Pll:
ELSEVIER
S0263-4368
Int. J. of Refractory Metals & Hard Materials 15 (1997) 81-87
© 1997 Elsevier Science Limited
Printed in Great Britain. All rights reserved
0263-4368/97/$17.00
(96)00016-9
The Erosion-corrosion Resistance of Tungstencarbide Hard Metals
E. J. Wentzel
Boart Longyear Research Centre, Krugersdorp, South Africa
&
C. Allen
Department of Materials Engineering, University of Cape Town, South Africa
(Received 22 November 1995; accepted 2 February 1996)
Abstract: A series of cemented tungsten-carbides with different binder phases
consisting of combinations of cobalt, nickel and chromium have been subjected to
erosion-corrosion testing using a silica-water slurry. The polarisation characteristics of these cermets have been investigated using a potentiodynamic technique.
The differences in binder composition influences the cermets' properties and
corrosion behaviour, which in turn affects the synergisticaction of erosion-corrosion. The inherent corrosion resistance of a pure nickel binder did not increase
the slurry erosion resistance of the cermets, but the nickel-chromium-cobalt
grades were found to improve the erosion-corrosion behaviour compared to the
pure cobalt binder grade. Comparisons are made between the properties and
behaviour of Ni-Cr-Co based cermets and pure metal grades with compositions
similar to those found in the binder phase of the corresponding cermet grades.
Explanations are advanced to explain the differences in behaviour linked to
composition and mechanical properties. © 1997 Elsevier Science Limited
1 INTRODUCTION
wear resistance in aggressive environments,
other metallic alloy compositions have been
formulated) In this work, slurry erosion rates
for cobalt, nickel, nickel-chromium and nickelchromium-cobalt based hard metals are compared with respect to their overall behaviour.
In order to establish the influence of binder
composition on the behaviour of cermets, the
behaviour of pure metal alloys with compositions similar to those of the binder phase, in the
corresponding cermets is examined. This will
determine whether pure binder metal behaviour
can be used to model cermet behaviour in
slurry erosion or pure corrosive environments.
It is apparent that materials subjected to slurry
environments must not only resist the erosion
of solid particles and liquids, but also be resistant to damage caused by corrosion. These two
factors may interact synergistically to produce
wear rates that are greater than the sum of
their separate effects. ''2 The ability of the
material to resist the synergistic action of slurry
erosion and corrosion is thus of the utmost
importance. The characteristic high hardness
and fracture toughness of W C - C o cermets have
in the past made them materials of choice for
use as cutting tools, mining bits, sandblasting
nozzles, and for a variety of other wear resistant
applications. In most cases cobalt has been used
as the tough metal binder phase, due to its
excellent wetting, adhesion and adequate
mechanical properties. Cobalt, however, has low
corrosion resistance and thus to improve the
2 EXPERIMENTAL PROCEDURES
A slurry jet erosion rig based on that of Zu 4
and Bester ~ was constructed from non-corrosive
plastic materials. This rig permits variations in
81
82
E. J. Wentzel, C. Allen
sT:
Fig. 1. A schematic representation of the slurry erosion
rig.
the impact angle, impact velocity, erodent size,
erodent concentration and carrier fluid. A schematic representation of the test apparatus is
shown in Fig. 1.
The carrier fluid is circulated by means of a
centrifugal pump from the holding tank, via a
bypass valve, a rotameter, a pressure gauge and
an ejector to the specimen surface. The ejector
ensures a pressure drop in the fluid, resulting in
a vacuum that is sufficiently strong to draw erodent particles up through a vertical suction
tube. The bottom of the suction tube is placed
in the submerged sand bed. The erodent and
carrier fluid is mixed in the ejector, thus causing
a slurry jet to be accelerated through the exit
nozzle onto the specimen surface. Tap water is
used as a carrier fluid to provide a base against
which to compare the erosion rate of a substitute ocean water based slurry and thus determine the degree that a corrosion environment
influences slurry erosion rate. The specimen is
held in a specimen holder that can be rotated
about the horizontal axis to any pre-set impact
angle between 15° and 90 °. The complete specimen and holder are enclosed in a test chamber.
The impact velocity was set at 7.0 m s - ' with an
erodent concentration of 6.3 wt% (37.8 kg of
erodent impacted the specimen surface per h).
