GB2328447A

GB2328447A - A desmutting solution for use prior to anodising

Info

Publication number: GB2328447A
Application number: GB9717310A
Authority: GB
Inventors: Peter Charles Morgan; Peter Nicholls
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1997-08-16
Filing date: 1997-08-16
Publication date: 1999-02-24
Also published as: EP0899361A2; GB9717310D0

Abstract

A desmutting solution for use in anodising an aluminium alloy surface comprises an aqueous solution containing from 0.01 to 5 percent by weight with at least rare earth metal salt such as cerium chloride or cerium nitrate and from 0.03 to 33 percent by weight of nitric acid. The desmutting solution may be used in conjunction with a sodium hydroxide etching step and an alkali di-sodium tetra borate anodising step to improve corrosion resistance of the anodic film.

Description

2328447 A DESNUTTING SOLUTION AND ANODISING TREATMENT PROCESS This

invention relates to a desmutting solution for treating a surf ace of an aluminium alloy work piece and a process for treatment of the work piece surface by anodising using the desmutting solution.

It is known to impart corrosion protection to aluminium alloys by the use of anodised films. - Conventionally aluminium alloy anodising processes involve the use of chromic acid which is environmentally undesirable because six valent chromium is carcinogenic. In order to improve the environmental acceptability of anodising processes it has been proposed that anodising of aluminium alloys should be carried out using alkali anodising and a process involving the use of an alkali sodium tetra borate anodising step has been disclosed in French patent application no. FR-A2714078. However this proposed technique is not suitable for all the aluminium alloys of use, for example, in the aerospace industry whilst the anodising process of FR-A-2714078 has been shown to protect a type 7075T6 aluminium alloy against a neutral salt spray test for over 336 hours which is the minimum European Aerospace Industrial Standard for this alloy, type 2014AT4 aluminium alloy could only be protected by this process against corrosion in a neutral salt spray test for a matter of 48 hours. Type 7075T6 aluminium alloy is an alloy of aluminium containing 0.10 percent silicon between 0. 2 and 1.9 percent copper, between 1.9 and 2.6 percent magnesium, between 0.18 and 0.25 percent chromium and between 5.2 and 8.2 percent tin with the balance apart from incidental constituents and impurities, being aluminium. Type 2014AT4 aluminium alloy contains, by weight, 0.50 to 1.2 percent silica, 0.7 percent iron, from 3.9 to 5 percent copper, from 0.4 to 1.2 percent manganese, from 0.2 to 0.8 magnesium with the balance, apart from impurities and incidental constituents being aluminium.

Whilst it is theoretically possible to improve the corrosion resistance of the known alkali anodising process with type 2014AT4 aluminium alloy by increasing the anodising voltage it has been found that in order to achieve corrosion resistance approaching the industry standard it is necessary to increase the voltage to in excess of safe levels f or a f actory environment so that any advantage gained through the removal of toxic chemicals from the factory floor is unacceptably achieved by introducing further hazards of high voltage usage.

There is thus a need for a generally improved anodising process for aluminium alloys which increases the corrosion resistance and is more suitable for use on a wider range of aluminium alloys than the existing processes but which does so in an environmentally safe manner.

According to one aspect of the present invention there is providing a desmutting solution for treating a surface of an aluminium alloy work piece, prior to anodising, to improve resistance of the surface to corrosion, comprising an aqueous solution containing from 0.01 to 5 percent by weight of at least one rare earth metal salt and f rom 0. 03 to 33 percent by weight of nitric acid.

Pref erably the at least one rare earth metal salt is a chloride and/or a nitrate, more preferably cerium chloride CeC13 and/or cerium nitrate Ce(N03)3.

Conveniently the desmutting solution contains 1 percent by weight of cerium chloride and/or cerium nitrate in a aqueous solution containing 50 percent by weight of nitric acid.

According to a further aspect of the present invention there is provided a process for treating the surface of an aluminium alloy work piece, including the steps of degreasing the work piece surface, etching the degreased surface with a sodium hydroxide solution, washing the etched surface, desmutting the washed etched surface using a desmutting solution according to the present invention, washing the desmutted surface, anodising the washed desmutted surface with an alkali borax solution, and washing the anodised surface.

Preferably the desmutting step includes treatment with a desmutting solution for 30 seconds at a temperature in the range of f rom 22 to 250C.

