Note: Descriptions are shown in the official language in which they were submitted.
~056702
COMPOSITION AND METHOD FOR CLEANING
ALUMINUM AT LOW TEMPE~ATURES
BACKGROUND OF INVENTION
Containers comprised of aluminum and alloys thereof are
produced in a drawing and forming operation, referred to as
drawing and ironing, which results in the deposition of lub-
ricants and forming oils on the surface. In addi~ion, residual
aluminum fines are deposited on the interior surfaces of the
container during the forming operation.
Prior to any processing steps, such as conversion coating
and sanitary lacquer deposition, the surfaces of the aluminum
containers must be clean and water-break-free so that there
are no contaminants which prevent further processing and which
render the containers unacceptable for use.
~ cid cleaners have been employed to clean the aluminum
surfaces and to remove aluminum fines deposited on the inter~
ior walls of aluminum containers. Acid cleaning is ordinarily
accomplished at temp~ratures from 185F to 200F in order to
remove or dissolve the aluminum fines and to remove the lub-
ricants and forming oils so that the surface is rendered
water-break-free. The cleanliness of the aluminum surface is
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~056702
measured by the ability of the interior and exterior surfaces
of the fcrmed alumirum container to support a con'cinuous film
of water, that is to be water-break-free.
Due to the high temperatures at which cleaning is ac-
complished, that is from 180F to 200F, and the acidity of
the cleaning composition, the processing equipment employed
to heat the cleaning composition, particularly the fire tubes
of gas fired heat eY~changers, are susceptable to corrosion.
Furthermore, the high temperatures increase operating costs
and fuel consumption.
Chromic acid or salts thereof have been utilized to
minimize the corrosion of the processing equipment by in-
hibiting the corrosive attack of the acid cleaning composition
on the processing equipment. An important shortcoming which
cleaners of this kind possess is the inheren~ toxicity of the
hexavalent and trivalent chromium compounds contained therein
and the resultant waste disposal problem created by the presence
of chromium in the cleaner effluent.
Attempts have been made to include other additives in
the cleaning compositions to prpvide efficient cleaning. Such
additives have included fluoride. Maintenance and control
of acidic cleaners at low pH's has proven impractical, espe-
cially when fluoxides are present. Furthermore, the fluoride
when present in high concentrations can attack the metal sur-
ace and etch the surface which is undesirable, especially when
cleaning of containers is to be effected.
The principal object of this invention is to provide a
cleaning composition for aluminum for removing and dissolving
aluminum fines and for cleaning lubricating oils fr~m the
aluminum surface,
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~OS6702
An object of this invention is to pro~ide a process for
eleaning aluminum surfaces at low temperatures thereby re-
ducing corrosion and attack of processing equipment and r~ducing
heating costs and heating fuel consumption.
An added object of this invention is to provide a
elbaning composition possessing good cleaning ability and
preventing corrosive attack of processing equipment while
havin~ no ehromate therein.
Another object is to provide a cleaning solution which
produces no sludge during operation.
A concomitant object of this invention is to provide a
eleaning solution having relatively low concentrations of
fluoride, which can be easily con,rolled and which enables
the cleaning of aluminum surfaces with little or no etching
of ~he surface.
DETAILED DESCRIPTION OF THE INVENTION
I have discovered a composition and process for the
eleaning of aluminum surfaces, said composition comprising
from about .005 to about .1 grams/liter of hydrofluoric acid
and from about 1 to aboutlO grams/liter of sulfuric acid.
When an aqueous cleaning composition comprising hydrofluoric
aeid and sulfuric acid is employed at the concentrations
speeified above, the aluminum surface is cleaned of lubricant
and metallie fines at temperatures as low as from about 110F
to about 135F.
It should be understood that the term aluminum surface
used herein includes aluminum and aluminum alloys in which
aluminum is the principal constit.lent. It should be understood
1056'702
that by "cleaning composition" or "cleaning solution" I mean the aqueous
acidic cleaning bath of the present invention comprising hydrofluoric acid
and sulfuric acid.
