US20140255611A1

US20140255611A1 - Process for the anticorrosion treatment of a solid metal substrate and treated solid metal substrate capable of being obtained by such a process

Info

Publication number: US20140255611A1
Application number: US14/351,430
Authority: US
Inventors: Florence Ansart; Rudina Bleta; Jean-Pierre Bonino; Julien Esteban; Olivier Jaubert; Marie Gressier; Pascal Lenormand; Marie-Joelle Menu; Elodie Xuereb; Pierre Bares; Celine Gazeau
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite Toulouse III Paul Sabatier; Mecaprotec Industries SA
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Toulouse III Paul Sabatier; Mecaprotec Industries SA
Priority date: 2011-10-14
Filing date: 2012-10-12
Publication date: 2014-09-11
Also published as: FR2981366A1; EP2766508B1; MX2014004512A; WO2013054064A1; ES2609581T3; FR2981366B1; BR112014008845A2; EP2766508A1; CA2851499A1; JP2014528520A

Abstract

An anticorrosion treatment process in which applied to an oxidizable surface of a solid metal substrate is a liquid solution, referred to as treatment solution, including: at least one alkoxysilane, and at least one cerium (Ce) cation; in a liquid aqueous-alcoholic composition, the treatment solution being suitable for being able to form, at the surface of the solid metal substrate, a hybrid matrix by hydrolysis/condensation of each alkoxysilane(s) and of each cerium (Ce) cation; the treatment solution having a molar ratio (Si/Ce) of silicon element of the alkoxysilane(s) with respect to the cerium (Ce) cation(s) of between 50 and 500; characterized in that the cerium (Ce) cation(s) has(have) a concentration between 0.005 mol/L and 0.015 mol/L in the treatment solution.

Description

The invention relates to a process for the anticorrosion treatment of a solid metal substrate, especially of a substrate made of aluminium or of aluminium alloy. The invention relates further to a solid metal substrate treated against corrosion, capable of being obtained by such a process.
Such a process for anticorrosion treatment has applications in the general field of the surface treatment of solid metal substrates, especially metal parts. Such a process is used in the field of transport vehicles, especially ships, motor vehicles and aircraft, where the problem of the fight against corrosion of metal parts arises.
Processes for the surface treatment of a solid metal substrate are already known, in which reagents based on chromium are used. Such reagents are toxic to the environment and to human health, and their use is regulated.
In order to remedy the disadvantages associated with the use of chromium, it has been proposed (Pepe et al., 2004, Journal of Non-Crystalline Solids, 348, 162-171) to form a coating based on silica gel on the surface of a substrate made of aluminium alloy by a sol/gel process. Such a treatment is carried out by dip-coating the substrate made of aluminium alloy in a hybrid solution of tetraethyl orthosilicate (TEOS) and methyltriethoxysilane (MTES) containing cerium nitrate. Such a process does not permit the obtainment of an anticorrosion coating that has both improved properties of mechanical resistance—especially resistance to tearing—and also improved healing properties and an improved barrier effect.
The invention aims to remedy the disadvantages described above by proposing a process for the anticorrosion treatment of a solid metal substrate which does not require the use of chromium derivatives—especially chromium VI—which is carcinogenic, mutagenic and reprotoxic.
There is also known from “Zhong et al., (2010), Progress in Organic Coatings, 69, 52-56” a process for the treatment of an aluminium alloy in which a hydroalcoholic solution of γ-glycidoxypropyltrimethoxysilane (GPTMS) and vinyltriethoxysilane (VETO) is prepared and then hydrolysis/condensation of said hydroalcoholic solution is carried out in order to form a gel, and then a quantity of cerium nitrate is added to the gel that has formed.
The invention aims to propose a process for anticorrosion treatment which is adapted to form on the surface of a solid metal substrate an anticorrosion coating that has high mechanical resistance.
The invention aims also to propose such a process for anticorrosion treatment which permits the obtainment of a layer of anticorrosion coating of controlled thickness—especially between 1 μM and 15 μM—and which is compatible with industrial recommendations, especially in the aeronautical field.
Another object of the invention is to propose a process for the anticorrosion treatment of a solid metal substrate—especially of metal parts for aeronautics—which is adapted to permit the formation of an anticorrosion coating for said solid metal substrate which is of substantially homogeneous thickness over the surface of the solid metal substrate, which has good coverage and which is also levelling. Levelling is understood as meaning that such a coating has a free outer surface—that is to say a surface remote from the substrate—that is substantially flat irrespective of the presence of structural defects on the surface of the underlying solid metal substrate. In particular, the invention aims to propose such a process which is adapted to permit the formation of an anticorrosion coating that does not exhibit cracking in the region of said structural defects.
The invention relates also to such a process which is adapted to permit the formation of an anticorrosion layer that is resistant to cracking.
The invention relates further to such a process which is adapted to permit the formation of an anticorrosion surface coating for a solid metal substrate that has both properties of passive protection—in particular by a barrier effect—of said substrate with respect to a corrosive external environment and properties of active protection of healing and limiting the progression of corrosion in the region of an accidental hole capable of affecting the anticorrosion coating.
The invention relates also to such a process for anticorrosion treatment which is adapted to be able to be applied to a polished solid metal substrate or to an unpolished solid metal substrate.
The invention aims also to achieve all these objects at low cost, by proposing a process which is simple and which requires for its implementation only steps of bringing into contact a solid metal substrate and liquid solutions.
The invention aims also and more particularly to propose such a process which is compatible with the constraints of safety and respect for the environment.
The invention additionally aims to propose such a solution which preserves employees' working practices, is simple to use, and involves only a small number of operations for its implementation.
The invention relates also to such a process for anticorrosion treatment which uses a treatment solution that is of simple composition as compared with the liquid treatment solutions of the prior art.
The invention therefore relates also to an anticorrosion coating having improved protective properties as compared with the anticorrosion coatings of the prior art, especially anticorrosion properties which are improved over time.
To that end, the invention relates to a process for anticorrosion treatment in which there is applied to an oxidizable surface of a solid metal substrate a liquid solution, called a treatment solution, comprising:

- at least one alkoxysilane, and
- at least one cerium (Ce) cation;
  in a liquid hydroalcoholic composition, said treatment solution being adapted to be able to form a hybrid matrix on the surface of the solid metal substrate by hydrolysis/condensation of each alkoxysilane and of each cerium (Ce) cation;
  the treatment solution having a molar ratio (Si/Ce) of silicon element of the alkoxysilane(s) to the cerium (Ce) cation(s) of between 50 and 500, especially between 80 and 250;
  wherein the cerium (Ce) cation(s) has(have) a concentration of between 0.005 mol/l and 0.015 mol/l—especially between 0.005 mol/l and 0.01 mol/l, preferably of approximately 0.010 mol/l—in the treatment solution.

In a process according to the invention, at least one alkoxysilane and at least one cerium (Ce) cation are mixed in a liquid hydroalcoholic solution under conditions capable of permitting hydrolysis/condensation of said at least one alkoxysilane and of said at least one cerium (Ce) cation, and said treatment solution is applied to the oxidizable surface of a solid metal substrate in order to be able to form a hybrid matrix on the surface of the solid metal substrate by hydrolysis/condensation of each alkoxysilane and of each cerium (Ce) cation.
In a process according to the invention, the hydrolysis/condensation of each alkoxysilane is carried out in the treatment solution in the presence of each cerium (Ce) cation, said treatment solution having a molar ratio (Si/Ce) of silicon element of the starting alkoxysilane(s) to the starting cerium (Ce) cation(s) of between 50 and 500, especially between 80 and 250.
The inventors have observed that the selection of a concentration value of the cerium cations in the treatment solution does not represent an arbitrary selection of concentration; instead, this selection yields a surprising, wholly unforeseeable result which is not described in the prior art and according to which the selected concentration of the cerium (Ce) cation in the solution for the anticorrosion treatment of a solid metal substrate simultaneously permits (1) the obtainment of optimum adherence of the treatment solution on the surface of the solid metal substrate, (2) the formation of a hybrid matrix for passive protection of said solid metal substrate by a barrier effect, adapted to limit the formation of corrosion products of the solid metal substrate—especially of a solid metal substrate having a hole—and (3) the formation of such a protective hybrid matrix having properties of physical resistance to mechanical stresses—especially resistance to delamination, resistance to cracking, and properties of plastic deformation—and of resistance to corrosion—after corrosive treatment for 1 day, 7 days and 14 days by immersion in a corrosive solution of NaCl at 0.05 mol/l in water—which are improved.
Such properties of physical resistance to mechanical stresses are evaluated especially by techniques which are known per se to the person skilled in the art, in particular by nano-indentation—for evaluating Young's modulus of elasticity and hardness (for example, Vickers nano-hardness)—or by nano-scratch for evaluating the adherence and resistance to delamination of the anticorrosion coating on the surface of the solid metal substrate.
Advantageously, the cerium cation is a single cerium cation and the concentration of the single cerium cation in the treatment solution is between 0.005 mol/1 and 0.015 mol/l, especially between 0.005 mol/1 and 0.01 mol/l, preferably approximately 0.01 mol/l.
Advantageously, the cerium cation is a composition comprising a plurality of distinct cerium cations, and the cumulative concentration of the plurality of distinct cerium cations in the treatment solution is itself between 0.005 mol/1 and 0.015 mol/l, especially between 0.005 mol/1 and 0.01 mol/l, preferably approximately 0.01 mol/l.
An anticorrosion coating according to the invention, that is to say a coating obtained with a treatment solution comprising a concentration of cerium cation—especially of Ce^III—of between 0.005 mol/1 and 0.015 mol/l, has:

