Received 19 January 2006, in final form 6 April 2006Published 21 July 2006Online at stacks.iop.org/JPhysD/39/3157
AbstractSlurry impingement is a type of tribocorrosion resulting from simultaneousmechanical and electrochemical surface degradation. In order to understandthe elementary process of slurry impingement and the effect of mechanicaldamage on passive films, electrochemical responses associated with passivefilm breakdown by particle impacts and following repassivation weremeasured during slurry impingement on Al and Al–1wt%Si alloymicroelectrodes. Measurements were made using a slurry jet system whichconsists of a pump, a microelectrode, a potentiostat suited for measurementsof fast transients of small currents and a high frequency data acquisitionsystem. Current transients corresponding to separated single particle impactshave been successfully measured. The current transient sharply rises andgradually decays following a high field model of oxide growth. It is shownthat pure Al repassivates faster after particle impact than the Al–1 wt% Sialloy. This type of erodent particle had an influence on the apparentrepassivation rate of these electrodes, and both Al and Al–1 wt% Si alloyshowed slower current decay after the impact of angular SiC particle thanafter the impact of spheroidal zirconia particle. The SiC particles made deepscars and scratches, and the zirconia particles made shallow depressions.
1. Introduction
Materials exposed to a corrosive environment with tribologicalcontact suffer from degradation. The chemical and mechanicalmechanisms do not independently affect the degradationbut a synergistic effect can act to accelerate degradation.Tribocorrosion is defined as ‘an irreversible transformationof a material resulting from simultaneous physico-chemicaland mechanical surface interactions occurring in a tribologicalcontact’ [1]. Slurry impingement or erosion–corrosion is onetype of tribocorrosion. A well-known example is the particleimpingement in pumps, pipes and valves carrying slurries. Tounderstand the slurry impingement phenomena and to know thecontrolling factor, it is important to investigate the elementaryprocess of impingement attack.
1 Authors to whom any correspondence should to be addressed.2 On leave from National Institute for Materials Science, 1-2-1 Sengen,Tsukuba 305-0047, Japan.
Postlethwaite et al have used an electrochemical techniqueto determine the corrosion component of erosion–corrosionof steel pipes carrying slurries [2–4] and have reported thatthe dominant mode of metal loss was corrosion and that therole of erosive action in the erosion–corrosion process was toprevent the formation of a complete rust film protective againstcorrosion [4]. The repassivation of a nascent metal surfaceis also an important aspect of the process of tribocorrosion.Early electrochemical studies of repassivation after abrasionof metal surfaces have been performed by several researchers,for instance, Ambrose and Kruger [5, 6], Alkire et al andBeck [7, 8]. Beck utilized a rotating disc of Ti scraped with asapphire cutter and analysed the dissolution products in situ.Ford et al also performed scratch experiments for measuringrepassivation transients on rotating disc electrodes [9]. Oltraet al used impingement of particles for the removal of passivefilms on stainless steel [10]. They have also reported acomparison of various techniques for passive film breakdown,
Figure 1. A schematic drawing of the microelectrode used. Thediameter of the Al and Al–1 wt% Si wires mounted in the glasscapillary was 125 µm. The Au counter electrode is coiled aroundthe top of the working electrode.
such as potential jump, particle impingement and focused laserpulse [11]. Burstein and co-workers carried out impingementexperiments in slurry erosion rigs [12–14] and have reportedthat the erosion rates of Al in NaCl and acetic acid solutioncontaining silicas were much higher than those in aqueousslurry without electrolyte additives even though the purecorrosion component was very small [12]. They reportedon the effect of the impact angle of the slurry jet and thenucleation of pitting on stainless steels [13, 14]. Neville andHu investigated the electrochemical response from high alloystainless steels during slurry impingement [15]. They showeda current transient associated with a single particle impact byusing a specially designed system. They used a glass sphereof 4 mm diameter as an erodent fired from an air pressure ata velocity of 17–20 m s−1 to break the surface film. Althoughthe particle size they used is by far larger than the actual sizeof particles in the slurry, the system is useful to measure sucha current transient.
