US3849264A - Production of stain resistant, clear, sealed anodized films - Google Patents

Production of stain resistant, clear, sealed anodized films Download PDF

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Publication number
US3849264A
US3849264A US00286595A US28659572A US3849264A US 3849264 A US3849264 A US 3849264A US 00286595 A US00286595 A US 00286595A US 28659572 A US28659572 A US 28659572A US 3849264 A US3849264 A US 3849264A
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Prior art keywords
solution
acid
anodized
coating
salt
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Expired - Lifetime
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US00286595A
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English (en)
Inventor
K Weber
D Crozier
W Hawley
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Lockheed Martin Corp
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Lockheed Aircraft Corp
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Application filed by Lockheed Aircraft Corp filed Critical Lockheed Aircraft Corp
Priority to US00286595A priority Critical patent/US3849264A/en
Priority to GB4090873A priority patent/GB1428048A/en
Priority to AU59841/73A priority patent/AU480772B2/en
Priority to IL43118A priority patent/IL43118A/en
Priority to CA180,279A priority patent/CA1005707A/en
Priority to IT52327/73A priority patent/IT997525B/it
Priority to JP10009173A priority patent/JPS5629760B2/ja
Priority to DE2345371A priority patent/DE2345371C2/de
Priority to BE1005339A priority patent/BE804462A/xx
Priority to FR7332001A priority patent/FR2208991B1/fr
Application granted granted Critical
Publication of US3849264A publication Critical patent/US3849264A/en
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
    • C25D11/02—Anodisation
    • C25D11/04—Anodisation of aluminium or alloys based thereon
    • C25D11/18—After-treatment, e.g. pore-sealing
    • C25D11/24—Chemical after-treatment
    • C25D11/246—Chemical after-treatment for sealing layers

