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Abstract

The Hot corrosion is the main and severe problem which can be controlled by thermal spray coatings. The various Corrosion control measures include Surface Heat Treatment, Engineering Paints, Vitreous Enamelling, Cladding, Powder coatings, Zinc coatings, Tin Plate, Electroplating, Cadmium Plating, Anodising (Anodizing), Thermal Spray Coatings., Plasma Nitriding/Carburising/Boronising., Pack Cementation, Ion Implantation, Ceramic and Cermet materials., Chemical Vapour Deposition, Physical Vapour Deposition. The demand for protective coatings has increased recently for almost all types of super alloys with improved strength, since high-temperature corrosion problems become much more significant for these alloys with increasing operating temperatures of modern heat engines. The Major areas where coatings have the application are Power generation Industries, Ceramics Industries, Chemical Industries, Iron & steel Industries and Mining Industries etc. Open or closed porosity in thermal spray coatings can originate from several different factors: partially or totally unmolten particles, inadequate flow or fragmentation of the molten particle at impact, shadowing effects due to lower than the optimal spray angle, and entrapped gas. The interconnected (open) porosity allows the corrosive media to reach the coating-substrate interface, which eventually leads to delamination of the coatings. Although the development of the modern thermal spray processes has decreased coating porosities, the transport of corrosive species to the substrate can still only be prevented by coating post treatment. Therefore it's of actual significance to develop an effective method to post treat the thermal spray coatings to enhance their life in corrosive environment. In this paper author has reviewed the significance of heat treatment in thermal spray coatings for improving their properties and has made an attempt to explore the potential of heat treatment process in thermal spray coatings.
National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8,2011)
Punjab Technical University, Jalandhar
HEAT TREATMENT OF THERMAL SPRAY COATINGS: A REVIEW
Kovid Sharmaa*, Sukhpal Singh Chathaa, Hazoor Singha , Harkulvinder Singha
a Department of Mechanical Engineering,Yadavindra College of Engineering, Punjabi University, Guru Kashi
Campus, Talwandi Sabo, Bathinda,Punjab-151302,India
*Corresponding author’s E-mail: kovids@yahoo.com
ABSTRACT:
The Hot corrosion is the main and severe problem which can be controlled by thermal spray
coatings. The various Corrosion control measures include Surface Heat Treatment, Engineering
Paints, Vitreous Enamelling, Cladding, Powder coatings, Zinc coatings, Tin Plate, Electroplating,
Cadmium Plating, Anodising (Anodizing), Thermal Spray Coatings., Plasma
Nitriding/Carburising/Boronising., Pack Cementation, Ion Implantation, Ceramic and Cermet
materials., Chemical Vapour Deposition, Physical Vapour Deposition. The demand for protective
coatings has increased recently for almost all types of super alloys with improved strength, since
high-temperature corrosion problems become much more significant for these alloys with
increasing operating temperatures of modern heat engines. The Major areas where coatings have
the application are Power generation Industries, Ceramics Industries, Chemical Industries, Iron &
steel Industries and Mining Industries etc. Open or closed porosity in thermal spray coatings can
originate from several different factors: partially or totally unmolten particles, inadequate flow or
fragmentation of the molten particle at impact, shadowing effects due to lower than the optimal
spray angle, and entrapped gas. The interconnected (open) porosity allows the corrosive media to
reach the coating-substrate interface, which eventually leads to delamination of the coatings.
Although the development of the modern thermal spray processes has decreased coating porosities,
the transport of corrosive species to the substrate can still only be prevented by coating post
treatment. Therefore it's of actual significance to develop an effective method to post treat the
thermal spray coatings to enhance their life in corrosive environment. In this paper author has
reviewed the significance of heat treatment in thermal spray coatings for improving their properties
and has made an attempt to explore the potential of heat treatment process in thermal spray
coatings.
Keywords: Corrosion, Coatings, Thermal Spray, Heat Treatment
1. Introduction:
Corrosion is a natural phenomenon. All natural
processes end toward the lowest possible energy states.
As described in the corrosion cycle of the steel shown
in Fig. 1. The iron and steel have a natural tendency to
combine with other chemical elements to return to their
lowest energy states & they frequently combine with
oxygen and water, both of which are present in most
natural environments, to form hydrated iron oxides
(rust), similar in chemical composition to the original
iron ore (ASM International, 2000).
