U.S. patent application number 11/031441 was filed with the patent office on 2006-06-29 for method for preparing pre-coated, metallic components and components prepared thereby.
This patent application is currently assigned to THE BOEING COMPANY. Invention is credited to Steven G. Keener.
Application Number | 20060141242 11/031441 |
Document ID | / |
Family ID | 33452122 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060141242 |
Kind Code |
A1 |
Keener; Steven G. |
June 29, 2006 |
Method for preparing pre-coated, metallic components and components
prepared thereby
Abstract
A high-strength, corrosion- and heat-resistant aircraft
structural component such as a fastener is prepared by providing a
corrosion-resistant stainless steel or heat-resistant superalloy
metallic component precursor that is not in its final heat-treated
state, and coating with an aluminum-containing, curable
polyaromatic phenolic coating material having a non-volatile
portion that is predominantly organic and is curable at about the
high-strength metallic alloy component's stress equalizing
tempering temperature. The coated, high-strength metallic-alloy
component is then thermally treated to concurrently impart
pre-determined metallurgical properties to the finished, metallic
substrate, and cure the organic, aluminum-containing coating.
Inventors: |
Keener; Steven G.; (Trabuco
Canyon, 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: |
33452122 |
Appl. No.: |
11/031441 |
Filed: |
January 7, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10453379 |
Jun 3, 2003 |
6953509 |
|
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11031441 |
Jan 7, 2005 |
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Current U.S.
Class: |
428/334 ;
428/457; 428/460 |
Current CPC
Class: |
B05D 3/0218 20130101;
Y10T 428/256 20150115; C22F 1/10 20130101; F16B 33/06 20130101;
B05D 2202/15 20130101; Y10T 428/20 20150115; B05D 7/16 20130101;
Y10T 428/31678 20150401; C09D 5/103 20130101; Y10T 428/24512
20150115; Y10T 428/31688 20150401; Y10T 428/263 20150115 |
Class at
Publication: |
428/334 ;
428/457; 428/460 |
International
Class: |
B32B 15/098 20060101
B32B015/098; B32B 15/18 20060101 B32B015/18 |
Claims
1-22. (canceled)
23. A stress equalized high-strength, corrosion-resistant aircraft
structural alloy component comprising a coating made from an
organic, polyaromatic phenolic resin mixed with at least one
plasticizer and an inorganic additive selected from the group
consisting of aluminum powder and chromate filler.
24. The component of claim 23, wherein the alloy is a nickel-base
alloy selected from the group consisting of Monel 400, Monel K-500,
Inconel 600, and Inconel X-750.
25. The component of claim 23, wherein the alloy component is Monel
400.
26. The component of claim 23, wherein the alloy component is
A-286.
27. The component of claim 23, wherein the coating material
comprises a polyaromatic resin-based compound.
28. 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, and mixtures thereof.
29. The component of claim 28, wherein the coating material
comprises a sprayable solution comprising about 30 weight percent
ethanol, about 7 weight percent toluene, about 45 weight percent
methyl ethyl ketone, about 2 weight percent aluminum powder and
about 2 weight percent strontium chromate.
30. The component of claim 23, wherein the coating is applied to
the component to a thickness of from about 0.0003 inch to about
0.0005 inch.
31. A stress equalized high-strength, corrosion-resistant aircraft
structural stainless steel alloy component comprising a coating
made from an organic, polyaromatic phenolic resin mixed with at
least one plasticizer and an inorganic additive selected from the
group consisting of aluminum powder and chromate filler.
32. The component of claim 31, wherein the component is made from a
material selected from the group consisting of 302, 303, 304, 305,
410, 416, 430, Custom 450, and 17-4PH stainless steels.
33. The component of claim 31, wherein the coating material
comprises a polyaromatic compound.
34. The component of claim 31, wherein the coating material is
dissolved in a solvent selected from the group consisting of
ethanol, toluene, methyl ethyl ketone, and mixtures thereof.
35. The component of claim 34, wherein the coating material
comprises a sprayable solution having about 30 weight percent
ethanol, about 7 weight percent toluene, about 45 weight percent
methyl ethyl ketone, about 2 weight percent aluminum powder and
about 2 weight percent strontium chromate.
36. The component of claim 31, wherein the coating is applied to
the component to a thickness of from about 0.0003 to about 0.0005
inch.
37. A high-strength, aircraft structural alloy component prepared
according to the method of claim 1.
38. A high-strength, aircraft structural nickel-base alloy
component prepared according to the method of claim 12.
39-47. (canceled)
48. An aircraft comprising an alloy aircraft structural component
comprising: an alloy precursor having a pre-determined stress
equalizing heat-treatment temperature, said precursor selected from
the group consisting of 302, 303, 304, 305, 410, 416, 430, Custom
450, and 17-4PH stainless steels, said stainless steels having a
pre-determined heat-treatment temperature; and a curable, organic
coating comprising a polyaromatic phenolic resin mixed with at
least one plasticizer, polytetrafluoroethylene and an inorganic
additive selected from the group consisting of aluminum powder and
chromate filler, said coating material having a non-volatile
portion that is curable at about the pre-determined alloy stress
equalizing heat-treatment temperature, said coating cured at a
temperature sufficient to substantially concurrently cure the
coating and heat-treat the alloy precursor.
49. An aircraft comprising an alloy aircraft structural component
comprising: an alloy precursor having a pre-determined stress
equalizing heat-treatment temperature, said precursor wherein the
alloy precursor is selected from the group consisting of Monel 400,
Monel K-500, Inconel 600, Inconel-X750, and A-286, said alloy
precursor having a pre-determined heat-treatment temperature; and a
curable, organic coating comprising a polyaromatic phenolic resin
mixed with at least one plasticizer, polytetrafluoroethylene and an
inorganic additive selected from the group consisting of aluminum
powder and chromate filler, said coating material having a
non-volatile portion that is curable at about the pre-determined
alloy stress equalizing heat-treatment temperature, said coating
cured at a temperature sufficient to substantially concurrently
cure the coating and heat-treat the alloy precursor.
