E. OZKAN: EFFECTS OF ALLOYING ELEMENTS AND SURFACE PROPERTIES ON THE CORROSION BEHAVIOR ...
291–298
EFFECTS OF ALLOYING ELEMENTS AND SURFACE
PROPERTIES ON THE CORROSION BEHAVIOR OF
HVOF-DEPOSITED WC COATINGS
VPLIV ZLITINSKIH ELEMENTOV IN LASTNOSTI POVR[INE NA
KOROZIJO PREVLEK WC IZDELANIH S POSTOPKOM HVOF
Erhan Ozkan
*
Dikkan R&D Center Izmir, Turkey
Prejem rokopisa – received: 2023-03-22; sprejem za objavo – accepted for publication: 2023-05-02
doi:10.17222/mit.2023.833
The main purpose of this study is to investigate the corrosion behavior of nickel-aluminum buffer and tungsten-carbide-based
ceramic-metal composite coated materials on EN 1.4404 quality stainless steels in a sulfuric acid (H2SO4) environment for pet-
rochemical industry applications. For this purpose, tungsten-carbide-based coatings were produced on nickel-aluminum-depos-
ited 1.4404 stainless-steel substrates using the HVOF (High Velocity Oxy-Fuel) technique. In the characterization of coatings,
X-ray diffraction (XRD) for phase analysis, optical microscope and scanning electron microscope (SEM) for surface morphol-
ogy, image analyzer for coating thickness measurements, energy distribution spectroscopy (EDS) for elemental analysis, and
roughness device for surface structures, were used. WC-, Co-, Ni-, and NiAl-based phases were observed in the coatings. Ac-
cording to metallographic studies, all the coatings had a similar coating microstructure and made good contact with the sub-
strate. Potentiodynamic polarization measurements and corrosion tests were carried out to determine the corrosion behavior of
HVOF plasma-sprayed coatings using a potentiostat/galvanostat. The results showed that the WCCo-NiAl-coated stainless-steel
substrate had a higher corrosion resistance to the H2SO4 environment than the NiAl and WCNi-NiAl samples.
Keywords: HVOF, stainless steel, coatings, corrosion
Namen {tudije je bil raziskati korozijsko obna{anje z Ni-Al bla`ilcem in s kompozitnim kerami~nim materialom na osnovi
volfram karbida (WC) opla{~enim nerjavnim jeklom EN 1.4404 v okolju `veplene kisline (H2SO4), namenjenih aplikacijam
v petrokemijski industriji. V ta namen so izdelali prevleke na osnovi WC na z Ni-Al prevle~enem nerjavnem jeklu vrste 1.4404
s postopkom napr{evanja z visoko hitrostjo v me{anici goriva in kisika (HVOF; angl.: High Velocity Oxy-Fuel). Po izdelavi
vzorcev s prevlekami so le-te ocenili. Z rentgensko difrakcijo (XRD) so izvedli fazne analize, z opti~nim in elektronskim
presevnim mikroskopom (SEM) so analizirali morfologijo povr{ine prevlek, z digitalnim analizatorjem pa so dolo~ili debeline
in hrapavost prevlek. Z energijsko disperzijsko spektroskopijo (EDS) so dolo~ili porazdelitev posameznih kemijskih elementov
(faz na osnovi WC, Co, Ni in NiAl) na povr{ini prevlek. Metalografske analize so pokazale, da imajo vse izdelane prevleke
podobne mikrostrukture in dober stik s podlago. S pomo~jo Potenciostata-Galvanostata so izvedli meritve potenciodinami~ne
polarizacije ter korozijske teste in dolo~ili korozijsko obna{anje izdelanih plazemsko napr{enih prevlek HVOF. Rezultati
raziskav so pokazali, da imajo vzorci nerjavnega jekla prevle~eni z WCCo-NiAl bolj{o odpornost proti koroziji v okolju H2SO4
kot prevleke na osnovi NiAl in WCNi-NiAl.