Definitions

  • ABSTRACT A stain resistant, substantially clear, anodized aluminum coating may be produced by: anodizing an aluminum body or article in a conventional manner such as in a sulfuric acid electrolyte; rinsing the anodized film produced; partially sealing this anodized film by contacting this film with a solution containing from about 0.1% to about 5% by weight chromic oxide either present as chromic acid or as a soluble salt of this acid and from about 0.5% to about 10% by weight of a soluble tungstate salt having a pH of from about 4.5 to about 8.0 at a temperature of from about 130F.
  • a solution containing from about 0. 1% to about 10% by weight of a soluble nickel or cobalt salt of a weak acid preferably an acid selected from the group consisting of boric, citric, formic and acetic acid having a pH of from about 4.5 to about 8 at a temperature of from about 150F to immediately below the boiling point of the solution.
  • a soluble nickel or cobalt salt of a weak acid preferably an acid selected from the group consisting of boric, citric, formic and acetic acid having a pH of from about 4.5 to about 8 at a temperature of from about 150F to immediately below the boiling point of the solution.
  • the final sealing solution is buffered so that its pH will be maintained within this range.
  • the present invention pertains to the sealing of anodized aluminum films so as to improve the corrosion and stain resistance of such films.
  • Such coatings or films are normally referred to as anodized coatings or films inasmuch as they are produced by electrolytic processes in appropriate electrolytes at the anodes of electrolytic cells or systems. Normally a sulfuric acid electrolyte is utilized in the production of these anodized films.
  • the oxide coatings produced in this manner are somewhat porous in character and are somewhat subject to various types of corrosion. Further, these anodized films are well recognized 'to be susceptible to staining as a result of various types of materials contacting these films.
  • Sealed anodized coatings produced in accordance with the process set forth in this co-pending application are considered to be highly desirable because they are substantially clear in character. They are also considered to be highly desirable because of their resistance to corrosion. Parts produced in accordance with the process of this prior application have withstood salt spray testing for significant periods without signs of the usual salt caused corrosion. However, in spite of these advantageous characteristics, sealed anodized coatings produced in accordance with this co-pending application are not considered to be as desirable as reasonably possible for certain commercial applications because they tend to lack adequate resistance to staining.
  • An objective of the present invention is to fulfill this need.
  • This invention is intended to provide a new and improved process for sealing anodized coatings which retain the benefits of sealing anodized coatings with tungstate-chromate solutions as indicated, but which also provides in such coatings a degree of stain resistance not achievable with such sealing alone.
  • An objective of the present invention is also to provide sealed, anodized films as indicated.
  • Another objective of the present invention is to provide a process as indicated which may be easily and conveniently carried out at a comparatively minimal cost so that the articles produced by this process can be economically competitive.
  • the word aluminum employed in this specification is used herein in a broad sense corresponding to the manner in which this word is conventionally utilized industrially.
  • the word aluminum as used herein does not only designate the metal known by this word and/or articles or bodies created from this metal, but it is also utilized to designate alloy compositions and/or articles or bodies created from this metal which are conventionally designated by the word aluminum since they predominantly contain this metal.
  • a number of such alloys are commonly known and used. They are frequently designated by trade designations such as 7075-T6, 7075-T76 and 2024-T3 used to designate their composition.
  • anodizing is carried out utilizing an aqueous electrolyte containing from about 7 to about 25% and preferably from about 1 1 to about 17% by weight of sulfuric acid at a temperature of from about 60F. to about 90F. and preferably from about 68F. to about 72F. at a current density within the range of from about 7 to about 20 amperes per square foot and preferably from about 10 to about amperes per square foot at a voltage normally from about 15 to about volts until an oxide film is produced which is sufficiently thick to withstand the physical abrasion or the like to which it is apt to be subject.
  • an anodized film may be neutralized in a weak alkaline solution such as a solution containing 5% by weight sodium bicarbonate and then is rinsed.
  • a weak alkaline solution such as a solution containing 5% by weight sodium bicarbonate
  • An anodized surface film produced in this manner is well recognized to be somewhat porous in character and to be somewhat susceptible to corrosion of various types and to be susceptible to being stained by contact with a wide variety of different materials.
  • such a film can be effectively sealed against corrosion by contacting it with an aqueous solution containing from about 1.5 to about 7% by weight of a soluble tungstate salt and from about 0.5 to about 5% by weight of chromic oxide either present as chromic acid or as a soluble salt of this acid having a pH of from about 3.2 to about 8.5 and preferably of from about 5.5 to 7 at a temperature of from about 150F. to about 212F.
  • This process is referred to as clear sealing" because it produces a clear, corrosion resistant oxide film.
  • the corrosion resistance and the stain resistance of such a coating may be improved or enhanced by contacting such a coating with a solution of a soluble nickel or cobalt salt as hereinafter indicated.
  • a soluble nickel or cobalt salt as hereinafter indicated.
  • Such a salt is considered to locate either the nickel or cobalt ion at the reactive sites in an inorganic complex as indicated so as to in effect block these sites in such a manner that materials which would normally tend to stain an anodized aluminum coating cannot be absorbed or held by such a coating, presumably at these same sites.
