Effect of Applied Cathodic Potential on Friction and Wear Behavior Of

Article Eﬀect of Applied Cathodic Potential on Friction and Wear Behavior of CoCrMo Alloy in NaCl Solution

Yong Sun * and Richard Bailey School of Engineering and Sustainable Development, Faculty of Computing, Engineering and Media, De Montfort University, Leicester LE1 9BH, UK; [email protected] * Correspondence: [email protected]

Received: 23 October 2020; Accepted: 19 November 2020; Published: 23 November 2020

Abstract: Most of the reported work on the effect of applied potential on tribocorrosion or corrosive wear of metallic alloys in a corrosive environment were conducted at anodic potentials. Limited tests have been conducted at cathodic potentials for comparison purposes or to derive the pure mechanical wear component in tribocorrosion. This work investigated the effect of cathodic potential on the friction and wear behaviour of an important biomedical alloy, CoCrMo, sliding against an Al2O3 slider in 0.9% NaCl solution at 37 ◦C. High friction was found at cathodic potentials close to the open circuit potential, where mechanical wear played a predominant role in material removal. At potentials more cathodic than the hydrogen charging potential, low friction and low wear were observed. The coefficient of friction (COF) and total material loss decreased with increasing cathodic potential, such that at 1000 mV (saturated calomel electrode, SCE), extremely low COF values, as low − as 0.02, and negligible material loss were obtained. Such reductions in friction and wear at increasing cathodic potentials were accompanied with the formation of parallel lines in the sliding track and were gradually diminished with increasing applied contact load. It is believed that hydrogen charging and hydrogen segregated layer formation at the surface are responsible for such a phenomenon. It can also be concluded that it is difficult to derive the pure mechanical wear component in tribocorrosion by simply conducting a test at an arbitrary cathodic potential.

Keywords: friction and wear; tribocorrosion; CoCrMo alloy; potential

1. Introduction Material degradation due to friction and wear in a corrosive environment is a common phenomenon that can be found in many engineering applications, such as in marine, mining, food processing, chemical processing and biomedical machineries and devices [1–3]. Studies of such a phenomenon, i.e., corrosive wear or tribocorrosion, have been focused on the interplay between mechanical wear and electrochemical corrosion in a tribo-electrochemical system [4–7]. This requires an understanding of the relative contribution of mechanical wear, corrosion and their synergism to total material removal from the system. While the contribution of corrosion can be derived through electrochemical measurements of current and potential, it is still debatable regarding how the mechanical wear and the synergism can be separated in a typical tribocorrosion study. A common approach used to derive the pure mechanical wear component involves friction and wear testing at a cathodic potential in the same electrolyte [2,8–10]. Because electrochemical corrosion is limited at cathodic potentials, material loss from the contact area is regarded as being due to mechanical wear only. However, it is very likely that such mechanical wear behaviour is dependent on potential even in the cathodic region because the cathodic reactions are potential-dependent, which can change the properties of the contact surface and modify the contact interface during friction and wear. Thus, uncertainties still exist regarding the selection of a cathodic potential in the test to derive the pure mechanical wear component during

Lubricants 2020, 8, 101; doi:10.3390/lubricants8110101 www.mdpi.com/journal/lubricants Lubricants 2020, 8, 101 2 of 15 tribocorrosion. Indeed, cathodic protection against tribocorrosion has been observed by several investigators [11–13]. In a recent study [14], it was found that when a biomedical CoCrMo alloy was tested at two different cathodic potentials in a NaCl containing electrolyte under sliding wear conditions, the resultant material removal rates were different by several times, indicating potential dependence of such cathodic protection. Cathodic polarisation of metals can induce hydrogen charging and hydrogen segregation on the surface. While the effect of hydrogen on the properties of metals, particularly hydrogen embrittlement and deformation behaviour, has been well documented [15,16], relatively little is known regarding how hydrogen affects the friction and wear behaviour. A reduction in coefficient of friction due to hydrogen charging during cathodic polarisation has been reported by several investigators [17–20]. However, contradictory results were published regarding the effect of hydrogen charging on wear rate: it may reduce wear rate [11,21] or increase wear rate [17–20], depending on material type, contact load, and tribological conditions. CoCrMo alloy is an important biomedical alloy widely used in medical implants. In such biomedical systems, the load-bearing surfaces are in contact with the host body fluids and suffer from combined mechanical wear and corrosion attacks, i.e., tribocorrosion. It is necessary to investigate the tribocorrosion behaviour of biomaterials under physiological conditions. Indeed, tribocorrosion of CoCrMo alloy has been the subject of many studies at a wide range of anodic potentials [12,22–25]. However, very few systematic studies have been reported on the effect of varying cathodic potentials. In most of the tribocorrosion studies of this alloy, a cathodic potential was arbitrarily selected for the purpose of comparison with the tribocorrosion behaviour at anodic potentials [12,23,24]. In this work, experiments were conducted to systematically study the effect of cathodic potential on the friction and wear performance of a CoCrMo alloy in a simulated physiological media, i.e., 0.9% NaCl solution, at the body temperature of 37 ◦C. A systematic investigation has been conducted at cathodic potentials ranging from 500 mV (saturated calomel electrode, SCE) to 1000 mV (SCE)) and − − under various loads (1 N to 10 N). The results are presented and discussed in this paper.

2. Materials and Methods The material investigated was a cast CoCrMo alloy grade ASTM F75 with composition listed below (in wt%): 29.03% Cr, 7.14% Mo, 0.23% C, and Co (balance). Specimens with dimensions of 11.8 mm diameter and 4.5 mm thickness were machined and the surfaces to be tested were smoothed by grinding and then polishing to obtain a fine surface finish of 0.04 µm (Ra). The specimens were then cleaned in methanol for 900 s in an ultrasonic bath before testing. A pin-on-disk sliding machine was used to perform the sliding tests, which allows for the continuous recording of frictional force during sliding. To facilitate the recording and application of electrochemical potential and current during the sliding process, an electrochemical potentiostat was incorporated in the tribometer through a specially designed test cell made of nylon, as reported elsewhere [10,14]. 0.9 wt% NaCl in deionised water was used as the electrolyte. An inert and insulating sintered Al2O3 ball of 7.9 mm diameter was the counterpart (slider) in the sliding pair. Prior to testing, an insulating lacquer was used to mask each specimen, leaving a test area of 10 mm diameter to be exposed to the electrolyte. An SCE was the reference electrode, which was inserted into 200 mL electrolyte contained in the test cell. A platinum mesh served as the counter electrode. During sliding, the specimen was rotating against the stationary alumina slider at 60 rpm for a sliding time of 5400 s, producing a 6 mm diameter circular sliding track. All tests were done at 37 ◦C electrolyte temperature. Two tests were conducted under each condition and the results of both tests are reported in this paper. The first set of sliding tests were designed to study the effect of cathodic potential on friction and wear. These were implemented under a small load of 1 N at open circuit potential (OCP) and at various constant cathodic potentials from 500 mV (SCE) to 1000 mV (SCE). This small load creates − − a maximum contact pressure of 628 MPa and a maximum shear stress of 196 MPa, thus avoiding severe bulk plastic deformation because the shear strength of the investigated alloy is about 250 MPa. Lubricants 2020, 8, 101 3 of 15

