EP0300846B1 - Process for modifying the surface of a substrate by the formation of an alloy, use of the process, paticularly in the preparation of ferrous alloys, and catalysts and alloys obtained by the process - Google Patents

Description

The present invention relates to a method of surface modification of the surface of a substrate by the formation of an alloy and the application of the method to the manufacture in particular of ferrous alloys and catalysts.

More particularly, the present invention applies to the formation of surface layers on metal parts based on iron and therefore to ferrous alloys, that is to say alloys whose iron content expressed in weight percent is greater than that of any other element of the alloy.

There are very numerous methods, which have been implemented for a long time, making it possible to improve the characteristics of the shaped or semi-shaped metal parts (by metal parts, we mean both parts made of elemental metal and those made of alloy) and in particular resistance to wear, scuffing, deformation, corrosion, heating and / or erosion.

These methods include coating the metal surface, or modifying the composition and / or microstructure of the metal surface by techniques such as carbon carburizing, nitriding, welding a highly alloyed layer onto the surface layer of a metal part, torch quenching, induction quenching and physical modification (e.g. hammering).

These methods of coating the surface layer also include chromium-plating or nickel-plating, spraying refractories with a plasma jet or a blowtorch onto the surface of a metal part, plating by rolling (in the case of rolling products occurring in the form of sheets or wires), the laser fusion of a cement and a predetermined thickness of immediately underlying metal for the manufacture of a cement alloy improved quality, or the laser fusion of a metal part and a solid powder covering it to obtain a coating.

The prior art is particularly illustrated by patent DE-A-2,362,026 which describes the deposition by plating of a thin layer of transition metals V, Cr, Mn, Fe, Co and Ni on aluminum substrate or aluminum alloy. The deposited metal is then melted by an electron beam or a concentrated energy beam and mixed with the substrate. In this document, the use of chemical compounds from one or more of these metals is suggested.

The prior art is also illustrated by the publication: "Platin and surface finishing" vol 73, n ° 2, February 1986 pages 57-64, Orlando, Florida US: Laser-induced deposition on semi-conductor and polymeric substrates. This document describes a metal deposit induced by a laser on gallium-based semiconductors and polymeric substrates (imides) immersed in solutions. A cyanide solution is especially cited there.

However, handling this type of solution poses security problems.

An object of the present invention is to provide a new method of forming an alloy surface layer with a substrate, such as a metal part.

The invention also makes it possible to obtain these surface layers of non-oxidized metal alloys in an atmosphere which is not necessarily controlled, such as air.

The surface layer of alloy formed makes it possible to impart particular properties to the metal part without deeply affecting the nature and characteristics of the substrate. These properties can be, for example, resistance to corrosion, wear, deformation, heating, erosion, modification of friction, catalytic activity and hardness.

• a) on recouvre la surface du substrat d'une pellicule contenant au moins un sel d'acide carboxylique choisi parmi les acides carboxyliques aliphatiques et aromatiques et les acides carboxyliques aliphatiques et aromatiques hydroxy-substitués, le sel d'acide carboxylique comportant au moins l'élément devant constituer l'alliage. Ladite pellicule a des capacités réductrices en présence du faisceau d'énergie.
• b) on irradie la surface ainsi recouverte avec ledit faisceau en adaptant les paramètres liés à cette irradiation pour qu'une épaisseur substantielle de substrat immédiatement sous-jacent fonde et pour que ledit élément provenant de la décomposition dudit sel et ladite partie fondue du substrat se mélangent.
• c) on resolidifie l'alliage ainsi obtenu.

The present invention provides a method of surface modification of a metal substrate containing mainly iron by forming an alloy with at least one chemical element, called an addition element, by means of a concentrated beam of energy, like a laser beam. More precisely, at least one treatment is carried out comprising the following steps:

• a) the surface of the substrate is covered with a film containing at least one carboxylic acid salt chosen from aliphatic and aromatic carboxylic acids and hydroxy-substituted aliphatic and aromatic carboxylic acids, the carboxylic acid salt comprising at least 1 'element to constitute the alloy. Said film has reducing capacities in the presence of the energy beam.
• b) the surface thus covered is irradiated with said beam by adapting the parameters linked to this irradiation so that a substantial thickness of immediately underlying substrate melts and so that said element originating from the decomposition of said salt and said molten part of the substrate is mix.
• c) the alloy thus obtained is solidified.

