Sol-gel method for producing an anti-corrosion coating on a metal substrate

Abstract

A sol-gel method for producing an anti-corrosion coating consisting of at least one layer of an oxide on a metal substrate. A non-aqueous solution of a precursor of the oxide is prepared and deposited on one surface at least of the metal substrate in order to cover said surface at least partially with a film comprising the precursor of the oxide. Hydrolysis-condensation of the precursor of the oxide is carried out by exposing the film to a humid atmosphere in order to form an oxide network in the film. Then, a treatment for stabilizing the film on the surface of the substrate is carried out, followed by a heat treatment of the surface of the metal substrate in order to crystallize the network of oxide and form the anti-corrosion coating.

Claims

1. A sol-gel method for producing an anti-corrosion coating consisting of at least one layer of an oxide on a metal substrate, with the method successively comprising: /a/ preparing a non-aqueous solution of a precursor of the oxide; /b/ depositing the non-aqueous solution on one surface at least of the metal substrate in order to cover said surface of the metal substrate at least partially with a film comprising the precursor of the oxide; and /c/ carrying out a hydrolysis-condensation of the precursor of the oxide by exposing the film to a humid atmosphere in order to form an oxide network in the film; /d/ carrying out a treatment for stabilizing the film on the surface of the substrate; /e/ carrying out a heat treatment of the surface of the metal substrate in order to crystallize the network of oxide and form the anti-corrosion coating; wherein the step /b/ is carried out by putting the surface into contact with a predetermined volume of solution confined at least partially by a sealed membrane, the sealed membrane being able to slide via translation along the surface, with a controlled displacement of the sealed membrane allowing for the formation of a controlled thickness of film on the surface.

2. The method according to claim 1, wherein the steps /b/ to /d/ are repeated in order to deposit more than one layer on the metal substrate.

3. The method according to claim 1, wherein the treatment for stabilizing comprises exposing the film to a flow of gas brought to a temperature greater than an ambient temperature and less than 200° C.

4. The method according to claim 1, wherein the treatment for stabilizing comprises exposing the film to ultraviolet radiation.

5. The method according to claim 1, wherein the treatment for stabilizing is chosen from a treatment of the film assisted by microwaves and a treatment of the film by induction, at a temperature greater than an ambient temperature and less than 200° C.

6. The method according to claim 1, wherein the precursor of the oxide is chosen from a precursor of titanium, a precursor of zirconium, a precursor of chromium, a precursor of yttrium, a precursor of cerium and a precursor of aluminum.

7. The method according to claim 1, wherein the precursor of the oxide is chosen from: titanium ethoxide, titanium n-propoxide, titanium s-butoxide, titanium n-butoxide, titanium t-butoxide, titanium isobutoxide, titanium isopropoxide, tetrabutyl orthotitanate, tetra-tert-butyl orthotitanate, poly(dibutyltitanate), zirconium n-propoxide, zirconium n-butoxide, zirconium t-butoxide, zirconium ethoxide, zirconium 2-methoxymethyl-2-propoxide, zirconium 2-methyl-2-butoxide, zirconium isopropoxide, yttrium isopropoxide, yttrium n-butoxide, titanium methacrylate triisopropoxide, titanium diisopropoxide bis(tetramethylheptanedionate), titanium 2,4-pentanedionate, diisopropoxy-bis(ethylacetoacetato)titanate, titanium di-n-butoxide (bis-2,4-pentanedionate), titanium 2-ethylhexoxide, titanium oxide bis(acetylacetonate), bis(2,2,6,6-tetramethyl-3,5-heptanedionato)oxotitane, titanium bis(ammonium lactato)dihydroxide, zirconium bis(diethyl citrato)dipropoxide, zirconyl propionate, chromium acetate, cerium t-butoxide, cerium methoxyethoxide, aluminum s-butoxide, aluminum n-butoxide, aluminum t-butoxide, yttrium isopropoxide, yttrium butoxide, yttrium acetylacetonate, yttrium 2-methoxyethoxide, aluminum isopropoxide, aluminum ethoxide, aluminum tri-sec-butoxide, aluminum tert-butoxide, cerium isopropoxide.

