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
  • C23C22/05—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 using aqueous solutions
  • C23C22/06—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 using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/34—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 using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides
  • C23C22/37—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 using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also hexavalent chromium compounds

Definitions

• the former method forms an oxide film having good corrosion resistance and paint adhesive properties thereon by the anodic oxidation treatment. It is, however, difficult to shorten the treating time at the present level of art. For instance, it requires at least 1 minute to form an oxide film having the minimum thickness of 2 p. in the continuous anodic oxidation treating line, so that when a workpiece is passed through as a strip a problem is presented in meeting todays demand for high speed productivity.
• the latter film forming method through chemical substitution may roughly be classified into one of using a treating solution mainly comprising phosphoric acid and hexavalent chromium and the other of using an acid treating solution mainly comprising hexavalent chromium and fluoride.
• the aluminium surface formed by the method using a treating solution containing mainly phosphoric acid and hexavalent chromium is covered with a thin green film and although it is effective as a ground coat and has improved paint adhesive property, the appearance is not very desirable.
• the aluminium surface obtained by the method using the acid treating solution mainly containing hexavalent chromium and flouride ranges widely from colorless to golden color. However, it is difficult to obtain corrosion resistance and paint adhesiveness expected when a material available in the market which may be able to form a colorless film is employed.
• a material such as disclosed in the specification of US. Pat. No. 2796371 or in the Japanese Patent Publication No. Showa 45-32922 (No. 32922/70) will improve the corrosion resistance and the material thus obtained is effective as a ground coat.
• its surface is covered with lusterous golden film, which is undesirable.
• it generally, requires more than 30 seconds for treating in order to give an excellent film with a solution of this kind.
• the above treating time is shortened, it is unavoidable that the surface will become dull yellow color and show varied patches of color.
• the present invention has been developed to obviate such difficulties in the art and is characterized in that ferricyanide or ferrocyanide has been further added to the aqueous solution comprising CrO trivalent chromium. fluorine ion and aluminium ion.
• the invention contemplates pre-treatment steps of de-oiling, rinsing with hot water and drying, with the water film remaining on the surface and, a post treatment step after a chemical treatment in which an aqueous solution containing'water soluble copolymer of itaconic acid and acrylonitrile is used.
• the object of this invention is to provide a uniform and stable chromate film on aluminium, aluminium alloy or aluminium coated steel.
• Another object of the present invention is to form DETAILED DESCRIPTION
• position of the treating solution Component of CrO 2 to l0g/l
• Trivalent chromium 0.05 to lg/l
• Fluorine ion 0.2 to Sg/l
• Aluminium ion 0.0l to lg/l
• Ferricyanide and/or ferrocyanide 0.01 to 5g/l
• the post-treating is carried out on the workpiece on which the above-mentioned chromate film is formed.
• the workpiece is processed in an aqueous solution containing a water soluble copolymer of itaconic acid and acrylonitrile of 10 to 50g/l.
• the treating solution used in the present invention is an aqueous acid solution which contains hexavalent chromium required for forming a desirable film on the surface of the workpiece.
• the amount thereof is 2 to l0g/l in the form of CrO If it is below 2g/l. sufficient corrosion resistance is not obtainable and if it exceeds l0g/l, an undesirable yellowish film tends to be formed on the surface.
• said treating solution contains trivalent chromium which is effective for forming a colorless film. That is to say, the film formed on the surface of the material does not become yellowish. although the amount of chromium increase on the surface. because of the existence of the trivalent chromium. It is desirable that the amount of the trivalent chromium should be 5 to 20% of the total amount of chromium in the solution, i.e. 0.05 to lg/l as Cr. If it is below 571, the effect is negligible while ifit exceeds 20%, reactivity ofthe solution is lowered.
• fluorine ion in an amount sufficient to dissolve the aluminium surface and form a good, sound and uniform film should be contained.
• the fluorine ion is added to the solution as at least one or more of those selected from the group consisting of HF, H SiF film and more than Sg/l will not form the film at all or cause an undesired dissolution of the surface.
• a ferricyanide such as potassium ferricyanide
• This ferricyanide not only promotes the reaction of forming a film but also, even when the treatment time is shortened considerably, brings a great improvement on the formation of a colorless chromate film showing excellent corrosion resistance.
• This is similar to the effect of adding trivalent chromium in the treating solution and it-is presumably the source of supplying Cr in the proximity of the surface where the treating reaction occurs. Accordingly, the amount of potassium ferricyanide of less than 0.0lg/l will decrease excessively the effect desired and conversely an amount exceeding 5g/l will not show any increase in effectiveness. If more is added, a good film cannot be obtained.
• potassium ferrocyanide in place of such potassium ferricyanide.
• This ferrocyanide is not limited to the salts of potassium, but any water soluble salt may be used.
