Stack 2003 Wear

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Wear 255 (2003) 456465

Mapping sliding wear of steels in aqueous conditionsM.M. Stack , K. Chi

Department of Mechanical Engineering, University of Strathclyde, James Weir Building, 75 Montrose St., Glasgow G1 1XJ, UK

Abstract

In studies of sliding wear in dry environments, there have been many attempts at mapping the processes. For steels, the classical wearmap has been developed and various other studies have extended the approach to composites, ceramics and coated materials. Despite thiswork, there have been few attempts to extend the methodology to wet conditions, where the wear process interacts with solutions that aredened by pH and electrochemical potential.

The object of this work was to study the sliding wearcorrosion behaviour of steels in a pin-on-disc apparatus in aqueous conditions.The effects of applied load and velocity were evaluated at various electrochemical potentials in carbonate/bicarbonate solution pH 9.8).The results were analysed using weight loss and scanning electron microscopy techniques.

Wear mechanisms were identied in the various environments and a method of identifying the wearcorrosion transitions, in aqueousconditions, was proposed in the work. These regimes were superimposed on wearcorrosion maps, where the change in wearcorrosionregime was identied as a function of velocity and electrochemical potential. Possible reasons for the differences in the boundaries of themap at various applied loads are discussed in this paper. 2003 Elsevier Science B.V. All rights reserved.

Keywords: Sliding wear corrosion; Maps; Wear mechanisms; Aqueous environments

1. Introduction

Signicant advances in recent years have been madein the study of tribo-corrosion in aqueous conditions.In particular, there has been much work carried out inthe eld of erosioncorrosion by solid particles [15] .Regimes of interaction have been identied, ranging fromerosion-dominated where the material loss is predomi-nately from the substrate to corrosion-dominated wherethe losses are as a result of dissolution or formation of corrosion product [68].

Various methodologies have now been developedfor characterising the tribo-corrosion interaction inerosioncorrosion environments, and in particular, in dryconditions [6]. There many schools of thought on thebest method of describing such interactions and thesehave been the subject of a recent review [9]. However,such mechanistic descriptions are important because theyenable the construction of a useful engineering tool forapplication to such environments, i.e. the engineeringmap.

Corresponding author. Tel.: + 44-141-5483754;fax: + 44-141-5525105. E-mail address: [email protected] (M.M. Stack).

Erosion [10] and wear maps [1114] are now part of

mainstream tribology but it is worth noting that there arestill many territories in this area that remain unchartedby such mapping approaches. In particular, although mapshave been constructed for aqueous-corrosion (e.g. Pourbaixdiagrams [15]) and erosion in such conditions [7,8] , therehas been no similar approach to characterising sliding wearin aqueous conditions. This is despite the fact that a mapfor dry sliding wear of steels (based on experimental resultsand constructed using analytical tools) [11] has been devel-oped for over a decade. Some recent work by Jiang et al.[16] has provided a basis for construction of a simple map,based on mathematical models of wearaqueouscorrosionin sliding contacts. However, there has been no labo-ratory work carried out to develop the map from rawdata.

In this work, the sliding wear behaviour in steels in anaqueous solution of buffered pH (carbonate/bicarbonate)was investigated at a range of velocities and appliedloads. The results were used to establish regimes of wearcorrosion. Wearcorrosion maps were constructed asa function of sliding velocity and applied potential and theeffects of increasing applied load on the wearcorrosionboundaries are discussed in terms of the competition be-tween the frictional heating effects in the aqueous solutionsversus the enhancement of mechanical wear.

0043-1648/03/$ see front matter 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0043-1648(03)00203-5

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M.M. Stack, K. Chi / Wear 255 (2003) 456465 457

