FR2838754A1 - Method of anodizing aluminum alloy part, e.g. aviation component, involves dipping part into bath comprising sulfuric acid, applying voltage to dipped part, and maintaining part in bath until coating of desired thickness has been obtained - Google Patents

Abstract

Aluminum alloy part is anodized by providing aqueous anodizing bath comprising 55-85 g/l sulfuric acid, and excluding presence of any phosphoric acid or any boric acid; maintaining bath at 15-27 degreesC; dipping part into bath; applying voltage of 5-30 V to dipped part with low current density on part; and maintaining part in bath until coating of desired thickness has been obtained, which is 1-3 microns.

Description

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 The present invention relates to the treatment of aluminum alloy parts, in particular parts intended to constitute aeronautical components, and more particularly to a method of anodizing an aluminum alloy part.

 To avoid the use of chemicals containing hexavalent chromium, methods of anodizing aluminum and its alloys have already been proposed using an aqueous anodizing bath containing sulfuric acid and boric acid. . A good illustration of such a sulfo-boric anodizing process is given in US-A-4,894,127. In this known process, an aqueous electrolytic bath comprising essentially sulfuric acid, with a concentration of between 30.5 g / l and 52 g / l, and boric acid, with a concentration of 5, are used. 2 g / l and 10.7 g / l.

 Such an anodizing process is relatively effective in applying an aluminum oxide coating to an aluminum alloy with a solution of sulfuric acid and boric acid. The anodized coating thus obtained is at least comparable and, in terms of corrosion resistance, equivalent to anodized and sealed coatings made in baths containing an aqueous solution of sulfuric acid and chromic acid. The superiority of the process of US-A-4,894,127 compared to other prior processes of sulfoboric anodizing is to obtain coatings of small thickness, in particular from 1 to 3 m, which is particularly interesting in the field of aeronautics. However, the compositions indicated for the implementation of such a method are very wide, which can lead to obtaining characteristics of great disparity for the layers obtained. In addition, it is difficult to control the thickness

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 of oxide obtained at the end of the treatment. It should be noted that in the context of this known method, a voltage which increases linearly from 5 V to 20 V, with a current density on said part which remains close to the same, is applied to the part which is immersed in the electrolytic bath. 100 A / m2.

 There are also other anodizing techniques using an aqueous anodizing bath comprising essentially sulfuric acid to the exclusion of any other acid, with for sulfuric acid a high concentration, generally of the order of 200 g / 1. Thus, in the aeronautical field, anodizing processes are used with electrolytic baths comprising sulfuric acid with a concentration of between 180 g / l and 250 g / l. Existing techniques have systematically been confined to this high range of sulfuric acid concentrations for the anodizing baths, due to a choice based on the curve giving the variations of the electrical conductivity as a function of the sulfuric acid concentration. Indeed, this curve of variations has substantially a parabolic shape facing downwards, and has a maximum in the zone corresponding to concentrations of between 180 g / l and 220 g / l for sulfuric acid. Specialists have therefore invariably relied on the search for maximum electrical conductivity for the electrolytic bath. Indeed, it is known that this high electrical conductivity is favorable to rapid growth of the oxide thickness. This explains the systematic choice of high concentrations (at least equal to 200 g / l) for the sulfuric acid present in the electrolytic bath.

 However, the known techniques of anodic oxidation sulfuric have the triple disadvantage of difficulty in controlling the thickness of the coating, and

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 obtaining both a porosity always high due to the high concentration of sulfuric acid and uncontrolled roughness. The control of the thickness or the weight of coating is certainly delicate, because there are two phenomena which develop concurrently with each other during the anodization process, namely an electrolytic phenomenon which corresponds to the growth of an interface layer, and a chemical dissolution phenomenon of the barrier layer formed at the interface between the substrate and the formed coating and on the surface of the porous layer in contact with the electrolyte. The porosity of the coating layer obtained, which is known to be dependent on the chemical composition of the electrolyte, and in particular the concentration of sulfuric acid, is therefore systematically high, which gives a generally unfavorable effect on the characteristics of the layer obtained.