50. An aircraft comprising an alloy aircraft structural component
comprising: an alloy precursor having a pre-determined stress
equalizing heat-treatment temperature, said precursor wherein the
alloy precursor is A-286, said alloy precursor having a
pre-determined heat-treatment temperature; and a curable, organic
coating comprising a polyaromatic phenolic resin mixed with at
least one plasticizer, polytetrafluoroethylene and an inorganic
additive selected from the group consisting of aluminum powder and
chromate filler, said coating material having a non-volatile
portion that is curable at about the pre-determined alloy stress
equalizing heat-treatment temperature, said coating cured at a
temperature sufficient to substantially concurrently cure the
coating and heat-treat the alloy precursor.
51. The aircraft of claim 48, wherein the aircraft structural
component is selected from the group consisting of fasteners,
fittings, hinges, bearings, gears, struts, and the mechanical
structures attached thereto.
52. The aircraft of claim 49, wherein the aircraft structural
component is selected from the group consisting of fasteners,
fittings, hinges, bearings, gears, struts, and the mechanical
structures attached thereto.
53. The aircraft of claim 48, wherein the coating is applied to the
component to a thickness of from about 0.0003 inch to about 0.0005
inch.
54. The aircraft of claim 49, wherein the coating is applied to the
component to a thickness of from about 0.0003 inch to about 0.0005
inch.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to the preparation of pre-coated,
high-strength stainless steel and superalloy metallic components.
More particularly, the present invention relates to the use of
organic, corrosion-inhibiting coatings containing aluminum pigment
to coat high-strength metallic corrosion-resistant stainless steel
and heat-resistant superalloy materials used as aircraft structural
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. Nos. 5,614,037; 5,858,133; 5,922,472;
5,944,918; 6,221,177, and 6,403,230 disclose methods for
pre-treating aluminum-alloy articles to obviate the use of
wet-sealants and other coatings for protection against corrosion
damage. Pre-coated ferrous-alloy articles such as carbon steels and
aircraft-quality low-alloy steels, for example; Aermet 100,
Hy-Tuf.TM., 300M, H-11, HP9-4-30, 52100, 1095, 4130, 4135, 4140,
4330V, 4340, 6150, 8740, etc. are often used as structural aircraft
components and are disclosed in U.S. Pat. Nos. 6,274,200 and
6,475,610. It had been known to protect ferrous-alloy components,
which include fasteners, bearings, struts, etc. from wear and
corrosion by applying an overplate of cadmium alone or in
combination with a chrome plate. Previously, these fasteners were
often installed using wet-sealant. While this use of wet-sealant
and/or plated overcoat protected the various material substrates
from corrosion, such wet-sealant installation and cadmium and
chrome-plating processes are time consuming, cumbersome, expensive,
and environmentally undesirable.
[0003] It would be extremely desirable to impart corrosion
resistance to the high-strength stainless steel and superalloy
metallic substrates obviating the need for the chrome and
cadmium-plating and/or wet-sealant installation processes. It would
be further highly desirable to incorporate the coating cure step
into an existing alloy fabricating process.
SUMMARY OF THE INVENTION
[0004] One embodiment of the present invention relates to a method
for providing a corrosion-resistant coating and applying it to
high-strength, nickel-base corrosion-resistant alloy and
heat-resistant superalloy aircraft structural components. With
regard to the nickel-base alloys, an aircraft structural component
made from a nickel-base corrosion-resistant or heat-resistant
superalloy precursor preferably is provided in an annealed,
untreated state. The nickel-base superalloy precursor is then
coated with an aluminum-containing, curable organic coating having
a non-volatile portion that is predominantly organic. The coating
is curable at a temperature approximately equal to the superalloy
component's stress equalizing heat-treatment tempering temperature,
and is applied to the superalloy precursor prior to the final
stress equalizing process step. In one embodiment, the coating
material is flash cured (200.degree. F. for about 1-2 minutes to
facilitate handling immediately following its application). The
coated, nickel-base superalloy precursor is then subjected to a
stress equalizing heat treatment to substantially concurrently 1)
impart pre-determined metallurgical properties or characteristics
to the finished nickel-base superalloy component, and 2) cure and
fully cross-link the organic, aluminum-containing coating. The term
"stress equalizing heat treatment" refers to a low-temperature
heat-treatment procedure used to balance stresses in cold-worked
material without an appreciable decrease in the mechanical strength
produced by cold-working.
[0005] More specifically, the present invention is directed to a
method for pre-treating a high-strength, corrosion-resistant
stainless steel alloy aircraft structural component comprising the
steps of providing an aircraft structural component made from a
high-strength, alloy precursor selected from the group consisting
of 302, 303, 304, 305, 410, 416,430, 440C, Custom 450, Custom 455,
17-7PH, and 17-4PH stainless steels, the stainless steels, each
having a pre-determined heat-treatment temperature, and subjecting
the component to a hardening treatment. A curable organic coating
material is provided comprising an organic, phenolic resin mixed
with at least one plasticizer and an inorganic additive selected
from the group consisting of aluminum powder and
chromate-containing filler, preferably strontium chromate. The
coating has a non-volatile portion that is curable at about the
pre-determined stainless steel alloy heat-treatment temperature.
The coating material is applied to the component and the coated
component is cured in a stress equalizing heat-treatment step to
substantially concurrently heat-treat the alloy to impart desired
characteristics and cure the applied coating.
[0006] The present invention is further directed to a method for
pre-treating a high-strength, nickel-base alloy corrosion- or
heat-resistant aircraft structural component by providing an
aircraft structural component made from a high-strength,
nickel-base alloy, or "superalloy" precursor, the nickel-base
alloys having a pre-determined heat-treatment temperature. The
preferred corrosion-resistant alloys include nickel-base alloys
Monel 400, Monel K-500, Inconel 600, Inconel X-750 and A-286. The
component is then subjected to a cold or hot working treatment. A
curable organic coating material is provided comprising an organic,
phenolic resin mixed with at least one plasticizer and an inorganic
additive selected from the group consisting of aluminum powder and
a chromate-containing filler, preferably strontium chromate, the
coating material having a non-volatile portion that is curable at
about the pre-determined nickel-base alloy heat-treatment
temperature. The coating material is applied to the component, and
the coated component is cured in a stress equalizing heat-treatment
step to substantially concurrently heat-treat the alloy and cure
the coating.
[0007] In addition, the present invention is directed to a stress
equalized high-strength, corrosion-resistant aircraft structural
stainless steel alloy component comprising a coating made from 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. Preferably the stainless steel alloy
is selected from the group consisting of 302, 303, 304, 305, 410,
416, 430, 440C, Custom 450, Custom 455, 17-7PH, and 17-4PH
stainless steels.
