A COMBINED OBJECT WITH CORROSION RESISTANT PROPERTIES

FIELD OF THE INVENTION

The present invention relates to a combined object, i.e. an object being formed by combining two or more original objects, and to a method of forming such a combined object. More particularly, the present invention relates to a combined object exhibiting relatively good corrosion resistant properties, in particular in zones where the at least two original objects have been joined in order to form the combined object.

BACKGROUND OF THE INVENTION

Objects which are meant to be positioned in highly corrosive environments must have an outer surface which is corrosion resistant in order to protect the object. Such a corrosion resistant outer surface may be provided by manufacturing the entire object from a corrosion resistant material. This may, however, be undesirable, e.g. due to the costs involved with manufacturing such an object, or because the corrosion resistant material may fail to meet other requirements or properties which the object has to fulfil or have, e.g. in terms of strength, magnetic properties, flexibility, durability, density, weight, thermal or electrical conductivity, workability (e.g. in with respect to pressing, stamping, welding, forging, screwing, soldering or gluing), elasticity, fatigue properties, lubrication related properties, hardness, roughness, etc. Accordingly, a corrosion resistant outer surface is often provided by coating the object with a layer of corrosion resistant material, such as tantalum (Ta).

In the case that it is necessary to join two or more coated objects, e.g. because it is desired to manufacture an object which is too large to be positioned in the coating equipment, or because it is desired to form a combined object from original objects made from different base materials,

there is a risk that the process of joining two or more objects, e.g. by means of welding, weakens the corrosion resistant properties of the coating material in a zone where the objects are joined. As a result, this zone will represent a weak zone or point with respect to corrosion, and there is a risk that the combined object will corrode when positioned in a corrosive environment. This is very undesirable.

SUMMARY OF THE INVENTION

It is, thus, an object of the invention to provide a combined object having enhanced corrosion resistance as compared to prior art combined objects.

It is a further object of the invention to provide a method for forming a combined object having enhanced corrosion resistance as compared to prior art combined objects.

It is an even further object of the invention to provide a method of joining two or more coated objects in a manner which, at least to a certain extent, maintains corrosion resistant properties of the coated objects.

According to a first aspect of the invention the above and other objects are fulfilled by providing a combined object comprising:

- a first object comprising a first body part being made from a first base material and having an outer surface, and a first coating layer of a corrosion resistant material covering at least a part of the outer surface of the first body part,

- a second object comprising a second body part being made from a second base material and having an outer surface, said second object being arranged adjacent to the first object, and

- a welding zone arranged at an interface defined between the first object and the second object, said welding zone joining the first object and the second object to form the combined object,

wherein the welding zone exhibits corrosion resistant properties which are significantly enhanced as compared to corrosion resistant properties of the first base material.

The first object comprises a first body part and the second object comprises a second body part. In the present context the term 'body part' should be interpreted to mean a part of the object which at least substantially defines the size and shape of the object. Thus, the body part constitutes a significant portion of the object.

The first body part is made from a first base material. The first base material may be one material making up the entire first body part. Alternatively, the first base material may be composed of two or more different materials in combination making up the first body part. In this case the two or more different materials may be mixed in such a manner that a substantially uniform first body part is obtained, or they may be non- uniformly distributed, e.g. forming sub parts of the first body part with more or less distinct interfaces there between. However, the first base material is preferably of a kind which has relatively poor corrosion resistant properties.

The first object comprises a first coating layer covering at least a part of the outer surface of the first body part. The first coating layer is made from a corrosion resistant material. In the present context the term 'corrosion resistant' should be interpreted to mean that the first coating layer is adapted to withstand corrosion, and that it is thereby capable of protecting the first body part when the first object is positioned in a corrosive environment. Furthermore, the corrosion resistant material forms a coating

layer on at least part of the outer surface part of the first body part. Thus, the thickness of the corrosion resistant material is very small as compared to the overall size of the first object.

