US3464870A

US3464870A - Aluminum polishing process

Info

Publication number: US3464870A
Application number: US467132A
Authority: US
Inventors: Gerald R Formsma; Louis G Gaspar
Original assignee: North American Aluminum Corp
Current assignee: North American Aluminum Corp
Priority date: 1965-06-25
Filing date: 1965-06-25
Publication date: 1969-09-02
Anticipated expiration: 1986-09-02
Also published as: DE1521908B2; GB1136588A; DE1521908A1

Description

United States Patent U.S. Cl. 156-21 5 Claims ABSTRACT OF THE DISCLOSURE The polishing of decorative and architectural aluminum alloy extrusions in an etchant solution results in removal of an extremely limited amount of material, of the order of .0005 in. to .001 in., for example. Hence the polishing process has been and is herein referred to as etching.

This invention relates to an improved process for etching aluminum alloys and, more particularly, relates to a process, as aforesaid, in which the etched products have a substantially uniform gloss finish from piece to piec and in which the etchant bath has a much longer useful life than it has heretofore been possible to achieve.

It is well known to etch aluminum alloy articles by immersing such articles in hot aqueous alkali solutions, e.g., a sodium hydroxide solution. The etched article is then treated with a suitable acid to remove smut, following which the etched article can be rinsed and dried or processed further such as in anodizing.

The following discussion with respect to aluminum alloy extrusions shall have reference to alloys of the following range of compositions:

Si, percent .2.8 Fe, percent (max.) .4 Cu, percent (max.) .2

The following discussion with respect to aluminum alloy sheets shall have reference to alloys of the following range of compositions:

Si-l-Fe, percent (max.)

Cu, percent (max.) .1

Mg, percent 2.2-2.8 15-.25

Cr All other elements, except aluminum, .l% max. for each element.

Al Balance A prime factor limiting the useful life of the etchant bath for etching aluminum extrusions of the composition aforesaid for decorative or architectural purposes is that sooner or later galvanizing? of the surfaces of the etched products will occur. Galvanizing as used herein refers to the phenomena according to which the grain located near the surface of the aluminum extrusion becomes relatively highly visible.

It may be noted here that the galvanizing problem appears to be limited to aluminum extrusions because aluminum alloy sheets, for example, as a result of their smaller grain size and other metallurgical factors, do not appear to be subject to galvanizing. Moreover, aluminum alloy castings, although having a larger grain size are subject to having nonuniform composition and porosity and therefore have only limited use for decorative or architectural purposes. The previous theory has been that 3,464,870 Patented Sept. 2, 1969 galvanizing is the result of aluminum build-up in the etchant bath. It has been considered that the maximum aluminum concentration in the etchant bath that can be tolerated, and still entirely avoid galvanizing, is approximately 25 g. of dissolved aluminum per liter.

While it has long been desirable that aluminum alloy extrusions and sheets of the compositions mentioned above, intended for architectural purposes, have a surface finish of uniform gloss, it has been diificult heretofore to achieve such a finish which is uniform from piece to piece. This is largely due to the fact that as the amount of dissolved aluminum in the bath increases, the activity of the bath changes. In working with this problem, we have discovered that etched finishes of substantially uniform gloss can be obtained on aluminum alloy extrusions and sheets, of the compositions mentioned above, provided that the etchant bath contains at least about 50 gms. of dissolved aluminum per liter and further that such etched finishes Will remain substantially uniform from piece to piece so long as the dissolved aluminum concentration in the bath is substantially constant during an extended production run. Uniform gloss is achieved using production baths having a substantially constant dissolved aluminum concentration so long as same is within the range of 50 to or more gms. per liter.

The gloss of aluminum extrusions can be measured by a suitable gloss meter, such as one manufactured by Photovolt Corporation. Such meters are graduated to indicate the gloss of the surface measured. Readings can be based on a perfect mirror finish having a reading of 1000. The aluminum alloy extrusions and sheets produced by the present invention under typical conditions have a gloss measured at 45 to the surface of about when the dissolved aluminum concentration in the bath is 50 g. per liter and they have a gloss of about 200 when the dissolved aluminum concentration is about 80 g. per liter. For high quality, uniform gloss work, the gloss reading should be maintained within a range of not more than about :20% of the median gloss reading.

In carrying out the etching process using an etchant bath containing at least 30 g. of dissolved aluminum per liter, the galvanizing problem on aluminum extrusions referredto above has been encountered and this has necessitated frequent changes of the etchant bath. This represents a substantial cost item not only because of the cost of the etchant material itself but also because of the time and the labor cost involved in changing the bath.