Klju~ne besede: postopek napr{evanja z visoko hitrostjo v me{anici goriva in kisika (HVOF), nerjavno jeklo, prevleke, korozija
1 INTRODUCTION
Petrochemical plants are installations for chemical
conversion processes. In these facilities, chemical raw
materials are produced using physical or chemical con-
version processes of inorganic or organic chemicals.
1–3
The most common of these raw materials is sulfuric acid
(H
2
SO
4
). H
2
SO
4
is produced from SO
2
that is derived
from a variety of sources, such as the combustion of ele-
mental sulfur or the heating of metal sulfides. SO
2
is then
converted to SO
3
using a catalyst in the gas phase chemi-
cal equilibrium reaction and reacts with water to form
H
2
SO
4
.
4–7
Recycling facilities have a critical structure in terms
of actively positioning sustainable environmental enter-
prises, which are today’s important facilities, in the orga-
nization.
8
H
2
SO
4
formed as a result of the above chemi-
cal reactions creates an aggressive, corrosive environ-
ment in the main lines of petrochemical industries and
especially in sulfuric acid recovery plants. In the petro-
chemical industry, surface modifications of metals have
come to the fore to protect metallic surfaces from corro-
sion. The corrosion resistance of materials can be in-
creased by changing and improving the surface morphol-
ogy and microstructure. Changing the surface chemistry
of materials is possible by carburizing, nitriding, carbo-
nitriding, boriding, siliconization and chroming. In addi-
tion, surface-coating processes with thermal spraying
have recently played a role in increasing the corrosion
resistance of metallic surfaces to slow the corrosion rate
and make the surface corrosion resistance more effec-
tive.
9,10
Thermal spray processes were made with spray guns
to protect the main material surface from environmental
Materiali in tehnologije / Materials and technology 57 (2023) 3, 291–298 291
UDK 665.71 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(3)291(2023)
*Corresponding author's e-mail:
erhan.ozkan@dikkan.com (Erhan Ozkan)

conditions, and to prevent corrosion by improving the
material surface properties.
11–13
After the coating mate-
rial is melted with the energy used in the gun, it is
thrown onto the material surface at high speed and the
coating is created. In particular, coatings produced with
HVOF (High Velocity Oxy-Fuel) show very good adhe-
sion and cohesion properties, as well as being very hard,
making this method more advantageous than other meth-
ods.
14,15
However, the formation of holes, pores and mi-
cro-cracks as a result of high-temperature particles hit-
ting the substrate surface at high speed can be counted
among the disadvantages of this method.
16
These pores,
cracks and holes have a negative effect on the corrosion
behavior of the material. In order to prevent this, systems
have been developed by creating a buffer layer on the
coatings. Buffer layers provide great advantages in pre-
venting formations such as capillary cracks and porosity,
which reduce the corrosion resistance.
17–24
EN 1.4402 quality stainless steel (commonly known
as 316L in AISI norm) is a low-carbon stainless steel
that is widely used in aggressive conditions such as sul-
furic acid, thanks to its chemical content. The weak side
of this stainless steel is that it cannot perform adequately
in environments with more aggressive corrosion, and
erosive environments. Surface coatings are applied to
improve this weakness and increase the service life.
In this study the corrosion behavior of EN 1.4402
stainless steels, which are widely used in the petrochemi-
cal industry, coated with NiAl, WC-Co and WC-Ni by
HVOF technique was evaluated in a 1-N H
2
SO
4
solution
with potentiodynamic methods. According to the results
obtained, the data on the prolongation of the service life
of surfaces with different alloy values were transferred in
detail, and technological solution methods were pro-
posed by disseminating the results. In addition, HVOF
coating parameters, porosities of coating surfaces, sur-
face thicknesses, microstructures, and their interactions
and changes with the corrosion behavior of the coatings
were presented to provide additional information.