The metal substrate may contain a proportion of iron greater than 60% by weight, for example from 60 to 100%, and advantageously from 90 to 99.9% by weight.

By film having reducing capacities in the presence of a beam of concentrated energy necessary for the decomposition of the salt comprising the addition element, it is meant that the film contains, in sufficient quantity, compounds capable of reducing the degree of oxidation d '' at least one other body, or to collect electrons from this other body, so that overall the degree of oxidation of the bodies constituting the alloy is reduced between the moment when the film is applied to the substrate and the moment where the alloy is formed. According to the process according to the invention, the reducing capacities of the film are substantially influenced by the atomic distribution of the compounds present in the film.

Also, the proportions of oxygen, carbon and hydrogen atoms present in the film may be such that the amount of oxygen atoms is less than the sum of twice the amount of carbon atoms and half the amount of hydrogen atoms.

When these reducing capacities of the film are more particularly conferred by one or more compound (s), this or these compounds will be qualified as reducing compounds. These compounds can be a film thickening agent, a salt comprising the addition element, a complexing agent, such as a carboxylic acid, ammonia, pyridine and / or the compounds derived therefrom.

Thus, by the method according to the invention, it will be possible to manufacture surface alloys comprising the addition element reduced to the state of zero oxidation. This manufacturing (irradiation and cooling) can be carried out in the presence of an uncontrolled atmosphere, for example in air or in a controlled atmosphere, for example in nitrogen or argon.

The parameters linked to irradiation, which are a function of the type of laser, will essentially be the power of the beam and the scanning time (or laser-matter interaction time, defined as the ratio of the diameter of the beam to the scanning speed. of this beam.

The use of salts makes it very practical to obtain a substantially homogeneous surface layer integrated into the surface of the substrate and consisting of intimate combinations between the element produced by the decomposition of the salt and the previously coated substrate.

The present invention also allows the production of a surface layer covering the substrate and comprising materials in an amorphous state.

When said alloy comprises at least two addition elements, these two elements may be present in at least one carboxylic acid salt according to the invention. The salt may be a complex salt. The film may include a complexing agent.

Salt complexation offers in particular the advantage of linking together the elements considered in a substantially homogeneous composition, without there being segregation of the metallic species present.

When the substrate is covered with the film, at least part of the salt may be in liquid solution, for example, in water or alcohol and / or at least part of the salt may be in powder form. , possibly dispersed in a liquid phase.

For example, to facilitate the maintenance or distribution of the film on the substrate, in particular before decomposing the salt, the film may be given a very viscous consistency, or even that of a gel. This consistency can be obtained by incorporation into the gelling or thickening solution, such as gums, such as gum tragacanth, Senegal gum, shellac, Dammar gum, Carob gum, or such as the substances industrial: alginic acid and alginate, polyvinyl alcohol, urea-formaldehyde resins, carboxyvinyl polymers, polyethylene oxide, carboxymethyl cellulose, methyl cellulose, polyglycols, polymethacrylates, polyethanolamines, oxide waxes, or an adhesive. These gelling agents can be considered as reducing compounds in the presence of a beam of concentrated energy, such as a laser beam.

Advantageously, a nonionic surfactant can be added thereto so as to have better spreading over the surface of the substrate.

The choice of gelling substance depends on its stability in the presence of the ions present in the film; the preceding list should in no way be considered as limiting.

The dynamic viscosity of the solution used for depositing the salt (s) on the substrate is generally greater than 10 mm² / s and preferably between 10 and 1000 mm² / s at room temperature. This solution can be deposited by any known method, for example by depositing with a brush, brush, roller, spray gun or by immersing the part to be covered in the solution.

According to a particularly advantageous embodiment of the method according to the invention, the deposition of the solution containing said carboxylic acid salts on the substrate can be followed by a dehydration operation leading to a deposition having a viscosity generally greater than 400 mm² / s and preferably between 8 × 10² and 10⁶ mm² / s.

The thickness of the film before dehydration is generally between 0.2 and 2 mm and preferably between 0.3 and 1 mm. After dehydration, it generally varies between 0.05 mm and 1 mm and preferably between 0.1 and 5 mm.

In particular, the salt may advantageously be a salt of at least one carboxylic acid such as formic, acetic, propionic, oxalic, citric, lactic, malic, salicylic and tartaric acids. The salts can also be alcoholates, such as methylate, ethylate, isopropylates (these alcoholates being moreover reducing compounds in the presence of the laser beam).

