Note: Descriptions are shown in the official language in which they were submitted.
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CASE 6143
1 This invention is directed to a simple method
of forming a hydrophobic coating on an aluminium surface.
Many processes are presently available for
coating aluminium surfaces, such as anodizing, plating,
chemical conversion coatings, painting and the like. The
coatings, although designed for long life, require extensive .
surface pretreatments and are quite expensive. However,
frequently, only short-term protection is needed or desired,
for example, in shipping or storing semi-fabricated
aluminTum products, such as coiled sheet and the like, to
prevent the formation of water staln or other oxidatlon
products. On other occasions, it is desirable to prevent
the gradual buildup of natural oxide on the aluminium
surface, for example, in welding applications and adhesive
bonding applications because the buildup of natural
oxide can interfere with these types of operations. How-
ever, no simple and inexpensive process is presently known
whtch will give short-term protection without interfering ~ -
wlth subsequent fabrication or surface treatments,
partlcularly when lubrlcants must be applied to the
surface.
A simple, inexpensive process for coating aluminium
is described by Wittrock et al in U.S. ~,726,721, assigned ~-
to the present assignee, but the friable coating formed by
thts process can interfere with subsequent forming or
coating operations.
Kubie in U.S. 2,963,391 describes a process
for forming a coating designed as an extrusion lubricant
wherein the alum;n;umsurface is first treated with an
L984
1 ammonia-laden alkaline solution containing a fatty acid
(or equivalent salt or ester thereof) and then baked at
about 400F to form a coating having unknown properties
except for lubrication.
~arosi in U.S. 3,8499207 describes and claims
a process for treating aluminium and other metals wherein
the surface is treated with an a1kaline sodium formate
solution and then coated with a clear resinous film to
form a sepia-colored coating. The nature of the coattng
formed during treatment in the alkaline sodium formate
solution is not describe~ in the reference. However,
it has been found that the aluminium surface underlyTng
such a coatlng 7s susceptible to water stain or other
oxldatlon in much the same manner as untreated alumlnium
because the coating is readily wet and penetrated by water
or aqueous solutions.
A simple, inexpensive method to temporarily
prevent extensive oxidation of an aluminlum surface by
water or other media particularly in coiled sheet has
been needed for many years, but has been theretofore
unavallable.
It is against thts background that the present
tnvention was developed. '
The invention generally relates to the
25 coating of an aluminium surface and particularly to th~ ~ '
treatment of an aluminium surface to form a tenacious
hydrophobic coating which protects the underlying
aluminium surface from oxidation and also'facilitates '
the application of lubricant for subsequent fabrication.
As used herein, alumimium refers to pure alumlnium,
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1 commercjally pure aluminium and aluminium alloys.
In accordance with the invention, an aluminium
surface is treated with an aqueous alkaline solution
containing a long chain aliphatic carboxylic acid, an
equivalent alkali metal salt thereof or a compound
which generates a long chain aliphatic carboxylate anion
in an alkaline solution at elevated temperatures greater
than 60C. Treatment times usually will be about one
second for a clean surface, but extended treatment times
do not seem to detrimentally affect the coating. The
surface coating is hyarophobic and usually highly
oleophil7c. Moreover, the coating Ts not usually affected
by mineral acids, such as nitric acld, hydrochlorlc acid
or sulfuric acid or by common polar solvents, such as
acetone or ethyl alcohol. The coating formed is very
difficul~ to analyze because under most circumstances,
it appears to be a monomolecular layer on the order of
lOO A thick. The carboxylate anion generating compound
Tn the alkaline solution is apparently either reacting
wtth the aluminium surface to form a type o~ aluminium soap
or at least strongly associating with the alumlnTum sur~ace.
The pH of the alkaline treating solutlon must
be from about 8.5 to about 10.0, preferably about 9-10.
At a pH much above 10, no coating occurs, only etchTng.
The temperature at the interface between the metal and the
bath should exceed 60C and preferably should be from
about 75C to the boiling point of the solutTon. For
optimum results, the temperature is maintained at
about 85C ~ 5C. Although generally it ~ill be most
convenient to treat the aluminium surface with a solutlon
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1 maintained at the prescribed tempe-ratures, it is con-
templated to treat an aluminium workpiece heated well
above the prescribed temperature with a so1ution at
1ess than the prescribed temperature to e~fect ~he
required interface tempera~ures. No significant coating
formation is found at interface temperatures much below
60C.
