WO2001002616A1 - Improved composition for surface coating - Google Patents

Abstract

The invention concerns a surface coating composition, in particular for heat-exchanger tubes in incineration installations, designed to operate in a high temperature oxidising environment (A). Said composition comprises in major part iron for crystallising along a centred cubic structure which has a high atomic diffusion speed. It comprises chromium (Cr) capable of migrating to the surface (CS) of the coating to combine with oxygen (O) and form a oxide layer (1) preventing oxygen from entering the coating volume (MV). Said oxide layer (1) is permeable to molybdenum (Mo) and niobium (Nb) which is contained in the inventive composition, said species being then capable of migrating to the surface (CS) and form with the chlorine (C1) in the environment (A) a surface layer (2) of chlorides preventing chlorine from entering the volume. Such a coating provides efficient protection against corrosion.

Description

Improved surface coating composition

The invention relates to a surface coating composition, in particular for supporting a corrosive environment of high temperature.

A composition of this type finds an interes-health application in the coating of heat exchanger tubes, in particular in power plants (boilers, waste incinerators, coal plants, biomass treatments, etc.).

Such heat exchangers are intended to operate in a corrosive atmosphere, capable of degrading the surface of the exchangers. The composition of the coating must be chosen to prevent any infiltration of corrosive elements such as oxygen, chlorine or even sulfur, through the heat exchange surfaces.

A composition may then be provided comprising an atomic species capable of spontaneously combining with oxygen in the atmosphere to form, on the surface of the coating, a thin layer of oxide which traps the oxygen. Thus, the coating acts as a barrier to oxygen. In addition, it protects itself from volume oxidation.

However, this oxide layer, to be effective, must be sufficiently dense, and the abovementioned atomic species must migrate rapidly towards the surface of the coating to form the surface layer.

Furthermore, although such an oxide layer constitutes a satisfactory barrier against oxygen, other substances present in the atmosphere can, on the other hand, pass through it and chemically attack the coating, or even the surfaces of the exchanger. .

The present invention improves the situation. To this end, it proposes a coating composition comprising, according to a general definition of the invention, a matrix formed:

- mainly of a first species, metallic, chosen to crystallize according to a structure allowing rapid atomic migration in the matrix,

- as well as a second and at least a third species, suitable for combining with one or more substances present in the aforementioned atmosphere, and chosen to spontaneously form, on contact with the environment, a surface complex comprising at least one inner thin layer and one outer thin layer,

the external layer being substantially impermeable to at least one substance present in the environment, capable of corroding the internal layer, while being permeable to the oxygen present in the environment, and

- The internal layer being an oxide layer, substantially impermeable to oxygen, while being permeable to the species generating the external layer.

According to an advantageous optional feature of the invention, the matrix also comprises a fourth species capable of combining with oxygen, in contact with the atmosphere, and chosen to form, in the surface complex, a second internal layer d oxide, promoting the formation of at least one of the internal or external layers in the complex.

Preferably, it also comprises at least one atomic element chosen from the group comprising yttrium and rare earths, capable of promoting the adhesion of the surface complex to the matrix.

Such a composition then makes it possible to obtain a surface coating material, mechanically robust and effective against any infiltration of oxygen and / or other substances of the environment, chemically active. As such, the present invention also relates to such a coating material, as well as a heat exchanger tube, coated with this material. Other characteristics and advantages of the invention will appear on examining the detailed description below, and the attached drawings in which:

- Figure 1 shows a surface complex of a coating comprising a composition according to the invention;

- Figure 2 schematically shows a sectional view of the coating on the surface of a tube T of a heat exchanger;

FIG. 3 graphically represents the variation in mass losses per surface, measured on a coating material according to the invention, immersed in a corrosive atmosphere between 400 ° C. and 700 ° C. for approximately 200 hours, as a function of the weight percentages of niobium Nb and molybdenum Mo which it contains;

- Figures 4A and 4B schematically show sectional views of heat exchangers carrying a coating according to the invention and produced in different shapes; and

- Figure 5 shows schematically and on a macroscopic scale the appearance of a MAT coating comprising a composition according to the invention.

The drawings essentially contain elements of a certain character. They can not only serve to better understand the detailed description below, but also contribute to the definition of the invention, if necessary.