Corrosion is the deterioration of a material by its
reaction with the surroundings. It adversely affects
those properties that are to be preserved. At higher
temperature, this mode of degradation is known as
oxidation or dry corrosion (Sidhu T. S. et al., 2006).
Metals and alloys sometimes experience
accelerated oxidation when their surfaces are covered
with a thin film of fused salt in an oxidizing atmosphere
at elevated temperatures. This mode of attack is called
Fig. 1 The corrosion cycle of steel (ASM 2000)
This paper has been published in a special issue of International Journal of
Materials Science and Engineering, Vol 2, No. 1-2, January-December 2011
National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8,2011)
Punjab Technical University, Jalandhar
hot corrosion (Sidhu T. S. et al., 2006; Sidhu, H.S. et
al., 2006).
Sidhu Buta Singh and Prakash S observe that
although corrosion problems cannot be completely
remedied, it is estimated that corrosion-related costs can
be reduced by more than 30% with development and
use of better corrosion control technologies (Sidhu Buta
Singh and Prakash S., 2006)
Xue-mei OU.et al evaluates that main reason for
hot corrosion on the boiler tube surface is the
impurities, such as Na, K, and S, present in the coal
being burned (Xue-mei OU.et al., 2008).
The demand for protective coatings has increased
recently for almost all types of super alloys with
improved strength, since high-temperature corrosion
problems become much more significant for these
alloys with increasing operating temperatures of
modern heat engines (Sidhu Buta Singh and Prakash S.,
2005).
Hot corrosion has been observed in boilers, internal
combustion engines, gas turbines, fluidized bed
combustion and industrial waste incinerators since the
1940s. However, it became a topic of importance and
popular interest in the late 1960s when gas turbine
engines of military aircraft suffered severe corrosion
attacks during the Vietnam conflict while operating
over and near sea water. During operation, blades and
vanes of gas turbines are subjected to high thermal
stresses and mechanical loads. In addition, they are
also attacked chemically by oxidation and/or high-
temperature corrosion. Only composite materials are
able to meet such a demanding spectrum of
requirements; the base material provides the necessary
mechanical properties and coatings provide protection
against oxidation and corrosion (Sidhu et al., 2006).
2. Thermal spray Coatings &
Processes
Methods for the deposition of protective coatings
on heat-resistance alloys (HRA) can be separated into
two basic groups : thermal diffusion, based on processes
leading to a change in the composition and structure of
the surface layer of the HRA as a result of its contact
and reaction with alloying chemical elements; and non-
diffusional, based on processes in which an external
(overlay) coating is deposited on the surface with little
inter-diffusion of elements only that necessary to
guarantee adherence (Podchernyaeva I. A. et al.,2000).
Sidhu T. S. et al. suggests that Nickel-based alloy
coatings show good high-temperature wear and
corrosion resistance. Wear resistance improves after
adding W and Mo elements to the alloy. Ni-based
coatings are used in applications when wear resistance
combined with oxidation or hot corrosion resistance is
required. When nickel is alloyed with chromium, this
element oxidizes to Cr2O3 at rates which could make it
suitable for use up to about 900°C (Sidhu T. S. et al.,
2006).
Abdi S. and Lebaili S. deposits NiCrBCSi metal
(Fe) to hard reset show better properties and
performance compared to hard chromium deposits.
Including the filing NiCrBCSi (Fe) type A, this may be
an appropriate alternative to hard chromium and enable
better protection of the environment. This is due to the
existence of microstructure, composed of the Ni3B
nickel boride and matrix reinforced by nano precipitates
rich in chromium (Abdi S. and Lebaili S. 2008).
Uusitalo M.A. deposits St35.8 steel, 13CrMo4-5
steel, St35.8 steel with chromium and aluminium
diffusion coatings, and St35.8 steel with different kinds
of thermal sprayed coatings were used as test materials.
In general, spraying systems using high particle
velocities produce dense coatings with small splat size,
high bonding strength and large contact area between
individual splats (M.A. Uusitalo, 2002).
The most common coatings are WC-Co, WC-CoCr,
and Cr3C2-NiCr systems. Cr3C2-NiCr coatings show
comparatively poorer tribological properties, but they
are much more resistant at high temperatures and in
aggressive environments: for these reasons they are
used, for example, in steam turbine blades or in boiler
tubes for power generation (Kaur Manpreet et al.,
2009).
