Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SURFACE PREPARATION FOR ALUMINUM PAR~S
Background of the Invention
This invention relates generally to the art of struc-
tural adhesive bonding of metals and more particularly to
preparation of the surfaces of aluminum, or aluminum alloy,
parts for structural adhesive bonding of such parts
As is known, proper preparation of the surfaces of the
aluminum parts to be adhesively bonded is necessary if a du-
rable bond between them is to be achieved. One surface
preparation technique is Phosphoric Acid Anodizing (PAA), a
process described in U.S. Patent No. 4,085,012, issued to
Marceau et al in which the aluminum parts are immersed in a
phosphoric acid solution while an electric current i5 passed
through the parts. This anodizing produces a film of aluminum
oxide on the surface of the parts which facilitates adhesive
bonding thereto. While the PAA process has yielded satisfac-
tory results, the produced aluminum oxide film is quite
fragile, making the PAA treated surface very sensitive to
handling.
Another method of preparing the surface of aluminum parts
for structural adhesive bonding is chromate conversion coating,
wherein a layer of chromate material is applied to the sur-
faces of the aluminum parts to be bonded. The applied chromate
coating converts each such surface to a superficial layer
containing a complex mixture of chromium compounds to provide
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he subsequently bonded joint with increased corrosion pro-
tection. An article entitled "Effect of Surface Preparation
on Stressed Aluminum Joints in Corrosive Saltwater Exposure",
appearing in ~ in October, 1980, noted that when
a coating of chromate is applied to the surfaces of aluminum
parts followed by a coating of high temperature curing adhe-
sive, and the adhesive cured by heating the thus coated
parts for a predetermined time, a durable adhesive bond
between them is obtained. However, high temperature curing
adhesives are generally inconvenient in a manufacturing
environmentc The article's author postulated that the high
bond durability was possibly due to a thermally induced struc-
tural change in the chromate. The author further theorized
that the use of a room temperature curing adhesive in place
lS of the high temperature curing adhesive would lead to different
results; however, he did not pursue this theory further.
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Summary of the Invention
In accordance with the present invention, a process is
described for preparing a surface of a metal part comprising alumi-
num for subsequent applicat.ion of a structural bonding adhesive,
comprising the steps of:
applying a coating of chromate to the surface of the part;
heating the chromate-coated part; and
subsequently cooling the chromate-coated part to substan-
tially room temperature for application of the bonding adhesive.
By the use of such process, an extremely durable structural bond to
t~e~. part may be achieved using a room temperature curing adhesive.
According to one aspect of the present inventio.n there is
provided a process for preparing a surface of a metal part compris-
ing aluminum for subsequent application of a structural bonding
adhesive, comprising the steps of:
applying a coating of chromate to the surface of the part;
heating the chromate-coated part at between substantially
250F and substantially 350F for approximately 2 hours; and
subsequently cooling the chromate-coated part to substan-
tially room temperature.
According to a further aspect of the present invention
there is provided a process for preparing a surface of a first part
comprising aluminum for adhesive ~onding to a second part comprising
aluminum, said process comprising the steps of:
applying a coating of chromate to the surface of the first
part;
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heating the chromate-coated first part at between substan-
tially 250F and subs-tantially 350F for approximately 2 hours;
subsequently cooling the chromate-coated first part; and
applying a layer of adhesive to the cooled, chromate-
coated first part.
According to another aspect of the present invention there
is provided a process for preparing a surface of a part for subse-
quent application of a structural bonding adhesive, said part being
a metal selected from the group consisting of aluminum and alloys
0 of aluminum, said process comprising the steps of:
applying a coating of chromate to the surface of the part;
applying a layer of primer over the chromate coating;
heating the chromate coated and primed part at between
substantially 250F and substantially 350F for approximately 2
hours; and
subsequently cooling the part to substantially room tem-
perature.
