Classifications

• • 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
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0222—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating in a reactive atmosphere, e.g. oxidising or reducing atmosphere
• • 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
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/026—Deposition of sublayers, e.g. adhesion layers or pre-applied alloying elements or corrosion protection

Definitions

• the present invention relates to a method for forming a zinc-aluminum dip coating on a steel strip.
• the dip coating of a continuous steel strip is a technique which has been known and widely applied for many years. Essentially, it consists of running a strip of steel through a bath of molten metal or alloy, then solidifying the coating after adjusting its thickness. Generally, the dip coating operation is preceded by annealing in a reducing atmosphere, in order to rid the surface of the strip of any trace of oxide which may compromise the adhesion of the coating.
• zinc-aluminum alloys are commonly used, typically containing, by weight, in addition to zinc, 55% of aluminum as well as 1.6% of silicon.
• Such alloys combine the high corrosion resistance of aluminum and the cathodic protection provided by zinc.
• the purpose of the addition of silicon is to moderate the reaction between the iron of the steel strip and the aluminum of the coating. In the absence of silicon, this reaction leads in fact to a very significant loss of iron and to a coating completely transformed into Fe-Al which has no adhesion.
• the coatings have a structure, essentially composed of a metastable mixture of two phases which do not solidify simultaneously; it thus appears rich areas made of aluminum and zones rich in zinc, which have distinct physical properties and are the seat of internal constraints.
• a layer of fragile intermetallic particles is formed at the interface between the steel substrate and the zinc-aluminum coating, which degrades adhesion.
• the silicon added to moderate the reaction between iron and aluminum does not remain entirely in solution; on cooling, it precipitates in the form of needles which are the source of stress concentrations and cause the fragility of the coating.
• the present invention provides a process for forming a zinc-aluminum dip coating on a steel strip which does not give rise to the aforementioned drawbacks and which allows, by simple and economically acceptable means in industrial operation, to give this coating excellent adhesion and ductility properties without impairing its corrosion protection power.
• a method for forming a zinc-aluminum dip coating on a steel strip is characterized in that an thin layer of a metal chosen from the nickel, cobalt and chromium, and in that said strip thus coated is passed through a zinc bath having an aluminum content of approximately 55% by weight.
• said steel strip is annealed under a protective atmosphere after the deposition of said thin layer and before the passage of the strip through said zinc-aluminum bath.
• the electrolytic deposition of said thin layer is carried out with a current density of at least 300 A / dm2, and preferably between 350 and 450 A / dm2.
• the thin layer of metal has in particular the effect of slowing down or preventing the exchanges between the steel substrate on the one hand and the zincaluminium coating on the other hand. It thus attenuates the reaction between the iron of the substrate and the aluminum of the coating and contributes to improving the adhesion of the latter. In addition, it avoids the presence of silicon in the zinc-aluminum bath; the zinc-aluminum coating therefore no longer contains embrittling silicon precipitates and its ductility is markedly improved.
• nickel was deposited in a thin layer, corresponding to approximately 3 g Ni / m2 in double face, on steel panels.
• An electrolytic solution containing 260 g / l of NiSO4.6H2O and 35 g / l of H3BO3 was used for this purpose, the pH of which was adjusted to 2.5 by addition of sulfuric acid.
• the current density was 330 A / dm2.
• the panels were then annealed at 720 ° C under an N2 - 5% H2 atmosphere for 3 minutes, then they were coated by immersion in a zinc bath containing 55% by weight of aluminum and no silicon.
• the innermost layers of the reaction zone i.e. on the steel side, are essentially made of Fe-Al, with very low percentages of nickel and zinc. Moving outwards, we then come across layers of aluminum enriched with nickel and zinc, with a very low iron content.
• the actual coating that is to say the outer layer, consists of two main phases, respectively enriched with aluminum and zinc, which still contain traces of iron. In places, the coating also has particles very rich in nickel.
• This structure is very resistant to bending at 180 ° over twice the thickness of the panel, as well as to profiling, without notable cracking or chipping.

Landscapes

• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Electroplating Methods And Accessories (AREA)
• Coating With Molten Metal (AREA)
• Heat Treatment Of Sheet Steel (AREA)

Applications Claiming Priority (2)

Publications (2)

Family

ID=3884763

Family Applications (1)

Country Status (5)

Citations (3)

• 1990
  • 1990-04-17 BE BE9000428A patent/BE1004079A6/fr not_active IP Right Cessation
• 1991
  • 1991-04-12 AT AT91870059T patent/ATE114739T1/de not_active IP Right Cessation
  • 1991-04-12 DE DE69105340T patent/DE69105340D1/de not_active Expired - Lifetime
  • 1991-04-12 EP EP91870059A patent/EP0453432B1/de not_active Expired - Lifetime
  • 1991-04-15 JP JP3176277A patent/JPH0726357A/ja active Pending

Patent Citations (3)

Non-Patent Citations (4)

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