WO1998031197A1

WO1998031197A1 - Improved coatings for electrical, metal sheathed heating elements

Info

Publication number: WO1998031197A1
Application number: PCT/US1998/000438
Authority: WO
Inventors: Thanh D. Huynh
Original assignee: Emerson Electric Co.
Priority date: 1997-01-07
Filing date: 1998-01-07
Publication date: 1998-07-16
Also published as: AU6239298A

Abstract

Heating elements having novel coatings of a refractory metal, refractory metal oxide, ceramic, inter-metallic, or a mixture thereof have been discovered. Such coatings provide the heating element with excellent corrosion protection and therefore, are especially useful for heating elements which will be subjected to highly corrosive environments. In addition, the coatings are easily applied and may be repaired if necessary.

Description

IMPROVED COATINGS FOR ELECTRICAL. METAL SHEATHED HEATING ELEMENTS

Field of the Invention The present invention relates to improved coatings for electrical, metal sheathed heating elements and, more particularly, coatings for such heating elements comprised of a refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixtures thereof.

Background of the Invention Heating elements are widely employed in order to raise the temperature of such media as liquids and gases. When utilizing a heating element to raise the temperature of a medium, the element is typically immersed in the liquid or gas which is desired to be heated. The immersed element is then heated via a power source such that the generated heat is radiated to the liquid or gas and the temperature of the liquid or gas rises. Unfortunately, in many instances the liquid or gas to be heated is itself corrosive to the heating element, contains corrosive substances, or contains substances which become corrosive upon heating. Corrosion of the heating element often results which necessitates excess repair and replacement costs to the unit which contains the heating element.

Because of the corrosion problem described above, heating elements are often encapsulated with a thick, i.e. from about 0.32 to about 0.65 centimeters, layer of a substance such as a synthetic fluorine containing resin. Such synthetic resins, for example, Teflon™, are well-known coating agents for many items, including cookware, because of their "non-stick" properties. The resin coating protects the heating element somewhat from corrosion but, unfortunately, has many deficiencies.

One such deficiency with a Teflon™ coating is that due to its thickness and relatively poor thermal conductivity, the coating decreases the efficiency of the heating element by reducing the transfer of heat from the element to the media to be heated. A related problem is that due to its non-stick properties, Teflon™ does not adhere well to the heating element surface. Therefore, the heating element must be encapsulated by the Teflon™. This requires sleeving a thick pre-formed, open-ended Teflon™ tube around the element. Often during such sleeving the heating element may be damaged. Additionally, the open-end of the Teflon™ sleeving allows for the possibility that corrosive materials may come into contact with part of the heating element because condensation is trapped between the Teflon™ sleeve and the surface of the heater.

Yet another deficiency of Teflon™ protective sleeves is that above about 260°C in air, the Teflon™ coating will often degrade. Such degradation may be difficult to detect before significant corrosion of the heating element occurs. However, even if the degradation is detected, the sleeve is not easily repaired and often must be replaced in its entirety via the sleeving process described above. Another manner of protecting heating elements from corrosion is to use a sheath made entirely of ceramic rather than coating a metal sheath with Teflon™. Such ceramic sheathed elements are disclosed in US Patent No. 5,084,606. Unfortunately, when the sheath is made of solid ceramic, the heating element is fragile due to the brittleness of the ceramic. Such elements are also expensive due to the amount of ceramic. Additionally, ceramic is not very conductive and thus heat is not readily transferred by the element. It would be desirable to discover a coating for an outer metallic shell of a heating element which is significantly thinner than the Teflon™ coating currently employed. If such a coating could be discovered then the heating elements would be efficient, as well as, less expensive and fragile than a ceramic sheathed heating element. It would further be desirable if such a coating would adhere to the heating element such that it could be applied via a simple process which would not harm the heating element and such that the coating did not leave an opening for corrosive materials to attack the heating element. It would yet further be desirable if such a coating would offer protection to the heating element at high temperatures and in corrosive environments and could be repaired easily if such repair becomes necessary.