[0008] Still further, the present invention is directed to a method
for assembling an aircraft component with a corrosion-resistant
coated fastener comprising providing a high-strength, aircraft
structural component made from an alloy precursor having a
pre-determined heat-treatment temperature and stress equalizing
tempering the precursor. An organic coating material is provided
comprising a phenolic resin mixed with at least one plasticizer,
and an inorganic additive selected from the group consisting of
aluminum powder and strontium chromate, the coating material having
a non-volatile portion that is organic and is curable at a
temperature about equal to the pre-determined heat-treatment
temperature. The precursor is then coated with the organic coating
followed by stress equalizing tempering the coated alloy precursor
to a finished alloy material by tempering to a temperature of from
about 450.degree. F. to about 600.degree. F. for a duration of from
about 1 hour to about 11/2 hours to substantially concurrently
temper the finished alloy material and cure the coating.
[0009] In addition, the present invention is also directed to a
method for assembling an aircraft component with a
corrosion-inhibiting coated fastener comprising providing a
high-strength, aircraft structural component made from an alloy
precursor having a pre-determined stress equalizing heat-treatment
temperature followed by austenitizing the precursor. An organic
coating is provided comprising a polyaromatic phenolic resin mixed
with at least one plasticizer, polytetrafluoroethylene and an
inorganic additive selected from the group consisting of aluminum
powder and strontium chromate. The coating material has a
non-volatile portion that is organic and is curable at a
temperature about equal to the pre-determined stress equalizing
heat-treatment temperature. The precursor is then coated with the
organic coating, followed by stress equalizing tempering the
coated, alloy precursor to a temperature of from about 450.degree.
F. to about 600.degree. F. for a duration of from about 1 hour to
about 11/2 hours to substantially concurrently stress equalizing
temper the finished alloy material and cure the coating. According
to one process of the present invention, the coated, superalloy
component is formed, cold-worked or thread rolled into a completed
or partially completed form prior to the coating being applied.
[0010] In a still further embodiment, the present invention relates
to a further method for coating a high-strength, superalloy
aircraft structural component with a corrosion-inhibiting coating.
A high-strength aircraft structural component made from a
superalloy precursor is provided and austenized/normalized,
followed by quenching. The component is cold-worked or otherwise
formed such as, by thread rolling, and then coated with an
aluminum-containing, curable organic coating material having a
non-volatile portion that is predominantly organic and is curable
at about the superalloy material's stress equalizing heat-treatment
tempering temperature. The superalloy precursor is then subjected
to the tempering temperature of from about 450.degree. F. to about
600.degree. F. for a duration of from about 1 hour to about 3
hours. The appropriate ranges depend on the specific nickel-base
superalloy being treated. Most preferably for the components of the
present invention, the tempering temperature is about 525.degree.
F. for about 1 hour to about 11/2 hours. This stress equalizing
heat-treatment tempering operation concurrently 1) imparts the
desired metallurgical characteristics to the nickel-base superalloy
material and 2) properly cures the coating.
[0011] These embodiments yield surprising and unexpected technical
and cost advantages when used in conjunction with high-strength,
stainless steel and nickel-base corrosion- and heat-resistant
superalloy aircraft structural components such as bearings, hinges,
fittings, gears, struts, fasteners, rivets, etc. Through the use of
the coating techniques of the present invention, the need to plate
the components with cadmium or chrome and/or use wet-sealant in
their installation for corrosion protection is obviated. Other
features and advantages of the present invention will be apparent
from the following more detailed description of the preferred
embodiments, taken in conjunction with the accompanying drawings,
which illustrate, by way of example, the principles of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1a-1c are process flow diagrams for the methods of the
invention for nickel, cobalt and iron-base heat-resistant
superalloys.
[0013] FIG. 2 is a process flow diagram for the method of the
invention for nickel-base corrosion- and heat-resistant
superalloys.
[0014] FIGS. 3a-3b are process flow diagrams for the method of the
invention for austenitic and ferritic/martenistic (respectively)
corrosion-resistant stainless steel and nickel-base alloys.
[0015] FIG. 4 is a schematic cross-sectional view of
protruding-head fastener used to join two pieces, without a female
component.
[0016] FIG. 5 is a schematic cross-sectional view of a flush-head
fastener used to join two pieces, without a female component.
[0017] FIG. 6 is a schematic view of the flush-head fastener of
FIG. 5, with a female component.
DETAILED DESCRIPTION OF THE INVENTION
[0018] FIGS. 1a-1c, 2, and 3a-3b, are schematic flow diagrams
outlining the method of the present invention when applied to
various corrosion- and heat-resistant high-strength, metallic
alloys. According to one preferred method of the present invention
shown in process 1a, a heat-resistant alloy work piece is supplied
and annealed 10. The partially-treated fastener is then hot-worked,
cold-worked, formed, or machined 12. The work piece is then
solution heat treated 14, followed by a tempering/aging step 16. A
coating material is provided and applied 18 to the work piece to a
specified, carefully controlled and pre-determined coating
thickness by any of various contemplated methods. Finally, the
coated work piece is exposed to a curing/stress equalizing
heat-treatment step 20 followed by installation (not shown).
[0019] FIG. 1b sets forth a step-wise process contemplated by a
further embodiment of the present invention that removes the
tempering/aging step 16 as was done in process shown in FIG. 1a,
and instead employs a combined tempering/aging and cure coating
final processing step 22.
[0020] FIG. 1c shows a further preferred process of the present
invention whereby a heat-resistant alloy is annealed or taken as is
10, followed by a solution heat treatment step 14 for approximately
2 hours to 4 hours. The work piece is then aged in an aging step 24
for a total period of time, preferably from about 24 hours to about
48 hours depending upon the material alloy. The work piece is then
hot-worked, cold-worked, formed, or machined 12. The coating is
then applied 18 followed by the final curing/stress equalizing
heat-treatment step 20.
[0021] The addition of magnesiumn, aluminum, silicon, titanium, and
certain other alloying elements to nickel and nickel-base alloys,
separately or in combinations, produces an appreciable response to
age hardening. The effect is dependant upon both chemical
composition and aging temperature; it is caused by precipitation of
submicroscopic particle throughout the grain, which results in a
marked increased in hardness and strength.