- a critical value (Hv) of plastic deformation, measured by nano-indentation, which is maximum and approximately 39 for a concentration of cerium of 0.01 mol/l in the treatment solution;
- a value of the critical load of delamination F_D(mN) of the anticorrosion coating on the solid metal substrate which is maximum and approximately 24 mN for a concentration of cerium of 0.01 mol/l in the treatment solution;
- a value of the critical load of cracking F_f(mN) of the anticorrosion coating which is maximum and approximately 15 mN for a concentration of cerium of 0.01 mol/l in the treatment solution; and
- a maximum value of the critical load of plastic deformation without cracking F_DP(mN) of approximately 6 mN for a concentration of cerium of 0.01 mol/l in the treatment solution.

The inventors have observed that a concentration of cerium cation in the treatment solution of between 0.005 mol/1 and 0.015 mol/l according to the invention confers upon the anticorrosion coating of a solid metal substrate resistance to corrosion that is optimum after deposition and before immersion in a corrosive solution. A concentration of cerium cation in the treatment solution of greater than 0.015 mol/l, on the other hand, leads to a significant degradation of the barrier effect of the protective layer and reduced resistance to corrosion before immersion in a corrosive solution. The surface resistance in a corrosive medium of such a protective layer having a thickness of 6.3 μM, obtained by an anticorrosion treatment of a solid metal substrate with a treatment solution comprising 0.05 mol/l of cerium cation, and immediately after immersion of said metal substrate in the corrosive medium is approximately 2.8×10⁶Ω·cm². The surface resistance in a corrosive medium of such a protective layer having thickness of 6.3 μM, obtained by an anticorrosion treatment of a solid metal substrate with a treatment solution comprising 0.1 mol/l of cerium cation, and immediately after immersion of said metal substrate in the corrosive medium is approximately 2.0×10⁵Ω·cm²as measured by electrochemical impedance spectroscopy (EIS).
Furthermore, a concentration of cerium cation in the treatment solution of between 0.005 mol/1 and 0.015 mol/l according to the invention permits:

- a reduction in the contact angle of the treatment solution on the solid metal substrate, reflecting improved wettability of the surface of the solid metal substrate with respect to the treatment solution and improved application of said treatment solution to the surface of the solid metal substrate;
- improved anchoring of the hybrid matrix obtained from the treatment solution on the surface of the solid metal substrate;
- improved gain in mass of the treatment solution on the surface of the solid metal substrate. Such a gain in mass shows a structuring effect of the treatment solution and of the hybrid matrix obtained from the treatment solution; and
- Ce^IIIto be given priority at the expense of Ce^IVin the treatment solution and in the hybrid sol. Ce^III/Ce^IVdistribution is measured by surface X-ray photoelectron spectrometry (XPS).

The inventors have observed that such a concentration of cerium cations of between 0.005 mol/l and 0.015 mol/l in the treatment solution is adapted to be able at least to preserve the mechanical properties of the hybrid matrix obtained from the treatment solution, to confer upon the treatment solution rheological qualities and qualities of adherence to the surface of the solid metal substrate that are improved as compared with a treatment solution that does not have such a concentration, while conferring upon said hybrid matrix properties of passive protection of the solid metal substrate by a barrier effect.
Advantageously and according to the invention, each alkoxysilane is chosen from the group formed of:

- the tetraalkoxysilanes of the general formula (I) below:

Si(O—R₁)₄ (I)
wherein:

- Si is the element silicon, O is the element oxygen;
- R₁is chosen from the group formed of:
  - a hydrocarbon group—especially a methyl or an ethyl—of the formula [—C_nH_2n+1], n being an integer greater than or equal to 1;
  - the group 2-hydroxyethyl (HO—CH₂—CH₂—); and
  - an acyl group of the general formula —CO—R₁′ wherein R₁′ is a hydrocarbon group—especially a methyl, an ethyl—of the formula [—C_nH₂₊₁], n being an integer greater than or equal to 1; and
- the alkoxysilanes of the general formula (II) below:

Si(O—R₂)_4−a(R₃)_a (II)
wherein:

- R₂is chosen from the group formed of:
  - a hydrocarbon group—especially a methyl, an ethyl—of the formula [—C_nH_2n+1], n being an integer greater than or equal to 1;
  - the group 2-hydroxyethyl (HO—CH₂—CH₂—); and
  - an acyl group of the general formula —CO—R₁′ wherein R₁′ is a hydrocarbon group—especially a methyl, an ethyl—of the formula [—C_nH_2n+1], n being an integer greater than or equal to 1; and
- R₃is an organic group—especially an organic group formed of carbon atom(s), hydrogen atom(s) and, where appropriate, nitrogen atom(s), oxygen atom(s) and optionally sulfur atom(s) and phosphorus atom(s)—bonded to the silicon element (Si) of the alkoxysilane by an Si—C bond;
- a is a natural integer of the interval]0; 4[—preferably 1.

Advantageously, in a first variant of a process according to the invention, each alkoxysilane is chosen from the group formed of tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), tetraacetoxysilane (TAOS) and tetra-2-hydroxyethoxysilane (THEOS).
Advantageously, in a second variant of a process according to the invention, the group R₃of each alkoxysilane is chosen from the group formed of the methacrylates, acrylates, vinyls, epoxyalkyls and epoxyalkoxyalkyls in which the alkyl group(s) has(have) from 1 to 10 carbon atoms and is(are) chosen from linear alkyl groups, branched alkyl groups and cyclic alkyl groups.
Advantageously, the group R₃of each alkoxysilane is chosen from the group formed of 3,4-epoxycyclohexylethyl and glycidoxypropyl.
Advantageously, each alkoxysilane is chosen from the group formed of glycidoxypropyltrimethoxysilane (GPTMS), glycidoxypropylmethyldimethoxysilane (MDMS), glycidoxypropylmethyldiethoxysilane (MDES), glycidoxypropyl-triethoxysilane (GPTES), methyltriethoxysilane (MTES), dimethyldiethoxysilane (DMDES), methacryloxypropyltrimethoxysilane (MAP), 3-(trimethoxysilyl)-propylamine (APTMS), 2-(3,4-epoxycyclohexyl)ethyl-triethoxysilane (ECHETES), 2-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane (ECHETMS) and 5,6-epoxyhexyl-triethoxysilane (EHTES).
In this second variant of a process according to the invention, there is formed, by hydrolysis condensation of each alkoxysilane, an organic/inorganic hybrid matrix.
Advantageously and according to the invention, the treatment solution comprises a single alkoxysilane.
Advantageously and according to the invention, the treatment solution comprises at least one metal alkoxide.
Advantageously and according to the invention, each metal alkoxide has the general formula (VII) below:
M′(O—R₉)_n″ (VII)
wherein:

- M′ is a metal element chosen from the group formed of aluminium (Al), vanadium (V), titanium (Ti) and zirconium (Zr);
- R₉is an aliphatic hydrocarbon group of the formula [—C₁H_2n+1—]—especially chosen from the group formed of a methyl, an ethyl, a propyl, a butyl, in particular a secondary butyl of the formula [CH₃—CH₂—(CH₃)CH—]—, wherein n is an integer greater than or equal to 1; and
- n″ is a natural integer representing the valence of the metal element M′.

In a process according to the invention, there are formed by hydrolysis of each alkoxysilane and of each metal alkoxide reactive species which are distributed homogeneously in the treatment solution and are capable of polymerizing and forming an organic/inorganic hybrid matrix. There is thus formed an organic/inorganic hybrid matrix by hydrolysis condensation of each alkoxysilane and of each metal alkoxide.
Advantageously, each metal alkoxide is chosen from the group formed of the aluminium alkoxides—especially aluminium tri(s-butoxide), aluminium tri(n-butoxide), aluminium tri(ethoxide), aluminium tri(ethoxyethoxyethoxide) and aluminium tri(isopropoxide)—, the titanium alkoxides—especially titanium tetra(n-butoxide), titanium tetra(isobutoxide), titanium tetra(isopropoxide), titanium tetra(methoxide) and titanium tetra(ethoxide)—, the vanadium alkoxides—especially vanadium tri(isobutoxide) oxide and vanadium tri(isopropoxide) oxide —, and the zirconium alkoxides—especially zirconium tetra(ethoxide), zirconium tetra(isopropoxide), zirconium tetra(n-propoxide), zirconium tetra(n-butoxide) and zirconium tetra(t-butoxide).
Advantageously, each metal alkoxide is an aluminium alkoxide of the general formula (III) below:
Al(OR₄)_n (III)
wherein:

- Al and O are the elements aluminium and oxygen, respectively; and
- R₄is an aliphatic hydrocarbon group having from 1 to 10 carbon atoms—especially chosen from the group formed of a methyl, an ethyl, a propyl, a butyl, in particular a secondary butyl of the formula [CH₃—CH₂(CH₃)—CH—]—;
- n is a natural integer representing the valence of the aluminium element (Al).

