U.S. patent number 3,862,851 [Application Number 05/337,163] was granted by the patent office on 1975-01-28 for method of producing magnesium-based coating for the sacrificial protection of metals.
This patent grant is currently assigned to The Chromalloy American Corporation. Invention is credited to Roy L. Blize, Kenneth K. Speirs, Martin Weinstein.
United States Patent |
3,862,851 |
Speirs , et al. |
January 28, 1975 |
Method of producing Magnesium-Based coating for the sacrificial
protection of metals
Abstract
A method is provided for improving the corrosion resistance of
material substrates. The method is particularly applicable to the
protection of metal substrates, such as ferrous metal articles,
among other metals, and, in particular, to the protection of low
alloy steel gas turbine engine components for aircraft in which the
surface is provided with an adherent protective layer of a
sacrificial coating rich in magnesium and also containing silicon,
oxygen and optionally iron. The coating is characterized by the
presence of magnesium silicide as a major active constituent.
Inventors: |
Speirs; Kenneth K. (Universal
City, TX), Weinstein; Martin (San Antonio, TX), Blize;
Roy L. (San Antonio, TX) |
Assignee: |
The Chromalloy American
Corporation (New York, NY)
|
Family
ID: |
26841785 |
Appl.
No.: |
05/337,163 |
Filed: |
March 1, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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144225 |
May 17, 1971 |
3748172 |
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Current U.S.
Class: |
427/252; 427/333;
427/419.2; 427/419.7; 427/403; 428/450 |
Current CPC
Class: |
C23C
10/52 (20130101) |
Current International
Class: |
C23C
10/52 (20060101); C23C 10/00 (20060101); C23c
009/00 () |
Field of
Search: |
;117/17.2P,7C,7S,135.1,169R,169A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rosdol; Leon D.
Assistant Examiner: Pitlick; Harris A.
Attorney, Agent or Firm: Sandoe, Hopgood & Calimafde
Parent Case Text
This application is a division of co-pending application Ser. No.
144,225, filed May 17, 1971, now U.S. Pat. No. 3,748,172.
Claims
What is claimed is:
1. In a method of protecting a metal substrate against the
corrosive effects of saline and marine environments, the
improvement which comprises, providing said substrate with a
coating selected from the group consisting of sodium silicate,
potassium silicate, lithium silicate and ethyl silicate and then
subjecting said coated substrate to a pack cementation process at
an elevated diffusion temperature comprising embedding said
substrate in a pack containing by weight about 5 percent to 100
percent magnesium in particulate form and up to about 95 percent by
weight of a refractory diluent together with a small but effective
amount of a halide energizer and then heating said pack to said
elevated diffusion temperature below the melting point of
magnesium, sufficient to effect transfer of magnesium to the
substrate, whereby a final coating is formed on said metal
substrate containing substantial amounts of magnesium silicide to
protect said metal substrate against corrosion.
2. The method of claim 1, wherein the pack contains by weight about
30 percent to 60 percent magnesium and about 70 percent to 40
percent by weight of the diluent.
3. The method of claim 1, wherein the diluent is alumina.
4. The method of claim 1, wherein the substrate is a ferrous
metal.
5. A method of improving the corrosion resistance of a metal
substrate against saline and marine environments which
comprises,
cleaning said substrate,
applying a uniform coating to said substrate with a silicate
selected from the group consisting of sodium silicate, potassium
silicate, lithium silicate and ethyl silicate,
subjecting said coated substrate to a pack cementation process
comprising embedding said substrate in a pack containing by weight
about 5 percent to 100 percent magnesium in particulate form and up
to about 95 percent by weight of a refractory diluent together with
a small but effective amount of a halide energizer,
heating said pack to a temperature of about 700.degree.F to
1,000.degree.F in a retort, and then removing the treated substrate
from the pack,
whereby an adherent sacrificial coating is obtained rich in
magnesium and also containing oxygen and silicon, a major portion
of said magnesium being in the form of magnesium silicide to
protect said metal substrate against corrosion.