 Those skilled in the art know that the anodization in an acidic medium (pH <2.5) is essentially porous, and that, in order to avoid a high porosity on the treated parts, it is then necessary to refer to anodizing techniques in a more neutral medium, to obtain a barrier anodization with a non-porous layer.

 The object of the present invention is to propose a more efficient anodizing process, which is essentially related to sulfuric anodic oxidation techniques, but which allows a better control of the coating thickness or weight, while avoiding obtaining a high porosity on treated parts.

 This problem is solved according to the invention by means of an anodizing process of an aluminum alloy part, comprising the following successive steps: - an aqueous anodizing bath comprising essentially sulfuric acid is provided, with a

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 concentration between 55 g / l and 85 g / l; the aforementioned bath is maintained at a constant temperature essentially between 15 C and 27 C; said part is immersed in said bath; said part immersed in said bath is applied with a tension essentially between 5 V and 30 V, with a low current density on said part; and said part is held in said bath until the desired coating thickness is obtained which is substantially between 1 m and 3 m.

 Particular modalities relating to the aforementioned steps of the anodizing process according to the invention will be described in more detail below, and it may in particular be provided that the aqueous anodizing bath also comprises an acid-alcohol having one to three acid functions, in order to limit the dissolution of the coating layer obtained, this to have a perfect homogeneity of the porosity throughout the thickness of the layer, without losing the electrical conductivity of the bath which promotes good growth of said layer .

 The anodizing process according to the invention will now be described in more detail, by exposing the different ranges associated with each of the parameters of the process, with each time an indication of the preferred values as they could be deduced from the tests carried out. by the plaintiff.

 The first step of the anodizing process according to the invention consists in providing an aqueous anodization bath essentially comprising sulfuric acid, with a concentration of between 55 g / l and 85 g / l.

Some authors prefer to mention the concentrations indicated in percentage by mass: in this case the aforementioned limits indicated in g / 1 correspond to values

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 their concentrations ranging from 5.36% to 8.2% by weight.

 It is important to note that the aforementioned range of concentrations is much lower than the concentrations used in the known sulfuric anodizing techniques mentioned above, which were between 180 g / l and 220 g / l. It was thus necessary to reverse a prejudice by not seeking a maximum electrical conductivity of the electrolytic bath, and consequently by leaving the domain systematically recommended high concentrations of sulfuric acid.

 The concentration of the sulfuric acid bath will preferably be substantially between 57 g / l and 67 g / l, a highly preferred value being in the vicinity of 62 g / l (ie slightly above 6 wt%).

 The second preparatory step of the anodizing process according to the invention consists in keeping the abovementioned aqueous anodizing bath at a constant temperature, which is essentially between 15 ° C. and 27 ° C. Preferably, the bath will be maintained at a constant temperature. close to 22 C.

 The aluminum alloy part to be treated is thus immersed in the aqueous anodization bath thus prepared.

 Basically, then applies to said piece dipped in said bath a voltage substantially between 5 V and 30 V, with a low current density on said part.

 The voltage applied to the part immersed in the bath can be set to a constant value throughout the duration of the anodizing treatment, said constant value then being between 5 V and 30 V. Advantageously then, one will choose a constant voltage value between 7 V and 20 V.

 It appeared however more interesting, for

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 to further increase the perfect control of the growth of the oxide coating, to provide a voltage applied to the part immersed in the bath which is first set to a first constant value, then, after a predetermined duration, to a second constant value higher than the first, said first and second constant values being between 5 V and 30 V.

 As such, it has been found particularly advantageous to choose a first constant voltage value between 5 V and 11 V, this low value making it possible to control the moderate growth of the oxide layer, and a second constant value of voltage between 15 V and 30 V, this to have the desired properties of the barrier layer. The use of two successive levels of tension, the duration of which will essentially depend on the desired thickness of coating, makes it possible to erect the barrier layer more rapidly while maintaining control of the growth of the coating.

The Applicant has carried out numerous tests, and in particular has found, in the light of the growth kinetics curves drawn, that excellent results are obtained with a first plateau at 10 V, for a duration of about twenty-five hours. five minutes, followed by a second level of 20 V for a period of fifteen minutes.

