Note: Descriptions are shown in the official language in which they were submitted.
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The invention concerns a corrosion-protective layer for
heat-resistant alloys, consisting of a compound material
(i'Verbundswerkstoff"), using as bases alloys possessing high-
temperature characteristics. The invention concerns further a
process for making such corrosion-protective layer and its
practical application.
Many proposals are known concerning corrosion-protective
layers for heat-resistant alloys which are subjected during opera-
tion to high thermal and mechanical as well as chemical stresses.
Such layers are particularly important for the protection of ma-
terials used for thermal machines, being exposed under high temper-
atures to creep and fatigue conditions and also being subjected to
the adverse influence of corroding gases. Specifically developed
heat-resistant alloys (for example super-alloys on a base of
nickel or cobalt) generally possess only moderate resistance to
oxidation ~t running temperatures, and their resistance to corro-
~sion is poor in many cases of applications, especially within theupper temperature range. Any homogeneous addition to these alloys
by use of corrosion-inhibitine components (such as chromium)
throughout the cross-section of the material is very limited in
lts appllcation because such admixtures will in tlme lower the
stability. ~or this reason attempts have been made to overcQme
this problem by the separation of functions. A highly heat-
resistant core material will take up the thermal-mechanical load,
~ with -the resistance to corrosion being insured by the surface
; area, or an area near the surface, of the work piece, disregard-
ing in the latter case high thermal stability. For this purpose
all feasible techniques have been tried, from the establishment
of a protective layer at the surface of the core material by
3 diffusion of corrosion-inhibiting substances to the multi-layered
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placement o~ external surface covers. There are known homogeneous
as well as heterogeneous layers, produced by metal spraying,
carburizing, sintering, cathode evaporation,electroplating,
metallurgical powder processes and other methods. Another group
is represented by protective layers applied metallurgica1ly in the
molten state, or by sintering in the fluid phase respectively (for
example Forbes M. Miller and `Nikola~5 T. Bredzs: ~'Protecting
metals in corrosive high-temperature environmentS~', Metal Pro-
gress, March 1973, pages 80 to 84~. In case of this process,
alloys with a relatively low melting point and additives which
will reduce the melting point stlll further (usually boron) are
solidly connected with the core material by thermal treatment in
a furnace at a temperature near their liquidus point, thereby
forming tight protective layers. The homogeneous protective
layers, applled to a heat-resistant alloy or produced at itS
~surface in accordance with the present state of art will, after a
relatively short period of operation, lose a great percentage of
their corrosion-inhibiting substances and thus become ineffective.
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~Their life-span, and consequently the durability of the core
material to be pratected, is therefore limited. For metallurglaal
and phy~ical rea80ns it i8 not feasible, irre8pective of the
process being used, to attain unlimited concentrations of
I ~ corrosion-impeding components within the homogeneous protective
` layers. On the other hand, protective covers, built up from
several layers of diverse chemical composition have the tendency
to become brittle and to flake off. Metallization and similar
processes result initially ln porous layers, requiring sintering
to produce solidity. In many instances they contain additives
to reduce the melting point, With detrimental effects on the
B ~ ~ characteris cs o r the final alloy. Here again, the e rre Ctiveness~
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of the protective layer and its life span are limited by the
type and the feasible amount of corrosion-inhibiting components.
It is the aim of the invention to develop compact, tightly
adherent co~rosion-protective layers, which will not chip off
the core material, and will possess a long life span and will
allow maximum choices as to materials as well as quantities
being used. The method of production should be simple,
economical and free of complicated and costly thermal treatments.
The invention solves this problem in that manner
that in case of a corrosion-protective layer of the above
defined type there are embedded in a basic substance (matrix),
consisting of a homogeneous alloy and forming a coherent body,
heterogeneously distributed islands of discrete elements which
form protective oxides and which are at least partially dis-
solved within the basic substance, and that the corrosion-pro-
tective layer is free from boron and boron compounds.
According to the invention this corrosion-protective
layer is made in this manner that the basic substance and the
discrete elements ("schutzoxydbildenden") which form protective
oxides are placed simultaneously onto the sllrface of the heat-
resistant alloy body, and are subsequently subjected to a
thermal treatment at a temperature range of 1,050C. to 1,150G.
during a period of one to four hours.
In particular there is pxovided in accordance with the
invention a corrosion-protective composition for heat-resistant
; alloys, consisting essentially of a compound material of a base
homogeneou~ alloy possessing high-temperature properties and
embedded therein heterogeneously distributed islands of dis-
crete elements which form protective oxides and which are at
least partially dissolved in solid solution in the base alloy,
; said corrosion-protective layer being free from boron or boron
compounds whereby during operation of said corrosion-protective
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layer in ~n oxidizing environment, the oxide-forming elements
of the distributed islands are supplied by diffusion into the
corrosion-protective layer.
In a particular embodiment of the invention there is
provided an article comprising a heat-resistant alloy covered
by a corrosion-protective layer of the composition.
