Electrochemical noise measurements on stainless steel during corrosion–wear in sliding contacts

Electrochemical noise measurements on stainless steel during corrosion–wear in sliding contacts

Wear 256 (2004) 480–490 Electrochemical noise measurements on stainless steel during corrosion–wear in sliding contacts Pei-Qiang Wu∗ , J.-P. Celis1 ...

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Wear 256 (2004) 480–490

Electrochemical noise measurements on stainless steel during corrosion–wear in sliding contacts Pei-Qiang Wu∗ , J.-P. Celis1 Department of Metallurgy and Materials Engineering (MTM), Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium

Abstract In this study, electrochemical noise measurements performed during corrosion–wear sliding tests with the working electrode coupled to a microelectrode, are presented. A microelectrode was used to record during fretting experiments current variations resulting from a modification of the working electrode induced by sliding. The tribocorrosion system investigated consists of an AISI 304 stainless steel plate sliding in a reciprocating mode against a corundum ball, both immersed in electrolytes of different pH. A detailed discussion of the in situ electrochemical noise measurements demonstrates that they are useful to identify and/or to unravel materials modification processes taking place during corrosion–wear sliding tests on passivating materials. © 2003 Elsevier B.V. All rights reserved. Keywords: Stainless steel; Electrochemical noise; Fretting corrosion; Corrosion–wear

1. Introduction Materials in passive state, such as stainless steel, usually have a very good corrosion resistance. Their corrosion resistance may significantly degrade under combined wear and corrosion conditions especially when the passive layer is destroyed by mechanical wear, thus producing a wear–corrosion synergism. Corrosion–wear phenomena have been studied by researchers using a variety of experimental setups [1–5]. Depassivation–repassivation phenomena of passive metals were studied under conditions of mechanical breakdown of the passive films in uni- and bidirectional pin-on-disk sliding [6–9], scratching [10,11], guillotining [12], and abrasion and straining [13,14]. In addition, the depassivation– repassivation phenomena were also investigated by electrochemical depassivation [9,11], pulsed laser depassivation [15,16] and micro-tribocorrosion electrochemical techniques [17,18]. Celis and co-workers [6,7] investigated the corrosion–wear of AISI 316 stainless steel and TiN coated tool steel. They proposed the concept of an “active wear track area” to explain the corrosion–wear of passive materials. A similar concept but expressed as a proportionality factor that represents the fraction of the wear track area be-

∗ Corresponding author. Tel.: +32-16-32-1292; fax: +32-16-32-1991. E-mail address: [email protected] (P.-Q. Wu). 1 Tel.: +32-16-32-1312; fax: +32-16-32-1991.

0043-1648/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0043-1648(03)00558-1

coming effectively depassivated, was proposed by Landolt and co-workers [9]. Some DC electrochemical methods used in corrosion–wear study are the monitoring of the open-circuit potential during corrosion–wear, the measurement of current changes under potentiostatic conditions combined with the investigation of friction and wear at applied potential, the comparison of potentiodynamic anodic polarization curves under wear and no-wear conditions, and the measurement of potential during wear tests performed under galvanostatic control. These electrochemical techniques generally provide data on potential or current variations measured during corrosion–wear tests. A critical review on the electrochemical methods used in tribocorrosion has recently been made by Landolt et al. [19]. Although electrochemical polarization allows a precise control of the redox potential of a rubbed metal, the application of an anodic polarization may accelerate the corrosion–wear process [4]. Therefore, electrochemical impedance spectroscopy (EIS) for monitoring the surface modification of passive materials during corrosion–wear is gaining interest [20,21]. Electrochemical noise (EN) technique consists in the analysis of spontaneous fluctuations of potential and current at electrodes, and was used to investigate corrosion–wear [8,22,23]. There are three major modes for measuring potential and current noise in a corrosion system, namely the two identical working electrode (WE) mode [24], one WE coupled to a microelectrode (e.g. Pt) [25], and two identical WEs with a bias potential [26]. Wood and co-workers [22] have

