Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/34—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/04—Anodisation of aluminium or alloys based thereon
  • C25D11/18—After-treatment, e.g. pore-sealing
  • C25D11/24—Chemical after-treatment
  • C25D11/243—Chemical after-treatment using organic dyestuffs
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/30—Anodisation of magnesium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2222/00—Aspects relating to chemical surface treatment of metallic material by reaction of the surface with a reactive medium
  • C23C2222/10—Use of solutions containing trivalent chromium but free of hexavalent chromium

Definitions

• the present invention relates to the field of the surface treatment of metal substrates, in particular made of aluminum alloy, more particularly of treatments of the type aimed at forming a chemical conversion layer on the surface of the substrate, with a view to improving it. the properties of corrosion resistance and adhesion to paint.
• the present invention relates to an aqueous composition for the surface treatment of a metal substrate, as well as to a process for the surface treatment of metal substrates using such a composition, and as well as to a substrate obtained by such a composition. process.
• metal parts in particular parts made of aluminum or aluminum alloy, or magnesium or magnesium alloy, must frequently be surface treated in order to improve their cooling properties. corrosion resistance.
• a technique commonly used for this purpose is that known under the name of 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 of hydroxides, oxyhydroxides and oxyfluorides of the aluminum constituting the part and of the corrosion-inhibiting metal cation used.
• the compositions for the chemical conversion of aluminum surfaces most commonly used at the present time in the aviation industry contain hexavalent chromium as a corrosion inhibiting metal cation.
• hexavalent chromium Due to the harmful nature of hexavalent chromium to the environment and its toxicity for living organisms, it has been sought for several 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. In the present description, in a conventional manner in itself, the term “hexavalent chromium” is understood to mean chromium in the +6 oxidation state, and, by trivalent chromium, chromium in the +3 oxidation state.
• compositions containing corrosion inhibitor 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.
• a coloring compound in 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 with regard to trivalent chromium.
• 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 fully 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.
• 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 impacting the corrosion resistance performance of these coatings, compared to those obtained by means of compositions converters devoid of coloring compound.
• 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 the coloring of which is visually detectable, since it is different from that of the treated substrate, and exhibiting a high degree of corrosion resistance.
• 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.
• Xylenol orange or tetrasodium salt of 3,3'-Bis [ N , N- bis (carboxymethyl) aminomethyl] - o -cresolsulfonephthalein, is a dye commonly used for complexometric assays, of the formula:
• xylenol orange does not have the capacity to complex chromium.
• 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 also “conversion composition”, contains, in a conventional manner in itself for this type of composition, a fluorinated compound and a compound.
• metallic corrosion inhibitor excluding hexavalent chromium compounds. It also contains, as a water-soluble coloring compound, xylenol orange.
• the corrosion inhibiting metal compound is a trivalent chromium salt and the pH of the composition is between 3 and 5.
• this composition makes it possible to form on metal substrates, in particular substrates made of aluminum or of an aluminum alloy, of magnesium or of a magnesium alloy, or of steel, a colored surface coating which has a high degree of corrosion resistance.
• This coating also has good uniformity over the entire surface. area of the substrate treated, and good adhesion to paint systems commonly used in the aircraft 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.
• substantially free is meant the fact that it does not contain hexavalent chromium, except in trace amounts, that is to say in an amount less than or equal to 0.1% by weight relative to the weight. total composition.
• the composition according to the invention does not contain any other coloring compound 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.
• a concentration range advantageously ensures, on the one hand that the coating formed on the surface of the metal substrate is sufficiently colored for this coloration to be detectable with the naked eye, and on the other hand that the resistance performance to maximum corrosion imparted to the treated substrate. In particular, above 1.5 g / l, this performance is less satisfactory.
• the conversion composition according to the invention may contain a single fluorinated compound, or a plurality of fluorinated compounds.
• 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.
• the fluorinated compound is a fluorinated salt of a transition metal and of an alkali metal, such as potassium.
• fluorinated compounds which 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 dihydrogen fluorophosphonate (H 2 PO 2 F), etc.
• a fluorinated compound sodium hexafluorosilicate (Na 2 SiF 6 ), dihydrogen fluorogermanate (H 2 GeF 6 ) or dihydrogen fluorophosphonate (H 2 PO 2 F), etc.
• a fluorinated compound which is particularly preferred 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 further contain a single corrosion-inhibiting metal compound, or a plurality of such compounds.
• a corrosion-inhibiting metal compound is understood to mean, in a conventional manner in itself, 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 salt of a corrosion-inhibiting transition metal, more particularly 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 of their mixtures.