[0022] Unlike precipitation hardening stainless steels and
aluminum-based alloys, the nickel and nickel-base alloys normally
do not require solution heat-treating in the upper annealing
temperature range prior to age hardening. However, solution
treating may be employed to enhance special properties. For
example, Inconel X-750 may be solution heat-treated for about 2 to
4 hours at approximately 1150.degree. C. (2100.degree. F.) and
air-cooled prior to a double or two-step, i.e. high and low
temperature, aging cycle to develop maximum creep, relaxation, and
rupture strength at temperatures above approximately 600.degree. C.
(1100.degree. F.). Specifically, this two-step aging process
entails exposure to about 845.degree. C. (1550.degree. F.) for 24
hours; air cooled; then re-heated to about 705.degree. C.
(1300.degree. F.) for approximately 20 hours, followed by an
air-cooling step. This combination of heat treatments is considered
essential for high-temperature components, fasteners, springs,
turbine blades, etc. made of Inconel X-750 and other heat-resistant
superalloys.
[0023] Aging treatments strengthen age-hardenable alloys by causing
the precipitation of one or more phases from the supersaturated
matrix that is developed by solution heat-treating and retained by
rapid cooling from the solution treating temperature. Factors that
influence the selection of number of aging steps and aging
temperatures include: (a) type and number of precipitating phases
available, (b) anticipated service temperature, (c) precipitate
size, and (d) the combination of strength and ductility desired and
heat treatment of similar alloys.
[0024] When more than one phase is capable of precipitating from
the alloy matrix, judicious selection of a single aging temperature
may result in obtaining optimum amounts of multiple precipitating
phases. Alternatively, a double or two-step aging treatment that
produces different sizes and types of precipitate at different
temperatures may be employed. The aging temperature also determines
not only the type but also the size distribution of precipitate.
Certain types of nickel-base alloys use this double aging
treatment, which represent this co-precipitation heat treatment
approach.
[0025] Exposure to temperatures higher than the optimum aging
temperature results in a decrease in strength through the process
of overaging. At still higher temperatures, re-solution may occur.
High aging temperatures will produce coarser gamma prime particles
than lower temperatures and result in higher creep-rupture
properties. For optimum product, short time elevated-temperature
properties, small, finely-dispersed particles of gamma prime
precipitate are desired. Therefore, lower final aging temperatures
than those used to obtain high creep-rupture properties preferably
are employed. A principal reason for two-step aging treatments, in
addition to gamma prime or gamma double prime control, is the need
to precipitate or control grain boundary carbide morphology.
[0026] A uniformly fine-grain microstructure can be produced in the
age-hardenable, heat-resistant alloys if final deformation is
carried out in the lower part of the hot-working range. This type
of metallurgical structure can be solution treated to a uniform
grain size. Alloys deformed in the upper hot-working range have a
coarser structure that cannot be refined by solution treating.
[0027] Cold-working is usually performed on alloys in the
solution-treated condition because of the markedly lower strength
and increased ductility of the material before aging. Cold-working
itself affects mechanical properties, through its influence on: (a)
grain growth during subsequent solution treatment, and (b) the
reaction kinetics of aging.
[0028] The age-hardenable, heat-resisting alloys are susceptible to
critical grain growth if they are solution treated after small
amounts of cold or hot work. Larger amounts of cold work refine the
grain size. Excessive grain growth may have deleterious effects on
creep and fatigue properties in all heat-resisting alloys.
Therefore, on parts subjected to cold or hot work prior to solution
treating, the critical amount of work (about 1% to about 6% cold
work, depending on the alloy, or about 10% hot work) must be
exceeded in all areas, to avoid the growth of abnormally large
grains. This rule applies to items such as cold-headed or
hot-worked bolts, fasteners, or other formed components.
[0029] FIG. 2 shows one process of the present invention where a
nickel-base corrosion-resistant alloy or heat-resistant superalloy
work piece is annealed or taken in the as-is condition 10 followed
by a hot-worked, cold-worked, formed, or machined stage 12. The
piece is then coated 18 followed by the final curing/stress
equalizing heat-treatment step 20.
[0030] The curing step takes place at specific and pre-determined
conditions of time, pressure and temperature for the specific alloy
being processed, such that the coating is cured concurrently with
the stress equalizing heat-treating of the nickel-alloy
article.
[0031] In one embodiment, the present invention contemplates using
the high-strength, corrosion-resistant stainless steel and
heat-resistant iron-, cobalt-, and nickel-base superalloy materials
compatible with the selected aluminum-containing, organic
corrosion-inhibiting coating formulation requiring a subsequent
aging/curing period. The preferred corrosion-resistant, nickel-base
materials include Monel 400, Monel K-500, and Inconel 600. The
preferred corrosion-resistant stainless steel materials include
302, 303, 304, 305, 410, 416, 430, Custom 450, 17-4PH, and 17-4PH.
The preferred heat-resistant nickel-base superalloy materials
include Inconel 600, 625, 718, X-750, and A286. The subsequent
aging/curing period can be conducted at an elevated temperature to
facilitate curing. In one embodiment, once cured, it is preferred
that the coating be tack-free to enable handling. The coating
thickness achievable by the present invention may vary according to
the preferred end-result characteristics of the coated component,
but preferably the coating thickness ranges from about 0.0003 inch
to about 0.0005 inch.
[0032] One embodiment of the invention relates to the preparation
of fasteners such as rivets and threaded bolts although the
invention is not limited to fasteners, and instead is more broadly
applicable. However, the use of the present invention relative to
fasteners offers particular advantages. The fasteners contemplated
by the present invention include, but are not limited to, screws,
bolts, lockbolts, threaded pins, rivets, etc., which may have
threads, or grooves, as well as female mating components such as
nuts, lock washers, collars, etc.
[0033] Fasteners are understood to mechanically join the various
structural elements and sub-assemblies of aircraft. With regard to
aircraft fasteners, the elimination of the practice of wet-sealant
installation approach for more than one million fasteners in a
large cargo aircraft offers a significant cost savings of several
hundreds of thousands of dollars per aircraft. As contemplated by
the present invention, the elimination of the use of wet-sealants
also improves the overall quality and workmanship in the fastener
installation, such as eliminating the possibility of missing or
overlooking some of the fasteners as the wet-sealant is applied
along with reducing the process variability associated with the
fastener installation. Further, the pre-coated fasteners provide
superior protection from corrosion during service than the
uncoated, wet-installed fasteners.