2. Experimental description

A pin-on-disc apparatus was used in this study ( Fig. 1).The asymmetric test rig employed a pin pressed againsta rotating ring both making conformal contact when thetrack rotates. Both the pin and ring were made of car-

bon steel of composition: 0.21% C, 0.38% Mn, 0.46%Si, 0.005% P and < 0.004% S. The pre-weighed ring wasxed into a perspex trough acting as the container forthe electrolyte (0.5M NaHCO 3 + 0.5M Na2CO3 solutionprepared using de-ionised water). Importantly, the rearface and edges of the ring, and the cylindrical circum-ference of the pin, were coated with a lacquer (Lacomit)so as to limit the electrochemically exposed area solelyto the abraded track (and also, necessarily, the pin face).The pin holder made it possible to apply variable forceusing a total weight of either 500 or 1000g and also toconnect the pin as the working electrode. A graphite aux-iliary electrode ring was concentric to, but insulated fromthe pin. The rig was connected to a reference electrode(standard calomel electrode, SCE) by a salt bridge witha capillary placed close to, but not in contact with, thering. The whole system was software controlled using aGillAC electrochemical interface from ACM Instruments(UK).

Following an initial stabilisation period at 1000 mV(SCE) for 120 s, polarisation curves during simultaneoussliding wear were measured from 1000 to + 500 mV(SCE) at a sweep rate of 1 mV s 1. Necessarily, in this ap-paratus, the solution was exposed to air. Thus, the measured

Fig. 1. Schematic diagram of experimental apparatus used for the wearcorrosion tests.

current densities were corrected, at all potentials, for thepresence of residual cathodic current by subtraction of theoxygen reduction current at 920mV (SCE). The polari-sation curves shown in this paper are presented after thiscorrection. Prior to this process, the currents at 920 mVwere plotted as a function of the square root of the rotation

speed; the expected linear relationship conrmed that diffu-sion controlled kinetics dominated at this potential and thatthe correction was justied. Although a resistive ( iR) dropmay also have been present, it was assumed, due to the rel-atively high conductivity of the 0.5 M electrolyte, that thiswas minimal and no further corrections to the polarisationdata were carried out.

Wearcorrosion tests were carried out rotation speedsequivalent to linear velocities at the pin of 0.157, 0.314,0.471, 0.628 m s 1 . The corrosion rates ( K c) during slid-ing wearcorrosion were obtained from the current densi-ties at potentials of 600, 400, 200, 0, 200 mV (SCE).Thus, the potential range covered active corrosion ( 600and 400 mV), pre-passivation ( 400 and 200mV) andpassivation ( 200 to 200mV), depending on the slidingwear conditions. The total mass losses due to sliding wearand corrosion ( K wc) at the above potentials after 1 h wereobtained from the weight change of the ring; this was mea-sured using a Mettler College 150 electronic balance, withan error of 0.1 mg (it should be noted that for these exper-iments, the corrosion and wear rates were not separated; thecorrosion rate refers to the corrosion of the pin and ring. Thetotal area of the wear track on the ring was 31.42 cm 2; thearea of the pin was 0.5cm 2 . The wear and wearcorrosion

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458 M.M. Stack, K. Chi / Wear 255 (2003) 456465

rates refer to the ring only; this was because tests on thepin showed that the wear and wearcorrosion losses werevery low). Thus, the mass loss due to sliding wear duringwearcorrosion ( K w) can be obtained from subtraction of K wc and K c . The background corrosion rate in the absence of wear (K co) was obtained under static conditions. Finally, the

background mass losses due to sliding wear in the absenceof corrosion ( K wo) were obtained during cathodic protectionat 800mV (SCE).

Fig. 2. Polarisation curves for steel at various sliding speeds and applied loads: (a) 500g; (b) 1000g.

3. Results

3.1. Polarisation curves

The polarisation curves for wear at 500 and 1000 g loadsdemonstrate the change in behaviour at various velocities

(Fig. 2(a) and (b) ).For loads of 500g ( Fig. 2(a) ), the steel shows the expectedactive-to-passive transition between 700 and 500mV

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SCE. As the sliding wear rate increases, the maximum cur-rent density in the active loop tends to increase while thepassive current density also increases. Although the corro-sion (zero current) potential values are obscured by the highnoise in the data, there is evidence that it tends to becomemore negative with increasing sliding wear.

For 1000 g loads ( Fig. 2(b) ), the trends are similar to thoseat 500 g. Thus, the passive current densities increase, andthe zero current density potentials become more negative,with increasing rotational velocity. Signicantly, the cur-rents were generally higher at all potentials with the higherload, this being particularly evident in the passive regime.Also, the potential at which the active-to-passive transitionoccurred was signicantly delayed to more positive values.These effects are presumably due to the considerably moreeffective removal of the passive lm under the higher loadconditions.

