EP0879901B1 - Protective coating for metal pieces with a good resistance against corrosion in a saline atmosphere and metal pieces with such a protective coating - Google Patents

Protective coating for metal pieces with a good resistance against corrosion in a saline atmosphere and metal pieces with such a protective coating Download PDF

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Publication number
EP0879901B1
EP0879901B1 EP98401213A EP98401213A EP0879901B1 EP 0879901 B1 EP0879901 B1 EP 0879901B1 EP 98401213 A EP98401213 A EP 98401213A EP 98401213 A EP98401213 A EP 98401213A EP 0879901 B1 EP0879901 B1 EP 0879901B1
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EP
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Prior art keywords
zinc
coating
tin
protective coating
alloy
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EP98401213A
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German (de)
French (fr)
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EP0879901A1 (en
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Michel Ruimi
Jean-Paul Guerbert-Jubert
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Safran Aircraft Engines SAS
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SNECMA Moteurs SA
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    • 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
    • C23C28/02—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 only coatings only including layers of metallic material
    • C23C28/021—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 only coatings only including layers of metallic material including at least one metal alloy layer
    • 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
    • C23C28/02—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 only coatings only including layers of metallic material
    • C23C28/023—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 only coatings only including layers of metallic material only coatings of metal elements only
    • C23C28/025—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 only coatings only including layers of metallic material only coatings of metal elements only with at least one zinc-based layer
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D3/00—Electroplating: Baths therefor
    • C25D3/02—Electroplating: Baths therefor from solutions
    • C25D3/56—Electroplating: Baths therefor from solutions of alloys
    • C25D3/565—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of zinc
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D3/00—Electroplating: Baths therefor
    • C25D3/02—Electroplating: Baths therefor from solutions
    • C25D3/56—Electroplating: Baths therefor from solutions of alloys
    • C25D3/60—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of tin
    • 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
    • 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.]
    • Y10T428/12535—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.] with additional, spatially distinct nonmetal component
    • Y10T428/12611—Oxide-containing 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.]
    • Y10T428/12708—Sn-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
    • Y10T428/12785—Group IIB metal-base component
    • Y10T428/12792—Zn-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
    • Y10T428/12785—Group IIB metal-base component
    • Y10T428/12792—Zn-base component
    • Y10T428/12799—Next to Fe-base component [e.g., galvanized]

