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Wear 271 (2011) 1314– 1324

Contents lists available at ScienceDirect

Wear

j o ur nal ho me p age: www.elsev ier .com/ locate /wear

he influence of carbide dissolution on the erosion–corrosion properties of castungsten carbide/Ni-based PTAW overlays

. Jonesa,∗, U. Waagb

National Research Council, 4250 Wesbrook Mall, Vancouver V6T 1W5, CanadaH.C. Starck GmbH, Kraftwerkweg 3, 79725 Laufenberg, Germany

r t i c l e i n f o

rticle history:eceived 8 September 2010eceived in revised form 5 January 2011ccepted 6 January 2011vailable online 27 March 2011

eywords:rosion–corrosionrosionorrosionC overlays

il sands

a b s t r a c t

WC/Ni-based plasma transfer arc welded (PTAW) overlays are frequently used in the oil sands industryfor applications requiring extremely high wear resistance.

These overlays usually consist of a dense distribution of either tungsten monocarbide (WC) or casttungsten carbides (WC/W2C) in a NiBSi or NiCrBSi matrix.

A previous erosion–corrosion (E–C) study of cast tungsten carbide/NiBSi-based PTAW overlays not onlyhighlighted the expected preferential E–C attack of the non Cr-bearing matrix but also the significantlygreater effect that carbide dissolution of the cast tungsten carbides had on the overall E–C performanceof the overlay.

It is recognised that cast tungsten carbide particles are more susceptible to dissolution during depo-sition than WC. As a result, a new process that modifies the outer periphery of cast tungsten carbidesby producing a tungsten monocarbide shell has been developed. Overlays consisting of these carbidesin a NiBSi binder were subsequently produced with the aim of reducing the extent of dissolution duringdeposition.

The aims of this study were (i) to assess the effectiveness of the tungsten monocarbide outer shellof cast tungsten carbides to resist dissolution during the PTAW deposition process; and (ii) to highlightany improvements in the E–C properties using a novel slurry pot erosion corrosion (SPEC) tester, whichincorporates a three-electrode cell to enable the separate components of erosion, corrosion and synergy,to be established. Results were compared with a commercially available NiBSi PTAW overlay containingunmodified cast tungsten carbides.

. Introduction

Tungsten carbide metal matrix composites (MMCs) are widelysed in the oil sands industry to provide protection in low stressliding abrasion, slurry abrasion and erosion–corrosion conditions1–4].

PTAW-deposited tungsten carbide MMCs containing Ni-basedatrices are commonly employed and consist of a high volume

raction of either tungsten monocarbide (WC) or cast tungsten car-ide (WC/W2C) particles in a self-fluxing NiBSi or NiCrBSi matrix.

t is recognised that during deposition, thermal decomposition ofhe carbides can occur due to interaction with the lower meltingoint matrix constituent [5], which can result in reduced wear per-

ormance of the deposit [6]. This has been demonstrated by Hartt al. [7] who related increasing carbide dissolution with higherow stress sliding abrasion rates.

∗ Corresponding author. Tel.: +1 604 221 3036; fax: +1 604 221 3001.E-mail addresses: mark.jones@nrc.ca, Mark.Jones@nrc-cnrc.gc.ca (M. Jones).

043-1648/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rioi:10.1016/j.wear.2011.01.046

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

The effects of tungsten carbide dissolution on theerosion–corrosion properties of two PTAW-deposited overlayswere recently studied [1]. It was shown that carbide dissolutionhad a significant effect on the E–C performance and that sphericaland angular cast tungsten carbides were more affected by carbidedissolution than tungsten monocarbides. The latter is in agreementwith other studies [2,8,9].

As a means of reducing or eliminating dissolution of cast tung-sten carbide during deposition, a process has been employed whichmodifies the carbide to provide an outer shell of tungsten mono-carbide. This is a patented process [10] that involves mixing casttungsten carbides with a carbon source (e.g. carbon black) and heat-ing the mixture within a temperature range of 1550–1900 ◦C ina hydrogen atmosphere. The carbon source then reacts with theW2C in the cast tungsten carbide, to form a WC outer shell. Thedesired carburising depth can be controlled by altering the time

and temperature of the process.

To assess the effectiveness of the carburisation process in reduc-ing carbide dissolution and hence improve the erosion–corrosionproperties, microstructural and E–C assessments were performed

ghts reserved.

M. Jones, U. Waag / Wear 27

Table 1SPEC test conditions.

