Int. J. Electrochem. Sci., 8 (2013) 1117 - 1137
International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.org
Review paper
Erosion - Corrosion of Cermet Coating
Magdy M. El Rayes1, 3, Hany S. Abdo1, 2, and Khalil Abdelrazek Khalil1, 2,*
1
Mechanical Engineering Department, King Saud University, P.O. Box 800, Riyadh 11421, Saudi
Arabia
2
Material Engineering and Design Department, Faculty of Energy Engineering, Aswan University,
Aswan, Egypt
3
On leave from Production Engineering Department, Faculty of Engineering, Alexandria University,
Egypt.
*
E-mail: kabdelmawgoud@ksu.edu.sa
Received: 25 November 2012 / Accepted: 18 December 2012 / Published: 1 January 2013
Cermet-based coatings are being increasingly used to combat erosion-corrosion in oil and gas
industries such that occurring in offshore piping, production systems and machinery involving fluid
and/or slurry flowing corrosive media which often contain solid particles such as sand. This leads to
material/ substrate damage caused by the combined surface degradation mechanisms of erosion and
corrosion. This review assesses the erosion-corrosion resistance and performance of cermet coatings
applied by different thermal spraying methods. Electrochemical measurements, which monitor the
erosion-corrosion mechanisms and coating integrity by themselves and when both erosion and
corrosion act simultaneously are considered. In addition, surface characterization, and the extent of
weight loss that covered through different combinations of cermet were reviewed. This paper also
discusses different types of substrates as well as the thermal spray coating processes that appeared in
the majority of publications such as atmospheric plasma spraying (APS) and electric arc spray (EArc)
with special emphasis on high velocity oxy-fuel (HVOF) with regard to cermets applied to enhance
erosion and corrosion resistance of the substrate. Electrochemical polarization measurements and salt
spray test to evaluate the erosion-corrosion mechanisms and coating integrity are used to quantify the
synergistic effects present when both erosion and corrosion acting simultaneously.
Keywords: HVOF; Corrosion; Erosion; Cermet coating
1. INTRODUCTION
Offshore oil and gas production environments represent aggressive conditions in terms of
erosion and corrosion. Consequently, materials selection must be given a detailed attention at every
stage of the design, construction and operation of systems and equipments including piping systems
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and their accessories, such as bends, elbows, tees, and valves. They also include devices that impart
movement to fluid such as pumps, impellers, propellers, and blowers, which are exposed to or
transport particle-laden fluids such as seawater. Therefore, full attention should be given to general
erosion and corrosion resistances, in order to minimize premature failures, which lead to loss of
production due to total shut downs and severe economic losses because of the inflating maintenance
costs. Even more important is the need to maintain offshore safety. Thus the specification and the use
of materials which combine erosion and corrosion resistances with high mechanical strength is a
fundamental requirement in industrial applications leading to be the focus of interest in numerous
researches. The electrochemical attack is caused by the surface condition and inherent nature of the
bare metal and the corrosive fluid. The protective film on the metal surface is swept away by rapid
movement of the processing fluid. [1, 2]. This review summarizes the various thermal spray coating
processes such as high velocity oxy fuel (HVOF), electric arc spray (EArc) and atmospheric plasma
spraying (APS) processes that are usually used in coating components subject to combined erosioncorrosion during service. Work in progress is reviewed to illustrate attempts being made to understand
the interaction between erosion and corrosion with the aim to allow robust surface selection for fluid
machinery and handling equipment, [3-5, 6-17]. Such coatings are applied by thermal spray
technologies. The microstructure of a thermally sprayed coating is usually inhomogeneous.
Discontinuities, such as pores, oxide lamellas or incomplete molten spray particles are typically
present in the sprayed materials. The deposition methods for the wear protective coatings are (APS)
and (HVOF) processes. Both of these methods have their own characteristics such as particle velocity
and flame temperature, which results a coating layer that has different microstructure and properties,
[3, 18].
The second purpose of this paper is to review: (a) the processing and characterization of
various thermal spray coating materials; (b) erosion properties and resistance of cermet composite
coatings; and (c) corrosion properties of the sprayed coatings strongly affect the materials loss rate
under wear corrosion conditions.
2. EROSION, CORROSION AND THEIR INTERACTION
Actually, both erosion and corrosion processes assist each other to bring about larger amount of
damage than the simple sum of the damage caused by each process separately [7]. The general area of
material selection for erosion-corrosion service therefore poses a complex problem and the potential
solutions offered in most literature are often reached by consideration of the independent erosion and
corrosion behavior [8]. By definition, erosion-corrosion is the acceleration in the rate of deterioration
of metal caused by the combined action of mechanical erosion and electrochemical attack. This
combined effect, often termed synergy, can lead to greater damage and higher metal loss rate beyond
that due to either erosion or corrosion alone and as a result can considerably shortens the service life of
components [9]. In addition, the erosion in specific not only affects the protective coatings itself but
may also damage the substrate, thus increasing the likelihood of substrate corrosion. This type of
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attack is known as erosion-corrosion [11, 14, 19]. The component of material loss under erosioncorrosion (T) is often represented by equation (1).
T=E+C+S
(1)
Where E is the material loss by pure mechanical erosion processes, C is the material loss by
electrochemical corrosion processes and S, the synergy, is the combined interaction between the two
processes. Thus, synergy is defined as “the difference between erosion-corrosion and the summation of
its two parts” and can be expressed by equations (2) and (3).
S = T − (E + C)
(2)
Synergy can be broken down into two components, ΔE and ΔC, where ΔE is the corrosionenhanced erosion (sometimes called enhanced erosion loss due to corrosion: ΔEC [18] and ΔC is the
erosion-enhanced corrosion (sometimes called enhanced corrosion loss due to erosion: ΔC E [18], as in
equation (3).
