A METHOD FOR COATING AN ELECTRODE USED IN THE ANNEALING PICKLING ELECTROLYSIS OF STAINLESS STEEL

Abstract: The invention realtes to a method for coating a steel electrode body (1) used in the annealing pickling electrolysis of stainless steel (40) with a coating agent (12a), the steel electrode body (1) being coated with an adhesive layer (2). The steel electrode body (1) coated with an adhesive (2) is placed in a casting mould (3), into which shielding gas is conveyed to prevent oxidisation of the steel electrode body (1). The temperature of the steel electrode body (1) and the adhesive layer (2) is increased by heating the casting mould (3) and the steel electrode body (1) is coated with a coating agent (12a) by pouring the fluid coating agent (12a) over the steel electrode body (1) arranged in the casting mould (3).

Description: A method for coating an electrode used in the annealing pickling electrolysis of stainless steel The invention relates to a method for coating a steel electrode body used in the annealing pickling electrolysis of stainless steel with a coating agent, the steel electrode body being coated with an adhesive layer. The invention can be used especially in the manufacture of the lead-coated electrodes used in the electrolytic tanks of annealing pickling lines in the steel industry. In known solutions, the electrode body is coated with a medium layer and the actual coating, which is, for example lead, is formed some time later, perhaps not until several hours or days later. Before the lead coating is formed on the adhesive-coated electrode body, the electrode must be heated. The electrode body is made of steel, that is, it has a great mass and thus the heating of the electrode body takes a rather long time, which is why the material of the adhesive layer, or medium layer, of the electrode body has time to oxidise during heating. The oxidisation of the medium layer, that is, the adhesive layer, prevents lead from adhering to the adhesive layer and this results in the formation of air pockets in the finished electrode, between the adhesive layer and the actual coating. When the electrode heats up in electrolysis, the air pockets in the electrode expand and result in the formation of holes in the lead coating, whereby the acid from the electrolysis is able to corrode the steel electrode body and the service life of the electrode in difficult working conditions, such as electrolysis, shortens substantially. According to conventional methods, the actual coating, such as a lead coating, is fixed by welding. Coating by welding is extremely slow and takes, for example, in the case of a 250- kilogram electrode, over thirty hours, incurring high costs. When lead is welded on the surface of an electrode body in several stages, inclusions and other welding defects remain in the welded coating accelerating the wearing of the lead coating, furthering the development of conditions in such a way that the acid is able to affect the steel electrode body and thus shortening the useful life of the electrode coated by welding. The aim of the invention is to develop a new method, which avoids the problems and disadvantages relating to known solutions and by means of which the useful life of the electrode is lengthened substantially compared with the useful life of electrodes coated by welding. To achieve the above-mentioned aim, the method relating to the invention is characterised in that a steel electrode body coated with an adhesive is placed in a casting mould, shielding gas is conveyed into the casting mould to prevent oxidisation of the steel electrode body, the temperature of the steel electrode body and the adhesive layer is increased by heating the casting mould, the steel electrode body is coated with a coating agent by pouring the fluid coating agent over the steel electrode body arranged in the casting mould. Due to the use of the shielding gas and the tight storage space, the steel electrode body and the adhesive layer are subjected to conditions furthering oxidisation for at most a very short, in practice insignificant, time immediately before the lead casting. Preferred embodiments of the invention are disclosed in the dependent claims. The invention is described in the following with reference to the accompanying drawings, in which: Figure 1 A shows a cross-section of an electrode body with an adhesive layer. Figure 1 B shows the electrode body according to Figure 1A from the side. Figure 2 shows one embodiment of the heating of the electrode body according to Figures 1A and 1B. Figure 3 shows another embodiment of the heating of the electrode body according to Figures 1A and 1B. Figure 4 shows the pouring of the lead over the electrode. Figure 5 shows the different stages of the method as a block diagram. Figure 6 shows the use of lead-coated electrodes in electrolysis. Figures 1A and 1 B show an electrode body, which is marked with reference numeral 1. The material used for the electrode body is preferably steel, but it is also possible to use another ferromagnetic material. On the electrode body 1 is formed an adhesive coating 2. Its formation is known and is not described in greater detail in this connection. As an adhesive 2 is preferably used, for example, tin or a tin-containing adhesive material. Figure 2 shows diagrammatically one embodiment for heating the electrode body 1 and the adhesive coating 2. The electrode body 1 is arranged in a cavity-type space 5. In this embodiment, the space 5 is formed in a casting mould 3, but at this stage the body 1 may also be located in another corresponding container space. Before heating, the body 1 is also stored in the casting mould or container space. Alongside the casting mould 3, in this case below it, is arranged a heater 7, preferably a gas heater. The heater 7 comprises a fuel gas source 7a, a feed channel 7b for conveying fuel gas to the burners 7c heating the casting mould 3. The flow of the fuel gas to the burner 7c is regulated by means of a valve 7d. The heater 7 thus heats the surface 