U.S. patent application number 10/374700 was filed with the patent office on 2004-08-26 for surface pre-treatment method for pre-coated heat-treatable, precipitation-hardenable stainless steel ferrous-alloy components and components coated thereby.
This patent application is currently assigned to The Boeing Company. Invention is credited to Keener, Steven G., Mendoza, Michael A..
Application Number | 20040163740 10/374700 |
Document ID | / |
Family ID | 32824725 |
Filed Date | 2004-08-26 |
United States Patent
Application |
20040163740 |
Kind Code |
A1 |
Keener, Steven G. ; et
al. |
August 26, 2004 |
Surface pre-treatment method for pre-coated heat-treatable,
precipitation-hardenable stainless steel ferrous-alloy components
and components coated thereby
Abstract
The present invention relates to the use, in a pre-coating
process, of a flash plating, without a subsequent chromate seal, as
a surface pre-treatment for a ferrous-alloy substrate prior to
applying a corrosion-inhibiting coating to improve the overall
corrosion protection of the pre-treatment component. Preferably the
ferrous alloy is a heat-treatable, precipitation-hardenable
stainless steel and the pre-treatement is a cadmium flash plate or
a zinc-nickel alloy flash plate.
Inventors: |
Keener, Steven G.; (Trabuco
Canyon, CA) ; Mendoza, Michael A.; (Huntington Beach,
CA) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
The Boeing Company
|
Family ID: |
32824725 |
Appl. No.: |
10/374700 |
Filed: |
February 25, 2003 |
Current U.S.
Class: |
148/537 ;
428/460; 428/623 |
Current CPC
Class: |
B05D 3/0254 20130101;
Y02T 50/60 20130101; Y10T 428/12569 20150115; Y10T 428/12785
20150115; Y10T 428/31688 20150401; C23C 28/00 20130101; C25D 5/48
20130101; C21D 6/004 20130101; B05D 2350/65 20130101; Y10T
428/12799 20150115; Y10T 428/12549 20150115; C23C 26/00 20130101;
Y10T 428/31678 20150401; B05D 7/14 20130101 |
Class at
Publication: |
148/537 ;
428/623; 428/460 |
International
Class: |
C21D 001/00; B32B
015/08 |
Claims
That which is claimed:
1. A method for coating a ferrous-alloy aircraft structural
component comprising the steps of: providing an aircraft structural
component made from a ferrous-alloy precursor having a
pre-determined treatment temperature; providing a flash plate
pre-treatment; subjecting the component to the flash plate
pre-treatment; providing a curable organic coating material having
a non-volatile portion that is curable at about the pre-determined
ferrous-alloy heat-treatment temperature; applying the coating
material to the component; and substantially simultaneously curing
the coating and the component by heat-treating the ferrous-alloy
precursor to a pre-determined heat-treatment temperature.
2. The method of claim 1, wherein the ferrous-alloy precursor is a
heat-treatable, precipitation-hardenable stainless steel.
3. The method of claim 1, wherein the pre-determined heat-treatment
temperature is selected to substantially simultaneously cure the
ferrous-alloy precursor and the organic coating.
4. The method of claim 1, wherein the flash plate pre-treatment is
selected from the group consisting of a cadmium flash plate and a
zinc-nickel alloy flash plate pre-treatment.
5. The method of claim 1, wherein the flash plate pre-treatment
applies a flash plating to the component to a thickness of from
about 0.0002 inch to about 0.0004 inch.
6. The method of claim 1, wherein the flash plate pre-treatment
does not comprise a subsequent chromate seal treatment.
7. The method of claim 1, wherein the curing step treats the
coated, ferrous-alloy component to impart pre-determined
metallurgical properties to the ferrous-alloy material, and
concurrently cure the coating.
8. The method of claim 1, wherein the ferrous-alloy precursor is
selected from the group consisting of 18-8, 17-4 PH, 17-7 PH, 15-5
PH, PH 13-8Mo, PH 15-7Mo, A-286, Custom 450, and Haynes 556
materials.
9. The method of claim 1, wherein the ferrous-alloy precursor is
A-286 alloy.
10. The method of claim 1, wherein the coating is applied to the
ferrous-alloy by a method selected from the group consisting of
dipping, spraying, brushing, and fluidized-bed deposition.
11. The method of claim 1, further comprising the step of snap
tempering the ferrous-alloy precursor before the flash plate
pre-treatment step.
12. The method of claim 1, further comprising a ferrous alloy
hardening treatment step wherein the hardening treatment step
comprises austenitizing or normalizing the ferrous-alloy
precursor.
13. The method of claim 1, wherein the aircraft structural
component is selected from the group consisting of fasteners,
fittings, hinges, bearings, gears, struts, and the mechanical
structures attached thereto.
14. The method of claim 1, wherein the organic coating material
comprises an organic, phenolic resin mixed with at least one
plasticizer and an inorganic additive selected from the group
consisting of aluminum powder and strontium chromate.
15. The method of claim 1, wherein the organic coating material
comprises polytetrafluoroethylene.
16. The method of claim 1, wherein the organic coating material is
dissolved in a solvent selected from the group consisting of
ethanol, toluene, methyl ethyl ketone (MEK), and mixtures
thereof.
17. The method of claim 1, wherein the organic coating material
comprises a sprayable solution having about 30 weight percent
ethanol, about 7 weight percent toluene, about 45 weight percent
methyl ethyl ketone (MEK), about 2 weight percent aluminum powder
and about 2 weight percent strontium chromate.
18. The method of claim 16, further comprising the step of exposing
the coated component to a temperature of from about 180.degree. F.
to about 220.degree. F. for about two minutes to liberate the
solvent from the coating.