S = ΔE + ΔC.
(3)
Erosion can mechanically strip the protective corrosion film creating fresh reactive corrosion
sites, i.e., producing ΔC, which depends on the integrity of the film formed. In reference to earlier
publications, Wharton et al [14] have summarized the possible erosion-enhanced corrosion
mechanisms which include: (i) increased mass transport by high turbulence levels; (ii) lowering of
fatigue strength by corrosion; and (iii) surface roughening of the specimen during particle impact
causing enhanced mass transfer effects and increased corrosion rate. In addition Corrosion-enhanced
erosion mechanisms (ΔE) are also possible, including: (iv) the removal of work hardened surfaces by
corrosion processes which expose the underlying base metal to erosion mechanisms; (v) preferential
corrosive attack at grain boundaries resulting in grain loosening and eventual removal. Most of the
above mechanisms, if dominant, would be expected to lead to positive synergy but in some instances
negative synergy can occur.
3. SUBSTRATES
The study of erosion-corrosion properties of materials in corrosive environments has been the
object of great attention in recent years [6]. Several work investigating erosion-corrosion rates [20] and
mechanisms has been focused towards metallic materials, ranging from cast iron [21], carbon steels [6]
to the higher grades of austenitic [6, 7], super duplex stainless steels [8, 9], cast nickel- aluminumbronze (NAB) [11, 14] high-grade nickel-base [20] and cobalt-base Stellite-6 alloys [8]. It is
acknowledged that corrosion resistant alloys generally do not resist erosion well and the interactions
(synergy effects) that exist between corrosion and erosion are not comprehensively covered. This
presents a problem when erosion–corrosion resistant surface are being selected.
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4. CERMET COATINGS
4.1 Definition, types and function of cermet coatings
On account of the vulnerability of metallic materials in aggressive erosion-corrosion
conditions, there is a strong incentive for alternative surface engineering options to be developed and
implemented to more efficiently resist damage by this cause. Ceramic- metallic "Cermets" materials
are often considered in service involving high erosion and corrosion conditions and there has been
extensive consideration of thermal-sprayed cermets as surface coatings on conventional metallic
materials [20]. The ceramic particles provide a high erosion resistance. The metallic binder phase
makes the coating more ductile than a pure ceramic coating. The success of applying thermal spray
coatings for improvement of wear resistance has been well documented [5, 13, 22, 23]. Examples of
applications where both erosion and corrosion properties are involved is in the offshore structures and
components. Valve components and certain parts of pipes like contractions, bends, T-connections etc.
may be exposed to erosion and corrosion. In extreme cases, valves have been severely eroded within
hours of service. Increasing the life times for these components by improving both the erosion and
corrosion resistances, will improve the safety conditions, fewer shut-downs and large reductions in
maintenance costs [24].
There are various types of cermet and also many different grades within the different types.
Although cermet coatings have been around since the mid 1960’s, the coatings have evolved greatly
over the past 15 years. Today’s coatings are superior in all aspects including cost, bond strength,
reduced friction as well as corrosion and erosion resistance, [25]. Protection of the metallic
components by cermet is an effective method to reduce erosion and corrosion, however, it has been
stated [3] that cermet carbide coating; Cr3C2-NiCr, is an excellent replacement to hard oxide; Cr2O3.
Generally the cermet coatings consist of WC or Cr C particles embedded in a metal binder, which can
be a pure metal or a mixture consisting of Ni, Cr and Co. WC-Co and CrC-NiCr systems constitute
two main carbide materials used in thermal spraying processes in order to improve the erosion/wear
resistance and decrease the friction coefficient between various sliding components. Coatings of the
WC-Co system generally have a higher hardness and wear resistance than CrC-NiCr coatings [26],
however, the decarburization of WC into W2C, W3C and even metallic W phase leads to the
degradation of coating properties and limit the application of these coatings as well as due to the
dissolution of Co phase leading to low corrosion resistance [18]. Such coatings are applied by thermal
spray technologies. The microstructure of a thermally sprayed coating is usually inhomogeneous and
contains discontinuities, such as pores, oxide lamellas or incompletely molten spray particles all of
which may be present in the sprayed coating materials [3].
4.2 Performance of cermet coatings in erosive-corrosive environments
The CrC-NiCr system coatings are widely used in high temperature-wear resistance and
corrosion-resistant applications in aggressive environments such as oil and gas, aerospace and power
generating industries. The CrC-NiCr coatings can be used in corrosive environments at service
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temperatures up to 800 to 900 oC. The main shortcoming of CrC-NiCr coatings is a lower hardness
than WC-Co system coatings. At low erosive conditions the erosion–corrosion and corrosion resistance
of WC-Co-Cr coatings increase when increasing the Cr content in the metallic binder from 5 to 8.5 w t.
% [12]. At high erosive conditions an increase of Cr content in the metallic binder from 5 t o 8.5 wt.%
did not give any increase of the erosion- corrosion and corrosion resistance of WC- Co- Cr coatings. A
reduction of the heat input during spraying reduced the degree of WC decomposition and improved the
coating properties when the spray powder contained a large fraction of small grains.
De Souza et al [27] have focused on understanding the synergy effect (defined as the
enhancement of erosion due to corrosion effects) on material loss of WC- Co- Cr thermally sprayed
coating when two different microstructures are formed and also the influence of chemical composition
of the coating. These microstructures resulted from the application of two thermal spraying techniques;
namely, HVOF and Super Detonation-Gun (D-gun) process. Experiments showed that HVOF coatings
have a slightly lower corrosion resistance than the (D-Gun) coatings but higher overall erosion–
corrosion resistance. They concluded that different microstructures of thermal spray coatings lead to
different erosion-corrosion resistances; hence the degradation rates and mechanisms are also different.