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recently used two nominally similar ring working electrodes to measure EN during erosion–corrosion. However, their setup cannot be easily adapted to fretting corrosion. Landolt and co-workers [27] used two originally identical samples of which one was subjected to reciprocating wear. The two samples were connected through a zero-resistance ammeter (ZRA) to monitor galvanic current during corrosion–wear. A reference electrode was used to measure the open-circuit potential of the worn sample. Apparently, the two originally identical samples may become significantly different once a wear process is started on one of them. A galvanic corrosion between the two samples may result from the wear on one sample, and this may significantly accelerate the corrosion–wear process on the worn sample. Similar problems may be encountered in applying the EN technique consisting of two identical WEs with a bias potential, to corrosion–wear. In addition, the application of a bias potential imposes the use of a more complex electrochemical setup. This problem may be overcome by using an appropriate microelectrode instead of a second identical electrode coupled to the working electrode through a ZRA. This method is known as electrochemical emission spectroscopy (EES) [25]. The integration of this EES technique into sliding corrosion–wear tests is not straightforward. Indeed, the microelectrode needs to be optimized, and counter-measures must be taken to combat external influences, especially those from the fretting wear test equipment, overwhelming the real electrochemical signal. Moreover, there are numerous differences between the EN in a corrosion system and in a corrosion–wear system. One of the major differences is that the mechanical interaction in a corrosion–wear test may induce EN that is not appearing in a corrosion test. In this paper, the corrosion–wear of AISI 304 stainless steel immersed in aqueous solutions and sliding against a corundum ball is reported. The electrochemical noise data and the wear of AISI 304 stainless steel are presented and compared. Corrosion–wear mechanisms in aqueous electrolytes of different pH are elaborated based on in situ friction and electrochemical measurements combined with ex situ wear track analyses by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX).

481

Fig. 1. Assembly of the Pt-microelectrode used in this study.

(3 M KCl) reference electrode (RE) was used to detect potential changes of the working electrode. The microelectrode used in this study consisted of a Pt electrode with a diameter of 0.25 mm and a tip length of 1.2 mm (Fig. 1). The experimental setup used for electrochemical noise measurements during corrosion–wear tests on immersed samples is schematically shown in Fig. 2. A potentiostat (Solartron electrochemical interface model 1287) was used, which allows voltage and current measurements at a resolution of 1 ␮V and 1 pA, respectively. The microelectrode coupled to the working electrode was used to sense the current flowing between them. To study the effect of that coupling on corrosion–wear, fretting corrosion tests on the stainless steel uncoupled to the microelectrode were also performed in the setup in Fig. 2. Coaxial cables were used for electrical connections wherever possible in order to reduce the noise coming from the surrounding environment. Data acquisition on potential and current was done for 3000 s at a sampling rate of 1 Hz, initiated 10 min after immersion of the samples into the test solution. The counterbody was lifted away immediately from the WE at the end of fretting tests. The electrochemical noise data are reported according to ASTM conventions [28]. For the potential of the working electrode, the positive direction is denoted as

2. Experimental AISI 304 stainless steel (SS) samples (25 mm × 25 mm × 1 mm) were polished to a roughness Ra < 0.01 ␮m. The nominal composition of the stainless steel is Cr 17–19 wt.%, Ni 8.5–10.5 wt.%, Si ≤1.0 wt.%, P ≤0.045 wt.%, and Fe balance. Corundum balls (φ 10 mm) were selected as counterbody material (Ceratec, The Netherlands) because of the high wear resistance, the chemical inertness, and the electric insulating properties of corundum. Stainless steel specimens to be used as WE were covered with an adhesive tape to leave an area of 1 cm2 exposed to the test solutions. A Ag/AgCl

Fig. 2. Schematic experimental setup combining a bidirectional sliding contact immersed in an electrolyte and the electrochemical noise measurements. The earthing of the fretting test machine is not shown in this illustration.