On the other hand Aalamialeagha M. E.et al.
reveals that high Velocity Oxy-Fuel (HVOF) spray
techniques can produce high performance alloy and
cermet coatings for applications that require wear
resistant surfaces. HVOF coatings require the careful
matching of the powder feed material to the process
variables e.g., fuel type, fuel/oxygen ratio, together with
the design and geometry of the spray gun.
(Aalamialeagha M. E.et al., 2003).
Among the different thermal spray processes, Super
D-Gun and HVOF have different features, such as
geometry and powder feed respectively. For the Super
D-Gun process 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. (V.A.D. Souza,
A. Neville, 2007).
So lot of techniques, such as Air plasma spray
(APS), Vacuum plasma spray (VPS), Solution-
precursor plasma spray (SPPS), Electron-beam physical
vapor deposition (EB-PVD), High velocity oxygen fuel
spray (HVOF), Magnetron sputtering, have been used to
deposit MCrAlY bond coat on super alloys (Zhiming
Li, Z et al. 2010).
Porosity or voids in the coating micro structure is
an important issue in thermal spraying, as due to this
physical property, corrosion resistance of different
thermal spraying coatings differs. Dense coatings
usually provide better corrosion resistance than porous
coatings (Sidhu et al., 2006).
National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8,2011)
Punjab Technical University, Jalandhar
To reduce the interconnected porosity and inter
splat boundaries, coatings are post treated by various
methods such as heat treatment, sealing, laser remelting
etc. (Sundararajan et al., 2009, Ahmaniemi et al., 2002,
Serresa, 2011).
The Heat Treatment process is one of the post
treatment processes and widely used to reduce the
interconnected porosity and inter splat boundaries.
Hence to be reviewed and further helpful in the post
treatments of thermal spray coatings.
3. Heat Treatment of thermal spray
coatings
Heat treatment is a process of heating the metals or
steel alloys at high temperature for some fixed time
which changes the microstructure of the substrate. With
increasing heat treatment temperature, the density of
weakly/ unbounded inter-splat boundaries and porosity
decrease with a corresponding increase in elastic
modulus (Sundararajan G. et al.2009).
When the alloys were thermally annealed, these
irregularities in the grain boundaries disappeared
(Gonzalez-Rodriguez J.G. et al., 2008). Generally, heat
treatment of thermally sprayed deposits can release
residual stress, decrease the porosity and improve the
microstructure and properties of the deposits (Wang
H.T., 2009)
4. Some studies on heat treatment of
thermal spray coatings
As we found that Porosity or voids in the coating
micro structure is an important issue in thermal
spraying, as due to this physical property, corrosion
resistance of different thermal spraying coatings differs
and to reduce the interconnected porosity and inter
splat boundaries, coatings are post treated by various
methods such as heat treatment. Here are some studies
on the heat treatment of thermal spray coatings.
Gff L.et al reveals the results regarding the effect
of both carburizing flame and argon atmosphere post-
heat treatments on the microstructure and corrosion
resistance of NiCrWBSi coatings are reported. Both
micro structural characterization and porosity
determination were carried out before and after the heat
treatments. It was determined that both treatments had
reduced the porosity considerably, and this reduction
was accompanied by pronounced micro structural
changes regarding the disappearance of the initial
lamellar structure, a more uniform distribution of the
hard phases, and a decrease in the number of micro
cracks and unmelted particles. Results from
potentiodynamic studies carried out in a 5% NaCl
solution have indicated an increase in the corrosion
resistance of both heat-treated coatings (Gff L.et al.,
2011).
The hardness and wear resistance of a thermal-
sprayed self-fluxing alloy (Ni-17wt. % Cr-3. 3wt.%B-4.
3wt.%Si-4.2wt.%Fe-0.9wt.%C) is improved by adding
20 wt.% of B13C2 to the powder and heating the coating
at 1030°C in a vacuum of 10-2 Torr. Porosity is
decreased from 20 to 0.3 vol. % by the heating if pre-
heating at 950°C is carried out to facilitate the escape of
trapped gases. The presence of numerous precipitates of
Cr3C2 and CrB in the coating is consistent with a
Rockwell hardness of HRC 63. The abrasive wear
resistance is much improved compared with that of
Stellite 6 (Shieh Yune-Hua et al., 1993).