[0034] Known wet-sealants include, two-part, manganese-cured,
polysulfide sealants containing an additional quantity of soluble
metallic chromates. These are flowable viscous materials, which are
applied by brush, spatula, roller, special applicator, or extrusion
gun. Examples are P/S 1422 or 870C corrosion-inhibiting sealants
produced by PRC-Desoto (a.k.a. PPG Aerospace, a division of
Pittsburgh Paint and Glass). By contrast, the process of the
present invention pre-coats the components with a pre-selected
organic coating and obviates the need for the use of wet-sealant
during the installation and assembly process.
[0035] The application of the organic coating obviates the need for
cadmium or chrome-plating and/or use of wet-sealant during
installation and does not adversely affect the desired final
properties or performance of the component. Indeed, it has now been
determined, through actual testing, that the corrosion-resistant
properties of the coated components are enhanced as compared to the
properties of plated and/or wet-sealant installed
corrosion-resistant, stainless steel alloy and heat-resistant
superalloy materials.
[0036] The preferred bolts are manufactured from any one of a
number of high-strength, corrosion-resistant, stainless steel alloy
and heat-resistant superalloy metallic materials. As used herein,
"corrosion-resistant" means that the metallic material is an
austenitic, martensitic, or ferritic stainless steel, or
nickel-base alloy. On the other hand, "heat-resistant" means that
the metallic material is an iron-, cobalt-, or nickel-base alloy
that can be strengthened by one of the following mechanisms:
solid-solution strengthening, precipitation hardening, and
oxide-dispersion strengthening, processing readily understood by
those skilled in the metallurgy field.
[0037] One embodiment of the present invention is directed to
aircraft components or fasteners made from either
corrosion-resistant alloy or heat-resistant superalloy metallic
materials. Specifically, these components are made of stainless
steels and non-ferrous alloy materials, preferably stainless steels
and nickel-, cobalt-, or iron-based alloys. Aerospace components,
such as fasteners, have been made from unusual materials, e.g.,
tantalum, but this invention is primarily directed to the use of
corrosion- and heat-resistant materials used in commercial and
aerospace fasteners that are readily available as standard
components such as those listed in Tables 1 and 2 below, or their
analogues.
[0038] A majority of all industrial fasteners classified as
`corrosion-resistant` are made of stainless steels. This general
designation covers austenitic, martensitic, and ferritic stainless
steels. Although the aerospace industry uses fasteners made from
all types of stainless steels, the 300 series austenitic types are
most widely used in the fabrication of components or fasteners. The
alloys in this austenitic group have at least 8% nickel in addition
to chromium. They offer a greater degree of corrosion resistance
than the martenistic and ferritic types, but less resistance to
chloride stress-corrosion cracking. Martensitic and ferritic
stainless steels contain at least 12% chromium, but contain little
or no nickel because it stabilizes austenite. Martensitic grades,
such as Types 410 and 416, are magnetic and can be hardened by heat
treatment. Ferritic alloys, such as Type 430, are also magnetic but
generally cannot be hardened by heat treatment, but rather develop
maximum ductility, toughness, and corrosion resistance in the
annealed and quenched condition. Therefore, the only heat treatment
applied to the ferritic alloys is annealing.
[0039] The fastener industry generally markets fasteners made of
Types 302, 303, 304, and 305 stainless steels as "18-8". Ferritic
and martensitic stainless steels for fastener use have included
alloy Types 410, 416, and 430. Corrosion-resistant, nickel-base
alloys characterized by good strength and good resistance to heat
and corrosion and that are used as fastener materials include Monel
400, Monel K-500, and Inconel 600. According to the present
invention, the pre-coating processes of the present invention
enhance the inherent level of corrosion protection for each
substrate listed in Tables 1 and 2 according to the steps outlined
in FIGS. 1a-1c, 2, 3a, and 3b for corrosion- and heat-resistant
alloys.
[0040] Similarly, corrosion-resistant stainless steel alloys
presented in Table 1 conform to the process steps outlined in FIGS.
3a and 3b. FIG. 3a shows a scheme for an austenitic
corrosion-resistant alloy. The alloy is supplied and annealed 10,
then hot-worked, cold-worked, formed or machined 12. A pre-selected
material is provided and applied 18 to the alloy to a specified,
carefully controlled and pre-determined coating thickness by any of
the various contemplated methods. Finally, the coated alloy is
exposed to a curing/stress equalizing heat treatment step 20. In
FIG. 3b for ferritic and martensitic alloys, an austenitizing heat
treatment 30 is conducted after the working or machining step 12
and the coating application 18. TABLE-US-00001 TABLE 1
Corrosion-Resistant Alloy Types Commercial Designation UNS No. ASTM
Specification Stainless steels Austenitic Type 302 S30200 F593
& F594 Type 303 S30300 '' Type 304 S30400 '' Type 305 S30500 ''
Ferritic & Martensitic Type 410 S41000 F593 & F594 Type 416
S41600 '' Type 430 S43000 '' Type 440C S44000 ''
Austenitc-Martensitic (Precipitation-Hardening) 17-4PH S17400 F593
& F594 17-7PH S17700 PH15-7Mo S15700 Custom 450 -- Custom 455
-- Nickel-base alloys Monel 400 N04400 F468 & F467 Monel R-405
N04405 '' Monel K-500 N05500 '' Inconel 600 N06600 ''
[0041] TABLE-US-00002 TABLE 2 Heat-Resistant Superalloy Types
Commercial Designation UNS No. ASTM Specification Nickel-base
alloys Inconel 600 N06600 F468 & F467 Inconel 625 N06625 --
Inconel 718 N07718 -- Inconel X-750 N07750 -- Hastelloy X N06002 --
Rene 41 N07041 -- Waspaloy N07001 -- Cobalt-base alloys MP-35N
R30035 -- MP-159 -- -- Haynes 25 R30605 -- Haynes 188 R30188 --
Iron-base alloys A-286 K66286 -- Haynes 556 -- --
[0042] The previous discussion dealt with those corrosion-resistant
stainless-steel alloys that are most often used for commercial and
aerospace fasteners whose compositions are recognized as standard
by the American National Standards Institute (ANSI) organization.
There are numerous other corrosion-resistant stainless steel
materials contemplated for use with the processes of the present
invention that are also used, either for standard fasteners or for
special components or parts, when dictated by strength
considerations, corrosion protection performance, or temperature
requirements, including, for example, 17-4PH, 17-7PH, Custom 450,
and Custom 455. These materials are used to obtain higher strength
properties than those available from "18-8" stainless steel
materials.
