Patent Application
20100077887
Formulations comprising pulverulent hard materials and pulverulent binder metals are used industrially to produce, inter alia, cemented hard materials or spray powders for surface coating. In the case of carbides, by far the most frequently used carbide is tungsten carbide; others such as titanium, vanadium, chromium, tantalum and niobium carbide or their mixtures with one another or with tungsten carbide are usually used only as additives. It is also possible to use nitrides. Cobalt is by far the most frequently used binder metal, but binder systems comprising 2 or 3 elements from among Fe, Co and Ni are also used; in spray powders also, for example, Mn, Al, Cr. Further possible inorganic additives are metal powders such as tungsten, molybdenum, and also elemental carbon. If cemented hard material contains titanium carbonitride instead of tungsten carbide as main component, it is referred to as “cermet”. Further possible hard materials are borides.
As binder metal in cemented hard materials and spray powders, use is usually made of cobalt, but nickel or an alloy of Fe, Co and Ni can also be used. In all cases, the binder phase after sintering or thermal spraying contains proportions of, for example, tungsten, chromium, molybdenum and carbon derived from the hard material as a result of exchange of elements with the carbide phase in liquid-phase sintering or fusion. Pulverulent binder metals used are either element powders such as iron, nickel or cobalt powders or else alloy powders.
The binder phase of spray powders can comprise not only the abovementioned elements and inorganic additives but also other elements such as Al, rare earths, yttrium.
Over the decades, a statistically significant increased occurrence of pulmonary fibrosis having a specific appearance pattern has been observed in the cemented hard materials industry and has been associated with handling dust-like cemented hard material or handling dust-like formulations for producing cemented hard material. The disease is also referred to as “cemented carbide lung” and was and is subject matter of numerous epidemiological studies and publications. In the customary production of cemented hard material by powder-metallurgical production processes, i.e. pressing and sintering of pulverulent hard material/binder formulations, respirable dusts are set free due to the nature of the process. If the cemented hard material is machined by grinding in the sintered or presintered state, very fine, respirable dusts (“grinding dusts”) are likewise formed.
Thermal spraying of carbide-containing spray powders likewise results in formation of very fine dusts (“overspray”).
It has likewise been known since about 5 years ago that cemented hard material dusts additionally have an acute toxic effect on rats after inhalation if the concentration is high enough. The precise mechanism of action has not been known hitherto. The two components tungsten carbide and cobalt do not have this effect on their own. In the interests of improving occupational hygiene, there is therefore strong interest in elucidating the mechanism of action and in substitutes which do not have an acute toxic effect or have a greatly reduced acute toxic effect.
It was an object of the present invention to provide cobalt in a formulation which reduces the inhalation toxicity both during thermal spraying of the formulation and during grinding machining of presintered cemented hard material parts (“grey machining”) and cemented hard materials. This object is achieved by a formulation comprising at least one hard material powder and at least 2 binder metal powders, characterized in that all the cobalt is present in the first binder metal powder and is prealloyed with one or more elements of groups 3 to 8 of the Periodic Table of the Elements and at least one further binder metal powder from the group consisting of powders of the elements Fe, Ni, Al, Mn, Cr and alloys of these elements with one another is present and the further binder metal powders do not contain any cobalt in unprealloyed form.
It has surprisingly been found that the acute toxic effect of dust-like formulations of tungsten carbide with cobalt is based on an electrochemical corrosion phenomenon which leads to increased bioavailability of the cobalt after inhalation.
Furthermore, it has surprisingly been found that cobalt as binder metal in hard material/binder formulations loses its inhalation toxicity when it is prealloyed with iron or another element of groups 3 to 8 (transition groups IIIa to VIIIa) of the Periodic Table of the Elements, but not when these are present in unalloyed form alongside the cobalt. In principle, all metals which are positioned to the left of cobalt in the Periodic Table and are preferably located in the same period produce a reduction in the corrosion tendency as a result of their less noble character while elements which are more noble, for example copper, have the opposite effect, which can even be confirmed in the case of alloyed-in copper which is present as an additional phase.
