in Table 2. A less broad but more intense reduction band, centred at 0.86 V/SCE, appears for 40 min treatment. A broad peak centred at 1.17 V/ ECS appears for 60 min treatment, consistent with the reduction of Fe(OH)3 to Fe(II). The intensity of the peak centred at 0.86 V/SCE increases with treatment time and follows the relation:

Table 2 Experimental and theoretical reduction Potentials of some iron oxides/ hydroxides in NaOH solution [4] Couple observed

Ered (V/ECS)

Etheo (V/ECS)

Fe2O3/Fe(0)

0.86

0.87

Fe3O4/Fe(0) Fe(OH)2/Fe

0.95

0.95

Fe2O3/FeO

1.01

0.97

Fe(OH)2/Fe (0)

1.05

1.05

Fe3O4/Fe(0)

1.08

FeOOH/Fe(OH)2 Fe(OH)3/Fe(II)

1.17

1.18

FeOOH/Fe(0)

0.75 0.97

0.78 0.95

FeOOH/Fe(OH)2

0.77 0.90 1.18 1.21

0.8 0.90 1.18 1.2

-1.30

-1.20

-1.10

1.00

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jJ j ¼ 0:1134t þ 3:1108;

R2 ¼ 0:9111

The relevant electronic transition can be attributed to the reduction of Fe2O3 to Fe(0). However, the height of the reduction wave linearly increases with exposure time in the range 0–60 min, but tends to be steady for treatments at least longer than 150 min (Fig. 11). The potential of the water reduction wall however progressively shifts toward more reductive poten-0.90

-0.80

-0.70

-0.60

t = 150

t = 60

J(A.cm-2)

E(V)/SCE

-0.50 0E+00 -1E-06 -2E-06 -3E-06 -4E-06 -5E-06 -6E-06 -7E-06 -8E-06 -9E-06 -1E-05 -1E-05 -1E-05 -1E-05 -1E-05 -2E-05 -2E-05 -2E-05 -2E-05

t=0

Fig. 12. Voltammograms of AISI 1018 steels treated under plasma. Rotation speed: 2400 rpm. Electrolyte: 0.025 mol/L NaOH solution, pH = 12.6; scan rate 1 mV min1: t = 0 min (a), t = 60 min (b), and t = 150 min (c).

-F e (1 1 0 ) -F e (2 2 0 )

Intensity (a.u.)

-F e (2 0 0 ) In cid e n ce ra sa n te (1°)

-F e (1 1 1 )

E 5

In cid e n ce n o n ra sa n te (15°- 60°) : co n fig u ra tio n B ra g g -B re n ta n o

30

40

-F e (2 0 0 )

50

60

-F e (3 1 1 )

-F e (2 2 0 )

70

80

90

100

110

-F e (2 2 2 )

120

2θ (°) Fig. 13. X-ray diffractogram of the AISI 1018 carbon steel treated by plasma for 5 min (E5).

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tials (Fig. 12). It is close to 1.20 V/SCE for untreated mild steel, 1.25 V/SCE for 60 min treated samples and 1.30 V/SCE for 150 min exposure to the discharge. For long treatments, compounds at the surface tend to transform into more stable species which are hardly reduced at the electrode

3.6. Mo¨ssbauer spectroscopy Mo¨ssbauer spectra of plasma 150 min treated (E150) and untreated (E0) samples were recorded at room temperature. The analysed depth is close to 10 nm. The spectrum of the E0 sample is characteristic of a AISI 1018 steel. It shows contributions from austenite

3.5. X-rays analysis Velocity (mm/s)

The mild steel samples were analysed with a Bruker D8 diffractometer. Various analyses were performed with variable incident angles fixed in the 15–60° range according to a Bragg–Brentano (h–2h) setting, i.e., a configuration which enables investigating the upper 5 lm layer of the sample. Then, the incident angle was set to low values (1° and 3°) to reduce the analysed depth and to increase the contribution of the surface in the diffractogram. Figs. 13 and 14 report the X-rays spectra of the 30 min plasma treated samples. All the results obtained in Figs. 13 and 14 are coherent. Diffractograms recorded in Bragg–Brentano configuration show the peaks of austenite c-Fe. Analyses realised with small incident angle (1°) show that the surface of the plasma treated AISI 1018 steel is enriched in ferrite phase a-Fe. These results suggest that austenite present at the solid surface transforms into ferrite during the plasma treatment. This observation is consistent with the following reaction: c  Fe þ H2 O þ O2 þ e

!

