Patent Application
20160047057
The present invention relates to the field of surface treatment of parts in aluminum or aluminum alloy, with the aim of improving their corrosion resistance properties. More particularly, it relates to a method for anodizing a part made of aluminum or an aluminum alloy, as well as to a more general method of surface treatment of such a part using said anodizing method followed by a sealing step.
Aluminum alloy parts intended to be used in the aeronautical sector, or in other sectors in which they are likely to be exposed to corrosion risks that may prove problematic, generally receive, prior to application, a surface treatment with the aim of protecting them against corrosion.
One of the techniques most widely used for this purpose is anodizing, also called anodic oxidation, which consists of forming, on the surface of the part, a porous layer of aluminum oxides/hydroxides, called an anodic layer, by applying a current to the part immersed in an electrolytic bath containing an electrolyte of the strong acid type, the part constituting the anode of the electrolytic device. The anodic layer thus formed on the surface of the part, after undergoing a sealing post-treatment, protects the part against corrosion. This anodic layer also constitutes a substrate suitable for conventional paint systems.
The electrolytic baths employed at present for anodizing aluminum alloy parts, which provide the most advantageous performance notably in terms of protecting the part against corrosion, mechanical bonding of paint coatings to the surface of the part, and reduction in fatigue strength, are formed on the basis of hexavalent chromium. However, chemicals containing hexavalent chromium have proved to be harmful to health and to the environment.
In order to avoid the use of substances based on hexavalent chromium for anodizing aluminum alloy parts, anodizing processes have been proposed in the prior art using other strong acids in the electrolytic bath, and in particular sulfuric acid. However, none of these baths gives satisfactory performance in terms simultaneously of protection of the part against corrosion, adherence of conventional paint systems on the part, and reduction in fatigue strength of the part. The performance proves insufficient notably in respect of the requirements imposed in the aeronautical sector.
The present invention aims to overcome the drawbacks of the methods for anodizing aluminum alloy parts of the prior art, notably those presented above, by proposing such a method that does not use any harmful substance, notably based on hexavalent chromium, while displaying performance at least equivalent to the methods of the prior art using hexavalent chromium, in particular in terms of corrosion resistance of the treated part, reduction in fatigue strength of the part and adherence of conventional paint systems on its surface.
It has now been discovered by the present inventors that an anodizing method of the sulfuric acid type, carried out in particular conditions, made it possible to achieve these objectives.
Thus, an anodizing method is proposed according to the present invention for anodizing an aluminum or aluminum-alloy part, according to which the part is immersed in an aqueous bath essentially comprising sulfuric acid at a concentration between 150 and 250 g/L and maintained at a constant temperature between 5 and 25° C. “Essentially comprising sulfuric acid” means that the bath does not contain any other electrolytically active substance, notably strong acid, in sufficient amount for it to be involved in the anodizing process. The bath notably does not contain phosphoric, boric, chromic or tartaric acid, or does, but only in trace amounts.
This method according to the invention is characterized by the application, to the part immersed in the bath, of a DC voltage according to a voltage profile comprising an increase in voltage, from a starting value of 0 V, at a rate between 1 and 32 V/min, then maintaining the voltage at a so-called plateau voltage value between 12 and 20 V for a sufficient time to obtain an anodic layer, of aluminum oxides/hydroxides, with a thickness between 3 and 7 μm, preferably between 3 and 5 μm, and/or a layer weight between 20 and 150 mg/dm2, on the surface of the part.
This anodic layer displays properties of adherence to paint and of corrosion resistance after sealing equivalent to those of the anodic layers obtained by the chromic acid anodizing processes of the prior art, and without employing a substance based on hexavalent chromium.
This result is moreover advantageously obtained with a small thickness of the anodic layer, i.e. less than or equal to 7 μm, preferably less than or equal to 5 μm, whereas the so-called standard sulfuric anodizing processes proposed in the prior art require, to attain acceptable performance, which nevertheless remains below that of the method according to the invention, forming an anodic layer of much greater thickness, typically between 8 and 12 μm, on the part. The method according to the invention therefore offers an additional advantage, which is that it does away with problems of redimensioning and reduction in fatigue strength caused by the standard sulfuric anodizing processes of the prior art.