Although the 500/~m diameter silica sand erodent was found not to degrade during a four
hour erosion test, new erodent was used for
each specimen. The impact angle was kept at
75 °, where the maximum slurry erosion rate
occurred. 6
The solutions used for the potentiodynamic
polarisation experiments were a 1N HzSO4
(27"2 ml of 98% H2SO4 per litre distilled water)
in accordance with the ASTM standard G5-827
and a substitute ocean water solution based on
ASTM specification Dl141-90 but containing
only the three main compounds. The salt water
solution for potentiodynamic testing is the same
as that used for the carrier fluid in slurry erosion testing. A standard potentiodynamic polarisation procedure was used. The specimen was
polished to a 1 ~m diamond finish and sealed in
a PTFE holder. This was the working electrode
of a cell consisting of a saturated calomel reference electrode (SCE) and a carbon rod counter
electrodes. The solution was deaerated with
bubbling argon for 60 rain before the specimen
was inserted and continued during the experiment. The specimen was allowed to reach equilibrium at E .... (corrosion potential) for 30 min
before polarisation began. The potential was
then anodically ramped from - 5 0 0 m V to
1500mV
by an Amel potentiostat
at
167gVs ~
3 MATERIALS
The materials used in this project were chosen
to examine the effects of compositional changes
within the binder on the slurry erosion-corrosion behaviour of WC-based hardmetals. The
materials were all manufactured by the Boart
Longyear Research Centre.
The target materials were separated into two
distinct groups, tungsten-carbid based cermets
and binder metal alloys containing no tungstencarbide particles. The tungsten-carbide based
cermets are again separated into two categories,
those with 10 wt% binder and those with 6 wt%
binder. The binder metal alloys have compositions similar to those of the binder phase of the
10 wt% cermets. These binder metal alloys were
manufactured by powder metallurgical techniques and were used to examine the behaviour
of the binders. In practice, however, liquid
phase sintering leads to some solution of tungsten and carbon in the binder phase which did
not occur here.
The different grades of materials can be identified as follows:
• 10 wt% binder cermets e.g. 833;
• 6 wt% binder cermets e.g. P6;
• 100 wt% binder alloys e.g. Uct 1.
The erosion-corrosion resistance of tungsten-carbide hard metals
The chemical compositions of the different
materials used in this work are tabulated in
Table 1. Note that the 439 grade and the 833
grade are both pure cobalt binder grades. The
mean WC grain size in all the alloys was
1-1.5/~m irrespective of the binder volume
fraction used.
Vickers hardness values were obtained, using
a 30 kg load for both the WC-based cermets
and the pure binder alloys, in order to facilitate
comparisons between hardness and slurry erosion resistance. The results are the average of
10 readings. The values of hardness and density
for all the grades tested are shown in Table 2.
4 RESULTS
Under steady-state conditions slurry erosion follows a linear relationship and is thus quoted as
volume loss per mass of impacting erodent. Linear regression analysis forcing the y-intercept
through the origin allows the slurry erosion
rates to be determined. For all the grades tested
the slurry erosion rates in the salt (substitute
ocean water) solution were greater than those
in the tap water solution. Figure 2 shows the
Table
1.
83
linearity of slurry erosion and the increase in
erosion rate caused by the more corrosive salt
water carrier fluid.
The slurry erosion rates were established for
all the materials tested in both carrier fluids and
the results are plotted in Figs 3-5. in all these
graphs the composition of the binders varies
from the high cobalt grades on the left to the
high nickel grades on the right. The intermediate grades have chromium additions of 5 wt%
of the binder phase. Note the pure nickel (Uct
7) does not fit the trend.
The slurry erosion rates shown in Figs 3 and
4 exhibit a trend with the lowest slurry erosion
rates occurring when the composition of the
binder phase has a high wt% cobalt and small
additions of chromium or chromium and nickel.
The influence of target material hardness on
the slurry erosion resistance of the hardmetals
and the binder metal grades is shown in Fig.