Similarly, the second body part is made from a second base material. The second base material may also be one material making up the entire second body part. Alternatively, the second base material may be composed of two or more different materials in combination making up the second body part. In this case the two or more different materials may be mixed in such a manner that a substantially uniform second body part is obtained, or they may be non-uniformly distributed, e.g. forming sub parts of the second body part with more or less distinct interfaces there between. The second base material may be of a kind having relatively good corrosion resistant properties, e.g. comparable to the corrosion resistant properties of the first coating layer. Alternatively, the corrosion resistant properties of the second base material may be somewhat poorer, e.g. comparable to the corrosion resistant properties of the first base material. In this case the second object may also be provided with a coating layer of a corrosion resistant material in order to provide corrosion resistant properties to the second object. This will be described in further detail below.

The first object and the second object are arranged adjacent to each other. Thereby an interface between the two objects is defined. At this interface a welding zone is arranged. The welding zone may be provided by means of a suitable welding technique, thereby joining the first object and the second object. Accordingly, once welded together, the first object and the second object together form the combined object, i.e. the combined object is formed by combining the first object and the second object at the interface, and by means of a suitable welding technique.

The welding zone is formed in such a manner that it exhibits corrosion resistant properties which are significantly enhanced as compared to corrosion resistant properties of the first base material. However, the corrosion resistant properties of the welding zone need not be as good as the corrosion resistant properties of the corrosion resistant material of the first coating layer. Accordingly, the welding zone will be capable of providing protection from corrosion to the first body part, and thereby to the first object. Thus, corrosion resistance will also be provided at the position of the welding zone, at least to some extent, and the welding zone therefore does not represent a weak spot in terms of corrosion. Thereby it is possible to weld objects together to form a larger, combined object, without compromising the corrosion resistance of the combined object. This is very advantageous and opens the possibility of manufacturing relatively large objects exhibiting a high degree of corrosion resistance without having to make the entire object from a corrosion resistant material, i.e. the combined object can be made in a cost effective manner.

The welding zone may, at a level corresponding to outer surfaces of the first and second objects, contain elements of the first coating layer material at a concentration of at least 8% by weight, such as between 8% and 15% by weight. In the present context the term 'outer surfaces of the first and second objects' should be interpreted to mean a boundary between the objects and the surroundings. Thus, it is preferably an outer surface of the first coating layer and the outer surface of the second body part or, if applicable, an outer surface of a second coating layer applied to the second body part. Thus, the 'outer surfaces' are the outer surfaces of the objects as defined prior to welding them together. The welding zone may have an outer boundary which is flush with these outer surfaces. However, it is very likely that when welding two objects together, the resulting welding zone will bulge, thereby defining an outer surface of the welding zone which protrudes from the combined object relatively to the

outer surfaces of the first and second objects. In this case a level corresponding to the outer surfaces of the first and second objects will be positioned below the outer surface of the welding zone, and the concentration mentioned above will, in this case, appear at a certain depth of the welding zone, rather than on the surface of the welding zone.

The fact that elements of the coating layer are present in the welding zone has the effect that the welding zone exhibits corrosion resistance, at least to a certain extent. In particular, the corrosion resistant properties of the welding zone will be enhanced as compared to the corrosion resistant properties of the first base material, due to the presence of the elements of the corrosion resistant material of the first coating layer.

Furthermore, the concentration of elements of the first coating layer material in the welding zone at a distance of 450 μm to 600 μm from an outer surface of the welding zone, may be between 6% and 10% by weight. According to this embodiment, the elements of the corrosion resistant material of the first coating layer are not only present at or near the surface of the combined object, but also at a certain depth of the object. Thereby the overall corrosion resistance of the combined object is even further improved.

In the situation described above it is important that the concentration of elements of the first coating layer material at a certain depth of the welding zone is sufficient to stop a possible corrosion at that depth. The corrosion resistant properties of the welding zone above this depth are of less importance as long as the corrosion is stopped when it reaches the depth defined above. Thereby the welding zone as such will have corrosion resistant properties which prevent corrosion beyond the specified depth, and the combined object will thereby exhibit sufficient corrosion resistance.

The material of the first coating layer may be a refractory metal, such as tantalum (Ta) or an alloy of tantalum, or titanium (Ti), or any other suitable refractory metal, or alloys thereof. As an alternative, the material of the first coating layer may be or comprise any other suitable material exhibiting a desired degree of corrosion resistance towards the environment where it is intended to position the combined object.