Sundararajan G. et al. evaluates the response of
cold sprayed SS 316L coatings on mild steel substrate
to aqueous corrosion in a 0.1 N HNO3 solution as
determined using polarization tests. The corrosion
behaviour of the SS 316L coating was studied not only
in the as-coated condition, but also after heat treatment
at 400, 800 and 1100°C. Heat treatment reduced the
porosity, improved inter-splat bonding, increased the
elastic modulus and more importantly increased the
corrosion resistance of the cold sprayed SS 316L
coating (Sundararajan G. et al.2009).
The in-situ co-deposition of Cr-Si into Cr17Ni2
stainless steel (similar to AISI 431) was achieved using
a pack cementation process. Through the optimum
parameters, a coating containing approximately 27 wt.
% Cr and 2 wt.% Si was obtained, with a layer
thickness of approximately 120 mm. Studies showed
that the thermal treatment of the coating resulted in a
reduction of tensile strength, but the improvement of
impact toughness, although the coating had little effect
on the mechanical properties of the bulk. Tempering at
300 or 450°C improved the tensile strength and the
impact toughness of the steel at 9 and -55°C, while
tempering at 550°C reduced these mechanical
properties. (Wei P., Wan X.R., 2000).
The corrosion performance of several NiAl alloys
in 62 mol% Li2CO338 mol% K2CO3 at 650 ◦C has
been studied using the weight loss technique. Alloys
included 50Ni50Al at. % (NiAl) and 75Ni25Al at. %
(Ni3Al) alloys with additions of 1, 3 and 5 at. % Li each
one, with or without a heat treatment at 400° C during
144 h. For comparison, AISI-316L type stainless steel
was also studied. The tests were complemented by X-
ray diffraction, scanning electronic microscopy and
micro-analyses. Results showed that NiAl-base alloy
without heat treatment presented the lowest corrosion
rate even lower than Ni3Al alloy but still higher than
conventional 316L-type stainless steel. In general terms,
by either by heat treating these base alloys or by adding
Li, the mass loss was increased. This effect was
produced because by adding Li the adhesion of the
external protective layer was decreased by inducing a
higher number of discontinuities inside the grain
boundaries. When the alloys were thermally annealed,
National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8,2011)
Punjab Technical University, Jalandhar
these irregularities in the grain boundaries disappeared,
decreasing the number of paths for the outwards
diffusion of Al from the alloy to form the external,
protective Al2O3 layer (Gonzalez-Rodriguez J.G. et al.,
2008).
It is difficult to deposit dense intermetallic
compound coatings by cold spraying directly using
compound feedstock powders due to their intrinsic low
temperature brittleness. A method to prepare
intermetallic compound coatings in-situ employing cold
spraying was developed using a metastable alloy
powder assisted with post heat treatment. In this study,
a nanostructured Fe (Al)/Al2O3 composite alloy coating
was prepared by cold spraying of ball-milled powder.
The cold-sprayed Fe (Al)/Al2O3 composite alloy coating
was evolved in-situ to FeAl/Al2O3 intermetallic
composite coating through a post heat treatment. The
effect of heat treatment on the phase formation,
microstructure and micro hardness of cold-sprayed Fe
(Al)/Al2O3 composite coating was investigated. The
results showed that annealing at a temperature of 600°C
results in the complete transformation of the Fe (Al)
solid solution to a FeAl intermetallic compound.
Annealing temperature significantly influenced the
microstructure and micro hardness of the cold-sprayed
FeAl/Al2O3 coating. On raising the temperature to over
950 °C, diffusion occurred not only in the coating but
also at the interface between the coating and substrate.
The micro hardness of the FeAl/Al2O3 coating was
maintained at about 600HV0.1 at an annealing
temperature below 500°C, and gradually decreased to
400HV0.1 at 1100°C (Wang Hong-Tao et al.2009).
A method to prepare intermetallic composite
coatings employing the cost-efficient electric arc
spraying twin wires assistant with suitable heat
treatment was developed. In this study, a FeAl
composite coating was produced by spraying twin
wires, i.e. a carbon steel wire as the anode and an
aluminum wire as the cathode by Chen Yongxiong et al.