19. The method of claim 1, wherein the organic coating material is
applied to the component to a thickness of from about 0.0003 inch
to about 0.0005 inch.
20. A method for improving the corrosion protection of a
ferrous-alloy substrate comprising the steps of: providing a
ferrous-alloy substrate; applying to the ferrous-alloy substrate a
flash plate pre-treatment without a subsequent chromate seal
treatment; and applying a curable organic coating material having a
non-volatile portion that is curable at about a pre-determined
heat-treatment temperature of the ferrous alloy.
21. The method of claim 20, wherein the ferrous alloy is a
heat-treatable, precipitation-hardenable, stainless steel.
22. The method of claim 20, wherein the flash plate pre-treatment
is selected from the group consisting of cadmium flash plate and
zinc-nickel alloy flash plate.
23. A corrosion resistant aircraft structural ferrous-alloy
component prepared according to a method comprising the steps of:
providing an aircraft structural component made from a
ferrous-alloy precursor having a pre-determined heat-treatment
temperature; providing a flash plate pre-treatment without a
subsequent chromate seal; subjecting the component to the flash
plate pre-treatment; providing a curable organic coating material
having a non-volatile portion that is curable at about the
pre-determined ferrous-alloy heat-treatment temperature; applying
the organic coating material to the component to form a coated
component; and curing the coated component to a pre-determined
temperature.
24. The component of claim 23, wherein the ferrous-alloy precursor
is a heat-treatable, precipitation-hardenable stainless steel.
25. The component of claim 23, wherein the organic coating applied
to the ferrous alloy and the ferrous alloy is heat-treated
substantially simultaneously.
26. The component of claim 23 wherein the curable organic coating
material is an organic, phenolic resin mixed with at least one
plasticizer and an inorganic additive selected from the group
consisting of aluminum powder and strontium chromate.
27. The component of claim 23, wherein the coating material is
dissolved in a solvent selected from the group consisting of
ethanol, toluene, methyl ethyl ketone (MEK), and mixtures
thereof.
28. A ferrous-alloy aircraft structural component comprising: a
ferrous-alloy precursor having a pre-determined heat-treatment
temperature; a flash plate pre-treatment without a subsequent
chromate seal; and a curable, organic coating made from a material
having a non-volatile portion that is curable at about the
pre-determined ferrous-alloy heat-treatment temperature, wherein
the precursor and the coating are substantially simultaneously
heat-treated.
29. The component of claim 28, wherein the ferrous-alloy precursor
is a heat-treatable, precipitation-hardenable stainless steel.
30. The component of claim 28, wherein the flash plate
pre-treatment is deposited onto the precursor to a thickness of
from about 0.0002 inch to about 0.0004 inch.
31. The component of claim 28, wherein the organic coating is
deposited onto the flash plate pre-treatment to a thickness of from
about 0.0003 inch to about 0.0005 inch.
32. The component of claim 28, wherein the precursor and organic
coating are substantially simultaneously heat-treated to a
temperature of from about 350.degree. F. to about 400.degree. F.
for a duration of from about 4 hours to about 5 hours.
33. The component of claim 28, wherein the flash plate
pre-treatment is selected from the group consisting of cadmium
flash plate and zinc-nickel alloy flash plate.
34. The component of claim 28, wherein the ferrous-alloy precursor
is selected from the group consisting of 18-8, 17-4 PH, 17-7 PH,
15-5 PH, PH 13-8Mo, PH 15-7Mo, A-286, Custom 450, and Haynes 556
materials.
35. The component of claim 28, wherein the ferrous-alloy precursor
is an A-286 alloy.
36. The component of claim 28, wherein the aircraft structural
component is selected from the group consisting of fasteners,
fittings, hinges, bearings, gears, and struts.
37. The component of claim 28, wherein the organic coating
comprises a phenolic resin mixed with at least one plasticizer, and
an inorganic additive selected from the group consisting of
aluminum powder and strontium chromate.
38. The component of claim 28, wherein the organic coating
comprises polytetrafluoroethylene.
39. The component of claim 28, wherein the organic coating is
dissolved in a solvent selected from the group consisting of
ethanol, toluene, methyl ethyl ketone (MEK), and mixtures
thereof.
40. The component of claim 28, wherein the organic coating
comprises a sprayable solution having about 30 weight percent
ethanol, about 7 weight percent toluene, about 45 weight percent
methyl ethyl ketone (MEK), about 2 weight percent aluminum powder
and about 2 weight percent strontium chromate.
41. The component of claim 39, wherein the organic coated component
is exposed to a temperature of from about 180.degree. F. to about
220.degree. F. for about two minutes to liberate the solvent from
the coating.
42. An aircraft comprising a ferrous-alloy aircraft structural
component comprising: a ferrous-alloy precursor having a
pre-determined heat-treatment temperature, said precursor
pre-treated with a flash plate pre-treatment deposited to a
thickness of from about 0.0002 inch to about 0.0004 inch without a
subsequent chromate sealant; and a curable, organic coating made
from a material having a non-volatile portion that is curable at
about the pre-determined ferrous-alloy heat-treatment temperature,
wherein the precursor and the coating are substantially
simultaneously heat-treated.
43. The aircraft of claim 42, wherein the ferrous alloy is a
heat-treatable, precipitation-hardenable stainless steel.
44. The aircraft of claim 42, wherein the flash plate pre-treatment
is selected from the group consisting of cadmium flash plate and
zinc-nickel alloy flash plate.
45. The aircraft of claim 42, wherein the precursor and the coating
are heat-treated to a temperature of from about 350.degree. F. to
about 400.degree. F. for a duration of from about 4 hours to about
5 hours.
