US 3365380 A
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United States Patent 3,365,380 ELECTROLYTIC POLISHING SOLUTION AND THE PROCESS OF POLISHING THEREWITH Yasuichi Shihasaki, Yokohama, Japan, assignor to Kyowa Hakko Kogyo Co., Ltd., Tokyo, Japan, a Japanese corporation No Drawing. Filed Feb. 24, 1964, Ser. No. 347,061
Claims. (Ci. 204-1405) The present invention relates to an electrolytic polishing solution and the process of polishing therewith, more particularly to the electrolytic polishing solution for effecting a pear-like appearance on a surface of aluminum and aluminum alloys and the usage thereof. The term pear-like appearance used in this specification means a surface similar to pear-skin, or uneven, and it is to be construed in a somewhat broader sense including coarsely uneven and finely uneven surfaces with or without luster. The term aluminum alloy means all-kinds of alloys composed of aluminum as a chief component.
One object of the present invention is to develop a pear-like appearance on the surface of aluminum and aluminum alloys by electrolyzing.
Another object is to provide a chemical composition of electrolytic solution useful for the production of pearlike appearance on the surface of aluminum and aluminum alloys.
Further object is to provide a novel and economical process for obtaining aluminum and aluminum alloys with pear-like surface by electrolytic polishing.
In the prior art for polishing an aluminum surface to a pear-like appearance, mechanical methods such as blasting of sand, Alundum, Carborundum or other particles onto the surface and brushing the surface with a turning wire brush have been widely adopted. These prior methods, however, are disadvantageous because they not only are intricate but also often give unfavorable stresses on the surface of metals.
The present inventor conducted research on electrolytic polishing of aluminum and aluminum alloys, and has found that when the electrolytic polishing of aluminum and aluminum alloys is carried out by using an electrolytic solution containing 3080% of sulfuric acid and a substance having the property of adhering to the surface of aluminum and aluminum alloys during electrolysis, the surface of the metal is polished to give pearlike appearance.
Electrolytic polishing by using sulfuric acid has already been known, but such polishing has been conducted to polish the metal surface to give mirror like appearance, and the application of such electrolytic polishing to pearlike appearance has never been reported.
As above-mentioned, the present inventor has found a novel process for applying electrolytic polishing to produce pear-like appearance.
The electrolytic solution in the present invention is a solution containing 30-80% of sulfuric acid and'a substance having the property of adhering to the surface of metal during electrolysis, and the electrolysis is carried out by using aluminum and aluminum alloys as anode. In the electrolysis of the present invention, the adhering substance adheres nonuniformly to the surface of the metal during electrolysis or maintains small bubbles of generated gas or oxygen at such surface, thus the surface of metal is dissolved nonuniformly and gives a pear-like appearance. This is the most important principal of the present invention. As to the concentration of sulfuric acid,
30-80% may be used and 5070% is most favorable.
When the concentration of sulfuric acid is lower, the surface of metal is covered with oxide film and does not give a good pear-like appearance.
3,305,380 Patented Jan. 23, 1968 As a result of studies, it was found that good pear-like appearance is not obtainable at a sulfuric acid concentration lower than 25%, and inversely at a concentration higher than 80% good pear-like appearance can be obtained technically but from the industrial viewpoint this condition has little practicability because the high viscosity of the sulfuric acid decreases the efiiciency of polishing work. Consequently, the concentration of sulfuric acid is suitable to be in the range of from 30% to 80% by weight and the concentration of from 50% to is recommended as an optimum range for practical purposes.
In some cases, a part of the sulfuric acid may be replaced by phosphoric acid, although it is necessary to keep the phosphoric acid concentration lower than 30%. The existence of sulfate, such as sodium sulfate and aluminum sulfate, in electrolytic solution is not injurious to developing of a good pear-like appearance, if the concentration of sulfate is not of a high value. As to the adhering substances, the substances added to the sulfuric acid solution are effective by themselves as the adhering substances but sometimes the added substances are converted in sulfuric acid and become effective.
