WO2020128353A1

WO2020128353A1 - Anticorrosion treatment solution and uses

Info

Publication number: WO2020128353A1
Application number: PCT/FR2019/053191
Authority: WO
Inventors: Jérôme FRAYRET; Mathieu POURRILLOU; Sandra ZOCCALI; Jean-Charles DUPIN; Arnaud UHART
Original assignee: Université De Pau Et Des Pays De L'adour; Centre National De La Recherche Scientifique; Soule Peintures Industrielles Aero
Priority date: 2018-12-20
Filing date: 2019-12-19
Publication date: 2020-06-25
Also published as: US20230193472A1; KR20210126552A; FR3090694A1; FR3090694B1; EP3899089A1; CN113544312A; CA3123826A1

Abstract

The present invention relates to a solution that is free of chromium in all its oxidising states, comprising: - at least one chemical oxidising compound, - at least one aluminium complexing agent, - at least one corrosion inhibiting compound, and - optionally, a chemical sealant compound, the solution having a pH in the range from 1 to 5. The present invention also relates to a method for treating a metal surface, comprising the application, on the surface, of a solution as defined above. The present invention additionally relates to a coating of a metal surface that can be obtained by the method for treating a metal surface as defined above, to a metal surface comprising the coating and to the use of the solution in an anti-corrosion treatment of a metal surface.

Description

TITLE OF THE INVENTION: ANTI-CORROSION TREATMENT SOLUTION AND

USES

[1] Technical area

[2] The present invention relates to a solution devoid of chromium in all its oxidation states, to a process for treating a metallic surface, comprising the application, on this surface, of the solution, as well as to a coating of a metallic surface capable of being obtained by the treatment process.

[3] In the description below, the references in square brackets ([]) refer to the list of references presented at the end of the text.

[4] State of the art

[5] Reducing the fuel consumption of aeronautical products is an important issue as much for equipment manufacturers and aircraft manufacturers as for engine manufacturers.

[6] In the race to evolve emerging materials, new, lighter, aluminum-based alloys have been developed.

[7] However, aluminum and its alloys are susceptible to corrosion. Therefore, objects based on aluminum or its alloys must be protected from attack by the external environment which can result in corrosion. Protective coatings must therefore be applied to them to protect the aluminum.

[8] The majority of current corrosion protection methods use compositions based on hexavalent chromium. Such compositions include, for example, chromium trioxide (CrC> 3), potassium dichromate K2O2O7, sodium dichromate Na ₂ Cr0 ₇ , or strontium chromate SrCr0 ₄ . However, hexavalent chromium is listed in the dangerous substances prohibited by the REACH regulation (Registration, Evaluation, Authorization and restriction of CHemicals), which aims to protect human health and the environment from the risks linked to chemical substances, while promoting the competitiveness of the chemical industry in the European Union. Its use has been completely prohibited since 2017. [9] Different treatments for protecting aluminum alloys from corrosion exist and depend on the composition of the alloy: electrochemical deposition, anodization, chemical conversion, gas deposition, sol-gel coatings or coatings by laser deposition. These various treatments are used on an industrial scale with a preference observed for the anodization and chemical conversion processes. In particular, the surface treatment by chemical conversion has several advantages such as its low cost, its ease of use and the properties of the protective layer obtained in accordance with the specifications of aeronautics for example. In fact, in the aeronautical environment, the metal parts treated against corrosion must have adhesion, coloring, conduction and bonding properties for a subsequent paint or varnish. In fact, a paint or varnish is generally applied after the conversion layer to improve protection against corrosion. The coloring makes it possible to carry out a visual control of the quality of the anticorrosion coating and represents a usual element in the specifications of certain customers. Furthermore, adhesion between the coating and the metal surface is necessary.

[10] Several types of chemical conversion treatments using solutions devoid of hexavalent chromium exist and are currently marketed. Generally, these are solutions based on trivalent chromium also noted Cr (III) (JT Qi et al.: "Trivalent chromium conversion coating formation on aluminum, Surface and Coatings Technology", 280 (2015) 317-329 ([ 1]); W.- K. Chen et al.: "The effect of chromic sulfate concentration and immersion time on the structures and anticorrosive performance of the Cr (lll) conversion coatings on aluminum alloys", Applied Surface Science, 256 (2010 ) 4924-4929 ([2])), such as SURTEC® 650 sold by the company SURTEC, or the Lanthanum solution 613.3 sold by the company COVENTYA or the TCS commercial by the company SOCOMORE.