46. The aircraft of claim 42, wherein the ferrous-alloy precursor
is selected from the group consisting of 18-8, 17-4 PH, 17-7 PH,
15-5 PH, PH 13-8Mo, PH 15-7Mo, A-286, Custom 450, and Haynes 556
materials.
47. The aircraft of claim 42, wherein the ferrous-alloy precursor
is an A-286 alloy.
48. The aircraft of claim 42, wherein the aircraft structural
component is selected from the group consisting of fasteners,
fittings, hinges, bearings, gears, struts, bolts, nuts, rivets,
washers, springs, and screws.
49. The aircraft of claim 42, wherein the organic coating comprises
a phenolic resin mixed with at least one plasticizer,
polytetrafluoroethylene and an inorganic additive selected from the
group consisting of aluminum powder and strontium chromate.
50. The aircraft of claim 42, wherein the aircraft structural
component is exposed to a temperature of from about 180.degree. F.
to about 220.degree. F. for about two minutes to liberate a solvent
from the coating material.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to the preparation of pre-coated
ferrous-alloy components. More particularly, the present invention
relates to the use of a surface preparation as a preliminary step
in a pre-coating process to improve the corrosion protection and
other properties of coated ferrous-alloy components.
[0002] Aircraft manufacturers use a variety of different ferrous
and non-ferrous metals in the fabrication of aircraft components.
Commonly assigned U.S. Pat. No. 5,614,037 discloses a method for
pre-treating aluminum and aluminum-alloy articles to obviate the
use of wet sealants and other coatings for protection against
corrosion damage.
[0003] Ferrous alloys such as carbon steels and aircraft-quality
low-alloy stainless steels, such as, for example, Aermet 100,
HY-TUF.TM., 300M, H-11, HP9-4-30, 52100, 1095, 4130, 4135, 4140,
4330V, 4340, 6150, 8740, 18-8, 17-4PH, 17-7PH, 15-5PH, PH 13-8Mo,
PH 15-7Mo, A-286, etc. are often used as primary structural
aircraft components. Typically, these ferrous-alloy components,
including fasteners, bearing, struts, etc., are often protected
from wear and corrosion by applying an overplate of cadmium alone
or in time-consuming combination with other protective finishes
such as chrome plate. These fasteners are often installed using a
labor-intensive, time-consuming, and, consequently, very costly
wet-sealant process.
[0004] However, cadmium is suspected of being a carcinogen and is a
known toxic element. Cadmium and cyanide, used in the
electrodeposition of cadmium, have been listed as two of the
seventeen chemicals targeted by the U.S. Environmental Protection
Agency (EPA) for reduction. Additionally, many cadmium-plating
specifications require a chromate coating or other subsequent
finish to improve corrosion resistance. This adds another toxic
metal (hexavalent chromium) that must be treated before discharge.
Many European nations have passed legislation restricting import of
products with cadmium, and the European Economic Community (EEC)
has prohibited use of cadmium-plated products. Such concerns have
resulted in the search for a replacement coating or finish.
[0005] Although several candidate coatings have been identified, no
single replacement coating or system has been found that meets all
of the engineering requirements. Earlier tests that formed the
foundation for the recently issued patent demonstrated an
equivalent level, if not improved, of corrosion protection was
achieved by pre-coating ferrous-alloy components in lieu of the
inferior practice of applying wet sealant to the component during
its assembly. See commonly assigned U.S. Pat. Nos. 6,274,200 and
6,494,972.
[0006] However, it has been shown that, with respect to pre-coated
components having different ferrous-alloy substrate materials,
while the surrounding structural components are adequately
protected to equivalent levels from corrosion attack, the coating
itself that is applied to some of the components in known
pre-coating processes may be adversely affected from an appearance
standpoint and may interact to a degree that is visually
perceptible. Over a prolonged period of time, the possibility
exists that the adverse effect of this interaction could not only
manifest itself in the coating's visual appearance but may also
have an effect on the coating's integrity, possibly leading to a
compromised corrosion protection condition.
SUMMARY OF THE PRESENT INVENTION
[0007] The present invention is related to the discovery that, by
utilizing a particularly selected surface pre-treatment process for
heat-treatable, precipitation-hardenable stainless steel
ferrous-alloy components prior to applying a corrosion-inhibiting
coating, a significantly improved, pre-determined, final coated
condition can be achieved. This improved or enhanced final
condition results from the improved compatibility or
inter-relationship afforded by the pre-treatment process between
the compositions of the subsequently applied protective coating and
the component substrate yielding an improved pre-coated
component.
[0008] More specifically, the present invention relates to the use,
in a pre-treatment process, of a flash plating without a chromate
seal as a surface treatment for a ferrous-alloy substrate prior to
applying a corrosion-inhibiting coating to improve the overall
corrosion protection of the pre-treatment process. Preferably the
flash plating is either a cadmium flash plate or a zinc-nickel
(Zn--Ni) alloy flash plate coating.
[0009] In addition, the present invention relates to a method for
coating a ferrous-alloy aircraft structural component comprising
the steps of providing an aircraft structural component made from a
ferrous-alloy precursor having a pre-determined heat-treatment
temperature and subjecting the component to a flash plate
pre-treatment. The flash plate-treated component is optionally
subjected to a hardening treatment. A curable organic coating
material is provided having a non-volatile portion that is curable
at about the predetermined ferrous-alloy heat-treatment
temperature, and is applied to the component. The coating and the
component are substantially simultaneously cured by heat-treating
the ferrous-alloy precursor. Cadmium (Cd) flash plating
pre-treatment is applied per the requirements of AMS-QQ-P-416A,
Type I, Class 3 specification, i.e., the flash plating thickness is
from about 0.0002 inch to about 0.0004 inch without the further,
subsequent application of a chromate seal finish. Alternative
pre-treatment processes may be utilized other than the cadmium
flash plating process, such as the zinc-nickel (Zn--Ni) alloy flash
coating process per the requirements of BAC 5637 specification.
