Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23Q—DETAILS, COMPONENTS, OR ACCESSORIES FOR MACHINE TOOLS, e.g. ARRANGEMENTS FOR COPYING OR CONTROLLING; MACHINE TOOLS IN GENERAL CHARACTERISED BY THE CONSTRUCTION OF PARTICULAR DETAILS OR COMPONENTS; COMBINATIONS OR ASSOCIATIONS OF METAL-WORKING MACHINES, NOT DIRECTED TO A PARTICULAR RESULT
  • B23Q1/00—Members which are comprised in the general build-up of a form of machine, particularly relatively large fixed members
  • B23Q1/70—Stationary or movable members for carrying working-spindles for attachment of tools or work
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/73—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process
  • C23C22/74—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process for obtaining burned-in conversion coatings
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S428/00—Stock material or miscellaneous articles
  • Y10S428/922—Static electricity metal bleed-off metallic stock
  • Y10S428/9265—Special properties
  • Y10S428/933—Sacrificial component
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S428/00—Stock material or miscellaneous articles
  • Y10S428/922—Static electricity metal bleed-off metallic stock
  • Y10S428/9335—Product by special process
  • Y10S428/934—Electrical process
  • Y10S428/935—Electroplating
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S428/00—Stock material or miscellaneous articles
  • Y10S428/922—Static electricity metal bleed-off metallic stock
  • Y10S428/9335—Product by special process
  • Y10S428/941—Solid state alloying, e.g. diffusion, to disappearance of an original layer
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12014—All metal or with adjacent metals having metal particles
  • Y10T428/12028—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, etc.]
  • Y10T428/12063—Nonparticulate metal component
  • Y10T428/12069—Plural nonparticulate metal components
  • Y10T428/12076—Next to each other
  • Y10T428/12083—Nonmetal in particulate component
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12014—All metal or with adjacent metals having metal particles
  • Y10T428/12028—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, etc.]
  • Y10T428/12063—Nonparticulate metal component
  • Y10T428/12104—Particles discontinuous
  • Y10T428/12111—Separated by nonmetal matrix or binder [e.g., welding electrode, etc.]
  • Y10T428/12118—Nonparticulate component has Ni-, Cu-, or Zn-base
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12736—Al-base component
  • Y10T428/1275—Next to Group VIII or IB metal-base component
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12771—Transition metal-base component
  • Y10T428/12861—Group VIII or IB metal-base component
  • Y10T428/12937—Co- or Ni-base component next to Fe-base component
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12771—Transition metal-base component
  • Y10T428/12861—Group VIII or IB metal-base component
  • Y10T428/12944—Ni-base component
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12771—Transition metal-base component
  • Y10T428/12861—Group VIII or IB metal-base component
  • Y10T428/12951—Fe-base component
  • Y10T428/12972—Containing 0.01-1.7% carbon [i.e., steel]
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12986—Adjacent functionally defined components