Slurry composition 3.5 wt.% NaCl solution + 35 wt.% AFS 50–70(212–300 �m) rounded silica sand

Slurry temp. (◦C) 30

ofiirmtd

2

2

oTaetip

2

tvtssf

cw

K

K

memcacTor

mli

containing Si (A), whilst the interdendritic phases consist of theaforementioned Ni-rich solid solution (A) and a Ni-rich phase (B)containing lower levels of Si (Fig. 4a). The distribution of nickelborides in the interdendritic regions is shown in Fig. 4b.

TN

Impeller speed (rpm) 900Test duration (h) 6

n two overlays consisting of a NiBSi matrix and either modi-ed or unmodified cast tungsten carbide. It is understood that

n more corrosive erosion–corrosion environments, a corrosion-esistant matrix is generally implemented, i.e. Cr bearing. As theain aim of this study was to assess the effects of carbide dissolu-

ion on erosion–corrosion resistance, non Cr-bearing matrices wereeemed suitable for assessment.

. Experimental procedure

.1. Sample preparation

Samples with dimensions, 18 mm × 6 mm × 5 mm werebtained, which constituted a test surface area of 1.08 cm2.his surface was prepared using an automatic polisher to produce

final surface finish obtained from a 1 �m diamond wheel. Anlectrical contact was attached to those samples requiring elec-rochemical assessment. All samples were subsequently mountedn epoxy to ensure that only the test surface was exposed to therevailing conditions.

.2. Test methodology

A schematic of the slurry pot erosion–corrosion (SPEC) test sys-em is shown elsewhere [11]. The unit consists of a 4-litre glassessel to hold the slurry, a neoprene-lined impeller that is rotatedo impel the slurry against the static test surfaces, a heating/coolingystem and a three-electrode cell with a Gamry PC4/750 potentio-tat to allow for electrochemical assessment. The test conditionsor erosion–corrosion assessment are shown in Table 1.

In order to obtain the separate components of erosion–orrosion, namely erosion, corrosion and total synergy, an approachas implemented with respect to Eq. (1).

ec = Keo + Kco + Ks (1)

where Kec = total erosion–corrosion; Keo = erosion-only;co = corrosion-only; Ks = total synergy.

Total erosion–corrosion, Kec, was determined by measuring theass loss using a microbalance with an accuracy of 0.01 mg. The

rosion-only rate, Keo (in the absence of corrosion), was deter-ined by measuring the mass loss following slurry erosion under

athodic protection conditions. The corrosion-only rate (Kco) wasssessed using the polarisation resistance technique in a separateorrosion cell with identical electrode distances to the SPEC tester.he synergistic effect was calculated by subtracting the erosion-nly and corrosion-only values from the total erosion–corrosionate. All wastage rates were converted to cm3/h cm2.

Surface examination using a Hitachi S-3500N scanning electronicroscope (SEM) provided images of specific phases in the over-

ay before and after each test at the same locations, to ascertainf there was any preferential damage under the various test con-

able 2ominal chemistry and hardness of both PTAW overlays.

Material Description and nominal

Cast tungsten carbide overlay 50 vol.% angular cast tungModified cast tungsten carbide overlay 45 vol.% angular cast tung

1 (2011) 1314– 1324 1315

ditions. Energy dispersive X-ray (EDX) analysis was also used toidentify constituent phases. This information was then combinedwith the SPEC test data to formulate a damage mechanism for thematerial. X-ray diffraction (XRD) was performed using a Bruker D8X-ray diffractometer.

Carbide volume fraction (CVF) of the tungsten carbide particu-lates was confirmed using ASTM Standard E 562-08 [12].

Macro- and nano-hardness values were obtained using a NewAge Indentron and a Fischerscope nano-hardness tester, respec-tively.

2.3. Materials

Two commercially available WC/NiBSi matrix PTAW overlayswere assessed in this study (Table 2). One contained a distributionof cast tungsten carbides in a NiBSi matrix, whilst the other overlayconsisted of a similar NiBSi matrix with a dispersion of ‘modified’cast tungsten carbides exhibiting an outer shell of tungsten mono-carbide. The overlays were produced by different manufacturers.Deposition parameters were not provided for either overlay.

2.3.1. Cast tungsten carbide (WC/W2C)/NiBSi PTAW overlayFig. 1 shows the carbide distribution of the cast tungsten carbide

overlay. It is evident that the carbides display considerable disso-lution, which is highlighted in further detail in Fig. 2. The EDX linescans of the carbide/matrix interface show the differing degrees ofcarbide dissolution, as exemplified by the extent of the W/Ni-richdissolution zone.

Further evidence of this dissolution is shown in Fig. 3, whereeta carbides (circled) have formed around the periphery of the casttungsten carbide, as they are a product of the carbide dissolution[1,8,9]. EDX spot analysis has confirmed that they are (W, Ni)-richcarbides.

