U.S. patent number 3,859,061 [Application Number 05/353,800] was granted by the patent office on 1975-01-07 for corrosion resistant coating system for ferrous metal articles having brazed joints.
This patent grant is currently assigned to Chromalloy American Corporation. Invention is credited to Michael F. Dean, Kenneth K. Speirs, Martin Weinstein.
United States Patent |
3,859,061 |
Speirs , et al. |
January 7, 1975 |
CORROSION RESISTANT COATING SYSTEM FOR FERROUS METAL ARTICLES
HAVING BRAZED JOINTS
Abstract
Ferrous metal articles, such as stainless steel aircraft
components, e.g., turbine vanes, vane/shrouds and the like,
characterized by at least one brazed joint, in which the braze is a
non-ferrous alloy are aluminized by first uniformly plating onto
the entire surface of said articles including the braze, a coating
of an aluminide-forming metal selected from the group consisting of
iron, nickel, cobalt and chromium and then thermally diffusing
aluminum over the entire surface of the article, including the
braze, such that a sacrificial corrosion resistant coating is
produced characterized by the presence of an aluminide compound
selected from the group consisting of iron aluminide, nickel
aluminide, cobalt aluminide and chromium aluminide. The aluminide
coatings are further enhanced by the application of a non-metallic
coating, e.g. a conversion coating.
Inventors: |
Speirs; Kenneth K. (San
Antonio, TX), Weinstein; Martin (San Antonio, TX), Dean;
Michael F. (San Antonio, TX) |
Assignee: |
Chromalloy American Corporation
(New York, NY)
|
Family
ID: |
23390629 |
Appl.
No.: |
05/353,800 |
Filed: |
April 23, 1973 |
Current U.S.
Class: |
428/614; 428/631;
428/651; 428/652; 428/671; 428/629; 428/633; 428/656; 428/941 |
Current CPC
Class: |
B23K
35/30 (20130101); C23C 10/02 (20130101); Y10S
428/941 (20130101); Y10T 428/12486 (20150115); F02B
2075/027 (20130101); Y10T 428/12618 (20150115); Y10T
428/12882 (20150115); Y10T 428/12778 (20150115); Y10T
428/1259 (20150115); Y10T 428/12604 (20150115); Y10T
428/12743 (20150115); Y10T 428/1275 (20150115) |
Current International
Class: |
C23C
10/02 (20060101); C23C 10/00 (20060101); B23K
35/30 (20060101); F02B 75/02 (20060101); B32b
015/18 () |
Field of
Search: |
;117/71M
;29/196.2,196.3,199,194,195M,195Y,195T |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rutledge; L. Dewayne
Assistant Examiner: Steiner; Arthur J.
Attorney, Agent or Firm: Sandoe, Hopgood & Calimafde
Claims
What is claimed is:
1. A ferrous metal article having at least one brazed joint,
the braze of said joint being formed of a non-ferrous brazing alloy
of melting point ranging from about 1,125.degree. to 1,925.degree.
F,
said article being characterized by a diffusion-bonded layer of a
metal aluminide selected from the group consisting of iron
aluminide, nickel aluminide, cobalt aluminide and chromium
aluminide over the entire surface thereof including the surface of
the brazed joint,
the corresponding metal of said metal aluminide selected from the
group consisting of iron, nickel, cobalt and chromium being
diffusion-bonded as a first layer into the surfaces of both the
braze and the ferrous metal and being enriched outward therefrom in
the corresponding metal aluminide by virtue of aluminum diffused
into said diffused first metal layer,
the thermally diffused aluminum coating also having bonded thereto
a cured non-metallic barrier layer formed from a silicate selected
from the group consisting of sodium silicate, potassium silicate,
lithium silicate and ethyl silicate,
whereby the aluminized brazed joint is characterized by an improved
combination of resistance to corrosion and fatigue.
2. The ferrous metal article of claim 1, wherein the non-ferrous
brazing alloy has a melting point of about 1,175.degree. to
1,850.degree. F and wherein said brazing alloy is selected from the
group consisting of a copper-base, silver-base or a gold-base
alloy.
3. The ferrous metal article of claim 1, wherein said non-metallic
barrier layer also includes a chromate and phosphate of at least
one metal.
4. The ferrous metal article of claim 3, wherein said chromates and
phosphates of said at least one metal are chromates and phosphates
of aluminum and magnesium.
5. The ferrous article of claim 4, wherein the silicate of the
non-metallic barrier layer is derived from sodium silicate.
6. The ferrous article of claim 4, wherein the silicate of the
non-metallic barrier layer is derived from potassium silicate.
7. A ferrous metal article having at least one brazed joint,
the braze of said joint being formed of a non-ferrous brazing alloy
of melting point ranging from about 1,125.degree. to 1,925.degree.
F,
said article being characterized by a diffusion-bonded layer of a
metal aluminide selected from the group consisting of iron
aluminide, nickel aluminide, cobalt aluminide and chromium
aluminide over the entire surface thereof including the surface of
the brazed joint,
the corresponding metal of the metal aluminide selected from the
group consisting of iron, nickel, cobalt and chromium being
diffusion-bonded as a first layer into the surfaces of both the
braze and the ferrous metal and being enriched outward therefrom in
the corresponding metal aluminide by virtue of aluminum diffused
into said first metal layer,
whereby the aluminized brazed joint is characterized by an improved
combination of resistance to corrosion and fatigue.
