Abstract:
This invention relates to an object having a corrosion resistant surface that is also sufficiently ductile to let the surface, or the whole object, be mechanically modified without creating cracks or other weaknesses undermining or damaging the corrosion resistance. The surface layer preferably contains at least 80% of a refractory metal, such as tantalum, and an alloy layer is created between a core element and the surface layer having the needed ductility and adhering abilities.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in International Patent Application No. PCT/DK2008/000414 filed on Nov. 20, 2008 and Danish Patent Application No. PA 2007 01652 filed Nov. 21, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to an object having a corrosion resistant surface that is also sufficiently ductile to let the surface, or the whole object, be mechanically modified without creating cracks or other weaknesses undermining or damaging the corrosion resistance. The surface layer preferably contains at least 80% of a refractory metal, such as tantalum, and an alloy layer is created between a core element and the surface layer having the needed ductility and adhering abilities. 
       BACKGROUND OF THE INVENTION 
       [0003]    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 in 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. 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). 
         [0004]    It is vital that such a surface layer is tight without pinholes creating exposed spots of the object under the coating to the highly corrosive environments, and a number of documents describes methods to apply such a pinhole free layer, such as EP0578605B1 describing a molten bath for plating with high-melting metals, in particular niobium and tantalum. The bath consists of an alkali metal fluoride melt, which contains oxide ions and ions of the metal to be precipitated. The molar ratio between the metal to be precipitated and the oxide ions, or the other cat ions in the melt, must be held within given ratios. The redox level must be held at a value which corresponds to that which is reached when the molten bath is in contact with the particular high-melting metal in the metallic form. 
         [0005]    Another example is EP1501962B1 relating to a method for modifying a metallic surface, the method comprising chemical vapour deposition on a substrate in a chamber adapted for CVD involving at least the steps of:
   subjecting the substrate to chemical vapour deposition with a flow of reactant gas comprising a metal compound to be incorporated in the metal surface; and interrupting the chemical vapour deposition by cutting off the flow of reactant   
 
         [0007]    A document U.S. Pat. No. 5,087,856 describes an object having a core of stainless steel covered by a surface layer of substantially tantalum, the object being a discharge electrode for a charger having a thin wire or core made of stainless steel or electrolytically polished tungsten, and a coating provided on the thin line. To form the coating, an amorphous alloy containing tantalum, niobium, zirconium, titanium or similar element belonging to the same group on the periodic table is deposited on the thin wire by sputtering, CVD (Chemical Vapor Deposition) or similar technology. The content of tantalum in the amorphous alloy is selected to be 10% to 70%. 
         [0008]    A content of even 70% is, however, not corrosion resistant enough for many corrosive environments, a concentration of at least 70%, or better more than 80% will often be required. 
         [0009]    A document U.S. Pat. No. 4,786,468 describes alloys highly resistant to corrosion by concentrated acid and having excellent adhering properties when coated on stainless steel, which are formed of 60 to 90 atomic percent tantalum or tungsten, with the remainder being iron, chromium and nickel in the proportions found in stainless steel, e.g., 304L stainless steel. They may be formed in situ on the surface to be coated by sputter deposition, using a sputter target which is part tungsten or tantalum, and part stainless steel. 
         [0010]    As revealed in e.g. this document it is a known problem to adhere such a tantalum rich surface coating to especially stainless steel, especially when it also needs to be pinhole free. In a document WO 98/46809 a solution is suggested relating to electroplating with refractory metal, mainly tantalum and niobium, from molten salts and can be applied in chemical, metallurgical, pharmaceutical, medical industries, turbine manufacture, air- and spacecraft, and other areas of engineering, in creation of corrosion-resistant and barrier coatings. The essence of the invention is that when the article to be coated is immersed into a molten electrolyte containing fluorides of both refractory and alkali metal and a eutectic melt of sodium, potassium and caesium chlorides, the article is warmed up to the working temperature of the electrolyte of 700-770° C. whereupon direct or reverse electric current is passed through the electrolyte, the current parameters being adjusted so that the quantity of electricity in the anodic Qa, and cathodic Qc, parts of the electroplating cycle corresponds to the ratio O≦Qa/Qc&lt;0.9. To improve the article quality it is desirable that the weight of the electrolyte exceeds that of the article by 5 times or more. The technical result attained is the production of uniform-thickness, high quality tantalum or niobium coatings on articles for industrial applications made of conventional materials. Open porosity of the resulting coatings is not higher than 0.001%, adhesion to the substrate is as high as 8 kg/mm. 
