US3010480A

US3010480A - Thermocouple tube and protective coating

Info

Publication number: US3010480A
Application number: US767000A
Authority: US
Inventors: Clifford A Ragsdale
Original assignee: Individual
Current assignee: Individual
Priority date: 1958-10-13
Filing date: 1958-10-13
Publication date: 1961-11-28
Anticipated expiration: 1978-11-28

Description

c. A. RAGSDALE 3,010,480

THERMOCOUPLE TUBE AND PROTECTIVE coATING Filed Oct. 13, 1958 F/gz Nov. 28, 1961 1N V EN TOR.

' CLIFFORD A. RAGSDALE MA1-) Kari' maf 3,010,480 THERMOCOUPLE TUBE AND PRTECTIVE CQA'IING Clifford A. Ragsdale, 214 Connecticut Ave., Lorain, Ghia Filed Oct. 13, 1958, Ser. No. 767,000 4 Claims. (Cl. 13S-89) The invention relates in general -to thermocouple tubes and a protective coating therefore which coating may also be utilized as a protective coating for other objects and devices which are subject to the same or similar conditions in their ultimate use.

Referring specifically to thermocouple and/or pyrometer tubes, it might be pointed out that they Vare used to contain and protect the means for indicating temperature (for example dissimilar wires in thermocouples) when the devices are subjected to a corrosive environment in their ultimate use. Thermocouple tubes in use today for immersion in aluminum, lead and zinc for example, are primarily constructed of cast iron or steel pipe closed at one end and open at the other. When these tubes are suspended substantially vertically in a container of molten metal the movement of the heated metal along with the corrosive nature of the material itself causes the closed end of the tube to fail first, washing out in a concave fashion. The next failure occurs in the cylindrical wall portion of the tube. The physical construction of the thermocouple tube of the present invention along with the protective coatings and modifications thereof enables the tube herein disclosed to stand up under operating conditions up to fifty times as long as standard iron and steel tubes. With the protective coatings herein disclosed, used on conventional iron and steel tubes, the

life of the same have been increased as much as fifteen to twenty times.

It is therefore an object of this invention to provide a new and improved thermocouple tube.

Another object of the invention is to provide a thermocouple tube having an end thereof which comprises a solid, substantially conical portion which is a separate unit Ifrom the thermocouple wires when installed. The solid conical portion serves to provide a surface which is at a small angle with respect to the normal ilow of metal. This serves to prevent washout which occurs in the conventional thermocouple tube.

Another object of the invention is to provide a new and novel method of making a thermocouple tube.

Another object of the invention is to provide a protective coating and method of making the same which includes a first coating of nickel-chromium with a fluxing agent, a second coating of nickel-chromium, and a third coating of aluminum or zirconium oxide with an aluminum or zirconium oxide or an aluminum sealer. When the article or device is to 4be immersed in a molten metal it is also preferably provided with a fourth coating of the molten metal.

Another object of the invention is to provide a protective coating and method of making the same which includes a first coating of nickel-chromium and a second coating of aluminum oxide with a sealer of aluminum oxide or aluminum or of zirconium oxide with a sealer of zirconium oxide or aluminum.

FIGURE l is a view of a steel or iron pipe partially in section and a closure member which is utilized in constructing the thermocouple tube of the present invention;

3,0l0,480 Patented Nov. 28, 1961 'ice FIGURE 2 shows the closure member welded in place in an open end of the pipe;

FIGURE 3 is a view similar to FIGURE 2` but showing the closure member machined to a generally conical shape and the weld being machined smooth;

FIGURE 4 is a view of the thermocouple tube of the present invention with the protective coatings thereon and indicating the tube in a vertical position which is its normal position within a container of molten metal;

FIGURE 5 is an enlarged view taken generally along the line 5 5 of FIGURE 4 and showing all four of the protective coatings slightly exaggerated -as compared to the pipe thickness for better ease in illustra-ting the same;

FIGURE 6 is a fragmentary View similar to that shown in FIGURE 5 but showing a modification thereof in Ithe sense that the fourth or outermost coating has been dispensed with; and

FIGURE 7 is a View similar to FIGURE 6 ybut illustrating a further modification in that the rst or innermost protective coating adjacent the surface of the pipe, and the fourth or outermost coating have been dispensed with.