• the amount to be added is the same as that of above mentioned ferricyanide and the two may be mixed and used together. However, the most important factor lies in the relation of concentration of the ferricyanide and the fluorine ion. For instance, it is desirable that when the solution contains more than 0.5g/l
• ferricyanide there should be a fluorine ion concentration of less than 2.4g/l.
• the forming of colorless and uniform film is stepped up by adding aluminium ion to the above mentioned solution.
• the amount thereof should be ,within the range of 0.01 to lg/l. If the amount is less than the above mentioned range, expected effects are not obtained while that exceeding above range will impair stability of the treating solution.
• a process for pre-treating in accordance with the present invention method is based on confirmation of the facts that the patches are mainly caused by the imperfect pre-treating.
• the treating solution 4 where the treatment is performed on the workpiece.
• These patches do not always occur owing only to the incompleteness of the pre-treating.
• an investigation of the phenomenon revealed that many of these-patches depend upon a thick and uneven water filmon the surface of the workpiece which had been caused'by the pretreating just before it is introduced into the chemical treating solution. That is to say, the difference thickness of the water film still lying on the surface causes a difference in thedissolution speed of the surface aluminium at the first stage of the chemical reaction and it was recognized that the shorter treating time was, the greater the difference in water thickness.
• the present invention aimed at improving the pretreating process conventionally used heretofore which comprises the steps of de-oiling. rinsing with water and chemical treating. That is, the water film still lying on the material surface after the washing with water is completely removed and the surface is dried. As one step of improvement, the materials was dried by the hot air at C and then passed through said treating solution. The above mentioned patches completely disappeared in this step. Further, investigations were made as to the use of rubber drawing rolls, for causing the water film lying on the surface to become extremely thin and uniform instead of said drying step prior to the chemical treating. The result obtained was the same. Then, it was found and confirmed that the simultaneous use of these two steps obtain still better results.
• the present invention then further proposed to perform the post-treating as aforementioned for stabilizing the painting property.
• the posttreating step is to use a neutral aqueous solution containing a water soluble copolymer of itaconic acid and acrylonitrile just after the chemical treating.
• the resin it is preferable to use at least 1% or more of said resin in order to sufficiently improve the paint adhesiveness, since an amount of less than the above will not provide effective results.
• the upper limit thereof is 5%, as an amount exceeding 5% will damage the appearance after treating.
• the treated material may be wetted with this post treating solution by spraying or immersion, the excess solution being removed by rubber drawing rolls and then the material may be dried by the hot air at 80 to 100C. Coating may be done with rolls as the case may be.
• Aluminium coated steel sheet Thickness of coated layer 20 p.
• Aluminium plate (118 ALPS Surface roughness H max 8 p.
• Aluminium plate JIS ALPS /2 H
• test pieces were de-oiled, rinsed with cold water, subjected to spraying for 5 minutes at 45C and under the spraying pressure of 0.6 Kg/cm and dried with hot air of 80C.
• test pieces thus obtained had a colorless film uniformly formed.
• EXAMPLE 5 The sheets treated in the Example 3 were immersed in the aqueous solution containing 1% of a solid resin of a water soluble copolymer of itaconic acid and acrylonitrile, then wiped by drawing rolls and dried by blowing hot air of C for 3 seconds. The product obtained had colorless transparent film on the surface.
• EXAMPLE 7 Colorless type chromate treating solution available in the market containing hexavalent chromium of about lg/l when calculated as chromic acid and borofluoric acid ion at a degree not to color the formed film was used in treating the test piece (2) of the Example 1 under the same condition as in the Example 1. The appearance of the resulting product was no different from that before the treatment.
• the Examples 1 to 5 are in accordance with the present invention while the Examples 6 and 7 are in accordance with the conventional art.
• the resulting products of respective examples were subjected to salt spray test in accordance with JlS-,Z -237l and the humidity cabinet test in accordance with J lS-Z-O228 for corrosion resistance of the film and they were coated with a baking paint of melamine resin to test the paint adhesive property.
• the following tables 1 and 2 show the results thereof.
• Examples 1 to 5 are in accordance with the present invention method and Examples 6 and 7 are in accordance with the conventional method.
• the method of painting used for test pieces in Table 2 was such that melamine resin was coated with a. bar coater and baked for 20 minutes at 140C with the thickness of painted layer being 20-31 p. and hardness of the same at H-ZH.
• the test method-employed was as follows:
• Cross-cut erichsen test Squares were marked as above and the test pieces were stretched for 5mm by Erichsen tester. A strip of cellophane tape was adhered on the surface and then peeled.

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• 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)
• Chemical Treatment Of Metals (AREA)

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Cited By (7)

Citations (4)

• 1974
  • 1974-04-11 CA CA197,387A patent/CA1024866A/en not_active Expired
  • 1974-04-12 US US460302A patent/US3907610A/en not_active Expired - Lifetime
  • 1974-04-18 FR FR7413523A patent/FR2226481B3/fr not_active Expired

Patent Citations (4)

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