Patent Application
20210025777
The invention relates to a method for producing a corrosion-resistant assembly of a field device for determining or monitoring a physical or chemical, process variable of a medium in an automated plant, wherein the assembly is composed of at least a first component and a second component, wherein the components are connected with one another in a connection region, wherein the first component is composed at least in the connection region of a non-corrosion resistant (i.e., corrodible) material and wherein the second component is composed at least in the connection region of a corrosion resistant material or of a corrodible material.
In automated plants, especially in process automation plants, field devices are often applied, which serve for registering and/or influencing process variables. Serving for registering process variables are sensors, which, for example, are integrated into fill level measuring devices, flowmeters, pressure and temperature measuring devices, pH-redox potential measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, pH value, and conductivity. Serving for influencing process variables are actuators, such as, for example, valves or pumps, via which the flow of a liquid in a pipe, tube or pipeline section, or the fill level in a container can be changed. Referred to as field devices are, in principle, all devices, which are applied near to the process and which deliver, or process, process relevant information. In connection with the invention, the terminology, field devices, thus, refers also to remote I/Os, radio adapters, and, in general, devices, which are arranged at the field level. A large number of such field devices are produced and sold by the firm, Endress+Hauser.
An important goal of measurement and/or automation technology is so to embody field device components, which are exposed to an aggressive measured medium and/or an aggressive atmosphere in a process, that they are reliably protected against corrosion. To be emphasized in this connection is especially protecting corrosion susceptible sensor elements. A concrete example is a diaphragm seal. A diaphragm seal comprises a flange and a thin metal membrane, which is secured to the flange. By means of the membrane, a process pressure is transferred via an inert pressure transfer liquid to a measurement cell. If the diaphragm seal is applied for pressure measurement in an aggressive measured medium, then both the membrane as well as also the flange must, at least to the extent that they are exposed to the measured medium, be made of corrosion resistant material. Suitable materials are preferably noble, and therewith expensive, metals, such as, e.g., gold, platinum, tantalum, titanium, zirconium and nickel, however, also alloys, such as, for example, chemically resistant Hastelloy® and copper alloys. A suitable copper alloy is known, e.g., under the designation, Monel®. Because these corrosion resistant materials are expensive, the sensor elements are usually not made of corrosion resistant material, but, instead, often of a stainless steel, which is provided with an appropriate corrosion resistant, protective cover coating or plating.
For applying a protective cover coating, various methods can be applied:
The coating methods known from the state of the art have advantages and disadvantages. Especially critical is the manufacture of coatings of tantalum, niobium, titanium or zirconium. For reasons of cost, a galvanic coating process for these metals is excluded. The coating methods ALD, CVD and PVD are too complex and expensive for producing field devices. The soldering process has the large disadvantage that the metals, tantalum, niobium, titanium and zirconium, preferred for coating foils have coefficients of thermal expansion, which differ greatly from the coefficient of thermal expansion of, e.g., stainless steel. Since the soldering takes place at high temperatures, the thin foils are greatly strained and deformed after the soldering and cooling. A resistant coating is thus not assured.
An object of the invention is to provide method and assemblies characterized by corrosion resistance produced in simple and reliable manner.
The object is achieved by a method comprising method steps as follows:
The corrosion resistant material is preferably gold, platinum, tantalum, zirconium, nickel or Hastelloy® or a chemically resistant copper alloy, such as e.g., Monel®. As already mentioned, a wide variety of corrosion resistant materials can be applied as coating on a corrodible component via a laser sintering method.
Preferably for applying the predetermined thickness profile, or the predetermined thickness profiles, of the corrosion resistant coating/coatings and/or, in given cases, for applying the functional intermediate layer/layers, a 3D printing method, especially a selective laser sintering method, is used. Selective laser sintering (SLS) is a generative manufacturing method, in the case of which 3-dimensional structures are produced by sintering of a powdered starting material with a laser. Selective laser sintering is suitable for processes, in the case of which plastic or metal powder is melted layer upon layer completely and without application of binders. After the solidification of the molten material, or processing of all layers, a homogeneous material of high density is obtained.
Advantages of the 3D printing method, and here especially laser sintering, are as follows:
According to the invention, a coating of corrosion resistant, or non-corroding, material is applied on at least one surface-portion of the corrodible component. An option is, however, also the applying of two or more layers, or coatings. For example, a component of a corrodible material is in a first method step precoated with a first metal; then a layer of a second metal is applied. In such case, e.g., the intermediate layer assumes the function of a bonding aid to provide an improved bonding of the corrosion resistant material of the protective layer on the component to be protected.
Additionally or alternatively, the functional intermediate layer of the first metal can serve for an optimized coupling between the corrosion resistant protective cover layer and the corrodible component in the case in which the two materials possess very different coefficients of thermal expansion. This is illustrated based on the following example: a corrodible stainless steel 1.4435 (316L) has a coefficient of thermal expansion of about 17×10{circumflex over ( )}−6 1/K, while corrosion resistant tantalum has a coefficient of thermal expansion of (7−8)×10{circumflex over ( )}−6 1/K. A functional intermediate layer of Hastelloy C22 with an intermediately lying coefficient of thermal expansion of (12−13)×10{circumflex over ( )}−6 1/K can significantly reduce thermal stresses arising in the tantalum layer in the case of a temperature change, so that the corrosion resistant coating remains durably bonded with the component.
In an advantageous, further development of the method of the invention, the coating/coatings or the functional intermediate layer on the first component and/or the second component are/is applied with an essentially homogeneous thickness; and, in such case, the predetermined thickness profile, or the predetermined thickness profiles, of the coating/coatings and/or, in given cases, the at least one functional intermediate layer are/is implemented via a grinding process using a grinder or a turning process using a lathe.
