US3524799A - Anodizing aluminum - Google Patents

Description

United States Patent O 3,524,799 ANODIZING ALUMINUM Kenneth Howard Dale, Henrico County, Va., assignor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware No Drawing. Application July 10, 1967, Ser. No. 652,020, which'is a continuation-in-part of application Ser. No. 559,389, June 22, 1966, which in turn is a continuationiu-part of application Ser. No. 306,999, Sept. 6, 1963. Divided and this application June 13, 1969, Ser. No.

Int. Cl. C23b 9/02 US. Cl. 20458 9 Claims ABSTRACT OF THE DISCLOSURE The invention described and claimed herein concerning anodizing in a four-component bath containing a water soluble alkali salt of a titanic acid complex of a hydroxy aliphatic carboxylic acid containing 2-8 carbon atoms was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautic and Space Act of 1958,

Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

Ownership of that invention and this patent is retained by Reynolds Metals Co. in accordance with the Administrators determination and waiver of title, subject to applicable regulations.

CROSS-REFERENCES TO RELATED APPLICATION This application is a division of application Ser.

No. 652,020 filed July 10, 1967, now abandoned, which in turn was a continuation-in-part of application Ser. No.'.559,389, filed .Tune 22, 1966, now abandoned,

which in turn was a continuation-in-part of application Ser. No. 306,999, filed Sept. 6, 1963, now abandoned.

BACKGROUND OF THE INVENTION The production of aluminum articles which possess thick, hard, dense anodic coatings, and which can readily he dyed and sealed, in arapid and economical manner, is of increasing industrial importance. Aluminum and aluminum base alloy sheets, extrusions, and castings thus jtreated are widely used for the production of exterior building trim, panelling, ornamental housewares, and

the like. I

It is known from the literature dealing with anodization that the controlling factor or limitation in anodizing with a mineral acid bath, such as surfuric acid or phosphoric acid, is the dissolution rate of the electrolyte on the aluminum oxide film. There are different theories concerning whether this represents a chemical reaction or an electrochemically catalyzed chemical reaction, but there is general agreement that there is a pronounced temperature and concentration differential between the bulk of the electrolyte and the pore base. It may be estimated that this differential represents a temperature of about 125 F. and, where sulfuric acid is the electrolyte, a sulfuric acid concentration greater than 50%, assuming hot concentrated sulfuric acid as the. reactant.

Patented Aug. 18, 1970 The methods now used in producing thick, hard anodic films in sulfuric acid electrolytes represent attempts to control this reaction through control of the bulk electrolyte with respect to temperature, concentration, violent agitation, additives which decarboxylate or form a gaseous product, alternating current impression for reversing polarity to expel reaction products, and/or refrigerant gases. It has also been proposed in US. Pat. No. 2,262,967 to apply hard coatings on aluminum by employing polyhydric alcohols as additions to electrolytic anodizing baths in which the anodizing acid is itself an organic carboxylic acid, including such acids as lactic, oxalic, or glycolic acid, together with a thorium salt. Such baths differ fundamentally from the novel anodizing baths of the present invention in composition, and moreover they are operated at ordinary current densities. These prior art methods are too slow, involved, and expensive for present-day needs.

In connection with space vehicle aluminum surfaces there has existed a need for a white coating which would provide a degree of whiteness and brightness required for optical tracking purposes, and which would at the same time meet the limitations placed upon film thickness and provide a high degree of corrosion resistance to the aluminum surface. The present practice has been to apply to the aluminum surface a conversion coating, followed by an application of a zinc chromate primer coating, and then a coating of a lusterless while enamel. The total coating thickness is of the order of 2 mils and adds weight to the vehicle to an undesirable extent.

SUMMARY OF THE INVENTION In accordance with the present invention there are provided novel methods and compositions for the rapid production of thick, hard, dense anodic coatings on aluminum and aluminum base alloys, and for the production of pigmented anodic coatings having a high degree of whiteness and brightness and corrosion resistance on aluminum and aluminum base alloy surfaces.