As to the added substances which give the adhering effect, many kinds of materials may be used both singly and conjointly, and the following are examples: gluconic acid; gluconates; natural high molecular weight substances such as glue and agar; carbohydrates such as starch, cellulose, glucose, fructose, sucrose and maltose; tannic acid; higher fatty acids; higher fatty acid derivatives, saccharin; polyhydric alcohols such as glycerine; synthetic high molecular weight substances such as polyethylene and polyvinyl alcohol. The substances abovementioned are generally effective as the adhering substance, but some substances change in the sulfuric acid and the changed substances are also effective; moreover, in some cases, the changed substances are more effective than the originals. Furthermore, substances such as polyethylene become effective when changed. When gluconic acid or gluconate is used as the additive substance, the total concentration is not critical and even a low concentration of 0.05% to 10% gives satisfactory results. Higher concentrations of gluconic acid are acceptable technically but inconvenient to practical operation because of the high viscosity of the solution. When other substances than gluconic acid and gluconate are used as additives, the concentration is also not critical but 0.01-10% is favorable in practical operation.
As to the electrolyzing conditions, the temperature of the electrolyte is not critical, however, 3090 C. may be suitable, the optimum range is 60-80 C., and 525 volts is the most formable bath voltage. As before-mentioned, aluminum and aluminum alloys are the anode, and as the cathode, many kinds of electrodes may be used. Carbon and lead are the most preferable cathodes.
The anodic current density may vary depending upon the composition and temperature of the electrolytic solution but usually it shows maximum amperage at the starting period of electrolysis and then diminishes with lapse of time. At about one minute after initiation of the electrolysis the current becomes two-thirds to one-fifth of the initial value and thereafter approaches an approximately constant value asymptotically. When the current is 30 a.dm.- to 50 a.dm.- at first and 5 a.dm.- to 15 a.dm.- one minute later, a good pear-like appearance is attainable on an aluminum surface. If the aluminum anode is vibrated, the current density increases unstably and electrolyzing sometimes exhibits a tendency to give the disappearance of the pear-like appearance.
The time required for the electrolysis is two to ten minutes normally but it should more definitely be determined with due consideration for uniformity and unevenness of pear-like surface.
The conditions of pear-like appearance obtained on the finishing surface vary in practice according to the composition of the electrolytic solution, liquid temperature, purity of aluminum, composition of aluminum alloy and other conditions. In order to produce a uniform and line surface of pear-like appearance the several conditions above-mentioned must be kept within the optimum range in consideration of the correlation between them. Pearlike appearance on aluminum metal surface developed in accordance with the present invention is fine and beautiful. Compared with mirror-surface polishing, moreover, pear-like appearance by electropolishing offers an advantage in that small scars on the aluminum surface are not distinguished and makes short or completely unnecessary the buffing of the aluminum surface prior to the electrolysis.
It falls Within the scope of knowledge of engineers concerned that post-treatment such as surface oxidizing, dyeing and sealing can be applied on the aluminum surface of pear-like appearance. Moreover, in the present process, the pear-like surface is often anodized and is dyed with dyestuif solutions; therefore, the anodic oxidation may be omitted.
The following examples show the nature of the present invention in greater detail. They are only for the understanding but not for the limitation of the present invention.
Example 1 A solution consisting of 300 kgs. of sulfuric acid, 200 kgs. of water and 2 kgs. of gluconic acid is used as the electrolyte (electrolytic solution). An aluminum plate having 2 mm. thickness and 30 dni. surface area is used as anode, and lead electrode is used as cathode. The electrolysis is carried out at 60 C., the bath voltage is 9-12 volts, and the current density is 60-20 a.dm.- After 3 minutes, the aluminum plate is taken out and washed with water. The surface of the plate shows fine pear-like ap- What I claim is:
1. A method for electrolytically polishing a metal selected from the group consisting of aluminum and alum-inuni alloys to effect a pear-like appearance thereon which comprises electrolyzing a solution consisting essentially of Water, from 30 to 80% by weight of sulfuric acid and an effective amount of an additament substance selected from the group consisting of gluconic acid, alkali metal gluconates, glue, agar, starch, cellulose, glucose, fructose, sucrose, maltose, polyethylene, polyvinyl alcohol, tannic acid, stearic acid, stearates, benzoates, glycerin and saccharin at a temperature of from 30 to 90 C. and a voltage of from 5 to volts, the metal to be polished constituting the anode, until the desired pear-like appearance is attained.
2. The method of claim 1, wherein the current density during the electrolysis is from 5 to 70 amperes per square decimeter.
3. The method of claim 1, wherein said solution also contains up to by weight of phosphoric acid.
4. The method of claim 1 wherein said additanient substance is a member selected from the group consisting of gluconic acid and alkali metal gluconates.