  • an initial clear sealing operation can be carried out utilizing an aqueous solution containing from about 0.1% to about 5% by weight of chromic oxide either present as a chromic acid or as a soluble salt of this acid and from about 0.5% to about l0% by weight of soluble tungstate salt having a pH of from about 4.5 to about 8.0 at a temperature of from about F. to a temperature immediately below the boiling point of the solution.
  • chromic oxide either present as a chromic acid or as a soluble salt of this acid and from about 0.5% to about l0% by weight of soluble tungstate salt having a pH of from about 4.5 to about 8.0 at a temperature of from about F. to a temperature immediately below the boiling point of the solution.
  • the bath used in this initial sealing can easily be created utilizing a conventional soluble tungstate salt such as a potassium, sodium, lithium or ammonium salt. It is also possible to utilize potassium, sodium or ammonium metatungstates or a combination of any of these salts.
  • the chromate used may be in the form of chromium trioxide (which of course forms chromic acid in solution) or a conventional chromate compound such as a sodium, potassium, lithium or ammonium chromate or dichromate or any mixture of these.
  • This initial sealing solution should consist essentially of these salts in the sense that it should'contain only a mixture of one or more tungstates and one or more chromates and if reasonably necessary, a suitable pH adjusting compound such as sodium hydroxide or the like containing ions such as alkali metals ions and hydroxide ions which will not significantly enter into the sealing action obtained, and, if desired one or more conventional wetting agents used in accordance with conventional practice so as to facilitate solution penetration into an anodized surface provided that such a wetting agent is of a known type not effecting the sealing action obtained.
  • a suitable pH adjusting compound such as sodium hydroxide or the like containing ions such as alkali metals ions and hydroxide ions which will not significantly enter into the sealing action obtained
  • a suitable pH adjusting compound such as sodium hydroxide or the like containing ions such as alkali metals ions and hydroxide ions which will not significantly enter into the sealing action obtained
  • one or more conventional wetting agents used in
  • the proportions of the tungstate (s) or chromate (s) used in the bath in this initial partial sealing are primarily dictated by practical considerations. If amounts of either of these two types of ions below those indicated are present in the sealing solution a partially sealed anodized film having less than a desired degree of corrosion resistance will be achieved. On the other hand, if greater amounts than are indicated are used, there tends to be an uneconomic or unnecessary use of material.
  • a pH within the range indicated is preferred during this initial sealing since a lower pH than 4.5 may result in some acidic-type attack on the aluminum treated and/or on the oxide film present on this aluminum. With higher pHs than are indicated there is also the possibility of alkaline attack on the aluminum and the oxide.
  • solution temperatures below the 130F. figure it appears that a desired partial type seal action is not effectively obtained. If a maximum temperature of the sealing solution is used which corresponds to the boiling point of this solution the vaporization of water may tend to interfere with the formation of a uniform, at least partially sealed coating.
  • the clear sealing step carried out in accordance with this invention may be performed utilizing various broad ranges of ingredients and conditions as indicated, it is preferred to carry out the invention utilizing the more specific ranges and conditions indicated in the preceding discussion as to prior, known clear seal process of sealing utilizing a combined tungstate-chromate bath. This is because it is considered that it is advisable to seal an anodized coating as completely as possible in the initial sealing step so as to avoid any chance of such a coating being completely sealed because of failure of any type of the secondary sealing operation performed in accordance with this invention. In essence, however, this is a matter of choice.
  • This initial sealing step or partial sealing step in accordance with this invention can be carried out in accordance with conventional practice by either dipping an article to be sealed in a bath as specified or by spraying such a bath on such an article.
  • the time of contact between this initial clear sealing solution will, of course, effect the completeness of the sealing accomplished during the treatment with it. It is considered that effective sealing can be accomplished with a contact period of at least one minute. Preferably, however, the contact time between an article being partially sealed in the solution should be longer in order to make sure that there is adequate, reasonably complete partial sealing during this step. It is considered that if a time period in excess of about twenty minutes is used that nothing is gained as a result of the use of this long a period. In general, the hotter the temperature of the sealing solution used the lower the time period required.
  • An anodized film or coating at least partially sealed with a tungstate-chromate bath as indicated in the preceding is further sealed in accordance with this invention with a solution containing from about 0.] to about 10% by weight of a soluble nickel .or cobalt'salt. in general, the lower the concentration of such a bath, the
  • the nickel or cobalt salt used with the present invention should be a salt of a weak acid such as the salt of either of these metals and an acid selected from the group consisting of boric acid, citric acid, formic acid and acetic acid. Salts of these metals with other acids than are indicated are considered to normally impart to a sealed anodized film as herein described a degree of color rendering an anodized, sealed film as herein indicated unacceptable for certain applications'where asubstantially clear, colorless film is required. Mixtures of various salts as indicated can be employed with the present invention.