By this process, surface alloys can be produced with at least one so-called addition element introduced at least in part in the form of at least one carboxylic acid salt.

The addition element may be at least one of the addition elements well known to those skilled in the art in the field of metallurgy or more particularly of the steel industry.

The addition element may be at least one element chosen from the group formed by the following elements: Si, Mg, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sn, La, W, Re, Os, Ir, Pt, Au, Bi, Ce and preferably Ni, Cr, Co and Mo.

• ― soit des acides organiques contenant deux ou plusieurs fonctions acides, tels les acides oxalique, malonique, succinique, glutarique...,
• ― soit des acides alcools, tels les acides glycolique, lactique,
• ― soit des acides aminés tels l'acide aminoacétique, l'alanine, la leucine,
• ― soit des acides plus complexes portant deux ou plusieurs fonctions acides et/ou autres fonctions alcool, ou amine ou carboxyle, tels les acides malique, tartrique, citrique, éthylène diaminotétracétique,
• ― soit encore de l'ammoniaque, la pyridine et les composés en dérivant.

The complexing substances which can be used are for example:

• - either organic acids containing two or more acid functions, such as oxalic, malonic, succinic, glutaric acids, etc.,
• - or alcoholic acids, such as glycolic and lactic acids,
• - either amino acids such as aminoacetic acid, alanine, leucine,
• - or more complex acids carrying two or more acid functions and / or other alcohol, or amine or carboxyl functions, such as malic, tartaric, citric, ethylene diaminotetracetic acids,
• - or again ammonia, pyridine and the compounds derived therefrom.

According to a characteristic of the process, two treatments can advantageously be carried out. Under these conditions, a richer alloy is obtained or the possibility of adding another additional metal at high concentration to the substrate, which makes it possible in particular to increase its resistance to corrosion.

According to another embodiment of the method according to the invention, a coating of the metallic substrate can be formed by forming an alloy with said addition element and with at least one infusible solid compound. The presence of this infusible compound makes it possible to obtain a substantially homogeneous surface layer integrated or not on the surface of the substrate and consisting of intimate combinations between the element produced by thermal decomposition of the carboxylic acid salt mentioned above. above and the infusible solid compound and the substrate. Corrosion resistance, in particular at high temperature under H₂S or under SO₂, is increased.

The solid infusible compound may be a carbide, such as B₄C, Cr₃C₂, NbC, HfC, MoC, SiC, TaC, TiC, WC, ZrC, or a silicide, such as MoSi₂, NbSi₂, TaSi₂, or a simple or mixed refractory oxide , such as Al₂O3, BeO, Cr₂O₃, MgO, SiO₂, TiO₂, ZrO₂, ZrO₂, Y₂O₃, Al₂O₃ TiO₂, CaZrO₂, MgO SiO₂, mullite (3Al₂O₃, SiO₂), spinel (MgO Al₂O₃, ZrO₂ Si such as those of aluminum or titanium or a mixture of these materials.

It will not be departing from the scope of the present invention to use any generator capable of supplying sufficient power for a time sufficient to produce the thermal decomposition of the salt (s), the melting of the surface of the substrate and production of the alloy. The power per unit area required is substantially between 10² W / cm² and 10⁷ W / cm² (Watt per square centimeter) and may preferably be between 10⁴ and 10⁵ W / cm² depending on the interaction time of the spot with the material .

In addition to the concentrated beam of energy produced by a laser, the irradiation parameters of which have been adapted, it is possible, for example, to use a concentrated beam of electrons. The process according to the invention can be applied to the manufacture of austenitic and / or ferritic and / or austenoferritic alloys. It is also possible to apply the process according to the invention to the manufacture of catalysts.

The treatment is carried out by a laser beam which produces a treatment bead of width 1 on the sample and a scanning of the entire sample is carried out by juxtaposition at least in part of two successive cords, with a preferably equal covering mode. to a half bead, that is to say that between two passes the laser beam is offset by 1/2.

The invention will be better understood and its advantages will appear more clearly on reading some examples of implementation of the method.

Example 1:

To enrich a surface of mild steel plate with nickel by weight composition: 0.17% carbon, 0.5% manganese, 0.25% silicon, 0.04% sulfur, 0.03% phosphorus , the complement to 100% being iron, a gel of nickel formate is used. The gel is prepared as follows: 40 ml of a 30% by weight solution of formic acid are added to 100 ml of distilled water, then 60 g of nickel formate Ni are dissolved at 40 ° C. (CHO₂) ₂, 2H₂O, and finally 4 g of a polysaccharide hydrocolloid called guar so as to produce a very viscous solution. The viscosity at 20 ° C of the solution is equal to 500 mm² / s.

