DE69424933T2 - METHOD FOR PRODUCING A METAL PART COATED WITH INORGANIC MATERIALS, PRODUCED PART AND ITS USE. - Google Patents

Abstract

Method of manufacture of a metal part (1) coated with several layers (4, 5, 6) of mineral materials, said method allowing some oxidation of the metal part's surface. The invention also concerns a hollow revolving part manufactured according to the above-mentioned method. The manufacture of the hollow revolving part comprises the following stages wherein a liquid casting is subjected to: a treatment using magnesium, an inoculation process and a centrifugal operation during which the first layer (4) of the coating (2) is applied to the metal part. The metal part of the invention is for use in the transport of aggressive fluids.

Description

The present invention relates to the manufacture of a metal pipe coated with inorganic materials. The invention particularly relates to the manufacture of a metal pipe made of ductile cast iron coated with at least two layers forming an inorganic coating based on silica and sodium oxide.

The manufacture of cast iron objects coated with glass is known (see, for example, the publication FR-A-2 495 190).

The inorganic coating provides effective protection for the metal part and has good chemical resistance to fluids that come into contact with it.

From the document DE-A-27 39 919 a process for enamelling a metallic surface by applying two powdered enamel layers is known. The first layer is applied at a temperature of at least 250°C to a support which has previously been treated with nickel, cobalt or manganese or several of these elements. This temperature is not specific to the temperature of a hot metal. In addition, this document contains no teaching regarding the thickness of the enamel with which the object according to the invention can be obtained.

The document US-A-3 484 266 also concerns the coating of steel pipes by enamelling. The problem to be solved in this document is, in general terms, to apply a second layer without damaging the surface of the first layer. The coating of a pipe made of cast iron is not addressed, nor is the connection (integration) of the process steps associated with the application of the coating and those associated with the manufacture of the pipe.

The object of the invention is to produce a metal pipe, its coating, which protects the metal pipe and chemically inert, adheres to the carrier and has few open pores, thus forming a passive barrier between the fluid and the carrier.

According to the invention, the process for manufacturing a metal pipe coated with at least two layers of silica and sodium oxide-based inorganic materials is a process in which three layers of an inorganic coating are applied one after the other to at least part of the surface of the metal pipe to be treated, which is a ductile cast iron pipe, which pipe has a temperature above 700°C, the constituents of the inorganic material being present in different proportions depending on the layer, the production of the first layer of the coating being carried out by applying the powdered material, which layer consists of an enamel having the following composition, expressed as a percentage by weight

SiO₂ 50 to 70%

CaO + MgO 5 to 20%

B₂O₃ less than 10%

CoO less than 2%

Fe₂O₃ less than 5%

Al₂O₃ less than 10%

Na2O + K2O 15 to 25%

F₂ less than 2

NiO less than 2%,

and is produced in a thickness of less than 200 µm and wherein the production of the second layer of the coating is carried out on a heated pipe by applying the powdery material and on a pipe which is at room temperature by applying a slip of this material, this second layer being produced in a thickness of more than 100 µm.

Several layers of the inorganic coating are applied one after the other to at least part of the surface of the metal pipe in order to reduce the porosity of the inorganic coating. to reduce their adhesion to the metallic substrate and to provide a very tight coating, thereby protecting the substrate from corrosion.

The first layer of the coating must cover the substrate sufficiently to form an anti-oxidation coating and must be thin enough to allow for degassing and the escape of gas bubbles formed during the reaction of the first layer with the metal. In addition, the first layer must be made of a material that has the required fluidity.

When producing the first layer of the coating, the flowability of the inorganic material must be adjusted so that the fusion of the powder is ensured, which is necessary for the formation of a covering coating and its adhesion to the carrier, and that chemical reactions between the carrier and the first layer of the coating only take place to a limited extent.

With a reducing carrier, e.g. a carrier made of cast iron, and an oxidizing first layer of the coating, there is a risk of a redox reaction with gas formation and the formation of bubbles.

A composition with too high a flowability can cause the coating material to flow away with the resulting disadvantages: the carrier is no longer completely covered and oxidation of the carrier occurs.

A composition with insufficient fluidity prevents good fusion of the powder, resulting in poor adhesion and poor coverage, thus risking oxidation of the support.

The oxidation of the carrier is particularly pronounced during heat treatments at high temperatures.

The flowability of the composition depends to a large extent on the silica content and the content of aggregates.

To limit redox reactions between the coating and the carrier, the inorganic material is reduced during its production.

