ARTICLE IN PRESS

Tribology International 43 (2010) 1218–1227

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/triboint

Interpretation of electrochemical measurements made during micro-scaleabrasion-corrosion

R.J.K. Wood a,�, D. Sun a,b, M.R. Thakarea,c, A. de Frutos Rozas a,d, J.A. Wharton a

a National Centre for Advanced Tribology at Southampton, School of Engineering Sciences, University of Southampton, Highfield, Southampton SO17 1BJ, UKb NIBEC, University of Ulster, Jordanstown Campus, Co. Antrim, BT37 0QB Northern Ireland, UKc Schlumberger Oilfield UK Plc, Stonehouse Technology Centre, Brunel Way, Stroudwater Business Park, Stonehouse, Glos GL10 3SX, UKd Department of Corrosion and Protection, National Centre for Metallurgical Research, CENIM-CSIC, Av. Gregorio del Amo 8, E-28040 Madrid, Spain

a r t i c l e i n f o

Article history:

Received 28 July 2009

Received in revised form

21 December 2009

Accepted 13 January 2010Available online 2 February 2010

Keywords:

Tribocorrosion

Microabrasion

Electrochemical

Synergy

9X/$ - see front matter & 2010 Elsevier Ltd. A

016/j.triboint.2010.01.004

esponding author. Tel.: +44 2380 594881.

ail addresses: rjw3@soton.ac.uk, R.Wood@solo

a b s t r a c t

This paper brings together and analyzes recent work based on the interpretation of the electrochemical

measurements made on a modified micro-abrasion-corrosion tester used in several research

programmes. These programmes investigated the role of abradant size, test solution pH in abrasion-

corrosion of biomaterials, the abrasion-corrosion performance of sintered and thermally sprayed

tungsten carbide surfaces under downhole drilling environments and the abrasion-corrosion of UNS

S32205 duplex stainless steel. Various abrasion tests were conducted under two-body grooving, three-

body rolling and mixed grooving-rolling abrasion conditions, with and without abrasives, on cast F75

cobalt–chromium–molybdenum (CoCrMo) alloy in simulated body fluids, 2205 in chloride containing

solutions as well as sprayed and sintered tungsten carbide surfaces in simulated downhole fluids.

Pre- and post-test inspections based on optical and scanning electron microscopy analysis are used to

help interpret the electrochemical response and current noise measurements made in situ during

micro-abrasion-corrosion tests. The complex wear and corrosion mechanisms and their dependence on

the microstructure and surface composition as a function of the pH, abrasive concentration, size and

type are detailed and linked to the electrochemical signals. The electrochemical versus mechanical

processes are plotted for different test parameters and this new approach is used to interpret tribo-

corrosion test data to give greater insights into different tribo-corrosion systems. Thus new approaches

to interpreting in-situ electrochemical responses to surfaces under different abrasive wear rates,

different abrasives and liquid environments (pH and NaCl levels) are made. This representation is

directly related to the mechano-electrochemical processes on the surface and avoids quantification of

numerous synergistic, antagonistic and additive terms associated with repeat experiments.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Many corrosion resistant metallic materials rely on a surfaceoxide film to provide protection against corrosion. However,under an externally applied stress or during wear, this passiveoxide film may become cracked and/or removed, exposing thesubstrate to a corrosive environment and leading to enhancedsurface degradation. Therefore, it is important to be able to selector design surfaces that have the correct balance between theability to repassivate in the service environment while not beingtoo reactive when corroding or in regimes where repassivation isnot complete (i.e. in wearing contacts). This is of particularconcern for many engineering and bioengineering applicationsthat are subjected to wear and corrosion such as pumps and

ll rights reserved.

n.ac.uk (R.J.K. Wood).

valves in the process, oil and gas and marine industries, as well asmetallic surgical implants used to replace human joints [1,2].Materials selected primarily on their corrosion resistance underbenign conditions can suffer highly interactive processes whensubjected to wear and corrosion together. These interactions leadto accelerated wall wastage and ion release rates which cancompromise component life and also induce adverse reactions ifreleased within the body [3].

Previous work at Southampton has investigated the interactionbetween abrasion and corrosion on various stainless and superduplex steels [4]. Both UNS S31603 and S32760 stainless steelsproduced interactions such that the abrasion–corrosion levelswere below the pure abrasion levels under two-body groovingabrasion conditions while the reverse was true for S30403. Theselarge negative interactions were due to the differences inrepassivation kinetics and/or composition of the passive filmsreducing the overall level of two-body abrasion [4]. However, allthree stainless steel types under three-body rolling abrasion

ARTICLE IN PRESS

R.J.K. Wood et al. / Tribology International 43 (2010) 1218–1227 1219

showed improved reproducibility with increasing abrasive vo-lume fraction with abrasion–corrosion levels greater than pureabrasion levels by 18%. Although this work did not utilize in-situelectrochemical measurement of tribocorrosion currents inducedduring abrasion–corrosion various studies have measured cur-rents during abrasion–corrosion tests of biomaterials and down-hole hardfacing materials as well as numerous engineeringcoatings [5–7]. These individually show different trends andsensitivity to test environment and conditions. Therefore the aimof this paper is to reanalyze and investigate the wear andelectrochemical interactions to allow more robust selection ofsurfaces for abrasion–corrosion resistant applications. The paperalso presents new data for UNS S32205 duplex stainless steelwhich is included as it is commonly found in marine applicationsor other high chlorine conditions due to its superior corrosionperformance. The other surfaces investigated were a sinteredWC–5.7Co–0.3Cr hard metal, a high velocity oxy fuel (HVOF)sprayed WC–10Co–4Cr coating and a cast F-75 CoCrMo bioengi-neering alloy. Tungsten carbide (WC)-based hardmetals andcoatings, CoCrMo based alloys and stainless steels are materialswhich readily form a passive oxide film and are therefore likely tohave interesting wear-corrosion properties.