In what follows, a surface coating material, in particular of tubes for heat exchanger, is described by way of non-limiting example in power plants (boilers, waste incinerators, etc.). In the atmospheres of power plants (for example by burning waste), the following types of corrosion have been observed:

- corrosion by gases;

- corrosion induced by deposits; and

- corrosion by molten salts.

In an oxidizing environment, the corrosive gases are mainly sulfur oxide SO2, hydrogen chloride HCl and / or chlorine CI2 in gaseous forms. The mechanism initiated by the reaction of CI2 (or HCl) gas with the metal components of one heat exchanger, leads to the formation of chlorides, for example iron chloride FeCl2 which has a gas phase at temperatures around 500 ° C, to which the tubes T of such an exchanger are generally exposed.

In addition, the thermodynamic conditions (temperature and pressure) are favorable in the vicinity of the tubes so that such chlorides combine with ambient oxygen to be oxidized according to one of the following reactions:

3 FeCl ₂ + 2 0 ₂ <=> Fe ₃ 0 ₄ + 3 Cl ₂ , and / or

2 FeCl ₂ + 3/2 0 ₂ <=> Fe ₂ 0 ₃ + 2 Cl ₂

By producing chlorine again in gaseous form, it is maintained a cycle of corrosion by chlorine and by oxygen, called "active oxidation".

Such heat exchangers are intended to operate in a corrosive atmosphere at high temperature. It is therefore advisable to protect their surface from any penetration of corrosive elements such as oxygen, chlorine, etc. The MV volume matrix of a coating material composition according to the invention mainly comprises iron Fe, which corresponds to the first metallic species mentioned above. Preferably, the composition comprises 50 to 90% iron (percentages of the total weight), for example about 60 to 70% iron.

In addition, this matrix typically comprises from 10 to 50% of chromium Cr, preferably approximately 23 to 29% of chromium. In the example described, chromium Cr corresponds to the second aforementioned species of the composition according to the invention.

The matrix also comprises molybdenum Mo which corresponds, in the example described, a. the third aforementioned species of the composition according to the invention. The composition contains between 0.5 and 10% molybdenum, preferably about 3 to 6% molybdenum.

According to one of the advantages which the present invention provides, the majority proportion of iron Fe (greater than 50%) contributes to crystallizing the matrix according to a crystallographic structure of centered cubic type.

Compared to other types of structures known in the field of heat exchanger coatings, such as a cubic structure with centered faces of the austenite type in which nickel in particular crystallizes, atomic diffusion is advantageously faster in the structure matrix centered cubic of the invention. In addition, such nickel-based alloys are of limited industrial application due to their high cost.

The tubes 1 of the heat exchanger are usually made of carbon steel, which has the ferritic crystallographic structure, of centered cubic type. According to another advantage which the present invention provides, the crystallographic structure in which the composition crystallizes gives the coating material good thermo-mechanical adaptation to the ST interface between the volume material MV and the tubes T (Figure 2).

The Applicant has found that a matrix having such a crystallographic structure promotes the migration of chromium Cr and molybdenum Mo atoms, in particular at the surface of the coating in a corrosive atmosphere at high temperature. Thus, the chromium atoms can migrate rapidly to the surface of the coating (arrow Cr in Figure 1), to combine with oxygen O present in the atmosphere. The molybdenum atoms Mo can also migrate rapidly towards the surface of the coating (arrow Mo) to combine, if necessary, with chlorine Cl present in the atmosphere. Referring to FIG. 1, a surface complex CS is formed, in contact with the atmosphere A, comprising a layer of oxides 1 and a layer of chlorides 2.

Advantageously, the composition according to the invention also comprises between 0.1 and 5% of niobium, preferably about 0.5 to 1.5% of niobium. The niobium atoms migrate to the surface of the coating to combine, with molybdenum, with chlorine from atmosphere A. The presence of niobium has the following advantages:

- It helps to trap atmospheric carbon at the surface, by forming niobium carbide, a compound more stable than chromium carbide, which makes it possible to make chromium available for the formation of the abovementioned oxide layer; and

- its association with molybdenum gives a synergistic effect for the formation of the chloride layer on the surface of the coating. Referring in fact to FIG. 3 representing the variation in mass losses per surface Dm / S, measured on a material in a corrosive atmosphere of high temperature, and comprising niobium Nb and molybdenum Mo, it is noted that the slope of the corrosion as a function of the proportion of molybdenum is notably modified according to the percentage of niobium that the composition comprises. It should be noted that tungsten can also be provided, in addition to or in substitution for these two species, to stabilize the layer of chlorides formed.