The gel obtained is then spread with a brush into a thin film of approximately 0.8 mm thick on a mild steel plate, then dehydrated, until the viscosity of the film is equal to 2000 mm² / s. .

In air, the sample is then subjected to irradiation with a YAG laser beam of 43 watts and having a laser-matter interaction time of 0.05 seconds, the interaction time being defined as being the ratio of the beam diameter at beam scanning speed. The diameter of the laser spot being 400 micrometers, the irradiation power is 3.4.10⁴ W / cm². This treatment is carried out by this laser beam which produces a treatment bead 400 mm wide on the sample and the scanning of the sample is carried out with a covering mode equal to half a bead. Between two passes, the laser beam is therefore offset by 200 mm.

After this treatment and after cooling, it is observed by micrography and X-ray diffraction analysis that a layer has been formed, the thickness of which varies between 20 and 22 micrometers, consisting of a homogeneous alloy of iron crystallized in gamma phase and nickel (γFe-Ni). This alloy, with a nickel content substantially equal to 70% by weight, does not contain nickel and iron in the oxidized state.

Comparative example 2:

A solution of nickel cyanide is prepared by dissolving an amount of freshly precipitated nickel cyanide containing 5 g of nickel in 80 ml of 3N ammonia. 4 g of a polysaccharide hydrocolloid called guar are added to this solution so as to produce a solution with a viscosity of 550 mm² / s.

The gel thus obtained is spread with a brush into a thin film of approximately 0.8 mm thick on a sheet of mild steel and then dehydrated until the viscosity of the film is equal to 2000 mm² / s.

In air, the sample is then subjected to irradiation by a YAG laser beam of 43 watts and having a laser material interaction time of 0.05 seconds. The irradiation power is 3.4 × 10⁴ W / cm².

After this treatment, it is observed by micrography and X-ray diffraction that a layer has been formed, the thickness of which varies between 12 and 14 micrometers, consisting of a homogeneous alloy of iron and nickel (60% nickel and 40% of iron) corresponding to only 25% of the nickel theoretically deposited, most of the nickel is in the oxidized state.

Example 3:

A mild steel plate from Example 1 having undergone the treatment of Example 1 is taken up and a second deposition is repeated under the same conditions of Example 1. Then, the substrate thus coated being in air, it is irradiated with a 43 watt laser having a spot diameter of 400 micrometers for an interaction time of 0.045 seconds.

After natural cooling, it is found that a layer 35 to 38 micrometers thick has formed. This layer comprises two zones: the first of these zones or the most external to the part with an average thickness of 20 micrometers consists of a homogeneous alloy of iron in gamma phase and of nickel having a nickel content substantially equal to 90 % by weight, the second of these zones, underlying the first, with a thickness of about 16 micrometers is made of a homogeneous alloy of iron in gamma phase and nickel having a nickel content close to 70% in weight.

Comparative example 4:

A solution of chromium nitrate containing 8 g of chromium in 50 ml of water is prepared. 2.5 g of a polysaccharide hydrocolloid called Guar are added to this solution so as to produce a very viscous solution with a viscosity of 500 mm² / s.

The gel thus obtained is spread with a brush into a thin film about 0.8 mm thick on a mild steel plate and then dehydrated until the viscosity of the film is equal to 1600 mm² / s. In nitrogen, the sample is then subjected to irradiation by a 50-watt YAG laser beam with a laser-matter interaction time of 0.07 seconds with a spot of 400 micrometers in diameter.

After cooling, the formation of a surface layer of thickness substantially between 32 and 35 micrometers, consisting of iron and chromium, is observed. The proportion of chromium alloyed with iron in the form of an alloy relative to total chromium is only by 10%, the vast majority of chromium is in the oxide state.

Example 5:

A saline solution of chromium tartrate is prepared from a hydrated chromium oxide, freshly precipitated and containing 8 g of chromium dissolved in 50 ml of a 5 M solution of tartaric acid. The solution obtained is concentrated by evaporation until its viscosity measured with the Hoppler viscometer with falling ball (Standard DIN 53 015) is 700 mm² / s. The gel thus produced is spread in a film 0.6 mm thick on a sheet of mild steel. After partial dehydration, the viscosity is 1600 mm² / s.

