US2461935A - Insulated electrical resistances - Google Patents

Description

Patented Feb. 15, 1949 2,461,935 INSULATED ELECTRICAL RESISTANCES John Stockdale, Clent, and Donald Eric Rison, Acocks Green, Birmingham, England, assignors to The Internalional Nickel Company, Inc., New York, N. Y., a corporation of Delaware No Drawing. Application July 19, 1944, Serial No. 545,742. In Great Britain August 14, 1943 6 Claims.

The present invention relates to insulated electrlcal resistances such as wires, ribbons, strips, tapes and the like.

It has been proposed heretofore to provide an insulating oxide coating on electrical resistance materials, but such insulating coatings have not been completely satisfactory in commercial pracparticularly when subjected to bending, etc..'

Although many attempts were made to remedy the aforementioned and other shortcomings, none, as far as we are aware, was entirely successful when put into commercial operation on an industrial scale in consistently producing satisfactory results.

We have discovered that if the surface of electrical resistance materials is made rich in nickel prior to oxidation, greatly improved insulating properties are obtained, provided the surface is substantially free from chromium and its oxides.

It is an object of the present invention to provide electrical resistances having an improved adherent insulating coating.

It is another object of the present invention to provide improved insulated electrical resistance materials having an insulating coating of nickel oxide substantially free from chromium and its oxides.

It is a further object of the present invention to provide chromium-containing electrical resistance materials having an improved insulating oxide coating which is adherent and does not tend to flake off.

It is still another object of the present invention to provide a method of producing improved insulating oxide coatings on electrical resistances.

It is also an object of the present invention to provide a method of producing improved insulating oxide coatings on chromium-containing resistance materials.-

It is also within the contemplation of the present invention to provide a method of producing an improved insulating coating on elec 2 trical resistance materials which is adherent and does not tend to flake 01f.

Other objects and advantages of the present invention will become apparent to those skilled in the art from the following description.

In general, according to the present invention the surface of the electrical resistance material is first made rich in nickel and then is subjected to an oxidizing treatment to produce an improved insulating coating. It is a feature of this invention that the enriched nickel surface prior to the oxidising treatment is clean and unoxidised or substantially unoxidised. It is another feature of the invention that the enriched nickel surface is free from or substantially free from chromium and its oxides.

The nickel-rich surface is provided by depositing nickel on the electrical resistance material or, if the material being treated contains nickel and chromium, by subjecting the surface to a. treatment which removes chromium, either solely or preferentially, and leaves an unoxidised nickel coating substantially free from chromium and its oxides. It is preferred to deposit nickel, advantageously electrolytically but any other method of deposition may be used. For example, nickel may be deposited from the vapour form or as a result of chemical reaction or from a suspension of powdered nickel in a vehicle which can be subsequently removed, e. g., by volatilisation, etc. Thus, the nickel coating can be provided by exposing the material to be coated, e. g., the wire, to nickel carbonyl vapours and decomposing the carbonyl to deposit nickel on the material being treated. However, in general the electrodeposited nickel coating has given satisfactory results.

After deposition of the clean unoxidised coating, it is preferred to heat the coated material in a reducing or inert atmosphere to bond the nickel in the coating to the surface of the material before oxidising the nickel. The bonding treatment diffuses the nickel into the immediate surface of the material and has the advantage of producing a strong keying effect, thereby binding the subsequently formed oxide firmly to the surface of the material being treated.

When the surface is to be rendered rich in nickel by removing the chromium substantially completely from the surface of a nickeland chromium-containing material, this can be ac complished, for example, by heating the material, for example, an alloy of 20% chromium, remainder substantially all nickel, in a bath of molten borax. This attacks the chromium, re-

. and then the oxidation can be combined, it desired, with the usual annealing of the material, e. g., wire. The present invention leaves the material in a clean unoxidised condition after the nickel-rich layer has been provided which permits a further drawing, etc., if desired, before the oxidising treatment. The reduction to the desired size may be effected before or after providing the nickel layer or coating. The nickel coating obtained by deposition, after drawing the material down to the desired size and prior to oxidation, is in the form of a continuous film preferably of the order of 4x to 5 10- inch thick.

