Method of anodising aluminum alloy pieces

0001The present invention is in the field of surface treatment of aluminum or aluminum alloy parts, aimed at improving their corrosion resistance properties. More particularly, it relates to a method of anodizing an aluminum part or one of its alloys, as well as a more general method of surface treatment of such a part employing said anodizing process followed by a clogging step.

0002Aluminum alloy parts intended to be used in the aeronautical sector, or in other sectors in which they are likely to be exposed to risks of corrosion that may be problematic, generally receive, before their implementation. , a surface treatment to protect them against corrosion.

0003One of the most widespread techniques for this purpose is anodization, also called anodic oxidation, which consists in forming on the surface of the part a porous aluminum oxide / hydroxide layer, called anodic layer, by applying a current to the piece immersed in an electrolytic bath containing a strong acid electrolyte, the piece constituting the anode of the electrolytic device. The anodic layer thus formed on the surface of the workpiece, after having been subjected to a clogging after-treatment, protects the workpiece against corrosion. This anodic layer is also a support for the attachment of conventional paint systems.

0004The electrolytic baths currently used for the anodizing of aluminum alloy parts, which provide the most advantageous performance in terms, in particular, of protection against corrosion of the part, mechanical grip of paint coatings on the surface of the workpiece, and fatigue abatement, are formed with hexavalent chromium. Chemicals containing hexavalent chromium are, however, harmful to health and the environment.

0005In order to avoid the use of hexavalent chromium-based substances for the anodizing of aluminum alloy parts, it has been proposed by the prior art anodizing processes using other strong acids in the process. electrolytic bath, and in particular sulfuric acid. None of these baths, however, has satisfactory performance in terms of both protection of the workpiece against corrosion, adhesion of conventional paint systems on the workpiece, and fatigue reduction of the workpiece. These performances are particularly insufficient compared to the requirements imposed in the aeronautical field.

0006The documents <patcit id="pcit0001" dnum="DE4213535"><text>DE 42 13535</text></patcit>, <patcit id="pcit0002" dnum="US2004050709A"><text>US 2004/050709</text></patcit>, <patcit id="pcit0003" dnum="FR1452852"><text>FR 1 452 852</text></patcit>, <patcit id="pcit0004" dnum="EP0232211A"><text>EP 0 232 211</text></patcit> and the publication of <nplcit id="ncit0001" npl-type="s"><text>Tharp and Tyminski in Plating, 1968, 55: 6, 580-583 </text></nplcit>describe for example such anodic oxidation processes of aluminum alloy parts, including immersing the workpiece in a bath containing sulfuric acid, and the application of a voltage in this bath.

0007The document <nplcit id="ncit0002" npl-type="s"><text>Erin Beck, in Proceedings 2003 AESF / EPA Conference for Environmental &amp; Process Excellence</text></nplcit>, describes methods of anodic oxidation of aluminum alloy parts, also using anodizing baths containing sulfuric acid.

0008The present invention aims to overcome the disadvantages of anodizing processes of aluminum alloy parts of the prior art, in particular to those described above, by proposing such a process that does not implement any harmful substance, particularly to Hexavalent chromium base, while having at least equivalent performance to the processes of the prior art employing hexavalent chromium, in particular in terms of corrosion resistance of the treated part, fatigue fatigue of the part and adhesion of conventional paint systems on its surface.

0009It has now been discovered by the present inventors that a sulfuric type anodizing process, implemented under particular conditions, made it possible to achieve these objectives.

0010It is thus proposed according to the present invention a method of anodizing a piece of aluminum or aluminum alloy as defined in claim 1, wherein the piece is immersed in an aqueous bath comprising essentially sulfuric acid at a concentration between 150 and 250 g / L and maintained at a constant temperature of between 5 and 25 ° C. By means essentially comprising sulfuric acid, it is meant that the bath contains no other active electrolytic substance, in particular strong acid, in an amount sufficient to intervene in the anodization. The bath contains in particular no phosphoric, boric, chromic or tartaric acid, or only in the form of traces.

0011This method according to the invention is characterized by the application to the immersed part in the bath of a DC voltage according to a voltage profile comprising a rise in voltage, from a starting value of 0 V, at a speed of between 1 and 6 V / min, then maintaining the voltage at a so-called plateau voltage value of between 12 and 20 V for a period of time sufficient to obtain an anode layer of aluminum oxides / hydroxides on the surface of the part, thickness between 3 and 5 microns, said duration being between 5 and 30 minutes.

0012Such an anodic layer has properties of adhesion to paint and corrosion resistance after clogging equivalent to those of the anodic layers obtained by prior art chromic anodizing processes, while not using any substance. based on hexavalent chromium.

0013This result is furthermore advantageously obtained with a thickness of the anodic layer which is weak, that is to say less than or equal to 5 μm, when the so-called standard sulfuric anodization processes proposed by the prior art require, in order to achieve acceptable performances, which remain however less than those of the process according to the invention, to form on the part an anode layer of much greater thickness, typically between 8 and 12 microns. In this, the method according to the invention has an additional advantage, which is to overcome the problems of resizing and fatigue abatement generated by standard sulfuric anodization processes of the prior art.

0014In particular embodiments of the invention, the voltage profile applied to the part comprises a rise in voltage at a speed of between 1 and 6 V / min until reaching the so-called plateau voltage value between 12 and 20 V, then maintaining the voltage at said plateau voltage value for a period of time sufficient to obtain an anodic layer of aluminum oxides / hydroxides of thickness between 3 and 5 on the surface of the part. .mu.m.

