GB2067958A

GB2067958A - Etching composition

Info

Publication number: GB2067958A
Application number: GB8102244A
Authority: GB
Original assignee: Albright and Wilson Ltd
Current assignee: Solvay Solutions UK Ltd
Priority date: 1980-01-24
Filing date: 1981-01-26
Publication date: 1981-08-05

Abstract

Alkaline aluminium etch baths contain alkali, chelating agents such as gluconate, and alkali metal dithionite. The etched aluminium articles have improved etch over those without dithionite with increased bath life.

Description

SPECIFICATION Aluminium etching bath and process for etching This invention relates to compositions for etching aluminium, methods of etching aluminium and articles etched therewith, in particular alkaline etching treatments.

In the commercial etching of aluminium and aluminium alloys to remove surface imperfections particularly lines caused by imperfect extruding dies, it is known to use alkaline etching baths containing sodium hydroxide. It is known to use such baths containing also chelating agents, preferred examples of which are sodium gluconate and sodium glucoheptonate, to prevent the formation of a hard alumina scale when the dissolved aluminium concentration reaches 20-30 gil Al. During use of such baths, the concentration of aluminium and other metals, which have been dissolved from the aluminium surfaces, increases and a coarsening of the etched finish is observed; this is commonly referred to as "galvanising".After extended use, e.g. after a week, the bath may contain sufficient dissolved aluminium, and particularly zinc an impurity in the aluminium, to produce etched articles with this coarse finish. Such a bath is often then discarded.

We have now found that addition of dithionite to an alkaline aluminium etching bath containing also a chelating agent can reduce the tendency to galvanising, thereby extending the life of the bath.

The present invention provides an alkaline etching bath, suitable for etching aluminium and alloys thereof, which comprises an alkali metal hydroxide, a chelating agent for aluminium and an alkali metal dithionite. There is also provided a method of etching an aluminium surface of an article which comprises treating said surface with an etching solution comprising an alkali metal hydroxide, a chelating agent for aluminium and an alkali metal dithionite.In particular there is provided a method of etching aluminium surfaces of articles on a continual basis, wherein the aluminium surfaces of said articles are continually treated with said etching solution in an etching bath, and the concentrations of alkali, chelating agent and dithionite in said solution are intermittently maintained in the desired range of concentrations by addition of further alkali, chelating agent and dithionite respectively. There is also provided an article having a surface of aluminium, said surface having been etched by the method of this invention or by means of an etching bath of this invention.Finally the present invention provides a solid product comprising an alkali metal hydroxide, a chelating agent for aluminium and an alkali metal dithionite, said product being capable of being dissolved in water to form an alkaline etching bath of the present invention; this product is usually a mixture of the ingredients but a 2 pack system may be used with the hydroxide in one pack and the dithionite in the other pack and the chelating agent in either pack, the 2 packs being such that on mixing of the contents of the packs the solid product is obtained or on addition of the contents of the packs to water an alkaline etching bath of the invention is obtained.

The alkali metal hydroxide may be a hydroxide of potassium or lithium, but sodium hydroxide is preferred. The etching solution may contain an amount of alkali metal hydroxide up to 200 g/l, e.g. 5-200 g/l but is usually 10-100 g/l, e.g.

20-60 g/l. The amount of alkali metal hydroxide in the etching bath quoted here excludes any which is in the form of an alkali metal aluminate i.e. only "free" hydroxide is considered.

The chelating agent is a compound which reduces the tendency of alkaline aluminium etching baths to deposit alumina during extended use; it is therefore a chelating agent for aluminium. Such compounds are often hydroxyacids such as salts, e.g. alkali metal salts of gluconic, tannic, heptonic, glucoheptonic, boroheptonic, tartaric, hexahydroxyheptonic, mucic, citric, itaconic, aconitic, lactic and.

saccharic acids, and polyhydroxy compounds such as mannitol, sorbitol, glucose, lactose, gum arabic, corn starch, and glycols; a salt of ethylene diaminetetraacetic acid may also be used, e.g.

sodium EDTA. Sodium gluconate or sodium glucoheptonate are preferred. The chelating agent is soluble in the etching bath.

The concentration of chelating agent in the etching solution may be 0.1-50 g/l, e.g.

0.1-5 g/l or 0.5-2.5 g/l (expressed by weight as sodium giuconate) and pro rata molar amounts of other chelating agents. The weight proportion of alkali metal hydroxide to chelating agent is usually 5:1 to 200:1, e.g. 10:1 to 200:1, particularly when the hydroxide is sodium hydroxide and especially when the chelating agent is a sodium salt of a hydroxy acid, e.g. gluconic acid.