Advantageously and according to the invention, the treatment solution comprises a single metal alkoxide. An inorganic hybrid matrix is thus formed by hydrolysis condensation of each alkoxysilane and of the single metal alkoxide. Advantageously, the treatment solution comprises a single metal alkoxide and a single alkoxysilane. An inorganic hybrid matrix is thus formed by hydrolysis condensation of the single alkoxysilane and of the single metal alkoxide.
Advantageously, the treatment solution comprises as the single metal alkoxide a single aluminium alkoxide.
There is thus formed a treatment solution comprising a single metal alkoxide—especially an aluminium alkoxide—and a single alkoxysilane, said treatment solution being of very simple composition and nevertheless being adapted to yield a very high-performance anticorrosion coating.
Advantageously, the single aluminium alkoxide is chosen from the group formed of aluminium tri(s-butoxide), aluminium tri(n-butoxide), aluminium tri(ethoxide), aluminium tri(ethoxyethoxyethoxide) and aluminium tri(isopropoxide).
Advantageously, the molar ratio of the alkoxysilanes to the metal alkoxides in the treatment solution is between 99/1 and 50/50. Advantageously, the molar ratio of the totality of the alkoxysilanes to the totality of the metal alkoxides in the treatment solution is between 99/1 and 50/50.
Advantageously and according to the invention, the molar ratio of the alkoxysilanes and the metal alkoxides—especially the aluminium alkoxide—in the treatment solution is between 85/15 and 6/4, especially between 8/2 and 64/36. Advantageously, the molar ratio of the totality of the alkoxysilanes to the totality of the metal alkoxides in the treatment solution is between 8/2 and 6/4.
Advantageously and according to the invention, the solid metal substrate is formed of a material chosen from the group formed of the oxidizable materials—especially aluminium (for example alloy 2024T3), titanium (for example alloy TA6V), magnesium (for example alloy AZ30) and their alloys.
Advantageously and according to the invention, the treatment solution is applied by dip-coating of the solid metal substrate in said treatment solution.
Advantageously, the solid metal substrate is withdrawn from the treatment solution at a predetermined speed of between 5 cm/minute and 10 cm/minute.
Advantageously and according to the invention, the treatment solution is applied to the surface of the solid metal substrate by atmospheric spray-coating.
The inventors have observed that it is possible to control the thickness of the hybrid matrix by the speed with which the solid metal substrate is withdrawn from the treatment solution. According to the Landau-Levich law (Landau L. D. and Levich B. G., (1942), Acta Physiochim. URSS, 17, 42-54), it is possible, for a treatment solution of known viscosity, to vary the thickness of the hybrid anticorrosion matrix from a value of 1 μm for a withdrawal speed of 1 cm/minute to a value of 14 μm for a withdrawal speed of 20 cm/minute. In particular, a withdrawal speed of 7 cm/minute allows a hybrid matrix having a thickness of 5 μm to be obtained.
The thickness of the hybrid matrix is measured by methods known per se to the person skilled in the art, especially by interferometric profilometry or by measurement of induced eddy currents.
Advantageously, the treatment solution further comprises a plasticizer chosen from the group formed of the PEGs.
Advantageously, the liquid treatment solution comprises a colorant. Such a colorant is chosen from the group formed of rhodamine B (CAS 81-88-9), brilliant green (CAS 633-03-4) and xylene cyanol (CAS 2850-17-1). Advantageously, rhodamine B is used at a concentration in the liquid treatment solution of between 5×10⁻⁴mol/l and 10⁻³mol/l, brilliant green is used at a concentration in the liquid treatment solution of between 5×10⁻⁴mol/l and 10⁻³mol/l, and xylene cyanol is used at a concentration in the liquid treatment solution of between 5×10⁻⁴mol/l and 10⁻³mol/l.
Advantageously, in a process for anticorrosion treatment according to the invention, the treatment solution comprises a load of nanoparticles formed of a colloidal dispersion of boehmite in the treatment solution, that is to say solid nanoparticles of boehmite of the general formula [—AlO(OH)—] forming a colloidal dispersion of boehmite nanoparticles in the treatment solution.
In a process for anticorrosion treatment according to the invention, in order to prepare a treatment solution, each alkoxysilane, each aluminium alkoxide and, where appropriate, each metal alkoxide is dissolved in an alcohol—especially chosen from the group formed of ethanol, 1-propanol and 2-propanol—and then a quantity of water or, where appropriate, a quantity of an aqueous solution comprising at least one lanthanide cation and/or colloidal boehmite is added in order to form an anticorrosion treatment solution.
Advantageously, alkoxysilane(s) and the metal alkoxide(s) and a quantity of water or, where appropriate, a quantity of an aqueous solution comprising at least one lanthanide cation and/or colloidal boehmite nanoparticles are added to an alcoholic solution in order substantially to preserve the rheological and thixotropic properties of the treatment solution.
Advantageously, in a third variant of a process for anticorrosion treatment according to the invention, a treatment solution comprising nanoparticles of boehmite of the general formula AlO(OH) and having a surface distribution of lanthanide cations—especially cerium cations—and/or of vanadate is formed. Such boehmite nanoparticles, called physisorbed boehmite nanoparticles, are obtained by a process which is known per se to the person skilled in the art and especially is adapted from a process described by Yoldas (Yoldas B. E. et al., (1975), J. Mater. Sci., 10, 1856).
In this third variant of a process according to the invention, the inventors have found an improvement in the resistance to corrosion of a solid metal substrate which has been treated with a treatment solution and subjected to immersion in a corrosive bath of NaCl at 0.05 mol/l.
Advantageously, in a fourth variant of a process for anticorrosion treatment according to the invention, there is formed a treatment solution comprising nanoparticles of boehmite, called doped boehmite, of the general formula (VIII) below:
Al_1−x(X)_x(O(OH) (VIII)
wherein:

- X is an element, called a doping element, chosen from the group formed of the trivalent lanthanides—especially trivalent cerium; and
- x is a relative number between 0.002 and 0.01.