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the noble direction and the negative direction as the active direction. Anodic currents at the working electrode are considered positive and cathodic currents are negative. The bidirectional sliding (fretting) test equipment was described earlier [29]. The sliding conditions correspond to a fretting test performed at a normal load of 5 N, an oscillating frequency of 10 Hz, and a linear displacement amplitude of 200 ␮m. These fretting tests were performed for 20,000 cycles at an ambient temperature of 23 ◦ C. The number of cycles, the tangential force, the normal force, the displacement amplitude, and the coefficient of friction were recorded at equally spaced time increments over the whole test duration. The electrolytes used are 0.5 M NaCl (pH 5.5), 0.02 M Na3 PO4 (pH 12.0), and 0.5 M H2 SO4 (pH 0.5). The neutral NaCl solution was selected as representative for the use of stainless steel in a saline environment. Alkaline Na3 PO4 with a high pH was selected as a corrosion inhibiting solution [30] and a buffer which might reduce the corrosion–wear. H2 SO4 is an aggressive acid media that might enhance the corrosion–wear process. The wear scars were investigated by reflected light microscopy with Nomarski contrast, laser profilometry (Rodenstock RM 600), and SEM–EDX. The wear volume was determined by a profilometric method described earlier [29]. The anodic current on coupling with the microelectrode was integrated during the corrosion–wear tests and converted into an equivalent volumetric material loss (Ve ) according to the following equation: t Wa toe (I(t) − I0 ) dt EW Q = (1) Ve = ndF dF with Wa is the atomic weight of the metal or alloy; n the number of electrons involved in the corrosion process; Q the total electrical charge (C) passed during the corrosive wear process; d the density of the metal or alloy (g/cm3 ); F the Faraday constant (96,500 C); I(t) the current measured at time t; I0 the background current when no fretting occurs; t0 and te the time at which a fretting test was started and ended, respectively; EW the equivalent weight of the metal or alloy. Assuming for AISI 304 stainless steel, d 7.94 g/cm3 and EW 25.12 g [31], Eq. (1) becomes Ve (␮m3 ) = 3.278 × 107 Q

(2)

Potential and current variations were acquired when the connections RE1, RE2 and WE were short-circuited, while the current imports Lo and WE were left open (cf. Fig. 2). A dummy fretting test was run in 0.02 M Na3 PO4 on an AISI 304 stainless steel sample used as working electrode, while the loading head was in contact with the solution but not with the sample. The test parameters in this dummy fretting test were the same as in fretting corrosion tests. The grounding of the fretting wear test machine and the insulation of its loading head in contact with the test solution, did not affect the measured potential. However, their influence on the measured current was significant (Fig. 3). With a non-insulated loading head, the current measured in

4

3

current (µA)

482

Loading head in contact with solution, fretting machine floating, loading head not insulated.

2 Fretting machine grounded, loading head insulated.

1

0 Fretting machine grounded, loading head not insulated.

0

200

400

600

800

1000

1200

time (s) Fig. 3. Effect of grounding the fretting test machine and insulating the loading head on the current noise during a dummy fretting test. An AISI 304 stainless steel sample was immersed in 0.02 M Na3 PO4 but the loading head was not in contact with the sample. Fretting parameters were 10 Hz and 200 ␮m.

this dummy test reached more than 2000 nA when the fretting test machine was floating, and only 400 nA on grounding. However, when the loading head was insulated and the fretting test machine grounded, the current reduced significantly to about 10 nA. In addition, the potentiostat must be run under grounding conditions, and coaxial cables should be used for electric connections wherever possible. With all these counter-measures taken, the system potential and the current noise levels were acquired in a dummy fretting test with the potential plugs RE1, RE2, and WE short-circuited, and the current plugs Lo and WE left open. The system potential and the current noise levels were then reduced to 8 ␮V and 1.5 nA, respectively. All potentials reported in this work are related to the AISI 304 stainless steel working electrode connected to the Pt electrode. All currents reported [32] are given for a defined area of the working electrode (1 cm2 ) coupled to the Pt-microelectrode (0.01 cm2 ).