The first layer of the coating is made in a thickness of 100 µm to allow the powder to fuse, which is necessary for good coverage and high adhesion.

To apply the inorganic material to a hot part, the second layer of the coating is produced by applying the inorganic material in the form of a dry powder.

The second layer of the coating, consisting of inorganic material, is produced by applying an inorganic material, which is an enamel with the following composition, expressed as a percentage by weight, in order to absorb the first layer of the coating, to dissolve the skin of iron oxides that forms on the surface of the metallic support when it is made of a ferrous metal, and to attack the surface of the support:

SiO₂ 40 to 60%

CaO + MgO less than 5%

Li₂O below 5%

TiO₂ 5 to 15%

NiO below 2%

ZnO below 2%

Al₂O₃ below 10%

Na2O + K2O 15 to 20%

B₂O₃ 5 to 15%

CoO below 2%

Sb₂O₃ below 2%.

The second layer of the coating is made in a thickness of more than 100 µm to achieve good adhesion of the coating by absorption, dissolution and surface attack as described above.

The second layer of the coating is produced in a thickness of 200 μm to avoid excessive porosity of this layer.

In order to ensure a tight-fitting coating and to obtain a coating with as uniform a thickness as possible, thereby limiting the number of open pores and giving the coating a smooth surface, a coating is created on the surface of the turned part by applying three layers of inorganic materials, the material forming the third layer being applied in the form of a powder or a slip.

The third layer is made by applying an enamel with the following composition, expressed in weight percent:

SiO₂ 40 to 60%,

CaO + MgO less than 5%,

Li₂O below 5%,

TiO₂ 5 to 15%,

NiO below 2%,

ZnO less than 2%,

Al₂O₃ less than 10%,

Na2O + K2O 15 to 20%,

B₂O₃ 5 to 15%,

CoO below 2%,

Sb₂O₃ below 2%.

This composition, which can be the same as the second layer or can be can differ from this composition, makes it possible to cover the previously described layers with a thin, covering coating.

The third layer is made with a thickness of more than 100 um to ensure tight sealing and coverage.

The third layer is produced with a thickness of 200 µm to achieve a good fusion of the coating.

The manufacture of tubular parts made of cast iron coated with glass is known (see, for example, the applicant's document FR-2 297 817).

The production of metallic turned parts according to the invention is carried out according to a process which comprises at least the following sequential steps:

- Treatment with magnesium,

- Inoculation,

- Centrifugal casting of the cast iron, during which process step the first layer of the coating, as defined above, is produced.

This process allows the coating to be applied during the production of the metal turned part and thus the use of the thermal energy of the metal for the melting of the first layer.

Combining the process steps involved in applying the coating with the process steps for producing the metal turned part makes it possible to reduce the number of production steps and thus make the production process cheaper.

The invention also relates to a hollow metal turned part which is coated on the inside of its surface with inorganic materials, said part being manufactured according to the method described above.

Aggressive fluids can be transported through this part without the properties of these fluids being impaired or the hollow rotating part being damaged.

The invention also relates to the use of the part produced by the above method for passing aggressive fluids.

The present invention is illustrated by means of an embodiment relating to a structural element made of ductile cast iron of a piping system for the supply of drinking water or the disposal of waste water, which is illustrated by the accompanying figures, of which

- Fig. 1 shows a structural element of a pipeline system produced according to the method according to the invention in a perspective view,

- Fig. 2 a micrograph (100x magnification) of the enamel coating together with the edge area of the cast iron support of the structural element of the piping system shown in Fig. 1.

In this example for the manufacture of a structural element of a piping system 1 having a coating 2 on the inside, the liquid cast iron is subjected to a treatment with magnesium and then to inoculation. The inoculation of the cast iron is carried out in a known manner with an inoculation material such as ferrosilicon, which is introduced into the liquid cast iron in powder form or as a wire, see for example the applicant's document FR-A-2 546 783.

The method includes a step in which the cast iron is centrifugally cast. During the centrifugal casting, the first layer 4 of the coating 2 is produced in a thickness of 100 > µm by applying a reduced enamel with the following composition:

SiO2 balance

CaO + MgO 10%

B₂O₃ 5%

CoO2 1%

Fe2O3 2%

Al₂O₃ 5%

Na₂O + K₂O 22%

F2 1%

NiO2 1%

This enamel is applied as a dry powder.