Such doped boehmite nanoparticles are obtained by a process in which a solution of at least one precursor of aluminium—especially Al(OC₄H₉)₃—in water and a solution of a cation of a doping element chosen from the group formed of a nitrate, a sulfate, an acetate and a chloride of the doping element are mixed.
In this fourth variant of a process according to the invention, the inventors have found an improvement in the resistance to corrosion of a solid metal substrate which has been treated with a treatment solution and subjected to immersion in a corrosive bath of NaCl at 0.05 mol/l.
Advantageously, the physisorbed boehmite nanoparticles and the doped boehmite nanoparticles have a larger dimension and two smaller dimensions, perpendicular to one another and perpendicular to said larger dimension, said larger dimension is less than 200 nm—especially less than 100 nm, in particular less than 50 nm, preferably between 5 nm and 20 nm—and the two smaller dimensions are less than 10 nm, preferably approximately 3 nm.
Advantageously and according to the invention, the treatment solution comprises a load of hollow boehmite nanoparticles.
Advantageously and according to the invention, after application of the treatment solution, heat treatment of the metal substrate adapted to permit the formation of the hybrid matrix and the evaporation of the solvents is carried out.
Advantageously, in a process according to the invention, before application of the treatment solution, said oxidizable surface of the solid metal substrate is immersed in a liquid solution, called a conversion solution, formed of at least one corrosion inhibitor in water, said corrosion inhibitor being chosen from the group formed of the lanthanide cations, and said oxidizable surface of the solid metal substrate is kept in contact with the conversion solution for a period of time adapted to form a conversion layer formed of said lanthanide bonded by at least one covalent bond to the oxidizable surface and extending over the surface of the solid metal substrate.
In a fifth variant of a process for anticorrosion treatment according to the invention, a conversion layer is first formed on the oxidizable surface of a solid metal substrate by bringing said oxidizable surface into contact with the conversion solution. Treatment with such a conversion solution constitutes an anticorrosion treatment in that it permits the formation of a conversion layer on the surface of the solid metal substrate, in place of an oxide layer of the metal of the solid metal substrate, said conversion layer having a resistance to corrosion—especially measured by electrochemical impedance spectroscopy (EIS)—that is increased as compared with the resistance to corrosion of the oxide layer formed naturally on the surface of the solid metal substrate.
The inventors have in fact observed that an anticorrosion treatment according to the invention in which a conversion layer is first formed on the surface of a solid metal substrate and then a treatment solution comprising at least one alkoxysilane, a cerium cation at a concentration between 0.005 mol/l and 0.015 mol/l and, where appropriate, at least one metal alkoxide is applied, permits an increase in the resistance to corrosion of the oxidizable surface of a solid metal substrate, even after immersion of the oxidizable surface of the solid metal substrate in a corrosion bath, especially an aqueous bath of NaCl 0.05 mol/l, for a predetermined period of time—especially a period of time greater than 1 hour.
The inventors assume that the treatment of the oxidizable surface of the solid metal substrate with the conversion solution leads to the formation of a conversion layer that has increased resistance to corrosion as compared with the metal oxide layer of the solid metal substrate not treated with the conversion solution. Such a conversion layer is characterized according to a representation, called the “Nyquist” representation, of the electrochemical impedance diagram by a value Z′(ω) of surface resistance (Ω·cm²) that is increased as compared with the value Z′(ω) of the surface resistance of a solid metal oxide layer formed naturally on the surface of a solid metal substrate. Measurements of the impedance Z(ω) are carried out in potentiostatic mode around the free potential, with a sinusoidal perturbation. The sinusoidal perturbation amplitude is fixed at 10 mV in order to satisfy the conditions of linearity. The frequencies scanned in the impedance measurements are between 65 kHz and 10 mHz with 10 points per decade.
The inventors have shown by energy dispersive spectroscopy (EDS) that this conversion layer is composed of mixed oxides of lanthanide and of the metal constituting the oxidizable surface of the solid metal substrate.
Advantageously, the conversion layer extending over the surface of the solid metal substrate has a mean thickness of between 1 nm and 200 nm.
The inventors have observed that increasing the immersion time of a solid metal substrate in a conversion solution according to the invention permits an increase in the value of the surface resistance which goes beyond the limiting value of the surface resistance of the layer of aluminium oxide formed naturally on the surface of a part made of aluminium alloy.
Furthermore, the inventors have also observed that such a treatment of the oxidizable surface of a solid metal substrate with a conversion solution does not lead to any detectable increase in the mass of the substrate. Such a conversion layer does not correspond to a crystallization of lanthanide oxides/hydroxides on the surface of the solid metal substrate.
In particular, treatment of the oxidizable surface of the solid metal substrate with the conversion solution permits the formation of a conversion layer for active protection and healing on the surface of the solid metal substrate by formation of a plurality of covalent bonds between the corrosion-inhibiting lanthanide element (Ln) and a metal element (M) of the solid metal substrate. The inventors have shown, by chemical analysis of the bonding energies—especially by X-ray photoelectron spectrometry (XPS)—that this covalent bond is of the type M-O-Ln-O— wherein M represents a metal element of the solid metal substrate, O is an oxygen atom, and Ln represents the corrosion-inhibiting element chosen from the lanthanides.
In a process according to the invention there is applied to the oxidizable surface of the solid metal substrate, and, where appropriate, to the surface of the conversion layer, a treatment solution formed of an organic/inorganic hybrid sol of at least one alkoxysilane—especially of an alkoxysilane carrying an organic group—, a cerium cation at a concentration of between 0.005 mol/l and 0.015 mol/l, where appropriate at least one metal alkoxide, adapted to form by hydrolysis/condensation of the alkoxysilane(s), of the metal alkoxide(s) and of the cerium cation an organic/inorganic hybrid matrix formed of inorganic atom chains (—Si—O—Si—) and organic hydrocarbon chains.
The inventors have observed that the immersion of a solid metal substrate in a conversion solution not only permits the formation of such a conversion layer and the active protection of the solid metal substrate from corrosion, but further permits an improvement in the adherence of a hybrid sol to the surface of the solid metal substrate and an improvement in the properties of passive protection of said solid metal substrate from corrosion.
Advantageously and according to the invention, each corrosion inhibitor of the conversion solution is chosen from the group formed of the lanthanum (La) cations, cerium (Ce) cations, praseodymium (Pr) cations, neodymium (Nd) cations, samarium (Sm) cations, europium (Eu) cations, gadolinium (Gd) cations, terbium (Tb) cations, dysprosium (Dy) cations, holmium (Ho) cations, erbium (Er) cations, thulium (Tm) cations, ytterbium (Yb) cations and lutetium (Lu) cations.
Advantageously and according to the invention, each corrosion inhibitor of the conversion solution is chosen from the group formed of the lanthanide chlorides, lanthanide nitrates, lanthanide acetates and lanthanide sulfates.
Advantageously, each corrosion inhibitor of the conversion solution is chosen from the group formed of lanthanum chloride (LaCl₃), cerium chloride (CeCl₃), yttrium chloride (YCl₃), cerium sulfate (Ce₂(SO₄)₃), cerium acetate (Ce(CH₃COO)₃), praseodymium chloride (PrCl₃), neodymium chloride (NdCl₃).
Advantageously and according to the invention, each corrosion inhibitor of the conversion solution is a cerium cation—especially cerium nitrate (Ce(NO₃)₃), cerium acetate (Ce(CH₃COO)₃), cerium sulfate (Ce₂(SO₄)₃) and cerium chloride (CeCl₃)— in which the cerium element is of valence III (Ce^III).
Advantageously and according to the invention, the cerium (Ce) cation of the treatment solution is chosen from the group formed of the cerium chlorides and cerium nitrates. In particular, the corrosion inhibitor of the conversion solution is cerium nitrate Ce(NO₃)₃.
The inventors have shown that the conversion layer is composed of mixed oxides of cerium and of the metal constituting the oxidizable surface of the solid metal substrate. Chemical analysis by energy dispersive spectroscopy (EDS) exhibits Lα and Mα lines characteristic of cerium bonded by covalent bonds at the surface of the solid metal substrate.
The inventors have observed that such a process for anticorrosion treatment permits the formation of an anticorrosion coating formed of a conversion layer which comprises at least one corrosion inhibitor and is adapted to permit self-healing of the solid metal substrate, said conversion layer itself being protected by the cerium-rich hybrid matrix, which exhibits an optimum barrier effect.
However, the inventors have also observed, wholly surprisingly, that the conversion layer:

- does not alter the mechanical properties of resistance to delamination and of adhesion of the hybrid matrix to the solid metal substrate;
- does not alter the barrier properties of the hybrid matrix, as measured by EIS;
- allows the loss of resistance to corrosion of the solid metal substrate during prolonged immersion thereof in a corrosion bath to be slowed down; and
- delays the appearance of corrosion products on metal substrates having a point of corrosion.

The presence of a conversion layer that is rich in corrosion inhibitor situated at the interface between the solid metal substrate and the hybrid matrix allows additional active protection to be provided, which is added to the protective effect of the hybrid matrix.
Advantageously and according to the invention, the conversion solution has a concentration of corrosion inhibitor—especially of cerium (Ce)—of between 0.001 mol/1 and 0.5 mol/l, especially between 0.05 mol/1 and 0.3 mol/l, in particular of approximately 0.1 mol/l.
Advantageously, the conversion solution has a concentration of corrosion inhibitor—especially of cerium (Ce)—of between 0.01 mol/1 and 0.5 mol/l, preferably between 0.1 mol/1 and 0.5 mol/l.
Advantageously, the oxidizable surface of the solid metal substrate and the conversion solution are kept in contact for a predetermined period of time of between 1 second and 30 minutes, especially between 1 second and 300 seconds, preferably between 1 second and 15 seconds, in particular between 1 second and 10 seconds, more preferably between 1 second and 3 seconds.
Advantageously, after the step of bringing into contact the oxidizable surface of the solid metal substrate and the conversion solution, the solid metal substrate is dried at a predetermined temperature of below 100° C.—especially of approximately 50° C.—in order to form on the surface of the solid metal substrate a layer, called a conversion layer, of the corrosion-inhibiting element Ln (lanthanide) bonded to a metal element M of the solid metal substrate by a bond of the type M-O-Ln-O—.
Advantageously, the conversion solution has a pH substantially of approximately 4. Advantageously, the pH of the conversion solution is adjusted by addition of a mineral acid—especially nitric acid—to the conversion solution.
The inventors have observed that a process for the anticorrosion treatment of a solid metal substrate in two steps according to the invention not only permits active protection, in particular by healing, of the solid metal substrate from corrosion but further allows passive protection from said corrosion to be obtained.
Advantageously and according to the invention, the liquid hydroalcoholic composition is formed of water and at least one alcohol—especially chosen from the group formed of ethanol, 1-propanol and 2-propanol.
Advantageously, the doped and/or physisorbed boehmite nanoparticles have a larger dimension and two smaller dimensions, perpendicular to one another and perpendicular to said larger dimension, said larger dimension is less than 200 nm—especially less than 100 nm, in particular less than 50 nm, preferably between 5 nm and 20 nm—and the two smaller dimensions are less than 10 nm, preferably approximately 3 nm.
Advantageously and according to the invention, the treatment solution comprises a load of hollow boehmite nanoparticles.
The invention relates also to an anticorrosion coating capable of being obtained by a process according to the invention.
The invention extends additionally to an anticorrosion coating for a solid metal substrate formed of a hybrid matrix which extends over the surface of the solid metal substrate and is obtained by hydrolysis/condensation of at least one alkoxysilane;
said hybrid matrix having a molar ratio (Si/Ce) of silicon element of the alkoxysilane(s) to at least one cerium (Ce) cation of between 50 and 500, especially between 80 and 250.
The Ce/Si ratio is determined by methods known per se to the person skilled in the art, in particular by RBS (Rutherford backscattering spectrometry) analysis of the elastic diffusion of the ions of an incident bundle of ions, which is adapted to be able to measure the quantity of a heavy element in a light hybrid matrix.
The invention extends in particular to an anticorrosion coating in which the hybrid matrix which extends in contact with a solid metal substrate and is obtained by hydrolysis/condensation of at least one alkoxysilane and, where appropriate, of at least one metal alkoxide and comprising:

- at least one inorganic group of the general formula (IX):

-A-O-B- (IX)
wherein

- O is the element oxygen,
- A and B are chosen independently of one another from the group formed of Si and M′; and
- at least one organic group of the general formula (XI):

-D-O—R₁₀—O-E- (XI)
wherein

- O is the element oxygen,
- D and E are chosen independently of one another from the group formed of Si, M′ and Ce; and
- R₁₀is a hydrocarbon group.

Advantageously, the anticorrosion coating has a thickness of between 1 μm and 15 μm
The invention extends additionally to an anticorrosion coating having at least one of the following features:

- the hybrid matrix of the anticorrosion coating is formed of a composite material comprising a hybrid xerogel—especially an organic/inorganic hybrid xerogel—and a load of physisorbed boehmite nanoparticles dispersed in the hybrid xerogel;
- the hybrid matrix of the anticorrosion coating is formed of a composite material comprising a hybrid xerogel—especially an organic/inorganic hybrid xerogel—and a load of nanoparticles of doped boehmite of the general formula (VIII):

Al_1−x(X)_xO(OH) (VIII)
wherein:

- X is an element, called a doping element, chosen from the group formed of the trivalent lanthanides—especially trivalent cerium; and
- x is a relative number between 0.002 and 0.01; said load being dispersed in the hybrid xerogel;
- the hybrid matrix of the anticorrosion coating is formed of a composite material comprising a hybrid xerogel—especially an organic/inorganic hybrid xerogel—and a load of hollow boehmite nanoparticles dispersed in the hybrid xerogel;
- the solid nanoparticles of the load of physisorbed boehmite nanoparticles and of the load of doped boehmite nanoparticles having a larger dimension and two smaller dimensions, perpendicular to one another and perpendicular to said larger dimension, the larger dimension is less than 200 nm—especially less than 100 nm, in particular less than 50 nm, preferably between 5 nm and 20 nm—and the two smaller dimensions are less than 10 nm, preferably approximately 3 nm;
- the solid nanoparticles of the load of hollow boehmite nanoparticles are of substantially spherical shape and have a mean diameter of approximately 30 nm.

Advantageously, the conversion layer of the anticorrosion coating has a thickness of between 1 nm and 20 nm.
The invention extends additionally to a metal surface coated with an anticorrosion coating obtained by a process according to the invention.
The invention relates also to a process characterized in combination by all or some of the features mentioned hereinabove or hereinbelow.
Other objects, features and advantages of the invention will become apparent upon reading the following description, which refers to the accompanying figures showing preferred embodiments of the invention, which are given solely by way of non-limiting examples and in which:

FIG. 1 is a schematic representation, not in proportion, of a variant of an anticorrosion coating according to the invention;

FIG. 2 is a view recorded by scanning electron microscopy (SEM) of a transverse section of an anticorrosion coating of a solid metal substrate obtained by a process according to the invention;

FIG. 3 is a comparative graphic representation of the evolution of the surface resistance to corrosion of a solid metal substrate treated according to two variants of a process according to the invention;

FIG. 4 is a graphic representation of the surface resistance (Ω·cm²) of an anticorrosion coating as a function of the concentration of cerium in the treatment solution;

FIG. 5 is a graphic representation of the Vickers nano-hardness of an anticorrosion coating as a function of the concentration of cerium in the treatment solution;

FIG. 6 is a graphic representation of the variation in the value of Young's modulus, in GPa, determined by nano-indentation measurements;

FIG. 7 is a graphic representation of the value of the critical load (mN) of delamination (∘), of cracking (▴) and of plastic deformation (□), determined by nano-nano-scratch, of an anticorrosion coating as a function of the concentration of cerium in the treatment solution;

FIG. 8 is a Nyquist representation of the electrochemical impedance of a solid metal substrate treated (∘) or not treated (▴) with a conversion solution;

FIG. 9 is a chemical surface analysis spectrum by energy dispersion spectroscopy (EDS) of a solid metal substrate treated with a conversion solution according to the invention.

An anticorrosion coating 1 according to the invention shown in FIG. 1 is supported on a metal substrate 2 formed of metal elements M. Such an anticorrosion coating is formed of an optional conversion layer 3 in which the corrosion-inhibiting elements Ln are bonded by covalent bonds M-O-Ln- to metal elements M of the metal substrate 2. Furthermore, the corrosion-inhibiting elements Ln of the conversion layer form covalent bonds with elements Si and, where appropriate, with metal elements M′ chosen from the group formed of aluminium (Al), vanadium (V), titanium (Ti) and zirconium (Zr) and with the element cerium (Ce) of the hybrid matrix 4 extending over the surface of the conversion layer 3.
FIG. 2 shows a scanning electron microscopy (SEM) section of an aluminium substrate 2 treated by a variant of a process according to the invention and comprising a conversion layer 3 (which is optional) extending at the interface between the aluminium substrate 2 and the hybrid matrix 4.
In a variant of a process for the anticorrosion treatment of a solid metal substrate according to the invention, a preparative surface treatment of a part of laminated 2024 T3 aluminium alloy is first carried out. The purpose of such a preparative treatment, which is given solely by way of a non-limiting example, is to remove from the surface of the solid metal substrate any trace of oxidation of the alloy or of contamination which may impair the homogeneous application of the conversion solution and of the treatment solution to the surface of the substrate during deposition (dip-coating, spray) thereof and the anchoring of the hybrid anticorrosion matrix obtained on the surface of the substrate.
Degreasing of the Solid Metal Substrate with an Organic Solvent
The preparative treatment comprises a first step of degreasing of the surface of the solid metal substrate, in which the surface of said substrate is brought into contact with a degreasing solvent. This degreasing step is carried out by methods known per se to the person skilled in the art, especially by soaking the surface of the substrate in the degreasing solvent or by spraying said surface with the degreasing solvent.
By way of examples, the degreasing solvent can be stabilized pure methylene chloride (marketed under the name Methoklone) or pure acetone. In this case, this degreasing step is carried out at a temperature below 42° C. and for a period of between 5 seconds and 3 minutes. It is possible to subject the solid metal substrate to ultrasound treatment during this first degreasing step.
Degreasing of the Solid Metal Substrate with an Alkaline Solution
The preparative treatment of the solid metal substrate comprises a second successive step of degreasing of the surface of said substrate, in which the surface of the substrate is brought into contact with an alkaline preparation, especially marketed under the name TURCO 4215 (HENKEL, Boulogne-Billancourt, France). This step of alkaline degreasing is carried out by methods known per se to the person skilled in the art, especially by soaking the surface of the substrate in the alkaline preparation or by spraying said surface with said preparation for a period of between 10 minutes and 30 minutes. Preferably, this step of alkaline degreasing is carried out at a temperature of between 50° C. and 70° C. It is possible to subject the substrate to ultrasound treatment during this second step of degreasing with an alkaline solution.
Scaling of the Solid Metal Substrate with an Alkaline Solution
The preparative treatment according to the invention comprises a third successive step of scaling of the surface of the substrate, in which the surface of the substrate is brought into contact with an alkaline preparation, especially an aqueous solution of sodium hydroxide at a concentration of from 30 g/1 and 70 g/l. This step of alkaline scaling is carried out by methods known per se to the person skilled in the art, especially by soaking the surface of the substrate in the concentrated alkaline preparation or by spraying said surface with said concentrated alkaline preparation for a period of between 10 seconds and 3 minutes. Preferably, this step of alkaline scaling is carried out at a temperature of between 20° C. and 50° C. It is possible to subject the solid metal substrate to ultrasound treatment during this second step of scaling with a concentrated alkaline solution.
At the end of this third successive step of treatment of the surface of the solid metal substrate, a pulverulent layer of oxides covering the surface of the solid metal substrate is observed.
Scaling of the Solid Metal Substrate with an Acidic Solution
The preparative treatment according to the invention comprises a fourth successive step of dissolving the layer of oxides extending over the surface of the solid metal substrate, in which the surface of said substrate is brought into contact with an acidic preparation, for example TURCO LIQUID Smut-Go NC (HENKEL, Boulogne-Billancourt, France) or ARDROX 295 GD (Chemetal GmbH, Frankfurt, Germany).
This dissolution step is carried out for a period of between 1 minute and 10 minutes at a temperature of between 10° C. and 50° C. using an aqueous solution comprising between 15% (v/v) and 25% (v/v) of TURCO LIQUID Smut-Go NC.
In a variant, this dissolution step is carried out for a period of between 1 minute and 10 minutes at a temperature of between 10° C. and 30° C. using an aqueous solution comprising between 15% (v/v) and 30% (v/v) of ARDROX 295 GD.
At the end of this step, the surface of the solid metal substrate is adapted to be able to be treated by an anticorrosion treatment according to the invention.
In a variant of a process for the anticorrosion treatment of a solid metal substrate according to the invention, a step of formation of a conversion layer on the surface of the solid metal substrate is carried out.
A part made of aluminium (Al 2024-T3) is immersed by dip-coating in an aqueous conversion solution comprising a concentration of between 0.001 mol/l and 0.5 mol/l of Ce(NO₃)₃, the pH of which is adjusted to a value of 4 by addition of nitric acid. After dip-coating, the aluminium part is dried for 10 minutes at 50° C. At the end of this treatment with the conversion solution, no gain in mass of said aluminium part is measured.
In a process for the anticorrosion treatment of a solid metal substrate, a treatment solution is prepared by the following steps:

- (a) preparation of a treatment solution as described in (A) below;
- (b) preparation of a colloidal dispersion of physisorbed boehmite nanoparticles as described in (B) below;
- (c) preparation of a colloidal dispersion of doped boehmite nanoparticles as described in (C) below;
- (d) preparation of hollow aluminium oxyhydroxide nanoparticles containing a corrosion inhibitor as described in (D) below;
- (e) preparation of a colloidal anticorrosion treatment dispersion from the compositions as described in (A), in (B), in (C) and in (D) below;
- (f) deposition of the colloidal anticorrosion treatment dispersion on the solid metal substrate;
- (g) heat treatment.

A—Preparation of a Treatment Solution—Epoxy Sol

A1—Epoxy Sol GPTMS/ASB/Ce(NO₃)₃

In a first embodiment, in order to prepare 1 liter of epoxy sol, 107.4 g (0.43 mol) of aluminium tri(s-butoxide) (ASB) are dissolved in 34.8 ml of 1-propanol by stirring—especially by magnetic stirring—for 10 minutes at ambient temperature. 470 ml (2.13 mol) of 3—(glycidoxypropyl)-trimethoxysilane (GPTMS) are then added. The molar proportion of GPTMS and ASB is 83/17. An aqueous solution of cerium(III) (Ce(NO₃)₃) at a concentration of between 0.02 mol/1 and 0.5 mol/l is also prepared, and one volume of this aqueous solution of cerium is added to the precursor solution (ASB/GPTMS) in order to bring about hydrolysis/condensation of the precursors. The epoxy sol obtained is stirred for a period of time necessary for a thermal decline to ambient temperature. The final concentration of cerium in the epoxy sol is 0.01 mol/l.
The epoxy sol is deposited on a substrate of Al 2024-T3 aluminium pretreated as described above by dip-coating of the substrate in said epoxy sol. The withdrawal speed is 20 cm/minute. The coated solid metal substrate is heated at a temperature of between 95° C. and 180° C. —in particular 110° C.—for a period of between 1 hour and 5 hours—in particular 3 hours. There is observed the formation on the surface of the solid metal substrate of a hybrid matrix having a thickness of 6 μM, which has a salt spray resistance time of between 96 and 800 hours.

A2—Epoxy Sol TEOS/MAP/Ce(NO₃)₃

In a second embodiment, in order to prepare 1 liter of hybrid sol, 230 ml of tetraethoxysilane (TEOS) are added to 600 ml of ethanol. 30 ml of methacryloxypropyltrimethoxysilane (MAP) are then added, followed by an aqueous solution of cerium(III) (Ce(NO₃)₃) at a concentration of 4.32 g/l in order to bring about hydrolysis/condensation of the precursors TEOS and MAP. The pH of the sol obtained is 4.5 and its viscosity is 3 mPa·s.

B—Preparation of Physisorbed Colloidal Boehmite Nanoparticles

Such a colloidal dispersion of boehmite nanoparticles functionalized at the surface (called physisorbed) is prepared in two steps described below, in which a colloidal solution of boehmite nanoparticles is first formed, and then said boehmite nanoparticles are functionalized by a corrosion inhibitor.

B1—Colloidal Boehmite

The hydrolysis condensation of aluminium tri-sec-butoxide (ASB, Al(OH)_x(OC₄H₉)_3−x) is carried out according to the method described by Yoldas B. E. (J. Mater. Sci., (1975), 10, 1856), in which a quantity of water previously heated to a temperature greater than 80° C. is added to aluminium tri-sec-butoxide. The solution obtained is stirred for 15 minutes.
By way of example, such a solution of aluminium tri(s-butoxide) at a concentration of 0.475 mol/l (˜117 g/l) is placed in water at a temperature of 80° C. for a period of 15 minutes. A step, called a peptization step, is then carried out, in which a volume of between 1.4 ml and 2.8 ml of a 68% nitric acid solution is added to the tri-sec-butoxide hydrolysis solution. The mixture is placed at 85° C. in an oil bath for a period of 24 hours. A colloidal dispersion of aluminium oxyhydroxide (boehmite) in water is obtained. The concentration of nitric acid in the colloidal dispersion is between 0.033 mol/1 and 0.066 mol/l. In a variant, it is possible to concentrate the colloidal dispersion to a concentration of aluminium oxyhydroxide of approximately 1 mol/l. Other mineral or organic acids can be used in this peptization step, especially hydrochloric acid and acetic acid. There is obtained a transparent and stable colloidal sol which has, by X-ray diffraction, the characteristic lines of boehmite, as described in the paper JCPDS 21-1307.
B2—Functionalization of the boehmite nanoparticles
To a colloidal dispersion as obtained according to the preceding step B1, having a concentration of aluminium of between 0.5 mol/1 and 0.8 mol/l, there is added, where appropriate, a non-ionic surfactant—especially a non-ionic surfactant chosen from Pluronic® P-123, Pluronic® F 127 (BASF, Mount Olive, N. J., USA), Brij 58 and Brij 52 in a final proportion by mass of between 1% and 5%. There is then added a quantity of a corrosion inhibitor, especially cerium(III) nitrate (Ce(NO₃)₃) or sodium vanadate, at a final concentration of between 0.001 mol/1 and 0.5 mol/l. This preparation is stirred at ambient temperature for a period of 6 hours. A colloidal dispersion of boehmite nanoparticles functionalized at the surface—called physisorbed boehmite nanoparticles—is obtained. In infrared spectroscopy using the diffuse reflection technique DRIFT (diffuse reflectance infra-red Fourrier transform), such a preparation has vibration bands at 1460 cm⁻¹and 1345 cm⁻¹characteristic of the coordination of cerium to the nitrate ions.

C—Preparation of a Colloidal Dispersion of Doped Boehmite Nanoparticles

Such a colloidal dispersion of doped boehmite nanoparticles is prepared in two steps described below, in which there is carried out (C1) the hydrolysis/condensation of a precursor—especially an alkoxide—of aluminium and a corrosion inhibitor. A step (C2), called a peptization step, of treatment in an acidic medium is then carried out in order to form doped boehmite nanoparticles.

C1—Hydrolysis/condensation of ASB and (Ce(NO₃)₃)

The hydrolysis/condensation of a mixture of aluminium precursor—especially an aluminium alkoxide, in particular aluminium tri(s-butoxide) (ASB)—and a corrosion inhibitor—especially cerium(III) nitrate (Ce(NO₃)₃)—is carried out by adding to this mixture a minimal amount of water heated to a temperature of 85° C. This hydrolysis/condensation mixture is stirred for 15 minutes. The final concentration of ASB in the hydrolysis/condensation mixture is 0.475 mol/l and the final concentration of corrosion inhibitor in the hydrolysis/condensation mixture is between 0.005 mol/l and 0.015 mol/l.

