EP0006046B1 - Verwendung einer Lösung zur elektrolytischen Bildung von Schutzüberzügen aus Pyrophosphat auf Zinkoberflächen - Google Patents

Verwendung einer Lösung zur elektrolytischen Bildung von Schutzüberzügen aus Pyrophosphat auf Zinkoberflächen Download PDF

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Publication number
EP0006046B1
EP0006046B1 EP79400312A EP79400312A EP0006046B1 EP 0006046 B1 EP0006046 B1 EP 0006046B1 EP 79400312 A EP79400312 A EP 79400312A EP 79400312 A EP79400312 A EP 79400312A EP 0006046 B1 EP0006046 B1 EP 0006046B1
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EP
European Patent Office
Prior art keywords
solution
fact
electrolysis
zinc
treated
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Expired
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EP79400312A
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English (en)
French (fr)
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EP0006046A1 (de
Inventor
Robert Rosset
Alain Jardy
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Chaffoteaux et Maury SAS
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Chaffoteaux et Maury SAS
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Priority to AT79400312T priority Critical patent/ATE6078T1/de
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Classifications

    • 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
    • C25D11/36—Phosphatising

Definitions

  • the invention relates to the formation of protective coatings on zinc surfaces. It also relates to the use of particular compositions for obtaining this training.
  • Zinc surfaces and particularly surfaces obtained by galvanizing ferrous metals are sensitive to corrosion. Particularly in prolonged use, natural waters constitute aggressive environments with regard to galvanized objects.
  • This coating in the techniques known up to now, is produced by contacting the parts with a treating solution containing phosphate ions.
  • the deposition of insoluble salts from this solution results from the chemical shift of an equilibrium in which, of course, the constituents of the solution participate, but also the metal of the treated surface.
  • the parameters which influence the result obtained are multiple.
  • the main ones are of course linked to the composition of the solution (nature and concentration), to its pH, but other factors such as temperature, duration of treatment, etc. are also involved.
  • the treating solutions for the formation of a protective deposit of phosphates contain one or more phosphates soluble in acid solution. This solution, in contact with the metal surface, produces a light attack of the metal and the precipitation of insoluble zinc phosphates.
  • Another type of difficulty is related to the fact that, for corrosion treatment, the time required to form the coating is usually several hours, or even several days. The length of treatment is all the more troublesome as it is most often carried out at temperatures above ambient temperature and requires permanent heating.
  • French patent FR-A-2 219 245 has proposed the formation of deposits by chemical or electrochemical route, from a solution containing aluminum phosphates. But they are also, in fact, generally intended to receive a layer of paint, and their anti-corrosion properties are still still insufficient.
  • protector essentially consisting of zinc pyrophosphate, a solution which is prepared from the dissolution of one or more soluble phosphates or their corresponding acids, chosen from the group comprising pyrophosphates or corresponding acids, phosphate ions or corresponding acids, of a degree of condensation higher or close to that of pyrophosphates, under conditions of concentration of pH and temperature such that they lead to pyrophosphates or to their corresponding acids.
  • oxygenated acid species of phosphorus at the degree of oxidation V and the corresponding ions are very varied. These are in particular orthophosphates, pyrophosphates, metaphosphates. Also known are associated acids in macromolecular form, with an average composition intermediate between that of pyrophosphoric and metaphosphoric acids. All these species in aqueous solution give rise to equilibria and, depending on the overall concentration, the pH, the temperature, the proportions of each of them can vary.
  • the protective coating formed is designated as being zinc pyrophosphate.
  • Analysis of the coating indeed shows that zinc pyrophosphate is the essential constituent of the deposit formed under the preferred implementation conditions of the process according to the invention.
  • other insoluble zinc phosphates can be precipitated with the pyrophosphate.
  • the electrolyte used for the formation of the pyrophosphate coating from other polyphosphates or from a mixture of polyphosphates capable of giving pyrophosphates under the conditions which are those of use of this electrolyte.
  • the polymetaphosphate ions possibly mixed with orthophosphate ions can lead, in solution and by equilibrium reactions, to mixtures of ions containing in particular pyrophosphates.
  • the solutions can also be prepared from the orthophosphates by dehydration using phosphoric anhydride P 2 0 5 .
  • the pyrophosphates are on the whole very poorly soluble, and in practice, in order to obtain sufficiently concentrated solutions, one is led to choose those whose solubility is the highest possible. For this reason, use is preferably made, according to the invention, of a solution of alkaline pyrophosphates, in particular those of sodium or potassium.
  • the content of polyphosphate ions and in particular pyrophosphates in the solution is limited in value by the solubility characteristics. This content can be increased without leading to the formation of precipitate by modifying for example the pH conditions of the solution. Conversely, the presence of cations capable of leading to very poorly soluble compounds, in particular of Zn 2+ ions, reduces the admissible content of pyrophosphate ions.
  • the conditions of solubility of the phosphates of cations deemed insoluble are a function not only of the respective ionic concentrations of phosphates and cations but also of the pH range of the solution, it being understood that, when one speaks of insoluble phosphates, one refers to solutions that are neutral or that do not stray too far from neutrality.
  • a solution is advantageously used, the overall phosphate content of which expressed in phosphorus is between 0.6 and 12.5 g / l.