The second object may comprise a second coating layer of a corrosion resistant material covering at least a part of the outer surface of the second body part. In this case the second base material is preferably of a kind having relatively poor corrosion resistant properties, and it is therefore necessary to provide the second coating to at least a part of the outer surface of the second body part in order to obtain a desired corrosion resistance of the second object. Thus, in this case both of the objects being joined at the welding zone to form the combined object have been coated prior to being joined, i.e. two coated objects are to be joined at the welding zone.

Alternatively, only the first object may be coated, while the second object is made entirely of a material having desired corrosion resistant properties.

In the case that the second object has also been provided with a coating layer, the first base material and the second base material may be the same. Alternatively, the first base material and the second base material may be two different kinds of material. Furthermore, the first coating layer and the second coating layer may be made from the same material, preferably a refractory metal, most preferably tantalum (Ta) or an alloy of tantalum.

The first base material and/or the second base material may be a stainless steel, e.g. a stainless steel comprising between 15% and 25% by weight of chromium (Cr). Stainless steels of this kind are relatively inexpensive.

However, they exhibit relatively poor corrosion resistant properties, and it is therefore necessary to provide them with a coating layer of corrosion resistant material in order to provide an object made from such material with sufficient corrosion resistant properties to allow them to be positioned in a corrosive environment. The table below illustrates various potential combinations of base material, coating material and composition of the welding zone. The rightmost column describes the overall corrosion resistance of the resulting combined object.

The first base material and/or the second base material may be a ferrite containing steel. Such steels are magnetisable, and according to this embodiment it is thereby possible to produce a magnetic actuator, e.g. for a valve, which can be used in a corrosive environment. This is a great advantage.

The first coating layer may have a thickness within the interval 5 μm-200 μm, such as within the interval 20 μm-100 μm. In the case that the second object is provided with a second coating layer, the second coating layer may similarly have a thickness within the interval 5 μm-200 μm, such as within the interval 20 μm-100 μm. Thus, the first coating layer, as well as a possible second coating layer, is preferably very thin, at least relatively to the overall dimensions of the corresponding object.

The welding zone may have been provided by means of laser welding. It has been found by the inventors of the present invention that laser welding is particularly suitable for ensuring that the welding zone exhibits corrosion resistant properties which are significantly enhanced as compared to corrosion resistant properties of the first base material. However, other welding techniques may alternatively be used.

The welding zone may be at least substantially continuous. In the present context the term 'continuous' should be interpreted to mean that there are no areas or points of the welding zone which are significantly different from the remaining welding zone in terms of corrosion resistant properties. Such areas or points may, e.g., arise at positions where a welding operation has been started or stopped, and they may represent 'weak spots' where corrosive material may gain access to the base material positioned beneath the coating layer(s). An at least substantially continuous welding zone may, e.g., be provided by ensuring that the welding seam is continued past a starting point. Thereby neither the

starting point nor the stopping point will have the form of a 'spot welding', and, accordingly, no weak points will be present in the welding zone.

The combined object may further comprise

- a third object comprising a third body part being made from a third base material and having an outer surface, said third object being arranged adjacent to the first object and/or adjacent to the second object, and

- at least a second welding zone arranged at an interface defined between the first object and the third object and/or between the second object and the third object, said second welding zone joining the first object and the third object and/or the second object and the third object to form the combined object.

According to this embodiment the third object is joined with the first object and/or the second object in a manner similar to the manner of joining the first object and the second object as described above, and the remarks set forth in this regard are accordingly equally applicable here. It should be noted, that in a similar manner a fourth, fifth, sixth and/or even further objects may be similarly joined to the combined object, thereby forming a combined object of a desired size, shape and constitution.

The third object may comprise a third coating layer of a corrosion resistant material covering at least a part of the outer surface of the third body part. This is very similar to what was described above, and the remarks above are accordingly equally applicable here.

The first body part may comprise at least two sub body parts, the sub body parts being made from different materials having different corrosion resistant properties. According to this embodiment the first body part is

itself a 'combined object', in the sense that it is composed of a number of parts made from various materials. However, the sub body parts in combination define an outer surface, and this outer surface is at least partly covered by the first coating layer. Thus, the sub body parts are preferably coated by the same coating layer.

