Materials Chemistry and Physics 159 (2015) 10e18
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Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
High temperature (salt melt) corrosion tests with ceramic-coated steel
Adelheid Schütz a, Martin Günthner b, Günter Motz b, Oliver Greißl c, Uwe Glatzel a, *
a
University Bayreuth, Metals and Alloys, Ludwig-Thoma-Str. 36b, D-95447 Bayreuth, Germany
University Bayreuth, Ceramic Materials Engineering, L.-Thoma-Str. 36b, D-95447 Bayreuth, Germany
c
EnBW Kraftwerke AG, Schelmenwasenstraße 13-15, D-70567 Stuttgart, Germany
b
h i g h l i g h t s
Corrosion wall thickness losses of 400 mm/2 weeks occurred in a waste incinerator.
Abrasion is a major problem on superheater tubes in waste incinerators.
Laboratory salt melt tests can simulate metal corrosion in waste incinerators.
Corrosion protection coatings for steel (temperature: max. 530 C) were developed.
Higher steam temperatures are possible in WIs with the developed coatings.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 August 2013
Received in revised form
2 March 2015
Accepted 11 March 2015
Available online 31 March 2015
Thermal recycling of refuse in waste-to-energy plants reduces the problems connected to waste disposal,
and is an alternative source of electric energy. However, the combustion process in waste incinerators
results in a fast degradation of the steam-carrying superheater steel tubes by corrosive attack and
abrasive wear. Higher firing temperatures are used to increase their efficiency but lead to higher
corrosion rates. It is more economical to apply protective coatings on the superheater steel tubes than to
replace the base material.
In-situ tests were conducted in a waste-to-energy plant first in order to identify and quantify all
involved corrosive elements. Laboratory scale experiments with salt melts were developed accordingly.
The unprotected low-alloyed steel displayed substantial local corrosion. Corrosion was predominant
along the grain boundaries of a-ferrite. The corrosion rate was further increased by FeCl3 and a mixture
of HCL and FeCl3. Coatings based on pre-ceramic polymers with specific filler particles were engineered
to protect superheater tubes. Tests proved their suitability to protect low-alloYed steel tubes from corrosive attack under conditions typical for superheaterS in waste incinerators, rendering higher firing
temperatures in waste-to-energy plants possible.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Corrosion (tests)
Interfaces
Polymers
Ceramics
Coatings
1. Introduction
Steel corrosion still leads to enormous economic losses and the
protection of steel constructions and industrial plants is therefore a
topic of current research. A multitude of laboratory corrosion tests
appear in the technical literature for both uncoated and coated steel
[[1e4]; to name but a few]. Varied test parameters include the
temperature, the chemical, physical and mechanical environment
as well as the testing time [5]. Material corrosion due to chemicals
or electrochemical influences (chlorides, nitrates, sulphates,
* Corresponding author.
E-mail address: [email protected] (U. Glatzel).
http://dx.doi.org/10.1016/j.matchemphys.2015.03.023
0254-0584/© 2015 Elsevier B.V. All rights reserved.
carbonates, hydroxides and corrosive gases) are described in
numerous publications [[6,7], etc.]. Few corrosion studies correlate
the results of the different tests [8e18].
Corrosion is a major problem in waste-to-energy incineration
plants (WI), too [19]. Different studies describe the corrosive gases
that are released in dependence of the waste fractions and the
temperature [20,21] and the origin of corrosive elements in WIs
[22]. Chlorine is the most detrimental of the gases [20]. WIs which
are similar in construction may suffer from different corrosive attacks, even when fired with the same type of waste [23]. Erosion
also plays a role in material loss of WI superheater tubes [24], as fly
ashes contain silicate spheres covered with sulphates and chlorides
[25].