The preferred method of making the thermocouple tube 10 of the present invention is best illustrated and seen in FIGURES 1-5. FIGURE l illustrates the step of selecting a piece of pipe 12, which may also be rcferred lto as an annular member, and which has iirst and second

open ends

13 and 14, respectively. The pipe is also provided with inside and

outside surfaces

16 and 17 respectively, which have inside and outside diameters. The pipe 12 is preferably of a steel or cast iron construction. A closure member 19 is also selected and is preferably of a steel or cast iron construction and this closure member includes a lirst cylindrical portion 20 ywhich has a diameter which is substantially equal to the outside diameter of the pipe. The closure member `also has a second cylindrical portion 21 which is smaller than the first and ywhich has a diameter substantially equal to the inside diameter ofthe pipe. The closure member is preferably initially a cylindrical member having a single diameter substantially equal to the irst cylindrical portion and the second cylindrical portion is provided preferably by machining or removing a part o-f the material. As a result of this construction, the iirst and second cylindrical portions can be said to be integrally connected together and this type of construction forms a shoulder 23 therebetween. The next step involved in making the thermocouple tube of the present invention is seen in FIGURE 2 and this comprises placing the second cylindrical portion 21 of the closure member into the first open end 13 of the pipe with the shoulder 23 abutting .the iirst end of the pipe. The closu-re member is then welded to the first end of the pipe as at 24. The first cylindrical portion o-f the closure member 19 is next machined to a substantially conical shape which is sho-Wn in FIGURE 3. The weld is also machined smooth with the outside surface of the pipe. This machining is accomplished by any method well known in the art and a tool 25 has been indicated schematically in FIGURE 2. The second end portion of the pipe is provided with threads 27 which may be provided thereon at nearly any stage in the making of the device. The threads are, however, preferably put on the second end portion of the pipe before the closure member is atiixed to the first end portion.

The -next steps in making the thermocouplle tube of the rosion and errosion resistant barrier.

present invention involves the applying of the successive protective coatings of the invention to the surface of the thermocouple tube. The thermocouple tube at this stage includes the assembled elements; namely, the pipe 12 and the closure member 19. The tube is cleaned by any method which'is necessary to remove dirt or foreign material from the outside of the same. This` can either be by physical methods such as wire brushing, wiping or by suitable chemical means if necessary. After the tube has been thoroughly cleaned, -it is grit blasted by means which are well recognized in the art. This serves to in effect roughen up the outside of the thermocouple tube. A nickelechromium alloy is then sprayed on the outside surface of the grit blasted tube. This nickel-chromium alloy contains suitable materials which act as a flux. The essential yelements contained in this alloy for acting as a flux are boron and silicon. The exact composition of an alloy which is suitable for this first coating comprises the following composition; namely, 15% chromium, 75% nickel, 3% silicon, 3% boron, 1% carbon, and 3% iron. This first coating is indicated by the reference numeral 30 (FIGURE 5) and is preferably on the order of .015 inch. This first protective coating 30 is applied by what is known in the art as metal spraying (metallizing), and

Vthe mechanics of this means of application are well known by those skilled in the art. A brief discussion, however of the means of applying and the apparatus used in this art are in order at this time. What is commonly referred to as a metallizing gun, is utilized in applying this first lcoating and this is primarily a device which has a hopper or canister located over the gun which contains the materials to be applied. These materials are in a finely divided or powder form and are generally gravity` fed through a metering tube into the tiring nozzle. This nozzle rin construction, is quite Vsimilar to a gas fired cutting torch nozzle such as is used in an acetylene cutting torch. The powder is fed through the center of the nozzle with the llame being fed through small hotles located near the outer rim of fthe nozzle which is generally circular in shape. As the material, which is in powder form enters the flame, it is melted 'and propelled to the object to be coated, which is in this case the thermocouple tube.