Following the pre-treatment, the component then is subjected to the
preferred pre-coating process of applying Hi-Kote.RTM. 1 coating
following the steps as previously claimed in the patents delineated
above. Thus the cadmium flash process of the present invention is
in strong contrast to the known cadmium plating that is necessarily
applied to thicknesses of 0.0005 inch to 0.0008 inch, and is
followed by a required subsequent chromate seal finish.
[0010] The present invention also relates to a corrosion-resistant
aircraft structural ferrous-alloy component prepared by providing
an aircraft structural component made from a ferrous-alloy
precursor having a pre-determined heat-treatment temperature and
subjecting the component to a flash plate pre-treatment. A curable
organic coating material is provided having a non-volatile portion
that is curable at about the predetermined ferrous-alloy
heat-treatment temperature and is then applied to the component.
The coated component is then heat-treated to substantially
simultaneously cure the coating and the component substrate.
[0011] Still further, the present invention relates to a
heat-treatable, precipitation-hardenable stainless steel
ferrous-alloy aircraft structural component comprising a
ferrous-alloy precursor having a pre-determined heat-treatment
temperature, a flash plate pre-treatment finish on the
ferrous-alloy precursor and a curable, organic coating covering the
flash plate. The organic coating is preferably made from a material
having a non-volatile portion that is curable at about the
pre-determined ferrous-alloy heat-treatment temperature, wherein
the precursor and the coating are substantially simultaneously
heat-treated.
[0012] The present invention also relates to an aircraft comprising
a ferrous-alloy aircraft structural component comprising a
ferrous-alloy precursor having a pre-determined heat-treatment
temperature. The precursor is pre-treated with a flash plate and
preferably followed by the application of a curable, organic
coating made from a material having a non-volatile portion that is
curable at about the predetermined ferrous-alloy heat-treatment
temperature, wherein the precursor and the coating are
substantially simultaneously heat-treated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 presents a flow chart, which delineates the steps
outlined by the process described in this disclosure including the
new step of performing the specified surface pre-treatment
operation.
[0014] FIG. 2 outlines alternative embodiments, which include an
optional forming or cold-working step either before or after the
surface pre-treatment process or operation.
[0015] FIG. 3 is a schematic cross-sectional view of
protruding-head fastener used to join two pieces, without a female
component.
[0016] FIG. 4 is a schematic cross-sectional view of a flush-head
fastener used to join two pieces, without a female component.
[0017] FIG. 5 is a schematic view of the flush-head fastener of
FIG. 4, with a female component.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0018] The present invention is directed to an improved method of
pre-treating a heat-treatable, precipitation-hardenable stainless
steel ferrous-alloy substrate by preparing the surface of the
ferrous-alloy substrate prior to the ferrous-alloy substrate
pre-coating process. Known patents disclose a ferrous-alloy
pre-coating process as it is directly applied to ferrous-alloy
substrates using any one of a variety of conventional surface
treatments primarily to satisfy industry accepted surface cleaning
requirements. See U.S. Pat. Nos. 6,274,200 and 6,494,972.
[0019] As shown by the process 10 outlined in FIG. 1 a
ferrous-alloy component 12 is annealed, normalized or austenitized
14 followed by an optional snap tempering step 16. The flash plate
surface pre-treatment 18 is then applied to the surface of the
component. An organic coating corrosion-resistant material is
provided 20 and then applied 22 to the flashed pre-treated
component followed by a curing or tempering treatment 24 designed
to substantially simultaneously treat both the component and the
organic coating. The treated component is then ready for
installation 26. FIG. 2 depicts a process 28 substantially similar
to that shown in FIG. 1 with the addition of a further forming or
fabrication step 30 to be included in the process 28. While the
cold-working forming step 30 may optionally occur before or after
the thermal treatment step 24, step 30 preferably occurs at some
point before the thermal treatment step 24.
[0020] As shown by the process outlined in FIGS. 1 and 2, the
coating material is applied to the pre-treated fastener in coating
step 22. Any suitable coating process can be used, such as, for
example, dipping, spraying, brushing, or a fluidized-bed method. In
one preferred process or approach, the solution of coating material
dissolved in a solvent is sprayed onto the pre-treated fasteners.
Once the fasteners are coated, the solvent is removed from the
as-applied coating by a quick drying or "flash cure" step, either
at room temperature or slightly elevated temperature, so that the
coated article is dried to a tack-free condition to enable
handling. Preferably, evaporation of solvent is accomplished by
flash cure or exposure to 200.degree. F. for about two minutes. The
coated component is still not suitable for service at this point,
because the coating is not sufficiently cured and adhered to the
pre-treated ferrous-alloy component and because the coating itself
is not sufficiently coherent to resist corrosion or mechanical
damage in service.
[0021] In the case of the preferred Hi-Kote.RTM. 1 coating, the
as-sprayed coating was analyzed by EDS analysis in a scanning
electron microscope. The heavier elements were present in the
following amounts by weight: Al, 82.4 percent; Cr, 2.9 percent; Fe,
0.1 percent; Zn, 0.7 percent; and Sr, 13.9 percent. The lighter
elements such as carbon and oxygen were detected in the coating but
were not reported because the EDS analysis for such elements is not
generally accurate.