High temperature corrosion studies of WIs are complex, due to
A. Schütz et al. / Materials Chemistry and Physics 159 (2015) 10e18
different plant systems and combustion methods [26]. The European Waste Framework Directive has set targets for municipal WI
plants of 60% recycling and 40% energy recovery by 2020 [27]. One
way to increase their efficiency is to raise the temperature of the
steam entering the turbines. Along these lines, a temperature raise
from 400 C to 500 C will result in a 20% increase of the generated
power [27] but also entails a higher corrosion rate of fireside materials, namely heat exchanger tubes, leading to shorter revision
intervals with shutdown periods of the whole incineration plant
[28]. Valid corrosion tests should therefore focus on one specific
section of the waste incinerator with a specific temperature range,
corrosive elements and mechanical load.
Great efforts are made to reduce corrosion in WIs. Ferritic steel
oxidises more easily than austenitic steel in chloride containing
high temperature atmospheres [29]. While steel shows a higher
corrosion rate than the more expensive nickel alloys, it tends to be
covered with a thick, non-protective Fe2O3 layer, austenitic highnickel alloys show localised pitting corrosion and the NiO scale is
not protective [30]. High-alloyed steel with high chromium and/or
nickel content shows a greater resistance to high-temperature
corrosion. However, sulphur can penetrate through the protective
chromium oxide scale making it less protective [31]. Therefore, the
use of the more expensive high-alloy steel in WIs is limited.
Approaches to protect the steel heat exchanger tubes from
corrosive attack include pack cementation coatings [32,33], thermally sprayed coatings [34,35], high velocity oxy-fuel nickelechromium containing coatings [29], or cladded coatings [36,24].
Furthermore, plasma sprayed coatings [37]; plasma immersion ion
implantation [38] and plasma nitriding [39] have been studied to
act as protective coatings. Despite great progress with these
advanced coatings there is still a need for further research in order
to enhance the durability of coatings under the extreme conditions
of WIs.
The objective of the presented research was to develop a cheap
coating system that could protect superheater tubes in WI plants
from corrosion for a period of one to two years. Corrosion mechanisms on uncoated steel tubes need to be understood to set up
laboratory-scale tests to investigate the protective properties of the
coatings. The present paper describes this development process
from determining the factors influencing corrosion on superheater
tubes in a WI, over developing lab tests to testing actual coatings.
The coatings were meant to equally protect steel tubes in biomass
incinerators, where corrosion caused by high alkali contents plays
also a major role [40,41]. Especially potassium chloride is detrimental for the steel in biomass incinerators [42]. Therefore, it was
used in the presented laboratory tests, too.
2. Experimental
2.1. Steel samples
All investigations were performed with the low alloy boiler steel
1.7335 (13CrMo4-5, mild steel, corresponding to ASTM A-213 T11;
AISI A182 e F11 or F12). Different sample geometries were used
(plates and tubes). The samples were ground to RA 0.6 mm (RZ 4 mm)
before their use.
A simple before-and-after-weighing of the samples and test
rings was not suitable to determine the corrosive loss, since mass
changes during the test can have different origins (deposition of
combustion residues, formation and/or loss of corrosion products,
and so on). Hence, inert markers of the original surface were
required to measure the material loss from the tubes diameter
during the probe tests. Therefore, nickel alloy rings were applied
directly on the probe ring surface by laser cladding using a fine
powder of Ni 748-3 (Praxair) and a Nd-YAG laser DY 044/DILAS DL
11
040H (Rofin Sinar) - without actually melting the probe ring surface
(Fig. 1).
The deposits on the probe ring samples were analysed after the
two-week test in the WI for the presence of chlorides and analysed
with inductively couple plasma optical emission spectroscopy (ICPOES).
Metallographic cross sections were prepared from all tested
rings. They were cut and embedded in the resins DUROFAST® or
Technovit. The cross sections were ground and polished with silicon carbide to mirror finish in ethanol. The metallographic crosssections were sputtered with graphite for all SEM investigations.
In some cases, the metallographic cross sections were etched using
an ethanol solution with 3 vol.% HNO3 to investigate the grain
structure and corrosion mechanisms.