The synergy of erosion-corrosion behavior of the coatings can change depending on the environment
(sand loading), composition and microstructure. The formation of different tungsten carbides and a
higher Cr amount can reduce the toughness of WC-Co-Cr coatings and reduce the corrosion rates
under erosion–corrosion.
In other work [28], the authors have isolated the electrochemical and mechanical factors which
affect the material degradation under erosion-corrosion environments as a means of understanding the
degradation mechanisms and therefore moving towards coating improvement. The coating was WCCo-Cr thermally sprayed using HVOF which was compared to austenitic and super duplex stainless
steels. In this work it was demonstrated that the benefits of this type of coatings is dependant on
environment severity and can provide good protection against erosion and corrosion in liquid–solid
impingement when compared with stainless steels [28, 29]. The role of corrosion and synergy in the
total damage on WC-Co-Cr- HVOF coating is more crucial than on the super duplex stainless steel. It
was also found that the corrosion of small hard phase particles WC can accelerate the material loss
under erosion–corrosion environments and is one important feature of the synergy effect. Results have
shown that the damage of WC-Co-Cr- HVOF coating moves towards and falls into the corrosiondominated regime meaning that there is an obvious potential benefit to be exploited if the corrosion
component of damage, and in turn the synergy can be reduced [28]. De Souza et al [29], on the other
hand, have concluded that the mechanisms of damage are dominated by erosion processes but
corrosion is affected by erosion processes and is more important at the lower solid levels.
R. Wood [18] has reviewed some practical concerns when using WC- Co- Cr - HVOF coatings
such as the level of porosity within the coatings which can accelerate crack propagation and coating
removal under erosion and also can be interconnected such that electrolyte can permeate into the
coating/substrate interface accelerating corrosion and corrosion driven coating-substrate de-bonding.
An additional concern with multiphase coatings (carbide/metal matrix) is the potential for de-bonding
between hard phase and softer matrix that can accelerate surface degradation. The role of corrosion and
synergy in the erosion-corrosion degradation of WC-Co-Cr- HVOF thermally sprayed coatings have
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been studied [28] using electrochemical polarization techniques in saline solutions (3.5% NaCl) at
varying temperatures. It was reported that corrosion proceeds, in the first instance, primarily by
dissolution of the Co phase, depending on the integrity and thickness of the passive film on the binder
phase (Cr2O3). Similar observations are reported for other compositions of thermally sprayed cermet
coatings (WC-CrNi and WC/CrC- CoCr) [20]. The dissolution of the binder matrix phase at the hard
phase/matrix interface is known mechanisms for synergy contributing to ΔE. The action of erosion is
also likely to weaken the hard phase/matrix interface and after cyclic loading from repeated solid
particle impacts could lead to crack initiation at the interface. Permeation of electrolyte into these
cracks could induce localized environments which are dramatically different (i.e low pH) from the
bulk conditions and induce crevice corrosion which in turn contributes to increased ΔC levels.
Improvements in coating durability for erosion-corrosion applications are therefore more likely
if the corrosion resistance can be enhanced. This can be achieved, as proposed [28], by improving the
binder integrity by alloying, improving the hard phase stability and improving the integrity of the hard
phase/binder interface. Coating Cr3C2-NiCr thermal spray coating using HVOF appear to be a better
alternative to WC- Co-Cr in most cases mainly when better erosion or corrosion is required and
therefore this type of coating was the focus of interest of some publications. N. Espallargas et al. [30]
have compared two HVOF thermal spray coatings (Cr3C2-NiCr and WC-Ni) with the conventional
hard chromium coatings. The coatings compositions were 80 Cr3C2-20 NiCr and 88 WC-12 Ni
respectively. Both of these coatings were found to be promising alternatives to hard chromium from
the point of view of erosion-corrosion resistance. At high erosive conditions, the coating structure and
hardness play an important role in the erosion-corrosion mechanisms. The anisotropic behavior of the
materials led to a higher material loss for lamellar structure; resulting from layer by layer deposition
parallel to the substrate, than for the columnar one; resulting from Cr growth perpendicular to the
substrate, when comparing hard chromium coatings and Cr3C2-NiCr coatings. WC-Ni coatings gave
the lowest material loss due to its high hardness. It was also found that at high erosive conditions, the
microstructure of WC coatings was responsible of its high erosion-corrosion resistance compared with
Cr3C2 and hard chromium coatings. The reason is due to the fine and well distributed WC particles in
the Ni binder. Electrochemical measurements, however, showed that Cr3C2-NiCr coatings were
superior with respect to corrosion resistance compared to WC-Ni under both erosive conditions.
The effect of coating thickness on the corrosion behavior of thermally sprayed Cr3C2-NiCr
HVOF coatings has been studied [15]. Thicker coatings permit the pass of the electrolyte due to the
stresses generated during coating deposition and the corresponding crack formation between different
layers. Thinner coating let the electrolyte go through the coating because it is not thick enough to
correctly protect the base steel. Hence, it was concluded that the optimization of spraying parameters
and stress relaxation processes will be as important as thickness when protection of the base steel is
needed with cermet coatings.
4.3 Mechanical properties of cermet coatings
The porosity and weak interface adversely affect the erosion property and the cracks allow
corrosive substance in the environment to attach the protective coating [31]. One of the important
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applications of HVOF coatings is their use in dynamic components in various off-shore and oil and gas
industrial equipments [32]. In service, these components are subjected to severe cyclic loading under
an aggressive environment. Consequently, investigations of fatigue properties of HVOF coating are of
utmost importance. This led to considerable research studies have been carried out to investigate
various affecting parameters on the fatigue properties of HVOF-coated surfaces.
