Understanding the electrochemical, microstructural and morphological changes during hot rolling from a corrosion perspective

Surface & Coatings Technology 201 (2006) 828 – 834 www.elsevier.com/locate/surfcoat

Understanding the electrochemical, microstructural and morphological changes during hot rolling from a corrosion perspective Premendra ⁎, Laetitia Philippe, Herman Terryn, J.H.W. de Wit, Laurens Katgerman Delft University of Technology, The Netherlands Received 14 September 2005; accepted in revised form 22 December 2005 Available online 8 February 2006

Abstract Thermo-mechanical processing of rolled aluminium alloys result in the formation of a deformed near-surface region with different filiform corrosion (FFC) behavior than the underlying bulk. This paper tries to correlate the microstructural, morphological and electrochemical changes occurring on the surface of aluminium sheet, during hot rolling, with the FFC behavior. The alloy under investigation is recycled AA5050. The electrochemical changes taking place in the roll-bite has been profiled and the result has been supported by electron microscopic and optical characterization for better understanding. Factors which may or may not be responsible for FFC susceptibility of rolled AA5050 have been discussed and importance of surface finish has been emphasized. © 2006 Elsevier B.V. All rights reserved. Keywords: Thermo-mechanical processing; Surface layer; Filiform corrosion

1. Introduction Rolling is an integral part of the fabrication of wrought aluminium alloy sheet. While the main purpose of hot rolling is gauge reduction, cold rolling additionally provides strain hardening. During rolling, the work rolls exert a load and a shear stress on the surface of the work piece, causing severe shear deformation of the near-surface region compared to the bulk microstructure. This results in the development of a surface layer (a few microns thick) [1] with different morphological [1– 3], optical [4–6], microstructural [7–9] and electrochemical [9– 12] properties compared to the bulk. The surface layer can control many important properties like corrosion resistance, adhesion and optical appearance and hence it is important to understand both its electrochemical behavior and microstructural details like the structure, precipitates and other features present. The deformed surface layer has extremely fine grains compared to the bulk [1,4]. Furthermore these ultra-fine grains are pinned by rolled-in oxides [1], preventing recrystallization of the surface layer during subsequent heat treatment. This ⁎ Corresponding author. E-mail address: [email protected] (Premendra). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.12.042

phenomenon is referred to as zener pinning [13]. Often, the surface layer comprises of a high density of sub-micron size precipitates in addition to surface features like cracks, rolling ridges, etc. [3]. Analysis using transmission electron microscopy (TEM) has revealed the composition, structure and thickness of the surface layer [7,8]. Optical reflectance of the surface layer has been found to be much less than the bulk alloy predominantly due to the presence of rolled-in oxides [4,5]. In the late nineties, it was reported that the surface layer is electrochemically active, i.e. the surface layer is more susceptible to corrosion attack, mainly filiform corrosion (FFC), than the underlying bulk [14]. FFC is usually an underfilm corrosion process which manifests itself in the form of thread-like filaments. Afseth et al. [8] combined the surface analytical and the electrochemical properties of the surface layer and formed a correlation between thermo-mechanical treatment and surface activation. They also found that hot rolling plays a more dominant role in activating the surface layer, as compared to cold rolling. Fishkis and Lin [1] had earlier reported that substantial morphological changes (cracks, holes, etc.) take place at high temperatures, i.e. initial hot rolling passes. While in the literature there is a fairly good understanding of the microstructural and morphological changes taking place in the surface layer during rolling, the detailed mechanistic

Premendra et al. / Surface & Coatings Technology 201 (2006) 828–834

Al strip A

B

Roll bite

D C

E FG Rolling direction

work roll

Fig. 1. Schematic representation of the roll-bite. Potentiodynamic polarization measurements using micro-cell were carried along the planes A, B, C, D, E, F and G. Planes A and G lie outside the roll-bite.

understanding of electrochemical activation is not well developed. This paper tries to develop an understanding of the electrochemical changes occurring in the near-surface after the initial hot rolling passes and also in the roll-bite. A schematic of the roll-bite can be seen in Fig. 1. The effect of mechanical deformation of the surface on the electrochemical as well as FFC behavior has been studied in detail.

