tap water filtrate

Author’s Accepted Manuscript On the tribocorrosion behavior of 304L stainless steel in olive pomace/tap water filtrate Fatma Ben Saada, Zied Antar, Khaled Elleuch, Pierre Ponthiaux www.elsevier.com/locate/wear

PII: DOI: Reference:

S0043-1648(15)00195-7 http://dx.doi.org/10.1016/j.wear.2015.03.023 WEA101391

To appear in: Wear Received date: 8 October 2014 Revised date: 24 March 2015 Accepted date: 28 March 2015 Cite this article as: Fatma Ben Saada, Zied Antar, Khaled Elleuch and Pierre Ponthiaux, On the tribocorrosion behavior of 304L stainless steel in olive pomace/tap water filtrate, Wear, http://dx.doi.org/10.1016/j.wear.2015.03.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

On the Tribocorrosion behavior of 304L stainless steel in olive pomace/tap water filtrate

Fatma Ben Saadaa, Zied Antara, Khaled Elleucha, Pierre Ponthiauxb a

Laboratoire de Génie des Matériaux et Environnement, LGME, Engineering school of Sfax, ENIS, University of Sfax B.P.X., 1173-3038 Sfax-Tunisia. b

Ecole Centrale Paris, Lab. LGPM, F-92290 Chatenay-Malabry, France

Corresponding author:

Pr. Khaled ELLEUCH Tel.: +216 74 274 088

E-mail: [email protected]

Highlights ▶ Tribocorrosion tests were performed on stainless steel sliding against alumina. ▶ Both the tribological and electrochemical responses of the contact were studied. ▶ Increasing latency time (interrupted sliding) affected the tribocorrosion behavior. ▶ In intermittent sliding the wear track alternates between active and passive states.

Abstract: Tribocorrosion is a concern in the operation of centrifuges used in olive oil separation from seeds. An investigation was conducted on the tribocorrosion behaviour of AISI 304L stainless steel, a typical centrifuge material, sliding against alumina in a mixture of olive pomace and tap water filtrate. A specially-configured pin-on-disc tribometer was used to couple electrochemical analyses with wear tests. The active and passive surface states involved in the tribocorrosion mechanisms for the steel are discussed. Mechanical and corrosion contributions to wear, which acted on the active and repassivated areas of the wear track, were quantified. Abrasive wear dominated in the AISI 304L. It was also found that the steel was more sensitive to tribocorrosion under intermittent sliding than continuous sliding. Keywords: Tribocorrosion, stainless Steel, Wear, Friction, Intermittent sliding.

1. Introduction Tribocorrosion is a form of solid surface alteration that involves the joint action of relatively moving mechanical contact with chemical reaction in which the result may be different in effect than either process acting separately [1]. It leads to a degradation phenomenon of material surfaces such as wear, corrosion and cracking. These degradation processes induce significant problems in olive oil industry, especially for maintenance. One of the steps in the olive oil extraction process is the physical separation of olive seeds from olive pulp by means of a horizontal centrifuge equipped with four metal rotary raclettes and a perforated cylinder made of AISI 304L (Figure 1.a.). The rotating speed of the raclettes varies from 1500 to 3000 rpm according to the designer of the machine. The centrifugal force applied by the seed particles on the raclettes is about 44 N. It was noticed that the raclettes are badly damaged after a short service period. In fact, mechanical wear and corrosion are the two main types of damage of the pit remover raclettes (figure 1.b.). The synergism between wear and corrosion may lead to an acceleration of the metals degradation. Therefore, it is important to identify the contribution of corrosion and wear to material removal in the tribocorrosion process in order to evaluate the material performance [2,3,4]. Perforated cylinder

(a)

Raclette

(b)

Figure 1. Horizontal centrifuge for pit removal before (a) and after (b) service period.