Summary of the Invention Surprisingly, it has now been discovered that a refractory metal, refractory metal oxide, ceramic, inter-metallic. or a mixture thereof may be utilized to coat heating elements. Such coatings offer excellent corrosion protection in many different environments including acidic environments. In addition, the coatings are very energy efficient in that heat is readily conducted through the coating. The numerous processes which may be utilized to apply the refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof to the heating element offer significant advantages. One advantage is that the processes do not damage the heating element. Yet another advantage is that the coatings do not require sleeving the heating element with an open-ended tubular structure such as Teflon™, but rather, coat the entire heating element. In this manner, if the coating becomes damaged, it may be repaired at the point of damage and replacement of the entire coating is not necessary. Also, by not having an open end, the heating element is better protected from corrosion.

Brief Description of the Drawings Figure 1 illustrates a typical heating element which is coated with a corrosion resistant coating in accordance with the instant invention.

Figure 2 illustrates the coatings of the instant invention. Detailed Description of the Invention

As used herein, the term "heating element" refers to any electrical resistance heating element used to raise the temperature of a liquid such as water or a gas such as air. Such electrical resistance heating elements are of many different shapes and sizes depending upon the intended application. Among such shapes include disks, tubes, filaments, and cartridges. The sizes may vary from a small heating element suitable for a personal hair dryer to a large heating element suitable for a commercial water heater. Thus, useful elements to be coated in this invention typically produce watt density of at least 5, preferably at least 50, more preferably at least 200 watts per square inch. A typical heating element which may be utilized in the instant invention is illustrated in Figure 1.

Preferably, the heating elements of this invention are useful in immersion heaters which are subject to corrosive environments. Such heaters are often used in heating chemicals and thus subject to the corrosive environments thereof. Such immersion heaters, for example as described in US Patent No. 5,013,890 incorporated herein by reference, typically are comprised of an electrically conductive resistive heating element and a metallic sheath covering the element, the sheath being insulated from the element by conventional insulation. The coatings of the instant invention are useful to cover the sheath and protect the element from corrosion even when the current flow through the element produces a watt- density of at least 5, preferably at least 50, more preferably at least 200 watts per square inch.

As used herein, the term "refractory metals" refers to those metals capable of withstanding temperatures to which a heating element may be subjected. Typically, such metals may withstand temperatures up to 100, preferably up to 500, more preferably up to 1000 °C. Among the refractory metals, those found most suitable for this invention include tantalum, niobium, hafnium, and rhenium. The most preferable refractory metal is tantalum.

As used herein, the term " refractory metal oxide" refers to a compound which contains oxygen and a refractory metal in any ratio sufficient for use as a coating in the instant invention. Among useful oxides of this invention include tantalum oxide, niobium oxide, hafnium oxide, and rhenium oxide such as, for example, Ta₂0₅, Nb₂0₅, Hf₂0₅, and Re₂0₅. As used herein, the term "ceramics" generally refers to metallic or non-metallic compounds which are capable of withstanding temperatures to which a heating element may be subjected. Typically, such materials may withstand temperatures up to 100, preferably up to 500, more preferably up to 1500 °C. Ceramics are described generally in, for example, Electric Process Heating by Maurice Orfeuil, Batelle Press, (1987) pp. 60-61, which is incorporated herein by reference. Typically, the term ceramics includes "carbides", "nitrides" and "oxides". The term "carbides" refers to a compound, metallic or non- metallic, associated with carbon such as silicon carbide (SiC), tantalum carbide (TaC), hafnium carbide (HfC), aluminum nitride (A1N), niobium carbide (NbC), rhenium carbide (ReC), and zirconium carbide (ZrC), as well as, diamond. The term "nitrides" refers to a compound, metallic or non-metallic, associated with nitrogen such as boron nitride (BN), tantalum nitride (TaN or Ta N), silicon nitride (Si₃N₄), and hafnium nitride (HfN). The term "oxides" refers to a compound, metallic or non-metallic, associated with oxygen such as silicon dioxide (Si0₂), tantalum pentoxide (Ta₂0₅) or lithium oxide. Other ceramic compounds of use in the instant invention include an oxidized form of the above compounds, for example, Si_xO_yN, Ta_xO_yN, and SiOC wherein x and y represent integers which provide a stable stoichiometric ratio of the formula. As used herein, the term "inter-metallics" is a two or more component system which has a semiconducting metal. A typical inter-metallic compound for use in the present invention includes nickel suicide (Ni₃Si).