In the present study, a slurry jet system has been assembledto monitor electrochemical responses from a microelectrode ofpure Al or Al–1 wt% Si alloy exposed to slurry impingementin order to understand the elemental process of impingementattack. The system had a high frequency data acquisitionsystem to collect data associated with a single particle impactto assess the repassivation kinetics through a single event [15].This work also aims at understanding the effect of the alloyingelement on the repassivation and to know the influence of thetype of particle impinging on the metal surface.
2. Experimental methods
Experiments of aqueous (acetate buffer) slurry jet were carriedout on a microelectrode of Al (>99.99%) and Al–1 wt% Si alloy(GoodFellow) whose diameter was 125 µm. Figure 1 shows aschematic drawing of the microelectrode used. A metal wirehaving a diameter of 125 µm was embedded into a polymerresin within a glass capillary with an inner diameter of 1 mm.The glass capillary was fixed in a thicker glass tube with an Auwire (ø 200 µm) as counter electrode. The Au wire was coiledaround the tip of the glass capillary. An aqueous slurry wasjetted onto the surface of the microelectrode. To analyse thekinetics of repassivation after a particle impact, it is important
to separate a current transient peak associated with a singleparticle impact in the time domain. One of the reasons forthe employment of electrodes with a small surface area wasto reduce the probability of simultaneous particle impacts onthe electrode and to obtain isolated signals of single impacts.Jetting a solution containing a small number of particles froma pump could be also effective in avoiding occurence ofoverlapping signals of multiple impacts, but it is difficult tocontrol the number of particles in the slurry using the currentsetup. Another reason for the employment of microelectrodeswas to reduce the background current in order to obtain abetter signal to noise ratio, which is the ratio of the currenttransients associated with particle impacts to the background.The background current has two major contributions; one isa random statistical noise and the other is an anodic currentof stationary corrosion that depends on electrode potential,electrode area and corrosivity of the electrolyte.
The experimental set-up used for measuring theelectrochemical response from the microelectrode exposed inthe slurry jet is shown in figure 2. The system consistedof a microelectrode, a pumping system for the slurry, apotentiostat and a high frequency data acquisition system. Ahigh performance pump (HNP Mikrosysteme GmbH, mzr-7205) controlled by a computer took the solution from a beakerand ejected particles onto the surface of the microelectrodemounted face-to-face with the nozzle. The acceleration ofthe pump was high enough to reach to the maximum rotationspeed in 3 ms after start. The impingement angle was 90◦ forall measurements in this study. The particles were loaded in apathway connected to the nozzle of the pumping system. Thenozzle of the pump was a glass tube with an inner diameter of1 mm. The particles were zirconia beads (Fuji ManufactoryCo., Ltd.) with a diameter in the range of 150–106 µm andSiC particles of grit 120 (BUEHLER). The shape of zirconiabeads was almost spheroidical as will be shown later. Thesolution carrying particles was a 0.1 M acetate buffer solution(50 mmol CH3COOH + 50 mmol CH3COONa, pH = 4.7).The solution was prepared from analytical grade reagents anddeionized water. This pH is near the minimum solubility ofthe aluminium oxide (pH = 6.0) and ensures a sufficientpassivation of the sample.
All experiments were performed at a temperature of 298 K.The flow velocity of the slurry jet ejected from the pump was6.1 m s−1 for all measurements reported here. Prior to theexperiment several tens of particles were loaded into a bypassof the pump system. After starting the pump the electrolytewas jetted against the electrode to ensure potentiostatic control.By switching a 3-way cock the electrolyte flow was redirectedthrough the bypass to take up the particles. Since the diameterof the target electrode is smaller than the jet diameter only asmall portion of the particles hit the surface to ensure that theyare sufficiently separated in time.
