materials of construction
33
Chemical Technology • September 2013
explained on the basis of its lower magnetic saturation
(Table 1) compared to the other test specimens. By
extension, the variation of magnetic saturation also
explains the variation of corrosion current densities,
i
corr
, in NaCl (Table 3), which increase with increas-
ing magnetic saturation, that is, decreasing W solute
atoms in Co. This variation of current density associ-
ated with changing W solute content in Co is, as has
already been mentioned, in line with literature, where
lower Co binder W solute content is associated with
poor corrosion resistance.
The above thinking where magnetic saturation/Co
binder W solute content explains corrosion behaviour
needs to be qualified, because it is generally not a
universal phenomenon. For example, corrosion of
WC-Co in nitric acid is not explained by the magnetic
saturation of the specimens. The solution can play a
role in accentuating the effect of the magnetic satura-
tion. For example, the pronounced pseudopassive
behaviour observed for corrosion of the 0VC specimen
in NaCl (Figure 1) could have been aided by the effect
of higher Cl ion concentration, which also encourages
pseudopassive behaviour.
The specificity of the solution-specimen depen-
dence of the effect of magnetic saturation on corro-
sion, referred to above, appears to partly apply to the
corrosion behaviour in SMW, where all specimens
continuously dissolved during potentiodynamic polar-
ization (Figure 2) and also where, generally, corrosion
current densities, i
corr
, decreased even as the mag-
netic saturation was increasing (Table 3). The Cl ion
concentration in the SMW was much less, preventing
the solution from predisposing specimens to pseudo-
passive behaviour. The variation of i
corr
, in SMW can
be loosely correlated with the variation of the volume
fraction of the binder, indicating that the effect of VC
on corrosion in SMW was probably through its effect
on the volume fraction of the binder.
The second possible impact of the VC on the cor-
rosion of WC-Co hardmetal would be expected to be
through its effect on the corrosion product. It has been
suggested that vanadium is naturally passivating.
However, this tendency did not manifest itself in this
study: for example, the potentiodynamic polarization
behaviour of specimens was not changed fundamen-
tally through for example, the emergence of true pas-
sivity, by the presence of VC (Figures 1 and 2). Also,
no vanadium-based compound was detected on the
corroded surfaces of specimens either by XRD studies
(Figures 6 and 7) or by Raman spectroscopy (Figures 8
and 9). The only vanadium-based corrosion compound
formed during corrosion was dissolved in the test
solutions (as shown by the presence of Vanadium ions
in the test electrolytes (Table 6)), and, in some cases,
precipitated out of the electrolytes and did not form
a barrier to curtail corrosion. This failure to form a
corrosion barrier explains the higher current densities
observed for the VC specimens during chronoampero-
metric testing (Figures 4 and 5).
The results of the 0,4wt%VC specimen deserve
some emphasis, given the importance of VC-grain
refinement of WC-Co materials. Fine-grained WC-Co
materials are important for wear applications. While
the wear resistance is high, our results show that the
corrosion resistance is poorer than for any other speci-
men in the study. This specimen had a higher corro-
sion current density, in both electrolytes (Table 3), and
was the only specimen that pitted during chronoam-
perometric testing (Figures 4 and 5). Even though this
has been reported in the literature of the corrosion of
WC-Co in neutral chloride solutions, the behaviour had
not been effectively explained. The detrimental effect
of VC stems, primarily, from the effect of VC on the
amount of W atoms in Co: the VC restricts the amount
of W that goes into solution in Co during sintering by
forming a (V,W)C film around the WC grains. This is the
mechanism by which VC grain refiners are known to
effect WC grain refinement. The effect of low W solute
content on corrosion has already been explained.
Conclusion
Investigations into the corrosion behaviour of WC-VC-
Co hardmetals using potentiodynamic scans, XRD and
Raman spectroscopy measurements in 1 M NaCl and
synthetic mine water (SMW) were undertaken. The
observations can be summarized as follows:
1. Specimens exhibit active-pseudopassive behaviour
in NaCl but the intensity of the pseudopassivation is
impaired by the presence of VC. In SMW, all speci-
mens actively corrode during anodic polarization with
no impact whatsoever from VC. The negative impact
in NaCl, and the lack of impact in SMW are caused,
primarily, by the fact that the VC does not participate
in the formation of a protective film on the surfaces of
the corroding samples, and, partly by the fact that the
VC prevents W atoms from going into solution in the
Co binder, that is, the VC fails to lower the magnetic
saturation of samples.
2. Additions of VC to WC-Co at levels used for WC grain
size refinement increase corrosion current densities
and predispose specimens to pitting corrosion. This
occurs because the VC reduces the amount of W
atoms dissolving in the Co binder (increases the mag-
netic saturation compared to specimen 0VC).
3. The effect of high VC content on the corrosion
resistance of WC-Co is solution dependent: high VC
contents are not beneficial for corrosion resistance in
NaCl, but greatly improve corrosion resistance in SMW.
Acknowledgement
This work was funded
by South Africa's
DST's Centre of
Excellence in Strong
Materials, based
at the University of
the Witwatersrand,
Johannesburg, and
the National Research
Foundation.
References
References for
this article and an
original of the paper
with citations are
available from the
editor, Glynnis Koch,
at chemtech@crown.
co.za.