The alloying partner of cobalt in the first binder metal powder is advantageously an element of the fourth period and of groups 3 to 8 of the Periodic Table. The alloying partner of cobalt in the first binder metal powder is particularly advantageously an element selected from the group consisting of Fe, Ni, Cr, Mn, Ti and Al. The first binder metal powder can also contain further elements such as aluminium and/or copper.
Apart from the first binder metal powder, further binder metals are usually necessary. These are particularly advantageously selected from the group consisting of iron powders, nickel powders, FeNi alloy powders and prealloyed FeNi alloy powders.
The hard material is usually titanium carbide, vanadium carbide, molybdenum carbide, tungsten carbide or a mixture of these with one another. These compounds are also known as catalysts for the reduction of oxygen and thus as catalysts for the oxidation of metals in aqueous media by the mechanism of oxygen reduction:
Co+1/2O2+H2O=CO(OH)2
In the case of spray powders, the at least one further added metal powder can contain Fe, Ni and, for example, further elements such as Al, Cr, Mn, Nb, Ta, Ti, but no cobalt except in the range of unavoidable and unintended impurities.
The first, cobalt-containing and completely alloyed binder metal powder preferably contains from 10% by weight to 50% by weight of cobalt. Particular preference is given to a ratio of iron to cobalt of 1:1 or more. Suitable compositions are, for example, FeCo 50/50, FeCoNi 90/5/5. This powder can additionally contain further elements of the iron group.
The further binder metal powder or powders which do not contain any cobalt in unprealloyed form is/are preferably iron- or nickel-based, i.e. the sum of the content of iron and nickel is at least 50%. The remainder of the further powder or powders comprises a total of at least 50% of iron and nickel. As further binder metal powders, it is advantageous to use alloy powders of the composition: for example, FeNi powders containing up to 30% of Fe, FeNi 50/50, FeNi 95/5.
The weight ratio of the first binder metal powder to the further powder or powders is preferably from 1:10 to 10:1, but particularly preferably from 1:5 to 5:1. A person skilled in the art can choose the required ratios on the basis of the desired stoichiometry and the alloy powders available.
The further binder metal powders advantageously have a BET surface area of greater than 0.3 m2/g, more advantageously greater than 0.5 m2/g, in particular greater than 1 m2/g.
In the cemented hard material and spray powder industry, the use of prealloyed powders which contain two or more elements from the group consisting of Fe, Co, Ni and represent the composition of the binder phase in respect of these elements is prior art as is the use of two or three element powders for producing this formulation. While the latter variant does not reduce the toxicity, the toxicity is reduced or eliminated by complete alloying of the binder system. Such alloy powders from hydrogen reduction of oxides or other compounds are commercially available, but have considerable disadvantages compared to the element powders, for example higher oxygen contents and poor pressability. Ni and Fe powders in particular can be produced by the carbonyl process and achieve very low oxygen contents since the reduction potential of carbon monoxide is greater than that of the hydrogen which is usually employed for producing fine alloy powders having specific surface areas of greater than 1 m2/g.
Advantageous formulations are therefore, for example, formulations which are obtained by a process for producing a hard material/binder mixture by use of a) at least one prealloyed powder selected from the group consisting of iron/cobalt and iron/nickel/cobalt; b) at least one element powder selected from the group consisting of iron and nickel or a prealloyed powder selected from the group consisting of iron/nickel which is different from component a); c) hard material powder, where the overall composition of the components a) and b) together contains a maximum of 90% of cobalt and a maximum of 70% by weight of nickel. The iron content is advantageously at least 10% by weight.
In an advantageous embodiment of the invention, it is a process for producing a hard material/binder mixture according to claim 1, in which the overall composition of the binder is max. 90% by weight of Co, max. 70% by weight of Ni and at least 10% by weight of Fe, where the iron content satisfies the inequality
(where Fe: iron content in % by weight, % Co: cobalt content in % by weight, % Ni: nickel content in % by weight),
and at least two binder powders are used, one binder powder is lower in iron than the overall composition of the binder and the other binder powder is richer in iron than the overall composition of the binder and at least one binder powder is prealloyed from at least two elements selected from the group consisting of iron, nickel and cobalt.