  • this second sealing solution has a pH below about 4.5 it is considered that there is a tendency for the acidity of this solution to attack the aluminum being treated with it and/or the treated oxide film on the aluminum.
  • the pH of this secondary treatment solution is relatively high such as at a pH of 8 or above, it is considered that there is normally a problem of maintaining a desired, effective concentration of the salt used. This is because nickel and cobalt salts as employed tend to precipitate out at relatively high pHs.
  • this secondary treatment solution employed have a pH of no greater than 7.
  • the solution used consist essentially of only one or more of the nickel or cobalt salts indicated or consist essentially of one or more of such salts and one or more buffering reagents as described.
  • the treatment solution used should be free from metallic ions other than alkali metal ions and from acidic ions other than those of the acids specifically enumerated.
  • the solution used may contain one or more conventional wetting agents used in accordance with conventional practice so as to facilitate solution penetration provided thatsuch-a wetting agent is of a known type not affecting the sealing action obtained. Suitable wetting agents may be anionic, cationic, ornonionic and may be conveniently used in amounts up to about 3% by weight of the solution.
  • This secondary sealing step may also be carried out by either dipping or immersing an at least partially clear sealed article as indicated in a bath as specified in the preceding or by spraying such solution on to an at least partially sealed oxide film or coating as indicated so that there is contact between the film and coating and the solution for a time period and at a temperature as specified.
  • a test panel of 7075-T6 aluminum material anodized in a sulfuric acid electrolyte in accordance with conventional practice can be partially sealed by immersing it in an aqueous solution containing by weight sodium tungstate and l% by weight chromic acid having I a pH of 6.5 at a temperature of 200F. for about 10
  • a test panel of 7075-T76 aluminum material anodized in a sulfuric acid electrolyte in accordance with conventional practice can be partially sealed by im- 6 15 minutes. Immediately following this the panel can be further sealed by immersing it in a solution containing about 3% by weight nickel acetate and 4% by weight boric acid at a temperature of about 160F. for a period of about 3 minutes.
  • said further solution consists essentially of water and said salt and said weak acid is an acid selected from a group consisting of boric, citric, formic and acetic acids.
  • said further solution consists essentially of water, said salt and a buffering compound
  • said weak acid is an acid selected from a group consisting of boric, citric, formic and acetic acids, and
  • said buffering compound is a compound selected from the group consisting of boric acid, citric acid, formic acid and acetic acid and ammonium and alkali metal salts of such acids.
  • said further solution contacts said anodized coating for a period of from about l to about 20 minutes and has a pH of from about 4.5 to about 8.
  • said first solution consists essentially of water and from about 0.5 to about 10% by weight of a soluble tungstate salt, from about 0.1 to about 5% by weight chromic oxide present as chromic acid or as a soluble chromate salt, has a pH of from about 4.5 to about 8 and is in contact with said anodized coating for a period of from about 1 to about 20 minutes at a temperature of from about F. to immediately below the boiling point of said solution,
  • said further solution is a solution selected from the group consisting of (1) aqueous solutions consisting essentially of said salt and of (2) buffered aqueous solutions consisting essentially of said salt and a buffering compound, said weak acid being selected from the group consisting of boric, citric, formic and acetic acids, said buffering compound being selected from the group consisting of boric acid, citric acid, formic acid and acetic acid and ammonium and alkali metal salts of such acids, said further solution having a pH of from about 4.5 to about 7 and contacting said anodized coating for a period of from about 1 to about 20 minutes.
  • the pH of said further solution is from about 5.5 to
  • said first solution includes a wetting agent serving to facilitate penetration of said first solution into said anodizedcoating.
  • said further solution includes a wetting agent serving to facilitate penetration of said further solution into said anodized coating.
  • both of said first and said further solutions include a wetting agent serving to facilitate penetration of said solutions into said anodized coating.
  • a process for sealing an anodized aluminum coating which comprises:
  • a further solution selected from the group consisting of (1) solutions consisting essentially of water and from about 0.1% to about 10% by weight of a salt of a metal selected from the group consisting of nickel and cobalt and of an acid selected from the group consisting of boric, citric, formic and acetic acids and (2) solutions consisting essentially of water and from about 0.1% to about 10% by weight of a salt of a metal selected from the group consisting of nickel and cobalt and of an acid selected from the group consisting of boric, citric, formic and acetic acids and of a buffering compound selected from the group consisting essentially of boric, citric, formicand acetic acids and ammonium and alkali metal salts of such acids, said further solution having a pH of from about 4.5 to about 8 and being at a temperature of from about F. to immediately below the boiling point of said solution.
  • said further solution has a pH of from about 5.5 to
  • said first solution includes a wetting agent serving to facilitate penetration of said first solution into said anodized coating.
  • said further solution includes a wetting agent serving to facilitate penetration of said further solution into said anodized coating.
  • both of said first and said further solutions include a wetting agent serving to facilitate penetration of said solutions into said anodized coating
  • said further solution has a pH of from about 5.5 to