3. Effect of coupling on the corrosion–wear In order to evaluate the effect of coupling on the corrosion–wear, fretting corrosion tests were performed on AISI 304 stainless steel in 0.5 M NaCl (pH 5.5). Three conditions were selected for the evaluation, namely non-coupled (open-circuit potential, Eoc ), coupled to the microelectrode, and coupled to an originally identical working electrode. Fretting test parameters were normal load 5 N, oscillating frequency 10 Hz, displacement amplitude 200 ␮m, and 20,000 cycles. The anodic current induced by a corrosion–wear process, cannot be measured in a test performed at Eoc . Indeed, in such circumstances, an eventual anodic current will be balanced by a cathodic current flowing across other parts of the

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working electrode. On coupling the WE with the microelectrode, the WE will be polarized at a mixed potential, Ecouple , and current variations between the WE and the microelectrode can be measured through a ZRA. To determine the effect of coupling a microelectrode to a working electrode during corrosion–wear, a switch was inserted between the WE and the microelectrode or an originally identical WE (cf. Fig. 2). When the switch is on, the coupling is activated; when it is off, the coupling is deactivated. The effect of coupling an originally identical working electrode and a Pt-microelectrode on the electrochemical noise measurements of an AISI 304 stainless steel WE sliding against corundum both immersed in 0.5 M NaCl is shown in Fig. 4. The coupling of an originally identical working electrode results in a positive potential shift of 60 mV and a current of 2 ␮A (Fig. 4a) during a fretting corrosion test. The coupling of the Pt-microelectrode causes a potential shift of only 8 mV and a current of 0.25 ␮A (Fig. 4b). Therefore, the effect of coupling an originally identical working electrode on the electrochemical noise measurements is much greater than that of coupling the WE to the Pt-microelectrode during fretting corrosion tests.

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Fig. 5. Effect of coupling on the wear volume on AISI 304 stainless steel sliding against corundum in 0.5 M NaCl (fretting parameters: 5 N, 10 Hz, 200 ␮m, 20,000 cycles). Three cases are shown: non-coupled condition (Free Eoc), coupled to a Pt-microelectrode (Coupled to ME), and coupled to an originally identical working electrode (Coupled to WE). The error bar was calculated from at least three duplicate tests.

The corrosion–wear of AISI 304 stainless steel sliding against corundum in 0.5 M NaCl is displayed in Fig. 5. The wear volume and its standard deviation were determined from at least three duplicate tests. The wear volume of stainless steel coupled to the Pt-microelectrode (ME) is comparable to the wear volume determined under non-coupled conditions (free Eoc ). However, the coupling of an identical working electrode to the stainless steel working electrode results in a 20% increase of the wear volume in comparison to the previous case.

4. Results 4.1. Electrochemical noise measurements

Fig. 4. Effect on the electrochemical noise of the coupling of either (a) an identical working electrode, or (b) a microelectrode with AISI 304 stainless steel sliding against corundum in 0.5 M NaCl (fretting parameters: 5 N, 10 Hz, 200 ␮m, 20,000 cycles) (on: coupling activated, off: coupling deactivated).

The evolution of the potential of an AISI 304 stainless steel sample, before, during, and after sliding against corundum in a 0.5 M NaCl solution (pH 5.5) was recorded by conventional open-circuit potential measurements, and on coupling with a Pt-microelectrode (Fig. 6). The potential of the stainless steel sample coupled or uncoupled to the Pt-microelectrode, undergoes a significant negative shift at the start of sliding. After that running-in phase, the potential stabilizes although some fluctuations with time are noticed. At the end of the fretting tests when the corundum counterbody is lifted away from the stainless steel surface, the potential of the stainless steel sample returns progressively to the potential level recorded before the start of the sliding test. The current measured on coupling to the Pt-microelectrode, evolves in phase with the potential variations (Fig. 6b). A background current of less than 50 nA was recorded before the sliding test was initiated. A significant rise of current takes place at the start of the sliding test. After a

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Fig. 6. Electrochemical noise measurements on AISI 304 stainless steel sliding against corundum in 0.5 M NaCl (fretting parameters: 5 N, 10 Hz, 200 ␮m, 20,000 cycles) for tests performed: (a) under open-circuit potential conditions; (b) on coupling to a Pt-microelectrode.