8. The method according to claim 1, wherein the solution of the precursor of the oxide comprises for one mole of the precursor of the oxide, 0 to 2 moles of complexing agent and 10 to 50 moles of ethanol.

9. The method according to claim 8, wherein the solution of the precursor of the oxide further comprises for one mole of the precursor of the oxide up to 0.2 mole of a surfactant.

10. The method according to claim 1, wherein the step /b/ is implemented by a technique chosen from: a dip-withdraw of the surface in the solution, the withdraw being carried out at a speed between 0.5 mm/s and 20 mm/s; a spraying of the solution onto the surface with a controlled spray flow rate and a controlled relative displacement speed of a sprayer with respect to the surface; an evaporation of the solution in an enclosure containing the surface and under controlled temperature and pressure.

11. The method according to claim 1, wherein the step /b/ is carried out by putting the surface into contact with a spongy element impregnated with the solution and diffusing the solution via capillarity on the surface.

12. The method according to claim 1, wherein the surface is an inside surface of a cylindrical substrate, with the sealed membrane being mobile in translation along an axis of the cylindrical substrate.

13. The method according to claim 1, wherein the steps /b/ to /e/ are implemented on a production line carrying out a relative displacement of the metal substrate with respect to animated modules arranged to carry out the depositing of the solution on the surface, the exposing of the film to a humid atmosphere, the exposing of the film to a treatment for stabilizing and the exposing of the film to a heat treatment.

14. The method according to claim 1, wherein the heat treatment is carried out at a temperature between 300° C. and 500° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The method object of the disclosure shall be better understood when reading the following description of embodiments presented for the purposes of information, and in no way limiting, and the observation of the drawings hereinafter wherein:

(2) FIG. 1 is a flowchart showing five steps of the method for producing an anti-corrosion coating according to an embodiment;

(3) FIGS. 2a and 2b diagrammatically show a method of the dip-withdraw type of a metal substrate in a non-aqueous solution for depositing a film of a precursor of oxides on a surface of the substrate;

(4) FIG. 3 diagrammatically shows a method for depositing the non-aqueous solution on the surface of the metal substrate by evaporation of the solution in an enclosure under controlled temperature and pressure;

(5) FIG. 4 diagrammatically shows the depositing of a non-aqueous solution of an oxide precursor on the inside surface of a cylindrical metal substrate by means of a membrane mobile in translation along the axis of the substrate;

(6) FIG. 5 diagrammatically shows a cylindrical metal substrate of the fluid circuit pipe type comprising an anti-corrosion coating on the inside surface thereof;

(7) FIG. 6 diagrammatically shows a production line that scrolls modules that apply a method for producing an anti-corrosion coating on a metal substrate;

(8) FIGS. 7a and 7b are graphs that respectively show the results of the electrochemical measurements at ambient temperature in a corrosive environment rich in chloride ions in the form of polarization curves (FIG. 7a) and in the form of a Bode diagram (FIG. 7b), for a substrate without coating, a substrate comprising a titanium oxide coating and a substrate comprising a zirconium oxide coating.

(9) For reasons of clarity, the dimensions of the various elements shown in these figures are not necessarily in proportion with their actual dimensions. In the figures, identical references correspond to identical elements.

DETAILED DESCRIPTION

(10) This disclosure proposes a method for producing an anti-corrosion coating consisting of at least one layer of an oxide on a metal substrate. A possible application of the disclosure is the protection of ducts of primary, secondary and tertiary circuits of thermal or nuclear power plants. In this particular context, an optimum protection is sought in order to prevent any degradation following corrosion that can increase the radiological risk or the environmental impact.

(11) Another application resides in the protection of installations subjected to a corrosive environment, such as motors in the aeronautical industry, installations on the seacoast (wind turbines, marine current turbines, subjected to humid and chlorinated environments) for example.

(12) The disclosure consists of a method that is simple to implement, which can be applied on large surfaces of metal substrate of any shape. Furthermore, the quality of the coating obtained makes it possible to improve by a factor 100 to 1000 the protection against corrosion of a metal substrate, extending by as much the service life of the metal substrate.