Definitions

  • the invention relates to a protective coating for parts. metal having good corrosion resistance in saline atmosphere and a metal part comprising such a coating. It applies in particular to protection aeronautical steel parts such as aircraft engines that require a high degree of safety and protection of aluminum alloy parts beforehand coated with a chemical zincate undercoat.
  • cadmium deposited by the electrolytic as a protective anodic coating. This coating can be used hot up to temperatures of around 235 ° C.
  • cadmium presents a risk of corrosion intergranular with cracks on contact with titanium and its alloys, and catalytic actions unfavorable in contact with synthetic oils and fuels.
  • Document JP 05033 188A describes a process for preparing a coating multilayer comprising on a steel substrate, a layer of tin, a layer of zinc and nickel alloy and a layer of tin and zinc.
  • Sheet steel coated is used to produce boxes.
  • the coating requiring three successive layers is expensive to make and increases the weight of coated parts significantly, which is detrimental in the case of application to aeronautical parts where one always seeks to obtain gains in mass.
  • the invention aims to develop a protective coating a metal part not containing cadmium, constituting an effective anodic protection against corrosion in a saline atmosphere and in alternating cycling, and having a low sensitivity to galvanic corrosion.
  • the invention relates to a binary coating of an alloy of tin and zinc comprising 8 to 35% by weight zinc.
  • the metal substrate is provided with a protective coating against corrosion resistance in saline atmosphere is characterized in that it comprises at minus a layer of a tin and zinc alloy containing between 8 and 35% by weight of zinc, an underlay of an alloy zinc and nickel containing between 10 and 16% by weight of nickel, the underlay being arranged between the part metal and the tin and zinc alloy layer, and the thickness proportion of the two alloys of the coating being two-thirds for the zinc-nickel alloy and one third for the tin and zinc alloy.
  • the tin and zinc alloy comprises between 12 and 25% by weight of zinc.
  • the coating further comprises a film chromate outer.
  • the layer of tin and zinc alloy and / or the zinc and nickel alloy underlay are deposited by electrolysis.
  • the invention also relates to a metal part comprising a protective coating against atmospheric corrosion saline.
  • coating To constitute an effective coating for the protection of metal parts against salt corrosion, coating must behave anodically with respect to the substrate metallic, i.e. it must have a behavior sacrificial in relation to the substrate.
  • the galvanic coupling between the coating and the substrate must be low to reduce the risk of sensitivity of the coating to galvanic corrosion and increase its duration of life.
  • a binary electrolytic coating consisting of a alloy of tin and zinc comprising between 8 and 35% in weight of zinc and preferably between 12 and 25% by weight zinc, exhibits salt corrosion behavior satisfactory even under severe cycling conditions alternating, and weak galvanic coupling with a substrate metallic.
  • the electrolytic coating of tin and zinc is used in a sandwich type coating. In that case it is deposited on a sub-layer of a zinc alloy of nickel comprising 10 to 16% by weight of nickel.
  • the alloy of zinc and nickel is electrolytically deposited on the metallic substrate.
  • the temperature range of the electrolysis bath is between 63 and 67 ° C; the range of cathodic current densities applied during electrolysis is between 1 and 3 A / dm 2 ; the range of applied voltages is between 2 and 5V.
  • the anodes used are preferably tin-zinc alloy anodes, comprising for example 75% by weight of tin and 25% by weight of zinc.
  • the composition of the electrolytic bath can be different; in particular, for health and safety reasons, the cyanide complexing agent can be replaced by an alkaline nitrogen complexing agent which is not cyanide comprising for example one or more amine functions and / or one or more amide functions.
  • the electrolytic coating of zinc and nickel (10 to 16% by weight of nickel) is carried out using an electrolytic bath known under the commercial name Slotoloy ZN50.
  • the additive with the trade name ZN51 is a complexing agent comprising amines; the trade name additives ZN52 and ZN53 are grain refiners.
  • Zinc is introduced in the form of zinc oxide ZnO; nickel is introduced in the form of NiSO 4 , 6H 2 O.
  • the anodes used are nickel anodes.
  • the temperature range of the electrolysis bath is between 63 and 67 ° C; the range of cathodic current densities applied during electrolysis is between 1 and 3 A / dm 2 ; the range of applied voltages is between 3 and 6 V.
  • Figure 1 represents a comparative table of the values of initial dissolution potentials and measured after one time t equal to 5 minutes, and the coupling value galvanic of different types of coatings made on steel substrates.
  • Measuring electrochemical dissolution potentials assesses the risks of sensitivity to galvanic corrosion that may exist between a coating and the substrate on which it is deposited.
  • galvanic coupling values greater than 250mV in the medium wet are likely to cause corrosion galvanic which results in a preferential attack of the coating if it has a sacrificial behavior by compared to the substrate on which it is deposited.
  • the measurement of electrochemical potentials for dissolving materials or coatings shown in the table in Figure 1, is performed using an electronic multi-meter in using a saturated calomel reference electrode (noted ECS).
  • the electrolyte used is a solution comprising 30 g / l of sodium chloride, 1.284 g / l of sodium phosphate and 0.187 g / l of boric acid.
  • the pH of the solution electrolytic is maintained at 8 ⁇ 0.1 and the measurements are performed at room temperature.
  • the coatings considered are a cadmium coating deposited on a XES steel substrate without chromic finish and followed by a chromic finish; a coating of a alloy of tin and zinc comprising 8 to 35% by weight of zinc deposited on an unfinished XES steel substrate chromic and followed by a chromic finish; a coating of an alloy of zinc and nickel comprising 10 to 16% in nickel weight followed by a chrome finish.
  • the coating of cadmium is used as a reference.
  • the values of measured electrochemical dissolution potentials show that all coatings have a sacrificial behavior, the steel substrate provided with one of the coatings considered being more anodic than steel alone.
  • the low galvanic coupling value between XES steel and a coating of a tin and zinc alloy containing 8 to 35% by weight of zinc suggests a long service life of this type of coating.
  • FIG. 1 also shows that the deposition of a film of chromate, called chromic finish, on the coating of protection is particularly advantageous because it allows significantly reduce the value of the galvanic coupling between the steel substrate and the coating and thereby increase considerably the life of the coating.
  • chromic finish a film of chromate
  • Coating resistance tests in the presence of fog saline and alternating cycling were performed for all coatings considered in Figure 1 as well as for a additional coating, called sandwich coating, comprising a first layer consisting of a coating electrolytic alloy of zinc and nickel comprising 10 to 16% by weight of nickel and a second layer made of an electrolytic coating of an alloy tin and zinc comprising 8 to 35% by weight of zinc.
  • the thicknesses of all the coverings considered are between 10 and 15 ⁇ m.
  • cadmium coatings have excellent behavior in the presence of fog saline. After 336 hours of exposure, no corrosion of the steel substrate is not observed, which confirms the protective effect of this coating with respect to steel.
  • the electrolytic coating of an alloy of zinc and nickel comprising 10 to 16% by weight of nickel and the electrolytic coatings of a tin and zinc alloy containing 8 to 35% by weight of zinc have behaviors similar in the presence of salt spray. From 216 hours of exposure to salt spray, fine drips of white corrosion appear, but these do not evolve not over time. After 336 hours of exposure to salt spray, no attack on the steel substrate is observed.
  • the Zn - Ni coatings (10 to 16% by weight Ni), Sn - Zn (8 to 35% by weight Zn) and sandwich 2/3 Zn - Ni (10 to 16% by weight Ni) + 1/3 Sn - Zn (8 to 35% by weight Zn) have very close behavior in salt corrosion up to 336 hours of exposure to salt spray.
  • Results obtained after exposure to salt spray are frequently different from the corrosion observed during exposure to the Earth's atmosphere. This is due to cyclical variations in climatic conditions and particular humidity, temperature, exposure to sunlight.
  • Each cycle consists of exposing a given material for 15 hours in salt spray at a temperature of 35 ° C, then at place this material at a predetermined high temperature for 6 hours.
  • High temperature is chosen lower than the melting point of the different elements of the coating.
  • the temperature high is chosen equal to 235 ° C; for coating containing a tin and zinc alloy and the coating sandwich, the high temperature is chosen equal to 150 ° C in because of the low melting point of tin.
  • the behavior, in alternating cycling, of the coating electrolytic alloy of tin and zinc comprising 8 to 35% by weight of zinc is similar to the behavior of electrolytic coating of zinc and nickel alloy.
  • 15 to 20% of the surface of the steel substrate is attacked by white corrosion.
  • the sandwich coating has the best behavior in saline corrosion and in alternating cycling compared to the zinc-nickel and tin-zinc coatings considered and constitutes an effective protection against corrosion of a steel part when the latter is used in severe conditions.
  • Zinc-nickel and tin-zinc coatings can also be used as protective coatings on steel parts, in cases where the conditions of use of the parts are less severe.
  • Zinc-nickel and tin-zinc coatings can also be applied to metal parts other than steel, such as, for example, aluminum alloy parts previously coated with an underlayer of chemical zincate.
  • the invention is not limited to the examples of embodiments precisely described; in particular the choice of an electrolytic route for depositing the alloys of the coating is advantageous in terms of the cost of producing the deposit and makes it possible to control the concentration of the elements of the alloy in a simple manner by choosing a density value of cathodic current applied during electrolysis and by the choice of an applied voltage value, but the deposition of the alloys considered can also be carried out by any other known method.

