Method for producing a silica-based sol-GEL coating on a metal substrate

Abstract

The present invention relates to a method for producing an organic-inorganic hybrid sol-gel coating on a metal substrate, said method comprising the following steps: a) preparing a sol through hydrolysis and condensation reactions between at least one silicon alkoxide having four hydrolysable bonds, at least one silicon alkoxide having at least one non-hydrolysable bond, and water; b) dispersing at least one inorganic iron-based additive selected from iron oxides, iron oxyhydroxides and iron hydroxides within the sol resulting from step a); c) applying the dispersion resulting from step b) onto said metal substrate; and d) heat treatment of the coating resulting from step c), obtaining said organic-inorganic hybrid sol-gel coating on the substrate.

Description

METHOD FOR PRODUCING A SILICA-BASED SOL-GEL COATING ON A METALSUBSTRATEDescription

[0001] Field of the invention

[0002] The present invention relates to a method for producing a sol-gel coating on a metal substrate, a metal substrate coated with a sol-gel coating, as well as a component of a car or a motorcycle, in particular a brake caliper or a brake disc, comprising a metal substrate coated with a sol-gel coating. The present invention also relates to the use of a sol-gel coating to provide corrosion resistance to the metal substrate on which it is applied.

[0003] State of the art

[0004] In the automotive field, the problem of corrosion of metal components of a vehicle, in particular metal components of the braking system of a vehicle, such as brake calipers and brake discs, is strongly felt.

[0005] Brake calipers of a braking system are typically made of cast iron or aluminium alloy, and brake discs are typically made of iron or steel.

[0006] The above-mentioned components are easily damaged following exposure to natural atmospheric conditions, for example, humidity, rain, marine environment, NOX, SOX. In particular, exposure of these components to corrosive environments, especially those containing chlorides, compromises the corrosion resistance of the metal surfaces, which will therefore be subject to a phenomenon of generalized corrosion. The consequence is the loss of structural integrity of the metal components, which results in safety issues and high costs for replacing damaged parts. All of this negatively affects the performance of the braking system of a vehicle.

[0007] This issue is more strongly perceived in luxury and high-performance vehicles,for example high-end cars or competition motorcycles, which must guarantee high performance.

[0008] Until now, corrosion protection of these metal components, especially components made of steel or aluminium alloys, has been achieved through painting the surfaces of such components, or through conversion processes such as anodization and ferritic nitrocarburization, or through chromate passivation. However, these techniques have proven to be energy-intensive, expensive and to have a high environmental impact.

[0009] Some solutions employing sol-gel coatings have also been proposed. Although less expensive and with reduced environmental impact, such solutions nevertheless show lower properties of corrosion resistance, for example due to reduced thicknesses. Some of these solutions use inorganic salts, typically salts of cerium, yttrium or manganese, as additives to confer active corrosion protection through redox processes. However, these additives are costly and have a negative impact both from an environmental and toxicological point of view.

[0010] Therefore, the problem underlying the present invention is to solve the aforementioned issues, namely to provide coatings for metal components of cars and motorcycles, in particular for components of the braking system of cars and motorcycles, capable of delaying corrosion of said components and prolonging their life, ensuring continuously improving braking system performance over a prolonged period of time, while maintaining a focus on environmental protection and human health.

[0011] Summary of the invention

[0012] The problem outlined above is solved by a method for producing a sol-gel coating on a metal substrate, by a metal substrate coated with a sol-gel coating, by a component of a car or a motorcycle comprising a metal substrate coated with a sol-gel coating, as well as by the use of a sol-gel coating to provide corrosion resistance to the metal substrate on which it is applied, as outlined in the appended claims, the definitions of which form an integral part of the present description.

[0013] A first object of the present invention is a method for producing an organic- inorganic hybrid sol-gel coating on a metal substrate, said method comprising the following steps: a) preparing a sol through hydrolysis and condensation reactions between at least one silicon alkoxide having four hydrolysable bonds, at least one silicon alkoxide having at least one non-hydrolysable bond, and water; b) dispersing at least one inorganic iron-based additive selected from iron oxides, iron oxyhydroxides and iron hydroxides within the sol resulting from step a); c) applying the dispersion resulting from step b) onto said metal substrate; and d) heat treatment of the coating resulting from step c), obtaining said organic- inorganic hybrid sol-gel coating on the substrate.