ToLubricants facilitate 2020 the, 8,continuous x FOR PEER REVIEW recording of potential and current before and after sliding, the specimen3 of 15 was rested in the electrolyte at the applied potential for 300 s before and after sliding. The second series of experiments were designed to study the effect of load on friction and wear at Thea cathodic second potential. series of experimentsThese were done were at designed the constant to study potential the e ffofect −900 of loadmV on(SCE) friction under and various wear at a cathodic potential. These were done at the constant potential of 900 mV (SCE) under various loads loads from 1 N to 10 N. The loads of 2 N to 10 N result in contact− pressures from 790 MPa to 1350 fromMPa, 1 N which to 10 N.would The loadsinduce of bulk 2 N toplastic 10 N deformation result in contact of the pressures contact fromsurface 790 and MPa increased to 1350 MPa, material which wouldremoval induce rates. bulk plastic deformation of the contact surface and increased material removal rates. TheThe electrochemical electrochemical currents currents and and coe coefficientfficient of frictionof friction (COF) (COF) were were recorded recorded during during the the test. test. After theAfter test, the test, sliding the wornsliding surfaces worn surfaces were microscopically were microscopically examined examined using opticalusing optical and scanning and scanning electron microscopeselectron microscopes (SEM). A (SEM). surface A profilometer surface profilometer was used was to used measure to measure the cross-sectional the cross-sectional profile profile of the slidingof the track. sliding This track. allows This forallows the for evaluation the evaluation of total of material total material loss (TML) loss (TML) from from the sliding the sliding track. track.

3.3. Results Results

3.1.3.1. Sliding Sliding at at Open Open Circuit Circuit under under 1 1 NN LoadLoad ExperimentsExperiments were were first first conductedconducted atat openopen circuitcircuit with with the the purpose purpose to to establish establish the the cathodic cathodic potentialpotential range range for for the the subsequent subsequent tests.tests. FigureFigure 11 showsshows the evolution of of OCP OCP recorded recorded during during two two repeatedrepeated tests tests under under 1 1 N N load. load. ItIt cancan bebe seenseen thatthat the two tests tests under under similar similar conditions conditions registered registered reproduciblereproducible OCP OCP evolution evolution curves. curves. PriorPrior toto sliding, the the OCP OCP increased increased with with time time due due to tothe the gradual gradual build-upbuild-up of of the the oxide oxide film film [ 3[3,4,7].,4,7]. ImmediatelyImmediately after the the start start of of sliding, sliding, the the OCP OCP dropped dropped by by 150 150 mV mV andand then then maintained maintained at at a a value value around around −450450 mV (SCE) during during the the sliding sliding period. period. This This is isowing owing to tothe the − periodicperiodic destruction destruction or or removal removal of of the the oxideoxide filmfilm in the contact contact area, area, a atypical typical phenomenon phenomenon observed observed inin tribocorrosion tribocorrosion of of passive passive metals metals [[5,6,10].5,6,10]. After the the termination termination of of sliding, sliding, the the OCP OCP rose rose to toreach reach valuesvalues even even higher higher than than that that recorded recorded beforebefore sliding.sliding. The The repassivation repassivation of of the the contact contact area area and and the the continuous thickening of the oxide film in areas outside the contact area during the test period are continuous thickening of the oxide film in areas outside the contact area during the test period are responsible for such an increase. The COF during sliding and the TML from the sliding track were responsible for such an increase. The COF during sliding and the TML from the sliding track were also also measured. The results are presented in the following sections. measured. The results are presented in the following sections.

−200-200 1 N, 60 rpm, 37 °C, OCP −250-250

−300-300 Test 2

−350-350 Sliding −400-400 OCP (mV/SCE) OCP −450-450 Test 1 −500-500

−550-550 0 1000 2000 3000 4000 5000 6000 Time (s) Figure 1. Recorded open circuit potential (OCP) before, during and after sliding under a contact load Figure 1. Recorded open circuit potential (OCP) before, during and after sliding under a contact load of 1 N in 0.9% NaCl solution at 37 ◦C, for two repeated tests. of 1 N in 0.9% NaCl solution at 37 °C, for two repeated tests. 3.2. Sliding at Cathodic Potential under 1 N Load 3.2. Sliding at Cathodic Potential under 1 N Load After establishing the OCP ( 450 mV (SCE)) during sliding, a series of sliding tests were conducted − potentiostaticallyAfter establishing at cathodic the OCP potentials (−450 mV ranging (SCE)) during from sliding,500 MV a series (SCE) of to sliding1000 tests mV were (SCE). conducted Cathodic potentiostatically at cathodic potentials ranging from −−500 MV (SCE) to −−1000 mV (SCE). Cathodic

