ES2745873B2 - Conversion layer generation procedure with corrosion resistance properties - Google Patents

Description

DESCRIPTION

Procedure for the generation of conversion layers with corrosion resistance properties

The present invention refers to a process for the formation of conversion layers with properties of resistance to corrosion through high pressure thermally activated chemical reactions for magnesium parts or powder or magnesium alloys, including pure or almost pure magnesium and all its combinations with other elements in any proportion, using a solution of one or more metallic salts, one of them being a salt of a rare earth in a deep eutectic solvent.

BACKGROUND OF THE INVENTION

Magnesium and its alloys have high mechanical strength and the lowest density among all metallic materials, which makes them very attractive. Unfortunately, its low corrosion resistance and high chemical reactivity have prevented its widespread use in many fields. The reason for the low resistance to corrosion of magnesium and its alloys lies in two aspects: the oxide layer that forms on the surface naturally is not protective; and, it is very susceptible to galvanic corrosion caused by impurities or secondary phases of the alloys (such as Mg17Al12, AlMn, Al8Mn5, etc.). These limitations therefore make it necessary to modify its surface to improve the corrosion resistance of magnesium alloys.

Conventionally, various techniques have been used for plating Magnesium alloys based on deposition [CD Gu et al. Electroless Ni-P plating on AZ91D magnesium alloy from a sulfate solution, J. Alloys Compd., 391 (2005), pp. 104-109], anodized [GL Song et al., Corrosion mechanism and evaluation of anodized magnesium alloys, Corros. Sci., 85 (2014), pp. 126-140], superhydrophobic coatings [Q. Chu et al., Facile fabrication of a robust superhydrophobic surface on magnesium alloy, Colloids Surf. A, 443 (2014), pp. 118-122], sputtering [SK Wu et al., A study of rf-sputtered Al and Ni thin films on AZ91D magnesium alloy, Surf. Coat. Technol., 200 (2006), pp. 2769-2774] and chemical conversion. The latter is one of the most common protection methods for increasing the corrosion resistance of magnesium alloys. These conversion layers are grown by chemical treatment of the metal surface to produce a surface layer of essentially different types of oxides chemically bonded to the surface [JE Gray et al., Pmtective coatings on magnesium and its alloys — a critique! review, J. Alloys Compd., 336 (2002), pp. 88-113]. Traditional conversion coatings were based on hexavalent chromium compounds and have been used extensively to protect magnesium alloys [P. Schmutz et al., Influence of Dichromate Ions on Corrosion Processes on Pure Magnesium, J. Electrochem. Soc. 2003 volume 150, issue 4, B99-B110]. However, increasing safety concerns regarding highly toxic chromate-based compounds have motivated researchers and industries to develop chromate-free conversion coating methods such as tin-based [F. Zucchi, et al., Stannate and permanganate conversion coatings on AZ31 magnesium alloy, Corros. Sci., 49 (2007), pp. 4542 4552], zinc phosphate [LY Niu et al., A study and application of zinc phosphate coating on AZ91D magnesium alloy, Surf. Coat. Technol., 200 (2006), pp. 3021-3026] and, very promisingly, those based on rare earth elements [T. Takenaka, et al., Improvement of corrosion resistance of magnesium alloys by surface film with rare earth elements, Mater. Trans., 49 (2008), pp. 1071-1076; X. Jiang et al., Microstructure and corrosion resistance of Ce-V conversion coating on AZ31 magnesium alloy, Appl. Surf. Sci., 341 (2015), pp. 166-174].

The typical process for the production of conversion layers of rare earth elements on magnesium or magnesium alloys consists of immersing the parts in an aqueous solution of a salt of the rare earth element, typically Ce (NO3) 3 or CeCl3, for a prolonged time of several minutes or hours that can be shortened by raising the temperature of the solution or adding an oxidant, typically H2O2, that accelerates the oxidation and precipitation of the oxides or cerium compounds. The growth of the conversion layer is closely related to the pH changes that occur in the metal substrate during immersion in the conversion bath and with the formation of cerium compounds with different valence states. The steps that take place during the formation of the conversion layer in a conventional aqueous medium are the following [MF Montemor et al., Characterization of rare-earth conversion films formed on the AZ31 magnesium alloy and its relation with corrosion protection, Appl. Surf. Sci., 253 (2007), p.6922]:

• Dissolution of the native oxide layer, accompanied by the formation of hydroxyl ions and an increase in pH.

