CA2425296A1 - Anodization process for an aluminum alloy part - Google Patents

Abstract

The invention relates to a method for anodizing a aluminum alloy part.
According to the invention, the method comprises the following successive steps:
- an aqueous anodization bath is provided comprising both essentially sulfuric acid, with a con-centering between 55 g / l and 85 g / l, excluding any presence of phosphoric acid or boric acid;
- the above-mentioned bath is maintained at a temperature constant essentially between 15 ° C and 27 ° C;
- said part is immersed in said bath;
- applying to said piece plunged into said bath a voltage essentially between 5 V and 30 V, with a low current density on said part; and - said part is maintained in said bath until obtaining the desired coating thickness which is felt between 1 µm and 3 µm.

Description

The present invention relates to the treatment of aluminum alloy parts, in particular parts intended to constitute aeronautical components, and more specifically a process of anodizing a pi ~: this in aluminum alloy.
BACKGROUND OF THE INVENTION
To avoid the use of chemicals containing hexavalent chromium, we have already proposed processes for anodizing aluminum and its alloys, using an aqueous anodizing bath containing sulfuric acid and boric acid. A good il-lustration of such a sulfo-boric anodization process is given in document US-A-4, 894, 127. In this pro-known transfer, an aqueous electrolytic bath is used carrying essentially sulfuric acid, with a concentration between 30.5 g / 1 and 52 g / 1, and boric acid, with a concentration between 5.2 g / 1 and 10.7 g / l.
Such an anodizing process is relatively ef effective at applying aluminum oxide coating on an aluminum alloy with an acid solution: sul furic and boric acid. The anodized coating as well obtained is at least comparable and, in terms of corrosion resistance, equivalent to coatings anodized and sealed in baths containing an aqueous solution of sulfuric acid and chromic acid nomic. The superiority of the process of document US-A-4,894,127 compared to other prior processes sulfo-boric anodization lies in obtaining coatings of small thickness, in particular from 1 ~ .m to 3 ~ .m, which is particularly interesting in the field of aeronautics. However, the compositions indicated for the implementation of such a process are very large, which can lead to of great disparity for the layers obtained

2 naked. In addition, it is difficult to control the thickness of oxide obtained at the end of the treatment. Note that in the context of this known process, we apply 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.
One can also cite the document EP-A-0 048 909 describing another anodization process using a anodizing bath which is essentially con: ~ titled sulfuric acid and phosphoric acid, with in part especially the respective concentrations of [50 g / 1; 50 g / 1], [63 g / 1; 37 g / 1] and [75 g / 1; 25 g / 1]. The presence sulfuric acid may, however, be undesirable in certain situations, if we are trying to get thin coating thicknesses, particularly inferior laughing at 3 ~ Cm.
We can also refer to document L: fS-A-4 861 440 describing the use of an anodizing bath containing ac: sulfuric acid and at least one acid because boxylic, with a high concentration of sulfuric acid than (112 g / 1 to 150 g / 1).
There are also other techniques anodizing using an aqueous anodizing bath com essentially carrying sulfuric acid to the exclusion of any other acid, with for sulfuric acid high concentration, usually around 200 g / 1. Document US-A-4,554,216 thus describes a program yielded from anodization using a bath comprising sulfuric acid at a concentration of 166 g / 1 ~ 230 g / 1. The aqueous bath is at a low temperature (0 ° C â
S ° C), and apply to the part immersed in said bath high current density (200 A / m2 to 300 A / m2). We commonly used in the aeronautical field of pro-ceded anodizing with electrolytic baths

3 carrying sulfuric acid with a concentrated taken between 180 g / 1 and 250 g / 1. Existing techniques are systematically confined to this high range view of sulfuric acid concentrations for baths anodizing, due to a choice based on the curve giving the variations of the electrical conductivity in depending on the sulfuric acid concentration. Indeed-fet, this curve of variations has a substantially shape dish facing down, and has a maximum in the zone corresponding to the concentrations included between 180 g / 1 and 220 g / 1 for sulfuric acid. The specialists have invariably based themselves on the re-seeks maximum electrical conductivity for the electrolytic bath. Indeed, it is known that this high electrical conductivity is favorable for rapid oxide thickness. This explains the systematic choice of high concentrations (at least equal to 200 g / 1) for sulfuric acid present in the electrolytic bath.
Known anodic oxidation techniques sul-furic, however, have the triple disadvantage of difficulty in controlling coating thickness, and obtaining both an always high porosity of the has a high concentration of sulfuric acid and an uncontrolled roughness. Thickness control ~ .seur or the coating weight is certainly delicate, because we are in the presence of two competing developing phenomena tell each other during the process anodizing, i.e. 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 on the surface of the porous layer in contact with the electrolyte. The porosity of the coating layer ob-held, which we know is dependent on the comp ~~ if-