Since chemical exchange between the binder phase and the carbide phase and between the melting particles of the binder metal powders occurs during thermal spraying and also during liquid-phase sintering of pressed formulations for producing cemented hard materials, it is sufficient to use element powders from a materials point of view, while from a toxicological point of view according to the above examples it is sufficient to prealloy only the cobalt content with a minimum content of iron, nickel, manganese, chromium or titanium and the remainder of the desired overall composition of the binder metal phase, by means of which, for example, the iron and/or nickel content or content of further metals is set, in the form of the corresponding element powder or, for example, an FeNi alloy powder. This novel procedure in the production of formulations makes it possible to satisfy both aspects (toxicology and oxygen content or control of the carbon content after sintering). It is also advantageous that the pressibility is significantly improved by the only partial use of prealloyed powders compared to the exclusive use of prealloyed powders.
Formulations as shown in Table 1, in which the first and second binder metal powders are present in a ratio of 1:1, are therefore particularly advantageous:
Preference is also give to formulations of Tables 2 and 3.
Table 2: Table 2 shows 54 formulations having the numbers 2.01 to 2.54 whose first binder metal powder, further binder metal powder and ratios of the alloy elements of the first binder metal powder and the second binder metal powder are identical to those of Table 1, with the first binder metal powder and the further binder metal powder being present in a ratio of 1:2. This means that in the case of the formulation 2.01, the first alloy powder is FeCo 50/50, the further alloy powder is FeNi 30/70 and the ratio of FeCo to FeNi is 1:2.
Table 3: Table 3 shows 54 formulations having the numbers 3.01 to 3.54 whose first binder metal powder, further binder metal powder and ratios of the alloy elements of the first binder metal powder and the second binder metal powder are identical to those of Table 1, with the first binder metal powder and the further binder metal powder being present in a ratio of 2:1. This means that in the case of the formulation 3.01, the first alloy powder is FeCo 50/50, the further alloy powder is FeNi 30/70 and the ratio of FeCo to FeNi is 2:1.
The present invention therefore provides metal formulations comprising at least one hard material powder and at least 2 binder metal powders, characterized in that all the cobalt is present in the first binder metal powder and is prealloyed with one or more elements of groups 3 to 8 of the Periodic Table of the Elements which are elements of the fourth period and at least one further binder metal powder from the group consisting of powders of the elements Fe, Ni, Al, Mn, Cr and alloys of these elements with one another is present and the further binder metal powders do not contain any cobalt in unprealloyed form;
or
metal formulations comprising at least one hard material powder and at least 2 binder metal powders, characterized in that all the cobalt is present in the first binder metal powder and is prealloyed with one or more elements of groups 3 to 8 of the Periodic Table of the Elements and at least one further binder metal powder from the group consisting of powders of the elements Fe, Ni, Al, Mn, Cr and alloys of these elements with one another is present and the further binder metal powders do not contain any cobalt in unprealloyed form, where the free corrosion potential between the hard material and the first binder metal powder, measured in air-saturated water at atmospheric pressure and room temperature, is less than 0.300 volt (preferably less than 0.280 volt), with the hard material having the positive polarity;
or
metal formulations comprising at least one hard material powder and at least 2 binder metal powders, characterized in that all the cobalt is present in the first binder metal powder and is prealloyed with one or more elements of groups 3 to 8 of the Periodic Table of the Elements and at least one further binder metal powder selected from the group consisting of iron powders, nickel powders, FeNi alloy powders and prealloyed FeNi alloy powders is used and the further binder metal powders do not contain any cobalt in unprealloyed form. In all these three above metal formulations, the hard material present can be, in particular, titanium carbide, vanadium carbide, molybdenum carbide or tungsten carbide, which advantageously has a BET surface area of greater than 0.3 m2/g, preferably greater than 0.5 m2/g, particularly preferably greater than 1 m2/g.