Description

L'invention concerne un revêtement de protection de pièces métalliques ayant une bonne résistance à la corrosion en atmosphère saline et une pièce métallique comportant un tel revêtement. Elle s'applique en particulier à la protection des pièces aéronautiques en acier telles que les pièces de moteurs d'avion qui requièrent un haut degré de sécurité et à la protection des pièces en alliage d'aluminium préalablement revêtues d'une sous-couche de zincate chimique.The invention relates to a protective coating for parts. metal having good corrosion resistance in saline atmosphere and a metal part comprising such a coating. It applies in particular to protection aeronautical steel parts such as aircraft engines that require a high degree of safety and protection of aluminum alloy parts beforehand coated with a chemical zincate undercoat.

Pour protéger les pièces en acier contre la corrosion saline, il est connu d'utiliser le cadmium déposé par voie électrolytique en tant que revêtement anodique protecteur. Ce revêtement est utilisable à chaud jusqu'à des températures de l'ordre de 235°C.To protect steel parts against salt corrosion, it is known to use cadmium deposited by the electrolytic as a protective anodic coating. This coating can be used hot up to temperatures of around 235 ° C.

Bien que le cadmium permette d'obtenir une bonne protection des pièces métalliques contre la corrosion, il présente un degré de toxicité élevé et des incompatibilités intrinsèques d'emploi avec les matériels actuels.Although cadmium provides good protection metal parts against corrosion, it has a high degree of toxicity and intrinsic incompatibilities of use with current equipment.

En particulier, le cadmium présente un risque de corrosion intergranulaire avec formation de criques lors du contact avec le titane et ses alliages, et des actions catalytiques défavorables au contact des huiles synthétiques et des carburants.In particular, cadmium presents a risk of corrosion intergranular with cracks on contact with titanium and its alloys, and catalytic actions unfavorable in contact with synthetic oils and fuels.

Différents types de revêtements ont été proposés pour remplacer le cadmium. Notamment les revêtements zinc-nickel comportant 6 à 8% de nickel et réalisés en milieu alcalin non cyanuré se sont avérés intéressants car ils offrent une bonne résistance à la corrosion saline, mais leur tenue en cyclage alterné est médiocre.Different types of coatings have been proposed for replace cadmium. In particular zinc-nickel coatings containing 6 to 8% of nickel and produced in a non-alkaline medium cyanide proved to be interesting because they offer good resistance to salt corrosion, but their resistance to cycling alternating is poor.

Il est également connu que dans le domaine de la connectique, les revêtements d'étain-nickel comportant 35 % de nickel et appliqués sur une sous-couche de cuivre offre de bonnes propriétés de résistance en corrosion. Cependant ce type de revêtement n'a pas un comportement sacrificiel par rapport aux substrats en acier ce qui limite sa durée de vie dans des conditions sévères telles que le cyclage alterné.It is also known that in the field of connectors, tin-nickel coatings containing 35% nickel and applied to a copper undercoat provides good corrosion resistance properties. However this type of coating does not have a sacrificial behavior compared steel substrates which limits its service life in severe conditions such as alternating cycling.

Le document JP 05033 188A décrit un procédé de préparation d'un revêtement multicouche comprenant sur un substrat en acier, une couche d'étain, une couche d'alliage de zinc et de nickel et une couche d'étain et de zinc. La tôle d'acier revêtue est utilisée pour produire des boítes. Le revêtement nécessitant trois couches successives est cher à réaliser et augmente le poids des pièces revêtues de façon importante, ce qui est préjudiciable dans le cas d'une application aux pièces aéronautiques où l'on recherche toujours à obtenir des gains en masse.Document JP 05033 188A describes a process for preparing a coating multilayer comprising on a steel substrate, a layer of tin, a layer of zinc and nickel alloy and a layer of tin and zinc. Sheet steel coated is used to produce boxes. The coating requiring three successive layers is expensive to make and increases the weight of coated parts significantly, which is detrimental in the case of application to aeronautical parts where one always seeks to obtain gains in mass.

L'invention a pour but d'élaborer un revêtement de protection d'une pièce métallique ne comportant pas de cadmium, constituant une protection anodique efficace contre la corrosion en atmosphère saline et en cyclage alterné, et présentant une faible sensibilité à la corrosion galvanique. Pour cela, l'invention a pour objet un revêtement binaire d'un alliage d'étain et de zinc comportant 8 à 35 % en poids de zinc.The invention aims to develop a protective coating a metal part not containing cadmium, constituting an effective anodic protection against corrosion in a saline atmosphere and in alternating cycling, and having a low sensitivity to galvanic corrosion. For this, the invention relates to a binary coating of an alloy of tin and zinc comprising 8 to 35% by weight zinc.

Selon l'invention, le substrat métallique est muni d'un revêtement de protection contre résistance à la corrosion en atmosphère saline est caractérisé en ce qu'il comporte au moins une couche d'un alliage d'étain et de zinc contenant entre 8 et 35 % en poids de zinc, une sous-couche d'un alliage de zinc et de nickel contenant entre 10 et 16 % en poids de nickel, la sous-couche étant disposée entre la pièce métallique et la couche d'alliage d'étain et de zinc, et la proportion en épaisseur des deux alliages du revêtement étant de deux tiers pour l'alliage de zinc et de nickel et de un tiers pour l'alliage d'étain et de zinc.According to the invention, the metal substrate is provided with a protective coating against corrosion resistance in saline atmosphere is characterized in that it comprises at minus a layer of a tin and zinc alloy containing between 8 and 35% by weight of zinc, an underlay of an alloy zinc and nickel containing between 10 and 16% by weight of nickel, the underlay being arranged between the part metal and the tin and zinc alloy layer, and the thickness proportion of the two alloys of the coating being two-thirds for the zinc-nickel alloy and one third for the tin and zinc alloy.

De préférence, l'alliage d'étain et de zinc comporte entre 12 et 25 % en poids de zinc.Preferably, the tin and zinc alloy comprises between 12 and 25% by weight of zinc.