As used herein, the term "coating" means a film or thin layer applied to an outer metallic shell, i.e., sheath, of a heating element. The sheath often encompasses components of an element which are in need of protection from corrosion. Such components may include thermocouples, resistance wires, and connectors. The thickness of such films or layers varies depending upon the particular coating material and the surface smoothness of the heating element. Typically, for most applications, the coatings of the instant invention are at least 1000, preferably at least 10,000, more preferably at least 50,000 Angstroms thick. Correspondingly, the coatings are not so thick that ductility is lost or the coating becomes fragile. Generally, the coating are less than 100, preferably less than 10, more preferably less than 1 microns thick.

The coatings of the instant invention are substantially pinhole and stress free. The coating adheres strongly to the surface of a metallic sheath of the heating element at temperatures ranging from - 20° to over 1000° C. After multiple thermal cycling, the coatings also conserves its strong adherence. Occasionally, it may be desirable to apply an undercoat on the surface of the metallic sheath to enhance the adhesion of the coating to the surface of the metallic sheath of the heating elements and/or fill any pinholes or cracks existing in the surface of the sheath. The undercoat may include any of the materials used for coatings herein. A particularly preferred undercoating material is tungsten. The thickness of the undercoat is typically less than 5, preferably less than 1, and more preferably less than 0.1 micrometers. Because the instant invention provides a tenacious, stress, and pinhole-free coating suitable for protecting heating elements, the invention is particularly applicable to heating elements which are in need of such protection. Typical heating elements in need of such protection include those heating elements used in immersion and gas heaters subjected to a corrosive environment. The coatings of the instant invention offer protection because the coatings are substantially impermeable to many corrosive environments which the heating element may be subjected.

A corrosive environment to which an immersion heater may be subjected often includes those environments where concentrated acids exist. Generally, the corrosive environment has a pH of from about less than 1 to about 6 and may comprise sulfuric acid, nitric acid, chromic acid, phosphoric acid, or other acids harmful to unprotected heating elements, as well as, mixtures of the aforementioned acids. Often, in addition to the aqueous, acidic environments described above, a heating element may be subjected to a corrosive gaseous environment, such as one comprising cyanide, chlorine, sulfur, or mixtures thereof. Furthermore, in some instances a corrosive environment may include high temperatures, for example, temperatures from about 100 to about 1000°C, pressures from about 0 to about 1 atmospheres, as well as, acidic pH's, for example, from about 0.01 to about 6, or basic pH's, for example, from about 9 to about 14. Before coating the heating element with the refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof, it is often preferable to prepare the surface of the heating element. Proper surface preparation fosters adhesion between the element and coating. Surface preparation also eliminates foreign objects and debris which could cause cracks or holes in the coating and allow corrosion of the heating element. Typical surface preparation may include a number of different steps or combination of steps. These steps, as one skilled in the art will appreciate, will vary depending upon the content and nature of the heating element^'s surface, as well as, upon the coating material and process to be utilized. Typical surface preparation steps comprise, for example, one or more of the following: electropolishing, mechanical polishing, chrome plating, deburring, electroless nickel plating, acid or chemical etching, plasma etching, utrasonical cleaning, or vapor degreasing.

A particularly preferred surface preparation procedure involves mechanically polishing the surface of the metal sheath of the heating element onto which the coating will be deposited. Depending on the degree of roughness of the metal sheath, the polishing may include removing from about 0.1 to about 30 micrometers or more of the surface of the metal sheath. One may often determine whether the mechanical polishing is sufficient examining the metal sheath under a microscope at 500x magnification. If the surface is substantially free from scratches, pits, and undercuts the mechanical polishing is sufficient. If further surface preparation is desirable for a given application, then a mirror-like surface may be achieved by further polishing the surface of the metal sheath with a polishing compound such as ultrafine diamond or A1 0₃ grit and paste. While not necessary, it is preferred that the above polishing be accomplished by use of an automated polishing process. In this manner, the polishing is highly precise and efficient, for example, often on the order of seconds for a typical polished tube of dimensions from about 35 to about 200 centimeters in length and from about 4 to about 6 centimeters in outside diameter.

Electropolishing is another preferred method of preparing the surface of the heating element. This method is of particular importance when minimizing the roughness profile of the metal surface.

Often, a mirror-like surface of the metal sheath may be achieved with an immersion time of from about 5 to about 15 minutes or longer.

After such surface preparation it is often preferable to ultrasonically clean or vapor degrease the polished surface of the metal sheath to be coated. This ensures a clean surface for coating the heating element.

