Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
METHOD OF PROTECTING AN ~LUMINUM SURFACE
This invention relates to a method of sealing and
protecting the surface of an aluminum base alloy from the
environment while at the same time creating a camouflage
05 color on the surface of the alloy. The aluminum base
alloy is derived from the following formula: Al, Fe, Si,
and at least one element or compound selected from a group
consisting of Mn; V; Cr; W; Nb; Ta; and SiC.
High strength and ductile rapid solidified
aluminum based alloys are replacing many specialty metals
in components for the aerospace projects. One such
aluminum based alloy is disclosed in U.S. Patent 4,729,790
which maintains both strength and ductility at both
ambient and at temperatures up to 400C in order to
protect the surface of such an alloy from corrosion, it is
normal to coat the surface with some type of protection.
The protection is achieved through an anodic coating such -~;
as disclosed in U.S. Patents 3,625,841; 3,761,362; and
4,571,287. The anodic oxide film or coating is developed
on the aluminum article by electrolysis while the article
is the anode in an aqueous solution containing one or more
acids and a metal salt as the electrolyte. The resulting
color of the coated article as disclosed in U.S. Patent
4,571,287 is essentially determined by time and the metal
salt used as an electrolyte. For instance, many outdoor
signs such as used as highway information conduits are
colored green. One such way that outdoor signs were
colored green was to apply an epoxy paint to such signs, ~-
this method was acceptable however the environment caused
30 the paint to fade and as a result the information -
contained thereon was hard to read after a period of time
without refurbishment or the paint was scraped off and the
surface directly exposed to the environment. Later, as
disclosed in U.S. Patent 4,043,880 the green color is made
35 part of the protection process for the aluminum through ~ -
the addition of copper ions during an anodization
process. This process of providing a dark green color is
an acceptable method of coloring many aluminum articles
2 ~ g
used in industry. Unfortunately, when copper ions are
used to color an aluminum article, the aluminum article
cannot be used in a fuel control of an aircraft without
causing problems. Thus, the use of this coloration
05 process and protection afforded thereby is limited to
components that are not in contact with fuel that is used
in an aircraft.
In an effort to develop a corrosion protection
system for the aluminum based alloy disclosed in U.S.
Patent 4,729,790 it was discovered that an olive drab
color was imparted to the alloy when the alloy was
anodized in a sulfuric acid bath, followed by
neutralization in a cold water bath of sodium bicarbonate
and sealed with a solution of sodium dichromate. The
15 olive drab color of the article was unexpected in the ~
absence of copper ions in any of the baths used in --~-
developing the protection system. On inspection of the
article was discovered that the olive drab color extended
throuqhout the entire anodized surface layer formed on the
aluminum based alloy.
An advantage of this method of this corrosion
protection for an aluminum based alloy resides in the
olive drab color imparted to the aluminum based alloy
during a direct current electrolysis and sealing process
wiihout the need of copper ions in a sulfuric acid bath.
It is an object of this invention to provide
corrosion protection for a rapid solidified aluminum alloy
to allow components made therefrom to operate in hostile
environments without oxidatizing.
It is another object of this invention to provide
an aluminum alloy with an anodization process wherein an
olive drab color imparted to the alloy is not effected by
corrosive environments under operating conditions wherein
temperatures reach 400C. -~
These advantages and objects should be apparent
from reading this specification.
Recognizing the strength and stability of the
aluminum based alloy disclosed in U.S. Patent 4,729,790
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when exposed to temperatures up to 225C, the mechanical
properties and fracture resistance thereof lends
application to many aerospace components. The strength
and stability of this alloy is due to the slower
05 coarsening rate of the silicides compared to the
dispersoids found in other high temperature aluminum
alloys. This alloy is devoid of any coarse needle or
plate-like intermetallic phases which degrade alloy
ductility and fracture toughness. Further, this ~ -
particular alloy contains high transition concentrations
of iron that form high volume fractions of silicides to -
maintain the mechanical properties during high temperature
exposure. For many aerospace applications corrosion
protection of components is required to meet operational
standards.
Because of the simplicity and safety involved it
was decided that the corrosion protection for this alloy
could best be achieved through anodization in an
electrolyte by direct current electrolysis. An article
made from this aluminum alloy is located in an aqueous
solution containing a low concentration sulfuric acid and
connected as the cathode to a voltage source. The
resulting anodic film formed on the surface of the ~
aluminum alloy while initially colorless when placed in a ;
bath containing sodium dichromate for about twenty minutes
changes to olive drab.
In order to evaluate the proposed process of
providing anodized protection for the aluminum base alloy,
samples of an aluminum based alloy as disclosed in U.S.
Patent 4,729,790 were obtained. The weight percentages of
the ingredients for the samples were:
Fe = 8.53;
V = 1.2;
Si = 1.8; and
Al = 88.47
Total = 100.00
2~?~r;3
Samples of this aluminum based alloy were
evaluated, without a coating, having a hard anodize
coating, according to the general accepted process of
sulfuric acid and oxalic acid anodize treatments and an
05 anodize coating according to the principals of this
invention using sulfuric acid treatments to establish
corrosion protection. The resulting treatments produced a
coating on the surface of the alloy samples approximately
0.0001" or 0.000254 cm. The samples with the hard anodize
coating and sulfuric acid coating of this invention which
unexpectedly changed to an olive drab color without the
addition of any dye or reaction with copper ions as was
previously required to obtain a green surface color. The -~
samples along with an treated or bare sample was subjected
to salt spra~ testing according to test procedure defined
by ASTM B117 and U.S. Mil-A-8625E standard. The bare
sample failed the test after 24 hours while the samples
with the hard anodized coating and sulfuric acid anodized
coating showed no visual effects after being subjected to -
336 hours of salt spray as required by the test
procedure. The olive drab color which was uniform
throughout the entire anodized coatings and did not fade
during the salt spray test. Further, this olive drab
color is substantially identical to the camouflage color
used on many military vehicles and installations to hinder
observation thereof.