3a of the casting mould 3, and further the electrode body 1 and the adhesive surface 2 in the cavity space. In conjunction with the mould 3 are arranged shielding gas supply means 6. They comprise a shielding gas source 6a arranged in the vicinity of the casting mould 3. From the source 6a, which is, for example, a pressure vessel, is arranged a feed channel 6b which is in contact with the space 5 being heated, by means of which channel the shielding gas is supplied to the space 5 being heated to protect, and to prevent oxidisation reactions on the surface of the steel electrode body 1 and, therefore, on the adhesive layer 2, as well as on their barrier layer. The gas flow is regulated by means of a valve 6c. As a shielding gas is preferably used an inert gas such as nitrogen or argon. Figure 3 shows an alternative heating method for heating the steel electrode body 1 and the adhesive layer 2, where the casting mould 3 is heated in a furnace 9, which has at least one heating resistor 10. The shielding gas is supplied to the space 5 being heated by the means described above. As a result of the heating, a gaseous substance with deteriorated functional properties, and other detrimental gaseous substances are formed in the space 5. To remove the gaseous substance, an exhaust device 11 for gaseous substances is arranged in conjunction with the furnace 9. It is preferably in contact with the interior 9a of the furnace 9, as shown in Figure 3, or directly with the space 5 being heated. The exhaust device 11 is preferably, for example, a fan, by means of which from the space 5 being heated is removed interior air from the furnace 15 and at the same time gaseous substances with deteriorated functional properties formed in the space, at the same time as new, clean shielding gas is supplied to the space 5 being heated by means of the shielding gas supply means 6. In this way, the concentration of clean shielding gas remains sufficient during heating. Figure 4 shows a coating container 15 and its heating device 13, which is gas-operated in the example shown in Figure 4, as is the heater 7 used for heating the container 5 shown in Figure 2. In a preferred embodiment, the molten material 12 in the container 15 is lead or a material containing mainly lead. The flow of the heating gas is regulated by means of a valve 13d. The heating gas source is marked with reference numeral 13a. In Figure 4, the device arrangement further comprises a pipe or other feed channel 14a and a valve 14b for regulating the flow of molten lead. The feed channel 14a is in contact with the casting mould 5, into which the molten material is thus conveyed, onto the adhesive-coated electrode body 1 to form the actual coating 12a, which solidifies in the mould 5. Before pouring, an aperture 4 is formed in the mould 3, through which the lead (12a) is able to flow into the cavity space 5 of the mould 3. The casting mould is heated further after casting. The surface of the molten coating agent 12 extends to the upper edge of the casting mould 3 to prevent the formation of cavities, such as air pockets, in the coating 12a while it solidifies in the casting mould 3. The mould 5 is then opened and the electrode 40 is finished, after which it is ready for use in electrolysis. Figure 5 shows the different stages of the method in block diagram form. In Figure 5, the borders of the blocks 5 to 10, which represent significant stages as concerns the invention, are marked with bolder lines. At first, in block 1 , takes place the cutting of the electrode body 1 blank, where an electrode body blank of the desired shape is cut from the desired material, such as steel. At the following stage 2 takes place the machining of the electrode body, in which the electrode body blank is machined in the desired manner, possibly making different shapes or openings in it. At stage 3 is carried out the heating of the electrode body to a suitable temperature, for example to the temperature of 220°C. The temperature is, however, selected so that it is sufficient for effecting melting of the adhesive material, such as powdered tin, or other sufficient fixing on the surface of the electrode body 1. At stage 4 is carried out the adhesive coating of the electrode body 1 to form the adhesive surface 2. At the following stage 5, the electrode body 1 is placed in the casting mould 3. Alternatively, the electrode body can be placed in a separate container. The adhesive-coated electrode body 1 is centred using centring devices 8, which can be seen in Figures 2 to 4. At stage 6, the casting mould 3 (or other container), which is possibly at least partly open, is closed and assembled and sealed so as to be gas tight. At stage 7 is carried out the shielding gas supply into the casting mould, as shown in Figure 2 or 3. At stage 8 is carried out the heating of the casting mould 3, for example, to the temperature of 220°C, even though the aim of the heating is to heat the adhesive-coated electrode body 1. Stage 8 corresponds to the stage shown in Figures 2 and 3. At stage 9 is carried out the melting of the coating, such as lead, in the container 15, to a temperature of approximately 700°C, which is also shown in Figure 4. Stages 7, 8 and 9 are preferably carried out essentially simultaneously. At stage 10 the supply of shielding gas is discontinued using a valve 7d. Furthermore, in accordance with stage 10 in Figure 5 and Figure 4, the coating agent 12, such as lead, is poured into the mould 3, whereby molten lead 12a surrounds the adhesive-coated electrode body 1 located in the cavity space 5 of the mould 3. At this stage, if the body 1 has been stored in a separate container, the body is first transferred from the container to the casting mould 3 and the pouring of the coating agent 12 is carried out after that. At stage 11 , the mould is cooled and at stage 12 the mould is opened, that is, disassembled. At the following stage 13, the ready-made electrode 40 is detached from the mould and at the last stage 14 the electrode is finished, that