6(a) and (b). The general trend of decreasing
erosion rate with increasing hardness is
followed in the cermets' grades. The influence
of binder composition can be seen in Fig. 7
where hardness values of the cermets and the
pure metal binder grades are compared. Again
the left side of the graph is the high cobalt and
Chemical compositions of the materials
Grade
Composition
(wt%)
WC
Ni
833
842
839
840
841
864
834
___90
+ 90
___90
___90
+ 90
+ 90
+ 90
0
0.13
2.13
4.57
5.49
7.42
9.15
10 wt% binder cermets
0
0.460
0.470
0.507
0.450
0.501
< 0-005
9.86
9.09
7.04
5.71
3.89
2.28
0
5.64
5.55
5.58
5.50
5.65
5.50
5.54
C6
P6
V7
V6
___94
+ 94
+ 94
+ 94
0
+ 3.0
5.67
+ 6.0
6 wt% binder cermets
0
_ 0"6
0.350
0
+ 6.0
+ 2.4
0.35
0
--5.79
--
0
0
0
0
0
0
0
0.01
0.01
33'0
60-0
70-0
95.2
99.91
100 wt% binder alloys
0.01
5.80
4.10
5.50
3.50
4.10
0.01
99.96
94.2
72.9
34-3
26.8
0.01
0.01
-------
Uct
Uct
Uct
Uct
Uct
Uct
Uct
1
2
3
4
5
6
7
Cr
Co
Total C
--
E. J. Wentzel, C. Allen
84
Table 2.
tested
Hardness and density values for the materials
Grade
Density
(g/cm~)
Hardness
HV30
10 wt%binder cermets
833
842
839
840
841
864
834
1473
1546
1498
1499
1424
1400
1409
14-53
14.53
14.51
14.58
14.48
14.58
14.55
0.00007
0.00006
~'E 0.00005
•
0.oooo4
.~
m
2
w
~
0.00003
0.11111102
(~
0.00001
0
6 wt%binder cermets
C6
P6
V7
V6
Uct
Uct
Uct
Uct
Uct
Uct
Uct
1550
14.97
1687
14.95
1674
14.95
1448
14-93
100 wt% binder alloys
258
8.52
227
8.25
146
8.00
91
7"71
87
7.64
80
7.65
95
8.54
1
2
3
4
5
6
7
the right side the high nickel. The trends for the
cermets and the pure metal binder grades are
superimposed and it must be noted that the
nickel rich binders are softer than the cobalt
rich binders influencing the cermets hardness in
the same manner, although to a lesser degree,
as a result of the low volume fraction of binder
in the cermets.
The influence of the more corrosive salt
water carrier fluid can be seen by plotting the
slurry erosion rate ratio of tests in tap water
solution to tests in salt water solution, as a percentage. Figure 8 shows this change in erosion
rate for the 10 wt%, 6 wt% and pure binder
metal grades. The average change is 52.2, 51.0
and 75.4% for the 10 wt%, 6 wt% and pure
binder and metal grades respectively.
833
842
839
840
841
864
834
Fig. 3. Slurry erosion rates and trends in erosion rate
for 10 wt% binder grade cermets in salt water solution
(SWS) and tap water solution (TWS).
Anodic polarisation curves for 10 wt% grades
and pure binder metal grades are shown in Figs
9(a) and (b). The expected increase in passivation behaviour, decreases in corrosion potential
and drop in current density occur as the nickel
volume fraction within the binder phase is
0 00025
E
g
~
re"
0.00015
•-~
0.0001
e
w
0.00005
0
C6
P6
V7
V6
Fig. 4. Slurry erosion rates and trends in erosion rate
for 6 wt% binder grade cermets in salt water solution
(SWS) and tap water solution (TWS).
0.0006
0.0005 1
0.009
0.008
e~
I:: 0.007
0.006-
~
0.005
E
O.003
O
~
re"
0.0003
1
P o.oo02
ul
ij
i
i
J
~
i
i
0.5
1
1.5
2
2.5
3
3.5
Exposure Time (hours)
Fig. 2.
0.1111114
._g
0.004-
~> 0.002
0.001 •
0-"
0
~
IncreasingNickel
[ ~
Linear nature of slurry erosion with substitute
ocean water and tap water carrier fluids.
o.
uctl
uct2
uct3
uct4
uct5
uct6
uct7
Fig. 5. Slurry erosion rates and trends in erosion rate
for pure metal binder grades in salt water solution (SWS)
and tap water solution (TWS).
The erosion-corrosion resistanceof tungsten-carbide hard metals
gO
&V6
0.0002
85
-Increasing Nickel
0.00015
i
Ig
.~
439 ~
~
8
3
3
&C61&6wt% Binder •
0.0001
O
2
u.l
~ 0.00005
F- 50
0
1350
1400
1450
1500
Hardness
1550 1600
(HV 30)
1650
1700
30
0.00055 m _ _
0.0005
E
E 0.00045
--I
[:-.e-10wt% .-B-6wt%
I
I
I
~-
Fig. 8. Ratio as a percentage of tap water slurry erosion
rate over salt water slurry erosion rate for all grades.