[11] Another alternative to using a composition based on hexavalent chromium consists in using solutions based on phosphate compounds (F. Andreatta et al.: “Addition of phosphates or copper nitrate in a fluotitanate conversion coating containing a silane coupling agent for aluminum alloy AA6014 ", Progress in Organic Coatings, 77 (2014) 2107-2115 ([3])), of compounds of titanium and zirconium in a fluorinated medium (P. Santa Coloma et al.: “Chromium- free conversion coatings based on inorganic salts (Zr / Ti / Mn / Mo) for aluminum alloys used in aircraft applications”, Applied Surface Science, 345 (2015) 24-35 ([6]); PD Deck et al.: “Investigation of fluoacid based conversion coatings on aluminum, Progress in Organic Coatings”, 34 (1998) 39-48 ([9]); .H. Nordlien et al.: "Formation of a zirconium-titanium based conversion layer on AA 6060 aluminum", Surface and Coatings Technology, 153 (2002) 72-78 ([10])), of molybdenum or manganese ([6]) , or rare earths such as cerium (B. Valdez et al.: "Cerium-based conversion coatings to improve the corrosion resistance of aluminum alloy 6061 -T6", Corrosion Science, 87 (2014) 141 -149 ([4] ); P. Campestrini et al.: “Formation of a cerium-based conversion coating on AA2024: relationship with the microstructure, Surface and Coatings Technology”, 176 (2004) 365-381 ([8])), or zirconium en m fluorinated area (Fl. R. Asemani et al. : "Effect of zirconium conversion coating: Adhesion and anti-corrosion properties of epoxy organic coating containing zinc aluminum polyphosphate (ZAPP) pigment on carbon mild Steel", Progress in Organic Coatings, 94 (2016) 18-27 ([5]); FO George et al. : “Formation of zirconium-based conversion coatings on aluminum and Al-Cu alloys”, Corrosion Science, 65 (2012) 231-237 ([7])).

[12] The advantages and disadvantages of the solutions mentioned above are indicated in Table 1. These advantages and disadvantages take into account the fact that they do not use hexavalent chromium.

[13] [Table 1] Different types of coatings

[14] Current solutions do not make it possible to satisfactorily replace the compositions based on hexavalent chromium. Indeed, these solutions are not as effective in terms of corrosion resistance. On the other hand, for parts with complex geometry, that is to say parts comprising recesses and / or internal areas not accessible such as pipes or internal reservoirs, or for parts having dimensional constraints, these solutions entail a variation in dimensions or dimensions greater than 1 or 2 microns. Such variation is not acceptable in areas such as aeronautics.

[15] The solutions used for the chemical conversion using solutions based on chromium III allow sufficient adhesion of the coatings. However, the corrosion protection obtained by means of these solutions does not make it possible to meet the requirements of the specifications of aeronautics in particular. On the other hand, these solutions based on chromium (III) do not exhibit a corrosion resistance similar to that of solutions based on chromium hexavalent. The corrosion resistance can be evaluated using a test which consists in exposing a test piece of standardized size to a salt spray for a duration of 168 h. The result of this test for a composition based on hexavalent chromium than Alodine 1200, is of the order of less than 2.5 punctures on the test piece. When this test is carried out with SURTEC ^® 650, the results vary according to the range of aluminum alloy treated. For example, for aluminum alloys of the 5000 and 6000 series, a resistance to salt spray greater than 168 hours is obtained and the corrosion resistance is therefore satisfactory. For aluminum alloys of the 2000 series, satisfactory protection by application of SURTEC ^® 650 is obtained for a shorter period, or in other words, the resistance to salt spray is less than 168 hours. For aluminum alloys of the 7000 series, corrosion protection is not viable (resistance to salt spray less than 100 hours) (C. Ham: Light Metal Surface Finishing, Treatment of light alloys, A3TS, 3-4 December 2013, Le Bourget, France ([11]); P. Frou, State of GIFAS work to support the Surface Treatments sector, faced with threats notably from REACH in a context of increased aeronautical speeds, Day surface treatment of the Aerospace Valley / DAS AMP cluster, March 18, 2016, Toulouse, France ([12]).