         [0011]    Some coated objects are subdued to a mechanical modification after applying the coating, this could e.g. because it is desired to manufacture an object which comprises grooves in the surface to be used as flow channels in such systems as fuel cells, heat exchangers, lab on a chip or the like. The process of modifying the object, like forming the grooves in the surface, may be at the risk that the process weakens the corrosion resistant properties of the coating material in a zone where the objects are modified. The modification may also be a result of e.g. drawing objects from a larger coated preform. Objects may also, either during operational use or just simply due to the operational environments, be mechanically subdued to impacts, blows, strokes, grinding, plastic or elastic deformations, this could be tools in general, rotor blades, fans, bellows, pistons etc. Other objects may be mechanically deformed unintentionally due to influence of tools during installation. E.g. a nut may be slightly deformed when tightened with a wrench. Further parts may be exposed to rough handling (e.g. strokes by a tool to ensure right placement in a setup) that may deform the coating and substrate. In all cases, the modified, deformed, or just affected zones 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. 
         [0012]    It is known to apply such a layer in order to create some mechanical properties besides the corrosion properties, such as giving hard wear resistant surfaces. This is described in e.g. U.S. Pat. No. 4,341,834, teaching how to create a cutting tool or a wear-resistant mechanical part that comprises: a substrate with or without an inner coating layer of TiC, TiN or TiCN; an intermediate layer of a titanium oxycarbide formed on the surface of the substrate or the inner coating layer by carrying out a reaction thereon at a temperature of 800° to 1,200° C. of a halide of titanium, hydrogen, and carbon monoxide or carbon dioxide or a mixture thereof; and an outer coating layer of aluminium oxide formed on the outer surface of the intermediate layer. The thicknesses of the inner layer, the intermediate layer, and the outer coating layer are of the order of 0.5 to 20 microns, 0.5 to 20 microns, and 0.5 to 10 microns, respectively. The substrate of a coated super-hard alloy article according to this invention comprises (1) at least one of carbides, nitrides, and carbonitrides of metals of Groups  4   a,    5   a,  and  6   a  of the periodic table and (2) at least one of Fe, Ni, Co, W, Mo, and Cr. Typical metals of the above group (1) are Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W. A super-hard alloy of this character is known and is disclosed in, for example, R. Kieffer: “Hartmetalle”, Springer-Verlag (Wien-NY), 1965. Examples of these alloys suitable for use in this invention are WC-TiC-TaC-Co alloy, WC-Co alloy, WC-TiC-Co alloy, WC-TiC-TaC-NbC-Co alloy, WC-TiC-Mo2 C—Ni—Co alloy, and TiC-Mo—Ni alloy. These super-hard alloys can be produced by known processes, such as, for example, a process comprising mixing powder of starting materials, pressing the mixture into a preform and sintering the preform. 
       SUMMARY OF THE INVENTION 
       [0013]    It is an object of this invention to make an object with a corrosion resistant and ductile coating, unlike e.g. the hard coating for a cutting tool described in e.g. U.S. Pat. No. 4,341,834. 
         [0014]    The object has to be corrosion resistant, even when subdued to a treatment that may imply plastic or elastic deformation. It is further an object of this invention to make an object having a corrosion resistant surface, where the surface of the object is subdued to some mechanical modification, or mechanically subdued to impacts, blows, strokes, grinding, plastic or elastic deformations. For example, the object may be subdued to a rolling or imprinting process forming surface structures, possibly in order to make the surface rough, thus increasing the surface area and thereby the adhesion of subsequent coating layers such as a spray coated ceramic layer.
   This is achieved by making a corrosion resistant object with a ductile surface, said object comprising:   a core element being made from a first base material and having an outer surface, and,   a coating layer comprising a concentration of at least 70% of a corrosion resistant material covering at least a part of the outer surface of the core element,
 
wherein an alloying zone exists between the core element and the coating layer, said alloying zone having a thickness from where the concentration of said corrosion resistant material is 90% of the concentration in the coating layer, to where the concentration of said corrosion resistant material is 10% of the concentration in the coating layer, from 0.1 micrometers to 10 micrometers.