The NiBSi matrix exhibits a dendritic structure and inter-dendritic phases (Fig. 4). EDX and XRD analysis of the matrixdetermined that the dendritic structure is a Ni-rich solid solution

Fig. 1. Microstructure of cast tungsten carbide overlay (Etchant: Glyceregia).

chemistry Hardness

sten carbide/Ni–3B–3Si–0.15C matrix 61 HRCsten carbide (WC outer shell)/Ni–3B–3Si matrix 60 HRC

1316 M. Jones, U. Waag / Wear 271 (2011) 1314– 1324

rbide

2o

bpced

spceso

Fb

Fig. 2. EDX line scans of two cast tungsten ca

.3.2. Modified cast tungsten carbide (WC/W2C)/NiBSi PTAWverlay

As mentioned previously, the cast tungsten carbides were car-urised to produce an outer shell of tungsten monocarbide torevent or reduce dissolution of the carbide during deposition. Thearbide distribution in this overlay is shown in Fig. 5, where it isvident that the carbides do not appear to have been affected byissolution.

Fig. 6 is a higher magnification image that highlights the tung-ten monocarbide shell and again illustrates its effectiveness inreventing carbide dissolution. EDX line analysis was utilised to

onfirm the chemistry of the outer shell as shown in Fig. 7. It isvident from the individual elemental traces that both the outerhell and inner cast tungsten carbide are W-rich with no evidencef nickel. There is also a clear boundary between the carbide and

ig. 3. Eta carbide formation (circled) around the periphery of a cast tungsten car-ide.

s displaying differing degrees of dissolution.

matrix due to the steep decline in W content and incline in Nicontent at the carbide/matrix interface.

An etched NiBSi matrix is shown in Fig. 8. EDX and XRD analysisconfirmed that the dendritic matrix regions consisted of a Ni solidsolution containing Si (C) (Fig. 8a), whilst the surrounding regionscontain of the aforementioned Ni solid solution (C), a Ni-rich phasewith trace levels of Si (D) (Fig. 8a) and a mixture of lighter nickelborides as shown in Fig. 8b.

3. Results and discussion

3.1. SPEC results

The total E–C rates for both overlays are shown in Fig. 9. It isclear that the modified cast tungsten carbide overlay exhibited asignificantly lower E–C rate. To further investigate the disparity inE–C performance between the two overlays, the individual contri-butions of erosion, corrosion and synergy were obtained and areshown in Fig. 10.

It is evident for both the cast tungsten carbide and modifiedtungsten carbide overlays, that synergy accounted for a large pro-portion of the total E–C rates (90% and 84%, respectively). Despiteexhibiting almost identical bulk hardness values and a lower casttungsten carbide volume fraction (45 vol.% cf. 50 vol.%), the mod-

ified cast tungsten carbide overlay exhibited a significantly lowererosion-only rate.

This increased erosion resistance is believed to be due to thehigher matrix hardness of the modified cast tungsten carbide over-

Table 3Microhardness values of the matrix phases of both overlays.

Material Microhardness (Hv1000)

Cast tungsten carbide overlay 586Modified cast tungsten carbide overlay 689

M. Jones, U. Waag / Wear 271 (2011) 1314– 1324 1317

F and ni(

ldd

iv

bope

3

3

aiaap

twcetra

distribution of reinforcing nickel borides.When subjected to corrosion-only conditions, there was evi-

dence that preferential attack of the Ni-rich phase (B) in the

ig. 4. Microstructure of NiBSi matrix highlighting (a) the nickel solid solution (A)

Etchant: Marbles).

ay as shown in Table 3. It is also believed that the level of carbideissolution of the cast tungsten carbide overlay will also have aetrimental effect on the overall erosion resistance.

As both overlays have very similar nominal matrix chemistriest was not surprising that they also exhibited similar corrosion-onlyalues (4.75E−07 cf. 5.16E−07 cm3/h cm2).

As the effect of synergy was the main cause for damage tooth overlays, SEM examination was performed on specific areasf the test surfaces before and after assessment to identify anyreferential damage that may have occurred in the various testnvironments.

.2. SEM examination

.2.1. Cast tungsten carbide (WC/W2C)/NiBSi PTAW overlayFig. 11 shows images of the exact test surface locations before

nd after erosion–corrosion assessment. It was evident that scal-ng and pitting of the matrix had occurred (Fig. 11b and d) as wells attack of the dissolution zone (highlighted in white) (Fig. 11and b) and removal of the eta carbides that had formed around theeriphery of the cast tungsten carbides (circled) (Fig. 11c and d).

