EP3757251B1 - Composition for surface treatment of a metal substrate and method for surface treatment using such a composition - Google Patents

Description

The present invention falls within the field of the surface treatment of metal substrates, in particular aluminum alloy, more particularly treatments of the type aimed at forming a chemical conversion layer on the surface of the substrate, with a view to improving it. corrosion resistance and paint adhesion properties.

More particularly, the present invention relates to an aqueous composition for the surface treatment of a metal substrate, as well as a process for the surface treatment of metal substrates implementing such a composition, and as well as a substrate obtained by such process.

With a view to their implementation in the aeronautical sector, metal parts, in particular parts made of aluminum or aluminum alloy, or of magnesium or magnesium alloy, must frequently be surface treated in order to improve their corrosion resistance.

A technique commonly used for this purpose is that known as chemical conversion, which consists in forming on the surface of the part, by means of a composition based on a corrosion-inhibiting metal cation, a protective coating composed mainly hydroxides, oxyhydroxides and oxyfluorides of the aluminum constituting the part and the corrosion-inhibiting metal cation used. The compositions for the chemical conversion of aluminum surfaces most commonly used at present in the aeronautical industry contain hexavalent chromium as a corrosion-inhibiting metal cation.

In the more specific context of an out-of-bath application of such compositions to the surface of the parts, in particular according to a so-called retouching mode, that is to say according to which only certain localized areas of the surface of the parts are treated, it It is interesting that the chemical conversion compositions make it possible to form on the surface of the treated parts a colored coating making it possible to easily visualize which zones have been treated and which have not. This is the case for chemical conversion compositions based on hexavalent chromium, which naturally make it possible to form, on aluminum or magnesium substrates, or their respective alloys, a color conversion layer different from that of the substrate.

Due to the harmful nature of hexavalent chromium towards the environment and its toxicity for living organisms, it has been sought for some years to replace it in chemical conversion compositions, with less harmful substances. By way of example, it has been proposed by the prior art to replace hexavalent chromium with trivalent chromium. Is meant in the present description, conventionally per se, by hexavalent chromium, chromium in the +6 oxidation state, and, by trivalent chromium, chromium in the +3 oxidation state.

However, the compositions containing corrosion-inhibiting metal cations other than hexavalent chromium, in particular trivalent chromium, do not make it possible to form a colored coating on the surface of the treated parts. In order to solve this problem, which is particularly penalizing for out-of-bath applications of chemical conversion compositions, it has been proposed by the prior art to add a coloring compound to these compositions based on a corrosion-inhibiting metal cation other than chromium hexavalent. By way of example, pyrocatechol violet has been proposed for this purpose, as described in particular in the document WO 2018/144580 for trivalent chromium.

However, when the compositions containing on the one hand corrosion-inhibiting metal cations other than hexavalent chromium, and more particularly trivalent chromium, and on the other hand the dyes proposed by the prior art, in particular pyrocatechol violet, are used for the chemical conversion of metal surfaces, in particular aluminum surfaces, it is observed that the corrosion resistance properties of the treated parts are insufficient to meet the requirements of the aeronautical field, in particular in terms of performance in the resistance test salt spray. Although not completely explained, this phenomenon seems to be due to a negative effect of the dye on the stability of the conversion layer formed by the chemical conversion composition on the surface of the treated parts.

There therefore remains a need for a coloring compound which can be used in chemical conversion compositions, in particular aluminum or magnesium, or their respective alloys, in particular in compositions based on trivalent chromium and a fluorinated compound, and which makes it possible to color the surface coating formed on the parts treated with this composition, so that this coating is visually detectable, this without however impacting the corrosion resistance performance of these coatings, compared to those obtained by means of compositions conversion devoid of coloring compound.

The documents WO 2013/185131 , WO 2014/151491 and WO 2014/151570 describe aqueous compositions for the surface treatment of substrates, containing a metal fluoride and a coloring compound such as catechol violet or xylenol orange. These compositions are devoid of trivalent chromium.

The present invention aims to provide a chemical conversion composition which makes it possible to form on a metal substrate a colored surface layer whose coloring is visually detectable, because it is different from that of the treated substrate, and which has a high degree of resistance to corrosion. The invention also aims for this surface layer to have good uniformity and good adhesion to paints, in particular to the primers commonly used in the aeronautical industry.

Additional objectives of the invention are that this composition be easy to prepare and to implement.

It has now been discovered by the present inventors that such advantageous results can be achieved, and this quite surprisingly in view of the prior art, by using, in the chemical conversion compositions, in particular aluminum or of its alloys, based on trivalent chromium as a corrosion-inhibiting compound and a fluorinated compound, a particular coloring compound: xylenol orange.

Xylenol orange, or tetrasodium salt of 3,3'-Bis[ N , N -bis(carboxymethyl)aminomethyl]-o-cresolsulfonephthalein, is a dye commonly used for complexometric assays, with the formula:

Nothing in the prior art suggested that, used in a chemical conversion composition based on trivalent chromium and a fluorinated compound, this particular compound would make it possible to obtain performance, in terms of corrosion resistance properties conferred to the treated part, which are particularly good, and which are much better than those obtained with the other coloring compounds proposed by the prior art, in particular catechol violet.