0015In different embodiments, the voltage profile applied to the part comprises a plurality of voltage increase phases, at least one of which is carried out at a speed of between 1 and 6 V / min, and which can be separated. two by two by a bearing during which the voltage is temporarily maintained at a fixed value, before the implementation of the final phase of maintaining the voltage at the plateau voltage value between 12 and 20 V.

0016It is within the competence of those skilled in the art to determine the duration of holding the voltage at the plateau value, in order to obtain the desired anodic layer thickness on the part, particularly as a function of the characteristics of the particular alloy and the conditions for subsequent implementation of the part.

0017According to the invention, the tension is maintained at the plateau value for a period of between 5 and 30 minutes, depending on the aluminum alloy and the thickness of the desired anodic layer.

0018According to an advantageous characteristic of the invention, in terms of protective performance of the part against corrosion, the rate of rise in voltage is equal to 3 V / min.

0019Preferably, the plateau voltage value is between 14 and 16 V. It is within the competence of those skilled in the art to determine the optimum voltage value within this range, depending in particular on the characteristics of the alloy. constituting the room.

0020The concentration of sulfuric acid in the bath is preferably from 180 to 220 g / l, for example equal to 200 g / l.

0021In 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.

0022All these preferential parameters ensure the best performance of the bath from the point of view of the properties of the anodic layer formed on the surface of the part.

0023The part may be subjected to a surface preparation step by degreasing and / or pickling prior to immersion in the bath, so as to remove grease, dirt and oxides present on its surface.

0024This prior surface preparation step can comprise one or more of the following operations:<ul><li>solvent degreasing, to dissolve greases present on the surface of the room. This operation can be carried out by dipping, spraying, or any other technique known in itself. It may for example be carried out by dipping in methoklone or acetone, at a temperature below 42 ° C, for a period of between 5 seconds and 3 minutes;</li><li>alkaline degreasing, to dissolve greases present on the surface of the room. This operation can be carried out by dipping, spraying, or any other technique known in itself. It may for example be carried out by dipping in a mixture of TURCO 4215 NCLT (Henkel), at 40 to 60 g / L, and TURCO 4215 additive (Henkel), at 5 to 20 g / L, at a temperature of between 50.degree. and 70 ° C, for a period of between 10 and 30 minutes;</li><li>alkaline pickling, to dissolve oxides naturally formed on the surface of the workpiece. This operation can be carried out by dipping, spraying, or any other technique known per se. It may for example be carried out by soaking in a solution of sodium hydroxide at 30 to 70 g / l, at a temperature between 20 and 60 ° C, for a period of between 10 seconds and 2 minutes. At the end of this operation, the part is covered with a powdery layer formed of oxidation products of the intermetallic compounds, which should be removed by an acid pickling step;</li><li>acid stripping, for dissolving the naturally formed oxides on the surface of the workpiece, and / or the oxidation layer formed on the surface of the workpiece during the alkaline stripping step. This operation can be carried out by dipping, spraying, or any other technique known in itself. It may for example be carried out by dipping in a 15 to 25% v / v solution of SMUT-GO NC (Henkel) at a temperature of between 10 and 50 ° C for a period of between 1 and 10 minutes; or by soaking in a solution of ARDROX 295GD (Chemetall) at 15 to 30% v / v, at a temperature between 10 and 30 ° C, for a period of between 1 and 10 minutes.</li></ul>

0025Interleaved rinses, especially with water, are preferably made between the successive steps above, and before the treatment of the piece by anodizing.

0026Another aspect of the invention is a more general method of surface treatment of an aluminum or aluminum alloy part, according to which the part is subjected to an anodizing process corresponding to one or more of the characteristics above, then to a sealing step of the anodic layer then formed on the workpiece.

0027The sealing step of the porous anodic layer may be of any type known to those skilled in the art. It can for example be a hydrothermal clogging, a hot clogging with hexavalent chromium salts or nickel salts, etc. Clogging processes not involving any substance harmful to the environment and / or health are particularly preferred in the context of the invention.

0028In advantageous embodiments of the invention, this sealing step comprises immersing the part in an aqueous bath containing a trivalent chromium salt and an oxidizing compound, of a temperature of between 20 and 80.degree. preferably between 20 and 60 ° C, more particularly between 35 and 45 ° C, and / or the immersion of the piece in water at a temperature between 98 and 100 ° C, and pH for example between 4 , 5 and 8.

0029In the present description, conventional trivalent chromium is understood to mean chromium in the +3 oxidation state. Hexavalent chromium means chromium in the +6 oxidation state.

0030The oxidizing compound may be of any type known in itself for post-anodizing clogging baths of aluminum or its alloys. Compounds having no adverse effect on the environment are particularly preferred in the context of the invention. Nonlimiting examples of such oxidizing compounds are fluoride-based substances, such as ammonium fluoride or potassium fluoro-zirconate K₂ZrF₆of permanganate, such as potassium permanganate, hydrogen peroxide H₂O₂etc. The concentration of oxidizing compound in the bath may especially be between 0.1 and 50 g / l.

0031The trivalent chromium salt and the oxidizing compound present in the bath may be constituted by two different compounds, or by one and the same compound capable of ensuring on its own the two functions of corrosion inhibition and oxidation, for example by trivalent chromium fluoride CrF₃.

0032The trivalent chromium salt can be brought in any conventional form in itself for post-anodizing aluminum clogging treatments, especially 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.

