Volume 6 Preprint 98
Corrosive Failure Analysis of Thermal Spray Coatings for Advanced Waste-to-Power Generation Plant at Elevated Temperatures
M. Yoshiba, A. Mikami, K. Shimada and T. Shimizu
Keywords: Waste-to-Power Generation, High-Temperature Corrosion, Thermal Spray Coating, HVOF, Plasma Spraying, Failure Analysis
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Volume 6 Paper H055
Corrosive Failure Analysis of Thermal Spray
Coatings for Advanced Waste-to-Power
Generation Plant at Elevated Temperatures
M. Yoshiba1, A. Mikami2, K. Shimada3 and T. Shimizu4
Department of Mechanical Engineering, Graduate School of
Engineering, Tokyo Metropolitan University, Tokyo 192-0397, Japan,
Present Affiliation: JEOL Ltd., Tokyo 196-8558, Japan
Present Affiliation: YKK Corporation, Kurobe 938-8601, Japan
4 Technology Development Laboratory, KUBOTA Corporation,
Amagasaki 661-8567, Japan
In order to assess an applicability of the thermal spray coating systems
as the corrosion-resistant components such as boiler superheater
tubes in highly aggressive environment for the high-temperature and
high-efficiency waste-to-power generation plant, the corrosive failure
analysis was conducted for different kinds of the thermal spray coating
systems by HVOF, APS and LPPS, with corrosion-resistant coating
alloys such as the 625 and 50Ni-50Cr alloys. On the basis of
corrosive failure analysis by means of the metallography and x-ray
microanalysis, the failure behaviour of coating systems was found to
be classified into three modes according to the morphology of
corrosive damage in the coating layer; a consumptive mode A with a
general corrosion dominating, an incidental mode B in which the
localized attack predominates, and another one is the mixed mode M
that is composed of both A and B. Most improved protection against
the oxy-chlorination dominating corrosion was found to be attainable
for the 625 alloy coating onto the 304 stainless steel substrate by
HVOF followed by the post spraying heat-treatment. Furthermore, it
was revealed that highly corrosion-resistant coating system should be
obtainable provided for a combination of both the coating processing
which can minimize the defect structures such as large numbers of
pores together with the pronounced oxide phases incorporated in the
coating layer and the post spraying heat-treatment suitable for the
compositional and microstructural homogenization of the coating
layer which is essential for preventing the localized attack. Coating
requirements for the much increased performance against the
corrosive attack were discussed from several aspects.
Keywords: Waste-to-Power Generation, High-Temperature Corrosion,
Thermal Spray Coating, HVOF, Plasma Spraying, Failure Analysis
Waste-to power generation is one of the most feasible energy recovery
means both in the existing waste incineration plants and recently in
the combined type systems of thermal decomposition (pyrolysis) and
gasification followed by ash melting. In order to improve the thermal
efficiency of these waste-to-power generation systems, it is required
that the superheated boiler steam temperature to be sent to a steam
turbine should be much increased up to around 500℃. This means for
the hot section components such as superheater tubes to be subject to
much severe and complicated corrosion attack which can be caused by
both the aggressive molten salt containing the multi-components of
chloride eutectics and the corrosive flue gases also rich in chlorides.
There are several approaches to prevent such a serious high-
temperature corrosion problem; (1) modification of boiler design to
minimize an ash deposition onto the alloy tube surface, (2)
environmental control to mitigate the corrosivity and (3) materials
innovation to overcome the corrosive damage. In particular, R&D of
the superheater tube materials is a key issue for the advanced wasteto-power generation plant. Consequently, large numbers of research
works about the development and evaluation of highly corrosionresistant superheater materials have been conducted during the last
decade in Japan, mainly as the national projects, to introduce
successfully in a practical service [#ref1-3]. However, only a limited
number of information has been obtainable about an applicability of
the protective coatings in spite of its practical importance [#ref3,4].
In the present study, in order to estimate an applicability and the
requirements of the thermal spray coatings in such an aggressive
environment, the corrosive failure analysis was conducted for three
kinds of thermal spray coating systems; high velocity oxygen fuel
flame (HVOF) spraying, atmospheric plasma spraying (APS) and lowpressure plasma spraying (LPPS), with highly corrosion-resistant
coating alloys onto the stainless steel substrate, by means of different
evaluation methodologies such as the corrosion mass change, the
metallography and x-ray microanalysis.