In literature, many studies on tribocorrosion have been performed to evaluate the combined action of wear and corrosion on engineered stainless steel components since 1983 by Noel and Ball [5]. Many previous studies to evaluate that combined action had been discussed earlier. The wear mechanism of passive film and the mechanistic models on tribocorrosion were reported by S. Mischler et al, [6], Stemp et al [7], P. Ponthiaux et al [8], J. P. Celis et al [9], Mischler [10] and Stojadinovic et al [11]. In a recent study, a new protocol which combines the sliding action and corrosion has been established in order to assess reliably the tribocorrosion behavior and the chemical-mechanical degradation of passive metallic materials in an aggressive medium. Indeed,

the electro/chemical growth of surface films

(passive films) between successive mechanical contacts at a given point on the wear track was the main reason of materials degradation [12,13]. One of the most frequently studied materials in tribocorrosion works is the stainless steel known for its passive behavior. A great number of controlled tribocorrosion experiments were carried out on stainless steel alloys immersed in H2SO4 [12,14-16,17-20] and NaCl solutions [14,20-24]. The aim of this work was to study tribocorrosion behavior [12,13] of AISI 304L immersed in olive pomace/tap water filtrate with pin on disc tribometer. The composition of this olive suspension is similar to that of the primary paste of the seed remover. The remaining of this paper was organized as follows: In section 2 we detailed our experimental study on sample preparation, testing devices required (potentiostat, reference and counter electrodes), tribological test conditions (applied load, frequency, rotational speed, track size), and electrochemical techniques (open circuit potential measurements, electrochemical impedance spectroscopy conditions). Section 3 was devoted to revealing our results and discussion. Tribocorrosion wear contributions were quantitatively performed and a qualitative approach was carried out to characterize the wear track. Finally main conclusions were drawn.

2. Experimental details

2.1.

Specimens and surface preparation

The chemical composition of AISI 304L is shown in Table 1. Cylindrical samples with a height of 10 mm and a diameter of 25 mm were used. They were mechanically polished until getting a final roughness (Ra) of about 0.11 µm. After grounding the samples were ultrasonically cleaned in acetone for 5 min and then dried. Table 1. Normalized chemical composition of AISI 304L [25]

Elements Chemical composition

C

Si

Mn

P

S

Cr

Ni

≤0.02

≤1

≤2

≤0.04

≤0.03

17-18

9-11

[wt%]

2.2.

Electrolyte : olive pomace/tap water filtrate

The test solution was prepared from the olive pomace in a solid state as produced by olive oil industry (Figure 2). A suspension was prepared mixing the olive pomace with tap water. After 24 hours, solid particles were separated from liquids by filtration through a paper filter to get the test solution. The average pH value of the prepared solution, used for the electrochemical and tribocorrosion tests, was about 5. The amount of chloride and sulfate in the filtrate is shown in Table 2.

Figure 2. Olive pomace

Table 2. Concentration of chlorides and sulfates in the prepared filtrate [Cl-]

[SO42-]

(mol/l)

(mol/l)

0.027

0.012

pH

5

2.3.

Potentiodynamic polarization measurements

The tests were performed using a cell which contains 100 ml of electrolyte (pomace olive / tap water filtrate) at 25°C. A three electrode set-up was used with a 1286 potentiostat and a 1250 Frequency Reponse Analyser Solartron. The working electrode consists of AISI 304L coated with resin to get a working area of 4.52 cm2. A platinum electrode was used as a counter electrode whereas the reference one was an Ag/Ag-Cl/KCl saturated electrode characterized by EAg-AgCl = +0.200 V/Standard hydrogen electrode (SHE). Potentiodynamic cyclic polarization curves were obtained for a scan rate of 0.5 mV.s-1 after immersing the sample three hours in the electrolyte and without sliding. Before starting the potential scan, cathodic pre-polarization of 5 min was performed at -1.5 V/Ag-AgCl to clean the working electrode surface.

2.4.

Tribocorrosion tests

The sliding test was combined with in situ electrochemical measurements which included free corrosion potential (Ecorr) as a time function and electrochemical impedance measurements. The same amount of electrolyte and the same cell were used during all measurements. The corrosion tests were performed on a pin-on-disc tribometer using the same potentiodynamic polarization measurement cell. A 7-mm diameter alumina pin with a 100-mm radius spherical tip was selected as a counterbody. A normal force and rotation speed of 5 N and 0.0628 m/s were applied on the contact to have 10 mm of wear track. Applying the moduli of elasticity

(Young’s moduli) and Poisson’s ratios of the pin and plate materials (alumina: 390 GPa, 0.23 and AISI 304L steel: 190 GPa, 0.3, respectively) a Hertz contact stress of 82 MPa and the corresponding contact radius of 0.14 mm were calculated. It is important to note that the chosen pin-on-disc tribometer allow just an approximate quantification of the wear rate in the pomace olive centrifuge. Two sliding types were performed, namely continuous and intermittent sliding tests. Under continuous sliding, the pin was animated by a permanent rotation during the whole test. To evaluate the combined action of corrosion and mechanical wear, an intermittent sliding was performed. The pin rotates for one period of time tr= 0.5 s .Then, the pin was kept immobile for another given period of time, tstop= 2 s. The length of the stop was selected to produce the same time between two successive seed impacts in the same location of the centrifuge raclette. The latency time of one cycle, tlat, corresponds to the time separating two successive contacts, so that tlat = tr + tstop [12, 13]. 3. Results and discussion 3.1.