4 chemical tion of the electrolyte, and in particular of the sulfuric acid concentration, is therefore systematic-high, which gives an overall unfavorable effect ble on the characteristics of the layer obtained:
Those skilled in the art know that anodization in ~ r ~ ilieu acid (pH <2.5) is essentially porous, and that if we want to avoid high porosity on processed parts tées, it is then necessary to refer to techniques anodization in a more neutral environment, making it possible to obtain 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 is connected essentially to anodic oxidation techniques sul furious, but allowing better control of coating thickness or weight, while avoiding to obtain a high porosity on the treated parts ~~~.
DETAILED DESCRIPTION OF THE INVENTION
This problem is solved in accordance with the invention thanks to an anodizing process of an aluminum part: Tying of aluminum, comprising the following successive stages your.
- an aqueous anodizing bath is provided both essentially sulfuric acid, with a con-centering between 55 g / 1 and 85 g / 1, excluding any phosphoric acid or boric acid than ;
the above-mentioned bath is maintained at a temperature constant essentially between 15 ° C and 27 ° C;
- said part is immersed in said bath;
- applying to said piece plunged into said bath a voltage essentially between 5 V and 30 v, with a low current density on said part;
and - said part is maintained in said juice bath only to obtain the desired coating thickness which

5 is substantially between 1 ~, m and 3 ~ .m.
Specific procedures relating to the stages mentioned above of the anodizing process according to the inventio.n se-will be described in more detail below, and we can particular provide that the aqueous anodizing bath also carries an acid-alcohol having one to three functions acid, in order to limit the dissolution of the layer of coating obtained, this to have a perfect homo-porosity in the entire thickness of the layer without losing electrical conductivity of the bath which promotes good growth of said cou-che.
We will now describe in more detail the process anodizing die according to the invention, by exposing the diff-different ranges associated with each of the program parameters assigned, with an indication of the pre-as they were deduced from the tests led by the plaintiff.
The first step in the anodizing process according to the invention consists in providing an anodization bath aqueous containing essentially sulfuric acid, with a concentration between 55 g / 1 and 85 g / 1, at excluding any presence of phosphoric acid or boric acid. Some authors prefer to mention concentrations indicated in percentage by mass. in the above-mentioned limits indicated in g / 1 correspond-lay at concentration values ranging from 5.36 â 8.2% by weight.
It is important to note that the aforementioned range of concentrations is much lower than the concentrations used in known sul anodizing techniques

6 mentioned above, which were located be 180 g / 1 and 220 g / 1. It was therefore necessary to overturn a pre-judged by not looking for electrical conductivity maximum of the electrolytic bath, and consequently when leaving the systematically recommended area of concentrations high in sulfuric acid.
The concentration of sulfuric acid in the bath will be preferably essentially between 57 g / 1 and 67 g / 1, a highly preferred value lying in the vicinity swim of 62 g / 1 (that is to say slightly above 6 ~ in weight).
The second preparatory stage of the process anodizing according to the invention consists in maintaining the The above-mentioned aqueous anodization bath at a constant temperature aunt, which is essentially between 15 ° C and 27 ° C. Preferably, the bath will be maintained at a temperature constant ture close to 22 ° C.
We therefore plunge the aluminum alloy part to treat in the aqueous anodizing bath thus prepared.
Basically we then apply to said piece immersed in said bath a tension basically included 5 V and 30 V, with low current density on said piece.
The voltage applied to the part plunged into the bath can be set to a constant value for the entire duration of the anodizing treatment, said value constant then being between 5 V and 30 V. Avanta then, we will choose a constant value of ten between 7 V and 20 V.
It appeared more interesting, however, for further increase the perfect control of the growth of the oxide coating, to provide a voltage applied to the piece immersed in the bath which is first wedged on a first constant value, then after a predefined duration finished, on a second higher constant value