6. The method of claim 5, wherein following the pack cementation
step, the coating is covered with an overcoat of silicate salt
selected from the group consisting of sodium silicate, potassium
silicate, lithium silicate and ethyl silicate.
7. The method of claim 5, wherein the substrate is a ferrous
metal.
8. The method of claim 5, wherein the pack contains by weight about
30 percent to 60 percent magnesium powder, and about 70 percent to
40 percent of the refractory diluent.
9. The method of claim 8, wherein the refractory diluent is
alumina.
10. A method of improving the corrosion resistance of a metal
substrate against saline and marine environments which
comprises,
cleaning said substrate,
applying a coating of a solution of a silicate salt selected from
the group consisting of sodium silicate, potassium silicate,
lithium silicate and ethyl silicate, to the surface of said
substrate, removing excess solution from said surface and drying
said surface,
thermally curing said silicate coated surface, repeating the
foregoing steps until a uniform silicate layer is formed of a
desired thickness,
subjecting said treated substrate to a pack cementation process
comprising embedding said substrate in a pack containing by weight
about 5 percent to 100 percent magnesium in particulate form and up
to about 95 percent by weight of a refractory diluent together with
a small but effective amount of a halide energizer,
subjecting the pack to a temperature of about 700.degree.F to
1000.degree.F in a retort,
and then removing the treated substrate from said pack,
whereby an adherent sacrificial coating is obtained rich in
magnesium and also containing oxygen and silicon, a major portion
of the magnesium being in the form of magnesium silicide to protect
said metal substrate against corrosion.
11. The method of claim 10, wherein the cementation pack contains
by weight about 30 percent to 60 percent magnesium and 70 percent
to 40 percent alumina.
12. The method of claim 10, wherein following the formation of the
magnesium-containing coating an overcoat of said silicate salt is
applied to the surface thereof and thereafter thermally cured.
13. A method of improving the corrosion resistance of a ferrous
metal substrate which comprises,
providing a ferrous metal substrate having a thermally cured
uniform silicate coating on the surface thereof selected from the
group consisting of sodium silicate, potassium silicate, lithium
silicate and ethyl silicate,
subjecting said treated substrate to a pack cementation process
comprising embedding said substrate in a pack containing by weight
about 5 percent to 100 percent magnesium in particulate form and up
to about 95 percent by weight of a refractory diluent together with
a small but effective amount of a halide energizer,
subjecting the pack assembly to a temperature of about 700.degree.F
to 1,000.degree.F in a retort,
and then removing the treated substrate from said pack,
whereby an adherent sacrificial coating is obtained rich in
magnesium and also containing oxygen, silicon and iron, the major
portion of the magnesium being in the form of magnesium silicide to
protect said metal substrate against corrosion.
14. The method of claim 13, wherein following the formation of the
magnesium-containing coating, an overcoat of said silicate salt is
applied to the surface thereof and thereafter thermally cured.
15. The method of claim 13, wherein the cementation pack contains
by weight approximately 50 percent magnesium and approximately 50
percent of alumina.
Description
This invention relates to the protection of material substrates,
particularly the protection of metal substrates, such as ferrous
and non-ferrous metals, from corrosion in highly saline and/or
marine atmospheres.
FIELD OF THE INVENTION
Jet and gas turbine engine compressor components, for example,
discs and blades, are subject to corrosion in highly saline
atmosphere at the air intake end of the engine and also to direct
impact of abrasive particulate matter, such as coral dust.
Additionally, compressor discs and blades among other components
are subjected to tremendous mechanical stresses from centrifugal
forces, thermal shock, vibration and other sources of stresses.
Thus, corrosion can accelerate catastrophic failure, since pits and
other corrosion defects can act as stress raisers.