The surprising results obtained with the use of the cleaning
solution include the removal and dissolution of aluminum fines from a formed
aluminum container, both on the interior walls and dome of said container, at
temperatures within the range of from about 110F to about 135F. In
addition, it has been found that with the use of the cleaning solution of the
present invention corrosive attack of the processing equipment, particularly
the fire tubes of gas fired heat exchangers, is reduced considerably. This
is accomplished without the use of any inhibitors in the cleaning solution,
such as hexavalent chromium.
I have found that active fluoride is a necessary constituent
which is responsible for assisting in dissolution of the aluminum fines and
oil film removal. In the present cleaning process it is essential that the
active fluoride be maintained within specified limits, since the active
fluoride affects the aluminum dissolution and oil film removal.
The term "active fluoride" means fluoride present in the
operating cleaning solution which, at a given pH is measured by a fluoride
sensitive electrode of the potentiometric type. For example, electrodes of
this type and their use are described in U. S. patent No. 3,431,182 dated
March 4th, 1969 in the name of Martin S. Frant. The electrodes described
therein are known to the art as fluoride specific ion electrodes.
" ~56702
Due to the low concentration of fluoride in the cleaning
solution of this invention, a potentiometric type electrode
is found to be preferable for measuring active fluoride.
Other fluoride measuring devices that are not of the potentio-
metric type are insensitive to the concentration of fluoride
in the cleaning solutions of the present invention, as they
are useful only in concentrated solutions or solutions having
higher concentrations of fluoride.
With the use of the potentiometric type electrode, the
active fluoride measurement is measured as a potential which
is proportional or related to the fluoride ion concentration
in tne solution.
It is known that in acid solutions hydrogen ion complexes
a portion of fluoride forming unassociated HF and HF2 . In
addition, when aluminum is dissolved in solution, Al+3 also
complexes fluoride. Due to the presence of these complexing
agents, it is difficult to measure fluoride ion concentration
without extensive sample manipulation. However, by the use
of the potentiometric type electrode, once a reference point
has been arrived at by measuring a make-up cleaning solution
potential and taking this potential as the zero point, it is
inconsequential how much fluoride is complexed. The potential
increases negatively in more concentrated solutions, that is
when the active fluoride concentration increases; and the
potential increases positively in more dilute solutions, that
is when the active fluoride concentration decreases. When
aluminum ions enter the solution as aluminum fines are removed
from the surface, the electrode potential becomes more positive
due to the presence of less active fluoride. When additions
105670Z
~ hydrofluoric acid are effected, the potential becomes less
posit~ve and approaches the zero point again, and aluminum
fine dissolution increases once again to its original rate.
Since the potentiometric type electrode will measure the
active fluoride in the solution, additions of hydrofluoric acid
can be made to the operating cleaning solution to return the
potential measurement to the original zero reference point.
It has been discovered that as a cleaning solution is
used, aluminum is dissolved at a specific rate. In general,
cleaning solu~ions of the present invention will have operating
characteristics such that at make-up the aluminum dissolution
rate is from about 8 to about 25 milligrams per s~uare foot of
aluminum surface treated. It has been observed that best re-
sults, with minimal etch of the surface, are obtained when
the aluminum dissolution rate is from 9 to 20 milligrams per
square foot of aluminum surface treated. This dissolution
rate occurs at make-up of a cleaning solution having from
about .005 to about .1 grams/liter of HF. By establishing a
zero potential point with a potentiometric type electrode at
make-up of the cleaning solution, and by recording the potential
measurements as metal surfaces are processed and cleaned, the
aluminum dissolution rateis maintained within the preferred
range by additions of Hydrofluoric acid. So, the potentio-
metric electrode is used not only as a guideline for determining
when to adjust the amounts of active fluoride in solution, but
also to maintain sufficient active fluoride therein to effect
a desirable aluminum dissolution rate.
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~0567VZ
The fluoride suitable for use herein is preferably added
to the cleaning composition as hydrofluoric acid. Complex
fluoride and fluoride salts can be employed, however, greater
concentrations will be necessary to yield desirable amounts of
active fluoride. The hydrolysis of complex fluorides is not
substantial enough to liberate the required active fluoride
due to the low concentrations of free fluoride liberated from
the complex fluoride. The amount of active fluoride in the
cleaning solution is expressed herein as a concentration of
hydrofluoric acid. This means that the active fluoride, present
as hydrofluoric acid or fluoride salt, is in the form of dis-
sociated fluoride whose concentration is expressed as the
concentration of hydrofluoric acid.