According to a s-till further aspect of the present inven-
tion there is provided a process for bonding a first part comprising
aluminum with a second part comprising aluminum, comprising the
steps of:
applying a coating of chromate having a predetermined
thickness to a surface of each of the parts;
heating the chromate-coated parts at between substantially
250F and substantially 350F for approximately 2 hours and subse-
quently cooling said parts to substantially room temperature;
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applying an adhesive to the chromate-coated surface of at
least one of the cooled parts; and
bonding the par-ts together.
According to another aspect of the present invention there
is provided a process for bonding a first metal part comprising
aluminum with a second metal part comprising aluminum, comprising
the steps of:
applying a coating of chromate comprising a nominal per-
centage of hexavalent chromium to a surface of each of said metal
parts;
heating the coated metal parts at a predetermined tempera-
ture for a predetermined time selected to substantially increase
the percentage o~ hexavalent chromium in the coating from the nomi-
nal percentage thereof;
applying an adhesive to said surface of at least one of
the metal parts; and
bondin~ said parts together.
According to a further aspect of the present invention
there is provided a process for bonding a first part comprising
aluminum with a second part comprising aluminum, such process com-
prising the steps of:
applying a coating of chromate comprising a nominal amount
of hexavalent chromium to a surface of each of the first and second
parts;
heating the chromate-coated first and second parts at a
predetermined temperature for a predetermined time, such predeter-
mined temperature and time being selected to substantially increase
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the amount oE hexavalent chromium in the coating from the nominal
amount thereof;
subsequently cooling the first and second parts to sub-
stantially room temperature;
applying a bonding adhesive to the chromate-coated surface
of one of the cooled Eirst and second parts; and
bonding the first and second parts together and curring
the adhesive.
In a preferred embodiment of -the present invention, the
process for preparing a sur:Eace of a part comprising aluminum for
subsequent application of a structural bonding adhesive first com-
prises the step of applying a coating of chromate to the surface of
the part. A layer of primer is then applied over the chromate
layer. The chromate-coated and primed aluminum part is heated to
thermally "age" the chromate and cure the primer. The part is sub-
sequently cooled to substantially room temperatuxe. The thusly
treated surface of the part may then have a layer of a room tempera-
ture curing adhesive applied thereto for structural bonding to a
similarly treated surEace of another part. In the interim, and un-
like Phosphoric Acid Anodizing (P~A) treated aluminum, the surfaceof the parts treated by the process of the present invention
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is quite insensitive to handling.
Additionally, the present invention provides a process
for bonding a first part comprising aluminum with a second
part comprising aluminum~ The flrst step in such process is
applying a coating of chromate having a predetermined thick-
ness to a surface of each of the parts. The chromate-coated
parts are then heated at a predetermined temperature for a
predetermined time and are subsequently cooled to substan-
tially room temperature. An adhesive is applied to the chro-
mate-coated surface of at least one of the cooled parts, and
the parts bonded together.
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Brief Description of the Drawings
The foregoing features of the present invention and the
advantages thereof may be more fully understood from the
following detailed description when read in conjunction with
the accompanying drawings wherein:
FIG. 1 is a flow diagram of a first embodiment of the
aluminum sur~ace preparation process of the present
invention; and
FIG. 2 is a flow diagram of a second embodiment of the
aluminum surface preparation process of the present
invention.
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Description of the Preferred Embodiments
Referring now to FIG. 1, the aluminum surface preparation
process of the present invention is illustrated. The first
step in this process consists of applying a coating of chromate
having a predetermined thickness to the sur-Eace of the alu-
minum, or aluminum alloy, parts which are to be adhesively
bonded together. As hereinafter used, the term aluminum
encompasses the metal aluminum and alloys thereof. One
method of applying the chromate coating is to immerse the
parts in a c~romate solution, although the chromate film may
also be applied by spraying, brushing or swabbing the parts.