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calculated from sputtering time using the multi-matrix calibration approach [15]. Microstructural characterization was carried out using backscattered electron imaging using a LEO 1455VP Scanning Electron Microscope (SEM) fitted with an Energy Dispersive X-ray (EDX) analyzer. Transmission Electron Microscopy (TEM) was carried out using CM30T from Philips. Crosssectional TEM specimens were prepared using a Gatan PIPS691 Ion Miller. The surface was protected using glue. Acidified salt spray (ASS) test was performed on the samples from different stages of fabrication in order to have a better estimation of the FFC susceptibility of the alloy under investigation. These tests were performed in accordance with European coil coating association (ECCA) T-8 test (EN 135238), at room temperature for 500 h. Test specimens were degreased in Z19 (phosphoric acid bath) at 50 °C for 20 s and rinsed. Thereafter the samples were pre-treated using Unicon 87 (Cr-free). Finally a transparent polyester topcoat was applied followed by curing. The coated specimens were then scribed and placed in the salt spray test chamber. Test solution, containing 1% CH3COOH and 5% NaCl, is continuously sprayed on the walls of the test chamber so as to have them suspended in the test environment. The samples were examined after 168 h and after 500 h. Corrosion number, a standard used in the industry, is defined in Eq. (1). A lower corrosion number implies a more corrosion resistant surface. Corrosion number ¼ ðfraction of scribe corrodedÞ ⁎ðaverage length of filamentÞ:

ð1Þ

2. Experimental The wrought architectural aluminium alloy, AA5050, was manufactured at the test mill facility of Hunter Douglas Europe BV, Rotterdam. The alloy was cast from recycled aluminium and hence had a high concentration of impurities like Fe and Si. The chemical composition of this alloy is shown in Table 1. Samples were provided from different stages of the fabrication (as-cast, hot rolled 1st and 2nd pass, final hot rolled, cold rolled) and also from the roll-bite of the first 3 passes of a 6-stand tandem hot rolling mill. The width of the cast bar was 23 cm and the gauge was 25 mm. The entry temperature in the hot rolling mill was around 525 °C and the gauge reductions in the first two hot rolling passes were 50% and 48% respectively. Glow discharge optical emission spectroscopy (GDOES) was used for depth profiling the concentration of various alloying elements. The instrument used was a Leco SDP-750 dc GDOES with 4 mm diameter copper anode. This instrument has a high sensitivity and can theoretically measure concentration as low as 10 ppm. A discharge voltage of 700 V and a discharge current of 20 mA were applied. The sputtered depth was

An electrolyte containing 5% NaCl + HCl (pH 2) was used for the electrochemical characterization. The pH of this electrolyte is similar to that at the head of a filiform filament [11]. Macro-electrochemical characterization was performed over an exposed area of 0.79 cm2 (ϕ = 1 cm). After a steadystate potential was developed, the test samples were polarized anodically with a scan rate of 10 mV/s. A standard 3-electrode set-up was used with Ag/AgCl (3 M KCl) as the reference electrode and platinum as the counter electrode. Due to their curved geometry, the roll-bite samples could not be characterized electrochemically on a macroscopic scale. This problem was overcome by characterizing the roll-bite specimen using a capillary-based micro-cell [16,17]. The micro-cell is used for small area measurement, in which a glass capillary is used to restrict the area of measurement. The electrolyte is filled up in the glass capillary and the size of the capillary tip determines the size of the working area because the capillary tip is in contact with the working electrode, i.e. the sample. The counter electrode and the reference electrode are connected to the other end of the glass capillary. This technique has been used in understanding the breakdown in association with

Table 1 Composition of alloying elements in the commercial AA5050 Element

Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

Pb

Be

Na

AA5050 (wt.%)

0.28

0.8

0.12

0.33

1.20

0.05

0.07

0.03

0.01

0.0006

0.003

3. Results 3.1. Depth profiling using GDOES GDOES depth profiles of certain alloying element concentration can be seen in Fig. 2. The as-cast bar had a segregation of alloying elements like Fe, Mn, Si on the surface (Fig. 2a). Although the alloy had 0.8 wt.% Fe, the concentration of Fe on the surface was approximately 16 wt.%. Surface segregation of alloying elements during casting is a well understood phenomenon and occurs due to the exudation of solute enriched liquid to the surface. However GDOES depth profiles fail to determine if the segregated alloying element is present in solid solution or as second phase particles. After the first hot rolling pass, the segregation of alloying elements on the surface was reduced (Fig. 2b). Also, the peak