The process used to coat the heating element with the refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof is not particularly critical so long as the coating is impermeable to corrosive fluids and adheres strongly to the surface of the metal sheath. It is preferable for most applications that the coating be of uniform thickness and adhere via physical means or chemical means to the heating element. Often the coating process utilized is based upon the size and shape of the heating element to be coated. Accordingly, such coating methods as electrolysis, electroplating, electrodeposition, conventional chemical vapor deposition such as the deposition of tantalum via a liquid or solid precursor, gas and solid diffusion such as pack cementation and chemical vapor deposition, physical vapor deposition such as RF and DC sputtering including both reactive and non-reactive processes (non-reactive processes generally consists of using inert or non-reactive gases such as Argon or Helium), plasma enhanced, or ion beam- assisted vapor deposition, as well as, vacuum or mechanical coating means including brushing, spraying such as thermal or plasma spraying, calendering, dip coating, roller coating, or any combination thereof may be utilized.

A particularly preferred coating process is that of pack cementation. This process consists of placing the heating element to be coated into a semi-permeable enclosure, for example, a box. The enclosure contains, or is subsequently filled, with the refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof which is to become the coating. The enclosure also contains, or is subsequently filled with, a halide which is volatile at the temperature of heat treatment. The pack cementation process of coating includes thermal decomposition of the metallic halide followed by a thermal deposition and diffusion of the refractory metal or oxide onto the surface of, and into the surface of, the outer metal sheath of the heating element. Typical deposition stages may be represented as follows:

MC12 + H2 -» M(solid) Formation + 2HC1 -» M(solid) via Diffusion + 2HC1 Another particularly preferable coating process is that of physical vapor deposition. As described above, it is preferable to surface treat the heating element and then clean it by a means such ultrasonic cleaning or vapor degreasing before coating by a means such as physical vapor deposition.

After cleaning, it is preferable to plasma etch the surface of the heating element in argon under ultra-high vacuum and then sputter coat the surface with reactive or non-reactive gases in conjunction with the desired coating material. The operation of the aforementioned process at ultra high vacuum avoids the introduction of most impurities onto the heating element's surface. In addition, low to high voltage biasing can be applied to promote a strong adhesion of the coating to the surface of the metal sheath of the heating element. The coating will also usually become highly dense when this biasing method is employed. Pulsing technique can also be utilized for coating of semi-conductive material such as silicon carbide. The above process often allows the heating element to be processed at low temperatures, preferably less than 400 degrees Celsius. Yet another preferable method of coating the heating element is by use of chemical vapor deposition. The chemical vapor deposition technique (CVD) is based upon a chemical reaction, such as thermal decomposition (pyrolysis), between a gaseous phase and a heated surface of the element to be coated. The surface of the heating element is first directly or indirectly heated via induction heating. A gaseous phase comprised of a non-chlorinated organo-metallic, metal carbonyls or metal hydrides compound comprising a refractory metal. It is necessary to select a non-chlorinated precursor, such as tetramethylsilane. which can generate the desired coating at reaction temperatures below 750° C due to the large mismatch in thermal expansion between the coatings and the materials of the heating element. Additionally, deposition temperatures below 750° Celsius eliminate formation of microcracks in the coatings during the cooling process. Typical deposition pressures are usually between 0.0132 atmosphere (10 torr) and 0.131 atmosphere (100 torr). Yet another preferred method of applying the coating to the surface of the metal sheath of the heating element is by gas diffusion via chemical vapor deposition. In this process, a halide of the solute metal is passed in vapor form over the metallic surface of the heating element to be coated, the surface being heated such that it is at a temperature at which diffusion can take place. Such temperatures are generally from about 400 to about 1000 degrees Celsius or more, depending on the precursor for the coating. The carrier gas for the halide vapor can be a reducing gas such as hydrogen, cracked ammonia, or the like or inert gases such as helium or argon. In this manner, the metal halide BX₂ in gaseous form is reduced to metal B which then diffuses into the solvent metal A. The three major chemical reactions for this gas phase diffusion process can be summarized as follows: Interchange reaction: A + BX₂ (gas) — > AX₂ + B Reduction reaction: BX₂ + H₂ — > 2HX + B

Thermal dissociation reaction: BX2 — > X2 + B

The interchange reaction implies the removal of one atom of A at the surface for each atom of B deposited.

