US3804731A

US3804731A - Process for forming hard anodic oxide film on aluminum base alloy

Info

Publication number: US3804731A
Application number: US00285595A
Authority: US
Inventors: K Yanagida; T Tsukiyasu; S Iwata
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 1971-09-07
Filing date: 1972-09-01
Publication date: 1974-04-16
Anticipated expiration: 1991-04-16
Also published as: IT965235B; DE2243178C3; DE2243178B2; FR2152678B1; JPS5140537B2; FR2152678A1; GB1366738A; JPS4834739A; DE2243178A1

Abstract

A HARD ANODIC OXIDE FILM IS FORMED UNIFORMLY ON AN ALUMINUM BASE ALLOY CONTAINING 1% BY WEIGHT OR MORE OF COPPER BY SUBJECTING THE ALUMINUM BASE ALLOY OT ANODIC OXIDATION USING AN ELECTROLYTE COMPRISING AN AQUEOUS SOLUTION CONTAINING 5 TO 40% BY WEIGHT OF AN AROMATIC SULFONIC ACID SUCH AS SULFOSALICYLIC ACID, NAPTHALENEDISULFONIC ACID, NAPHTHALENE-TRISULFONIC ACID, PHENOL SULFONIC ACID AND SULFOPHTHALIC ACID, AND 15 TO 40% BY WEIGHT OF SULFURIC ACID.

Description

United States Patent M 3,804,731 PROCESS FOR FORMING HARD ANODIC OXIDE FIIM 0N ALUMINUM BASE ALLOY Kiyomi Yanagida and Tadashi Tsukiyasu, Nagoya, and Seitaro Iwata, Ibaragi, Japan, assignors to Sumitomo Chemical Company, Limited, Osaka, Japan No Drawing. Filed Sept. 1, 1972, Ser. No. 285,595

Claims priority, application Japan, Sept. 7, 1971, 46/69,528 Int. Cl. C231! 9/02 US. Cl. 204-58 6 Claims ABSTRACT OF THE DISCLOSURE A hard anodic oxide film is formed uniformly on an aluminum base alloy containing 1% by weight or more of copper by subjecting the aluminum base alloy to anodic oxidation using an electrolyte comprising an aqueous solution containing to 40% by weight of an aromatic sulfonic acid such as sulfosalicylic acid, napthalenedisulfonic acid, naphthalene-trisulfonic acid, phenol sulfonic acid and sulfophthalic acid, and 15 to 40% by weight of sulfuric acid.

This invention relates to a process for forming on an aluminum base alloy an anodic oxide film which is hard and excellent in corrosion resistance and rigidity.

Heretofore, there have been proposed many processes for forming anodic oxide films on aluminum and aluminum base alloys, in general. For example, processes using an aqueous sulfuric acid solutions or oxalic acid solutions have been well known. For formation of hard anodic oxide films, there have been well known a Kalcolor process using a mixed aqueous electrolyte of sulfosalicylic acid and sulfuric acid (U.S. Pat. 3,031,387), a Duranodic 300 process using a mixed aqueous electrolyte of sulfophthalic acid and sulfuric acid (British Pat. 962,048), a Veroxal process using a mixed aqueous electrolyte of sulfosalicylic acid, maleic acid and sulfuric acid (British Pat. 973,391), a Sumitone process using a mixed aqueous electrolyte of p-phenolsulfonic acid and sulfuric acid (Japanese patent publication No. 29,954/64) and a process using a mixed aqueous electrolyte of naphthalenedisulfonic acid and sulfuric acid (U.S. Pat. No. 3,486,991). These processes are characterized, as is clear from the patent specifications, in that aluminum as an anode is charged in an electrolyte comprising a mixed aqueous solution of a suitable aromatic sulfonic acid and a low concentration sulfuric acid, a cathode is also charged in the electrolyte and then an electric current is flowed to the electrolyte to form an anodic oxide film on the surface of aluminum and aluminum base alloys. However, the said processes have not been satisfactory as processes for forming anodic oxide films on copper-containing aluminum base alloys. That is, when copper-containing aluminum alloys, particularly those containing more than 1% by weight of copper, are treated according to the abovementioued processes, elution of copper takes place during the anodic oxidation treatment to give anodic oxide films which are imperfect and soft to be put into practical use. For example, Al-Si-Cu alloys have been widely used for various aluminum alloy castings, and have been frequently used die castings or sand castings for cylinders, engine parts, bearings and the like products which are required to be high in abrasion resistance. Nevertheless, anodic oxide films formed by subjecting the said aluminum alloy products to anodic oxidation treatment are not uniform, thin in film thickness and low in hardness.