High strength ferrous alloys are employed in the construction of
compressor discs, spacers, blades and other aircraft engine
components (e.g. Society of Automotive Engineers alloy designation
AMS 6304, SAE 4340, AMS 5508, AMS 5616, and others) but, but,
because of their low resistance to saline corrosion, they are
generally subjected to a protective surface treatment. One in
particular, is the provision of an aluminum-base diffusion coating
on the ferrous substrate by pack-aluminizing at coating
temperatures ranging up to about 1,000.degree. F (538.degree.C) and
preferably not higher so as to avoid undesired crystallographic or
metallurgical changes in the substrate during coating, which might
have an adverse or undesired effect on the mechanical properties of
the parts. While such coatings provide advantageous oxidation and
erosion resistance and minimize the production of pulverous
corrosion products on alloys, such as AMS 5615 (12 percent chromium
steel), they are not sufficiently anodic with respect to low alloy
steel substrates, such as AMS 6304 (less than 3 percent chromium
and less than 1 percent nickel), to offer the desired sacrificial
or anodic protection thereof against saline corrosion.
A surface treatment involving a magnesium-based coating has now
been discovered which provides optimum resistance to corrosion for
prolonged periods of time. As far as is known, this novel treatment
for protecting metal substrates was not known prior to this
invention.
OBJECTS OF THE INVENTION
It is thus the object of this invention to provide a secrificial
coating for the protection of material substrates, such as metal
substrates.
Another object is to provide a method for further enhancing the
corrosion resistance of ferrous and non-ferrous metal substrates,
particularly low alloy steel substrates.
A still further object is to provide an improved substantially
sacrificial coating system for protecting ferrous and non-ferrous
metal surfaces.
Another object is to provide an improved sacrificial coating system
for protecting certain anodically active metals, such as aluminum
and aluminum alloys.
A further object is to provide a method of protecting materials of
construction which are subject to galvanic corrosion at their
contacting substrates by interposing between said materials a
sacrificial coating that preferentially corrodes anodically
relative to both substrates.
These and other objects will more clearly appear when taken in
conjunction with the following description and the appended
claims.
STATEMENT OF THE INVENTION
This invention is based on the discovery that a highly adherent
composite sacrificial coating rich in magnesium as an essential
ingredient provides unexpectedly improved galvanic protection for a
wide range of substrate materials, such as ferrous and non-ferrous
metal substrates, for example, low alloy steel, aluminum, aluminum
alloys, titanium, titanium alloys and other metals. Normally, in
the case of stainless steels, galvanic protection has been achieved
with a thermally diffused aluminum coating based on an
iron-aluminum intermetallic. However, the required electromotive
force difference between the aluminum coating and low alloy steel
substrates is not always sufficient for severe corrosion
environmental conditions. It has been found that the behavior of a
protective coating based on thermally diffused aluminum or noble
coating of nickel can be detrimental in that localized breakdown of
the coating can often lead to aggravated sub-surface pitting
attack.
While it is apparent that pure magnesium would provide excellent
sacrificial protection for steels, a main disadvantage of pure
magnesium is that it tends to react spontaneously with water and,
as a consequence, such coatings would not have sufficient
environmental stability. Moreover, it is practically impossible to
transfer magnesium onto steel by the pack cementation process
because of its insolubility in iron. The invention overcomes the
foregoing difficulties and provides a magnesium-rich coating having
excellent sacrificial properties.
Thus, stating it broadly, one embodiment of the invention resides
in providing a substrate to be coated, applying a
silicon-containing coating of finite thickness to the substrate,
e.g. a coating of sodium silicate, subjecting the coated substrate
to a pack diffusion process by embedding the substrate in a pack
comprising a magnesium-containing constituent, preferably in
particulate form, and a small but effective amount of a halide
energizer, subjecting the pack assembly to a temperature sufficient
to cause transfer of magnesium to the substrate, for example, at a
temperature of about 700.degree.F (400.degree.C) to 1,000.degree.F
(538.degree.C) in a retort, and then removing the treated substrate
from the pack, such that an adherent sacrificial coating is
obtained rich in magnesium and also containing oxygen, silicon and
possibly iron, where the substrate is a ferrous metal. In a
preferred embodiment, after the application of the magnesium
coating, a silicate overcoat (e.g. sodium silicate) is applied to
the surface thereof and thereafter thermally cured. An analysis of
the coating indicates that a major portion of the magnesium is
present as a magnesium silicide. The pack into which the part is
embedded preferably contains an inert powder diluent, such as
alumina.