Préférentiellement, le revêtement comporte en outre un film externe de chromate. Avantageusement, la couche d'alliage d'étain et de zinc et/ou la sous-couche d'alliage de zinc et de nickel sont déposées par électrolyse.Preferably, the coating further comprises a film chromate outer. Advantageously, the layer of tin and zinc alloy and / or the zinc and nickel alloy underlay are deposited by electrolysis.

L'invention concerne aussi une pièce métallique comportant un revêtement de protection contre la corrosion en atmosphère saline. The invention also relates to a metal part comprising a protective coating against atmospheric corrosion saline.

D'autres particularités ou avantages de l'invention apparaítront clairement dans la suite de la description donnée à titre d'exemple non limitatif et faite en regard des figures annexées qui représentent :

  • la figure 1, un tableau comparatif indiquant les valeurs des potentiels de dissolution et les valeurs de couplage galvanique de différents types de revêtements réalisés sur des substrats en acier ;
  • la figure 2, un tableau rappelant la composition de deux types d'aciers considérés ;
  • la figure 3, un tableau comparatif résumant les résultats obtenus pour les différents types de revêtements considérés lors d'essais de tenue en présence de brouillard salin et en cyclage alterné.
Other features or advantages of the invention will appear clearly in the following description given by way of non-limiting example and made with reference to the appended figures which represent:
  • FIG. 1, a comparative table indicating the values of the dissolution potentials and the values of galvanic coupling of different types of coatings produced on steel substrates;
  • Figure 2, a table recalling the composition of two types of steel considered;
  • FIG. 3, a comparative table summarizing the results obtained for the different types of coatings considered during resistance tests in the presence of salt spray and in alternating cycling.

Pour constituer un revêtement efficace pour la protection des pièces métalliques contre la corrosion saline, le revêtement doit se comporter anodiquement par rapport au substrat métallique, c'est à dire qu'il doit avoir un comportement sacrificiel par rapport au substrat. Par ailleurs, le couplage galvanique entre le revêtement et le substrat doit être faible pour diminuer les risques de sensibilité du revêtement à la corrosion galvanique et augmenter sa durée de vie.To constitute an effective coating for the protection of metal parts against salt corrosion, coating must behave anodically with respect to the substrate metallic, i.e. it must have a behavior sacrificial in relation to the substrate. In addition, the galvanic coupling between the coating and the substrate must be low to reduce the risk of sensitivity of the coating to galvanic corrosion and increase its duration of life.

Après avoir effectué une étude comparative des propriétés de différents types de revêtements binaires par rapport à un revêtement électrolytique de cadmium, nous avons déterminé qu'un revêtement électrolytique binaire constitué d'un alliage d'étain et de zinc comportant entre 8 et 35 % en poids de zinc et préférentiellement entre 12 et 25 % en poids de zinc, présente un comportement en corrosion saline satisfaisant même dans des conditions sévères de cyclage alterné, et un faible couplage galvanique avec un substrat métallique. After carrying out a comparative study of the properties of different types of binary coatings compared to a electroplated cadmium coating we have determined that a binary electrolytic coating consisting of a alloy of tin and zinc comprising between 8 and 35% in weight of zinc and preferably between 12 and 25% by weight zinc, exhibits salt corrosion behavior satisfactory even under severe cycling conditions alternating, and weak galvanic coupling with a substrate metallic.

Le revêtement électrolytique d'étain et de zinc est utilisé dans un revêtement de type sandwich. Dans ce cas il est déposé sur une sous-couche d'un alliage de zinc de nickel comportant 10 à 16 % en poids de nickel. L'alliage de zinc et de nickel est déposé par voie électrolytique sur le substrat métallique.The electrolytic coating of tin and zinc is used in a sandwich type coating. In that case it is deposited on a sub-layer of a zinc alloy of nickel comprising 10 to 16% by weight of nickel. The alloy of zinc and nickel is electrolytically deposited on the metallic substrate.

La proportion en épaisseur des deux alliages du revêtement sandwich est la suivante : 2/3 Zn-Ni + 1/3 Sn-Zn.
Le revêtement sandwich permet d'obtenir une double protection des pièces métalliques contre la corrosion saline, il permet d'augmenter la résistance à la corrosion en diminuant le couplage galvanique du revêtement par rapport au substrat métallique. L'alliage de zinc et de nickel est utilisé de préférence en sous-couche pour améliorer l'adhérence du revêtement sur la pièce métallique.
Le revêtement d'étain et de zinc ou de type sandwich peut comporter en outre un film externe de chromate permettant d'améliorer encore la tenue du revêtement en corrosion saline.
Les dépôts électrolytiques de l'alliage d'étain et de zinc et/ou de l'alliage de zinc et de nickel sont réalisés en utilisant des bains électrolytiques ne comportant aucun agent d'addition de type brillanteur organique ou métallique, car ces agents d'addition sont source de fragilisation par l'hydrogène.
Le revêtement électrolytique d'étain et de zinc est déposé en utilisant un bain dont un exemple de composition est donné ci-dessous :

  • stannate de sodium : de 30 à 75 g/l et de préférence 67 g/l
  • cyanure de zinc : de 2 à 10 g/l et de préférence 5,4 g/l
  • soude : de 2 à 10 g/l et de préférence 5 g/l
  • cyanure de sodium : de 15 à 45 g/l et de préférence 28 g/l
The thickness proportion of the two alloys of the sandwich coating is as follows: 2/3 Zn-Ni + 1/3 Sn-Zn.
The sandwich coating makes it possible to obtain double protection of the metal parts against salt corrosion, it makes it possible to increase the resistance to corrosion by reducing the galvanic coupling of the coating with respect to the metal substrate. The zinc and nickel alloy is preferably used as an undercoat to improve the adhesion of the coating to the metal part.
The coating of tin and zinc or of the sandwich type may also comprise an external chromate film making it possible to further improve the resistance of the coating to salt corrosion.
The electrolytic deposits of the tin and zinc alloy and / or of the zinc and nickel alloy are carried out using electrolytic baths containing no addition agent of organic or metallic brightening type, because these agents 'addition are source of embrittlement by hydrogen.
The electrolytic coating of tin and zinc is deposited using a bath, an example of composition of which is given below:
  • sodium stannate: from 30 to 75 g / l and preferably 67 g / l
  • zinc cyanide: from 2 to 10 g / l and preferably 5.4 g / l
  • soda: from 2 to 10 g / l and preferably 5 g / l
  • sodium cyanide: from 15 to 45 g / l and preferably 28 g / l