The electrochemical control and measurement of currenttransient for the microelectrode was carried out by usinga bespoke high speed potentiostat optimized for measuringa fast transient of small current. Current transients wererecorded by a computer equipped with a board for highfrequency data acquisition (maximum 20 MHz) to isolate thecurrent transients associated with a single particle impact fromthe multiple impact data. In the case of a two-electrode
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Figure 2. A schematic diagram of the system used for the measurement of current transients of a microelectrode exposed under a slurry jet.The flow velocity of the jet was 6.1 m s−1 and the impact angle is 90◦ for all measurements. The system set up in the three-electrodeconfiguration is shown on the top of the diagram.
system, a voltage of 2 V was applied between the workingand the Au counter electrodes. Before jetting the aqueousslurry jet to the microelectrode, the working electrode waspassivated by applying 2 V between two electrodes under slowsolution flow free from particles. When operated in a three-electrode configuration the system employed consisted of themicroelectrode, the top of the nozzle of the pump system and aLuggin capillary connected to an Ag/AgCl reference electrode.A sketch of the system is given in the upper part of figure 2. Thecurrent transients measured by the potentiostat were recordedfor 0.1 s at the data acquisition rate of 1 MHz. The data wereFourier transformed and were inverse Fourier transformed aftercutting the data in the high frequency region from 20 to 500 kHzto filter noise signals.
To observe the depressions or scars produced by impactsof zirconia and SiC particles, mirror finished Al plateswere exposed under the slurry jet containing these particles.Scanning electron microscopy (SEM) observation was carriedout by means of CamScan (Elektronen-Optik) at 20 keV forobservations of zirconia and SiC particles and the depressionsand scars on the Al plates.
3. Results and discussion
An example of a current transient of Al microelectrodeobtained under impingement of 0.1 M acetate buffer solutioncomprising zirconia particles with 150–106 µm diameter isshown in figure 3. The applied voltage between the workingand the counter Au electrodes was 2 V. Each peak in figure 3can be assigned to an individual particle impact on theAl microelectrode. The background current was very lowbecause the surface area of the microelectrode was small.Representative current transients of Al and Al–1 wt% Si alloyelectrodes caused by single zirconia particle impacts arecompared in figures 4(a) and (b). In these figures, the current
Figure 3. A representative current transient of the Almicroelectrode obtained under impingement of 0.1 M acetate buffersolution comprising zirconia beads with 150–106 µm diameter.Data acquisition frequency was 1 MHz.
is normalized to the height of the peak to allow a comparisonof the transient behaviour of different current transients. Thetime of the peak current was set to 0 and the time scale wasadjusted to this maximum. The current transients show a sharprise in a few tens of microseconds followed by a gradual decayto the background level in several milliseconds correspondingto the repassivation of the Al or Al–1 wt% Si alloy electrode.Obviously, pure Al shows a faster decay of the current than Al–1 wt% Si alloy. It can therefore be said that the repassivationprocess of pure Al is faster than the Al–1 wt% Si alloy. It mayalso be possible to consider the influence of the difference inthe dissolution currents of nascent metal surfaces on Al and theAl-Si alloy and so on, but further investigation of the surfaceproperties is required to explain the difference in the rate ofcurrent decay.
In figure 4(b), the changes in the normalized current as afunction of time since impact for Al and Al-Si alloy electrodes
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(a)
(b)
Figure 4. Representative current transients of pure Al andAl–1 wt% Si alloy microelectrodes caused by zirconia particle in0.1 M acetate buffer solution in linear–linear scale (a) and in log–logscale (b). The current was normalized to the peak height of thetransient and the time of the peak was set to 0.
