Procedure for anodising aluminium or aluminium alloys

Description

Field of the Invention

[0001] This invention refers to a process for anodising aluminium or aluminium alloy parts, including pure or almost pure aluminium and all its combinations with other elements in any proportion.

Background

[0002] Traditionally, the acidic solutions used in anodising procedures are composed of sulphuric acid in high concentrations, or of chromic acid. The latter is the main component used in the aerospace industry. Sulphuric acid is not used in the aerospace industry due to the low adherence in the treated parts, while chromic acid has a high toxicity in live beings and is hazardous for the environment. The aqueous tartaric-sulphuric acid solution is an alternative method to anodise parts through an electrolytic process with low environmental impact. This method is described in patent number US 2002/0157961 A1. Another alternative method is an aqueous solution of sulphuric acid and boric acid described in US patent 4894127. These methods do not provide the aluminium or aluminium alloy parts with the same properties pertaining to corrosion as the chromic acid treatment.

[0003] The procedure in this invention uses the aluminium or aluminium alloy parts, described as anodes in an electrolytic cell with an aqueous acidic electrolyte, in order to create a superficial layer of aluminium oxide on said parts. This superficial aluminium oxide improves the properties pertaining to resistance against corrosion and surface layer adherence of an aluminium or aluminium alloy part.

Detailed Description of the Invention

[0004] This invention refers to an anodising procedure for aluminium or aluminium alloys in which the aluminium or aluminium alloy parts are submerged in an aqueous solution at a temperature between 0°C and 130°C, and where said solution includes:

• sulphuric acid,
• tartaric acid, and
• at least one inorganic salt of an element selected between at least one transition metal, one lanthanide element, one actinide, and combinations of them, and apply a controlled potential difference.

[0005] The inorganic salt of the transition metal may be present in a concentration between 5·10^-7 and 1.5 M, preferably in a concentration between 1·10^-6 and 1 M.

[0006] In the invention's procedure, said inorganic salt of the transition metal may be a salt of at least one metal selected between metals from the IIIB, IVB, VB, VIB, VIIB, VIIIB, IB. and IIB groups, a salt from a lanthanide or actinide element, combinations of the previous ones, and preferably a molybdenum salt.

[0007] The previously mentioned inorganic salt or salts behave as corrosion inhibitors by preventing the development of the different corrosion reactions (depending on the type of inorganic salt), therefore improving the behaviour of the parts pertaining to corrosion.

[0008] The same aluminium or aluminium alloy parts to be anodised can be used as an anode.

[0009] According to particular embodiments, the electrolyte is an aqueous acidic electrolyte; preferably it is an aqueous solution of tartaric-sulphuric acid.

[0010] According to a preferred embodiment, the aqueous solution that acts as an electrolyte has a concentration of sulphuric acid between 0.1 and 1.5 M, preferably between 0.2 M and 0.9 M, and a concentration of L(+)-tartaric acid between 0.1 and 1.5 M, preferably between 0.2 and 0.8M.

[0011] According to the most preferable embodiment, said solution has a concentration of sulphuric acid between 0.2 M and 0.9 M, L(+)-tartaric acid with a concentration between 0.2 and 0.8M, and one or several inorganic salts composed of at least one or several transition metals in a concentration between 1·10^-6 and 1 M.

[0012] During the anodising process, the temperature of the aqueous solution is kept between 0°C and 140°C, preferably between 0°C and 130°C, even more preferably between 5°C and 80°C, and most preferably between 30 and 40°C.

[0013] The electrolytic cell is subject to a potential difference 0.5V and 130V, preferably between 1 V and 120 V, even more preferably between 2 V and 100 V, and most preferably between 10 and 30 V according to the procedure.

[0014] The duration of the anodising procedure is between 1 and 130 minutes, preferably between 5 and 120 minutes, and more preferably between 5 and 40 minutes.

[0015] This procedure has a duration cycle of about 40% less time as regards traditional chromic acid anodising.

[0016] The toxicity and hazardousness to the environment of the residues created by this invention is greatly reduced when compared to those created by the traditional anodising procedures.