La plage des températures du bain d'électrolyse est comprise entre 63 et 67°C ; la plage des densités de courant cathodiques appliquée pendant l'électrolyse est comprise entre 1 et 3 A/dm2 ; la plage des tensions appliquées est comprise entre 2 et 5V.
Les anodes utilisées sont de préférence des anodes d'étain-zinc alliés, comportant par exemple 75 % en poids d'étain et 25 % en poids de zinc.The temperature range of the electrolysis bath is between 63 and 67 ° C; the range of cathodic current densities applied during electrolysis is between 1 and 3 A / dm 2 ; the range of applied voltages is between 2 and 5V.
The anodes used are preferably tin-zinc alloy anodes, comprising for example 75% by weight of tin and 25% by weight of zinc.

Il est également possible d'utiliser deux anodes d'étain et deux anodes de zinc en alternance en tenant compte du fait que les anodes de zinc se dissolvent plus vite que les anodes d'étain, ce qui provoque un enrichissement progressif du bain en zinc.
La composition du bain électrolytique peut être différente ; en particulier, pour des raisons d'hygiène et de sécurité, le complexant cyanure peut être remplacé par un complexant alcalin azoté non cyanuré comportant par exemple une ou plusieurs fonctions amines et/ou une ou plusieurs fonctions amides.
Le revêtement électrolytique de zinc et de nickel (10 à 16 % en poids de nickel) est effectué en utilisant un bain électrolytique connu sous le nom commercial Slotoloy ZN50.It is also possible to use two tin anodes and two zinc anodes alternately, taking into account that the zinc anodes dissolve faster than the tin anodes, which causes a progressive enrichment of the bath with zinc. .
The composition of the electrolytic bath can be different; in particular, for health and safety reasons, the cyanide complexing agent can be replaced by an alkaline nitrogen complexing agent which is not cyanide comprising for example one or more amine functions and / or one or more amide functions.
The electrolytic coating of zinc and nickel (10 to 16% by weight of nickel) is carried out using an electrolytic bath known under the commercial name Slotoloy ZN50.

La composition de ce bain est la suivante :

  • soude :   12,5 g/l
  • zinc :   7,5 g/l
  • nickel :   1,3 g/l
  • ZN51 :   40 ml/l
  • ZN52 :   75 ml/l
  • ZN53 :   5 ml/l
The composition of this bath is as follows:
  • soda: 12.5 g / l
  • zinc: 7.5 g / l
  • nickel: 1.3 g / l
  • ZN51: 40 ml / l
  • ZN52: 75 ml / l
  • ZN53: 5 ml / l

L'additif de nom commercial ZN51 est un complexant comportant des amines ; les additifs de noms commerciaux ZN52 et ZN53 sont des affineurs de grain. Le zinc est introduit sous la forme d'oxyde de zinc ZnO ; le nickel est introduit sous la forme de NiSO4, 6H2O. Les anodes utilisées sont des anodes de nickel. La plage des températures du bain d'électrolyse est comprise entre 63 et 67°C ; la plage des densités de courant cathodiques appliquée pendant l'électrolyse est comprise entre 1 et 3 A/dm2 ; la plage des tensions appliquées est comprise entre 3 et 6 V.The additive with the trade name ZN51 is a complexing agent comprising amines; the trade name additives ZN52 and ZN53 are grain refiners. Zinc is introduced in the form of zinc oxide ZnO; nickel is introduced in the form of NiSO 4 , 6H 2 O. The anodes used are nickel anodes. The temperature range of the electrolysis bath is between 63 and 67 ° C; the range of cathodic current densities applied during electrolysis is between 1 and 3 A / dm 2 ; the range of applied voltages is between 3 and 6 V.

La figure 1 représente un tableau comparatif des valeurs des potentiels de dissolution initial et mesuré au bout d'un temps t égal à 5 minutes, et de la valeur de couplage galvanique de différents types de revêtements réalisés sur des substrats en acier.Figure 1 represents a comparative table of the values of initial dissolution potentials and measured after one time t equal to 5 minutes, and the coupling value galvanic of different types of coatings made on steel substrates.

La mesure des potentiels électrochimiques de dissolution (notés pdd) permet d'évaluer les risques de sensibilité à la corrosion galvanique qui peuvent exister entre un revêtement et le substrat sur lequel il est déposé. En particulier, les valeurs de couplage galvanique supérieures à 250mV en milieu humide sont susceptibles de provoquer une corrosion galvanique qui se traduit par une attaque préférentielle du revêtement si celui-ci a un comportement sacrificiel par rapport au substrat sur lequel il est déposé. La mesure des potentiels électrochimiques de dissolution des matériaux ou des revêtements indiqués dans le tableau de la figure 1, est effectué à l'aide d'un multimesureur électronique en utilisant une électrode de référence au calomel saturé (notée ECS).Measuring electrochemical dissolution potentials (noted pdd) assesses the risks of sensitivity to galvanic corrosion that may exist between a coating and the substrate on which it is deposited. In particular, galvanic coupling values greater than 250mV in the medium wet are likely to cause corrosion galvanic which results in a preferential attack of the coating if it has a sacrificial behavior by compared to the substrate on which it is deposited. The measurement of electrochemical potentials for dissolving materials or coatings shown in the table in Figure 1, is performed using an electronic multi-meter in using a saturated calomel reference electrode (noted ECS).