In one embodiment of the invention as applied to a bare steel
specimen, the specimen was coated with sodium silicate and the
coated specimen placed in a cementation pack containing about 20
percent magnesium, 78 percent aluminum oxide and 2 percent aluminum
chloride. The pack, which was confined in a retort, was placed in a
furnace and heated to 900.degree.F for about 16 hours. A coating
was produced as a reaction product of the silicon-containing
coating with the active ingredients in the pack, the coating being
enriched in magnesium, a major portion of which is in the form of
magnesium silicide, the coating also containing oxygen and
iron.
DETAILS OF THE INVENTION
In producing the coating, three steps may be employed, to wit:
preparing the substrate to receive the coating, applying the
silicon-containing layer (e.g. sodium, potassium or lithium
silicate) and then thermally treating the coated substrate in a
magnesium pack. As an optional fourth step, an overcoat of silicate
or other conversion coating is preferably applied.
In preparing a ferrous metal substrate to receive the
silicon-containing layer, the surface is preferably first cleaned.
A preferred method is to hone it with finely divided aluminum oxide
as this yields the best surface consistent with assuring good
fatigue properties. While chemical cleaning can be employed, it is
not preferred since it can have some effect on lowering the fatigue
properties. The silicon-containing layer, e.g. sodium silicate,
should be applied as uniformly as possible using, for example, a
sodium silicate or potassium silicate solution of predetermined
concentration. The part is immersed in the silicate solution and
the excess removed by first draining it off and then blowing air
over the surface. After drying the layer, it is subjected to a
curing cycle at, for example, 400.degree.F (205.degree.C) to expel
excess moisture. The part is allowed to cure and an additional
layer applied in the same manner. Additional coats may be applied,
depending upon the thickness to be achieved.
As stated above, the pack may comprise a mixture of a
magnesium-containing constituent in particulate form (e.g. from
about 30 to 325 mesh) and an inert powder diluent, such as
refractory oxide, e.g. alumina, plus a small but effective amount
of a halide energizer. A pack composition which has been found
particularly useful is one containing 40 percent by weight of
nominally 100 mesh magnesium and 60 percent by weight of aluminum
oxide normally about 200 mesh. To the mixture is added about 2
percent by weight of, for example, ammonium chloride. The pack is
placed in a two-part retort, the part embedded in it and the retort
lid secured to the retort flanges with layers of aluminum inserted
therebetween as a gasket and the lid then bolted to the flange.
The part is subjected to pack diffusion for about 16 hours in an
oven maintained at an elevated temperature conducive to effect
transfer of the magnesium to the substrate, e.g. between about
800.degree.F to 900.degree.F (about 427.degree.C to 482.degree.C).
The retort is then removed from the oven and allowed to air cool,
any excess powder adhering to the part being removed by blowing off
with air or by immersing the part in a solution of water and
silicate in an ultrasonic cleaning tank. The part is then dried
and, as an optional step, a silicate overcoat applied. In one
embodiment, the overcoat is applied with a nominal 1 percent by
volume silicate solution of, for example, sodium silicate (diluted
from 41.5 Baume solution), by immersion in the silicate solution,
the excess solution being thereafter removed from the part followed
by drying and then curing at about 400.degree.F (205.degree.C).
Normally, three layers of silicate overcoat are applied in this
manner. The foregoing silicate solution amounts to about 0.3
percent by weight of SiO.sub.2 equivalent. Broadly speaking, the
silicate solution, (e.g. sodium silicate, potassium silicate,
lithium silicate and ethyl silicate) may range by weight from 0.1
percent to 17.5 percent SiO.sub.2 equivalent. At the higher end of
the range, the silicate solution is preferably applied by
spraying.