Corrosion tests without mechanical loading

The polarization curve of AISI 304L immersed in olive pomace/tap water filtrate is presented in Figure 3. The polarization curve shows four domains where the potential increases: the cathodic domain below corrosion potential Ecorr; the narrow domain at the vicinity of Ecorr (transition from cathodic to anodic current); the large passive domain and the anodic domain. It can be seen that there is a considerable difference of about 1 V/Ag-AgCl between pitting (Epit) and corrosion (Ecorr) potential. Chen et al. found a similar behavior, with a narrower passive domain, when they established potentiodynamic polarization (PDP) curve of 316L stainless steel immersed in 3.5% sea water (pH=8.2) [2]. Moreover, a large hysteresis which indicates the disruption of a passive film by pitting corrosion [26] was observed when the potential decreases. These results were confirmed by an optical microscopy examination that

showed a sample surface damaged by pitting corrosion, Figure 4. Also, some additional information about the surface topography, shown on Figure 5 and Table 3, was determined by profilometry. 10 Log(i) (mA/cm2)

1

Eprot

0.1 0.01 0.001

Epit

0.0001 1E-05

Ecorr

1E-06 1E-07 -2

-1

0 E (V/Ag-AgCl)

1

2

Figure 3. Cyclic polarization curve of AISI 304L: Epit = pitting potential; Eprot = protection potential.

Figure 4. Pitting corrosion on the sample surface after polarization test.

Figure 5. 3D dimensional image of pits

Table 3. Pit parameters Pit parameters Mean values

3

width (µm)

Depth (µm)

Volume (µm )

Ra (µm)

13

40

1376

6,86

The evolution of the open-circuit potential OCP, shown in Figure 6, provides information about the electrochemical reactivity of the sample surface. 0

E (V/Ag-AgCl)

-0.05 -0.1 -0.15 -0.2 -0.25

treac

-0.3 -0.35 0

2000

4000

6000

8000

10000

12000

Time (Sec)

Figure 6. Evolution of open circuit potential of AISI 304L without mechanical loading. An increase of the potential in the noble direction is required to achieve a passive film growth on the surface reflecting a balance between anodic and cathodic reaction. The evolution of the open-circuit potential EOCP over immersion time indicates that a stable passive surface state is reached on the AISI 304L after 3600 s (treac) when EOCP variation is smaller than 60 mV.h-1. 3.2.

AISI 304L tribocorrosion behavior. 3.2.1. Corrosion tests under mechanical loading

Sliding tests were started when the open-circuit potential (OCP) was stabilized. The results are given in Figures 7 and 8. These finding show the OCP and friction coefficient variation with the cycle numbers.

Friction applied

0.00

Intermittent sliding Continuous sliding

EOCP (V/Ag-AgCl)

-0.10 -0.20 -0.30 -0.40 -0.50 -0.60 0

1000

2000

3000

4000

Cycle numbers

Figure 7. Evolution of the open-circuit potential, before and during continuous and intermittent sliding tests performed at 5 N and 0.0628 m/s on AISI 304L immersed in filtrate

Friction coefficient µ

at 25 ◦C. 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

Intermittent sliding Continuous sliding

0

500

1000

1500

2000

2500

3000

3500

Cycle numbers

Figure 8. Friction coefficient as a function of cycle numbers of AISI 304L/ alumina ball contacts under continuous and intermittent sliding. At the beginning of sliding, the open-circuit potential drops down sharply to more negative potential values revealing a modification of the surface state in the sliding track area. Under continuous sliding, the evolution of potential versus time is similar to that observed under intermittent sliding with more negative potentials. By comparing friction coefficients with EOCP (Figures 7 and 8), we can deduce that the corrosion state in the contact area of the material affects the tribological conditions. The high friction coefficient 0.19 is obtained with the more negative potentials (E = - 0.37 V/Ag-AgCl) under continuous sliding. This indicates the local destruction of the passive film in the contact area that increases the corrosion of the