Advantageously the process is utilised to treat an aluminium alloy having the composition containing from 0.5 to 6 percent by weight copper and from 0.1 to 4 percent by weight magnesium, with the balance, apart f rom impurities and incidental constituents, being aluminium.

Preferably the degreasing step is carried out in an acqueous solution containing sodium hydroxide and sodium nitrate.

Conveniently the etching step is carried out in an aqueous solution containing 3 grams of sodium hydroxide per one litre of deionised water for 8 minutes at a temperature of substantially 25'C.

Conveniently the etched surface is washed in running water for substantially 2 minutes.

Advantageously the desmutted surface is washed in running water for substantially 5 minutes.

Conveniently the washed desmutted surf ace is anodised in a solution containing 175 grams of di-sodium tetra borate dissolved in 3.5 litres of deionised water at a temperature of 650C with the work piece forming an anode and with a sheet of stainless steel forming a cathode, at a voltage of up to 75 volts for a time of up to 30 minutes.

Preferably the anodised surface is washed in running water.

Advantageously the washed anodised surface is sealed by immersion in deionised water at a temperature of at least 960C for substantially 10 minutes.

Preferably the sealed surface is oven dried in air at a temperature in the range of from 80 to 1OWC for substantially 8 minutes.

It has been shown experimentally that the alkali anodising process of FRA-2714078 does not provide adequate corrosion protection against corrosion by salt spray for type 2014AT4 aluminium alloys and for other type 2000 aluminium alloys. It is thought that this is because the known process does not remove the second phase precipitate particles which are present in type 2000 aluminium alloys and that the level of corrosion produced by the neutral salt spray test is directly related to the number of these particles left in the aluminium oxide anodising film after anodising. This does not appear to be a problem if the desmutting solution according to the present invention is utilised in the treatment process additionally according to the present invention.

According to the present invention a desmutting solution for treating a surface of an aluminium alloy, such as a type 2000 aluminium alloy, prior to anodising to improve the resistance of the surface to corrosion comprises an aqueous solution containing from 0.01 to 5 percent by weight of at least one rare earth metal salt and f rom 0. 03 to 33 percent by weight of nitric acid. The at least one or more metal salts is a chloride and/or a nitrate. Preferably the at least one rare earth metal salt is cerium (triple valent) chloride CeC13 and/or cerium (triple valent) nitrate Ce(N03)3. Preferably the solution contains 1 percent by weight of cerium chloride and/or cerium nitrate in an aqueous solution containing 50 percent by weight of nitric acid (0.5M).

The process of the present invention for treating a surface of an aluminium alloy, such as type 2000 alloy, work piece, includes the steps of degreasing the work piece surface, etching the degreased surface with a sodium hydroxide solution, washing. the etched surface, desmutting the washed etched surface using a desmutting solution according to the present invention as set forth above, washing the desmutted surf ace, anodising the washed desmutted surface with an alkali borax solution, and washing the anodised surf ace. If necessary the washed anodised surf ace may be sealed and the sealed surfaces air dried. The desmutting step includes treatment with - the desmutting solution of the present invention for 30 seconds at a temperature in the range of f rom 22 to 25'C. Preferably the type of alloy treated by the process of the invention is an aluminium alloy having a composition containing from 0.5 to 6 percent by weight copper and from 0.1 to 4 percent by weight magnesium with the balance, apart from impurities and incidental constituents, being aluminium.

Degreasing is carried out in an aqueous solution of sodium hydroxide and sodium nitrate and the etching step is carried out in a aqueous solution containing 3 grams of sodium hydroxide per 1 litre of deionised water for 8 minutes at a temperature of substantially 250C.

The etched surface is washed in running water which preferably is cold for substantially 2 minutes. The surface is then desmutted using the solution of the invention as described above by treatment by 30 seconds in the desmutting solution at a temperature in the range of from 20 to 250C. The desmutted surface is then washed in running water, preferably cold, for substantially 5 minutes.

The washed desmutted surface is then anodised in a solution made up of 175 grams of di-sodium tetraborate dissolved in 3.5 litres of deionised water at a temperature of 650C with the work piece forming an anode- and with a sheet of stainless steel forming a cathode, at a voltage of up to 75 volts for a time of up to 30 minutes. Preferably the work piece is held at the process voltage for 30 minutes with the anodising starting within 5 minutes of the work piece being immersed in the anodising solution. A direct current power supply was utilised.