[0014] A second object of the present invention is a metal substrate coated with an organic-inorganic hybrid sol-gel coating comprising at least one inorganic iron-based additive selected from iron oxides, iron oxyhydroxides, and iron hydroxides, wherein said coating is obtainable by the method described above.

[0015] A third object of the present invention is a component of a car or a motorcycle comprising said metal substrate.

[0016] An additional object of the present invention is the use of a coating obtainable by the above method to provide corrosion resistance to the metal substrate on which it is applied, in particular to the metal substrate of a component of a car or a motorcycle.

[0017] The substrates on which the sol-gel coatings obtained with the method according to the present invention are applied, as well as the components of cars andmotorcycles comprising substrates on which said coatings are applied, show remarkable corrosion resistance compared to uncoated substrates, avoiding the use of inorganic salts, but rather using materials and reagents that are harmless or, in any case, significantly less impactful, both for the environment and for human health.

[0018] Furthermore, it has surprisingly been found that the sol-gel coatings obtainable with the method of the invention are such as to impart specific aesthetic properties to the substrates on which they are applied, in terms of colour and surface finish, which are particularly desired in the substrates of components of cars and motorcycles, especially high-end ones.

[0019] Further features and advantages of the invention will be more apparent from the description of some illustrative embodiments, provided below by way of nonlimiting example.

[0020] Brief description of the figures

[0021] Figure 1 shows the linear sweep voltammetry (LSV) profiles obtained for a cast iron substrate coated with a coating obtainable by the method of the invention and for an uncoated cast iron substrate.

[0022] Figure 2 shows the X-ray diffraction spectrum of a coating obtainable by the method of the invention, comprising hematite (a-Fe2O3).

[0023] Detailed description of the invention

[0024] An object of the present invention is a method for producing a coating on a metal substrate, in particular on a metal substrate of a component of a car or a motorcycle, through a sol-gel process starting from silicon alkoxides. More particularly, said sol-gel process comprises a step of applying onto said substrate a sol obtained from silicon alkoxides within which at least one inorganic iron-based additive selected from iron oxides, iron oxyhydroxides and iron hydroxides isdispersed.

[0025] It has been observed that the metal substrate on which the above organic- inorganic hybrid sol-gel coating, silica-based, is applied possesses high corrosion resistance. In particular, it has surprisingly been observed that, thanks to the presence of at least one of the above-mentioned additives, the sol-gel coating has a thickness capable of ensuring high corrosion resistance, which is obtained through the combined barrier and thermodynamic effect. In greater detail, the above-mentioned inorganic iron-based additives act specifically and simultaneously to make the metal substrate less susceptible to the corrosive phenomenon, as well as to limit its propagation rate.

[0026] Furthermore, it has surprisingly been observed that, depending on the type of additive that is dispersed and on its particle size, it is possible to modulate the properties of the sol-gel coating, in particular to vary its thickness, colour and brightness. This makes the method of the invention extremely versatile, allowing modulation of the coating properties according to the needs.

[0027] The sol-gel process involves the synthesis of a sol, which constitutes the precursor for the subsequent formation of an inorganic network, called gel, through hydrolysis and condensation reactions. The term “sol” used in the present application denotes the product obtained from the hydrolysis and condensation reactions occurring between silicon alkoxides and water. More specifically, the term “sol” refers to a colloid consisting of a fine dispersion of solid particles in a liquid medium.

[0028] As already mentioned above, the method according to the present invention comprises a step a) of preparing a sol through hydrolysis and condensation reactions between at least one silicon alkoxide having four hydrolysable bonds (also referred to as “hydrolysable alkoxide” in the present application), at least one silicon alkoxide having at least one non-hydrolysable bond (also referred to as “alkoxide having a non-hydrolysable bond” in the present application) and water.

[0029] The method according to the present invention comprises a subsequent step b) of dispersing at least one inorganic iron-based additive within the sol resulting from step a).

[0030] The colloidal dispersion resulting from step b) is applied onto a metal substrate during a step c), obtaining a coating on said substrate, and said coating is subsequently subjected to a step d) of heat treatment. At the end of step d), an organic-inorganic hybrid sol-gel coating is obtained on said substrate.

[0031] The expression “silicon alkoxide having four hydrolysable bonds” means a silicon alkoxide having four hydrolysable Si-0 bonds.