As i5 known, chromate is a complex mi~ture of chromium with
additional compounds, and is cornmercially available under
several trade ~ , such as Iridite, manufactured by the
Allied-Kelite Division of Witco Chemical Corporation of Des
Plaines, Iowa, and Alodine, manufactured by Amchem Products
Inc. of Ambler, Pennsylvania.
Proper chromate thickness is necessary to pro~ide an
optirnum bond. Sufficient chromate is needed to ~reYent
mo~sture from affecting the bonded joint and thus corroding
the bond and reducing durability. However, if the chromate
coating is too thick, the bonded joint may fail in the chromate
coating itself. The concentration of the chromate solution,
its degree of acidity (pH) and temperature, the irnmersion
time, and the alloy composition of the part being chromated
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all influence the thickness of the chromate coating. An
acceptable concentration range for the chromate solution,
using Iridite inaterial as the chromate, i5 between 1.0 and
1.5 ounces of Iridite material per gallon of solvent (here,
water)O This yields a chromate solution with a pH of between
1.3 and 1.6. The temperature of the solution should be
between 60F and 100F. Proper immersion time is between 25
and ~5 seconds~
Proper chromate thickness is on the order of a few
millionths of an inch; therefore, it is impractical to measure
absolutely the thickness of the chromate coating. However,
it is possible to determine whether the chromate coating
thickness of a given chromated part is within an acceptable
range, that is, neither too thin nor too thick. This can be
done by comparing the color of such chromated part with
color standards. The color of a chromate-coated part ranges
from clear, for a very thin chromate coating, to brown for a
much thicker coating. Thus, color standards representing a
color range corresponding to a thickness range can be obtained.
The first color standard, representing the thinnest acceptable
chromate coating, is obtained by immersing sample sections
(called coupons) of aluminum into the weakest chromate solu-
tion to be used (here 1.0 oz. of Iridite chromate/gal. of water)
for the minimum practlced time (here 25 sec.). The second
color standard, representing the thickest acceptable chromate
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coating, is produced by dipping aluminum coupons into the
strongest chromate solution to be used (here 1.5 oz. of Iridite
chromate/gal.) for the maximum practiced time (here 45 sec.).
Thus, a standard color range is found, to which the color of
the parts chromated for bonding are compared to determine if
the chromate coating of such parts is of the proper thickness.
By thermally "aging" the chromate a strong and durable
adhesive bond may be made to the chromated aluminum parts.
Thermally "aging" the chromate is the second step of the
surface preparation process of the present invention and
is performed by heating the chromate-coated aluminum parts
at a predetermined temperature, for example, 3S0F, for a
predetermined time, for example, two hours. The chromated
aluminum parts are then cooled to substantially room tempera-
ture. The "aged" chromated aluminum parts were examined usiny
Electron Spectroscopy for Chemical Analysis (ESCA) (also
called photoelectron spectroscopy), a generally known surface
analysis technique in which x-rays having known energy are
directed at the surface of the "aged" chromated parts and
the energy of electrons emitted therefrom in response to the
x-rays analyzed to identify the element ~rom which the elec-
trons came and the molecular configuration of such element.
The use of ESCA on the "aged" chromàted parts reveals that
the thermal "aging" converts the normally trivalent chro~ium
atom (Cr~3) of the chromate to hexavalent chromium (Cr~6).
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As is known, hexavalent chromium resists corrosion better
than does trivalent chromium; thus, a more durable bond can
be obtained by bondlng to the "aged" chromate surfaces of the
parts. As the table below shows, the higher the aging tem-
perature, the more complete the conversion. However, care
should be taken not to expose the aluminum parts to excessive
temperatures, as this may mechanically weaken the metal.
The table also shows that the conversion process will take
place in an inert atmosphere of nitrogen, as well as in
air. This indicates that the conversion reaction is internal
to the chromium and is not part of an oxidation process.