2 EXPERIMENTAL PART
EN 1.4402 stainless-steel samples with the dimen-
sions of (50 × 50 × 2) mm were used as a substrate mate-
rial. The surface preparation of the parts used in the
HVOF technique is an important step. This is because
the structure of the coatings spreads directly with the
surface roughness and is controlled by the type of blast,
pressure, angle, width, duration and sand-blast nozzle
(hole size). The material surface was sandblasted prior to
the HVOF thermal spraying for the purpose of maintain-
ing the strength durability of the coating. Aluminas at
high outputs were impacted on the material at high
speeds, increasing the roughness of the surface and notes
the number and size of surface asperities with increasing
roughness. During the spraying process, it attached itself
to this surface mechanically and physically by clinging
to these asperities coming from the earth. A high bond
strength could only be achieved with the expected high
roughness, and this caused incresing resistance of the
coating powders to withstand the substrate values. This
process was carried out with 35-grit alumina powders for
30 s using 400-kPa air. Then, via Metco Diamond Jet
(DJ) 2600 brand system, a coating was obtained with tra-
ditional sintered powders having a grain size of 14 μm in
the ratio of 13 w/% Co, 87 w/% WC and 13 w/% Ni and
87 w/% WC, but the cracks on the surfaces of the ob-
tained coatings had a negative effect on the wear. An
interlayer coating containing nickel and aluminum was
carried out to ensure the resistance of the WC-based
cermet coatings to the substrate. To limit the content of
the intermediate layer, the bond from the literature and
previous properties was used and accordingly NiAl con-
taining 95 w/% Ni, 5 w/% Al was used as the powder
coating. The NiAl coating and WC basis, bond parame-
ters of the two coatings are given in Table 1.
24
Table 1: HVOF coating process parameters
Parameters Values
NiAl
WC-based
coatings
Rate of oxygen flow (L min
–1
) 135 245
Rate of hydrogen as fuel gas
(L min
–1
)
410 650
Rate of nitrogen as carrier gas
(L min
–1
)
20 18
Distance of spray (mm) 260 310
Velocity of horizontal substrate
(m s
–1
)
11
Speed of traverse vertical gun
(mm s
–1
)
55
Layer number 30 30
A Rigaku D/Max-2200/PC Model diffractometer was
used for the X-ray diffraction (XRD) of the as-sprayed
coatings using Cu-K
 radiation (0.15406 nm), a 0.05°
step size, and a 2-s dwell duration to confirm the
crystallinity of the coatings. The coatings were cold
molded in the way of using the cross-sectional areas of
the samples for optical microscope examinations. In or-
der to obtain better optical microscope results, specimens
were grinded and polished to a 0.1-μm finish surface.
Using a spark wire cutter, coating cross-sections were
separated from the batch. A Nikon Eclipse LV 150 opti-
cal microscope and a LUCIA 4.21 image analyzer were
used for the measurement of the coating thickness. The
surface morphology of the coatings was investigated
with a JEOL JJM 6060 scanning electron microscope
(SEM) coupled with energy-dispersive spectroscopy
(EDS). Surface-roughness measurements were per-
formed with a Mitutoyo SJ-301 surface-roughness tester.
The corrosion behavior of the coatings was evaluated
using a standard potentiodynamic polarization technique
with a GAMRY PC4/750 Potentiostat/Galvanostat
Model. The NiAl-, WCNi-NiAl- and WCCo-NiAl-
coated EN 1.4402 stainless-steel samples were ultrasoni-
E. OZKAN: EFFECTS OF ALLOYING ELEMENTS AND SURFACE PROPERTIES ON THE CORROSION BEHAVIOR ...
292 Materiali in tehnologije / Materials and technology 57 (2023) 3, 291–298

cally cleaned in acetone for 15 min before the experi-
ments. Prior to the corrosion tests, uncoated parts of the
substrate were lacked with polimeric materials to only
have a coating surface. The electrochemical system con-
sisted of a corrosion cell, a potensiostat/galvonostat and
a computer. The corrosion cell used was GAMRY’s
newly designed cell. The reference electrode used in this
study was a saturated calomel electrode (SCE). The
working solution was selected as 1-N H
2
SO
4
due to the
petrochemical industry applications. The tests were con-
ducted at room temperature, which varied from 24 °C to
27 °C. During this time, the corrosion potential of the
samples was also monitored. Potential scans for the po-
larization were initiated at –500 mV and end at 4000 mV.