(13) FIG. 1 shows a flowchart that illustrates five steps of the method of the disclosure. This method is a sol-gel method that implements a first step S1 or preparing the sol-gel solution comprising a precursor of an oxide intended to form a coating on the metal substrate, a second step S2 of depositing the solution on a surface of the metal substrate in order to form a film of the precursor of the oxide, a third step S3 of initiating the hydrolysis-condensation by exposing the film to a humid atmosphere, for the purpose of creating an oxide network in the film. Then, a fourth step S4 of treatment for stabilizing aims to evaporate any organic component that may be present in the film, and to favor the condensation reactions that also make it possible to eliminate organic compounds. Finally, a step S5 corresponding to a thermal treatment of crystallization of the oxide network in order to form the anti-corrosion coating.

(14) In a first step S1, a non-aqueous solution containing a precursor of the oxide is prepared. The precursor of the oxide is typically an oxide precursor of a transition metal of the alkoxide type of general formula [M(OR).sub.z].sub.n, where M is a metal of valency z, R is an organic compound. It is also possible to produce a composition that comprises several different oxide precursors, for example a mixture comprising a zirconium oxide precursor and a titanium oxide precursor.

(15) The oxide precursor can typically be a precursor of titanium oxide or of zirconium oxide, which are metals that are particularly suited for use as a coating in nuclear installations. Zirconium oxide furthermore has the advantage of having a high coefficient of expansion, which naturally protects it from the appearance of cracks during the crystallization of the oxide network on metal substrates, which is carried out at a temperature between 300° C. and 500° C.

(16) Other oxide precursors can be used as chromium or yttrium precursors. Yttrium can be used to stabilize the zirconium in the cubic phase in particular.

(17) The group R is generally an alkyl group preferably comprising 1 to 4 carbon atoms such as a methyl, ethyl, n-propyl, i-propyl, n-butyl, s-butyl or t-butyl group.

(18) In particular, the precursor can for example be chosen from the following compounds: titanium ethoxide Ti(OC.sub.2H.sub.5).sub.4, titanium propoxide Ti(OC.sub.3H.sub.7).sub.4, titanium isopropoxide Ti[OCH(CH.sub.3).sub.2].sub.4, titanium butoxide Ti(OCH.sub.2CH.sub.2CH.sub.2CH.sub.3).sub.4, zirconium butoxide Zr(OC.sub.4H.sub.9).sub.4, zirconium propoxide Zr(OCH.sub.2CH.sub.2CH.sub.3).sub.4, chromium acetylacetonate Cr(C.sub.5H.sub.7O.sub.2).sub.3, yttrium butoxide Y(OC.sub.4H.sub.9).sub.3, yttrium isopropoxide Y(OCH(CH.sub.3).sub.2).sub.3.

(19) Furthermore, the precursor can be chosen from: titanium isobutoxide, poly(dibutyltitanate), zirconium ethoxide, zirconium 2-methoxymethyl-2-propoxide, zirconium 2-methyl-2-butoxide, zirconium isopropoxide.

(20) The non-aqueous solution typically comprises a mixture in which for one mole of metal oxide precursor 10 to 50 moles of ethanol (non-aqueous solvent) are added and advantageously 0 to 2 moles of complexing agent.

(21) The complexing agent is an additive that makes it possible to stabilize the precursor in the solution, in that the alkoxides are very reactive, which is detrimental to the quality of the oxide network obtained during the hydrolysis-condensation of the solution.

(22) In the presence of a complexing agent, the metal oxide precursor has for general chemical formula L.sub.x[M(OR).sub.z].sub.n-x, where L is a monodentate or polydentate ligand such as carboxylic acid in C.sub.1-18 such as acetic acid, a β-dicetone, preferably in C.sub.5-20, such as acetoacetone or dibenzoylmethane, a β-cetoester preferably in C.sub.5-20 such as methyl acetoacetate, a β-cetoamide, preferably in C.sub.5-20 such as an N-methylacetoacetamide, an α or β-hydroxiacid preferably in C.sub.3-20 such as lactic acid or salicylic acid, an amino acid such as alanine or a polyamine such as diethylenetriamine (DETA).