are compared in log–log scaled plot. Both electrodes showstraight lines with a similar slope, (d log i)/(d log t), ofabout −1. Assuming that the measured current was usedfor the formation of the oxide film and the current consumedfor anodic dissolution was negligible or constant with time,the dependence of current decay on time indicates the rate-determining movement of ions within the oxide film underhigh field conditions. When the movement of ions in the oxidefilm is the rate-determining step, the oxide formation current,i, is a function of the potential drop, �U , and the thickness ofthe oxide, d [16]:
i = i0 exp(βE) = i0 exp
(β
�U
d
), (1)
where i0 and β are material-dependent constants and E is theelectric field strength. The increase of the oxide thickness withtime t is expressed by
dd
dt= M
ρnFi0 exp
(β
U − U0
d
), (2)
where M and ρ are the molecular weight and the density ofthe oxide, respectively and n is the valence of the ions and F isthe Faraday constant. The current calculated numerically fromintegration of equation (2) is constant in the beginning and thefield strength is then reduced by the increase in oxide thickness.This results in a slope (d log i)/(d log t) close to −1 [16]. The
Figure 5. The relation between charge and peak height of currenttransient peaks associated with single particle impacts for pure Aland Al–1 wt% Si alloy microelectrodes under zirconia particleimpingement.
experimental data of figure 4(a) show a −1 slope after about200–300 µs which is in agreement with the high field modelof oxide growth.
The charge obtained from numerical integration from anumber of current transients is plotted as a function of thecorresponding peak height for pure Al and Al–Si alloy infigure 5. Although the data points scatter to some extent, thereis a clear trend to increase with peak height. Both data setswere fitted to straight lines which are shown in the graph. Thissuggests that the size of the current transient that corresponds tothe destructed area does not affect the repassivation kinetics ofeither pure Al or the Al–Si alloy. The slope of the regressionlines has a unit of seconds thus suggesting that this value isrelated to the repassivation rate. The slope is lower for Al thanfor the Al–Si alloy indicating that the repassivation rate of Alis higher than the Al–Si alloy irrespective of the size of currenttransients corresponding to single events. The differencesin the size of the current transients are more pronounced aswould be expected from the distribution of kinetic energy ofthe particles with respect to their size or mass respectively. Itwas shown for the zirconia beads used here that the diameterranges from 82–146 µm. As the mass of the particle scaleswith a power of 3 the resulting impulse varies by a factor ofmore than 3 [17]. For angular shaped particles this effectmust be expected to be even more pronounced taking intoaccount the various orientations during impact. Other effectssuch as partial energy transfer while hitting the rim of themicroelectrode seem to be of minor importance.
Since the diameter of the zirconia particle is roughlyidentical to the diameter of the microelectrode it is unlikelythat two transients overlap in a way that one transient hides theother entirely. The average diameter of depression made bythe zirconia particles is about 20 µm as it will be shown later.Note that the data points in figure 5 are only for the currenttransient peaks which could be isolated from other peaks andpeaks overlapping with other peaks were omitted with respectto the difficulties in a proper charge calculation.
Figure 6 compares the charge-peak height plot for currenttransient of pure Al and Al–Si alloy generated by SiC particleimpacts. The scattering of data points is more pronounced thanthat in the plot for zirconia particles in figure 5. SiC particleshave angular and non-uniform shapes and the size distribution
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Figure 6. The relation between charge and peak height of currenttransient peaks associated with single particle impacts for pure Aland Al–1 wt% Si alloy microelectrodes under SiC particleimpingement.
of SiC particles is even wider than that of zirconia spheroidparticles. The non-uniform shape and size of the SiC particlesis responsible for the lower reproducibility of the currenttransient peaks. Although the data points scatter significantlythe same trend to higher charges at higher peak heights isobvious. The linear regression of the experimental data showssteeper slopes corresponding to a prolonged repassivation timethan those for the zirconia particle impacts shown in figure 5.Figures 7(a) and (b) show representative current transients ofpure Al generated by impact of zirconia and SiC particles.Figure 7(a) shows the plot in linear–linear scale and figure 7(b)the transients in the log–log scale. Apparently, the currentdecay of Al after zirconia impact is faster than that after SiCimpact.