   
 
         [0018]    The present invention further concerns an object having an iron containing core element with a substantially pinhole free surface coating layer having good corrosion resistance, where the surface layer preferably is tantalum or a metal having a corrosion resistance significantly larger than steel, like e.g. reactive or refractory metals or just of the same group of metals as tantalum, such metals including W, Nb, Mo, Ti, Hf, Zr. The core element itself is substantially without tantalum or the metal(s) that otherwise makes up the surface coating. The core element further preferably contains Ni at a concentration by weight not more than 50%. 
         [0019]    It is especially an object of this invention that the iron containing core element is a steel, preferably stainless or carbon steel. 
         [0020]    To ensure good corrosion resistance of the surface, the metallised component must have a composition (metallic purity, meaning that any content of e.g. non-metals, Oxygen, Nitrogen, Carbon and so on, is ignored) with a tantalum content of 80% or higher. With a tantalum content of 80% or more, the ability of the surface is substantially identical to that of pure tantalum. 
         [0021]    The object of the invention is further to create an object where the surface coating is ductile and has a good adhesion. It has been experienced, that the ability to attach to an iron containing core element, is highly affected by the structure of the interface between the core element and the tantalum surface. 
         [0022]    This is achieved by a central feature of the present invention, to provide the object with an alloying zone being between a core element and a corrosion resistant surface layer. For example, if the core element is austenitic stainless steel (like AIS 316L), especially the distribution of the concentration of the alloying elements Ni, Cr and Fe is important to the adhesion. 
         [0023]    The interface contains tantalum at an increasing concentration from the core element to the surface layer. The transition between the tantalum surface and the interface, or alloying zone, is defined by the depth where the content of tantalum is 90% of the surface concentration. The transition from the alloying zone to the core element is defined as the depth where the tantalum concentration is 10% of the surface concentration. The alloying zone is in general from 0.1 micrometers to 10 micrometers into the object, or more preferred from 0.3 to 2.0 micrometers. 
         [0024]    In order to ensure an alloying zone with a suitable composition, the process temperature is a critical factor when using a CVD process. At temperatures below 500° C. the diffusion speed of the alloying substances in the object in general is too low to be significant. When using temperatures of 1200° C. and above on a core of stainless steel, it has been experienced that the diffusion speed of Nickel is too high to achieve a suitable structure of the alloying substances. In the interface alloying layers are formed containing high contents of Nickel. Such alloys having high Nickel contents have proven too brittle to give a good attachment or adhesion. As a rule of thumb, to ensure a good adhesion, tantalum containing phases containing more than 20% Nickel may not exist, and the Nickel content in the alloy has to be lower than that of Iron. If the content of Nickel in the alloying zone at some spot is higher than 10 times the content of Iron, there is a risk of a poor adhesion because of the formation of tantalum/Nickel alloys. In the same manner, the Nickel content may nowhere be higher than the tantalum content. For iron based substrates with a nickel content lower than 1% (e.g. Carbon steels) good results are obtained up to a temperature of 1200° C. 
         [0025]    It therefore is a further object of this invention to make an alloying zone between the core element and the coating, wherein the alloying zone comprises the alloying elements Ni, Fe and Ta, but where the concentration by weight of Ni is nowhere higher than 20%, more preferably less than 15%, more preferably less than 10%. 
         [0026]    It is further an object of the present invention to introduce a method to produce such an object, the method comprising the steps of:
   providing a core element ( 2 ) made from a first base material and having an outer surface,   applying a coating layer ( 4 ) of a corrosion resistant material to at least a part of the outer surface of the core element by a CVD process at a temperature between 700 and 1200° C.,   applying said coating layer at a rate that ensures the formation of an alloying zone ( 3 ) between the core element ( 2 ) and the coating layer ( 4 ) having a thickness from where the concentration of said corrosion resistant is 90% of the concentration in the coating layer, to where the concentration of said corrosion resistant material is 10% of the concentration in the coating layer, of at least 0.1 micrometers.   