2.2. Coatings
Coated steel plates and tubes were initially sand blasted and
cleaned with acetone before the coatings were applied. Three
different types of coating were developed and tested on the probe
rings to investigate the effect of the WI conditions on the actual
coating. Coatings consisted of two layers that were consecutively
applied onto 1.7335 substrates. The bond coat was always prepared
from a commercial pre-ceramic polymer as a precursor (polysilazane PHPS, supplied as a 20 wt.% solution in di-n-butylether by
Clariant Advanced Materials GmbH) that forms a pre-ceramic layer
after a heat treatment at 110 C for 1 h in air [43].
Coating type one: BN (Henze BNP GmbH, Germany) particles or
alumina particles (Alfa Aesar, 042572, D50 ¼ 5.14 mm) were added
to a polymer suspension described in the previous section.
Coating type two: A subsequently applied topcoat consisted of
an equal volume fraction mixture of borosilicate particles (8470
glass particles, Schott AG) and commercial barium silicate particles
(G018-311 glass particles, Schott AG) which have similar thermal
expansion coefficients (9e10 106/K [44]) as the steel substrate
(14.5 106/K [45]). Homogeneous coating slurries were achieved
by dispersing the different powders in ether using a commercial
dispersing agent (disperk ether). The coatings were applied by a dip
or spray coating process.
Coating type three: In some cases, zirconia particles (bought
from Alfa Aesar, USA), alumina particles (Alfa Aesar, 042572,
D50 ¼ 5.14 mm) as well as the polycarbosilazane PSZ (Clariant
Advanced Materials) were used in addition to the mixture of glass
particles (same particles as in coating type 2).
After coating with the suspensions, all steel samples were heat
treated in air at 700 C for 1 h. This allowed the interface to form
and the glass particles to completely soften. The preparation of the
coatings is described in more detail in reference [46]. The coatings
and their interface with the steel substrates have been thoroughly
Fig. 1. Steel probe ring with surface markers for corrosion test in the WI plant.
12
A. Schütz et al. / Materials Chemistry and Physics 159 (2015) 10e18
characterised, the results have been published in [47]. It was shown
that this type of precursor system in combination with specific
glass filler particles leads to compact coatings with a thickness of
up to 100 mm that adhere well [47].
2.3. Analytical procedures
To investigate corrosion at the interface regions between
coating and substrate, the coated samples were sectioned before
and after the corrosion tests, embedded into a phenolic hot
mounting resin with carbon filler (Polyfast, Struers GmbH) and
mechanically ground up to 1000 grit emery paper and finally polished to a mirror finish with 1 mm silicon carbide powder in an
ethanol suspension. The samples were cleaned in ethanol using an
ultrasonic bath for 15 min, then degreased with acetone and dried.
The WI residues as well as some coatings were examined using
thermogravimetry (TGA) and differential thermal analysis (DTA)
(thermobalance TG L81-I STA Linseis GmbH). Investigation of the
thickness and the composition of the coatings as well as of the
number and size of defects caused by corrosive attack were performed using optical microscopy (Axioplan 2, ZEISS), and a scanning electron microscope (SEM, 1540EsB Cross Beam, ZEISS) with
energy dispersive X-ray spectroscopy (EDS). The compositional
analysis was performed using EDS. The crystal phases present in
the corroded regions as well as the deposits were examined by xray diffraction (XRD). XRD patterns were recorded using a Seifert
XRD 3000 P set-up in Bragg-Brentano geometry with copper Ka
radiation (k ¼ 0.15418 nm) and a Germanium monochromator. The
XRD data were analysed using CrystalDiffract software version 1.1.2
(CrystalMaker Software Ltd., UK).
FactSage™ is an integrated database computational system for
chemical thermodynamics that has been developed by Thermfact/
CRCT and GTT-Technologies [48]. Christoph Wieland of Technische
€t Munich contributed the FactSage™ calculations. The
Universita
melting points of the oxides and their mass fractions according to
the ICP-OES results were used for this calculation.