The article may suitably take the form of a turbine
or impeller blade a turbine wheel, a compressor rotor or a
component for the combustion chamber of a gas turbine or for
the guiding vanes of a gas turbine.
In another aspect of the invention there is pro-
vided a process for the manufacture of a corrosion-pxotective
layer on a surface o a body of a heat-resistant alloy, which
comprises placing simultaneously on the surface of the body of
heat-resistant alloy a basic substance and discrete elements
which form protective oxides, and subjecting the so-treate~
body to a thermal tre~tment at a temperature of from l,OSODC.
to 1,250C., during a period of from 1 to 4 hours to at least
partially dissolve the protective oxides in solid solution in
a base alloy foxmed from the basic substance whereby, during
operation of said corrosion-protective layer in an oxidizing
environment, the oxide-fo~ming elements of the distributed
islands are supplied by diffusion into the corrosion-protective
layer.
The corrosion-protective layer may suitably be
formed in a thickness of 0.1 to 0.5 mm, preferably 0.18 to
0.22 mm,
In a particular embodiment the basic substance is
precipitated electrolytically from an electrolyte solution
which is suitably at a bath temperature of 32 to 45C. and a
cathodic current density of 150 to 250 A/m .
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The discrete elements may suitably be in 9US-
pension in the electrolyte solution, in a powder form having
a particle size of 1~ to 15~.
The thermal treatment is more preferably carried
out at a temperat~re of 1,100 to l,120C. during a period of
2 to 3 hours under an argon atmosphere or a vacuum.
The invention is based on the main concept o build-
ing up the corrosion-protective layer from one single homog-
geneous alloy, serving as the carrier for the elements which
:10 form protective oxides and which are present in the form of
~ discretely distributed particles, whereby the latter are
:~ partially dissolved within the basic substance. The discretely
embedded particles serve, so to speak, as ~tatistically dis-
tributed "stores" for ~ .
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1131947
¦ subsequent delivery by diffusion to the protective layer, so that
¦ the latter will not lack protective-oxide forming elements and its
composition will remain substantially uniform throughout a long
period of time, corresponding to the life span of the work piece.
The concentration o~ the elements which form protective oxides and
are in solution within the basic substance, measured across the
depth of the protective layer, will not change as time goes on,
as in case of the conventional, known protective layers but will
remain constant.
Further details of the invention are disclosed by the
practical example given below and partly explained also in the
appended drawing, in which
Fig. 1 shows a cross-section of the corrosion-protective
layer after its deposit at the core material; and
Fig. 2 shows the crosa-section of the corrosion-protec-
tive layer after the thermal treatment.
Specific Example of the Process
B A gas turbine blade, caSt from a nickel super-alloy
~"Inconel 738"), is first degreaged for three minutes at 40 C.in
a 10% solution of soda. The blade is then pickled anodically in
. a 20% solution of sulphuric acid at room temperature, and for
~ this purpose the work piece i8 connected With the posltive
:~ I pole of a direct current source. The current density relative to
the work piece surface may be approximately 400 A/m2. After this
preliminary treatment the blade is washed quickly and transferred,
while the surface ls still wet, to the electrolytic bath where the
corrosion-protective layer 1s applied. The bath has the following
composition:
Per 1 liter of water: NiS04 : 300 g
3o NiC12 : 80 g
~fD~ ~ B203 :8 g.
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Pulverized solid matter forming protective oxides in suspension:
Cr3Si (15% by weight of Si) : 125 g (grain size 3 - 8 ~u)
TaC : 10 g (grain size 3 - 12 ~u)
FeSi (35% by weight of Si) : 15 g (grain size 3 - 7 ~u)
The temperature of the bath is 32 to 36 C.
The bath is always kept in motion by means of compressed air, fed
from below through the bottom of the vessel and the solid matter
is thus kept in suspension. The content of solid matter can
generally range from 5 to 20% by weight.
The electrolytic deposition of the protective layer at
~; the gas turbine bl~ade takes place at a current density which
varies between 150 and 2~Y~ A/m . After four hours of processing
the protective layer reaches a thickness of approximately 0.12
to 0.20 mm, with the particles of solid material, admixed to the
electrolyte, finely dispersed within the layer. It is found that
the finer the admixed~particles of solids, the greater is the
uniformity of~ the dispersion. After completion of the electrolytic
process the bl!ade is washed and dried.
Fig. 1 shows a oross-section of the corrosion-protective
~layer lmmediately after its application onto the core material.
Nume~ral~ indicates~the heat-resistant alloy being coated
("Inconel 738~ in case of the example shown). Numeral 2 denotes
the basic substance (matrix) of the protective layer, consisting
here of nickel. Numeral 3 denotes collectively the embedded
substances which form the islands and produce the protective oxide~
~; and which can have chemical compositions in variance from each
other. This fact is indicated in Fig. 1 by means of circles,
squares and tri ~ es. In the example shown, 3a denotes Cr3Si;
3b TaC; and 3c ~4~i.
After drying, the so-coated gas turbine blade is
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subjected to a thermal treatment in a vacuum furnace, the residual
pressure being 10 5 bar. This treatment is carried out at a
temperature of 1 120 C., maintained for a period of three hours.