The effects of Cr3C2-25NiCr and WC-10Ni- HVOF coatings and hard chromium electroplating
on the fatigue strength, abrasive wear and corrosion resistance of AISI 4340 steel was evaluated [33].
Cr3C2-25NiCr results in higher fatigue strength when compared to chromium electroplated coatings.
With respect to WC-10Ni thermal spray coated, insignificant influence on the fatigue strength was
detected. Salt spray test results showed that Cr3C2-25NiCr HVOF thermally sprayed coating has better
corrosion resistance in comparison to WC-10Ni. It was also concluded that coatings Cr3C2- 25NiCr
and WC-10Ni presented better abrasive wear resistance with lower wear weight loss than chromium
electroplated.
In the HVOF process, it is the end-product quality that matters; in this case, coating structural
homogeneity, adherence to the substrate underneath, and operational durability are the main concerns
in ensuring the coating quality [16]. Consequently, investigations into the mechanical properties of
coating becomes important for improving the durability of coating and also in order to better
understand the influence of composition and microstructure on these properties, it is necessary to
evaluate quantitatively the mechanical properties of the coatings [17]. Mechanical properties of HVOF
coating were investigated by Brandt [34]. He showed that carbide coatings by the HVOF process with
porosity levels of less than 1% behave like a homogeneous material with improved ductility. Fracture
toughness of HVOF-sprayed WC–Co coating was investigated by De Palo et al. [35]. They indicated
that Vickers indentation method was useful and it became a convenient technique for fracture
toughness measurement of coatings. Fatigue properties of a 4340 steel with HVOF coating were
studied by Herna´ndez et al. [36]. They showed that crack nucleation sites are associated with the
presence of alumina particles left over from the grit blasting prior to coating. Mechanical properties
and residual stress distribution of thermal spray coating were examined [37]. They showed that HVOF
coating had more rigidity than coating by the atmospheric plasma spraying process. Fatigue behavior
of HVOF-coated 4140 steel was investigated [38]. It was indicated that the possible existence of
tensile residual stresses in the vicinity of the substrate–coating interface would assist in the
propagation of the fatigue cracks nucleated at the alumina particles.
4.4. Applications of cermet coating
A new application for thermally sprayed cermet coatings is as replacements for hard chrome
plating. Hard chrome plating can produce a wear resistant coating with good surface finish at cost
effective price. However, there are growing environmental concerns associated with the disposal of the
effluents from the used plating solution and these concerns have caused the cost of the process to
increase. Cermet coatings have a wear resistance which is between 2.5 and 5 times better than hard
chrome plating and do not suffer from effluent disposal problems. They are therefore finding
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increasing use at the expense of hard chrome plating, particularly if wear resistance is important or if a
thick coating is required on a large part, [25]. In corrosive media the wear resistance of cermet coatings
depends on the corrosion resistance of the metallic binder. Other factors influencing the coating
deterioration are the corrosivity of the media and any galvanic interaction from the surrounding
material. The corrosion resistance of the metallic binder should be comparable to the material of the
rest of the system. This is especially important when the surrounding materials are corrosion resistant
alloys as stainless steels, where the coatings otherwise will act as an anode. [24]
5. TYPES OF THERMAL SPRAY COATING PROCESSES
Here are some of thermal spray coating processes, for example: APS is the most common
thermal spray process to deposit ceramic coatings. However, HVOF methods can produce metallic
coatings with low porosity and excellent wear resistance. The deposition methods most frequently used
for the erosion protective coatings are super detonation gun (D-gun), the least appearing in literature,
air plasma spraying (APS), and high- velocity oxygen-fuel (HVOF) flame spray processes. In the Dgun the gases (acetylene and oxygen) are mixed along with a pulse of powder introduced into the
barrel. Detonation using a spark generates waves of high temperature and pressure which heat the
powder particles to their melting point or above. Particle velocities of about 750 m/s, can be achieved.
This process is a non-continuous process by the fact that after each detonation the barrel is purged with
nitrogen and the process is repeated at up to 10 times per second [27]. The APS process is basically the
spraying of molten material onto a surface to form a coating [47]. Sprayed material in the form of
powder is injected into a very high temperature plasma flame, where it is rapidly heated and
accelerated. The melted droplets would impact on the substrate surface, flatten, spread and rapidly cool
down, forming the so-called splats. The final coating consists of number of splats. However, the layers
deposited by plasma spray process have some disadvantages, e.g. micro-cracks, poor adhesion between
the coating and substrate, phase changes due to high-temperature exposure, non-uniformity in the
coating density, and improper microstructural control, which could result in failure of the implanted
system. The HVOF process [27] comprises a mixture of fuel (propane, propylene, hydrogen or
acetylene) and oxygen which are burned in the chamber and because of the expansion the gas velocity
can become supersonic. Powder is introduced axially, heated, melted and accelerated. The powder
normally reaches velocities of around 550 m/s. A great advantage of HVOF on conventional thermal
spraying such as APS is high particle velocity and low thermal energy [10] (lower temperature (1900–
3000K)) which reduces the chance of carbide particles changing or oxidizing during the process
(decarburization) [27].
Generally these methods have their own characteristics such as different spray particle
velocities and temperatures which results in coatings having different microstructure and properties
[3]. During the spraying processes and the cooling of deposits complex chemical transformations of
the materials occur. The main phenomena which occur during APS or HVOF spraying of cermets are
the thermal decomposition of the tungsten monocarbide WC or chromium carbide Cr3C2 and the
carbide reactions with the metallic binder [3]. The decarburization of WC, followed by the formation
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of undesirable carbides like W2C, complex Co-W-C and metallic tungsten occur often during APS
because of the high temperature of the plasma flame and the oxidizing spray atmosphere. With a
significant lower flame temperature and higher particle velocity the HVOF spraying leads to less phase
transformation and produces denser coatings with lower porosity. Due to the technology improvement
that has occurred during the last years, HVOF provides coatings with better compaction and low
chemical decomposition, especially for WC- based coatings [5]. In addition, one of the great
advantages of the HVOF process is the higher velocity reached by the particles and the low
temperatures involved which minimizes any potentially damaging effects to the coating and substrate
[28].