  • the solution is prepared using sodium pyrophosphate, 0.01 to 0.2 mol / l, and preferably about 0.1 mol / l, is used, ie 44.6 g / l of Na 2 P 2 0 7 , 10 H 2 0.
  • a weakly acidic electrolyte it is preferable to use a weakly acidic electrolyte.
  • a basic solution could lead to a modification of the structure of the deposit by the formation of basic salts or zinc hydroxides, and, ultimately, could lead to the dissolution of the zinc of the object treated. It is advantageous to operate with an electrolyte whose pH is less than 6.
  • a very strongly acidic solution is also not desirable.
  • Such a solution by promoting the solubility of zinc phosphates, would delay the formation of the desired coating. Furthermore, if the solution is very acidic, it can attack the zinc surface to be coated inappropriately.
  • the electrolytes used advantageously have a pH which is not less than 2.5.
  • the preferred pH conditions vary a little.
  • the pH is between 2.7 and 3, and if one operates in alternating current, between 4.5 and 5.
  • the choice of acid used to adjust the pH is not critical. The most advantageous is to acidify by means of phosphoric acids and in particular orthophosphoric acid, these acids contributing to the establishment of the desired phosphate ion content, but it is also possible to use other acids, in particular sulfuric acid.
  • an electrolyte is advantageously used, the Zn 2 content of which can range up to 8.10 - 3 ion g / I.
  • zinc chloride or zinc oxide As a compound introduced into the electrolyte to release Zn 2 ions therein, it is advantageous to use zinc chloride or zinc oxide. When using zinc oxide, it is necessary to take into account its own basicity and to modify in sequence the quantities of acid used to maintain the pH of the solution within the preferred values.
  • the temperature at which the electrolysis according to the invention is carried out is not critical. This temperature is not, however, indifferent. An increase in temperature results in two antagonistic effects, on the one hand, an increase in the solubility of phosphates, which consequently is contrary to the formation of the precipitate, on the other hand, an increase in the chemical kinetics of the corresponding process displacement of the equilibria, which promotes the formation of the precipitate. Overall, however, it seems that the second effect predominates and that an increase in temperature accelerates the formation of the deposit.
  • the operation is advantageously, according to the invention, at a temperature between 20 and 70 ° C, and preferably at about 60 ° C.
  • the electrolytic baths having a tendency to heat up by the Joule effect, when they are in service, one can operate at the temperature which establishes itself, without the need to heat them or cool them.
  • electrolysis is carried out in the traditional way, with an electrolyte as defined above.
  • Electrolysis can be carried out by direct or alternating current. In both cases, a deposit of zinc pyrophosphates generated by the surface anodic oxidation of the zinc of the treated object is obtained.
  • the treated object When operating with direct current, the treated object is placed at the anode of the electrolysis device.
  • the electrolytic yield is all the better as the electrolyte used is closer to the conditions for precipitation of insoluble phosphates. Even very limited oxidation of the anode, by modifying the equilibrium conditions of the electrolyte, then allows the desired deposit to be formed.
  • the cathodic reaction modifies the equilibrium conditions of the electrolyte, and in particular it is necessary to avoid depositing zinc.
  • the anode and cathode compartments are separated using a membrane which does not allow the Zn 2+ ions to pass.
  • Membranes of this type are well known in the fields of industrial electrochemistry; they may in particular be asbestos membranes or membranes of anion exchange resins.
  • An additional advantage provided by the use of alternating current according to the invention is that it becomes superfluous to use a diaphragm. This is particularly advantageous when the treatment envisaged requires the use of devices (tanks, electrodes, etc.) of complex shapes.
  • the cathode is chosen from a material such that it remains stable in contact with the electrolytic solution and, consequently, does not modify the composition of the latter.
  • Cathodes of steel, lead or graphite can in particular be used.
  • the only condition necessary for the electrical parameters is that the voltage applied to the electrodes is sufficient, under the operating conditions, for the surface electrochemical oxidation of the zinc of the treated objects to occur.
  • This voltage depends on many factors: nature of the electrolysis, nature and configuration of the electrodes, arrangement of the electrodes, resistance of the diaphragm, etc. In practice, the voltage is fixed so that there is oxidation of the zinc of the objects treated regularly, which can be controlled by the density of the anode current.
  • the applied voltage should not exceed that at which one would lead to the electrolysis of the water of the solution used, failing which, in addition to harmful modifications of the bath, the faradaic yield of the operation would drop significantly.
  • current densities of the order of 40 A / m 2 are used . It is of course possible to operate with lower current densities, but the formation of the deposit is then slower and the treatment is longer for a deposit of the same size.
  • the effective current and voltage densities applied are higher.
  • the current density is adjusted to values of 40 to 100 A / m 2.
  • the applied voltages are between 5 and 10 V.
  • the duration of the treatment varies according to the size of the deposit sought, but also according to many factors such as the current density, the temperature, the concentration of the solution, etc.
  • the treatment according to the invention aims at the complete formation of a protective coating, given as we have indicated the insulating nature of the layer of pyrophosphates deposited, the treatment is continued until the necessary voltage applied becomes too large, possibly leading to electrolysis of the water or "breakdown" of the protective coating.
  • objects of the most diverse forms can be treated according to the invention.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Electroplating Methods And Accessories (AREA)
  • Electroplating And Plating Baths Therefor (AREA)
  • Electrochemical Coating By Surface Reaction (AREA)
  • Chemical Treatment Of Metals (AREA)
  • Electrolytic Production Of Metals (AREA)