At least one sub body part may be made from the material of the first coating layer. Such a sub body part may advantageously be arranged at the part of the first object which is to be positioned adjacent to the second object, i.e. the part of the first object which, in combination with a corresponding part of the second object, defines the interface between the first object and the second object. In this case the welding zone will not be in direct contact with parts of the first body part being made from a material having poor corrosion resistant properties, and the corrosion resistant properties of the welding zone will therefore be enhanced as compared to overall corrosion resistant properties of the first body part. In fact, the welding zone will, in this case, probably exhibit corrosion resistant properties which are identical to, or at least comparable with, the corrosion resistant properties of the first coating layer. This is very advantageous, since the welding zone will thereby not introduce a point having poorer corrosion resistant properties than the coated parts of the first object. Accordingly, the combined object will, in this case, exhibit corrosion resistant properties which are comparable to corrosion resistant properties of an object which has first been manufactured, welded, etc., and then provided with a coating layer of a corrosion resistant material covering the entire outer surface of object. Furthermore, this has been achieved without having to provide coating equipment being sufficiently large to accommodate the finished object. This is particularly an advantage when it is desired to manufacture large object for use in corrosive environments, e.g. tower constructions for off shore wind turbines.

According to a second aspect of the invention the above and other objects are fulfilled by providing a method of forming a combined object, the method comprising the steps of:

- providing a first object comprising a first body part being made from a first base material and having an outer surface,

- applying a first coating layer of a corrosion resistant material to at least a part of the outer surface of the first body part,

- providing a second object comprising a second body part being made from a second base material and having an outer surface,

- arranging the first object and the second object adjacent to each other, and

- forming a welding zone at an interface defined between the first object and the second object, thereby joining the first object and the second object to form the combined object, in such a manner that the welding zone exhibits corrosion resistant properties which are significantly enhanced as compared to corrosion resistant properties of the first base material.

It should be noted that a person skilled in the art would readily recognise that any feature described in combination with the first aspect of the invention could equally be combined with the second aspect of the invention, and vice versa. Thus, the method according to the second aspect of the invention may suitably be used for forming a combined object according to the first aspect of the invention.

The first coating layer may be applied using any suitable coating technique known in the art, such as, e.g., electrochemical deposition or vapour deposition.

The step of forming a welding zone may be performed by means of laser welding as described above. However, it may alternatively be performed using any other suitable welding technique.

The step of forming a welding zone may be performed in such a manner that an at least substantially continuous welding zone is formed. In this case the step of forming a welding zone may advantageously comprise the steps of:

- starting the welding step at a starting point,

- welding along the interface until the starting point is reached,

- continuing the welding, thereby passing the starting point, and

- stopping the welding at a stopping point being positioned beyond the position of the starting point.

As described above, this approach ensures that the starting point as well as the stopping point are arranged at a position where 'continuous' welding is also performed, and it is thereby avoided that the starting point and/or the stopping point for the welding process represent(s) weak points with respect to corrosion resistance.

The method may further comprise the step of applying a second coating layer of a corrosion resistant material to at least a part of the outer surface of the second body part. According to this embodiment both of the objects

have been provided with a coating layer prior to joining the objects to form the combined object. This has already been described above.

It should be noted that the method according to the second aspect of the invention is equally suitable for joining three, four, five or even further objects to form a combined object.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further details with reference to the accompanying drawings in which

Fig. 1 is a schematic drawing of three objects prior to joining them into a combined object according to an embodiment of the invention,

Figs. 2-5 show SEM-EDX analyses of a welding zone of a combined object according to an embodiment of the invention and corresponding line scans of the welding zone,

Fig. 6 shows an SEM-EDX analysis of the welding zone of Figs. 2-5 with specific areas marked,

Figs. 7a and 7b are schematic illustrations of a combined object according to an embodiment of the invention before and after forming the welding zone, respectively,

Figs. 8-11 are schematic illustrations of combined objects according to various embodiments of the invention, and

Fig. 12 is a schematic drawing of a combined object according to an embodiment of the invention, and illustrating a method of providing an at least substantially continuous welding zone.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic drawing of a first object 1 , a second object 2 and a third object 3. At least one of the objects 1 , 2, 3, possibly all of them, has been provided with a coating layer of a corrosion resistant material. The objects 1 , 2, 3 are positioned next to each other and spaced apart. This is in order to illustrate that the objects 1 , 2, 3 have been separately manufactured and coated, if applicable. The objects 1 , 2, 3 should now be moved closer to each other until they are arranged adjacent to each other, and then welded together to form one combined object.