A typical gas phase diffusion of tantalum onto and into the metallic surface of the heating element can typically be processed at temperatures of less than about 800 degrees Celsius. Tantalum can be introduced into the deposition chamber by reacting heated tantalum chips or pellets with hydrochloric gas. Another preferred method of introducing tantalum into the deposition chamber consists of heating the tantalum chips or pellets to its sublimation temperature to form tantalum vapor. The tantalum vapor is then transported into the deposition chamber with an inert carrier gas such as argon. The deposition of tantalum can also be processed via gas or liquid precursor such as tantalum pentachloride or other derivatives of tantalum, provided that the thermodynamics for thermal decomposition of tantalum precursor can occur at temperatures less than 1000 degrees Celsius or preferably less than 500 degrees Celsius. The pressure used for this process can range from about full vacuum to about 0.98 atmospheres. As described above, the heating elements of this invention are particularly useful in corrosive environments. The heating elements of the invention will typically not show any pits, crevices, holes, or cracks indicating degradation of the coating even when subjected to pH's of less than about 1.0 (such as concentrated chromic plating solution and nitric acid) at boiling temperatures for more than 60, preferably more than 100 hours. The coatings of this invention may be modified by addition of such substances such as metals, non-metals, alloys, as well as, inorganic and organic compounds so long as such substance does not destroy the functionality of the protective coating. Such modifications may often be desirable in order to alter one or more of the properties, for example electrical, thermo-mechanical, or chemical properties, of the coating for a particular application. For example, if a more ductile coating is desirable, then the coating may be doped with boron or titanium to promote such ductility. As another example, it may be desirable for some applications to add materials such as inter-metallic substances, for example, nickel suicide, which have a coefficient of thermal expansion which is relatively equivalent to the thermal expansion coefficient of a base material of the heater. In this manner, efficient heating is accomplished. In addition to being modifiable by the addition of other substance, the coatings of this invention may have more than one layer. This may be desirable if the heating element is to be subjected to a particularly corrosive environment for an extended length of time or if the heating element is unusually fragile. The use of layered coatings may reinforce the failure of the outer layers in case of a severe electrochemical attack. Often if multiple layers are to be utilized, the number of layers will range from one (1) to one hundered (100) or more, preferably one (1) to ten (10), depending on the specific application. The thickness of each layer may generally vary from about 100 to 5000 Angstroms and it may be desirable to use a material such as tungsten between the layers to enhance the adhesion of the layers to each other. Each layer may be of different or identical materials and optimum performance may be achieved via routine experimentation.

The coated heating elements of this invention may be easily inspected for defects such as microcracks, cracks, holes, or other failures in the coating. Such inspection may be accomplished in numerous ways. In one method used most often for oxide coatings, leakage current is tested by immersing the coated area of the heating element in deionized water and acidic solution in conjunction with a metallic electrode probe. A current, for example 12 volts, is applied to the uncoated area of the heater and metallic electrode probe. If the probe detects a voltage change, then a defect exists within the coating. Typically, the magnitude of the voltage change increases as the size of the defect. A second method useful for coatings which are conducting, semi-conducting, or insulating is that of potentiodynamic polarization.

If a coated heating element has a defect then it may be repaired in a similar manner to which it was coated. Briefly, the repair steps include surface preparation, cleaning, and then reapplying the coatings onto the clean surface of the heating element as described above. When cleaning the surface of a damaged heating element, it is preferable to ultrasonically clean the element with deionized water in order to neutralize any acid which may have become trapped in the element.

A schematic of a resistance heating device of the instant invention is described in Figure 1. The internal elements of the heating device comprise of a resistance wire (1), refractory or ceramic insulation (2), a Chromalox patented connector (3) used to control the overheating of the resistance wire (1) and a terminal block (4). The internal elements are protected by an outer metallic shell or sheath (5). The base (6) of the metallic sheath comprises a heavy-gauge disk which is securely welded. A corrosion-resistant coating of the instant invention (7) surrounds the sheath.

Figure 2 shows as an embodiment of this invention, the high performance corrosion resistant coating (7) of refractory metals, refractory metal oxides, ceramics, inter-metallics. or mixture thereof which is applied onto and into the outer surface of the metallic sheath of the heating device is illustrated in 2a of Figure 2.