[0022] The base, ferrous-alloy metal of the pre-treated fastener
and the applied coating are together heated to a suitable elevated
temperature in a cure/temper step 24, to achieve two results
substantially simultaneously. In this single step, the
ferrous-alloy material is treated to its final, desired strength
state, and the coating is cured to its desired final cross-linked
or bonded state.
[0023] Preferably, the temperature and time associated with the
treatment of step 24 is selected to be that required to achieve the
desired properties of the ferrous-alloy metal, as provided in the
industry-accepted and proven process standards for that particular
ferrous-alloy material. Surprisingly, this treatment is typically
not that specified by the coating manufacturer and may not produce
the most optimal cure state for the coating, but it has been
determined that the thermal treatment of the metal is less
forgiving of slight variations from the optimal treatment than is
the curing treatment of the organic coating. That is, according to
the present invention, the curing of the coating can sustain larger
variations in time and temperature with acceptable results than can
the heat-treatment process of the ferrous-alloy material. Contrary
to expectations and manufacturer's specifications, the coating
cured by the non-recommended procedures exhibits acceptable
adhesion to the ferrous-alloy substrate. The coating also exhibits
other desirable properties during the life of the coated component.
Thus, the use of the recommended embrittlement relief thermal
treatment process of the metal yields the optimal physical
properties of the metal, and acceptable coating properties. In the
case of one preferred A-286 heat-treatable,
precipitation-hardenable stainless steel ferrous alloy and
Hi-Kote.RTM. 1 coating, the preferred thermal treatment is the
embrittlement relief treatment process of the A-286 alloy, namely
about 4 hours to about 5 hours at about 350.degree. F. to about
400.degree. F.
[0024] Thus, the thermal treatment procedure 24 involves a
significantly different temperature than is recommended by the
manufacturer for the organic coating. There was initially a concern
that the higher temperature, beyond that required for the standard
curing of the coating, would degrade the coating and its properties
during service. Surprisingly, this concern proved to be unfounded.
The final coating 48, shown schematically in FIGS. 3-5, is strongly
adherent to the ferrous-alloy metal substrate and is also strongly
coherent and cross-linked. In FIGS. 3-5, the thickness of the
coatings 48 and 148 is exaggerated so that it is visible. In
reality, the coating 48 (FIG. 3) is typically about 0.0003 inch to
about 0.0005 inch thick after treating in step 24.
[0025] After coating and drying, the coated and fully-treated
component is ready for the installation step, (See 28, FIGS. 1 and
2). The component is installed in the manner appropriate to its
type. In the case of the fastener, such as the depicted bolt 40,
the bolt is placed through aligned bores in the two mating pieces
42 and 44 placed into intimate contact, as shown in FIG. 3. As
shown in FIG. 5, the remote protruding threaded end 150 of the bolt
140 has a female component, such as nut or collar installed so that
the pieces 142 and 144 are mechanically captured between the
pre-manufactured head 146 and a female component or threaded nut
152 of the bolt. FIG. 5 illustrates the threaded nut 140 for the
case of the flush head, and the general assembly configuration of
the bolts of the other types of bolts is similar. The coating 148
remains tightly adherent on the bolt even after assembly, as shown
in FIG. 5.
[0026] As mentioned above, the installation step reflects one of
the advantages of the present invention. If the coating were not
applied to the fastener, it would be necessary to place a viscous
wet-sealant material into the hole and onto the bolt prior to its
installation, which in turn coats the contacting adjacent surfaces
of the fastener and surrounding, adjacent structure. The
wet-sealant material is potentially toxic to workers, messy,
difficult to work with, and necessitates the use of extensive
cleanup tools as well as exposing surfaces of the pieces 42 and 44
to caustic chemical solutions after installation of the fastener.
Moreover, it has been observed that the presence of residual wet
sealant inhibits the adhesion of later-applied paint and other
topcoats applied over the fastener heads and surrounding
structure.
[0027] The coating process of the present invention overcomes these
problems confronted by the use of wet sealants. According to the
process of the present invention, use of wet sealant is not needed
or applied during fastener installation. Additionally, the
later-applied paint or other top coats adhere well over the
pre-coated fastener heads.
[0028] By performing a specific surface preparation of the
ferrous-alloy material prior to the pre-coating process, adverse
interaction between coating material and substrate is significantly
reduced or eliminated. Instead of simply cleaning or stripping the
component's surfaces prior to pre-coating, according to one aspect
of the invention, using a cadmium flash plating, without a
subsequent chromate seal, exhibits not only excellent additional
corrosion protection but reduces or eliminates the interaction
between the subsequently applied coating and the component's
substrate. The process of applying an intermediate surface
preparation, such as cadmium flash plating, in lieu of simply
cleaning or stripping the substrate's surface prior to the
pre-coating revealed superior results when compared to the known
pre-coating process. In addition, it has also been discovered that
a zinc-nickel (ZnNi) pre-plating surface preparation, also can
mitigate effects of the coating and substrate interaction.
[0029] The present invention contemplates using any high-strength,
ferrous-alloy material compatible with the selected
aluminum-containing, organic corrosion-inhibiting coating
formulation requiring a subsequent aging/curing period for the
pre-coated component. The subsequent aging/curing period can be
conducted at an elevated temperature commensurate with the
ferrous-alloy material's thermal treatment protocol to facilitate
curing of the coating. The coating thickness achievable by the
present invention may vary slightly according to the preferred
end-result characteristics of the coated component, but preferably
coating thicknesses range from about 0.0003 inch to about 0.0005
inch.