5.1 High Velocity Oxy-fuel (HVOF) spray coating
This method is the state-of-art-technology in the heat spray sphere where oxygen and kerosene
are the heat energy source. Typical materials for HVOF spraying are cermets (ceramic-metal), most
often tungsten carbides and chrome based. Wear and tear resistant and corrosion resistant coatings in
different environments are typical applications, [20–39]. The high velocity oxygen fuel (HVOF)
powder spray process represents the state-of-the-art for thermal spray metallic coatings and can result
in very dense, tightly adherent coatings with little or no oxidation during the application and low
residual stresses, [23]. Deposition of coatings by thermally activated processes like HVOF thermal
spraying has been successfully used for producing nanocrystalline (NC) coatings. Nanostructures
promote selective oxidation, forming a protective oxide scale with superior adhesion to the substrate,
[4]. Ceramic coatings are attractive as they possess good thermal and electrical properties, and are
more resistant to oxidation, corrosion, erosion and wear than metals in high-temperature environments.
Nanoparticles of diamond as well as chemical compounds used for hard coatings (SiC, ZrO2, and
A12O3) are commercially available, with typical particle sizes in the range 4-300 nm. Within tribology,
a new development has been to deposit nano-coatings from colloids, e.g. of graphite. Nano-sized silica
has proved to be an alternative to toxic chromate conversion coating. [4, 22]
The HVOF spraying WC-based cermet hard coatings such as WC-Co, WC-CoCr and others
have been investigated by W. Fang et al [5] for obtaining the coatings of high hardness, wear
resistance, thermal stability and corrosion resistance. The surface properties, such as microstructure,
hardness and porosity of WC-CrC-Ni coatings prepared by optimal coating process (OCP) have been
investigated. In particular, the friction and wear behaviors are analyzed for the WC-CrC-Ni coatings,
EHC (electrolytic hard chrome) and the substrate Inconel 718 (IN 718) both at 25 and 450 ◦C. They
found that the HVOF WC–CrC–Ni coating is very protective for alloy surface. [5]
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Figure 1. HVOF spray coating device [40].
1. Added material – Powder
2. Kerosene inlet
3. Oxygen inlet
4. Spray
5. Background
6. Sparkling plug
Substitution of hard chromium coatings with new HVOF cermet coatings has been studied by
L. Fedrizzi et al [23] and they found that this process involves very high benefits for the environment,
as the proposed HVOF technique allows to substitute some highly polluting surface treatment
technologies, such as chromium-plating, with a perfectly “clean” process from an environmental point
of view. In addition, the replacement should bring some important benefits such as sensible reduction
in wastewater pollution caused by chromium-plating processes and the increase of the performance
(corrosion and wear resistance) with respect to chromium plated. Tribo-corrosion phenomena
involving mechano-chemical degradation were studied using electrochemical and weight loss
measurements. The apparatus used to study wear-corrosion has been very effective because the
combination of both electrochemical and mechanical analyses allowed analyzing the degradation
mechanisms, [41-43]. Hard chromium degradation was found to be determined mainly by a wear of an
adhesive type mechanism. But weight loss measurements clearly showed a synergistic effect due to the
combined wear and corrosion degradation. Electrochemical data suggested that the corrosion rate of
chromium coatings is increased by almost one order of magnitude by the mechanical damage.
Degradation mechanisms of the HVOF coatings appeared to be quite different. In this case the
presence of a large ceramic component in the composite coating made the corrosion degradation less
important. The active-passive behavior is really important for the hard chromium coating and is no
more fundamental in the case of the HVOF coating even if the metal matrix is NiCr made.
The use of nano-sized powders improves the good behavior of the conventional powders
mainly because of a decrease of the interconnected porosity, a lower roughness, and a better
distribution of the chromium carbides in the metal matrix, [23].
5.2 Electric Arc Spray (E Arc) Coating
In the electric arc spray process (also known as the wire arc process), two consumable wire
electrodes connected to a high-current direct-current (dc) power source are fed into the gun and meet,
establishing an arc between them that melts the tips of the wires. The molten metal is then atomized
and propelled toward the substrate by a stream of air. The process is energy efficient because all of the
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input energy is used to melt the metal. Spray rates are driven primarily by operating current and vary
as a function of both melting point and conductivity. Generally materials such as copper-base and ironbase alloys spray at 4.5 kg (10 lb)/100 A/h. Zinc sprays at 11 kg (25 lb)/100 A/h. Substrate
temperatures can be very low, because no hot jet of gas is directed toward the substrate. Electric arc
spraying also can be carried out using inert gases or in a controlled-atmosphere chamber, [44]. Another
consequence of the highly localized heating is that the heat input to the substrate is low, primarily
because there is no hot gas or plasma jet directed at the substrate. Hence, wire arc spraying can be used
to form coatings on materials such as polymers that would not withstand the heat input from other
thermal spray processes, [45].
As shown in Fig 2 during the electric arc spray is the added material melted while coming into
the spray gun as two wires. The compressed air flow accelerates the melted material and sprays it on
the prepared surface of machine part. Added material wires can be either from the same material or
different composition. Using nitrogen or argon can reduce the oxidation of such surface. [40]
Figure 2. Electrical arc coating device.