(23) The compounds that incorporate a ligand can in particular be chosen from: titanium methacrylate triisopropoxide, titanium diisopropoxide bis(tetramethylheptanedionate), titanium 2,4-pentanedionate, diispropoxy-bis(ethylacetoacetato)titanate, titanium di-n-butoxide (bis-2,4-pentanedionate), titanium 2-ethylhexoxide, titanium oxide bis(acetylacetonate), bis(2,2,6,6-tetramethyl-3,5-heptanedionato)oxotitane, titanium bis(ammonium lactato)dihydroxide, zirconium bis(diethyl citrato)dipropoxide, zirconyl propionate, chromium acetate.

(24) It is suitable to note that the presence of complexing agents acts not only to stabilize the solution, but also makes it possible to cause a micro-porosity to appear in the oxide precursor film, with pores of a size typically less than about 2 nm.

(25) The non-aqueous solution can in particular further contain a surfactant element, used to modify the porosity of the film of metal oxide obtained. The surfactant is present in the solution in proportions such that for one mole of the precursor of the oxide there is up to 0.2 mole of the surfactant.

(26) The surfactant is typically chosen from non-ionic amphiphilic surfactants. These can be amphiphilic molecules or macromolecules such as polymers.

(27) The molecular non-ionic amphiphilic surfactants can for example be ethoxylated linear alcohols in C.sub.12-22, comprising from 2 to 30 ethylene oxide units, or esters of fatty acids comprising from 12 to 22 carbon atoms, and sorbitane. For example, the surfactants available under the name of Brij®, Span® and Tween® can be used.

(28) The polymeric non-ionic amphiphilic surfactants can be any amphiphilic polymer that has both a hydrophilic nature and a hydrophobic nature. By way of example, these surfactants can be chosen from fluorinated copolymers such as CH.sub.3—[CH.sub.2—CH.sub.2—CH.sub.2—CH.sub.2—O].sub.n—CO—R.sub.1 with R.sub.1═C.sub.4F.sub.9 or C.sub.8F.sub.17, with the block copolymers comprising two blocks, three blocks of the A-B-A or A-B-C type or four blocks.

(29) Among the surfactants that are particular suited to this disclosure, the following compounds can be retained: copolymers with a poly((meth)acrylic acid) base, copolymers with a polydiene base, copolymers with a hydrogenated diene base, copolymers with a poly(propylene oxide) base, copolymers with a poly(ethylene oxide) base, copolymers with a polyisobutylene base, copolymers with a polystyrene base, copolymers with a poly(2-vinyl-naphthalene) base, copolymers with a poly(vinyl pyrrolidone) base, and block copolymers formed from chains of poly(alkylene oxide), with each block being formed from a chain of poly(alkylene oxide), with the alkylene comprising a different number of carbon atoms according to each chain.

(30) In order to guarantee the presence of both hydrophilic and hydrophobic groups, one of the two blocks can comprise a chain of poly(alkylene oxide) of a hydrophilic nature while the other block can comprise a chain of poly(alkylene oxide) of hydrophobic nature. For a tri-block copolymer, two of the blocks can be of a hydrophilic nature while the other block, located between the two hydrophilic blocks, can be of a hydrophobic nature. Preferably, in the case of a tri-block copolymer, the chain of poly(alkylene oxide) of a hydrophilic nature are chains of poly(ethylene oxide) noted as (POE).sub.u and (POE).sub.w and the chains of poly(alkylene oxide) of hydrophobic nature are chains of poly(propylene oxide) noted as (POP)v or chains of poly(butylene oxide), or mixed chains in which each chain is a mixture of several monomers of alkylene oxides. In the case of a tri-block copolymer, it is possible to use a compound of formula (POE).sub.u-(POP).sub.v-(POE).sub.w with 5<u<106, 33<v<70 and 5<w<106. Commercialized compounds such as Pluronic® P123 (u=w=20 and v=70) or Pluronic® F127 (u=w=106 and v=70) can be favored.