Thermal Spray Coating Systems
Three kinds of the thermal spray coatings; HVOF, APS and LPPS, were
adopted onto the annealed 304 stainless steel substrate with the form
of the basically disk-shaped specimen. Specimen was machined to
the form of not only a basic geometry with 16mm diameter and 3mm
thickness, but also the specially designed geometry of both with a
parallel portion of the width 15mm, and with the chamfer of 0.5mm in
all the corner (edge), so as to make easily a preparation of crosssectioning for the metallographic examination without a coating
damage, and to avoid inducing preferentially the coating failure such
as cracking and spalling from the edge of specimen, respectively. Two
kinds of the corrosion-resistant coating alloys; 625 and 50Ni-50Cr
alloys, were used as the spraying materials, and the coating thickness
was approximately 200µm. Alloy compositions of these coating alloys
are listed along with 304 steel for the substrate in Table 1.
A series of the coating processing with different spraying
methodologies and materials are denoted with the coating system
codes in Table 2. Influence of adopting the post-spraying heattreatment (annealing) was considered, as also shown in Table 2.
Furthermore, three kinds of the thermal spraying conditions basically
according to their practical conditions are summarized in Table 3.
Table 1 Chemical composition of 304 stainless steel substrate
and coating alloys (mass%).
Ni Cr Mo Fe Co Nb+Ta
Substrate SUS 304 0.05 0.39 1.35 0.03 0.003 8.1 18.2
50Ni-50Cr 0.01 0.66 0.19 0.001 0.002 Bal. 47.8
Alloy 625 0.04 0.11 0.33 0.005 <0.001 Bal. 21.7 8.82 3.36 0.27 3.75
Table 2 Coating processings.
Al 0.17 Ti 0.32
Table 3 Spraying conditions in
HVOF, APS and LPPS.
High-Temperature Corrosion Testing
High-temperature corrosion testing was conducted in the laboratory
by means of the salt coating methodology [#ref5], using the corrosion
testing apparatus mainly composed of both the main testing zone
within a quartz tube heated externally by an electric furnace and the
gas supplying and cleansing systems, as schematically shown in Fig. 1.
Then, a specimen was pre-coated with a given amount of the actual
plant ash corrected from four principal cities of the municipal solid
waste (MSW) incineration plants in Japan and blended evenly, of which
chemical composition is listed in Table 4. This actual plant ash is
characterized from the DTA-TG analysis by having the melting points
at 286℃, 342℃, 378℃ and 513℃, suggesting the melting points of a
variety of the mainly chlorides and/or partly chlorides-sulphates
eutectics. The quantity of ash pre-coated is 40mg/cm2 onto the entire
NO2 / N2
SO2 / N2
CO2 / N2
HCl / N2
N 2 + O2
Fig.1 Schematic flow of apparatus for high-temperature corrosion
Table 4 Chemical composition of ash collected from actual four
The specimens pre-coated were settled by putting on the alumina boat
into the quartz tube and then heated up. Prior to heating, vacuuming
and introducing the argon gas was made for purging air from the
quartz tube, and then heating was started up. When the temperature
was attained to the testing range, the synthetic gas mixture was
supplied into the quartz tube with the flow rate of 500cm3/min.
Chemical composition of the flow gas mixture is N2-10% O2-10% CO2-
20%H2O-1000ppm HCl-100ppm CO-100ppm NO-50ppm SO2 in
volume fraction, which is the representative flue gas composition
commonly experienced in the actual MSW incineration plants in Japan,
and characterized by much higher HCl concentration.
High-temperature corrosion test was carried out at 450℃ and 550℃
which should correspond for enduring to the steam temperatures of
400℃ and 500℃, respectively. Both the isothermal corrosion tests for
up to 200h and the thermal cycle corrosion tests of 50h-4cycles by
the repeated coating with the same amount of ash were conducted.
The corrosion testing was conducted by three times; using three
specimens for each coating system, for the same testing condition.