The dithionite is an alkali metal salt, e.g. a sodium salt; dithionite salts are often also called hydrosulphite salts and have an anion of formula S204=. The etching solutions may contain 0.1-50 g/l dithionite (expressed in terms of the weight of sodium dithionite) or pro rata molar amounts of the other dithionites, e.g. 0.2-1 0 g/l, such as 0.5-5 g/l dithionite.

The invention is of particular use in relation to etching baths containing aluminium, e.g. those achieved after a fresh bath has been used for a period of time. Particularly important are those baths, which if they did not contain the dithionite would give galvanizing. The baths preferably contain at least 25 g/l Al e.g. 25-80 g/l Al, 3.g.

30-60 g/l Al, and usually amounts of zinc, copper, magnesium, iron and/or silicon at least those which will be produced by dissolving N3 or H9 aluminium alloy in an alkaline bath of 50 gjl NaOH and 2 g/i sodium gluconate. Such baths containing at least 25 g/l Al tend to give a better finish to the etched surface than corresponding baths without aluminium, and hence, in a commercial operation, in order to optimize the finish from start up with a fresh bath, it is desirable to dissolve scrap aluminium to the bath to adjust the Al level from the start or aiternatively only a portion of a used bath is discarded and the remainder is mixed with fresh bath in order to provide the bath of desired aluminium content.

The etching bath (or the solid product for making it) may also contain other additives such as an etch modifier, e.g. a metallic salt or a fluoride or nitrate, or a spray suppressant such as an anionic surfactant. Preferably, however, oxidizing etch additives such as nitrate are substantially absent.

The aluminium forming the surface to be treated is usually an aluminium alloy or commercial purity aluminium the latter containing 60.1% Cu, 0.5% Si,#0.7% Si, S0-7% Fe, Fe,60.1% Mn and 0.10/0 Zn, to a total of 1%, the remainder being Al. Alloys are made by addition to that alloy of either up to 6% Mg, or up to 6% Si or up to 2% Fe or up to 2% Mn; or 61 % Cr and/or 60.1 % Ti may be present in the alloys. Alloys N3 and N4 (according to BS1470) and H9 and H30 (according to BS 1474) are preferred; the alloys are also internationally designated 3103, 5251, 6063 and 6082 respectively.

Particularly important are alloys with 0.32% Mn, such as N3 and N4. Examples of suitable aluminium surfaces are those of sheets made e.g.

of alloy N3 or N4, or extrusions made e.g. of alloy H9 or H30. The aluminium surface is preferably cleaned before the etching method of the invention, and then rinsed with water; the cleaning may be with a conventional acid or alkali aluminium cleaner.

In the method of the invention the aluminium surface to be treated is contacted, e.g. by immersion or spraying with the etchant solution, which may be at 40-1 000C preferably 60-800C. The treatment time at the above temperatures depends on the depth of etch wanted and the temperature and concentration of the etchant solution; however 1-20 mins, is a usual treatment time. At the end of the treatment the aluminium surface is preferably rinsed with water, before usually being desmutted often with acid, and anodized.

In the etching the treatment is usually by immersion of the article having the aluminium surface in the bath of solution, protecting if necessary any surfaces of the article not to be treated. During continual commercial operation from start-up, the metal content of the bath increases until equilibrium conditions are reached when the loss of metal due to drag-out due to liquid adhering on the surface of the withdrawn articles equals gain in metals, particularly aluminium and zinc, etched from surfaces of incoming articles. The drag-out of the alkali, compiexing agent and dithionite is compensated for by intermittent addition of the appropriate fresh compounds to maintain the content of each in the bath within the desired range.

The method of the invention may give an article with an aluminium surface having a pleasing etched finish, which may have a more matt finish and with better die line hiding than a bath based on alkali and gluconate only, particularly with the lower purity N3 and N4 alloys and with a bath containing 25-50 g/l sodium hydroxide and at least 25 g/l dissolved aluminium, e.g. 30-80 g/l Al at 70-800C. This improved die line hiding may be carried out with a shallower etch (the same contact time as without the dithionite but a slower etch rate) and hence less metal removed leading to a longer bath life and reduced consumption of bath ingredients.

The invention is illustrated in the following Examples, in which the alloys H9 (extrusion HE9) and N3 (sheet NS3) had the following analysis: H9. 60. 1% Cu, 0.4-0.9% Mg, 0.30.7% Si, 60.4% Fe, 60. 1% Mn, 0.2% Zn, #0.2% Ti, 60. 1% Cr the remainder aluminium, and N3 0.1% C, Q0.1% Mg, Q0.6% Si, Q0.7% Fe, 0.81.5% Mn, 0.2% Zn, 60:2% Ti and Cr combined and the remainder aluminium.