The anodised surface is then washed in running water, preferably cold water, to remove surplus solution from the surface followed by a rinse using a cold water hand spray. The washed anodised surface may then if desired be sealed by immersion in deionised water at a temperature of at least 960C for substantially 10 minutes. This hot seals the oxidic anodic coating forming the anodised surface which may then be hot air dried in an oven at a temperature in the range of from 80 to 1000C for substantially 8 minutes.

-g- An accelerated corrosion assessment carried out on samples of type 2014AT4 aluminium alloy anodised according to the process of the present invention using the desmutting solution of the present invention and assessed using an electro chemical potentio dynamic curve has shown that the corrosion resistance of the anodised surface produced at 20 volts is as good as that of an anodised surf ace of the same alloy produced by the hereinbefore described conventional process when worked at the environmentally unacceptable anodising voltage of 60 volts. It is possible that the improved performance of the process of the present application is due to the rare earth metal such as cerium changing the chemistry of the precipitates in the alloy therefore activating them for oxidation in the subsequent anodising step. The rare earth metal deposits during the desmutting step as a thin f ilm on the activated surf ace of the work piece.

Claims

A desmutting solution for treating a surface of an aluminium alloy work piece, prior to anodising, to improve resistance of the surface to corrosion, comprising an aqueous solution containing from 0.01 to 5 percent by weight of at least one rare earth metal salt and from 0.03 to 33 percent by weight of nitric acid.

A desmutting solution according to claim 1, in which the at least one rare earth metal salt is a chloride and/or a nitrate.

A desmutting solution according to claim 1 or claim 2, in which the at least one rare earth metal salt is cerium chloride CeC13 and/or cerium nitrate Ce(N03)3 4. A desmutting solution according to any one of claims 1 to 3, including 1 percent by weight of cerium chloride and/or cerium nitrate in a aqueous solution containing 50 percent by weight of nitric acid.

5.

A desmutting solution for treating the surface of an aluminium alloy work piece, substantially as hereinbefore described.

A process for treating a surface of an aluminium alloy work piece, including the steps of degreasing the work piece surface, etching the degreased surface with a sodium hydroxide solution, washing the etched surf ace, desmutting the washed etched surf ace using a desmutting solution according to any one of claims 1 to 5, washing the desmutted surface, anodising the washed desmutted surf ace with an alkali borax solution, and washing the anodised surface.

A process according to claim 6, in which the desmutting step includes treatment with the desmutting solution f or 30 seconds at a temperature in the range of f rom 22 to 250C.

8. A process according to claim 6 or claim 7, when used to treat an aluminium alloy having the composition containing from 0.5 to 6 percent by weight copper and from 0.1 to 4 percent by weight magnesium, with the balance, apart from impurities and incidental constituents, being aluminium.

9. A process according to any one of claims 6 to 8, in which degreasing is carried out in an aqueous solution containing sodium hydroxide and sodium nitrate.

10. A process according to any one of claims 6 to 9, in which etching is carried out in an aqueous solution containing 3 grams of sodium hydroxide per one litre of deionised water for 8 minutes at a temperature of substantially 25'C.

11. A process according to any one of claims 6 to 10, in which the etched surface is washed in running water for substantially 2 minutes.

12. A process according to any one of claims 6 to 11, in which the desmutted surface is washed in running water for substantially 5 minutes.

13.

A process according to any one of claims 6 to 12, in which the washed desmutted surface is anodised in a solution made up of 175 grams of disodium tetra borate dissolved in 3.5 litres of deionised water at temperature of 650C with the work piece forming an anode and with a sheet of stainless steel forming a cathode at a voltage of up to 75 volts for a time of up to 30 minutes.

14. A process according to any one of claims 6 to 13, in which the anodised surface is washed in running water.

15. A process according to any one of claims 6 to 14, in which the washed anodised surface is sealed by immersion in deionised water at a temperature of at least 960C for substantially 10 minutes.

16. A process according to claim 15, in which the sealed surf ace is oven dried in air at a temperature in the range of f rom 80 to 1000C for substantially 8 minutes.

17. A process for treating a surface of an aluminium alloy work piece according to claim 6 substantially as hereinbefore described.