-10

!

Absorption (%)

a  FeOOH þ OH

1.00 1.09

Untreated AISI 1018 carbon steel (E0) Paramagnetic contribution from austenite γ-Fe δ = - 0,10 ± 0,01 mm/s Δ = 0,15 ± 0,01 mm/s 61 % of the spectrum surface area

a  Fe2 O3

However, there is not a clear difference between the diffractograms of treated steels, whatever the treatment duration is. The accuracy of the X-rays diffractometry technique is limited in measuring the influence of the plasma treatment on the surface change of the steel. This implies that a passive film is first formed during the sample polishing. Exposing to the discharge then induces the formation of a thin (few nanometers) passive layer [17,18], which is mainly composed of amorphous or not well crystallized oxides/ hydroxides.

Paramagnetic contribution from ferrite α-Fe δ = - 0,02 ± 0,02 mm/s 2ε = - 0,04 ± 0,03 mm/s B = 25,5 ± 0,2 T 39 % of the spectrum surface area Fig. 15. Mo¨ssbauer spectra at room temperature of the untreated AISI 1018 carbon steel (E0). Contributions from the two phases present at the surface are displayed.

-F e (1 1 0 )

-F e (2 2 0 )

Intensity (u.a.)

-F e (2 0 0 ) I n ci d e n c e ra sa n te (1 °)

I n c i d e n c e ra sa n te (3 °)

-F e (1 1 1 )

E 150

I n c id e n c e n o n ra sa n te (1 5 °- 60 °) : c o n fi g u ra tio n B ra g g -B re n ta n o

30

40

-F e (2 0 0 )

50

60

+10

1.00

At high temperature a  FeOOH

0

1.09

-F e (2 2 0 )

70

80

90

-F e (3 1 1 )

100

110

-F e (2 2 2 )

120

2θ (°) Fig. 14. X-ray diffractogram of the AISI 1018 carbon steel treated by plasma for 150 min (E150).

F. Depenyou Jr. et al. / Corrosion Science 50 (2008) 1422–1432

c-Fe and ferrite a-Fe. The austenite is adjusted with a singlet, corresponding to a paramagnetic contribution. The ferrite is adjusted with a sextuplet, corresponding to a magnetic contribution. These contributions can be observed in Fig. 15. The adjustment is coherent with X-rays analyses showing that the studied steel contains mostly austenite, with an important proportion of ferrite at the surface. The spectrum of the sample treated for 150 min (E150) is adjusted with the contributions of austenite (c-Fe) and ferrite (a-Fe). The spectra of samples E0 and E150 are compared in Fig. 16. A supplementary paramagnetic contribution constituted by a doublet was adjusted in order to take into account a slight broadening at the basis of the austenite contribution. Hyperfine parameters of this contribution correspond to iron oxyhydroxides such as FeOOH. However, the very low Mo¨ssbauer intensity of this signal is close to the detection limit of the analysis. An acceptable adjustment of the spectrum can be obtained without this contribution As observed for the treatment of stainless steels [5] the humid air plasma interacts with the steel surface and mod-

Velocity (mm/s) -10

0

+10

Absorption (%)

1.09

1.00 1.08

1.00

Comparison of samples E0 and E150 Sample E0 (upper) austenite : 61 % ferrite : 39 % Sample E150 (lower) austenite : 63 % ferrite : 35 % FeOOH : 2% Fig. 16. Mo¨ssbauer spectra at room temperature of the AISI 1018 carbon steel. Untreated sample (E0) and sample treated during 150 min ( E150). The oxyhydroxide FeOOH contribution is shown on the spectrum of the sample E150.

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ifies the phase distribution, or transforms the (a-Fe) phase into an oxide. The ratio of the (a-Fe) phase in the untreated AISI 1018 mild steel is 39%, while for 150 min exposure time to the discharge, it falls down to 35%. This feature may suggest that of the (a-Fe) phase transforms into the (c-Fe) phase, as observed lepidocrocite (c-FeOOH), or the formation of an Iron oxide. The latter suggestion is more likely owing to the appearance of reduction peaks in the electrochemical investigation of the treated surfaces. The Mo¨ssbauer spectra of treated and untreated samples are not very different, which may be related to the limited changes in the surface composition resulting from the plasma treatment and to the intrinsic sensitivity of the method.