In accordance with a first aspect of the invention, there is provided a three-component anodizing bath comprizing a mineral acid, at least one polyhydric alcohol containing from 3 to 6 carbon atoms, and at least one organic carboxylic acid containing a reactive group in the alpha-position to the carboxylic acid group. There is also provided a novel method of anodizing aluminum and aluminum alloys with said three-component bath at extremely high current densities to form thick, hard, dense anodic films rapidly on the metal surfaces.

It has been found, in accordance with the invention, that a polyhydric alcohol containing from 3 to 6 carbon atoms, and an organic carboxylic acid containing a reactive group in alpha-position to the carboxylic acid group will react with the hot reaction products formed during anodizing with or adjacent to the surface of the pore base, and thereby suppress the attack or dissolution of the forming oxide film by these products.

The mineral acid component of the electrolyte is advantageously sulfuric acid or phosphoric acid. Where sulfuric acid is used, the bath concentration is generally maintained between about 8% and about 30% by weight,

.preferably about 15%. Where phosphoric acid is employed, the working range of concentration is somewhat higher, being between about 10% and 45%, With the preferred concentration being about 30%.

Polyhydric alcohols containing from 3 to 6 carbon atoms which may be employed in the practice of the invention, singly or in admixture, include glycerol, butanediol-1,4, pentanediol-1,5, mannitol and sorbitol. The total amount of polyhydric alcohol employed ranges from about 1% about 4% by weight of the anodizing electrolyte. The preferred polyhydric alcohol is glycerol, for economic considerations.

The organic carboxylic acids containing a reactive group in alpha-position to the carboxylic acid group include acids in which the reactive group is hydroxy, amino, keto, or carboxyl. Examples of such acids include glycolic (hydroxyacetic), lactic (hydroxypropionic), malic (hydroxysuccinic), oxalic, pyruvic, and aminoacetic acids. Acyclic carboxylic acids such as lactic, malic, and aminoacetic (glycine) acids are preferred. A mixture of two or more of these acids may be employed in combination with the mineral acid and the polyhydric alcohol. The amount of carboxylic acid included in the electrolyte is preferably between about 1% and about 4% by weight of the bath.

The temperature of the electrolyte is not critical, and in general, an operating temperature range of about 65 F. to 95 F. will give good results with sulfuric acid, whereas with phosphoric acid, temperatures up to about 110 F. may be used.

Direct current may be employed, and current densities may range from about 40 amperes per square foot, up to about 500 amperes per square foot, or even higher. Anodizing is very rapid, the anodizing times ranging from about 5 minutes to about 40 minutes, with the production of film thicknesses ranging from about 1 to 5 mils. Thus, in the anodization of small articles, there can be typically produced anodic films of greater than 5 mils thickness in minutes.

While not wishing to be bound by any particular theory, it is believed that in the three-component electrolyte system of the invention, the polyhydric alcohol functions both as a catalyst and as a chemical reactant, entering into the reaction, to form, in combination with the organic carboxylic acid, products which control the thermal and chemical properties of the oxidizing mineral acid of the electrolyte, which facilitate the formation of anodic pores. Thus, for example, sulfuric acid may act to dehydrate or decarboxylate the organic acid, and this action is promoted by the polyhydric alcohol in the amounts employed in the bath. This action of the polyhydric alcohol is in marked contrast to the function of glycerol in a sulfuric acid bath of protecting the anodic coating or the metal itself against corrosive attack, as taught in US. Pat. No. 1,869,042. It could not have been predicted that when employed in conjunction with a mineral acid and an organic carboxylic acid of the character described, as a component of a three-component electrolyte system, the polyhydric alcohol would coact with the first two ingredients to promote formation of an initially porous coatmg.