• Growth of the first layers composed of hydroxides of Ce (IV) and Ce (III), mixed with hydroxides of Mg

• Thickening of the surface film and slowing down of the increase in pH that lead to the preferential deposition of Ce (OH) 4 and its conversion into CeO2, forming an outer layer, richer in Ce (IV) species.

In aqueous media, the growth of the first layers and the subsequent thickening of the film are favored in CeCl3 solutions. This behavior suggests that chloride ions induce faster surface attack and pH rise compared to the action of other salt anions [M.F. Montemor et al., Composition and corrosion resistance of cerium conversion films on the AZ31 magnesium alloy and its relation to the salt anion, Applied Surface Science, Volume 254, Issue 6, 2008, Pages 1806-1814].

The processes for obtaining conversion layers with rare earths on magnesium or its alloys in aqueous media are very inefficient and the thicknesses obtained are very limited, on the order of nanometers.

Ionic Liquid (IL) is a class of organic salts that are composed entirely of ions. Due to its organic nature, typically at least one of the cations or anions has a bulky and asymmetric structure that prevents strong interaction between the ions [M. Armand et al. lonic-liquid materials for the electrochemical challenges of the future. Nature Materials 8 (2009) 621-629]. Unlike sodium chloride, which melts at high temperatures (800 ° C), ILs can exist in a liquid state at temperatures below 100 ° C. A particular class of IL with a melting point below 25 ° C is called Ionic Liquids at Room Temperature (RTIL). Because ILs are composed solely of ions, they have very interesting properties compared to water or other solvents, such as having low melting points, very low volatility (negligible vapor pressure), good thermal stability, and high ionic conductivity. , making them ideal electrolytes for many applications.

Recent work has shown new applications for IL, including protection against reactive metal corrosion, the advantages of which lie in the intrinsic properties of IL. Almost all early work involving IL treatments for corrosion protection focused on trihexyl (tetradecyl) phosphonium bis (trifluoromethylsulfonyl) amide ([P6,6,6,14] [NTf2]) IL, due to its commercial availability and to its advantageous electrochemical properties, using a simple immersion treatment. Subsequently, and as new ILs were synthesized, numerous works have been developed using various anions and processes to accelerate the formation of the conversion layer by increasing the temperature, the treatment time, the percentage of water in the IL or the application. of anode potentials. Among all of them, the fluorinated and organophosphate ILs the mentioned trihexyl (tetradecyl) phosphonium bis (trifluoromethylsulfonyl) amide ([P6,6,6,14] [NTf2] and trihexyl (tetradecyl) phosphonium diphenyl phosphate [P6,6,6 , 14] [dpp]) are among the best performers. These studies have also shown that there is a critical treatment time, as long treatment times often lead to thick, cracked and unprotective films. The presence of equilibrium amounts of water with the IL and the treatment at 50 ° C improved the quality and protection against corrosion of the film generated [P. Huang et al. A review of ionic liquid surface film formation on Mg and its alloys for improved corrosion performance, Electrochimica Acta, Volume 110, 2013, pages 501-510].

Particularly interesting is the proposal to increase the temperature of the process until reaching the decomposition temperature of IL [US9435033B2]. The method consists of contacting the magnesium-containing surface with an ionic liquid under conditions that result in decomposition of the ionic liquid to produce a conversion layer on the magnesium or magnesium alloy with a significant improvement in resistance to corrosion. corrosion. The temperatures for the degradation of the ionic liquid are between at least 100 ° C and 300 ° C. For example, the conversion layer can be obtained by applying a layer of the ionic liquid tetraoctylammonium bis (2-ethylhexyl) phosphate [N8888] [DEHP] on the surface of the sample and then heat treating the sample at 300 ° C in an oven for 7 minutes. The conversion layer has a thickness of 80-100 nm and is composed of magnesium alloy elements, oxides and phosphates.