After centrifugal casting, the structural element of the piping system 1 is graphitized. The second layer 5 of the coating 2 is then produced in a thickness of 200 µm by applying a powdered enamel having the following composition, given in weight percent:

SiO2 balance

CaO + MgO 0.5%

Li₂O 4%

TiO2 6.5%

NiO2 0.5%

0.5%

Al₂O₃ 6%

Na₂O + K₂O 19%

B₂O₃ 11%

CoO2 1.5%

Sb₂O₃ 1%

The second layer 5 of the coating 2 is applied to the structural element of the piping system 1 at a temperature of 800 to 700ºC. Subsequently, ferrite formation takes place in the structural element of the piping system 1 in a temperature range below 800ºC, during which the second layer 5 of the coating 2 is baked. The third layer 6 of the coating 2 is then produced in a thickness of 200 µm by applying a powdered enamel whose composition corresponds to the composition of the above-mentioned enamel of the second layer 5 of the coating 2. This powder is applied to the structural element of the piping system 1 at a temperature below 800ºC.

After application of the third layer 6, glazing of the third layer is carried out, i.e. firing for a period of less than 5 minutes at a temperature of 750 to 700ºC.

According to a first modification, the cast iron obtained after centrifugal casting is pearlitic gray cast iron with spheroidal graphite. The process then differs from the previously described process in that no graphitization of the structural element of the piping system 1 is carried out.

Under these conditions, it is possible to produce the second layer 5 of the coating 2 during centrifugal casting or subsequently by applying the enamel in the form of a dry powder.

The structural element of the piping system is subsequently subjected to a treatment for ferrite formation in a temperature range below 800°C, and the firing of the second layer 5 of the coating 2 takes place simultaneously with the ferrite formation in the structural element of the piping system 5.

According to a second modification, the cast iron obtained after centrifugal casting is ferritic gray cast iron with spheroidal graphite. The process differs from the process of the first modification in that no ferrite formation takes place in the structural element of the piping system 1 and that the firing of the second layer 5 of the coating 2 at a temperature of 800 to 700 °C of the construction element 1 takes place after the application of the second layer 5 of the coating 2.

According to a third variation, the third enamel layer is applied in the form of a slip, the temperature of the structural element of the piping system 1 corresponding to the ambient temperature.

The structural element of the piping system 1, which can be manufactured according to the method according to the invention, including the three modifications described above, is a pipe made of ductile cast iron 3, which has an enamel coating 2 on the inside of its surface, which consists of three layers 4, 5, 6. This pipe has a simple or smooth end 8 at one end and a socket 7 at the other end, which can receive the smooth or simple end of another pipe of the same construction.

The micrograph of the enamel coating 2 in the edge area of the cast iron carrier (Fig. 2) shows the carrier, the interface between the carrier 3 and the coating 2, the second layer 5 of the coating 2 and the third layer 6 of the coating 2. The carrier 3 consists of ferritic cast iron with spheroidal graphite. In the area of the interface between the carrier and the coating, the skin consisting of oxides has been dissolved. The first layer 4 of the coating 2 has been absorbed by the second layer 5 of the coating 2. The interface between the coating and the cast iron carrier 3 is continuous and contains only a few pores. The superficial and locally limited attack on the carrier leads to the formation of anchoring points for the coating 2.

The second layer has limited porosity.

The third layer 6 of the coating 2 adheres well to the second layer 5 of the coating 2. It has a very low porosity, forms a smooth surface and ensures a very tight seal of the coating.

The method according to the invention even enables the coating of oxidized metal parts that have a firmly adhering oxide skin with a thickness of less than 20 µm.

By combining the process steps for producing the enamel coating 2 with the process steps for producing the pipe 2, the invention enables an improvement in productivity and lower heat energy consumption. The invention makes it possible to omit one or more process steps that serve to burn in the enamel.

The variations of the process in which the enamel is applied in the form of a slip make the production process more versatile.

The pipes obtained enable the production of piping systems for the transport of aggressive fluids, such as acid solutions, solutions with a high content of dissolved CO2, abrasive fluids, industrial waste, waste water or sewage sludge.

