Anodization process for an aluminum alloy part

CA 02425296 2003-04-17 1 The present invention relates to the treatment of aluminum alloy parts, in particular parts intended to constitute aeronautical components, and more especially a process of anodizing a pi ~: this in alloy aluminum.

BACKGROUND OF THE INVENTION To avoid the use of chemicals containing hexavalent chromium, processes for 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 anodization process is given in document US-A-4, 894, 127.

In this known process, use is made of an aqueous electrolytic bath comprising essentially sulfuric acid, with a concentration of between 30.5 g / 1 and 52 g / 1, and boric acid, with a concentration of between 5, 2 g / 1 and 10.7 g / l.

Such an anodizing process is relatively efficient for 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 the anodized and sealed coatings produced in baths containing an aqueous solution of sulfuric acid and chromic acid.

The superiority of the process of document US-A4,894,127 over other prior sulfo-boric anodizing processes lies in obtaining low thickness coatings, in particular from 1 ~ .m to 3 ~ .m, this which is particularly interesting in the field of aeronautics.

However, the compositions indicated for the implementation of such a process are very broad, which can lead to obtaining characteristics of great disparity for the layers obtained CA 02425296 2003-04-17 2 bare.

In addition, it is difficult to control the thickness of oxide obtained at the end of the treatment.

It should be noted that within the framework of this known process, a voltage is applied to the part which is immersed in the electrolytic bath, a voltage which increases linearly from 5 V to 20 V, with a current density on said part which remains close to 100 A / mz.

Mention may also be made of document EP-A-0 048 909 describing another anodizing process using an anodizing bath which is essentially con: ~ titué of sulfuric acid and phosphoric acid, in particular with respective concentrations of [50 g / 1; 50 g / 1], [63 g / 1; 37 g / 1] and [75 g / 1; 25 g / 1].

The presence of sulfuric acid may however prove to be undesirable in certain situations, if one seeks to obtain low coating thicknesses, in particular less than 3 ~ Cm.

Reference may also be made to document L: fS-A-4 861 440 describing the use of an anodizing bath containing sulfuric acid and at least one carboxylic acid, with a high concentration of acid. sulfuric acid (112 g / 1 to 150 g / 1).

There are also other anodizing techniques using an aqueous anodizing bath comprising essentially sulfuric acid to the exclusion of any other acid, with the sulfuric acid in a high concentration, generally sulfuric acid. order of 200 g / 1.

Document US Pat. No. 4,554,216 thus describes an anodizing process using a bath comprising sulfuric acid at a concentration of 166 g / 1 ~ 230 g / l.

The aqueous bath is at a low temperature (0 ° C to 5 ° C), and a high current density (200 A / m2 to 300 A / m2) is applied to the part immersed in said bath.

Anodizing processes with electrolytic baths com CA 02425296 2003-04-17 3 carrying sulfuric acid with a concentration of between 180 g / 1 and 250 g / 1 are commonly used in the aeronautical field.

The existing techniques have systematically confined themselves to this high range of sulfuric acid concentrations for anodizing baths, due to a choice based on the curve giving the variations in electrical conductivity as a function of the sulfuric acid concentration.

In fact, this variation curve has substantially the shape of a parabola turned downwards, and has a maximum in the zone corresponding to the concentrations of between 180 g / 1 and 220 g / 1 for sulfuric acid.

Specialists have therefore invariably based themselves on the search for maximum electrical conductivity for the electrolytic bath.

Indeed, it is known that this high electrical conductivity is favorable to a rapid growth of the oxide thickness.

This explains the systematic choice of high concentrations (at least equal to 200 g / 1) for the sulfuric acid present in the electrolytic bath.

The known techniques of sulfuric anodic oxidation, however, present the threefold drawback of a difficulty in controlling the coating thickness, and of obtaining both an always high porosity due to the high concentration of sulfuric acid and 'uncontrolled roughness.