A method similar to the one previously described has recently been proposed, but performing the treatment in deep eutectic solvents (DES). Unlike conventional ILs, DES are commonly defined as systems composed of a mixture of at least two components, a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD), which are capable of self-associating to form a new eutectic phase characterized by a melting point (below 100 ° C) lower than that of each individual component. These constituent components of DES must also have safe, low toxicity, renewable and biodegradable characteristics, in addition to a significantly lower cost than ILs. The number of possible combinations of HBA and HBD that can form eutectic mixtures is extremely high, and in order to rationalize the properties and behavior of these solvents, the classification into four types of DES according to the two components that make it up has often been considered: Type I (quaternary salt and metal halide), Type II (quaternary salt and hydrated metal halide), Type III (quaternary salt and hydrogen bonding donor) and Type IV (metal halide and hydrogen bonding donor) [LIN Tomé et al., Deep eutectic solvents for the production and application of new materials, Applied Materials Today, Volume 10, 2018, Pages 30-50].

Said proposed process for the formation of conversion layers with DES on magnesium alloys is based on the interaction of a deep eutectic solvent (DES) with the substrate. Upon heating, the DES based choline chloride / urea mixture reacts with the magnesium alloy surface to form a promising anti-corrosion conversion film. The chemical conversion film is mainly composed of the MgH2 and MgCO3 phases. Electrochemical tests and visual inspections indicated that the corrosion resistance of the magnesium alloy was improved by the presence of a conversion layer [Gu et al., Corrosion resistance of AZ31B magnesium alloy with a conversion coating produced from a choline chloride — Urea based deep eutectic solvent, Corrosion Science 106, May 2016, Pages 108-116].

As an alternative to its application to generate coatings or conversion layers, different hydrothermal synthesis processes [M. Hirano et al., Hydrothermal Synthesis of Nanocrystalline Cerium ( IV) Oxide Powders, Journal of the American Ceramic Society 82, 3, (1999) pp 786-788], solvothermal [RI Walton, Solvothermal synthesis of cerium oxides, Progress in Crystal Growth and Characterization of Materials, Volume 57, Issue 4, 2011, Pages 93-108] and more recently, ionothermic in ionic liquids [T. Brezesinski et al. , Mesostructured crystalline ceria with a bimodal pore system using block copolymers and ionic liquids as rational templates, Chem Mater, 17 (2005), p. 1683] and in deep eutectic solvents [Hammond et al., Deep eutectic-solvothermal synthesis of nanostructured ceria, Nature Communications volume 8, Article number: 14150 (2017)], have proposed routes for the generation of nano-crystalline Ce oxide powder. (IV), CeO2, for various applications. In all of these synthetic processes, including recent work on DES, the cerium or rare earth precursor in the form usually of chloride, sulfate, or nitrate is dissolved in the solvent (water, an organic solvent, an IL, or a DES) and is Raise the temperature and pressure until nanometric crystals of CeO2 or some cerium compound are precipitated, which are later calcined in an oven to form CeO2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention refers to a process for generating conversion layers with, mainly, corrosion resistance properties on the surface of magnesium or magnesium alloys, characterized in that it comprises immersing said magnesium or said magnesium alloy, in any geometric shape including in powder form of any particle size, in a non-aqueous solution formed by one or more metallic salts, one of which is a salt of a rare earth element, at a temperature equal to or greater than 100 ° C in a reactor high pressure, preferably at pressures between 1 and 30 atmospheres.

Said dissolution comprises:

- a deep eutectic solvent of the general formula Cat + X-zY, where Cat + is an organic cation such as ammonium, sulfonium or phosphonium and X - is a Lewis base, generally a halide anion of the salt, Y is a Lewis base or Bronsted, and z is the number of molecules of Y,

- one or more metal salts of the metal or metals of the general formula M + X- where M is the metal with which the conversion layer is to be formed on the part and X - is the same halide anion of the deep eutectic solvent, being al minus one of the salts a salt of the elements known as rare earths (cerium, lanthanum, etc.)

A sufficiently high temperature must be applied under conditions of high pressure and limited humidity for decomposition of the solution of the metal salt (s) in the deep eutectic solvent to occur, in turn reacting with the surface of the magnesium alloy to form a layer. of metal oxides, oxy-carbonates and hydroxides of the metals present in the solution and of the metallic elements of the magnesium alloy themselves, obtaining conversion layers of homogeneous thickness and composition that have properties as good or even higher than those obtained by other processes in eg corrosion protection applications for magnesium alloys or in oxide coated powder manufacturing applications for catalysis applications.