5 4

80 60

3 silicon

manganese

2

40

magnesium

1

20

iron

0

conc (wt%) of aluminium

100

(a)

aluminium

0 0

1

2

3

4

5

depth (microns)

5

100 aluminium

(b)

4

80

3

60 silicon

manganese

2 1 0

40

magnesium

20

iron

0

1

2 3 depth (microns)

4

5

conc (wt%) of aluminium

individual second phase particles in iron [17,18] as well as aluminium [19–22] alloy because the area of measurement can be reduced to the order of 20 μm2 (approx.). The electrochemical characterization, in the present study, was carried out over an exposed area of 0.20 mm2 (ϕcapillary = 500 μm). The tip of the glass capillary was coated with silicone [16] to prevent electrolyte leakage at the glass–sample interface and to prevent the formation of crevice, attributed to the hydrophobic nature of silicone. A battery-operated potentiostat (Jaissle IMP 83PC TBC) with a current resolution of 10 fA was used. The electrolyte, cell set-up and operating parameters were the same for both macro- and micro-electrochemical measurements. For a capillary size of 500 μm in diameter, the ohmic drop can be considered negligible [21]. Mechanical deformation during hot rolling not only results in the formation of a surface layer but also affects the surface finish. The effect of mechanical deformation was studied on a laboratory scale by mechanically grinding the as-cast AA5050 to different surface roughness. The corrosion behavior of the as-cast AA5050 was compared with that of the as-cast commercially pure Al (99.94% Al, 0.04% Fe, 0.02% Si), with similar surface finish. This experiment was performed on an as-cast specimen because it has no pre-existing surface layer. Surface segregation of intermetallics and alloying elements were removed by grinding, before attaining the required surface finish. Commercially pure Al was chosen as a reference because the influence of second phase particles on the corrosion behavior would be the least in this case. Corrosion testing was carried out in the acidified salt spray test cabinet for a period of 500 h, the samples being analyzed at frequent intervals. For each of the 2 alloys mentioned above, 2 different surface finish was analyzed (a) ground unidirectionally using grit 320 grinding paper, and (b) 1 μm diamond polished. Furthermore, the electrochemical response of rolled AA5050 was also compared with that of commercially pure Al (ground) to analyze the effect of surface finish on electrochemical behavior, if any. Surface roughness was measured using the Laser Scanning Confocal Microscopy (LSCM).

conc (wt%) of alloying elements

Premendra et al. / Surface & Coatings Technology 201 (2006) 828–834

conc (wt%) of alloying elements

830

0

Fig. 2. Elemental depth profiling using GDOES for key alloying elements like Mg, Fe, Mn, Si: (a) as-cast, (b) hot roll 1st pass.

concentration was less than that observed for the as-cast sample. The change in the depth profile after rolling is due to the distribution of segregated alloying elements over an increased surface area. The reproducibility of GDOES measurement was confirmed by measuring over 2–3 different areas on the same sample. The measurements were performed far away from the edge of the samples. 3.2. Microstructure SEM backscattered electron micrographs can be seen in Fig. 3. This analysis was carried out to identify the various phases present and to observe the microstructural changes due to rolling. A dense network of ‘inter-dendritic’ intermetallics was observed on the as-cast alloy surface (Fig. 3a). Most of the precipitates analyzed using EDX were quaternary Al–Fe–Mn– Si phases. These intermetallics are mostly the eutectics [23], formed during alloy solidification. Extensive EDX analyses show that intermetallics in the near-surface region contained 2–8% Si, 1–3% Mn and 10–20% Fe. After the first hot rolling pass, morphological surface features like shingles could be observed on the surface (Fig. 3b). The intermetallics observed were fine and irregularly shaped. This is due to the breaking-up and smearing-out of the intermetallics during rolling. EDX analysis showed that almost all the intermetallics were rich in Al, Fe, Mn and Si. Dark patches of oxide, rich in carbon, were observed on the surface. Carbon contamination is from the emulsion used in the hot rolling mill. Extensive EDX analyses show that intermetallics in the near-surface region, after first hot rolling pass, contained 2–7% Si, 2–3% Mn and 10–20% Fe.