[0043] Additionally, A-286, an iron-base heat-resistant superalloy
that has greater corrosion resistance than the 18-8 types, as well
as good mechanical properties at elevated temperatures, is intended
for use with the processes of the present invention, for
applications requiring resistance to both heat and corrosive media
environments.
[0044] More specifically, the A-286 and Monel 400 materials have
provided the focus of significant cost savings on cargo aircraft
applications. The processes of the present invention are also
contemplated for use for components in areas such as cargo floors,
door areas, engine nacelles, fuel tanks, or areas containing
hydraulic fluids. Other particular alloys have also been utilized
in the pre-coating process of the present invention resulting in
successful improvements in their corrosion resistance.
[0045] The high-strength, corrosion-resistant stainless steel alloy
and heat-resistant superalloy metallic components of the present
invention achieve their full, required strength and other
metallurgical properties produced by the stress equalizing or aging
heat-treatment as well as curing of the coating. Predictably,
achieving a specified strength level of the substrate is important,
because users of the components, such as the customers of aircraft,
can not permit or allow a sacrifice of mechanical performance in
order to achieve improved corrosion protection. High-strength
fasteners are understood to be those fasteners which possess an
ultimate tensile strength value of greater than 150,000 psi.
[0046] The present invention preferably is used with a rivet, bolt,
fastener, or other article or component manufactured to any
conventional shape and size. FIGS. 4-6 illustrate two preferred
embodiments with two types of bolts 40, 140, at an intermediate
state of their installation to join a first piece 42, 142 to a
second piece 44, 144, after installation to the first and second
pieces but before use of the female component, collar, or nut 152.
The bolt 40 of FIG. 4 has a manufactured protruding head 46 on one
end and a threaded portion 50, at the opposite end. The bolt 140 of
FIG. 5 has a manufactured flush head 146 on one end that resides in
a countersink 141 in the piece 142. The present invention may be
used with these and other types of fasteners. FIG. 6 shows a female
component, collar, or nut 152 engaged with the grooved or threaded
portion 150 of bolt 140.
[0047] Typically, the high-strength heat-resistant superalloy
materials have at least about 50 percent by weight of either iron,
cobalt, or nickel 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
superalloy materials as desired. Alloying elements that are added
to either the iron-, cobalt-, or nickel-base metal to modify its
properties include, for example, carbon, manganese, silicon,
nickel, chromium, and molybdenum.
[0048] The present invention contemplates the use of heat-treatable
nickel-base superalloy materials. Preferably, the article is first
fabricated to a desired shape through such steps as hot-working,
cold-working, forming, or machining, either separately or in some
combination thereof, for example, a fastener such as a bolt. These
nickel-base alloys that have been work-hardened or formed by the
hot- or cold-working operations, such as rolling, drawing, spinning
or severe bending, require softening before further processing can
be continued. One of the thermal treatments that will produce this
condition is known as stress equalizing heat treatment. In order to
impart the required strength to the fastener or article, the
article must then be heat-treated. In the stress equalizing heat
treatment process, the article is heated to an elevated
temperature, which is used to balance stresses in the cold-worked
or hot-worked material without an appreciable decrease in the
mechanical strength produced from the forming processes.
[0049] Nickel and nickel-base alloys may be subjected to one or
more of five principal types of thermal treatment, depending upon
chemical composition, fabrication requirements and intended
service. One of these thermal treatment methods is "stress
equalizing". Stress equalizing is a relatively low temperature heat
treatment process that effects what is known as "partial recovery."
This recovery, which precedes any detectable microscopic structural
changes, consists of a considerable increase in the proportional
limit, slight increases in hardness and tensile strength, no
significant change in elongation or reduction of area, balancing of
stresses, and return of electrical conductivity toward its
characteristic value for the alloy in the annealed condition. The
temperature required for stress equalizing depends upon the
composition of the alloy. For cold drawn, nickel-base Monel 400
alloy rod material, the optimum temperature range is about
450.degree. F. to about 600.degree. F. A normal temperature of
about 525.degree. F. is recommended. Even though a long treatment
period of up to 3 hours is recommended at this temperature, it has
no detrimental effect on the base metal. According to the present
invention, the treatment period is recommended between 1 hour to
1.5 hours in order to achieve substantially concurrent curing and
cross-linking of the coating.
[0050] Collectively, all of the thermal processing steps leading to
the strengthening of the material or article are generally termed
"heat-treating" or "thermal-treating", wherein the article 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 article'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 corrosion-resistant stainless
steel and superalloy materials.
[0051] A preferred, specific high-strength, corrosion-resistant
nickel-base alloy material for fastener applications is the Monel
400 alloy (UNS N04400), which has a nominal composition of 66.0
percent nickel, 0.12 percent carbon, 0.90 percent manganese and
0.15 percent silicone, 31.5 percent copper, and 1.35 percent iron
plus minor impurities. Other contemplated heat-resistant
nickel-base superalloys include, but are not limited to, Inconel
600, 625, 718, and X-750, Hastelloy X, Rene 41, Waspaloy, and Monel
400 and 500 series heat-treatable, nickel-base alloys.
[0052] The Monel 400 alloy is available commercially. After
fabricating the alloy to the desired shape such as a fastener like
those shown in FIGS. 4-6, the Monel 400 alloy is ready for coating
and stress equalizing heat treatment. This state is usually
obtained following fabrication including any of the processes of
machining, forging, drawing, cold-working, or otherwise forming the
fastener into the desired shape. During the fabrication, the
article may be subjected to multiple forming operations and
periodically re-annealed as needed, prior to the coating and stress
equalizing heat-treatment process steps.
[0053] According to one embodiment of the present invention, a
coating material is provided, 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 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. 4-6.
[0054] The use of such a curable coating is distinct from known
non-curable coatings, such as a lacquer, which has different
properties and is not suitable for the present,
corrosion-protection application. Indeed, many non-curable lacquers
will degrade upon exposure to elevated temperatures. Thus, the
over-aging problems associated with the use of non-curable and even
many curable coating materials, and which necessitate the present
invention, simply do not arise. It is further understood that
optional cleaning steps may be required to prepare the base metal
for coating. 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.