 As has been said above, a relatively low current density is used on the part which is immersed in the electrolytic bath. The weak term means in this case that this current density is substantially less than 100 A / m2.

 In practice, it appears advantageous to provide a current density substantially less than 80 A / m 2, preferably being between 30 A / m 2 and 70 A / m 2. Optimum values for the current density, with the above-mentioned voltage values, are around 34 to 35 A / m2.

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 In the context of the anodizing process as previously described, the workpiece is finally maintained in the electrolytic bath until the desired coating thickness is obtained, which is substantially between 1 m and 3 m.

 In this respect, it is interesting to note that relatively low concentrations of sulfuric acid had already been used in hard anodising techniques, but the coating thicknesses concerned had nothing to do with the values currently envisaged. since we were interested in very thick coatings of the order of 250 m. The other hard anodizing parameters were also largely outside the range of the present process (high voltage up to 120 V, high current densities of the order of 250 A / m2, and low temperatures ranging from -5 C to +5 C): therefore, the present anodizing process can not be compared with the prior art of hard anodizing.

 In the context of more advanced developments of the anodization process previously described, it has furthermore proved advantageous to envisage adding to the aqueous anodizing bath an acid-alcohol having one to three acid functions, with a corresponding concentration which remains low, but between 12 g / l and 22 g / l.

 This addition of an acid-alcohol with a low concentration seems interesting to limit the dissolution of the layer and to increase the wettability of the electrolyte, and thus obtain a perfect homogeneity of porosity throughout the thickness of the layer, and all this without altering the electrical conductivity of the electrolytic bath. The acid-alcohols are particularly interesting in this case because they are completely miscible with the electrolyte at room temperature. We thus obtain a great stability in the dissolution rate

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 with a very satisfactory maintenance of the electrical conductivity.

 Preferably, the acid-alcohol used in the bath will be tartaric acid (acid-alcohol having two acid functions, of formula C4H606) or citric acid (acid-alcohol having three acid functions, of formula C6H8O7). The concentration of tartaric acid or citric acid will then preferably be essentially between 12 g / l and 17 g / l, the optimum concentration found in the tests carried out being close to 17 g / l.

 The choice of tartaric acid and citric acid also appears to be particularly advantageous insofar as it is desired, on the one hand, to have a stabilized pH at a low value and, on the other hand, to have a strong oxidizer. , whose electrochemical activity is high without being aggressive, and therefore not generating punctures. It can thus be seen that the anodic sulfo-tartaric or sulpho-citric oxidation is much more attractive than the anodic sulpho-boric oxidation of the prior art, in particular that described in US-A-4,894,127. Indeed, here we obtain excellent stability of the dissolution of the alumina layer.

 The addition of tartaric acid or citric acid in the bath in the context of the invention makes it possible to have a dissolution of the alumina layer which is lower than with sulfuric acid alone, and moreover the current density in the bath does not fall as it would be the case with other acids, such as boric acid, because of the action on surface tension.

 It is also important to provide that the workpiece undergoes a preliminary treatment degreasing / stripping or deoxidation before being immersed in the bath.

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 In traditional techniques, a first degreasing step was used, followed by rinsing, followed by a second etching step in an alkaline medium or, more generally, in an aqueous solution of sulfuric acid and chromic acid, finally followed. a new rinse. However, it appears more advantageous to use a product capable of directly performing a satisfactory degreasing / pickling in a single step, and it is advantageous to use a solution composed of phosphoric acid and anionic surfactants, such as the product marketed under the reference NOVACLEAN AL-85 by the German company HENKEL SURFACE TECHNOLOGIES. The use of such a product makes it possible to have a uniform etching, that is to say without the presence of any surface attack.

 Finally, it is advantageous to provide that the treated part undergoes a subsequent treatment clogging coating.

 The clogging of the coating must provide a dual function which is to develop both the promotion of adhesion and the corrosion resistance. This clogging is conventionally carried out by dipping in hot water at a temperature of at least 97 ° C., or in a dilute solution of potassium dichromate. A solution of deionized water at a temperature of between 85 ° C. and 98 ° C. is preferred, the soaking being carried out for a time which is a function of the coating thickness obtained. Alternatively, it is also possible to use sodium molybdate (MoNa 2 O 4, 2H 2 O) or manganese sulphate (MnO 4 S, H 2 O), or else immersion in a particularly effective clogging product based on nickel acetates or acetates. lithium, for example using respectively the product marketed by the company HENKEL SURFACE TECHNOLOGIES supra under

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 the reference ANOSEAL 1000 or ENVIROSEAL 2500.