Examination of erosion-only test surfaces (Fig. 12) revealed thathe corrosion pitting witnessed on the E–C test surfaces (Fig. 11)as not evident and therefore confirmed the effectiveness of the

athodic protection applied. Whilst the carbides did exhibit some

rosion damage, there was no evidence of preferential removal ofhe dissolution zone or the eta carbides (circled). Therefore, theemoval of these phases witnessed under E–C conditions cannot bepportioned solely to the erosive component.

Fig. 5. Carbide distribution of modified cast tun

ckel-rich phase (B) (Etchant: Glyceregia), and (b) the distribution of nickel borides

Examination of the matrix (Fig. 13) highlighted the preferentialerosion attack of the light dendritic phase. This observation wasconfirmed with nano-hardness testing which showed that the darkinterdendritic regions exhibited more than twice the hardness ofthe dendritic phases (816 cf. 383 Hv). This was not surprising asthe interdendritic regions were reported previously to contain a

Fig. 6. High magnification image of modified cast tungsten carbides (Etchant: Mar-bles).

gsten carbide overlay (Etchant: Marbles).

1318 M. Jones, U. Waag / Wear 271 (2011) 1314– 1324

itrav

3tbtItre

9.269E-05

2.99E-05

0.000E+00

1.000E-05

2.000E-05

3.000E-05

4.000E-05

5.000E-05

6.000E-05

7.000E-05

8.000E-05

9.000E-05

1.000E-04

To

tal

E-C

ra

te (

cm

3/h

.cm

2)

HRC61 HRC60

Cast tungsten carbide

/ NiBSi overlay Modified cast tungsten car bide

/ NiBSi overlay

Fig. 9. Total erosion–corrosion rates.

9.269E-05

2.99E-05

4.75E-07 5.16E-07

8.59E-06

4.33E-06

2.51E-05

8.36E-05

0.000E+ 00

1.000E- 05

2.000E- 05

3.000E- 05

4.000E- 05

5.000E- 05

6.000E- 05

7.000E- 05

8.000E- 05

9.000E- 05

1.000E- 04

E-C

ra

te (

cm

3/h

.cm

2)

Total E-C rate

Corrosion only

Erosion only

Synergy

Cast tungsten carbide

/ NiBSi overlay

Modified cast tungsten carbide

/ NiBSi overlay

F(

Fig. 7. EDX line analysis of outer shell of modified cast tungsten carbide.

nterdendritic regions occurred (Fig. 14a), as well as removal ofhe reinforcing nickel-based phase in the dissolution zones, thusevealing an unsupported network of carbides (Fig. 14b). Corrosivettack of the eta carbides and reinforcing matrix phase was alsoisible (circled) (Fig. 15).

.2.1.1. Degradation mechanism. It is evident from Figs. 14 and 15hat the dissolution zones and eta carbides were not removed solelyy the corrosive action. Test surfaces shown in Fig. 12 also highlighthat these features were not removed in erosion-only conditions.

t can therefore be concluded that, in E–C conditions, the combina-ion of erosion and corrosion, i.e. synergy, played a key role in theiremoval. When subjected to an E–C environment, the corrosivelement removed the supporting Ni-based phase in both the disso-

ig. 8. Microstructure of NiBSi matrix highlighting the (a) nickel solid solution (C) and nicEtchant: Marbles).

Fig. 10. Separate components of erosion–corrosion.

lution zone and around the eta carbides, to reveal an unsupportedcarbide network that was subsequently removed by the impingingparticles.

kel based phase (D) (Etchant: Glyceregia), and (b) the distribution of nickel borides

M. Jones, U. Waag / Wear 271 (2011) 1314– 1324 1319

F ples

peTmedi

ig. 11. SEM images of test surfaces before and after E–C assessment showing exam

Synergy also resulted in increased damage of the matrixhase (Fig. 11) when compared with the damage caused byrosion-only (Fig. 13) and corrosion-only (Fig. 14) environments.he proposed degradation mechanism is also supported by theeasured values (Fig. 10) where the contribution from syn-

rgy was reported to be 90% of the total E–C rate. A schematic

iagram of the proposed degradation mechanism is shown

n Fig. 16.

Fig. 12. Test surfaces before and af

of removal of the dissolution zone (highlighted in white) and eta carbides (circled).

3.2.2. Modified cast tungsten carbide (WC/W2C)/NiBSi PTAWoverlay

SEM images of test surfaces subjected to erosion–corrosion areshown in Fig. 17. The main damage was by attack of the non Cr-bearing matrix and of the modified cast tungsten carbides, althoughit is immediately evident that the extent of carbide damage shown

is much less than the unmodified carbides. This of course, is due tothe absence of dissolution of the cast tungsten carbides due to the

ter erosion-only assessment.