[0055] The present process contemplates the use of a number of
curable organic coating materials. Preferred coating materials have
a polyaromatic-based resin, such as, for example, phenolics,
polyimides, polybenozazoles, or polytetrafluoroethylenes, and may
be mixed with one or more plasticizers, and inorganic additives
such as, for example, aluminum powder and/or strontium chromate.
These coating materials are preferably dissolved in a suitable
solvent present in an amount to produce a desired consistency based
upon the desired end use.
[0056] For the phenolic-based coating material just discussed, the
solvent preferably 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 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 polyaromatic phenolic
product is available commercially as "Hi-Kote.RTM.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.
[0057] As shown by the various process methods outlined in FIGS.
1a-1c, the coating material is applied to the untreated fastener in
coating step 18. Any suitable coating approach, such as dipping,
spraying, brushing, or a fluidized bed method can be used. In one
preferred approach, the solution of coating material dissolved in
solvent is sprayed onto the untreated bolts. The solvent is removed
from the as-applied coating by drying or "flash cure", either at
room temperature or slightly elevated temperature, so that the
coated article is dry to the touch. Preferably, a flash cure
(exposure at approximately 200.degree. F. for about two minutes)
accomplishes evaporation of the solvent. The coated component is
still not suitable for service at this point, because the coating
is not sufficiently cured and adhered to the corrosion-resistant
alloy or heat-resistant superalloy component and because the
coating itself is not sufficiently coherent to resist corrosion or
mechanical damage in service.
[0058] 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.
[0059] As shown in FIG. 2, once coated, the base, nickel-alloy
metal of the fastener and the applied coating are together heated
to a suitable elevated temperature in a cure/stress equalizing step
20, to achieve the two results substantially concurrently. In this
single step, the nickel-alloy material substrate is stress
equalized heat-treated to its final, desired strength state, and
the coating is aged to its desired final cured or bonded state.
[0060] According to the present invention, preferably, the
temperature and time associated with the treatment of step 20 is
selected to be that required to achieve the desired properties of
the nickel-base alloy or stainless steel metal, as provided in the
industry-accepted and proven process standards for that particular
material. Surprisingly, stress-equalized heat-treatment is
typically not that specified by the coating manufacturer and may
not produce the most optimal cure state for the coating. However,
contrary to expectations and manufacturer's specifications, the
coating cured by the non-recommended procedures exhibits desirable
adhesion to the alloy substrate. That is, according to the present
invention, the curing of known coatings can sustain larger
variations in time and temperature with acceptable results than can
the heat-treatment tempering or hardening process of the alloy
material.
[0061] In the case of the preferred Monel 400 nickel-base alloy and
Hi-Kote.RTM. 1 coating discussed above, the preferred
heat-treatment is the stress equalizing tempering treatment process
of the Monel 400 nickel-base alloy, namely about 11/2 hours at
about 525.degree. F. Thus, the cure/stress equalizing step 20
involves a significantly different temperature than is recommended
by the manufacturer for the organic coating.
[0062] The final coating 48, shown schematically in FIGS. 4-6, is
strongly adherent to ferrous and nickel-base alloy metal substrates
and is also strongly coherent and cross-linked. In FIGS. 4-6, the
thickness of the coatings 48 and 148 is exaggerated so that it is
visible. In reality, the coating 48 (FIG. 4) is preferably from
about 0.0003 inch to about 0.0005 inch thick after treating in step
26.
[0063] 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 either chrome
plate and/or 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. The wet-sealant material is
potentially toxic to workers, messy, requires constant
refrigeration prior to use and is difficult to work with when
applying, and necessitates the use of extensive clean-up tools as
well as exposing surfaces of the pieces 42 and 44 to caustic
chemical solutions after use in the 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 bolt heads.
[0064] The coating process of the present invention overcomes these
and other problems confronted by the use of chrome plating and/or
wet-sealants. According to the process of the present invention,
wet-sealant is not needed or used during installation.
Additionally, the later-applied paint or other topcoats adhere well
over the pre-coated bolt heads. The following example serves only
to further illustrate aspects of the present invention and should
not be construed as limiting the invention.
EXAMPLE 1
Monel 400 Corrosion-resistant Alloy
[0065] The present invention also has been reduced to practice with
one-piece rivets, and other fastener types, made of Monel 400
corrosion-resistant nickel-base alloy metal. Specifically,
1/8-inch, 3/16-inch and 1/4-inch diameter Monel 400 rivets,
initially in the untreated state, were anodized and spray-coated
with a corrosion-inhibiting, phenolic-based coating, e.g.,
Hi-Kote.RTM. 1. The coated rivets were then thermally treated to
achieve an ultimate shear strength of between about 49,000 psi and
about 59,000 psi with the stress equalizing thermal treatment of
approximately 1.5 hours at about 525.degree. F. followed by an
ambient, forced-air cooling period.
[0066] The coated rivets were mechanically tested in accordance
with NASM5674 and NASM1312-20 and ASTM B 565 for driveability and
shear testing, respectively, to verify that they met the required
upset and ultimate shear strength requirements of the
specifications. The ultimate shear strength range for standard,
uncoated Monel 400 rivets is 49,000 psi to 59,000 psi. From the
test results, the ultimate shear strength of the coated rivets
ranged from 51,500 psi to 58,000 psi, depending upon the rivet
diameter, well within the required allowable limits.
[0067] Rivets were also installed and subsequently removed to
evaluate driveability and coating integrity characteristics using
both macroscopic streoscope and scanning electron microscope (SEM)
techniques. The driveability characteristics were acceptable
without any indications of cracking or other material flaws. The
coating itself exhibited no signs of cracking, spalling, or any
other unacceptable conditions or abnormalities. The coatings were
uniformly adherent and retained on the surface of the rivets even
after the assemble process. Thus, the coating remained in place and
tightly affixed to the rivets' surfaces to protect the components
against corrosion after installation, obviating any need for the
use of wet sealants.
EXAMPLE 2
A-286 Heat-resistant Superalloy
[0068] A comparative 2000-hour salt spray exposure test was
performed on A-286 heat-resistant superalloy Hi-Set.RTM. one-piece
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-pigmented
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, and various other aerospace and industrial solvents when
applied on a wide variety of metallic substrates.
[0069] The salt spray evaluation testing was performed in
accordance with ASTM B 117 apparatus and associated standard test
method procedures. Aluminum-alloy test specimen 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.