An HVOF WC–Co coating was chosen as this type of coating isincreasingly being used as a chromium coating replacement.However, HVOF WC–Co coatings are microstructurally complexpotentially leading to internal micro-galvanic corrosion activityand thus destablising the overall coating surface integrity. Thishas been seen in the corrosion and corrosion–wear behaviour ofHVOF sprayed WC cermet coatings with metallic binders of Co,Co–Cr, CrC–Ni and Ni, in strong acidic environments by Cho et al.[8]. Considerable micro-galvanic corrosion occurred between theWC particles and the binder, and uniform corrosion occurred inthe binder materials of WC–Co and WC–Ni. These coatings areoften bench marked against sintered hardmetal of similarcomposition therefore this paper has included analysis of asintered WC–CoCr material to allow comparisons. The WC-basedhardmetals and coatings are commonly utilized for their superiorwear resistance in downhole drilling components. Downholeenvironments subject these materials to harsh tribologicalconditions, such as two- and three-body abrasion in the presenceof alkaline drilling fluids.

Recently, micro-abrasion tests coupled with an electrochemi-cal cell, have been deployed to study the abrasion–corrosionbehaviour of these engineering materials under conditions wherecorrosive electrolyte and third body abrasives co-exist [9–11]. Themicroscale abrasion–corrosion performance of two sinteredhardmetals, WC–6Co and WC–11Ni, and two WC–10Co–4Crsprayed coatings using both NaOH (pH 11) and neutral SiCaqueous slurries. Contrary to expectations, micro-abrasion underalkaline conditions generally resulted in lower wear ratesproducing a negative 8–18% abrasion–corrosion interaction, withthe exception of the sintered WC–11Ni [9,10].

The CoCrMo alloys, on the other hand, are commonly used inmetal-on-metal (MoM) hip replacement surgery and are exposedto oxygenated and chloride containing body fluids and tissues.The relative motion of femoral ball and cup will generate weardebris, some of which can act as third body abrasives yieldingfurther abrasive wear of the bearing surfaces. The depassivation ofthe surfaces does promote metal ion (cobalt and chromium)release both have biocapatability issues and can inflame thetissues in the joint area which may subsequently cause pain andhypersensitivity for patients [12–14]. Retrieved metal-on-metalhip replacements have revealed that two and three-body abrasiveare dominant wear mechanisms. Therefore the need to under-stand the tribocorrosion performance of these surfaces is criticalto the continued use of the MoM joint replacements.

Micro-abrasion–corrosion testing has also been conducted in0.9% NaCl, phosphate buffered saline solution, 25% and 50%bovine serum solutions with 0 or 1 g cm�3 SiC abradant at 37 1C.The results show the presence of proteinaceous materialincreased the total specific wear rate (SWR). Conversely, electro-chemical noise measurements indicated that the average anodiccurrent levels were appreciably lower for the proteinaceoussolutions when compared with the inorganic solutions [11].

Therefore, this paper will assess the wear–corrosion perfor-mance of four metallic surfaces with different repassivationcharacteristics to see if the trends between mechanical andelectrochemical responses are affected by repassivation charac-teristics.

2. Experimental

The test materials studied include a cast cobalt-basedbioengineering alloy (ASTM F75 CoCrMo—25–27% Cr, 5–7% Moand the balance Co) used in metal-on-metal replacement joints; asintered hardmetal (WC-CoCr); a thermally sprayed hard coating(WC-CoCr); as well as corrosion resistant alloy. The corrosionresistant alloy chosen was UNS S32205 duplex stainless steel asthis has superior corrosion resistance, particularly againstseawater, compared with Grade 316. It has excellent resistanceto localized corrosion including intergranular, pitting and crevicecorrosion; the critical pitting temperature of 2205 is generally atleast 35 1C.

Fig. 1(a) shows that sintered WC-based hardmetals have atypical skeletal carbide structure. The size of the carbides was2–3mm. Fig. 1(b) shows the SEM (backscattered electron image)image of a polished WC-10Co–4Cr surface. The darker regionsrepresent the heavier elements, such as tungsten in thecomposition and the lighter regions represent the relativelylighter elements, such as cobalt, chromium and nickel. Themicrography shows an inhomogeneous distribution of carbiderich and binder rich areas along with the presence of some voidsand cracks formed on the surface during the cooling of thecoating. Fig. 1(c) shows that cast CoCrMo (consists of blocky-shape M23C6 primary carbides (10–30mm in size, where M is Cr,Mo and Co [15]) incorporated in the cobalt rich matrix. Thecarbides are standing proud of the Co-matrix (�200 nm higheraccording to atomic force microscopy analysis). It is believed thatthese hard asperities can protect the softer matrix and reducewear in the event of lubrication film starvation [16]. Fig. 1(d)shows the 2205 stainless steel microstructure after electrolyticetching in 40% KOH at 1.5 V for 1 min. The elemental compositionof the 2205 duplex stainless steel was Fe, o0.03% C, 21–23% Cr,4.5–6.5% Ni, 2.5–3.5% Mo, 0.8–2.0% N, o2% Mn, o1% Si, o0.03%P and o0.02% S. The lighter areas correspond to d – ferrite anddarker regions are g – austenite. The ferrite and austenite contentsare nearly 50%, with the austenite evenly distributed within theferrite. Additionally, some black spots can be observed, which areprobably sites of MnS inclusions that probably have beendissolved during the etching procedure.