8. The ferrous metal article of claim 7, wherein the non-ferrous
brazing alloy has a melting point of about 1,175.degree. to
1,850.degree. F, and wherein said brazing alloy is selected from
the group consisting of a copper-base, silver-base or gold-base
alloy.
Description
This invention relates to the protection of ferrous metal articles
against corrosion in highly saline and/or marine or other corrosive
environments by employing thermally diffused aluminum as the main
sacrificial coating, the invention being particularly applicable to
ferrous metal articles, such as stainless steel articles,
characterized by at least one brazed joint, wherein the braze is a
non-ferrous alloy. The invention also relates to the production of
aluminized coatings on ferrous metal articles comprising brazed
joints, in which the aluminized layer also has a barrier-type
non-metallic overcoat.
FIELD OF THE INVENTION
Jet and gas turbine engine compressor components are subject to
corrosion in highly saline environments at the air intake end of
the engine and also to the direct impact of abrasive particulate
matter, such as coral dust. Additionally the compressor blades 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 blades and other aircraft engine components such as
vane/shrouds (e.g. Society of Automotive Engineers alloy
designation AMS 5508, AMS 5616, AMS 6304 and others), but, because
of their low resistance to saline corrosion, they are generally
provided with 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
1,000.degree. F and preferably not higher so as to avoid undesired
crystallographic or metallurigcal changes in the substrate during
coating which might have an adverse or undesired effect on the
mechanical property of the parts. Such coatings have provided
advantageous oxidation and erosion resistance and have minimized
the production of pulverous corrosion by-products and have been
very useful in extending the operating life of jet engine
components.
However, where the jet and gas turbine engine components are
comprised of welds and/or at least one brazed joint, such as
vane/shroud assemblies in which the braze is a non-ferrous alloy
based on a metal selected from the group consisting of copper,
silver and gold, e.g. copper-, silver-, and/or gold-base brazing
alloys, certain problems arise in the production of a uniform
aluminized coating having requisite physical characteristics. The
braze, being markedly different than the ferrous metal surface,
reacts differently during the diffusion of aluminum therein. The
extent of diffusion of aluminum in different materials is governed
mainly by the aluminide phases that are formed which act as
diffusion barriers. Generally speaking, the lower the melting point
of a particular braze alloy, the deeper is the diffusion of
aluminum into it. Moreover, some metal substrates do not provide
good adherent aluminide coatings.
Thus, in the case of aluminum diffusion in a brazed component in
which stainless steel elements are brazed together to form the
joints, the aluminum case depth on the stainless steel element
might be 0.0005 inch, whereas the aluminum may penetrate as much as
0.007 inch deep in the brazed portions of the joints for a braze
alloy comprising by weight 50% Ag, 18% Cd, 16% Zn and 16% Cu; or as
much as 0.002 inch in a braze composition comprising by weight 54%
Ag, 40% Cu and 6% Zn. The ratio of penetration between the braze
metal and stainless steel for example, may vary between 3:1 to
15:1, depending upon the braze composition.
Mechanical tests have indicated that the uncontrolled diffusion of
aluminum into the braze alloy tends to degrade the brazed joint as
evidenced by fatigue test data.
Thus, it would be desirable to provide a method of controlling the
diffusion at the brazed joint commensurate with the diffusion at
the unbrazed portion of the component such as to insure uniform
properties over the whole surface of the component.
OBJECTS OF THE INVENTION
It is thus the object of the invention to provide a sacrificial
coating of aluminum on ferrous metal parts characterized by the
presence of at least one brazed joint comprising a non-ferrous
brazing alloy and wherein the properties of the coating are
substantially uniform at both the unbrazed areas and the brazed
joint.
Another object of the invention is to provide a method of
aluminizing a ferrous metal part having at least one brazed joint,
wherein the entire metal part including the braze is coated with an
aluminide forming metal selected from the group consisting of iron,
nickel, cobalt and chromium prior to aluminizing of the part such
that the fatigue properties at the joint are maintained at
acceptable levels or are enhanced.
A further object is to provide, as an article of manufacture, an
aluminized ferrous metal part, characterized by at least one brazed
joint of non-ferrous metal or alloy, the entire metal part being
first provided with a layer of aluminide-forming metal, such that
the aluminum coating on the article including the braze is
characterized by the presence of a substantially uniform layer of
an aluminide compound selected from the group consisting of iron
aluminide, nickel aluminide, cobalt aluminide and chromium
aluminide.
These and other objects will more clearly appear from the following
disclosure and the accompanying drawing.
DRAWINGS
FIG. 1 represents a vane/shroud assembly of stainless steel in
which the vanes are brazed across the annular space between inner
and outer rings making up the shroud;
FIG. 2 is a close-up of a fragment of the shroud showing the
plurality of brazed joints making up the vane/shroud assembly;
FIG. 3 is a representation of a photomicrograph taken at 100 times
magnification showing the contour of an aluminide coating based on
an applied nickel plate followed by thermal diffusion therein of
aluminum, the coating being disposed uniformly over the braze and
along the steel surface;
FIG. 4 is an electron microprobe trace showing the distribution of
nickel and aluminum in the coating with respect to the braze metal
joint to which the coating adheres;
FIG. 5 is a representation of a photomicrograph at 500 times
magnification showing a nickel-aluminum coating adjacent the steel
substrate;
FIG. 6 is an electron microprobe trace showing the distribution of
the nickel and aluminum in the coating adhering to the steel
substrate,
FIG. 7 is a representation of a photomicrograph at 500 times
magnification showing a chromium-aluminum coating adjacent the
steel substrate;
FIG. 8 is an electron microprobe trace showing the distribution of
chromium and aluminum in the coating adhering to the braze;
FIG. 9 is a representation of a photomicrograph at 500 times
magnification showing a chromium-aluminum coating adjacent to the
steel substrate; and
FIG. 10 is an electron microprobe trace showing the distribution of
chromium and aluminum in the coating adhering to the steel
substrate.