Premendra et al. / Surface & Coatings Technology 201 (2006) 828–834

(a)

Al-Fe-Mn-Si

10 µm

(b)

831

specimens and maximum for the specimen from initial hot rolling passes (approximately 7.5 μm/h). The effect of surface finish on FFC can be seen in Fig. 6, where the corrosion number (refer to Eq. (1)) has been plotted against time (in days). It can be seen in Fig. 6 that irrespective of the surface finish, FFC initiation did not take place on commercially pure Al even after 500 h of testing. The ground AA5050 was more susceptible to FFC attack than the polished AA5050. While FFC initiation on polished AA5050 took place after 4 days of testing, it took only 2 days for the initiation of FFC attack on ground AA5050. 3.4. Macro-electrochemical characterization In this article pitting potential will be stated as breakdown potential because on commercial AA5050 there is never a stable passive film formed and breakdown takes place close to the open circuit potential (OCP). Hence the situation is different from classical pitting where a localized dissolution of metal takes place due to the breakdown of an otherwise passive film.

Oxide and C Al-Fe-Mn-Si

10 µm

Fig. 3. SEM backscattered micrograph of the surface: (a) as-cast, (b) hot roll 1st pass.

Cross-sectional TEM images illustrating the morphology of the surface layer before and after the first hot rolling pass can be seen in Fig. 4. The as-cast specimen (Fig. 4a) had a homogenous near-surface region with the grain boundaries extending all the way to the surface. However after the 1st pass of hot rolling, a heavily deformed surface layer was observed and the grain boundaries could no longer be seen (Fig. 4b). 3.3. Acidified salt spray (ASS) test on AA5050 The test result (Fig. 5) shows the fraction of surface corroded and average filament length as a function of exposure time and thermo-mechanical treatment. Fraction of the surface corroded is the ratio of the length of scribe corroded to the total length of scribe. After 168 h of test (Fig. 5), initiation of FFC attack was yet to occur on the as-cast specimen. On the other hand, rolled specimens were almost completely corroded indicating that the as-cast surface had the slowest FFC initiation compared to the rolled surface. The average filament length for the hot rolled and cold rolled specimens appeared to be comparable at this stage (2–3 mm). After 500 h of test (Fig. 5), all the specimens were completely corroded. However the difference lay in the filament length. Hot rolled samples were observed to have longer filaments compared to the as-cast and cold rolled specimens. The rate of FFC propagation, i.e. increase in filament length per unit time between 168 h and 500 h, was higher for the hot rolled

Fig. 4. Cross-section TEM images showing the morphology of the near-surface: (a) as-cast, (b) hot roll 1st pass.

1.2

6

1.0

5

0.8

4

0.6

3

0.4

2

0.2

fractional area attacked: 168hrs fractional area attacked: 500hrs longest filament length: 168hrs longest filament length: 500hrs

0.0

Average filament length (mm)

Premendra et al. / Surface & Coatings Technology 201 (2006) 828–834

Fraction of scribe corroded

832

1 0 -1

as-cast

hot roll 1st pass

hot roll 2nd pass

hot rolled

cold rolled

Different stages of fabrication

Fig. 5. Acidified salt spray (ASS) test results showing the fraction of the scribe corroded and filament length as a function of thermo-mechanical treatment for different test periods.

Fig. 7 shows the plot of breakdown potential versus surface roughness for commercial AA5050 and commercially pure (ground) aluminium. The as-cast surface had a breakdown potential of − 810 mV (Fig. 7). However for all the rolled specimens, the breakdown potential was observed to be around − 730 mV. The RMSroughness values varied between 0.81 μm (cold rolled) and 2.37 μm (hot rolled 2nd pass). Surface roughness was highest after initial hot rolling passes and thereafter decreased monotonically in subsequent passes. The breakdown potential on ground, pure Al varied from − 810 mV (grit 60) to − 740 mV (diamond polished). For grinding paper finer than grit 360 (RMSroughness = 2.32 μm), breakdown potential almost stabilized. The main purpose of using commercially pure Al was to isolate the effect of surface finish on surface activity by neglecting the effect of alloying element and thermo-mechanical treatment. 3.5. Electrochemical characterization of roll-bite specimen The breakdown potential distribution on the surface of rollbite specimen, for the 1st and 2nd hot roll pass, can be seen in Fig. 8a and b respectively. Measurements were performed along