Definitions

• the present invention relates to the protective coating of ferrous metals to prevent corrosion thereof, and relates particularly to the protective coating of steel to inhibit corrosion and stress corrosion cracking.
• the invention further relates to such protected ferrous metals.
• the corrosion of ferrous metals in corrosive atmospheres is a problem that has long faced the metal working arts.
• the problem of preventing corrosion is particularly acute in the manufacture of metal fasteners, such as bolts, where corrosion of the fastener when under stress can result in stress corrosion cracking and sudden failure of the fastener.
• metal fasteners such as bolts
• the problem of stress corrosion cracking and failure are particularly troublesome. While stress corrosion cracking in metals is not limited to steels, ultra-high strength steels are more susceptible than most other alloys because of their highly stressed condition and low load bearing area after pit initiation. Nevertheless, the use of ultrahigh strength steels for fasteners in the aerospace industry is still widespread because of their low initial cost.
• the electroplating of fasteners with a metal such as nickel in thicknesses greater than about 0.5 mil is not feasible because the plated layer is unevenly deposited. If a bolt is electroplated, for example, the thread roots are areas of lower current density than the thread crests, leading to a preferential deposit of the plating metal on the crests. This may cause difficulties in mating the bolts with nuts, particularly if the nuts have been similarly electroplated.
• nickel-cadmium coatings lose their sacrificial property toward underlying and surrounding base metals at relatively low temperatures, i.e. at temperatures between 650 900F., although temperatures within this range are commonly encountered in aerospace applications, for example in certain parts of jet engines.
• temperatures i.e. at temperatures between 650 900F.
• the nickel When diffused nickelcadmium protective coatings are heated to these temperatures, the nickel apparently diffuses upwardly through the coating, making the surface of the coating more noble and less sacrificial.
• cadmium can escape from the plating and may cause stress-alloy cracking, either of the metal structure surrounding a plated article such as a bolt, or of the article itself if the nickel undercoating is defective. Under these circumstances, cadmium acts as a corrosive medium attacking the grain boundaries of the metal with which it alloys.
• a protective coating which is highly effective in preventing corrosion and stess corrosion cracking in ferrous metals, particularly high strength steel, to which the coating is applied.
• the coatings are of particular utility when employed to protect articles exposed to high temperatures and stresses, e.g. ultra-high strength steels fashioned into components for jet engines or metal fasteners used in aerospace applications.
• the protective coating comprises a first coating of a metal more noble than the ferrous metal to be protected and a coating of phosphate chromate bonded aluminum (PCBA) over said first coating.
• PCBA phosphate chromate bonded aluminum
• the first property is of particular importance in the aerospace industry where steel fasteners are often employed in contact with surrounding aluminum structures which, because they are of a metal more active than the metal of the fastener, are often undesirably corroded.
• the PCBA coating is more sacrificial than the diffused nickel-cadmium alloy coatings heretofore used in the art and is free of cadmium which could cause stress-alloy cracking. More importantly, the PCBA coating remains sacrificial even after exposure to elevated temperature conditions under which nickel-cadmium coatings would lose this protective property, as hereinbefore discussed.
• any metal more noble than the ferrous metal being coated can be employed, including tin, tantalum, titanium, copper, nickel, cobalt and the conventional noble metals or precious metals such as silver, gold, platinum, palladium, etc.
• the use of precious metals is impractical because of cost, however, Metals such as tantalum and titanium cannot be applied as coatings by electroplating and their use as coatings is, thus, inconvenient and- /or expensive.
• a metal such as copper may tend to form intermetallic compounds with a ferrous substrate at high temperatures after long periods of time.
• the noble metals most practical for use are nickel and cobalt. Those metals are relatively inexpensive, can be electroplated, and are particularly suitable for use at high temperature. Nickel is presently preferred as the plating metal because of its lower cost, and is shown in this specification as the preferred embodiment. However, the cost of this metal is increasing to values at which cobalt, with properties comparable with those of nickel, is becoming economically competitive.
• phosphate chromate bonded aluminum materials employed according to the present invention are described and claimed in US. Pat. No. 3,248,251, where their use as protective coating and bonding compositions for metals is taught.
• the use of such coatings alone on ferrous metals such as steels subject to stress corrosion cracking is less satisfactory than when used as an overcoating on a noble metal.
• the PCBA coating remains sacrificial at high or low temperatures, it is not a moble material (with respect to ferrous metals) and cannot per se prevent pitting. Such a coating used alone merely delays failure of the underlying ferrous alloy.
• the PCBA materials employed together with a noble metal according to the present invention to form a protective coating on ferrous metals comprise an aqueous dispersion of metallic aluminum having a grain size less than 325 mesh in an aqueous solution of a combination of inorganic compounds from the group consisting of phosphoric acid, chromic acid, molybdic acid, and metal salts of said acids.
• the solution of inorganic materials comprises at least one mol per liter of dissolved phosphate, at least 0.3 mol per liter of dissolved chromate or molybdate, and at least 0.5 mol per liter of dissolved metal such as magnesium, zinc, aluminum, iron, calcium, lithium, sodium, silver, etc.
• the suspended aluminum powder is present in an amount of from about to 2000 grams per liter of solution.
• the composition is commonly applied by conventional techniques such as spraying, dipping, rolling, or brushing to the surface to be coated and is subsequently cured to waterinsolubility by heating at a temperature of about 500 1000F. for periods of time variable with the temperature employed, but generally at least about minutes.
• any ferrous metal can be protected from corrosion according to the present invention, including stainless steels and those with high nickel content.