STATEMENT OF THE INVENTION
One aspect of the invention resides in a method of aluminizing a
brazed ferrous metal article, such as an aircraft engine component
comprised of at least one brazed joint. The brazing material in the
joint is made of a non-ferrous brazing alloy having a melting point
ranging from about 1,125.degree. F. (607.degree. C) to
1,925.degree. F (1,052.degree. C), such as an alloy based on at
least one metal selected from the group consisting of copper,
silver and gold. Because the aluminum tends to diffuse more deeply
into the non-ferrous brazing alloy than in the steel substrate, the
method resides in coating the entire article including the braze
with an aluminide-forming metal and then thermally diffusing the
aluminum, preferably by pack cementation, into the entire surface
of the ferrous article thereby producing a substantially uniform
coating of aluminum over substantially the entire surface, wherein
the thermally diffused aluminum coating is characterized by the
presence of a substantially uniform layer of an aluminide compound
selected from the group consisting of iron aluminide, nickel
aluminide, cobalt aluminide and chromium aluminide. Fatigue tests
on cantilevered specimens have indicated that the fatigue
properties of the aluminized surface at particularly the braze are
comparable to uncoated specimens and superior to aluminized brazed
specimens.
The foregoing method is also applicable to the coating of ferrous
metal articles which uses a protective duplex coating system, that
is, a system in which following the production of a thermally
aluminized coating on the ferrous metal substrate, a non-metallic
silicate overcoat is applied to the aluminum coating which is
highly adherent. With respect to the non-metallic overcoat,
reference is made to co-pending application Ser. No. 143,842, filed
May 17, 1971, now U.S. Pat. No. 3,729,295. The disclosure of said
application relating to the production of such non-metallic
overcoats on aluminized ferrous metal surfaces is incorporated
herein by reference.
As stated hereinbefore, the non-ferrous brazing alloys which
present the problem of uncontrolled diffusion of aluminum therein
include those which are based on one or more metals selected from
the group consisting of copper, silver and gold. When the
expression "based on one or more metals of the group copper, silver
and gold" is employed, what is meant is that the non-ferrous
brazing alloy contains at least one of the foregoing copper group
metals as a main ingredient, with one or more of other non-ferrous
metals making up substantially the balance, such as zinc, nickel,
palladium, cadmium, tin, manganese and the like.
Apparently, the lower the melting point of the braze alloy, the
deeper is the aluminum diffusion into it. Thus, it may be stated
generally that the non-ferrous brazing alloys include those having
melting points ranging from about 1,125.degree. F (607.degree. C)
to 1,925.degree. F (1,052.degree. C) and preferably 1,175.degree. F
(635.degree. C) to 1,850.degree. F (1,010.degree. C). As will be
understood by those skilled in the art, the non-ferrous brazing
alloy must have a lower melting point than the metal substrates
being joined and yet must be high enough to resist softening at
elevated temperatures to which the aircraft part (e.g. vane/shroud
assemblies) is subjected during use.
Examples of non-ferrous brazing alloy compositions commonly
employed for producing brazed joints in aircraft components made of
ferrous metals, e.g. stainless steel are by weight as follows: (1)
50% Ag, 18% Cd, 16% Zn and 16% Cu [designated as AMS 4770 C]; (2)
54% Ag, 40% Cu, 6% Zn and up to 1% Ni [designated as AMS 4772B];
(3) 82% Au, 18% Ni [designated as PWA 698], (4) 54% Ag, 25% Pd and
21% Cu [designated as PWA 706], and (5) 55% Cu, 35% Mn and 10% Ni,
among many others. The diffusion of aluminum in alloy (1) may
proceed up to a depth of 0.007 inch and in the others up to 0.002
inch. Such diffusion by aluminum into the braze has a weakening
effect on the joint and adversely affects its resistance to
fatigue.
Illustrative brazing alloy compositions range in weight percent
(for brazing stainless steel) including their solidus, liquidus and
brazing temperatures are as follows:
Example Temperature .degree.F Nos. % Ag % Cu % Zn % Cd % Ni Solidus
Liquidus Brazing
__________________________________________________________________________
1A 44 - 46 14 - 16 14 - 18 23 - 25 -- 1125 1145 1145 - 1400 2A 49 -
51 14.5 - 16.5 14.5 - 18.5 17 - 19 -- 1160 1175 1175 - 1400 3A 49 -
51 14.5 - 16.5 13.5 - 17.5 15 - 17 2.5 - 3.5 1170 1270 1270 - 1500
4A 39 - 41 29 - 31 26 - 30 -- 1.5 - 2.5 1240 1435 1435 - 1650 5A 44
- 46 29 - 31 23 - 27 -- -- 1250 1370 1370 - 1550 6A 54 bal. 5 -- 1
1325 1575 1575 - 1785 7A 60 bal. -- -- 10% Sn 1115 1325 1325 - 1550
8A 92.5 bal. -- -- 0.2 Li 1435 1635 1610
__________________________________________________________________________
- 1800
Another brazing alloy based on gold comprises 81.5% Au and the
balance nickel. This alloy has a solidus-liquidus temperature of
1,740.degree. F and is employed over a brazing temperature of about
1,740.degree. F to 1,840.degree. F.