In further embodiments of the invention, the alloying partner of the cobalt in the first binder metal powder in the above metal formulations is an element of the fourth period;
or the alloying partner of the cobalt in the first binder metal powder in the above metal formulations is an element selected from the group consisting of Fe, Ni, Cr, Mn, Ti and Al;
or
the first binder metal powder in the above metal formulations can contain further alloyed elements, with aluminium and/or copper (Cu) being able to be used as further elements.
In further embodiments of the present invention, one or more further binder metal powders selected from the group consisting of iron powders, nickel powders, FeNi alloy powders and prealloyed FeNi alloy powders are present in addition to the first binder metal powder.
In all these above metal formulations, the free corrosion potential between the hard material and the first binder metal powder, measured in air-saturated water at atmospheric pressure and room temperature, is less than 0.300 volt, with the hard material having the positive polarity.
Hard materials which can be present are, in particular, titanium carbide, vanadium carbide, molybdenum carbide or tungsten carbide, which advantageously have a BET surface area of greater than 0.3 m2/g, preferably greater than 0.5 m2/g, particularly preferably greater than 1 m2/g.
In all such metal formulations, the weight ratio of the first binder metal powder to the further binder metal powder or powders is advantageously from 1:10 to 10:1. All such metal formulations can advantageously contain: a) at least one prealloyed powder selected from the group consisting of iron/cobalt and iron/nickel/cobalt; b) at least one element powder selected from the group consisting of iron and nickel or a prealloyed powder comprising iron and nickel which is different from component a); c) hard material powder, where the overall composition of the components a) and b) together contains a maximum of 90% of cobalt and a maximum of 70% by weight of nickel. In such a metal formulation, the iron content is advantageously at least 10% by weight.
In such a metal formulation, the overall composition of the binder is advantageously max. 90% by weight of Co, max. 70% by weight of Ni, and at least 10% by weight of Fe, where the iron content satisfies the inequality
(where Fe: iron content in % by weight, % Co: cobalt content in % by weight, % Ni: nickel content in % by weight) and at least two binder powders of which one binder powder is lower in iron than the overall composition of the binder and the other binder powder is richer in iron than the overall composition of the binder and at least one binder powder is prealloyed from at least two elements selected from the group consisting of iron, nickel and cobalt are used.
Such metal formulations are advantageous for various applications and such metal formulations can be used for producing cemented hard material or for producing porous sintered agglomerates.
Such a porous agglomerate can be obtained by sintering without pressing of one of the above metal formulations.
Thermal spray powders containing such a porous agglomerate which can be obtained by sintering without pressing of one of the above metal formulations and contains Al, yttrium and/or rare earths are also suitable.
The present invention further provides a method of controlling the toxic effect of cobalt-containing metal formulations, characterized in that one of the above metal formulations, advantageously metal formulations as shown in Tables 1 to 3, is used for producing cemented hard material or porous sintered agglomerates.
In general, the present invention provides a method of controlling the toxic effect of cobalt-containing metal formulations, which is characterized in that the cobalt is prealloyed with one or more elements of groups 3 to 8 of the Periodic Table of the Elements in the metal formulation.
The present invention therefore also provides a method of controlling the toxic effect of cobalt-containing metal formulations, in which a metal formulation according to the invention, a porous agglomerate according to the invention or a thermal spray powder according to the invention is used for producing shaped bodies or coatings. In the method of controlling the toxic effect of cobalt-containing metal formulations, the toxicological effect is, in particular, pulmonary fibrosis and/or the disease cemented carbide lung.
Since the high bioavailability of cobalt is based on an electrochemical corrosion phenomenon, the free corrosion potential between the hard material and the first binder metal powder, measured in air-saturated water at atmospheric pressure and room temperature, is, according to the invention, less than 0.380 volt, preferably less than 0.330 volt, in particular less than 0.300 and very particularly advantageously less than 0.280 volt, with tungsten carbide having the positive polarity.
However, the contact voltage surprisingly decreases when the cobalt is alloyed with iron, even though iron is less noble than cobalt. The reason for this phenomenon is not known. It is easy to see that the decreasing free corrosion potential results in the driving force of the corrosion phenomenon decreasing or corrosion proceeding more slowly, and the bioavailability likewise decreasing. The free corrosion potential of the measurement set-up described in Example 4 can therefore serve as an indicator of the inhalation toxicity of a hard material/binder metal formulation which is to be expected. A further indicator of the inhalation toxicity to be expected is the amount of dissolved binder metal which goes into solution as soon as a corresponding contact element is in contact with water in the presence of oxygen over a defined period of time.