3.2. Weight change data

If K wc = K w + K c (i)

where, K wc is the total wearcorrosion rate (measured asdescribed above); K w the total wear rate; K c is the totalcorrosion rate.

K w can be written as K wo + K w (ii)

where K wo is the wear rate in the absence of corrosion; K wis the synergistic effect of corrosion on the wear rate.

K c can be written as K co + K c

where K co is the corrosion rate in the absence of wear; K cis the enhancement of corrosion due to wear, the so-calledadditive effect.

The above analysis is based on an earlier approach used tocharacterise the various components of the wearcorrosionprocess.

The results for the various contributions to the weightchange are given in Tables 18 and Figs. 34 . The corrosionrate data, K c and K co , were derived using Faradays law, e.g.

K c =Q

ZF (iii)

Kc

=M wit

ZF (iv)

Table 1Mass loss data for corrosion, wear and wearcorrosion contributions at0.157 m s 1 and at 500g load

Potential K wc K co K c K c K wo K w K w K c / K w

0.6 1.1 0.133 0.587 0.454 1 0.514 0.487 1.142 0.4 0.7 0.052 0.159 0.108 1 0.541 0.469 0.299 0.2 0.9 0.045 0.201 0.156 1 0.699 0.301 0.288

0 1.2 0.043 0.198 0.156 1 1.001 0.002 0.1980.2 2.0 0.042 0.199 0.156 1 1.801 0.801 0.110

Table 2Mass loss data for corrosion, wear and wearcorrosion contributions at0.314ms 1 and at 500g load


0.6 1.7 0.133 0.963 0.830 1.100 0.737 0.363 1.305 0.4 1.5 0.052 0.203 0.151 1.100 1.297 0.197 0.156

0.2 1.1 0.045 0.244 0.200 1.100 0.856 0.244 0.2860 0.9 0.042 0.243 0.201 1.100 0.657 0.443 0.3700.2 1.6 0.042 0.246 0.203 1.100 1.354 0.254 0.181



0.6 1.3 0.133 0.620 0.487 0.400 0.680 0.280 0.912 0.4 3 0.052 0.224 0.172 0.400 2.776 2.376 0.081 0.2 1.1 0.045 0.270 0.226 0.400 0.830 0.430 0.326

0 0.9 0.042 0.263 0.221 0.400 0.637 0.237 0.4130.2 1.1 0.042 0.263 0.220 0.400 0.837 0.437 0.313



0.6 0.9 0.137 0.588 0.455 2.3 0.312 1.988 1.89 0.4 1.7 0.052 0.259 0.207 2.3 1.44 0.858 0.180 0.2 0.9 0.045 0.323 0.279 2.3 0.577 1.723 0.560

0 2.2 0.043 0.314 0.272 2.3 1.886 0.414 0.167

0.2 2.1 0.042 0.312 0.269 2.3 1.789 0.512 0.174



0.6 1.0 0.133 0.551 0.419 1.200 0.449 0.751 1.229 0.4 1.1 0.052 0.181 0.129 1.200 0.919 0.281 0.197 0.2 0.7 0.045 0.214 0.169 1.200 0.486 0.714 0.439

0 0.8 0.042 0.212 0.170 1.200 0.588 0.612 0.3610.2 1.3 0.042 0.206 0.164 1.200 1.094 0.106 0.189



0.6 1.7 0.133 0.506 0.374 0.700 1.194 0.494 0.424 0.4 1.5 0.052 0.197 0.146 0.700 1.303 0.603 0.152 0.2 1.2 0.045 0.275 0.230 0.700 0.925 0.225 0.297

0 2.0 0.042 0.256 0.214 0.700 1.744 1.044 0.1470.2 1.5 0.042 0.258 0.216 0.700 1.242 0.542 0.208

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0.6 0.9 0.900 0.133 0.738 0.606 0.500 0.162 0.338 0.4 2.2 2.200 0.052 0.266 0.215 0.500 1.934 1.434

0.2 1.7 1.700 0.045 0.283 0.238 0.500 1.417 0.9170 0.8 0.800 0 .042 0 .277 0 .235 0 .500 0 .523 0.0230.2 0.9 0.900 0.042 0.280 0.238 0.500 0.620 0.120

Fig. 3. Variation of weight change loss due to corrosion, K c , due to wear, K w , and due to wearcorrosion, K wc , with velocity at applied loads of 500g:(a) 0.6V; (b) 0.4V; (c) 0.2V; (d) 0 V; (e) 0.2V.