Most stainless steels can be brazed by any one of several different
filler metals, including silver-base alloys, gold-base alloys,
copper-base alloys and the like. Stating it another way, the
non-ferrous brazing alloy may contain at least about 40% by weight
of the copper group metals and preferably at least about 50% and
the balance non-ferrous alloying ingredients so long as the melting
point of the alloy ranges from about 1,125.degree. F to
1,925.degree. F and preferably from about 1,175.degree. F to
1,850.degree. F. The melting point is generally taken as the
liquidus temperature of the alloy. The minimum of at least 40% of
the copper group metal is met where the alloy contains at least 40%
Ag, or 40% Cu or 40% Au, or at least 40% of a combination of two or
more of the foregoing copper group metals. As stated hereinbefore,
the other non-ferrous alloying elements may comprise one or more of
Zn, Cd, Ni, Sn, Mn, Pd and the like. Thus, the brazing alloy may
contain 40 to 95% of at least one of the copper group metals and
the balance at least one other non-ferrous metal.
When the foregoing brazing alloys are employed in the production of
certain aircraft components, it has been found essential, in order
to assure substantially uniform corrosion and oxidation resistant
properties over the entire surface thereof, to coat said surface
including the braze with an aluminide-forming metal prior to
thermally aluminizing the whole component. Various methods may be
employed in coating the component. A preferred method is
electroplating. Another method is to deposit the metal from a
chemical plating bath, such as a nickel hypophosphite bath, where
nickel is the aluminide-forming metal.
In plating complex shapes like a vane/shroud component, an
insoluble anode is employed which has a configuration corresponding
to the overall shape of the shroud so as to assure substantially
uniform throwing power of the bath to the complex surface being
plated.
The plating metals selected are those which combine with aluminum
to form thermally stable aluminides. Metals which are particularly
preferred are iron, nickel, cobalt and chromium. The most useful
systems are nickel or chromium plate followed by aluminum diffusion
therein. The sacrificial behavior of nickel aluminide and chromium
aluminide can be evaluated by salt spray testing and also indicated
by EMF measurements.
For example, coated coupons of stainless steel with 1/4 inch strips
of the coating removed have been placed in a salt spray cabinet
operated in accordance with ASTM B117. Examination of the specimens
has shown production of corrosion products on the coating with the
base stainless steel protected against corrosion. As illustrative
of the EMF readings obtained for various substrates using a calomel
electrode in a 3% sodium chloride solution, the following data are
given:
Substrate EMF Reading (Volts)
______________________________________ 410 Stainless Steel -0.30
Chromium Aluminide -0.58 Nickel Aluminide -0.43 AMS 4770 Braze
-0.14 AMS 4772 Braze -0.23 Aluminum Diffusion) In AMS 4770 ) -0.68
Aluminum diffusion) In AMS 4772 ) -0.91
______________________________________
The foregoing values indicate that chromium aluminide and nickel
aluminide are both sacrificial (that is, anodic) relative to the
stainless steel substrate. While the diffusion of aluminum into the
braze indicates sacrificial properties; nevertheless, such
diffusion adversely affects the fatigue properties at the brazed
joint. However, by plating the whole component including the braze
with an aluminide-forming metal, substantially uniform chemical and
physical properties are assured for the treated component. The
thickness of the applied aluminide-forming metal is at least about
0.0002 inch.
The plating of the entire surface including the braze is
advantageous in that the results obtained can now be independent of
the prevailing ferrous substrate being coated. For example, a
ferrous substrate of one analysis might react differently with
diffused aluminum as compared to a ferrous substrate of another
analysis, such that the aluminized layer might have different
characteristics. By coating the surface substantially uniformly
with an aluminide-forming metal, such as nickel or chromium, the
process is rendered independent of the metal substrate being
protected.
As stated hereinbefore, the aluminide-forming metal may be applied
by various methods, such as by electroplating, electroless plating
from a chemical plating bath, vacuum plating from a vapor, etc. For
example, nickel may be applied electrolytically from a sulfamate
bath or from an electroless nickel bath. Following the plating or
coating of the part with the aluminide-forming metal, the part,
e.g. the vane/shroud component, is embedded in an
aluminum-containing cementation pack comprising by weight, for
example, 80% aluminum powder and 20% aluminum oxide having mixed
therein about 2% of dry aluminum chloride.
One method comprises preparing an aluminizing pack comprised, for
example, of 800 lbs. of -60+140 mesh aluminum powder blended with
200 lbs. of Al.sub.2 O.sub.3, also -60+140 mesh size. To the 1,000
lb. mixture is added 20 lbs. of dry AlCl.sub.3 under a humidity
preferably not exceeding 45%.
The pack is mixed in a vibrating blender under dry conditions for
about 5 to 10 minutes. If the charge is a fresh charge, it is
subjected to burn-out at 795.degree.-825.degree. F (425.degree. to
440.degree. C) for 36 hours. However, where a charge has already
been used and is recycled for another pack, burn-out is not
required. The pack is placed in a dry condition in a retort with
the vane/shroud component of AMS 5616 steel to be treated, the
vane/shroud being completely embedded in the pack using vibration
to fill in the spaces between the vanes. The cover is sealed to the
retort body with multiple layers of aluminum foil in the form of a
gasket sufficient to prevent air from getting in but to allow
out-gassing of gaseous by-products.