The cause of the phenomenon of inhalation toxicity, which can only be explained by a high degree of interaction of the organism with the inhaled dust, has to be a synergy between the two components cobalt and hard material, since either of them alone have been found not to display this behaviour, which is known from the literature. In addition, since a dependence on the intensity of geometric contact of the two components has been found in the present invention, an increased bioavailability caused by corrosion appears to be the likely explanation for the toxicological effect. Cemented hard material has long been known as a contact corrosion element. For example, it is known that water-based cooling fluids as are used for the grinding of cemented hard materials preferentially dissolve cobalt from the cemented hard material. The thesis by Megede (Universität Frankfurt a. Main, 1985) examines the mechanism in detail: cobalt corrodes in the presence of water and oxygen by the principle of oxygen reduction and forms a hydroxide layer which has a passivating effect on the surface. Tungsten carbide catalyses the electron transfer in the formation of the hydroxide anion, so that corrosion is greatly accelerated and proceeds topotactically. The passivating effect of the hydroxide layer is thus undermined. This likewise explains why tungsten carbide but no cobalt is found in sections of cemented carbide lungs—the cobalt has obviously been corroded and resorbed in an accelerated fashion. The resulting increased bioavailability of cobalt in small doses/concentrations leads to chronic disorders (pulmonary fibrosis or “cemented carbide lung”), and in the case of high concentrations to acute toxic phenomena. The bioavailability of cobalt has a negative effect on the organism which has not been fully elucidated hitherto. Attempts at explanations include addition of ionic cobalt onto DNA or stabilization of reactive oxygen species such as the hyperoxide anion by complex formation, for which cobalt is known.
In the case of cemented hard materials and carbidic spray powders, the corrosion resistance, which is determined by the chemical attack on the binder, can be improved by adding Cr carbide or Cr metal to the formulation. In both cases, the Cr is partly present in alloyed form in the binder after sintering or thermal spraying. If the Cr concentration in the binder is sufficiently high, which can be controlled by means of the carbon balance, the cemented hard material or the spray layer is then considerably more corrosion resistant, from which it can be concluded that the dust in the case of grinding such cemented hard materials or the overspray has to be significantly less toxic than pure WC—Co. A further improvement in the corrosion resistance can be achieved by partial replacement of the cobalt by nickel, which is likewise industrial practice in the case of cemented hard material.
In conclusion, the acute toxic action of cemented hard material dusts can be correlated with the corrosion rates in the presence of water and oxygen. The free corrosion potential can be reduced by alloying the cobalt with, for example, iron, as a result of which a cobalt-containing formulation in which the cobalt is prealloyed with iron is significantly less acutely inhalation toxic. This is supported by the finding that cemented hard materials whose binder phase contains cobalt and iron have a better corrosion resistance against oxidizing acids in the presence of air than purely cobalt-bonded materials (TU Wien, thesis by Wittmann, 2002).
It can be predicted that some intermediates in the production of cemented hard material will be particularly inhalation toxic, including, in particular, the dust from the grinding machining of presintered cemented hard material parts (“grey machining”). Here, the formulation is pressed and sintered at a temperature below the melt eutectic (“presintering”) so that sufficient mechanical strength for the sintered body to be machined by grinding is obtained as a result of sinter bridges. In this state, the sintered body is still porous, no longer contains any organic additives and the powders used have not yet equilibrated in the formulation, so that cobalt is still largely present in elemental form. This combined with the porous structure of the grinding dust means that a very high inhalation toxicity is to be expected. Even in the case where not only cobalt metal powder but also iron metal powder has been used for producing the formulation, no reduction in the toxicity can be expected since virtually no interdiffusion (=alloy formation) between cobalt and iron particles occurs during presintering.