It should be noted that xylenol orange does not have the ability to complex chromium.

Thus, according to a first aspect, there is proposed according to the present invention an aqueous composition for the surface treatment of a metal substrate, in particular of the type by chemical conversion. This composition, which will be designated in the present description by the expression “chemical conversion composition”, or even “conversion composition”, contains, conventionally in itself for this type of composition, a fluorinated compound and a compound metallic corrosion inhibitor, excluding compounds based on hexavalent chromium. It also contains, as a water-soluble coloring compound, xylenol orange. The corrosion inhibiting metallic compound is a trivalent chromium salt and the pH of the composition is between 3 and 5.

As indicated above, this composition makes it possible to form on metal substrates, in particular aluminum or aluminum alloy, magnesium or magnesium alloy, or steel substrates, a colored surface coating which has a high degree of corrosion resistance. This coating also has good uniformity over the entire area of the substrate treated, and good adhesion to the paint systems commonly used in the aeronautical industry.

The conversion composition according to the invention can also meet one or more of the characteristics described below, implemented in isolation or in each of their technically effective combinations.

The conversion composition according to the invention is preferably substantially free of hexavalent chromium. By substantially free is meant the fact that it does not contain hexavalent chromium, except in trace form, that is to say in an amount less than or equal to 0.1% by weight relative to the weight composition total.

Preferably, the composition according to the invention contains no coloring compound other than xylenol orange.

The xylenol orange is preferably present in the composition according to the invention in a concentration of between 0.3 and 2 g/l, preferably between 0.5 and 1.5 g/l. Such a narrow concentration range advantageously ensures, on the one hand, that the coating formed on the surface of the metal substrate is sufficiently colored for this coloring to be detectable with the naked eye, and on the other hand that the performance of resistance to corrosion imparted to the treated substrate are maximal. In particular, above 1.5 g/l, these performances prove to be less satisfactory.

The conversion composition according to the invention may contain a single fluorinated compound, or a plurality of fluorinated compounds.

Preferably, at least one fluorinated compound is a fluorinated salt, preferably a fluorinated salt of a metal, in particular of a transition metal. This transition metal is preferably chosen from zirconium and titanium. Preferably, the fluorinated compound is a fluorinated salt of a transition metal and an alkali metal, such as potassium.

Examples of fluorinated compounds that can be used in the conversion composition according to the invention are potassium hexafluorozirconate (K ₂ ZrF ₆ ), dihydrogen hexafluorozirconate (H ₂ ZrF ₆ ), potassium hexafluorotitanate ( K ₂ TiF ₆ ) and dihydrogen hexafluorotitanate (H ₂ TiF ₆ ).

The conversion composition according to the invention may otherwise contain, as as a fluorinated compound, sodium hexafluorosilicate (Na ₂ SiF ₆ ), dihydrogen fluorogermanate (H ₂ GeF ₆ ) or even dihydrogen fluorophosphonate (H ₂ PO ₂ F), etc.

A particularly preferred fluorinated compound in the context of the invention, since it confers particularly advantageous properties on the coating formed on the surface of the treated metal substrate, is an alkali metal hexafluorozirconate, in particular potassium hexafluorozirconate.

The conversion composition according to the invention preferably contains a concentration of between 1 and 15 g/l of fluorinated compound(s), in particular a concentration of between 1 and 15 g/l of potassium hexafluorozirconate.

The conversion composition according to the invention may also contain a single corrosion-inhibiting metal compound, or a plurality of such compounds. In the present description, the term “corrosion-inhibiting metal compound”, in a conventional manner per se, means a compound containing a corrosion-inhibiting metal ion, in particular a corrosion-inhibiting metal cation.

A corrosion-inhibiting metal compound contained in the conversion composition according to the invention is a trivalent chromium salt.

This corrosion-inhibiting trivalent chromium salt can be chosen from fluorides, sulphates, chlorides, iodides, nitrates, acetates, carbonates, phosphates, etc., or any one of their mixtures. This salt may contain, in addition to the corrosion-inhibiting trivalent chromium ion, one or more counterions. As corrosion-inhibiting metal compound, trivalent chromium fluorides and trivalent chromium sulphates are particularly preferred within the scope of the invention.

The trivalent chromium salts used can for example be chosen from the group consisting of chromium trifluoride CrF ₃ .xH ₂ O, chromium sulphate Cr ₂ (SO ₄ ) ₃ .xH ₂ O, potassium chromium sulphate CrK(SO ₄ ) ₂ .xH ₂ O, chromium trichloride CrCl ₃ .xH ₂ O, chromium nitrate Cr(NO ₃ ) ₃ .xH ₂ O, chromium acetate (CH ₃ CO ₂ ) ₂ Cr,xH ₂ O, chromium hydroxide acetate (CH ₃ CO ₂ ) ₇ Cr ₃ (OH) ₂ ,xH ₂ O, etc.

