CA1056702A

CA1056702A - Composition and method for cleaning aluminum at low temperatures

Info

Publication number: CA1056702A
Application number: CA217,499A
Authority: CA
Inventors: Robert E. Binns
Original assignee: Amchem Products Inc
Current assignee: Henkel Corp
Priority date: 1974-02-14
Filing date: 1975-01-07
Publication date: 1979-06-19
Also published as: DE2506066C3; FR2261349A1; BR7500430A; JPS5328245B2; CS179934B2; JPS50115630A; SE7501629L; ZA7547B; HK24482A; BE825546A; PL94962B1; DE2506066B2; DE2506066A1; GB1454974A; FR2261349B1; IT1030139B

Abstract

ABSTRACT OF THE INVENTION

Acid cleaning of aluminum surfaces is accomplished employing a composition comprising Hydrofluoric acid and Sulfuric acid. More specifically, this invention relates to a composition and method for treating aluminum containers at low temperatures.

Description

~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, ~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 ~ 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.

~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.

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 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.

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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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.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An aqueous composition suitable for removing and dissolving aluminum fines and cleaning lubricating oils from aluminum surfaces containing from about 0.005 to 0.1 grams/liter of hydrofluoric acid and from about 1 to about 10 grams/liter of sulfuric acid.

2. A composition as defined in claim 1 having a pH from about 1.0 to about 1.8.

3. A composition as defined in claim 1 having a pH from about 1.2 to about 1.5.

4. A composition as defined in claim 1 wherein the hydrofluoric acid is present in an amount from about 0.01 to about 0.03 grams/liter.

5. A composition as defined in claim 1 wherein the sulfuric acid is present in an amount from about 3 to about 5 grams/liter.

6. A composition as defined in claim 1 wherein the hydrofluoric acid is present in an amount from about 0.01 to about 0.03 grams/liter and the sulfuric acid is present in an amount from about 3 to about 5 grams/liter.

7. An aqueous composition suitable for removing and dissolving aluminum fines and cleaning lubricating oils from aluminum surfaces containing hydro-fluoric acid in an amount from about 0.005 to about 0.1 grams/liter, sulfuric acid in an amount from about 1 to about 10 grams/liter and a surface active agent in an amount from about 0.1 to about 10 grams/liter.

8. A composition as defined in claim 7 wherein the surface active agent is selected from the group consisting of an anionic surfactant and a nonionic surfactant.

9. A process for cleaning an aluminum surface which comprises contact-ing said surface with a composition as defined in claim 1.

10. A process for cleaning an aluminum surface which comprises contact-ing said surface with a composition as defined in claim 1 at a temperature of about 100°F to about 135°F.