The inter-deposited FeAl coating was transformed in-
situ to FeAl intermetallic composite coating after a
post annealing treatment. The effect of annealing
treatment conditions on phase composition,
microstructure and mechanical properties of the coating
was investigated by using XRD, SEM, EDS and OM as
well as micro hardness tester. The results show that the
desirable intermetallic phases such as Fe2Al5, FeAl and
Fe3Al are obtained under the annealing condition. The
main oxide in the coating is FeO which can partially
transform to Fe3O4 up to the annealing condition (Chen
Yongxiong et al., 2009).
G. Bolelli et al. evaluated the effect of a 600°C, 1 h
heat treatment on the corrosion performance of three
HVOF-sprayed metal alloy coatings by electrochemical
corrosion tests and corrodkote test. In general, the heat
treatment has two major effects on the tested coatings: it
improves interlamellar cohesion, reducing active
corrosion along interlamellar boundaries, but can also
trigger galvanic microcells at intralamellar level,
because of the formation of secondary phases. The first,
beneficial effect prevails in the case of Co800 and
D4006 coatings, so that an overall improvement in their
corrosion resistance is found and they have lower
corrosion current density, less active corrosion at
interlamellar boundaries and improved corrodkote test
resistance. The heat treatment is therefore an effective
way to improve the overall performance of the Co800
and D4006 coatings. The properties of the heat-treated
Co800 coating are particularly significant when
compared to those of electrolytic hard chrome (EHC).
By coupling the corrosion test outcomes to former
results on tribological behaviour, we find that the
corrosion resistance of heat-treated Co800 is
comparable to that of EHC and its tribological
characteristics far surpass EHC under various contact
conditions. By contrast, the effects of the heat treatment
on the corrosion resistance of Ni700 are less obvious.
Most importantly, after the heat treatment, the Ni700
coating shows greater sensitivity to crevice corrosion,
so that its overall corrosion resistance may seem to be
reduced by the heat treatment (G. Bolelli et al. 2008).
G. Bolelli and L. Lusvarghi examined the
tribological behavior of HVOF sprayed Co-28%Mo-
17%Cr-3%Si coatings, both as deposited and after heat
treatments, correlating it with microstructural and
micromechanical features. A significant degree of splat
boundary oxidation exists in the as-sprayed coating,
because of exothermic oxidative reaction occurring at T
> 810°C. This coating is mainly amorphous due to splat
quenching; thus, it has low hardness and toughness,
resulting in poor tribological performance
particularly, its low hardness promotes adhesive wear
against 100Cr6 steel pins. Adhesion causes a rapid
increase in friction coefficient, and consequently the
contact point temperature reaches a critical value where
rapid oxidation occurs. Oxides decrease the friction
coefficient, but they are not particularly adherent to the
contacting surfaces and mostly form debris. Therefore,
friction increases again and continues to oscillate
periodically because adhesive wear continues to raise
flash temperature up to the critical value. Most of the
wear loss occurs in the first stage, where adhesion is
particularly severe due to direct contact between
metallic surfaces. In the tests against alumina pin, the
sample wear rate is smaller because less adhesion takes
place; abrasive wear is prevalent, but the Co-base alloy
has sufficient intrinsic plasticity to withstand it without
undergoing too much cutting wear. However, the fast
oxidation process, with peculiar friction coefficient
behavior, still takes place. While the 200 and 400°C
heat treatments do not cause any major change (the
former one even degrading the coating properties), the
600°C treatment causes the appearance of sub-
micrometric crystalline regions improving hardness and
elastic modulus. Adhesive phenomena between coating
and steel pin are thus definitely reduced; the wear loss
National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8,2011)
Punjab Technical University, Jalandhar
is negligible for the coating and decreased by two
orders of magnitude for the pin; no friction coefficient
peaks occur nor is fast oxidation started. Instead,
friction coefficient soon gets to a steady value. The
coating wear rate against alumina pin is not
significantly changed because abrasive wear still
prevails, so there are no major changes in the wear
process. However, adhesive phenomena are further
reduced, preventing the appearance of friction
coefficient peaks and of fast oxidation. Thus,
performing a 600°C, 1 h heat treatment in air could be
suggested as a way to improve the sliding wear
performance of the present alloy at room temperature.