2.4. Corrosion tests in the waste incineration plant (probes)
In the designated superheater partition of the WI surface temperatures of 400e500 C are expected. Furthermore, abrasive particles (silicates) travel at a velocity of approximately 8 m/s and hit
the tubes surface at a small incident angle.
The probe rings material was mild steel 1.7335. A period of two
weeks was chosen for the probe tests to achieve a representative
sampling of different types of waste. The maximum temperature to
be attained in the near future at this type of superheaters is 530 C,
therefore, 530 C was chosen as testing temperature for the probe
tests in the actual power plant. Temperature was controlled with
thermocouples NiCrNi on the inside wall of the specimen, air was
used to cool if necessary. The temperature at the outside of the
mounted probe rings was adjusted between 510 C and 550 C. The
design of the probe holder was such that four rings with a width of
30 mm each, a wall thickness of 4 mm, and a diameter of 44 mm
could be mounted on the probe head of a water-cooled support
lance (a schematic drawing of the probe holder is depicted in Fig. 2
[23,49]).
2.5. Laboratory scale corrosion tests
Laboratory scale corrosion tests were developed from the results
of the corrosion probe tests in the waste incinerator. To evaluate the
corrosion resistance, coated and uncoated samples of 1.7335
(13CrMo4-5; mild) steel were tested in a salt melt at 530 C for
168 h in refractory crucibles. The chemical composition of the
Fig. 2. Schematic view of the probe head with electrical wiring; from [23].
standard salt melt is shown in Table 1. To extrapolate the measured
corrosion rates, lab scale corrosion tests with duration of 2.5 h, 20 h,
24 h and 336 h were conducted as well (same salt melt composition: Table 1). The temperature of the samples (530 C) was
controlled using NiCrNi thermocouples. The lids were sealed onto
the crucibles using M.E.Schupp® e FiberPlast C 1800 D High Temperature Glue.
3 wt.% FeCl3 and 3 wt.% HCl were added to 100 g of the standard salt mixture as shown in Table 1 in a separate corrosion test
series to increase the corrosion rate in a controlled way (test
duration: 168 h and 336 h).
For a 20 h corrosion test series, only 0.5, 1 or 5 wt. ppm FeCl3
was added to 100 g of the standard salt mixture as shown in Table 1.
1.7335 steel plates with a weight of 9 g each were used. Residues
from the salt melt as well as brittle and porous metal oxides were
mechanically removed from the samples surface. The corrosion rate
was determined by before-and-after test weighing to detect even
small corrosive losses.
3. Results and discussion
3.1. Probe tests in the superheater section of the waste incinerator:
corrosive attack
This test was done to identify the chemical elements that lead to
corrosion. Abrasive wear is considerable in WIs, too. The resistance
against abrasive wear had been tested throughout the coatings
development process and is published in [47]. The whole probe
mount was covered with a 10 mm thick layer of deposits after the
two weeks test in the waste incinerator. The outer layer was
crumbling off, whereas the inner, darker layer was consolidated
(Fig. 3). Samples for element analysis were taken from the outer
deposits.
The following elements were detected in the deposits (Table 2;
apart from chloride which could not be detected with this method
and had been analysed separately using an aqueous AgNO3 acidic
solution).
Table 1
Composition of the salt melt corrosion tests.
Initial salt mixture
NaCl
Na2SO4
KCl
K2SO4
ZnCl2
ZnSO4
wt.%
Gas atmosphere
vol.%
19.0
N2
71.2
22.8
O2
18.2
24.7
H2O
10.0
28.5
SO2
0.02
2.5
HCl
0.08
2.5
A. Schütz et al. / Materials Chemistry and Physics 159 (2015) 10e18
13
Fig. 3. Deposits on probe rings after a test in the WI.
Fig. 4. Optical micrograph of the GRAIN BOUNDARY corrosion.