Fig. 2 shows an SEM image 4 of a welding zone 5 of a combined object according to an embodiment of the invention. The combined object comprises a first object 1 and a second object 2 with the welding zone 5 arranged in between. Both of the objects 1 , 2 have been provided with a coating layer 6 of a corrosion resistant material. The corrosion resistant material in this case is tantalum (Ta). It is clear from the SEM image 4 that the coating layer 6 has been destroyed in an area corresponding to the welding zone, and that the surface of the combined object 'bulges' in the area corresponding to the welding zone 5.

A line 7 is marked on the SEM image 4, indicating the position of a line scan performed on the combined object. The line 7 is positioned approximately 250 μm from an outer surface defined by the welding zone 5. The result of the line scan is shown on the right part 8 of Fig. 2. The amounts of chromium (Cr) 9, iron (Fe) 10 and tantalum (Ta) 11 present in the combined object at the positions indicated by the line 7 have been investigated, and the results are shown as graphs in the right part 8 of Fig. 2. It is clear that the amount of tantalum 11 in the combined object significantly increases in the welding zone 5 as compared to outside the welding zone 5, i.e. corresponding to base materials of the objects 1 , 2.

Since tantalum is a corrosion resistant material the corrosion resistant properties of the welding zone 5 are thereby enhanced as compared to the corrosion resistant properties of the base materials of the objects 1 , 2. This is a great advantage because the destruction of the coating layer 6 in the welding zone 5 is thereby compensated, at least to a certain extent. The tantalum in the welding zone 5 originates from the coating layer 6. During the welding process the tantalum mixes with the base materials of the objects 1 , 2, in this case steel. The welding process has to be carefully performed in order to achieve this. The inventors have found that laser welding is suitable for the purpose.

Fig. 3 shows the SEM image 4 of Fig. 2, and parts which have already been described above will therefore not be described in detail here. In Fig. 3 a line 12 is also marked on the SEM image 4, indicating the position of a line scan performed on the combined object. The line 12 is positioned approximately 650 μm from an outer surface defined by the welding zone 5, i.e. at a deeper level than the line 7 shown in Fig. 2. The result of the line scan is shown in the right part 8 of Fig. 3. The amounts of chromium (Cr) 9, iron (Fe) 10 and tantalum (Ta) 11 present in the combined object at the positions indicated by the line 12 have been investigated, and the results are shown as graphs in the right part 8 of Fig. 3. It is clear that the amount of tantalum 11 , also in this case, increases in the welding zone 5 as compared to outside the welding zone 5, i.e. corresponding to the base materials of the objects 1 , 2. However, in this case the increase is not as significant as it was the case in the situation illustrated in Fig. 2. However, Fig. 3 illustrates that tantalum is present at a significant depth of the combined object.

Fig. 4 also shows the SEM image 4 of Figs. 2 and 3. In Fig. 4 a line 13 is also marked on the SEM image 4, indicating the position of a line scan performed on the combined object. The line 13 is positioned approximately

1000 μm from an outer surface defined by the welding zone 5, i.e. at an even deeper level than the line 12 shown in Fig. 3. The result of the line scan is shown in the right part 8 of Fig. 4. The amounts of chromium (Cr) 9, iron (Fe) 10 and tantalum (Ta) 11 present in the combined object at the positions indicated by the line 13 have been investigated, and the results are shown as graphs in the right part 8 of Fig. 4. In this case the amount of tantalum in the welding zone 5 is similar to the amount of tantalum outside the welding zone 5, i.e. corresponding to the base materials of the objects 1 , 2. Thus, at this depth tantalum has not mixed with the base materials.