The hardness values of each material are shown in Table 1along with the hardness of the counter face (a zirconia ball) andthe silicon carbide (SiC) and alumina (Al2O3) abradants. Theabradants were chosen either to mimic the carbide hardness ofthe CoCrMo or to conform to previous test conditions wherehighly reproducible results were obtained with effective particleentrainment into the contact. Abradant concentrations werechosen to control the abrasion mechanism from two (grooving)to three body (rolling) with increasing concentrations.

A modified Phoenix Tribology TE/66 microabrasion testerincorporating a liquid tank and a 3-electrode electrochemical cell

ARTICLE IN PRESS

Fig. 1. SEM analysis showing the microstructure of (a) sintered WC-5.7Co–0.3Cr hard metal, (b) sprayed WC-10Co–4Cr coating, (c) cast F-75 CoCrMo alloy and

(d) UNS S32205 duplex stainless steel.

Table 2Test conditions for sintered WC-5.7Co–0.3Cr and sprayed WC-10Co–4Cr coating.

Load 0.2 N

Abrasive slurry 4.5mm SiC suspended in solutions of NaOH+NaNO3

(pH 9, 11, 13) or NaNO3 of pH 7

Slurry volumeconcentration

0.166

Sliding distance/speed 30 m/0.05 m s�1

Exposure time Freshly polished or exposed (in pH 7 and 11

solutions) for 168 h

Table 3Test conditions for cast CoCrMo and 2205 duplex stainless steel.

Load 0.25 N

Abrasive slurry 4.5mm SiC or 1.7mm Al2O3 suspended in 0.9%

NaCl or 3.5% NaCl

Slurry volumeconcentration at pH 7

0.006, 0.072, 0.030, 0.12 (0.238 for CoCrMo

only)

Sliding distance/speed 38 m/0.05 m s�1

Exposure time Immerse in testing solution for 30 min prior to

tests

Table 1Vickers microindentation hardness (2.94 N) of the counterface, abrasive and

materials used in this study in GPa.

Zirconia

ball

counterface

SiC

abrasive

4.5mm

Al2O3

abrasive

1.7mm

Sintered

WC-

5.7Co–

0.3Cr

Sprayed

WC-

10Co–

4Cr

Cast

CoCrMo

2205

duplex

stainless

steel

13.0 21–26 18–20 17.7 11.1 3.3 2.9

R.J.K. Wood et al. / Tribology International 43 (2010) 1218–12271220

was used, the detailed structure of the rig can be found elsewhere[11]. All samples were machined to a dimension of 20 mm�10mm�3 mm and were ground/polished down to 1mm diamondpaste. A pre-conditioned zirconia ball was rotated against thesample under a specified load and in the presence of abrasiveslurry. The detailed test conditions for each material can be foundin Tables 2 and 3, respectively.

The abrasives used for the micro-abrasion corrosion tests are4.5mm SiC (Grade F1200, Washington Mills Ltd., Manchester, UK)and/or 1.7mm Al2O3 (Logitech Ltd., UK). Their microstructures areshown in Fig. 2(a) and (b), respectively.

In situ electrochemical current-noise was measured underan applied potential (the open circuit potential of the testingmetal) using a Gamry potentiostat and ESA 400 software. Thisfollows other workers in the field of tribocorrosion [17]. Theelectrochemical current-noise sampling rate was 2 Hz. Each testhas been repeated at least twice to gauge repeatability. The size ofthe wear scar can be measured in situ using a calibrated eye pieceon the microabrasion rig and wear volume and wear rate can becalculated using Eq. (1) [18]:

V ¼pb4

64Rðfor b{RÞ ð1Þ

where V is the wear volume (m3), b the diameter of wear scar (m)and R the radius of the ball (m). Assuming the specimen ishomogeneous (i.e. the wear rate is constant such that the wearvolume is proportional to the load and sliding distance), thespecific wear rate SWR (m3 N�1 m�1) can be calculated by theArchard equation [19,20] (Eq. (2)):

SWR¼V

WLð2Þ

where SWR (mm3 N�1 m�1) is a dimensional wear coefficient andrepresents the volume of material removed by wear per unitdistance (L in m) slid per unit normal load (W in N) on the contact.

The average current was calculated from the current versustime curve using Eq. (3):

Iave ¼

R t0 I dt

tð3Þ

where I is the current and t the duration of the test. It isproportional to the total charge transfer that occurs during theabrasion-corrosion process and can be correlated to the materialloss, Wm, due to the electrochemical dissolution using Faraday’slaw, see Eq. (4):

Wm ¼MrQ

zFð4Þ

where Mr is the molar mass of the material, z the number ofelectrons transferred in the reaction, F the Faraday’s constant andQ the total charge given by Eq. (5):

Q ¼

ZI dt ð5Þ

ARTICLE IN PRESS

Fig. 2. (a) 4.3 SiCmm and (b) 1.67mm Al2O3 abrasives used in this study.