While it would appear, from the prior knowledge, that the problems of eliminating galvanizing and at the same time achieving a uniform gloss finish on aluminum alloy extrusions of the composition referred to above using a long-lived etchant bath involved mutually inconsistent solutions, we have discovered that these problems canv be solved simultaneously if there is added to the etchant bath a suitable material for insuring the presence of sulfide ions therein. We have discovered that the cause of the galvanizing problem in etchant baths having a high dissolved aluminum concentration is the dissolved zinc which also becomes present in the etchant bath. As little as four or five parts per million of zinc in the etchant bath containing a high dissolved aluminum concentration is capable of causing galvanizing. The aluminum alloy that is being etched is the major source of the dissolved zinc, although other possible sources include the make-up water supplied, and impurities in the proprietary etchant. A typical value for zinc which is present in 6063 aluminum alloy is'.02%. It has been found that the presence of small amounts of sulfide ions in the etchant bath will precipitate the zinc sulfide and that such substantially eliminates the problem of galvanizing in etchant baths having a high dissolved aluminum concentration.

According to the invention,'there is provided a method of etching an aluminum alloy product which comprises treating the product with an alkali etching solution having dissolved therein at least about 50 g. of aluminum per liter and also having present therein a positive concentration of sulfide ions. For uniform gloss products of either the extrusion or sheet type having the compositions referred to above, the minimum dissolved aluminum concentration is 50 g. per liter. The etched surface of an extrusion thus treated is free of galvanizing effects caused by the etch bath and is smooth and has a uniform gloss as the bath ages further. This gloss will be substantially uniform for all aluminum alloy extrusions and sheets treated by the same type of bath under similar conditions of time, temperature and alkali concentration in the bath. Moreover, the useful life of the bath will be of indefinite duration, provided that suitable amounts of make-up chemicals are added thereto from time to time to maintain the desired concentrations of such chemicals.

The method according to the invention can be carried out on aluminum sheets or extrusions of the compositions referred to above, it being noted here that the invention does not pertain to the treatment of other aluminum alloys. An aluminum alloy which provides especially good results is the commercially available alloy 6063. However, other 6063 type aluminum alloys can be etched according to the invention to obtain similar improved results.

The etching treatment is carried out in a hot aqueous alkali solution containing, for example, sodium or potas sium hydroxide, or other alkaline materials, sodium hydroxide, being preferred. The alkali concentration of the etchant bath is not critical and such concentration can range from 2 oz. alkali/ gallon of Water to oz. alkali/ gallon of water. The alkali solution also can contain various conventional noncritical additives, such as sodium gluconate, which are added to improve the etching effect, prevent scale formation, etc.

The alkali solution must contain at least about 50 g. of dissolved aluminum per liter in order to achieved the desired uniform gloss on the surface of the treated aluminum alloy products. Lesser amounts of dissolved aluminum in the etchant bath will cause variations in the gloss of the etched extrusion which are diflicult to control. The maximum dissolved aluminum concentration in the etching bath is limited only by the ability of the etchant solution to retain the aluminum in solution. This is affected by a variety of factors, including particularly the temperature of the solution. It has been found that when a suitable temperature is maintained, the concentration of the dissolved aluminum in the etchant bath can be as high as about 90-105 g. per liter Without difficulty, and even higher dissolved aluminum concentrations can be used in appropriate cases.

As indicated above, there will be some variation in the gloss level depending on the concentration of the dissolved aluminum in the etchant bath. During start up with a fresh etchant bath, it will be advisable to build up the concentration of the dissolved aluminum to the desired level before etching the parts which require a uniform gloss. This can be done by etching scrap aluminum pieces, by using the bath to etch pieces which do not require a uniform gloss, or by employing part of an existing bath as make up for the new bath.

While aluminum is continuously being dissolved in the etchant bath as the extrusions are etched so that the aluminum concentration of the bath tends to rise, this presents no serious problem because some of the dissolved aluminum is continually being removed from the etchant bath with the etched products. That is, a certain amount of the etchant bath will adhere to the etched extrusion or will collect in pockets therein and the same will be removed from the remainder of the bath along with the extrusion. This removal of the part of the etchant is commonly termed drag out. It is customary to add fresh etching solution to the bath from time to time in order to make up for the part that is removed from the bath with the etched product and to make up for that consumed in one chemical reaction. Since the make-up etching solution is free of aluminum, the aluminum concentration in the etchant bath is thereby maintained within fairly constant limits because the rate at which the aluminum is dissolved from the etched products becomes more or less equal to the amount of dissolved aluminum that is removed from the bath as a result of the drag out.

The temperature of the etchant bath may range from F. to 180 F. It has been found satisfactory to use etching times of from 1 to 30 minutes.