[0032] The expression “silicon alkoxide having at least one non-hydrolysable bond” means a silicon alkoxide comprising at least one non-hydrolysable Si-C bond, preferably a silicon alkoxide comprising two or three hydrolysable Si-0 bonds and one or two non-hydrolysable Si-C bonds.

[0033] Step a) of preparation of the sol

[0034] According to an embodiment, step a) in turn comprises:- a step a1) of mixing said at least one hydrolysable alkoxide and said at least one alkoxide having a non-hydrolysable bond in a mixture comprising a solvent and at least one thickener, obtaining a mixture of said alkoxides, wherein said solvent consists of at least one Ci-C4alcohol, and said at least one thickener is selected from the group consisting of polyvinyls, polyphenols, polyalcohols and derivatives thereof;- a step a2) during which water and an organic or inorganic acid catalyst selected from CH3COOH, HNO3, HCI and H2SO4are added to the mixture resulting from step a1), obtaining a reaction mixture; and- a step a3) during which said reaction mixture resulting from step a2) is stirred for atleast 10 hours at a temperature between 10 and 50°C.

[0035] Preferably, said Ci-C4alcohol is selected from methanol, ethanol, isopropanol, isobutanol, n-butanol and mixtures thereof.

[0036] According to a preferred embodiment, said hydrolysable alkoxide has formula (I):Si(OR1)(OR2)(OR3)(OR4) (I) wherein R1, R2, R3and R4are independently selected from the group consisting of linear or branched Ci-Cio alkyls, C5-C6aryls, linear or branched C2-Ci0alkenyls, and linear or branched C2-Ci0alkynyls.

[0037] Preferably, said hydrolysable alkoxide is tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS).

[0038] According to a preferred embodiment, said alkoxide having a non- hydrolysable bond has formula (II):R8-(CH2)n-Si(OR5)(OR6)(OR7) (II) where:R5, R6and R7are independently selected from the group consisting of linear or branched Ci-Cio alkyls, C5-C6aryls, linear or branched C2-Ci0alkenyls, and linear or branched C2-Ci0alkynyls, n is an integer between 0 and 6, preferably 2, andR8is selected from the group consisting of linear or branched Ci-Cio alkyls, linear or branched C2-Ci0alkenyls, CH2=C(CH3)COO-, and

[0039] According to this embodiment, said alkoxide having a non-hydrolysable bond of formula (II) is preferably selected from vinyltriethoxysilane (VTES), vinyltrimethoxysilane (VTMS), 3-(glycidoxypropyl)triethoxysilane (GPTES), 3- (glycidoxypropyl)trimethoxysilane (GPTMS), methyltriethoxysilane (MTES),methyltrimethoxysilane (MTMS), ethyltriethoxysilane (ETES), ethyltrimethoxysilane (ETMS), propyltriethoxysilane (PTES), propyltrimethoxysilane (PTMS), butyltriethoxysilane (BTES), butyltrimethoxysilane (BTMS), isopropyltriethoxysilane (I PTES), isopropyltrimethoxysilane (I PTMS), 3-(trimethoxysilyl)propyl methacrylate (yMPS).

[0040] Preferably, said alkoxide having a non-hydrolysable bond of formula (II) is selected from methyltriethoxysilane (MTES), methyltrimethoxysilane (MTMS), 3- (trimethoxysilyl)propyl methacrylate (yMPS), or 3-(glycidoxypropyl)trimethoxysilane (GPTMS).

[0041] According to a preferred embodiment, at least one alkoxide having a non- hydrolysable bond of formula (II) is GPTMS. Advantageously, the epoxy groups present in the GPTMS increase the adhesion of the organic-inorganic hybrid sol-gel coating obtained with the method of the invention to the substrate, and facilitate the painting of components of cars and motorcycles.

[0042] Preferably, the molar ratio between said at least one hydrolysable alkoxide of formula (I) and said at least one alkoxide having a non-hydrolysable bond of formula (II) is between 1:0.1 and 1:10, preferably between 1:0.2 and 1:6, more preferably between 1:0.2 and 1:5, even more preferably between 1:0.3 and 1 :3. For example, said molar ratio is about 1 : 1 or about 1 : 1.5 or about 1 :2 or about 1 :2.5.

[0043] According to an embodiment, the molar ratio between said at least one hydrolysable alkoxide of formula (I) and said solvent, preferably ethanol, is between 1 :5 and 1 :50, for example between 1 :8 and 1 :20.