EXAMPLE 1 AND COMPARATIVE EXAMPLE A 2 fresh aqueous etchant baths were made up.

Solution 1 for comp. Ex. A contained sodium hydroxide (50 g/l) and sodium gluconate (5 g/l) and solution 2 Ex. 1 contained sodium hydroxide (50 g/i), sodium gluconate (5 g/l) and sodium dithionite (2 g/l).

Cleaned extrusions of aluminium alloy HE9 were immersed in solution 1 or solution 2 which were maintained at 650C, with a treatment time of 10 mins. After the etch the extrusion was rinsed in water, treated in 1:1 HNO3/water to remove smutt and rinsed again.

The same baths, solutions 1 and 2, were used to etch many extrusions, fresh alkali being added to compensate on an equimolar amount with the gram atoms of aluminium etched. Fresh gluconate and, for solution 2 dithionite were added as necessary to maintain their concentrations.

The finish of the extrusions was periodically monitored after rinsing with nitrice acid (a 1:1 mixture of conc. HNO3 and water) to desmut it and then rinsing with water, prior to inspecting it.

When the concentration of aluminium in the solutions had reached 45 g/l, the finish on the extrusion etched in solution 1 was coarse and bright due to "galvanizing" and unacceptable while the finish on the extrusion etched in solution 2 (Ex. 1 of the invention) was fine and more matt and acceptable. The level of smutt on the extrusion treated with solution 2 (of the invention) was less than for the extrusion treated with solution 1. The solution 1 would in commercial operation have to have been discarded before this Al level was reached, while solution 2 could still be used.

EXAMPLE 2 The etching process as described in Ex. 1 was used with an etching bath (solution 3) containing sodium hydroxide (50 g/l), sodium gluconate (2.5 g/l) and sodium dithionite (2 g/l). Without the sodium dithionite "galvanizing" occurred after a few days continual operation, but with solution 3, continual operation was possible for at least 2 weeks, with periodic additions of fresh components for the solution to maintain the concentrations, and the extrusions produced had a consistent acceptable fine matt etch.

EXAMPLE 3 AND COMPARATIVE EXAMPLE B Aqueous etching baths were freshly made up containing the following compounds; Solution 4 (Comparative Ex. B), 35 g/l NaOH and 1 g/l sodium glucoheptonate; Solution 5, 35 g/l NaOH, 1 g/l sodium glucoheptonate and 1 g/l sodium dithionite.

Cleaned sheets of NS3 aluminium alloy were immersed in the above solutions which were maintained at 700 C, with a treatment time of 5 mins. After etching the sheets were rinsed in water, treated with dilute nitric acid (1:1 conc.

HNO3 :water) to desmutt them and rinsed again.

Solution 4 gave a moderately bright, smooth etch while solution 5 gave a noticeably more uniform, matt, white finish with less smutt.

EXAMPLE 4 AND COMPARATIVE EXAMPLES C AND D Solutions 4 and 5 were artificially aged by adding to each scrap HE9 aluminium extrusion (50 g/l), NaOH (74 g/l) and the appropriate amounts of sodium glucoheptonate and (for solution 5 only) sodium dithionite to make respectively solution 6 (Comparative Example C) and solution 7 (Ex. 4). Pieces of aluminium extrusion HE9 were processed as described for Ex. 3 including desmutting. The extrusions process in solution 6 showed considerable galvanizing i.e.

the surface was coarse and unacceptable, but those processed in solution 7 were entirely free from this defect and had a uniform matt appearance.

The etched aluminium from Solution 7 was compared with that obtained by etching pieces of aluminium extrusion HE9 with a portion of solution 6 to which sodium sulphide (1 g/l) had been added to make solution 8 (comp. Ex. D). The etch in the intrusion from solution 7 (of the invention) was more matt than that etched with solution 8 the difference being amplified when both extrusions were subsequently anodized in 10% v/v sulphuric acid to give a 25 micron anodic film thickness.

Solutions 7 and 8 were also used to etch pieces of aluminium sheet NS3 under the conditions as in Ex. 3 with desmutting. The etch from solution 7 (of the invention) was much more matt and had a whiter finish that that from solution 8.