The metal salt selected from the rare earth elements may be present in a concentration between 5-10'7 and 1.5 M, preferably in a concentration between 0.01 and 1 M, and more preferably between 0.1 and 0.5 M.

In the process of the invention said inorganic rare earth salt may be a lanthanide or actinide element, preferably a salt of a halide of the chemical element Lanthanum or Cerium, and more preferably CeCl3.

The process can be carried out with only said salt of a rare earth element or in a mixture of said salt with other metal salts. The selected inorganic metal salt may be present in a concentration between 5-10.7 and 1.5 M, preferably in a concentration between 0.01 and 1 M, and more preferably between 0.1 and 0.5 M.

The metal salt may be of a metal selected from alkali, alkaline-earth or transition metals, or a salt of another lanthanide or actinide element, and preferably a salt of a transition metal halide.

In the process of the invention, the deep eutectic solvent DES where the metal salts are dissolved is a mixture of at least two components, a hydrogen bond acceptor compound (HBA) and a hydrogen bond donor compound ( HBD) that form a new phase Eutectic with a melting temperature lower than the melting temperature of both compounds and lower than 100 ° C or even lower than 25 ° C.

The deep eutectic solvent of the process of the invention can be described with the general formula Cat + X "zY, where Cat + is the cation of any ammonium, sulfonium or phosphonium salt (ammonium, sulfonium, or phosphonium) and X - is a base of Lewis, generally a halide anion of the salt, Y is a Lewis or Bronsted base, and z is the number of Y molecules. In the process of the invention the DES used comprises a halide salt of a quaternary amine and an amide, carboxylic acid or alcohol as the hydrogen bond donor, and preferably, the DES formed by choline chloride and urea as HBD and preferably, in a 1: 2 molar ratio to form the eutectic phase. Both components are safe, cheap and non-toxic.

In the process of the invention DES is generated in the usual way by mechanically mixing the two elements (HBA and HBD) at a temperature between 50 and 80 ° C until the formation of the eutectic phase.

In the process of the invention the salt of the rare earth element, together with the additional metal salt or salts, are added to the previously formed DES in anhydrous or hydrated form at room temperature or preferably at a temperature between room temperature and 100 ° C or more preferably between 50 and 80 ° C to accelerate dissolution.

In the process of the invention, the solution formed by DES and the metal salt or salts are subjected to a vacuum drying process at a temperature between 50 and 80 ° C for at least 12 hours, preferably 24 hours or the time necessary to reach the minimum amount of dissolved water, and in any case less than 500 ppm.

As indicated above, the procedure is applied to the surface of magnesium or magnesium alloys, which can be:

- in the form of a solid or porous piece with dimensions greater than one millimeter. - in the form of a powder of any size between one millimeter and one nanometer.

In a possible embodiment of the process of the invention, the part to be coated is chemically pure magnesium or magnesium with a purity greater than 99%. Alternatively, the part is made of a magnesium alloy.

In some embodiments, the magnesium or magnesium alloy is subjected to a stage, prior to treatment with the dissolution of metal salts in the DES, of grinding and / or sanding and / or polishing by mechanical means until a homogeneous surface is obtained and free from organic or inorganic contamination.

In some embodiments, the magnesium or magnesium alloy is subjected to an acid pickling process, prior to treatment with dissolving the metal salts in the DES. In this acid pickling step, the magnesium or magnesium alloy surface is contacted with one or more acids, which can be inorganic or organic acids. Some examples of inorganic pickling acids include hydrohalogenated acids (eg, HF, HCl, HBr, HI), nitric acid, sulfuric acid, and phosphoric acid. Some examples of organic pickling acids include carboxylic acids (eg, acetic or propionic acid and fluorinated versions, eg, trifluoroacetic acid) and sulfonic acids (eg, methylsulfonic or triflic acids). In further embodiments, an alkaline conditioning step may be employed after the acid pickling step and prior to treatment with dissolving the metal salts in the DES. In the alkaline conditioning step, the magnesium or magnesium alloy surface is contacted with one or more alkaline substances, typically in the form of an alkaline aqueous solution. Some examples of alkaline compounds that can be used in the alkaline conditioning step include metal hydroxides (eg, sodium hydroxide), carboxylate salts, metal carbonates or bicarbonates (eg, sodium carbonate or bicarbonate), and amines. (for example, dimethyl or trimethylamine). In further embodiments, a surface activation step may be employed after the acid pickling or alkaline conditioning step and immediately prior to treatment with the dissolution of the metal salts in the DES. In the surface activation step, the magnesium or magnesium alloy surface is contacted with hydrohalogenated acids (eg, HF).