UCT5 t
2 o.0003
LU
CT4
_~_._*UCT3 __~cT1
~ 0.00025
_=
e~CT7
0.05O2
,-A.- Binders l
~UCT6
0~ 0.0004
._~ 0.0111135
4O
I
50
I -
100
5 DISCUSSION
UCT2•
I
I
--- I
150 200 250 300
(HV30)
Hardness
Fig. 6. (a) Salt water slurry erosion rates for 10 and 6
wt% grades related to hardness; (b) salt water slurry erosion rates for pure metal binder grades related to hardness.
The loss of material during a wear process is a
complex relationship between many interacting
variables. Thus the changing of one variable can
1500
increased. A more distinct passivation region
can be seen in the pure metal binder grades
than in the cermet grades, although the passivation current density of the cermets is lower than
that of the pure binder metals. This is not
unexpected as the cermets have high volume
fractions of the more corrosion resistant WC
phase. The passivation behaviour of the 6 wt%
grades follows the same trends highlighted by
the two categories shown.
1000
g
lu
k~
c
O 5O0
Q.
c
._o
2i_
O
O
0
-
l
1600
eerme,;1
1550
S"
Binder Grades
1511 c
1450
'1-
\
1.o~
I11
1350
01
511
842
Uctl
Uct2
839
Uct3
840
Uct4
841
Uct5
~
:
I
105O0UCt 00000
i
-5oo
1300
033
0
2511
I
200 >
-~
0
300
v
Increasing Ni
5
834
Uct6 Uct7
Fig. 7. The influence of binder hardness on the hardness
of the cermet grades. Note the left side of the graph is
high cobalt and the right high nickel.
0.1
~Z
I
10
100
1000 10000 100000 1000000
(pA/cm 2)
Current Density
Fig. 9. (a) Anodic polarisation curves for the 10 wt%
binder grades in 1N H2SO4; (b) Anodic polarisation
curves for the pure metal binder grades in 1N H2SO4.
86
E. J. Wentzel, C. Allen
have important consequences on the behaviour
pattern of the total system. In this work, cemented carbides have been subjected to slurry erosion conditions where impact velocity, impact
angle, erodent concentration and erodent
characteristics remain constant. Thus only the
effect of changing the binder composition on
the erosive-corrosive wear process has been
investigated. The relationship between corrosion resistance and/or erosion-corrosion resistance of the binder and that of the cermet is not
simple, as related factors such as interface
strength and the sinterability of the composite
also influence the behaviour of the cermet. In
this work however all the related factors are
included and the performance of the cermet is
considered in its entirety.
When discussing the effects of binder composition it is important to bear certain fundamentals of cermet erosion-corrosion in mind. The
mechanism of material removal is one of binder
loss followed by WC grain pullout, ''8 thus the
ability of a binder to resist removal by either
improved mechanical properties or improved
corrosion resistance"'"' is fundamental to the
success of that material as a cermet binder.
Slurry erosion is a balance between the erosion by solid particles and the corrosion of the
target. The importance of the corrosion resistance of the binder is only of significance when
the corrosion aspect of the process plays the
dominant role or is the rate controlling factor.
When the erosive component of the slurry process is dominant, the target material mechanical
properties will be of more importance than the
corrosion resistance of the binder material. The
severity of the erosive component relative to the
corrosive component is critical and it is thus
very difficult to generalise as to the behaviour
of cermets in slurry erosion conditions.
Intuitively it might be expected that a cermet's slurry erosion resistance would improve
with the addition of more corrosion resistant
WC phase. In this work, however this is not the
case as can be seen from Fig. 6(a) where the 10
wt% cermets outperform the 6 wt% cermets.
This could be due to a dynamic situation that
exists where the rate controlling factor for
material removal is the corrosion of the binder
phase, although the mechanical properties of
the cermet must not be ignored, and thus the
more binder phase present in the material the
longer it will take to be removed to the extent
where WC grain pullout is possible. This could
however also be due to a dynamic situation
where the erosion component is dominant and
the small amount of binder in the 6 wt% grades
causes more severe binder extrusion between
WC grains and thus increased wear rates. A
balance of these two conditions is also possible.