 
         [0030]    It is further an object of this invention, to make such a corrosion resistant object with a surface sufficiently ductile to be subdued to mechanical processing, such as plastic or elastic deformations, mechanical deformations, rolling, imprinting, drawing etc. 
         [0031]    It is further an object of this invention to provide a method to produce an object with a corrosion resistant surface, and where the surface of the object is subdued to a mechanical modification, such as rolling, imprinting, by stroke or impact. This is achieved by providing a method comprising the steps of:
   providing a core element ( 2 ) made from a first base material and having an outer surface,   applying a coating layer ( 4 ) of a corrosion resistant material to at least a part of the outer surface of the core element by a CVD process at a temperature between 700 and 1200° C.,   applying said coating layer at a rate that ensures the formation of an alloying zone ( 3 ) between the core element ( 2 ) and the coating layer ( 4 ) having a thickness from where the concentration of said corrosion resistant is 90% of the concentration in the coating layer, to where the concentration of said corrosion resistant material is 10% of the concentration in the coating layer, of at least 0.1 micrometers,   mechanically modifying the surface of the object so that the surface of the core element, the alloying zone and the coating layer are affected by the modification.   
 
         [0036]    It is further an object of this invention to provide an object with a corrosion resistant surface, and where the surface of the object is subdued to a mechanical modification, such as rolling, imprinting, by stroke or impact. This is achieved by providing: 
         [0037]    a core element being made from a first base material and having an outer surface, and, 
         [0038]    a coating layer comprising a concentration of at least 70% of a corrosion resistant material covering at least a part of the outer surface of the core element, 
         [0000]    wherein an alloying zone exists between the core element and the coating layer, said alloying zone having a thickness from where the concentration of said corrosion resistant material is 90% of the concentration in the coating layer, to where the concentration of said corrosion resistant material is 10% of the concentration in the coating layer, from 0.1 micrometers to 10 micrometer, and where the surface of the object has been mechanically modified in such a manner that the surface of the core element, the alloying zone and the coating layer are affected by the modification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]      FIG. 1 : is a schematic view of the invention where an alloying zone exists between the core element and the coating. 
           [0040]      FIG. 2 : is a schematic view of porosities in the alloying zone. 
           [0041]      FIG. 3  A&amp;B: is a schematic view of a first embodiment of a surface modification of the object of the invention. 
           [0042]      FIG. 4  A&amp;B: is a schematic view of a second embodiment of a surface modification of the object of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]      FIG. 1  shows is a schematic view of an object ( 1 ) of the invention, where the object comprises the core element ( 2 ) having a surface, a corrosion resistant coating ( 4 ) covering at least part of the surface of the core element ( 2 ), where the corrosion resistant coating consists of at least 80% by weight of tantalum or preferably of a metal of the same group of metals as tantalum, like W, Nb, Mo, Ti, Hf. Between the core element ( 2 ) and the coating ( 4 ) is an interface, or alloying, section ( 3 ) ensuring a good adhesion of the coating ( 4 ). 
         [0044]    The diffusion is controlled by the temperature, otherwise unfavourable diffusion parameters may result in Kirkendall porosity at the coating-base material interface, meaning that if the diffusion fluxes of the alloying elements from the core element ( 2 ) are different from the diffusion fluxes of the alloying elements from the coating ( 4 ), there will be a net flow of matter. Given that there is a net flow of matter there will be an equal and opposite net flow of vacancies, being missing atoms in a crystal structure, and forming pores or porosities. 
         [0045]      FIG. 2  illustrates this general problem, especially being the case when the core element ( 2 ) is steel, or just an Ni containing element, where porosities ( 5 ), being empty pockets or vacuums, exists in the alloying layer ( 3 ) These porosities ( 5 ) give weaknesses in the adhesion of the coating layer ( 4 ) to the core element ( 2 ), because they are weak points where , when the coated object ( 1 ) is being subdued to mechanical deformations, possibly as part of the shaping/manufacturing of the object, or as part of the use of the object, cracks may appear in the coating layer at these weaknesses, thereby creating pinholes to the porosities. 
         [0046]    Such an object ( 1 ) having a sufficient ductile corrosion resistant coating layer ( 4 ) to withstand mechanical deformations, is ensured by forming an alloying zone ( 3 ) between the core element ( 2 ) and the coating ( 4 ) that comprises especially the alloying elements Ni, Fe and Ta, but where the concentration by weight of Ni is nowhere higher than 20%, more preferably less than 15%, more preferably less than 10%. 