Examples

Example 1

Anodising a 2000 series aluminium alloy in a tartaric-sulphuric acid bath with molybdenum salt

[0017] A 2000 series aluminium alloy part, 150x100x2 mm, is subjected to a conventional cleaning and surface layer removal treatment: degreasing by immersion for approximately 10 minutes, rinse in distilled water for approximately 5 minutes, surface layer removal for approximately 10 minutes, and rinse in distilled water for 5 minutes.

[0018] Then the part is completely submerged in an electrolytic cell, where the part functions as an anode; the cathode is composed of AISI 321 stainless steel, and it has a geometric area equal to or larger than the anode's geometric area. The electrolyte is an aqueous acidic solution made of 0.40 M sulphuric acid, 0.53 M L (+)-tartaric acid, and 0.25M sodium molybdenum. The cell's temperature is at 37°C ± 1°C. The potential difference increases from 0 to 14 V at a rate of 2.8 V·min-1, and it stays at 14 V for 20 minutes, creating an oxide layer of approximately 2 µm.

[0019] The part is rinsed in anodised water for approximately 5 minutes, and it is sealed in anodised water at boiling point for approximately 40 minutes. Then it is dried with hot air.

Example 2

Anodising a 2000 series plaqued aluminium alloy part in a tartaric-sulphuric acid bath with molybdenum salt.

[0020] A 2000 series plaqued aluminium part, 150x100x2 mm, is subjected to a conventional cleaning and surface layer removal treatment, as described in Example 1.

[0021] The part is completely submerged in an electrolytic cell, where the part functions as an anode; the cathode is made of AISI 321 stainless steel, and it has a geometric area equal to or larger than the anode's geometric area. The electrolyte and the anodising conditions are the same as those described in Example 1, obtaining an oxide layer of approximately 2 µm.

[0022] The anodised part is rinsed and sealed in the same manner as that described in example 1.

Example 3 (comparative)

Anodising a 2000 series aluminium alloy in a tartaric-sulphuric acid bath.

[0023] A 2000 series aluminium alloy part, 150x100x2 mm, is subjected to a conventional cleaning and surface layer removal treatment, as described in Example 1.

[0024] The part is completely submerged in an electrolytic cell, where the part functions as an anode; the cathode is made of AISI 321 stainless steel, and it has a geometric area equal to or larger than the anode's geometric area. The electrolyte and the anodising conditions are the same as those described in Example 1, obtaining an oxide layer of approximately 3 µm.

[0025] The anodised part is rinsed and sealed in the same manner as that described in example 1.

Example 4 (comparative)

Anodising 2000 series aluminium in a tartaric-sulphuric acid bath.

[0026] A 2000 series plaqued aluminium part, 150x100x2 mm, is subjected to a conventional cleaning and surface layer removal treatment, as described in Example 1.

[0027] Then the part is completely submerged in an electrolytic cell, where the part functions as an anode; the cathode is made of AISI 321 stainless steel, and it has a geometric area equal to or larger than the anode's geometric area. The electrolyte is an aqueous acidic solution made of 0.40 M sulphuric acid and 0.53 M L (+)-tartaric acid. The cell's temperature is maintained at 37°C ± 1°C.

[0028] The potential difference increases from 0 to 14 V at a rate of 2.8 V·min-1, and it is maintained at 14 V for 20 minutes, creating an oxide layer of approximately 3 µm.

[0029] The anodised part is rinsed and sealed as in Example 1.

Example 5 (comparative)

Anodising with chromic acid

[0030] A 2000 series plaqued aluminium part, 150x100x2 mm, is subjected to a conventional cleaning and surface layer removal treatment, as described in Example 1.

[0031] Then the part is completely submerged in an electrolytic cell, where the part functions as an anode; the cathode is made of AISI 321 stainless steel, and it has a geometric area equal to or larger than the anode's geometric area. The electrolyte is an aqueous acidic solution with chromic acid. The cell's temperature is maintained between 35°C and 40°C.

[0032] The potential difference increases from 0 to 40 V at a rate of 5 V·min-1, and it stays at 14 V for 45 minutes, creating an oxide layer of approximately 3 µm.