The present inventors made extensive studies or processses for forming anodic oxide films on aluminum base alloys. As the result, the inventors have found that a uniform and hard anodic oxide film is formed on a copper- 3,804,731 Patented Apr. 16, 1974 containing aluminum base alloy by subjecting the alloy to anodic oxidation treatment in an aqueous electrolyte containing an aromatic sulfonic acid and a high concentration sulfuric acid which solution has not been known hitherto. Particularly, it has been clarified that a hard anodic oxide film having an excellent quality is formed on an Al-Si-Cu alloy containing 6% by weight or more of silicon and 1% by weight or more of copper.

It is therefore the object of the present invention is to provide a process for forming a hard anodic oxide film on an aluminum base alloy, characterized by subjecting an aluminum alloy containing 1% by weight or more of copper to anodic oxidation treatment using as an electrolyte an aqueous solution containing 5 to 40% by weight of an aromatic sulfonic acid and 15 to 40% by weight of sulfuric acid.

In the present invention, the aromatic sulfonic acid is an essential component for forming an anodic oxide film on an aluminum base alloy. If the concentration of the aromatic sulfonic acid is less than 5% by weight, no effect of increasing the hardness of the resulting film can be observed. Further, the aromatic sulfonic acid is difficultly dissolved to a concentration of more than 40% by weight in an aqueous sulfuric acid solution of such a concentration as adopted in the present invention. If the concentration of sulfuric acid is less than 15% by weight, not only no uniform anodic oxide film can be formed on a copper-containing aluminum alloy or a high silicon content aluminum alloy, but also the cell voltage becomes higher with formation and growth of the film to easily cause burning. If the concentration of sulfuric acid is more than 40% by weight, the organic acid is lowered in solubility to decrease the hardness-improving effect of the aromatic sulfonic acid. A part of the sulfuric acid may be replaced by a water-soluble sulfate or bisulfate such as sulfate or bisulfate of an alkali metal. The use of an electrolyte comprising an aqueous solution containing 10 to 15% by weight of aromatic sulfonic acid and 15 to 25% by Weight of sulfuric acid gives particularly excellent effects. The aromatic sulfonic acid used in the present invention is sulfosalicylic acid, naphthalenedisulfonic acid, napthalene-trisulfonic acid, phenolsulfonic acid, sulfophthalic acid or the like aromatic sulfonic acid having at least one sulfone group. The electrolyte used in the present invention may be incorporated with a small amount of a carboxylic acid such as oxalic or maleic acid. 1

Aluminum-copper alloys containing 1% by weight or more of copper which are to be treated according to the present process are not particularly limited in form but may be products of any forms such as castings, sheets, extrusions and the like. According to the present process, hard and uniform anodic oxide films can be formed on aluminum-silicon-copper alloys containing 6% by weight or more of silicon and 1% by weight or more of copper on which such films have been diflicultly formed hitherto.

In the present process, the hardness of the resulting film tends to lower with increasing anodic oxidation temperature. Accordingly, the electrolyte temperature is desirably below 20 C., more advantageously within the range from 0 to 15 C. The current density is in the range from 1 to 4 amp./dm. and the treatment period varies depending on the thickness of the resulting film.

Generally, anodic oxide films formed according to the present process are subjected to conventional sealing treatment. In case aluminum alloy products having anodic oxide films formed according to the present process are to be used for pistons, cylinders, bearings, etc., however, it is sometimes preferable that the anodic oxide films are not subjected to sealing treatment, since the films, which have not been subjected to sealing treatment, are high in oil retainability.

The present invention is illustrated in detail below with thickness and hardness from those formed according to reference to examples, but it is needless to say that the the known processes, and that the present process can invention is not limited to the examples. form hard and uniform anodic oxide films on copper- Aluminum alloy materials used in the examples and containing aluminum base alloys on which complete thick in comparative examples are shown in Table 1. 5 films have been difiicultly formed hitherto.

TABLE 1.CHEMICAL COMPOSI'IXggQ ND SHAPE OF TEST ALUMINUM Alloy composition (weight percent) Zn Mg Mn Cr Fe Ni Ti B Al Shape 0.25 0.29 0.06 0.22 0.005 Balance. Casting. do Sheet.

Casting. Do.