4. Conclusion Using a gliding discharge technique for the treatment of carbon steel provided interesting results which are directly related to the chemical properties of the instable species formed in the discharge. The highly oxidative properties of the OH radicals alter the surface sample and form more resistive barrier layers which improve the corrosion potential and dramatically decrease the corrosion current for 30 min plasma treatment. The contact angle measurement showed that the hydrophilic character of the mild steel surface was largely increased, which confirmed the effects already observed for AISI 304 stainless steel [19]. This hydrophilic character is related to the presence of a passive film, involving polar species at the surface. Polarization curves showed that the presence of a passive film increased the corrosion potential and reduced the corrosion current density in 0.5 mol L1 NaCl solution. Better corrosion protection was obtained on sample exposed on a rotating sample holder, which confirms the positive effect of sample rotation on improving the plasma-surface interaction. The large passive domain of the treated samples showed by the polarization curves in 0.01 mol L1 NaNO3 at pH: 11.8 suggest the presence of Fe(II) oxides or hydroxides, as Fe3O4 and Fe(OH)2. Cathodic polarization curves recorded in 0.025 mol L1 NaOH pH: 12.8 showed reduction peaks that are tentatively assigned to compounds such as Fe(OH)3, FeOOH, Fe3O4, Fe2O3. The presence of lepidocrocite FeOOH on treated samples was also suggested by Mo¨ssbauer spectroscopy. X rays analysis showed the presence of a very thin surface layer enriched in ferrite (a-Fe) phase. X rays analysis and the decrease of the reduction wave intensity in NaOH suggest that the difficult reducible compound observed on the plasma treated surface for long time (150 min) could be a-Fe2O3. Identification of the hydroxides/oxides formed, which are likely Fe(OH)3, FeOOH, Fe3O4 or Fe2O3, needs confirmation by more sensitive surface techniques.

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Acknowledgements The authors are grateful to the French Government for a Grant attributed to one of them (F. Depenyou Jr.). This financial support allowed performing the experimental part of F. Depenyou’s co-directed Thesis (University of Yaounde-I, Cameroon and University of Rouen, France) and strengthening the co-operation research program between these universities. References [1] J. Fanmoe, J.O. Kamgang, D. Moussa, J.-L. Brisset, Application de l’arc glissant d’air humide au traitement des solvants industriels: cas du 1,1,1-trichloroethane, Phys. Chem. News 14 (2003) 1–4. [2] F. Abdelmalek, S. Gharbi, B. Benstaali, A. Addou, J.-L. Brisset, Plasma chemical degradation of azo dyes by humid air plasma: yellow Supranol 4 GL, Scarlet Red Nylosan F3 GL and industrial waste, Water Res. 38 (2004) 2339–2347. [3] D. Moussa, J.-L. Brisset, E. Hnatiuc, G. Decobert, Plasma-chemical destruction of Trilaurylamine from nuclear laboratories of reprocessing plants, J. Ind. Eng. Chem. Res. 45 (2006) 30–33. [4] M. Moreau, M. Feuilloley, N. Orange, J.-L. Brisset, Lethal effect of the gliding arc discharge on Erwinia spp, J. Appl. Microbiol. 98 (2005) 1039–1046. [5] B. Benstaali, A. Addou, J.-L. Brisset, Electrochemical and X-rays investigation of austenitic 304L and 316L stainless steels treated by a gliding arc in humid air, Mater. Chem. Phys. 78 (2002) 214–221. [6] N. Bellakhal, K. Draou, J.-L. Brisset, Plasma and wet oxidation of 63Cu37Zn Brass, Mater. Chem. Phys. 73 (2002) 235–241. [7] A. Anders, Plasma and ion sources in large area coating: a review, Surf. Coat. Technol. 200 (2005) 1893–1906. [8] B.J. Larson, J.M. Helgren, S.O. Manolache, A.Y. Lau, M.G. Lagally, F.S. Denes, Cold-plasma modification of oxide surfaces for covalent biomolecule attachment, Biosen. Bioelectron. 21 (2005) 796–801.

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