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Lubricants 2020, 8, x FOR PEER REVIEW 4 of 15 potentials more negative than 1000 mV (SCE) were not used in this work because tribocorrosion tests potentials more negative than− −1000 mV (SCE) were not used in this work because tribocorrosion tests are rarely conducted under such conditions. Figure2a shows the current transient curves recorded at are rarely conducted under such conditions. Figure 2a shows the current transient curves recorded at 1 1 N load. Clearly, at all the applied potentials, the recorded currents were negative. At each potential, N load. Clearly, at all the applied potentials, the recorded currents were negative. At each potential, duringduring the the ﬁrst first 300 300 s immersions immersion without without sliding,sliding, thethe value of the the negative negative current current increased increased with with time time (i.e.,(i.e., the the current current shifted shifted downwards downwards inin FigureFigure2 2a).a). DuringDuring sliding,sliding, the value value of of the the negative negative current current increasedincreased ﬁrst first and and then then decreased. decreased. Figure Figure2 a2a also alsoshows shows thatthat despitedespite the noticeable noticeable scatter scatter in in cathodic cathodic currentcurrent at at each each potential, potential, there there is is a a clear clear trendtrend thatthat thethe value of the cathodic cathodic current current increased increased with with increasingincreasing cathodic cathodic potential. potential. At At −500500 mVmV (SCE),(SCE), which was was close close to to the the OCP OCP and and was was in in the the transition transition − regionregion between between anodic anodic dissolution dissolution and and cathodiccathodic reduction,reduction, the the value value of of the the negative negative current current was was very very small,small, around around 0.01−0.01 mA. mA. Increasing Increasing thethe valuevalue ofof cathodiccathodic potential to to −10001000 mV mV (SCE) (SCE) increased increased the the − − negativenegative current current to to around around 0.4−0.4 mA, mA, which which isis 4040 timestimes the value at at −500500 mV mV (SCE). (SCE). Figure Figure 2b2 bshows shows the the − − averageaverage negative negative current current measured measured during during slidingsliding atat various applied applied cathodic cathodic potentials. potentials. As As expected, expected, thethe value value of of the the negative negative current increased increased with with increasing increasing cathodic cathodic potential potential in the in range the rangebetween between −500 500mV mV (SCE) (SCE) and and−10001000 mV (SCE) mV (SCE) tested tested in this inwork. this work. − −

(a) 00 −500 mV −600 mV -0.1−0.1

−700 mV -0.2−0.2 −800 mV

−900 mV -0.3−0.3 Current (mA) Current

-0.4−0.4 −1000 mV

-0.5−0.5 0 1000 2000 3000 4000 5000 6000 Time (s) 0.1 1 (b) Anodic 00 OCP Cathodic -0.1−0.1

-0.2−0.2

-0.3−0.3 Avereage (mA) Current Avereage −0.4-0.4 1 N, 60 rpm, 37 °C -0.5−0.5 −1100-1100−900 -900 −700 -700 −500 -500−300 -300 Potential (mV/SCE)

Figure 2. (a) Recorded currents at various cathodic potentials under a contact load of 1 N in 0.9% NaCl Figure 2. (a) Recorded currents at various cathodic potentials under a contact load of 1 N in 0.9% NaCl solution at 37 C, and (b) Average current during sliding as a function of applied cathodic potential. solution at 37◦ °C, and (b) Average current during sliding as a function of applied cathodic potential. Figure3a shows typical COF curves measured during sliding at various applied cathodic potentials. Figure 3a shows typical COF curves measured during sliding at various applied cathodic Severalpotentials. interesting Several observations interesting observations can be made can here. be made At OCP, here. the At measuredOCP, the measured COF curve COF was curve similar was to thatsimilar reported to that previously reported [ 14previously], i.e., after [14], the i.e., initial after quick the initial rise, thequick COF rise, increased the COF slowlyincreased with slowly sliding with time

Lubricants 2020, 8, 101 5 of 15 to reach values around 0.8. At 500 mV (SCE), the COF followed the same trend as that at OCP, but the Lubricants 2020, 8, x FOR PEER REVIEW− 5 of 15 overall COF values were much higher, reaching around 1.2 at the later stage of sliding. Higher friction whensliding sliding time at cathodic to reach values potentials around has 0.8. been At observed−500 mV (SCE), during the tribocorrosion COF followed testingthe same of trend several as that passive metalsat [OCP,10,14 but,26], the and overall this COF phenomenon values were is much attributed higher, to reaching the lack around of an oxide 1.2 at the film later at cathodicstage of sliding. potentials, whichHigher could friction serve aswhen a lubricant sliding at tocathodic reduce potentials friction. has However, been observed this was during true tribocorrosion only when the testing applied cathodicof several potential passive was metals close to[10,14,26], OCP, e.g., and 500this mVphenomen (SCE).on When is attributed the cathodic to the potentiallack of an wasoxide increased film at to − morecathodic negative potentials, than 600 which mV (SCE),could serve the friction as a lubricant behaviour to reduce of the friction. sliding However, couple became this was quite true only different. − At 600when mV the (SCE), applied the cathodic COF was potential initially was similar close toto thatOCP, recorded e.g., −500 at mV OCP, (SCE). but startedWhen the to declinecathodic after − aboutpotential 3000 s sliding. was increased The most to more significant negative observation than −600 wasmV (SCE), that at the cathodic friction potentials behaviour between of the sliding700 mV couple became quite different. At −600 mV (SCE), the COF was initially similar to that recorded− at (SCE) and 1000 mV (SCE), the COF followed a very interesting characteristic pattern. At the early OCP, −but started to decline after about 3000 s sliding. The most significant observation was that at stagecathodic of sliding, potentials the COF between increased −700 withmV (SCE) time and to reach−1000 amV maximum (SCE), the value,COF followed then it decreaseda very interesting gradually to reachcharacteristic a relatively pattern. low and At the stable early value. stage of The slidin stableg, the COF COF valueincreased achieved with time decreased to reach witha maximum increasing cathodicvalue, potential, then it decreased i.e., at 1000gradually mV (SCE),to reach the a relatively stable COF low valueand stable achieved value. was The asstable low COF as 0.02. value From − Figureachieved3a, it can decreased also be with seen increasing that the criticalcathodic sliding potential, time, i.e., afterat −1000 which mV (SCE), the stable the stable low frictionCOF value value was achieved,achieved was decreased as low as with 0.02. increasing From Figure cathodic 3a, it can potential. also be seen The that maximum the critical COF sliding value time, measured after duringwhich sliding the atstable the earlylow friction stage alsovalue decreased was achieved, with decreased increasing with cathodic increasing potential. cathodic Clearly, potential. the The present workmaximum demonstrates COF thatvalue increasing measured cathodicduring sliding potential at th hase early the stage benefit also of decreased reducing with friction increasing during the cathodic potential. Clearly, the present work demonstrates that increasing cathodic potential has the entire sliding period, and such a friction-reducing ability becomes more effective after sliding for a benefit of reducing friction during the entire sliding period, and such a friction-reducing ability certain period. becomes more effective after sliding for a certain period.