The active fluoride in the cleaning solution aids in the
removal of aluminum fine6 on the metal substrate which have
formed during the forming operation. A surprising aspect of
this invention is that the cleaning process can be effected
when the amount of hydrofluoric acid present in the solution,
is as low as .005 grams/liter. I have found that by employing
the preEerred amount of hydrofluoric acid, resulting in the
presence of sufficient active fluoride, removal of the aluminum
fines is accomplished without vigorous attack of the underlying
aluminum surface. Of course, should the active fluoride be
depleted in the cleaning solution, it can be replenished by
addition of hydrofluoric acid.
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1(~5670Z
Sulfuric acid in the cleaniny solution should be present
in an amount ~rcm about 1 to about 10 grams/liter. It is
preferred that the sulfuric acid be present in an amount rom
about 3 to about 5 grams/liter.
The hydrofluoric acid should be present in the cleaning
solution in an amount from ahout .005 to about .1 grams/liter.
For optimum results it is preferred that the hydrofluoric acid
be present in an amount from about .01 to about .03 grams~liter.
In the preferred embodiment of this invention an operating
cleaning solution is employed comprising hydrofluoric acid
and sulfuric acid wherein the concentration of constituer;ts,
at the beginning of operation as well as upon rep]enishment,
is maintained such that the sulfuric acid is present in amount
from about 3 to about 5 grams/liter, and ~he active fluoride
.is maintair.ed at the level measured at makeup by employing a
pOtentiGmetric type electrode, that is when the hydrofluoric
acid concentration at make-up is about .01 to about .03 grams/
liter. When a cleaning solution is operated and maintained
within these preferred limits it has been found that excellent
cleaning-of the aluminum surface will result. A mGst sur-
prising result is that the surface will be free of oils and
aluminum fines without the corrosive attack of processing equip-
ment occuring.
The cleaning solution preferably is prepared by employing
aqueous concentrates consisting of sulfuric acid and water,
and hydrofluoric acid and water. The aqueous concentrates can
be added to an appropriate amount of water to prepare a working
cleaning solution having constituent concentrations within the
~ ~ . ~
~ 0sl6702
--~erative ranges set forth herein. Alternatively, the hydro-
fluoric acid and sulfuric acid can be prepared as a concentrate
and can be added simultaneously as one component to water to
form the aqueous cleaning composition at concentrations for
use.
Surfact:ants are desirably included in the cleaning
composition. Such materials enhance the cleaner performance
considerably. The surface active agents to be employed herein
can be anionic, cationic, or nonionic. It has been observed
that the use of surfactants in the cleaning solution assist in
more rapid wetting of the surface. The surface active agent
can be prese~t in the cleaning solution in an amount from about
.1 to about 10 grams/liter.
A cleaning composition having the following formula can
be employed:
Grams
Hydrofluoric Acid 0.02
Sulfuric Acid 4.0
Anionic surfactant 1.0
Water to make 1 liter
The metal surface should be cleaned employing techniques
that result in a completely water-break-free surface.
The cleaning solution can be applied to the aluminum sur-
face utilizing any of the contacting techniques known to
the art. Preferably, application will be effected by con-
ventional spray or immersion me~hods. The time of treatment
of the surface ~ith the cleaning solution need only be long
enough to insure complese wettiny of the surface and can be as
_g_
105670;~:
~ng as 10 minutes. Preferably, the surface should be
treated for a time from about 15 seconds to about 2 minutes.
The aluminum fines and formlng oils are removed from
the aluminum surface by the cleaning solution at tempera-
tures lower than ordinarily expected. The cleaning process
can be operated at temperatures from about 110F to about
135F. It is preferred that the cleaning process be operated
at temperatures from about 120F to about 125F. This is a
distinct advantage of the present invention over prior art
processes, as the low operating temperatures with good clean-
ing results prevents accelerated corrosion and attack of
processing equipment.