[0044] According to an embodiment, the molar ratio between said at least one hydrolysable alkoxide of formula (I) and water is between 1:1 and 1 :10, for example between 1:2 and 1 :5.

[0045] According to an embodiment, the molar ratio between said at least onehydrolysable alkoxide of formula (I) and said inorganic acid catalyst is between 1 :0.01 and 1 :0.1 , for example between 1 :0.02 and 1 :0.05.

[0046] Preferably, said at least one hydrolysable alkoxide of formula (I) is present in the reaction mixture in an amount by weight between 3 and 30% by weight, preferably between 5 and 20% by weight, more preferably between 8 and 20% by weight, for example between 12 and 15% by weight, with respect to the total weight of the reaction mixture.

[0047] Preferably, said at least one alkoxide having a non-hydrolysable bond of formula (II) is present in the reaction mixture in an amount by weight of at least 5% by weight, or at least 8% by weight, or at least 10% by weight, or at least 12% by weight, or at least 20% by weight, or at least 25% by weight, or at least 30% by weight, and / or in an amount by weight not higher than 60% by weight, or not higher than 55% by weight, or not higher than 50% by weight, or not higher than 45% by weight, or not higher than 40% by weight, or not higher than 35% by weight, with respect to the total weight of the reaction mixture.

[0048] Preferably, the Ci-C4alcohol is present in an amount by weight between 30 and 80% by weight, for example between 30 and 65% by weight, with respect to the total weight of the reaction mixture.

[0049] Preferably, the thickener selected from the group consisting of polyvinyls, polyphenols, polyalcohols and derivatives thereof is present in an amount by weight between 1 and 40% by weight, for example between 10 and 30% by weight, or between 15 and 25% by weight, with respect to the total weight of the reaction mixture.

[0050] Preferably, water is present in an amount by weight between 1.5 and 30% by weight, for example between 2 and 20% by weight, or between 3 and 15% by weight, or between 7 and 12% by weight, or between 7 and 10% by weight, with respect tothe total weight of the reaction mixture.

[0051] Preferably, the organic or inorganic acid catalyst is present in an amount by weight between 0.004 and 18% by weight, preferably between 0.004 and 0.1% by weight, for example between 0.005 and 0.08% by weight, with respect to the total weight of the reaction mixture. Said percentage amount by weight of the organic or inorganic acid catalyst refers to an aqueous solution of said acid, preferably a 0.1 M solution of said acid, for example a 0.1 M solution of HNO3or HCI.

[0052] The above percentages by weight are with respect to the total weight of the reaction mixture, that is the total weight of the components that are mixed to give the reaction mixture.

[0053] Preferably, during said step a3), the reaction mixture is stirred for at least 10 hours, or for at least 15 hours, or for at least 20 hours, or for at least 24 hours, or for at least 48 hours, or for at least 72 hours, or for at least 96 hours. More preferably, during said step a3), the reaction mixture is stirred for a period between 10 hours and 96 hours, even more preferably between 15 and 96 hours.

[0054] Preferably, during said step a3), the reaction mixture is stirred at room temperature. The expression “room temperature” denotes a temperature between 18 and 25°C.

[0055] Preferably, during said step a3), the reaction mixture is stirred at a speed of at least 400 rpm, more preferably between 450 rpm and 900 rpm, or between 500 and 800 rpm, for example 600 rpm.

[0056] Step b) of dispersing the inorganic iron-based additive

[0057] As already mentioned above, said at least one inorganic iron-based additive is selected from iron oxides, iron oxyhydroxides and iron hydroxides.

[0058] Preferably, said inorganic iron-based additive is added to the sol in an amountbetween 5% and 40% by weight, for example between 10 and 30% by weight, or between 15 and 25% by weight, with respect to the total weight of the sol.

[0059] Preferably, the concentration of said inorganic iron-based additive within the dispersion resulting from step b) is between 50 g / L and 300 g / L, preferably between 100 g / L and 250 g / L, even more preferably between 150 g / L and 200 g / L.

[0060] Preferably, the total solid concentration within the dispersion after addition of the inorganic iron-based additive is between 30 g / L and 350 g / L, preferably between 150 g / L and 250 g / L.

[0061] Preferably, the iron oxides are selected from the group consisting of: FeO (wustite), Fe3O4(magnetite), a-Fe2O3(hematite), p-Fe2O3, y-Fe2O3(maghemite) and £-Fe2O3.

