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@PHDTHESIS{Naji:730991,
author = {Naji, Ammar},
othercontributors = {Schütze, Michael and Beck, Wolfgang},
title = {{D}esign of {A}l diffusion coatings for {F}e-based and
{N}i-based alloys; 1st ed.},
volume = {16},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {Shaker Verlag},
reportid = {RWTH-2018-227226},
isbn = {978-3-8440-6204-5},
series = {Schriftenreihe des DECHEMA-Forschungsinstituts},
pages = {XII, 117 Seiten : Illustrationen},
year = {2018},
note = {Zweitveröffentlicht auf dem Publikationsserver der RWTH
Aachen University; Dissertation, RWTH Aachen University,
2018},
abstract = {Components, which consist of metallic materials and operate
at high temperatures, can degrade due to high temperature
corrosion attack. The most frequently occurring phenomenon
is oxidation. When metallic materials are exposed to an
oxidizing atmosphere, it is desired that elements, which are
called protective oxide forming elements, diffuse from the
material interior to the material surface and form a dense
and slow-growing oxide scale, which acts as a diffusion
barrier and decelerates further oxidation. Since for several
reasons most high temperature alloys contain only a limited
amount of protective oxide forming elements, coatings are
applied with higher amount of these elements. The concept of
the diffusion coatings is to enrich the substrate surface
with one or more protective oxide forming elements (e.g.
Al). Aluminization of Fe- and Ni-based alloys leads to Al
diffusion coatings, which can consist of one phase or
several stacked phases, depending on the Al activity within
the intermetallic phase, according to the Fe-Al and Ni-Al
phase diagrams. In this work, the aim was to develop a
predictive design procedure for the manufacturing of pack
cementation Al coatings on austenitic steels and Ni-based
alloys. The pack cementation process is a CVD (chemical
vapour deposition) process, where the substrate to be coated
is embedded in a powder mixture, consisting of the
deposition element (e.g. Al), an activator (e.g. NH4Cl) and
a filler (e.g. Al2O3) and is heated in a tube furnace for
several hours in an $Ar/5\%H2$ inert atmosphere. The coating
design is based on thermodynamic and kinetic considerations
of the pack cementation process. Thermodynamic
considerations were conducted by calculations with the
thermodynamic software FactSage®, to determine the Al
activity (total partial pressure of Al carrying halides)
within the pack powder as a function of process temperature
and powder composition. Furthermore, the determination of
the full range of the binary phase diagram of the Fe-Al and
Ni-Al systems and the Al activities of these systems were
calculated as a function of temperature and mole fraction.
Kinetic values, as the diffusion coefficient, which affect
the resulting coating thickness, have been determined via a
limited amount of experiments for each alloy system,
followed by Matano analysis. It was shown that based on the
model considerations and the collection of the thermodynamic
and kinetics data for a material/deposition element couple,
the coating design approach developed enables a quantitative
prediction and adjustment of the resulting coating
properties (intermetallic phases and coating thickness) for
a wide range of process parameters. Coating experiments on
austenitic steels (AISI 321, AISI 314 and Alloy 800) and a
Ni-based alloy (Alloy 601) have shown that low pack process
temperatures (up to 900°C) promote the formation of HA
coatings. This observation is in agreement with the
thermodynamic calculations, but also kinetic considerations
show that a high process temperature promotes the
interdiffusion of Al from the coating to the interior of the
material during the coating process, which promotes the
formation of LA coatings. The coating design postulates that
it is possible to determine the entire kinetic values (the
pre-factor D0, the activation energy EA and the constant q)
for a deposition element/substrate couple by means of three
“calibration” pack experiments at three different
temperatures. The determined diffusion coefficients for the
coating procedure on AISI 321, AISI 314, Alloy 800 and Alloy
601 at 800, 900 and 1000°C showed good agreement with
literature values. The collection of the entire
thermodynamic and kinetic information made it possible to
predict the coating microstructure for these four materials
and to compare the predicted and experimentally formed
coating properties, which showed good agreement. On the
other side, the coating design contains limitations. For
example, an extensive activator amount in the pack, which
would theoretically cause a higher Al activity in the pack,
leads to an attack of the substrate by the hydrogen halides.
The co-deposition of another element to the main deposition
element Al reduces the Al activity within the pack, since
the activator is consumed by both deposition elements.
Experiments have shown that Si and Hf can be co-deposited to
an Al coating. The coating thickness is reduced in
comparison to a mono-element Al coating, which is not only
caused by the Al activity reduction due to co-deposition.
Diffusion coefficient determinations of Al and the
co-deposition elements (Si and Hf) have shown that also the
diffusion coefficient of Al was reduced, because the
co-deposition element occupies Al lattice sites. Cyclic
oxidation experiments in an oxidizing and reducing
atmosphere at 1000°C have shown that Si co-deposited Al
coatings enhance the high temperature corrosion resistance,
since the Al activity within this two-element coating is
lower in comparison with the mono-element Al coating. Thus,
the Al interdiffusion to the interior substrate and the
coating brittleness is reduced. Also, Hf co-deposited Al
coatings have shown an enhancement compared to the
mono-element Al coating in a way that oxide scale thickness
is lower, which indicates a slower oxide scale growth rate.
The design concept has successfully been applied to a
combustion chamber in a reformer system and is available for
further use in coating technology.},
cin = {522110 / 520000},
ddc = {620},
cid = {$I:(DE-82)522110_20140620$ / $I:(DE-82)520000_20140620$},
typ = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2018-227226},
url = {https://publications.rwth-aachen.de/record/730991},
}
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