After the test, almost all the specimens were subjected to an electrochemical descaling in the molten Na2CO3-NaOH salt followed by the
mass measurement to determine the conventional corrosion mass
change, except for the specimen sending to the x-ray microanalysis
without descaling. Furthermore, the metallographic examination also
was conducted at the cross-sections of all the specimens to determine
quantitatively the different aspects of the corrosive damages, as
described later in detail.
Corrosion Mass Loss for Different Coating Systems
Figure 2 shows the corrosion mass losses for different kinds of the
coating systems along with no-coated 304 steel obtained from both
the isothermal corrosion test for 200h and the thermal cyclic corrosion
test of 50h for 4 cycles, at 450℃ and 550℃. Here, the symbol *
denotes that a part of the coating layer has experienced an exfoliation
during mainly the descaling process. It is apparent that the corrosion
mass loss is much significant under the conditions both at higher
temperature of 550℃ and of the thermal cycling, suggesting the
harmful effect of both an increased temperature and a thermal cycle
on the coating failure. So long as the comparison of coating systems
is concerned, HVOF and LPPS systems followed by the heat-treatment
mainly in vacuum; such as 62HV and 62LV, exhibit generally a sound
performance with the relatively reduced mass loss and without a
According to such an evaluation methodology based of the mass
change, however, no information can be obtainable at all about the
corrosion morphology and more importantly about the steel substrate.
:Exfoliated during Descaling
* : Exfoliated
Fig.2 Corrosion mass loss of different coating systems and the uncoated 304 steel (B) .
Morphological Features of Corrosive Failure
From the metallographic examination at the cross-sections of all the
specimens in detail, the corrosive failure behaviour of different coating
systems was revealed to be classified generally into three modes by
virtue of the failure morphology in such an aggressive environment.
These three modes of failure behaviour are summarized schematically
in Fig. 3. Thus, the mode A is of the consumptive manner in which a
general corrosion is dominating in the coating layer. This mode was
representative for the HVOF systems with both 50Ni-50Cr (55H) and
625 (62H) alloys.
The mode B is of rather localized manner in which a localized
corrosion can occur preferentially along a number of defects such as
the pores and oxide micro-channels pre-existing in the coating layer
to be attainable rapidly to the steel substrate, prior to the general
attack progresses. In this case, a premature degradation in the
protective function of coating should be most significant, since the
localized attack toward the steel substrate and along the
coating/substrate interface tends to bring about a premature
exfoliation at the interfacial region, in spite of for the other coating
Fig.3 Three modes of the corrosive failure behaviour observed in
different coating systems at 450ºC and 550ºC.
portion remaining in sound. APS system with alloy 625 (62A)
exhibited this mode of failure behaviour. Third one is the mode M
having commonly the mixed characteristics of both modes A and B. A
part of the LPPS systems with alloy 625 (62L) is included in this mode.
Metallographic Evaluation of Corrosive Failure in Coating Systems
In order to evaluate appropriately both a variety of the coating failure
modes and the information about the corrosive damage of steel
substrate, a reasonable measurement methodology should be
proposed with taking into account for the information both about the
coating failure and about the depth of attack into the steel substrate.
Figure 4 denotes the schematic illustration showing the measurement
concept proposed in this study for different aspects of the corrosive
failure observed in the coating systems. Then, the coating/substrate
interface can be regarded as an apparent base line on the cross-
section, and the outward and inward measurements from there should
be indicative of the remaining coating thickness and the depth of
attack into the steel substrate, respectively. A microscopic
measurement was actually made for the representative ten fields of
view on the whole specimen cross-section at a magnification x100.
Coating / Substrate Interface (Base Line)
Remaining Coating Thickness (at View i )
Depth of Attack into Substrate (at View i )
Average Remaining Coating Thickness
C ave = ( ∑ C i ) / n
C min : Minimum Remaining Coating Thickness
(C1 in Case Fig.4)
D ave : Average Depth of Attack
D ave = ( ∑ D i ) / n
Dmax : Maximum Depth of Attack
(D1 in Case Fig.4)
(Magnification : x100)
Fig.4 Concept of measuring methodology for different aspects of the
corrosive failure in coating systems.
