US3447953A - Method of vapor sealing anodically produced oxide films on aluminum and aluminum alloys - Google Patents

Description

United States Patent US. 'Cl. 1l7106 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to scaling a porous anodized coating on a body of aluminum or aluminum alloy by exposing the coating to mixed vapors of water and a chlorinated hydrocarbon such as trichloroethylene or perchloroethylene.

This invention relates to the anodizing of aluminum and aluminum alloys, and particularly to a method of sealing a porous anodized coating on aluminum, the latter term as employed hereinafter including alloys in which aluminum is the predominant constituent.

Oxide coatings produced on aluminum by anodizing are usually porous and have to be sealed if highest corrosion resistance and other desirable properties are to be achieved. Exposure of the anodized surface to steam in closed chambers is generally accepted to produce the best sealing effect, but steam scaling is a time consuming operation. The sealing time can be reduced to some extent by the use of superheated steam, but the high pressure of the superheated steam must be contained in costly vessels so that this method has not found wide acceptance.

It is a common drawback of all steam sealing methods that they operate at or above the boiling point of water so that the steam in the sealing chamber must be vented to the atmosphere or condensed before a batch of scaled material can be removed from the chamber to avoid scalding the operator. The resulting consumption of steam and of thermal energy makes the process economically inefficient.

It has now been found that vapors of chlorinated hydrocarbons substantially saturated with water produce an adequate or even a superior sealing effect even at temperatures lower than 100, and that such a sealing process can be performed very adequately in a container open to the atmosphere. The process of the invention produces adequate sealing in a fraction of the time required for steam sealing at about 100 C., and the thermal efficiency of the new process is high.

The mixture of water and chlorinated hydrocarbon vapors can be prepared by jointly heating water and the liquid hydrocarbon to the necessary temperature. The two liquids being practically insoluble in each other, it is preferred vigorously to stir or otherwise to agitate the liquids while they are being heated so that water and the hydrocarbon are present in the gas phase in vapor pressure equilbrium with the liquid phases.

A gaseous mixture of water and chlorinated hydrocarbon in which anodized aluminum may be sealed quickly and completely is also produced by introducing saturated, and preferably superheated steam, into a body of liquid chlorinated hydrocarbon. The steam partly evaporates the solvent and provides the mechanical agitation necessary for equilibrium saturation of the gas phase with the constituent elements.

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It is also possible to evaporate the chlorinated hydrocarbon by suitably supplied thermal energy, to feed steam into the solvent vapor near the surface of the hot liquid, and to stir the liquid while an anodized aluminum object is immersed in the layer of relatively heavy water-bearing solvent vapor.

The chlorinated solvents containing more than one chlorine atom per molecule and having a boiling point near that of water, that is, at l00i25 C. have been found to be most effective. Trichloroethylene and perchloroethylene, the conventional dry cleaners solvents, meet these requirements and are readily available at low cost. The rapidly produce the desired results in the sealing method of the invention.

The effect of sealing on freshly anodized porous aluminum surfaces is readily evaluated by a dye absorption test. Aluminum anodized by most conventional methods, when not sealed, avidly adsorbs most wool dyes, particularly acid dyes, but also certain dyes which do not readily color wool. Fully sealed aluminum does not take up such dyes from an aqueous solution, and the ability of anodized aluminum to adsorb or absorb a certain green dye is the basis of a widely accepted standardized sealing test (see Aluminum, 38 (1962), 150-154 and 161). It permits an approximate quantitative evaluation of the efiiciency of a sealing method, and was employed in the following examples which further illustrate this invention.

EXAMPLE 1 An upright cylindrical container having a capacity of about 200 liters and an open top was charged with 25 liters distilled water and 25 liters perchloroethylene. A fairly uniform mixture of the two liquids was maintained in the container by vigorously stirring with an agitator driven at high speed by an electromotor.

A steam coil was immersed in the liquid and controlled by a thermostat arranged in the vapor phase above and set for -90 C. A cooling coil upwardly spaced from the liquid surface vertically confined the vapor by condensation and made it unnecessary to cover the open top of the container.

The vapor of perchloroethylene saturated with water quickly reached a state of equilibrium. Cups of 300 ml. capacity made from commercial aluminum metal (99.5% Al) were anodically coated with a thin oxide layer in a conventional sulfuric acid electrolyte, carefully rinsed, and immersed in the vapor layer between the liquid surface and the condensation zone on the level of the cooling coil. Groups of samples were withdrawn every minute and tested for dye adsorption.

Excellent sealing was achieved within eight minutes. percent of the tested samples gave a negative dye adsorption test after only six minutes.

Steam sealing tests performed at ambient pressure with anodized cups from the sample batch indicated that a sealing time of more than 20 minutes was necessary to achieve sealing comparable to the results achieved in 8 minutes in the gaseous mixture of water and perchloroethylene.

EXAMPLE 2 The procedure of Example 1 Was repeated with tri chloroethylene instead of perchloroethylene, and operating at a temperature of 70-80 C. The results obtained were similar to those achieved with perchloroethylene, but the sealing period required was somewhat longer than was necessary with perchloroethylene, twelve minutes being required on an average for complete sealing.

Analogous results were obtained with all commercial aluminum alloys which are usually anodized, and the alloying elements effected sealing with moist chlorinated hydrocarbon vapors in the same manner in which they are known to affect steam sealing or sealing in boiling water with or without addition agents. The superiority of the solvent vapor sealing method over the known methods varied in magnitude from alloy to alloy, but was always significant.

The electrolyte employed in anodizing was not found to have a significant effect on the relationship between the time required for sealing anodized coating by the method of the invention and the time required for steam sealing.

What is claimed is:

1. A method of sealing a porous anodized coating on a body of aluminum or aluminum alloy which comprises exposing said coating to mixed vapors of water and of a chlorinated hydrocarbon, said hydrocarbon being normally liquid.

2. A method as set forth in claim 1, wherein the boiling point of said hydrocarbon is between 75 C. and 125 C., each molecule of said hydrocarbon containing at least two chlorine atoms.

3. A method as set forth in claim 2, wherein the temperature of said mixed vapors is lower than the boiling point of said water and the boiling point of said chlorinated hydrocarbon, but not lower by more than 25 C. than the lower one of said boiling points.

4. A method as set forth in claim 3, wherein said mixed vapor is in contact with liquid water and liquid hydrocarbon, and said liquid water and hydrocarbon are agitated 4 at a rate sufiicient to substantially maintain vapor pressure equilibrium between said vapor, said water, and said liquid hydrocarbon.

5. A method as set forth in claim 3, wherein said vapor is produced by introducing steam into a liquid body of said chlorinated hydrocarbon.

6. A method as set forth in claim 3, wherein said chlorinated hydrocarbon is trichloroethylene or perchloroethylene.

References Cited UNITED STATES PATENTS 2,662,034 12/1953 Mason et al. 117-49 2,837,451 6/1958 Hannon 1486.3 X 3,000,346 9/1961 Hnilicka 117--106 X 3,365,377 l/1968 Michelson 204-35 OTHER REFERENCES Metal Finishing, vol. 54, No. 8, August 1956, pp. 53-57 relied upon.

ALFRED L. LEAVI'IT, Primary Examiner.

A. GOLIAN, Assistant Examiner.

US. Cl. X.R.