Fig. 7. Electrochemical noise measurements on AISI 304 stainless steel sliding against corundum in 0.02 M Na3 PO4 (fretting parameters: 5 N, 10 Hz, 200 ␮m, 20,000 cycles) for tests performed: (a) under open-circuit potential conditions; (b) on coupling to a Pt-microelectrode.

running-in phase, the current stabilizes although it fluctuates slightly with time. The current flowing between the working electrode and the Pt-microelectrode returns at the end of the sliding tests to the original level. A similar sharp potential drop or current increase was related by Oltra et al. to the depassivation of a surface by laser irradiation [15,33], by Isaacs and Ishikawa to the initiation of pitting [34]. On the contrary, a progressive potential rise or current decrease was related by them to the repassivation of an active surface. The potential and current variations in Fig. 6 thus suggest that stainless steel sliding against corundum in 0.5 M NaCl under the selected test conditions mainly undergoes a removal of its passive surface film during the running-in phase, remains partly active during the sliding test, and finally progressively repassivates on unloading. In analogy to this case, the potential and current variations recorded on AISI 304 stainless steel sliding against corundum in 0.02 M Na3 PO4 (Fig. 7), indicate that the passive film on stainless steel is removed during the running-in of the sliding tests. That AISI 302 material remains active during the sliding test, and repassivates after unloading. However, the electrochemical data recorded on sliding in 0.5 M H2 SO4 , differ significantly from those recorded in NaCl or Na3 PO4 . Indeed,

at the start of the sliding test, a significant negative shift of the potential occurs accompanied by a current increase, but this current decreases exponentially on further testing (Fig. 8b). Current spikes regularly emerge after a maximum current was reached, and these spikes appear till the end of the test. Such current spikes did not appear on immersion of AISI 304 stainless steel coupled to a Pt-microelectrode in 0.5 M H2 SO4 for 3 h, but not subjected to a sliding contact. It means thus that under sliding in 0.5 M H2 SO4 , a tribo-activated corrosion process takes place on stainless steel. When the corundum counterbody is lifted away from the stainless steel sample, the potential of the stainless steel sample remains far away from the potential before the start of the sliding test. At the same time, only a slight lowering of the current is observed. It seems thus that some area remains active on the stainless steel after the end of the fretting test. 4.2. Tribological properties The coefficient of friction of AISI 304 stainless steel sliding against corundum was measured at the open-circuit potential or on coupling to the Pt-microelectrode (Fig. 9). Each curve is the average of three duplicate tests. The coupling

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Fig. 8. Electrochemical noise measurements on AISI 304 stainless steel sliding against corundum in 0.5 M H2 SO4 (fretting parameters: 5 N, 10 Hz, 200 ␮m, 20,000 cycles) for tests performed: (a) under open-circuit potential conditions; (b) on coupling to a Pt-microelectrode.

to the microelectrode has no effect on friction in the three test solutions. The coefficient of friction is 0.54 in 0.02 M Na3 PO4 (pH 12.0), 0.31 in 0.5 M NaCl (pH 5.5), and 0.25 in 0.5 M H2 SO4 (pH 0.5). These different coefficients of friction can be related to different surface states of stainless steel in electrolytes of different composition and pH. The wear volume determined by profilometry on AISI 304 stainless steel after sliding against corundum is displayed in Fig. 10 for the three electrolytes of different pH. The wear volume in the wear scar was calculated from at least three duplicate tests performed either at open-circuit potential or coupled to the Pt-microelectrode for 20,000 fretting cycles. The smallest wear scar was noticed in 0.02 M Na3 PO4 (Fig. 10b), while the largest one was obtained in 0.5 M H2 SO4 (Fig. 10c). As can be seen, the coupling to the Pt-microelectrode did not enlarge the corrosion–wear on the stainless steel. An electrochemically equivalent volumetric material loss (Fig. 10) was calculated based on Faraday’s law from the current measured between the WE and the Pt-microelectrode (see Eq. (2)). That current corresponds to the net current flowing through the anodic and cathodic areas of the working electrode. Therefore, the material loss calculated by the time integration of such a current may not be

Fig. 9. Coefficient of friction of AISI 304 stainless steel sliding against corundum in: (a) 0.5 M NaCl (pH 5.5); (b) 0.02 M Na3 PO4 (pH 12); and (c) 0.5 M H2 SO4 (pH 0.5) (fretting parameters: 5 N, 10 Hz, 200 ␮m, 20,000 cycles). Sliding tests were performed at open-circuit potential (Eoc ), and on coupling AISI 304 SS to a Pt-microelectrode (ME).

exclusively related to a material loss in the wear scar. That material loss calculated from the current integration differs significantly from the one measured by profilometry. It is very small in 0.5 M NaCl, but quite large in 0.5 M H2 SO4 , even larger than the wear volume determined in the wear scar!