In the example shown in Figure 1, the surface complex CS has two thin layers: an inner layer 1 and an outer layer 2. The inner layer 1 is that formed by the combination of chromium atoms Cr to oxygen atoms of atmosphere A. It additionally contains chromium oxide Cr2C> 3. This thin layer of oxide

1 then traps the oxygen atoms O and thus prevents oxidation, in volume, of the underlying material MV. On the other hand, this oxide layer is permeable to molybdenum Mo, niobium Nb and / or tungsten. These species, for their part, form the outer thin layer 2, by combining with the chlorine Cl present in the atmosphere A. This layer of chlorides 2 traps the chlorine in the atmosphere A, while being relatively permeable to oxygen. O.

In the application to the coating of the targeted heat exchangers, the heat exchangers operate in an environment comprising sulfur oxide SO2, hydrochloric acid in gaseous form HCl, air at a temperature in the region of 1000 ° C. , as well as incineration ash containing minerals such as silica, iron oxides, zinc, lead, etc. Chlorine, oxygen and sulfur attack the coating of the heat exchanger, while the incineration ash, projected on the tubes, mechanically attack their surface.

The Applicant has found that the formation of the surface complex CS constitutes a particularly effective protection of the tubes of the exchanger against the active oxidation described above. According to tests carried out by the Applicant in a corrosive atmosphere comprising gaseous hydrochloric acid and sulfur oxide in air between 400 ° C and 700 ° C, the coatings comprising a composition of type described above undergo corrosion, removing on average 1.5 milligrams / cm ² in 200 hours (Figure 3).

Advantageously, the composition according to the invention also comprises between 1 and 5% of silicon, preferably about 2 to 4% of silicon Si. The silicon atoms Si leave the matrix M to migrate towards the surface of the coating by combining to oxygen atoms O, and form a layer of silica SiC> 2. This layer of silica advantageously promotes the transit of chromium to the surface complex CS. Referring to FIG. 2, the silica layer 3 is located under the chromium oxide layer 1. It also makes it possible to trap the oxygen atoms liable to escape from the layer 1 and thus protects the material under -jacent to oxidation. In substitution or in addition to silicon, aluminum may also be provided in similar proportions in the composition according to the invention.

The composition according to the invention also comprises yttrium Y and / or rare earths, in proportions of between 0.01 and 0.5%, preferably between 0.05% and 0.15% approximately. These elements migrate towards the surface of the coating and form, at the interface between the volume material MV and the surface complex CS, a thin adaptation layer 4, which in particular makes it possible to prevent the surface complex from blistering.

Thus, the overall structure of the surface complex CS, starting from the coating material in volume MV, towards atmosphere A, comprises:

- A thin adaptation layer 4 of yttrium and / or rare earths promoting the adhesion of the complex CS to the material in volume MV. Its thickness is typically close to a few tens of Angstroems (a few atomic layers);

a thin layer 3 of Siθ2 silica (and / or of Al2O3 alumina), typically of the order of a tenth of a micron thick, which promotes the migration of chromium Cr atoms to the CS complex and traps oxygen capable of penetrating in volume MV;

- A layer of chromium oxide C ^ C ^, typically of the order of ten microns thick, capable of trapping oxygen O; and

a surface layer 2 of chlorides, typically with a thickness of the order of a tenth of a micron, and formed by the combination of chlorine from atmosphere A with atoms of the matrix M such as molybdenum, niobium , and, if necessary, tungsten.

It should be noted that these thin layers 1, 2, 3, 4 form spontaneously in contact with the atmosphere A. Of course, the thickness values of the thin layers of the surface complex CS are given above according to means d thicknesses measured (in practice by scanning electron microscopy) on a plurality of samples. In practice, these thin layers are not regular, certain layers interpenetrating locally into one another. The term "surface complex" is therefore understood to mean an irregular stacking of these thin layers.