[16] Only one permanganate ion solution has been listed but not identified ([11]). In addition, although there are solutions for treating objects based on magnesium or magnesium alloy, it should be noted that these solutions are not suitable for the treatment of objects made of aluminum alloy or aluminum .

[17] There is therefore a real need for anti-corrosion coatings without hexavalent chromium, overcoming these defects, drawbacks and obstacles of the prior art.

[18] Description of the invention

[19] It is after extensive research that the Applicant has developed an anti-corrosion protection without chromium, in particular in the form of a treatment bath, aluminum alloys, as well as an associated treatment method. .

[20] The invention thus relates to a chemical conversion solution devoid of chromium and which advantageously has good properties, namely: - good corrosion resistance, in particular a variation in the corrosion potential of the protected alloy compared to the raw aluminum alloy (DE) equal to + 0.3V, a DE = + 0.45V compared to the coating obtained from Alodine 1200, annual corrosion of less than 5 pm / year whereas protection by a bath of Alodine 1200 limits annual corrosion to only about 15 pm / year,

- good adhesion of varnish and / or paint,

- an absence of dimensional constraint: obtaining a layer of 1 -2, or even less than 1 pm thick,

- obtaining a colored and conductive deposit: yellow-orange coloring,

- a self-repairing property.

The treatment of an aluminum or aluminum alloy part with the solution of the invention advantageously allows:

- the oxidation of the surface of the aluminum alloy allowing the formation of a thick protective layer,

- the formation of a colored passivating layer which may include manganese oxides (MnC> 2 (s), MnO (s)),

- the treatment of porosity problems encountered for example with a layer of manganese dioxide MnÜ2 (s) alone,

- the limitation of aluminum concentration in the deposit thanks to the action of aluminum complexing agents,

- the incorporation of corrosion inhibitors improving corrosion resistance.

- the trapping of potassium permanganate giving the deposit self-healing properties.

[21] The invention is of interest for the aeronautical sector (civil and military), impacted by the REACH regulation, in particular equipment manufacturers, aircraft manufacturers and engine manufacturers, as well as for sectors which use chemical conversion to chromium VI, such as automotive, building, street furniture.

[22] Thus, a first object of the invention relates to a solution devoid of chromium in all its oxidation states comprising:

- at least one oxidizing chemical compound, - at least one aluminum complexing agent,

- at least one corrosion-inhibiting compound, and

- possibly a clogging chemical compound,

said solution having a pH ranging from 1 to 5.

[23] By "solution" is meant, within the meaning of the present invention, a composition in the liquid state, in which parts comprising aluminum or an aluminum alloy can be dipped. The solution of the invention is advantageously a chemical conversion solution, that is to say adapted to be used in the context of a chemical conversion treatment, or allowing the chemical conversion of aluminum and its alloys.

[24] The term “aluminum alloy”, within the meaning of the present invention, an alloy whose main constituent is aluminum. The alloy can also comprise at least one other component chosen from copper, silicon, magnesium, titanium and zinc. At least one other component may be present in the alloy at a weight percentage of between about 0.10 to about 21.00% by weight relative to the weight of the alloy. The aluminum alloy can be, for example, an alloy from the 2000 series of the classification of aluminum alloys (Aluminum Association, Washington DC 2006, United States), such as, for example, alloy 2024 or 2618, from the series 6000 or 7000 series, such as 7075 or 7175, for example.

[25] The solution of the invention may "include", or "be made up", of the elements indicated above. In the event that it "includes" the elements indicated, then it may include these elements as well as other elements, with the exception of chromium. If it is "made up of" these elements, then it includes only the elements listed above, to the exclusion of any other element.

[26] By “free of chromium in all its oxidation states” is meant, within the meaning of the present invention, a total absence of chromium in the solution of the invention. In other words, there is a total absence of chromium in its observable oxidation states ranging from -II to VI, and in particular of hexavalent chromium, in the solution of the invention. The chromium ion can be in particular the chromate or dichromate. The absence of chromium may be due to the fact that no element containing chromium, in particular hexavalent chromium, is added during the process for preparing the solution of the invention.