1. Added material - Wire No. 1
2. Added material - Wire No. 2 (same or different as wire No. 1)
3. Background
4. Surface
5.3 Atmospheric Plasma Spray (APS) Coating
This is the state-of-art-technology of surfacing which subject matter is the electric arc
generated between tungsten electrode and surfacing material. During this generation is from incoming
inert gas - argon - created a high concentrated plasma flow of high temperature. The powder material
is added in this flow and creates the surface, [39,46]. The conventional plasma spray process is
commonly referred to as air or atmospheric plasma spray (APS). Plasma temperatures in the powder
heating region range from about 6000 to 15,000 °C (11,000 to 27,000 °F), significantly above the
melting point of any known material. To generate the plasma, an inert gas-typically argon or an argonhydrogen mixture is superheated by a dc arc. Powder feedstock is introduced via an inert carrier gas
and is accelerated toward the workpiece by the plasma jet. Provisions for cooling or regulating the
spray rate may be required to maintain substrate temperatures in the 95 to 205 °C (200 to 400 °F)
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range. Commercial plasma spray guns operate in the range of 20 to 200 kW. Accordingly, spray rates
greatly depend on gun design, plasma gases, powder injection schemes, and materials properties,
particularly particle characteristics such as size, distribution, melting point, morphology, and apparent
density, [44]. Ying Chun Zhu et al, [46] have characterized nanostructured WC–Co coating deposited
by (APS). The result shows that the structure of the plasma sprayed WC–Co coating is very
complicated. The main structure of the coating is composed of WC grains with a mean particle size of
35 nm. In some regions, the structure is composed of WC grains with a mean particle size of 10 nm
embedded in an amorphous matrix, which is formed by the melting of the WC–Co powders. Moreover,
some regions of the coating are constituted completely of amorphous phase. It was also found that WC
grains have grown to 100 nm in some regions of the coating. Second recrystallization occurred, stripshaped and square shaped structures are formed in some regions of the nanostructured WC coating.
The as-prepared WC-Co coating is composed mainly of WC phase with minor phases of a-W2C, bWC1-x, and W3Co3C. The hardness of nano WC-Co coating is about 18 GPa, which is apparently
improved comparing with conventional WC-Co coatings, [46].
Figure 3. APS coating principles.
1. Added material – powder, 2. Plasma gas – argon
3. Gaseous shield - nitrogen
4. Tungsten electrode
4. Surface
5. Background
Table 1. Comparison of Thermal Spraying Processes and Coating Characteristics
Process
Particle
Velocity
(m/s)
Adhesion
(MPa)
Oxide
Content
(%)
Porosity
(%)
Arc
Plasma
HVOF
100
200 – 300
600 – 800
10 – 30
20 – 70
> 70
10 – 20
1–3
1–2
5 – 10
1–8
1–2
Depositio Typical
n
Rate Deposit
(kg/hr)
Thickness
(mm)
6 – 60
0.2 – 10
1–5
0.2 – 2
1–5
0.2 – 2
Int. J. Electrochem. Sci., Vol. 8, 2013
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The influence of metallic matrix composition and spray powder size distribution of different
WC-Co-Cr coating compositions deposited by HVOF process on the erosion-corrosion properties have
been studied [12]. It has been found that powders having a narrow powder grain size distribution give
coatings of higher quality than powders with wider grain size distributions. This is explained by the
different melting behavior of powder grains of different size. Small grains are more easily over heated
than larger grains. Over heating may give phases with low erosion resistance and therefore coatings of
poor quality.
Nanocrystalline coatings with grain sizes in the nanometer range are also known to exhibit
superior hardness and strength. The search for nanostructured coatings is driven by the improvement in
coating technologies and the availability of various kinds of synthesized nanopowders. Such
nanopowders can be used as feedstock materials for thermal spray processes; these include plasma
spraying and HVOF spraying. Thermal spraying involves particle melting, rapid cooling and
consolidation in a single-step operation. Thermal-sprayed nanocrystalline coatings with moderate
hardness are found to possess better wear performances than their counterparts fabricated from
microcrystalline powders. HVOF is particularly suited to deposit dense nanocrystalline ceramic
coatings as opposed to plasma spraying because of its lower spraying temperature. Today, HVOF
allows tailoring nanocrystalline coatings with low porosity, higher bond strength and increased wear
properties, [48].
5.4. Benefits of Thermal Spraying
Among all thermal spraying processes, the main benefits of these processes can be summarized
as follows:
1.
Comprehensive choice of coating materials: metals, alloys, ceramics, cermets and
carbides.
2.
Thick coatings can be applied at high deposition rates.
3.
Coatings are mechanically bonded to the substrate-can often spray coating materials
which are metallurgically incompatible with the substrate, e.g., materials with a higher melting point
than the substrate.
4.
Components can be sprayed with little or no pre- or post-heat treatment, and component
distortion is minimal.
5.
Parts can be rebuilt quickly and at low cost, and usually at a fraction of the price of a
replacement.
6.
By using a premium material for the thermal spray coating, coated components can
outlive new parts.
7.
Thermal spray coatings may be applied both manually and automatically.
5.5. Advantages of coating
The general advantages of coating applied by thermal-spraying processes have been
summarized in earlier work [49]. These advantages include:
Int. J. Electrochem. Sci., Vol. 8, 2013
1130
1.
Protection of equipment and structures from the environment by acting as a barrier
between the substrate and the aggressive environment, such as the marine and industrial environments.
2.
Control of marine fouls; certain constituents in coating control the growth of mildew
and marine fouling in seawater.
3.
Reduction in friction; coating reduces friction between two contacting surfaces.
4.
Pleasant appearance; certain types of coatings provide a pleasant appearance and
produce attractive surroundings.
5.
Visibility; many combinations of colors because of their visibility from large distances
are used on TV and radio towers to warn aircraft.
6.
Modification of chemical, mechanical, thermal, electronic and optical properties of
materials.
7.