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  • Chemical & Material Sciences (AREA)
  • General Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Chemical Treatment Of Metals (AREA)
  • Electrochemical Coating By Surface Reaction (AREA)
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US00286595A 1972-09-05 1972-09-05 Production of stain resistant, clear, sealed anodized films Expired - Lifetime US3849264A (en)

Priority Applications (10)

Application Number Priority Date Filing Date Title
US00286595A US3849264A (en) 1972-09-05 1972-09-05 Production of stain resistant, clear, sealed anodized films
GB4090873A GB1428048A (en) 1972-09-05 1973-08-30 Production of stain resistant clear sealed anodized aluminium oxide coatings
AU59841/73A AU480772B2 (en) 1972-09-05 1973-08-30 Production of stain resistant, clear, sealed anodized films
IL43118A IL43118A (en) 1972-09-05 1973-08-31 Production of anodized membranes that are stain resistant, transparent and opaque
CA180,279A CA1005707A (en) 1972-09-05 1973-09-04 Production of stain resistant, clear, sealed anodized films
IT52327/73A IT997525B (it) 1972-09-05 1973-09-04 Produzione di pellicole anodizzate protettive trasparenti resistenti alla macchiatura
JP10009173A JPS5629760B2 (de) 1972-09-05 1973-09-05
DE2345371A DE2345371C2 (de) 1972-09-05 1973-09-05 Verfahren zum fleckfesten Versiegeln von anodischen Oxidschichten auf Aluminium
BE1005339A BE804462A (fr) 1972-09-05 1973-09-05 Production de films ansdifs non poreux, transparents et inalterables
FR7332001A FR2208991B1 (de) 1972-09-05 1973-09-05

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Application Number Priority Date Filing Date Title
US00286595A US3849264A (en) 1972-09-05 1972-09-05 Production of stain resistant, clear, sealed anodized films

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US3849264A true US3849264A (en) 1974-11-19

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US00286595A Expired - Lifetime US3849264A (en) 1972-09-05 1972-09-05 Production of stain resistant, clear, sealed anodized films

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US (1) US3849264A (de)
JP (1) JPS5629760B2 (de)
BE (1) BE804462A (de)
CA (1) CA1005707A (de)
DE (1) DE2345371C2 (de)
FR (1) FR2208991B1 (de)
GB (1) GB1428048A (de)
IL (1) IL43118A (de)
IT (1) IT997525B (de)

Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4288299A (en) * 1978-05-22 1981-09-08 Alcan Research And Development Limited Enhanced hydrothermal sealing of anodized aluminum
EP0468883A1 (de) * 1990-07-27 1992-01-29 Nippon Dacro Shamrock Co. Antikorrosive Überzugszusammensetzung
US5411607A (en) * 1993-11-10 1995-05-02 Novamax Technologies Holdings, Inc. Process and composition for sealing anodized aluminum surfaces
US5731124A (en) * 1992-04-22 1998-03-24 Agfa-Gevaert, N.V. Method for preparing an aluminum foil for use as a support in lithographic printing plates
US6537678B1 (en) 2000-09-20 2003-03-25 United Technologies Corporation Non-carcinogenic corrosion inhibiting additive
US6613390B2 (en) * 2000-12-19 2003-09-02 United Technologies Corporation Compound, non-chromium conversion coatings for aluminum alloys
US8512872B2 (en) 2010-05-19 2013-08-20 Dupalectpa-CHN, LLC Sealed anodic coatings
US8609254B2 (en) 2010-05-19 2013-12-17 Sanford Process Corporation Microcrystalline anodic coatings and related methods therefor
CN103614757A (zh) * 2013-12-09 2014-03-05 海安县申菱电器制造有限公司 铝合金表面多孔膜稀土的一种封闭方法
US10138566B2 (en) * 2017-01-13 2018-11-27 Macdermid Acumen, Inc. Sealing anodized aluminum using a low-temperature nickel-free process
US10214827B2 (en) 2010-05-19 2019-02-26 Sanford Process Corporation Microcrystalline anodic coatings and related methods therefor