Chromium Oxidation State Percentages
Chromium 2P
~ Hexavalent Triv
Non-thermally aged 36.6~ 63.4~
2 hrs. at 250F in air 41.8~ 54.2%
2 hrs. at 300F in air 48,5~ 45.3~
2 hrs. at 350F in air 55.9% 30.2%
2 hrs. at 250F in N2 44 7% 52.6%
2 hrs. at 300F in N2 47O3% 41.8%
2 hrs. at 350F in N2 52.4~ 31.6~
The next step in the aluminum surface preparation process
is to apply a layer of primer to the chromated aluminum parts.
As is known, primer is an epoxy-based material containing
hexavalent chromium and assists the chromate layer in resisting
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corrosion of the subsequently bonded joint. ~ere, the primer
i5 EA-9228, manufactured by the Hysol Division of the Dexter
Corp. of Pittsburg, California, although other primers may
be used. The thickness of the primer layer must be controlled
for the same reason as for chromate thickness: sufficient
primer must be applied to provide, along with the chromate
coating, corrosion protection to prevent moisture from
a~fecting the bonded joint; however, the primer layer must
not be ~o thick that it becomes weaker than the bonded joint
itsel. An acceptable range of primer thickness is between
0.2 mil to Or4 mil (i.e., 0.0002 to 0.0004 inches). The
primer can be applied either by spraying or by dipping the
chromated aluminum parts in a primer solution and conventional
techniques may be used to measure primer thickness.
The primer must be cured. Conventionally, this is done
by heating the primed aluminum parts at about 250F for about
one hour, although other temperatures and times may be used.
While this extra heating cycle will not adversely affect the
"aged" chromate (in fact, it may convert more trivalent
chromium to hexavalent chromium), the cycle is redundant
given the prior step of thermally aging the chromate. There-
fore, the preferred method, depicted in the flow diagram of
FIG. 2, is to apply the primer layer to the chromated alu-
minum parts prior to thermally aging the chromate. Thus,
heating the chromated and primed aluminum parts for two
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hours at 350F, for example, both ages the chromate (converts
trivalent chromium to hexavalent chromium) and cure~ the
primer. The chromated and primed aluminum parts are cooled
to room temperature and are ready to be bonded to~ether. In
fact, this process yields even higher adhesive bond strengths
than does ~he process of chromating, aging, priming and
primer curing.
Whichever of the two methods is used, once the primer is
cured the parts can be stored for long periods of time (up
to at least one year) and handled without special sanitary
precautions without contaminating the treated surfaces. The
surfaces need only be cleaned with, for example, an ordinary
solvent prior to bonding.
The surfaces of the aluminum parts having been prepared
lS by the process of the present invention, a room temperature
curing adhesive is applied to at least one of the prepared
surfaces of the par~s, and the surfaces of the parts placed
together and held in place by conventional means, such as
clamps, while the adhesive cures. Here, the adhesive is
EA-93Z0NA, manufactured by the Hysol Division of the Dexter
Corporation of Pittsburg, California, although other adhesives
may be used. Typically, a room temperature curing adhesive
obtains its maximum adhesive quality after about seven days
at room temperature. Once the adhesive has cured, the clamps
are removed and the parts durably held together by the adhesive
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bond.
The parts need not be bonded with room -temperature curing
adhesive, however. Higher temperature curing (i.e. oven-cured)
adhesives may be used as well, with equivalent or better bond
strength and durabi.lity being achieved, although high temperature
curing adhesives are generally inconvenient in a manufacturing
environment.
It is noted that the processes of the present invention
are non-electrolytic processes, that is, such processes do not
require an anodizing step, such as phosphoric acid anodizing (PAA)
discussed in the above-identified Marceau Patent No. 4,085,012.
Having described preferred embodiments of the present
invention, minor modifications may beccme apparent to those skilled
in the art without departing from the spi.rit of the invention. I-t
is therefore lmderstood that the scope of the present inven-tion
is to be limited only by the scope of the appended claims.
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