L'électrolyte employé est une solution comportant 30 g/l de chlorure de sodium, 1,284 g/l de phosphate dissodique et 0,187 g/l d'acide borique. Le pH de la solution électrolytique est maintenu à 8 ± 0,1 et les mesures sont réalisées à température ambiante.The electrolyte used is a solution comprising 30 g / l of sodium chloride, 1.284 g / l of sodium phosphate and 0.187 g / l of boric acid. The pH of the solution electrolytic is maintained at 8 ± 0.1 and the measurements are performed at room temperature.

Les potentiels électrochimiques de dissolution sont mesurés au temps t = 0 (mesure instantanée) et au bout de 5 minutes après stabilisation de la solution électrolytique pour deux types d'acier différents connus respectivement sous les noms commerciaux d'acier XES et d'acier 15CDV6, et pour différents types de revêtement déposés sur ces aciers. Electrochemical dissolution potentials are measured at time t = 0 (instant measurement) and after 5 minutes after stabilization of the electrolytic solution for two different types of steel known respectively as XES steel and 15CDV6 steel, and for different types of coating deposited on these steels.

La composition dès deux types d'acier considérés est rappelée dans le tableau représenté à la figure 2.The composition from two types of steel considered is recalled in the table shown in Figure 2.

Les revêtements considérés sont un revêtement de cadmium déposé sur un substrat en acier XES sans finition chromique et suivi d'une finition chromique ; un revêtement d'un alliage d'étain et de zinc comportant 8 à 35 % en poids de zinc déposé sur un substrat en acier XES sans finition chromique et suivi d'une finition chromique ; un revêtement d'un alliage de zinc et de nickel comportant 10 à 16 % en poids de nickel suivi d'une finition chromique. Le revêtement de cadmium est utilisé comme référence. Les valeurs des potentiels électrochimiques de dissolution mesurées montrent que tous les revêtements ont un comportement sacrificiel, le substrat en acier muni de l'un des revêtements considérés étant plus anodique que l'acier seul.The coatings considered are a cadmium coating deposited on a XES steel substrate without chromic finish and followed by a chromic finish; a coating of a alloy of tin and zinc comprising 8 to 35% by weight of zinc deposited on an unfinished XES steel substrate chromic and followed by a chromic finish; a coating of an alloy of zinc and nickel comprising 10 to 16% in nickel weight followed by a chrome finish. The coating of cadmium is used as a reference. The values of measured electrochemical dissolution potentials show that all coatings have a sacrificial behavior, the steel substrate provided with one of the coatings considered being more anodic than steel alone.

Par ailleurs, la faible valeur de couplage galvanique entre l'acier XES et un revêtement d'un alliage d'étain et de zinc comportant 8 à 35 % en poids de zinc, laisse présager une longue durée de vie de ce type de revêtement.Furthermore, the low galvanic coupling value between XES steel and a coating of a tin and zinc alloy containing 8 to 35% by weight of zinc, suggests a long service life of this type of coating.

La figure 1 montre également que le dépôt d'un film de chromate, appelé finition chromique, sur le revêtement de protection est particulièrement avantageux car il permet de diminuer notablement la valeur du couplage galvanique entre le substrat en acier et le revêtement et d'augmenter ainsi considérablement la durée de vie du revêtement.FIG. 1 also shows that the deposition of a film of chromate, called chromic finish, on the coating of protection is particularly advantageous because it allows significantly reduce the value of the galvanic coupling between the steel substrate and the coating and thereby increase considerably the life of the coating.

Des essais de tenue du revêtement en présence de brouillard salin et en cyclage alterné ont été effectués pour tous les revêtements considérés sur la figure 1 ainsi que pour un revêtement additionnel, appelé revêtement sandwich, comportant une première couche constituée d'un revêtement électrolytique d'un alliage de zinc et de nickel comportant 10 à 16 % en poids de nickel et une deuxième couche constituée d'un revêtement électrolytique d'un alliage d'étain et de zinc comportant 8 à 35 % en poids de zinc. Coating resistance tests in the presence of fog saline and alternating cycling were performed for all coatings considered in Figure 1 as well as for a additional coating, called sandwich coating, comprising a first layer consisting of a coating electrolytic alloy of zinc and nickel comprising 10 to 16% by weight of nickel and a second layer made of an electrolytic coating of an alloy tin and zinc comprising 8 to 35% by weight of zinc.

Les épaisseurs de tous les revêtements considérés sont comprises entre 10 et 15 µm.The thicknesses of all the coverings considered are between 10 and 15 µm.

Les résultats obtenus au cours de ces essais sont résumés dans un tableau comparatif représenté à la figure 3. Les essais de tenue en corrosion saline ont été réalisés conformément à la norme AFNOR NFX4/.002, c'est à dire en exposant les revêtements dans un brouillard contenant du chlorure de sodium à 5 % ; de pH 7 ± 0,1 ; à une température de 35° ± 2°C. La durée de l'exposition est de 336 heures.The results obtained during these tests are summarized in a comparison table shown in Figure 3. The salt corrosion resistance tests have been carried out in accordance with standard AFNOR NFX4 / .002, i.e. in exposing the coatings in a mist containing 5% sodium chloride; pH 7 ± 0.1; at a temperature 35 ° ± 2 ° C. The duration of the exhibition is 336 hours.

Avec ou sans finition chromique, les revêtements de cadmium ont un excellent comportement en présence de brouillard salin. Après 336 heures d'exposition, aucun point de corrosion du substrat en acier n'est observé, ce qui confirme l'effet protecteur de ce revêtement vis à vis de l'acier.With or without a chromic finish, cadmium coatings have excellent behavior in the presence of fog saline. After 336 hours of exposure, no corrosion of the steel substrate is not observed, which confirms the protective effect of this coating with respect to steel.