Images of zirconia and SiC particles were taken by meansof a SEM. Figure 8 shows the secondary electron (SE) imageof the zirconia particle. The influence of the diameter on themass and impulse has been mentioned before. The imagesgive a good impression of the actual shape of these sphericalparticles. In contrast, the shape of the SiC particle is notuniform as can be seen in figure 9. The particles are angularand much sharper than the zirconia particles. In order tostudy the influence of the type of particle, zirconia beads andSiC particles were jetted onto a mirror-finished macroscopicsurface of aluminium under open circuit conditions. The scarscreated this way were then visualized by means of SEM.Figure 10(a) shows a SE image of a zirconia blasted area.The depressions in the SE image are recognized from the(slight) white circle surrounding the depression. However,the depressions are not very clearly seen, probably becausethe depressions are rather shallow. In contrast to the SEimage, back scattering electron (BSE) image of the depressionsshows a much higher contrast for recognizing the depressionsas shown in figure 10(b), although the contrast on the BSEimage does not directly show the morphology of the surfacerather the changes in crystallography changes resulting fromthe mechanical deformation of the surface. This effect is due toa different degree of interaction of the electrons with the surfacematter. The depressions accumulate around the central partof the flow with 1 mm diameter because of the hydrodynamiclensing effect that aims particles with a density higher than thatof water into the centre of the jet stream. This hydrodynamic
(a)
(b)
Figure 7. Representative transients of normalized current of the Almicroelectrode caused by the zirconia and SiC particle impact in thelinear–linear scale (a) and in log–log scale (b). The current isnormalized by the peak height of the transient and the time of thepeak was set to 0.
lensing effect results from the expansion of the slurry afterejection from the nozzle. The pressure remains highest in thecentre of the jet while it is equal to the environmental pressureat its edges. This pressure gradient causes a density gradientin which particles with a density higher than that of mediumare held in the centre.
The size of the depression observed was roughly 20 µm indiameter. Assuming that a part of the surface of the spheroidfits to the depression, the depth of the depression can becalculated from equation (3),
AL = 2πRh = π(r2B + h2), (3)
where AL is the surface area of a crater, R is the diameterof a spheroid which makes the depression, rB and h are thediameter and the depth of the depression, respectively. Foran assumed diameter of 20 µm and taking into account thediameter reported by the producer (150–106 µm) the depthof the depression is calculated to be in the range from 950to 670 nm. On the other hand the depth might be lower ifelastic recovery leads to a partial relaxation. In any case, thedepth is less than 1 µm, and this is presumably the reasonwhy the depressions are not clearly seen in the SE images(figure 10(a)). These findings are in agreement with staticindentation experiments in which the same particles wereemployed and a similar mechanical work was done [18]. Incontrast to the depressions created by the zirconia particleimpact, the scars made by the SiC particle impact are angular,
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(a) (b)
Figure 8. SE image of zirconia particles used for the measurements.
(a) (b)
Figure 9. SE image of SiC particles used for the measurements.
(a) (b)
Figure 10. An SE image (a) and a BSE image (b) of the surface of an Al plate exposed to zirconia particle impingement. The flow velocityof the slurry jet was 6.1 m s−1.
deep and inhomogeneous as seen in the SE image in figure 11.Some of them look like scars created by indentation of the edgeof the SiC particle and are lengthy scratches. The depth of thescars cannot be determined reliably from the SE images but it is
obviously higher than that of the craters created by the zirconiaimpact. As shown in figure 6, the data points on the chargeversus peak height plot for the SiC particle impact scatter morethan those for the zirconia particle impact. The heterogeneous
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Figure 11. An SE image of the surface of an Al plate exposed to SiCparticle impingement. The flow velocity of the jet was 6.1 m s−1.
destruction mechanism for the SiC particle is held responsiblefor the strong scattering of the data points. It is consideredthat when an SiC particle slides on the electrode surface andscratches along the surface continuously it produces nascentsurfaces resulting in a retardation of the repassivation. Thismechanism must be assumed as most of the scratches are longerthan the size of particles. In case of the spheroidical zirconiaparticles on the other hand a somewhat instant destruction canbe expected.