(31) The molar proportion of surfactant in the non-aqueous solution makes it possible to control the quantity of pores in the film of oxide precursor. Typically, adding surfactant in proportions such that for one mole of the precursor of the oxide there is up to 0.2 mole of the surfactant, leads to a porosity that can reach up to 50% by volume in the anti-corrosion coating, for an average pore size of 2 nm to 10 nm thick.

(32) The presence of complexing agents and of surfactants can also contribute to increasing the thickness of the anti-corrosion coating while rendering it more porous.

(33) The non-aqueous solution 1 can furthermore also comprise nanoparticles of titanium oxide or of zirconium oxide. These nanoparticles can be used as a seed crystal in order to favor the crystallization of the anti-corrosion coating during the step of heat treatment of the latter. Adding nanoparticles can also contribute to densifying the anti-corrosion coating and limiting the formation of cracks.

(34) The second step S2 consists in depositing the non-aqueous solution on one surface at least of the metal substrate in order to form a film comprising the precursor of the oxide on this metal substrate. This step can be implemented in different ways, as disclosed in particular in FIGS. 2a, 2b, 3 and 4.

(35) As indicated in FIG. 2a, a means for depositing the non-aqueous solution 1 on the surface 11 of the metal substrate 10 consists in carrying out a dip-withdraw (or dip-coating). In FIG. 2a, this step is carried out by plunging the metal substrate into the solution, then by removing the metal substrate from the solution, as shown in FIG. 2b. The thickness of the film comprising the precursor of the oxide depends in particular on the speed of the step of withdraw of FIG. 2b. Typically, in order to obtain films that have a thickness between 50 nm and 150 nm it is suitable to provide a withdraw speed between 0.5 mm/s and 20 mm/s. The dip-withdraw at these speeds is carried out by draining, not by capillarity. Thus, the higher the withdraw speed is, the thicker the film obtained is.

(36) Another parameter than influences the thickness of the film obtained is the molar proportion in ethanol in the non-aqueous solution 1. Indeed, the more ethanol there is in the solution, the less precursor of the oxide there is per volume unit in the solution and the thinner the deposited film is.

(37) Although FIGS. 2a and 2b show a dip-withdraw wherein the solution is maintained in a fixed position while the substrate is displaced in the solution, an alternative configuration that allows for a relative movement of the substrate in relation to the solution can also be implemented. The non-aqueous solution 1 can for example be displaced a first time until it coats the surface 11 of the metal substrate 10 then again displaced in order to release the metal substrate 10 of the non-aqueous solution 1, at a controlled speed.

(38) Dip-withdraw is a method for depositing the solution on metal substrates of simple geometrical shape. However, more complex surfaces can benefit from more suitable methods such as spraying.

(39) Spraying can be carried out in an alternative manner to the means of a sprayer mobile in relation to the metal substrate 10, of which the displacement speed and the ejected flow rate can be controlled in order to obtain a desired thickness of film 20.

(40) FIG. 3 diagrammatically shows an example of a depositing of the non-aqueous solution 1 on the metal substrate 10 by evaporation of the solution in an enclosure containing the surface 11 and under controlled temperature and pressure. A carrier gas inlet 2 can contribute to conveying the solution and bringing it on the surface 11 of the metal substrate 10.

(41) According to another alternative that is particularly suited for a fine control of the thickness of the film 20 deposited on the metal substrate 10, it is possible to use a sealed membrane mobile in relation to the metal substrate as shown in FIG. 4.

(42) FIG. 4 shows a cylindrical tank 100 comprising the metal substrate 20 as a cylindrical pipe. A sealed membrane 44 is fastened to an axis 45 of the cylindrical tank 100. A section 412 located above the membrane comprises a predefined volume of non-aqueous solution 1. The membrane 44 slides in translation along the axis 45 by maintaining a sealed contact with the walls of the metal substrate 10. The displacement of the sealed membrane 44 can in particular be provided by a traction ring 43 connected to the sealed membrane 44. The mass of this traction ring 43, as well as the volume of solution in the section 41 are parameters that make it possible to control the thickness of the film 20 deposited onto the metal substrate 10. The section 40 located above the section 41 is devoid of non-aqueous solution 1 but is already coated with the film 20. The section 42 located under the section 41 will be treated by the depositing of a film 20 when the sealed membrane 44 will be displaced to the level thereof.