material. The lowest coefficient of friction 0.16 is noticed at an intermittent sliding when the EOCP is more important (E = - 0.33 V/Ag-AgCl). The increase of EOCP can be explained the effect of repassivation in the contact area during each cycle stop time. The passive layer acts as a third body in the contact to generate a solid lubricant [27], which might the decrease of the friction coefficient. The effect of friction on the corrosion behavior of the material is shown in Figure 9 revealing a superposition of friction coefficient and open circuit potential evolutions during one cycle. During continuous sliding, the EOCP and µ are stable. In fact the pin was rotated during all the test time to generate a permanent damage of the sliding track. During intermittent sliding with an off time toff of 2 s, the evolution of EOCP is opposite to µ. The EOCP decreases during trot due to mechanical removal of the oxide layer. The repassivation inside the sliding track takes place during toff inducing a rise of the EOCP, where the decline of µ is detected. µ, intermittent E, intermittent

-0.3

0.4

-0.31

0.35

-0.32

0.3

-0.33

0.25

-0.34

0.2

-0.35

0.15

-0.36

0.1

-0.37

0.05

-0.38 3000.4

3000.6

3000.8

3001

3001.2

Friction coefficient µ

E (V/Ag-AgCl)

µ, continuous E, continuous

0 3001.4

Cycle numbers

Figure 7. Evolution of open circuit potential during one cycle under continuous and intermittent sliding tests.

The surface state of AISI 304L was investigated by an electrochemical impedance spectroscopy performed when a stable open-circuit potential was reached. The impedance measurement was performed under continuous sliding. In fact the measurement of the

electrochemical impedance was performed using a sinusoidal voltage signal with a small amplitude of 20 mV. This has the advantage of generating only a negligible fluctuation of EOCP of the sample and this is the case of the continuous sliding and not the intermittent one due to the instability of EOCP. The impedance diagram is presented in a Nyquist plane in Figure 10. With sliding -40000

Without sliding

0,1 Hz

Zim (Ohm cm2)

-35000 -30000 -25000 -20000 0,1 Hz

-15000

0,04Hz

-10000 1Hz

-5000 0 0

10000

20000

30000

40000

Zre (Ohm cm2)

Figure 8. Electrochemical impedance spectra measured at the mean open-circuit potential value during continuous sliding tests performed at 5 N and 0.0628 m/s on AISI 304L immersed in filtrate at 25 °C. The polarization resistance is directly related to the size of the single arc of circle (diameter of semi circle). The size of impedance diagrams differs by several orders of magnitude. Under sliding, the diagram is characterized by a low polarization resistance corresponding to a damaged surface in sliding track under continuous sliding [13]. The passivation current density ipass is calculated using equation 1 where Rp, A0, and B are the polarization resistance, surface area of 4.521 cm2 and a 26 mV constant, respectively.
 



 



The value of ipass of about 21 10-9 A/cm2 indicates that AISI 304L is covered with a dense Cr2O3 film in contact with the solution in the stationary state. Such a film is characterized by a high polarization resistance [14].

Based on the value of the polarization resistance of the active material Ract and the active area Aact under continuous sliding, the current density iact caused by the corrosion of the active material is given by the following expression [12]: i 

B 2 R  A

It assumed that during continuous sliding, the whole wear track is in an electrochemically active state. Despite the sliding track width increases progressively due to wear during sliding tests, a mean sliding track area ∗ is calculated as follows [12]:

∗       + 
! 1 With #$%  &#$% ', the maximum area value measured at the end of the test, #() 

&#() ', the minimum area value obtained at the end of the first cycle and, L the length of the sliding track . For Atrmax, the width of the sliding track, emax, measured at the end of the sliding

tests, is used. For Atrmin, the width emin is taken as the diameter of the Hemrtzian static contact area [12]: e+,-.