Figures 5 and 6 show the results obtained from the metallographic
measurement for all the coating specimens at 450℃ and 550℃,
respectively. Here, the information both about three modes of the
corrosive failure denoted in Fig. 3, and about the corrosion
morphology in the steel substrate are also noted. It was revealed that
higher temperature testing of 550℃ results in the more significant
corrosive damage both in the coating layer and namely in the steel
substrate as compared to the case at 450℃. As regards an influence
of the coating system, HVOF systems with alloy 625 which were
follower by an appropriated heat-treatment in air or in vacuum; such
as 62HA and 62HV, showed the minimized corrosive damage into the
steel substrate, which is corresponding to the relatively mitigated
coating failure. It is also apparent that the beneficial effect of HVOF is
more pronounced in the combination with alloy 625 as compared with
50Ni-50Cr alloy. APS and LPPS systems with 625 alloys; such as 62A
and 62L series, on the contrary, very rapid coating failure and
consumption occurred according to the modes of either B or M to
cause subsequently the significant corrosion attack into the steel
It is also indicative from Fig. 6 that an isothermal corrosion condition
tends to bring about for the coating systems rather increased
corrosive attack into the steel substrate as compared to the case of a
A :: Mode
B :: Mode
50 h x×4c
50 h ×
II :: Intergranular
i i/ /mm
Attack,DDi i/ /mm
Fig.5 Result of the microstructural evaluation of corrosive failure both
in coating layer and into steel substrate of various coating
systems at 450ºC.
50 h ×
A :: Mode
B :: Mode
Thickness, CCi i / /mm
I : I:Intergranular
i i / mm
Fig.6 Result of the microstructural evaluation of corrosive failure both
in coating layer and into steel substrate of various coating
systems at 550ºC.
cyclic corrosion, while for the no-coated 304 steel (B) the reversed
result is obtainable. This might be attributed to the fact that a
corrosive attack can be active for extending the corrosion front
persistently for 200h under the isothermal condition, whilst under the
thermal cycle condition the corrosive action must be interrupted at
every thermal cycling of 50h by virtue of the solidification and
modification of the molten phase in the ash constituents so as for the
renewed ash to penetrate once again from the coating surface toward
the corrosion front of the steel substrate. Anyway, it appears certain
that the thermal cycling should be least harmful to the coating
degradation such as an exfoliation for the thermal spray coating layer.
Figure 7 shows the representative micrographs by the backscattered
electron (BSE) image at the cross-sectional surface zone of the two
kinds of coating systems; 62HV and 62L, after the isothermal
corrosion test at 550℃ for 200h. It is evident that both the integrity
and protective effect of coating layer is superior for 62HV rather than
for 62L. In 62HV which showed the failure behaviour of mode A,
namely, distribution of the large scale defects was minimized,
although even in this coating system the chloride-induced corrosion
can be already attainable partly to the coating/substrate interface
probably along the micro-channels such as the incorporated oxides
lamella in the coating layer. In 62L which showed the mode M of
failure behaviour, on the contrary, there are a number of the relatively
Fig.7 Backscattered electron (BSE) images at the cross-sectional
surface zone of coating system 62HV and 62L specimens
subjected to high-temperature corrosion at 550ºC for 200h.
large pores in the coating layer capable of providing the preferential
penetration path for the corrosive constituents such as chlorides. This
situation observed in 62L is essentially similar to the case of APS
system; such as 62A, which has shown the mode B of failure behaviour
Influence of Coating Materials on the Corrosion Protection
From the result of quantitative corrosion failure analysis in Fig. 6, a
protective effect of the coating materials by the same HVOF spraying
was found to be different between alloy 625 and 50Ni-50Cr alloy; the
former is much superior to the latter. In general, 50Ni-50Cr alloy is
certainly regarded as one of the most corrosion-resistant alloys in the
oxidation-dominant corrosive environments. In the aggressive
environment which an oxy-chlorination may dominate as in this study,
however, 50Ni-50Cr alloy becomes highly reactive to form easily the
very volatile oxy-chlorides such as for example CrO2Cl2 [#ref2,6]. This
suggests that increasing merely in the Cr content should be less
effective for preventing the corrosive failure in the waste-to-power
generation plants, because of the preferential reaction and
consumption of the beneficial Cr [#ref2]. On the contrary, alloy 625
has been evaluated as the highly corrosion-resistant alloy in such an
aggressive environment mainly by virtue of the good compositional
balance composed mainly of Cr-Ni-Mo [#ref6]. Consequently, alloy
625 appears to be effective also as the spray coating material,
provided for an appropriate control of the composition and
microstructure in the coating layer.