C2—Peptization

An acidified aqueous solution—especially from 1.4 ml to 2.8 ml of a 68% nitric acid solution—is added to the hydrolysis/condensation mixture and the acidified mixture is placed at a temperature of 85° C. in an oil bath for a period of 24 hours. The concentration of nitric acid in the acidified mixture is between 0.033 mol/l and 0.066 mol/l. In a variant, it is possible to concentrate the colloidal dispersion to a concentration of aluminium oxyhydroxide of approximately 1 mol/l. There is obtained a transparent and stable colloidal sol which has, by X-ray diffraction, the characteristic lines of boehmite as described in the paper JCPDS 21-1307.

D—Preparation of Hollow Aluminium Oxyhydroxide (Boehmite) Nanoparticles Containing the Corrosion Inhibitor

Such hollow aluminium oxyhydroxide nanoparticles containing the corrosion inhibitor are prepared by formation of an inverse microemulsion (Daniel H., et al. (2007), Nano Lett., 7; 11, 3489-3492) and simultaneous encapsulation of the corrosion inhibitor.
An apolar phase is prepared by mixing an alcohol—especially hexanol—, an alkane—especially dodecane—and a surfactant—especially hexadecyltrimethylammonium bromide (CTAB). A polar phase comprising water, an alcohol—especially methanol—and a corrosion inhibitor—especially cerium nitrate—is also prepared. The polar phase and the apolar phase are mixed, and the mixture is stirred for 30 minutes in order to form an inverse microemulsion of water in the apolar phase. A solution of an aluminium alkoxide—especially aluminium tri(s-butoxide) (ASB)—in one volume of the alkane—especially dodecane—is prepared. The aluminium alkoxide solution is introduced into the inverse microemulsion, with stirring. The mixture is allowed to rest for 12 hours. A residue containing hollow aluminium oxyhydroxide nanoparticles is separated off by centrifugation. After washing the residue with diethylene glycol, there is obtained a powder of hollow aluminium oxyhydroxide nanoparticles containing the corrosion inhibitor.

E—Preparation of a Colloidal Anticorrosion Treatment Solution

Such a colloidal anticorrosion treatment dispersion is prepared by mixing a quantity of a treatment solution (hybrid sol) as prepared in (A), a quantity of colloidal dispersion of physisorbed boehmite nanoparticles as prepared in (B) and/or a quantity of a colloidal dispersion of doped boehmite nanoparticles as prepared in (C) and/or a quantity of a dispersion of hollow boehmite nanoparticles. The concentration of aluminium and silicon in the colloidal anticorrosion treatment dispersion is between 1.66 mol/1 and 2 mol/l. The concentration of aluminium supplied by the colloidal dispersion of boehmite nanoparticles functionalized at the surface in the hybrid sol is between 0.1 mol/l and 0.13 mol/l. The concentration of aluminium supplied by the colloidal dispersion of doped boehmite nanoparticles in the hybrid sol is between 0.1 mol/l and 0.13 mol/l. The hybrid sol so obtained is allowed to rest at ambient temperature for a period of 24 hours.
In an advantageous variant according to the invention, an alcoholic solution comprising at least one alkoxysilane and at least one aluminium alkoxide is prepared, and there is then added to said alcoholic solution a quantity of the colloidal dispersion of physisorbed boehmite nanoparticles and/or of doped boehmite nanoparticles and/or of hollow boehmite nanoparticles.
In this manner, the viscosity of the treatment solution (dispersion), which falls with the addition of the colloidal dispersion of boehmite, is controlled. The thickness of the hybrid gel deposited on the surface of the solid metal substrate is thus controlled in particular as a function of the speed of withdrawal of the solid metal substrate from the treatment solution (dispersion).

F—Deposition of the Colloidal Dispersion on the Solid Metal Substrate

A step of deposition of the colloidal anticorrosion treatment dispersion on a surface of a solid metal substrate, especially of a part of laminated 2024 T3 aluminium alloy which has previously undergone degreasing and scraping, is carried out. In this deposition step, part of the solvent of the composite hybrid sol evaporates, and at the same time the hydrolysis/condensation of the alkoxysilane(s) and metal alkoxide(s) permits the formation of a composite hybrid anticorrosion matrix on the surface of the solid metal substrate.
The presence of cerium as corrosion inhibitor, especially of free cerium (Cer^III), in the treatment solution permits the formation, during the deposition of said solution, of a conversion layer that is chemically stable in a corrosive medium. Such a conversion layer is formed in particular from the hydroxylated groups of an element M constituting the solid metal substrate and forming a bond M-O—Ce— with the cerium.
The presence of physisorbed boehmite nanoparticles and of doped boehmite nanoparticles in the treatment solution is adapted to permit the formation of reservoirs of corrosion inhibitor in the composite hybrid matrix constituting the anticorrosion coating, said reservoirs being adapted to permit the controlled release of the corrosion inhibitor over time.
Application of the treatment solution to the surface of the solid metal substrate is carried out by any means known per se to the person skilled in the art, especially by dip-coating, by spray-coating or by application by means of a paint brush, a pad or a brush for localized uses as spreading of the coating of the surface of the solid metal substrate.
For the dip-coating technique, the withdrawal speed allows the thickness of the deposit of the treatment solution to be controlled for a given viscosity of the treatment solution. Typically, the withdrawal speed varies between 2 and 53 cm/minute. The application of a plurality of successive layers by successive dip-coating operations, each dip-coating operation being followed by a drying step, permits the formation, where appropriate, of a coating of increased thickness.
It is also possible, in the dipping step, to impose a residence time of the solid metal substrate in the treatment solution which is prolonged in order to assist the chemical reactions between the solid metal substrate and the treatment solution. By way of example, the prolonged residence time can vary between 1 and 300 seconds.
For the spray-coating technique, the thickness of the deposits is controlled by the viscosity of the treatment solution, by the spray-coating parameters, especially the pressure, the flow rate, the geometric characteristics of spray nozzles, as well as by the speed of displacement of the nozzles opposite the surface of the solid metal substrate and the number of passages of the nozzles in front of the surface of the solid metal substrate. Application of the treatment solution can be carried out manually or can be automated according to conventional techniques.
For the technique of manual application using a paint brush, a pad or a brush, the thickness of the deposit is controlled by the viscosity of the treatment solution and by the number of successive applications to the surface of the solid metal substrate.
It is possible to carry out this step of deposition of the treatment solution on a surface of a solid metal substrate in a vessel under a controlled atmosphere and controlled humidity, especially in order to limit the too rapid evaporation of the solvent(s) and in order to limit the pollution of the atmosphere.
In a process according to the invention, it is also possible to carry out this step of deposition of the treatment solution in the open air, in particular by spray-coating in the open air.

G—Heat Treatment

A heat treatment of the treatment solution applied to the surface of the solid metal substrate is carried out in order to remove by evaporation the residual solvent(s) of the treatment solution and to permit its polymerization to a composite hybrid matrix. In particular, such a heat treatment comprises two successive steps in which the solid metal substrate coated with the treatment solution is subjected first to a first heating step at a temperature of between 50° C. and 70° C. for a period of between 2 hours and 24 hours, said first heating step being adapted to permit the removal of aqueous and/or organic solvents, and then to a second heating step at a temperature of between 110° C. and 180° C. for a period of between 3 hours and 16 hours, said second heating step being adapted to complete the polymerization of the treatment solution and to improve the mechanical properties of the composite hybrid matrix.
FIG. 3 shows the variation of the surface resistance of a solid metal substrate treated by a process according to the invention as a function of the immersion time of the solid metal substrate in a corrosion bath (NaCl 0.05 mol/l in water). Curve () represents the variation of the surface resistance of a solid metal substrate treated by a process according to the invention consisting in the successive application of a conversion solution rich in cerium (0.1 mol/l) and then of a treatment solution comprising cerium (0.01 mol/l). It is observed that the surface resistance of the treated solid metal substrate () decreases more slowly than the surface resistance of a solid metal substrate (Δ) treated with the same treatment solution (0.01 mol/l of Ce) but without a conversion layer.
The numerical values are given in Table 1 below.