1320 M. Jones, U. Waag / Wear 271 (2011) 1314– 1324

Fig. 13. SEM images of matrix before and after erosion-only assessment.

rix ph

pd

alw

Fig. 14. Preferential attack of (a) Ni-based mat

rotection offered by the tungsten monocarbide outer shell duringeposition.

Higher magnification images of a modified cast tungsten carbidere shown in Fig. 18. The W2C component of the carbide appearsighter when compared with the WC phase due to its higher atomic

eight. When the test surfaces were compared before and after E–C

Fig. 15. Attack of eta carbides (circ

ase and (b) dissolution zone and eta carbides.

assessment, it was evident that the W2C phase was preferentiallyremoved.

Examination of erosion-only test surfaces confirmed that thematrix was predominantly affected by the impinging particles(Fig. 19). Identification of specific matrix phase attack was not pos-sible. Erosive damage of the modified cast tungsten carbides was

led) and supporting matrix.

M. Jones, U. Waag / Wear 271 (2011) 1314– 1324 1321

Fig. 16. Schematic diagram of the proposed degradation mechanism.

Fig. 17. Test surfaces of the modified cast tungsten carbide overlay subjected to an E–C environment.

1322 M. Jones, U. Waag / Wear 271 (2011) 1314– 1324

Fig. 18. Higher magnification image highlighting the preferential attack of the W2C component of the modified carbide.

Fig. 19. Erosion-only test surfaces before and after assessment.

Fig. 20. SEM images highlighting the preferential erosive attack of the W2C phase of the modified carbides.

M. Jones, U. Waag / Wear 271 (2011) 1314– 1324 1323

Fig. 21. Corrosion-only test surfaces before and after assessment.

n mec

mwo

ibNp

3jeptmsb

Fig. 22. Schematic diagram of the proposed degradatio

inimal, but preferential attack of the W2C phase of the carbidesas evident (Fig. 20) which is likely due to the relative brittleness

f the W2C phase [9].Surfaces subjected to corrosion-only test conditions are shown

n Fig. 21. The Ni solid solution containing silicon (C) and the nickelorides were not affected by the corrosive environment, but thei-rich phase in the surrounding regions (D) appears to have beenreferentially attacked.

.2.2.1. Damage mechanism. Examination of test surfaces sub-ected to erosion-only (Fig. 20) and corrosion-only (Fig. 21)nvironments clearly showed a lower degree of damage when com-ared with the erosion–corrosion test surfaces (Fig. 17). It can

herefore be concluded that the increased level of attack to the

atrix and modified carbides is due to the combination of ero-ion and corrosion, i.e. synergy. This conclusion is also supportedy the measured values shown in Fig. 10, where the synergy com-

hanism for the modified cast tungsten carbide overlay.

ponent accounts for approx. 84% of the overall E–C rate. A schematicdiagram of the proposed degradation mechanism is shownin Fig. 22.

4. Conclusion

SEM analysis confirmed the effectiveness of the carburised outerlayer of the modified carbide to resist dissolution during deposition.

E–C assessment revealed the considerable influence that car-bide dissolution had on the erosion–corrosion performance of theoverlays with the modified cast tungsten carbide overlay exhibit-ing an E–C rate that was approximately a third of the rate for theunmodified cast tungsten carbide overlay.

Measurement of the separate components of erosion–corrosionfor both overlays highlighted the very large influence of synergy.

The degradation mechanism for the unmodified cast tungstencarbide overlay was preferential attack of the dissolution zone and

1 ear 27

ea

bcu

oa

A

lrNs

ference Proceedings, Essen, Germany, March, 2002, pp. 278–283.

324 M. Jones, U. Waag / W

ta carbides, which are products of carbide dissolution, and thettack of the NiBSi matrix.

The damage mechanism for the modified cast tungsten car-ide overlay was E–C attack of the Ni-based matrix. The modifiedarbides were also attacked but to a much lesser extent than thenmodified carbides.

Pronounced attack of the Ni-based matrices highlighted thebvious necessity for a more corrosion-resistant matrix, i.e. theddition of chromium.

cknowledgements

The authors would like to record their appreciation to col-

eagues in the Wear and Corrosion Group, particularly Ting Li. Theyecognise management at NRC and members of the CollaborativeRC/Industry Mining Wear Materials for their support and spon-

orship of the work.

[[[

1 (2011) 1314– 1324

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