The material having the bonded nickel-rich surface layer or coating is subsequently subjected to oxidation whereby a nickel oxide surface layer or coating firmly bonded to the material and having good electrical insulating properties is produced. Any suitable manner of oxidising the nickel may be used, the preferred treatment being to heat in an oxidising atmosphere. The oxide films produced in accordance with the present invention have good insulating properties, are tenacious and do not flake off from wires and the like when subjected to severe bending.

We have found that the insulating surface coating must be substantially all nickel oxide, 1. e., oxidised nickel, and that it must be substantially chromium-free. When we refer herein to a nickel-rich surface layer or coating, we mean an outer zone which is substantially all nickel and which after the oxidation treatment is substantially all nickel oxide, i. e., oxidised nickel. Other oxide coatings or surface layers such as copper oxide or iron oxide may not be used in place of nickel oxide as such coatings or surface layers are very inferior as insulators and do not produce the desired results.

In order that those skilled in the art may have a better understanding of the invention, the following illustrative examples are given.

Example I A nickel-chromium-iron resistance wire made of an alloy containing about 14% chromium,

iron, 2% manganese and balance all nickel and having a diameter of 0.0253 inch was cleaned by running in a strand at a speed of about 50 feet per minute through an aqueous bath containing about ounces per gallon of nickel chloride and about 16 fluid ounces per gallon of concentrated hydrochloric acid. In this bath the wire was treated as the anode using a current density of about 32 amperes per square foot, the time of treatment being about 30 seconds. The wire on leaving the bath was washed in a stream of water and then led to a plating bath where it was treated for about 5 minutes at a current density of about 28 amperes per square foot, the aqueous electrolyte consisting essentially of about 300 grams per litre of nickel chloride, about 30 grams per litre of boric acid and sufficient hydrochloric acid to give a pH of about 3 to 4. The operating temperature was about 65 to 70 C. and a nickel deposit approximately about 0.00007 inch thick was obtained which did not crack or flake ofi when the wire was twisted round its own diameter. The plated wire was then washed, annealed at about 1000 C. in a reducing atmosphere, e. g., hydrogen, and drawn down to 0.016 inch diameter, the annealing treatment serving to diffuse the nickel into the immediate surface of the wire as well as softening the material for drawing. The wire was then oxidised by heating in an open electric muille furnace for about 40 minutes at 850 C.

Example If A hard drawn 0.020 inch diameter wire of an alloy of 20% chromium, balance substantially all nickel, was annealed for 5 minutes at 1000 C., in molten borax. The wire was quenched in water and then boiled in water until all traces of salt had been removed. This left a clean wire substantially free from oxide which was drawn to 0.0179 inch diameter. On oxidising for 30 minutes at 900 0., a good insulating oxide was produced.

' Example III A hard drawn wire, made from an alloy of 20% iron, 15% chromium, balance substantially all nickel, having a diameter of 0.0253 inch, was annealed for 5 minutes at 1000 C. in molten borax. The material, after quenching in water, was treated as in Example II and subsequently drawn down to 0.0226 inch diameter. On oxidising for 30 minutes at 900 C., a satisfactory insulating oxide layer was obtained.

The foregoing illustrative examples have been described with particular reference to wires, but it is to be understood that other forms of resistances, including ribbons, tapes, strips and the like, are within the scope of the invention and that when the term wire or wires is used in describing or defining the invention, it is to be construed to include other forms of electrical resistance materials, including ribbons, tapes, strips, and the like.