The high current densities in the locally depassivatedcrater may be made responsible for a significant ohmic dropthat would cause a dependence on the potential applied.In order to study the influence of the applied potential,measurements of current transients caused by zirconia particleimpact were performed in a three-electrode system. Infigures 12(a) and (b), representative normalized currenttransients are compared. The applied potential was variedfrom 0.5 to 2.0 V versus Ag/AgCl. No obvious dependenceon the applied potential was found. In figure 13, the relationbetween the charge of current transients and their peak heightis plotted. Some scattering is seen in the plot but the data pointsfor all potential applied can be fitted to the same straight line.There is no distinct potential dependence as seen from the factthat points below the solid line correspond to both the lowestand the highest potential whereas the data points above the linecorrespond to both the lowest and the second highest potential.
Since the oxide film thickness formed on Al increaseslinearly with applied potential, the film thickness at agiven potential can be calculated [16, 19]. The high fieldtheory predicts that the charge of a current transient peakcorresponding to a single particle impact is consumed foroxide film formation; this allows an estimation of the area.For example, an area where repassivation took place wasestimated for a relatively large current transient at 2.0 V versusAg/AgCl (2.22 V versus SHE) whose charge was about 4 nC.The redox potential of Al is −1.662 V versus SHE. Therefore,the potential drop across the oxide thickness is 3.88 V. Thefilm formation factor of Al in a neutral solution (1 M acetatebuffer, pH = 6.0) is 1.6 nm V−1, and the charge consumedfor forming an oxide film in unit area and unit thickness is3.17 mC cm−2 V−1 [20]. Consequently, the charge for formingthe passive film in a unit area at 2.0 V versus Ag/AgCl is
(a)
(b)
Figure 12. Representative current transients of an Al microelectrodenormalized by peak height; in linear–linear scale (a) and in log–logscale (b). The current transients were caused by zirconia particleimpact at 0.5–2.0 V versus Ag/AgCl reference electrode.
Figure 13. Relation between change and peak height of currenttransient peaks associated with single particle impacts for pure Almicroelectrode under zirconia particle impingement at 0.5–2.0 Vversus Ag/AgCl.
12.3 mC cm−2. The area where repassivation took place for thediscussed current transient with a charge of 4 nC is, therefore32.6 µm2. This corresponds to a diameter of 6.44 µm for aflat circle. The area estimated for this relatively large currenttransient with a charge of 4 nC is much smaller than thearea observed for the depressions made by zirconia particles(figures 12(a) and (b)). This fact suggests that the passivefilm in the depression is not broken or removed completely bythe impact of zirconia particles. Evidence for this observationcan be found when an experiment is performed in which only
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one particle hits the surface and an exact correlation betweenthe observed current transient and the crater size becomespossible [17].
4. Conclusions
A slurry jet system has been constructed and was employedto measure the fast current transients associated with thebreakdown and repassivation of passive film on Al and an Al–1 wt% Si alloy. Current transients corresponding to discreteparticle impacts have been successfully measured. Thefindings of this study are as follows.
(1) The repassivation kinetics of Al and Al–1 wt% Si alloyafter the breakdown of passive film by particle impact arewell described by the high field model of oxide growth.
(2) Pure Al repassivates faster than the Al–1 wt% Si alloy.(3) The reproducibility of the current transients caused by
zirconia particles is significantly better than that of SiCparticles with respect to the destruction mechanism andhigher uniformity of zirconia particle impacts. This canbe seen directly when looking at the surface morphology.Zirconia particles with a spheroid shape make relativelyuniform and shallow depressions. In contrast, angular SiCparticles make deep scars or scratches.
(4) The apparent repassivation rate for both pure Al andthe Al–1 wt% Si alloy is slower after the SiC particleimpact than that after the zirconia particle impact. This isattributed to the fact that the particle transfers its energymuch faster during impact while scratching is much slowerresulting in a retardation of the activation.
(5) The estimated impacted area as calculated from thecharge consumed through the current transient of Alis significantly smaller than the observed impact areasuggesting that only parts of the passive film are brokenor removed.
Acknowledgments
The authors gratefully acknowledge the financial support of theJapan Society for the Promotion of Science through a researchfellowship (E.A.). The authors thank A J Smith for the helpin establishing the high frequency data acquisition system andM Nellessen for assistance with the SEM.
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