Considerable concentrations of calcium, sulphur, potassium,
sodium, silicon, aluminium, zinc, iron, magnesium, and phosphor
were found. Corrosive phases include calcium sulphate, sodium
chloride, zinc chloride and potassium chloride. These corrosive
compounds should be used in lab scale corrosion tests as well. The
measured lead concentration is low. Lead was therefore excluded
from the laboratory testing mixture.
Chlorides and sulphates were identified as the main corrosive
species in the WI. An initial salt mixture (Table 1) of chlorides and
sulphates was used to set up lab scale corrosion tests from the
probe test results. Similar corrosion products and corrosion
mechanisms were observed in the laboratory and the probe tests. A
preferential corrosion of cementite and along the grain boundaries
of a ferrite was observed (Fig. 4).
This is in accordance with high temperature corrosion literature: chlorides attack carbides in steels first [50]. Corrosion starts
with intergranular attack in high alloyed steel such as FeeNieCrsteel [50]. The corrosion rate in the laboratory tests needed to be
adapted. This was done by adding various amounts of FeCl3 and
HCl.
Active oxidation of the steel tubes takes place from 450 C onwards [28], whenever chlorine (from alkali chlorides, HCl or Cl2)
diffuses back to the oxide/metal interface and is hence available for
the formation of further iron chloride [41]. FeCl2 acts as a catalyst
for the oxidation of steel in an HCl atmosphere [25,51]. A typical
corrosion rate of carbon steel in a molten eutectic LiCleKCl mixture
is 63 mm/year (stainless steel: 20 mm/year) versus 15 mm/year in an
eutectic 86.3 mol% NaNO3, 8.4 mol% NaCl, 5.3 mol% Na2SO4 mixture
(stainless steel: 1 mm/year) [52]. Since the FeCl2 is reused in the
active corrosion process, only Fe2O3 and Fe3O4 are to be found on
the corroded surfaces [53] and inside the deposits. This highlights
the role of the chloride anion in the active oxidation processes.
Hence, active oxidation entails a higher corrosion rate of fireside
materials, namely heat exchanger tubes, leading to shorter revision
intervals with more frequent shutdown periods of the whole
incineration plant.
Liquid phase corrosion is faster than corrosion originating from
solid deposits. Deposits on superheater tubes can have a melting
point as low as 272 C [21]. Magnesium sulphate addition can raise
the melting point of the ash and consequently depress corrosion
[54]. The actual deposits from the waste incineration plant, however, start to melt at 600 C and evaporate at 1200 C (Fig. 5).
The mass fractions of solid and liquid deposits were calculated
using FactSage. This method for the calculation of similar deposits
is described in [55]. According to these calculations, the oxide deposits start to melt at approximately 900 C. Equal mass fractions of
the oxide deposit components are present in molten state and in
gaseous state at 1250 C. These temperatures are well above the
temperatures that are actually attained on the superheater surface;
therefore, liquefaction or even evaporation of these oxide deposits
is highly unlikely. Hence, as oxide deposits form, they could protect
the ceramic coating from abrasive wear and lower the surface
temperature of the steel tubes. The uncoated 1.7335 ring showed
severe pitting corrosion in vicinity to the Ni 748-3 rings (Fig. 6). A
more uniform corrosion was observed over the whole surface of the
ring. The areas distant from the In686 rings were used to measure
the corrosive loss.
From these micrographs (Fig. 6) and further samples a corrosion
rate corresponding to a material loss of 350e450 mm/2 weeks was
calculated for superheater tubes on this position in the WI plant.
This corrosion rate would lead to an annual corrosion of 9e12 mm
per year. However, the corrosion rate should increase with the
increased thickness of deposits (if corrosive gases can penetrate) as
discussed in [56]. The authors report diffusion controlled kinetics
underneath the deposits, leading to low pO2 and high pCl2, which
are important factors for corrosion. Indeed, Fe2O3 residues are
found in the outer layers of the crumbly deposits. However, experience has shown that corrosion of the superheater tubes slows
down after an initial period with this high corrosion rate. It is not
fully understood whether this is because of a passivation effect of
the deposits or whether limited diffusion of corrosive elements
lowers the corrosion rate. Thick deposits of fly ash or firing residues
Table 2
Element concentration (weight ppm) in the deposits on probe rings 1 and 4 from a WI plant (ICP-OES).
