US2460897A

US2460897A - Rendering aluminum and aluminum alloy surfaces corrosion resistant

Info

Publication number: US2460897A
Application number: US554147A
Authority: US
Inventors: Walter R Meyer
Original assignee: Enthone Inc
Current assignee: MacDermid Enthone Inc
Priority date: 1944-09-14
Filing date: 1944-09-14
Publication date: 1949-02-08
Anticipated expiration: 1966-02-08
Also published as: GB601736A; FR916540A

Description

Patented Feb. 8, 1949 RENDERING AND AL ALULIINUM UMINUM ALLOY SURFACES .CORROSION RESIST- ANT Walter R. Meyer, Hamden, Conn., assignor. by mesne assignments, to Enthone, Inc., New Haven, Conn, a corporation of Connecticut No Drawing. Application September 14, 1944, Serial No. 554,147

Claims. (01. 148-620) This invention relates to a method for produc-' ing corrosion-resistant films on aluminum and aluminum alloy surfaces and encompasses compositions employed in the practice of the method. The invention is particularly valuable in that it eliminates the necessity for anodizing metal surfaces comprising aluminum.

It is known to coat aluminum or aluminumcontaining alloys by means of an aqueous solution of an alkali metal chromate and an alkali metal carbonate. The coatings provided by such solutions give only a fair measure of protection to surfaces of pure aluminum or non-cuprous alloys of aluminum while the protection afforded surfaces of copper-aluminum alloy is very poor.

I have discovered that the inclusion of an alkali metal chlorite in the solution markedly improves the quality of the protective coating as to surfaces of pure aluminum or non-cuprous alloys of aluminum; also, that the solution comprising an alkali metal chlorite will give a satisfactory coating on a copper-aluminum alloy surface.

In the preparation of a coating solution according to my invention, I may supply part or all of the chromate radical in the solution through the addition of an alkali metal dichromate since the alkali metal dichromate will immediately react with the alkali metal carbonate to form alkali metal chromate. I have found that the chromate concentration in the solution, calculated as mono-chromate, is most suitably kept between 1 and 20 g./l. I have also found it advantageous v to maintain the concentration of alkali metal chlorite within that same range. The most advantageous concentration of alkali metal carbonate appears to be interrelated with the concentrations of the other components. Usually the carbonate concentration is best maintained,

between 8 and 50 gf/l. It is my preferred practice to use sodium salts in the preparation of the coating solution, but the corresponding salts of other alkali metals, for example potassium salts, may be used.

The coating operation is most suitably carried out by immersing the work piece in the solution. Any operating temperature promoting the formation of a corrosion resistant film on the aluminum or aluminum alloy surface may be employed, but I prefer to operate the solution or bath at a temdesired film is a function of the concentration of '2 perature between 200 F and its boili I ng oin The color of the coatings produced may var from a purplish iridescent color to a dark greenish gray color depending upon the com the surface contacted with the solutio ffi of In coating surfaces of substantially pure aluminum or of alloys of aluminum with manganese magnesium, or chromium, I preferably employ a bath of pproximatel th 1 trations: y e onowmg en- Sodium chlorite gJ15. Sodium chromate 5-6 Sodium carbonate 20 In treating copper-aluminum alloy surfac I have found that a bath containing from 4 t 16 g./l. of sodium chlorite, 4 to 5 g./l. of sodium chromate, and from 12 to 16 g./l.v of sodium carbonate gives particularly good results. Such baths may, with advantage, be prepared by dissolving in water a solid composition in divided form consisting essentially of the three salts in the relative proportions desired in the bath.

My investigations indicate that whenever the chromate concentration is increased, it is advisable to increase the carbonate concentration.