In particular embodiments of the invention, the voltage profile applied to the part comprises an increase in voltage at a rate between 1 and 32 V/min until the so-called plateau voltage value is reached between 12 and 20 V, then maintaining the voltage at said plateau voltage value for a sufficient time to obtain an anodic layer, of aluminum oxides/hydroxides, of thickness between 3 and 7 μm, preferably between 3 and 5 μm, and/or of layer weight between 20 and 150 mg/dm2, on the surface of the part.
In different embodiments, the voltage profile applied to the part comprises a plurality of phases of voltage increase, at least one of which is carried out at a rate between 1 and 32 V/min, and which may be separated two by two by a plateau during which the voltage is maintained temporarily at a fixed value, before application of the final phase of maintaining the voltage at the plateau voltage value between 12 and 20 V.
A person skilled in the art is able to determine the time for maintaining the voltage at the plateau value, to obtain the desired thickness of anodic layer on the part, notably as a function of the characteristics of the particular alloy and the conditions of subsequent use of the part.
In some embodiments of the invention, the voltage is maintained at the plateau value for a time between 5 and 30 minutes, according to the aluminum alloy and the desired thickness of the anodic layer.
According to an advantageous feature of the invention, in terms of performance of protection of the part against corrosion, the rate of increase in voltage is between 1 and 6 V/min, preferably equal to 3 V/min.
Preferably, the plateau voltage value is between 14 and 16 V. A person skilled in the art is able to determine the optimal voltage value within this range, notably in relation to the characteristics of the alloy of which the part is constituted.
The concentration of sulfuric acid in the bath is preferably between 180 and 220 g/L, for example equal to 200 g/L.
In some embodiments of the invention, the bath temperature is between 15 and 25° C., preferably between 18 and 20° C., and for example equal to 19° C.
All these preferred parameters ensure the best performance of the bath from the standpoint of the properties of the anodic layer formed on the surface of the part.
The part may be submitted to a step of surface preparation by degreasing and/or pickling prior to its immersion in the bath, so as to remove grease, dirt and oxides present on its surface.
This preliminary step of surface preparation may comprise one or more of the following operations:
Interposed rinsings, notably with water, are preferably carried out between the aforementioned successive steps, and prior to treatment of the part by anodizing.
Another aspect of the invention is a more general method of surface treatment of a part in aluminum or aluminum alloy, according to which the part is submitted to an anodizing method corresponding to one or more of the features mentioned above, then to a step of sealing the anodic layer thus formed on the part.
The step of sealing the porous anodic layer may be of any type known by a person skilled in the art. It may for example be hydrothermal sealing, hot sealing with hexavalent chromium salts or with nickel salts, etc. The methods of sealing that do not employ any substance that is harmful to the environment and/or health are particularly preferred in the context of the invention.
In advantageous embodiments of the invention, this sealing step comprises immersion of the part in an aqueous bath containing a trivalent chromium salt and an oxidizing compound, with a temperature between 20 and 80° C., preferably between 20 and 60° C., more particularly between 35 and 45° C., and/or immersion of the part in water at a temperature between 98 and 100° C., and with pH for example between 4.5 and 8.
In the present description, trivalent chromium means, conventionally per se, chromium in the +3 oxidation state. Hexavalent chromium means chromium in the +6 oxidation state.
The oxidizing compound may be of any type known per se for baths for post-anodizing sealing of aluminum or aluminum alloys. Compounds that do not have a harmful effect on the environment are particularly preferred in the context of the invention. Nonlimiting examples of these oxidizing compounds are substances based on fluorides, such as ammonium fluoride or potassium fluozirconate K2ZrF6, on permanganate, such as potassium permanganate, on hydrogen peroxide H2O2, etc. The concentration of oxidizing compound in the bath may notably be between 0.1 and 50 g/L.
The trivalent chromium salt and the oxidizing compound present in the bath may consist of two different compounds, or of one and the same compound that is able just by itself to provide the two functions of inhibition of corrosion and of oxidation, for example trivalent chromium fluoride CrF3.