The foregoing coating provides markedly improved protection of the
substrate when subjected to the salt spray test, complete
protection having been observed for times up to 500 hours in the
salt spray cabinet. This test is based on the procedure outlined in
ASTM B 117-64.
The ASTM salt spray test (Designation B 117-64) employed in testing
the resistance to corrosion of the various coating systems
disclosed herein comprises a fog chamber, a salt solution
reservoir, a supply of suitably conditioned compressed air, one or
more fog nozzles, specimen supports, provisions for heating the
chamber and control means. The specimens are supported or suspended
between 15.degree. and 30.degree. from the vertical (out of contact
with each other) and preferably parallel to the principal direction
of horizontal flow of fog through the chamber. The salt solution is
made up of 5.+-.1 parts of salt to 95 parts of distilled water
containing not more than 200 ppm, of total solids. The condensed
fog should have a pH of about 6.5 to 7.2. The temperature within
the chamber is maintained at 95.degree.F plus 2.degree. or minus
3.degree.F. For the specimens in this case, the salt spray testing
is carried out for a period stated herein, precautions being taken
to avoid dripping of condensed solution from one specimen to
another.
In using the test to evaluate the quality of the sacrificial
coating, specimens comprising 1/2 inch cylinder or 1 inch strip of
the substrate are employed. In the case of the strip, a section of
the coating is abraded from the specimen to be tested. In the case
of the cylindrical specimen, one edge is bevelled by abrasion on a
belt or grinding wheel to expose the substrate. The specimens with
the partially exposed substrate are then subjected to the
aforementioned ASTM salt spray test. The sacrificial coating gave
excellent results after 500 hours of testing as evidenced by the
complete freedom of substrate deterioration.
Very good protection has even been observed after exposure of the
coated part at 900.degree.F (482.degree.C) followed by the salt
spray test. However, the coating is more effective at temperatures
up to 850.degree.F (455.degree.C). Excellent salt spray protection
has been obtained after exposure at temperatures up to about
800.degree.F (427.degree.C).
The coating thickness has been found to vary between 0.2 to 1 mil
(0.0002 to 0.001 inch) and is a function of the silicate thickness
applied. Very good protection occurs at a nominal thickness of
about 0.5 mil. Metallographically, the coating has a dark structure
which is free of microcracks.
Fatigue tests have indicated that the coating does not adversely
affect the endurance limit of the substrate metal. Thus, high cycle
fatigue tests using rotating beam specimens have substantiated that
the bare and coated specimens have the same endurance limit.
EMF measurements in a 3 percent sodium chloride solution with a
calomel reference electrode showed 1.2 volts for the
magnesium-based coating as compared to 0.56 volt for Aluminum Alloy
No. 2016, 0.45 volt for low alloy steel designated as AMS 6304 and
0.11 volt for stainless steel bearing the designation AMS 5616.
A chemical profile of the coating as determined by the electron
microprobe has established that the coating can have levels of
approximately 50 percent Mg, 20 percent Si, 20 percent oxygen and
varying levels of iron on a steel substrate. An analysis has
indicated that a major portion of the magnesium is present in the
form of magnesium silicide. This has been confirmed by X-ray
diffraction.
The coating is apparently a reaction product resulting from the
pack cementation process. In some cases, the iron compositional
profile is such that an iron solid solution occurs at the substrate
coating interface, indicating that diffusion of iron from the steel
substrate into the coating has occurred. The magnesium level,
according to the electron microprobe, has a peak associated with
the silicon peak and both are at lower levels at the outer surface
of the coating.
The exceptionally high electrochemical potential of the coating
(1.2 volts as compared to about 1.3 volts for pure magnesium) and
the analyzed 50 percent magnesium in the coating indicates that the
magnesium in the coating exhibits a high positive deviation from
ideality. This behavior is very unique in this type of coating.
Generally, the coating may contain about 20 percent to 50 or 60
percent by weight of magnesium, and the balance essentially
silicon, oxygen and some constituents derived from the
substrate.
As illustrative of the invention, the following example is
given.