A constant potential scan rate of 5 mV s
–1
was used. To
observe the characteristics of the corrosion test, the dif-
ferences in the pre- and post-corrosion surface images of
the samples and the changes in coating thickness and
surface roughness were observed. The electrochemical
method was used the observe the amount of porosity for
the HVOF-coated specimens.
3 RESULTS
Figure 1 illustrates XRD patterns of NiAl pre-coated
and WC with Co and WC with Ni coatings deposited by
HVOF technique on 1.4402 SS substrates. According to
XRD results, different forms of tungsten carbide and its
derivatives, tungsten carbide cobalt alloys and nickel alu-
minum were determined.
Even though WC, W
2
C, W, and FeNi phases were
found with the WCNi-NiAl coatings, WC, W
2
C, W, and
Co phases were determined from the WCCo-NiAl-
coated stainless steel substrate. W
2
C and metallic W
phases, which were obtained in both WC-based layers
after deposition process, were formed by carbon losses
during the coating step. A NiAl phase was found from
both of the samples because of the NiAl bond layer on
the substrate. However, it is important to express that the
electrochemical corrosion behaviours of the WCCo-NiAl
and WCNi-NiAl layers are complex because these coat-
ings consist of different phases.
SEM was used for the larger-scale microstructure
analysis. Figure 2 demonstrates surface morphologies of
the NiAl, WCCo and WCNi coatings produced using the
HVOF system. It was determined that the coatings usu-
ally exhibit porosity, microcracks and unmelted particles
according to the SEM evaluations. As reported in
23
,
SEM images showed that the bond between the substrate
and the coating was strong, due to the dense layer of the
coating. Due to the application principles of the HVOF
process, incompletely melted particles caused plastic
damage on the surface, causing micro-cracks and pores
in general.
The weakness of he HVOF-sprayed coatings, i.e.,
pores, microcracks and unmelted particles, are the areas
that activate corrosion, especially in H
2
SO
4
solutions.
Even if these inhomogeneties are very small structures,
unfortunately some of them are interconnected in these
coatings, allowing penetration of the coating by a fluid.
Additionally, since this technique has these kinds of ad-
vantages compared to other thermal spray techniques,
such as electric arc, flame spray, plasma spray and deto-
nation gun, WC-based HVOF-sprayed coatings have be-
come important in petrochemical applications. The other
significant issue here is the surface morphology of the
coatings for corrosion behaviours. Upon investigating the
surface morphologies, the WCCo coating has similar
features to WCNi, but NiAl possess distinctly different
microstructures. The NiAl metallic coatings have very
large splats with microcracks, comparied to the
WC-based coatings. The splat sizes of the NiAl, WCCo
E. OZKAN: EFFECTS OF ALLOYING ELEMENTS AND SURFACE PROPERTIES ON THE CORROSION BEHAVIOR ...
Materiali in tehnologije / Materials and technology 57 (2023) 3, 291–298 293
Figure 2: Surface morphologies of: a) NiAl, b) WCCo-NiAl and c) WCNi-NiAl layers produced by HVOF system prior to corrosion tests. The
scale bars of all coatings are 100 μm.
Figure 1: XRD patterns of the HVOF-coated NiAl, WC, WCCo,
WCNi specimens

and WCNi coatings are approximately (50, 10 and
15) μm, respectively.
From the aspect of corrosion, the porosity, surface
roughness, coating thickness, bond layer, pinhole and
microcrack are significant issues for HVOF-sprayed
coatings. The coating itself may be very resistant to cor-
rosion, but if the corrosion media can penetrate to the
substrate from the microstructural defects in the coat-
ings, the corrosion resistance of the substrate also be-
comes important. The cathodic and anodic electrochemi-
cal behaviors of the NiAl-, WCCo-NiAl-, and
WCNi-NiAl-coated stainless-steel samples were investi-
gated in 1-N H
2
SO
4
solutions. The electrochemical mea-
surements of the NiAl-, WCCo-NiAl-, WCNi-NiAl-
coated stainless-steel samples and the bare substrate are
presented in Table 2.