1.4 ° (a) 1N, 60 rpm, 37 C −500mV 1.2

1.0 OCP 0.8

0.6 −600mV

0.4

Coefficient of Friction −700mV 0.2

0.0 0 1000 2000 3000 4000 5000 6000 Time (s) 1.4 (b) 1.2

0.8

OCP 0.6

0.4

Average Average Coefficient of Friction 0.2 1 N, 60 rpm, 37 °C 0 −1100-1100−900 -900−700 -700−500 -500−300 -300 Potential (mV/SCE) Figure 3. (a) Recorded coeﬃcient of friction (COF) curves during sliding at OCP and at various applied cathodic potentials under 1 N load; and (b) measured average COF versus applied cathodic potential. Lubricants 2020, 8, x FOR PEER REVIEW 6 of 15

Figure 3. (a) Recorded coefficient of friction (COF) curves during sliding at OCP and at various

Lubricantsapplied2020, cathodic8, 101 potentials under 1 N load; and (b) measured average COF versus applied cathodic6 of 15 potential.

InIn FigureFigure3 b,3b, the the average average COF COF calculated calculated for for the the entire entire sliding sliding period period in in each each test test was was plotted plotted againstagainst appliedapplied cathodic cathodic potential. potential. It It clearly clearly shows shows that that sliding sliding at at500 −500 mV mV (SCE) (SCE) led led to anto an increase increase in − averagein average COF, COF, and and at furtherat further higher higher cathodic cathodic potentials, potentials, the the average average COF COF decreased decreased with withincreasing increasing potentialpotential toto reachreach anan averageaverage valuevalue belowbelow 0.10.1 at at −10001000 mVmV (SCE).(SCE). − InIn lineline withwith thethe reducedreduced COFCOF asas thethe appliedapplied cathodiccathodic potentialpotential waswas increased,increased, thethe TMLTML fromfrom thethe slidingsliding tracktrack was was also also reduced. reduced. As As shown shown in in Figure Figure4, the4, the TML TML decreased decreased continuously continuously as theas the value value of cathodicof cathodic potential potential was was increased. increased. The The TML TML at at900 −900 mV mV (SCE) (SCE) and and −10001000 mV mV (SCE) (SCE) waswas anan orderorder ofof − − magnitudemagnitude lowerlower thanthan thethe TMLTML atat OCP.OCP. ItIt isis also also notednoted thatthat although although the the COF COF measured measured at at −500500 mVmV − (SCE)(SCE) waswas muchmuch largerlarger than than that that measured measured at at OCP OCP (Figure (Figure3 ),3), the the TML TML at at −500500 mVmV (SCE)(SCE) waswas smallersmaller − thanthan thatthat atat OCPOCP (Figure (Figure4 ).4). The The inset inset in in Figure Figure4 shows 4 shows typical typical sliding sliding track track proﬁles profiles measured measured using using a proﬁlometer.a profilometer. Clearly, Clearly, the the sliding sliding depth depth decreased decreased with with increasing increasing value value of cathodic of cathodic potential. potential. At OCP, At theOCP, sliding the sliding depth wasdepth about was 1.4aboutµm, 1.4 whilst μm, whilst at 900 at mV −900 (SCE) mV it(SCE) was it less was than less 0.2 thanµm. 0.2 μm. −

0.4 −900mV −600mV 0.00 m) 1.2×101.2E-04-4 μ −0.4-0.4 −0.8-0.8 Depth (

) −500mV 3 1.0×101.0E-04-4 −1.2-1.2 OCP

−1.6-1.6 8.0×108.0E-05-5 0.0 0.1 0.2 0.3 0.4 Horizontal Distance (mm)

6.0×106.0E-05-5 CoCrMo, 37 °C, 1N 60rpm, 5400s 4.0×104.0E-05-5 Total Material Loss (mm 2.0×102.0E-05-5 Cathodic OCP

0.0E+000.0 −1100-1100−900 -900−700 -700 −500 -500 −300 -300 Applied Potential (mV(SCE))

FigureFigure 4.4. TotalTotal materialmaterial loss loss (TML) (TML) versus versus applied applied cathodic cathodic potential potential after after sliding sliding at at 1 N1 N load load for for 5400 5400 s. Ths. Th inset inset shows shows typical typical cross-sectional cross-sectional proﬁles profiles of theof the sliding sliding tracks tracks measured measured by aby proﬁlometer. a profilometer.

ItIt hashas commonlycommonly beenbeen observedobserved duringduring tribocorrosiontribocorrosion testingtesting ofof passivepassive metalsmetals thatthat thethe TMLTML measuredmeasured at at cathodic cathodic potentials potentials is is less less than than the the TML TML measured measured at at OCP OCP and and at anodicat anodic potentials, potentials, due due to theto the diminished diminished contribution contribution of corrosion of corrosion and theand synergism the synergism between between corrosion corrosion and mechanical and mechanical wear towear TML to TML at cathodic at cathodic potentials. potentials. Thus, Thus, TML TML measured measured at at a cathodica cathodic potential potential has has been been commonlycommonly regardedregarded asas beingbeing mainlymainly duedue toto mechanicalmechanical wear.wear. ThisThis purepure mechanicalmechanical wearwear component,component, togethertogether withwith thethe measuredmeasured TMLTML andand thethe currentcurrent duringduring sliding,sliding, hashas beenbeen usedused toto derivederive thethe contributioncontribution ofof corrosion-wearcorrosion-wear synergismsynergism toto TMLTML atat OCPOCP andand atat anodicanodic potentialspotentials [[8–10,26].8–10,26]. However,However, thethe presentpresent workwork demonstratesdemonstrates thatthat thethe TMLTML atat cathodiccathodic potentialspotentials doesdoes notnot havehave aa singlesingle valuevalue eveneven underunder thethe same mechanical loading and sliding conditions and it is potential-dependent. The TML at 500 mV same mechanical loading and sliding conditions and it is potential-dependent. The TML at− −500 mV (SCE) is 6 times larger than that at 1000 mV (SCE). Thus, there are other factors that can contribute to (SCE) is 6 times larger than that at− −1000 mV (SCE). Thus, there are other factors that can contribute theto the mechanical mechanical wear wear at cathodic at cathodic potentials, potentials, as discussedas discussed in Section in Section4. 4.