[27] The pH of the solution can be from 1.0 to 5.0, the limits being included. It can for example be from 1.2 to 4.8, or from 1.5 to 4.5, or from 2.0 to 5.0, or from 2.2 to 4.8, or from 2.5 to 4.5, or from 3.0 to 4.0, or from 3.2 to 3.8, the limits being included. In one embodiment, if the pH of the solution fluctuates beyond these values, it is possible to bring the pH back to the values indicated, for example by adding a strong acid such as sulfuric acid (H ₂ SO ₄ ) to lower the pH or a strong base such as potash (KOH) to increase the pH. Advantageously, a pH close to 4.0 can make it possible to obtain the best performance in terms of protection against corrosion, in particular a DE = + 0.3V compared to the crude alloy, that is to say without coating due to the solution of the invention.

[28] The term "oxidizing chemical compound", within the meaning of the present invention, any chemical compound capable of receiving at least one electron of another chemical species during a redox reaction. The oxidizing chemical compound of the solution of the invention can be chosen from the group comprising permanganate salts, molybdate salts, persulfate salts and hydrogen peroxide, and mixtures thereof. Advantageously, the concentration of the oxidizing chemical compound in the solution can be between 0.01 and 0.45 mol / L, the limits being included, for example ranging from 0.05 to 0.40, or from 0.1 to 0 , 4, or from 0.2 to 0.3 mol / L, the limits being included.

[29] In the case of permanganate or molybdate salts, the permanganate or molybdate ions contained in the solution of the invention can be associated with any suitable type of counterion, for example potassium permanganate KMnÜ4 or permanganate of sodium NaMn0 ₄ and molybdate sodium Na2Mo04, potassium K2M0O4 or ammonium (NH4) 2Mo04. Advantageously, the permanganate ion is used because it is a source of manganese and the molybdate ion is used as a source of molydbene.

[30] In the case of persulfate salts, the persulfate ion can be peroxomonosulfate SOs ^2- or peroxodisulfate S ₂ O8 ^{2 ·} . The persulfate salt can for example be chosen from all the known persulfate salts, and for example among ammonium persulfate, sodium persulfate, potassium persulfate, potassium hydrogen sulfate and the triple salt of potassium monopersulfate.

[31] The term "complexing agent", within the meaning of the present invention, any compound making it possible to react with a metal, in particular aluminum and its alloys, and thus form a complex soluble compound. Advantageously, the aluminum complexing agent also plays the role of corrosion inhibitor. As such, it can advantageously make it possible to prevent or limit corrosion on a metal part, in particular aluminum and its alloys, with the exception of the chromium ion. Corrosion can be evaluated by measuring the number of pits on the surface of the metal part, in a given time and under given conditions. The required properties are an absence of pitting after 168 hours of exposure to a salt spray test according to standard ASTM B117.

[32] The aluminum complexing agent contained in the solution of the invention may be a fluorinated salt or a mixture of fluorinated salts, an organic compound chosen from gluconates, citrates, oxalates, acetates and formates, or a mixture any of these. The fluorinated salt may for example be chosen from hexafluorozirconates, hexafluorotitanates, hexafluorosilicates and any mixture thereof. Among the gluconates, it may for example be sodium gluconate, potassium gluconate, calcium gluconate or ammonium gluconate. Among the citrates, it may for example be sodium citrate, potassium citrate or ammonium citrate. Among the oxalates, it can be sodium oxalate, potassium oxalate or ammonium oxalate. Among the acetates, it can be sodium acetate, potassium acetate or ammonium acetate. Among the formates, it may be sodium formate, potassium formate or ammonium formate.

[33] “Corrosion inhibiting compound” is understood to mean, within the meaning of the present invention, any compound capable of reducing the rate of corrosion of a metal surface under usual conditions of use. The corrosion inhibitor compound can be chosen from the rare earth, tungstate, vanadate, phosphate and cerium III salts, zirconium, titanium or silicon salts. These compounds can be introduced in a minority dose, for example from 0.1 to 5% by mass, in particular from 0.5 to 4.0% by mass, or from 1.0 to 3.0% by mass. mass. A single inhibitor or a mixture of inhibitors can be used to improve the corrosion resistance of the coating.