In summary, therefore, it is believed that the polyhydric alcohol and the organic acid together act as a thermochemical regulator controlling the dissolution rate of the oxidizing acid within the pore walls and endothermally dissipating the localized heat. The additives and their reaction and combination products variously interact with the oxidizing acid to establish an equilibrium. The acid concentration and the temperature Which exist in the pore base aid in furnishing water from dehydration and cooling from decarboxylation. The polyhydric alcohol also catalyzes the reaction of the alpha-reactive carboxylic acid with the oxidizing acid at lower temperature and concentration. The additives have the further advantage of facilitating the use of normal anodizing solutions in the formation of hard, dense anodic coatings, at low voltages mainly between 20 and volts and at conventional operating temperatures, to produce films which can be dyed.

The invention also contemplates the provision of an anodizing concentrate or additive for incorporation in a mineral acid anodizing electrolyte,'which concentrate consists essentially of at least one polyhydric alcohol containing from 3 to 6 carbon atoms, and at least one organic carboxylic acid containing a reactive group in the alphaposition to the carboxylic acid group, the proportion of polyhydric alcohol to carboxylic acid ranging from about 1:4 to about 4:1. Such a concentrate may be in the form of a substantially homogeneous dry mixture or a solution of the ingredients, or-an'aque'ous or a mineral acid solution thereof including a suitable diluent,;preferably, in the instance of aqueous mineral acid diluents, a solution of'the same mineral acid as thatemployed in the anodizing electrolyte in which the concentrate is to be incorporated.

In accordance with a second aspect of the invention, there is provided a novel four-component anodizing bath comprising a mineral acid, at least one polyhydric alcohol containing from 3 to 6 carbon atoms, at least one organic carboxylic acid containing a reactive group in alpha-position to the carboxylic acid group, and at least one water-soluble alkali salt of a titanic acid complex of a hydroxyaliphatic carboxylic acid containing from 2 to 8 carbon atoms, and a method of anodizing alumimum and aluminum alloys with said bath.

When utilized for anodizing the surface of aluminum and aluminum alloys, the four-component anodizing bath of the invention produces a pigmentized anodic coating which exhibits extraordinary whiteness and brightness and which is especially suitable for application to space vehicle aluminum surfaces. The white coating provides the necessary degree of whiteness and brightness for optical and visual tracking, and at the same time it imparts a high degree of corrosion resistance.

The novel four-component anodizing electrolyte of the invention permits elimination of the need for conversion coatings, zinc chromate primers, and white enamel coatings hitherto employed on space vehicles. It provides a coating which meets the specifications requiring passage of 1000 hours in the salt spray test, and which exhibits a minimum luminous apparent reflectivity value of at least 50, with values up to 75. The electrolyte permits a reduction in coating thickness compared with previously employed methods, with attendant saving in weight.

The four-component anodizing electrolyte including the titanium complex compounds is applicable to aluminum metal and to aluminum base alloys of all kinds, but is particularly adapted to the anodic coatings of copper-bearing aluminum alloys such as Nos. 2014 and 2219, to zinc-bearing alloys such as No. 7006, and to magnesium-bearing alloys such as No. 6063.

The critical factor in the pigmentizing of the anodic coating appears to reside in a combination of high current density employed with oxidation of the titanium.

In addition, the inclusion of the titanium complex compound unexpectedly results in a saving in net power consumption during anodizing. This is manifested, for example, in requiring lower voltages for the same current density which would be employed in a three-component bath previously described, without the titanium complex compound.

Hydroxyaliphatic carboxylic acids containing from 2 to 8 carbon atoms, which may be employed in the preparation of the titanic acid complex compound include, for example, glycolic (hydroxyacetic), malic, lactic citric, tartaric, alpha-hydroxybutyric, glyceric, and mandelic (hydroxyphenylacetic) acids.

The titanic acid complexes utilized in the practice of the invention are alkali salts. Examples of alkali salts include ammonium, sodium, potassium, and lithium salts, but ammonium salts are preferred. The preferred titanic acid complex is titanium dilactate ammonium salt.