C
400
Ag
40
Al
32,000
Co
40
B
5500
Li
160
Mg
11,000
Ba
3300
Mn
850
Bi
41
Na
72,000
Ca
135,000
Ni
450
Cd
120
Cr
900
Cu
1000
P
6300
Pb
700
S
104,000
Fe
17,000
Si
57,000
Ga
110
In
50
K
77,000
Sr
300
Ti
9500
Zn
17,000
14
A. Schütz et al. / Materials Chemistry and Physics 159 (2015) 10e18
Fig. 5. DTA and TGA graphs of the deposits from a WI plant.
Fig. 6. Metallographic cross section of the probe ring (Fig. 1) after two weeks of corrosive/abrasive wear in the WI plant. The horizontal line marks the original steel surface.
can actually decrease the corrosion rate (and lower abrasion) since
they reduce the metal surface temperature [21]. In those deposits,
HCl is the predominant species over Cl2 (i.e. at low oxygen partial
pressure [51]). A further factor in corrosive decay is the removal of a
possible passivation layer due to abrasive forces of the particle
freight. Deposits on the superheater tube would prevent the mechanical removal.
The measured element concentrations and corrosion rates are
subject to temperature fluctuation and varying waste composition.
A corrosion rate of 350e450 mm/2 weeks was determined in the
present study. For comparison with current literature values: The
corrosion rates of the ferritic steel Fe2Cr1Mo in circulating fluidised
beds burning refuse derived fuel at 460 C are 5 mm/year; at 516 C
corrosion rates of 13 mm/year were measured [57]). This highlights
the influence of an increased temperature on the corrosion rate.
Hence, the temperature for laboratory corrosion tests should be
chosen at the maximum temperature that is aspired to be attained
in the specific part of the power plant.
Although the waste is being homogenised before being fired in
WI power plants, the two weeks probe test will only reflect a
fraction of reactions which occur on the superheater tubes of
different WI plants over a two-year period of time (optimum
revision interval).
One important finding of the probe tests was the discovery of
the abrasive loading of the silicate particle freight contained in the
flue gas/ashes. Developed protective coatings need to have a high
abrasion resistance at elevated temperatures, for example at
530 C; otherwise a good corrosion resistance is not of much worth
during operation. Particles are deposited onto the superheater
tubes and act as abrasion protection. This could be seen from the
probe tests.
The examined scales contained oxides of the alloys components.
Other researchers reported Fe3O4 deposits close to the metal, and
ۧbauer spectroscopy allowed the
Fe2O3 further out [57]. Mo
detection of even more phases in literature: a-Fe2O3, Fe3O4, gFe2O3, d-FeOOH, a-FeOOH, Fe(OH)2, Fe(OH)3 (180 h steam oxidation
at 660 C [58]). Unfortunately, this could not be verified in the
present study.
3.2. Laboratory corrosion test results
From findings after probe tests, laboratory salt melt corrosion
test have been developed (Table 1). Severe corrosion was observed
on uncoated 1.7335 steel samples after the 168 h corrosion tests in
the salt melt (Table 1). A preferential corrosion of cementite and
along the grain boundaries of a ferrite was measured in the laboratory tests, too. Besides Fe2O3 the following phases were detected
using XRD (test corrosion products are shown in Fig. 7).
The corrosion salt melt test has been altered by the addition of
different concentrations of FeCl3 to the mixture stated in Table 1.
The addition of FeCl3 leads to an increase in corrosion rates (single
tests, see Fig. 8).
Adding HCl to the FeCl3 containing salt mixture increases the
corrosion rate. A comparison of all measured local corrosion rates
with corresponding error bars (for the different repetitions of the
corrosion tests) is shown in Fig. 9.