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
IT1228581B (it) * 1982-06-29 1991-06-24 Italtecno Srl Procedimento per il fissaggio protettivo dell'ossido anodico di alluminio e sue leghe conferente particolare resistenza agli agenti aggressivi alcalini
DE4000821C2 (de) * 1990-01-13 2000-07-06 Bosch Gmbh Robert Verfahren zur Verbesserung der Korrosionsbeständigkeit anodisch erzeugter Oxidschichten auf Aluminiumlegierungswerkstoffen

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2888388A (en) * 1957-08-26 1959-05-26 Sandoz Ag Sealing of dyed anodized aluminum
US3257244A (en) * 1964-10-14 1966-06-21 Reynolds Metals Co Sealing and inhibiting corrosion of anodized aluminum
US3616298A (en) * 1968-11-22 1971-10-26 Philco Ford Corp Sealing anodic films
US3704210A (en) * 1969-07-21 1972-11-28 Cegedur Gp Process for coloring aluminum objects

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2888388A (en) * 1957-08-26 1959-05-26 Sandoz Ag Sealing of dyed anodized aluminum
US3257244A (en) * 1964-10-14 1966-06-21 Reynolds Metals Co Sealing and inhibiting corrosion of anodized aluminum
US3616298A (en) * 1968-11-22 1971-10-26 Philco Ford Corp Sealing anodic films
US3704210A (en) * 1969-07-21 1972-11-28 Cegedur Gp Process for coloring aluminum objects

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4288299A (en) * 1978-05-22 1981-09-08 Alcan Research And Development Limited Enhanced hydrothermal sealing of anodized aluminum
EP0468883A1 (de) * 1990-07-27 1992-01-29 Nippon Dacro Shamrock Co. Antikorrosive Überzugszusammensetzung
US5731124A (en) * 1992-04-22 1998-03-24 Agfa-Gevaert, N.V. Method for preparing an aluminum foil for use as a support in lithographic printing plates
US5411607A (en) * 1993-11-10 1995-05-02 Novamax Technologies Holdings, Inc. Process and composition for sealing anodized aluminum surfaces
US5478415A (en) * 1993-11-10 1995-12-26 Novamax Technology Holdings, Inc. Process and composition for sealing anodized aluminum surfaces
US6537678B1 (en) 2000-09-20 2003-03-25 United Technologies Corporation Non-carcinogenic corrosion inhibiting additive
US6613390B2 (en) * 2000-12-19 2003-09-02 United Technologies Corporation Compound, non-chromium conversion coatings for aluminum alloys
US8512872B2 (en) 2010-05-19 2013-08-20 Dupalectpa-CHN, LLC Sealed anodic coatings
US8609254B2 (en) 2010-05-19 2013-12-17 Sanford Process Corporation Microcrystalline anodic coatings and related methods therefor
US9260792B2 (en) 2010-05-19 2016-02-16 Sanford Process Corporation Microcrystalline anodic coatings and related methods therefor
US10214827B2 (en) 2010-05-19 2019-02-26 Sanford Process Corporation Microcrystalline anodic coatings and related methods therefor
CN103614757A (zh) * 2013-12-09 2014-03-05 海安县申菱电器制造有限公司 铝合金表面多孔膜稀土的一种封闭方法
US10138566B2 (en) * 2017-01-13 2018-11-27 Macdermid Acumen, Inc. Sealing anodized aluminum using a low-temperature nickel-free process

Also Published As

Publication number Publication date
JPS4986232A (de) 1974-08-19
AU5984173A (en) 1975-03-06
FR2208991B1 (de) 1979-06-01
JPS5629760B2 (de) 1981-07-10
GB1428048A (en) 1976-03-17
DE2345371C2 (de) 1982-04-22
IL43118A (en) 1977-02-28
CA1005707A (en) 1977-02-22
IL43118A0 (en) 1973-11-28
IT997525B (it) 1975-12-30
DE2345371A1 (de) 1974-03-14
BE804462A (fr) 1974-03-05
FR2208991A1 (de) 1974-06-28

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