Preferably, the conversion composition according to the invention contains at least potassium chromium sulphate, preferably in the form of CrK(SO ₄ ) ₂ .xH ₂ O hydrate, in particular in the form of CrK(SO ₄ ) ₂ dodecahydrate, 12H ₂ O. The conversion composition according to the invention preferably contains a concentration of between 1 and 10 g/l of corrosion-inhibiting metallic compound(s), in particular a concentration of between 1 and 10 g/l of potassium chromium sulfate dodecahydrate CrK(SO ₄ ) ₂ .12H ₂ O.

The pH of the conversion composition is between 3 and 5.

The pH may have been adjusted within such a range of values in any manner conventional per se. In particular, the conversion composition may contain a pH adjuster, in particular of the basic type, for example sodium hydroxide NaOH, in an adequate quantity to adjust the pH to the desired value.

A particularly preferred conversion composition according to the invention contains at least one zirconium fluoride, a trivalent chromium salt, optionally sodium hydroxide, and xylenol orange.

• de l'hexafluorozirconate de potassium, de préférence à une concentration comprise entre 1 et 15 g/l,
• du sulfate de chrome trivalent, en particulier du sulfate de chrome de potassium, notamment sous forme dodécahydrate, de préférence à une concentration comprise entre 1 et 10 g/l,
• de l'orangé de xylénol, de préférence à une concentration comprise entre 0,5 et 1,5 g/l,
• et optionnellement de l'hydroxyde de sodium en quantité adéquate pour obtenir un pH compris entre 3 et 5.

An example of a particularly preferred composition according to the invention contains, in solution in water:

• potassium hexafluorozirconate, preferably at a concentration of between 1 and 15 g/l,
• trivalent chromium sulphate, in particular potassium chromium sulphate, in particular in the dodecahydrate form, preferably at a concentration of between 1 and 10 g/l,
• xylenol orange, preferably at a concentration of between 0.5 and 1.5 g/l,
• and optionally sodium hydroxide in an adequate quantity to obtain a pH of between 3 and 5.

Preferably, the conversion composition according to the invention contains concentrations of fluorinated compound(s) and of corrosion-inhibiting metallic compound(s) such as the mass ratio of fluorinated compound(s) (s) relative to the corrosion-inhibiting metal compound(s), these compounds being distinct from each other, is between 1 and 2.5, preferably between 1 ,5 and 2. In the case where the composition contains several fluorinated compounds, it is the total mass of these fluorinated compounds which is taken into account here, likewise for the metallic compounds which inhibit corrosion. Such a characteristic advantageously makes it possible to obtain the best performance in terms of corrosion resistance of the treated parts.

The corrosion-inhibiting metallic compound and the fluorinated compound present in the conversion composition according to the invention can consist of two different compounds. In variants of the invention, they consist of one and the same compound, capable of providing the two functions on its own, for example by trivalent chromium fluoride CrF ₃ .

The viscosity of the conversion composition according to the invention, formed solely from the compounds described above, is approximately 1 mPa.s.

The conversion composition according to the invention may optionally contain one or more thickening agent(s), preferably soluble(s) in water, making it possible to increase its viscosity.

Thickening agents which can be used for this purpose are, for example, cellulose or its derivatives, starch, fumed silica, polyacrylic acid, xanthan gum, etc.

The thickening agent(s) are preferably contained in the composition in an adequate amount to give the composition a Brookfield viscosity of between 1000 and 4000 mPa.s, preferably between 2000 and 3000 mPa.s, and preferably between 2100 and 2500 mPa.s, this viscosity being measured with a spindle type LV01 at a temperature of 23° C. and at a speed of 12 rpm.

By way of example, the conversion composition according to the invention may contain microcrystalline cellulose or a cellulose derivative, at a concentration of between 5 and 15 g/l, for example approximately 10 g/l.

The conversion composition according to the invention may additionally contain one or more conventional additives per se in the field of chemical conversion compositions, for example surfactants, wetting agents, pH stabilizers, additional corrosion inhibitors , chelating/complexing agents, etc.

A process for preparing the conversion solution according to the invention includes the incorporation of the various compounds in the solid state in an aqueous vehicle.

Preferably, the order of incorporation of the compounds into the vehicle is as follows: metal compound(s) corrosion inhibitor(s), then fluorinated compound(s), then coloring compound, then pH adjuster . If necessary, the thickening agent is introduced last.

The process for preparing the composition according to the invention is preferably carried out at ambient temperature, that is to say at a temperature of between 20 and 25° C. approximately.

According to another aspect, the present invention relates to a process for the surface treatment of a metallic substrate, by forming a conversion coating on the surface of this substrate. This method comprises the application of an aqueous composition according to the invention to an area of the surface of the substrate. This zone corresponds, where appropriate, to the entire surface of the substrate.

The metallic substrate on which the surface treatment method according to the invention is applied is in particular a substrate made of aluminum or of aluminum alloy, of magnesium or of magnesium alloy, or of steel.