[34] The term "clogging agent", within the meaning of the present invention, any compound making it possible to avoid the presence of porosities or heterogeneity in thickness of the layer of precipitate deposited _. The sealing agent capable of being contained in the solution of the invention may be a compound based on phosphate ions, phosphonates or polyphosphates or iron. As such, the phosphate ion can be combined with any suitable type of counterion. It can be, for example, potassium or sodium hydrogenphosphates KH2PO4 _, K2HPO4, NaFhPC or Na2HPC> 40 or phosphoric acid H3PO4. Advantageously, the phosphate ion can be used as a sealing agent, that is to say that it has the function of standardizing the thickness and the profile of chemical composition of the layer of manganese and aluminum oxides formed in order to make it more exciting. The sealing agent can also be an iron salt of the iron sulphate type Fe2 (SC> 4) 3, ferric chloride FeCb, potassium ferricyanide (K3Fe (CN) 6), gluconate or iron oxalate. In the solution of the invention, the concentration of clogging agent may for example be between 0.001 and 0.20 mol / L, the limits being included, in particular from 0.010 to 0.18 mol / L, or from 0.050 to 0, 18 mol / L, or from 0.08 to 0.18 mol / L, or from 0.10 to 0.15 mol / L, the limits being included.

[35] For example, the chemical conversion solution of the invention can be a solution in which:

- the oxidizing chemical compound is potassium permanganate,

- the sealing agent, when present, is a compound chosen from potassium hydrogen phosphate, phosphoric acid or an iron salt, and

- the complexing agent is a mixture of hexafluorozirconic acid, hexafluorotitanic acid, and hexafluorosilicic acid.

[36] In the solution of the invention, the concentration of permanganate ion can be between 0.01 and 0.45 mol / L, the limits being included. The concentration can be, for example, from 0.05 to 0.40, or from 0.1 to 0.4, or from 0.2 to 0.3 mol / L, the limits being included. [37] In the solution of the invention, the concentration of phosphate ions can be between 0.001 and 0.20 mol / L, the limits being included. The concentration may for example be from 0.010 to 0.18 mol / L, or from 0.050 to 0.18 mol / L, or from 0.08 to 0.18 mol / L, or from 0.10 to 0.15 mol / L, the limits being included.

[38] In the solution of the invention, the concentration of complexing agent can be between 0.001 and 0.15 mol / l, the limits being included. The concentration may for example be from 0.005 to 0.15 mol / L, or from 0.010 to 0.15 mol / L, or from 0.05 to 0.15 mol / L, or from 0.08 to 0.12 mol / L, the limits being included.

[39] Another object of the invention relates to a method of treating or coating a metal surface, comprising the application, on said surface, of a solution as defined above. The treatment may for example be an anticorrosion treatment.

[40] The method may further comprise at least one step of pretreatment of the surface. Thus, the method of the invention can consist of a single or a succession of pre-treatment steps, followed by a step of treatment with the solution of the invention.

[41] The preprocessing step can be of type (1), (2) or (3) below, and successively include the following steps:

(1) an alkaline degreasing of the surface, followed by a nitric pickling of the surface then a hydrofluoric pickling of the surface,

(2) alkaline degreasing of the surface and sulfo-nitro ferric pickling of the surface, or

(3) alkaline degreasing of the surface, followed by a basic sodium stripping of the surface under ultrasonic field or a basic sodium stripping of the surface without ultrasonic field.

[42] Each type of pre-treatment (1), (2) or (3) can include or consist of immersion in a bath maintained at a fixed temperature and for a defined time and then in two rinses in cascade with Demineralized Water.

[43] The treatment step may comprise or consist of immersion in a bath comprising or consisting of the solution of the invention, maintained at a fixed temperature and for a defined time and then in two rinses in cascade with water demineralized. [44] The concentrations of the different species during the preparation of the conversion bath can be as defined above in the context of the definition of the solution of the invention. For example the ion concentrations can be the following:

- The concentration of MnCV ions can be equal to 0.01-0.45 mol / L,

- The concentration of H2PO4 ions can be equal to 0.05-0.2 mol / L,

- The concentration of FhZrFe ions can be equal to 0.005-0.1 mol / L,

- The concentration of Ce ions (lll) can be equal to 0.003-0.3 mol / L

which can correspond for example to masses initially introduced of approximately:

- 1.5 to 75 g / L of potassium permanganate KMnÜ4,

- 5 to 30 g / L of potassium hydrogen phosphate KH2PO4,

- 1, 4 to 27 ml / L of 50% hexafluorozirconic acid,

- 1 to 10 g / L of cerium nitrate III.