is, any casting residues and burrs are removed. Figure 6 shows an electrolytic tank 20 with electrolytic fluid 21. In Figure 6, the electrodes are marked with reference numeral 30 and they are shaped, and there are two of them on top of each other, whereas in Figure 1 A, 1 B, 2 to 4, the question is of an electrode shown in block-shaped form as a diagram in principle. It can be seen that on the edges of the electrodes 30, that is, outside the electrolytic tank 20, there is a free, uncoated area, where there is thus nothing on top of the electrode body. The electrolytic tank 20 shown in Figure 6 is an electrolytic tank 20 comprised in the annealing pickling line in a steel industry plant. The annealing pickling line is a production line used in the steel industry, where steel material, such as steel band 40, is coated or otherwise surface-treated by means of electrolysis. In Figure 6, a steel band 40, which is, for example, a rolling band of steel, travels either away from the viewer or towards the viewer, that is, the steel band 40 travels between the electrodes 30. Other implementations are obviously also possible. As appears from Figure 4, it is advantageous that the electrode body 1 coated with an adhesive layer 3 is coated by casting the actual surface layer 12a on it in the same space 5, in which the adhesive-coated electrode body 1 is stored and heated in shielding gas. In this way is achieved the advantage that the electrode body does not have to be moved anywhere and nor is another container needed. Since the coating 12a is formed by casting, the coating 12a can be formed much quicker, compared with, for example, the welding technique. In this case, the body 1 is not at any stage in conditions which further oxidisation, or is in such conditions for a very short time. Both the adhesive layer 2 and the actual coating 12a are formed on both surfaces of the electrode body 1 and also on the, possibly narrow, ends between surfaces. It is obvious to a person skilled in the art that the basic idea of the method relating to the invention can be implemented in many ways. The invention and its embodiments are thus not limited to the examples described above, but may vary within the scope of the claims.

Claims: Claims 1. A method for coating a steel electrode body (1) used in the annealing pickling electrolysis of stainless steel (40) with a coating agent (12a), the steel electrode body (1) being coated with an adhesive layer (2), characterised in that a steel electrode body (1) coated with an adhesive (2) is placed in a casting mould (3), shielding gas is conveyed into the casting mould (3) to prevent the oxidisation of the steel electrode body (1), the temperature of the steel electrode body (1) and the adhesive layer (2) is increased by heating the casting mould (3), the steel electrode body (1) is coated with a coating agent (12a) by pouring the fluid coating agent (12a) over the steel electrode body (1) arranged in the casting mould (3). 2. A method as claimed in claim 1 , characterised in that the material of the adhesive layer (2) is preferably tin or a tin-containing material. 3. A method as claimed in claim 1 , characterised in that the shielding gas is an inert gas, such as nitrogen or argon. 4. A method as claimed in any of the claims 1 to 3, characterised in that the material of the coating (12a) is preferably lead or a material containing mainly lead. 5. A method as claimed in any of the claims 1 to 4, characterised in that the temperature of the heated steel electrode body (1) is lower than the combustion temperature of the shielding gas and the melting temperature of the adhesive (2). 6. A method as claimed in any of the claims 1 to 5, characterised in that after casting, the casting mould (3) is heated in such a way that the surface of the coating agent (12a) extends to the upper edge of the casting mould (3) to prevent the formation of cavities in the coating agent (12a) while it solidifies in the casting mould (3). 7. A method as claimed in any of the claims 1 to 6, characterised in that the casting mould (3) is heated by means of a heater (7) or in a heating furnace (9)- 8. A device for coating a steel electrode body (1) used in the annealing pickling electrolysis of stainless steel with a coating agent, the steel electrode body (1) being coated with an adhesive layer (2), characterised in that the device comprises a casting mould (3), in which casting mould (3) is formed a space (5) for placing the steel electrode body (1) coated with an adhesive in the casting mould (3); heated surfaces (3a) for heating the casting mould (3) space (5) and the steel electrode body (1); shielding gas supply means (6) for supplying shielding gas into the heated space (5); an opening (4) for supplying coating agent (12a) into the heated space (5) in the casting mould (3); casting mould (3) heating means (7, 9). 9. A device as claimed in claim 8, characterised in that the heating means (7) comprise a heating gas source (7a), a heating gas feed channel (7b), a heating gas flow regulator (7d), and gas burners (7c). 10. A device as claimed in claim 8, characterised in that the heating means (7) comprise a furnace (9) in which is arranged one or more heating resistors (10). 11. A device as claimed in claim 10, characterised in that in conjunction with the furnace (9) is arranged an air exhaust device (11). 12. A device as claimed in claims 8-11, characterised in that in the space (5) of the casting mould (3) are arranged supports (8) for positioning the steel electrode body (1). 13. A steel electrode body (1) used in the annealing pickling electrolysis of stainless steel, which is coated with an adhesive layer (2) and a coating agent (12a), characterised in that around the adhesive layer (2) is one of the coating agent layers (12a) formed by casting according to any of the preceding claims 1 to 12.

Inventors: RIIHIMAA SIMO, RIIHIMAA, SIMO
Applicants: LAPMET OY, RIIHIMAA SIMO, RIIHIMAA, SIMO
Application year: 2001
Main classification: C23C6
IPC codes: B22D19, C23C6, C25F7