The subsequent cooling down to room temperature is accomplished
approximately within one hour. The temperature of the thermal
treatment can range from 1 050 C. to 1 250 C., depending on the
alloy as well as the size of the work piece, and its duration can
range from 1 to 4 hours. It can also be carried out under an
argon atmosphere in place of a vacuum.
Fi~. 2 shows, at a scale greater than the scale used in
Fig. 1, a cross-section of the corrosion-protective layer after
the thermal treatment. The numeral 1 again denotes the core
material being coated, numeral 2 the basic substance of the protec-
tive layer, which is nickel in case of this example. 3a, 3b, and
3c correspond to the islands as shown in Fig. 1, which however in
actuality are smaller than illustrated. The thermal treatment has
caused one portion of the elements forming protective oxides (3a,
3b, 3c) to enter the matrix 2 by diffusion and to dissolve there.
This is indicated in Fig. 2 by dotted lines. The dissolved element
will form initially a "halo" t4a, 4b, 4c) around the undissolved
particle (3a, 3b, 3c) and will eventually impregnate the entire
area of the matrix 2 at greater or lesser uniformity, indicated
by the general numeral 4. It can be determined metallographically
that the particles of the elements 3, forming protective oxides,
have definitely become smaller after the thermal treatment, and
that one portion of the, - origlnally smaller, - particles has
disappeared completely. The metallographically detectable profile
of the elements forming protective oxides has become smaller
collectively. The changes in the protective layer caused by
3o the annealing can a_so be determined by magnetic measurements.
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The deposited nickel shows ferromagnetic characteristics prior to
the thermal treatment, while the nickel alloy produced by the
annealing is nonmagnetic.
After completion of the processing, the protective layer
will have a high resistance against corrosion and has a long life
span at temperatures ranging from 700 C.to 900 C. The elements
forming protective oxides which are consumed by chemical reactions
in the course of operations are replenished continuously by por-
tions of these elements present in the form of particles, so thak
there will be no lack of such substances throughout the entire
life span of the blade.
The composition of the protective layer is not limited
to the substances 11sted in the above given example. In place of
nickel, there can also be selected for the matrix 2 either cobalt
or iron or an alloy of at least two of the three above-mentioned
~metals, preferably a nickel/i~ron alloy. rrhe same holds true for
~the protective-oxide-formin~g islands 3 which can consist basically
o~ Cr, S1, Al, Ti, Ta, Be, a rare earth or a mixture of at least
two of these substances. It is also possible to use the substance~
listed above ln admixture with nickel and~or iro~, or carbides of
these elementa either in pure form or as admlxtures.
~ ~ ~ This generalization applies also aacordingly to the pro-: ~ ~
cess of the electrolytlc deposltion. The electrolyte can consist
generally of a fluid whlch contains~nickel or cobalt salts and
boron compounds. Preferred electrolytea are aqueous solutions
where 300 to 350 g of nickel or cobait sulphate~ 75 to 80 g of
nickel or cobalt chloride and 6 to 10 g of borontrioxide are
apportioned to one liter of water. If cobalt is used, the composi-
tion of the bath can be similar to the composition with nickel,
with the Ni-salts replaced by corresponding Co-salts. If iron is
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1131947
to be depo ited, ~t will be expedient to use a concentrated solu- j
tion of iron ammonium sulphate. Furthermore, there are mixtures
commercially available for certain alloys, especially for the
system Fe-Ni.
~ The corrosion-protective layer produced by the invention
¦ as well as the associated process are definitely not limited to
gàs turbine blades. They are generally applicable to components
of thermal machinery, such as impeller blades, turbine wheels,
compressor rotors, components of combustion chambers and guiding
vanes.
The novel corrosion-protective layers, as proposed by
the invention, represent compound materials which adhere fixedly
to the core material due to their chemical affinity. Since the
diffusion-voids which will arise during operation by consumption
of the corrosion-inhibiting substances, are uniformly distributed
in statistical respect throughout the longitudinal and the trans-
verse section of the protective layer and since there is thus no
danger that bands and/or layers will form within the material,
; ~ which would lead to a mechanical weakening and a reduction in
elasticity, the protective layer W111 neither Chip nor flake off.
In this manner there is in5ured a long life span for the protec-
tive layer and for the core material covered by it. Furthermore,
the discrete distribution of the protective oxide-forming elements
within the matrlx makes possible a practically unlimited margin in
variations concerning the selection of these substances in quali-
tative as well as in quantitatlve respect.
The manufacturing process proposed by the invention
produces solid protective layers in a direct manner. There is no
need for costly subsequent treatments for the purpose of increas-
3 ing the density of the material, such as the dlffiCult sintering
ll~ 1131947
of layers produced by metal spraying to attain solidification.The thermal treatment for the partial solution of the corrosion-
inhibiting substances within the matrix of the protective layer
is to a large extent coincidental with the final thermal treatment
of the core material (super-alloy) which must be carried out in
any event, so that an additional processing step can be saved.
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