Another type of metallizing gun which may be used in producing the results ofthe present invention employes an air or an electric motor to drive two notched wheels, one located above the other. The material to be applied, which is in Wire form passes between these two notched wheels which serves to move the wire material intothe tiring nozzle. l

After this first coating 30 has been applied, it is heat treated at approximately 1900 F. and because of the use of the uxing material in the first coating, the rst coating flows or becomes quite smooth and conforms very closely to the base material of the thermocouple tube. As an `attempted explanation, but without limitation, it is believed that this irst coating creates in substance a oor- It also provides a casing, in effect, which serves to correlate the expansion between the thermocouple tube and other coatings which are to be applied and which will be described hereinafter.

After the iirst coating has been completed, the thermocouple tube is preferably cooled and it again is grit blasted. After this yhas been accomplished a second coating 3-1 is applied by the same means as the first coating is applied namely by utilizing a metallizing gun. The metals of this coating comprise an alloy of nickel and chromium, the preferred mixture being substantially 75% nickel and 25% chromium. This coating is applied to a thickness on the order of .010 inch. This coating does not contain any iluxing agent and as a result, produces a protective coating which acts to provide a cushion between the first coating and the thermocouple tube and a third coating 32 which is applied land will be described hereinafter. This coating appears to compensate for the difference in coeicient of expansion between the thermocouple tube and the third coating. Y

The third coating 32 is next applied to the thermocouple tube on top of the 'second coating 31. If the thermocouple tube is to be used in aluminum, lead or zinc, for example, or any other mass of molten met-al of a temperature up to about 2600 F., the third coating comprises 1an aluminum oxide. This material is app-lied as discussed hereinabove being originally in a powder form and being sprayed through the metallizing gun. This coating of laluminumoxide is preferably applied to a thickness on the order of .020 inch. Where the thermocouple tube is to be subjected to temperatures of about 2600 F. and above, a different material is preferably utilized which will'be discussed hereinafter. It would be well to point out at thisv time that the third coating 32 of of aluminum oxide, when applied in this hereinabove described manner, to a certain extent is porous. Y Molten metals, especially aluminum, lead and zinc, will penetrate the aluminum oxide coating and attack the metals underneath, which eventually leads to the destruction of these metals and a thermocouple tube failure. Therefore, it is of importance that the porosity be eliminated as much las possible. To accomplish this end, a sealer is utilized which comprises aluminum oxide suspended in a suitable vehicle such as lacquer or any material that will burn olf orV evaporate without creating an undesirable residue. This sealer material penetrates the somewhat porous surface of the third coating and when subjected to any heat, forms a film that greatly decreases Y the porosity. c

In the event the thermocouple tube or other obJects are to be subjected to temperatures of about 2600 F.

Vand above the third coating, instead of Acomprising of aluminum oxide, comprises Ya zirconium oxide. The

is substantially the same and the material is applied in substantially the same manner when the third coating is of zirconium oxide. The sealer which is Vpreferably used, comprises zirconium oxide carried by a suitable vehicle such as lacquer. Water is also a suitable vehicle for carrying the aluminumand zirconium oxide Sealers.

After the third coating 32 has been applied, which includes the sealer, a fourth coating 33 is applied. Before this coating is applied, the solvent or carrier for the sealer mentioned above is preferably removed. This isreadily accomplished by applying heat to the third coating such as by a torch or by placing the thermocouple tube in a heat treating oven toremove the carrier. If the fourth coating is not to be applied, as will be described hereinafter, then the removing of the solvent or carrier can be dispensed with unless the same would contaminate a molten metal mass intowhich it is to be immersed. The