These titanic acid alkali salt complexes are known compounds and their preparation by a variety of methods is described in the chemical literature. They can be prepared, for example, by reacting a titanium salt, such as titanium sulfate, in aqueous solution, with an equivalent quantity (one or two moles) of a hydroxyaliphatic carboxylic acid calcium salt, removel of precipitated calicum sulfate, and neutralization with an alkali such as ammonium hydroxide or sodium hydroxide. They can also be made by reacting an aqueous solution of titaniumpotassium oxalate with, for example, calcium lactate, to form titanium-potassium lactate, as described in US. Pat. 760,319. Another well-known method of preparation is the reaction of a tetra-alkyl titanate with an alpha-hydroxyaliphatic carboxylic acid in an organic solvent, such as acetone or isopropyl alcohol, removal of the solvent, and neutralization with a suitable alkali such as sodium or ammonium hydroxide to yield the alkali salt of the titanium complex;

The resulting alkali salts of the titanic acid complexes are water-soluble, and acid-soluble, and are stable in acid solutions at low pH values, for example as low as 1-2, and up to about 7.5. They are fully compatible with mineral acid anodizing electrolytes, particularly with sulfuric acid anodizing electrolytes, and with the threecomponent electrolytes of the present invention, in which they dissolve freely. Their presence during anodizing results in the deposition of titanium dioxide in the absorbent anodized aluminum film, imparting to the anodic film good pigment loading and hiding power.

'When employing the titanium complexes, it is desirable, although not essential, to select as the alpha-hydroxycarboxylic acid employed as the complex former the same alpha-reactive acid as is employed in the anodizing bath itself, e.g. lactic acid where the complex is a titanium lactate.

When employing the four-component anodizing electrolyte including the titanic acid complex salt, the electrolyte temperature will advantageously range from about 70 F. to about 100 F., preferably about 85 F. Direct or alternating current may be employed, with voltages between about 12 and 25 volts. Current density ranges from about 12 to about 70 amperes per square foot.

Sulfuric acid is the preferred mineral acid, with a concentration ranging from about 15% to 30% by weight, preferably about 26% The proportion of polyhydric alcohol preferably ranges from about 1% to 2% by weight of the bath, preferably about 2%; the proportion of organic carboxylic acid from about 1% to 5%, preferably about 2%,.

The proportion of titanium complex salt ranges from about 1% to 5% by weight of the bath, preferably about 3% The preferred electrolyte has the composition:

Percent by wt.

Sulfuric acid 26 Glycerol 2 Lactic acid 2 Titanium dilactate ammonium salt 3 Water, balance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples serve to illustrate the practice of the invention, but are not to be regarded as limiting:

Example 1 An extrusion of No. 6063 aluminum alloy was anodized for 7 minutes at 80 F. in an anodizing bath having the following composition:

Percent Sulfuric acid Glycerol 1 Glycine 1 with DC current at a current density of 300 amperes per square foot. A light grey coating was obtained having 6 a film thickness of 3.3 mils, which could readily be dyed a dark red with the dye Rubine YX.

Example 2 An aluminum casting was anodized for 8 minutes at 85 F. in an anodizing bath having the composition:

Percent Sulfuric acid l5 Glycerol 1 Pyruvic acid 1.5

with DC current at a current density of 480 amperes per square foot. A coating having a thickness of more than 5 mils was obtained, which was dyed a deep black with Black SN.

Example 3 A drawn sheet of No. 5557 alloy was anodized for 5 minutes at F. in an anodizing bath having the composition:

Percent Sulfuric acid 15 Lactic acid 2 Glycerol 2 Example 4 A casting of aluminum was anodized for 5 minutes at 65 F. in an anodizing bath having the composition:

Percent Sulfuric acid 15 Glycine 1 Pyruvic acid l Glycerol 3 with DC current at a current density of 400 amperes per square foot. An anodic coating having a thickness of 3 mils was obtained.

