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
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/78—Pretreatment of the material to be coated

Definitions

• the present invention relates to the chemical surface treatment of a metallurgical product made of aluminum alloy, or even of technically pure aluminum. More particularly, the invention relates to such products forming a coating on a substrate which, most often, is a steel sheet.
• these aluminous compounds have on the surface a thin film of oxide of these metals, and particularly of alumina, which forms naturally.
• the presence of these oxides on the surface disrupts the subsequent surface treatment operations to which the sheet may be subjected, and in particular the phosphating operation.
• French patent application 2,567,158 describes a process for preparing a hot-dip galvanized steel product by soaking in a bath containing zinc or a Zn-Al alloy, and intended to be phosphated.
• This process consists in forming on the product an electrolytic deposit of a metal chosen from a determined group of metals, this metal possibly, in particular, being zinc.
• the zinc layer thus deposited on the oxide film formed on the surface of the coating, ensures better adhesion of the phosphate layer deposited later.
• the process of electrolytic deposition in an acid medium has the disadvantage, on the industrial level, of requiring a modification of the whole of the usual phosphating chain, due to the introduction of a treatment. additional involving the use of an electrolytic tank, and its power supply.
• the electrolytic deposition is preceded by an electrolytic pickling bath (electrochemical polishing) necessary to remove the layer of oxides formed on the surface without polluting the zinc deposition bath. It is easily understood that, because of the two electrolytic baths, this process is particularly greedy in electrical energy, which represents a big disadvantage from the economic point of view.
• patent application GB 2 152 955 also relates to a process for treating sheets coated with a zinc-aluminum alloy in order to facilitate a subsequent phosphating operation.
• the sheet is treated in an alkaline solution intended to strip the aluminum existing on the surface.
• the coating of the treated sheets is relatively poor in aluminum (less than 10%).
• the solution used contains ions of at least one of the following metals: nickel, iron, cobalt, manganese. These metals can deposit on the surface of the coating and, although they are likely to facilitate subsequent phosphating, they form with the zinc of the coating, a heterogeneous surface layer. Within this layer can be created electrochemical phenomena (phenomena of batteries between different metals) leading to a subsequent degradation of it.
• the present invention aims to facilitate the phosphating of an aluminous coating, and more particularly of a coating with a high aluminum content, while avoiding the phenomena described above.
• Another object is to improve the bonding and the behavior of the layer of phosphates deposited subsequently during phosphating.
• Yet another object is to provide pretreatment before phosphating at a reduced cost.
• the object of the invention is to produce on the coating intended to be phosphated, a deposit of metal other than aluminum and in particular zinc, having properties for bonding phosphates, this deposit being sufficient to ensure a good adhesion of the phosphate layer deposited later.
• the present invention relates to a process for the surface treatment of an aluminous product intended to be subjected to a phosphating operation, process during which said product is covered with a layer of metal other than the aluminum, and having phosphate bonding properties, and characterized in that to produce this layer of metal, an etching treatment of said product is carried out by means of a strongly alkaline solution containing said metal in solution.
• the alkaline solution has a pH greater than 11, and preferably at least equal to 12.5 /
• the alkaline solution is a caustic soda lye which contains at least five moles of NaOH per liter of water.
• the treatment is carried out at a temperature of approximately 10 to 65 ° C., and preferably at ambient temperature (ie of the order of 15 to 20 ° C.).
• the treatment is carried out for a time sufficient to obtain a deposit of metal in the form of a substantially continuous layer, the latter preferably having a thickness of approximately 1 ⁇ m.
• the invention also relates to the application of the method described above to the phosphating of coated sheets by dipping in a bath of molten metal comprising a high proportion of aluminum.
• a possible first degreasing of the sheet with solvents it is passed, in accordance with said process, into a strongly alkaline degreasing bath containing the metal in solution and maintained at ambient temperature, then it is passed through a cold rinsing bath, followed by a refining bath containing titanium or nickel, then in the phosphating bath.
• a decisive advantage of the method according to the invention is that it consumes no electrical energy, which makes it economically particularly advantageous.
• no significant modification is necessary to adapt a conventional phosphating chain to the process, since only the composition of the alkaline degreasing bath is modified.
• the inventors discovered, during a study on the influence of surface treatments of galvanized coatings on the adhesion of paints, that, while the problems of phosphating of sheets coated with Zn-Al alloys high proportion of Al were well known, there was no need to resort, to solve these problems, expensive methods of electrolytic deposition of zinc.