[0030] One preferred embodiment of the invention relates to the
preparation of fasteners, such as rivets and threaded bolts, and
the following discussion will emphasize such components. The use of
the invention is not limited to fasteners, and instead is more
broadly applicable to a larger group of components. However, its
use with fasteners offers particular advantages that will be
discussed. The fasteners contemplated by the present invention
include screws, bolts, pins, rivets, etc., which may have threads,
and may have female mating components such as nuts, collars, lock
washers, etc.
[0031] The process of the present invention is also useful for
ferrous-alloy components used in aircraft construction such as, for
example, landing gears, machined fittings, as well as other
high-strength structural components such as fasteners and bearings.
Fasteners are understood to mechanically join the various
structural elements and subassemblies of aircraft. For example, a
large transport aircraft, such as the C-17 typically includes over
1,000,000 total fasteners such as bolts, screws, and rivets. When
such fasteners are formed from a ferrous-containing alloy, to
insure protection from corrosion, each fastener must be coated with
a suitable protective plating such as cadmium or chrome. In
addition, to further facilitate corrosion protection, such
fasteners are typically installed with a wet sealant that is toxic
and requires special handling during application. The wet sealant
further requires careful and expensive cleaning and special removal
and handling as a hazardous waste.
[0032] Typical wet sealants include, two-part, manganese-cured,
polysulfide sealants containing an additional quantity of soluble
metallic chromates. Wet sealants require refrigeration storage
until such time when they are required for use on the shop floor,
which contributes to their costly use. These are flowable viscous
materials which are applied by brush, spatula, roller or extrusion
gun. Examples are P/S 1422 or 870 C corrosion-inhibiting sealants
produced by PRC-Desoto, Glendale, Calif. By contrast the process of
the present invention pre-treats the components first with a
surface pre-treatment in the form of a flash plate, preferably a
cadmium flash plate without any chromate sealant, followed by the
pre-coating process, which applies a corrosion-resistant organic
coating. This pre-coating process obviates the need for the use of
wet sealant during the fastener installation and component assembly
process.
[0033] The ferrous-alloy components of the present invention
achieve their full, required strength and other metallurgical
properties produced by a thermal treatment as well as curing of the
coating. Achieving a specified strength level of the substrate is
important, because users of the components, such as the customers
of aircraft, will not permit a sacrifice of mechanical performance
in order to achieve improved corrosion protection. In the past,
they have required both acceptable mechanical performance and also
the use of various harsh full-up production plating treatments in
addition to the use of wet-sealant to achieve acceptable corrosion
protection. In the present approach, on the other hand, the
aircraft structural components have both acceptable mechanical
performance and a less toxic and costly method for providing
acceptable corrosion protection. It is known to those skilled in
the field of metals finishing, and in particular plating processes,
that minimal benefits for wear and corrosion protection are
associated with relatively thin, flash plate finishes. Yet the true
benefit as incorporated in the processing methodology of the
present invention is the ability that is afforded to facilitate the
more beneficial aspects of the subsequent pre-coating process. In
other words, the general use of various relatively thin, flash
plate finishes has shown to have greatly reduced benefits from wear
resistance and corrosion protection standpoints among other
considerations. However, when the pre-treatment flash plating
process is used in conjunction with the subsequent application of a
corrosion-inhibiting coating, as is the case in the pre-coating
process of the present invention, superior corrosion protection is
achieved, while mitigating the adverse interaction between the
coating and substrate, which the flash-plate pre-treatment is
intended to achieve.
[0034] With regard to aircraft bearings and fasteners, the
elimination of the requirement for the wet-sealant installation
approach for more than 1,000,000 fasteners in a large cargo
aircraft offers a significant cost savings of several hundreds of
thousands of dollars per aircraft. The elimination of the use of
wet sealants also improves the overall quality and workmanship in
the fastener installation, as there is no possibility of missing or
overlooking some of the fasteners as the wet sealant is applied.
Further, the pre-coated, fully-treated fasteners provide equivalent
or enhanced protection from corrosion during service than the
uncoated, wet-installed fasteners.
[0035] The preferred bolts, such as those represented in FIGS. 3-5,
preferably are manufactured from a heat-treatable,
precipitation-hardenable stainless steel ferrous alloy material. As
used herein, "ferrous alloy" or "ferrous-containing alloy" means
that the material has more than about 50 percent by weight iron.
Typically, the ferrous-alloy material has at least about 50 percent
by weight of iron, with the balance being alloying elements and a
minor amount of impurities. Alloying elements are added in
precisely controlled amounts to modify the properties of the
ferrous-alloy material as desired. Alloying elements that are added
to iron to modify its properties include, for example, carbon,
manganese, silicon, nickel, chromium, and molybdenum.
[0036] In one embodiment, the ferrous-alloy material is a
heat-treatable, precipitation-hardenable stainless steel material.
The component is first fabricated to a desired shape, for example,
a fastener such as a bolt. In order to impart strength to the
component, the component must then be heat-treated. In the solution
heat-treating or austenitizing/hardening process, the component is
heated to an elevated temperature where a Face-Centered-Cubic phase
called "austenite" is formed. While still at this elevated
temperature, the component is rapidly quenched, reverting the
austenite to a Body-Centered-Tetragonal phase called "martensite."
Untempered martensite is a hard, brittle phase and must be softened
by a process called "tempering." In the tempering process, the
austenitized and quenched component is subjected to an elevated
temperature, which is much lower than the austenitizing
temperature. This tempering process softens the component and
imparts toughness.