5. The method of claim 1, wherein said additament substance is a member selected from the group consisting of glue and agar.
6. The method of claim 1, wherein said additament substance is a member selected from the group consisting of starch, cellulose, glucose, fructose, sucrose and maltose.
7. The method of claim 1, wherein said additament substance is a member selected from the group consisting of polyethylene and polyvinyl alcohol.
8. The method of claim 1, wherein said additament substance is a member selected from the group consisting of tannic acid, stcaric acid, stearates and benzoates.
9. The method of claim 1, wherein said additament substance is glycerin.
pearance with luster.
Example 2 Run Composition of Electrolytic solution Temp. Voltage Current Time Material to be No. (ratio by weight) C.) (v.) (D insity (min.) polished Results 1 Sulfuric.3 acid, 56; Water, Gluconic 70 9-12 54-18 3 99.9% A1 Lustrous pear appearance.
Duralumin.. Grey-colored, coarse, slightly uneven pear appearance. 2 Phosphoric acid, 24; Sulfuric acid, 52; 72 8 67-18 3 99.5% Al Semi-lustrous, some-what non- Water, 90; Sodium gluconate, 10. uniform peer appearance. 3 Sulfuric acid, 85; Water, 45; Gluconic 45 8 20-30 10 99.7% Al Semi-lustrous, non-uniform acid, 2. slightly uneven pear appearonce.
4 Sulfuric acid, Water, 69; Gluconic 62 7. 5 40-24 3 99.9% Al Lustrous, somewhat nonacid, 20. uniform pear appearance.
5 Sulfuric acid, 60; Water, 40; Glycerine, 70-80 10-12 35-9 3 99.7% A1 Lustrous, good, fine, uniform 5-10. pear appearance.
6 Sulfuric acid, Water, 45; Z-ethylhex- 70 10 55-19 3 99.7% Al Lustrous, good, uniform pear ylammonium stearate, 0.5. appearance.
7 Sulfuric acid, Water, 40; Saccharin, 70-75 11 50-8 3 99.7% Al Lustrous, good, finely shallow,
1.0. h uniform pear appearance.
8 511611511110 acid, 60; Water, 40; Stearic acid, 70 9 50-10 4 99.7% Al Lustrous, good, uniform pear a carancc.
9 SuIlfuric acid, 60; Water, 40; Tannic acid, -75 10 45-6 4 99.7% Al 3 1 30.
10 Sulfuric acid, 55; Water, 45; Agar, 0.5.-.. 60-75 12 -12 3-4 99.7% A1 Semi-lustrous, good, slightly coarse, uniform pear appearonce.
11 Sulfuric acid, 55 ;Water, 45; Glue, 0.5 72 11-12 56-21 3 09.7% Al Lustrous, good, slightly coarse,
uniform pear appearance. 12 Sulfuric acid, 55; Water, 45; 2-ethylhex- 70 10 40-20 3 Duralumin.. Semi-lustrous, shallow, coarse, ylainmonium benzoate, 0.5. somewhat non-uniform pear appearance. 13 Sulfuric acid, 55; Water, 45; Sawdust 11 50-20 3 99.7% Al Lustrous, good, uniform pear (cellulose), 1. appearance. 14 Sulfuric acid, 60;Water, 40; Polyethylene 65-70 10 55-10 3 99.7% Al Lustrous, good, fine, uniform (but slightly soluble), 0.1. pear appearance.
15 Sulfuric acid, 55; Water, 45; Polyvinyl 70-75 5-10 40-10 3 99.9% AL..- Lustrous, good, uniform pear alcohol, 0.15. appearance.
16 ..do 70 5-10 40-10 3 Duralumin.... Grey-colored, slightly uneven pear appearance.
10. The method of claim 1, wherein said additament substance is saccharin.
References Cited UNITED STATES PATENTS Chester 204145 Lagut 260-301 Easley 252-79.4 Hesch 204-140.5 McDonald 252-794 Turner 204140.5
6 9/1956 Easley et a1 252-79.4 1/1961 Cybriwsky et a1. 204140.5 10/1963 Parkinson 25279.4 11/1964 Neunzig 204140.5
FOREIGN PATENTS 7/1952 Great Britain.
ROBERT K. MIHALEK, Primary Examiner. JOHN H. MACK, Examiner.