28
Chemical Technology • September 2013
materials of construction
T
ungsten carbide (WC) – cobalt (Co)-based
hardmetals corrode when applied in situations
that contain fluids, for example metal machin-
ing and in the mining industry, where the fluids act as
electrolytes. The Co constituent is the least corrosion
resistant. Efforts to improve their corrosion resistance
have involved reducing the Co content by introduc-
ing more corrosion resistant elements like nickel,
chromium and ruthenium. Other efforts have involved
the introduction of small amounts of some transition
carbides. For example, the addition of 0,5 wt% chro-
mium carbide (Cr
3
C
2
), to WC-Co significantly improves
corrosion resistance.
Earlier studies on the effect of vanadium carbide
(VC), another transition carbide that South Africa has
substantial ore deposits of have produced inconsis-
tent results. Small additions have at best a neutral
effect on the WC-CO corrosion resistance; large ad-
ditions (10 wt%VC) improve the corrosion resistance
only in the case of high Co content WC-Co. However,
work by the current authors showed that increasing
VC contents are accompanied by increasing corrosion
resistance, in hydrochloric and sulphuric acids. The
current article is extending the corrosion study of the
materials to neutral chloride electrolytes to determine
whether the improved corrosion resistance is solu-
tion independent, which in literature is not always the
case.
Experimental
Materials
Table 1 gives the nominal composition (wt%) (VC
content 0, 0,4, 10 and 27 wt%) of the test materials
Corrosion of WC-VC-Co hardmetal in
neutral chloride containing media
The effect of varying VC content on the corrosion behaviour of WC-10wt%Co hardmetals in
sodium chloride (NaCl) and synthetic mine water (SMW) solutions have been investigated
using anodic polarization scans and surface analytical methods. It is shown that the polari-
zation behaviour is active-pseudopassive in NaCl and active in SMW regardless of the VC
content, while the corrosion resistance is poorer and independent of VC content in NaCl but
better at high VC contents in SMW. The corrosion behaviour of samples is explained using
the effect of VC on the chemical composition of the binder.
This article was
first published in
"ISRN Corrosion"
the details of which
are are follows: C N
Machio, "Corrosion of
WC-VC-Co Hardmetal
in Neutral Chloride
Containing Media,"
ISRN Corrosion, vol.
2013,
Article ID 506759,
10 pages, 2013.
doi:10.1155/2013
/506759.
by C N Machio, Council for Scientific and Industrial Research (CSIR), Pretoria, SA, D S Konadu, DST/NRF Centre of
Excellence in Strong Materials, and School of Chemical and Metallurgical Engineering, University of Witwatersrand,
Johannesburg, SA, and Materials Science and Engineering Department, University of Ghana, Legon, J H Potgieter,
School of Research, Enterprise and Innovation, Manchester Metropolitan University, UK, S. Potgieter-Vermaak,
School of Science and the Environment, Manchester Metropolitan University, UK, and School of Chemistry, University
of the Witwatersrand, Johannesburg, SA, and J Van der Merwe, DST/NRF Centre of Excellence in Strong Materials,
and School of Chemical and Metallurgical Engineering, University of Witwatersrand, Johannesburg, SA
and their magnetic saturation levels. The microstruc-
tures of the specimens are fully described elsewhere.
At the 10 and 27wt% levels, the added VC occurs as
(W,V)C because WC dissolves in VC during sintering.
For easier referencing, the samples will be referred to
using their VC content. The target binder content was
10wt%Co for all the specimens, but samples 10VC
and 27VC had 12 and 11wt%Co respectively. Compari-
son between the samples is still possible because the
effect of cobalt content on the corrosion behaviour of
WC-Co hardmetals is known. For example, Co content
and corrosion current densities increase in tandem
during corrosion in acidic solutions.
Table 1: Nominal chemical composition and magnetic properties
of test specimens
Specimen
Approx.
vol% Co
Magnetic saturation
(4
πσ
) (G cm
3
g
)
WC-10Co
16
172
WC-0.4VC-10Co
16
185
WC-10VC-12Co
17
230
WC-27VC-11Co
13
194
The magnetic saturation of the specimens in-
creased with the introduction of VC and with Co con-
tent (Table 1). Cobalt is the magnetic phase and the
higher values with higher Co contents are expected.
The magnetic saturation measures the amount of
solutes in Co and is a quick technique for determining
the quality of sintered WC-Co. In general, the magnetic
saturation of WC-Co decreases linearly as the amount
of W in Co increases.
-1 3