	TABLE 1

	Surface resistance, Ω · cm²

Immersion time, h	With conversion layer	Without conversion layer

1	7870000	8080000
5	7710000	7760000
24	5280000	3980000
48	4050000	2120000
72	2750000	1111000
148	1310000	1080000
240	1180000	1030000
336	1190000	1020000

It is observed in particular that after immersion for 48 hours in the corrosion bath, the surface resistance of the untreated substrate (Δ) reaches a value of approximately 2.12×10⁶Ω·cm², while the surface resistance of the treated substrate () remains approximately 4.05×10⁶Ω·cm². After immersion for 72 hours in the corrosion bath, the surface resistance of the untreated substrate (Δ) reaches a limiting value of approximately 1.1×10⁶Ω·cm², while the surface resistance of the treated substrate () remains approximately 2.75×10⁶Ω·cm².
The inventors have also observed, wholly surprisingly and unexpectedly, that the anticorrosion treatment of a solid metal substrate according to the invention with a treatment solution comprising a concentration of cerium of between 0.005 mol/1 and 0.015 mol/l not only permits the obtainment of a surface resistance of the anticorrosion coating, measured by electrochemical impedance spectroscopy (FIG. 4), which is optimal for an immersion time of the solid metal substrate in a corrosion bath of 1 day (Δ), 7 days (∘) and 14 days () in an aqueous solution of NaCl 0.05 mol/l, but that such a treatment also permits the obtainment of a nano-hardness (FIG. 5), a Young's modulus (FIG. 6) and a resistance to delamination (∘, FIG. 7), a resistance to cracking (▴, FIG. 7) and a limiting value of resistance to plastic deformation (□, FIG. 7) which are also maximum.
The resistance to corrosion of a solid metal Al 2024-T3 substrate which has or has not been treated with a conversion solution and has then been exposed to a step of corrosion by immersion in an aqueous solution of NaCl 0.05 mol/l is analyzed by electrochemical impedance spectroscopy. The results are given in FIG. 8 according to Nyquist's representation.
Treatment of a part made of 2024-T3 aluminium, which has been treated with a conversion solution, by immersion in a corrosion solution (NaCl 0.05 mol/l) for 30 minutes at ambient temperature confers upon the part a surface resistance Z′ in “Nyquist” representation of approximately 4×10⁴Ω·cm²(∘, FIG. 8). By way of comparison, treatment of a part made of crude 2024-T3 aluminium (that is to say which has not been treated with a conversion solution) by immersion in a corrosion solution confers upon the part a surface resistance Z′ of approximately 5×10³Ω·cm²(Δ, FIG. 8).
Treatment of a part made of 2024-T3 aluminium which has previously undergone immersion in a conversion solution formed of water and cerium (0.01 mol/l) and without immersion in a corrosion bath confers a surface resistance Z′ having a value greater than 6×10⁵Ω·cm². Increasing the immersion time in the corrosion bath to 168 hours brings the surface resistance of the aluminium part back to the characteristic value of the aluminium oxide layer of approximately 5×10³Ω·cm².
Increasing the concentration of cerium in the conversion solution and increasing the immersion time of the aluminium metal part in the conversion solution lead to an increase in the surface resistance of the aluminium metal part.
In particular, the residual surface resistance Z′ of such an aluminium part after immersion for 1 hour in the corrosion solution is approximately 1.1×10⁴Ω·cm²for a concentration of cerium of 0.01 mol/l in the conversion solution and a conversion treatment time of 1 second, approximately 2×10⁴Ω·cm²for a concentration of cerium of 0.05 mol/l in the conversion solution and a conversion treatment time of 1 second, and approximately 3.3×10⁴Ω·cm²for a concentration of cerium of 0.1 mol/l in the conversion solution and a conversion treatment time of 1 second.
The residual surface resistance Z′ of an aluminium part after immersion for 1 hour in the corrosion solution is approximately 1.1×10⁴Ω·cm²for a concentration of cerium of 0.01 mol/l in the conversion solution and a conversion treatment time of 1 second, approximately 2×10⁴Ω·cm²for a concentration of cerium of 0.01 mol/l in the conversion solution and a conversion treatment time of 60 seconds, and approximately 3.8×10⁴Ω·cm²for a concentration of cerium of 0.01 mol/l in the conversion solution and a conversion treatment time of 300 seconds.
The residual surface resistance Z′ of an aluminium part after immersion for 1 hour in the corrosion solution is approximately 3.2×10⁴Ω·cm²for a concentration of cerium of 0.1 mol/l in the conversion solution and a conversion treatment time of 1 second, approximately 4.0×10⁴Ω·cm²for a concentration of cerium of 0.1 mol/l in the conversion solution and a conversion treatment time of 60 seconds, and approximately 9.0×10⁴Ω·cm²for a concentration of cerium of 0.1 mol/l in the conversion solution and a conversion treatment time of 300 seconds. Prolonged immersion in the corrosion bath leads to a decrease in the surface resistance value, which reaches the value of the surface resistance of the aluminium oxide in 10 hours.
The surface resistance Z′ of an aluminium part treated with a conversion solution comprising cerium at a concentration of 0.5 mol/l for a period of 1 second, 60 seconds and 300 seconds remains greater than 1×10⁴Ω·cm²after immersion of the aluminium part in the corrosion bath for 40 hours, 70 hours and 90 hours, respectively.
Chemical analyses (FIG. 9) by EDS reveal the presence of cerium Ce(III) on the surface of the solid metal substrate treated for 300 seconds with a conversion solution comprising cerium at a concentration of 0.5 mol/l. The majority signal is characteristic of the aluminium substrate.

Claims

1. A process for anticorrosion treatment in which there is applied to an oxidizable surface of a solid metal substrate a liquid solution, called a treatment solution, comprising:

at least one alkoxysilane, and

at least one cerium (Ce) cation;

in a liquid hydroalcoholic composition, said treatment solution being adapted to be able to form a hybrid matrix on the surface of the solid metal substrate by hydrolysis/condensation of each alkoxysilane and of each cerium (Ce) cation;

the treatment solution having a molar ratio (Si/Ce) of silicon element of the alkoxysilane(s) to the cerium (Ce) cation(s) of between 50 and 500;

wherein the cerium (Ce) cation(s) has(have) a concentration of between 0.005 mol/1 and 0.015 mol/l in the treatment solution.

2. The process as claimed in claim 1, wherein each alkoxysilane is chosen from the group formed of:

the tetraalkoxysilanes of the general formula (I) below:

Si(O—R₁)₄ (I)

wherein:

Si is the element silicon, O is the element oxygen;

R₁is chosen from the group formed of:

a hydrocarbon group of the formula [—C_nH_2n+1], n being an integer greater than or equal to 1; and

the group 2-hydroxyethyl (HO—CH₂—CH₂—); and

an acyl group of the general formula —CO—R′₁wherein R′₁is a hydrocarbon group of the formula [—C_nH_2n+1], n being an integer greater than or equal to 1; and

the alkoxysilanes of the general formula (II) below:

Si(O—R₂)_4−a(R₃)_a (II)

wherein:

R₂is chosen from the group formed of:

the group 2-hydroxyethyl (HO—CH₂—CH₂—); and

R₃is an organic group bonded to the silicon element (Si) of the alkoxysilane by an Si—C bond;

a is a natural integer of the interval]0; 4[.

3. The process as claimed in claim 1, wherein the treatment solution comprises at least one metal alkoxide.

4. The process as claimed in claim 3, wherein each metal alkoxide has the general formula (VII) below:

M′(O—R₉)_n″ (VII)

wherein:

M′ is a metal element chosen from the group formed of aluminum (Al), vanadium (V), titanium (Ti) and zirconium (Zr);

R₉is an aliphatic hydrocarbon group of the formula [—C_nH_2n+1] wherein n is an integer greater than or equal to 1; and

n″ is a natural integer representing the valence of the metal element M′.

5. The process as claimed in claim 3, wherein each metal alkoxide is an aluminum alkoxide of the general formula (III) below:

Al(OR₄)_n (III)

wherein:

Al and O are the elements aluminium and oxygen, respectively; and

R₄is an aliphatic hydrocarbon group having from 1 to 10 carbon atoms;

n is a natural integer representing the valence of the aluminum element (Al).

6. The process as claimed in claim 1, wherein the solid metal substrate is formed of a material chosen from the group formed of the oxidizable materials.

7. The process as claimed in claim 1, wherein, before application of the treatment solution, said oxidizable surface of the solid metal substrate is immersed in a liquid solution, called a conversion solution, formed of at least one corrosion inhibitor in water, said corrosion inhibitor being chosen from the group formed of the lanthanide cations, and said oxidizable surface of the solid metal substrate is kept in contact with the conversion solution for a period of time adapted to form a conversion layer formed of said lanthanide bonded by at least one covalent bond to the oxidizable surface and extending over the surface of the solid metal substrate.

8. The process as claimed in claim 7, wherein the conversion solution has a concentration of corrosion inhibitor of between 0.001 mol/1 and 0.5 mol/l.

9. The process as claimed in claim 1, wherein the treatment solution is applied by dip-coating of the solid metal substrate in said treatment solution.

10. The process as claimed in claim 1, wherein the treatment solution is applied by atmospheric spray-coating of the treatment solution on the surface of the solid metal substrate.

11. The process as claimed in claim 1, wherein the hydroalcoholic composition is formed of water and at least one alcohol.

12. The process as claimed in claim 1, wherein the cerium cation of the treatment solution is chosen from the group formed of the cerium chlorides and cerium nitrates.

13. The process as claimed in claim 4, wherein each metal alkoxide is an aluminum alkoxide of the general formula (III) below:

Al(OR₄)_n (III)

wherein:

Al and O are the elements aluminium and oxygen, respectively; and

R₄is an aliphatic hydrocarbon group having from 1 to 10 carbon atoms;

n is a natural integer representing the valence of the aluminum element (Al).