The porosities of the coatings were calculated using
electrochemical methods and the following equation.
25
F
R
R
E
=−
⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ p,m
p
cor
a
log
Δ
 (1)
In this equation F represents the total coating poros-
ity, R
p,m
is the polarization resistance of the base mate-
rial, R
p
is the measured polarization resistance of the
coated samples,  E
cor
is the difference between the cor-
rosion potentials of the surfaces with and without the
coating, and  a
is the anodic Tafel slope of the base ma-
terial.
The anodic polarization behaviours of the layers on
the stainless steel are given in Figure 3. The corrosion
potentials of the NiAl, WCCo-NiAl, WCNi-NiAl coat-
ings are (–340, –250, and –300) mV, respectively. More-
over, the corrosion current values of the NiAl, WCCo-
NiAl, and WCNi-NiAl coatings were (459 × 10
–6
, 727 ×
10
–7
and 929 × 10
–6
) A, respectively. The WCCo-NiAl
layered sample was the most stable coating with respect
to the H
2
SO
4
corrosive environment due to the pas-
sive-film-forming effect of Co and its low potential.
As shown in Figure 3, the curves for the WCCo-
NiAl and WCNi-NiAl samples can be divided into two
areas. In the first field where the potential value is lower
than 1 V, the current values increase exponentialy with
the increasing potential. Despite the fact that the first
field behavior resembles a passive layer, which differs
from the overall passive behavior. The current field I is
observed for currents above about 100 mA. This is high
compared to the typical passive current of about 10 mA.
This also means that alloying has a great influence on the
coating layer’s anodic reaction. Region I was formed as a
result of IR degradation due to surface corrosion prod-
ucts’ deposition of the layer combined with the binder
material’s dissolution. For the WCCo-NiAl deposit, re-
gion I is formed by Co and W oxidation. Interestingly,
the NiAl coating exhibited a general passivation behavior
similar to the stainless-steel samples containing active,
passive and transpassive regions.
Figure 4 shows SEM micrographs taken from the
surfaces of the NiAl, WCCo-NiAl and WCNi-NiAl coat-
ings after the corrosion tests. A few holes and pits were
observed on the surfaces of all the coatings. Notably, an
examination of the corroded samples revealed localized
corrosion spots on the surface. In addition to these, be-
fore the corrosion tests, cross-sectional optical
microstructural observations included many scratches
and it was difficult to separate the coating and base mate-
rial with the naked eye. On the other hand, after the cor-
rosion tests, separation became easier, the coating ap-
peared, and the scratches removed owing to the acidic
conditions. Therefore, it can be said that the corrosion
tests acted as an electro-polishing process, which caused
to the coatings to be seen more clearly and formed
unscratched structure. SEM micrographs of the coatings
strongly supported the anodic polarization results. Al-
though several holes and pits were observed on the sur-
faces of the NiAl and WC-Ni coatings, uniform corro-
sion was found for the WCCo coating. As mentioned
before, a H
2
SO
4
solution was our corrosive environment
used in the experiments. On the other hand, S and O
E. OZKAN: EFFECTS OF ALLOYING ELEMENTS AND SURFACE PROPERTIES ON THE CORROSION BEHAVIOR ...
294 Materiali in tehnologije / Materials and technology 57 (2023) 3, 291–298
Table 2: Electrochemical measurements of the coatings in 1-N H
2
SO
4
solution at 25
o
C
Samples
Ec
(mV vs SCE)
 c
(mV)
 a
(mV)
I
cor
(A)
Pol. Res. (R)
( )
Porosity
(%)
NiAl –340 309,4 312,3 459 × 10
–6
1.90 × 10
5
8.24
WCCo-NiAl –250 375,0 283,9 727 × 10
–7
9.66 × 10
5
5.58
WCNi-NiAl –300 207,9 172,6 929 × 10
–6
3.18 × 10
5
7.60
Figure 3: Anodic polarization curves of NiAl, WCCo-NiAl and
WCNi-NiAl coatings

were the most effective elements for the corrosion reac-
tions in this acidic environment.