It is preferable in all cases that the aluminum or aluminum alloy surface before treatment according to my invention be cleaned with an alkali that etches the surface to provide clean active metal for coating. An aqueous solution of trisodium phosphate or caustic soda heated to about 180 to 212 F. has been found suitable for the cleaning operation. In preparing the tri-sodium phosphate solution, I usually employ about 6 oz. of the tri-sodium phosphate for each gallon of water. In preparing the caustic solution, I customarily use about lb. of caustic soda for each gallon of water. Following the treatment with the warm alkaline solution, the surface should be dipped in strong nitric acid (50 or stronger) to remove any copper or iron smut. The work is then ready for immersion in the oxidizing solution containing a chlorite, chromate, and carbonate of an alkali metal.

In general, the time required to produce the the bath and of the operating temperaure.

of solutions of low concentration than in the case or solutions of relatively higher concentration to produce a coating in the same length of time. Certain aluminum alloy surfaces may require slightly highertemperatures and concentrations than surfaces of pure aluminum.

The invention is further illustrated by the following examples which are not to be considered as in any way limiting the scope thereof.

Example I A composition containing 1 part by weight of sodium chlorite, 1.1 parts by weight of sodium chromate (about 1.013 parts by weight of sodium dichromate, if used) and 4 parts by weight of sodium carbonate was dissolved in sufficient hot water (200 F.) to give an aqueous solution in which the concentrations ofthe salts were approximately as follows:

Grams per liter Sodium chlorite 5 Sodium chromate 5.5

Sodium carbonate 20 A strip of commercially pure aluminum was immersed in the solution and allowed to remain therein for a period of about minutes. This resulted in the production of a film on the aluminum strip which withstood 263 hours of a salt spray at 95 F. without a sign of corrosion.

Example II The coating solution was prepared as in Example I. A strip of 38 aluminum was immersed in the solution for 15 minutes. The coating produced on the strip did not break down when subjected for 263 hours to a 20% salt spray at 3S aluminum is an alloy of aluminum with manganese, containing 1.25% of the latter metal.

Example III The coating solution was prepared as in Example I. A strip of 52S aluminum, an alloy of aluminum with magnesium and chromium containing 2.5% and 0.25% respectively of these metals, was immersed in the solution for 15minutes. The resulting film also withstood 263 hours of a 20% salt spray at 95 F. without a sign of corrosion. v

Example IV A strip of 24S aluminum was immersed in the solution and allowed to remain therein for a period of about 15 minutes. The film produced on the strip did not break down after 40 hours of a 20% salt spray at 70 F.

245 aluminum contains 4.2% copper, 0.5% manganese and 1.5% magnesium, the balance being aluminum and impurities.

Example V One part by weight of a composition containing 1 part by weight of sodium dichromate, 4 parts 4 by weight of sodium chlorite, and 3 parts by weight of sodium carbonate was dissolved in about 30 parts by weight of hot water (200 F.) to give a solution of the following concentrations:

Grams per liter Sodium chlorite 16 Sodium chromate 4.34 Sodium carbonate 12 A strip of 24S aluminum was immersed in the solution and allowed to remain therein for a period of about 15 minutes. 48 hours of a 20% salt spray at 70 F. was required to break down the film produced on the strip.

- Example VI One part by weight of a composition contain ing 1 part by weight of sodium dichromate, 2 parts by weight of sodium chlorite, and 4 parts by weight of sodium carbonate was dissolved in about 36 parts by weight of hot water (200 F.) to give a solution of the following concentrations:

Grams per iiter Sodium chlorite 8 Sodium chromate 4.34 Sodium carbonate 16 A strip of 24S aluminum was immersed in the so lution and allowed to remain therein for a period of about 15 minutes. 48 hours of a 20% salt spray at 70 F. was required to break down the film produced on the strip.

It is to be understood that my invention is not restricted to the use of sodium compounds but may be effectively practiced using other alkali metal salts, for example potassium chlorite, potassium chromate and/or dichromate, and potassium carbonate.

A composition according to my invention contains from 1-20 parts of an alkali metal chlorite, 1-20 parts of an alkali metal chromate or dichromate, and 8-50 parts of an alkali metal carbonate. Anhydrous materials in a state of fine subdivision are preferred for use in the preparation of the composition. The materials may be mixed using any suitable mixing device, for example an ordinary mixing drum of the type designed for the intimate admixing of fine powders.