0.6 1.7 0.133 0.738 0.606 2.500 0.962 1.538 0.768 0.4 1.0 0.052 0.266 0.215 2.500 0.734 1.766 0.363

0.2 1.9 0.045 0.283 0.238 2.500 1.617

0.883 0.1750 0.7 0.042 0.277 0.235 2.500 0.423 2.077 0.6550.2 1.2 0.042 0.280 0.238 2.500 0.920 1.580 0.305

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Fig. 3. (Continued ).

where Q is the charge passed; F the Faradays constant(96494); n the number of electrons involved in corrosionprocess; i the current density; t the exposure time; M w is themolecular weight of material

The weight loss due to wear in the absence of corrosion,K wo , was estimated by measuring the weight change in ca-thodic conditions.

The results at the lower load of 500g ( Fig. 3) shows thatat 0.6 V (Fig. 3(a) ), the corrosion rate, K c, exhibited a peak at intermediate velocities. There also appeared to be a peak in the wear rate, K w and K wc , at intermediate velocities. Athigher potentials ( Fig. 3(b) ) 0.4 V, the corrosion rate wassignicantly lower with very little difference between thevalues of K w and K wc , with the peak in these values occurringat intermediate speeds. At higher potentials Fig. 3(c) 0.2 V,the values of K c were higher than at 0.4V (Fig. 3(b) ).

At potentials of 0.2V (Fig. 3(d) ), there was also evi-dence of a peak in the weight loss as a function of increasingvelocity. At higher potentials of 0 and 0.2 V ( Fig. 3(e) and(f)), there was an initial reduction in the K w and K wc valueswith increasing velocity, with these values increasing againat the highest rotational speeds, where the corrosion rateswere at a maximum.

The results at the higher load 1000g ( Fig. 4(a) ), showa largely similar pattern to that observed at lower loads, at

potentials of 0.6 V, with a peak in the wear rate recordedas a function of increasing velocity; however in this casethe values of K w and K wc attained minimum values at in-termediate speeds and subsequently increased at higher val-ues. The K c value was high; it approached the value of K wat the maximum rotational velocity. At higher potential val-ues, 0.4 V, the characteristic peaks in the values of K w andK wc were recorded. At higher values, however ( Fig. 4(c) ),the peak disappeared and there was a continuous increasein these values with increasing rotational speed. The resultsat potentials of 0 and 0.2V ( Fig. 4(d) and (e) ) showed thatpeaks in the values of K w and K wc as a function of veloc-ity were also recorded; however, at the highest potential thewear rate appeared to attain a minimum value at intermedi-ate velocities; further increases in rotational velocity causedan increase in the recorded weight changes.

3.3. Interactions of corrosion and wear on thesteel surface

The results show that there was a positive enhancementof corrosion rate due to wear, K c , at the exposure poten-tials (Tables 18 ), for each of the applied loads. This wasconsistent with the polarisation data ( Fig. 2), which showedthat increasing the sliding speed shifted the passive current

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Fig. 4. Variation of weight change due to corrosion, K c , due to wear, K w , and due to wearcorrosion, K wc with velocity at applied loads of 1000 g: (a) 0.6V; (b) 0.4V; (c) 0.2V; (d) 0 V; (e) 0.2V.

densities to higher values. The corrosion enhancement washighest at the active-to-passive transition, 0.6 V. Abovethis value, where the passive lm formed, the values of K cwere lower. The K c is the so-called additive effect of wear on corrosion and can be measured in situ from theFaradaic conversion of current density to weight change asdescribed above.