Application of thin coatings on low-cost substrates results in increased efficiency and
cost savings.
5.6. Erosion Test
Erosion–corrosion related problems occur in power plants, oil and gas processing and chemical
plants where there is an interaction between solid particles, corrosive fluid and a target material. The
problem has been reported to affect static equipment for example pipelines, valves, heat exchangers,
pressure vessels and various rotating equipment namely compressors, turbines and pumps. The
importance of material selection for applications in these environments cannot be overstated as
component wear can be accelerated by the aggressive conditions in these harsh environments. Synergy
is the additional wear rate experienced by a metal under the combined action of erosion-corrosion
conditions which is higher than the sum of wear rate due to pure erosion and flow corrosion, [19].
Figure 4. Assembled slurry pot erosion tester enclosed in a Faraday cage (pot capacity 4.0 L) [19].
S.S. Rajahram et al [19] used a slurry pot erosion tester to perform erosion- corrosion
experiments. Fig. 4 [19] shows the diagram of the assembled slurry pot enclosed in the Faraday cage.
Int. J. Electrochem. Sci., Vol. 8, 2013
1131
The rig is driven by a 3.5kW motor which is connected to the slurry shaft through a toothed belt and
two pulleys (on the shaft and on the motor).
Cylindrical test samples are inserted between two nylon-coated arms at the end of the shaft as
shown in Fig. 5 [19]. The speed of the motor is controlled through a variable speed drive with
maximum rotation speed of up to 3500rpm. The pot is made of uPVC with a maximum capacity of 4
litres and has a cup type design copper cooler which allows the temperature of the slurry to be
controlled by the circulation of hot/coldwater. The pot is designed with baffles in it, to allow mixing of
solid particles in the slurry, preventing it from settling at the bottom of the pot. The rig assembly is
enclosed within a Faraday cage which allows electrochemical measurements to be made and also acts
as a protective safety barrier when running experiments [19]. It was found that the measurements and
slurry pot erosion rig provide repeatable and reproducible test results with high confidence levels.
Increasing the velocity and the sand concentration produced higher mass loss rates in erosion
conditions. The increase in kinetic energy of the particles is suggested to be the reason for the higher
mass loss rates.
Figure 5. Placement of samples on two nylon-coated arms, secured with o-rings to prevent slurry
ingress [19].
6. ELECTROCHEMICAL CORROSION TEST
With the increased use of cermet coatings and solid cermets in applications where corrosion
can play a part in the degradation process, it is becoming increasingly important to be able to assess the
effects of the joint corrosion as the same in erosion processes.
V.A. de Souza, A. Neville [29] used electrochemical analysis in conjunction with weight-loss
analysis to determine the total material loss (TWL) and to isolate the contributions due to pure
corrosion (C). The corrosion rate was measured in situ using a three-electrode electrochemical cell
comprising a Ag/AgCl reference electrode connected by means of a salt bridge and a platinum counter
electrode. DC anodic polarization tests (in static conditions or under the impinging jet) involved
scanning the potential of the working electrode (the specimen under examination) from the free
corrosion potential (Ecorr) in the more noble (positive) direction at a fixed rate of 0.25 mV/s. The
Int. J. Electrochem. Sci., Vol. 8, 2013
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potential was scanned in the positive direction until the current flowing in the external circuit between
the working and counter electrodes reached a value of 500 A/cm2. The anodic polarization tests were
started after 30 min exposure to static saline solution or the impinging jet. [29]
The impingement apparatus comprised a liquid–solid jet generated using a re-circulating rig as
shown in Fig. 6 and described elsewhere [29,50]. The rig comprised a dual nozzle system. They
demonstrated that WC–CoCr thermal sprayed coatings can provide good protection against wear and
corrosion in liquid–solid impingement when compared with stainless steels.
Figure 6. (a) The rig configuration used in the experiments and (b) the electrochemical set up on the
nozzle [29,50].
7. EROSION-CORROSION TEST
It was found in various publications [28–30, 49, 51] that the total degradation due to erosioncorrosion has been divided into three principal components as defined below:
1133
Int. J. Electrochem. Sci., Vol. 8, 2013
(a) Corrosion (C): electrochemical charge transfer leads to material loss. The charge transfer
rate can be accentuated by the increased mass transfer or mechanical impacts as a result of an
impinging flow.
(b) Erosion (E): mechanical damage due to impacts of a high-energy flow or suspended solids
within a flow. The material removal does not involve any corrosion processes.
(c) Interactions or synergy (S): the enhanced material damage as a result of corrosion
enhancing erosion. In this instance the corrosion processes affect the integrity of the material and
render it more susceptible to mechanical damage.
6.1. Experimental details
N. Espallargas et al [30] used in their investigation the erosion–corrosion equipment shown in
Fig. 7 for testing Cr3C2–NiCr and WC–Ni coatings obtained by HVOF.
Figure 7. Scheme of the erosion–corrosion equipment, [30].
Six cylindrical samples were fixed to a rotating disk. The disk was rotated in a 65 L mixture
made of 3.4 wt.% NaCl solution and 0.25 wt.% silica sand with average grain size 250 μm. The
samples were electrically insulated from the disk and from each other. For each sample a conductor
through an Hg cup was connected to a potentiostat for electrochemical measurements. The erosivity
was varied by applying different rotation velocities: 14.3 and 22.9 m s-1.