Lubricants 2020, 8, 101 7 of 15

3.3. SlidingLubricants at 2020900, 8, x mV(SCE) FOR PEER REVIEW under Various Contact Loads 7 of 15 − The3.3. resultsSliding at presented −900 mV(SCE) in Section under Various3.2 were Contact obtained Loads under a relatively small load of 1 N. A question can be raised here, i.e., whether the observed low COF and low TML at large cathodic potentials The results presented in Section 3.2 were obtained under a relatively small load of 1 N. A could also apply at higher contact loads. Thus, further experiments were conducted at 900 mV (SCE) question can be raised here, i.e., whether the observed low COF and low TML at large− cathodic underpotentials various could loads also from apply 1 N at to higher 10 N. contact Figure lo5 ads.shows Thus, the further recorded experiments COF curves were conducted (Figure5a) at and−900 the variationmV (SCE) of average under various COF with loads load from (Figure 1 N to 510b). N. AtFigure OCP, 5 shows friction the decreased recorded COF with curves increasing (Figure contact5a) load, withand the the variation average of COF average decreased COF with from load about (Figure 0.8 5b). under At OCP, 1 N load friction to 0.65decreased under with 10 N increasing load. On the othercontact hand, at load,900 with m Vthe (SCE), average friction COF decreased was always from lower about than 0.8 under thatat 1 N OCP load under to 0.65 all under employed 10 N load. contact − loadsOn and the the other variation hand, at of −900 COF mV with (SCE), load friction followed was always an opposite lower than trend: that COF at OCP increased under all with employed increasing load.contact The COF loads curves and recordedthe variation at of900 COF mV with (SCE) load under followed small an loads, opposite e.g., trend: 1 N and COF 2 N,increased follow with a similar − pattern:increasing the COF load. rose The rapidly COF curves during recorded the early at stage −900 tomV reach (SCE) a under maximum small value,loads, ande.g., then1 N and it decreased 2 N, follow a similar pattern: the COF rose rapidly during the early stage to reach a maximum value, and gradually to reach low values below 0.2 under 2 N load, and below 0.05 under 1 N load. The COF then it decreased gradually to reach low values below 0.2 under 2 N load, and below 0.05 under 1 N underload. 2 N The load COF was under always 2 N larger load was than always that underlarger than 1 N load.that under However, 1 N load. when However, the high when contact the high load of 10 N wascontact applied, load of the10 N COF wasvalue applied, was the maintained COF value was at a maintained constant value at a constant of around value 0.4 of throughout around 0.4 the slidingthroughout period. the sliding period.

1.0 (a) −900 mV 1N

0.8 2N

5N OCP 0.6 10N

0.4 10N

5N Coefficient Friction of 0.2 2N −900mV 1N 0.0 0 1000 2000 3000 4000 5000 6000 7000 Time (s) 1 (b) −900 mV

0.8

OCP 0.6

0.4 −900 mV

0.2 Average Average of Coefficient Friction

0 024681012 Contact Load (N)

Figure 5. (a) Recorded COF curves at OCP and at 900 mV (saturated calomel electrode, SCE) under Figure 5. (a) Recorded COF curves at OCP and at− −900 mV (saturated calomel electrode, SCE) under variousvarious contact contact loads; loads; and and (b) ( measuredb) measured average average COF COF asas aa function of of load. load.

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Figure6 6 shows shows the the measured measured TML TML from from the the sliding slidin trackg track as aas function a function of contact of contact load load at OCP at OCP and andat 900at − mV900 (SCE).mV (SCE). The TMLThe TML increased increased with loadwith at load both at potentials, both potentials, which iswhich expected is expected from the from general the − generalprinciple principle of wear: of increased wear: increased mechanical mechanical loading loading leads toleads increased to increased mechanical mechanical wear. wear. At 900At − mV900 − mV(SCE), (SCE), the TML the TML under under all loads all loads was much was smallermuch smaller than that than measured that measured at OCP, at indicating OCP, indicating the cathodic the cathodicprotection protection ability of ability the alloy. of the In alloy. the inset In the of inset Figure of 6Figure, the ratio 6, the of ratio TML of at TML OCP at toOCP that to at that900 at − mV900 − mV(SCE) (SCE) was plottedwas plotted as a function as a function of load. of Clearly, load. Clea thisrly, ratio this decreased ratio decrease with increasingd with increasing load, indicating load, indicatingdecreasing decreasing cathodic protection cathodic asprotection load was as increased. load was Underincreased. 1 N load,Under the 1 N TML load, at OCPthe TML was at 9 timesOCP waslarger 9 times than thatlarger at than900 that mV at (SCE). −900 IncreasingmV (SCE). theIncreasing load to 5the N load and 10to 5 N, N the and TML 10 N, at the OCP TML was at about OCP − was4 times about larger 4 times than larger that at than900 that mV at (SCE). −900 mV (SCE). −

2.1×102.1E-04-4 CoCrMo, 37oC -4 60rpm, 5400s

) 1.8×101.8E-04 3 at OCP 1.5×101.5E-04-4

10 1.2×101.2E-04-4 8 6 Ratio 4 -5 2 9.0×109.0E-05 TML 0 024681012 Contact Load (N) 6.0×106.0E-05-5 Total Material Loss (mm 3.0×103.0E-05-5 at −900mV

0.0E+000.0 024681012 Contact Load (N)

Figure 6. TML versus contactcontact load,load, measured measured from from the the sliding sliding tracks tracks after after sliding sliding at at900 −900 mV mV (SCE) (SCE) for − for5400 5400 s under s under various various contact contact loads loads from from 1 N to1 N 10 to N. 10 The N. insetThe inset shows shows the ratio the ratio of TML of TML at OCP at OCP to TML to TMLat 900 at − mV900 (SCE) mV (SCE) as a functionas a function of load. of load. − The above observation on the effect eﬀect of contact load on cathodic protection agrees with the observation of Akonka et. et al.al. [[11]11] onon stainlessstainless steelsteel slidingsliding at cathodiccathodic potentials, who observed that sliding atat cathodiccathodic potentials potentials resulted resulted in muchin much lower lower wear wear rates rates than slidingthan sliding at OCP, at and OCP, such and cathodic such cathodicprotection protection disappeared disappeared under high under contact high loads. contact Akonka loads. et al. Akonka [11] attributed et. al. [11] such attributed a phenomenon such toa phenomenonthe hydrogen to charging the hydrogen at the cathodic charging potential. at the cathodic potential.