The 600°C heat treated coating wear rates are lower
than those recorded by the authors for hard chrome
platings at room temperature under the same testing
conditions (G. Bolelli and L. Lusvarghi, 2006).
Hence it has been observed that with the heat
treatment of thermal spray coatings better results can be
obtained in post treatment of the coatings for enhancing
their life for different applications but not much work
has been done in this field to post treat the coatings and
by changing the parameters like Temperature and time
of heat treatment better results can be obtained in post
treatment of the coatings. Fig. 2 describes the some of
the work done in the heat treatment in the shape of
pyramid.
5. Conclusion
Thermal Spray Coatings are very effective for
corrosion, erosion and wear applications, but due
to interconnected porosity the corrosive species
(
(
1
(
(
(Wei P., Wan X.R.,
2000), 27 w t. % Cr and
2 wt. % Si (in-situ co-
deposition of Cr-Si into
Cr17Ni2 stainless steel
(similar to AISI 431)),
Heat Treatment,
Reduction of tensile
strength, but the
improvement of impact
toughness
(Shieh Yune-Hua et
al., 1993), Thermal
sprayed self fluxing
alloy (N i-17Cr-
3.3B-4.3Si-4.2Fe-
0.9C), Heat
Treatment,
Improved hardness
& wear Resistance
(G. Bolelli and L.
Lusvarghi, 2006), Co-
28%Mo-17%Cr-3%Si
HVOF Sprayed, Heat
Treatment, Reduced wear
rate & increased hardness
(Gonzalez-Rodriguez J.G. et
al., 2008), Ni-Al alloys and
AISI-316L in molten (Li +
K) carbonate, Heat
Treatment, Disappearance of
irregularities in grain
boundaries
(G. Bolelli et al., 2008),
Co-28%Mo-17%Cr-3%Si
Flame sprayed alloy
coatings , Heat
Treatment, Hot corrosion
resistance increased
(Wang Hong-Tao et al.,
2009), Nanostructured Fe
(Al)/Al2O3 Composite alloy
coatings, Heat T reatment,
Intermetallic compound
formation i.e. interface of
coating & substrate
(Chen Yongxiong et al., 2009),
Fe-Al composite coatings, Post
Annealing Treatment, Inter
splat diffusion & intermetallic
phase distribute micro hardness
homogeneous
(Sundararajan G. et al., 2009),
Cold Spray SS 316L Coatings,
Heat Treatment, Reduced
porosity, improved inter splat
Bonding, increase in elastic
modulus & increased in
corrosion resistance
(Gff L.et al., 2011), NiCrWBSi
coatings, Heat Treatment, Reduced
the porosity , micro structural
changes regarding the
disappearance lamellar structure, a
more uniform distribution of the
hard phases, and a decrease in the
micro cracks and unmelted
particles
National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (Oct. 7-8,2011)
Punjab Technical University, Jalandhar
are able to penetrate and attack the substrate via
interconnected network of voids and oxide at
splat boundaries, hence their life is reduced.
Further the various Post treatments of thermal
spray coatings are effective methods to improve
their life.
Heat treatment is one of them which found to
give better results in reducing the porosity
considerably and improves interlamellar
cohesion.
From the literature it has been observed that not
much work has been done in this field to post
treat the coatings and by changing the parameters
like Temperature and time of heat treatment
better results can be obtained in post treatment of
the coatings for enhancing their life for different
applications.
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... This observation is not considered to be critical since the heat treatment leads to a better layer cohesion and a decrease in the crack susceptibility, which is mainly attributed to stress relief (promotion of compressive stresses), phase transformation, sintering, and elements diffusion [36][37][38]. As seen in Figure 9a, the microhardness of the substrate decreased during the heat treatment due to a softening of the former precipitation-hardened material. ...
... This observation is not considered to be critical since the heat treatment leads to a better layer cohesion and a decrease in the crack susceptibility, which is mainly attributed to stress relief (promotion of compressive stresses), phase transformation, sintering, and elements diffusion [36][37][38]. Failures such as cracking or delamination are mostly prevented as a result of the strong metallurgical bonding formed at the interface region due to the thermal exposure associated with diffusion and interdiffusion processes. ...