[0033] The anodised part is rinsed and sealed as in Example 1.

Table 1. Comparison of properties in the parts treated according to the previous examples

Part	Thickness (µm)	(1)96 hours of exposure	(1)336 hours of exposure	(2)Adherence of dried paint (Gt)	(2)Adherence of damp paint /Gt)
Ex. 1	2	Pass	Pass	0	0
Ex.	2	Pass	Pass	0	0
Ex.	3	Pass	Fail	0	0
Ex. 4	3	Pass	Fail	0	0
Ex. 5	3	Pass	Pass	0	0

(1) Assay in saline fog chamber according to standard ASTM B 117. (2) Assay of paint adherence according to standard ISO 2409 (before and after 14 days of immersion in distilled water).

[0034] The parts treated according to the invention exceed 336 hours in saline fog according to the requirement established in section 3.7.1.2 of the military standard MIL-A-8625-F for IC type anodic layers.

[0035] The comparison of the obtained results for the tests done on the example parts according to the invention and the comparative example conclude that the oxides created by the invention have better properties pertaining to corrosion than the oxides created in aqueous acidic mediums without inorganic salts. These properties are equal to or better than the ones obtained through anodising in chromic acid.

Claims

1. An anodising procedure for aluminium or aluminium alloys characterised in that it comprises in submerges an aluminium or aluminium alloy part in an aqueous solution, said solution comprising:

- sulphuric acid,

- tartaric acid, and

- at least one inorganic salt of an element selected between at least one transition metal, one lanthanide element, one actinide, and a combination of them, and apply a controlled potential difference.

2. An anodising procedure according to claim 1, characterised in that said aqueous solution also includes one inorganic salt of a transition metal with a concentration between 1·10^-6 and 1 M.

3. An anodising procedure according to claim 1, characterised in that said inorganic salt is a molybdenum salt.

4. An anodising procedure according to claim 1, characterised in that as the anode is the same aluminium or aluminium alloy parts to be anodised.

5. An anodising procedure according to claim 1, characterised in that one aqueous acidic electrolyte is used.

6. An anodising procedure according to claim 1, characterised in that said aqueous acidic electrolyte is an aqueous, tartaric-sulphuric solution.

7. An anodising procedure according to claim 6, characterised in that said solution has a concentration of sulphuric acid between 0.2 M and 0.9 M, L(+)-tartaric acid with a concentration between 0.2 and 0.8M.

8. An anodising procedure according to claim 6, characterised in that said solution has a concentration of sulphuric acid between 0.2 M and 0.9 M, L(+)-tartaric acid with a concentration between 0.2 and 0.8M, and one or several inorganic salts with at least one or several transition metals with a concentration between 1·10^-6 and 1 M.

9. An anodising procedure according to claim 1, characterised in that the temperature of the aqueous solution is maintained during the anodising process between 0°C and 130°C.

10. An anodising procedure according to claim 1, characterised in that a difference of electrical potencial between 1V and 120V is applied to the electrolytic cell.

11. An anodising procedure according to claim 1, characterised in that it takes between 5 and 120 minutes of time.

12. An anodising procedure according to claim 1, characterised in that a steel plate is used for the cathode.

13. An anodising procedure according to claim 1, characterised in that an aqueous solution of tartaric-sulphuric acid and a molybdenum salt are used.

14. An anodising procedure according to claim 1, characterised in that the aqueous solution is made up of sulphuric acid whose concentration is between 0.20 and 0.50 M, L (+)-tartaric acid with a concentration between 0.4 and 0.55 M, and molybdenum salt whose concentration is between 0.20 M and 0.30 M.

15. An anodising procedure according to claim 1, characterised in that it comprises:

- a cathode made of AISI 321 stainless steel whose area is equal to or larger than the area of the anode,

- an aqueous solution made up of 0.40 M sulphuric acid, 0.53 M L (+)-tartaric acid, and 0.25 M molybdenum salt, and

- an electrolytic cell's temperature maintained at 37°C ± 1°C., comprising a potential difference of 0 to 14 V applied at rate of 2.8 V·min^-1 and maintained during 20 minutes.