[0037] Tempering must occur shortly after the austenitizing and
quenching procedure, or fissures or cracking may occur leading to
component failure. The present invention contemplates providing, as
a tempering step, the necessary and required hydrogen embrittlement
relief (not specifically just conventional "tempering" of the metal
substrate) to cure the metal as well as the subsequent organic
coating. To achieve the desired hydrogen embrittlement, the
duration for the treatment will range from about 4 to up to about 9
hours depending upon the alloy selected. Further, if the part must
be processed (i.e., straightened, coated, etc.) prior to a full
temper, the component can be given an intermediate and abbreviated
"snap" temper. This snap temper softens the ferrous alloy slightly
and reduces the likelihood of cracking.
[0038] According to one embodiment of the present invention, all
parts having hardness, i.e. Rockwell "C" scale, of 36.0 or greater
are preferably embrittlement relieved following pickling, plating,
or electrolytic cleaning. Aerospace components, such as fasteners,
typically fall into this hardness category due to strength
requirements. The subsequent thermal treatment or baking allows for
release of hydrogen.
[0039] The ingress of hydrogen into a component, an event that can
seriously reduce the ductility and load-carrying capacity, can
result in cracking and catastrophic brittle failures at stresses
well below the yield stress of susceptible materials. Hydrogen
embrittlement occurs in a number of forms, but the common features
are an applied tensile stress and hydrogen dissolved in the metal.
An example of hydrogen embrittlement is cracking of hardened steels
when exposed to conditions, which inject hydrogen into the
component. Presently, the phenomenon is not completely understood
and hydrogen embrittlement detection, in particular, is
problematic. Further, hydrogen embrittlement does not affect all
metallic materials equally. The most vulnerable materials are
high-strength stainless steels, titanium alloys, and aluminum
alloys.
[0040] Sources of hydrogen causing embrittlement have been
encountered in the making of steel, in processing parts, in storage
or containment of hydrogen gas, and related to hydrogen as a
contaminant in the environment that is often a by-product of
general corrosion. Hydrogen entry, the obvious pre-requisite of
embrittlement, can be facilitated in a number of ways. One example
is by manufacturing operations, such as welding, electroplating,
pickling, etc. If a material subject to such operations is
susceptible to hydrogen embrittlement, then a final, baking thermal
or heat treatment to expel any hydrogen is required. Another
example is a by-product of a corrosion reaction, such as in
circumstances when the hydrogen production reaction acts as the
cathodic reaction since some of the hydrogen produced may enter the
metal in atomic form rather than evolving as a gas into the
surrounding environment.
[0041] Hydrogen diffuses along the grain boundaries and combines
with the carbon, which is alloyed with the iron, to form methane
gas. The methane gas is not mobile and collects in small voids
along the grain boundaries where it builds up enormous pressures
that initiate cracks. If the metal component is under a high
tensile stress, brittle failure can occur. At normal room
temperatures, the hydrogen atoms are absorbed into the metal
lattice and diffused through the grains, tending to gather at
inclusions or other lattice defects. If stress induces cracking
under these conditions, the path is transgranular. At high
temperatures, the absorbed hydrogen tends to gather in the grain
boundaries and stress-induced cracking is then intergranular. The
cracking of martensitic and precipitation hardened steel alloys is
believed to be a form of hydrogen stress corrosion cracking that
results from the entry into the metal of a portion of atomic
hydrogen that is produced in the following corrosion reaction.
[0042] To address the problem of hydrogen embrittlement, emphasis
is placed upon controlling the amount of residual hydrogen in the
steel, controlling or limiting the amount of hydrogen pick-up in
processing, developing alloys with improved resistance to hydrogen
embrittlement, developing low or no embrittlement plating or
coating processes, and restricting the amount of in-situ hydrogen
introduced during the service life of a component.
[0043] Collectively, all of the processing steps leading to the
strengthening of the material or component are generally termed
"heat-treating" or "thermal treatment", wherein the component is
subjected to one or more periods of exposure to an elevated
temperature for a duration of time, with heating and cooling rates
selected to aid in producing the component's desired final,
metallurgical properties. The temperatures, times, and other
parameters required to achieve particular properties are known to
those skilled in the metallurgy field and are available in
reference documents for standard ferrous-alloy materials.
[0044] A preferred, specific heat-treatable,
precipitation-hardenable stainless steel ferrous-alloy material for
fastener applications is the A-286 alloy (UNS K66286) per AMS5731,
which has a nominal composition of 0.03 to 0.05 percent carbon,
15.0 percent chromium, 26.0 percent nickel and 1.25 percent
molybdenum, with the balance being iron plus minor impurities.
Other contemplated heat-treatable, precipitation-hardenable
stainless steel ferrous-alloys include, but are not limited to,
18-8, 17-4 PH, 17-7 PH, 15-5 PH, PH 13-8Mo, PH 15-7Mo, Custom 450,
and Haynes 556 series heat-treatable precipitation-hardenable,
stainless steel ferrous alloys. The A-286 alloy is available
commercially from several companies. After fabricating the alloy to
the desired shape such as a fastener like those shown in FIGS. 3-5,
the A-286 alloy may be fully annealed, normalized and stress
relieved. This state is usually obtained following fabricating
including machining, forging, or otherwise forming the fastener
into the desired shape. Following these steps, the ferrous-alloy
material is hardened or austenitized, quenched and, if necessary,
"snap" tempered. This condition is termed the "untreated state"
herein, as it precedes the final, full-tempering heat-treatment
soak required to optimize the strength and other properties of the
material. The component may be subjected to multiple forming
operations and periodically re-annealed as needed, prior to the
strengthening or hardening, heat-treatment processes.
[0045] A coating material is provided 20, preferably in solution so
that it may be readily and evenly applied. The usual function of
the coating material is to protect the base metal to which it is
applied from corrosion, including, for example, conventional
electrolytic corrosion, galvanic corrosion, and stress corrosion.