Electron microscopy of the wear surfaces of the
target materials in this work indicates that the
first option is the most probable, with corrosion
of the binder phase being the rate determining
or dominating factor.
In both tap water and the salt water carrier
fluids, the slurry erosion resistance of the 10
wt% grades decreased when additions of chromium and chromium-nickel were made to the
predominantly cobalt binder. As the nickel content in the binder increased, the erosion rate
also increased thus having little positive effect
on retarding the erosive-corrosion process. High
nickel content in the binder adversely affected
the hardness of the cermet and that of the pure
metal binder grades. The trend seen in the 10
wt% binder cermets is replicated in the 6 wt%
binder grades and the pure metal binder grades.
The exception to this rule is the pure nickel
binder metal that shows decreased slurry erosion rates in both carrier fluids.
Figure 8 indicates that the influence of the
more corrosive carrier fluid on the slurry erosion rate is lessened as the binder wt%
increases, and as the binder composition tends
toward the high nickel binder grades. As nickel
is the more corrosion resistant metal this influence is expected.
The polarisation behaviour of the different
grades indicates that the corrosion resistance of
the grades improves with the addition of the
more corrosion resistant nickel to the binder.
Not only do the pure binder grades show this
improvement, but the cermets also show this
behaviour. The importance of this similarity in
behaviour between the pure binder grades and
the related cermet grades is the possibility of
modelling the corrosion resistance of the cermets on the corrosion resistance of the pure
binder grades.
It is interesting that this improvement in the
corrosion resistance of the cermets with the
increase in nickel content does not necessarily
give rise to an improvement in the slurry erosion resistance of the cermets. In all cases the
high nickel grades have better passivation
The erosion-corrosion resistance of tungsten-carbide hard metals
behaviour than the C o - C r - N i grades, but the
slurry erosion behaviour deteriorates. It should
be noted that the passivation behaviour of the
nickel binder may well negate its superior corrosion resistance under conditions when any
passive film formation is continuously being
removed, such as in slurry erosion. In such conditions the hardness, deformation characteristics
and possible phase transformations of the
binder may be more important in determining
the wear rates. These factors will be investigated in future work.
Nevertheless, the nickel-chromium-cobalt
and in particular the chromium-cobalt,
although not always having improved pure corrosion properties, do show a distinct improvement over a cobalt binder cermet in slurry
erosion-corrosion conditions. The influence of
the binder composition is an important factor in
the design of a long life cermet for erosioncorrosion conditions as the importance of synergistic wear must not be neglected.
6 CONCLUSIONS
Based on the results of this work on slurry erosion-corrosion of hard metals and the influence
of binder composition, we conclude the following:
(1) The slurry erosion resistance of WC
based cermets with pure metal binders of either
Co or Ni can be improved by alloying.
(2) Of the grades tested the Co-Cr grade
with 10 wt% showed the lowest slurry erosion
rates in both a salt water and a tap water solution.
(3) No simple relationship exists between any
one tested property and the slurry erosion
resistance of the hard metals.
87
(4) An improvement in the passivation
behaviour of a binder metal does not show any
relationship to an improvement in the slurry
erosion-corrosion resistance of the grades tested.
ACKNOWLEDGEMENTS
The financial support of Boart Longyear
Research Centre, Eskom and the FRD are
gratefully acknowledged.
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abrasion and corrosion during wear. Wear, 87 (1983)
351-61.
2. Madsen, B. W., Measurement of erosion-corrosion
synergism with a slurry wear test apparatus. Wear, 123
(1988) 127-42.
3. Human, A. M., Northrop, I. T., Luyckx, S. B. &
James, M. N., A comparison between cemented carbides containing cobalt- and nickel-based binders. J.
Hard Mater., 2 (3-4) (1991) 245-55.
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selection of AI alloys, AI MMC's and steels. MSc
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3, G5-82, No. 1, 1985.
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erosion of WC-Co cermets and its relationship to
material properties. Proc. 6th Intern. Conf. on Erosion
by Liquid and Solid Impact, (1983) pp. 1-8.
9. Tomlinson, W. J. & Linzell, C. R., Anodic polarisation and corrosion of cemented carbides with cobalt
and nickel binders. J. Mater. Sci., 23 (1988) 914-18.
10. Tomlinson, W. J. & Molyneux, I. D., Corrosion, erosion-corrosion and the flexural strength of WC-Co
hardmetals. J. of Mater. Sci., 26 (1991) 1605-8.