... Especially in the oxidized state at the interface region, the influence of diffusion processes and the reactive layer growth on the hardness are obvious (climax/low point in coatings profile). Decreases in the hardness measured for the oxidized coatings in comparison with those of the heat-treated ones is explained as a result of diffusion processes and a ceramic and metallic phase separation, combined with some changes concerning the degree of internal stress [36]. Nevertheless, the presence of the SiO 2 could not be identified as a weakening factor in the coating. ...
Article
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Article
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Cr3C2-NiCr coating was deposited on SAE-347H boiler steel by high velocity oxy fuel (HVOF) spray process. Subsequently, high-temperature corrosion behavior of the bare and coated boiler steel was investigated at 700 °C for 50 cycles in Na2SO4-82Fe2(SO4)3 molten salt, as well as air environments. Weight-change measurements after each cycle were made to establish the kinetics of corrosion. X-ray diffraction, field emission-scanning electron microscopy/energy dispersive spectroscopy, and x-ray mapping analyses were performed on the exposed samples to analyze the oxidation products. The bare 347H steel suffered accelerated oxidation during exposure at 700 °C in the air as well as the molten salt environment in comparison with its respective coated counterparts. The HVOF-spray Cr3C2-NiCr coating was found to be successful in maintaining its adherence in both the environments. The surface oxide scales were also found to be intact. The formation of chromium rich oxide scale might have contributed for the better hot corrosion/oxidation resistance in the coated steel.
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The influence of microstructure on the overall material loss in erosion–corrosion environments is presented for WC–Co–Cr coatings applied by (i) High Velocity Oxy-Fuel (HVOF) and (ii) Super Detonation-Gun (D-Gun) processes. The study is focused on understanding the synergy effect (here defined as the enhancement of erosion due to corrosion effects) on material loss when two different microstructures are formed and also the influence of chemical composition of the coating. Experiments showed that HVOF coatings have a slightly lower corrosion resistance than the Super Detonation-Gun (D-Gun) coatings but higher overall erosion–corrosion resistance. It is important to point out that HVOF and Super D-Gun coating microstructures vary depending on parameters of application and therefore the results presented in this paper cannot be generalised. In this work a particular case is presented to establish a link between the coating composition, microstructure and erosion–corrosion performance for WC–Co–Cr coatings when different microstructures are formed.
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Stellite-6 (St-6) coating was obtained on boiler tube steels namely ASTM-SA210-Grade A1 (GrA1), 1Cr–0.5Mo steel ASTM-SA213-T-11 (T11) and 2.25Cr–1Mo steel ASTM-SA213-T-22 (T22) through plasma spray process. Ni–Cr–Al–Y was used as a bond coat before applying St-6 coating. Nd:YAG laser has been used for the post-coating treatment. As sprayed and laser remelted steels were subjected to molten salt environment (Na2SO4–60%V2O5) at 900°C under cyclic conditions. The samples were visually examined and subjected to weight change measurements at the end of each cycle of study.Techniques like XRD, SEM/EDAX and EPMA analysis have been used to analyse the oxide scale. The coating was found to be effective in decreasing corrosion rate of the boiler tube steels. Protection is higher when GrA1 steel was the substrate steel and lower for T22 base steel. Due to the formation of vertical cracks, laser remelted coatings have indicated slightly higher corrosion rate in the given molten salt environment.
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The corrosion performance of several Ni–Al alloys in 62mol% Li2CO3–38mol% K2CO3 at 650°C has been studied using the weight loss technique. Alloys included 50Ni–50Alat.% (NiAl) and 75Ni–25Alat.% (Ni3Al) alloys with additions of 1, 3 and 5at.% Li each one, with or without a heat treatment at 400°C during 144h. For comparison, AISI-316L type stainless steel was also studied. The tests were complemented by X-ray diffraction, scanning electronic microscopy and micro-analyses. Results showed that NiAl-base alloy without heat treatment presented the lowest corrosion rate even lower than Ni3Al alloy but still higher than conventional 316L-type stainless steel. In general terms, by either by heat treating these base alloys or by adding Li, the mass loss was increased. This effect was produced because by adding Li the adhesion of the external protective layer was decreased by inducing a higher number of discontinuities inside the grain boundaries. When the alloys were thermally annealed, these irregularities in the grain boundaries disappeared, decreasing the number of paths for the outwards diffusion of Al from the alloy to form the external, protective Al2O3 layer.