• This salt may contain, in addition to the corrosion inhibitor trivalent chromium ion, one or more counterions.
• trivalent chromium fluorides and trivalent chromium sulphates are particularly preferred in the context 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.
• the conversion composition according to the invention contains at least potassium chromium sulfate, preferably in the form of CrK (SO 4 ) 2 hydrate, xH 2 O, 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 metal compound (s) which inhibit corrosion (s), in particular a concentration of between 1 and 10 g / l potassium chromium sulfate dodecahydrate CrK (SO 4 ) 2 , 12H 2 O.
• the pH of the conversion composition is between 3 and 5.
• the pH can have been adjusted within such a range of values in any manner conventional in itself.
• the conversion composition may contain a pH adjuster, in particular of the basic type, for example sodium hydroxide NaOH, in an amount suitable for adjusting 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.
• the conversion composition according to the invention contains concentrations of fluorinated compound (s) and of corrosion-inhibiting metal compound (s) such as the ratio by mass of fluorinated compound (s) (s) relative to the metal compound (s) corrosion inhibitor (s), these compounds being distinct from each other, is between 1 and 2.5, preferably between 1 , 5 and 2. If the composition contains several fluorinated compounds, it is the total mass of these fluorinated compounds which is taken into account here, the same for the metal 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 metal compound and the fluorinated compound present in the conversion composition according to the invention may 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 is approximately 1 mPa.s.
• the conversion composition according to the invention may optionally contain one or more thickening agent (s), preferably soluble in water, making it possible to increase its viscosity.
• thickening agent preferably soluble in water
• 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 of between 2000 and 3000 mPa.s, and preferably of between 2100 and 2500 mPa.s, this viscosity being measured with an LV01 type spindle at a temperature of 23 ° C and at a speed of 12 rev / min.
• 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 of approximately 10 g / l.
• the conversion composition according to the invention may further contain one or more additives conventional in themselves in the field of chemical conversion compositions, for example surfactants, wetting agents, pH stabilizers, additional corrosion inhibitors. , chelating / complexing agents, etc.
• additives conventional in themselves 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 comprises the incorporation of the various compounds in the solid state in an aqueous vehicle.
• the order of incorporation of the compounds into the vehicle is as follows: corrosion inhibitor metal compound (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 room temperature, that is to say at a temperature of between 20 and 25 ° C approximately.
• the present invention relates to a method for treating the surface of a metallic substrate, by forming a conversion coating on the surface of this substrate.
• This method comprises applying 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 metal substrate to which the surface treatment method according to the invention is applied is in particular a substrate made of aluminum or of an aluminum alloy, of magnesium or of a magnesium alloy, or of steel.
• the conversion composition is applied to the area of the surface of the substrate in an amount of between 5 and 100 mg / cm 2 of said surface.
• the conversion composition contains a thickening agent, and is therefore in the form of a gel
• it is preferably applied to the area of the surface 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.
• 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.
• the choice of values within 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 visibility. corrosion resistance properties of this coating.
• the application of the conversion composition to the surface of the metal substrate can be carried out in any conventional manner.
• the conversion composition is applied to the surface of the substrate in a mode known as retouching, by spraying, brushing or wiping.
• retouching by spraying, brushing or wiping.
• it can be implemented any means of application conventional in itself, such as a pen or 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.
• the application is preferably carried out by several series of crossed passes on the surface of the treated zone, a waiting time preferably being spared between series of passes. successive.
• the application can be carried out by a single continuous contact of the conversion composition on the surface of the treated zone, 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 between 5 and 40 ° C, preferably between 15 and 30 ° C and preferably 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 prior surface preparation step that is conventional in itself, in particular of cleaning, degreasing, pickling, mechanical, acidic or basic. , etc., so as to remove dirt, grease, oxides, etc. which may be present there.
• the surface treatment process 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 can also comprise a step of drying the surface of the treated substrate.
• a post-treatment process can comprise the application to the surface of the treated substrate of a composition based on a rare earth salt which inhibits corrosion and on an oxidizing compound.
• a metal 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 orange amount of xylenol it contains.
• the xylenol orange can be present there in any chemical form, in particular in complexed form with the transition metal which can enter into the composition of the fluorinated compound contained in the conversion composition according to the invention used, for example in complexed form with some zirconium.
• the metal substrate according to the invention is preferably formed from aluminum or from an aluminum alloy, from magnesium or from a magnesium alloy, or from steel.