Example 5 Eight 5005 alloy aluminum coasters 3' OD. were anodized for 45 minutes at 45-55 F. in an anodizing bath having the composition:

Percent Sulfuric acid l5 Malic acid l Glycerol 1 with DC current at a current density of 30 amperes per square foot. A dense black anodic coating having a thickness of 1.5 mils was obtained.

Example 6 Four pieces 6063 T6 alloy aluminum extrusions were anodized for 60 minutes at 70 F. in an anodizing bath having the composition:

Percent Sulfuric acid 15 Malic acid l Glycerol 1 With a constant voltage of 19 DC volts and a DC current density of 25 amperes per square foot. An anodic coating having a thickness of 2.2 mils was obtained.

Example 7 Four panels each of 5657 alloy aluminum were anodized for 10, 15, 20 and 30 minutes respectively at 70 F. in an anodizing bath having the composition:

Percent Sulfuric acid l5 Glycolic acid 1 Glycerol 1 with a constant DC voltage of 18 volts and DC current at a current density of 20 amperes per square foot. Anodic coatings of .3 mil for 10 minute anodize, .45 mil for 15 minute anodize, .6 mil for 20 minute anodize, and .85 mil for the 30 minute anodize were obtained.

Example 8 An aluminum casting containing chromium as an alloying ingredient was anodized for 10 minutes at 70 F. in

an anodizing bath having the composition:

Percent Sulfuric acid 15 Glycolic acid 2 Glycerol 2 with DC current at a current density of 493 amperes per square foot. An anodic coating with a thickness greater than 5 mils was obtained.

Example 9 Three panels of alloy 5052-H14 were anodized for 12, 18, and 24 minutes, respectively, at 85 F. at a voltage of 22-48 DC volts, and a current density of 90 amperes per square foot, in an anodizing bath having the composition:

Percent Phosphoric acid 30 Lactic acid 1 Mannitol 1 The respective film thicknesses obtained were 1.5, 2.3, and 3.0 mils.

Example 10 Alloy 7002 aluminum was anodized for 30 minutes at 70 F. in an anodizing bath having the composition:

Percent Sulfuric acid 1S Lactic acid l Sorbitol l with DC current at a current density of 105 amperes per square foot. A coating having a thickness of 4 mils was obtained.

Example 11 An extruded section of 6063-T6 alloy was anodized for minutes at 70 F. with DC current, at a current density of 200 amperes per square foot, in an anodizing bath having the composition:

Percent Sulfuric acid 15 Lactic acid 1 Mannitol 1 A coating having a thickness of 4 mils was obtained.

Example 12 Alloy 2014 aluminum plate was anodized at 85 F. for 30 minutes, with DC current at a current density of 60 amperes per square foot, in an anodizing bath having the following composition:

Percent Sulfuric acid 26 Malic acid 2 Mannitol 2 The film obtained had a thickness of 2.7 mils. The anodizing of alloys 2219 and 7006 gave similar results.

Example 13 Panels of three alloys 5052, 3003, and 5005 were anodized at current densities of 45, 60, and 90 amperes per were anodized at a current density of 60 amperes per square foot, at 70 F. in a bath having the composition:

Percent Sulfuric acid 17 Malic acid 2 Pentanediol-1,5 2

The anodizing results were satisfactory, although fumes from the bath were very irritating to the eyes.

Example 15 Four pieces of 3003-H14 sheet 4" x 6" x .050" were anodized at 60 amperes per square foot for 30 minutes, 24 minutes, 18 minutes, and 12,minutes, respectively, at 70 F. in a bath having the composition:

Percent Sulfuric acid 17 Lactic acid 1.0

Mannitol 1.0

Glycerol 1.0

The film thicknesses were: 2.6 mils for the 30 minute anodize, 2.1 mils for the 24 minute anodize, 1.7 mils for the 18 minute anodize, and 1.1 mils for the 12 minute anodize.