3.4. Morphology Morphology of of Sliding Sliding Tracks Microscopic images images of of the sliding tracks generate generatedd at various cathodic potentials are shown in Figure 77.. The morphologicalmorphological featuresfeatures ofof thethe slidingsliding trackstracks cancan bebe divideddivided intointo twotwo groups.groups. AtAt potentialspotentials less negative than 600 mV (SCE), including OCP and 500 mV (SCE), the sliding tracks were less negative than −600 mV (SCE), including OCP and −500 mV (SCE), the sliding tracks were characterized by by the the existence of of many scratch marks, i.e., i.e., grooves of of abrasive wear, parallel to the sliding direction (Figure 77c,d).c,d). SomeSome ofof thethe scratchesscratches werewere notnot continuous,continuous, butbut terminatedterminated withinwithin thethe track with some third-body particles (arrowed in Figure7 7c,d).c,d). ItIt isis believedbelieved thatthat thethe wearwear particlesparticles werewere accumulated on on the sliding track track and and stuck stuck on on the co counterfaceunterface to to cause cause abrasive abrasive wear wear of of the the surface. surface. There were no cracks observed in the sliding tracks produced at OCP, 500 mV (SCE) and 600 mV There were no cracks observed in the sliding tracks produced at OCP, −−500 mV (SCE) and −−600 mV (SCE). However, the scratches in the wear track produced at OCP were much wider and deeper than those produced at 500 mV (SCE) and 600 mV (SCE). The corrosion-wear products resulted from those produced at −500 mV (SCE) and −−600 mV (SCE). The corrosion-wear products resulted from removing the the oxide oxide film ﬁlm at at OCP OCP may may be be responsi responsibleble for for the the increased increased abrasion abrasion of of the the surface. surface.

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Figure 7. Microscopic images of the sliding tracks generated under 1 N load at OCP and various applied Figure 7. Microscopic images of the sliding tracks generated under 1 N load at OCP and various applied cathodic potentials. The small arrows indicate the terminated scratches with third-body particles. cathodic potentials. The small arrows indicate the terminated scratches with third-body particles. (a) (a) 1000 mV, (b) 700 mV, (c) 500 mV and (d) OCP. −1000− mV, (b) −700− mV, (c) −500− mV and (d) OCP. Another group of sliding tracks include those produced at potentials more negative than 600 mV Another group of sliding tracks include those produced at potentials more negative than− −600 (SCE) (Figure7a,b). These sliding tracks were characterized by two distinct features, i.e., (1) the mV (SCE) (Figure 7a,b). These sliding tracks were characterized by two distinct features, i.e., (1) the existence of fine scratch marks caused by the micro-abrasion of the counterface, and (2) the presence of existence of fine scratch marks caused by the micro-abrasion of the counterface, and (2) the presence many parallel lines which were inclined to the sliding direction. Most of these lines could hardly be of many parallel lines which were inclined to the sliding direction. Most of these lines could hardly observed under microscopes. Only through very careful examination by slightly adjusting the focus be observed under microscopes. Only through very careful examination by slightly adjusting the could the lines be revealed clearly. These lines were formed across the sliding track and did not extend focus could the lines be revealed clearly. These lines were formed across the sliding track and did not far beyond the track. Figure8a shows a high magnification SEM image revealing two parallel lines extend far beyond the track. Figure 8a shows a high magnification SEM image revealing two parallel with a spacing about 9 µm on the 1000 mV (SCE) sliding track. It is also noted from Figure8a that the lines with a spacing about 9 μm −on the −1000 mV (SCE) sliding track. It is also noted from Figure 8a µ linethat opening the line wasopening very was narrow, very aboutnarrow, 0.1 aboutm, which 0.1 μm, has which imposed has imposed some di ffisomeculty difficulty in revealing in revealing the lines under microscopes. Figure8b shows an SEM image of the sliding track generated at 500 mV (SCE). the lines under microscopes. Figure 8b shows an SEM image of the sliding track generated− at −500 Clearly,mV (SCE). no such Clearly, lines no were such observed lines were in the observ slidinged trackin the which sliding was track populated which was with populated abrasion markswith withabrasion accumulated marks with wear accumulated particles which wear could particles serve wh asich third-body could serve abrasives as third-body during the abrasives sliding during process. the sliding process.

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Figure 8. Scanning electron microscope (SEM) images of the sliding tracks generated under 1 N load at Figure 8. Scanning electron microscope (SEM) images of the sliding tracks generated under 1 N load (a) 1000 mV(SCE) revealing the parallel lines (arrowed), and (b) 500 mV (SCE) showing no such at (−a) −1000 mV(SCE) revealing the parallel lines (arrowed), and (b−) −500 mV (SCE) showing no such line formation. line formation. Similarly, the sliding tracks produced at 900 mV (SCE) under various contact loads can also be Similarly, the sliding tracks produced at− −900 mV (SCE) under various contact loads can also be divided into two groups (Figure9). The sliding tracks produced under relatively small loads, e.g., divided into two groups (Figure 9). The sliding tracks produced under relatively small loads, e.g., 1 1 N and 2 N, belong to one group, which were characterized by the existence of ﬁne and parallel N and 2 N, belong to one group, which were characterized by the existence of fine and parallel micro- micro-abrasion marks and the presence of parallel lines across the sliding track (Figure9a,b). On the abrasion marks and the presence of parallel lines across the sliding track (Figure 9a,b). On the other other hand, the sliding tracks produced under higher loads, e.g., 5 N and 10 N, belong to another hand, the sliding tracks produced under higher loads, e.g., 5 N and 10 N, belong to another group group (Figure9c,d), which were characterized by the existence of wider and deeper scratch marks in (Figure 9c,d), which were characterized by the existence of wider and deeper scratch marks in the the central region of the track, where no such lines were observed. However, near to the edges of these central region of the track, where no such lines were observed. However, near to the edges of these sliding tracks, where the scratch marks were ﬁner and abrasive wear was less severe, such lines could sliding tracks, where the scratch marks were finer and abrasive wear was less severe, such lines could be observed (arrowed areas in Figure9c,d). Thus, it is likely that such lines also formed across the be observed (arrowed areas in Figure 9c,d). Thus, it is likely that such lines also formed across the sliding tracks under 5 N and 10 N loads, but in the central area of the track, the lines were removed sliding tracks under 5 N and 10 N loads, but in the central area of the track, the lines were removed due to enhanced material removal in this region. due to enhanced material removal in this region.

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FigureFigure 9.9. Microscopic imagesimages ofof thethe slidingsliding trackstracks generatedgenerated atat −900 mV (SCE) underunder variousvarious contactcontact − loads,loads, ((aa)) 11 N,N, ((bb)) 2N,2N, ((cc)) 55 NN andand ((dd)) 1010 N.N.