The control of the thickness or of the coating weight is certainly delicate, because we are in the presence of two phenomena which develop concurrently with each other during the anodizing process, namely an electrolytic phenomenon which corresponds to the growth of an interface layer, and a chemical phenomenon of dissolution of the barrier layer formed at the interface between the substrate and the coating formed and at 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 comp ~~ if CA 02425296 2003-04-17 4 chemical tion of the electrolyte, and in particular on the concentration of sulfuric acid, is therefore systematically high, which gives an overall unfavorable effect on the characteristics of the layer obtained:

Those skilled in the art know that the anodization in ~ r ~ ilieu acid (pH <2.5) is essentially porous, and that, if we want to avoid high porosity on the treated parts, it is then necessary refer to anodizing techniques in a more neutral medium, making it possible to obtain a barrier anodization with a non-porous layer.

The technological background of the invention is also illustrated by documents US-A-3,563,867, US A-6,149,795, US-A-4,968,389 and JP-A-2000/026997.

OBJECT OF THE INVENTION The object of the present invention is to provide a more efficient anodizing process, which relates essentially to sulfur anodic oxidation techniques, but allowing better control of the thickness or weight of the coating, while by avoiding obtaining a high porosity on the parts treated ~~~.

DETAILED DESCRIPTION OF THE INVENTION This problem is solved in accordance with the invention by means of a process for anodizing a part made of al: Aluminum binder, comprising the following successive steps.

- An aqueous anodizing bath containing essentially sulfuric acid is provided, with a concentration of between 55 g / 1 and 85 g / 1, excluding any presence of phosphoric acid or boric acid;

the aforementioned bath is maintained at a constant temperature essentially between 15 ° C and 27 ° C;

- Said part is immersed in said bath;

- Applying to said part immersed in said bath a voltage essentially between 5 V and 30 CA 02425296 2003-04-17 v, with a low current density on said part;

and - said part is maintained 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 inventio.n will be described in more detail below, and provision may in particular be made for the aqueous anodizing bath also to include an acid-alcohol having a with 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 however 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 various ranges associated with each of the process parameters, each time with 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 anodizing bath comprising essentially sulfuric acid, with a concentration of between 55 g / 1 and 85 g / 1, to the exclusion of any presence of phosphoric acid or boric acid.

Some authors prefer to mention concentrations indicated in percentage by mass. in this case, the aforementioned limits indicated in g / l correspond to concentration values ​​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 techniques of anodizing sul CA 02425296 2003-04-17 6 furic mentioned above, which were between 180 g / 1 and 220 g / 1. It was therefore necessary to reverse a prejudice by not seeking maximum electrical conductivity of the electrolytic bath, and consequently by leaving the systematically recommended range of high concentrations of sulfuric acid.

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

The second preparatory step of the anodizing process according to the invention consists in maintaining the aforementioned 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 therefore immersed in the aqueous anodizing bath thus prepared.

Basically, a voltage is then applied to said part immersed in said bath essentially 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 anodization treatment, said constant value then being between 5 V and 30 V.

Advantageously then, a constant voltage value of between 7 V and 20 V. will be chosen.

However, it appeared more interesting, 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 on a first constant value, then, after a predetermined duration, on a second constant value higher CA 02425296 2003-04-17 7 than the first, said first and second constant values ​​being both between 5 V and 30 V.

As such, it appeared 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, in order to have the desired properties of the barrier layer. The use of two successive stages of tension, the duration of which will be essentially a function of the desired thickness of the coating, makes it possible to build up the barrier layer more quickly while retaining control of the growth of the coating.

The Applicant has carried out numerous tests, and in particular has observed in the light of plotted growth kinetics curves which were obtained with excellent slow results with a first plateau at 10 V, for a period of the order of twenty-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” seed means in this case that this current density is appreciably less than 100 A / m '.

In practice, it appears advantageous to provide a current density essentially less than 80 A / m 2, preferably being between 30 A / m 2 and 70 A m 2.

Optimal values ​​for current density, with the above voltage values, are around 34 to 35 A / m2.

As part of the anodization process as described above, the part to be treated is finally maintained in the electrolytic bath until the desired coating thickness is obtained, which is substantially between 1 ~ .m and 3 ~ .m.

CA 02425296 2003-04-17 ô In this regard, it is interesting to note that relatively low concentrations of sulfuric acid had already been used in hard anodizing techniques, but the coating thicknesses concerned had not been nothing to do with the values ​​currently considered, since we were interested in very thick coatings of the order of 250 gyms.