The present invention is applicable to electrical resistance materials and electrical resistances in general. It is particularly and advantageously applicable to chromium-containing electrical resistance materials such as nickel-chromium alloys, including nickel-chromium-iron alloys, and such as iron-chromium alloys, including ironchromium-aluminium alloys. Cobalt-containing alloys ofchromium are also known to have suitable properties for electrical resistance purposes. The electrical resistance alloys are well-known in themselves. Two common types are the nickel-20% chromium and the 65% nickel-15% chromium-20% iron types. The chromium containing alloys usually contain 10% to 35% chromium and may optionally contain from a small amount, say 0.01%, to about 20% of one or more metals from the group consisting of molybdenum, tungsten, titanium, columbium, zirconium, aluminium, silicon and manganese, and the balance substantially all metal of the iron group, i.e., metal of the group consisting of nickel, iron and cobalt. The total of iron-group metals will usually exceed 50%. The nickel-containing alloys generally fall within the range of 10% to 30% chromium, 20% to nickel and 0 to 50% iron. As will be apparent to those skilled in the art, the alloys may also contain small amounts of minor or incidental elements and impurities, e. g., sulphur, phosphorus.

arsenic, antimony, carbon, calcium, rare earth metals, etc.

It is to be observed that the present invention provides improved electrical resistances, having an adherent insulating surface layer or coating of nickel oxide substantially free from chromium and its oxides. The surface layer or coating is characterised by an improved combination of properties, including reliable or consistently good insulating properties combined with resistance to flaking off when subjected to bending and the like. Preferably, the electrical resistances provided by the invention comprise an insulating nickel-rich surface layer, a core or body, and an intermediate diffusion zone firmly bonding said surface layer to said core or body. Thus, an electrical resistance made of a chromium-containing alloy, e. g., nickel-chromium alloy, preferably is comprised of an insulating surface layer of nickel oxide, a core of the chromium-containing alloy, and an intermediate diffusion zone firmly bonding the surface layer to the core.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will understand. Such modifications and variations are considered to be within the purview and scope of the invention and of the appended claims.

We claim:

1. The method of coating electrical resistances with an insulating oxide layer which comprises depositing on a resistance material made of a nickel-chromium alloy a substantially unoxidised surface layer of nickel, subjecting said material with said deposited surface layer to a diffusion treatment by heating in a substantially nonoxidising atmosphere to obtain diffusion, and thereafter oxidising said deposited surface layer by heating in an oxidising atmosphere.

2. An electrical resistance element made of a chromium-containing alloy having an adherent insulating surface layer comprising substantially all nickel oxide.

3. An electrical resistance element made of a nickel-chromium alloy having a firmly bonded insulating surface layer of nickel oxide, said surface layer being devoid of chromium and its oxide:

4. An electrical resistance element comprising an insulating surface layer, a core of nickelchromium alloy, and an intermediate diffusion zone firmly bonding said surface layer to said core, said insulating surface layer being substantially free from chromium and its oxides and com prising substantially all nickel oxide.

5. The method of producing electrical resistance materials having an insulating oxide layer which comprises providing a resistance material made of a chromium-containing alloy with an unoxidised nickel-rich surface layer substantially free from chromium, heating said resistance material with said surface layer in a substantially non-oxidising atmosphere to obtain diffusion, and thereafter oxidising said surface layer by heating in an oxidising atmosphere.

6. The method of coating electrical resistance chromium-containing alloys with an insulating oxide layer which comprises electrodepositing on a chromium-containing electrical resistance alloy a substantially unoxidised nickel-rich surface layer, heating said alloy with said electrodeposited unoxidised surface layer in a substantially nonoxidising atmosphere to obtain diffusion, and thereafter oxidising said electrodeposited layer of nickel by heating in an oxidising atmosphere.

JOHN STOCKDALE. DONALD ERIC RISON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 734,522 Edison July 28, 1903 1,547,395 Hoyt July 28, 1925 1,578,254 Bennett Mar. 30, 1926 1,608,694 Cain Nov. 20, 1926 1,792,082 Fink et a1 Feb. 10, 1931 2,081,051 Freidrick May 18, 1937 2,156,262 Fink et al May 2, 1939 2,402,834 Nachtman June 25, 1946 FOREIGN PATENTS Number Country Date 115,700 Great Britain May 21, 1918