3F. R  2/ 4 4E

57 6

4

With Fn the applied normal load, R the radius of the tip of the curved counterbody, and E the equivalent elastic modulus given by: − : − :  + ; 8 8 8 With <5 and <= the Poisson’s ratios, and E1 and E2 the elastic module of the stainless steel test sample and alumina pin respectively. Aact is defined as the active area in the sliding track, and is characterized by a polarization resistance Ract. The area outside the sliding track, Ao−Aact, is considered to be in a passive

state characterized by a polarization resistance Rpass. Consequently, Rps is given by the following equation [12]:  + >       Rpass is calculated from the specific polarization resistance, rp, for the material in a passive electrochemical state with an open-circuit potential before sliding given as [12]:   

 

B  − ?∗@A

According to Equation 2, the corrosion current density iact is 64.5 10-6 A/cm2 considering that the active area $C  ∗ is 0.065 cm2 (Equation 3) and the specific polarization resistance Rps is 7100 Ohms. The current density of the active material iact is 1000 times higher that of the passive material ipass indicating an active material state due to the total destruction of the protective passive film in the sliding track of AISI 304L under sliding in filtrate. Moreover, a galvanic coupling between passive and active areas was established with the ridges acting as an anode inducing a mixed potential. Thus, the mixed potential corresponds to EOCP measured under friction and depends on the state (ratio) of unworn and worn areas in the sample surface in contact with the solution [9,12]. 3.2.2. Wear Analysis The total wear loss after sliding in filtrate could be calculated according to the typical tribocorrosion protocol [12,13]. N. Diomidis et al. described the total wear loss in the wear C track, Wtr, as the sum of four contributions according to Equation 8. Where D$C is material C loss due to corrosion of active material in the wear track, DEF$GG is the material loss due to

# corrosion of repassivated material in the wear track,D$C is material loss due to mechanical # wear of the active material in the wear track and DEF$GG is the material loss due to the

mechanical wear of repassivated material in the wear track.

C # C # D  D$C + D$C + DEF$GG + DEF$GG 8

After tribocorrosion tests, the total wear loss was determined by the profilometric measurements in the sliding track. The wear volume of the sliding track is D  I'. Where, L and S are respectively the length and the average area of the cross section of the sliding track, determined using a high-resolution optical profilometry. C # and DEF$GG are zero because the active material is the Under continuous sliding (cs), DEF$GG

only state on the sliding track due to the permanent material degradation in the contact. The C JK , material loss due to the corrosion of the active material D$C given by Equation 9 [13],

can be calculated relying on Faraday’s law for a given tribocorrosion test from the corrosion current density iact deduced from the polarization resistance by impedance measurement (Equation 2). C D$C JK  LM N$C $C ∆P 9

C Where D$C represents the specific wear per cycle (cm3/cycle) for the cycle duration ∆P equal

to tlat = 0.5 s. KF is the Faraday factor, as follows: LM 

1 TME UME TW UW TX( UX( Y5 / + + 4 10 RS VME VW VX(

With F the Faraday constant, S the density, xFe, xCr and xNi the molar weight, nFe, nCr and nNi the number of electrons, and MFe, MCr and MNi the molar mass suggested as follows: - F = 96500 C mol-1 - S = 7.9 g cm-3 - xFe = 0.72; xCr = 0.18; xNi = 0.10 - nFe = 2; nCr = 3; nNi = 2 - MFe = 55.8 g mol-1 MCr = 52 g mol-1 MNi = 58.7 g mol-1 # The material loss due to the mechanical wear of the active material, D$C , can be calculated

based on the wear track volume, Wtr, measured after the continuous sliding test:

# C D$C  D − D$C 11

Under intermittent sliding (is), the wear track can be decomposed into two distinct zones, namely: the active state area Aact corresponding to a fraction of the sliding track not covered by the passive film during sliding and the repassivated state area Arepass corresponding to the remaining sliding track area covered by a surface passive film. The area functions occupied by the two zones in the wear track vary with time between two successive contact events, P[$ . The fraction of the surface covered by the passive film

\]^_`aa \b]



is proportional to the ratio  c`b . ]^`d

The track area is:   $C + EF$GG 12 The material loss due to the corrosion of the active material after intermittent sliding, C  D$C NK , is calculated from Equation 13 based on the two sliding tests performed at two

different latency times tlat (cs) = 0.5 s and tlat (is) = 2.5 s [11]. C C D$C(G  D$CCG e

5Y)

$C (G P[$ (G fe f $C CG P[$ CG

13

Where Aact (cs) and Aact (is) are the active areas of the sliding track under continuous and intermittent sliding, respectively. Where n is taken equal to 0.7 according to previous studies on the tribocorrosion of passivating materials in sulfuric acid solution [12;28]. # , is The mass loss due to the mechanical wear of the active material in sliding track, D$C(G

# calculated from the value of D$CCG supposing that the mechanical wear is not affected by

latency time, given by the following equation from [12,13]: $C (G # # D$C(G  D$CCG e f 14 $C CG

