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US 2003/0196907 A1Published application
Method of anodizing a part made of aluminum alloy
AssigneeMessier-Bugatti
InventorAlain Viola
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AssigneeMessier-Bugatti
InventorAlain Viola
Estimating the term…
The invention provides a method of anodizing a part made of aluminum alloy. According to the invention, the method comprises the following steps in succession: providing an aqueous anodizing bath essentially comprising sulfuric acid at a concentration lying in the range 55 g/l to 85 g/l, and excluding the presence of any phosphoric acid or any boric acid; maintaining said bath at a constant temperature essentially lying in the range 15° C. to 27° C.; dipping said part into said bath; applying a voltage to said part dipped in said bath, the voltage lying essentially in the range 5 V to 30 V, with low current density on said part; and maintaining said part in said bath until a coating of desired thickness has been obtained, which thickness lies substantially in the range 1 μm to 3 μm.
1/ A method of anodizing an aluminum alloy part, the method comprising the following steps in succession:
2/ A method according to claim 1, in which the concentration of sulfuric acid in the bath lies essentially in the range 57 g/l to 67 g/l, and is preferably close to 62 g/l.
3/ A method according to claim 1, in which the bath is maintained at a constant temperature close to 22° C.
4/ A method according to claim 1, in which the voltage applied to the part dipped in the bath is fixed on a constant value throughout the duration of the anodizing treatment, said constant value lying in the range 5 V to 30 V.
5/ A method according to claim 4, in which the constant value of the voltage lies in the range 7 V to 20 V.
6/ A method according to claim 1, in which the voltage applied to the part dipped in the bath is initially fixed to a first constant value, and then after a predetermined duration, to a second constant value that is higher than the first, said first and second constant values both lying in the range 5 V to 30 V.
7/ A method according to claim 6, in which the first constant voltage value lies in the range 5 V to 11 V, and the second constant voltage value lies in the range 15 V to 30 V.
8/ A method according to claim 1, in which the current density on the part dipped in the bath remains well below 100 A/m2.
9/ A method according to claim 8, in which the current density is essentially less than 80 A/m2, preferably lying in the range 30 A/m2 to 70 A/m2.
10/ A method according to claim 1, in which the aqueous anodizing bath also contains an acid alcohol having one to three acid functions, at a concentration lying in the range 12 g/l to 22 g/l.
11/ A method according to claim 10, in which the acid alcohol used in the that is tartaric acid or citric acid, and the concentration of said acid alcohol lies essentially in the range 12 g/l to 17 g/l.
12/ A method according to claim 1, in which the part to be treated is subjected to preliminary degreasing/pickling treatment prior to being dipped in the bath.
13/ A method according to claim 1, in which the treated part is subjected to subsequent treatment of sealing the coating.
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Characterised by the electrolytes used: containing inorganic acids
Characterised by the electrolytes used: containing organic acids
Of aluminium or alloys based thereon: anodising more than once, e.g. in different baths
Processes for the electrolytic or electrophoretic production of coatings
Term end not estimated
The present invention relates to treating aluminum alloy parts, in particular parts for use in making aviation components, and more specifically the invention relates to a method of anodizing a part made of aluminum alloy.
In order to avoid using chemicals containing hexavalent chromium, proposals have already been made for methods of anodizing aluminum and its alloys by using an aqueous anodizing bath containing sulfuric acid and boric acid. A good illustration of such a sulfuric-boric anodization method is given in document U.S. Pat. No. 4,894,127. In that known method, an aqueous electrolytic bath is used that essentially comprises sulfuric acid at a concentration lying in the range 30.5 grams per liter (g/l) to 52 g/l, and boric acid at a concentration lying in the range 5.2 g/l to 10.7 g/l.
Such an anodization method is relatively effective in applying a coating of aluminum oxide on an aluminum alloy with a solution of sulfuric acid and boric acid. The resulting anodized coating is at least comparable, and in terms of resistance to corrosion, equivalent to the anodized and sealed coatings made in baths containing an aqueous solution of sulfuric acid and chromic acid. The superiority of the method of document U.S. Pat. No. 4,894,127 over other prior methods of sulfuric-boric anodization lies in the thinness of the coatings obtained, in particular coatings having a thickness of 1 micrometer (μm) to 3 μm, which is particularly advantageous in the field of aviation. Nevertheless, the compositions specified for implementing such a method are very broad, and that can lead to characteristics being obtained that present a wide range of variation in the resulting layers. Furthermore, it is difficult to control the thickness of the oxide obtained at the end of treatment. It should be observed that in the context of that known method, a voltage is applied to the part which is dipped in the electrolytic bath, which voltage increases linearly from 5 volts (V) to 20 V, with current density over said part remaining close to 100 amps per square meter (A/m2).
Particular features relating to the above-specified steps of the anodizing method of the invention are described in greater detail below, and provision can be made in particular for the aqueous anodizing bath also to contain an acid alcohol having one to three acid functions in order to limit dissolution of the resulting coating layer, for the purpose of having perfectly uniform porosity throughout the thickness of the layer while not losing electrical conductivity in the bath which favors good growth of said layer.
The anodizing method of the invention is described in greater detail below, setting out the various ranges associated with each of the parameters of the method, each time giving preferred values as they have been determined in testing carried out by the Applicant.
The first step of the anodizing method of the invention consists-in providing an aqueous anodizing bath essentially comprising sulfuric acid at a concentration lying in the range 55 g/l to 85 g/l, and excluding the presence of any phosphoric acid or boric acid. Certain authors prefer the specified concentrations to be given in weight percentages: specifically, the above-mentioned limits given in g/l correspond to concentration values lying in the range 5.36% to 8.2% by weight.