Claims (20)

1. Process for producing a metal tube coated with three layers of inorganic materials based on silica and sodium oxide, characterized in that three layers (4, 5, 6) of an inorganic coating (2) are applied one after the other to at least part of the surface of the metal pipe to be treated, which is a pipe (1) made of ductile cast iron, this pipe having a temperature of more than 700ºC, the constituents of the inorganic material being present in different proportions depending on the layer, the first layer (4) of the coating (2) being produced during the manufacture of the pipe by centrifugal casting by applying the powdered material, the layer consisting of an enamel having the following composition, given in percent by weight: SiO₂ 50 to 70% CaO + MgO 5 to 20% B₂O₃ less than 10% CoO less than 2% Fe₂O₃ less than 5% Al₂O₃ less than 10% Na2O + K2O 15 to 25% F₂ less than 2% NiO less than 2%, and is produced in a thickness of less than 200 µm and wherein the production of the second layer (5) of the coating (2) on the hot pipe takes place during its cooling by applying the powdery material, wherein this second layer is produced in a thickness of more than 100 µm and is baked after application. 2. Process according to claim 1, characterized in that the enamel is reduced during its production. 3. Method according to claim 2, characterized in that the first layer (4) of the coating (2) is produced in a thickness of 100 µm. 4. Method according to one of claims 1 to 3, characterized in that the inorganic material applied to form the second layer (5) of the coating (2) is an enamel with the following composition, expressed as a percentage by weight: SiO₂ 40 to 60% CaO + MgO less than 5% Li₂O below 5% TiO₂ 5 to 15% NiO below 2% ZnO below 2% Al₂O₃ below 10% Na2O + K2O 15 to 20% B₂O₃ 5 to 15% CoO below 2% Sb₂O₃ below 2%. 5. Method according to one of the preceding claims, characterized in that the second layer (5) of the coating (2) is produced in a thickness of 200 µm. 6. Method according to one of the preceding claims, characterized in that the material forming the third layer is applied in powder form or as a slip. 7. Method according to claim 6, characterized in that the third layer (6) of the coating (2) is produced by applying an enamel with the following composition, given in percent by weight: SiO₂ 40 to 60% CaO + MgO less than 5% Li₂O below 5% TiO₂ 5 to 15% NiO below 2% ZnO below 2% Al₂O₃ below 10% Na2O + K2O 15 to 20% B₂O₃ 5 to 15% CoO below 2% Sb₂O₃ below 2%. 8. Method according to one of claims 6 and 7, characterized in that the third layer (6) of the coating (2) is produced in a thickness of more than 100 µm. 9. Method according to the preceding claim, characterized in that the third layer (6) of the coating (2) is produced in a thickness of 200 µm. 10. Process according to claim 1, characterized in that the production of the metal tube from a liquid cast iron comprises at least the following successive steps: - Treatment with magnesium, - Inoculation, - centrifugal casting of the cast iron, during which the first layer (4) of the coating (2) is produced as in one of claims 1 to 3. 11. Method according to claim 10, characterized in that the cast iron pipe has a temperature of 800 to 700°C when the second layer (5) of the coating (2) is applied as in one of claims 4 and 5. 12. Method according to claim 11, characterized in that after the production of the first layer (4) of the coating (2), the second layer (5) of the coating (2) is applied to the pipe after the centrifugal casting. 13. Method according to claim 11, characterized in that after the production of the first layer (4) of the coating (2), the second layer (5) of the coating (2) is produced during the spindle casting. 14. Method according to one of claims 11 to 13, characterized in that after the production of the second layer (5) of the coating (2), a heat treatment of the tube for ferrite formation is carried out in a temperature range below 800°C, wherein the thermal treatment for ferrite formation is followed by the production of the third layer (6) of the coating (2) as in one of claims 6 to 8. 15. Method according to one of claims 11 and 12, characterized in that after the centrifugal casting of the pipe and before the production of the second layer (5) of the coating (2), a heat treatment of the cast iron for graphitization is carried out. 16. Method according to one of claims 11 to 13, characterized in that after the production of the second layer (5) of the coating (2), a baking treatment of the pipe is carried out at a temperature of 800 to 700°C. 17. Method according to the preceding claim, characterized in that after firing, a third layer (6) of the coating (2) as in one of claims 6 to 8 is applied. 18. Method according to claim 14 or 17, characterized in that after the application of the third layer (6) of the coating (2) in the form of a slip, the coating is baked at a temperature of 750 to 700°C. 19. Metal pipe (1) coated on the inside of its surface with three layers of inorganic material, characterized in that it is manufactured as in one of claims 1 to 18. 20. Use of the metal pipe (1) according to claim 19 for the transport of aggressive fluids, in particular acid solutions, solutions with a high content of dissolved CO2, abrasive fluids, industrial discharge material, waste water or sewage sludge.