-10

0

10

20

30

40

50

60

70

80

90

100

0 200

Time / s

Cur

rent

/µA

WC-5.7Co-0.3Cr pH 7 WC-5.7Co-0.3Cr pH 11 WC-5.7Co-0.3Cr pH 7-exp

WC-5.7Co-0.3Cr pH 9 WC-5.7Co-0.3Cr pH 11-exp WC-5.7Co-0.3Cr pH 13

Depletion of binder

400 600 800 1000 1200 1400 1600

Fig. 3. Current-noise response for sintered WC-5.7Co–0.3C under different test conditions.

R.J.K. Wood et al. / Tribology International 43 (2010) 1218–1227 1221

3. Results and discussion

Fig. 3 shows the current-noise (It) response for fresh andexposed (-exp) sintered WC-5.7Co–0.3C samples obtained duringmicro-abrasion tests using alkaline (pH 9, pH 11 and pH 13) andneutral (pH 7) abrasive slurries. The current gradually increasedthroughout the wear test duration associated with the increase inthe wear scar area with time. The current levels and the curveshapes observed for pH 7, pH 9, pH 11 and pH 11-exp samples aregenerally similar (less than 30mA). However, higher currentvalues are observed for pH 7-exp (80mA) and pH 13 (90mA)samples. The shape of the It curves for pH 7-exp and pH 13samples are distinctly different from the conditions. A steep risein current is observed between 100 and 200 s after abrasionbegan, particularly for the pH 7-exp sample. This can be explainedby the Pourbaix diagram of Co (Fig. 4), where the Co binder ismore prone to dissolution at pH 7 and pH 13 [21]. It is also likelyto be due to the increased electrochemical dissolution of thebinder phase due to micro-galvanic coupling between the nascentwear scar and the unworn area of the passive surrounding (due toexposure). The other distinctive feature of the pH 7-exp and pH 13

curves is the reduction in current approximately half way throughthe test. This is thought to be due to binder phase depletionbetween the carbides and/or local pH variation within the cavitycreated by binder depletion between carbides. The partial decayof currents (i.e. the currents do not reduce to original pre-testlevels) seen in Fig. 3 after the ball has stopped rotating suggeststhat the wear scar did not readily repassivate when in contactwith neutral and alkaline slurries due to the depletion of thebinder phase and the local environment within cavities betweencarbides. The partial decay effect is seen for all pH conditionstested but is not seen on the other materials tested in this work asthey have dispersed carbides which do not suffer such localizedcorrosion.

Fig. 5 shows a plot of the average current, Iave, values measuredduring micro-abrasion against the SWR sintered WC-5.7Co–0.3Cr.The SWR is predominately influenced by mechanical wearprocesses but will contain a small component of materialremoved by corrosion. No obvious relationship between SWR

and Iave is seen under neutral and weak alkaline conditions.Overall, mechanical wear processes occurred by fragmentationand loosening of carbide grains but only the top surface carbides

ARTICLE IN PRESS

0 2x10-13 4x10-13 6x10-13

Aver

age

curr

ent /

µA

0

10

20

30

40

50

60

70

pH 7-exppH 7pH 9pH 11pH 11-exppH 13

SWR / m3 N-1 m-1

Fig. 5. Average current and SWR relationship for sintered WC-5.7Co–0.3Cr.

Fig. 4. Pourbaix diagram for cobalt [21].

R.J.K. Wood et al. / Tribology International 43 (2010) 1218–12271222

were damaged determined by SEM and focused ion beam analysis(results not shown here). This is probably due to the skeletalstructure of the carbides supporting the abrasive loading on thetop carbides while carbide fracture occurs via individualtransgranular cracking with carbide boundaries inhibitingintergranular fracture. A maximum current is observed for thepH 13 samples which also resulted in the highest SWR, possiblydue to the excessive rate of binder depletion observed under pH13. The increase in wear rate is likely to be due to excessive binderdepletion around the top carbides that enhances unsupportedcarbide fracture and pull-out. The results can be correlated withthe wear scar morphology as shown in Fig. 6.

To study the wear mechanisms occurring on the sintered WC-5.7Co–0.3C samples, typical SEM images of worn sample wearscars generated in neutral (pH 7) and alkaline (pH 11 and pH 13)slurries are presented in Fig. 6. For consistency, all images wereobtained from the wear scars centres for all samples. The wornsurfaces of the sintered samples appeared to be similar for othertest conditions and are devoid of any binder, compared with theunworn surface (Fig. 6a). The carbide grains within the wear scarappear to have suffered from repeated fragmentation andfracture. The size of the fragmented carbides in neutral and weakalkaline conditions (pH 7, pH 9 and pH 11) is similar, whichindicates that the rate of binder dissolution suffered bythese samples is also similar. However, pH 13 sample (Fig. 6d)shows severely fragmented carbide grains on the surface which islikely to be caused by excessive binder depletion during micro-abrasion. Hence, for this surface there is no obvious relationshipbetween average current and SWR as the wear mechanism is notaffected by the subtle variation in current associated withdifferent levels of passivation of the binder phase betweencarbides on the surface. Only under pH 13 where the binderphase is actively corroding does the surface layer of carbidesbecome undermined by excessive binder removal and fracturescausing an increase in SWR.