0033In preferred embodiments of the invention, the trivalent chromium salt present in the bath is a fluoride. This is for example chromium trifluoride CrF₃.

0034In particular embodiments of the invention, the immersion step in the aqueous bath meets one or more of the following operating parameters:<ul><li>the bath temperature is between 20 and 80 ° C, preferably between 20 and 60 ° C, more preferably between 35 and 60 ° C, and preferably between 35 and 45 ° C, for example equal to 40 ° C;</li><li>the pH of the bath is between 3 and 4.5, preferably between 3 and 4, for example equal to 3.5;</li><li>the immersion time in the bath is between 5 and 40 min, preferably between 10 and 30 minutes, for example equal to 15 or 20 minutes.</li></ul>

0035The concentration of trivalent chromium salt in the bath is preferably between 0.5 and 50 g / l.

0036The immersion of the piece in water at a temperature of between 98 and 100 ° C. can be carried out with an immersion time of between 10 and 60 minutes, in accordance with the operating parameters of the so-called traditional hydrothermal sealing methods.

0037In 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 of between 98 and 100 ° C. These steps can be carried out in any order, and in particular be separated by one or more interleaved water rinses.

0038For example, the clogging step may comprise immersing the part in the aqueous bath containing a trivalent chromium salt and an oxidizing compound, and then, after rinsing (s), in water at a temperature of 98 ° C. at 100 ° C. Otherwise, the clogging step may include immersing the piece in water at a temperature of 98 to 100 ° C, and then, after rinsing (s), in the aqueous bath containing a trivalent chromium salt and an oxidizing compound.

0039The features and advantages of the invention will emerge more clearly in the light of the following examples of implementation, provided for illustrative and not limiting purposes of the invention, with the support of the <figref idref="f0001">FIGS. 1A to 1E</figref>, which show micrographs of anodic layers formed on the surface of aluminum parts by, <figref idref="f0001">figure 1A</figref>chromic anodizing (OAC), <figref idref="f0001">Figure 1B</figref>, standard sulfuric anodization (OASstandard), <figref idref="f0001">Figure 1C</figref>, sulfo-tartaric anodizing (OAST), <figref idref="f0001">figure 1D</figref>, sulfoboric anodizing (OASB) and <figref idref="f0001">figure 1E</figref>anodizing according to an embodiment of the invention.

EXAMPLE 1

1.1 /

Anodizing processes of aluminum alloy parts

0040Parts of 2024 T3 aluminum alloy laminated 120x80x2 mm dimensions are treated by anodizing according to the methods below.

0041Steps for surface preparation of the part are firstly carried out successively:<ul><li>alkaline degreasing, by soaking the workpiece in a mixture of TURCO 4215 NCLT at 50 g / L and TURCO 4215 additive at 10 g / L, at a temperature of 60 ° C, for 20 min;</li><li>rinsing with water;</li><li>acid etching, by soaking the part in a solution of SMUT-GO NC at 19% v / v, at a temperature of 20 ° C, for 5 min;</li><li>rinsing with water.</li></ul>

0042Parts are then subjected to an anodizing process according to an embodiment of the invention, as follows.

0043A bath is prepared by diluting a solution of sulfuric acid in water to obtain a sulfuric acid concentration of 200 g / L, excluding any other compound. This bath is heated and maintained at a temperature of 19 ° C.

0044The part is immersed in the bath, and it is applied a DC voltage according to the following voltage profile: voltage rise, from an initial value of 0 V, at a speed of 3 V / min, to a value called 16 V tray. The tension is maintained at the plateau value for 16 minutes.

0045On the surface of the part, an anodic oxide / aluminum hydroxide layer approximately 4 to 5 μm thick is formed.

0046By way of comparative examples, identical parts having been subjected to the same surface preparation operations are anodized according to the conventional methods of chromic anodization (OAC), standard sulfuric anodization (standard OAS), sulfo-tartaric anodizing (OAST) and sulfoboric anodizing (OASB).

0047The operating parameters for standard OAS, OAST, OASB and OAC are shown in Table 1 below.<tables id="tabl0001" num="0001"><table frame="all"><title><u>Table 1</u> operating parameters used for the various anodising processes of the standard OAS, OAST, OASB and OAC prior art</title><tgroup cols="5"><colspec colnum="1" colname="col1" colwidth="48mm" /><colspec colnum="2" colname="col2" colwidth="30mm" /><colspec colnum="3" colname="col3" colwidth="30mm" /><colspec colnum="4" colname="col4" colwidth="30mm" /><colspec colnum="5" colname="col5" colwidth="30mm" /><thead><row><entry valign="top" /><entry align="center" valign="top">Standard OAS</entry><entry align="center" valign="top">OAST</entry><entry align="center" valign="top">OASB</entry><entry align="center" valign="top">OAC</entry></row><row rowsep="0"><entry morerows="1" rowsep="1" valign="top">Composition of the bath</entry><entry morerows="1" rowsep="1" align="center" valign="top">H₂SO₄ : 200 g / L</entry><entry align="center" valign="top">H₂SO₄: 40 g / L</entry><entry align="center" valign="top">H₂SO₄: 45 g / L</entry><entry align="center" valign="top">CrO₃: 60 g / L</entry></row><row><entry align="center" valign="top">C₄H₆O₆: 80 g / L</entry><entry align="center" valign="top">H₃BO₃: 8 g / L</entry><entry align="center" valign="top">C₂H₂O₄: 2 g / L</entry></row></thead><tbody><row><entry>Bath temperature (° C)</entry><entry align="center">16 - 20</entry><entry align="center">36 - 39</entry><entry align="center">25 - 28</entry><entry align="center">38 - 42</entry></row><row><entry>Voltage rise (V / min)</entry><entry align="center">3.4</entry><entry align="center">2.8</entry><entry align="center">5.3</entry><entry align="center">4</entry></row><row><entry>Voltage and plateau time</entry><entry align="center">17 V 40 min</entry><entry align="center">14 V 25 min</entry><entry align="center">15 V 23 min</entry><entry align="center">20V 50 min</entry></row><row><entry>Thickness of the anodic layer formed on the part (μm)</entry><entry align="center">8 to 10</entry><entry align="center">2 to 5</entry><entry align="center">1 to 3</entry><entry align="center">3 to 5</entry></row></tbody></tgroup></table></tables>