My invention is not limited to coating articles of solid aluminum or aluminum alloys but may be used to coat surfaces of other materials which have been plated with aluminum or aluminum alloys when the coating thickness is greater than 0.00005" and the aluminum content is greater than about 60%.

I claim:

1. The method of treating a surface composed of aluminum or an aluminum alloy having a content of aluminum greater than about 60%, the surface having a thickness of at least 0.00005 inch when it represents a coating, which comprises contacting the surface with an aqueous solution consisting essentially of water and an alkali metal chlorite, an alkali metal chromate and an alkali metal carbonate containing from 1 to 20 grams per liter each of the chlorite and chromate and from 8 to 50 grams per liter of the carbonate, the temperature and duration of the treatment being such as to result in the formation of a corrosion resistant 'film on the surface.

2. The method of treating a surface composed of aluminum or an aluminum alloy having a content of aluminum greater than about 60%, the

surface having a thickness of at least 0.00005 inch when it represents a coating, which comprises contacting the surface with a hot aqueous solution consisting essentially of water and sodium chlorite, sodium chromate and sodium carbonate containing from 1 to 20 grams per liter each of the chlorite and chromate and from 8 to 50 grams per liter of the carbonate, the duration of the treatment bein such as to result in the formation of a corrosion resistant film on the surface.

3. The method of treating a surface composed of aluminum or a non-cuprous alloy of alumlnum having an aluminum content greater than about 60%. the surface having a thickness of at least 0.00005 inch when it represents a coating, which comprises contacting the surface with a hot aqueous solution consisting essentially of water and sodium chlorite, sodium chromate and sodium carbonate containing about grams per liter each of the chlorite and chromate and about 20 grams per liter of the carbonate, the duration of the treatment being such as to result in the formation of a corrosion resistant film on the surface.

4. The method of treating a surface composed of a copper-containing alloy of aluminum having a content of aluminum greater than about 60%, the surface having a thickness of at least 0.00005 inch when it represents a coating, which comprises contacting the surface with a hot I aqueous solution consisting essentially of water and sodium chlorite, sodium chromate and sodium carbonate containing from about 4 to about 16 grams per liter of the chlorite, about 4 to 5 grams per liter of the chromate, and from about 12 to about 16. grams per liter of the carbonate, the duration of the treatment being such as to result in the formation of a corrosion resistant film on the surface. Y

5. The method of claim 3 where the treating solution is maintained at a temperature between 200 F. and its boiling point.

6. The method of claim 4 where the treating solution is maintained at a temperature between 200 F. and its boiling point.

7. A solid composition in divided form adapted for use in aqueous solution to produce a corrosion resistant film on a surface composed of aluminum or a non-cuprous alloy of aluminum having a content of aluminum greater than about 80%, consisting essentially of an intimate mixture of sodium chlorite, sodium chromate and sodium carbonate in approximately the proportions given below:

8. A solid composition in divided form adapted for use in aqueous solution to produce a corrosion l0 resistant film on a surface composed of a copper-aluminum alloy having a content of aluminum greater than 60%, consisting essentially of an intimate admixture of sodium chlorite, so-

dium chromate and sodium carbonate in approx- 15 imately the proportions given below:

Parts by weight Sodium chlorite 4to 16 Sodium chromate 4 to 5 Sodium carbonate 12 to 16 20 9. A solid composition in divided form consisting essentially of an intimate admixture of an alkali metal chlorite, an alkali metal chromate an an alkali metal carbonate, the relative proin the following indicated ranges:

Parts Alkali metal chlorite 1 to 20 Alkali metal chromate 1 to 20 Alkali metal carbonate 8 to 50 Parts Sodium chlorite 1 to 20 Sodium chromate 1 to 20 Sodium carbonate 8 to 50 WALTER R. MEYER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,838,633 Pacz Dec. 28, 1931 2,205,708 Zurbrugg June 25, 1940 2,364,993 Meyer Dec. 12, 1944 portions of the three salts by weight being with-