(43) An excess of non-aqueous solution 1 can be removed by openings provided in a central element 46 of which the dimensions are adapted to the maintaining of a predetermined volume of non-aqueous solution above the membrane 44.

(44) Of course, using the displacement of a sealed membrane can be carried out for other geometries of the metal substrate 10, non-cylindrical, in which case the arrangement of the various elements described hereinabove can be adapted.

(45) Another possibility for carrying out the depositing of the non-aqueous solution 1 on the surface 11 of the metal substrate 10 consists in using a spongy element impregnated with the non-aqueous solution 1 and diffusing while depositing the solution via capillarity on the surface 11.

(46) A third step S3 of the method for producing an anti-corrosion coating consists in initiating the hydrolysis of the oxide precursor in the film 20 by exposing the film to water in gaseous form present in a humid atmosphere. An originality of the disclosure resides in the fact that this step S3, that makes it possible to increase the viscosity of the film 20 and form an oxide network in the film 20, is carried out after the film 20 has been deposited on the surface 11 of the metal substrate 10 in the step S2. Thus, the diffusion of the water in gaseous form prevents the local appearance of a large quantity of water that can produce a heterogeneous oxide network during the hydrolysis and the condensation that follows. The disclosure also eliminates recourse to a step of maturation in the non-aqueous solution, which is a long step of the methods of prior art. Moreover, initializing the hydrolysis via the gaseous route under a humid atmosphere makes it possible to offset the high reactivity of the oxide precursors by exposing them progressively and in a controlled manner to the moisture that is diffused through the thicknesses of the film 20. Furthermore, initiating the hydrolysis via exposure to a humid atmosphere is particularly effective due to the thicknesses of the film 20 that intervene in the method, of about a hundred nanometers, which facilitates the permeation of the water through the film 20.

(47) It is suitable to note that the moisture content of the atmosphere can be controlled, and is advantageously between 20% and 80%. A higher level of humidity can lead to the formation of condensation on the film 20, which is not desired. A range of moisture content corresponding to the ambient humidity, typically between 40% and 70% is preferred.

(48) The duration of the exposure to this humid atmosphere can typically be between 30 seconds, in particular for high levels of humidity, and 5 minutes, in particular for low moisture content.

(49) The temperature during the step S3 is a parameter than can influence the kinetics of the hydrolysis-condensation. A temperature between 15° C. and 35° C. is preferred.

(50) In the step S4 of FIG. 1, the film 20 that comprises the oxide network is subjected to a treatment for stabilizing that makes it possible to favor the condensation reactions that lead to an elimination of any organic component that remains in the film 20, and to prevent the appearance of cracks in the film during the later step of heat treatment S5.

(51) The treatment for stabilizing of the step S4 can be carried out in different ways. For example, this treatment can be carried out by means of a simple exposure at a temperature greater than the ambient temperature, and advantageously less than 200° C., in an oven. Such an approach is suitable in particular for a treatment for homogeneous stabilizing in the case of metal substrates 10 of complex shape, such as for example bent pipes of a length that can reach 10 m.

(52) Another approach consists in circulating a gas brought to a temperature greater than the ambient temperature, and advantageously less than 200° C. around the metal substrate 10.

(53) According to another alternative, this stabilization of the film in order to consolidate the inorganic portion of the oxide network can be carried out by application of microwaves or by induction, at a temperature higher than the ambient temperature and less than 200° C.

(54) According to another alternative, the consolidation of the film can be carried out by applying ultraviolet radiation. This solution has the advantage of furthermore allowing for a reduction in the porosity of the film, and therefore to densify the film 20 that comprises the oxide network. A duration of exposure between 30 seconds and 10 minutes to radiation with a wavelength between 280 nm and 400 nm (referred to as UVa and UVb radiation) with an irradiance of about 225 mW/cm.sup.2 is particularly effective for stabilizing a film 20 of a thickness of about 100 nm.

(55) The step S4 can be implemented at least partially although the step S3 is still being carried out.