In the process of the invention, the part or the magnesium or magnesium alloy powder, previously subjected to the mechanical or chemical cleaning process, is introduced into a high pressure reactor with an internal Teflon coating and is filled up to half of its volume with the solution of the metal salt or salts in the DES previously dried in vacuum until obtaining a humidity value of less than 500 ppm. The reactor is then closed and heated to a temperature at which the decomposition of DES and the metal salt or salts occurs, preferably at a temperature between 100 and 300 ° C, and more preferably between 150 and 180 ° C, and more preferably still at 160 ° C, for a time between 1 and 12 hours, preferably between 1 and 6 hours, and more preferably between 2 and 3 hours.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 . Image obtained by a Scanning Electron Microscope of the surface of the AZ31 piece treated in Example 1. a) x1000 magnification, b) x3000 magnification, c) x10000 magnification, d) x30000 magnification.

Figure 2. Image obtained by a Scanning Electron Microscope of the surface of the AZ31 piece treated in Example 1. a) x8000 magnification, b) x5000 magnification.

Figure 3. Image and elemental composition obtained by a Scanning Electron Microscope of the surface of the AZ31 piece treated in example 1. a) Image at x8000 magnification with the relative composition profiles obtained by X-ray Energy Dispersion Spectroscopy EDX, b) EDX carbon profile, c) EDX oxygen profile, d) EDX magnesium profile, e) EDX cerium profile.

Figure 4. X-ray diffraction diagram at grazing angle incidence of the sample of example 1.

Figure 5. XPS spectra of the energy levels (a) C1s (b) Ce3d of the sample of example 1.

Figure 6. Polarization curves in NaCl 1% by weight of a sample of AZ31 treated according to example 1 and a sample of untreated AZ31.

Figure 7. Volume of hydrogen evolved during immersion in NaCl 1% by weight of a sample of AZ31 treated according to example 1 and a sample of untreated AZ31. Average values of 3 samples.

Figure 8. Image obtained by a Scanning Electron Microscope of the surface of the AZ31 piece treated in Example 3. a) x1000 magnification, b) x3000 magnification.

Figure 9. EDX spectra obtained by a Scanning Electron Microscope of the surface of: a) the piece of AZ31 treated in example 1, b) the piece of AZ31 treated in example 3.

EXAMPLES

Example 1.

Conversion layer on a piece of magnesium alloy AZ31 obtained with a 0.1 M solution of CeCl3-7H2O in the DES choline chloride-urea.

A 24mm diameter, 2mm thick disc of AZ31 magnesium alloy is subjected to a conventional solvent cleaning treatment with acetone for 5 minutes, manually roughened with # 2000 grain size SiC paper, rinsed with distilled water for about 2 minutes and with ethanol for 30 seconds.

DES is prepared by mixing choline chloride, solid, with urea, solid, in a proportion of 1 mole of choline chloride with 2 moles of urea and heating it to 50 ° C while stirring mechanically until a liquid phase is formed. colorless from the eutectic choline-urea chloride.

100 cm3 of the resulting DES are measured and 3.74 g of CeCl3 heptahydrate are added to it to obtain a solution with a concentration of 0.1M CeCl3 in DES. The dissolution process is carried out at a temperature of 60 ° C with mechanical stirring until complete dissolution.

The solution of CeCl3 in the DES choline chloride-urea is heated at 80 ° C under vacuum <10 mbar for 24 hours to minimize, by selective evaporation, the water contained in the solution.