The other parameters of the hard anodization were also largely outside the ranges provided for in the present process (high voltage up to 120 V, high current densities of the order of 250 Ajm2, and low temperatures ranging from -5 ° C to + 5 ° C). therefore, the present anodizing process cannot be compared with prior hard anodizing techniques.

In the context of more advanced developments of the anodizing process described above, it has also appeared advantageous to consider 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 / 1 and 22 g; 1.

This addition of an acid-alcohol with a low concentration seems advantageous 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 of particular interest in this case because they are completely miscible with the electrolyte at room temperature.

A great stability in the dissolution rate is therefore obtained 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 CA 02425296 2003-04-17 9 (acid-alcohol having three functions acid, of formula C6Ha07).

The concentration of tartaric acid or citric acid will then preferably be essentially between 12 g / l and 17 g / l, the optimum concentration observed during the tests carried out being in the region of 17 g / l.

The choice of tartaric acid and citric acid also appears 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 oxidant. , whose electrochemical activity is high without being aggressive, and therefore not generating pitting.

It can thus be seen that the sulpho-tartaric or sulpho-citric anodic oxidation appears much more attractive than the sulpho-boric anodic oxidation of the prior techniques, in particular that which is described in document US Pat. No. 4,894,127.

In fact, excellent stability of the dissolution of the alumina layer is obtained here.

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 weaker than with sulfuric acid alone, and moreover. the current density in the bath does not drop as it would with other acids, such as boric acid, due to the action on surface tensions.

I1 is also important to provide that the part to be treated undergoes a preliminary treatment of lubrication / pickling or deoxidation before being immersed in the bath.

In traditional techniques, a first degreasing step was used, followed by rinsing, then followed by a second pickling step in an alkaline medium or more generally in an aqueous solution of sulfuric acid and chromic acid, followed by c = _nfin CA 02425296 2003-04-17 of a new rinsing. However, it appears more advantageous to use a product capable of directly carrying out a satisfactory degreasing / stripping in a single step, and one could advantageously use a solution composed of phosphoric acid supplemented with anionic surfactants, such as that 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 pickling, that is to say without the presence of any superficial attack.

It is finally advantageous to provide that the treated part undergoes a subsequent treatment of clogging of the coating.

The sealing of the coating must perform a dual function which is to develop both the promotion of adhesion and the resistance to corrosion.

This clogging is conventionally carried out by soaking in hot water at a temperature at least equal to 97 ° C., or in a dilute solution of potassium dichromate.

Preference will be given to a solution of deionized water at a temperature of between 85 ° C. and 98 ° C., the soaking being carried out for a period which depends on the thickness of the coating obtained.

As a variant, it is also possible to use sodium molybdate (MoNa204, 2H20) or manganese sulphate (Mn04S, H20), or else an immersion in a particularly effective sealing product based on nickel acetates or acetates of nickel. lithium, for example by using respectively the product marketed by the aforementioned company HENKEL SURFACE TECHNOLOGIES under the reference “ANOSEAL 1000” or “ENVIROSEAL 2500”.

Among the many 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 CA 02425296 2003-04-17 11 concentration of 62 g / 1 .

Regular growth of the coating layer was then observed, with perfect control: ~ e low thicknesses, without modification of the initial roughness of the substrate.

Other tests with a sulfur dioxide anodization bath can also be mentioned, with a concentration of 62 g / 1 of sulfuric acid and 17 g / 1 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 voltage applied was 10 V during a first level of 25 minutes then 20 V during a second level. 15 minutes.

We then succeeded in obtaining a coating thickness of low porosity, between 2.8 ~ m and 3.2 ~, m.

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

After sealing, it was found that the parts thus treated still had a very satis aspect making after 500 hours of exposure to the mist its linen.

In addition, the abatement characteristics in. fatigue remained very satisfactory compared to known techniques of anodic sulfo-boric or chromic oxidation.

It has thus been possible to achieve a very efficient anodizing process which makes it possible both to control the thickness or the weight of the coating and the roughness, and to obtain a low porosity at the surface of the treated parts.

The invention is not limited to the process which has just been described, but on the contrary encompasses any equivalent anodizing process falling within the general definition given at the head of the description.