The SEM micrographs of the coating layers and the
maps for the sulphur and oxygen as corrosion products
are depicted in Figure 5. It is possible to see from this
figure that the S and O distributions are the most dense
and regular in the NiAl coating (see Figure 5a). The S
and O are rarely and irregularly distributed in the WCNi
and WCCo coatings (see Figure 5b and 5c).
In the corrosion of the coatings, there were a lot of
parameters influencing the corrosion behaviours. These
parameters were coating structures such as porosity, sur-
face roughness, and thickness. The porosity values of the
coatings are listed in Table 2 and the porosity percent-
ages of the NiAl, WCCo and WCNi coatings were deter-
mined to be 8.24, 5.58 and 7.60, respectively. The NiAl
coatings possessed the highest porosity content. During
the corrosion tests, the porosities became potential zones
E. OZKAN: EFFECTS OF ALLOYING ELEMENTS AND SURFACE PROPERTIES ON THE CORROSION BEHAVIOR ...
Materiali in tehnologije / Materials and technology 57 (2023) 3, 291–298 295
Figure 4: Surface morphologies of: a) NiAl, b) WCCo-NiAl and c) WCNi-NiAl coatings after electrochemical corrosion tests. The scale bars of
all coatings are 100 μm
Figure 5: The SEM micrographs of the coatings and X-ray maps of sulphur and oxygen elements as corrosion products The SEM micrographs of
the coatings and X-ray maps of sulphur and oxygen elements as corrosion products after electrochemical corrosion tests. The corroded samples
are: a) NiAl, b) WCCo-NiAl and (c) WCNi-NiAl coatings.

to the corrosive environment and reacted first. Because
of this the corrosion resistance of the coatings decreased
with increasing porosity.
The changes in the coatings’ surface roughness be-
fore and after the corrosion tests are presented in Ta-
ble 3. The surface structure of the coatings before the
corrosion tests was relataed to the HVOF process param-
eters. The particle distribution, shape, size and parame-
ters of the atomization could form open porosities, caus-
ing accelerated corrosion speed on the surface of the
coatings. Additionally, the surface-roughness values (R
a
)
are important to determine some corrosion phenomena.
The R
a
values of the coatings increased after the corro-
sion tests, except the sample with the WCCo layer. The
reason for this phenomenon was the amount of porosity
that the coating contains. Also, the polarization resis-
tance can be explained as a function of the surface
roughness. Polarization means a deviation from the ideal
situation. Polarization resistance is the prevention of the
polarization of the materials. Surface roughness is an im-
portant parameter on the polarization resistance of the
coatings. The general rule is an increase of the surface
roughness casuses a decrease of the polarization resis-
tance.
Table 3: Surface roughness of the coatings before and after corrosion
tests
Coatings
Before Corrosion Tests After Corrosion Tests
Ra
R
z
R
q
R
a
R
z
R
q
NiAl 8.61 57.59 10.77 8.63 58.77 11.29
WCCo-NiAl 7.73 45.04 9.41 6.17 48.82 8.42
WCNi-NiAl 8.01 48.08 9.71 8.05 52.43 10.22
Figure 6 shows the change in polarization resistances
of the surface roughness for the NiAl-, WCCo-NiAl-,
and WCNi-NiAl-coated samples. It is clear from the fig-
ure that the resistance of the corrosion decreased with in-
creasing surface roughness for the different coatings.
That is to say, surface roughness is one of the important
parameters to determine the polarization resistance of the
coatings.
The coating thicknesses before and after the corro-
sion tests are given in Table 4.