The trivalent chromium salt may be supplied in any conventional form per se for treatments of post-anodizing sealing of aluminum, notably in the form of fluoride, chloride, nitrate, acetate, acetate hydroxide, sulfate, potassium sulfate, etc., of trivalent chromium, for example CrF3,xH2O, CrCl3,xH2O, Cr(NO3)3,xH2O, (CH3CO2)2Cr,xH2O, (CH3CO2)7Cr3(OH)2,xH2O, Cr2(SO4)3,xH2O, CrK(SO4)2,xH2O, etc.
In preferred embodiments of the invention, the trivalent chromium salt present in the bath is a fluoride. It is for example chromium trifluoride CrF3.
In particular embodiments of the invention, the step of immersing in the aqueous bath corresponds to one or more of the following operating parameters:
The concentration of trivalent chromium salt in the bath is preferably between 0.5 and 50 g/L.
The immersion of the part in water at a temperature between 98 and 100° C. may be carried out with a duration of immersion between 10 and 60 minutes, according to the operating parameters of the so-called conventional hydrothermal methods of sealing.
In particular embodiments of the invention, the sealing step comprises immersing the part successively in the aqueous bath containing a trivalent chromium salt and an oxidizing compound, and in water at a temperature between 98 and 100° C. These steps may be carried out in any order, and notably may be separated by one or more interposed rinsings with water.
For example, the sealing step may comprise immersing the part in the aqueous bath containing a trivalent chromium salt and an oxidizing compound, then, after optional rinsing(s), in water at a temperature from 98 to 100° C. Otherwise, the sealing step may comprise immersing the part in water at a temperature from 98 to 100° C., then, after optional rinsing(s), in the aqueous bath containing a trivalent chromium salt and an oxidizing compound.
The features and advantages of the invention will become clearer from the embodiment examples given below, supplied purely for purposes of illustration and not in any way limiting the invention, with the aid of
Parts in rolled aluminum alloy 2024 T3 with the dimensions 120×80×2 mm are treated by anodizing by the following methods.
Steps of surface preparation of the part are first carried out successively:
Some parts are then submitted to an anodizing process according to an embodiment of the invention, as follows.
A bath is prepared by diluting a sulfuric acid solution in water to obtain a concentration of sulfuric acid of 200 g/L, excluding any other compound. This bath is adjusted to and maintained at a temperature of 19° C.
The part is immersed in the bath, and a DC voltage is applied to it according to the following voltage profile: voltage increase, from an initial value of 0 V, at a rate of 3 V/min, up to a so-called plateau value of 16 V. The voltage is maintained at the plateau value for 16 minutes.
An anodic layer of aluminum oxide/hydroxide with a thickness of about 4 to 5 μm forms on the surface of the part.
As comparative examples, identical parts that have undergone the same operations of surface preparation are anodized according to the conventional methods of chromic anodizing (OAC), standard sulfuric anodizing (OAS standard), sulfo-tartaric anodizing (OAST) and sulfo-boric anodizing (OASB).
The operating parameters for OAS standard, OAST, OASB and OAC are shown below in Table 1.
The various parts thus obtained are submitted to the following tests.
A morphological analysis of the anodic layer formed on the surface of each of the parts thus treated is carried out by field-effect electron microscopy (FEG-SEM). The micrographs are shown in
It can be seen in this table that the morphology of the anodic layer formed on the parts by the method according to an embodiment of the invention is more similar to that of a layer obtained by chromic anodizing, relative to the other anodizing processes using sulfuric acid proposed in the prior art.
The various anodized parts are submitted to a fatigue test in order to evaluate the reduction in fatigue strength associated with formation of the anodic layer on their surface. The parameters of the fatigue test are as follows:
The results of this test, in terms of fatigue limit and reduction relative to the unanodized parts, for parts treated by the method according to an embodiment of the invention and by different conventional methods, are shown in Table 3 below.
These results clearly show that the reduction in fatigue strength caused by the method according to an embodiment of the invention is far less than that generated by the conventional anodizing processes, whether standard sulfuric anodizing (OAS) or chromic anodizing (OAC), for an equivalent thickness of the anodic layer. The parts treated by the anodizing method according to an embodiment of the invention notably have better resistance to stresses than those treated by the anodizing processes of the prior art. In particular, in comparison with standard sulfuric anodizing, they allow weight reduction of the structures in which they are used. These parts may moreover be substituted advantageously for the parts treated by chromic anodizing already used, notably in aircraft, without the need for any redimensioning of them.