[0062] Preferably, the iron oxyhydroxides are selected from the group consisting of: a-FeO(OH) (goethite), y-FeO(OH) (lepidocrocite), b-FeO(OH) (feroxyhyte) and FeO(OH,CI) (akaganeite).

[0063] Preferably, the iron hydroxides are selected from the group consisting of: Fe(OH)2and Fe(OH)3.

[0064] Preferably, said at least one inorganic iron-based additive has a particle size distribution (“Particle Size Distribution”, PSD), expressed as D90, between 1 nm and 50 pm, preferably between 5 nm and 10 pm. In the present application, the term “granulometry” will refer to the particle size.

[0065] In the present application, a micrometric particle size (D90) means a particle size of at least 1 pm, and a nanometric particle size (D90) means a particle size below 1 pm.

[0066] It has surprisingly been found that, depending on the granulometry of the additive dispersed in the sol, both the colour and the brightness of the final coatingmay vary. In particular, for the same additive, a micrometric particle size (D90) advantageously allows obtaining a coating with higher brightness compared to a nanometric particle size (D90).

[0067] It has also surprisingly been found that the dispersion of one or more of the above-mentioned additives allows obtaining final coatings with colours varying, for example, from black to grey to brown to green to orange to red to pink.

[0068] Depending on the granulometry of the dispersed additive, it is also possible to vary the thickness of the final coating.

[0069] Table 1 below reports some examples of colour variation depending on the type of additive and its particle size (D90).Table 1

[0070] The possibility of modulating the properties of the final coating, in terms of thickness, colour and brightness, is extremely advantageous in the automotive sector, especially in the luxury vehicle sector, where, in addition to performance, aesthetics also play an important role.

[0071] Step c) of applying the colloidal dispersion

[0072] The step of applying the colloidal dispersion resulting from step b) onto the substrate may be carried out using various technologies selected from spray deposition, dip-coating, spin coating and electrodeposition.

[0073] According to a preferred embodiment, said step c) of applying the colloidal dispersion onto the substrate is carried out by dip-coating. In this embodiment, the substrate is immersed in the colloidal dispersion and withdrawn at a constant speed to allow solvent evaporation and enable its gelation.

[0074] Preferably, the withdrawal speed of the metal substrate from said colloidal dispersion is between 20 and 1500 mm / min. According to some embodiments, said withdrawal speed is at least 50 mm / min, or at least 100 mm / min, or at least 150 mm / min, or at least 200 mm / min, or at least 250 mm / min, or at least 500 mm / min, for example about 1000 mm / min.

[0075] Together with the type of dispersed additive and its granulometry, the withdrawal speed of the metal substrate from said colloidal dispersion contributes to determining the thickness of the resulting coating. In particular, higher withdrawal speeds result in greater coating thicknesses compared to thicknesses obtainable with lower withdrawal speeds.

[0076] Step d) of heat treatment

[0077] According to a first embodiment, step d) of heat treatment is carried out at atemperature below 80°C for at least 12 hours, for example at 60°C for a time between 15 and 20 hours, for example 17 hours. This treatment is also referred to as “low- temperature heat treatment”.

[0078] According to a second embodiment, step d) of heat treatment is carried out at a temperature of at least 80°C for a time between 30 and 180 minutes, for example at a temperature between 80 and 150°C for a time between 60 and 120 minutes, or at a temperature above 150°C, for example 300°C, for a time between 30 and 60 minutes. This treatment is also referred to as “high-temperature heat treatment”.

[0079] Preferably, said step d) of heat treatment is carried out at a temperature of 120°C for a time between 120 and 180 minutes.

[0080] Without being bound by theory, it is believed that said step of drying and solidification heat treatment allows elimination of the liquid phase from the gel, further promoting condensation reactions and hence the crosslinking of the coating.

[0081] Preferably, between said step c) of applying the colloidal dispersion onto the substrate and said step d) of heat treatment, the coating is kept at room temperature for at least 30 minutes, for example for 60 minutes, with consequent evaporation of at least part of the solvent.

[0082] Preferably, the coating obtained with the method of the present invention has a thickness of at least 3 microns, more preferably at least 5 microns, even more preferably greater than 5 microns. Preferably, the coating obtained from said step b) has a thickness between 5 and 10 microns, or between 6 and 10 microns, or between 7 and 10 microns, or between 8 and 10 microns, or between 9 and 10 microns.