Corrosion rates of tests in the actual deposits and in the initially
used salt mixture are of the same magnitude. Annual corrosion
losses of several mm due to molten salts were already found at
300e350 C in literature [26]. This is in good agreement with
present findings (several mm when extrapolated to one year). The
measured decrease in the corrosion rate when HCl was added
might not be significant due to the comparatively large experimental error. At 500 Ce600 C, HCl in the presence of moisture
accelerates corrosion, especially in the case of ferritic steel, by
causing “active oxidation” [59]. The corrosion products were
similar to the products found after the probe tests in a WI. Corrosion rates in the laboratory tests could be significantly increased by
the addition of FeCl3 and even further increased when FeCl3 and
HCl were added to the initial salt mixture. It is concluded that
laboratory test can be adapted to be more aggressive than conditions on superheater tubes in WI plants and hence they can be used
A. Schütz et al. / Materials Chemistry and Physics 159 (2015) 10e18
15
Fig. 7. Crystalline phases detected on samples after 168 h corrosion test of uncoated 1.7335 steel in salt melt (composition: see Table 1) A: Fe3O4; B: NaCl; C: KCl; D: ZnFe2O4; E:
K2SO4; F: Fe2(SO4)3; G: Cr2(SO)4; H: Na2SO4; I: CrCl3.
3.3. Protective coatings
Fig. 8. Comparison of corrosion rates for uncoated 1.7335 steel in single laboratory salt
melt tests.
to predict suitability of the developed protective layers. PbCl2 can
be added to further increase the corrosion rate [60], but was not
used in the present studies since only minor amounts of Pb were
detected in the deposits from the probe tests.
Using precursor derived ceramic coatings as corrosion protection for WI tubes is a completely new approach. It was assumed
that the coatings should have a critical thickness of at least 100 mm
to withstand the test of time. Different coatings have been developed using the corrosion tests results in an iterative process. Results are summarised in Table 3. Initially unfilled and BN- or Al2O3powder filled precursor based coatings (type one) were tested as
protective coatings. They were permeable to the ions of the corrosive salts due to shrinkage porosity of the precursor. Hence, those
coatings offered no protection of 1.7335 steel in the salt melt tests
(Fig. 10).
Only samples without open porosity offered sufficient protection from corrosive attack in the laboratory corrosion tests
(Table 1). Corrosion occurs at sites with permeable coating defects
(as shown in [61]: Al2O3e13%TiO2 tested with electrochemical
impedance spectroscopy). A compensation of the shrinkage
porosity during the ceramisation regime of the precursor was
achieved when a mixture of glass particles was employed (coating
type two) [46]. Still, coating defects such as delamination or very
large gas inclusions were a major issue to be addressed during the
development process. Only intact, gas tight coatings showed no
16
A. Schütz et al. / Materials Chemistry and Physics 159 (2015) 10e18
Fig. 9. Comparison of corrosion rates in different tests.
corrosion of the metal substrate after the laboratory corrosion tests.
Tests in WIs showed furthermore, that the coatings need to have
a certain abrasion resistance. Otherwise, the coating is removed
under the extremely abrasive conditions in the section of the superheaters. Although the porosity of the present coating could be
reduced by using glass filler particles, their abrasion resistance at
high temperatures is poor and needed to be improved first. This
was achieved by adding zirconia and alumina particles or zirconia
particles and PSZ fillers to the glass mixture. The abrasion resistance tests described in [47] showed that a coating with ZrO2 and
Al2O3 in the standard mixture of glass particles has a very high
abrasion resistance, making it suitable to withstand WI conditions.
Similar results were found for a coating containing 25 vol% ZrO2
and 10 vol.% PSZ in addition to the mixture of glass particles [47].
This coating showed a very good corrosion resistance in laboratory
corrosion tests, too (Fig. 11).