Parts anodized by the method according to an embodiment of the invention, as indicated above, are submitted to tests of adherence of conventional paint systems.
Two paint systems are tested: a water-dilutable epoxy-based system (P60+F70) and a solvent-treated polyurethane-based system (PAC33+PU66). The tests are carried out according to standard ISO 2409, for dry adherence, after drying of the paint system, and for wet adherence: after drying of the paint system, the samples are immersed in demineralized water for 14 days, then dried before undergoing the adherence test according to the standard.
The results are shown in Table 4 below.
For comparison, similar tests are carried out on parts treated by standard sulfuric anodizing (OAS standard) as indicated above. The results of these tests are given in Table 5 below.
These results show that the parts treated by the method according to an embodiment of the invention display adherence to the paint systems, whether they are of the water-dilutable or solvent-treated type, equivalent to those treated by the conventional anodizing processes OAST and OASB, which, as is known, also give results expressed as Grade 0 in the aforementioned adherence tests. This adherence, for either of the two paint systems, is far higher than that obtained by the standard sulfuric anodizing process proposed in the prior art.
The parts treated by the method according to an embodiment of the invention, by OAC, OAST or OASB, as indicated above, are submitted to the sealing process C1 according to an embodiment of the invention, as follows:
As comparative examples, anodized parts are also submitted to the following different conventional methods of sealing: hydrothermal sealing, hot sealing with hexavalent chromium salts, hot sealing with nickel salts, according to the operating conditions shown in Table 6 below.
A sealed anodic layer is obtained on each treated part.
The parts thus treated are submitted to a salt spray test according to standard ISO 9227.
Preliminary approximate average results, obtained on a small number of parts, are shown in Table 7 below.
More precise average results relating to appearance of the first corrosion pits (more precisely of the 1st corrosion pit (“1st”) and generalized corrosion (“Gon”)), obtained on a larger number of parts, are shown in Table 8 below.
These results clearly demonstrate that the anodizing process according to an embodiment of the invention, followed by a sealing step, of whatever type, makes it possible to endow the treated part with corrosion resistance at least equivalent to that obtained by the conventional anodizing processes followed by the same sealing.
In particular, the anodizing process according to an embodiment of the invention displays anticorrosion performance equivalent to chromic anodizing (OAC) combined with hydrothermal sealing or hot sealing with hexavalent chromium salts, and far superior to diluted sulfo-tartaric anodizing (OAST) or sulfo-boric anodizing (OASB).
This capacity of the anodic layer formed by the method according to the invention to be sealed in a post-treatment to give it corrosion resistance properties might notably be explained by its morphology with pores larger than 10 nm, which facilitates its hydration during hydrothermal sealing for example, leading to blocking of the pores and protection against corrosion by a barrier layer effect.
Finally it is observed that the particular combination of the anodizing process according to an embodiment of the invention, with the sealing process C1 according to an embodiment of the invention, makes it possible to obtain results in terms of corrosion resistance of the treated part that are far superior to those obtained for any other anodizing/sealing combination.
Various parameters of the anodizing process according to the invention are varied relative to the preceding Example 1.
Aluminum alloy parts similar to those of Example 1, having previously been submitted to steps of surface preparation as indicated in Example 1 above, are submitted to an anodizing process according to the invention by immersion in a bath at 19° C. containing sulfuric acid at a concentration of 150 or 250 g/l, excluding any other compound. A DC voltage is then applied to each part according to the following voltage profile: voltage increase, from an initial value of 0 V, at a rate of 6 V/min, up to a so-called plateau value of 16 V. The voltage is maintained at the plateau value for 16 minutes.
The anodic layer is then sealed by immersing the part in a water bath at a temperature between 98 and 100° C., for 40 min.
An anodic layer of aluminum oxide/hydroxide with a thickness of about 3.5 to 4.5 μm forms on the surface of each part.
As a comparative example, the same method of treatment by anodizing and then sealing is applied to a similar part, but using a concentration of sulfuric acid in the bath of only 100 g/l.
The parts thus treated are submitted to a salt spray test according to standard ISO 9227. The results obtained are shown in Table 9 below.