The long chain aliphatic compound in the treat-
ment bath ne~d only be present in smal1 quanti~ies,
10 usually greater than 1 part per millicn by weight. How- -
ever, it is preferred to maintaTn the compound in a slTght
excess of saturation to form an emulsion for the convenlence
of compositlon con~rol. The compound can be a long chain
allphatlc carboxyltc acld ~a fatty acid), an alIsall metal
salt or ammonium salt thereof or other compounds whlch
generate a long chain carboxylate anion in an alkafine
solution. The long chain carboxylate anion should have
from 10-20 carbon atoms, preferably 12-18. Below 10
carbon atoms, the coatings are not sufficiently hydrophobic
to be of any value In preventing the wetting of the coating
and the penetration thereof by water or other aqueous
solutions which leads to water stain. Compounds wlth
more than 20 carbon atoms in the chain usually Just coat
the aluminiumsurface and neither react with nor strongly
associate with the alumin~m substrate. This latter
feature is readily shown by removing the coating with
polar solvents, such as acetone or ethyl alcohol.
Addtt7Onally, excessively long chain carboxylate components
are usually too difficult to maintain as an emulsion to
b~ effective for treating the sur~ace-
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1 Suitable long chain aliphatic carboxylic acids
include lauric acid, myristic acid, palmitic acid, stearic
acid, oleic acid, ricinoleic acid, linoleic acid, arachidic
acid and the like. Preferably, alkali metal salts of the
above acids are used. Although the sodium and potassiumsalts are most desirable in the present invention, lithium,
cesium and ammonium salts are functional. Other compounds
which generate the appropriate carboxylate anion in an
alkaline solution can also be used. The aliphatic component
of the carboxylate generating compound can be saturated
or unsaturated; however, unsaturated compounds tend to be
less effective than saturated compounds. SubstitutTons on
the aliphatic carbon chaln can be made provided they do not
prevent the carboxylate compound from forming the desired
coatlng-
If desired, ~etting agents, such as Emsorb 6903(sold by Emery Industries, Inc.), Tween 85 tsold by ICI
American, Inc.) and Ultrawet (sold by the Armour and Company)
can be added to ths solution in amounts up to 3~ by weight
to facilitate the wetting of the alumi~um surface by the
alkalIne solutlon durlng treatment. Use of wettlng agents
tends to render the coatTng more ol'eophillc. Other
components, such as emulsifTers and the like, can be added
up to 3% by weight to render the fatty acid component
2S mlscible or at least emulsifiable with the aqueous
solution. Mechanical dispersions can also be employed, for
example, when the emulsification of ~he carboxylate generat-
lng componsnt in the alkaline solution; is difficulto
The treatment solution may be rendered alkaline
by the addTtion of appropriate sapon7fyin~ agents, such as
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1 an alkali ~etal hydroxide or ammonium hydroxide. However,
in many instances, such as when utilizing sodium stearate
or sodium palmitate, the solution will be sufficiently
alkaline so that additional hydroxide additions become
unnecessary. Although ammonium salts and ammonium hydroxide
can be employed in the alkaline treatment solution, these
compounds are not particularly desirable because at the
required elevated interface temperatures, ammonia is driven
from the solution rendering the maintenance of a suitably
alkaline pH at the~interface very difficult.
When the aluminjum surface is flrst contacted by
the alkalIne solution of the inventlon, an Initial, very
short burst of ef~ervescence occurs Indicatlng that the
alumlnlum substrate is belng etched. The effervescence
qùickly subsides, however, and the desired hydrophobic
coating forms, usually within a few seconds. A heavy
oxide layer can retard coating formation and apparently
most, if not all, of oxide coating must be etched away
~efore any reaction or strong association can occur
20'~between the carboxylate anion and the aluminum substrate.
The etchlng whlch occurs inltlally may leave or generate
a very thin layer oF oxide on the metal surface, and in
all likelihood, the carboxylate anion may be reactlng
with basic sites on this thin oxide coating to form the
soap.