The coating can be single (2b of Figure 2) or layered (2c of Figure 2). According to this invention, the coating diffuses into the surface or grain boundaries of the metallic sheath or tube to promote a strong adhesion. The coating is 100 percent (%) dense. Each layer of the multi-layer coatings (2c of Figure 2) can be of identical or different materials selected from refractory metals, refractory metal oxides, ceramics, inter-metallics, or a mixture thereof. Example One

RF Bias sputtering was used to apply 100% dense amorphous silicon carbide to the surface of four steel tubes. On each steel tube, an undercoating layer of tungsten of 100 Angstroms thick was deposited prior to the silicon carbide coating to enhance adhesion. Silicon carbide was then deposited on each of the four steel tubes. The first steel tube was coated with a single layer of silicon carbide which was 3 micrometers in thickness. The second steel tube was coated with 10 layers of silicon carbide, each layer being 1000 Angstroms. The third steel tube was coated with 20 layers of silicon carbide, each layer being 1000 Angstroms. The fourth steel tube was coated with 30 layers of silicon carbide, each layer being 1000 Angstroms. Upon visual inspection, each steel tube was found to be free of microcracks and pinholes.

The silicon carbide coated steel tubes were tested by immersing them in chromic plating solution at boiling temperature for 48 hours. The chromic plating solution had a pH near 0.0 at boiling temperature. The average weight loss was 0.1 percent (%). Example Two

DC Bias magnetron sputtering was used to apply 100% dense tantalum to the surface of steel disks, tubes, and shells of cartridge and tubular type heaters. A typical heating element having a tantalum coating is shown in Figure 2. The sputtering atmosphere was argon. The temperature of the surface of the substrate was 250°C. The DC bias voltage was 500 volts to 4 kilovolts. The argon pressure was 1.31 x 10^" atmospheres. The sputtering time ranged from 60 to 90 minutes. The resulting tantalum coatings were about 3 micrometers in thickness.

The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were examined for defects under a microscope at a magnification of 500 times and 5000 times. No defects- were observed and the tantalum coating adhered strongly to the surface of the metallic sheath.. The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at 67° C for 100 hours. The chromic plating solution had a pH of 1.0 at 67° C. The average weight loss was 0.005 percent (%).

The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at boiling temperature for 100 hours. The chromic plating solution had a pH near 0.0 at boiling temperature. The average weight loss was 0.09 percent (%).

Example Three

Ion beam-assisted deposition, in particular, electron beam evaporation is useful to apply 100% dense tantalum to the surface of steel disks, tubes, and shells of cartridge and tubular type heaters. The metal substrate was biased to enhance the adhesion of the coating. The bias voltage was 500 volts to 2 kilovolts. Low temperature processing of less than 250 degrees Celsius is useful for the heat sensitive surfaces of the outer sheaths. Tantalum chips, pellets, or a mixture thereof are evaporated under high vacuum by an electron beam. An ion beam of argon via an ion gun bombards the tantalum vapors onto and into the surface to be coated. Tantalum films which are 100 percent (%) dense, about 3 micrometers thick, defect-free, and have strong adhesion to the outer metal sheath are obtained using this deposition method of coating.

Example Four

A gas diffusion method was used to apply 100 percent (%) dense tantalum to the surface of steel disks, tubes, and shells of cartridge and tubular type heaters. The surface was first heated to temperatures ranging from 300 to 500 degrees Celsius. Tantalum chips or pellets were then heated to their sublimation temperature. Tantalum vapor was then transported to the deposition chamber with a carrier gas such as Argon. Hydrogen was also introduced directly into the deposition chamber during the gas diffusion process. A typical in-depth diffusion of 0.5 to 1 micrometers of tantalum into the surface of the shell heater was achieved within 30 minutes to two hours. Tantalum was also introduced into the deposition chamber by using tantalum pentachloride (gas) as a precursor associated with tantalum. Hydrogen was used to decompose tantalum pentachloride into tantalum at temperatures of less than 900 degrees Celsius.