In particular embodiments of the invention, the conversion composition is applied to the surface area of the substrate in an amount of between 5 and 100 mg/cm ² of said surface. In the configurations in which the conversion composition contains a thickening agent, and is therefore in the form of a gel, it is preferably applied to the surface area of the substrate in an amount of between 10 and 100 mg/cm ² of said surface, preferably between 30 and 80 mg/cm ² of said surface, and preferably between 40 and 60 mg/cm ² of said surface. In the configurations in which the conversion composition is devoid of thickening agent, and is therefore in liquid form, it is preferably applied to the area of the surface of the substrate in an amount of between 5 and 50 mg/cm ² of said surface. Here again, the choice of values in such ranges advantageously makes it possible to ensure, on the one hand, good visibility of the coloring of the coating formed on the surface of the substrate, and, on the other hand, good corrosion resistance properties of this coating.

The application of the conversion composition to the surface of the metallic substrate can be carried out in any conventional manner.

Preferably, the conversion composition is applied to the surface of the substrate according to a so-called retouching mode, by spraying, brushing or wiping. For this purpose, any conventional means of application can be used per se, such as a pen or a pencil, a brush, a pad, a sponge, a wipe, etc.

The application of the conversion composition can otherwise be carried out by immersing the substrate, in whole or in part, in said composition.

The contact time of the conversion composition with the metal substrate is preferably between 10 seconds and 1 hour, preferably between 30 seconds and 20 minutes, preferably between 2 and 15 minutes, preferably between 3 and 10 minutes and preferably between 3 and 8 minutes.

In the configurations in which the conversion composition is applied in touch-up mode, the application is preferably carried out by several series of crossed passes over the surface of the treated zone, a waiting time being preferentially provided between series of passes. successive.

Alternatively, the application can be carried out by a single continuous contact of the conversion composition on the surface of the treated area, for a contact time preferably between 30 seconds and 20 minutes, preferably between 2 and 15 minutes, preferably between 3 and 10 minutes and preferably between 3 and 8 minutes.

The application of the conversion composition to the surface of the substrate is preferably carried out at a temperature comprised between 5 and 40°C, preferably comprised between 15 and 30°C and preferentially comprised between 20 and 25°C.

The method according to the invention may comprise, before the application of the conversion composition to the surface of the substrate, any preliminary step of conventional surface preparation in itself, in particular cleaning, degreasing, pickling, mechanical, acid or basic , etc., so as to remove dirt, grease, oxides, etc. who may be present there.

• un dégraissage au solvant, notamment au moyen du produit commercialisé sous la dénomination Diestone DLS/UV par la déposante. Cette opération est notamment réalisée par essuyage, à température ambiante ;
• et un décapage mécanique, au moyen d'un tampon abrasif, de préférence imprégné d'eau déminéralisée.

Preferably, the preliminary surface preparation step comprises:

• solvent degreasing, in particular by means of the product marketed under the name Diestone DLS/UV by the applicant. This operation is in particular carried out by wiping, at ambient temperature;
• and mechanical blasting, using an abrasive pad, preferably impregnated with demineralised water.

These various operations can be separated and/or followed by rinsing steps, in particular with clean water, and, where appropriate, drying.

The surface treatment method according to the invention can also be carried out on a substrate which has previously undergone an anodization treatment. It can then be qualified as a sealing process, the composition according to the invention then also being able to be designated by the terms “sealing composition”.

The method according to the invention may further comprise a step of drying the surface of the treated substrate.

It may also or otherwise comprise a step of post-treatment of the treated substrate, for example with the aim of reinforcing the layer formed on the surface of the substrate. By way of example, such a post-treatment method may comprise the application to the surface of the treated substrate of a composition based on a corrosion-inhibiting rare earth salt and an oxidizing compound. Another aspect of the invention relates to a metallic substrate obtained at the end of a surface treatment process according to the invention. This substrate comprises, on at least part of its surface, a coating, more precisely a colored conversion coating containing xylenol orange and chromium oxyfluorides and metal constituting the metal substrate. This coating has a color ranging from pink to purple, depending on the amount of xylenol orange it contains.

The xylenol orange may be present therein in any chemical form, in particular in the form complexed with the transition metal which may enter into the constitution of the fluorinated compound contained in the conversion composition according to the invention implemented, for example in complexed form. with some zirconium.

The metal substrate according to the invention is preferably formed from aluminum or an aluminum alloy, magnesium or a magnesium alloy, or steel.

The conversion coating present on its surface is in particular formed of oxyfluorides, oxyhydroxides and hydroxides of the metal constituting the substrate and of chromium. It can for example be formed from oxides, hydroxides and oxyfluorides of aluminum and of chromium, as well as, where appropriate, of the transition metal entering into the constitution of the fluorinated salt, for example of zirconium.

The coating is also preferably present at the surface of the substrate in an amount of between 5 and 100 mg of coating per cm ² of said coated surface.

In variants of the invention, the coating is present on the surface of the substrate in an amount comprised between 10 and 100 mg/cm ² of said surface, preferably comprised between 30 and 80 mg/cm ² of said surface, and preferentially between 40 and 60 mg/cm ² of said surface. This may in particular correspond to the configurations in which the conversion composition used for the surface treatment contained a thickening agent, and was therefore in the form of a gel.