Table 4: Change in coating thickness
Samples
Before Corrosion
Tests
After Corrosion
Tests
NiAl 719 μm 626 μm
WCCo-NiAl 514 μm 507 μm
WCNi-NiAl 513 μm 471 μm
Figure 7 denotes a decrease of the coating thickness
depending on the corrosion time, which took 167 min
because the scan rate was constant (5 mV s
–1
). The thick-
nesses of the NiAl, WCCo-NiAl and WCNi-NiAl layers
were measured to be (719, 514 and 513) μm before the
corrosion tests, respectively. These thickness values of
the NiAl, WC–Co/NiAl and WC–Ni/NiAl coatings de-
creased to (626, 507 and 471) μm after the corrosion
tests, respectively. Therefore, the WCNi coatings had the
highest corrosion rate. This is an expected result, be-
cause the WCNi coating has the highest negative corro-
sion potential.
4 DISCUSSION
The corrosion behaviors of the HVOF sprayed NiAl,
WCCo-NiAl and WCNi-NiAl coatings on EN 1.4402
substrates were investigated for petrochemical industries.
The obtained resultes are summarized as follows;
The WCCo and WCNi coatings without buffere ex-
hibited porosity, microcracks and unmelted particles ac-
cording to SEM evaluations. Even if these inhomo-
geneties were small structures, some of them are
interconnected in these coatings, allowing penetration of
the coating by a fluid. Therefore, a NiAl buffer layer was
E. OZKAN: EFFECTS OF ALLOYING ELEMENTS AND SURFACE PROPERTIES ON THE CORROSION BEHAVIOR ...
296 Materiali in tehnologije / Materials and technology 57 (2023) 3, 291–298
Figure 7: Change in coating thickness versus time
Figure 6: Polarization resistance versus surface roughness for the
coatings

applied to the coatings and no cracks were observed on
the surfaces.
W, C, WC, W
2
C, and NiAl phases were detected as
common in WCCo and WCNi coatings. The NiAl phase
originated from the buffer layer, while the W, C, W
2
C
phases originated from the carbide coating HVOF pro-
cess.
The corrosion potentials of the NiAl, WCCo-NiAl,
WCNi-NiAl coatings are -340, -250, and -300 mV, re-
spectively. Of these materials, the WCCo-NiAl layered
sample is the most stable coating with respect to the
H
2
SO
4
corrosive environment. After the corrosion tests, a
number of holes and pits were observed from the sur-
faces of all the coatings. The S and O distribution is the
most dense and regular in the NiAl coating. The S and O
are rarely and irregularly distributed in the WCNi and
WCCo coatings.
Porosity was measured by using the electrochemical
technique. Depending on this, the porosity percentages
of NiAl, WCCo and WCNi coatings were determined to
be 8.24, 5.58 and 7.6, respectively.
An increase of the surface roughness casused a de-
creasing polarization resistance. The corrosion resistance
decreased with an increasing surface roughness for the
different coatings.
The thicknesses of the NiAl, WCCo-NiAl and
WCNi-NiAl layers were measured to be 719, 514 and
513 μm before the corrosion tests, respectively. These
thickness values of NiAl, WCCo-NiAl and WCNi-NiAl
coatings decreased to (626, 507 and 471) μm after the
corrosion tests, respectively.
5 CONCLUSIONS
The results showed that the WCCo-NiAl-coated
stainless-steel substrate had a higher corrosion resistance
to the H
2
SO
4
environment than the NiAl and WCNi-NiAl
samples. While the close densities of the cobalt and
nickel did not allow a realistic comparison of the pro-
cess, it was observed that cobalt had smaller particles in
the HVOF process due to its slightly higher melting
point. This resulted in a lower porosity rate in the coat-
ing. Moreover, increasing the porosity caused an increas-
ing corrosion rate. Therefore, the WCNi-NiAl showed a
lower corrosion resistant compared to the WCCo-NiAl
coatings, despite its emf series upper position. As a re-
sult, this article will increase the gains of the sector, with
the efforts to improve the erosion and corrosion behavior
of EN 1.4402 stainless steels, which are widely used in
the petrochemical industry, and to improve their service
life.
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