The substrate thus coated, being in nitrogen, is subjected by scanning to irradiation, for 0.07 seconds, of a spot 400 micrometers in diameter generated by a laser of power of 50 watts. The specific power is therefore approximately 4 × 10⁴ W / cm².

After cooling, there is the formation of a surface layer of thickness substantially between 30 and 32 micrometers consisting of an alloy of iron and chromium, each comprising metals in the metallic state, and whose chromium content is equal to 40% by weight.

Example 6:

A solution of chromium salt is prepared by adding 5 g of tartaric acid (acting as complexing agent) to 100 ml of a solution of chromium acetate containing 5 g of chromium, then the volume is reduced to 8 ml. of this salt solution so that it is viscous. The dynamic viscosity of the deposited solution is 100 mm² / s at 20 ° C.

A mild steel plate from Example 1 is coated with a film 0.8 mm thick. After partial dehydration, a dynamic viscosity of 1100 mm² / s is reached, then the air is subjected to the substrate thus covered on irradiation with a spot 400 micrometers in diameter with a 50 watt laser beam scanning the surface at a speed such that the interaction time is 0.04 seconds.

After cooling, it is observed that a surface zone 30 micrometers thick has formed, consisting of an alloy of iron and chromium containing 90% by weight of chromium, the iron and the chromium being both in the form of metal. .

Example 7:

A solution of chromium acetate and nickel acetate is prepared by dissolving, in acetic acid, a coprecipitate of chromium and nickel hydroxide containing by weight twice as much chromium as nickel.

The coprecipitate containing 10 g of chromium and 5 g of nickel is dissolved in 100 ml of an aqueous solution containing 30 ml of pure acetic acid. To the acetate solution thus obtained, 5 g of tartaric acid are added and then 8 ml of ammonia at 28% by weight. Acetic acid and ammonia complex the acetate salts.

By evaporation, the volume of the solution is reduced to 10 ml so as to make it viscous, ie a viscosity at 20 ° C of 120 mm² / s. After partial dehydration, 900 mm² / s is reached.

A sheet of mild steel from Example 1 is covered with a film 0.4 mm thick with the thickened solution, then the film is irradiated in the air by scanning for 0.05 seconds with a spot 400 micrometers in diameter and 50 watts produced by a laser.

After cooling, we observe the formation of a surface zone 50 to 55 micrometers thick and consisting of an austenitic Fe-Cr-Ni alloy, with a nickel content by weight equal to 10% and chromium content equal to 20% .

Example 8:

A solution of chromium acetate and of nickel acetate is prepared, containing respectively 10 g of chromium and 3.2 g of nickel, by dissolving in 100 ml of aqueous solution containing 30 ml of pure acetic acid, a coprecipitate of chromium hydroxide and nickel. To this acetate solution are added: 10 ml of a solution of ammonium paramolybdate (NH₄) ₆ Mo₇O₂₄, 4H₂O (containing 1.2 g of molybdenum), then 7 g of tartaric acid. The saline solution produced is brought back by evaporation to a volume of 15 ml so as to increase its viscosity up to a value of 100 mm² / s. After partial dehydration, a viscosity at 20 ° C of 900 mm² / s is reached.

A sheet of mild steel from Example 1 is covered with a 0.4 mm film of this solution. It is irradiated under nitrogen by scanning for 0.05 seconds with a spot 400 micrometers in diameter and 50 watts produced by a laser.

After cooling, there is the formation of a surface zone with a thickness between 45 and 50 micrometers, consisting of an austenoferritic alloy with contents by weight of chromium equal to 25%, of nickel equal to 8%, of molybdenum equal at 3%.

Example 9:

A nickel acetate solution is prepared by dissolving, in 20 ml of water and 10 ml of glacial acetic acid at 96% by weight brought to the boil, 8 g of basic nickel carbonate which contains 4 g of nickel.

To this solution is added 4.5 g of acetic acid. The volume of this solution is reduced to 5 ml by evaporation, then to this solution which has become very viscous is added 1 g of powdered silicon carbide of dimension less than 4 micrometers. The silicon carbide is then dispersed within the liquid by mixing.

The solution obtained is spread with a brush on a mild steel plate of Example 1 in a thin film of about 0.8 mm, then is dehydrated. The dynamic viscosity at 20 ° C of the mixture thus deposited is equal to 1100 mm² / s.