Fig. 5 also shows the SEM image 4 of Figs. 2-4. In Fig. 5 a line 14 is also marked on the SEM image 4, indicating the position of a line scan performed on the combined object. The line 14 in this case is positioned corresponding to the welding zone 5 and runs from the surface defined by the welding zone 5 and into the combined object until a certain depth has been reached. The result of the line scan is shown in the right part 8 of Fig. 5. The amounts of chromium (Cr) 9, iron (Fe) 10 and tantalum (Ta) 11 present in the combined object at the positions indicated by the line 14 have been investigated, and the results are shown as graphs in the right part 8 of Fig. 5. It is clear that the amount of tantalum 11 increases significantly immediately below the surface defined by the welding zone 5, and then gradually decreases as the line scan reaches deeper levels of the combined object. However, a significant amount of tantalum 11 is present even at relatively deep levels, and an efficient corrosion resistance must therefore be expected in the welding zone 5.

Fig. 6 also shows the SEM image 4 of Figs. 2-5. In Fig. 6 seven different areas have been marked, and the amounts of various compounds have been investigated at each area. The results are given in the table below. For each compound the percentage of that compound by weight is indicated.

The table confirms what was found in the analyses described with reference to Figs. 2-5, i.e. that the amount of tantalum is very low at areas corresponding to the base materials of the objects 1 , 2 (points 1 and 7), that the amount of tantalum is relatively high near the surface defined by the welding zone 5 and remains high until a certain depth (points 2 and 3), and that the amount of tantalum decreases with the distance from the surface defined by the welding zone 5 (points 4-6).

Fig. 7a is a schematic illustration of a combined object 15 comprising a first object 1 and a second object 2. The first object 1 comprises a first body part 16 made from a first base material, and a first coating layer 6 of a corrosion resistant material. The second object 2 comprises a second body part 17 made from a second base material, and a second coating layer 18 of a corrosion resistant material. The first object 1 and the second object 2 are positioned adjacent to each other, thereby defining an interface 19, but have not yet been joined, i.e. the combined object 15 has in reality not been formed yet. In Fig. 7a no coating layers 6, 18 are

illustrated at the end parts of the objects 1 , 2 defining the interface 19. It should, however, be noted that in some situations it will be appropriate to provide coating layers 6, 18 at these end parts.

Fig. 7b is a schematic illustration of the combined object 15 of Fig. 7b. However, in Fig. 7b a welding zone 5 has been provided at the interface 19 between the first object 1 and the second object 2. Accordingly, the first object 1 and the second object 2 have been joined to form the combined object 15. The welding zone 5 has been provided in such a manner that it exhibits enhanced corrosion resistant properties as compared to the corrosion resistant properties of the first base material, and preferably also as compared to the second base material. As mentioned above, this is obtained by welding the objects 1 , 2 together in such a manner, that elements of the corrosion resistant material of the coating layers 6, 18 mix with the base materials in the welding zone 5. In the case that the corrosion resistant material of the coating layers 6, 18 is tantalum, it has been found by the inventors, that the tantalum surface concentration is above 5% at the welding zone 5, and above 95% in non-welded areas covered by coating layers 6, 18. It has also been found, that the corrosion resistance of the alloy formed at the welding zone 5 is comparable to the corrosion resistance of the areas covered by the coatings 6, 18.

Fig. 8 is a schematic illustration of a combined object 15 according to another embodiment of the invention. According to this embodiment a recess 20 has been formed at the end parts of the objects 1 , 2 prior to providing the coating layers 6, 18. Accordingly, the welding process is performed at a position which is inwards in the combined object 15 as compared to outer surfaces defined by the objects 1 , 2. The welding process is otherwise performed as described above. Following the welding process the recess 20 is filled with a corrosion resistant material, possibly the material of the coating layers 6, 18. Thereby a corrosion firm

connection is provided. In the case that tantalum is used for the coating layers 6, 18 as well as for filling the recess 20, the surface concentration of tantalum in an area corresponding to the welding zone 5 may be more than 95%. In the situation where the tantalum in the filled recess is not fully dense, e.g. due to micro-porosity, the created welding zone 5 has an enhanced corrosion resistance (minimum 5% tantalum) as compared to a situation where the welding zone 5 does not contain tantalum. It should be noted, that the embodiment shown in Fig. 8 introduces the risk of micro- porosity of the material filled in the recess, since the filling will have to be performed in open air due to the size of the combined object 15, and vacuum processes may therefore not be applied.