(56) If more than one layer of protection has to be carried out, which is particularly advantageous in order to ensure good protection against pitting corrosion, it is possible to repeat the preceding steps on an existing coating. Furthermore, the method can even be repeated entirely (steps S1 to S5) during the depositing of each layer of coating. It is in particular possible to deposit several types of transition metal oxides that are different from one layer to another. FIG. 5 diagrammatically shows the section of a tube 3 of a fluidic circuit as a top view. The metal substrate 10 is covered on an inside surface of the tube 3 by two layers 31, 32 of different metal oxides. It is also possible to provide only a single layer for the anti-corrosion coating, which can be beneficial in order to prevent an excessive thickness for the anti-corrosion coating, and reduce the time and cost required for the total treatment of the substrate.

(57) The step S5 consists in applying to the film 20 comprising the stabilized oxide network a heat treatment at a temperature typically between 300° C. and 500° C. This step is carried out preferably under a controlled atmosphere in order to prevent an oxidation of the substrate which disturbs the crystallinity of the coating. Thanks to this step the oxide network of the film 20 is crystallized in order to form the final anti-corrosion coating.

(58) The method described hereinabove eliminates recourse to a long step of maturation of the non-aqueous solution 1, thanks to a hydrolysis-condensation in the vapor phase under a humid atmosphere. The disclosure can in particular be carried out on an industrial production line, such as that shown diagrammatically in FIG. 6.

(59) FIG. 6 proposes to place the metal substrate 10 in a fixed position, and to scroll the modules animated by a line 60 in the direction of the metal substrate 10. Thus, a first module 61 can for example be used to polish the surface 11 in order to prepare it for treatment. This preparation can be a mechanical stripping, a mechanical polishing or chemical stripping for example. A module 62 can then proceed with a cleaning of the polished surface 11, for example via rinsing. The module 63 carries out the depositing of the sol-gel solution by one of the methods described hereinabove for example. In FIG. 6, this depositing is carried out by a spongy element. A module 64 exposes the film 20 of the surface 11 to a humid atmosphere. The module 65 proceeds with the treatment for stabilizing (for example via exposure to ultraviolet radiation), then the module 66 carries out the treatment for crystallizing the anti-corrosion coating.

(60) In the example of FIG. 6, the line 60 can be used to transport the various modules to the metal substrate 10, and can also include inlets for water, electricity and non-aqueous solution for example, in order to eliminate the various modules.

(61) As an alternative, it is also possible to provide a displacement of the metal substrate 10 along a line 60 that comprises fixed modules.

(62) The metal substrate provided with the anti-corrosion coating obtained thanks to the method described hereinabove has a resistance to corrosion 100 to 1000 greater than a metal substrate devoid of such a coating.

(63) In particular, the corrosion current of a metal substrate comprising the anti-corrosion coating is less by at least a factor of 10 than a corrosion current of a metal substrate that does not comprise any anti-corrosion coating.

(64) Comparative measurements were taken on a metal substrate of inconel 690, without anti-corrosion coating and with an anti-corrosion coating in TiO.sub.2 and in ZrO.sub.2, in the presence of a corrosive environment comprising chloride ions. FIG. 7a shows polarization curves for these three samples in a solution containing NaCl at a concentration of 0.05 mol/L. In FIG. 7a a cathodic Tafel region can be seen on the left portion of the figure and an anodic Tafel region on the right portion of the figure. The Tafel straight-line method makes it possible to determine the corrosion current density, indicated for each sample in FIG. 7a by the name I.sub.corr. These curves reveal a corrosion current density and a corrosion potential that are clearly lower in the presence of anti-corrosion coating, which confirms the effectiveness of the method described hereinabove. In particular, the corrosion current density is 10 to 100 times smaller in the presence of a titanium oxide and zirconium oxide coating.

(65) FIG. 7b shows impedance spectroscopy measurements taken on these same samples. The effectiveness of the coating with respect to corrosion is revealed in particular by an increase in the impedance modulus Z at low frequencies.

(66) Other measurements, not shown, confirm the effectiveness of using several superposed layers of coating in order to reduce pitting corrosion. Additional tests carried out in an acidic medium confirm the results of FIGS. 7a and 7b.