Another factor that influences corrosion is the existence of
joined micro-pores [62]. This has to be avoided at all costs. Oxygen
diffusion and iron diffusion through the coating and scale determine the rate of the whole corrosion process. Oxygen and iron
migrate independently and the reactions at phase boundaries are
fast [63]. Again, their number and size was lower, when ZrO2 and
Al2O3 particles were used in addition to the glass filler particles in
the precursor derived ceramic coatings.
The present coating system is easier to apply and less expensive
than thermal spray coatings, for example. With this optimised
coating the steam temperature could be increased in WI power
plants and hence the efficiency could be further increased as stated
in the introduction. However, as soon as coating defects occur;
accelerated corrosion will take place at higher steam temperatures
leading to failure of the whole superheater unit.
Table 3
Developed and tested coating systems.
Substrate
Bond coat
Functional coat
Lab corrosion tests: coating protects steel?
Abrasion resistance
1.7335
PHPS
Precursor
Precursor
Precursor
Precursor
Precursor
Precursor
Precursor
No (shrinkage porosity)
No (shrinkage porosity)
No
yes
Yes
Yes
Yes
Not tested
Not tested
Not tested
Low
Medium
High
High
(type 1)
þ BN (type 1)
þ Al2O3 (type 1)
þ molten glass particles
þ molten glass particles
þ molten glass particles
þ molten glass particles
(type 2)
þ ZrO2 (type 3)
þ ZrO2 þ Al2O3 (type 3)
þ ZrO2 þ PSZ (type 3)
Fig. 10. Element distribution of a porous precursor derived coating on 1.7335 steel after a corrosion test in the initially used salt mixture. Corrosive elements penetrate the coating
easily and can be detected in the underlying steel substrate, too.
A. Schütz et al. / Materials Chemistry and Physics 159 (2015) 10e18
Fig. 11. An optimised composite coating with a high abrasion resistance (at 530 C) on
a 1.7335 steel substrate after one week salt melt test at 530 C.
3.4. Probe tests in the superheater section of the waste incinerator:
coatings
Large abrasive particle freight in the superheater partition leads
to considerable material loss on the probe rings coated with glass
particles. Only the tested coating type three with ZrO2 addition to
the mixture of glass particles resisted the abrasive wear (Fig. 12) as
predicted by the abrasion resistance tests discussed in [47].
No corrosive attack was detected underneath the ceramic
coating containing glass particles and ZrO2 particles. This layers
offer sufficient corrosion protection (Fig. 13). Hence or otherwise,
the coatings have to have a high abrasion resistance if they are to be
used in this section of the WI.
4. Conclusions
Initial corrosion rates are extremely high in the waste incineration plants in the superheater section. A wall thickness loss of
approximately 400 mm/2 weeks occurred in a waste incinerator.
Although the corrosion rate slows down, protection is required for
superheater tubes.
17
Fig. 13. Metallographic cross section of a coating containing glass and ZrO2 particles on
1.7335 steel after a two week test in a WI.
Abrasion is a major problem on superheater tubes in waste incinerators. Corrosion protection coatings must show a high abrasion resistance.
Laboratory salt melt tests can simulate metal corrosion in waste
incinerators. They should contain large amounts of chlorines and
sulphates and have a low melting point.
Corrosion protection coatings for steel (temperature: max.
530 C) were developed. Higher steam temperatures are possible in
WIs with the developed coatings.
Acknowledgements
The authors acknowledge the financial support of the Bavarian
State Ministries of Science, Research and Art, and that of Economy,
Infrastructure, Transport and Technologies, Germany and the
companies Clariant AG and EnBW Kraftwerke AG as a part of the
research initiative “KW21” (Project Nr. BY 7 DE). Christoph Wieland
€t Munich contributed the FactSage calcuof Technische Universita
lations. The authors acknowledge this contribution. The authors
also thank Tobias Katzmann and Eva Eisinger for their support of
some of the experiments (Tobias Katzmann (corrosion rate of
probes); Eva Eisinger (tests with FeCl3 addition).
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