In accordance with the invention the cleaning solution
is highly acidic, having a pH below 2Ø The amount of
sulfuric acid and hydrofluoric acid can be varied within
limits in accordance with the ranges set forth hereinabove so
that the pH of the cleaning solution is adjusted from about
1.0 to about 1.8 under the temperature conditions employed in
the process. Preferably the pH of the cleaning solution should
be from about 1.2 to about 1.5.
In accordance with broader aspects of the invention,
there is considerable flexibility available with respect to
portions of the overall processing of the aluminum substrate.
In particular, chemical processing steps can be effected prior
to cleaniny such as, for example, a hot water prerinse of the
surface. Following application of the cleanlng solution, the
surface can be rinsed with water and then dried. Ordinarily a
water rinse is necessary to remove any remaining residues which
may have remained after the cleaning step. After the rinse
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1~)5670Z
~ep the aluminum surface may be contacted with coating
solutions or siccat:ive finish coating compositions well
known to the art. Generally, the coating solution will
be applied directly after the cleaning operation or a short
period of time therafter. However, as specified hereinabove,
the cleaned surface can be dried and the coating steps may be
accomplished at a later time.
The following examples are illustrative of this invention
and are not considered as limiting for other materials and
operating conditions falling within the scope of this invention
which might be substituted.
E X A M P L E
Aluminum container test specimens of 3004 alloy, drawn
into single piece containers, were employed in this procedure.
The containers had been subjected to a drawing operation and
wexe covered with aluminum fines and drawing oils.
The test specimens were treated as follows:
1. Treated for 60 seconds by spraying the interior and
exterior of the specimens with the solutions listed in Table 1.
2. -Rinsed with water by immersion in cold water for 30
seconds at a~bient temperature.
Control specimens were treated with aqueous compositions
comprising acids, as indicated in Table 1, such as sulfuric,
hydrochloric, phosphoric, and nitric acid, as well as specified
combinations or mixtures of these acids. The concentrations
of the aqueous solutions and the constituents therein are
listed in Table 1. The temperature of each of the respective
solutions when employed to treat the test and control specimens
is listed in Table 1.
105670Z
Except when an anionic surfactant was employed in the
solutions as indicated in Irable 1, all other compositions
contained .1 grams/liter of a nonionic surfactant.
The aluminum surfaces were tested for water-break following
cleaning. The container surfaces were rated by measuring the
percent of water-break on the surface, that is the percent of
the total surface area which did not support a continuous film
of water. The results are listed in Table 1.
The results reported in Table 1 include a determination
of the presence of aluminum fines remaining on the surface
after processing was completed. The brightness and appearance
of the test specimens at ~he end of the processing procedure
was also observed. The brightness quality was determined by
visually rating the degree of brightness of the surface from
1 to 5 wherein the brightlless ra~ing of 1 represeilts best
performance and appearance and rating of 5 represents poor
appearance. The presence of aluminum fines on the interior
surface was determined by rubbing the surface with a clean
white cloth and observing the fines deposited on the cloth.
The presence of fines is evidenced by a dark black residue on
the shite cloth. The amount of fines present is expressed in
Table 1 in gradations from excellent (E) for no residue, good
(G) for very light xesidue, fair tF) for moderate residue, to
poor (P) for very heavy residue.
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1056~'0Z
E X A M P L E II
An aqueous acidic cleaning bath was prepared by adding
4.0 grams of sulfuric acid, 0.02 grams of hydrofluoric acid,
and ].0 gram of an anionic surfactant to each one liter of
water. The temperature o~ the bath was elevated to and main-
tained at 120 F. A fluoride specific Ion electrode and a
saturated calomel reference electrode were coupled to a
potentiometric meter capable of discerning changes in electrode
potential of + l.0 millivolt. The electrodes were immersed
into the bath and the meter adjusted, by the zero offset
control, to read on the center of the scale.
Aluminum containers of 3004 alloy drawn into single piece
containers were sprayed with the prepared bath and the meter
readings became more positive. When the electrode potential
reached +3 millivolts from center scale, hydrofluoric acid
additions ware made to restore the electrode potential to
center scale. Electro metric pH measurements, and sulfuric
acid additions were made to maintain the pH at 1.3 to 1.5.
The containers were observed to be water break free and
bright in appearance.
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