Claims (8)

1. Verwendung einer Lösung von Pyrophosphat zur elektrolytischen Bildung eines im wesentlichen aus Zinkpyrophosphat bestehenden Schutzüberzuges auf einer Zinkoberfläche, dadurch gekennzeichnet, daß man die Lösung von Pyrophosphat aus einer Lösung eines oder mehrerer löslicher Phosphate oder deren korrespondierenden Säuren herstellt, ausgewählt aus der Gruppe enthaltend die Pyrophosphate oder deren korrespondierende Säuren, die Phosphationen oder deren korrespondierende Säuren, wobei der Kondensationsgrad oberhalb oder benachbart dem der Pyrophosphate ist, und mit einem Phosphationengehalt, ausgedrückt als Phosphor, zwischen 0,6 und 12,5 g/Liter und mit einem pH-Wert zwischen 2,5 und 6, und einer Temperatur zwischen 20 und 70° C.
2. Verwendung der elektrolytischen Lösung nach Anspruch 1, dadurch gekennzeichnet, daß die Phosphate oder ihre korrespondierenden Säuren ausgewählt sind aus der Gruppe umfassend die Pyrophosphate, Metaphosphate, Polyphosphate oder ihre korrespondierenden Säuren.
3. Verwendung der elektrolytischen Lösung nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß sie weiters Zn2+-lonen bis zu einem Gehalt von 8 x 10-3 g-ion/Liter enthält, aber unterhalb des die Löslichkeit begrenzenden korrespondierenden Wertes, bezogen auf die Phosphatkonzentrationen und den pH-Wert der Lösung.
4. Verwendung der elektrolytischen Lösung nach einem der Ansprüche 1-3, dadurch gekennzeichnet, daß man die Gegenstände, deren Oberfläche behandelt werden soll, in Kontakt mit einer Lösung nach einem der Ansprüche 1-3 bringt, und dadurch, daß man sie als Anode einer mit Gleichstrom betriebenen Elektrolyseanordnung plaziert.
5. Verwendung der elektrolytischen Lösung nach Anspruch 4, dadurch gekennzeichnet, daß man die kathodische Reduktion der Zinkionen verhindert, indem man die Kathode vom Elektrolyten durch eine Membran trennt, die ein Hindernis für den Durchgang dieser Ionen darstellt.
6. Verwendung der elektrolytischen Lösung nach einem der Ansprüche 1-3, dadurch gekennzeichnet, daß man die Gegenstände, deren Oberfläche behandelt werden soll, in Kontakt mit einer Lösung gemäß einem der Ansprüche 1-3 bringt, und dadurch, daß man die Elektrolyse mit Wechselstrom durchführt.
7. Verwendung einer elektrolytischen Lösung nach einem der Ansprüche 1-3, dadurch gekennzeichnet, daß man die Gegenstände, deren Oberfläche behandelt werden soll, in Kontakt mit einer Lösung gemäß einem der Ansprüche 1-3 bringt, und dadurch, daß man die Elektrolyse bis zur Bildung eines isolierenden Überzuges auf der behandelten Zinkoberfläche durchführt.
8. Verwendung der elektrolytischen Lösung nach einem der Ansprüche 1-3, dadurch gekennzeichnet, daß man die Gegenstände, deren Oberfläche behandelt werden soll, in Kontakt mit einer Lösung gemäß einem der Ansprüche 1-3 bringt, und dadurch, daß die Elektrolyse unter Anwendung einer Spannung durchgeführt wird, die für eine oberflächliche Oxydation des Zinks des behandelten Gegenstandes ausreicht, aber geringer ist als diejenige, die die Elektrolyse des Wassers der Lösung bewirkt.
EP79400312A 1978-05-19 1979-05-17 Verwendung einer Lösung zur elektrolytischen Bildung von Schutzüberzügen aus Pyrophosphat auf Zinkoberflächen Expired EP0006046B1 (de)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AT79400312T ATE6078T1 (de) 1978-05-19 1979-05-17 Verwendung einer loesung zur elektrolytischen bildung von schutzueberzuegen aus pyrophosphat auf zinkoberflaechen.