Table 2. Conditions used for erosion-corrosion tests, [30]
Test Solution
Velocity (m s-1)
Condition A
Condition B
Mixture of 0.25 wt.% sand and 3.4 wt.%
NaCl
22.9
14.3
Temperature (oC)
20
20
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The total material loss caused by erosion–corrosion was determined by weighing the samples
before and after the tests. All tests were run for 24 h. The experimental conditions of erosion–corrosion
experiments are summarized in Table 1. The samples were exposed to 0–90° impact angles due to their
cylindrical shape and the impeller-like flow. [30]
The corrosion resistance of all coatings was evaluated by electrochemical measurements. The
electrochemical measurements were done both during and after rotation. A saturated calomel electrode
(SCE) was used as a reference electrode and the 22Cr–5Ni (wt.%) duplex stainless steel wall of the
apparatus was the auxiliary electrode. Each coated sample (working electrode) was fixed at the rotating
disk, exposing an area of 4.71 cm2 to the solution, [30].
They clearly notify in their study that the coating structure and hardness play an important role
in the erosion–corrosion mechanisms at high erosive conditions. At high erosive conditions, the
microstructure of tungsten carbide coatings was responsible of its high erosion–corrosion resistance
compared with chromium carbide and hard chromium coatings. The relative importance of erosion and
corrosion should be considered when selecting coating material for erosion–corrosion resistance. WCbased coatings show better wear resistance, resulting on a better erosion–corrosion resistance under the
most erosive conditions. The best corrosion performance of Cr3C2–NiCr coatings also places them as a
good alternative to hard chromium coatings under lower erosive erosion–corrosion conditions.
Chengzhi Zhuo et al [51] are focused in their study on investigating the corrosion and erosioncorrosion behaviors of two kinds of nano-particle-reinforced Ni-Cr-Mo-Cu alloying layers in slurry
flow environment. An electrochemical impedance spectroscopy (EIS) was measured to study the
effects of the different particulates on corrosion and erosion-corrosion behaviors. They found that with
increasing the impact velocity and content of sand particles under hydrodynamic conditions, the
current densities increase with fluctuations appearing for 316L stainless steel, single alloying layer and
composite alloying layer. Also the results of polarization curve measurements obtained under slurry
flow conditions exhibit an increase in corrosion current density and decrease in corrosion potential
compared with that of obtained under static state condition, [51]. Furthermore, V.A.D. Souza, A.
Neville [28] have reported the following:
•
WC Co Cr thermal sprayed coatings can provide good protection against wear and
corrosion in liquid–solid impingement when compared with stainless steels. The extent of the benefits
offered by WC Co Cr is dependent on environment severity.
•
The role of corrosion and synergy in the total damage is important on WC Co Cr HVOF
coating, much more so than on the Super duplex stainless steel.
•
The corrosion of small hard phase particles (WC) can accelerate the material loss under
erosion–corrosion environments and is one important feature of the synergy effect.
•
Because corrosion and synergy play an important role in the total damage of WC Co Cr
there is scope to improve the overall erosion–corrosion performance by enhancing the corrosion
behavior.
•
They also demonstrated in another study [27, 29] that different microstructures of these
thermal spray coatings mean that in erosion–corrosion the degradations rates and mechanisms are
different. It is important to understand how erosion and corrosion factors interact. In particular:
Int. J. Electrochem. Sci., Vol. 8, 2013
1135
•
Synergy (Ec) behavior of the coatings can change depending on the environment
(sand loading), composition and microstructure.
•
The formation of different tungsten species, eta phases and a higher Cr amount can
reduce the toughness of WC–Co–Cr coatings and reduce the corrosion rates under erosion–corrosion.
G.C. Saha et al [51] used in their investigation the impingement jet system shown in Fig. 8
which developed to perform erosion–corrosion tests. The system consisted of a plastic tank used as a
reservoir, a high pressure pump, a flow velocity controller, a sand concentration controller, a stirrer,
and valves. When the fluid entered the ejector at a high speed, it produced a partial vacuum due to the
venting effect. The sands underneath the valve could be mixed with the flowing fluid by means of
suction. A speed-adjustable mechanical stirrer was used to ensure the homogeneous mixing of sands in
the solution. An electrochemical cell was incorporated into the test rig to enable in-situ electrochemical
measurements. In their study the erosion–corrosion resistant behavior of a near-nanocrystalline ‘duplex
Co coated’ WC-17Co coating produced by HVOF spraying was compared with a microcrystalline
WC-17Co coating and an uncoated AISI 1018 carbon steel. The results showed that the combined
erosion–corrosion resistance of the coated coatings was significantly higher than that of the uncoated
steel. Furthermore, the near-nanocrystalline coating showed approximately 1/3 lower erosion–
corrosion rate than that of the microcrystalline coating. Preliminary results showed that the erosion–
corrosion mechanism in the coatings was dominated by pure erosion in the microcrystalline coating
and the corrosion- enhanced erosion in the near-nanocrystalline coating. [51]
Figure 8. Schematic diagram of the impingement jet loop system (RE, reference electrode; WE,
working electrode; CE, counter electrode). [51].
8. SUMMARY AND CONCLUSIONS
There are several papers in the literature dealing separately with the study of corrosion and
wear resistance for different thermal spray coatings. Corrosion properties of thermal spray coatings in
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1136
a corrosive media have been previously studied by different authors. They found that the absence of
pores and cracks (micro and macro) is very important when corrosion resistance is required because
the electrolyte penetrates through these defects to reach the substrate. When the substrate is less noble
than the coating, galvanic effects can be found between coating and substrate, resulting in a significant
attack of the substrate material. On the other hand, if the substrate is more noble than the coating (i.e.
stainless steel) the coating acts as a sacrificial anode accelerating its corrosion.
It was concluded that chemical composition of metallic binder materials and the occurrence of
micro cracks were the most important factors influencing the corrosion resistance of the HVOF
sprayed WC cermet coatings in the strong acidic environment.
ACKNOWLEDGEMENTS
The authors would like to express their sincere thanks to the National Plan for Science and Technology
(NPST), King Saud University for financially supporting this work in the Project No. 10-ADV1033-02
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