Effect of Spraying Methodology on the Corrosion Protection
It was clarified that the protective effect in the coating system is
strongly dependent on the defect structure such as pores and oxide
phases incorporated and its population in the coating layer. Namely,
HVOF spraying produces the highly densified coating layer with the
minimized oxidation reaction mainly by virtue of the very high speed
impingement of the half-melting splat onto the substrate surface.
Figure 8 shows the comparison of the cross-sectional coating
microstructures made by three kinds of the thermal sprayings. It was
revealed from the image analysis on the coating cross-sections that
the average values of an apparent density of the defects such as pores
and including the grey phase of oxides are approximately 13%, 37%
and 16% for HVOF, APS and LPPS systems with alloy 625, respectively.
It is surprising for the LPPS system has relatively higher density of
defects as compared with the HVOF system. However, there are also a
number of the experimental evidences that the coating layer made by
HVOF spraying can be much densified so as for the distribution of
through-porosity to be negligible [#ref7]. Anyhow, the HVOF spraying
should be able to bring about the coating layer with a high
performance barrier function against the penetration attack even in
such an aggressive environment [#ref4].
Fig.8 Comparison of the cross-sectional microstructures for different
coating systems after spraying.
Effect of Heat-Treatment after the Spraying
From the corrosive failure analysis for different kinds of the coating
systems, a beneficial effect of adopting the heat-treatment after the
spraying was found to be pronounced only for the HVOF systems with
alloy 625. For APS and LPPS systems with alloy 625; such as 62A and
62L series, on the contrary, an improvement of the corrosion
protection by heat-treatment was hardly expected. These facts
suggest that the beneficial effect of heat-treatment is strongly
dependent on the inherent nature of coating layer; such as the density
and population of different kinds of defects. Then, the inherently
high-quality coating layer such as by HVOF spraying can be additively
modified by adopting the heat-treatment both through the
compositional equalization and microstructural control, which should
be much effective to remove the short circuit or easy penetration path
for the corrosive constituents such as chlorides.
(1) The corrosive failure behaviour of the thermal spray coating
systems by HVOF, APS and LPPS with 625 and 50Ni-50Cr alloys can be
classified into three modes, according to the morphological features of
corrosive damage in the coatings.
(2) The most improved corrosion performance was obtainable for the
HVOF systems with alloy 625, with annealing heat-treatment after the
(3) Coating requirements for the much improved performance against
the corrosive failure were discussed from the compositional and
microstructural aspects of the coating through the spraying
methodologies and materials along with heat-treatment.
Authors wish to acknowledge Mr. K. Tokushima of KUBOTA
Corporation for the competent experimental work in this study.
!ref1 ‘Application of new corrosion-resistant superheater tubings for a
550ºC, 9.8MPa high-efficiency waste-to-energy plant’, Y. Kawahara, N.
Orita, M. Nakamura, S. Ayukawa and T. Hosoda, Corrosion 99, NACE
Int’l., Paper No. 91, pp1-19, 1999.
!ref2 ‘Materials innovation toward establishment of the advanced
waste-to-energy recovery system’, M. Yoshiba, Trans. MRS Japan, 24,
!ref3 ‘Complicated high-temperature corrosive damage and
countermeasure of heat-resisting alloy systems in aggressive
environment of advanced waste-to-power plant’, M. Yoshiba, Materia
Japan, 38, pp203-211, 1999.
!ref4 ‘Corrosion of coating materials in oxidizing and hydrogen
chloride containing atmospheres’, S. C. Cha and P. Woelpert, Mater.
and Corros., 53, pp886-892, 2002.
!ref5 ‘Test methods for fuel oil ash corrosion of heat resisting alloys’,
R. Tanaka and O. Miyagawa, JEMT, Trans. ASME, Oct., pp322-329,
!ref6 ‘The role of chlorine in high temperature corrosion in waste-to-
energy plants’, G. Sorell, Mater. at High Temperatures, 14, pp137-150,
!ref7 ‘Evaluation of through-porosity of HVOF sprayed coating’, J.
Kawakita, S. Kuroda and T. Kodama, Surf. Coat. Technol., 166, pp17-