C , is The material loss due to corrosion of repassivated material in the sliding track, DEF$GG

determined in analogy to Equation 9 supposing that the corrosion behaviors of the starting C passive film A0 and the repassivated fraction material Arepass are similar. DEF$GG is calculated

for the cycle duration ∆P equal to tlat (is) = 2.5 s and with the current density ipass under the repassivated surface Arepass, as follows [2,13]: C  LM NF$GG EF$GG ∆P 15 DEF$GG

# The material loss due to mechanical wear of repassivated material in the wear track, DEF$GG

can be calculated relying on the wear track volume, Wtr, measured after the intermittent sliding test: C # # C  D − D$C + D$C + DEF$GG

16 DEF$GG

The obtained values of the different active, passive and repassivated areas of wear track and of the relative contributions of the different wear contributions are shown in Tables 4 and 5, respectively. Then the different wear contributions are presented in Figure 11. Table 4. Active, passive and repassivated fraction areas of wear track. Latency time

#()

#$%

$C

EF$GG

[s]

[cm2]

[cm2]

[cm2]

[cm2]

0.5

0.047

0.068

0.057

0

2.5

0.047

0.398

0.222

1.5435 10-4

Table 5. Contributions of wear obtained on AISI 304L under continuous (tlat(cs) = 0.5 s) and intermittent (tlat(is) = 2.5s) sliding against alumina pins (5 N, 0.0628 m/s) in filtrate olive pit/tap water. Latency time

D

C D$C

# D$C

C DEF$GG

# DEF$GG

[s]

[cm3/cycle]

[cm3/cycle]

[cm3/cycle]

[cm3/cycle]

[cm3/cycle]

0.5

1.85 10-9

5.92 10-11

1.79 10-9

0

0

2.5

8.53 10-9

37.5 10-11

6.99 10-9

2.7 10-16

1.17 10-9

The total wear of the material under intermittent sliding is significantly larger than that under continuous sliding, due to the increase of tribochemical reactivity of wear track with filtrate.

The duration of the latency time affects the mechanical wear resistance of the active material. # In fact the active area mechanical wear D$C (Table 5) and active area Aact increases with tlat C (Table 4). The corrosion wear of the active area D$C is smaller compared to the total wear, C C even if it is larger than the corrosion wear of repassivated area DEF$GG . DEF$GG is negligible

compared to the total wear (Table 5). However, the mechanical wear of the repassivated # area DEF$GG represents by itself 13.7 % of the total wear under intermittent sliding.

Under continuous sliding, the wear track is at an electrochemically active state during the entire test. The wear debris consists mainly of fine metallic particles that are continuously created, ejected from the wear track by an abrasive mechanism and then oxidized by the electrolyte. In fact, discontinuous scratching marks and grooves are observed in the wear tracks (Figure 12). With the increasing sliding distance under continuous sliding, the amounts of wear debris increased with a stabilized wear.

Figure 9. Contribution of wears obtained on AISI 304L under continuous (tlat(cs) = 0.5 s) and intermittent (tlat(is) = 2.5 s) sliding against alumina pin (5 N, 0.0628 m/s) in filtrate olive pit/tap water.

While under intermittent sliding, a transition of wear behavior is accompanied by a change of the wear track state. In fact during each cycle, the wear track alternates between active and repassivated states during trot and toff, respectively. Obviously, the passive oxide layer which was created during toff will be destroyed during trot. Consequently, corrosive wear debris were ejected and caused then the appearance of deep abrasive scratches by a third body mechanism shown in Figures 13 and 15, respectively. Then during the 2s off period, the repassivation of the wear track occurs to protect the surface. The intermittent sliding could be compared to a continuous cycle performed 5000 times successively. For each cycle, the establishment of contact at the beginning, the debris formation during trot and the wear track repassivation during toff make the wear more important compared to the stabilized wear of surface under continuous sliding. This could be explained by the relatively wider wear track under intermittent sliding compared to its width under continuous sliding, shown in the microtopographic surface of the wear track in Figures 14 and 15. Discontinuous scratching marks

(a)

(b)

Figure 10. Image surface of the wear track after continuous sliding test on AISI 304L in filtrate, with 5N sliding force and 0.0628 m/s, after 5000 cycles: (a) Optic image surface; (b) Scanning electron microscope observation