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR7814950A FR2426097A1 (fr) 1978-05-19 1978-05-19 Procede et composition pour la formation de revetements protecteurs sur des surfaces de zinc, et objets ainsi proteges
FR7814950 1978-05-19

Publications (2)

Publication Number Publication Date
EP0006046A1 EP0006046A1 (de) 1979-12-12
EP0006046B1 true EP0006046B1 (de) 1984-02-01

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EP79400312A Expired EP0006046B1 (de) 1978-05-19 1979-05-17 Verwendung einer Lösung zur elektrolytischen Bildung von Schutzüberzügen aus Pyrophosphat auf Zinkoberflächen

Country Status (6)

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US (1) US4243496A (de)
EP (1) EP0006046B1 (de)
JP (1) JPS552793A (de)
AT (1) ATE6078T1 (de)
DE (1) DE2966600D1 (de)
FR (1) FR2426097A1 (de)

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
IL69507A (en) * 1983-08-16 1986-11-30 Chromagen Metal Works Selective surfaces for collectors of solar and other radiation
JPS6148597A (ja) * 1984-08-14 1986-03-10 Nippon Paint Co Ltd リン酸亜鉛化成処理法
US4663000A (en) * 1985-07-25 1987-05-05 Kollmorgan Technologies, Corp. Process for electro-deposition of a ductile strongly adhesive zinc coating for metals

Family Cites Families (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2132438A (en) * 1933-12-11 1938-10-11 American Chem Paint Co Method of coating metal
US2125387A (en) * 1936-10-02 1938-08-02 Pittsburgh Plate Glass Co Protective coating
GB876127A (en) * 1960-04-04 1961-08-30 Cons Mining & Smelting Co Anodic treatment of zinc and zinc-base alloys
GB1090743A (en) * 1965-02-10 1967-11-15 Council Scient Ind Res Anodic phosphating
JPS536945B2 (de) * 1973-02-27 1978-03-13
US3898139A (en) * 1973-05-11 1975-08-05 Nippon Steel Corp Process for surface treatment of zinc-plated steel plates

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
Die Phosphatierung von Metallen (Werner Rausch) p 78, 90 *
The condensed Chemical Dictionary, p. 799-800 *

Also Published As

Publication number Publication date
ATE6078T1 (de) 1984-02-15
US4243496A (en) 1981-01-06
FR2426097B1 (de) 1980-11-07
DE2966600D1 (en) 1984-03-08
FR2426097A1 (fr) 1979-12-14
JPS552793A (en) 1980-01-10
JPS624480B2 (de) 1987-01-30
EP0006046A1 (de) 1979-12-12

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