sea-ler is best applied by either brushing on, dipping or spraying. A The fourth protective coating which is applied, is applied primarily when the thermocouple tube or other I device to be protected is to be immersed in a molten metal bath. This fourth coating-comprises preferably the metal into which it is to be immersed in its nal use. In other words, if the thermocouple tube is going to be yused in molten aluminum, then aluminum is preferably sprayed on after the fourth coating.- This material is applied preferably in the same manner as the above mentioned rst, second and third protective coatings. If the `tube is to be used in lead, then a lead coating would It has been found,'however, that Satisfactory results` can be obtained Without the use of the fourthV coating 33 in some instances particularly where the thermocouple tube is not to be subjected to the action of molten metals but only to a very corrosive atmosphere. In this instance then, the thermocouple tube and protective coating would be such as shown in FIGURE 6 and only coat-ings 3i), 31 and 32 would be utilized.

A modification of the protective coating structures has produced very good results and FIGURE 6 may be resorted to in describing this modification. In this modification, the first and second coatings 3) and 31 are as described hereinabove and also as described hereinabove, the third coating may comprise either aluminum or zirconium oxide. The essential difference in this modification is in the sealer. In this instance, the sealer which is utilized is aluminum rather than aluminum oxide and `is preferably in the powder form. This aluminum is suspended in a suitable vehicle or carrier as described hereinabove, which carrier will burn off or be disposed of when heated. A suitable example of such a carrier which might be utilized is aluminum paint, and aluminum powder may be added to the paint, to provide the proper concentration `of the aluminum in the vehicle or carrier. It has been found that a thermocouple tube or other article which has been coated in -this manner can be heated to 2600 F. and over and immersed at this heat into cold `water with substantially no ill effects upon the coating. The aluminum or zirconium oxide of the third coating will withstand this thermal shock with no damage to this coating and no fractures whatever to the coating. This coating may be 'reheated a number of times with no effect whatever. if the sprayed third coating of aluminum or zirconium oxide is utilized without this sealer and is heated to the temperature mentioned and then immersed in cold water, the coating will fracture and fly off. If it is exposed to this temperature in a corrosive atmosphere and kept at this temperature for a period of time, it will deteriorate completely. lf the sealer, such as described hereinabove, is utilized, the coating will not deteriorate and will withstand this temperature. 'It is sometimes desirable to heat the third coating after the sealer has been applied to drive oft the vehicle or carrier. However, this may be dispensed with whenever the vehicle will evaporate of its own initiative or if it will not be detrimental to the material into which it is to be immersed.

It has also been found that a thermocouple tube, covered with only two protective coatings as shown in FIGURE 7, will provide the device `with a life which is many times the life of the ordinary present day thermocouple tubes. The two coatings which may be satisfactorily utilized with beneficial results are the

coatings

31 and 32 which have been described hereinabove and which have been illustrated in FIGURES 5 and 6. In other words, the nickel-chromium alloy, without a fiuxing agent or agents is applied as the first coating on the outside of the tube. The second coating 32 would then comprise either aluminum or zirconium oxide. suitable sealer would be utilized and these sealers have been appropriately described hereinabove. To repeat, however, the aluminum oxide coating with the aluminum oxide sealer may be utilized and the zirconium oxide coating with the zirconium oxide sealer may be used. The alternate sealer of aluminum may be utilized for both the aluminum oxide and zirconium oxide coatings. This coating does not produce as good results as do the coatings described in conjunction with FIGURES 5 and 6. This type of protective coating does, however, produce very marked results from anything which is at all known or available at the present time.

