Patent Application
20080057275
In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art, having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as not to obscure the description of the present invention. Finally, wherever applicable, like reference numerals refer to like elements.
Piping and other vessels (e.g. fining vessel, stir chamber, etc) used in the glass making industry are often fabricated from high melting temperature, oxidation resistant refractory metals. Such metals are typically so-called precious or noble metals, and therefore expensive. Of particular importance as refractory metals are the platinum group metals: ruthenium, rhodium, palladium, osmium, iridium and platinum, and alloys thereof. Their resistance to chemical attack and their excellent high temperature performance have lead to extensive use of platinum group metals in various refractory applications. However, it should be understood that the methods described herein are applicable to other non-platinum group metals, including many of the transition metals, such as, for example, molybdenum or rhenium, or alloys thereof.
Illustrated in
In spite of the precautions noted above, it has been found that such secondary coatings may themselves be susceptible to cracking, and while effective in preventing overall corrosion of the vessel surface, the secondary coating may in fact, in some cases, exacerbate corrosion of the refractory metal vessel surface by inducing oxygen pitting (i.e. pitting of the surface of the refractory metal via selective and accelerated oxidation).
For example, volatilization of platinum above 1200° C., as an oxide, is proportional to the partial pressure of oxygen to which the platinum is exposed. Thermodynamically, the equilibrium between the metal, oxygen and the gaseous oxide may be expressed as
xM+½yO2⇄MxOy (1)
where M represents the metal. The equilibrium constant k may be written as
k=p(MxOy)/(p(O2)1/2ya(M)x) (2)
where a(M) is the activity of the metal M and p represents a partial pressure.
When a steady, measured flow of an inert carrier gas is passed over a refractory metal specimen maintained at a constant temperature, vapor of the metal is removed at a rate which is dependent upon the partial pressure of the vapor. At low flow rates, the carrier gas is more likely to be saturated with the vapor because the contact time between the carrier gas and the sample is longer. If, over a range of flow rates the same mass of volatile metal species is transported for a given volume of gas, then the carrier gas is considered to be perfectly saturated. Put another way, if the mass loss is directly proportional to the flow rate, then the gas is saturated. If the mass loss is independent of the gas flow rate, then the gas is unsaturated.
Without wishing to be bound by theory, it is believed that under low flow or quasi static conditions, the refractory metal oxide saturates the volume around the test sample, providing an “equilibrium” loss rate, and the loss of metal is directly proportional to the gas flow rate in this regime. As long as the gas remains saturated, the amount of metal loss remains a function of how quickly the saturated gas is being removed. As flow rates are increased, a point may be reached where the metal oxide vapor cannot maintain saturation of the volume surround the sample and the regime switches to a “non-equilibrium” regime. In the non-equilibrium regime the metal loss rate is determined by other mechanisms, such as surface desorption rate. The non-equilibrium loss rate is independent of gas flow rate but dependant on the geometry of the setup: (1) the free space volume above the refractory metal surface; and (2) the amount of open, non-occluded metal surface area bordering the free space volume.
Refractory metal samples with larger surface areas of exposed metal and smaller free space volumes will maintain a saturated environment more easily and require a higher gas flow rate before switching to a non-equilibrium metal loss regime. In contrast, large free space volumes relative to the area of non-occluded metal surface will switch to non-equilibrium metal loss rates even at low gas flow rates. This latter case is representative of the situation when cracks are present in the protective ceramic layer. Cracks represent a small area of exposed metal which favors metal loss at the non-equilibrium rate. Thus two samples under the same temperature, flow rates and oxygen partial pressures can have very different metal loss rates, since the metal loss rate is also dependent on the geometrical setup variables mentioned above. Indeed, it has been found that a refractory metal vessel (e.g. a platinum rhodium alloy) covered by a protective ceramic coating can experience metal loss at sites where the ceramic coating is cracked which are 5× higher than unprotected areas of the vessel. It is believed that this high rate of loss is due to the environment surrounding the region of the crack being in the “non-equilibrium loss regime. Such a metal loss can be seen by examining
In accordance with an embodiment of the present invention, a method to drive the metal loss into a slower “equilibrium” loss rate regime and reduce oxidation of the refractory metal layer in contact with the glass is presented whereby a free space volume above a protective coating or barrier layer is saturated with an oxide of the refractory metal. Thus, cracks which may later form in the barrier layer expose the underlying refractory metal inner layer to the refractory metal oxide saturated atmosphere.
Barrier layer 32 is disposed adjacent to outer surface 44 of refractory metal layer 30. Barrier layer 32 may be a flame sprayed or a plasma sprayed refractory oxide, or any other coating intended to provide oxidation protection of the refractory metal inner layer. For example, barrier layer 32 may comprise alumina or zirconia. Structural jacket 42 is disposed about barrier layer 32 and largely in contact with barrier layer 32. Structural jacket 42 is preferably formed from a refractory oxide suitable for slurry casting. A suitable refractory oxide may be, for example, alumina or zirconia
In accordance with an embodiment of the present invention and depicted in
It should be understood that although
Sacrificial metal member 50 should have as a constituent a metal which comprises inner layer 30, preferably a metal selected from ruthenium, rhodium, palladium, osmium, iridium, platinum, rhenium and alloys thereof. Sacrificial member 50 may be in the form of a sheet of metal disposed proximate barrier layer 32, or sacrificial metal member 50 may, for example, be a mesh or screen, paste or foil. If inner layer 30 is an alloy, sacrificial metal member 50 should include a majority constituent which is the same as the majority constituent of the inner layer alloy. For example, in the case where inner layer 30 is an 80% platinum 20% rhodium alloy, the majority constituent of the sacrificial metal member should be platinum. The weight percent makeup of sacrificial metal member 50 need not be the same as the composition of inner layer 30 however. In this particular example, the sacrificial metal member could be 100% platinum.
It is not necessary that the sacrificial metal member be excessively thick, and may be, for example, less than about 50 microns thick. However, the sacrificial metal member or members may have a thickness on the order of hundreds of microns, or more, albeit at higher cost. Preferably, the sacrificial metal member, particularly if in the form of a layer, is less than about 500 μm thick.
Indeed, the form and placement of the sacrificial metal member is a tradeoff between the ability to provide the sacrificial metal member proximate a location on barrier layer 32 most likely to develop a crack which may expose the underlying inner refractory metal layer 30 to oxidation, and the added cost of the additional metal. Ideally, one may wrap the entire system in a layer of sacrificial metal member, thus anticipating a crack at any point. However, this practice must be weighed against the cost of the additional, and generally expensive, refractory metal. To this end, several solutions may be applied, including reducing the amount of sacrificial metal member while still maintaining a significant surface area coverage, such as by employing a metal mesh, or applying the sacrificial metal member layer only in those areas where cracks are most likely to occur (e.g. transition joints/couplings, areas prone to vibration or other mechanical shock, etc.). These two philosophies are not mutually exclusive and may be applied simultaneously. That is, the sacrificial metal member 50 that is applied to selected high-risk areas prone to cracking could be a metal mesh which minimizes the amount of sacrificial metal member needed, while maximizing the overall protected surface area as depicted in
The sacrificial metal member may be applied in a number of ways, depending upon the form and the composition of the metal. For example, as previously described the metal could be a layer, and applied by plasma spraying, flame spraying, sputtering, or even wrapping as a foil layer.
In another variant, depicted in
In still another embodiment of the present invention illustrated in
It should be emphasized that the above-described embodiments of the present invention, particularly any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.