The AZ31 disk is introduced into a high pressure reactor lined internally with Teflon with a capacity of approximately 100 cm3 and approximately 50 cm3 of the previously prepared solution of CeCl3 is poured into the DES of cholineurea chloride. The rector is hermetically closed and placed in an oven previously heated to 160 ° C. It is kept at 160 ° C for 2 hours. It is removed from the oven and cooled with water to room temperature.

The AZ31 disk is removed from the reactor and the remains of the CeCl3 solution in the DES choline chloride-urea are cleaned with distilled water and dried with warm air.

After cleaning, the surface of the AZ31 disc has a homogeneous blackish-brown color. Figure 1 shows an image taken in a SEM Scanning Electron Microscope where it is observed that the surface is covered by a homogeneous layer that presents a network of cracks typical of the rare earth conversion layers on magnesium or aluminum alloys obtained by methods. conventional in aqueous solutions.

Figure 2 shows an image taken by SEM where it is observed that the coating obtained in AZ31 has a thickness of approximately 4 microns, substantially greater than the thickness obtained by conventional means in aqueous solutions. The cracks observed on the surface go through the entire coating. The microstructure of the coating is dense and there is a good bond with the AZ31 substrate.

Figure 3 shows superimposed on image 2 the relative distribution of the chemical elements that make up the coating obtained by X-ray Energy Dispersion Spectroscopy in the SEM. It is observed that the coating is composed of Magnesium, Cerium, Oxygen and Carbon. The most superficial part of the coating is relatively richer in cerium and the inner part is richer in magnesium, which shows that there has been a reaction between CeCl ³ and DES with the magnesium alloy surface itself.

Figure 4 shows the X-ray Diffraction (XRD) diagram obtained by grazing angle of the sample. The coating is predominantly amorphous with three crystalline phases that can be identified as cerium, magnesium, and oxycarbonate. cerium. Semiquantitatively, the composition of the crystalline phases is 66.3% cerium oxy-carbonate, 25.8% magnesium and 8.1% cerium dioxide.

The surface analysis of the samples by X-ray photoelectron spectroscopy (XPS) shows that the cerium present on the surface is found exclusively as Ce (III), while most of the carbon is in the form of carbonate (Figure 5 ).

The results indicate that a homogeneous, dense and adherent coating of approximately 4 microns thickness of amorphous and nano-crystalline character is obtained, composed mainly of cerium (III) oxy-carbonate.

Example 2.

Corrosion resistance properties in NaCl of an AZ31 magnesium alloy disc treated in Example 1.

An AZ31 magnesium alloy disc treated following the method described in Example 1 herein was subjected to electrochemical measurements of corrosion resistance in an aggressive medium consisting of a 1% by weight solution of NaCl in distilled water. A conventional three-electrode cell was used for this type of electrochemical measurements, the disk treated according to example 1 being the working electrode, a platinum wire the counter electrode and a silver / silver chloride electrode the reference electrode. After 900 s of immersion in the NaCl solution, a potential sweep was performed from the open circuit potential towards more anodic (positive) values collecting the current density. In another area of the disk treated according to example 1, the same procedure was carried out, but with the potential sweep from the open circuit potential towards more cathodic (negative) values. For reference, the same procedures were performed on a previously untreated AZ31 alloy disc.

Figure 6 shows the results of current density versus potential (polarization curves or potentio-dynamic curves) of a disc treated according to example 1 and of an untreated AZ31 alloy disc. The corrosion potential of the sample treated according to example 1 is observed to be more negative than that of the AZ31 alloy, -1.66 V vs REF and -1.52 V vs REF respectively. It is also observed that the corrosion density, taken as the extrapolated current density of the branch The cathodic corrosion potential is significantly lower in the AZ31 sample treated according to example 1 compared to an AZ31 sample without treatment, 1.6 10-5 A / cm2 and 1.310-4 A / cm2 respectively. This implies that the corrosion rate of the AZ31 alloy is reduced by an order of magnitude thanks to the method described in Example 1 of this invention.