different planes, as shown schematically in Fig. 1. Along each plane, 6–8 measurements were performed. Breakdown potential, being a distributive parameter, has been represented using error bars in Fig. 8. The hot rolled surface has a non-uniform oxide thickness and this factor was kept in mind while carrying out the measurements. The test specimen was kept at OCP and the anodic polarization was performed only after a sudden change in OCP was observed, representing the dissolution of oxide. The as-cast surface had a breakdown potential around − 820 mV (plane A, Fig. 8a). Just after the first impact with the work rolls, a permanent shift in breakdown potential was observed. The breakdown potential at B was around − 700 mV and was observed to remain the same throughout the roll-bite of the first hot roll pass. The breakdown potential after the first hot roll pass was around − 670 mV (plane G, Fig. 8a) and remained almost the same in subsequent passes. No substantial change in the electrochemical activity was observed in the roll bite of the second (Fig. 8b) and third (data not presented) hot roll pass. In the case of the roll-bite of the second hot roll pass, the error bars had a slightly higher scatter but there was enough overlap in the breakdown

5 RMS roughness (microns)

Corrosion number (mm)

1.2 pure Al - polished (1 µm) pure Al - ground (grit 320) AA5050 (cast) - polished (1 µm) AA5050 (cast) - ground (grit 320)

1.0 0.8 0.6 0.4

pitting potential range

grit 60

grit 120

1

5

10

15

20

25

hot rolled (1st pass)

grit 360

2

grit 1200

as-cast

polished

0 -850 0

hot rolled (2nd pass)

3

0.2 0.0

Pure Al AA5050

of rolled surface

4

-800

-750

hot rolled

roughness range of rolled surface

cold rolled

-700

-650

-600

E (mV) vs. Ag/AgCl (3M KCl)

days

Fig. 6. ASS test result showing the effect of surface finish on as-cast AA5050 and as-cast commercially pure Al.

Fig. 7. Breakdown potential profile as a function of surface roughness on AA5050 as well as commercially pure Al (dmacro-cell = 1 cm; reference: Ag/AgCl (3 M KCl)).

Premendra et al. / Surface & Coatings Technology 201 (2006) 828–834 -550 -600

entry in the roll-bite

exit from the roll-bite

(a)

E (mV)

-650 -700 -750 -800

pitting potential

-850 A

B

C

D

E

F

G

distance in the roll bite -550 -600

entry in the roll bite

exit from the roll bite

(b)

E (mV)

-650 -700 -750 -800

pitting potential

-850 A

B

C

D

E

F

G

distance in the roll bite

Fig. 8. Breakdown potential profile along various planes (refer to Fig. 1) in the roll bite (dmicro-cell = 500 μm; reference: Ag/AgCl (3 M KCl)): (a) hot roll 1st pass (AB = 1 cm), (b) hot roll 2nd pass (AB = 0.6 cm).

potential values at different points to conclude any substantial change. 4. Discussion The as-cast surface has the most negative breakdown potential. SEM micrograph coupled with EDX and GDOES analyses has revealed the presence of an extensive network of quaternary Al–Fe–Mn–Si intermetallics (cathodic to the matrix [24]) on the surface of as-cast aluminium bar. Therefore a high cathodic surface area results in a more negative breakdown potential. A permanent shift in the breakdown potential (in a noble direction) upon rolling takes place at the very first impact with the work roll of the first hot roll pass. The breakdown potential gradient across the roll bite is maximum for the first hot roll pass and almost no gradient is observed across subsequent passes. Hence the electrochemical activity changes only in the first hot roll pass. When the ascast bar enters the roll-bite, shearing of the surface takes place. As a result, the intermetallics present on the as-cast surface are broken, smeared out and rolled into the surface. Therefore due to rolling the intermetallics once present on the surface has been spread over a volume, comprising of an elongated surface as well as the near-surface region. A shift in the breakdown potential in a noble direction, just after entering the roll-bite, reflects a decrease in the cathodic activity on the surface. Possibility of precipitation of intermetallics during hot rolling cannot be ignored [25] but the extent of precipitation may be insignificant compared to the amount of intermetallics already present on the surface of the as-cast aluminium bar,