0048The different pieces thus obtained are subjected to the following tests.

1.2 /

Morphological analysis of the anodic layer

0049Morphological analysis of the anodic layer formed on the surface of each of the pieces thus treated is carried out by field-effect electron microscopy (SEM-FEG). Micrographs are shown on<figref idref="f0001">FIGS. 1A to 1E</figref>. The<figref idref="f0001">figure 1E</figref>, corresponding to the anode layer obtained by a method according to an embodiment of the invention, shows a homogeneous morphology in the thickness of the layer, with the absence of micro-precipitates from the substrate within the layer. From the micrographic observations, the pore diameters were measured for each of the anode layers and the results are shown in Table 2 below.<tables id="tabl0002" num="0002"><table frame="all"><title><u>Table 2</u> - Pore diameter of the anodic layer formed on the 2024 T3 aluminum alloy parts rolled according to the anodizing process used</title><tgroup cols="5"><colspec colnum="1" colname="col1" colwidth="48mm" /><colspec colnum="2" colname="col2" colwidth="26mm" /><colspec colnum="3" colname="col3" colwidth="26mm" /><colspec colnum="4" colname="col4" colwidth="26mm" /><colspec colnum="5" colname="col5" colwidth="41mm" /><tbody><row><entry>Anodizing process</entry><entry align="center">OAC</entry><entry align="center">OAST</entry><entry align="center">OASB</entry><entry align="center">Anodizing according to the invention</entry></row><row><entry>Pore ​​diameter of the anodic layer (nm)</entry><entry align="center">20-30</entry><entry align="center">5-10</entry><entry align="center">5-10</entry><entry align="center">10-20</entry></row></tbody></tgroup></table></tables>

0050It can be seen from this table that the morphology of the anodic layer formed on the parts by the process according to an embodiment of the invention is similar to that of a layer obtained by chromic anodizing, compared with the other processes of the invention. anodizing using sulfuric acid proposed by the prior art.

1.3 /

Fatigue fatigue tests

0051The various anodized parts are subjected to a fatigue test in order to evaluate the fatigue reduction related to the formation of the anodic layer on their surface. The parameters of the fatigue test are as follows:<ul><li>solicitation: rotary bending</li><li>temperature: 20 ° C</li><li>R = -1</li><li>Frequency: 100 Hz</li><li>Kt = 1.035</li><li>type of test specimens: FFRT16</li><li>number of test pieces: 12</li></ul>

0052The results of this test, in terms of fatigue limit and abatement with respect to non-anodized parts, for parts processed by the method according to an embodiment of the invention and by various conventional methods, are shown in the Table. 3 below.<tables id="tabl0003" num="0003"><table frame="all"><title><u>Table 3</u> - Fatigue reduction evaluated by a fatigue test for 2024 T3 aluminum alloy parts according to the anodizing process used</title><tgroup cols="5"><colspec colnum="1" colname="col1" colwidth="44mm" /><colspec colnum="2" colname="col2" colwidth="30mm" /><colspec colnum="3" colname="col3" colwidth="23mm" /><colspec colnum="4" colname="col4" colwidth="26mm" /><colspec colnum="5" colname="col5" colwidth="44mm" /><thead><row><entry align="center" valign="top" /><entry align="center" valign="top">Untreated piece</entry><entry align="center" valign="top">Item processed by OAC</entry><entry align="center" valign="top">Standard OAS processed part</entry><entry align="center" valign="top">Piece treated by anodization according to the invention</entry></row></thead><tbody><row><entry align="center" valign="middle">Fatigue limit (10⁷ cycles) 90% survival (MPa)</entry><entry align="center" valign="middle">147</entry><entry align="center" valign="middle">120</entry><entry align="center" valign="middle">107</entry><entry align="center" valign="middle">136</entry></row><row><entry align="center" valign="middle">dejection</entry><entry align="center" valign="middle">Reference</entry><entry align="center" valign="middle">- 22.5%</entry><entry align="center" valign="middle">- 37%</entry><entry align="center" valign="middle">- 8%</entry></row></tbody></tgroup></table></tables>

0053These results clearly demonstrate that the fatigue reduction generated by the process according to an embodiment of the invention is significantly lower than that generated by conventional anodizing processes, whether anodizing. sulfuric acid (OAS) standard but even chromic anodizing (OAC), for an equivalent anodic layer thickness. The parts treated by the anodizing method according to an embodiment of the invention have a better resistance to efforts than those treated by the anodizing processes of the prior art. In particular, in comparison with standard sulfuric anodization, they allow a lightening of the structures in which they are implemented.

1.4 /

Adhesion tests of paint coatings

0054Anodized parts by the method according to an embodiment of the invention, as indicated above, are subjected to adhesion tests of conventional paint systems.