• the inventors have in fact discovered that, unlike the electrolytic deposition of zinc in an acid bath, the deposition of zinc can be carried out by simple passage of the coated sheet metal to be phosphated in a strongly basic pickling bath containing zinc in solution.
• the results of the tests carried out have shown that the deposition rate and the thickness of the zinc layer thus formed essentially depend on the NaOH concentration of the alkaline bath, the deposition being facilitated when the pH of the bath is high.
• the tests carried out also made it possible to show that the zinc concentration of the bath influences the characteristics of the deposit: an increase in the quantity of zinc in solution promotes the deposit, and makes it possible to compensate, to a certain extent, for the effects of a possible reduction in the pH of the bath.
• the deposited zinc layer may not be absolutely continuous, that is to say that if slight imperfections of this layer appear, they will not disturb the germination of the phosphates, during the phosphating operation, these- these are inserted into any imperfections in the zinc layer. This tolerance should not, of course, be extended, otherwise the anti-corrosion properties of the product may deteriorate rapidly.
• the inventors have tried to understand and explain the surprising effect of the deposition of zinc in solution in an alkaline pickling bath on a Zn-Al substrate.
• the alkaline pickling bath also called degreasing
• the alkaline pickling bath insofar as its pH is sufficiently high, chemically attacks the substantially continuous surface layer of alumina present on the surface of the Al-Zn alloy, thus as the immediately underlying layer of zinc and aluminum oxides, until the substrate surface is completely deoxidized.
• Metallic zinc and aluminum then appear, which form with pickled zinc and aluminum in solution in ionic form, two red-ox couples. Due to the differences in respective potentials of these two pairs, the zinc in solution has a strong tendency to redeposit on the substrate and in particular on the aluminum visible on its surface, thus quickly forming a continuous layer of zinc.
• the dissolved aluminum remains in solution or can be complexed and deposited in the form of sludge at the bottom of the bath.
• the method according to the present invention uses the pickling of surface aluminum with an alkaline solution notably revealed by the document GB 2 152 955 already cited, it differs greatly in principle. It is important to remember beforehand that, at high concentrations of alkaline agent, the pH of a solution is not representative of said concentration. In addition, it is likely to vary greatly during treatments such as those considered here, this being due in particular to the reactions occurring in the solution during the treatment and also to carbonation, resulting from contact of the solution with the ambient medium and the carbon oxides it contains.
• the inventors have been able to establish that, at the concentrations referred to in document GB 2 152 955 (1 to 60 g of sodium hydroxide per liter), the solution used for pickling the coating would see its pH drop rapidly during treatment, greatly reducing this does the pickling efficiency. These concentrations may however prove to be sufficient to surface-pickle the surface alumina and allow the deposition on the remaining alumina of metals, such as Ni, Cr, Mn, contained in the solution in ionic form.
• a primary advantage of this process is to provide a surface layer containing only zinc, which avoids the problems of dissolution caused by the electrochemical phenomena described in the first part of this thesis.
• the method according to the invention makes it possible to dispense with the operations of protecting a front face galvanizing, or eliminating by scraping the layer deposited during the galvanizing, operations which were necessary to produce the sheets. galvanized on one side, which are used in this field of activity.
• a phosphating process including the surface treatment method comprises the following steps: - the sheet goes into a degreasing bath of the solvent type to remove most of the surface oils or greases; - then, after a rinsing bath, the sheet passes into an alkaline pickling bath (commonly called "degreasing" bath, this bath being strongly basic, with a pH greater than 12, such as, for example, a sodium hydroxide solution 400 g of NaOH per liter of water.
• This bath contains an aqueous solution of zinc introduced in the form of zinc powder or zinc oxide, at the rate of a few grams per liter.
• the process described above is not limitative of the invention, and numerous variants can be applied without departing from the field covered by the invention.
• the invention can be applied to all products or coatings comprising an alloy of zinc and aluminum, from low Al percentages of the order of 5%, up to practically 100%, i.e. pure aluminum. It is obvious, however, that the main interest of the method is revealed for alloys comprising a high proportion of aluminum, greater than about 50%.
• the characteristics of the alkaline bath used can also vary within the limit where it remains strongly alkaline.

Landscapes

• Chemical & Material Sciences (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Coating With Molten Metal (AREA)
• Chemical Treatment Of Metals (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=9349679

Family Applications (1)

Country Status (3)

Families Citing this family (2)

Citations (3)

Family Cites Families (3)

• 1987
  • 1987-03-27 FR FR8704583A patent/FR2612947A1/fr active Pending
• 1988
  • 1988-03-21 EP EP88440022A patent/EP0285535A1/de not_active Withdrawn
  • 1988-03-28 US US07/174,294 patent/US4844748A/en not_active Expired - Fee Related

Patent Citations (3)

Non-Patent Citations (2)

Also Published As

Similar Documents

Legal Events