[0070] 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 heat-resistant superalloy
material for the control specimens. Additional derivative test
specimen samples were processed with modifications to the A-286
material preparation prior to the application of the 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, specimen no. HS-2,
containing a different type of production Hi-Kot.RTM. 1l-coated
titanium-alloy material fastener installations was selected for a
known comparative baseline for corrosion protection results and
characteristics.
[0071] Six (6) A-286 Hi-Set.RTM. fasteners were installed in test
specimen assemblies having total thickness of 0.500 inch and
comprised of two 0.250-inch thick components. One component was
made from 2024-T351 Alclad aluminum-alloy and the other was made
from 7075-T651 bare aluminum-alloy materials. The fasteners were
installed in the test specimen assemblies using a Drivmatic.RTM.
automated assembly machine that drilled, countersunk, and installed
the Hi-Set.RTM. fasteners in one operation. The test specimen
assembly components were chromic-acid anodized and sealed in
accordance with the requirements and procedures of DPS-11.01-3
prior to drilling and countersinking the holes. The hole size that
was drilled was 0.185 inch. The flush head fasteners were installed
flush with the surface of the test specimen components within
.+-.0.010 inch after installation. All fastener installations were
spaced such that the center of the installed fastener heads were at
least 1.000 inch away from the next adjacent fastener (i.e.,
center-to-center) with a 0.500-inch edge margin. For comparison,
some of the fasteners were wet-sealant installed in accordance with
DPS 4.50-36-17.
[0072] After installation of the fasteners, the test specimen
assemblies were solvent cleaned with methyl ethyl ketone (MEK) to
remove all grease, oils, wet-sealent residue, and fingerprints. The
test specimen assemblies were placed at a 15.degree. angle to the
horizontal on a non-reactive plastic rack inside the salt spray
chamber per ASTM B 117. The test chamber's environment as salt
solution were monitored daily throughout the test for acceptable
sodium chloride limits. The atmosphere was maintained
satisfactorily while the pH of the salt solution was within the
specification limits of 6.5 to 7.2 with a constant temperature of
+95.degree. F., +2.degree. F./-3.degree. F., per ASTM B 117
guidelines.
[0073] Salt spray (fog) testing was performed at the Hi-Shear
Corporation's testing laboratory. The test specimen assemblies with
fastener installations were exposed for 2000 hours to the salt
spray environment in accordance with ASTM B 117. Observations were
made at 24, 168, 300, 500, 700, 1000, 1500, and 2000 hours. All
samples were visually examined and photographed throughout the
2000-hour salt spray exposure. The fastener heads, fastener upsets,
and countersinks periphery surface areas on the aluminum-alloy test
specimen components showed noticeable corrosion by-products except
for the test specimen assembly containing the A286 HSR217AP6-9
fasteners (test specimen assembly number HS-5). These fasteners
were prepared using a cadmium flash plate pre-treatment prior to
being coated with the corrosion-inhibiting coating, Hi-Kote.RTM. 1.
The current method of wet-sealant installing the Hi-Kote.RTM. 1
fasteners (test specimen assembly number HS-13) with DPS 2.50-17
Type 18 wet sealant applied prior to their installation showed more
corrosion by-products around the fastener head, countersink, and
upset than the HS-2 or HS-5 test specimen assemblies.
[0074] After the 2000-hour exposure, the leading candidates (test
specimen assembly nos. HS-2, HS-5, and HS-13) selected due to
minimal levels of corrosion activity were metallurgically sectioned
and examined. All fasteners were removed from each of their
respective test specimen assemblies in such a manner as to prevent
deformation or other damage to the fasteners or the surfaces of the
surrounding, adjacent holes. Loose corrosion and salt by-products
were removed by de-ionized water and allowed to fully dry.
Countersink areas and areas on the surfaces of the test specimen
assembly components around the fastener heads were compared for
appearance and condition. One fastener from each test specimen
assembly was examined for corrosion or exfoliation attack by
sectioning longitudinally while still installed in the
aluminum-alloy test specimen components. Each individually mounted,
metallurgical specimen contained one fastener from the previously
mentioned test specimen assemblies along with both mating 2024 and
7075 aluminum-alloy components of the adjacent test specimen
assembly. The specimens ere polished and then examined
microscopically at 15X and 400X magnification in the un-etched
condition. A Keller's reagent for the aluminum-alloy test specimen
components was used to reveal the grain structure. This type of
etchant does not aggressively attack the A286 or titanium alloy
fastener materials. Alternate etchants could be used for the
fastener material but may obscure the aluminum-alloy component's
grain structure for proper evaluation.
[0075] Upon sectioning the 2024/7075 aluminum-alloy test specimen
components with the fasteners still installed, the following was
revealed. The wet-sealant installed HSR217AP6-9 Hi-Kote.RTM. 1
coated fastener installations in test specimen assembly number
HS-13 revealed localized corrosion damage to a depth of 0.0412 inch
along the grain boundary of the 7075 aluminum-alloy test specimen
component starting adjacent to the fastener shank. The 2024-T351
Alclad aluminum-alloy component showed no signs of localized,
intergranular corrosion activity. The test specimen assembly, no.
HS-5, having fasteners with cadmium flash, Hi-Kote.RTM. 1 coated
fasteners installed, revealed some localized corrosion activity in
the fastener hole along the grain boundary in the same area as the
HS-13 test specimen assembly, but with a reduced depth of only
0.0086 inch. The 2024-T351 Alclad aluminum-alloy component did not,
as with the HS-13 test specimen baseline assembly, show any signs
of localized intergranular corrosion activity. The HS-2 test
specimen baseline assembly, which contained dry-installed coated
titanium lockbolt fasteners, S4932868C06-08, as a control, did not
display localized or intergranular corrosion in the 7075
aluminum-alloy component, but did display slight indications of
corrosion pitting at the edge of the hole's countersink in the
2024-T351 Alclad aluminum-alloy test specimen component.
[0076] The test results indicated conclusively that the pre-coated
A-286 heat-resistant superalloy HSR217 Hi-Set.RTM. fasteners, which
employed the pre-treatment cadmium flash plate finish process 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 wet-installed Hi-Set.RTM. fasteners pre-coated
with Hi-Kote.RTM. 1 coating on standard prepared A-286 base
material as well as the baseline production titanium-alloy
fasteners pre-coated with Hi-Kote.RTM.1.
[0077] In summary, 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 DPS 2.50-17, Type 18.
[0078] 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.
[0079] 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.
* * * * *