In other variants of the invention, the coating is present on the surface of the substrate in an amount of between 5 and 50 mg/cm ² of said surface. This may in particular correspond to configurations in which the conversion composition used for the surface treatment was devoid of thickening agent, and was therefore in liquid form.

The characteristics and advantages of the invention will appear more clearly in the light of the examples of implementation below, provided purely by way of illustration and in no way limiting the invention.

A/ Materials and methods

The substrates used are rectangular 2024-T3 aluminum alloy plates with dimensions of 125 mm×75 mm and thickness of 1 mm.

Surface treatment process in retouching mode

Unless otherwise specified, the substrate is first subjected to a step of pre-treatment, by successive immersion in aqueous alkaline degreasing baths, then sodium attack then acid deoxidation, the substrate being, between each bath and after immersion in the last bath, rinsed by immersion in demineralized water for 3 min, then by spraying with demineralised water. The alkaline degreasing bath is more specifically formed from the product marketed by the applicant under the name Sococlean A3432, at 10% by volume, the immersion being carried out at 55° C. for 15 min. The acid deoxidation bath is more specifically formed from the product marketed by the applicant under the name Socosurf A1858/A1806, 40/10% vol, the immersion being carried out at 50° C. for 5 min.

The substrate is then, in all cases, subjected to a surface preparation step, by degreasing using a wipe impregnated with the product marketed by the applicant under the name Diestone DLS; then stripping in a crossed or circular manner using a fine-grained abrasive pad, impregnated with demineralized water, until a surface without breaking the water film is obtained; finally cleaning the sanded area with demineralized water by spraying, and drying by wiping.

The conversion composition is then applied to the chosen area of the surface of the substrate by means of crossed passages, by means of a sponge. For this purpose, the composition is first of all applied to the entire area concerned, along a first axis of application, then it is applied along a second axis of application perpendicular to the first. After waiting 2 or 3 minutes, these successive operations are repeated, for a total of 2 or 3 series of applications. The surface thus coated is then left to dry in the open air. These operations are carried out at a temperature between 15 and 30°C.

Immersion surface treatment process

Unless otherwise indicated, the substrate is first subjected to a surface preparation step, by degreasing with the product marketed by the applicant under the name Sococlean A3432, at 10% by volume, at 55° C. for 15 min, then immersion in demineralised water for 3 min, and rinsing by spraying demineralised water. The substrate is then subjected to deoxidation using the product marketed by the applicant under the denomination Socosurf A1858/A1806, 40/10% vol, at 50° C. for 5 min, then rinsed again by immersion in demineralised water for 3 min and then by spraying with demineralised water.

The substrate thus prepared is immersed in the chemical conversion composition, at 40° C. for 10 min, then post-treated by immersion in demineralised water for 3 min and then spraying with demineralised water.

Salt spray test

This test is carried out according to the protocol described in the ASTM B117 standard. The substrates are exposed to salt spray, and the number of corrosion pits that appear on their surface after various exposure times is counted. For a retouching mode, the criterion applied here is not to exceed 5 pits on the surface of the substrate after 96 hours of exposure to salt spray.

Grid paint adhesion test

This test is carried out according to the protocol described in standard NF EN ISO 2409 (“cross cut test”). Unless otherwise indicated, the primer used is that marketed under the name MAPAERO P60. Are carried out: a dry evaluation, after 7 days of drying of the primer; and a wet evaluation, for which the substrates are immersed for 14 days in water at room temperature, then the surface is dried just before making the grid. Typical requirements are grade 0 for the dry test and grade 1 for the wet test.

Bending paint adhesion test

This test is carried out according to the protocol described in the ISO 6860 standard, using a conical mandrel having an end with a smaller diameter of 7 mm. The substrate is bent by 180 degrees in 2 to 3 s, at around 23° C. and at a relative humidity of around 50%, then exposed to salt spray for 3000 h. The evolution of pitting corrosion at the bending zone is evaluated.

Color appearance and visibility

The appearance (uniformity) of the coating formed on the surface of the substrate following the treatment process is assessed visually, as well as the visibility of its coloring (Visib., noted out of 5, the assigned value being all the higher that the coloring is better visible).

Coloring compounds tested

The characteristics (name and Chemical Abstracts Service (CAS) number) of the dye compounds tested are indicated in Table 1 below. <u>Table 1</u> Family Coded Dye CAS number Complexometric indicators Inv Xylenol orange, tetrasodium salt 3618-43-7 C1 Pyrocatechol violet 115-41-3 C2 Alizarin Red 72-48-0 C3 Eriochrome Black T 1787-61-7 C4 2-pyridylazo resorcinol 1141-59-9 C5 Arsezano III 1668-00-4 Natural extracts C6 Unoxidized logwood extract 8005-33-2 C7 Tannic acid 1401-55-4 Metal cations C8 Sodium iron citrate C9 Sodium molybdate dihydrate 10102-40-6 Xylenol orange precursor C10 Imminodiacetic acid 142-73-4

Conversion comps

• K₂ZrF₆ (N° CAS 16923-95-8 ) à différentes concentrations variant de 1 g/l à 8,5 g/l
• KCr(SO₄)₂.12H₂O (N° CAS 7788-99-0 ) à différentes concentrations variant de 2,5 g/l à 4 g/l
• NaOH en quantité adéquate pour obtenir le pH souhaité. Sauf indication contraire, ce pH est fixé à 3,9.