Le revêtement électrolytique d'un alliage de zinc et de nickel comportant 10 à 16 % en poids de nickel et les revêtements électrolytiques d'un alliage d'étain et de zinc comportant 8 à 35 % en poids de zinc ont des comportements similaires en présence de brouillard salin. A partir de 216 heures d'exposition au brouillard salin, des fines coulures de corrosion blanche apparaissent, mais celles-ci n'évoluent pas au cours du temps. Au bout de 336 heures d'exposition au brouillard salin, aucune attaque du substrat en acier n'est observée.The electrolytic coating of an alloy of zinc and nickel comprising 10 to 16% by weight of nickel and the electrolytic coatings of a tin and zinc alloy containing 8 to 35% by weight of zinc have behaviors similar in the presence of salt spray. From 216 hours of exposure to salt spray, fine drips of white corrosion appear, but these do not evolve not over time. After 336 hours of exposure to salt spray, no attack on the steel substrate is observed.

En ce qui concerne le revêtement sandwich Zn - Ni (10 à 16 % en poids Ni) + Sn - Zn (8 à 35 % en poids Zn), des fines coulures de corrosion blanche sont observées à partir de 192 heures d'exposition au brouillard salin, mais ces défauts sont insignifiants et n'évoluent pas jusqu'au temps d'exposition égal à 336 heures. Aucun point de corrosion du substrat en acier n'est observé.Regarding the Zn - Ni sandwich coating (10 to 16% by weight Ni) + Sn - Zn (8 to 35% by weight Zn), fines white corrosion drips are observed from 192 hours of salt spray exposure but these flaws are insignificant and do not evolve until time exposure equal to 336 hours. No corrosion point of steel substrate is not observed.

Par conséquent, les revêtements Zn - Ni (10 à 16 % en poids Ni), Sn - Zn (8 à 35 % en poids Zn) et sandwich 2/3 Zn - Ni (10 à 16 % en poids Ni) + 1/3 Sn - Zn (8 à 35 % en poids Zn) ont un comportement très proche en corrosion saline jusqu'à 336 heures d'exposition au brouillard salin.Consequently, the Zn - Ni coatings (10 to 16% by weight Ni), Sn - Zn (8 to 35% by weight Zn) and sandwich 2/3 Zn - Ni (10 to 16% by weight Ni) + 1/3 Sn - Zn (8 to 35% by weight Zn) have very close behavior in salt corrosion up to 336 hours of exposure to salt spray.

Les résultats obtenus après exposition au brouillard salin sont fréquemment différents de la corrosion observée lors de l'exposition à l'atmosphère terrestre. Ceci est dû aux variations cycliques des conditions climatiques et en particulier de l'humidité, la température, l'exposition à la lumière du soleil.Results obtained after exposure to salt spray are frequently different from the corrosion observed during exposure to the Earth's atmosphere. This is due to cyclical variations in climatic conditions and particular humidity, temperature, exposure to sunlight.

Des essais en cyclage alterné ont donc été effectués pour évaluer le comportement de tous les revêtements considérés sur la figure 1 ainsi que pour le revêtement sandwich 2/3 Zn - Ni (10 à 16 % en poids Ni) + 1/3 Sn - Zn (8 à 35 % en poids Zn).Alternating cycling tests were therefore carried out to evaluate the behavior of all the coatings considered in Figure 1 as well as for the 2/3 Zn sandwich coating - Ni (10 to 16% by weight Ni) + 1/3 Sn - Zn (8 to 35% by weight Zn).

Chaque cycle consiste à exposer un matériau donné pendant 15 heures en brouillard salin à une température de 35°C, puis à placer ce matériau à une température élevée prédéterminée pendant 6 heures. La température élevée est choisie inférieure à la température de fusion des différents éléments du revêtement.Each cycle consists of exposing a given material for 15 hours in salt spray at a temperature of 35 ° C, then at place this material at a predetermined high temperature for 6 hours. High temperature is chosen lower than the melting point of the different elements of the coating.

Pour les revêtements ne contenant pas d'étain, la température élevée est choisie égale à 235 °C ; pour le revêtement contenant un alliage d'étain et de zinc et le revêtement sandwich, la température élevée est choisie égale à 150°C en raison du bas point de fusion de l'étain.For coatings not containing tin, the temperature high is chosen equal to 235 ° C; for coating containing a tin and zinc alloy and the coating sandwich, the high temperature is chosen equal to 150 ° C in because of the low melting point of tin.

En ce qui concerne le revêtement de cadmium, après huit cycles d'essais, aucune attaque du substrat en acier n'est observée. le comportement de ce revêtement est excellent.As for the cadmium coating, after eight test cycles, no attack on the steel substrate is observed. the behavior of this coating is excellent.

En ce qui concerne le revêtement électrolytique d'un alliage de zinc et de nickel comportant 10 à 16 % en poids de nickel, après quatre cycles d'essais, la corrosion blanche occupe 50 % de la surface du revêtement. Au cinquième cycle d'essais, la corrosion blanche a progressé et s'étend sur la totalité de la surface du revêtement. Des points de corrosion du substrat en acier apparaissent au sixième cycle d'essais.Regarding the electrolytic coating of an alloy zinc and nickel comprising 10 to 16% by weight of nickel, after four test cycles, white corrosion occupies 50 % of the surface of the coating. In the fifth test cycle, white corrosion has progressed and extends over the whole of the coating surface. Corrosion points of the steel substrate appear in the sixth test cycle.

Le comportement, en cyclage alterné, du revêtement électrolytique d'un alliage d'étain et de zinc comportant 8 à 35 % en poids de zinc est similaire au comportement du revêtement électrolytique de l'alliage de zinc et de nickel. Au sixième cycle d'essais, 15 à 20 % de la surface du substrat en acier est attaquée par la corrosion blanche.The behavior, in alternating cycling, of the coating electrolytic alloy of tin and zinc comprising 8 to 35% by weight of zinc is similar to the behavior of electrolytic coating of zinc and nickel alloy. In the sixth test cycle, 15 to 20% of the surface of the steel substrate is attacked by white corrosion.