The surface coating wh1ch forms ;n the Inven~ion
Ts neither readily wet nor penetrated by water or other
nonalkaline aqueous solutions. The surface 7s usually
oleophilic and is generally compatible with most~ Jf
not all, m~tal-working lubricant~ includ7ng water-based
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1 emulsions~ This compatibility of the coating with
lubricants is enhanced considerably by the use of wetting
agents in the alkaline treating solution and particularly
by treating the formed hydrophobic coating with a hot
(greater than 60C) alkaline solution (pH 8.5-10, pre-
ferably 9.0-10.0) of a wetting agent. Nonionic and
cationic wetting agents are preferred.
The hydrophobic coating has an electrical
resistance initially of about 15 micro-ohms per cm2 which
remains relatively stable for at least 2-3 weeks. This
indicates that essentially no oxidation of the underlytng
alum7rium surface is occurr7ng. These resistance levels
are to be compared w7th a natural oxide coating wh7ch has
an inftial reslstance of about 30 micro ohms per cm2 and
which can gradually tncrease to well over 1000 micro-ohms
per cm2 Tn a matter of days. The relatively stable re-
slstance exhibited by the coating of the invention is a
very advantageous feature. For example, in welding
alumTn~m, the oxide coating is not desirable because
it Tnterferes with the welding operation, particularly
spot reststance welding. By maintain1ng the reslstance
at a relatively constant level, there is no need to clean
the surface pr70r to welding.
Moreover, in manufacturlng facilities utilizing
the adhesive bonding of 21uminjum components, the coating
of the 7nven-tTon prevents oxidation during the various ~ '~
fabrication or assembly procedures, ye~ it provides an
excellent foundation for adhesion between the alumin;um
suhstrate and o~her materials.
The coating of the invention can be readily
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1 removed by treatment with alkaline cleaning solutions
normally employed to industrially c1ean aluminium surfaces
prior to various surface treatments, such as anodizing,
painting and the like.
It has also been found that the process of the
invention can be employed to treat aluminium surfaces which
are oxidized or otherwise contaminated with oxidized pro-
ducts prior to subsequent fabrication. The etchlng whlch`
occurs in the process removes the contaminated oxide surface
and the coating formed is readily compatible with and
wet by metal-working lubricants. For example, the surface
of aluminium alloy sheet used for maklng drawn and Troned
can bod7es is frequently contamTnated with water staln
(oxiciation product from the condensatlon of water on the
surface during transporting or storing) or pick-up (highly
oxidized metal particles embedded in the surface during
rolling), which interferes with the drawing and ironing
operations. By treating such sheet in accordance with
the Tnvention9 the etching removes substantially all of
the oxidized surface contaminants. The coating formed
readily accepts the draw and Iron lubricant, part7cularly
when subsequently treated w7th a hot alkaline solution
containing a wetting agent. Treatment times with heavy
natural oxide coatings which have been agad ~end to be
in the order of several minutes rather than a few seconds
as when a fresh natural oxide-coated alumi~7u~ surface is
- treated.
The following examples are given to fur~her
illustrate the invention.
In each of the Examples 1-7, a clean 3004-Hl9
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71984
1 aluminium alloy sheet was treated. The treating solutions,
which were maintained at 85 + 5C~ were prepared by adding
l.O gram/liter of the noted acid to deionized or dis~illed
water and then adjusting the pH to 9.0 ~ Q.l with NaOH if
needed. Treatment time in each case was 30 seconds. Each
treated specimen was checked for water wettability after
treatment in the alkaline solution, after a 30-second dlp
in a 35% (by weight) nitric acid solution and then after an
acetone-ethyl alcohol rinse.
Example l
Lauric acid (C-12) formed a hydrophobtc, oleo-
philic surface which remained hydrophobic a~er a 30-second
d1p in the nitric acid. However, aPter the nitric acld
treatment, the acetone-ethyl alcohol rinse apparently
lS removed the hydrophobic coat1ng because the treated surface
could then be wet with water.
Example 2
Myristlc acid (C-14) formed a hydrophobic, oleo-
philic surface which remained hydrophobic after a 30-second
dip in the nitrTc acid. However, after the nttrtc acid
treatment, the acetone-ethyl alcohol solution apparently
removed the hydrophobic coatlng because the treated surface
could then be wet with water.
Example 3 ~i~
Palmitic acid (C-16) formed a hydrophobic,
oteophilic surface which remained so after both ~he
nitric acid dip and the acetone-ethyl alcohol rinse. ;
Exampl~ 4
Stearic acid (C-18) formed a hydrophobic, oleo- I
phTlic surface whtch remained so after both the nitric
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1 acid dip and the acetone-ethyl alcohol rinse.