• the conversion coating present on its surface is in particular formed of oxyfluorides, oxyhydroxides and hydroxides of the metal constituting the substrate and chromium. It may for example be formed from oxides, hydroxides and oxyfluorides of aluminum and chromium, as well as, where appropriate, of the transition metal entering into the composition of the fluorinated salt, for example of zirconium.
• the coating is further preferably present on the surface of the substrate in an amount of between 5 and 100 mg of coating per cm 2 of said coated surface.
• the coating is present on the surface of the substrate in an amount 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.
• This may in particular correspond to the configurations in which the conversion composition used for the surface treatment contained a thickening agent, and therefore was in the form of a gel.
• 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 the configurations in which the conversion composition used for the surface treatment was devoid of thickening agent, and therefore was in liquid form.
• the substrates used are rectangular 2024-T3 aluminum alloy plates with dimensions of 125 mm x 75 mm and a thickness of 1 mm.
• the substrate is first subjected to a step of pretreatment, by immersion successively 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 demineralized water.
• the alkaline degreasing bath is more precisely 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 precisely 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 of the sanded area with demineralized water by spraying, and drying by wiping.
• the conversion composition is then applied to the selected area of the surface of the substrate by cross passes, using a sponge.
• the composition is first of all applied to the entire area concerned, along a first application axis, then it is applied along a second application axis 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 air dry. These operations are carried out at a temperature between 15 and 30 ° C.
• the substrate is first of all subjected to a surface preparation step, by degreasing with the product sold by the applicant under the name Sococlean A3432, at 10% by volume, at 55 ° C for 15 min, then immersion in demineralized water for 3 min, and rinsing by spraying with demineralized water.
• the substrate is then subjected to deoxidation using the product sold by the applicant under the Socosurf name A1858 / A1806, 40/10% vol, at 50 ° C for 5 min, then rinsed again by immersion in demineralized water for 3 min then by spraying with demineralized water.
• the substrate thus prepared is immersed in the chemical conversion composition at 40 ° C. for 10 min, then post-treated by immersion in demineralized water for 3 min then spraying with demineralized water.
• 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.
• This test is carried out according to the protocol described in standard ISO 6860, using a conical mandrel having an end of smaller diameter of 7 mm.
• the substrate is bent, 180 degrees in 2-3 s, at about 23 ° C and about 50% relative humidity, then exposed to salt spray for 3000 h.
• the development of corrosion pitting at the bending zone is evaluated.
• the appearance (uniformity) of the coating formed on the surface of the substrate as a result of the treatment process is evaluated visually, as well as the visibility of its coloring (Visib., Rated out of 5, the value attributed being even higher that the coloring is better visible).
• the coloring compound used is xylenol orange, in accordance with the invention, and the treatment process is carried out in retouching mode.
• Xylenol orange is tested at different concentrations ([dye]) in the conversion composition: 0.5 g / l, 1 g / l, 2.5 g / l.
• 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 observed visually, then subjected to a salt spray exposure test.
• 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]).
• 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 with a pH equal to 3.9 or 5.5, are used to treat substrates in accordance with the invention, directly or after pretreatment.
• the coated substrates are subjected to the salt spray test.
• the results obtained (in number of bites for each exposure time to the BS salt spray) are shown in Table 4 below.
• compositions according to the invention For two of the compositions according to the invention exhibiting the best results, tests of resistance to salt spray 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 exhibit good visual surface homogeneity, and a violet coloration clearly visible to the naked eye.
• Substrates treated with 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 squaring or folding.
• the thickness of primer deposited on the surface of the substrate is between 25 and 40 ⁇ m.
• 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 primer without hexavalent chromium, a chromated primer (containing hexavalent chromium) water-soluble and a chromated primer (containing hexavalent chromium) solvent.
• the test carried out is a dry or wet cross-cut adhesion test. The results obtained are shown in Table 7 below.
• the thickness deposited on the surface of the substrate is specified therein.
• Table 7 ⁇ /u> Primary Primer thickness ( ⁇ m) Dry test (Grade) Wet test (Grade) Primary without Cr (VI) 70 to 80 0 1 0 0 0 1 Primer containing water-soluble Cr (VI) 30 to 40 0 0 0 0 0-1 0 0 Primer containing solvent-based Cr (VI) 10 to 20 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
• Substrates are treated by a conversion process in touch-up 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.
• Substrates are treated in touch-up mode or in immersion mode, with pretreatment, by chemical conversion processes using 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.
• pyrocatechol violet at 0.01% w / w gives good results in the salt spray test, but does not color the plate. At 0.05% w / w, it colors the plate well but gives very poor results in salt spray.

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