Preferably, a laser welding method is used as welding method.
The object is achieved, furthermore, by an assembly for determining and/or monitoring a physical or chemical, process variable of a medium in an automated plant, wherein the assembly is composed at least of a first component and a second component, wherein the components are welded together in a connection region, and wherein at least one of the two components is composed of a corrodible material, wherein the assembly is produced via at least one of the above described methods.
For example, the assembly is a diaphragm seal for determining and/or monitoring pressure of a medium, wherein the two components of the diaphragm seal to be welded together are a flange of a corrodible material and a measuring membrane of corrosion resistant material. Provided between the flange and the measuring membrane is a chamber for accommodating a pressure transfer liquid. Via the bond between the corrosion resistant coating on the flange and the measuring membrane of corrosion resistant material, the chamber is durably sealed from the measured medium.
Preferably, the coating on the flange, on the one hand, and the measuring membrane, on the other hand, are of the same corrosion resistant material, wherein the material is preferably tantalum. The flange is preferably manufactured of stainless steel.
Preferably, the thickness of the coating in the connection region is in the range, 0.1-5.0 mm, preferably 0.1-0.5 mm. As already stated above, the method of the invention is implementable for any thickness of the coating/coatings. The thickness of the coating/coatings is, however, for reasons of cost so selected that it always lies within a range as here specified and the connection region after the welding process is free of a mixing of corrodible material into the corrosion resistant material.
The thickness of the measuring membrane lies, moreover, preferably in the range, 0.025-0.2 mm.
As further example of an assembly, which can be used in an automated plant and which is produced via the method of the invention, is provided by a vibronic sensor for determining fill level, density and/or viscosity of a medium, wherein the medium is located in a container. The two components of the vibronic sensor to be welded together are a flange of a corrodible material and a sensor element of corrosion resistant material. The sensor element comprises a pot-shaped housing, which is sealed with a membrane on an end region facing the medium. Formed on the membrane is, in given cases, at least one oscillatory tine. Provided in the connection region and also in the flange areas, which come in contact with the measured medium or with the atmosphere in the container, is a corrosion resistant coating. The two components—flange and sensor element—are welded together in the connection region, wherein, in turn, by a correspondingly predetermined thickness of the coating, it is assured that during the welding process only corrosion resistant materials of coating and sensor element are connected with one another. A mixing in of corrodible material in the connection region can, in this way, be effectively prevented.
Preferably, the sensor element is composed of stainless steel, and also the corrosion resistant coating on the flange is a coating of stainless steel.
The invention will now be explained in greater detail based on the appended drawing. The figures of the drawing are all sectional views and show as follows:
Illustrated in
The other components of the sensor element 10, especially the sensor pot 11, or the pot-shaped housing 11, and the oscillatable unit 12 composed of the membrane 13 on the process facing end region of the sensor pot 11 and the oscillatory fork 13 with two tines 14—thus, all components, which come in contact with the medium or the atmosphere in the container—are manufactured of corrosion resistant material. Such material is often Hastelloy®. Furthermore, a welded connection 17 between the corrosion resistant material of the plated layer 16 on the flange 25 and the corrosion resistant material of the sensor pot 12 can be provided. Of course, the plated layer 16 does not necessarily have to be welded with the flange 25. Because of the plated layer 16, which best serves as a suitable sealing surface, the flange 25 same as, in given cases, the connection region 17 between the flange 25 and the sensor pot 11 are protected against corrosion.
In the visualized case, a functional intermediate layer 46 is located between the component 41; 42 and the coating 45. The functional intermediate layer 46 acts, for example, as a bonding aid and enables a lasting connection between the material of the component 41, 42 and the material of the coating 45. Via a suitable choice of the material of the functional intermediate layer 46, also, e.g., a suitable buffering between different coefficients of expansion of the material of the component 41; 42 and the coating 45 can be achieved.
In
Applied on the coating 45 is the measuring membrane 48 of corrosion resistant material. The two components 41, 42; 45, 48 are preferably welded together in the connection region 43. Especially a laser welding method is applied for this. Located between the measuring membrane 48 and the sensor bed 26 is a chamber 49. Such is, same as the connecting line 63, filled with pressure transfer liquid 50. The bores 33 in the flange 52 serve for securing the flange assembly 40 on a process flange (not shown).
The following example serves for purposes of illustration. Of course, instead of the corrosion resistant material, tantalum, also other corrosion resistant materials can be used. The same holds for the material of the corrodible component(s). Likewise the numerical values are examples.
The coating 45 applied in the 3D printing method on the flange 52 can be as thick as desired, e.g., 0.1-5.0 mm. For many applications, however, a tantalum layer of 0.1-0.5 mm is sufficient completely to avoid corrosion of the component 41; 42 to be protected. The membrane 48 is, e.g., a tantalum foil, which is welded on the coating 45 in the connection region 43, e.g., by laser welding. Tantalum foils can have, for example, a thickness of 0.025 to 0.200 mm. Preferably, the thickness of the tantalum foil is, e.g., 0.10 mm when the coating 45 has a thickness of, e.g., 0.20 mm.
Flange 56 is provided with corrosion resistant coating 45 in the region, which can—directly or indirectly—come in contact with a medium 51. The thickness of the coating 45 applied via a 3D printing method is, in such case, so selected and/or structured that in a following joining process, e.g., a welding, of the two components 55, 56 only the corrosion resistant material, or the corrosion resistant materials, are melted and connected with one another. A mixing in of corrodible material into the joined connection region 43 of the two components 41, 42; 55, 56 of the assembly 40 is, thus, safely excluded.
In summary, advantages of the solution of the invention include the following:
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2017 115 651.8 | Jul 2017 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/068466 | 7/9/2018 | WO | 00 |