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Fig. 10. Wear volume and equivalent volumetric material loss determined on AISI 304 stainless steel after sliding against corundum in: (a) 0.5 M NaCl (pH 5.5); (b) 0.02 M Na3 PO4 (pH 12); and (c) 0.5 M H2 SO4 (pH 0.5). Fretting parameters were 5 N, 10 Hz, 200 ␮m, 20,000 cycles.

The shape and size of the wear scars are quite similar for tests performed in NaCl and H2 SO4 , but slightly smaller in Na3 PO4 (Fig. 11). The surface condition of stainless steel in and outside the wear scars differs in the three test solutions. In 0.5 M NaCl, numerous grooves are visible in the wear scar, but no corrosion pits are detected outside the wear scar. The morphology of this wear scar indicates that the debris

Fig. 11. Reflected light micrographs with Nomarski contract of wear scars on AISI 304 stainless steel after sliding against corundum in: (a) 0.5 M NaCl (pH 5.5); (b) 0.02 M Na3 PO4 (pH 12); and (c) in 0.5 M H2 SO4 (pH 0.5). Fretting parameters were 5 N, 10 Hz, 200 ␮m, 20,000 cycles.

generated in NaCl is abrasive. On the contrary, the wear scar in 0.02 M Na3 PO4 is bright and relatively smooth. Corrosion pits are not found outside the wear scar but a slight color change is noticed at the edges of the wear scar along the sliding direction. Finally, the wear scar in 0.5 M H2 SO4 is rather smooth except for some grooves in the center of the fretting wear scar. Remarkably is that outside the wear scar

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487

Table 1 EDX analyses of a wear scar on AISI 304 stainless steel after a fretting corrosion test in 0.02 M Na3 PO4 a Element (wt.%) Fe Nominal compositions Spot inside the wear scar At the edge of the wear scar a

Cr

Ni

Si

P

Balance 17–19 8.5–10.5 ≤1.0

Na

≤0.045 0

Balance 19.15

8.97

0.80

1.05

2.05

Balance 16.29

7.39

0.81

16.34

3.61

Analysis of base material is also given for reference.

5. Discussion

Fig. 12. Reflected light micrographs with Nomarski contract of: (a) an area outside the wear scar on AISI 304 stainless steel after sliding against corundum in 0.5 M H2 SO4 (fretting parameters: 5 N, 10 Hz, 200 ␮m, 20,000 cycles); and (b) an AISI 304 stainless steel sample only immersed for 3 h without any fretting test performed on it.

numerous corrosion pits are found spread over the exposed stainless steel working electrode (Fig. 12a). An immersion test for 3 h in 0.5 M H2 SO4 under no fretting conditions was done for comparison with an AISI 304 stainless steel sample coupled to the Pt-microelectrode. Current peaks were not noticed during this 3 h immersion test, and after this 3 h immersion test, corrosion pits were not noticed on the AISI 304 sample (Fig. 12b). EDX analyses were done at the edge and inside the wear scar after a fretting test in Na3 PO4 (Fig. 11b, see white crosses). The data are listed in Table 1. The nominal composition of the stainless steel is also given for comparison. High amounts of phosphorus and sodium are detected at the edge of and inside the wear scar, but not outside the wear scar. This reveals the formation of a tribo-activated reaction layer in and around the wear scar.