The conversion compositions used in the examples contain, in solution in water, in addition to the dye compound, the following compounds:

• K ₂ ZrF ₆ (No. CAS 16923-95-8 ) at different concentrations ranging from 1 g/l to 8.5 g/l
• KCr(SO ₄ ) ₂ .12H ₂ O (No. CAS 7788-99-0 ) at different concentrations ranging from 2.5 g/l to 4 g/l
• NaOH in sufficient quantity to obtain the desired pH. Unless otherwise indicated, this pH is fixed at 3.9.

B/ Surface treatment with compositions according to the invention

In the experiments below, the coloring compound used is xylenol orange, in accordance with the invention, and the treatment process is carried out in retouching mode.

Salt spray tests

The xylenol orange is tested at different concentrations ([dye]) in the conversion composition: 0.5 g/l, 1 g/l, 2.5 g/l. The concentration of KCr(SO ₄ ) ₂ .12H ₂ O is 2.5 g/l. The substrates obtained at the end of the surface treatment process are visually observed, then subjected to a salt spray exposure test.

Each condition tested is carried out in triplicate. The results obtained for visual observation, and for the salt spray test, expressed as the number of corrosion pits observed on the treated surface after different times of exposure to salt spray (BS), are shown in table 2 below, for different concentrations of K ₂ ZrF ₆ ([K ₂ ZrF ₆ ]). <u>Table 2</u> BS exposure time (h) [Colorant] (g/l) [K ₂ ZrF ₆ ] (g/l Aspect Visib. 21 87 115 156 181 0.5 4.5 Homogeneous 2 0 0 0 0 0 0 0 0 1 2 0 0 0 0 0 1 4 Not homogeneous 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 4.5 Homogeneous 3 0 0 >10 >10 >10 0 0 0 0 3 0 0 0 0 2 2 4 Not homogeneous 3 0 0 >10 - - 0 0 >10 - - 0 2 >20 - - 2.5 4 Not homogeneous 5 0 2 4 >10 - 0 0 0 0 - 0 1 4 >10 -

As can be seen, for all concentrations of xylenol orange tested, far less than 5 corrosion pits formed on the treated substrate after 87 h of salt spray exposure. The best results are obtained for concentrations between 0.5 and 2 g/l. Moreover, even at a concentration of 0.5 g/l, the coloring of the coating formed on the substrate is visible to the naked eye.

Similar results, a little less good in terms of resistance to salt spray, are obtained for the substrates treated in accordance with the invention, but without prior pretreatment steps (alkaline degreasing, sodium attack, acid deoxidation).

Variation in the mass ratio of fluorinated compound relative to the corrosion-inhibiting metal compound (“Zr/Cr ratio”).

Salt spray tests are carried out after treatment with compositions in accordance with the invention containing 0.5 g/l of xylenol orange and different concentrations of K ₂ ZrF ₆ ([K ₂ ZrF ₆ ]) and KCr(SO ₄ ) 2.12H ₂ O ([KCr ₍ SO ₄ ) _{2.12H 2} O]).

The results obtained (in number of bites for each salt spray exposure time) are indicated in Table 3 below. <u>Table 3</u> Exposure time to salt spray (h) [ _{K2ZrF6 ]} (g/l) [KCr(SO ₄ ) ₂ .12H ₂ O] (g/l) Zr/Cr ratio 45 88 119 165 186 253 7.5 4 1.9 0 0 0 0 2 4 6.5 4 1.6 0 0 0 0 3 >5 8.5 3 2.8 0 5 >10 - - - 4.5 2.5 1.8 0 0 0 0 0 1 1 4 0.25 0 >10 >10 - - -

It is observed there that the corrosion protection performances of the treated substrates are particularly good when the mass ratio K ₂ ZrF ₆ /KCr(SO ₄ ) ₂ .12H ₂ O is between 1.5 and 2.

pH variation

Compositions in accordance with the invention, containing 1 g/l of xylenol orange, 4.5 g/l of K ₂ ZrF ₆ and 2.5 g/l of KCr(SO ₄ ) ₂ .12H ₂ O, and of pH equal to 3.9 or 5.5, are used to treat substrates in accordance with the invention, directly or after pretreatment. Coated substrates are salt spray tested. The results obtained (in number of bites for each time of exposure to BS salt spray) are indicated in Table 4 below. <u>Table 4</u> BS exposure time (h) pH Pretreatment Aspect Visibility 46 112 158 186 250 5.5 No Homogeneous orange 3 0 0 >10 - - 0 0 4 - - 5.5 Yes Homogeneous orange 3 0 0 0 0 - 0 2 7 >10 - 3.9 No purple homogeneous 5 0 0 1 5 >10 0 0 2 5 >10 3.9 Yes purple homogeneous 5 0 0 0 1 >10 0 0 0 1 6

Very satisfactory results are obtained for the two pHs tested. The pH value of 3.9 proves to be the most efficient.