0055Two paint systems are tested: a water-based epoxy-based system (P60 + F70) and a solvent-based polyurethane system (PAC33 + PU66). The tests are carried out according to the ISO 2409 standard, for dry adhesion, after drying of the paint system, and for wet adhesion: after drying of the paint system, the samples are immersed in demineralised water for 14 hours. days and then dried before undergoing the adhesion test according to the standard.

0056The results are shown in Table 4 below.<tables id="tabl0004" num="0004"><table frame="all"><title><u>Table 4</u> Results of adhesion tests of two paint systems on rolled 2024 T3 aluminum alloy parts treated by a method according to an embodiment of the invention</title><tgroup cols="4"><colspec colnum="1" colname="col1" colwidth="46mm" /><colspec colnum="2" colname="col2" colwidth="37mm" /><colspec colnum="3" colname="col3" colwidth="42mm" /><colspec colnum="4" colname="col4" colwidth="42mm" /><thead><row><entry namest="col1" nameend="col2" align="left" valign="top">Paint system</entry><entry align="center" valign="top">Dry adhesion</entry><entry align="center" valign="top">Wet adhesion</entry></row></thead><tbody><row><entry morerows="1">Solvent base</entry><entry>PAC33</entry><entry align="center">Grade 0</entry><entry align="center">-</entry></row><row><entry>PAC33 + PU66</entry><entry align="center">Grade 0</entry><entry align="center">Grade 0</entry></row><row><entry morerows="1">Water-thinnable base</entry><entry>P60</entry><entry align="center">Grade 0</entry><entry align="center">-</entry></row><row><entry>P60 + F70</entry><entry align="center">Grade 0</entry><entry align="center">Grade 0</entry></row></tbody></tgroup></table></tables>

0057By way of comparison, similar tests are carried out on parts treated with standard sulfuric anodizing (standard OAS) as indicated above. The results of these tests are shown in Table 5 below.<tables id="tabl0005" num="0005"><table frame="all"><title><u>Table 5</u> - Results of adhesion tests of two paint systems on laminated 2024 T3 aluminum alloy parts treated by a standard sulfuric anodising process</title><tgroup cols="4"><colspec colnum="1" colname="col1" colwidth="46mm" /><colspec colnum="2" colname="col2" colwidth="37mm" /><colspec colnum="3" colname="col3" colwidth="42mm" /><colspec colnum="4" colname="col4" colwidth="42mm" /><thead><row><entry namest="col1" nameend="col2" align="left" valign="top">Paint system</entry><entry align="center" valign="top">Dry adhesion</entry><entry align="center" valign="top">Wet adhesion</entry></row></thead><tbody><row><entry morerows="1">Solvent base</entry><entry>PAC33</entry><entry align="center">Grade 0</entry><entry align="center">-</entry></row><row><entry>PAC33 + PU66</entry><entry align="center">Grade 0</entry><entry align="center">Grade 1</entry></row><row><entry morerows="1">Water-thinnable base</entry><entry>P60</entry><entry align="center">Grade 1</entry><entry align="center">-</entry></row><row><entry>P60 + F70</entry><entry align="center">Grade 1</entry><entry align="center">Grade 2</entry></row></tbody></tgroup></table></tables>

0058These results show that the parts treated by the process according to one embodiment of the invention have an adhesion to the paint systems, whether of the water-soluble or solvent-borne type, equivalent to those treated by conventional OAST anodizing processes. and OASB, which also show, in a known manner, Grade 0 results in the adhesion tests above. This adhesion, for one or the other of the two paint systems, is much greater than that obtained by the standard sulfuric anodization process proposed by the prior art.

1.5 /

Corrosion resistance after clogging

0059The parts treated by the method according to an embodiment of the invention, by OAC, OAST or OASB, as indicated above, are subjected to the clogging process C1 according to an embodiment of the invention. following invention:<ul><li>immersion in an aqueous bath of composition: CrF₃: 6 g / L and K₂ZrF₆: 1 g / L, in water, at a pH of 3.5 and a temperature of 40 ° C, for 15 minutes,</li><li>then immersion in water at a pH of 6.5, at a temperature of 98 ° C, for 40 minutes.</li></ul>

0060By way of comparative examples, anodized parts are also subjected to the following conventional sealing methods: hydrothermal clogging, hot clogging with hexavalent chromium salts, hot clogging with nickel salts, according to the operating conditions indicated in Table 6 below.<tables id="tabl0006" num="0006"><table frame="all"><title><u>Table 6</u> operating parameters used for different clogging processes</title><tgroup cols="4"><colspec colnum="1" colname="col1" colwidth="41mm" /><colspec colnum="2" colname="col2" colwidth="37mm" /><colspec colnum="3" colname="col3" colwidth="47mm" /><colspec colnum="4" colname="col4" colwidth="43mm" /><thead><row><entry valign="top" /><entry align="center" valign="top">Hydrothermal clogging</entry><entry align="center" valign="top">Clogging with chromium VI salts</entry><entry align="center" valign="top">Clogging with nickel salts</entry></row></thead><tbody><row><entry>Composition</entry><entry align="center">H₂O</entry><entry align="center">K₂Cr₂O₇: 30 mg / L</entry><entry align="center">(CH₃COO)₂Ni: 10 g / L</entry></row><row><entry>pH</entry><entry align="center">6.5</entry><entry align="center">6</entry><entry align="center">5.5</entry></row><row><entry>Temperature (° C)</entry><entry align="center">98</entry><entry align="center">98</entry><entry align="center">98</entry></row><row><entry>Immersion time (min.)</entry><entry align="center">40</entry><entry align="center">20</entry><entry align="center">30</entry></row></tbody></tgroup></table></tables>

0061On each treated part, an anodic layer clogged.