Fig. 9 is a schematic illustration of a combined object 15 according to yet another embodiment of the invention. The embodiment shown in Fig. 9 is very similar to the one shown in Fig. 8, the only difference being that the coating layers 6, 18 have, in this case, been removed from the recess 20. Thereby the welding zone 5 is formed directly in the base materials of the body parts 16, 17. It has been observed, that in some cases brittle alloys may be formed in the welding zone 5 when the coating material is present. This is, e.g., the case when the base materials have a high concentration of nickel (Ni), e.g. nickel based alloys with a nickel concentration exceeding 50%, and the coating material is tantalum or a tantalum alloy, since nickel-tantalum alloys may be brittle. In such cases the embodiment of Fig. 9 is suitable. Another situation where the embodiment of Fig. 9 is suitable is when the coating material has limited solubility with the base materials at high temperatures. This is, e.g., the case when the coating material is tantalum and the base material has a high concentration of nickel. It should be noted that it could also be envisaged that the coating layers 6, 18 have only been removed from a bottom part of the recess 20, i.e. that coating material is still present at the sides of the recess 20. The remarks set forth above are equally applicable in this case.

Fig. 10 is a schematic illustration of a combined object 15 according to yet another embodiment of the invention. In the embodiment of Fig. 10, layers 21 of a corrosion resistant material have been applied to end surfaces of the objects 1 , 2 forming the interface 19. The material of the layers 21 is preferably a refractory metal, e.g. tantalum (Ta), Titanium (Ti), chromium (Cr), or any other metal or alloy having desired corrosion resistant properties. It may even be the same material as the corrosion resistant material of the coating layers 6, 18. The objects 1 , 2 as well as the layers 21 are then provided with the coating layers 6, 18. When the welding process is subsequently performed, the welding zone 5 is formed at a position where plenty of corrosion resistant material is present, i.e. the coating layers 6, 18 as well as the layers 21. The idea is that the layers 21 should be sufficiently thick to prevent that the welding zone 5 enters the body parts 16, 17, i.e. the welding zone 5 will only comprise compounds of corrosion resistant material, and it is thereby ensured that no weak spots appear.

In the embodiment shown in Fig. 10 the body parts are each made from two sub body parts as described above, i.e. the original body parts 16, 17 and the layers 21.

Fig. 11 is a schematic illustration of a combined object 15 according to yet another embodiment of the invention. The embodiment shown in Fig. 11 is very similar to the embodiment shown in Fig. 7b. However, in the embodiment shown in Fig. 11 the second body part 17 has not been provided with a coating layer. In this case the second base material, which the second body part 17 is made from, is preferably a corrosion resistant material, thereby making it unnecessary to provide the second body part 17 with a coating layer in order to obtain desired corrosion resistant properties of the resulting combined object 15. Corrosion resistant elements from the first coating layer 6 as well as from the second base

material will in this case mix with the first base material in the welding zone 5. Thereby the welding zone 5 will exhibit enhanced corrosion resistant properties as compared to the corrosion resistant properties of the first base material, as described above.

Fig. 12 is a schematic drawing of a combined object 15 according to an embodiment of the invention. Fig. 11 illustrates a method of providing an at least substantially continuous welding zone 5.

Two kinds of welding positions may be defined, spot welding being when special welding conditions are present at a point or spot, and continuous welding being when welding conditions are present in a larger, continuous area. Spot welding occurs, e.g., at starting points and stopping points of a continuous welding. It is often desirable to prevent spot welding, since such points or spots will often represent weak points of the welding. Fig. 11 illustrates a method of preventing this.

A welding zone 5 in the form of a welding seam is started at starting point 22 and continued around the entire combined object 15 along arrow 23 until the starting point 22 is once again reached. The welding is then continued past the starting point 22 to a stopping point 24 positioned well past the starting point 22. Thereby the positions of the spot weldings defined by the starting point 22 and the stopping point 24 are both subjected to continuous welding, and the problems described above are accordingly avoided.