Optimized compositions

For two of the compositions according to the invention exhibiting the best results, salt spray resistance tests over longer periods and over a larger number of samples are carried out for substrates treated in retouching mode, directly or after pretreatment. These two compositions contain 1 g/l of xylenol orange, 2.5 g/l of KCr(SO ₄ ) ₂ .12H ₂ O and 4 g/l or 4.5 g/l of K ₂ ZrF ₆ .

All of the treated substrates show good visual surface homogeneity, and a violet coloration clearly visible to the naked eye.

The results obtained, expressed as the number of corrosion pits on the surface of the substrate for each time of exposure to salt spray, are shown in Table 5 below, as a function of the concentration of K ₂ ZrF ₆ ([K ₂ ZrF ₆ ]) and the presence ("Yes") or absence ("No") of pretreatment. <u>Table 5</u> Exposure time to salt spray (h) [ _{K2ZrF6 ]} (g/l) Pretrait. 89 113 138 165 184 253 284 300 4.5 Yes 0 0 6 7 7 >10 - - 0 0 3 6 10 >10 - 0 0 0 0 0 9 - - 0 0 0 0 0 0 6 6 0 0 0 0 0 0 6 6 0 0 0 0 2 >10 >10 >10 0 0 0 0 0 >10 - - 0 0 0 0 0 >10 - - 0 0 0 0 1 >10 - - 4.5 No 0 3 6 6 6 >10 - - 0 2 4 4 5 >10 - - 0 0 0 0 4 >10 >10 >10 0 0 0 0 0 0 4 7 0 0 0 0 2 6 8 10 4 Yes 0 0 0 0 3 >10 - - 0 0 0 0 2 >10 - - 0 0 2 2 2 6 8 10 0 0 0 0 2 4 7 10 0 0 0 0 0 0 0 0 0 0 0 5 11 >10 - - 0 0 0 3 6 >10 - - 4 No 0 0 2 2 3 >10 - - 0 0 0 0 1 >10 - - 0 0 0 0 0 0 0 0 0 0 0 1 1 4 7 7 0 0 0 0 0 4 7 7

These results indisputably show the high corrosion protection performance conferred on the substrates treated by the surface treatment methods according to the invention, both with and without steps of surface pretreatment of the substrates.

Paint Adhesion Tests

The substrates treated with the compositions containing 1 g/l or 2 g/l of xylenol orange, 2.5 g/l of KCr(SO ₄ ) ₂ .12H ₂ O and 4 g/l of K ₂ ZrF ₆ , are subjected to the paint adhesion test by gridding or bending.

The thickness of primer deposited on the surface of the substrate is between 25 and 40 μm.

The results obtained, for the dry test and for the wet test in the grid test, and for the folding test, as a function of the concentration of coloring compound ([dye]), are indicated in Table 6 below. Concerning the folding test, the presence of a cross means an absence of cracking at the time of folding. A dash indicates that the condition has not been tested. <u>Table 6</u> [dye] (g/l) Dry adhesion test (grade) Wet adhesion test (grade) Bend test 1 0 1 - 0 2 - 0 2 - - - x - - x - - x 2 0 5 - 0/1 5 - 0 5 - - - x - - x - - x

The adhesion performances in dry conditions prove to be very satisfactory for the two concentrations of xylenol orange tested. For the wet test, the concentration of 1 g/l is found to be much better than the higher concentration.

A similar experiment was carried out with 1 g/l of xylenol orange and 4 g/l of K ₂ ZrF ₆ . The results obtained are similar to those obtained with 4.5 g/l of K ₂ ZrF ₆ , and even better for the wet adhesion test.

Substrates treated with a composition in accordance with the invention, containing 1 g/l of xylenol orange, 4.5 g/l of K ₂ ZrF ₆ and 2.5 g/l of KCr(SO ₄ ) ₂ .12H ₂ O, are evaluated in a paint adhesion test using different primers: a hexavalent chromium-free primer, a chromated primer (containing hexavalent chromium) waterborne and a chromated primer (containing hexavalent chromium) solvent. The test carried out is an adhesion test by dry or wet grid. The results obtained are shown in Table 7 below. For each primer tested, the thickness deposited on the surface of the substrate is specified. <u>Table 7</u> Primary Primer thickness (µm) Dry test (Grade) Wet test (Grade) Cr(VI)-free primer 70 to 80 0 1 0 0 0 1 Primer containing waterborne Cr(VI) 30 to 40 0 0 0 0-1 0 0 Primer containing solventborne Cr(VI) 10 to 20 0 0 0 0 0 0

These results are very satisfactory regardless of the primer used.