Example 16 A cast aluminum piston (12% Si alloy) was anodized for 30 minutes at 60 amperes per square foot at 65 F. to a film thickness of 2.6 mils, in a bath having the composition:

Percent Sulfuric acid 17.3 Oxalic acid 2.0 Glycerol 2.0

Also, using the same bath compoistion, a 6463 aluminum alloy extrusion with convex and concave flutes was anodized for 15 minutes at 150 amperes per square foot at 70 F. to a film thickness of 3.2 mils. The extrusion was dyed gold in FAQ.

Example 17 A panel of 2219 alloy was anodized for 18 minutes at a temperature of F. in a four-component anodizing bath having the composition by weight:

Percent Sulfuric acid 26 Glycerol 2 Lactic acid 2 Titanium ammonium lactate 3 with DC current at a current density of 60 amperes per square foot. A hard, very white anodic coating having a thickness of 0.0008 was obtained.

Example 18 Using the anodizing electrolyte of Example 17, a. panel of 2014 alloy was anodized for 18 minutes at a temperature of 85 F. and a constant current density of 60 amperes per square foot. A hard white coating having a thickness of 0.0010" was produced.

Example 19 Utilizing the electrolyte of Example 17, a panel of 7006 alloy was anodized for 18 minutes at a temperature of 85 F. and a constant current density of 30 amperes per square foot, yielding a white coating of thickness 0.0010".

A panel of 6063 alloy was anodized for 20 minutes at a temperature of 84 F. in a bath having the composition, by weight:

at a current density of 15 amperes per square foot, yield- .ing a white coating of thickness about 0.0004 inches.

Example 21 A panel of 2219 allow was anodized for 15 minutes at a temperature of 88 F. in an electrolyte having the composition, by weight:

Percent Sulfuric acid 26 Glycerol 1 Malic acid 1 Titanium ammonium lactate 3 at a current density of 60 amperes per square foot, yielding a dense white coating of thickness 0.0008".

What is claimed is:

1. Method of forming a hard, dense, white anodic coating upon an aluminum surface which comprises anodizing the aluminum in an aqueous acid electrolyte consisting essentially of about 15% to about 30% by weight of sulfuric acid, from about 1% to about 2% by weight of the electrolyte of at least one polyhydric alcohol containing from 3 to 6 carbon atoms, from about 1% to about by weight of the electrolyte of at least one organic carboxylic acid containing a reactive group in the alpha-position to the carboxylic acid group, and from about 1% to about 5% by weight of the electrolyte of an alkali salt of a titanic acid complex of a hydroxyaliphatic carboxylic acid containing from 2 to 8 carbon atoms.

2. The method of claim 1 in which the organic carboxylic acid is lactic acid.

3. The method of claim 1 in which the titanic acid complex is titanium dilactate ammonium salt.

4. The method of claim 1 in which the polyhydric alcohol is glycerol.

5. The method of claim 1 in which the anodizing current density is between about 12 and about amperes per square foot.

6. An aqueous anodizing electrolyte consisting essentially of about 15% to about 30% by weight of sulfuric acid, from about 1% to about 2% by weight of at least one polyhydric alcohol containing from 3 to 6 carbon atoms, from about 1% to about 5% by weight of at least one organic carboxylic acid containing a reactive group in alpha-position to the carboxylic acid group, and from about 1% to about 5% by weight of an alkali salt of a titanic acid complex of a hydroxyaliphatic carboxylic acid containing from 2 to 8 carbon atoms.

7. The composition of claim 6 in which the titanic acid complex is titanium dilactate ammonium salt.

8. The composition of claim 6 in which the organic carboxylic acid is lactic acid.

9. The composition of claim 6 in which the polyhydric alcohol is glycerol.

References Cited UNITED STATES PATENTS 1,869,041 7/ 1932 Bengston 204-5 8 XR 2,231,373 2/1941 Schenk 20458 XR 2,260,278 10/1941 Schenk 204-58 2,262,967 11/ 1941 Schenk 20458 2,356,575 8/1944 Frasch 204--58 XR FOREIGN PATENTS 455,412 10/ 1936 Great Britain.

GERALD L. KAPLAN, Primary Examiner