US3140245A - Dulling hard aluminum oxide coatings on aluminum-rich basis metals - Google Patents

Description

United States Patent 3 140,245 DULLING HARD ALIiMINUM OXIDE COATINGS ON ALUMINUM-RICH BASIS METALS Anthony Gallaccio, Havertown, and Reyburn W. Wick, Philadelphia, Pa., assignors to the United States of America as represented by the Secretary of the Army No Drawing. Filed Aug. 23, 1961, Ser. No. 134,055 3 Claims. (Cl. 20435) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Goverment for governmental purposes without the payment to us of any royalty thereon.

This invention relates to aluminum and alloys thereof in which aluminum is the principal ingredient. It is particularly directed to the provision of a process and an aqueous acid solution therefor which produce a dull, non-reflective, dye-receptive surface on hard anodized aluminum oxide coatings on aluminum.

It is well known that metal articles having dark lustrous finishes are desirable and suitable for numerous applications and uses, particularly for decorative purposes, to promote radiation, etc. Too, matte black finishes are important in providing non-reflecting surfaces in industrial and military instruments of various types. However, on hard anodized coatings which have been electrolytically applied to aluminum and its alloys, a brownish, compact, high corrosion and erosion resistant coating has resulted. Since these coatings are dense and compact, they are glossy and appreciably reflective, and hence are generally non-receptive to subsequent dyeing processes. In varied military applications, the reflectivity of these hard anodized coatings is undesirable.

It is recognized that the dulling of hard anodized aluminum oxide coating surfaces might be accomplished by mechanical means, i.e., by abrasive blasting and the like, but a non-uniform reduction of coating thickness or complete removal of the coating in areas frequently results. Additionally, the dye receptivity of mechanically dulled surfaces is not considered good.

It is therefore a broad object of the invention to provide means for dulling hard anodized aluminum oxide coatings on aluminum rich basis metals.

Another object of the invention is to provide a process and bath for rendering aluminum oxide coatings on aluminum-rich basis metals dull and dye-receptive.

Other and further objects of the invention will be apparent to those skilled in the art upon study of this disclosure.

We have discovered that black dyes, such, for example, as nigrosine and Aluminum Black 3B, are extremely adherent to hard anodized aluminum oxide surfaces when dulled by the bath process of our invention.

One process, in accordance with our invention, referred to by Way of example, consists of cleaning the article and degreasing if necessary, and immersing in our treating solution for approximately 80 seconds, the solution being maintained at about 95 C. and containing phosphoric acid, chromium tri-oxide and a suitable surface-active agent, such that the pH of the solution does not exceed about 2.0. Our process has been successfully applied to a variety of commonly used alloys consisting largely of aluminum as well as pure aluminum. A few of the well known aluminum-rich alloys include at least one "ice of the metals selected from manganese, magnesium, silicon, chromium and copper.

The solution used in practicing our improved dulling process are characterized by a content of acid, chromium tri-oxide and a surface-active agent which are within a rather Well-defined area, the following being typical examples:

Formula No. 1

Phosphoric acid (H PO ml./l 30 Chromium tri-oxide (CrO g./l 15 Water, to make 1 liter Balance Surface-active agent percent by weight 0.01

The surface-active agent may suitably be an alkylaryl polyether alcohol, such as Triton, or an alkylarylsulfonate such as Ultrawet.

A number of alternative bath formulas are given below illustrating a few of the many variations in compositions Which our improved solutions may take Within their operating range:

Formula N0 2 Phosphoric acid (H PO 85% ml./l 24 Chromium tri-oxide (CrO g./l 18 Water, to make 1 liter Balance Surface-active agent percent by weight 0.01

Formula No. 3

Phosphoric acid (H PO 85% ml./l 36 Chromium tri-oxide (CrO g./l 12 Water, to make 1 liter Balance Surface-active agent percent by weight 0.01

It is therefore possible to vary the phosphoric acid range between about 24 to 36 ml./l. (equivalent to a molarity of about 0.38 to 0.57) and the chromium trioxide between about '12 to 18 g./l (equivalent to a molarity of about 0.12 to 0.18), the surface-active agent remaining about 0.01% by weight.

Of the examples given, Formula No. 1 produced excellent dulling of the hard anodized aluminum oxide coatings whereas Formulas No. 2 and No. 3 produced equally good dulling surfaces but required about 3 to 4 seconds additional immersion time. By excellent dulling is meant reflectivity is almost totally eliminated as indicated by visual observation.

If the bath temperature is raised above C. the immersion time will be less than 80 seconds when a solution as typified by Formula No. 1 is employed. Conversely, when the bath temperature is lowered from 95 C., the immersion time necessary to produce good dull surfaces will be increased. After immersion the article is rinsed and dried, and if a dyeing operation is required, may now be taken to the dye bath without the necessity of being dried.

It is apparent that through the practice of our invention, hard anodized aluminum oxide coatings may successfully, economically and rapidly be treated to produce a dull, non-reflective, dye-receptive surface. Further, our solution will not attack the basis aluminum where exposed, such, for example, as at racking points. It is also to be understood that our invention is not limited to use with black dyes only, but that other suitable dyes may be employed if desired.

We claim:

1. A method of minimizing the reflectivity of hard anodized aluminum oxide coatings on an aluminum-rich basis metal article which comprises treating said article with an aqueous solution consisting essentially of phosphoric acid of about 0.38 to 0.57 molarity, chromium tri-oxide of about 0.12 to 0.18 molarity about 0.01 weight percent of a surface active agent.

2. A method according to claim 1 which comprises cleaning and degreasing said article prior to treating.

3. A method according to claim 1 which comprises rinsing and dyeing said article subsequent to treating.

References Cited in the file of this patent UNITED STATES PATENTS 2,438,877 Spruance Mar. 30, 1948 4 Jones Aug. 25, Newhard Nov. 26, Beiswanger et al. Nov. 26, Newhard et a1. Oct. 20, Newhard et al May 10, Newhard et al Oct. 30,

OTHER REFERENCES Henley: Light metals, Temple Press, London, October 10 1959, pages 536542.

Kurz: Journal of the Electrochemical Society, vol.

103, No. 4, April 1956.

Claims (1)

1. A METHOD OF MINIMIZING THE REFLECTIVITY OF HARD ANODIZED ALUMINUM OXIDE COATINGS ON AN ALUMINUM-RICH BASIS METAL ARTICLE WHICH COMPRISES TREATING SAID ARTICLE WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF PHOSPHORIC ACID OF ABOUT 0.38 TO 0.57 MOLARITY, CHROMIUM TRI-OXIDE OF ABOUT 0.12 TO 0.18 MOLARITY ABOUT 0.01 WEIGHT PERCENT OF A SURFACE ACTIVE AGENT.