Spray powders sintered from granulated formulations are difficult to disperse in air because of their size, but the respirable fines formed as a result of internal friction during handling of the powders are very toxic (see Example 1e).
The formulations according to the invention can, for example, be used for producing cemented hard material or porous sintered agglomerates, with the porous sintered agglomerates being able to be advantageously used in thermal spray powders. Cemented hard materials having binder systems based on FeCoNi, in particular, offer, depending on the composition, technical advantages over purely cobalt-bonded materials in many applications and are therefore advantageous according to the invention.
Prealloyed powder is, according to the invention, a metal powder which contains the composition of the binder in respect of the Fe, Co and Ni contents in atomically dispersed form in each powder particle. Prealloyed powders within the meaning of the invention can be alloy powders atomized from the melt or alloy powders obtainable by precipitation and reduction, for example as described in U.S. Pat. No. 6,554,885, EP-A-1079950 and the documents cited there, or be produced by other processes which are suitable in principle, e.g. carbonyl processes, plasma processes, CVD, etc., with alloy powders obtainable by precipitation and reduction, for example as described in U.S. Pat. No. 6,554,885, EP-A-1079950 and the documents cited there, being advantageous. The production of carbidic spray powders corresponds to the production of the granulated formulation in the production of cemented hard materials, but the granules are not pressed but instead sintered as such at temperatures either below or slightly above the lowest eutectic temperature and then classified. The organic additives present are removed in the step. The particles obtained in this way are still porous and have sinter necks between the particles representing the binder metal phase and the hard materials.
Spray powders can contain other elements such as Al, rare earths, yttrium, in addition to the abovementioned elements and inorganic additives in the binder phase.
Formulations for producing cemented hard materials and spray powders usually contain not only the abovementioned inorganic constituents but also organic additives such as paraffins, polyethylene glycols, inhibitors, which aid further processing and handling but are no longer present in the cemented hard material or after ignition in the spray powder. These formulations can have been granulated, e.g. by spray drying. It is also possible for plasticizers as are used in extrusion, for example polyethylenes and paraffin waxes, and bonding agents such as carboxylic acids and dispersants to be present.
Industrially customary formulations composed of hard materials and binder metals always also contain oxygen, since the surface of the powders becomes coated with water and hydroxides as a result of handling in air, milling and mixing in aqueous liquids and subsequent drying. The oxygen present reacts with the carbon present in the carbides or in elemental form in the formulation during subsequent thermal treatment to form carbon monoxide and carbon dioxide and thus upsets the equilibrium between metal content and carbon content of the sintered body or the spray powder which has to be maintained precisely. In general, the oxygen content of a formulation has to be kept as low as possible in order to be able to control the metal/carbon equilibrium better.
All examples were carried out as inhalation studies as prescribed by the EEC (annex II.5.2.3) by Huntingdon Life Sciences Ltd., Cambridge, GB, on behalf of the applicant. The powders to be examined were atomized as aerosol and this was blown into a chamber in which 10 rats were present. Aerosol concentrations are reported in mg/l, and the average particle size in μm. Proportion>7 μm in percent; hours are abbreviated to h. The dust concentration and the size distribution of the particles in the chamber were determined (Marple Cascade Impactor Mod. 298, manufactured by Graseby Andersen Inc, Atlanta, Ga.). After 4 h, the number of dead or dying rats was determined and the total number is reported as mortality.
The results show that the inhalation toxicity of WC/Co formulations depend on various influencing factors.
The highest toxicity is shown by Example a). Due to the way in which it is produced, it gives a maximum measure of contacts between the cobalt particles and the tungsten carbide particles.
Example b), which as a powder mixture has far fewer contacts between cobalt particles and WC particles, is less toxic.
Example c) likewise displays, as a powder mixture but with a reduced cobalt content, a once again reduced effect.
Example d), carried out using 2 concentrations, shows a further reduced toxic effect. Since the contact between the cobalt particles and the tungsten carbide particles would be very intensive due to the attritor milling, the reduced toxicity is attributed to hydrophobicization by the paraffin wax present (2% by weight corresponding to 25% by volume).
