Chemical Bulletin of Politehnica University of Ti misoara, ROMANIA [623659]

Chemical Bulletin of “Politehnica” University of Ti misoara, ROMANIA
Series of Chemistry and Environmental Engineering
Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 60(74), 2, 2015

60 Corrosion Behavior of WC-Ni Coatings Deposited by D ifferent Thermal
Spraying Methods

I. Hulka *, D. UTu *, V.A. Serban *, M.L. Dan *, V. Matikainen * and P. Vuoristo *

* Politehnica University of Timisoara, P-ta Victoriei , Nr 2, 300006, Timisoara, Romania
email: [anonimizat]

** Tampere University of Technology, Department of Mat erials Science, FI-33720, Finland

Abstract: This study compares the properties of WC-Ni coatin gs manufactured from fine powder particles using hi gh-
velocity oxygen fuel (HVOF) and high-velocity air f uel (HVAF) spraying processes. The results indicate d that the
decarburization of carbide phase is influenced by t he spraying equipment. It was found out that the de position velocity is
an important factor influencing the bonding between splats and furthermore the coating density. The WC -Ni coating
sprayed with the supersonic M3 HVAF spraying gun re vealed the lowest degree of porosity and degree of decarburization,
achieving the best properties in terms of electroch emical corrosion resistance when compared to the HV OF sprayed WC-
Ni coating.

Keywords: WC-Ni, hard coatings, cermet, corrosion studies, t hermal spraying

1. Introduction

Thermal spraying is a deposition technique in which
molten, semi-molten or solid particles are deposite d on a
certain surface [1]. The coating is formed by conti nuous
impacts of accelerated droplets on the substrate. A t impact
the particles deform into thin lamellar shapes (spl ats),
overlap and interlock as they solidify and cool dow n [2]. A
device called spray gun is used to accelerate the h eated
particles. The feedstock material can be in the for m of
stick, wire or powder which is fed into the flame p roduced
by the spraying gun. The high temperature and kinet ic
energy of the particles is obtained by burning mixt ures of
different fuel gases or kerosene with oxygen, or by using
electrical power sources.
The high-velocity oxygen fuel (HVOF) spray
processes uses thermal and kinetic energy to melt t he
feedstock powder and to accelerate the particles to wards
the surface, respectively. In HVOF spraying the fue l is
burnt with oxygen at a high pressure generating a h igh
velocity jet. For HVOF spraying the typical fuels a re gases
such as propane, ethylene, propylene, hydrogen and
methylacetylene-propadiene-propane (MAPP) mixture o r
liquid kerosene. Usually the temperature varies bet ween
2500 °C and 3200 °C depending on the fuels [3] whic h
generate particle velocities up to 750 m/s [4]. Hig h-velocity
air fuel (HVAF) spray process is a similar but more
economical method compared to HVOF spraying [5]. Hi gh
velocity air-fuel (HVAF) thermal spraying is a newe r
developed spraying process for deposition of metall ic and
carbide based coatings from powder feedstock [6]. D uring
the spraying process the powder particles are heate d below
their melting point (under 2000°C) and accelerated to
velocities above 700 m/s which makes the process a solid
spraying technology in which spraying temperature i s an
important parameter for coating deposition [5, 7]. The new M3 supersonic spray gun represents the state of the art in
HVAF spraying. It can produce high particle velocit ies up
to 1200 m/s and high particle temperatures compared to
single nozzle HVAF spraying guns which allows produ cing
dense and hard coatings [7].
The objective of this study is to investigate the
influence of deposition method on the microstructur e and
corrosion properties of the HVOF and HVAF sprayed
coatings.