Le comportement du revêtement sandwich en cyclage alterné est nettement meilleur. Aucune corrosion blanche n'est observée après huit cycles d'essais. Cependant, quelques piqûres de corrosion de dimension voisine de 0,5 mm2 apparaissent en surface à l'issue des huit cycles d'essais.The behavior of the sandwich coating on alternating cycling is clearly better. No white corrosion is observed after eight test cycles. However, a few pits of corrosion with a dimension close to 0.5 mm 2 appear on the surface at the end of the eight test cycles.

Par conséquent, le revêtement sandwich possède le meilleur comportement en corrosion saline et en cyclage alterné par rapport aux revêtements zinc-nickel et étain-zinc considérés et constitue une protection efficace contre la corrosion d'une pièce en acier lorsque celle-ci est utilisée dans des conditions sévères.
Les revêtements zinc-nickel et étain-zinc peuvent également être utilisés en tant que revêtements protecteurs de pièces en acier, dans des cas où les conditions d'utilisation des pièces sont moins sévères.
Les revêtements zinc-nickel et étain-zinc peuvent également être appliqués sur des pièces métalliques autres que l'acier, telles que par exemple, des pièces en alliage d'aluminium préalablement revêtues d'une sous-couche de zincate chimique. L'invention n'est pas limitée aux exemples de réalisations précisément décrits ; en particulier le choix d'une voie électrolytique pour déposer les alliages du revêtement est avantageuse au niveau du coût de réalisation du dépôt et permet de contrôler de façon simple la concentration des éléments de l'alliage par le choix d'une valeur de densité de courant cathodique appliquée pendant l'électrolyse et par le choix d'une valeur de tension appliquée, mais le dépôt des alliages considérés peut également être effectué par toute autre méthode connue.Consequently, the sandwich coating has the best behavior in saline corrosion and in alternating cycling compared to the zinc-nickel and tin-zinc coatings considered and constitutes an effective protection against corrosion of a steel part when the latter is used in severe conditions.
Zinc-nickel and tin-zinc coatings can also be used as protective coatings on steel parts, in cases where the conditions of use of the parts are less severe.
Zinc-nickel and tin-zinc coatings can also be applied to metal parts other than steel, such as, for example, aluminum alloy parts previously coated with an underlayer of chemical zincate. The invention is not limited to the examples of embodiments precisely described; in particular the choice of an electrolytic route for depositing the alloys of the coating is advantageous in terms of the cost of producing the deposit and makes it possible to control the concentration of the elements of the alloy in a simple manner by choosing a density value of cathodic current applied during electrolysis and by the choice of an applied voltage value, but the deposition of the alloys considered can also be carried out by any other known method.

Claims (9)

  1. Metal substrate provided with a coating for protection against corrosion in a saline atmosphere, characterized in that it includes at least one layer of a tin/zinc alloy containing between 8 and 35% by weight of zinc and a sublayer of a zinc/nickel alloy containing between 10 and 16% by weight of nickel, the sublayer lying between the metal component and the layer of tin/zinc alloy and the thickness proportion of the two alloys of the coating being two thirds in the case of the zinc/nickel alloy and one third in the case of the tin/zinc alloy.
  2. Metal substrate provided with a protective coating for metal components according to Claim 1, characterized in that the layer of tin/zinc alloy contains between 12 and 25% by weight of zinc.
  3. Metal substrate provided with a protective coating for metal components according to either one of the preceding claims, characterized in that it furthermore includes an external chromate film.
  4. Metal substrate provided with a protective coating for metal components according to any one of the preceding claims, characterized in that the layer of tin/zinc alloy and/or the sublayer of zinc/nickel alloy are deposited by electrolysis.
  5. Metal substrate provided with a protective coating for metal components according to Claim 4, characterized in that the tin/zinc alloy and/or the zinc/nickel alloy are deposited electrolytically using plating solutions which contain no addition agent of the brightener type, whether organic or metallic.
  6. Metal substrate provided with a protective coating for metal components according to Claim 5, characterized in that the composition of the plating solution used for depositing the tin/zinc alloy is as follows:
    sodium stannate:   67 g/l
    zinc cyanide:   5.4 g/l
    sodium hydroxide:   5 g/l
    sodium cyanide:   28 g/l.
  7. Metal substrate provided with a protective coating for metal components according to Claim 6, characterized in that the cyanide complexant, used in the zinc cyanide and the sodium cyanide, is replaced by a noncyanide nitrogen-containing alkaline complexant.
  8. Metal substrate provided with a protective coating for metal components according to any one of Claims 4 to 6, characterized in that the tin/zinc alloy is deposited electrolytically using tin/zinc alloyed anodes.
  9. Metal component having a coating for protection against corrosion in a saline atmosphere, according to any one of the preceding claims.
EP98401213A 1997-05-22 1998-05-20 Protective coating for metal pieces with a good resistance against corrosion in a saline atmosphere and metal pieces with such a protective coating Expired - Lifetime EP0879901B1 (en)

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FR9706232A FR2763605B1 (en) 1997-05-22 1997-05-22 PROTECTIVE COATING OF METAL PARTS HAVING GOOD CORROSION RESISTANCE IN SALINE ATMOSPHERE, AND METAL PARTS COMPRISING SUCH A PROTECTIVE COATING

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JPH10330964A (en) 1998-12-15
FR2763605A1 (en) 1998-11-27
EP0879901A1 (en) 1998-11-25
CA2238061A1 (en) 1998-11-22
FR2763605B1 (en) 1999-07-02
JP3340386B2 (en) 2002-11-05
DE69804267D1 (en) 2002-04-25
CA2238061C (en) 2005-07-12
DE69804267T2 (en) 2002-11-21
ES2171003T3 (en) 2002-08-16
US5989735A (en) 1999-11-23

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