EXAMPLE 5 AND COMPARATIVE EXAMPLE E Aqueous etching solutions were made up to contain the following ingredients. Solution 9 (Comparative Example E) 45 g/l NaOH 2 g/l Na gluconate 0 g/l Aluminium 600C time 10 mins.

Solution 10 (Example 5) 28 g/l NaOH 2 g/l Na gluconate 2 g/l Na dithionite 45 g/l Aluminium 800Ctime 10 mins. The aluminium content of the solution 10 was obtained by dissolving up scrap HE9 aluminium extrusion in the solution.

Pieces of HE9 aluminium extrusion were processed through the above solutions as in Example 3 but with 10 mins etch at 600C for solutions 9 and 10 mins at 800C for Example 5.

Afterdesmutting in 1:1 HNO3 and anodizing in 10% v/v H2SO4 to give 25 microns anodic film thickness the pieces were examined. Those processed through solution 10 of the invention had a more matt white appearance and the extrusion die lines were less visible than those processed through solution 9.

Measurement by weighing of the metal removed carried out on two identical pieces of H9 extrusion etched in the above solutions and desmutted but not anodized showed the metal removal to be the same in both cases despite the higher temperature, and despite the different finish obtained from solution 10.

Claims

1. An aqueous alkaline etching solution bath for etching aluminium and alloys thereof, which bath comprises an alkali metal hydroxide, a chelating agent for aluminium and an alkali metal dithionite.

2. A bath according to claim 1 which comprises 0.1-50 g/l alkali metal dithionite (expressed as sodium dithionite).

3. A bath according to claim 1 which comprises 0.2-10 g/l alkali metal dithionite (expressed as sodium dithionite).

4. A bath according to claim 3 which comprises 0.5-5 g/l alkali metal dithionite (expressed as sodium dithionite).

5. A bath according to any of claims 1 4 wherein the chelating agent is a salt of a hydroxy acid.

6. A bath according to claim 5 wherein the chelating agent is sodium gluconate or sodium glucoheptonate.

7. A bath according to claim 5 or 6 which comprises 0.150 g/l of chelating agent (expressed by weight as sodium gluconate).

8. A bath according to claim 7 which comprises 0.1-5 g/l of chelating agent (expressed by weight as sodium gluconate).

9. A bath according to any of claims 1-8 which comprises a weight proportion of alkali metal hydroxide (expressed as sodium hydroxide) to chelating agent (expressed as sodium gluconate) of 5:1 to 200:1.

10. A bath according to any of claims 1-9 which comprises 5-200 g/l alkali metal hydroxide (expressed as sodium hydroxide).

11. A bath according to claim 10 which comprises 20-60 g/l alkali metal hydroxide (expressed as sodium hydroxide).

12. A bath according to claim 1 substantially as described in any one of Examples 1-5.

13. A solid mixture comprising an alkali metal hydroxide, a chelating agent for aluminium and an alkali metal dithionite.

14. A mixture according to claim 13 wherein the amounts of hydroxide dithionite and chelating agent are such that when said mixture is dissolved in water, an alkaline etching bath as claimed in any one of claims 1-11 1 is obtained.

1 5. A two pack system comprising a first pack comprising an alkali metal hydroxide and a second pack comprising an alkali metal dithionite, with a chelating agent for aluminium in at least one of said first and second packs.

16. A two pack system according to claim 15 wherein the amounts of hydroxide, dithionite and chelating agent are such that when the whole contents of the packs are dissolved in water, an alkali etching bath as claimed in any one of claims 1-11 1 is obtained.

17. A method of etching an aluminium (or aluminium alloy) surface of an article, which comprises treating said surface with an etching bath as claimed in any one of claims 1-12.

18. A method according to claim 17 wherein said bath contains at least 25 g/l Al.

19. A method according to claim 18 wherein said bath contains 30-60 g/l Al.

20. A method according to any one of claims 1 7-19 wherein the aluminium surface is of aluminium alloy N3, N4, H9 or H30.

21. A method according to any one of claims 1 7-20 wherein said bath contains 25-50 g/l sodium hydroxide, sodium gluconate, sodium dithionite and 30-80 g/l Al and the treatment is at 60-800C.

22. A method according to any one of claims 17-21 wherein in a continual process said etching bath treats continually the aluminium surface of articles, and the concentrations of alkali metal hydroxide, chelating agent and dithionite are maintained in the desired range of concentrations by addition of further alkali metal hydroxide, chelating agent and dithionite.

23. A method according to claim 17 substantially as described in any one of Examples 1-5.

24. An article having a surface of aluminium, said surface having been etched by a method according to any one of claims 17-23, or by a bath according to any one of claims 1-12.