EXAMPLE 1
A compressor disc of AMS 6304 low alloy steel and sample coupons
thereof are cleaned by honing with finely divided aluminum oxide
powder. The disc is then immersed in a heated potassium silicate
solution of 10 percent by volume concentration derived from a 30.2
Baume solution. The 10 percent by volume concentration corresponds
to about 2.1 percent by weight of SiO.sub.2 equivalent. The
solution is maintained in the temperature range of about
140.degree.F (60.degree.C) to 160.degree.F (70.degree.C), and the
disc and coupon alternately immersed and raised above the solution
to enable the liquid to drain off. Any excess liquid is blown off
the surface with compressed air after which the disc and coupons
are again immersed in the solution, followed by draining and
drying. The foregoing is repeated seven times to provide seven
layers of the silicate. The coated parts are then cured in an oven
for 30 minutes at about 400.degree.F (205.degree.C). After the
foregoing treatment, the parts are embedded in a pack comprising 50
percent by weight of 30 mesh magnesium (U.S. Standard) and about 50
percent by weight of 30 mesh aluminum oxide. The pack is sealed in
a retort which is then placed in an oven and heated for 30 hours at
800.degree.F (427.degree.C). After completion of the coating, the
parts are removed from the powder and cleaned by washing in a hot
solution of potassium silicate maintained at a temperature of about
140.degree.F (60.degree.C) to 160.degree.F (71.degree.C).
The coupons treated together with the discs in the foregoing manner
provided a coating which exhibited excellent sacrificial properties
when subjected to the salt spray test.
The corrosion resistant properties were even further enhanced by
applying an overcoat of potassium silicate from a solution
containing about 25 percent by volume of 30.2 Baume potassium
silicate, (about 5.2 percent by weight of SiO.sub.2 equivalent),
the overcoat being applied in three cycles by dipping, draining,
drying and curing until the desired thickness is obtained or by
spraying and curing. Where the silicate is applied by dipping, the
solution concentration may range from about 0.2 percent to 5
percent SiO.sub.2 equivalent. Where the coating is sprayed, the
solution concentration may range by weight from about 5 percent to
17.5 percent SiO.sub.2 equivalent. Broadly, the solutions may range
from about 0.2 percent to 17.5 percent SiO.sub.2 equivalent.
EXAMPLE 2
A group of compressor blades of AMS 5616 stainless steel is cleaned
by honing with aluminum oxide powder. The blades are then racked
and the rack immersed in a tank containing an aqueous solution of
10 to 25 percent by volume of 41.5 Baume sodium silicate (about 2.9
percent to 7.3 percent by weight of SiO.sub.2 equivalent). The
temperature of the bath is maintained at about 80.degree.F
(26.degree.C) to 100.degree.F (38.degree.C). After the blades have
been immersed to completely wet the surface, the rack is removed
and excess liquid allowed to drain off and the blades dried by
blowing with air. The blades are then recycled in the solution and
dried by blowing off with air for a total of three applications.
The dried blades are then subjected to a curing cycle at
400.degree.F (205.degree.C) for 15 minutes. This constitutes one
silicate application. The blades are then allowed to cool and two
additional applications of silicate made in the same manner. The
foregoing silicate cycles provide a silicate thickness of about 0.4
mil.
After the blades have been silicate treated, they are embedded in a
pack containing about 50 percent by weight of 50 mesh magnesium
powder (U.S. Standard), about 50 percent by weight of alumina (200
mesh) and ammonium iodide added in amounts of about 2 percent of
the total weight of the pack, the pack being confined in a retort
as described herein. The blades are then subjected to pack
diffusion by placing the retort in an oven maintained at about
900.degree.F (482.degree.C) and the blades processed for about 24
hours. Following the pack cementation process, the blades were
cooled inside the retort to substantially ambient temperature and
the blades then ultrasonically cleaned in water containing about 25
percent potassium silicate (30.2 Baume).