The protective coatings are applied to the thermocouple tube from the conical tip of the same back to the point at which the threads A27 begin. The thermocouple wires which are actually utilized to determine the tempera-ture of a molten bath or for other systems, do not form a part or are not secured to any of the elements shown in the drawings. The wires are, however, inserted into the tube and may bear against the end of the closure Thena r member. The thermocouple tube is preferably mounted in a vertical manner in the upper portion of a bath of molten metal. Referring to FIGURE 4, this would be the general direction which the thermocouple tube would be immersed into a pot or bath of molten metal and would assume this vertical position. The heating means in such a bath would preferably be located at the lowermost vertical position of the bath and as a result the natural flow of molten metal would be in a vertical direction where it would become colder than the rest and would recirculate again to the bottom of the bath. Due to this movement of the molten metal, the metal has a natural tendency to attack portions of the thermocouple tube which offer the greatest resistance to its movement. This action is greatest on the end or the tip of the thermocouple tube and when a flattened or blunt end is presented to this material flow which end is substantially normal thereto, this offers a great deal of resistance. Because of the construction of the herein disclosed thermocouple tube with the solid conical shaped end portion or tip, this conical end will tend to deflect the molten metal as it rises upwardly thus eliminating as much as possible, any horizontal areas which offer a resistance to the movement of the molten metal. Even after an extended use of the instant thermocouple tube and after a breakdown of the coating occurs, the life of the device is further enhanced due to the end of the tube being of the solid construction. This is because the errosion in the normal thermocouple tube tends to wear the metal out in a concave fashion. The increased life is due to the increased length of time which is required for the molten metal to wash out or errode the entire solid conical end of the tube. This solid conical portion, as far as dimensions are concerned, is usually never shorter than at least 3 or 4 inches as compared to 1A to 3/s of an inch thickness for the ends of standard thermocouple tubes.

Although the protective coating of the present invention has been described primarily in connection with thermocouple tubes, it will be readily appreciated that such a coating has utility as a protective coating for any metals and materials which are to be used in corrosive environments as described hereinabove. The protective coating, however, is very peculiarly adapted for use in connection with thermocouple tubes.

What is claimed is:

l. A thermocouple tube comprising a hollow cylindrical member having first and second end portions, said first end portion being closed by a generally conical shaped closure member, a corrosion resistant coating on the outside of said tube including a first layer on said tube of an alloy which comprises Cr and Ni and a fluxing agent, a second layer on said first layer of an alloy which comprises Ni and Cr, a third layer on said second layer, said third layer comprising a porous material selected from the group consisting of aluminum and zircomum oxide and a sealer of aluminum.

2. A thermocouple tube comprising a hollow cylindrical member having a first and second end portions, said first end portion being closed by a conical shaped closure member, a corrosion resistant coating on the outside of said tube including a first layer on said tube of an alloy which comprises Cr and Ni, a second layer on said rst layer, said second layer comprising a porous zirconium oxide with the porous zirconium oxide being filled with zirconium oxide.

3. A corrosion resistant article comprising a member having a first protective coating of chromium and nickel with a uxing agent, a second protective coating comprising chromium and nickel, a third protective coating comprising porous material selected from the group consisting of zirconium and aluminum oxide with the pores sealed with aluminum.

4. A corrosion resistant article comprising a member having a first protective coating of chromium and nickel with a uxng agent, a second protective coating comprising chromium and nickel, a third protective coating comprising aluminum oxide.

References Cited in the le of this patent 5 UNITED STATES PATENTS Christman Nov. 15, 1904 McIlory May 12, 1914 McIlory June l5, 1937 10 Simpson et al June 24,

Claims

1. A THERMOCOUPLE TUBE COMPRISING A HOLLOW CYLINDRICAL MEMBER HAVING FIRST AND SECOND END PORTIONS, SAID FIRST END PORTION BEING CLOSED A GENERALLY CONCIAL SHAPED CLOSURE MEMBER, A CORROSION RESISTANT COATING ON THE OUTSIDE OF SAID TUBE INCLUDING A FIRST LAYER ON SAID TUBE OF AN ALLOY WHICH COMPRISES CR AND NI AND A FLUXING AGENT, A SECOND LAYER ON SAID FIRST LAYER ON SAID SECOND WHICH COMPRISES NI AND CR, A THIRD LAYER ON SAID SECOND LAYER, SAID THIRD LAYER COMPRISING A POROUS MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ZIRCONIUM OXIDE AND A SEALER OF ALUMINUM.