2. Experimental

The WC-Ni coatings were deposited onto low carbon
steel substrates by HVOF and HVAF spraying techniqu es
using a commercial WC-Ni (Ni ~ 10%, C ~5.4%, W –
balance) powder manufactured by Buffalo Tungsten In c.,
US, as the feedstock material. Angular alumina was used as
an abrasive media for substrate preparation in orde r to
increase the surface roughness before spraying. The HVAF
spraying was carried out using a Supersonic Air Fue l
HVAF (SAF) M3 (Uniquecoat Technologies LLC, US)
spray gun while the HVOF spraying was carried out w ith a
DJ 2700 (Sulzer Metco, US) spraying gun. The coatin g
cross-sections were examined using a Quanta FEG 250
(FEI, The Netherlands) scanning electron microscope
(SEM) equipped with EDAX analyzer. The porosity
within the coatings was determined by image analysi s
using image processing software, Image Tool 3.00, o nto 12
backscattered electron micrographs at 2000X
magnification. Micro-hardness was measured with a
Matsuzawa MMT X7 tester using 300g load for 10
indentations per sample. X-ray diffractometry (XRD) of
powder and as-sprayed coatings was performed on an
Empyrean diffractometer (PANAnalytical, The
Netherlands) using Cu-K α radiation. The measurements
were performed at 2 θ diffraction angle in the range of

Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 60(74), 2, 2015

61 20°–100° and with 0.02° step size. Phase identifica tion was
performed using the PANAnalytical X'Pert High Score
Plus software using the ICDD JCPDF-2 database
(International Centre for Diffraction Data, Newtown
Square, US).
Electrochemical tests were performed at room
temperature using a SP-150 potentiostat/galvanostat (Bio-
Logic, SAS, France) using a typical glass cell equi pped
with three electrodes: working electrodes consistin g of
WC-Ni coated samples deposited by HVOF and HVAF
spraying processes, Ag/AgCl reference electrode and a
platinum mesh used as counter electrode. For the
experiments the exposed surface of specimens was 1 cm 2,
submerged in a 3.5 wt. % NaCl solution. The electro de
potential was stabilized for 60 min to achieve the
equilibrium.

3. Results and Discussion

3.1. Powder characterization

Figure 1a shows a representative image of the WC-Ni
powder particles used in the present study. The pow der was
manufactured by agglomeration and sintering process . The
size distribution of the particle granules was in t he range of
7-21 µm and the powder morphology consisted of sphe rical
and irregular particles. For thermal spraying the p referable
shape of the powder particles is spherical in order to
increase the flowability during deposition. The ir regular
shape and small size of the particles can lead to p oor
powder feeding during the spraying process which ca n
have a strong influence on the coatings properties. The
EDX spectrum of the powder is presented in Figure 1 b and confirms the powder chemical composition, the ident ified
elements being carbon, tungsten and nickel.

3.2. Coating microstructure

Metallographic images of cross-sectional
microstructure of the two sprayed coatings are pres ented in
Figure 2. Both coatings are dense and a good adhesi on to
the substrate can be noticed. Some cracks can be ob served
in the upper region of the HVOF coating which might be
attributed to the rapid cooling and solidification process. In
comparison, the HVAF coating looks denser and free of
major defects but due to the very fine powder and t he
presence of irregular shape powder particles the fl owability
during deposition was affected and the deposition
efficiency was lower which conducted to a thinner c oating.
Using the HVOF thermal spraying method a thicker
coating was obtained with a thickness of about 260
micrometers compared to the HVAF deposited coating
with a thickness of 100 micrometers.
The micro-hardness determined on the cross section of
the coatings showed different results regarding the
recorded values. The coatings produced by HVAF ther mal
spraying had higher hardness values than the HVOF
sprayed coatings, as listed in Table 1, which might be
attributed to hard phases formed during spraying du e to
higher intralamellar hardness [8]. The porosity of the
coatings was analyzed from the metallographic image s
with the corresponding software and the results wer e
1.85% for the HVOF coating and 0.23% for the HVAF
coating, as listed in Table 1. This indicates that the HVAF
coating had a denser structure due to the M3 gun wh ich
produces higher particle velocities compared to the DJ
2700 gun.