         [0047]    This interface or alloying zone ( 3 ) contains tantalum at an increasing concentration from the core element to the surface layer. The transition between the tantalum surface, or the coating, ( 4 ) and the interface, or alloying zone, ( 3 ), is defined by the depth where the content of tantalum is 90% by weight of the content of tantalum in the coating ( 4 ). The transition from the alloying zone ( 3 ) to the core element ( 2 ) is defined as the depth where the tantalum concentration is 10% by weight of the content in the coating ( 4 ). The alloying zone ( 3 ) is in general from 0.1 micrometers to 10 micrometers into the object, or more preferred from 0.3 to 2.0 micrometers. 
         [0048]    Since the temperature is the predominant parameter used to control the diffusion of elements in the alloying zone, where the process temperatures would be in the range from 700° C. to 1200° C., a ‘cold process’ such as sputtering would not be suitable to form the desired alloying zone ( 3 ). Therefore, to apply the coating layer ( 4 ) of a corrosion resistant material to at least a part of the outer surface of the core element, a CVD process at a temperature between 700 and 1200° C. is preferred. 
         [0049]    The coating layer is applied at a rate that ensures the formation of an alloying zone ( 3 ) between the core element ( 2 ) and the coating layer ( 4 ) having a thickness from where the concentration of said corrosion resistant material is 90% of the concentration in the coating layer, to where the concentration of said corrosion resistant material is 10% of the concentration in the coating layer, of at least 0.1 micrometers. 
         [0050]    The process time typically is in the range of 1-20 hours, or more preferably 5-10 hours. 
         [0051]    One critical factor to give the process temperature is the concentration of Ni in the core element ( 2 ), where, the more Ni, the lower temperature is needed, and the less Ni, the higher temperature is tolerable.. 
       EXAMPLE 1 
       [0052]    It was for example found that, when a core element ( 1 ) was made up of austenitic stainless steel (AISI 304 or 316) and a coating was deposited at 950° C., then non-porous, well-adhering coatings were obtained, where the interdiffusion of tantalum and the stainless steel elements, the alloying zone, was roughly 1.5 μm based on visual observation on a microscopical picture. 
       EXAMPLE 2 
       [0053]    Coating a carbon steel substrate with up to 0.5% C at temperatures from 625 to 900° C. gives coatings that are similar to those on stainless steel, but where good adherence is more easily obtained. A coating deposited at 875° C. for 195 min revealed a 1-1.5 μm diffusion zone, or alloying zone, found visually on microscopical pictures. 
         [0054]      FIGS. 3 and 4  are illustrations of a further aspect of the object ( 1 ) of the invention, where the object ( 1 ) is subdued to mechanical processing after the coating ( 4 ) has been applied to the core element ( 2 ). 
         [0055]      FIG. 3A  shows a core element ( 2 ) with some kind of protrusions ( 6 A) at the surface, where a corrosion resistant surface coating ( 4 ) is deposited on at least a part of the surface of the core element ( 2 ), and where an alloy zone ( 3 ) is formed between the core element ( 2 ) and the coating ( 4 ).  FIG. 3B  shows that these protrusions ( 6 A) have then been reshaped by some not further specified mechanical process. 
         [0056]    An example is that structures are formed into the surface of the object ( 1 ) after the tantalum/refractory layer is deposited. This could e.g. be to shape flow channels in the surface for fuel cells. Therefore it is essential that the object has a dense and ductile surface, meaning that at least the surface layer ( 4 ) and the alloy layer ( 3 ) are ductile.  FIG. 4A  illustrates such an embodiment, where an object ( 1 ) is seen formed with a substantially flat surface. By any known means, channels ( 7 ), or other surface structures, are formed into the surface of the object ( 1 ) as seen in  FIG. 4B . 
         [0057]    For all of the objects of the illustrations in  FIGS. 3 and 4 , it is essential that the surface layer ( 4 ) and the alloy zone ( 3 ) are sufficiently ductile to absorb or withstand the forces from the mechanical processing, without cracking or otherwise loosing the corrosion resistance. 
         [0058]    While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present.