Furthermore, the compositions given above have been checked by the Applicant by X-ray diffraction techniques, which can be supplemented, if necessary, by luminescent discharge spectroscopy analyzes. These composition values are, of course, averages measured in different samples of coating materials.

Referring to FIG. 5, the coating material of the tubes T is in the form of an agglomeration MAT of fine grains G, on the surface of the tubes T. Such a grain structure (generally generating defects) advantageously contributes to further increase the atomic migration rates towards the surface of the MAT coating. Initially, the crystal resulting from the mixture based on iron and comprising chromium, molybdenum, niobium, etc., is mechanically fragile and breaks easily. The tubes T of a heat exchanger cannot therefore be designed in this alloy, and the material must be applied in the form of coating of these tubes. It is generally in the form of a fine grain powder, which should be sprayed onto the surface of the T tubes.

It can then be provided to spray such a powder onto the tube T from a plasma torch, or from any other powder spraying technique. The Applicant has however noted that the addition of a proportion of nickel Ni to the composition according to the invention, of approximately 2 to 4%, makes it possible to make the mixture obtained malleable. Thus, the coating material can be cast in the form of wires FI, F2 (FIG. 5), these wires being able to be deformed without breaking.

Two wires are then provided, the ends of which are preferably profiled in the form of points (wires FI and F2 in FIG. 5), substantially facing the tube T to be coated. It is then planned to circulate a current i in one of the wires to create an electric arc ARC between the wires F1 and F2. Finally, a compressed air blower projects the grains G that this arc generates on the surface of the tube T. The coating obtained is in the form of a MAT material which has the appearance of an agglomerate of fine grains .

A coating material is thus obtained comprising a composition according to the invention. As such, the present invention also relates to a method comprising steps as described above, of coating with this material (Figure 5).

Referring to FIG. 4A, the tubes T of an exchanger can be coated over their entire surface with a MAT material according to the invention, sprayed by a method of the above type. In the variant shown in FIG. 4B, it can be provided for putting the coating material in the form of a TO sheet and then pressing it onto one face of the exchanger, for example by surface welding of the TO coating and / or of the T tubes.

In another variant, provision may be made to produce composite tubes from a carbon steel preform and comprising the coating material, according to a powder metallurgy process of the type comprising a step of stretching T tubes.

Of course, the present invention is not limited to the embodiment described above; it extends to other variants.

Thus, it will be understood that the proportions of the species in the composition described above are likely to vary. However, the proportion of iron in the composition must be greater than 50%, which makes it possible to obtain an alloy whose matrix crystallizes according to a structure of centered cubic type. Advantageously, this structure allows rapid atomic migration in the matrix, as well as good adaptation to the ferritic structure of the tubes. Thus, it will be understood that in a variant of the composition according to the invention, the latter may comprise a majority species different from iron, but which crystallizes according to a structure which allows rapid atomic diffusion.

The proportions of chromium between 23 and 29% are described above by way of example. It is however desired to have at least 10% chromium in the composition of the coating intended to be used in an incinerator. Ultimately, the higher the proportion of chromium, the more effective the coating material will be. However, the addition of chromium tends to weaken the coating mechanically. This is why the optimal proportions of chromium are close to 25%.

It will also be understood that the composition of the material comprising niobium and molybdenum is described above by way of example. These two species can be replaced (and / or supplemented) with tungsten and aluminum, which are also likely to migrate to the surface of the material to form a surface layer of chlorides. However, molybdenum in the proportions of 3 to 6% described above, in combination with niobium in the proportions of 0.5 to 1.5%, quickly forms this surface layer of chlorides, according to a synergistic effect that has noted the Applicant.

In the example described above, the chlorine in atmosphere A helps maintain the corrosion mechanism of the walls of the incinerator. In another application, another substance from atmosphere A can participate in this corrosion. In the composition according to the invention, atomic species capable of migrating towards the surface of the coating to combine with this substance may be provided, in order to prevent corrosion in volume of the underlying coating.

The formation of the silica layer, under the chromium oxide layer, is, although advantageous, described above by way of example. It promotes the transit of chromium atoms to the surface of the material. However, in a simplified variant of the composition according to the invention, the incorporation of silicon can be omitted.