Figure 1. Morphology (a) and EDX spectra (b) of WC- Ni powder

Figure 2. SEM micrographs of HVOF (a) and HVAF (b) WC-Ni coatings in cross-sections
a)
b)
a) b)

Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 60(74), 2, 2015

62 TABLE 1. Thickness, hardness and porosity of therma lly sprayed
WC-Ni coatings

Material Coating
thickness [µm] Micro-hardness
HV0.3 Porosity [%]
WC-Ni HVOF 262.16 ± 18.92 747 ± 115 1.85
WC-Ni HVAF 99.02 ± 21.2 1395 ± 47 0.23

The XRD patterns of the used WC-Ni powder and the
two sprayed coatings are presented and compared in Figure
3. The analysis of the XRD patterns confirms that t he
powder consists of WC phase and metallic γ-Ni phase. The
spectrum for the coatings predominantly consists of WC
phase while also W 2C can be detected. The intensity of the
W2C peaks is lower for the HVAF sprayed coating
indicating lower degree of decarburization. The M xC peaks
might correspond to phase dissolution of the carbid e phase
in the molten Ni matrix. It is thought that a reduc tion in
flame temperature during thermal spraying led to a
decrease of decomposition of WC phase which means t hat
no significant transformation took place during dep osition.

Figure 3. X-ray diffraction (XRD) patterns of inves tigated WC-Ni powder
and deposited HVOF and HVAF coatings

3.3. Corrosion behavior

3.3.1. Open circuit potential

The evolution in time of the equilibrium potential of
the tested samples (electrodes) is twofold: first, it allows an
assessment of the corrosion behavior of the exposed surface to the corrosive media, and secondly allows the
estimated time to reach a stationary or quasi-stati onary
state. In Figure 4, open cell potentials of the two sprayed
coatings are compared. At the beginning of the tes t the
coatings present an unstable open cell potential in dicating
corrosion activity afterwards the potentials starte d to
stabilize. According to the obtained chart, it can be noticed
that after 1 hour of testing, the system had reache d a quasi-
stationary state, enough to start the linear polari zation
measurements.

Figure 4. Open circuit potential ( EOCP ) measurements of WC-Ni coatings

Within the WC coating, the open circuit potential o f
the carbide phase is much nobler than that of the m etallic
matrix and substrate. The open circuit potential di fference
between WC and Ni causes micro-galvanic corrosion w hen
the electrolyte is in contact with the surface of t he coating.
In that case the WC phase becomes cathodic and Ni
becomes anodic, resulting in corrosion of anodic bi nder
material. Also, when the electrolyte infiltrates al ong the
micro-cracks and pores existing in the coating, the macro-
galvanic corrosion can occur [9].
The values obtained after measurements reaching the
quasi-stationary state are shown in Table 2. The pr esented
data confirms that the electrode movement towards a more
positive value leads to a decrease in the speed of
polarization which means better corrosion behavior. The
HVAF coating indicated better corrosion behavior du e to
lower porosity which might be attributed to a dense r
coating deposited at higher velocities.

TABLE 2. E OCP at 25˚C

Electrolyte Sample Temperature [˚C] E OCP [V]/Ag/AgCl
WC-Ni HVOF -0,271 NaCl 3,5% WC-Ni HVAF 25˚C -0,344

Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 60(74), 2, 2015

63 3.3.2 Potentiodynamic polarization studies

This method enables immediate corrosion rate
determination via the intensity of the corrosion cu rrent and
Tafel slopes but also shows possible changes at the
electrode surface or changes in the mechanism of co rrosion
process. In the present study, a sweep potential of ±250 mV
was applied against the open cell potential and a s can speed
of 1 mVs -1 was used. By this method the corrosion rate can
be calculated using the direct substitution of Tafe l slope
values (cathodic – bc and anodic – ba) using the following
equation:

.
.(1)

were represents the anodic slope and
respectively the cathodic slope; Tafel slope consta nts
associated with anodic ( ba) and cathodic ( bc) processes; E
and Ecor – potential and corrosion potential ; icor – corrosion
current density. Linear polarization curves recorded for
WC-Ni coatings in 3.5% NaCl at 25°C are presented i n
Figure 5.
In order to determine the electrochemical parameter s
the polarization curves were fitted and the results are
presented in Table 3. The HVAF coating exhibits bet ter
corrosion resistance indicated by the lower passive current
density and wide passive region due to a denser coa ting,
reduced porosity and micro-cracks compared to the H VOF
coating.