A positive sulfide ion concentration is maintained in the etchant bath at all times in order to precipitate Zinc as zinc sulfide. Because of the low solubility of zinc sulfide in alkaline aqueous solutions, it is possible to main tain the residual zinc concentration in the bath at a sufficiently low level that galvanizing does not occur. The sulfide ion concentration is continuously maintained at a level of from four to eight gms. per liter (calculated as sodium sulfide) in order to assure precipitation of the zinc. A higher level of the sulfide concentration can be maintained, if desired, but such is not necessary. Similarly, lower concentrations of the sulfide ions can be used, provided that at all times there is a positive sulfide concentration in the etchant bath, but lower amounts ncrease the risk that galvanizing will occur. Desirably the sulfide material is added to the bath from time to time to make up for the part thereof which precipitates with the zinc and other alkali insoluble metallic sulfides and, also, the part that leaves the bath with the etched product as a result of the drag out. The sulfide material will be required at a rate of one to ten gms. for each 100 gms. of the make-up etchant material added to the bath. Examples of the sulfide ions supplying material that can be used include hydrogen sulfide, alkali sulfides and polysulfides, sodium sulfide being preferred. These materials may be employed individually or as a mixture of two or more of these materials.

It will be understood that the so-called smut remaining on the surface of the extrusion after etching can be removed in any suitable convenient fashion. As this procedure is conventional and well known and forms no part of the inventionya detailed description thereof is believed unnecessary.

Gloss differences can be minimized by anodizing or other after treatment.

Examples (I) An extrusion of 6063 alloy was etched for 10 minutes at F. in an aqueous solution containing 5 oz. per gallon of NaOH and 2 gms. per liter per gallon of sorbitol. No dissolved aluminum was present in the solution. The extrusion was then rinsed in tap water and desmutted at ambient temperature in 30% by weight nitric acid. No galvanizing occurred and the gloss measurement was 215.

(II) Another sample of the same extrusion was etched for 10 minutes at 130 F. in an aqueous solution containing the same etchant as above but with 20 gms. per liter dissolved aluminum. The extrusion was then rinsed in tap water and desmutted at ambient temperature in 30% by weight nitric acid. No galvanizing occurred and the gloss measurement was 120.

(III) A third sample of the same extrusion was etched for 10 minutes at 130 F. in an aqueous solution containing the same etchant and 40 gms. per liter dissolved aluminum. The extrusion was then rinsed in tap water and desmutted at ambient temperature in 30% by weight nitric acid. Galvanizing occurred which resulted in scrap material.

(IV) A fourth sample of the same extrusion was etched for 10 minutes at 130 F. in an aqueous solution containing the same etchant, 40 gms. per liter dissolved aluminum, and 4 gms. per liter sodium sulfide. This solution was the same as that used in Example III with 4 gms. per liter sodium sulfide added. The extrusion was then rinsed in tap water and desmutted at ambient temperature in 30% by weight nitric acid. No galvanizing occurred and the gloss measurement was 150.

(V) A fifth sample of the same extrusion was etched for minutes at 130 F. in a duplicate of the solution used in Example II, which had added to it 6 parts per million zinc. The extrusion was then rinsed in tap water and desmutted at ambient temperature in 30% by weight nitric acid. Because of galvanizing, the sample was rejected.

(VI) A sixth sample of the same extrusion was etched for 10 minutes at 130 F. in the same solution used in Example V, which had added to it 4 gms. per liter sodium sulfide. The extrusion was then rinsed in tap water and desmutted at ambient temperature in 30% by weight nitric acid. No galvanizing occurred and the gloss measurement was 105 as measured in the same manner as sample I.

(VII) A seventh sample of the same extrusion was etched for 10 minutes at 130 F. in an aqueous solution containing 5 oz. per gallon of NaOH, 2 gms. per liter sorbitol, 50 gms. per liter dissolved aluminum, and 4 gms. per liter dissolved Na S. The extrusion was then rinsed in tap water and desmutted in 30% by weight HNO acid. No galvanizing occurred and the gloss measurement was 170 as measured in the same manner as sample I.

(VHI) Repeat of VII using 70 gms. per liter aluminum. Gloss 200.

(IX) Repeat of VII using 90 gms. per liter aluminum. Gloss 220.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of polishing aluminum alloy extrusions which contain minute traces of zinc for decorative or architectural purposes in an etching solution to produce a uniform gloss finish thereon, to substantially prevent galvanizing of the surfaces thereof and to substantially extend the useful life of said etching solution, which includes immersing said extrusions in an alkali etching solution having dissolved therein at least about grams per liter of aluminum and having present therein sulfide ions sufficient to limit the presence of zinc to less than five parts per million.

2. A method according to claim 1, in which the maximum aluminum concentration in the etching solution is about gms. per liter.

3. A method according to claim 1, in which the sulfide ions are obtained by dissolving in the etching solution materials selected from the group consisting of sulfides and polysulfides of alkali metals.

4. A method according to claim 1, in which the sulfide ions are obtained by dissolving sodium sulfide in the etching solution.

5. A method according to claim 1, in which the temperature of the etching solution is maintained between 100 F. and F. and the aluminum alloy extrusions are immersed in the etching solution for from 1 to 30 minutes.

References Cited UNITED STATES PATENTS 3,300,349 1/1967 Tershin et al. 156-22 3,356,550 12/1967 Stiflier et al. 15622 JACOB H. STEINBERG, Primary Examiner