0062The parts thus treated are subjected to a salt spray test according to the ISO 9227 standard.

0063First approximate average results, obtained on a small number of pieces, are shown in Table 7 below.<tables id="tabl0007" num="0007"><table frame="all"><title><u>Table 7</u> - Salt spray resistance of 2024 T3 aluminum alloy rolled parts treated by anodization and clogging, the anodization being carried out by a method according to an embodiment of the invention or by anodizing processes of prior art</title><tgroup cols="5"><colspec colnum="1" colname="col1" colwidth="40mm" /><colspec colnum="2" colname="col2" colwidth="24mm" /><colspec colnum="3" colname="col3" colwidth="31mm" /><colspec colnum="4" colname="col4" colwidth="36mm" /><colspec colnum="5" colname="col5" colwidth="36mm" /><thead><row><entry align="center" valign="top" /><entry namest="col2" nameend="col5" align="center" valign="top">Resistance to salt spray (appearance of the 1^era puncture of corrosion) (h)</entry></row><row><entry morerows="1" align="center">Type of anodizing</entry><entry namest="col2" nameend="col5" align="center" valign="top">Clogging type</entry></row><row><entry align="center" valign="top">Hydrothermal clogging</entry><entry align="center" valign="top">Clogging with chromium VI salts</entry><entry align="center" valign="top">Clogging with nickel salts</entry><entry align="center" valign="top">Clogging C1</entry></row></thead><tbody><row><entry align="center">OAC</entry><entry align="center">300</entry><entry align="center">1,500</entry><entry align="center">-</entry><entry align="center">-</entry></row><row><entry align="center">OAST</entry><entry align="center">96</entry><entry align="center">1,300</entry><entry align="center">450</entry><entry align="center">550</entry></row><row><entry align="center">OASB</entry><entry align="center">96</entry><entry align="center">1,000</entry><entry align="center">336</entry><entry align="center">450</entry></row><row><entry align="center">Anodizing according to the invention</entry><entry align="center">300</entry><entry align="center">1,500</entry><entry align="center">850</entry><entry align="center">1,600</entry></row></tbody></tgroup></table></tables>

0064More precise average results concerning the appearance of the first corrosion pits (more precisely the 1^era corrosion puncture ("1^era ") And the generalization of corrosion (" G^we ")), Obtained on a larger number of pieces, are shown in Table 8 below.<tables id="tabl0008" num="0008"><table frame="all"><title><u>Table 8</u> - Salt spray resistance of 2024 T3 aluminum alloy rolled parts treated by anodization and clogging, the anodization being carried out by a method according to an embodiment of the invention or by anodizing processes of prior art</title><tgroup cols="9"><colspec colnum="1" colname="col1" colwidth="33mm" /><colspec colnum="2" colname="col2" colwidth="18mm" /><colspec colnum="3" colname="col3" colwidth="18mm" /><colspec colnum="4" colname="col4" colwidth="17mm" /><colspec colnum="5" colname="col5" colwidth="17mm" /><colspec colnum="6" colname="col6" colwidth="17mm" /><colspec colnum="7" colname="col7" colwidth="17mm" /><colspec colnum="8" colname="col8" colwidth="17mm" /><colspec colnum="9" colname="col9" colwidth="17mm" /><thead><row><entry align="center" valign="top" /><entry namest="col2" nameend="col9" align="center" valign="top">Salt spray resistance (h)</entry></row><row><entry morerows="1" align="center">Type of anodizing</entry><entry namest="col2" nameend="col9" align="center" valign="top">Clogging type</entry></row><row><entry namest="col2" nameend="col3" align="center" valign="top">Hydrothermal clogging</entry><entry namest="col4" nameend="col5" align="center" valign="top">Clogging with chromium VI salts</entry><entry namest="col6" nameend="col7" align="center" valign="top">Clogging with nickel salts</entry><entry namest="col8" nameend="col9" align="center" valign="top">Clogging C1</entry></row></thead><tbody><row><entry align="center" /><entry align="center">1 ^era</entry><entry align="center">G^we</entry><entry align="center">1 ^era</entry><entry align="center">G^we</entry><entry align="center">1 ^era</entry><entry align="center">G ^we</entry><entry align="center">1 ^era</entry><entry align="center">G^we</entry></row><row><entry align="center">OAC</entry><entry align="center">336</entry><entry align="center">1056</entry><entry align="center">1320</entry><entry align="center">2136</entry><entry align="center">-</entry><entry align="center">-</entry><entry align="center">-</entry><entry align="center">-</entry></row><row><entry align="center">OAST</entry><entry align="center">72</entry><entry align="center">192</entry><entry align="center">1176</entry><entry align="center">1368</entry><entry align="center">336</entry><entry align="center">840</entry><entry align="center">480</entry><entry align="center">1344</entry></row><row><entry align="center">OASB</entry><entry align="center">48</entry><entry align="center">168</entry><entry align="center">912</entry><entry align="center">1056</entry><entry align="center">288</entry><entry align="center">744</entry><entry align="center">384</entry><entry align="center">1128</entry></row><row><entry align="center">Anodizing according to the invention</entry><entry align="center">312</entry><entry align="center">1008</entry><entry align="center">1296</entry><entry align="center">2064</entry><entry align="center">792</entry><entry align="center">1344</entry><entry align="center">1488</entry><entry align="center">2520</entry></row></tbody></tgroup></table></tables>

0065These results clearly demonstrate that the anodizing method according to an embodiment of the invention, followed by a clogging step, of any type whatsoever, makes it possible to give the treated part a corrosion resistance. at least equivalent to that obtained by conventional anodising processes followed by the same blocking.