3.3.3 Electrochemical Impedance Spectroscopy

This method represents a stationary electrochemical
technique based on overlapping a low amplitude alte rnative
signal (which contains an excitation frequency) abo ve the
electrode potential and tracking the electrode resp onse at
this perturbation. While the electrochemical system
behaves like a nonlinear system its impedance will be
dependent on the potential. Analyzing the response
provided by the system the interface structure can be
determined and also the reactions which take place at the
coating/electrolyte interface by proposing and equi valent
electric circuit for process simulation. In the cas e of
corrosion, the polarization resistance of working e lectrode
depends on the micro-cracks and porosity within the
coating. The obtained results by carrying out this type of
measurement are expressed by so called Nyquist and Bode
diagrams [10, 11] presented in Figure 6.

Figure 5. Polarization curves of WC-Ni coatings

The Nyquist plot for the HVAF coating shows the
presence of an incomplete semicircle attributed to a
depressed capacitive loop at high to intermediate f requency
range [12]. Deviation of recorded circle from the p erfect
circular shape can be associated with electrode sup erficial
inhomogeneity, which might be noticed for the HVOF
coating. This superficial inhomogeneity, is caused as effect
of surface roughness or different interfacial pheno mena.
These spectra were fitted using the equivalent circ uit
shown in Figure 7 in order to determine the experim ental
values presented in Table 4.
When analyzing the equivalent circuit presented in
Figure 7 it can be observed that the system consist s of four
components: Rs – solution resistance, CPE – constant phase
element which represent a modified capacitance that is a
frequency dependent element and is related to the s urface
roughness [14], Rct – charge transfer resistance and W –
Warburg impedance which is associated with mass
transport process. Presence of Warburg impedance
suggested that the corrosion reaction is limited by mass
transport. Based on presented circuit were obtained the
parameters associated with corrosion process. Elect rical
circuit used for modeling of experimental EIS data consist
of a serial connection between solution resistance Rs and a
parallel connection of the constant phase connectio n CPE
and the charge transfer resistance Rct in series with a
Warburg impedance.

TABLE 3. Electrochemical parameters obtained from p olarization curves

TS M echiv.
[g] Dens.
[g cm -3] icor
[μA cm -2] Ecor
[mV] -bc
[mV dec -1] ba
[mV dec -1] Rp
[Ω] vcor
[mm an -1]
HVOF 38.56 4.0 6.21 -435 96 208 3017 0.20
HVAF 38.56 4.0 3.57 -346 256 281 7514 0.11

Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 60(74), 2, 2015

64

Figure 6. Nyquist (a) and Bode (b) plots obtained i n NaCl 3,5% solution at 25°C for WC-Ni coatings

Rs CPE
Rct W

Figure 7. Equivalent circuit for spectra fitting

TABLE 4. Experimental values of EIS during the corr osion process

TS Rs [Ω cm 2] T [F cm -2 s n-1] n Rct [Ω cm 2] C dl [μF cm -2] Chi 2
HVOF 9,14 (0,7%) 3,2 10 -4 (3,16%) 0,83 (0,62%) 182,4 (1,32%) 114 7,3 10 -4
HVAF 36,9 (1,43%) 1,63 10 4 (0,61%) 0.71 (0,61%) 433 (2,62 %) 516 8,8 10 -4

Impedance of constant phase element is described by :

ZCPE = 1/ T(j ω)n (2)

where 0 < n <1 and is describing the constant phase angle
of CPE, Y0 is a parameter related to the double lay er
capacitance [6].
Impedance of Warburg element in case of a finite
length thickness of diffusion layer δ can be expressed:

Zw = ( Rw(j ωτD)-φ) tan (j ωτD)φ (3)

where Rw – diffusion resistance; τD – diffusion time
constant given by τD = δ2/D, where δ – diffusion thickness
and D – diffusion coefficient; φ – an exponent whit values
between 0 and 1 [14].
After modeling of experimental data using CNLS
procedure for both superficial layers are presented in Table
4. The charge transfer resistance is associated wit h the
occurrence of the charge transfer reaction at the b ottom of
pores present in the coating layer. In the case of HVAF
coating the Rct is more than two times higher in comparison
with HVOF coating. The higher value can be attribut ed to
the effective barrier behavior. EIS measurements th erefore
show that the HVAF coating possesses less interconn ected porosity and micro-cracks which indicate through
electrochemical characteristics that the coating is nobler
than HVOF coating.

4. Conclusions

In the present study the properties of WC-Ni coatin gs
manufactured by DJ 2700 HVOF gun and M3 HVAF torch
were assessed and compared. The experimental result s lead
to the following conclusions:
– Hard and dense WC-Ni coatings can be obtained by
using the HVOF and HVAF processes to spray fine
feedstock powder. The coatings sprayed by HVAF meth od
are denser and exhibit less pores and cracks.
– WC dissolution is reduced in the metallic matrix and
no significant decarburization took place during th e coating
deposition with HVAF spraying.
– Due to the supersonic speed of HVAF spraying the
powder particles are projected with higher velocity leading
to denser coatings with reduced porosity (<0.5%) an d less
cracks within the coatings. This has a positive eff ect on the
corrosion behavior of the coating compared to HVOF
manufactured coating.

Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 60(74), 2, 2015

65 – The thickest coating (260 µm) deposited by HVOF
spraying is less protective than the thinner HVAF s prayed
coating (100 µm) in 3.5% NaCl solution.

ACKNOWLEDGEMENT

This work was supported by the strategic grant
POSDRU/159/1.5/S/137070 (2014) of the Ministry of
National Education, Romania, co-financed by the Eur opean
Social Found – Investing in people, with the Sector ial
Operational Program Human Resources Development
2007-2013.

REFERENCES

1. Pawlowski L., The science and engineering of the rmal spray coatings,
Ed. John Wiley and Sons, 2008 .
2. Schneider K.E., Belashchenko V., Dratwinski M., Siegmann S. and
Zagorski A., Thermal spraying for Power Generation Components,
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2006 .

3. Turunen E., Diagnostic tools for HVOF process op timization,
Dissertation for the degree of Doctor of Science in Technology, VTT
Industrial Systems, Helsinki, 2005 .
4. Davis J R., Handbook of Thermal Spray Technology , ASM
International, 2004 .
5. Zhou X.I. and Zhang J.S., Transaction of Nonferrous Metals Society of
China , 18, 2008 , 262-269.
6. Verstak A. and Baranovski V., Thermal Solution: Advances in
Technology and Applications, 10-14 May, Osaka, DVS: German welding
society, 2004 , 551-555.
7. Uniquecoat Technologies Inc. website: www.unique coat.com
8. Verdom C., Karimi A. and Martin J.L., Materials Science and
Engineering A , 246, 1998 , 11-24.
9. Cho J. E., Hwang S.Y. and Kim K.Y., Surface and Coatings
technology , 200, 2006 , 2653-2662.
10. Orazem M.E. and Trobollet B., Electrochemical i mpedance
spectroscopy, Ed. Wiley, 2008 .
11. Barsoukov E. and Macdonald J. R., Impedance spe ctroscopy theory,
Experiment and Applications, Ed. Wiley Interscience , 2005 .
12. Ashassi-Sorkhabi H., Ghalebsaz-Jeddi N., Hashem zadeh F. and Jahani
H., Electrochimica Acta , 51, 2006 , 3848-3854.
13. Lecante A., Robert F., Blandinieres P.A. and Ro os C., Current
Applied Physics , 11, 2011 , 714-724.
14. Kellenberger A. and Vaszilcsin N., Micro si Nan omateriale,
Electrochimia starii solide, Ed. Politehnica, 2013 .

Received: 12 October 2015
Accepted: 24 November 2015

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