• the problem of stress corrosion cracking is most important, as discussed above, in high alloy steels.
• high alloy steels include those known in the art as H-1 1, A15] 4340, AlSl 4130, the maraging-type steels such as Maraging 300, and, in general, any ultra-high strength alloys having a minimum ultimate tensile strength of at least about 220,000 psi.
• the surface to be coated is cleaned, preferably by a technique such as dry blasting with particles of alumina.
• pickling of the alloys for purposes of cleaning is to be avoided to preclude hydrogenembrittlement of the alloy which may result from treatment with acid.
• the metal to be protected is then coated with the noble metal by an suitable method such as electroplating vacuum deposition or the like.
• the thickness of the noble metal is between about 0.1 mil and about 0.4 mil, preferably between about 0.1 mil and about 0.3 mil when fasteners are being coated.
• electroplating from a sulfamate bath is particularly suitable.
• electroplated articles are then preferably heated to drive off any hydrogen codeposited during the electroplating step.
• Such heating may be for a period between 2 and 24 hours at moderate temperatures, for example about 375F.
• the coated surface may next optionally be lightly blasted (for example with alumina particles) to toughen it and thus to improve the physical adhesion of the PCBA coating to be applied thereover.
• the parts Before or after blasting, the parts may also be subjected to degreasing in an alkaline bath, particularly if they have been stored for any length of time.
• the PCBA solution is next applied in an amount sufficient to give a cured coatiing at least about 0.3 mil thick.
• the coating is preferably between about 0.3 mil and about 0.5 mil thick, but for other articles an upper limit on coating thickness may be dictated only by economy, and the coating may be several mils thick.
• the noble metal undercoating serves important functions, e.g. as a noncorrosive barrier which also prevents diffusion of aluminum into the ferrous substrate at high temperatures.
• the PCBA coating is preferably applied by spraying.
• the coating is preferably gently heated slowly to drive off water and to render the coatings dry to the touch.
• This drying prior to curing is optional, but is convenient.
• coated articles such as bolts
• drying at temperatures between 200F., preferably about F., for about 10-20 minutes is sufficient.
• the coating is cured to waterinsolubility by heating at higher temperatures to remove any remaining water and to drive off any hydrogen which may have been generated by reaction of the metals with the acid components of the bath.
• such curing may be at temperatures as low as 350 375F. for 24 hours or more, or may be accomplished in as little time as 15 minutes at 650F., for example.
• the electrical conductivity of the cured coatings can be improved to make then more capable of providing galvanic protection to surrounding structures by heating them at temperatures of 850 to 1000F. for a period of at least 1 1 /2 hours.
• conductivity in the coatings can alternatively be increased by mechanically working the coatings, for example by blasting or burnishing.
• the coatings may be mechanically worked by blasting with glass beads (e.g. 150 200 mesh size), or can, alternatively, be burnished by a fine wire wheel. Wheels comprising stainless steel wires are preferred to preclude any possible embedment of corrodable wire particles in the coatings.
• coated articles are now ready for use, or still further coatings may be applied thereover, such as conventional conversion coatings.
• the PCBA-coated samples included samples in which the coating was: (1) cured at 375F. for 24 hours and then burnished with glass beads; (2) cured at 650F. for 15 minutes and then burnished with a wire wheel; and (3) cured at 650F. for 15 minutes and then at 1000F. for 90 minutes.
• the PCBA coatings were between 0.3 and 0.5 mil thick.
• PCBA on nickel all had PCBA coatings from 0.3-0.5 mil thick but different samples had nickel plating either 0.1, 0.2, or 0.4 mil thick: in each case the PCBA coating thereover was cured at 1000F. for 90 minutes.
• the diffused Ni-Cd samples had 0.10.2 mil cadmium deposited on 0.20.4 mil nickel. Diffusion was effected by heating at 630F. for one-half hour.
• the bolt samples were first exposed to a temperature of 900F. for 8 hours, followed by a 16 hour exposure to 5 percent salt spray. This treatment (one cycle) was repeated for 20 cycles, with color photographs being made on the coatings after 5, l0, l5, and 20 cycles. The observations are summarized in Table 1 below.
• the method of protecting ferrous metal to inhibit corrosion which comprises coating said ferrous metal with a layer, from about 0.1 mil to about 0.4 mil thick, of a more noble metal, and then coating said more noble metal with a sacrificial layer, at least about 0.3 mil thick, by applying thereto a composition consisting essentially of about 10 to 2000 grams of alumiunum powder, having a grain size less than 325 mesh, per liter of an aqueous solution of a combination of phosphoric acid, chromic acid, or molybdic acid, or salts of said acids, and then heat curing said composition at a temperature from 350 to 1000F. for a time between at least 24 hours at the lowest temperature in the range specified to about 15 minutes at the highest temperature in the range specified, until said composition is water-insoluble.
• said layer of more noble metal is an electroplated layer of nickel or cobalt.
• the method of inhibiting stress corrosion cracking in a fastener of high-alloy steel which comprises coating said fastener with nickel in a thickness from about 0.1 mil to 0.3 mil, then coating said nickel by applying thereto a composition consisting essentially of about 10 to 2000 grams of aluminum powder, having a grain size less than 325 mesh, per liter of an aqueous solution of a combination of phosphoric acid, chromic acid, or molybdic acid, or salts of said acids, and then heat curmethod of claim 1.
• a protected ferrous metal article prepared by the method of claim 3.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Paints Or Removers (AREA)
• Chemically Coating (AREA)
• Electroplating Methods And Accessories (AREA)

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Cited By (8)

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Citations (7)

• 1972
  • 1972-05-26 FR FR7218835A patent/FR2139950B1/fr not_active Expired
  • 1972-05-26 GB GB2489772A patent/GB1361101A/en not_active Expired
  • 1972-06-01 SE SE7207198A patent/SE377947B/xx unknown
• 1973
  • 1973-09-19 US US398578A patent/US3897222A/en not_active Expired - Lifetime

Patent Citations (7)

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