[0083] Preferably, the coating obtained with the method of the invention has a high corrosion resistance. In particular, the coating obtained with the method of the invention has corrosion current values typically lower than nA / cm2(10“9A / cm2),preferably of the order of pA / cm2(10“12A / cm2), for example of the order of a few tens of pA / cm2, for example between 1 and 100 pA / cm2, or between 1 and 80 pA / cm2, or between 1 and 50 pA / cm2. Furthermore, the coating obtained with the method of the invention has a corrosion potential significantly less negative than that of an uncoated metal substrate, preferably up to +100 mV, for example +80 mV, compared to the corresponding uncoated metal substrate.

[0084] An additional object of the present invention is a metal substrate coated with an organic-inorganic hybrid sol-gel coating obtainable by the method described above.

[0085] Another object of the present invention is a component of a car or a motorcycle comprising said metal substrate.

[0086] According to a preferred embodiment of the invention, said component is the brake caliper of the braking system of a car or a motorcycle, or a part of said brake caliper.

[0087] According to an embodiment, said component is the brake disc of the braking system of a car or a motorcycle, or a part of said disc. For example, said component is the bell of said disc. For example, said component consists of the ventilation channels of said disc, which can advantageously be coated with the coating obtainable by the method of the invention.

[0088] Experimental part

[0089] Coatings were prepared for a cast iron substrate and for a carbon steel substrate, for example of a disc for disc brakes, starting from TEOS and GPTMS in a molar ratio of 1 :2. a-Fe2O3(hematite) with D90 < 5 pm was dispersed within said coating.

[0090] The experimental procedure used for its synthesis is reported below.

[0091] Surface pretreatment of the substrate

[0092] The carbon steel substrate was cleaned before application of the respective coating. In particular, it was washed with acetone, distilled water and ethanol twice and placed in an ultrasonic bath with ethanol for 5 minutes. Finally, the substrate was washed with acetone, water and ethanol, and left to dry at room temperature.

[0093] An analogous procedure was carried out for the cast iron substrate.

[0094] Preparation of the colloidal dispersion

[0095] TEOS (15.2% v / v) and GPTMS (30% v / v) in a molar ratio of 1 :2 were mixed within a mixture consisting of ethanol (40% v / v) as a solvent and a polyphenol (0.1 g / mL) as a thickener. The resulting mixture was then activated by addition of water (14.5% v / v) and HNO3(2 10-5M) as an inorganic acid catalyst, obtaining a reaction mixture. The latter was stirred for about 24 hours at about 25°C. Subsequently, a- Fe2O3(hematite) with D90 < 5 pm was dispersed, obtaining a corresponding dispersion having a concentration of hematite of 150 g / L.

[0096] Coating depositionUsing a dip coater, the substrate was immersed in the resulting colloidal dispersion and withdrawn at a constant speed of about 250 mm / min to allow solvent evaporation and enable its gelation.

[0097] Heat treatment (curing) of the coating

[0098] The wet coating was kept at room temperature for about 2 hours and then placed in an oven at a temperature of about 130°C for about 4 hours.

[0099] Corrosion resistance

[0100] The corrosion resistance properties of the cast iron substrate coated with the coating obtained with the above procedure and of an uncoated cast iron substrate were tested by linear sweep voltammetry (LSV) measurements.

[0101] The linear sweep voltammetry measurement was carried out using the Bio Logic VMP 300 instrument. The configuration of the electrochemical cell comprises a graphite counter-electrode, a calomel electrode serving as a reference electrode, and a working electrode represented by the sample to be tested. Before scanning, the open circuit potential was measured using the same instrument and the same configuration of the electrochemical cell. The scan was carried out in a range between -180 mV and +300 mV around the open circuit potential. The scan rate was set to 0.167 mV / s.

[0102] The LSV profiles obtained for the two substrates are shown in the graph of Figure 1.

[0103] In particular, the coated substrate showed a corrosion potential of -540 mV relative to a saturated calomel electrode (SCE) and a significant increase of the corrosion potential, equal to about +100 mV, compared to the uncoated substrate.

[0104] The coated substrate also showed a corrosion current of 20 pA / cm2, i.e. , 6 orders of magnitude lower than that of the uncoated substrate, which instead showed a corrosion current of 40 pA / cm2.