Example 5
Oleic acid (unsaturated C-18) formed a hydrophobic,
oleophilic surface which remained hydrophobic after a 30-
second dip in the nitric acid. However, after the nitricacid treatment, the acetone-ethyl alcohol solution apparently
removed the hydrophobic coating because the treated surface
could then be wet with water.
Example 6
Linoleic acid (unsaturated C-18) formed a hydro-
phobic, oleophilic surface which remained hydrophobTc after
a 30-second dip in the nTtric acid. However, after the
nitrtc acid treatment, the acetone ethyl alcohol solution
apparently removed the coating because the treated surface
could then be wet with water.
Example 7
Arachidic acid (C-20) formed a hydrophobic,
oleophilic surface but after the nltric acid dip, the surface
became hydrophilic.
Example 8
A clean 3004-H32 alumin1um alloy sheet was
treated for 5 seconds In an aqueous alkaline solution main-
tained at 80OC which contained 1.0 gram~liter sodium
stearate. The pH of the solution was 9.3. Initially,
a burst of effervescence occurred but the effervescence
quickly subsided and the desired hydrophobic, oleophilic
coating formed. The treated sheet withstood 20 hours of I'
continuous water-fog exposure with no evidence of water
stain or other surface defects. The coating was $u11y
compatible with varîous metal-working lubricants, such as
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1 are used in rolling, forging, drawing and ironing, shap-
ing, stamping and the like. Initially, the treated sur- -
face had an electrical resistance of 16 micro-ohms/cm~
and after 6 weeks of laboratory exposure (23C and 70%
humidity) had an- electrical resistance of only 30 micro-
ohms/cm2 .
Example 9
A steel sheet was treated in accordance with
the conditions set forth above for E~amples 1-7. However,
no significant coating formation was noted. The surface
of the ferrous product was readily wet by water after
treatment. There was some evidence (dTscoloration of
the solutlon) that the solutlon was merely dissolvin~
the surface.
Example 10
A plurality of clean, closely packed 3004-H32
aluminilm sheets were treated in the manner set forth above
In Example 8 except that 2.0 grams/liter of a polyoxy-
ethylene sorbltan trioleate soid under the brand name
Tween 85 was added to the solution as a wetting agent.
The coatlng formed was fully equivalent to the coatlng formed
in Example 8. The wetting agent allowed the sclution to
penetrate in between closely packed aluminium sheet and
react with the surfaces thereof.
Example 11
A 3004-H32 aluminium alloy sheet was treated with
a solution containing a polyoxyethylene sorbitan trioleate
alone (Tween 85) as the source for carboxylate anions. The
solution which contained 2 grams/liter of the trioleate
(Tween 85) was at a pH of 9.5 and a temperature of 80C.
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1 The coating formed was hydrophobic and oleophi1ic.
Example 12
Clean 3004-H32 aluminium alloy sheets were
treated for 30 seconds in a hot aqueous alkaline solution
containing 1 gram/liter of sodium stearate. The pH of
the so1ution was 9.5 and the temperature was 80C. After
treatment2 the sheets were rinsed and ~hen separate sheets
were treated for 5, 15, 25 and 35 seconds in a second hot,
aqueous a1kaline solution containing 2 grams/liter of a
polyoxyethylene sorbitan trioleate (Tween 85). The pH
and temperature of the second solution were also 9.5 and
80C, respectively. The coatings formed were hydrophobic
and hlghly oleophilTc. Treated sheets were then evaluated
for compatibility wTth mineral oil. The evaluation was
conducted by placing a drop of mineral oTl on a treated
surface inclined about 70C from the horizontal and then
determining the time required for the drop of oil to
travel 3 inches on the inclined surface. Longer times
indicate greater wettability and thus greater compatibility
with the lubricant. The results are as follows:
Duration of Second Treatment Time2 Sec
0 8-10
25-35
` 60
25 25 9
3~ 90
Similar results were obtained with a drop of a 30~ by
volume oil-in-water emulsion of Texaca 591, a common
alùminium metal-working lubricant.
In other tests, C-8 acids, such as caprylic
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1 acid, and C-22 acids, such as behenic acid, were found to
form hydrophilic coating.
It is obvious that various modifications and
improvements can be made to the invention described herein
without departing from the spirit thereoF and the scope
of the appended claims.
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