7 that the first, said first and second values constants both being between 5 V and 30 V.
As such, it appeared particularly before tageux to choose a first constant value of ten between 5 V and 11 V, this low value per putting to control the moderate growth of the layer of oxide, and a second constant voltage value taken between 15 V and 30 V, this to have the properties barrier layer. The use of two successive voltage steps, the duration of which will be essential depending on the desired coating thickness, allows the barrier layer to be built more quickly while retaining control over the growth of the coating.
The Applicant has carried out numerous tests, and has particular observed in light of kinetic curves that growth plotted that we got from excel.lents results with a first plateau at 10 V, during a 'du-twenty-five minutes, followed by a second 20 V step for fifteen minutes.
As mentioned above, we use relatively low current density on the workpiece which is immersed in the electrolytic bath. Germ "Low" means in this case that this density of rant is significantly less than 100 A / m '.
In practice, it seems interesting to pre see a substantially lower current density â
80 A / mz, preferably being between 30 A / m2 and 70 Ajmz. Optimal values for neck density rant, with the aforementioned voltage values, lie around 34 to 35 A / m2.
As part of the anodization process such as previously described, we finally maintain the piece ter in the electrolytic bath until the desired coating thickness, which is sensitive between 1 ~ .m and 3 ~ .m.

oh As such, it is interesting to note that we had previously used relatively low concentrations sulfuric acid in anodizing techniques hard, but the coating thicknesses concerned had nothing to do with the values currently envi-wise, since we were interested in very thick around 250 gym. The other parameters of hard anodizing was also largely in outside the ranges provided in this process (ten-high sion up to 120 V, current densities high of around 250 Ajm2, and low temperatures from -5 ° C to + 5 ° C). we cannot therefore compare the present anodizing process with prior techniques hard anodizing.
As part of more advanced developments in previously described anodization process, it appeared further worth considering adding in the bath aqueous anodizing 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 low con-centration seems interesting to limit the dissolution layer and to increase the wettability of the electrolyte, and thus obtain perfect homogeneity porosity throughout the thickness of the layer, and all without altering the electrical conductivity of the bath electrolytic. Acid alcohols are particularly interesting in this case, because they are totally misci-the electrolyte at room temperature. We get therefore great stability in the dissolution rate with very satisfactory maintenance of conductivity electric.
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 C6Ha07). The concentration of tartaric acid or citric acid will then preferably be essentially between 12 g / 1 and 17 g / 1, the optimum concentration mate observed during the tests carried out while driving sinage of 17 g / 1.
The choice of tartaric acid and the above acid also appears particularly interesting insofar as one wishes on the one hand to have a pH
stabilized at a low value, and on the other hand have a strong oxidant, whose electrochemical activity is high without being aggressive, and therefore not general puncture scorer. We see that anodic sulfo-tartaric or sulfo-citric oxidation appears much more attractive than anodic oxidation than sulfo-boric of prior techniques, in particular link the one described in document US-A-4,894 127. Indeed, we obtain here an excellent stability of the dissolution of the alumina layer.
Addition of tartaric or citric acid in the bath in the context of the invention makes it possible to have dissolution of the weaker alumina layer ble than with sulfuric acid alone, and moreover the densi-current in the bath does not fall as it would the case with other acids, like boric acid, in because of the action on surface tensions.
It is also important to provide that the workpiece undergoes a preliminary treatment of dë
greasing / pickling or deoxidation before being plon aged in the bath.
In traditional techniques, we used a first degreasing step, followed by rinsing, followed by a second stripping step in the middle alkaline or more generally in an aqueous solution sulfuric acid and chromic acid, followed by c = _nfin a new rinse. It appears more interesting, healthy to use a product capable of performing direct-in one step, a satisfactory degreasing / pickling health, and one can advantageously use a solution 5 tion composed of phosphoric acid added agents anionic surfactants, such as the commercial product sold under the reference "NOVACLEAN AL-85" by the company al-lenande HENKEL SURFACE TECHNOLOGIES. The use of a such a product makes it possible to have a uniform pickling, that is to say 10 say without the presence of any superficial attack cial.
Finally, it is interesting to foresee that the room treated undergoes a subsequent clogging treatment of the coating.
The clogging of the coating must ensure a double ble function which is to develop both promotion adhesion and corrosion resistance. This colma-tage is conventionally carried out by soaking in hot water at a temperature of at least 97 ° C, or in a dilute solution of potassium dichromate. We prefer a solution of deionized water at a temperature between 85 ° C and 98 ° C, soaking is done:
for a period which is a function of the thickness of re-garment obtained. Alternatively, we can also use sodium molybdate (MoNa204, 2H20) or sulfate of manganese (Mn04S, H20), or even immersion in a particularly effective sealant based nickel acetates or lithium acetates, for example ple using respectively the product marketed by the company HENKEL SURFACE TECHNOLOGIES mentioned above under the reference "ANOSEAL 1000" or "ENVIROSEAL 2500".
Among the many tests carried out by the request there is a first example of oxidation sulfuric anodic, with an aqueous anodizing bath containing sulfuric acid in a concentration of 62 g / 1. We then observed a regular growth of the coating layer, with perfect control: ~ e des small thicknesses, without modification of the initial roughness tial of the substrate.
Other tests with a sulfo- anodization bath citric can also be mentioned, with a con-concentration of 62 g / 1 in sulfuric acid and 17 g / 1 in citric acid. For the other operating conditions, we can mention that the constant temperature at which the bath was maintained was 22 ° C, and that the applied voltage was 10 V during a first level of 25 min then of 20 V during a second stage of 15 min.
We then managed to obtain a coating thickness ment of low porosity, between 2.8 ~ m and 3.2 ~, m.
As an indication, the tests carried out were made on aluminum alloys of references 2024 T 351 and 7175 T 7351.
After clogging, we could see that the parts thus treated still had a very satisfied aspect after 500 hours of exposure to fog its linen. In addition, the abatement characteristics in. fatigued that remained very satisfactory compared to the technical ques known anodic sulfo-boric or chromic oxidation nomic.
We thus managed to carry out a process very efficient anodizing which allows both check the coating thickness or weight and the gosity, and to obtain a low porosity on the surface of treated parts.
The invention is not limited to the process which has just been described, but on the contrary includes all cédë of equivalent anodization falling within the definition general given at the beginning of the description.