A passive material such as AISI 304 stainless steel fretted against corundum in a corrosive environment is characterized by a heterogeneous surface condition. Indeed, the tribo-activated wear scar acts as a small anode, whereas the mostly remaining passive unworn surface acts as a large cathode. A galvanic cell is thus established between the active wear scar and the remaining passive unworn surface, inducing a galvanic corrosion in the wear scar. At the open-circuit potential, Eoc , the anodic current generated at the tribo-activated wear scar is balanced by a cathodic current on the passive surface. These anodic current and cathodic currents cannot be measured at Eoc . Hereto, a microelectrode or an originally identical WE can be coupled to the WE through a zero-resistance ammeter (ZRA). The measured current is the net current between the anodic and cathodic current components at the working electrode at the mixed potential resulting from the coupling of the WE with the microelectrode. Landolt and co-workers demonstrated the possibility of measuring a galvanic corrosion current between two originally identical working electrodes in a corrosive wear system [27]. However, once fretting is started, these two originally identical working electrodes become significantly different. The coupling of an originally identical WE doubles the passive surface area, and thus may accelerate the galvanic corrosion in the wear scar as noticed in Fig. 5. The galvanic corrosion may be alleviated by coupling a smaller working electrode like an optimized microelectrode. The results of Fig. 5 indeed show that the coupling of an optimized microelectrode can mitigate the galvanic corrosion in comparison with the coupling of an identical working electrode (Figs. 4 and 5). This investigation reveals that a low coefficient of friction does not always correspond to a low corrosive wear rate. Indeed, AISI 304 stainless steel tested in 0.02 M Na3 PO4 (Figs. 9 and 10) shows the highest coefficient of friction and the lowest wear loss. On the contrary, stainless steel tested in 0.5 M H2 SO4 has the lowest coefficient of friction, but the highest wear loss. These results suggest that different corrosion–wear mechanisms are active in different tribocorrosion systems.

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The pH of the test solution may affect the passivation of stainless steel, and thus influences the coefficient of friction. According to the potential–pH diagrams for Fe–Cr–Ni [35], the stainless steel at the low pH of 0.5 of the sulfuric acid solution may enter into the corrosion zone at free Eoc . At higher pH, the stainless steel may become passive. From Fig. 9 it appears that a higher pH corresponds to a higher coefficient of friction. Therefore, it may be concluded that when stainless steel passivates or repassivates more easily, the passive surface induces a higher coefficient of friction. However, as revealed by EDX (Table 1), a tribochemical reaction probably takes place during the corrosion–wear test in 0.02 M Na3 PO4 . Reaction products are pushed aside and accumulate along the edges of the wear scar during the fretting test (Fig. 11b). Therefore, notwithstanding the high coefficient of friction in Na3 PO4 , the reaction products affect beneficially the corrosive wear on stainless steel. On the contrary, such a beneficial tribochemical reaction seems not to occur on stainless steel sliding against corundum in 0.5 M NaCl. The synergistic effect of wear and corrosion in NaCl increases the wear loss (Fig. 10). The same is noticed in 0.5 M H2 SO4 although the lowest coefficient of friction was recorded in this test solution. The discrepancy between the wear volume in the wear scar and the one calculated from a current integration (Fig. 10) has to be attributed to different corrosion–wear mechanisms operative in the different test solutions. In the cases of AISI 304 stainless steel sliding against corundum in 0.5 M NaCl and in 0.02 M Na3 PO4 , the stainless steel surface is in a passive state except the tribo-activated wear zone during the entire fretting test. Therefore, part of the anodic current generated in the tribo-activated wear scar will be compensated by a cathodic current on the passive surface. The material loss calculated from the current integration flowing between the WE and the microelectrode should thus be smaller than the material loss measured by profilometry in the wear scar. In the case of AISI 304 stainless steel sliding against corundum in 0.5 M H2 SO4 , a “zoom-in” of the exposed surface outside and inside the wear scar (Figs. 11c and 12a) reveals that indeed a tribo-activated corrosion process occurs not only in the wear scar but also over the whole sample surface. The large value of the equivalent volumetric material loss in H2 SO4 (Fig. 10c), even larger than the wear volume in the wear scar, is thus due to the tribo-activated corrosion on the unworn stainless steel. Relative variations of potential and current are of interest in electrochemical noise analyses. For example, Bertocci et al. [36–38] often present their EN data in time-domain by arbitrarily setting the zero of scale. A “zoom-in” on the noise data presented in Fig. 7b, reveals a cyclic variation of potential and current in the range of 3 mV and 10 nA, respectively, during the steady-state phase of fretting tests performed in Na3 PO4 (Fig. 13a). A potential drop always corresponds to a current increase, and vice versa. It is known that the potential of an electrode shifts in noble direction when a passive film grows on the electrode material, and consequently the