Typical deposition parameters useful for high density Tantalum coating onto and into carbon steel sheath via gas diffusion - chemical vapor deposition are as follows: Etching gas: Hydrochloric (HC1)

HC1 Pressure: 0.0065 atmosphere (5 torr)

Etching time: 5 minutes

Etching temperature: 400 degrees Celsius

Flushing gas and pressure: Argon @ 0.026 atmosphere (20 torr) Flushing time: 15 minutes

Tantalum deposition parameters: Total Pressure: 0.013 atmosphere (10 torr)

Argon Pressure: 0.0066 atmosphere (5 torr)

HC1 Pressure: 0.0066 atmosphere (5 torr)

Tantalum chips to sublimation temperature Deposition temperature: 500 degrees Celsius

Deposition time: 30 minutes

Diffusion temperature: 800 degrees Celsius

Diffusion time: 1 hour

Diffusion Pressure: 0.65 atmosphere (500 torr). The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters of

Example Three were examined for defects under a microscope at a magnification of 500 times and 5000 times. No defects were observed.

The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at 67° C for 100 hours. The chromic plating solution had a pH of 1.0 at 67° C. The average weight loss was 0.1 percent (%).

The tantalum coated steel disks, tubes, and shells of cartridge and tubular type heaters were tested by immersing them in chromic plating solution at boiling temperature for 100 hours. The chromic plating solution had a pH near 0.0 at boiling temperature. The average weight loss was 0.5 percent (%).

Claims

What is Claimed is:

1. In a heating device comprising an electrically conductive heating element with an outer metallic shell having a coating encompassing the surface of said outer metallic shell, the improvement which comprises a coating selected from the group consisting of refractory metals, refractory metal oxides, ceramics, inter-metallics, and mixtures thereof.

2. The device of Claim 1 wherein the refractory metal is tantalum, hafnium, niobium, or rhenium.

3. The device of Claim 1 wherein the ceramic coating is silicon carbide.

4. The device of Claim 1 wherein the inter-metallic coating is nickel suicide.

5. The device of Claim 1 wherein the heating device is an immersion heater.

6. The device of Claim 1 wherein the coating is from about 100 Angstroms to about 100 micrometers in thickness.

7. The device of Claim 1 wherein the coating comprises from about 2 to about 10 layers.

8. The device of Claim 7 wherein each layer of the coating is independently selected from the group consisting of refractory metals, refractory metal oxides, ceramics, inter-metallics, and mixtures thereof.

9. The device of Claim 1 wherein the coating is applied by DC Bias or pulse sputtering.

10. The device of Claim 9 wherein the coating is tantalum or nickel suicide.

1 1. The device of Claim 1 wherein the coating is applied by ion-beam assisted vapor deposition with bias substrate.

12. The device of Claim 1 1 wherein the coating is tantalum.

13. The device of Claim 1 wherein the coating is applied by gas diffusion.

14. The device of Claim 13 wherein the coating is tantalum.

15. The device of Claim 1 wherein the coating is applied by RF. DC, or pulse sputtering.

16. The device of Claim 15 wherein the coating is silicon carbide.

17. The device of Claim 1 wherein the weight loss of the coating is less than about 0.005 percent when subjected to an acidic solution at 67┬░ C for 100 hours.

18. The device of Claim 1 wherein the weight loss of the coating is less than about 0.1 percent when subjected to a boiling acidic solution for 100 hours.

19. The device of Claim 1 wherein the surface of the outer metal sheath is prepared by mechanical polishing, electropolishing, electroplating chromium, or a combination thereof prior to coating the sheath with a refractory metal, refractory metal oxide, ceramic, inter-metallic, or mixture thereof.

20. The device of Claim 1 wherein an undercoating is applied to the metallic shell before the coating.

21. The device of Claim 20 wherein the undercoating is tungsten and is from about 100 to about 10,000 Angstroms thick.

22. In a heating device comprising an electrically conductive heating element with an outer metallic shell having a coating encompassing the surface of said outer metallic shell, the improvement which comprises a coating selected from tantalum or silicon carbide wherein said coating is from about 100 Angstroms to about 100 micrometers in thickness.

23. A heating element comprising:

(a) a nickel-chromium resistance wire;

(b) a metal shell which surrounds the resistance wire and has refractory insulation between the resistance wire and the sheath;

(c) a connector operably connected to the resistance wire and adapted to connect to a power source;

(d) a corrosion-resistant coating encompassing the surface of said outer metallic shell wherein said coating is selected from from tantalum or silicon carbide and wherein said coating is from about 100 Angstroms to about 100 micrometers in thickness.