[0105] The significant increase of the corrosion potential (+100 mV) and the simultaneous significant decrease of the corrosion current (reduced by six orders of magnitude) shown by the coated substrate indicate its greater corrosion resistance.

[0106] Thickness measurement

[0107] The thickness of the coating obtained with the above procedure was measured by analysis of the cross-section performed with the Tescan Amber X scanning electron microscope. The cross-section was obtained by preparation of the sample carried out in the same microscope. The sample was cut by focusing on its surface a plasma powered by xenon. The resulting cross-section was polished using the same technology by applying plasma currents around 400 pA. The cross-sectionwas analysed for the measurement of the thickness using the electron microscope by applying a voltage of 10 keV and a current of 300 pA.

[0108] A thickness of 10 pm was measured. This thickness of the coating proved to be one order of magnitude greater than the thickness of a corresponding coating without additive (10 pm vs. 1 pm).

[0109] Hardness and elastic modulus measurement

[0110] The mechanical properties (hardness and elastic modulus) of the coating obtained with the above procedure were measured by nano-indentation analysis using a nano-indenter (NHT3 HEAD FOR Step X00) equipped with a Berkovich tip. A surface detection method, labelled Adjust Depth Offset (ADO), was performed by setting an approach speed of 100 pm / min, a contact stiffness threshold of 500 pN / pm and a contact force of 25 mN. Subsequently, an indentation matrix was performed to generate a two-dimensional map of hardness and elastic modulus. The tip penetrated the coating to a maximum depth between 300 nm and 1000 nm with a loading and unloading rate of 30 mN / min. All indentation tests were performed using a Poisson’s ratio of 0.3, an acquisition frequency of 10 Hz, an approach speed of 2000 nm / min and an approach distance of 1500 nm.

[0111] The hardness of the coating was found to be 6.5 HV(IT> and the elastic modulus was found to be 0.60 GPa.

[0112] X-ray diffraction

[0113] X-ray diffraction (XRD) analyses of the coating applied to the carbon steel substrate were carried out.

[0114] In particular, these analyses were performed with a Rigaku SmartLab diffractometer equipped with a rotating anode and a Copper source. The measurement was conducted in para-focusing geometry (Bragg-Brentano) in the angular range between 10 and 65°, with incremental steps of 0.02° and an acquisitionspeed of 0.25 7min. During acquisition, the sample was rotated around its own axis at a speed of 10 rpm, in order to avoid possible effects of preferential orientation.

[0115] The XRD spectrum is shown in Figure 2 and revealed the presence of both an amorphous phase (sol-gel matrix) and a crystalline phase (hematite). In addition, it was possible to identify low-intensity peaks belonging to iron phases corresponding to the substrate.

[0116] Colour and finish

[0117] The coating obtained with the above procedure was found to be red in colour and with a glossy finish.

[0118] It is evident that those described are only particular embodiments of the present invention. The person skilled in the art will be able to introduce into the method, the substrate and the component of the invention all those modifications necessary for their adaptation to particular conditions, without however departing from the scope of protection as defined in the appended claims.

Claims

CLAIMS1. Method for producing an organic-inorganic hybrid sol-gel coating on a metal substrate, said method comprising the following steps: a) preparing a sol through hydrolysis and condensation reactions between at least one silicon alkoxide having four hydrolysable bonds, at least one silicon alkoxide having at least one non-hydrolysable bond, and water; b) dispersing at least one inorganic iron-based additive selected from iron oxides, iron oxyhydroxides and iron hydroxides within the sol resulting from step a); c) applying the dispersion resulting from step b) onto said metal substrate; and d) heat treatment of the coating resulting from step c), obtaining said organic- inorganic hybrid sol-gel coating on the substrate.

2. Method according to claim 1 , wherein the concentration of said inorganic iron-based additive within the dispersion resulting from step b) is between 50 g / L and 300 g / L, preferably between 100 g / L and 250 g / L, or between 150 g / L and 200 g / L.

3. Method according to claim 1 or 2, wherein said at least one inorganic ironbased additive has a particle size, expressed as D90, between 1 nm and 50 pm, preferably between 5 nm and 10 pm.