Claims (13)

1. Method of anodizing an alloy part of aluminum, comprising the following successive stages your:

- an aqueous anodization bath is provided comprising both essentially sulfuric acid, with a con-centering between 55 g / 1 and 85 g / 1, excluding any phosphoric acid or boric acid than ;

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

- said part is immersed in said bath;

- applying to said piece plunged into said bath a voltage essentially between 5 V and 30 v, with a low current density on said part;
and said part is maintained in said bath until only to obtain the desired coating thickness which is substantially between 1 µm and 3 µm. 2. The method of claim 1, wherein the concentration of sulfuric acid in the bath is essential between 57 g / 1 and 67 g / 1, being of preferably around 62 g / 1. 3. Method according to claim 1 or the res-dication 2, in which the bath is maintained at a temperature constant temperature close to 22 ° C. 4. Method according to one of claims 1 to 3, in which the voltage applied to the part plunged into the bath is set to a constant value throughout the duration of the anodization treatment, said value con-aunt 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. 6. Method according to any one of the claims 1 to 3, in which the tension applied to the workpiece diving in the bath is first set on a first constant value, then, after a predetermined period, on a second constant value higher than the first mière, said first and second constant values both being between 5 V and 30 V. 7. The method of claim 6, wherein the first constant voltage value is included in tre 5 V and 11 V, and the second constant value of voltage sion is between 15 V and 30 V. 8. Method according to any one of the claims 1 to 7, in which the current density on the room immersed in the bath remains significantly less than 100 A / m2. 9. The method of claim 8, wherein the current density is essentially less than 80 A / m2, preferably between 30 A / m2 and 70 A / m2. 10. Method according to any one of the claims.
1 to 9, in which the aqueous anodizing bath also has an acid-alcohol having one to three acid functions, with a concentration between 12 g / l and 22 g / l. 11. The method of claim 10, in the-which acid-alcohol used in the bath is tar-acid or citric acid, and the concentration of this acid-alcohol is essentially between 12 g / l and 17 g / l. 12. Method according to any one of the claims 1 to 11, in which the part to be treated undergoes preliminary degreasing / stripping treatment to be immersed in the bath. 13. Method according to any one of the claims 1 to 12, in which the treated part undergoes processing subsequent clogging of the coating.