Example e) shows the toxicity of a typical powder for thermal spraying. It should be noted here that only part of the powder can get into the lungs because of the comparatively coarse particles but a significant mortality nevertheless occurs.
When Examples a) to f) are compared, it can be seen that, in addition to the ability to get into the lungs and any content of hydrophobicizing agents, the intensity of contact between Co and WC is the main influencing parameter on the degree of inhalation toxicity.
As a typical industrial grinding dust from final machining by grinding of cemented carbide, Example a) displays a comparatively very high toxicity. The iron content of 12% is due to abrasion of grinding disks and other contamination, but not to final machining of cemented carbides having an iron-containing binder system. The iron content is thus not prealloyed with the cobalt content. This grinding dust is not a formulation within the meaning of the invention, since it has not been produced in a targeted manner and the cobalt content is not prealloyed with iron.
Example b), produced using element powders Fe and Co, displays a toxicity of a similar order of magnitude to that of an attritor-milled formulation containing 5% of Co without further additives.
Example c) does not display any toxicity, even at 5 mg/l, although in this case the contact between the WC particles and the prealloyed FeCo particles is just as intensive as in Example 1a) and the composite was produced analogously.
An inhalation experiment was carried out using a mixture of 10% of a prealloyed FeNi 50/50 with 90% of tungsten carbide. In this, a mortality rate of 0% occurred even at an effective aerosol concentration of from 0.53 to 5.22 mg/l.
The example shows that no acute inhalation toxicity occurs, which is attributed to the absence of cobalt.
Tungsten carbide powder was hot pressed at 2200° C. in a hot press to produce a solid body having a density of 15.68 g/cm3, which corresponds to the theoretical density. In addition, cobalt metal powder and a prealloyed iron-cobalt metal powder (cobalt content: 50%) were pressed at 1000° C. to give dense bodies having virtually the theoretical density. In a first experiment, the contact voltage of the electrochemical couple tungsten carbide/cobalt was measured by providing two solid pieces with power outlet electrodes for measuring the contact voltage and dipping this arrangement partly into air-saturated tap water. Without mutual contact of the two solid bodies, a difference of 0.330 volt was measured, with cobalt having a negative polarity relative to the tungsten carbide. This difference represents the free corrosion potential. When the solid bodies were in contact (short circuit), a difference of 0.04 mV was measured, with reversal of the polarity being observed.
The measurement was repeated with the cobalt piece being replaced by that produced from FeCo. The measured value for the free corrosion potential was then 0.240 volt with the polarity being preserved. When the solid bodies of FeCo and tungsten carbide were in contact, a difference of 0.007 mV was measured, with reversal of the polarity occurring.
When Examples 1) to 3) are compared with one another, it becomes clear that the presence of elemental cobalt in contact with tungsten carbide is a necessary prerequisite for inhalation toxicity to occur, but that the concentration required is greater by a factor of at least 20 or more when the cobalt is prealloyed with equal parts of iron.
Example 4) demonstrates that the contact voltage or free corrosion potential between WC and cobalt, which according to the laws of electrochemistry which are known to those skilled in the art depends critically on the concentration of molecular oxygen in the water, makes an appreciable contribution. The 0.33 V measured here compare well with the value of Mori et al. of from 0.301 to 0.384 V (R&HM 21, 135 (2003)) obtained from potentiometric measurements on cemented carbides. However, the contact voltage surprisingly drops when the cobalt is alloyed with iron, although iron is less noble than cobalt. The reason for this phenomenon is not known. It can easily be seen that the decreasing free corrosion potential results in the driving force of the corrosion phenomenon decreasing or corrosion proceeding more slowly and the bioavailability likewise decreasing. The free corrosion potential of the measurement set-up described in Example 4 can therefore serve as an indicator of the inhalation toxicity of a hard material/binder metal formulation which is to be expected. A further indicator of the inhalation toxicity to be expected is the amount of dissolved binder metal which goes into solution as soon as a corresponding contact element is in contact with water in the presence of oxygen over a defined period of time.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 004 937.6 | Jan 2007 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2008/050845 | 1/25/2008 | WO | 00 | 7/27/2009 |