However, magnesium and magnesium alloys are known to have an anomalous corrosion mechanism that does not allow a corrosion rate to be directly inferred in terms of loss of mass or thickness from electrochemical values [A. Atrens et al., Review of Recent Developments in the Field of Magnesium Corrosion, Advanced Engineering Materials, vol 17, number 4 (2015) pp 400-453]. Therefore, the method of measuring the evolution of hydrogen during immersion in a solution of NaCl 1% by weight in distilled water was used. Discs of AZ31 treated by the process described in Example 1 of the present invention were immersed in the NaCl solution at room temperature and the evolved hydrogen was collected for a total of 72 hours. Figure 7 shows the volume of accumulated hydrogen formed by the corrosion process of AZ31 untreated and treated according to the method of example 1. It is observed that the evolution of hydrogen is significantly lower on the surface of the alloy AZ31 treated according to example 1 Since each mole of hydrogen formed by the corrosion reactions corresponds to one mole of metallic magnesium that has been oxidized, the values in figure 7 can be transformed into values of magnesium thickness lost due to corrosion per year using the expression given by Atrens et al. to the. [TO. Atrens et al., Review of Recent Developments in the Field of Magnesium Corrosion, Advanced Engineering Materials, vol 17, number 4 (2015) pp 400-453]. Therefore, it is concluded that the treatment described in Example 1 notably reduces the corrosion rate to 0.32 mm / year with respect to the value of 0.75 mm / year of the AZ31 alloy without treatment.

Example 3.

Conversion layer on a piece of AZ31 magnesium alloy obtained with a 0.1 M solution of CeCl3-7H2O and 0.1 M of CaCl2 in the DES choline chloride-urea.

An AZ31 magnesium alloy disc was treated following the method described in Example 1 hereof except that 1.11 g of CaCl2 are added to the DES of choline chloride and urea to obtain a solution with a concentration of 0.1M of CeCl3 and 0.1M CaCl2 in DES. The dissolution process is carried out at a temperature of 60 ° C with mechanical stirring until complete dissolution.

After treatment with the same parameters as example 1, the AZ31 disk is removed from the reactor and the remains of the CeCl ³ and CaCl ² solution in the DES choline chloride-urea are cleaned with distilled water and dried with air. tempered.

Figure 8 shows an image taken in a SEM Scanning Electron Microscope where it is observed that the surface is covered by a homogeneous layer that presents a network of cracks typical of the rare earth conversion layers similar to that obtained following the procedure of the example 1 (figure 1).

Figure 9 shows by way of comparison the results of X-ray Energy Dispersion Spectroscopy (EDX) in the SEM of the layer obtained on an AZ31 disk treated according to example 1 and treated according to example 3. It is observed that the coating obtained in Example 1 is composed of Magnesium, Aluminum, Cerium, Oxygen and Carbon, while in the coating obtained in Example 3 the presence of Calcium is seen in addition to these elements. This shows that there has also been a reaction between CaCl2 and DES with the very surface of the magnesium alloy through which Calcium has become part of the coating formed.

The semi-quantitative composition of the coating of Example 1 obtained by EDX is (in atomic%) 60.5% Oxygen, 21.8% Carbon, 8.1% Cerium, 7.0% Magnesium, and 2.7% Aluminum. The semi-quantitative composition of the coating of example 3 obtained by EDX is (in atomic%) 63.1% Oxygen, 21.6% Carbon, 11.6% Cerium, 1.9% Magnesium, 1.2% Aluminum, and 0, 6% Calcium.

Claims (26)