In air, the sample is then subjected to irradiation by a 43-watt YAG laser beam for a laser-matter interaction time of 0.03 seconds, the interaction time being defined as the ratio of the diameter. of the beam at the scanning speed. The diameter of the laser spot being 400 micrometers, the irradiation power is 4.3.10⁴ W / cm².

After this treatment and air cooling, it is observed by micrography that a layer 25 to 30 micrometers thick has been formed consisting of nickel in the metallic state in which the particles of silicon carbide are dispersed. represent approximately 25% by weight of the coating thus formed.

Example 10:

A solution of chromium tartrate is prepared by dissolving hydrated chromium oxide freshly precipitated in tartaric acid. This solution is then concentrated so as to obtain a chromium concentration of 400 g per liter. To 10 ml of this solution, 1 g of zirconium oxide (ZrO₂) in powder with a particle size less than 3 micrometers is added, while kneading.

The viscous solution thus obtained is then spread in a film 0.6 mm thick on a mild steel plate, then is partially dehydrated.

In air, the sample is then subjected to irradiation, for an interaction time of 0.05 seconds, of a spot 400 micrometers in diameter produced by a YAG laser beam of 43 watts.

After this treatment and after cooling in air, it is observed by micrography and X-ray diffraction, that a layer has been formed, the thickness of which varies from 30 to 35 micrometers, consisting of a homogeneous alloy of iron and chromium of chromium content substantially equal to 50% and in which the ZrO₂ particles are dispersed homogeneously and represent approximately 10% by weight of coating thus prepared.

Claims (15)

1. Method for surface modification of the surface of a metal substrate containing largely iron by forming an alloy with at least one chemical element known as the addition element, by means of a concentrated energy beam such as a laser beam, characterized by at least one treatment comprising the following stages being carried out:

a) said surface of the substrate is coated with a film containing at least one salt of a carboxylic acid chosen from the aliphatic and aromatic carboxylic acids and the hydroxy-substituted aliphatic and aromatic carboxylic acids, said carboxylic acid salt having at least the element which will form said alloy, said film having reducing capacities in the presence of said energy beam;

b) said surface so coated is irradiated with said beam, with the parameters of this irradiation being adjusted such that a substantial thickness of the immediately underlying substrate melts and such that said element coming from the decomposition of said salt and said molten part of the substrate mix with each other.

c) The alloy thus obtained is resolidified.

2. Method according to claim 1, characterized by said addition element being at least one metal chosen from the group formed of the following elements: Si, Mg, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sn, La, W, Re, Os, Ir, Pt, Au, Bi, Ce.

3. Method according to claims 1 or 2 characterized by said film containing at last two addition elements in the form of at least one salt in order to form an alloy having at least the two addition elements.

4. Method according to one of claims 1 to 3 characterized by said film having at least one complexing agent.

5. Method according to one of claims 1 to 4 characterized by the proportions of oxygen, carbon, and hydrogen atoms present in the film being such that the quantity of oxygen atoms is less than the sum of half the quantity of the hydrogen atoms and double the quantity of carbon atoms.

6. Method according to one of claims 1 to 5 characterized by the salt being a carboxylic acid salt chosen from the group formed by formic, acetic, propionic, lactic, salicylic, malic, oxalic, citric, and tartaric acids.

7. Method according to one of claims 1 to 6 characterized in that, when said substrate is coated with said film, at least part of said salt is in liquid solution.

8. Method according to one of claims 1 to 7 characterized in that, when said substrate is coated with said film, at least part of said salt is in the powder form.

9. Method according to one of claims 1 to 8 characterized in that, after said surface of said substrate has been coated with said film containing said salt having at least said addition element, and before said salt has been decomposed, the film containing said salt is concentrated by evaporation.

10. Method according to one of claims 1 to 9 characterized by said film having a viscosity higher than 400 mm²/s and preferably between 800 and 10⁶ mm²/s before being irradiated.

11. Method according to one of claims 1 to 10, characterized in that the substrate is a mild steel and in that the addition element is at least one metal chosen form the group formed by nickel, chromium and molybdenum.

12. Method according to one of claims 1 to 11 characterized in that the surface of the substrate is coated with said film containing at least said salt and at least one non-melting solid compound.

13. Use of the process according at one of claims 1 to 12 to manufacturing surface alloy which contains the addition element reduced to oxidation state zero.

14. Use of the process according to one of claims 1 to 13 to manufacturing austenitic and/or ferritic and/or austenoferritic alloys.

15. Use of the process according to one of claims 1 to 14 to manufacturing catalysts.