 Among the numerous tests carried out by the applicant, there may be mentioned a first example of sulfuric anodic oxidation, with an aqueous anodizing bath comprising sulfuric acid in a concentration of 62 g / l. A regular growth of the coating layer was then observed, with perfect control of the small thicknesses, without modifying the initial roughness of the substrate.

 Other tests with a sulfocitric anodization bath can also be mentioned, with a concentration of 62 g / l of sulfuric acid and 17 g / l of citric acid. For the other operating conditions, it can be mentioned that the constant temperature at which the bath was maintained was 22 C, and that the applied voltage was 10 V during a first stage of 25 mn then 20 V during a second stage of 15 minutes.

It was then possible to obtain a coating thickness of low porosity, between 2.8 m and 3.2 m.

 As an indication, the tests were carried out on the aluminum alloys of references 2024 T 351 and 7175 T 7351.

 After clogging, it was found that the parts thus treated still had a very satisfactory appearance after 500 hours of exposure to salt spray. In addition, the fatigue abatement characteristics remained very satisfactory compared with known techniques of anodic sulfoboric or chromic oxidation.

 It has thus been possible to achieve a high performance anodizing process that allows both to control the thickness or the coating weight and roughness, and to obtain a low porosity on the surface of the treated parts.

 The invention is not limited to the process which

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 has just been described, but on the contrary covers any equivalent anodizing process falling within the general definition given at the head of description.

Claims (13)

1. A method of anodizing an aluminum alloy part, comprising the following successive steps: - an aqueous anodizing bath comprising essentially sulfuric acid, with a concentration of between 55 g / l and 85 g / 1; the aforementioned bath is maintained at a constant temperature essentially between 15 C and 27 C; said part is immersed in said bath; said part immersed in said bath is applied with a tension essentially between 5 V and 30 V, with a low current density on said part; and said part is held in said bath until the desired coating thickness is obtained which is substantially between 1 m and 3 m.  2. The method of claim 1, wherein the concentration of the sulfuric acid bath is essentially between 57 g / 1 and 67 g / 1, being preferably close to 62 g / 1.  3. Method according to claim 1 or claim 2, wherein the bath is maintained at a constant temperature close to 22 C.  4. Method according to one of claims 1 to 3, wherein the voltage applied to the coin immersed in the bath is set to a constant value throughout the duration of the anodizing treatment, said constant value being between 5 V and 30 V.  5. The method of claim 4, wherein the constant voltage value is between 7 V and 20 V.  The method of any one of claims 1 to 3, wherein the voltage applied to the workpiece <Desc / Clms Page number 13>  immersed in the bath is first set to a first constant value, then, after a predetermined duration, to a second constant value higher than the first, said first and second constant values being between 5 V and 30 V.  The method of claim 6, wherein the first constant voltage value is between 5V and 11V, and the second constant voltage value is between 15V and 30V.  The method of any one of claims 1 to 7, wherein the current density on the coin immersed in the bath remains substantially less than 100 A / m2.  The method of claim 8, wherein the current density is substantially less than 80 A / m2, preferably ranging from 30 A / m2 to 70 A / m2.  The process according to any one of claims 1 to 9, wherein the aqueous anodizing bath also comprises an acidic alcohol having one to three acid functions, with a concentration of between 12 g / l and 22 g / l.  11. The method of claim 10, wherein the acid-alcohol used in the bath is tartaric acid or citric acid, and the concentration of this acid-alcohol is essentially between 12 g / 1 and 17 g / 1. 1.  12. Method according to any one of claims 1 to 11, wherein the workpiece undergoes a preliminary degreasing / pickling treatment before being immersed in the bath.  13. A method according to any one of claims 1 to 12, wherein the treated part undergoes a subsequent treatment of clogging of the coating.