4.4. Discussion TheThe investigatedinvestigated CoCrMo alloy is a passive alloy with an oxide ﬁlmfilm before the sliding tests. DuringDuring sliding,sliding, thethe oxideoxide ﬁlmfilm isis damageddamaged oror eveneven removedremoved suchsuch thatthat thethe OCPOCP isis cathodicallycathodically shiftedshifted from around 320 mV (SCE) to 450 mV (SCE). Sliding tests at cathodic potentials ranging from from around −320 mV (SCE) to −450− mV (SCE). Sliding tests at cathodic potentials ranging from −500 500 mV (SCE) to 1000 mV (SCE) in this work illustrate that the friction and wear behaviour depends −mV (SCE) to −1000− mV (SCE) in this work illustrate that the friction and wear behaviour depends on onthe the cathodic cathodic potential potential and and the the load load applied. applied. The The fric frictiontion behaviour behaviour can can be bedivided divided into into two two types, types, as asevidenced evidenced in inFigure Figure 3a3 aand and schematically schematically illustrated illustrated in in Figure Figure 10.10. Type Type 1 isis characterizedcharacterized byby thethe gradualgradual increaseincrease inin COFCOF duringduring thethe earlyearly stagestage ofof slidingsliding toto reachreach aa steadysteady state.state. ThisThis frictionalfrictional behaviour was observed at OCP and 500 mV (SCE). Type 2, which is observed at potentials more behaviour was observed at OCP and −−500 mV (SCE). Type 2, which is observed at potentials more cathodic than 600 mV (SCE), is characterised by 3 distinct frictional zones: in Zone I, COF increases cathodic than −−600 mV (SCE), is characterised by 3 distinct frictional zones: in Zone I, COF increases withwith slidingsliding timetime duringduring thethe earlyearly stage;stage; inin ZoneZone 2,2, COFCOF decreasesdecreases graduallygradually withwith timetime asas slidingsliding continues;continues; andand inin Zone Zone 3, 3, relatively relatively stable stable and and low low COF COF is achieved.is achieved. The The COF COF values values in each in each zone zone and theand time the time to reach to reach each each zone zone decrease decrease with with increasing increasing cathodic cathodic potential. potential.

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Type 1 OCP, −500mV(SCE) COF

Type 2 < −600 mV(SCE)

III I II Sliding Time

FigureFigure 10. 10. SchematicSchematic diagram diagram showing showing the the two two differ diﬀerentent categories categories of of friction friction curves. curves.

InIn the the cathodic cathodic potential potential range range tested tested in in this this work, work, there there are are three three possible possible cathodic cathodic reduction reduction reactions,reactions, i.e., i.e., oxygen, oxygen, proton proton and and water water reductions reductions [24,27]. [24,27]. Oxygen Oxygen reduction reduction can can occur occur at at potentials potentials belowbelow OCP, OCP, thus thus the the prior prior existing existing oxide oxide film film ma mayy be be dissolved dissolved at at applied applied cathodic cathodic potentials. potentials. However,However, the the oxide oxide film film on on CoCrMo CoCrMo alloy alloy mainly mainly comprises comprises chromium chromium oxide oxide that that cannot cannot be be dissolved dissolved cathodicallycathodically [27,28]. [27,28 ].Thermodynamically, Thermodynamically, the the proton proton reduction reduction can can only only occur occur at at potentials potentials below below the the reversible potential, i.e., more negative than 650 mV (SCE) for the present pH (7.2) and temperature reversible potential, i.e., more negative than −−650 mV (SCE) for the present pH (7.2) and temperature (37(37 °C).◦C). Proton Proton reduction reduction can can lead lead toto the the adsorpti adsorptionon of ofhydrogen hydrogen on onthe the cathode cathode surface, surface, forming forming a hydrogena hydrogen segregation segregation layer layer at the at metal the metal surface surface [29,30 [29]. ,This30]. critical This critical cathodic cathodic potential potential coincides coincides with with the experimental result that only at potentials below 600 mV (SCE) is the characteristic low the experimental result that only at potentials below −600 mV− (SCE) is the characteristic low friction behaviourfriction behaviour observed observed(type 2 in (typeFigure 2 10). in Figure Thus, 10hydr). Thus,ogen charging hydrogen seems charging have seems played have a key played role here. a key However,role here. the However, concentration the concentration of protons at of pH7.2 protons is ve atry pH7.2 low and is very its contribution low and its contributionto overall current to overall and hydrogencurrent and charging hydrogen is also charging low. The is also most low. likely The mostreduction likely reaction reduction is water reaction reduction, is water reduction,which is kineticallywhich is kineticallyinhibited inhibiteduntil a sufficient until a su overpotentialfficient overpotential is applied is appliedbelow the below reversible the reversible potential. potential. It is It is expected that at potentials below 700 mV (SCE), water reduction occurs, which starts with the expected that at potentials below −700− mV (SCE), water reduction occurs, which starts with the adsorptionadsorption of of H H on on the the metal metal surface. surface. The The recombination recombination of two of two adsorbed adsorbed H leads H leads to the to theformation formation of Hof2 molecule H2 molecule and hydrogen and hydrogen evolution evolution [31]. In [31 many]. In cases, many depending cases, depending on the nature on the of nature the metal of the surface, metal a surface,H segregated a H segregated layer, in the layer, form in of the either form a of solid either solution a solid or solution metal hy ordrides, metal hydrides,is formed isat formed the surface, at the leadingsurface, to leading changes to in changes structures in structures and properties and properties of the surface. of the Accordingly, surface. Accordingly, the two types the two of friction types of behaviourfriction behaviour observed observedin this work in this(Figure work 10) (Figure can be 10 explained) can be explainedas follows. as follows. AtAt OCP OCP and and −500500 mV mV (SCE), (SCE), hydrogen hydrogen reduction reduction is isinhi inhibitedbited and and thus thus the the surface surface is is free free from from − hydrogenhydrogen segregation. segregation. Sliding Sliding under under these these two two conditions exhibits similarsimilar frictionalfrictional behaviourbehaviour (Type (Type 1). 1).COF COF increases increases with with time time during during the the early early stage stage of sliding,of sliding, because because of the of gradualthe gradual removal removal of the of priorthe priorexisting existing oxide oxide film film and theand gradualthe gradual increase increase in contact in contact area, area, until until a steady a steady stage stage is reached. is reached. At OCP, At OCP,the oxide the oxide film isfilm removed is removed or damaged or damaged after eachafter slidingeach sliding cycle butcycle is but reformed is reformed during during each sliding each slidinginterval, interval, while while at 500 at mV−500 (SCE), mV (SCE), once theonce prior the prior existing existing oxide oxide film isfilm removed, is removed, reformation reformation of the − ofoxide the oxide film during film during sliding sliding interval interval is inhibited is inhibited because atbecause this cathodic at this potential cathodic oxygen potential reduction oxygen is reductionpreferred. is Thus, preferred. the COF Thus, at OCPthe COF is smaller at OCP than is thatsmaller at 500than mV that (SCE) at −500 because mV (SCE) the formation because ofthe an − formationoxide film of helps an oxide to reduce film frictionhelps to [ 10reduce]. However, friction when [10]. theHowever, TML from when the the sliding TML trackfrom isthe concerned, sliding trackFigure is 4concerned, clearly shows Figure that 4 theclearly TML shows at 500 that mV the (SCE) TML isat actually−500 mV smaller (SCE) is than actually that at smaller OCP. This than can that be − atexplained OCP. This by can the lackbe explained of contribution by the of lack corrosion of contribution and the synergism of corrosion between and corrosionthe synergism and mechanical between corrosionwear to TML and atmechanical cathodic potentials. wear to TML Thus at the cathodic TML at potentials.500 mV (SCE) Thus canthe beTML treated at −500 as the mV result (SCE) of can pure − be treated as the result of pure mechanical wear without much of a contribution from corrosion and