In addition, the addition of yttrium or rare earths to the composition according to the invention is described above by way of example. Although advantageous, this addition in a simplified variant of the composition according to the invention may not be envisaged.

The incorporation of nickel into the composition described above makes the coating material obtained malleable. In a variant according to which it is desired to obtain the coating material in particular in the form of a grain powder intended to be sprayed onto the surface to be coated, this incorporation of nickel is not necessary.

In the example described above, the material comprising the composition according to the invention is used as the material for coating the heat exchanger tubes, in particular in a boiler, an incinerator or, in general, in a power plant. Of course, such a material can also be used as a coating for any other surface. It may be intended to coat materials other than carbon steel, for example ceramics or the like. Furthermore, the general shapes of the tubes of the above heat exchangers are cylindrical in the examples shown. Alternatively, they can be flat.

Claims

claims 1. Composition for surface coating, in particular for supporting a corrosive atmosphere (A) of high temperature, characterized in that it comprises a matrix (MV) comprising: - mainly a first, metallic species, chosen to crystallize according to a structure allowing rapid atomic migration in said matrix (MV), - as well as a second and at least a third species, suitable for combining with one or more substances present in said environment, and chosen to spontaneously form, in contact with the environment, a surface complex (CS) comprising at least one inner thin layer (1) and one outer thin layer (2), - the external layer (2) being substantially impermeable to at least one substance present in the environment, capable of corroding the internal layer, while being permeable to the oxygen present in the environment, and - the internal layer (1) being an oxide layer, substantially impermeable to oxygen, while being permeable to the species generating said external layer (2). 2. Composition according to claim 1, characterized in that the first species is iron (Fe), while the general crystallographic structure of the matrix is of centered cubic type. 3. Composition according to claim 2, characterized in that it comprises between 40 and 90% of iron (percentages of the total weight), preferably, about 60 to 70% of iron. 4. Composition according to one of claims 1 to 3, characterized in that the second species is chromium (Cr), while the inner layer (1) comprises chromium oxide. 5. Composition according to claim 4, characterized in that it comprises between 10 and 50% of chromium, preferably, approximately 23 to 29% of chromium. 6. Composition according to one of claims 1 to 5, characterized in that the third species is at least one atomic element chosen from the group comprising niobium (Nb), molybdenum (Mo) and tungsten, while the layer external is impermeable to chlorine (Cl) present in the environment. 7. Composition according to claim 6, characterized in that it comprises between 0.5 and 10% of molybdenum, preferably about 3 to 6% of molybdenum. 8. Composition according to one of claims 6 and 7, characterized in that it comprises between 0.1 and 5% of nobelium, preferably, about 0.5 to 1.5% of niobium. 9. Composition according to one of claims 6 to 8, characterized in that it comprises two atomic elements chosen from the group comprising niobium (Nb), molybdenum (Mo) and tungsten, in proportions chosen to form said outer layer (2), according to a synergistic effect between said two elements (Nb, Mo). 10. Composition according to one of the preceding claims, characterized in that the matrix also comprises a fourth species capable of combining with oxygen, in contact with the atmosphere (A), and chosen to form, in said complex (CS), a second internal oxide layer (3), promoting migration of at least one of the second and third species, from the matrix (MV) to the complex (CS). 11. Composition according to claim 10, characterized in that the fourth species is at least one atomic element chosen from the group comprising silicon and aluminum. 12. Composition according to claim 11, characterized in that the composition comprises between 1 and 5% of silicon, preferably, approximately 2 to 4% of silicon, while said second internal layer (3) comprises of silica. 13. Composition according to one of the preceding claims, characterized in that it also comprises at least one atomic element chosen from the group comprising yttrium and rare earths, capable of forming a thin layer (4) favoring the adhesion of the surface complex (CS) to the matrix (MV). 14. Composition according to claim 13, characterized in that it comprises between 0.01 and 0.5% of yttrium and / or rare earths, preferably, approximately 0.05 to 0.15% of yttrium and / or rare earths. 15. Material of a surface coating, characterized in that it comprises a composition according to one of the preceding claims. 16. Material according to claim 15, characterized in that it is produced in the form of an agglomerate (MAT) of fine grains (G). 17. Heat exchange surface, characterized in that it is coated with a material (MAT) according to one of claims 15 and 16.