The retort is placed in an oven at ambient temperature and the
temperature allowed to rise to the desired coating temperature by
the application of heat. As the temperature rises, it is preferred
that it go through an endothermic arrest at about 350.degree. F
(176.degree. C) due to vaporization of AlCl.sub.3 to effect further
cleansing of the surface of the component of any oxide film thereon
and then allowed to reach a temperature not exceeding about
1,000.degree. F (538.degree. C), for example, a range of about
795.degree. F to 825.degree. F (425.degree. C to 440.degree. C) and
the retort maintained at substantially that temperature range for
about 36 hours. Upon completion of the heating cycle, the retort is
removed from the oven and allowed to cool approximately to
400.degree. F (205.degree. C), after which it is placed in a dry
environment for cooling to ambient temperature.
The cooled retort is then placed in a humidity control cabinet, the
cover removed and the aluminum-coated vane/shroud taken out of the
cementation pack. The part is cleaned of adhering coating compound
by blowing with dry air and immersed in water to remove fine dust
and other residues to provide a very clean aluminum deposit
containing an iron aluminide intermetallic compound, such as
FeAl.sub.3, on the braze-free portion of the component and a nickel
aluminide intermetallic compound on the nickel-plated brazed
portion of the component. The aluminized surface, like other
thermally diffused aluminum coatings, is characterized by
sacrificial properties in that it will corrode in preference to the
ferrous substrate in saline environments and, therefore,
substantially protect the ferrous substrate against corrosion.
Generally speaking, the pack composition may comprise by weight
about 60% to 100% aluminum, about 1% to 5% dry AlCl.sub.3 and the
balance essentially an inert refractory metal oxide, such as oxides
of aluminum, magnesium, titanium, zirconium, etc.
An electroplating bath that may be employed in providing a ferrous
metal component with a substantially uniform plate of nickel is as
follows:
______________________________________ Nickel Sulfamate
[Ni(NH.sub.2 SO.sub.3).sub.2 ] 450 gpl* Boric Acid 30 gpl pH --
3.0-5.0 Temp -- 40.degree.-60.degree. C Current Density -- 2-30
Amps/dm.sup.2 ______________________________________ *gpl is grams
per liter
Another bath comprises:
Nickel Sulfamate [Ni(NH.sub.2 SO.sub.3).sub.2 ] -- 300 gpl Nickel
Chloride (NiCl.sub.2.6H.sub.2 O) -- 6 gpl Boric Acid -- 30 gpl pH
-- 3.5-4.2 Temp. -- 28.degree.-60.degree.C Current Density --
2-25
With regard to plating with nickel and other aluminide-forming
metals, reference is made to the book "Modern Electroplating"
edited by Frederick A. Lowenheim (published 1942, 1953, 1963 by
John Wiley & Sons, Inc.).
A typical chromium plating bath is as follows:
(1) Chromic Acid (CrO.sub.3) -- 400 gpl Sulfate (SO.sub.4 .sup.=)
-- 4 gpl
The sulfate ion is employed as an acid radical catalyst.
Another composition is:
(2) Chromic Acid (CrO.sub.3) -- 250 gpl Sulfate (SO.sub.4 .sup.=)
-- 2.5 gpl
Most commercial baths contain 150 to 400 gpl of chromic acid. Thus,
bright deposits are produced from bath (2) above at 40.degree. C
and cathode current densities of 3.1 amps/dm.sup.2 to 15.5
amps/dm.sup.2. A plating speed of about 1 mil (0.001 inch) per hour
can be achieved.
One electroless plating bath for producing a substantially uniform
coating of nickel over the whole surface of a ferrous metal part
comprises the following:
Nickel Chloride (NiCl.sub.2.6H.sub.2 O) -- 25 gpl Sodium
Hypophosphite (NaH.sub.2 PO.sub.2.H.sub.2 O) -- 25 gpl Sodium
Pyrosphosphate (Na.sub.4 P.sub.2 O.sub.7) -- 50 gpl pH -- 10-11
NH.sub.4 OH -- for neutralizing
A cobalt electroless plating bath is as follows:
Cobalt Chloride (CoCl.sub.2.6H.sub.2 O) -- 30 gpl Sodium
Hypophosphite (NaH.sub.2 PO.sub.2.H.sub.2 O) -- 20 gpl Sodium
Citrate (Na.sub.3 C.sub.6 H.sub.5 O.sub.2.51/2H.sub.2 O) -- 35 gpl
Ammonium Chloride (NH.sub.4 Cl) -- 50 gpl pH -- 9-10 NH.sub.4 OH --
for neutralizing
A stainless steel part that has been successfully treated in the
foregoing manner is the vane/shroud component shown in FIGS. 1 to
3. FIG. 1 shows a vane/shroud 10 of AMS 5616 steel comprising inner
and outer rings 11 and 12, respectively, with vanes 13 brazed
therebetween.
The brazed joints 14 are clearly shown in FIG. 2. FIG. 3 shows the
steel substrate 15 as well as the fillet 16 of the braze at the
joint coated uniformly with nickel by electroplating into which
nickel coating 17 has been diffused with aluminum to form a
protective layer of substantially nickel aluminide in coating 17.