Continuous scratching marks

(a)

(b)

Figure 11. Image surface of the wear track after the intermittent sliding test on AISI 304L in filtrate, with 5N sliding force and 0.0628 m/s, after 5000 cycles: (a) Optic image surface; (b) Scanning electron microscope observation Alpha = 187°

Beta = 30°

µm 8

8.21 µm

7.5 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5

1000 µm

2 1.5 1 0.5 1000 µm

0

Figure 14. 3D dimensional image of the wear track determined by optical profilometry under continuous sliding

Figure 15. 3D dimensional image of the wear track determined by optical profilometry under intermittent sliding

The passive film effect on the mechanical wear under intermittent sliding can be evaluated based on the parameter Km which is defined by the ratio between the specific mechanical # ⁄ # ⁄EF$GG parts of the sliding wear on the active D$C $C and on the repassivated DEF$GG

track, as shown in Equation 17 [12]. The parameter Km has a value of 0.004, lower than 1, indicates that the formation of the passive film accelerated the mechanical material removal. In fact under intermittent sliding (tlat=2.5 s) the repassivated film had the time to re-grow during toff of each cycle. With this parameter Km the synergy between corrosion and wear effects is deconvoluted for the first time into wear contributions from the different states of the wear track. At a second time it provides information on the surface state evolution with testing time. Synergism between wear and corrosion of AISI 304L in 1 M HNO3 was reported by Priya et al. [29], based on the standard guide for determining the synergism between wear and corrosion established by Madsen [30]. The contribution of pure corrosion loss to total material loss of AISI 304L was less than the pure mechanical wear loss which is in agreement with the present paper results. As observed in the work of Priya et al., the accumulated wear debris particles might have acted as a third body in the tribo contact and induced abrasion on the sliding surface. Nevertheless, the mechanical sliding would remove a larger amount of material than when there is no passive film on the wear track under continuous sliding (tlat=0.5 s). The higher is the latency time, the more the sensitivity to tribocorrosion will increase. L# 

# D$C ∗ EF$GG 17 # DEF$GG ∗ $C

The effect of the mechanical action is shown by the distribution of microhardness in the wear track in Figure 16. It was clearly noted that microhardness is more important in worn surface than in unworn one. In fact, microhardness is about 370 HV0.05 and 267 HV0.05 in worn and unworn surface, respectively. The surface layer of the sample is severely work hardened. It

seems that prows with grooved and scratched surfaces were harder than the non-grooved surface. The hardness of the sheared material at the base of the scratch was in the range of 400 HV0.05 which correspond to a heavily work-hardened metal. 450

Scratch marks

Microhardness (HV0,05)

400 350 300 250 200 150 100 50

Wear track

0 0

100

200

300

400

Distance (µm) Continuous sliding

(a) 400

Scratch marks

Microhardness (HV0,05)

350 300 250 200 150 100 50

Wear track

0 0

100

200

300

400

500

600

700

Distance (µm) Intermittent sliding

(b)

Figure 16. Microhardness profile in the wear track width under continuous (a) and intermittent sliding (b).

4. Conclusions The objective of this work was to study the tribocorrosion behavior of AISI 304L rubbed against an alumina pin under both continuous and intermittent sliding conditions in an olive pomace/tap water filtrate. (a)

In the case of corrosion tests without mechanical loading, the polarization curves of

AISI 304L immersed in olive pomace/tap water filtrate shows that pitting was responsible of the passivated film damage. (b)

Under mechanical loading, OCP measurements reveal a permanent and total

destruction of passive film in the wear track during a continuous sliding. As a result, an electrochemically active state of the track was obtained. However, a different behavior was recorded under an intermittent sliding characterized by the destruction of the passive film and the occurrence of an electrochemically active state of the wear track, followed by a repassivation during outage duration (passive state). (c)

The wear modelization shows that mechanical wear was the predominant contributor

under continuous and intermittent sliding. Further, it was clearly found that wear is stronger under intermittent sliding because of the difference of wear track state. In fact, under continuous sliding the wear track is at an active state during the entire test where debris consists mainly on fine metallic particles that are continuously created then oxidized in the electrolyte. Nevertheless, under intermittent sliding, the wear track alternates between active and repassivated states during trot and toff. From repassivated film, oxidized wear debris are created and ejected to cause an abrasive wear mechanism.

Acknowledgment This work was partially supported by EU-FP7 grant Oil&Sugar (295202).

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