833

which has not been scalped before being fed into the hot rolling mill. Contrary to the findings of electrochemical analysis, salt spray test result showed that the initiation (fraction of the scribe corroded) and propagation (filament length) of FFC attack was the slowest on as-cast specimen. Hence the as-cast bar is the most corrosion resistant. FFC susceptibility on rolled surface is governed, to a great extent, by the filament length owing to the observation that initiation is almost the same on both the hot rolled and cold rolled specimens (refer to Eq. (1)). However a much better estimation of FFC initiation can be obtained by a more frequent analysis of test samples towards the beginning of the test, as done for the other set of samples aimed at studying the influence of surface finish (Fig. 6). The longest filiform filaments have been observed on hot rolled specimens compared to the cold rolled and as-cast specimens. Hence hot rolled specimens exhibit maximum FFC susceptibility. Hot rolling results in the formation of a heavily deformed near-surface region, as can be seen from TEM images in Fig. 4, which in turn is a major contributing factor in enhancing the FFC susceptibility [14]. The absence of a deformed near-surface explains the slow propagation of FFC on as-cast specimens even if they are more susceptible to pitting. The electrochemical activity of the surface and the near-surface region has been observed to be comparable for both the hot rolled and the cold rolled specimens [22]. The composition and the electrochemical nature of the intermetallics in the near-surface region have been found to be very similar for both hot and cold rolled specimens [26]. In spite of the presence of a deformed near-surface region, of comparable electrochemical activity, the difference in corrosion behavior of the hot rolled and cold rolled specimens does indicate the presence of other factor(s) also responsible for FFC susceptibility. On AA3005 (with 1.1% Mn), Afseth et al. [27] have found that depletion of Mn from solid solution into the second phase particles in the deformed surface layer, during the thermomechanical processing, enhances FFC susceptibility. However on AA5050 (with 0.33% Mn), Mn content in the second phase particle, in the near-surface region, does not change substantially during thermo-mechanical processing. This indicates that Mn depletion from the solid solution took place before the alloy was subjected to thermo-mechanical processing. Hence Mn depletion in the near-surface region, during thermo-mechanical processing, may not be the contributing factor responsible for enhanced FFC susceptibility of hot rolled sheet. With increase in surface roughness, the FFC susceptibility has been found to increase. The main reason behind this is not known yet but an increase in roughness will result in the increase of exposed cathodic area and hence higher corrosion rate. Scamans et al. [14] have shown that as the surface finish gets finer, the extent of FFC is reduced because filiform filaments tend to follow the rolling/grinding direction. However surface finish has no impact on the breakdown potential (Fig. 7), at least in the roughness range in which the rolled samples lie. This indicates that electrochemical characterization does not address the effect of surface roughness on rolled aluminium

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alloys although this aspect can have a considerable impact on corrosion susceptibility. Therefore, a rough surface morphology together with an active near-surface region enhances the FFC susceptibility of the Al-strip after 1st hot roll pass and this effect presumably predominates the nobility caused as a result of a decrease in cathodic activity on the surface. 5. Conclusion Electrochemical investigation of the roll-bite specimen along with electron microscopic analysis and corrosion testing, before and after the first pass of hot rolling, helps in understanding the microstructural, electrochemical and corrosion response due to hot rolling. The as-cast specimen has a better FFC resistance than rolled specimen even though it has the most negative breakdown potential. Surface roughness does play an important role in enhancing the FFC susceptibility of an alloy. However electrochemical characterization on aluminium alloy, to some extent, fails to distinguish between 2 similar samples with varying surface roughness, which would otherwise have an influence on FFC results. The Al-strip after the initial hot roll pass is the most FFC susceptible. FFC susceptibility decreases in subsequent passes, which can also be attributed to improvement in surface finish. Acknowledgement The authors are thankful to Hunter Douglas for the production of the alloy under study and also for performing the salt spray test. Geoff Scamans (Innoval Technology) is acknowledged for invaluable inputs in this work and Frans Tichelaar (NCHREM, TUDelft) is acknowledged for the TEM analysis. This work has been performed with financial support from the Dutch Ministry of Economic Affairs under the Innovation Oriented Research Program (IOP project code: IOT00006). References [1] M. Fishkis, J.C. Lin, Wear 206 (1997) 156. [2] L. Gjonnes, Wear 192 (1995) 216.

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