[45] In the context of the process of the invention, the object to be treated can have a metallic surface made of aluminum or an aluminum alloy.

[46] Advantageously, the method of the invention can make it possible to produce a coating on a metal surface.

[47] Thus, another object of the invention relates to a coating of a metal surface capable of being obtained by the method of treatment of a surface as defined above. Advantageously, the coating of the invention can be a compact layer, of thickness less than 1 μm and adherent for the application of a varnish or a paint. Advantageously, other characteristics of this coating are possibly all or part of the following:

- present a visible, uniform coloring free from defects,

- be continuous, uniform, adherent, without discontinuity (cracks, holes ...), without dusting, smooth and identifiable (that is to say colored or easily identifiable),

- have perfect temperature resistance up to 80 ° C and not deteriorate, - resist at least 168 hours against salt spray with less than 1.5 bites / dm ² and no bites with a diameter greater than 0.8 mm in diameter after 168 hours,

- have a layer weight of between 0.42 g / m ² and 1.2 g / m ² ,

-have good dry and wet adhesion for any corrosion primer,

- be insoluble in alcohols, water and solvents, but be soluble in alkaline products and strong acids,

- have an electrical continuity of less than 5000 pOhms / inch2 in the initial state and not exceed 10,000 pOhms / cm2 after a 168 h exposure to salt spray.

[48] Another object of the invention relates to a metal surface, in particular aluminum or aluminum alloy, comprising a coating as defined above.

[49] Another object of the invention relates to the use of a solution as defined above, for treating a metal surface, in particular aluminum or aluminum alloy.

[50] The treatment can be chosen from:

- anticorrosion treatment (preventive),

- a pretreatment for a paint application,

- local repair of a coating of parts already treated,

- clogging of parts having undergone anodization.

[51] Other advantages may also appear to the skilled person at the

read the examples below.

[52] EXAMPLES OR EMBODIMENTS [53] Example 1: Preparation of a chemical conversion solution of the invention

[54] The preparation of a chemical conversion solution consists in dissolving in water several salts of potassium permanganate, potassium hydrogen phosphates, cerium nitrates and hexafluorozirconic acid in the following proportions:

- 1.5 to 75 g / L of potassium permanganate KMnC> 4,

- 5 to 30 g / L of potassium hydrogen phosphate KH2PO4,

- 1, 4 to 27 ml / L of 50% hexafluorozirconic acid,

- 1 to 10 g / L of cerium nitrate III.

[55] The preparation is carried out at 60 ° C with a dissolution time of all the salts of approximately 1 h.

[56] Example 2: Treatment of a metallic surface of aluminum or alloy using the chemical conversion solution of the invention

The treatment protocol for an aluminum or aluminum alloy part is broken down into several stages:

- immersion of the part for a few minutes (2-6 minutes) in an alkaline degreasing bath which is chosen from the various solutions existing in the surface treatment workshop,

- rinsing in a dead bath then in a demineralized water bath,

- immersion of the part for a few minutes (2-6 minutes) in an acid pickling bath of composition available in surface treatment workshops (TS),

- rinsing in a dead bath then in a demineralized water bath,

- immersion in the chemical conversion bath which is the subject of the present invention for a time of between 2 and 10 minutes depending on the alloy to be treated,

- rinsing in a dead bath then in a bath of demineralized water. List of references

1. J.T. Qi et al. : "Trivalent chromium conversion coating formation on aluminum, Surface and Coatings Technology", 280 (2015) 317-329.

2. W.-K. Chen et al. : "The effect of chromic sulfate concentration and immersion time on the structures and anticorrosive performance of the Cr (lll) conversion coatings on aluminum alloys", Applied Surface Science, 256 (2010) 4924-4929.

3. F. Andreatta et al. : "Addition of phosphates or copper nitrate in a fluotitanate conversion coating containing a silane coupling agent for aluminum alloy AA6014", Progress in Organic Coatings, 77 (2014) 2107-2115.

4. B. Valdez et al. : "Cerium-based conversion coatings to improve the corrosion resistance of aluminum alloy 6061 -T6", Corrosion Science, 87 (2014) 141 -149.

5. H. R. Asemani et al. : "Effect of zirconium conversion coating: Adhesion and anti-corrosion properties of epoxy organic coating containing zinc aluminum polyphosphate (ZAPP) pigment on carbon mild Steel", Progress in Organic Coatings, 94 (2016) 18-27.