1. A process for generating conversion layers with corrosion resistance properties on a magnesium or magnesium alloy surface comprising a step of immersing a magnesium or magnesium alloy part or powder in a non-aqueous solution of at least one metal salt, at a temperature equal to or greater than 100 ° C in a high pressure reactor, characterized in that said non-aqueous solution comprises: - a deep eutectic solvent of the general formula Cat + X-zY, where Cat + is the cation of any ammonium, sulfonium or phosphonium salt and X - is a Lewis base, preferably a halide anion of the salt, Y is a base of Lewis or Bronsted, and z is the number of molecules of Y, - at least one metal salt of the general formula M + X - where M is the metal with which the conversion layer is to be formed on the part and X - is the same Lewis base, preferably the same halide anion of the deep eutectic solvent and where M is a rare earth element. 2. Process for generating conversion layers according to claim 1, characterized in that said deep eutectic solvent comprises a halide salt of a quaternary amine and an amide, carboxylic acid or alcohol. 3. Process for generating conversion layers according to any one of claims 1 or 2, characterized in that said DES is the one formed between choline chloride and urea in a 1: 2 molar ratio to form the eutectic phase. 4. Process for generating conversion layers according to any one of claims 1 to 3, characterized in that the rare earth element salt is a halide of a lanthanide or actinide element. 5. Process for generating conversion layers according to claim 4, characterized in that the rare earth element salt is a cerium or lanthanum halide. 6. Process for generating conversion layers according to claim 5, characterized in that the salt of rare earth elements is cerium chloride. 7. Process for generating conversion layers according to any one of claims 1 to 6, characterized in that, in addition to the salt of a rare earth element, a halide salt of an alkaline element is added. 8. Process for generating conversion layers according to any one of claims 1 to 6, characterized in that, in addition to the salt of a rare earth element, a halide salt of an alkaline earth element is added. Process for generating conversion layers according to any one of Claims 1 to 6, characterized in that, in addition to the salt of a rare earth element, a halide salt of a transition metal is added. 10. Process for generating conversion layers according to any one of claims 1 to 9, characterized in that the concentration of the rare earth element salt is between 0.01 and 1 M. 11. Process for generating conversion layers according to claim 10, characterized in that the concentration of the salt of the rare earth element is between 0.1 and 0.5 M. 12. Process for generating conversion layers according to any one of claims 1 to 11, characterized in that the concentration of the additional salt of halides of alkali elements, alkaline earth elements or of transition metal is between 0.01 and 1 M. 13. Process for generating conversion layers according to claim 12, characterized in that the concentration of the additional salts of alkali, alkaline-earth elements or transition metal is between 0.1 and 0.5 M. 14. Process for generating conversion layers according to any one of claims 1 to 13, characterized in that the decomposition of the solution of salts in DES and the reaction with the surface of magnesium or magnesium alloy occurs at a temperature between 100 ° C and 300 ° C in a high pressure reactor. 15. Process for generating conversion layers according to claim 14, characterized in that the decomposition of the salt solution in DES and the reaction with the magnesium or magnesium alloy surface occurs at a temperature between 150 and 180 ° C. 16. Process for generating conversion layers according to claim 15, characterized in that the decomposition of the salt solution in DES and the reaction with the magnesium or magnesium alloy surface occurs at a temperature of 160 ° C. 17. Process for generating conversion layers according to any one of claims 14 to 16, characterized in that the decomposition of the solution of salts in DES and the reaction with the magnesium or magnesium alloy surface occurs for a time reaction time between 1 and 12 hours. 18. Process for generating conversion layers according to claim 17, characterized in that the decomposition of the salt solution in the DES and the reaction with the surface of magnesium or magnesium alloy occurs during a reaction time between 2 and Three hours. 19. Process for generating conversion layers according to any one of claims 1 to 18, characterized in that the part to be coated is chemically pure magnesium or magnesium with a purity greater than 99%. 20. Process for generating conversion layers according to any one of claims 1 to 18, characterized in that the part to be coated is a magnesium alloy. 21. Process for generating conversion layers according to any one of claims 19 or 20, characterized in that the magnesium or magnesium alloy is in the form of a solid or porous piece with dimensions greater than one millimeter. 22. Process for generating conversion layers according to any one of claims 19 or 20, characterized in that the magnesium or the alloy of Magnesium is in the form of a powder of any size between a millimeter and a nanometer. 23. Process for generating conversion layers according to any one of claims 1 to 22, characterized in that the magnesium or magnesium alloy is previously subjected to a surface preparation by mechanical means until a clean and homogeneous surface is obtained. 24. Process for generating conversion layers according to any one of claims 1 to 22, characterized in that the magnesium or magnesium alloy is previously subjected to cleaning and pickling in an acid bath. 25. Process for generating conversion layers according to any one of claims 1 to 22, characterized in that the magnesium or magnesium alloy has been previously cleaned and pickled in an acid bath and conditioned in an alkaline bath. 26. Process for generating conversion layers according to any one of claims 23 to 25, characterized in that the magnesium or the magnesium alloy, previously cleaned and mechanically or chemically pickled, is activated in hydrofluorinated acid.