0066In particular, the method of anodizing according to an embodiment of the invention has anticorrosion performance equivalent to chromic anodization (OAC) in combination with a hydrothermal clogging or hot clogging with hexavalent chromium salts, and much better than dilute sulfo-tartaric (OAST) or sulfoboric (OASB) anodizations.

0067This ability of the anodic layer formed by the process according to the invention to be clogged during a post-treatment in order to provide it with properties of resistance to corrosion could notably be explained by its pore morphology of size greater than 10. nm, which facilitates its hydration during a hydrothermal clogging for example, resulting in the sealing of the pores and protection against corrosion by the barrier layer effect.

0068Finally, it is observed that the particular combination of the anodizing method according to an embodiment of the invention, with the clogging method C1 according to an embodiment of the invention, makes it possible to obtain results. in terms of corrosion resistance of the treated part, which are significantly higher than those obtained for any other combination anodizing / clogging.

EXAMPLE 2

0069Various parameters of the anodizing process according to the invention are varied with respect to Example 1 above.

2.1 /

Variations of sulfuric acid concentration

0070Aluminum alloy parts similar to those of Example 1, having previously been subjected to surface preparation steps as indicated in Example 1 above, are subjected 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, to the exclusion of any other compound. It is then applied to each piece a DC voltage according to the following voltage profile: voltage rise, from an initial value of 0 V, at a speed of 6 V / min, up to a plateau value of 16 V. The tension is maintained at the plateau value for 16 minutes.

0071The anode layer is then sealed by immersing the part in a water bath at a temperature between 98 and 100 ° C for 40 min.

0072On the surface of each part, an anodic oxide / aluminum hydroxide layer with a thickness of approximately 3.5 to 4.5 μm is formed.

0073As a comparative example, the same method of treatment by anodization and clogging is applied to a similar room, but using a concentration of sulfuric acid in the bath of 100g / l only.

0074The parts thus treated are subjected to a salt spray test in accordance with the ISO 9227 standard. The results obtained are shown in Table 9 below.<tables id="tabl0009" num="0009"><table frame="all"><title><u>Table 9</u> - Salt spray resistance of laminated 2024 T3 aluminum alloy parts treated by anodizing and clogging, for different concentrations of sulfuric acid in the anodizing bath</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="59mm" /><colspec colnum="2" colname="col2" colwidth="49mm" /><colspec colnum="3" colname="col3" colwidth="59mm" /><thead><row><entry align="center" valign="top" /><entry namest="col2" nameend="col3" align="center" valign="top">Salt spray resistance (h)</entry></row><row><entry align="center" valign="top">Concentration of sulfuric acid in the anodizing bath (g / l)</entry><entry align="center" valign="top">Appearance of the 1^era corrosion puncture</entry><entry align="center" valign="top">Generalization of corrosion</entry></row></thead><tbody><row><entry align="center">100</entry><entry align="center">120</entry><entry align="center">288</entry></row><row><entry align="center">150</entry><entry align="center">264</entry><entry align="center">888</entry></row><row><entry align="center">250</entry><entry align="center">264</entry><entry align="center">864</entry></row></tbody></tgroup></table></tables>

0075These results show the effectiveness, in terms of corrosion resistance of the treated parts, anodizing processes according to the invention using a concentration of sulfuric acid in the bath of between 150 and 250 g / l. This efficiency is notably greater than the comparative method using a sulfuric acid concentration of 100 g / l, lower than that recommended by the present invention.

2.2 /

Voltage rise speed variants

0076Aluminum alloy parts similar to those of Example 1, having previously been subjected to surface preparation steps as indicated in Example 1 above, are subjected to an anodizing process by immersion in a bath at 19 ° C containing sulfuric acid at a concentration of 200 g / l, excluding any other compound. It is then applied to each part a DC voltage according to the following voltage profile: voltage rise, from an initial value of 0 V, to a plateau value of 16 V. The voltage is then maintained at the value of plateau for 16 minutes. Different speeds of rise in voltage are tested: 1 V / min, 20 V / min, 32 V / min.

0077The anode layer is then sealed by immersing the part in a water bath at a temperature between 98 and 100 ° C for 40 min.

0078On the surface of each part is formed an anodic layer of aluminum oxide / hydroxide with a thickness of approximately 4 to 4.5 μm.

0079The parts thus treated are subjected to a salt spray test in accordance with the ISO 9227 standard. The results obtained are shown in Table 10 below.<tables id="tabl0010" num="0010"><table frame="all"><title><u>Table 10</u> - Salt spray resistance of 2024 T3 aluminum alloy parts laminated treated by anodizing and clogging, for different voltage rise rates</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="50mm" /><colspec colnum="2" colname="col2" colwidth="55mm" /><colspec colnum="3" colname="col3" colwidth="61mm" /><thead><row><entry align="center" valign="top" /><entry namest="col2" nameend="col3" align="center" valign="top">Salt spray resistance (h)</entry></row><row><entry align="center" valign="top">Rise speed (V / min)</entry><entry align="center" valign="top">Appearance of the 1^era corrosion puncture</entry><entry align="center" valign="top">Generalization of corrosion</entry></row></thead><tbody><row><entry align="center">1</entry><entry align="center">312</entry><entry align="center">984</entry></row><row><entry align="center">20</entry><entry align="center">288</entry><entry align="center">960</entry></row><row><entry align="center">32</entry><entry align="center">288</entry><entry align="center">984</entry></row></tbody></tgroup></table></tables>

0080These results show the effectiveness, in terms of corrosion resistance of the treated parts, of the anodizing process according to the invention implementing a rise in voltage at a speed of 1 V / min.