Fig. 7 shows the electrochemical current noise response for theWC-10Co–4Cr coating during micro-abrasion tests for differentpH. The It curves show a sharp rise in the current immediatelyafter the abrasion test begins. This sharp rise in current isobserved due to the removal/damage of the passive Co(OH)2 filmon the binder rich areas. After the initial rise in the current itattains a relatively steady value. The current levels do not changesubstantially for the remaining duration of the micro-abrasiontest despite the increase in the wear scar area. The fluctuation inthe current is due to the spontaneous repassivation within thewear scar and subsequent depassivation due to the action ofabrasive particles. At the end of each test the current levelssharply drop to similar values as seen before abrasion. This is dueto the repassivation of the wear scar after the ball rotationstopped. This further corroborates that binder-rich areas withinthe wear scar readily passivate in the absence of abrasion. This isin contrast to the sintered hardmetal which has a small surfacearea of binder phase exposed compared with the sprayed coatingwith far grater exposed binder phase and does not have a skeletalcarbide matrix and therefore does not readily form cavitiesbetween carbides. A similar current-noise behaviour wasobserved during the reciprocating-sliding wear of passive metals(316 stainless steel, chromium and nickel) by Mischler et al. [22].It was reported that the variation of the measured anodic currentwas the measure of the electrochemical metal removal rate in thewear scar.

Fig. 7 shows that the maximum current levels observed for pH7, pH 9, pH 11 and pH 11-exp are similar (less than 5mA).However, higher current values are observed for pH 7-expsamples (10mA) and pH 13 samples (25mA). Interestingly, thecurrent observed for pH7-exp samples was higher than for the pH7 samples. This is likely to be due to the microgalvanic couplingbetween the nascent surface in the wear scar and the surroundingarea of the pH 7-exp sample leading to increased electrochemicaldissolution. The wear scar is expected to be more active ascompared with the relatively passive surrounding (due toexposure) leading to higher corrosion within the wear scar.Micro-galvanic coupling between the active wear scar and therelatively passive unworn areas is likely to occur for all samples.The high current values observed for pH 13 samples are expecteddue to the greater tendency of W, Co and Cr to corrode at pH 13.

Fig. 8 shows the average current measured during microabrasionagainst SWR for the fresh and exposed WC-10Co–4Cr coatings

ARTICLE IN PRESS

Fig. 6. SEM micrographs of wear scars on the sintered WC-5.7Co–0.3 samples (a) unworn surface and (b) worn in pH 7, (c) worn in pH 11 and (d) worn in pH 13 conditions.

-5

0

5

10

15

20

25

30

0

Time / s

Cur

rent

/ µA

WC-10Co-4Cr pH 7-exp WC-10Co-4Cr pH 13 WC-10Co-4Cr pH 11

WC-10Co-4Cr pH 7 WC-10Co-4Cr pH 9 WC-10Co-4Cr pH 11-exp

200 400 600 800 1000 1200 1400 1600

Fig. 7. Current-noise response of sprayed WC-10Co–4Cr coating under different test conditions.

R.J.K. Wood et al. / Tribology International 43 (2010) 1218–1227 1223

abraded using neutral and alkaline slurries. No obvious relationshipexists between increases in the average current and the SWR underneutral and wear alkaline conditions. However, the lowest currentand SWR are observed for pH 11 and pH 11-exp samples while thehighest current and SWR observed for the pH 13 sample. As shown inFig. 4, at pH 11 Co is expected to form a passive oxide film Co(OH)2,which is likely to lower the rate of binder removal. It is also seen fromFig. 8 that the use of pH 9 and 11 has resulted in a lower SWR as

compared with pH 7. This indicates that pH 7 should not beconsidered as a corrosion-free condition. Therefore, as with the WC-5.7Co–0.3C samples, the binder is passivated under the pH 9–11range while at pH 7-exp and 13 the binder is actively corroding whichexposes the surface carbides and enhances carbide release andthereby the SWR levels.

SEM micrographs of worn samples were studied to determinethe wear mechanisms that occurred on the sprayed coatings.

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R.J.K. Wood et al. / Tribology International 43 (2010) 1218–12271224

Fig. 9 compares the unworn surface of the WC-10Co–4Cr coatingto the wear scars on pH 11 sample. Due to the non-uniformmicrostructure of the coating the number of carbides exposed ineach case is expected to differ. In general for all samples, the SEMmicrographs reveal that wear has occurred by preferentialremoval of binder around the carbide grains, leading to thecracking and subsequent removal of the carbide grains. Thepreferential binder removal leads to the formation of a ‘moat-like’feature around the carbide grains. This feature is observed on allworn surfaces and appears to be principally in the direction of themotion of abrasive slurry and is formed due to the preferentialdepletion of the binder phase around the carbide grains.