Lubricants 2020, 8, 101 13 of 15 mechanical wear without much of a contribution from corrosion and hydrogen charging. The principle wear mechanism is abrasive wear, evidenced by the sliding track morphology (Figures7c and8b). At cathodic potentials from 700 mV (SCE) to 1000 mV (SCE), type 2 frictional behaviour was − − observed (Figure3a), which can be related to hydrogen charging. The amount of H adsorption depends on the electrolyte, the applied potential, the electrocatalyst nature of the metal surface, and particularly the number of active sites on the surface [31,32]. It is understood that mechanical loading, shear deformation and surface roughening can accelerate the adsorption of hydrogen due to increased active sites [33,34]. At a constant cathodic potential, during the early stage of sliding, hydrogen segregation is not built up yet. As the slider slides over the surface, the contact area is increased with time and so does friction, leading to the observed Zone I in Figure 10. Meanwhile, a sliding track is gradually generated on the surface with increased surface damage and roughness, accelerating the formation of a hydrogen segregated layer inside the sliding track, which leads to the gradual reduction in friction (Zone II). Once a hydrogen segregated layer is established in the sliding track, a stable and low friction region is reached, i.e., Zone III. At each sliding cycle, the hydrogen segregated layer is damaged or removed due to the sliding action, and reformed at each sliding interval. Increasing cathodic potential accelerates the formation of the hydrogen segregated layer and moves the friction curve downwards, reducing the length of Zone I and Zone II and the COF in Zone III (Figure3). Increasing contact loads increases the material removal rate, thus the hydrogen segregated layer is removed at a higher rate, resulting in the shift of the friction curve upwards (Figure5). The parallel lines formed at high contact loads may not diminish, but the abrasive wear prevents the direct observation of the cracks. Although no chemical and composition analysis was conducted due to the dynamic nature of the H-segregated layer and the small size of the sliding track area, the formation of an H-segregated layer can be confirmed positively by the formation of many parallel lines across the sliding tracks produced at potentials more negative than 700 mV (SCE), see Figures7–9. It is well known that − hydrogen adsorption leads to the embrittlement of metals. Under the conditions where hydrogen charging is prohibited, i.e., at OCP and 500 mV (SCE), no such lines were observed in the sliding − tracks. Thus, parallel line formation at larger cathodic potentials may be the result of hydrogen charging and embrittlement. It is believed that these parallel lines are twins and stacking-faults due to strain-induced martensitic transformation assisted by hydrogen charging. It is possible that martensite phase transformation by dynamic recrystallisation occurs due to hydrogen charging during the sliding process [35,36]. The effect of cathodic hydrogen charging on the friction and wear of metals has been the subject of several recent studies [17–20]. Although hydrogen adsorption can lead to embrittlement of the sliding surface, it may have some beneficial effect on tribological properties. Recently, Georgiou et al. [18] found that hydrogen incorporation under in-situ cathodic polarization can significantly reduce the COF of an aluminium alloy under reciprocating conditions, but increase the wear rate of the alloy due to embrittlement. Pokhmurskii et al. [21] found that electrolytic hydrogenation can increase the surface hardness and reduce the friction of α-titanium, although its ductility was reduced. The possible formation of metal hydrides with crystal structures favourable for friction reduction can also account for the observed reduction in friction.

5. Conclusions The effect of applied cathodic potential on the sliding friction and wear behaviour of CoCrMo alloy in NaCl solution has been systematically studied over a wide range of cathodic potentials and applied contact loads. It can be concluded that applied cathodic potential affects the friction and wear behaviour of the alloy in two different ways. At cathodic potentials close to the open circuit potential, high friction and relatively large material loss results, where material loss is predominantly due to mechanical wear. At potentials more cathodic than the hydrogen charging potential, low friction and low wear result. The COF and TML decrease with increasing cathodic potential, such that at large cathodic potentials, e.g., 1000 mV (SCE), COF values as low as 0.02, and negligible material loss − Lubricants 2020, 8, 101 14 of 15 are obtained. Such reductions in friction and wear at increasing cathodic potentials are accompanied with the formation of parallel lines in the sliding track and are gradually diminished with increasing applied contact load due to increased material removal. Hydrogen charging and the formation of a hydrogen-segregated layer at the surface are believed to be responsible for such a phenomenon. It can also be concluded that it is difficult to derive the pure mechanical wear component in tribocorrosion by simply conducting a test at an arbitrary cathodic potential. The results have a further implication that material degradation during sliding wear in a corrosive environment could be minimised through appropriate cathodic protection.

Author Contributions: Conceptualization, Y.S.; methodology, Y.S. and R.B.; experimentation, Y.S. and R.B.; formal analysis, Y.S.; investigation, R.B.; writing—original draft preparation, Y.S.; writing—review and editing, R.B. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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