In FIG. 3, an overplate 18 of nickel and copper is shown which is
added to the specimen to preserve the coating during mounting and
polishing of the specimen.
FIG. 4 is a microprobe trace of the coating of FIG. 3 showing a
coating up to approximately the braze interface of thickness of
about 0.0008 inch. As will be noted, the outer 0.0005 inch thick
coating contains about 36.7% Al and about 50 to 57.5% nickel, the
nickel aluminide indicated being Ni.sub.2 Al.sub.3. The small
amount of copper in the coating is the result of outward diffusion
from the braze. The aluminum did not penetrate into the braze.
The micrograph of FIG. 5 at 500 times magnification adjacent the
steel substrate 15 shows more clearly the nickel aluminide layer
17A adjacent unreacted nickel layer 17 next to the steel
substrate.
The microprobe trace of FIG. 6 is taken across the coating to the
steel substrate, certain portions of the trace being indicated with
the approximate composition of the diffused coating. It will be
noted that the whole coating thickness, including the unreacted
nickel layer, is approximately 0.0008 inch.
FIGS. 7 to 10 are similar figures illustrating the use of chromium
as an aluminide-forming metal; FIGS. 7 and 8 showing the effect of
chromium at the braze and FIGS. 9 and 10 as to the steel
substrate.
In FIG. 7 (500 times magnification) braze 18 is depicted showing a
portion of unreacted chromium plate 19 thereon and a layer of
chromium aluminide 20 (Cr.sub.4 Al.sub.9) adjacent the unreacted
chromium. Referring to the microprobe trace of FIG. 8, the
combination of the chromium aluminide layer and the unreacted
chromium layer adjacent the braze provides a thickness of about
0.0007 inch. The composition of the aluminum and chromium at
indicated portions of the coating is indicated, the aluminide layer
containing approximately 53.3% aluminum, 42.7% chromium, 3% copper
and the balance residuals.
The coating on the stainless steel portion of the article is shown
in FIG. 9 (500 times magnification) as comprising the chromium
aluminide layer 20A, unreacted chromium layer 19A on steel
substrate. The various layers are indicated graphically in the
microprobe trace of FIG. 10 together with the approximate analysis
of each portion.
As will be noted from the foregoing, a substantially uniform
sacrificial coating is provided over substantially the whole
surface and particularly at the braze where the aluminum is
inhibited from diffusing into the braze and thus adversely affect
the fatigue properties of the joint as aluminide precipitates in
the braze tends to weaken the braze structure.
As stated hereinbefore, unless the whole surface of the ferrous
component, including the braze, is plated with an aluminide-forming
metal prior to aluminizing said component, the fatigue properties
at the joint are generally degraded. This has been confirmed by
tests in which specimens of the joint have been subjected to
cantilever loading to provide maximum bending stress at the joint
during fatigue testing. In a test series conducted, the fatigue
samples were shaped into the form of a "tee" from type 410
stainless steel and a joint produced with braze AMS 4772. This was
done by brazing a machined cantilever arm to the sample. Prior to
brazing, the specimens were solution treated, quenched and
tempered.
One sample was aluminized without first applying a layer of
aluminide-forming metal. Another was substantially covered with
nickel and another with chromium and subsequently aluminized. The
specimens were then subjected to cantilever fatigue testing at a
stress of 50,000 psi. The following results were obtained:
Fatigue Cycles Item Specimens to Failure Remarks
______________________________________ (1) As brazed 300,000
Average of 7 samples (2) Aluminum-coated 180,000 Average of 3
samples (3) Nickel-Plated + Aluminum 350,000 Average of 3 samples
(4) Chromium Plated + Aluminum 240,000 1 sample
______________________________________
As will be noted from Item (1), the as brazed specimen exhibits
good resistance to fatigue until it is thermally coated with
aluminum as shown in Item (2) wherein the specimen failed at
180,000 cycles, a drop of about 40% from the higher value.
However, when the specimen is first nickel plated and then
aluminized as shown by Item (3), the number of cycles at failure is
greatly enhanced to 350,000. In Item (4), chromium plate protects
the brazed joint from aluminum to the extent that the fatigue life
is substantially less degraded and, in fact, is much better than
when the part is only aluminized.
A more telling test is the oxidation/corrosion fatigue test. This
test involves the following cycle:
(1) 10,000 fatigue cycles at 50,000 psi root load (2) 5 hours
oxidation at 700.degree.F (3) 16 hours salt spray
The salt spray test used is ASTM B 117.
The foregoing cycle is repeated until the sample fails. The results
obtained on the bare brazed joint, the aluminized joint and the
nickel-plated aluminized joint are as follows:
Total Fatigue Cycle Salt Oxi- da- No. of to Failure at Spray tion
Specimen Cycles 50,000 psi Hours Hrs. Braze Joint 5 58,000 80 25
Aluminized Joint 10 106,000 160 50 Nickel-Plated Aluminized Joint
20 210,000* 320 100 * - No Failure Improvements were also indicated
with chromium as the aluminide-forming metal. Thus,
aluminide-forming metals may be the iron-group metals iron, nickel
and cobalt and also chromium.
One of the attributes of an aluminide coating on a metal substrate
is its ability to absorb readily a silicate liquid in the
production of a protective non-metallic overcoat.
As pointed out in the aforementioned copending application Ser. No.