Microabrasion-corrosion of cast CoCrMo has been carried outusing different saline based abrasive slurries at various concen-trations of SiC and Al2O3 abrasives (0.238, 0.120, 0.072, 0.030 and0.006 vol%). Fig. 10 shows the typical current-noise curves seenfor cast CoCrMo with 0.072 vol% SiC and Al2O3 abrasive slurries.The current-noise level under SiC test conditions is generallyhigher than that for the Al2O3 test conditions. From the start ofthe test, the current shows a sharp increase which corresponds tothe rupture of the surface oxide film. The current is thenmaintained at a relatively constant level throughout the testdue to the stable competition between the depassivation/repassivation processes. In contrast to WC-5.7Co–0.3Cr, thecurrent after the CoCrMo abrasion recovers to the pre-abrasion

0 10-12 1.5x10-12 2.0x10-12 2.5x10-12

Ave

rage

cur

rent

/ µA

0

5

10

15

20

25

30

pH 7-exppH 7pH 9pH 11pH 11-exppH 13

5.0x10-13

SWR / m3 N-1 m-1

Fig. 8. Average current and SWR relationship for sprayed WC-10Co–4Cr coating.

Fig. 9. SEM micrographs of wear scars on sprayed coatings

level indicating the repassivation has effectively inhibited thewear-induced corrosion current.

Fig. 11 shows the interdependence of the SWR and the averagecurrent for cast CoCrMo abraded in 0.9% NaCl based slurries. Forboth SiC and Al2O3, the average current decreases as the SWR

decreases. A linear correlation was found for data sets of bothabrasives (R2 values greater than 0.93). The slope of the linear fitfor Al2O3 data (2.1�1013mA m�2 N�1) is approximately twicethat for SiC (0.88�1013mA m�2 N�1), indicating that for the samevolume of alloy removed, the smaller sized abrasives couldgenerate a greater electrochemical response (current) under theabrasion-corrosion. This could be attributed to the fact that thetime for repassivation between particle impacts was reducedsignificantly by a reduction in particle size [23]. The abovestatement is supported by the SEM investigation, see Fig. 12,where the wear scars generated by Al2O3 contained morenumerous and finer indents (for 0.238 vol%) or grooves (for0.072 vol%).

Fig. 13 shows the current-noise curves for the 2205 duplexstainless steel at different SiC volume concentrations in 3.5% NaClsolution. A sharp increase in the current is noticed when themicroabrasion-corrosion test starts due to the partial rupture ofthe surface oxide film by abrasion and the subsequent exposure ofthe metal surface to the chloride solution. Recorded current levelincreases with the SiC volume fraction. For the 0.120 and0.072 vol% the current level increases gradually during thewhole test. However, a slight decrease in the current noise levelis noticed after 400 s in the case of the 0.030 vol%, the detailed

(a) unworn surface and (b) worn in pH 11 condition.

-5

0

5

10

15

20

25

30

0

Time / s

Cur

rent

/ µA

SiC Al2O3

200 400 600 800 1000

Fig. 10. Typical current-noise curves of cast CoCrMo at 0.072 vol% abrasive

concentration.

ARTICLE IN PRESS

R.J.K. Wood et al. / Tribology International 43 (2010) 1218–1227 1225

reason for this remains unknown. The fact that the averagecurrent level decreases with decreasing SiC volume concentrationmay be due to the different competition between depassivation/repassivation during the abrasion under various test conditions.At relatively high SiC vol%, the higher number of entrainedparticles could reduce the time for repassivation between particleimpacts/indents, and therefore the material has less chance torepassivate.

The interdependence between the average current and SWR isshown in Fig. 14. A linear correlation was found (R2=0.9912) with aslope of 3.36�1013mA m�2 N�1. This linear correlation indicatesthat the (percentage) contribution of the electrochemical volumeloss to the total wear loss due to the microabrasion-corrosionprocess remains the same although test condition changes.

Fig. 15 shows micrographs of the abrasion-corrosion wearscars for different concentrations of SiC. Fig. 15(a) shows three-

0 5.0x10-13 10-12 1.5x10-12 2.0x10-12 2.5x10-12

Ave

rage

cur

rent

/ µA

0

5

10

15

20

25

30

SiCAl2O3

Decreasing slurry vol.%Decreasing slurry vol.%

SWR / m3 N-1 m-1

Fig. 11. Average current and SWR relationship for cast CoCrMo.

Fig. 12. Wear scars on cast CoCrMo produced by (a) 0.238 vol% SiC,

body rolling damage dominates while Fig. 15(b) shows bothindentation damage and grooving indicating a mixed two/three-body mode occurs at 0.072% SiC. Lowering the concentrationfurther to 0.03% changes the abrasive mechanism topredominately two-body grooving as seen in Fig. 15(c).Fig. 15(d) shows the scar for pure sliding under the no abrasivecase and only light polishing and some light scratches created byasperities on the ball are seen. The level of indentation (rolling) inthe contact, therefore, seems to correlate with the average currentmeasurements.

Fig. 16 compares the predominately mechanical wear andcorrosion current induced for the four metallic surfacesinvestigated. Four different trends are seen with the corrosion ofthe sprayed surface being insensitive to wear rate until1.5�10�12 m3 N�1 m�1 then increased currents are seen with aslope of 3�1013mA m�2 N�1. The cast CoCrMo has two

(b) 0.072 vol% SiC, (c) 0.238 vol% Al2O3 and (d) 0.072 vol% Al2O3.