6. P. Santa Coloma et al. : "Chromium-free conversion coatings based on inorganic salts (Zr / Ti / Mn / Mo) for aluminum alloys used in aircraft applications", Applied Surface Science, 345 (2015) 24-35.

7. F.O. George et al. : “Formation of zirconium-based conversion coatings on aluminum and Al-Cu alloys”, Corrosion Science, 65 (2012) 231-237.

8. P. Campestrini et al. : "Formation of a cerium-based conversion coating on AA2024: relationship with the microstructure, Surface and Coatings Technology", 176 (2004) 365-381.

9. P.D. Deck et al. : "Investigation of fluoacid based conversion coatings on aluminum, Progress in Organic Coatings", 34 (1998) 39-48.

10. H. Nordlien et al. : "Formation of a zirconium-titanium based conversion layer on AA 6060 aluminum", Surface and Coatings Technology, 153 (2002) 72-78.

11. C. Ham: Light Metal Surface Finishing, Processing of light alloys, A3TS, December 3-4, 2013, Le Bourget, France. 12. P. Frou, Progress report on GIFAS to support the Surface Treatments sector, faced with threats notably from REACFI in the context of an increase in aeronautical speeds, TS Day of the Aerospace Valley / DAS AMP pole, March 18 2016, Toulouse, France.

Claims

[Claim 1] Solution devoid of chromium in all its oxidation states including

at least one oxidizing chemical compound, at least one aluminum complexing agent, at least one corrosion inhibiting compound, and

- possibly a clogging chemical compound,

said solution having a pH ranging from 1 to 5.

[Claim 2] The solution of claim 1, wherein the oxidizing chemical compound is selected from the group consisting of permanganate salts, molybdate salts, persulfate salts and hydrogen peroxide.

[Claim s] Solution according to claim 1, in which the complexing agent is chosen from fluorinated salts and their mixtures, the organic compounds chosen from the group comprising gluconates, citrates, oxalates, acetates and formates, and a mixture of at least one fluorinated salt and at least one of said organic compounds.

[Claim 4] The solution according to claim 3, wherein the complexing agent is a fluorinated salt selected from hexafluorozirconates, hexafluorotitanates, hexafluorosilicates and a mixture thereof.

[Claim 5] Solution according to any one of the preceding claims, in which the corrosion-inhibiting agent is chosen from rare earth salts, tungstate, vanadate, phosphate and cerium III, zirconium salts, titanium or silicon.

[Claim 6] A solution according to any one of the preceding claims, further comprising a clogging chemical compound based on phosphate ions, phosphonate ions or polyphosphates or iron ions.

[Claim 7] Solution according to claim 6, in which:

- the concentration of permanganate ion is between 0.01 and 0.45 mol / L,

- The concentration of phosphate ion is between 0.001 and 0.20 mol / l, and the concentration of complexing agent is between 0.001 and 0.15 mol / L.

[Claim 8] A solution according to claim 6, in which:

the chemical compound comprising permanganate ions is potassium permanganate,

the chemical compound comprising phosphate ions is chosen from potassium hydrogen phosphate, phosphoric acid and an iron salt, and

[Claim 9] A method of treating a metal surface, comprising applying, on said surface, a solution as defined in any one of claims 1 to 8.

[Claim 10] The method of claim 9, further comprising a step of pretreatment of said metal surface.

[Claim 11] The method of claim 10, wherein said preprocessing step successively comprises the following steps

an alkaline degreasing of the surface, a nitric pickling of said surface and a hydrofluoric pickling of said surface, or

an alkaline degreasing of said surface and a sulfo-nitro ferric pickling of said surface, or

- an alkaline degreasing of said surface and a basic sodium stripping of said surface possibly under ultrasonic field.

[Claim 12] A method according to any of claims 9 to 11, wherein said metal surface is made of aluminum or is an aluminum alloy.

[Claim 13] Coating of a metallic surface capable of being obtained by the method of treating a metallic surface as defined in any one of claims 9 to 11.

[Claim 14] Metal surface comprising a coating as defined in claim 13.

[Claim 15] Use of a solution as defined in any one of claims 1 to 8, in an anticorrosion treatment of a metal surface.