Fig. 13. “Zoom-in” on potential and current noise measured on AISI 304 stainless steel sliding against corundum in: (a) 0.02 M Na3 PO4 (pH 12); and (b) 0.5 M H2 SO4 (pH 0.5) during the steady-state phase (fretting parameters: 5 N, 10 Hz, 200 ␮m).

anodic current decreases. On the contrary, the potential shifts in the negative direction on removal of a passive film, and the anodic current increases. The fluctuations of potential and current in Fig. 13a reveals the successive removal and regrowth of the passive film in the tribo-activated wear scar on stainless steel during the steady-state phase of fretting corrosion tests. The frequency at which these fluctuations take place, is 0.05 Hz, and is thus much lower than the fretting test frequency of 10 Hz. It seems that a breakdown of the passive film does not occur during each fretting cycle, but takes place at the time a critical thickness of the passive film is reached. These results provide an in situ experimental proof for the oxidational wear mechanism proposed by Quinn [39] and further developed by Abd-El-Kader and El-Raghy [40]. That model assumes that a gradual growth of the oxide layer is removed instantaneously by the rubbing action when a threshold thickness is reached. Indeed, a dynamic equilibrium between depassivation and repassivation processes is noted in our fretting corrosion experiments in Na3 PO4 . In the case of AISI 304 stainless steel sliding against corundum in 0.5 M H2 SO4 , a tribo-activated corrosion (Fig. 12a) is superposed onto the fretting corrosion process in the wear scar during the steady-state phase. An anodic current resulting from the tribo-activated corrosion is thus

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added to the anodic current generated in the wear scar. Therefore, the equivalent volumetric material loss calculated from the current integration is larger than the material loss measured in the wear scar (Fig. 10c). A “zoom-in” on the electrochemical noise data of Fig. 8b for AISI 304 stainless steel in 0.5 M H2 SO4 is displayed in Fig. 13b. A sharp increase of both potential and current is followed by a slow decrease. Such a noise pattern is typical for activation-controlled corrosion with hydrogen evolution [25,37,41]. Because the stainless steel working electrode under sliding against corundum has a potential of −0.2 V SHE (Fig. 8), below the standard hydrogen redox potential, the reduction of protons may occur on the electrode surface. Bubbles were indeed observed during the corrosion–wear experiments. In addition, the analysis of specimens tested under corrosion–wear and under static immersion conditions, confirms that a tribo-activated corrosion process takes place over the entire specimen surface in 0.5 M H2 SO4 solution only under fretting corrosion conditions (Fig. 12a and b).

6. Conclusions Electrochemical noise can successfully be applied to the investigation of corrosion–wear of AISI 304 stainless steel under oscillating sliding contacts (fretting). Interference from the fretting wear test machine itself was identified as a major obstacle. Grounding of the fretting test machine and of the electrochemical instrument significantly reduces the system current noise level. Moreover, an insulation of the loading head of the fretting test machine further reduces the system current noise level. The use of a microelectrode has to be preferred to the use of a counter electrode similar to the working electrode. Electrochemical noise obtained on three typical tribocorrosion systems show that the technique can be used to identify the finger prints of different corrosion–wear processes. AISI 304 stainless steel sliding against corundum in 0.5 M NaCl (pH 5.5) and 0.02 M Na3 PO4 (pH 12.0) undergoes a removal of its passive film during the running-in phase of the fretting corrosion tests. In the steady-state, depassivation and repassivation alternately occur in the tribo-activated wear area. At the end of the fretting tests, the stainless steel repassivates progressively on unloading. During sliding in Na3 PO4 , a tribochemical reaction takes place that reduces the corrosive wear. On the contrary, the synergistic effect of wear and corrosion in NaCl enhances the corrosive wear. In 0.5 M H2 SO4 (pH 0.5), AISI 304 stainless steel undergoes a removal of its passive film during the running-in phase of fretting tests. A tribo-activated corrosion process takes place during the steady-state period that even may induce localized corrosion outside the wear track area. This tribo-activated corrosion process may remain active after unloading. The interaction of wear and corrosion accelerates the corrosive wear process. Detailed analysis of electrochemical noise

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data will assist in unraveling wear and corrosion processes and their synergisms in material degradation in sliding contacts.

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