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

0Time / s

Cur

rent

/ µ

A0.00 vol %

0.030 vol %

0.072 vol %

0.120 vol %

200 400 600 800 1000

Fig. 13. Typical current-noise curves for UNS S32205 duplex stainless steel at

different SiC abrasive concentrations in 3.5% NaCl solutions.

ARTICLE IN PRESS

R.J.K. Wood et al. / Tribology International 43 (2010) 1218–12271226

sensitivities (slopes of 1 and 2�1013mA m�2 N�1) but has lowercorrosion levels compared with the 2205 duplex steel with a slopeof 3�1013mA m�2 N�1. The sintered WC-CoCr has a near verticaltrend with the corrosion rates varying without altering the wearrate. The slopes associated with the sprayed WC-CoCr coating, thecast CoCrMo and 2205 duplex stainless steel may reflectthe tenacity of repassivation on the materials within theenvironments tested. However, the currents measured are likelyto be from mechanically mixed layers or microstructurallyaffected regions within the wear scar. Thus, further work isrequired to identify these tribologically induced surfacemicrostructural and compositional changes to better understandthe significance of these slopes.

Fig. 15. Wear scar of 2205 duplex stainless steel produced by (a) 0.12 vol% SiC,

0 5.0x10-13 10-12 1.5x10-12 2.0x10-12 2.5x10-12

Ave

rage

cur

rent

/ µA

0

10

20

30

40

50

60

70

0.12 vol.%

0.072 vol.%

0.03 vol.%

Pure sliding

Decreasing slurry vol.%

Decreasing slurry vol %

SWR / m3 N-1 m-1

Fig. 14. Average current and SWR relationship for the UNS S32205 duplex

stainless steel in 3.5% NaCl.

This type of representation does not mask the physicalmeaning of the abrasion-corrosion data and is perhaps a betterway to present the results compared with quantifying synergisticlevels or percentages of the total wall wastage due to synergisticterms which are difficult for the designers to use. The errors barsseen in both x and y axes in Fig. 16 show the extent of datavariation within each test.

The analysis, thus far, has focused on the average current levelsbut the fluctuations around the mean values (standard deviation)were also noted to vary and could be related to the depassivation/repassivation activity within the wear scar as well as with thewear mechanism–electrochemical interactions. The 2 Hz current-noise sampling rate used (approximately twice per revolution ofthe ball) meant that individual abrasive indents or grooves couldnot be resolved. Thus further work is required to investigate thisobservation and tests with increased sampling rates are requiredcoupled with more detailed understanding of the degradationprocesses occurring within the scar.

4. Summary

The present study shows that the abrasion-corrosion responseof sintered and thermally sprayed WC-CoCr, Cast CoCrMo and2205 duplex stainless steel can be dependent on the test solutionchemistry, type of abrasives and the abrasive wear mechanisms.The in situ electrochemical current-noise indicates the rate andcorrosion process of the material as abrasive wear progresses. Italso indicates susceptibility to corrosion under abrasive wear andthe extent and effectiveness of passivity/repassivation. Plots ofinterdependence between predominately mechanical processes(SWRs) and electrochemical processes (mean current level)against different testing parameters can be useful to interpretwear-corrosion performance. Four different trends are seen withthe tribocorrosion of the sprayed surface being insensitive tocurrent levels associated with passivation until 1.5�10�12 m3

(b) 0.072 vol% SiC, (c) 0.03 vol% SiC and (d) pure sliding (without abrasives).

ARTICLE IN PRESS

0 5.0x10-13 10-12 1.5x10-12 2.0x10-12 2.5x10-12

Ave

rage

cur

rent

/ µA

0

10

20

30

40

50

60

70CoCrMo - SiCRegression lineCoCrMo - Al2O3

Regression line2205 - SiCRegression linepH 7-exp sinteredpH 7 sinteredpH 9 sinteredpH 11 sinteredpH 11-exp sinteredpH 13 sinteredpH 7-exp sprayedpH 7 sprayedpH 9 sprayedpH 11 sprayedpH 13 sprayed

y = 3x1013x+ 1.9036R2 = 0.9912

y = 2x1013x - 1.376R2 = 0.9947

y = 1x1013x + 1.8947R2 = 0.9556

SWR / m3 N-1 m-1

Fig. 16. Current versus SWR for sintered WC-5.7Co–0.3Cr hard metal, sprayed WC-10Co–4Cr coating, UNS S32205 duplex stainless steel and cast F-75 CoCrMo.

R.J.K. Wood et al. / Tribology International 43 (2010) 1218–1227 1227

N�1 m�1 then increased currents are seen with a slope of3�1013mA m�2 N�1. The cast CoCrMo has two sensitivitiesdepending on particle size and the number of particles entrainedinto the contact (slopes of 1 and 2�1013mA m�2 N�1) but haslower corrosion levels compared with the 2205 duplex steel witha slope of 3�1013mA m�2 N�1. The sintered WC-CoCr has a nearvertical trend with the corrosion rates varying without alteringthe wear rate due to the skeletal nature of the carbides onlyallowing corrosion of the near surface binder phases betweensurface carbides. The slopes associated with the sprayed WC-CoCrcoating, the cast CoCrMo and 2205 duplex stainless steel mayreflect the tenacity of repassivation on the materials within theenvironments tested. The current levels observed on all fourmaterials appear related to the number of indents (frequency orrate) generated in 3-body abrasion and the repassivation time ofthe worn surface. Owing to the close proximity of the carbides onthe surface of the sintered WC-CoCr, crevice type corrosiondeveloped within the localized environments between surfacecarbides once the binder had been removed or dissolved. Thisresulted in the current levels remaining high after the tribocorro-sion tests. This was not seen for the other surfaces tested as theyhad dispersed carbides and current levels returned to pre-testlevels after testing.