The specific sulfuric acid anodize treatment
process performed on the samples is as follows:
Samples were passed through a vapor degreasing
chamber of 1,1,1 trichloroethane to remove any oils that
may have been deposited on the surface of the sample after
the manufacture thereof. After passing through the
degreasing chamber the sample was placed in an inhibited
alkaline bath to further clean the alloy. The alkaline
bath includes a cleansing product sold under the name of
Aldet. 4-6 oz/gal or 50-80 gmfl of Aldet produces a
concentration to maintain a 12 pH in the alkaline bath.
The temperature in the al~aline ~ath was maintained
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between 48-60C and the samples were cleaned in about 5
minutes. After the samples were removed from the
inhibited alkaline bath they were rinsed in a water bath
to stop the cleaning action and substantially return the
05 alloy to a neutral state before being placed in a
deoxidizing bath having a minimum temperature of 20C ~ ~
containing a deoxidizing agent sold under the name of -
Alutone for 3 minutes . A concentration 10% + 1% of
Alutone is sufficient to remove any oxidation and
contamination that may be located on the surace of the
alloy either during manufacture or as a result of exposure
to the environment. After deoxidation, the cleaned
samples were placed in a bath containing sulfuric acid
having a concentration of from 150-250 g/liter and a
temperature of between 20-25C. At room temperature and
concentration the sulfuric acid bath is a relative safe -
operation. The samples were connected to the anode of a
voltage source. The voltage between the anode and cathode -~
is gradually increased from zero to 20 volts at a slow
rate of approximately 3 volts per minute and this voltage
is maintained for about 30 minutes. The voltage between
the anode and cathode can vary between 18-25 volts
dispensing on the time available for the development of a
columnar substrate on the surface of the alloy. The
columnar substrate which has a nominal height of 0.00016
inches or 0.0004 cm forms an initial level of protection
on the surface of this alloy.
After the substrate has been formed, the alloy is
removed from the sulfuric bath and rinsed in a cold water
bath to interrupt the reaction between the sulfuric acid
and alloy and to terminate the development of the columnar
substrate. Throughout this specification where the term
cold water bath is used the temperature of the water is
below 20C while a room or warm water bath indicates the
temperature to be above 2GC. The cold water bath
effectively removes the sulfuric acid from the surface of
the alloy however some sulfuric acid may be trapped
between the individual columns in the columnar substrate.
To assure total reaction termination, the samples were
passed through a room temperature bath with a
concentration of between 40-60 grams/liter of sodium ~
bicarbonate. After about one minute the sodium ~ --
05 bicarbonate should have reacted with and neutralized any
sulfuric acid that may have been retained in the ~-
interstices of the columnar substrate on the surface of
the samples.
The reaction between the sodium bicarbonate and
sulfuric acid produces a sodium sulphate residue which
along with any residual sodium bicarbonate is rinsed away
in a warm water bath. On removal of the samples from this
bath, the anodized surface layer is essentially gray in
color.
The gray colored samples were placed in bath
containing sodium dichromate and a surfactant. A 5%
concentration of sodium dichromate adjusted with sodium
hydroxide to create a pH of between 5-6 in the bath when a
minimum temperature of 90C is maintained should be
sufficient to seal the columnar substrate. A 0.01%
concentration of the surfactant, a product sold under the
trade name of Intravon was used, low~red the surface
tension on the columnar substrate such that after about 20
minutes, the sodium dichromate flowed into and filled the
interstices and sealed the surface of the samples from the
surrounding environment. This bath because of the sodium -
dichromate, surfactant and water has an orange color
however on removal of the samples from the bath, its color
was surprising olive drab.
The samples now colored olive drab on its removal
from the sodium dichromate bath were first placed in a
warm water bath, below 20C, bath to remove and excess
sodium dichromate from the now sealed columnar substrate.
Thereafter the samples were then placed in a hot water
bath, where the temperature of the water is maintained
above 50C, to uniformly raise the temperature of the
samples. When a desired temperature is reached, the
samples were removed from the hot water bath and
2 ~ ~ ~ s.3 ~
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transported to a chamber where circulating warm air above
25C, air dried the samples.
This anodizing process was used to seal and
protect the samples from corrosion as required by ASTM
05 B117 and Mil A-8625E. On inspection of the samples after
the salt spray test was performed, no measurable weight
loss was found in the samples nor was visible corrosion
observed.
In order to confirm the results and protection
for the aluminum based alloy using this anodizing process,
different samples based on the composition disclosed in
U.S. Patent 4,729,790 was obtained however 11% by volume
of silicon carbide was included in the basic composition.
The samples of this alloy were anodized in accordance with
the process set forth above and subjected to the ASTM B117
salt fog test for the required 336 hours. On visual
inspection, no corrosion was observed on the samples and
no weight change occurred as a result of the salt spray
test. The measured thickness of the anodized layer was
found to be 0.00016 inches or 0.0004 cm. The olive drab
color was consistent throughout the layer of anodic film
or coating on these samples and the only way the coating
was removed from the samples was by a mechanical scraper.
Thus, the method of anodizing an aluminum based
alloy as described above in addition to providing for
corrosion protection also colors the alloy without the
need for dye to an olive drab color which is universally
accepted as a camouflage for most military vehicles and
installations.