143,842 (U.S. Pat. No. 3,729,295), it is believed that the high
affinity of the thermally aluminized coating or surface for the
silicate is associated with the physical-chemical character of the
aluminized surface arising out of the method of growth of the
aluminide. The expression "thermally aluminized coating or surface"
is meant to cover the thermal diffusion of aluminum in a metal
surface in which an aluminide compound is formed at the
surface.
In one embodiment, the non-metallic overcoat or barrier layer is
formed by applying to the thermally aluminized surface of the
article a solution of a soluble silicate salt at a temperature
ranging up to about 100.degree. C, for example, about 70.degree. to
95.degree. C (about 160.degree. to 200.degree. F), removing excess
liquid from the surface, such as by blowing it off with air, to
form a uniform layer of said silicate salt, and then drying the
layer on said surface.
While the aluminized surface in and of itself exhibits resistance
to saline corrosion, a typical salt spray test shows that
sacrificial products form on the aluminized surface after
approximately 15 hours of testing, whereas times in excess of 200
hours have been obtained when the aluminized surface is coated with
a uniform silicate layer.
A wide range of sodium silicate solutions can be employed in
producing the non-metallic overcoat. For example, the solutions can
be prepared from solutions of 50 to 100% concentrations of Na.sub.2
O.sup.. 3.22 SiO.sub.2. Various other sodium silicates can be
employed to prepare solutions such as Baume 40, 45, 47 and 50.
Potassium silicate may be similarly employed. Lithium silicate and
also organic silicates can be used, such as ethyl silicate.
A preferred solution for producing a uniform pre-coat or barrier
layer on the intermetallic iron aluminide substrate is one
containing by weight about 0.05 to 2% SiO.sub.2 equivalent, for
example, a soluble silicate in the form of Na.sub.2 O .sup.. 3.22
SiO.sub.2. The temperature of the substrate during application
should preferably range from about 70.degree. to 95.degree. C.
A preferred method for applying the silicate solution pre-coat at
the foregoing concentration comprises immersing the thermally
aluminized ferrous component in a tank maintained at a temperature
of about 70.degree. to 95.degree. C with sufficient time in the
bath to bring the component to temperature and assure absorption of
the solution into the aluminized surface. The excess liquid is then
blown off with air and the part allowed to dry. It is immersed
again for a brief period for a time sufficient to allow the article
to be covered with liquid, after which it is removed, blown off
with air and air dried. The steps may be repeated until the desired
thickness is obtained. It has been found that when using the
air-drying technique, only short dips in the tank need be employed
to ensure a continuous build-up of the silicate layer. Leaving the
part in the bath too long can result in the layer being redissolved
in the solution. To assure wettability of the coating on the
intermetallic substrate, an ionic surfactant or wetting agent may
be employed, for example, an anionic phosphate surfactant such as
Triton QS-30 (manufactured by Rohm & Haas).
An alternate method which yields a more stable silicate coating
resides in applying a succession of layers as described hereinabove
followed by curing in an oven. Infra-red or forced air heated ovens
may be employed in the temperature range of about 150.degree. to
430.degree. C (about 300.degree. to 805.degree. F) with enhanced
corrosion protection. The silicate coating applied by any of the
methods described herein will produce a uniform layer with a
thickness of approximately 0.0001 inch (0.1 mil) while avoiding as
far as is possible areas of excess silicate on the surface. A
preferred method is to apply at least one pre-coat from a dilute
silicate bath containing 0.05% to 2% by weight of SiO.sub.2
equivalent by a series of dipping, drying and curing steps followed
by at least one spray coating of silicate from a more concentrated
solution containing about 2.5% to 17.5% (e.g. 6.8%) by weight of
SiO.sub.2 equivalent.
The advantage of curing the silicate coating which allows multiple
layers to be formed is that the cured coating can withstand ten
oxidation-corrosion cycles comprising heating the coated substrate
to 1,000.degree. F (about 538.degree. C) for 1 hour followed by 5
hours of salt spray testing, the foregoing test being repeated for
10 cycles. Applications of the silicate coating followed by curing
at about 400.degree. F (205.degree. C) have yielded high degrees of
protection and, in many cases, very little sacrificial products
have been observed after 10 cycles of heating to 1,000.degree. F
(538.degree. C) followed by the salt spray test. The foregoing
tests are helpful as controls in assuring the quality of the
silicate coating before the next coating treatment is applied.
The corrosion resistance of the intermetallic layer is further
enhanced by applying a conversion coating to the cured silicate
layer. The conversion coating in turn may be covered by a silicate
layer. The conversion coating may be applied by spraying, using
commercially available reciprocating guns. Following the
application of the conversion coating, the silicate solution
containing 6.8% by weight equivalent of SiO.sub.2 and containing
about 0.002% by weight of an anionic phosphate surfactant may
optionally be sprayed over the conversion coating having a surface
temperature not exceeding about 150.degree. F (65.degree. C)
followed by curing at temperatures from about 300.degree. F
(150.degree. C) to about 805.degree. F (430.degree. C) for 10
minutes in an infra-red furnace. Another method of covering the
conversion coating is to dip the article in a hot solution of about
180.degree. to 200.degree. F (82.degree. to 93.degree. C), using a
sodium silicate concentration of about 0.9% to 2.4% by weight of
SiO.sub.2 equivalent with a 0.002% addition of an anionic phosphate
surfactant.