2.3 /

Variations of plateau voltage value

0081Aluminum alloy parts similar to those of Example 1, having previously been subjected to surface preparation steps as indicated in Example 1 above, are subjected 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. It is then applied to each part a DC voltage according to the following voltage profile: voltage rise, from an initial value of 0 V, with a speed of 3 V / min, up to a plateau value of 14 V or The voltage is then held at the plateau value for 16 minutes.

0082The anode layer is then sealed by the clogging method C1 described in Example 1 above.

0083On the surface of each part, an anodic oxide / aluminum hydroxide layer approximately 4 to 5 μm thick is formed.

0084The parts thus treated are subjected to a salt spray test in accordance with the ISO 9227 standard. The results obtained are shown in Table 11 below.<tables id="tabl0011" num="0011"><table frame="all"><title><u>Table 11</u> - Salt spray resistance of 2024 T3 aluminum alloy parts laminated treated by anodizing and clogging, for different plateau voltage values</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="52mm" /><colspec colnum="2" colname="col2" colwidth="54mm" /><colspec colnum="3" colname="col3" colwidth="60mm" /><thead><row><entry align="center" valign="top" /><entry namest="col2" nameend="col3" align="center" valign="top">Salt spray resistance (h)</entry></row><row><entry align="center" valign="top">Tray tension value (V)</entry><entry align="center" valign="top">Appearance of the 1^era corrosion puncture</entry><entry align="center" valign="top">Generalization of corrosion</entry></row></thead><tbody><row><entry align="center">14</entry><entry align="center">1176</entry><entry align="center">2376</entry></row><row><entry align="center">16</entry><entry align="center">1320</entry><entry align="center">2544</entry></row></tbody></tgroup></table></tables>

0085These results show the effectiveness, in terms of corrosion resistance of the treated parts, anodizing processes according to the invention implementing the final maintenance of the voltage at a plateau value of between 14 or 16 V.

2.4 /

Temperature variations of the anodizing bath

0086Aluminum alloy parts similar to those of Example 1, having previously been subjected to surface preparation steps as indicated in Example 1 above, are subjected to an anodizing process according to the invention. by immersion in a bath containing sulfuric acid at a concentration of 200 g / l, to the exclusion of any other compound. Several bath temperatures are tested, more particularly 6 ° C, 12 ° C and 25 ° C.

0087It is then applied to each piece a DC voltage according to the following voltage profile: voltage rise, from an initial value of 0 V, at a speed of 3 V / min, up to a plateau value of 16 V. The voltage is maintained at the plateau value for a period of between 10 and 60 minutes, depending on the temperature value of the bath. This time is set to obtain on the surface of each piece an anodic layer thickness of oxide / aluminum hydroxide of about 4 to 5 microns thick.

0088The anode layer is then sealed by the clogging method C1 described in Example 1 above.

0089As a comparative example, the same method of treatment by anodization and clogging is applied to a similar piece, but using an anode bath temperature of 30 ° C.

0090The parts thus treated are subjected to a salt spray test according to the ISO 9227 standard. The results obtained are shown in Table 12 below.<tables id="tabl0012" num="0012"><table frame="all"><title><u>Table 12</u> - Salt spray resistance of 2024 T3 aluminum alloy parts treated by anodizing and clogging, for different temperatures of the anodizing bath</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="61mm" /><colspec colnum="2" colname="col2" colwidth="44mm" /><colspec colnum="3" colname="col3" colwidth="62mm" /><thead><row><entry align="center" valign="top" /><entry namest="col2" nameend="col3" align="center" valign="top">Salt spray resistance (h)</entry></row><row><entry align="center" valign="top">Anodizing bath temperature (° C)</entry><entry align="center" valign="top">Appearance of the 1^era corrosion puncture</entry><entry align="center" valign="top">Generalization of corrosion</entry></row></thead><tbody><row><entry align="center">6</entry><entry align="center">1272</entry><entry align="center">2304</entry></row><row><entry align="center">12</entry><entry align="center">1224</entry><entry align="center">2280</entry></row><row><entry align="center">25</entry><entry align="center">1320</entry><entry align="center">2424</entry></row><row><entry align="center">30</entry><entry align="center">624</entry><entry align="center">1536</entry></row></tbody></tgroup></table></tables>

0091These results show the effectiveness, in terms of corrosion resistance of the treated parts, anodizing processes according to the invention using an anode bath temperature of between 5 and 25 ° C. This efficiency is notably superior to the comparative method using a bath temperature of 30 ° C., higher than that recommended by the present invention.

0092The above description clearly illustrates that by its different features and advantages, the present invention achieves the objectives it has set for itself. In particular, it provides a method of anodizing aluminum alloy parts which avoids the use of substances based on hexavalent chromium, while having performance, in particular in terms of corrosion resistance of the treated part, fatigue reduction and adhesion of paint coatings on the surface of the part, which are at least equivalent to those of chromic anodizing processes, and superior to those sulfuric anodizing processes proposed by the prior art .