The predominately mechanical versus electrochemical repre-sentations can serve as tools to better understand and predictperformance and design surfaces for various tribo-corrosionsystems. The representation also has advantages over the presentpractice of running multiple tests to determine various synergis-tic, antagonistic and additive terms and there quantificationwhich involves introducing significant errors and uncertainties.This approach has also been shown to cause difficulty in materialselection and life prediction for wear-corrosion applications. Thusthe mechano-electrochemical plots may offer advantages whendealing with surfaces subjected to tribocorrosion.

References

[1] Vivien B. Selecting process valves for in-service reliability. Proc Eng 1988:33–6.[2] Kembaiyan KT, Keshavan K. Combating severe fluid erosion and corrosion of

drill bits using thermal spray coatings. Wear 1995;186–187:487–92.[3] Mischler S, Spiegel A, Landolt D. The role of passive oxide films on the

degradation of steel in tribocorrosion systems. Wear 1999;229:1078–87.(Part 2 APR).

[4] Bello JO, Wood RJK, Wharton JA. Synergistic effects of micro-abrasion-corrosion of UNS S30403, S31603 and S32760 stainless steels. Wear2007;263:149–59.

[5] Wood RJK. Tribo-corrosion of coatings: a review. J Phys D: Appl Phys2007;40:5502–21.

[6] Sun D, Wharton JA, Wood RJK. Effects of proteins and pH on tribocorrosionperformance of cast CoCrMo—a combined electrochemical and tribologicalstudy. Tribol – Mater Surf Interfaces 2008;2(3):150–60.

[7] Sun D, Wharton JA, Wood RJK. Abrasive size and concentration effects on thetribo-corrosion of cast CoCrMo alloy in simulated body fluids. Tribol Int2009;42(11–12):1595–604.

[8] Cho JE, Hwang SY, Kim KY. Corrosion behaviour of thermal sprayed WCcermet coatings having various metallic binders in strong acidic environ-ment. Surf Coat Technol 2006;200:2653–62.

[9] Thakare MR, Wharton JA, Wood RJK, Menger C. Exposure effects of alkalinedrilling fluid on the microscale abrasion-corrosion of WC-based hardmetals.Wear 2007;263(1–6):125–36.

[10] Thakare MR, Wharton JA, Wood RJK, Menger C. Exposure effects of strongalkaline conditions on the microscale abrasion-corrosion of D-gun sprayedWC-10Co–4Cr coating. Tribol Int 2008;41(7):629–39.

[11] Sun D, Wharton JA, Wood RJK, Ma L, Rainforth WM. Microabrasion-corrosion of cast CoCrMo alloy in simulated body fluids. Tribol Int2009;42(1):99–110.

[12] Jacobs JJ, Gilbert JL, Urban RM. Current concepts review: corrosion of metalorthopaedic implants. J Bone Jt Surg (Am) 1998;80:268–82.

[13] Park JB, Lakes RS. Metallic implant materials in biomaterials—an introduc-tion. New York: Plenum Press; 1992. p. 75–115.

[14] Urban RM, Jacobs JJ, Gilbert JL, Galante JO. Migration of corrosion productsfrom modular hip prostheses—particle microanalysis and histrophathologi-cal findings. J Bone Jt Surg 1994;76(A):1345–59.

[15] Hernandez-Rodriguez MAL, Mercado-Solis RD, Perez-Unzueta AJ, Martinez-Delgado DI, Cantu-Sifuentes M. Wear of cast metal–metal pairs for totalreplacement hip prostheses. Wear 2005;259(7–12):958–63.

[16] Metcalf JEP, Cawley J, Band TJ. Cobalt chromium molybdenum metal-on-metal resurfacing orthopaedic hip devices. Bus Briefing: Med Device ManufTechnol 2004:1–7.

[17] Landolt D, Mischler S, Stemp M. Electrochemical methods in tribocorrosion: acritical appraisal. Electrochim Acta 2001;46:3913–29.

[18] Sinnett-Jones PE, Wharton JA, Wood RJK. Micro-abrasion-corrosion of aCoCrMo alloy in simulated artificial hip joint environments. Wear2005;259(7–12):898–909.

[19] Archard JF. Contact and rubbing of flat surfaces. J Appl Phys 1953;24(8):981–8.[20] Archard JF, Hirst W. The wear of materials under unlubricated conditions.

Proc Royal Soc 1958;A-236:71–3.[21] Pourbaix M. Atlas of electrochemical equilibria in aqueous solutions. New

York: Pergamon Press; 1966.[22] Mischler S, Debaud S, Landolt D. Wear-accelerated corrosion of passive

metals in tribo-corrosion systems. J Electrochem Soc 1998;145(3):750–8.[23] Wright LG, Sethi VK. Some thoughts on the contributions of bed particle

properties to the FBC tube wastage problem. In: Proceedings of thesymposium on corrosion and particle erosion at elevated temperatures,TMS-ASM, Las Vegas, 1989.