The article or component is immersed in the bath and allowed to
come to temperature and excess liquid removed rapidly by means of
an air gun. It is then immersed again and immediately pulled out of
the solution and air dried. A third application is made in the same
manner. It is important that excess liquid be removed from the part
to avoid foaming during curing. The purpose of repeated immersion
and drying is to assure uniform coating of the surface. As stated
above, the curing is preferably carried out at about 800.degree. F
in an infra-red furnace.
A simple production procedure which has been found successful for
applying uniform layers of silicate is as follows:
A component of AMS 5616 steel is subjected to 4 cycles of treatment
in the solution by supporting the component, e.g. a vane/shroud
component, on a rack which is immersed in the solution and
immediately withdrawn. The liquid is allowed to drain for
approximately 15 seconds, after which it is immersed again and
withdrawn. Following the second dip, air pressure means is disposed
about the rack to blow off the excess liquid. This group of steps
constitutes one cycle. Four cycles are employed to produce the
desired silicate layer. If necessary, an air gun can be used to
remove excess liquid from the root of the blades. After the fourth
cycle, the blades are dried free of moisture by, for example,
blowing with air. The temperature at which the silicate layers are
applied may range from about 160.degree. F (70.degree. C) to
200.degree. F (90.degree. C). Following completion of the four
cycles, the coating on the dried blade is then cured at about
800.degree. F (425.degree. C) in an infra-red oven.
Apparently the combination of the silicate coating and the
aluminide compound in the aluminized surface markedly improves the
resistance of the aluminized surface to corrode sacrificially,
wherein the life of the sacrificial coating is unexpectedly
extended for longer periods of time in saline environments than
obtained with the aluminized surface alone.
However, as stated in co-pending application Ser. No. 143,842, the
life of the silicated sacrificial aluminized coating is further
enhanced by the application of a conversion coating from a solution
in substantially the manner in which the silicate coating is
applied. An aqueous conversion coating solution which is preferred
may range by weight from about 5% to 30% phosphoric acid
(preferably 10 to 30%), about 0.0235% to 3% aluminum, about 3% to
8% chromic acid (CrO.sub.3), about 0.75% to 6% magnesium, and the
balance essentially water. A formulation found particularly
preferred in producing the solution is as follows:
Wt. % Phosphoric acid 15.0 Aluminum powder 0.225 chromic acid
(CrO.sub.3) 5.0 Magnesium turnings 1.5 A non-anionic surfactant
compris- ing a condensation product of ethylene oxide with an
alkyl- phenol (Triton X-100 by Rohm & Haas 0.1 Water 78.175
100.00
The aluminum and magnesium are dissolved in the solution by virtue
of the free acid present.
In conversion coating a silicated steel substrate, the substrate is
sprayed, then dried and cured in the oven which heats the substrate
to a temperature of about 800.degree. F (427.degree. C). The
substrate is then cooled prior to the next application of the
coating. The steps of spraying, baking and cooling constitute one
spray cycle. Three spray cycles are normally used in applying the
conversion coating.
The application of the conversion coating as described above
results in a smooth uniform surface layer which provides
oxidation-corrosion protection without the need for supplementary
surface finishing. A build-up of approximately 0.1 mil can be
obtained by employing a plurality of silicate and conversion
coating applications.
It is believed that the baking of the duplex silicate-conversion
coating results in a reaction product which provides new and
improved resistance to corrosion in saline environments. While the
silicate is preferably first applied to the thermally aluminized
surface, it is appreciated that it can be applied as a solution
together with the conversion coating materials. Thus, the
conversion coating solution prior to spraying may contain about
0.05 to 2% by weight SiO.sub.2 equivalent as sodium silicate,
potassium silicate, ethyl silicate, and the like.
It will be noted from co-pending application Ser. No. 143,842 that
in addition to the conversion coating formulation described herein,
various conversion coatings of the phosphate-chromate types may be
employed in conjunction with the soluble silicate salt. Stating it
broadly, the conversion coating comprises phosphates and chromates
of at least one metal, for example, Al, Mg, Zn, Be, Ba, Sr, Ce,
group metals and other metals. As a preferred embodiment, a
conversion coat containing phosphates and chromates of aluminum and
magnesium is particularly desirable.
Broadly speaking, the conversion solutions may range in composition
by weight of at least about 0.5% of at least one phosphate and
chromate-forming metal, e.g. about 0.5 to 10%, about 5% to 30%
phosphoric acid, about 3% to 8% chromic acid (CrO.sub.3) and the
balance essentially water. A preferred conversion solution is one
containing by weight about 0.02% to 3% dissolved aluminum, about
0.75% to 6% dissolved magnesium, about 5% to 30% phosphoric acid
(preferably 15% to 30%), about 3% to 8% chromic acid and the
balance essentially water. A more specific composition is one
containing by weight about 1/4% aluminum, about 1.5% magnesium,
about 15% phosphoric acid, about 5% chromic acid and the balance
essentially water.
Examples of steels to which the invention is particularly
applicable include AMS 5616. This steel comprises 13% Cr, 2% Ni, 3%
W and the balance essentially iron; type 410 comprises about 11.5
to 13.5% Cr, 1% Si max., 1% Mn max., 0.15% C max., and the balance
essentially iron; and 17-4 PH comprises about 17% Cr, 4% Ni, 3% Cu,
small amounts of Co, Mn, Si, etc., and the balance essentially
iron. Broadly speaking, the steels may comprise about 5% to 25% Cr,
up to 5% W, up to 25% Ni, up to 4% Cu, up to 3% Al, up to 2% Ti and
the balance essentially iron.
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.
* * * * *