The alumina is used in the pack as an inert diluent. Besides
alumina, other inert and temperature stable diluents can be
employed, such as zirconia, titania, hafnia, thoria, rare earth
oxides, silicon carbide, titanium carbide, tungsten carbide, and
the like. The inert diluent employed is generally refractory in
nature and has a melting point about 1,300.degree.C.
The magnesium in the pack may range by weight from about 5 to 100
percent, the refractory diluent up to about 95 percent by weight,
and the halide energizer in small but effective amounts, such as
from about 1/4 percent to 5 percent by weight of the total weight
of the pack. The halide energizer may comprise metal and ammoniacal
halides and halide formers, such as iodine. Examples of halides are
NH.sub.4 Cl, NH.sub.4 F, NH.sub.4 I, NH.sub.4 Br and AlCl.sub.3,
among others.
In curing the silicate layer, whether applied as a foundation coat
or an overcoat, the temperature may range from about 200.degree.F
(93.degree.C) to 800.degree.F (426.degree.C). When applying the
silicate layer as a foundation coat prior to pack cementation, the
solution, whether it is sodium silicate, potassium silicate or
ethyl silicate, may range by weight from about 0.2 percent to 17.5
percent SiO.sub.2 equivalent. Where the silicate solution is
applied as an overcoat, that is, over the pack cementation coating,
the amount of silicate material in the solution may range over the
foregoing composition and, more preferably, from about 5 percent to
17.5 percent by weight of SiO.sub.2 equivalent.
As additional examples, the following are given.
EXAMPLE 3
Compressor blades of a titanium alloy containing by weight about 6
percent Al, 4 percent V and the balance essentially titanium are
honed with finely divided alumina followed by cleaning in an
ultrasonic tank containing 10 percent by volume solution of
potassium silicate (produced from 30.2 Baume solution), the
temperature of the solution being about 80.degree.F (27.degree.C).
The ultrasonic cleaning action removes fine debris and enables a
layer of the silicate to form on the blades. The blades are raised
out of the solution and allowed to drain and dry by blowing with
compressed air. The blades are then sprayed with a solution of 37.5
volume percent potassium silicate (produced from 30.2 Baume
solution) which corresponds to about 7.8 percent by weight of
SiO.sub.2 equivalent. The sprayed coating is cured and the step
repeated of coating and curing. The treated parts are then placed
in a pack mixture containing 40 percent magnesium powder of 30 mesh
(U.S. Standard) size and 60 percent alumina, also of 30 mesh size.
To the mixture is added 2 percent by weight of ammonium iodide.
Where the pack is being used over again from a prior charge, about
a 10 percent addition is made of magnesium and alumina. After
blending the powder, the compressor blades are embedded in the pack
and the assembly subjected to pack cementation in a sealed retort
at about 750.degree.F (400.degree.C) for about 48 hours.
After the blades have been coated, they are removed and cleaned in
a hot potassium silicate solution (1 percent by volume from Baume
30.2 solution) at a temperature ranging from about 140.degree.F
(60.degree.C) to 160.degree.F (71.degree.C).
EXAMPLE 4
An aluminum oxide disc is immersed in a 10 percent by volume sodium
silicate solution (derived from a 41.5 Baume solution) at a
temperature of about 80.degree.F (27.degree.C), the solution
concentration corresponding by weight to 2.9 percent SiO.sub.2
equivalent. The disc is removed from solution and excess solution
allowed to drain off. After drying, the disc is cured at
600.degree.F (315.degree.C) for 15 minutes, cooled and again dipped
in the solution and dried and cured, until the cycle has been
repeated three times.
The thus treated aluminum oxide disc is then embedded in a pack
containing 20 percent magnesium of 100 mesh size (U.S. Standard)
and 80 percent aluminum oxide of 200 mesh size, 2 percent iodine
being added as the halide-forming transfer agent. The pack which is
sealed in a retort is then heated at 850.degree.F (455.degree.C)
for 24 hours. After the coating has been completed, the disc is
removed from the pack and cleaned in a 10 vol. percent sodium
silicate solution (based on Baume 41.5).