4. Method according to any one of the preceding claims, wherein said iron oxides are selected from FeO, Fe3O4, a-Fe2O3, p-Fe2O3, y-Fe2O3, £-Fe2O3, and / or said iron oxyhydroxides are selected from a-FeO(OH), p-FeO(OH), y-FeO(OH), 6-FeO(OH), FeO(OH,CI), and / or said iron hydroxides are selected from Fe(OH)2and Fe(OH)3.

5. Method according to any one of the preceding claims, wherein step a) of preparing the sol comprises the following steps:- a step a1) of mixing said at least one silicon alkoxide having four hydrolysable bonds and said at least one silicon alkoxide having at least one non-hydrolysable bond in a mixture comprising a solvent and at least one thickener, wherein said solvent consists of at least one Ci-C4alcohol and said at least one thickener is selected from the group consisting of polyvinyls, polyphenols, polyalcohols and derivatives thereof;- a step a2) during which water and an organic or inorganic acid catalyst selected from CH3COOH, HNO3, HCI and H2SO4are added to the mixture resulting from step a1), obtaining a reaction mixture; and- a step a3) during which said reaction mixture resulting from step a2) is stirred for at least 10 hours at a temperature between 10 and 50°C.

6. Method according to claim 5, wherein during step a3), said reaction mixture is stirred for at least 15 hours, or at least 20 hours, or at least 24 hours, or at least 48 hours, or at least 72 hours, or at least 96 hours, and / or at a temperature between 18 and 25°C.

7. Method according to any one of the preceding claims, wherein step d) of heat treatment is carried out at a temperature below 80°C for at least 12 hours, or at a temperature of at least 80°C for a time between 30 and 180 minutes, preferably at a temperature of 120°C for a time between 120 and 180 minutes.

8. Method according to any one of the preceding claims, wherein said silicon alkoxide having four hydrolysable bonds has formula (I):Si(OR1)(OR2)(OR3)(OR4) (I) wherein R1, R2, R3and R4are independently selected from the group consisting of linear or branched C1-C10 alkyls, C5-C6aryls, linear or branched C2-Ci0alkenyls, and linear or branched C2-Ci0alkynyls, preferably said silicon alkoxide of formula (I) being tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS).

9. Method according to any one of the preceding claims, wherein said silicon alkoxide having at least one non-hydrolysable bond has formula (II): R8-(CH2)n-Si(OR5)(OR6)(OR7) (II) where:R5, R6and R7are independently selected from the group consisting of linear or branched Ci-Cio alkyls, C5-C6aryls, linear or branched C2-Ci0alkenyls, and linear or branched C2-Ci0alkynyls, n is an integer between 0 and 6, preferably 2, andR8is selected from the group consisting of linear or branched C1-C10 alkyls, linear or branched C2-C10 alkenyls, CH2=C(CH3)COO-, and ;preferably said silicon alkoxide of formula (II) being methyltriethoxysilane (MTES), methyltrimethoxysilane (MTMS), 3-(trimethoxysilyl)propyl methacrylate (yMPS), or 3- (glycidoxypropyl) trimethoxysilane (GPTMS).

10. Method according to any one of the preceding claims, wherein step c) of applying the dispersion resulting from step b) onto the substrate is carried out by dipcoating of the substrate into the dispersion, preferably the withdrawal speed of the substrate from said dispersion being between 20 and 1500 mm / min, more preferably being at least 50 mm / min, or at least 100 mm / min, or at least 150 mm / min, or at least 200 mm / min, or at least 250 mm / min, or at least 500 mm / min, for example about 1000 mm / min.

11. Method according to any one of the preceding claims, wherein said substrate is made of aluminium or aluminium alloy; or iron or iron alloy, for example steel or cast iron; or titanium or titanium alloy.

12. Metal substrate coated with an organic-inorganic hybrid sol-gel coating comprising at least one inorganic iron-based additive selected from iron oxides, iron oxyhydroxides, and iron hydroxides, wherein said coating is obtainable by the method according to any one of the preceding claims.

13. Component of a car or a motorcycle, comprising a metal substrate according to claim 12.

14. Component according to claim 13, said component being the brake caliper of the braking system of a car or a motorcycle, or a part thereof, or said component being the brake disc of the braking system of a car or a motorcycle, or a part thereof, for example the bell of said disc.

15. Use of a coating obtainable by the method according to any one of claims 1 to 11 to provide corrosion resistance to a metal substrate on which it is applied, in particular to a metal substrate of a component of a car or a motorcycle.

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