The values of K w, the so-called true synergistic ef-fect, varied as a function of potential, for the conditionstested (Tables 18 ). This indicated that for tests carried out

at 500 g loads, enhanced corrosion during the wear process,impeded wear at intermediate potentials ( Tables 13 ), and atlow applied loads and velocities, i.e. less than 0.471m s 1.At this velocity, corrosion appeared to enhance wear at thehigher potentials, thus indicating that, at least for these con-ditions, corrosion was not effective in reducing wear.

At the higher applied load of 1000g, the values of K wwere negative at high velocities and at high potentials, i.e.0.628ms 1. This was attributed to the formation of a pro-tective tribo-chemical lm in these conditions.

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Fig. 4. (Continued ).

3.4. Scanning electron microscopy of worn surfaces

The micrographs ( Fig. 5), show typical examples of theworn morphologies at various applied potentials. The results(at loads of 500g) ( Fig. 5(a) ) showed that at low poten-tials of 0.8 V, at 0.314m s 1, there was extensive plasticdeformation on the surface. As the potential was increasedto 0.4 V, there was some evidence of oxide akes on thesurface consistent with the increase in corrosion at this po-tentials ( Fig. 5(b) ). At higher potentials of 0.2 V, the surfacewas heavily deformed ( Fig. 5(c) ). The corrosion on the sur-face appeared to be conned to the deformed surface proudof the wear track; the fact that there appeared to be no in-crease in corrosion rate compared to that at lower potentials(Fig. 5(b) ) was consistent with the polarisation data ( Fig. 2),and mass loss results ( Table 3 ).

4. Discussion

4.1. Trends in wearcorrosion data as a function of increasing velocity and applied potential

The results showed that the trends in wear rate data as afunction of increasing velocity were similar to that in dryconditions, in which the classical peak in the wear rate has

been observed for steels [17]. This peak may be due to

the effect of frictional heating, leading to formation of anoxide which provides some protection against wear ( Figs. 3and 4). That this peak changes at higher potentials at thelower load ( Fig. 3(c)-(e) ) may be indicative of the deleteriouseffects of aqueous-corrosion on the surface of this normallyprotective tribo-chemical lm.

The results at the higher load of 1000 g ( Fig. 4(a) and(b)) at the lower potential values are largely similar to thoserecorded at 500 g ( Fig. 3(a) and (b) ) with the classical peak in the wear rate recorded with increasing rotational speedof the pin. The patterns at the higher loads ( Fig. 4(c)-(e) )differ to those observed at the lower loads ( Fig. 3(c)-(e) ).The continuous increase in wear rate with increasing veloc-ity, at 0.2V (Fig. 4(c) ) is not observed at high potentials(Fig. 4(d) and (e) ) where the peak in the wearcorrosionrates is again observed. In these conditions ( Fig. 4(d) ), whenthe corrosion rates at high velocities, K c, approach the wearrates, K w, a reduction in the wearcorrosion rate, K wc , isrecorded. At 0.2 V ( Fig. 4(e) ), there appears to be range of rotational velocities in which the wearcorrosion rate is at aminimum indicating the ability of the corrosion lm to pro-vide protection against wear; at higher velocities, this lm isremoved again due mechanical wear. This indicates that theoverall wearcorrosion rate K wc can be reduced at highervelocities due to the protective nature of the tribo-chemical

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Fig. 5. Scanning electron micrographs of worn surfaces at 500g loads and at 0.314 m s 1: (a) 0.8V; (b) 0.4V; (c) 0.2 V.

lm formed; wear occurring at higher loads tends to pro-mote the development of this layer.

Hence, the results indicate that a signicant differencebetween the wear process in these passivating carbon-ate/bicarbonate conditions and wear in dry conditions, isthe competition between frictional heating and corrosionof the iron oxide lm which provides protection at highvelocities. In these conditions, the iron oxide lm appearsto loose protection in aqueous conditions at low loads and

high velocities and in the highest potential range studied.At high loads and velocities, the lm which may be con-siderably thicker as indicated by the higher corrosion rates,tends to provide better protection during sliding, albeit in anarrow range of conditions.

4.2. Wearcorrosion regimes and wear maps inaqueous conditions

The concept of wearcorrosion regimes can be ap-plied to these conditions to distinguish between wear- andcorrosion-dominated conditions.

The regimes can be dened as follows:K cK w

0.1 (wear ) (v)

0.1

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