EXAMPLE 1 EXAMPLE 8 The aluminum alloy 1 in Table 1 was degreased, etched and desmutted according to ordinary procedures, and then subjected to anodic oxidation treatment at a current den- 20 o 2 i of 2 2 for 60 minutes at a temperature of xldation treatment at a current denslty of 2.5 A./dm.

for 60 minutes in an electrolytic bath at 5 to 7 C. 10 C. in an aqueous electrolyte containing by weight of naphthalene-disulfonic acid, by weight of Contammg by welgtft sulfosahicychc acld and sulfuric acid and 1% by weight of oxalic acid. In this 20% by welght of su'lfunc acldcase, the case, the cell voltage was gradually increased from 20 v. Voltage was gradually Increased from 15 50 The The aluminum alloy 4 in Table l was degreased according to an ordinary procedure, and then subjected to anodic to 25 The fil fo m d by Said anodic oxidation treat. film formed by said anodic oxidation treatment was subment was subjected to sealing by means of boiling water. J' ed t0 Seali g treatment by use of boiling water. The The film had a thickness of about 60,5 and a hardness of film had a thickness of about 60,71 and a hardness of about about Hvz370 in terms of Vickers hardness. HvzSOO in terms of Vickers hardness.

glgg f fg EXAMPLES 9-15 AND COMPARATIVE EXAMPLES 3-5 Example 1 was repeated, except that the electrolyte composition was varied. The results obtained were as set Example 8 Was repeated, except that the elfictrolyte forth in Table 2. composition, the treatment temperature and the treatment TABLE 2 Current Electrolysis Thick- Hardness Kind density ness of of oxide of Electrolyte composition (amp/ Temp. Time oxide film alloy (wt. percent) dmfl) C.) (min.) film (p) (Hv) Example number:

2 1 {gapgthalene-trisulfonic acid 13 2'5 10 60 60 400 1 HZSO 2.5 10 e0 50 320 1 {H230 2. 5 1o 60 40 300 1 2 5 10 5o 40 350 H250 2.5 10 5o 20 150 {H280 2.5 10 25 150 From Table 2, it is understood that the films formed time were varied. The results obtained were as set forth according to the present process are greatly difierent in in Table 3.

TABLE 3 Current Electrolysis Thiek- Hardness Kind density ness of of oxide 0! (amp./ Temp. Time oxide film alloy Electrolyte composition (wt. percent) dmfl) 0.) (111111.) film (p) (Hv) Example number:

Naphthalene-disuli'onic acid 15 9 igrso u nn a3} a one sn 0 c ac 1o 4 5 2.5 5-7 40 40 450 4 {H280 2. 5 5-7 40 40 470 4 {H250 2. 5 10-12 55 420 {H2804 20 2. 5 5-7 60 60 510 C00 1% {H2804 2. 5 5-7 00 45 480 {H250 20 2. 5 10-12 50 440 15 2. 5 10-12 60 0-50 410 4 llgzsggfiin ainifnnauniaap ene su o eac 4 {H280 2.5 5-7 50 e 380 The aluminum alloys 2, 3 and 5 shown in Table 1 were subjected to anodic oxidation treatment according to the present invention. The results obtained were as set forth in Table 4.

2 A process according to claim 1, wherein the electrolyte contains 10 to 25% by weight of the aromatic sulfonic acid and to by weight of sulfuric acid.

3. A process according to claim 1, wherein the aluminum alloy contains 6% by weight or more of silicon and 1% by weight or more of copper.

4. A process according to claim 1, wherein the anodic oxidation treatment is effected at a temperature of from 0 to 15 C.

5. A process according to claim 1, wherein the anodic oxidation treatment is efiected at a current density in the range from 1 to 4 amp./dm.

TABLE4 Current Electroysis Thick- Hardness Kind density ness of of oxide What is claimed is:

1. A process for forming a hard anodic oxide film on an aluminum alloy, characterized by subjecting an aluminum base alloy containing 1% by weight or more of copper to anodic oxidation treatment at a temperature below 20 C. using an aqueous electrolyte containing 5 to 40% by weight of an aromatic sulfonic acid selected from the group consisting of sulfosalicyclic acid, naphthalene-disulfonic acid, naphthalene-trisulfonic acid,

6. A process according to claim 1, wherein the electrolytic bath is further incorporated with oxalic or maleic acid.

References Cited phenol-sulfonic acid and sulfophthalic acid and 15 to GERALD KAPLAN Primary Examiner by weight of sulfuric acid.

R. L. ANDREWS, Assistant Examiner