These results show the effectiveness, in terms of corrosion resistance of the treated parts, of the anodizing processes according to the invention using a concentration of sulfuric acid in the bath between 150 and 250 g/l. This effectiveness is notably far better than the comparative method using a concentration of sulfuric acid of 100 g/l, which is lower than that recommended by the present invention.
Aluminum alloy parts similar to those of Example 1, having been submitted beforehand to steps of surface preparation as indicated in Example 1 above, are submitted to an anodizing process according to the invention by immersion in a bath at 19° C. containing sulfuric acid at a concentration of 200 g/l, excluding any other compound. A DC voltage is then applied to each part according to the following voltage profile: voltage increase, from an initial value of 0 V, up to a so-called plateau value of 16 V. The voltage is then maintained at the plateau value for 16 minutes. Different rates of voltage increase are tested: 1 V/min, 20 V/min, 32 V/min.
The anodic layer is then sealed by immersing the part in a water bath at a temperature between 98 and 100° C., for 40 min.
An anodic layer of aluminum oxide/hydroxide with a thickness of about 4 to 4.5 μm forms on the surface of each part.
The parts thus treated are submitted to a salt spray test according to standard ISO 9227. The results obtained are shown in Table 10 below.
These results show the effectiveness, in terms of corrosion resistance of the treated parts, of the anodizing processes according to the invention using a rate of voltage increase between 1 and 32 V/min.
Aluminum alloy parts similar to those of Example 1, having been submitted beforehand to steps of surface preparation as indicated in Example 1 above, are submitted to an anodizing process according to the invention by immersion in a bath at 19° C. containing sulfuric acid at a concentration of 200 g/l, excluding any other compound. A DC voltage is then applied to each part according to the following voltage profile: voltage increase, from an initial value of 0 V, at a rate of 3 V/min, up to a so-called plateau value of 14 V or 16 V. The voltage is then maintained at the plateau value for 16 minutes.
The anodic layer is then sealed by the sealing process C1 described in Example 1 above.
An anodic layer of aluminum oxide/hydroxide with a thickness of about 4 to 5 μm forms on the surface of each part.
The parts thus treated are submitted to a salt spray test according to standard ISO 9227. The results obtained are shown in Table 11 below.
These results show the effectiveness, in terms of corrosion resistance of the treated parts, of the anodizing processes according to the invention, finally maintaining the voltage at a plateau value between 14 or 16 V.
Aluminum alloy parts similar to those of Example 1, having been submitted beforehand to steps of surface preparation as indicated in Example 1 above, are submitted to an anodizing process according to the invention by immersion in a bath containing sulfuric acid at a concentration of 200 g/l, excluding any other compound. Several bath temperatures are tested, more particularly 6° C., 12° C. and 25° C.
A DC voltage is then applied to each part according to the following voltage profile: voltage increase, from an initial value of 0 V, at a rate of 3 V/min, up to a so-called plateau value of 16 V. The voltage is maintained at the plateau value for a time of between 10 and 60 minutes, depending on the value of bath temperature. This duration is fixed to obtain an anodic layer of aluminum oxide/hydroxide with a thickness of about 4 to 5 μm on the surface of each part.
The anodic layer is then sealed by the sealing process C1 described in Example 1 above.
As a comparative example, the same method of treatment by anodizing and then sealing is applied to a similar part, but using an anodizing bath temperature of 30° C.
The parts thus treated are submitted to a salt spray test according to standard ISO 9227. The results obtained are shown in Table 12 below.
These results show the effectiveness, in terms of corrosion resistance of the treated parts, of the anodizing processes according to the invention using an anodizing bath temperature between 5 and 25° C. This effectiveness is notably far better than the comparative method using a bath temperature of 30° C., which is higher than that recommended by the present invention.
The above description clearly illustrates that, owing to its various features and the advantages thereof, the present invention achieves the stated objectives. In particular, it provides a method for anodizing aluminum alloy parts which avoids the use of substances based on hexavalent chromium, while giving performance, in terms notably of corrosion resistance of the treated part, reduction in fatigue strength and adherence of the paint coatings on the surface of the part, which are at least equivalent to those of the chromic acid anodizing processes, and better than those of the sulfuric anodizing processes proposed in the prior art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1251273 | Feb 2012 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP13/52686 | 2/11/2013 | WO | 00 |