While the silicate coating is a good source for silicon in
producing the magnesium silicide coating by pack cementation, other
silicon-containing pre-coats can be employed, such as a fine
dispersion of a silica gel. In employing the silicate coating
technique, the amount employed is determined in accordance with the
percent by weight of SiO.sub.2 equivalent in the coating. Thus,
silicate solutions containing about 0.2 percent to 17.5 percent of
SiO.sub.2 equivalent may be employed in building up a foundation
pre-coat preparatory to pack cementation in a magnesium-containing
pack.
The markedly improved electro-negative potential and the remarkable
oxidation stability of the composite coating make the coating
suitable for the protection of low alloy steels, mild steels,
titanium, titanium alloys, aluminum and aluminum alloys and
ceramics, such as shapes made from alumina.
The invention is particularly applicable to low alloy steels, e.g.
AMS 6304, employed in jet or gas engine compressor components
operating at over 300.degree.F (150.degree.C). The protection of
aluminum and aluminum alloys is another very good application. By
applying the magnesium coating of the invention to high strength
steel or titanium rivets, fasteners, blading and other elements
used in direct contact with aluminum and aluminum alloy sheet,
castings, and other aluminum shapes or structural elements, the
deterioration of the aluminum, as well as the contacting metal, can
be greatly inhibited. By employing the invention in situations in
which magnesium is in direct contact with steel, the galvanic cell
potential developed between these materials can be significantly
reduced by producing generalized rather than localized corrosion
attack. Thus, as stated above, the invention may be applied to
titanium, aluminum, iron, iron alloys, magnesium alloys and
numerous ceramic materials, e.g. Al.sub.2 O.sub.3 of MgO where EMF
potential associated with the magnesium-rich coating can be
desirably utilized. Thus, a direct titanium-aluminum couple can be
avoided in aluminum-titanium structural systems by providing one of
the metals with the sacrificial coating of the invention by
interposing the coating between the two metals, such that the
magnesium-rich coating corrodes anodically in preference to either
of the substrate metals.
As has been stated herein, the cementation pack may range by weight
from about 5 percent to 100 percent of the magnesiumcontaining
material and up to about 95 percent by weight of a refractory
diluent, the pack also containing a small but effective amount of a
halide energizer. Based on the total weight of the pack, the
energizer may range from about 1/4 percent to 5 percent by weight.
A preferred pack is one containing about 30 percent to 60 percent
of the magnesium material and about 70 percent to 40 percent by
weight of the diluent, e.g. aluminum oxide.
The cementation process is generally carried out in a retort at a
temperature ranging from about 700.degree.F (about 425.degree.C) to
about 1000.degree.F (about 538.degree.C) for about 1/4 hours to 60
hours.
As will be apparent from the description, the invention provides as
an article of manufacture a thermally coated substrate comprising a
reaction product between a silicon-containing material, e.g. a
silicate salt, and magnesium consisting essentially of magnesium,
silicon and oxygen, the major portion of the magnesium being in the
form of magnesium silicide. Where the substrate is a ferrous metal,
a small portion of the coating will contain iron. Likewise, where
the substrate is aluminum or titanium, a small amount of such
elements may appear in the coating. Generally speaking, the coating
will consist essentially of magnesium, silicon and oxygen plus
small amounts of substrate material.
Usually, the coating contains at least about 20 percent by weight
of magnesium, such as about 20 percent to 50 percent and, more
preferably, 30 percent to 50 percent by weight of magnesium, a
major portion of which is in the form of magnesium silicide.
As stated hereinbefore, the invention is applicable to the coating
of a wide diversity of substrates and, in particular, jet and gas
engine turbine components or parts, such as discs, spacers, blades,
tie bolts, casings, shrouds, vanes, shafts, and the like.
Although the present invention has been described in conjunction
with preferred embodiments, it is to be understood that
modifications and variations may be resorted to without departing
from the spirit and scope of the invention as those skilled in the
art will readily understand. Such modifications and variations are
considered to be within the purview and scope of the invention and
the appended claims.
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