The coating material is a formulation that is primarily of an
organic composition, but which may contain additives to improve the
properties of the final coating. In one preferred embodiment, the
coating is initially dissolved in a carrier or solvent liquid so
that it can be applied to a substrate. After applying, the coating
material is curable to effect structural changes within the organic
component, typically cross-linking of the organic molecules to
improve the adhesion and cohesion of the coating. The coating layer
48, 148 on the preferred fastener is shown in FIGS. 3-5.
[0046] Such a curable coating is distinct from a non-curable
coating, such as a lacquer, which has different properties and is
not as suitable for the present, corrosion-protection application.
With a non-curable coating such as a lacquer, there is no need to
heat the coated article to elevated temperatures for curing. Thus,
the over-aging problems associated with the use of curable-coating
materials, and which necessitate the present invention, simply do
not arise. It is further understood that optional industry accepted
cleaning steps may be required to prepare the base metal for the
flash plate. Such cleaning procedures are those well known to those
skilled in the coating field and include the use of solvents,
acids, alkalines, and mechanical methods.
[0047] The present process contemplates a number of curable organic
coating materials. A typical and preferred coating material has a
phenolic resin mixed with one or more plasticizers, other organic
components such as polytetrafluoroethylene, and inorganic additives
such as aluminum powder and/or strontium chromate. These coating
components are preferably dissolved in a suitable solvent present
in an amount to produce a desired consistency based upon the
desired end use.
[0048] For the coating material just discussed, one useful
preferred solvent is a mixture of ethanol, toluene, and methyl
ethyl ketone (MEK). A typical, sprayable coating solution has about
30 percent by weight ethanol, about 7 percent by weight toluene,
about 45 percent by weight methyl ethyl ketone (MEK) as the
solvent, about 2 percent by weight strontium chromate, and about 2
percent by weight aluminum powder, with the balance being phenolic
resin and plasticizer. Optionally, a small amount of
polytetrafluoroethylene may be added. Such a product is available
commercially as "Hi-Kote 1" from Hi-Shear Corporation, Torrance,
Calif. The coating material has a standard elevated temperature
curing treatment of 1 hour at 400.degree. F..+-.25.degree. F., as
recommended by the manufacturer.
[0049] The following examples serve only to further illustrate
aspects of the invention and should not be construed as limiting
the present invention.
EXAMPLE
[0050] A comparative 2000 hour salt spray exposure test performed
on A-286 stainless steel Hi-Set.RTM. fasteners having various
surface preparation methods employed prior to the application of
Hi-Kote.RTM. 1 aluminum pigmented coating. Hi-Kote.RTM. 1 is a
phenolic resin-based aluminum coating as described above, and has
been demonstrated to possess excellent corrosion protection when
subjected to 2000-hour salt spray corrosion evaluations, as well as
high temperature resistance (to 400.degree. F.), excellent
resistance to fuel, hydraulic fluids, solvents when applied on a
wide variety of metallic surfaces.
[0051] The salt spray evaluation testing was performed in
accordance with ASTM B117 apparatus and standard test method
procedures. Aluminum-alloy test speciment assemblies, each
containing six fastener installations, were placed at a 15-degree
angle to the horizontal inside the salt spray test chamber for a
period of 2000 hours.
[0052] All fasteners used in the evaluation were selected from the
same manufacturing lot of Hi-Set.RTM. fasteners and represented
standard manufacturing processes, which included the application of
Hi-Kote.RTM. 1 coating onto A-286 stainless steel material for the
control specimens. Additional derivative samples were processed
with modifications to the A-286 material preparation prior to the
application of Hi-Kote.RTM. 1 coating, which included various
plating alternatives as well as wet-sealant installation of the
standard production Hi-Set.RTM. fasteners. A separate test coupon
assembly containing installations of a different type of production
Hi-Kote.RTM. 1-coated titanium-alloy material fastener was selected
for a comparison baseline of corrosion prevention results and
characteristics.
[0053] The test results indicated conclusively that the pre-coated
A-286 stainless steel HSR217 Hi-Set.RTM. fasteners, which employed
the pre-treatment cadmium flash plate finish prior to the
application of the Hi-Kote.RTM. 1 coating, exhibited excellent
corrosion protection performance. The pre-treatment process of
applying the cadmium flash plate finish on the fasteners in lieu of
stripping standard, full-up production cadmium plating prior to
pre-coating with Hi-Kote.RTM. 1 revealed superior results when
compared to results for wet-installed Hi-Set.RTM. fasteners
pre-coated with Hi-Kote.RTM. 1 coating on a standard prepared A-286
base material as well as the production titanium-alloy fasteners
pre-coated with Hi-Kote.RTM. 1.
[0054] The A-286 HSR217AP6-9 Hi-Set.RTM. fasteners processed with
the pre-treatment cadmium flash plate finish prior to being
pre-coated, out-performed the wet-installed, standard processed
A-286 material Hi-Set.RTM. fasteners pre-coated with Hi-Kote.RTM.
1. The wet-sealant installation was performed in accordance with
DPS2.50-17, Type 18.
[0055] Furthermore, the Hi-Set.RTM. A-286 fasteners processed with
the pre-treatment cadmium flash plate finish and subsequently
pre-coated with Hi-Kote.RTM. 1 performed equivalently in protecting
against exfoliation and pitting corrosion to that exhibited by the
dry-installed, standard production titanium-alloy fasteners, which
were pre-coated with Hi-Kote.RTM. 1.
[0056] Although a particular embodiment of the invention has been
described in detail for purposes of illustration, various
modifications and enhancements may be made without departing from
the spirit and scope of the invention. Accordingly, the invention
is not to be limited except as by the appended claims.
* * * * *