C/ Comparison of xylenol orange / catechol violet

Substrates are treated by a conversion process in retouching mode, directly or after pretreatment, with compositions containing 7.5 g/l of K ₂ ZrF ₆ and 4 g/l of KCr(SO ₄ ) ₂ .12H ₂ O , and 0.5 g/l of coloring compound, xylenol orange or pyrocatechol violet.

The substrates thus coated were subjected to the salt spray test. The results obtained, expressed as the number of pittings on the surface of the substrate after various times of exposure to salt spray, are indicated in Table 8 below. <u>Table 8</u> Exposure time to salt spray (h) Dye Pretrait. Color 21 44 111 142 158 209 275 301 xylenol orange Yes Light purple 0 0 0 0 0 2 4 >10 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 2 xylenol orange No Light purple 0 0 0 0 4 5 5 7 0 0 0 0 0 0 0 2 0 0 0 0 0 1 1 2 Pyrocatechol violet (C1) Yes Blue 0 >10 >20 >20 - - - - 0 >10 >20 >20 - - - - 0 >10 >20 >20 - - - - Pyrocatechol violet (C1) No Blue 0 >10 >20 >20 - - - - 0 >10 >20 >20 - - - - 0 >10 >20 >20 - - - -

It is observed that the corrosion protection performance of the compositions in accordance with the invention, containing xylenol orange, are much higher than those obtained with the coloring compound of the prior art pyrocatechol violet.

D/ Surface treatment by comparative compositions

Substrates are treated in retouching mode or in immersion mode, with pretreatment, by chemical conversion processes implementing different dyes at different concentrations in a composition containing 4.5 or 4.8 g/l of K ₂ ZrF ₆ and 2.5 or 1.9 g/l of KCr(SO ₄ ) ₂ .12H ₂ O. The visual appearance of the coated substrates is evaluated, and the substrates are subjected to the salt spray test on the one hand, and to a paint adhesion test (wet adhesion grid) on the other hand.

The results obtained in terms of visibility of the colored coating, for the salt spray tests (exposure time causing the appearance of 5 corrosion pits) and the paint adhesion tests, are summarized in table 9 below. . <u>Table 9</u> Dye Concentration (% w/w) How to apply Coating coloring Time (h) of exposure to BS for 5 bites Paint adhesion C1 0.05 Retouch Blue < 44 - C1 0.01 Retouch Non visible 162 Grade 5 C2 0.05 Retouch Not colored < 60 - C3 0.05 Retouch Not colored <94 - C4 0.05 Retouch Not colored < 94 - C5 0.05 Retouch Light purple < 60 - C6 0.01 Immersion Purple < 72 - C7 0.1 Immersion Yellow < 76 - C8 0.05 Immersion Pale yellow < 28 - C9 0.8 Retouch Light brown < 24 - C10 1 Retouch Not colored Total corrosion at 90 h -

These results show that none of the dyes tested makes it possible to combine good visibility of coloring of the coating formed on the surface of the substrate and good corrosion resistance performance.

In particular, pyrocatechol violet at 0.01% w/w gives good results. to the salt spray test, but does not stain the plate. At 0.05% w/w, it colors the plate well but gives very poor results in salt spray.

All of the above results demonstrate that xylenol orange is the only coloring component to combine the advantages of good homogeneity and good visibility of the coloring of the coating formed on the surface of the substrate, and good performance in protecting the substrate treated against corrosion.

Claims (12)

1. Aqueous composition for the surface treatment of a metal substrate, said composition comprising a fluorinated compound, a corrosion-inhibiting metal compound excluding compounds based on hexavalent chromium, and a water-soluble colouring compound, characterised in that the colouring compound is xylenol orange, the corrosion-inhibiting metal compound is a trivalent chromium salt and the pH of said composition is between 3 and 5.
2. Composition according to claim 1, wherein the fluorinated compound is a fluorinated salt of a transition metal.
3. Composition according to claim 2, wherein said transition metal is selected from zirconium and titanium.
4. Composition according to any one of claims 1 to 3, wherein the trivalent chromium salt is selected from the fluorides and sulfates of trivalent chromium.
5. Composition according to claim 4, wherein the trivalent chromium salt is chromium potassium sulfate.
6. Composition according to any one of claims 1 to 5, containing a thickening agent.
7. Composition according to any one of claims 1 to 6, containing 0.5 to 1.5 g/l of xylenol orange.
8. Composition according to any one of claims 1 to 7, wherein the ratio by mass of fluorinated compound with respect to the corrosion-inhibiting metal compound is between 1 and 2.5.
9. Composition according to claim 8, wherein the ratio by mass of fluorinated compound with respect to the corrosion-inhibiting metal compound is between 1.5 and 2.
10. Method for surface treatment of a metal substrate, characterised in that it comprises the application, to a zone of the surface of said substrate, of a composition according to any one of claims 1 to 9.
11. Method according to claim 10, according to which said composition is applied to said zone of the surface of the substrate in a quantity of between 5 and 100 mg/cm² of said surface.
12. Metal substrate obtained by a method according to either one of claims 10 to 11, including, on at least a part of its surface, a coloured coating containing xylenol orange and oxyfluorides of chromium and of the metal constituting said metal substrate.