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
  • C23C22/82—After-treatment
  • C23C22/83—Chemical after-treatment
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/38—Chromatising

Definitions

• the invention relates to a method for the aftertreatment of phosphate coatings applied to metal surfaces by means of a solution containing chromium VI and chromium III ions and subsequent painting.
• the object of the invention is to provide a process for the aftertreatment of phosphate coatings applied to metal surfaces, which leads to uniformly good results, is as little dependent on the previously produced phosphate coating, improves the corrosion resistance and the paint adhesion and requires little procedural effort.
• the problem is solved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the treatment with the solution containing chromium VI and chromium III ions is carried out by cathodic electrolysis, the concentration of chromium VI ions to a value in the range from 0.05 to 10 g / l and the ratio of chromium III / chromium VI to less than 1.0.
• the nature of the phosphate coating produced is of minor importance.
• the process according to the invention can be followed by any phosphating process which permits the formation of a dense phosphate coating. It is advisable to subsequently carry out a multi-stage water rinse. Even after the electrolytic treatment with the solution containing chromium VI and chromium III ions, rinsing should be carried out with water and, if possible, also with completely deionized water. If necessary, it is then dried. The final stage of the process is painting.
• the method according to the invention which is designed in particular for the aftertreatment of phosphated automobile bodies, is suitable both for phosphated steel surfaces, e.g. Made of cold-rolled steel, and also suitable for phosphated steel surfaces with zinc or zinc alloy coatings.
• the workpieces can also partially have aluminum surfaces or surfaces of other phosphatable metals.
• Solution containing chromium VI / chromium III ions is advantageously prepared by dissolving anhydrous chromic acid and / or its alkali, alkaline earth or ammonium salts. If the concentration falls below 0.05 g / l, the current yield is greatly reduced and the chromium / chromate separation is insufficient. In terms of corrosion resistance and paint adhesion, this leads to results that are far below what can be achieved. At concentrations above 10 g / l, a complex water rinse is required, although blistering is not guaranteed after the subsequent painting. The paint adhesion can also decrease.
• the preferred range for the concentration of chromium VI ions is in the range of 0.2 to 3.0 g / l.
• chromium III ions stabilizes the properties of the painted workpiece even at very low concentrations. Concentrations corresponding to a ratio of equal to or greater than 1 lead to unstable aftertreatment solutions and the formation of sludge. A ratio of chromium III to chromium VI in the range from 0.1 to 0.5 is particularly favorable.
• the treatment is carried out with a solution containing chromium VI and chromium III ions, which additionally contains colloidal silica in a concentration of 0.01 to 5.0 g / l.
• a solution containing chromium VI and chromium III ions which additionally contains colloidal silica in a concentration of 0.01 to 5.0 g / l.
• a further advantageous embodiment of the invention provides for the treatment to be carried out with a solution which additionally contains phosphate ions in a concentration of 0.01 to 2 g / l.
• a solution which additionally contains phosphate ions in a concentration of 0.01 to 2 g / l This is associated with an increase in the electrical conductivity of the aftertreatment solution, and the adhesion of the chromate layer is also improved. This in turn improves the "wet adhesion" after painting and the corrosion resistance of the paint film.
• lower phosphate concentrations have practically no effect; larger concentrations are not associated with any additional improvement. Instead, there is a risk of blistering due to insufficient water rinsing. Concentrations in the range from 0.2 to 1.0 g / l are particularly advantageous.
• the levels of phosphate and silica can have a synergistic effect.
• fluoride and boron compounds, sulfuric acid or sulfates, if compatible with the solution can be added to the aftertreatment solutions in addition to the aforementioned inorganic compounds.
• An advantageous embodiment of the invention consists in carrying out the treatment with a solution which additionally contains water-soluble organic polymer in a concentration of 0.01 to 5.0 g / l (calculated as dry substance).
• This configuration results in a particularly strong adhesion of the chromate layer in the pore area of the phosphate layer. The reason is in that the polymer is practically not removed even when rinsed with water, and the adhesion is further increased when drying. At a concentration below 0.0l g / l this effect practically does not yet occur, at concentrations above 5 g / l the appearance of the paint film can be impaired.
• the content of water-soluble polymer with the simultaneous presence of silica and / or phosphate has a noticeable additive effect.
• an aftertreatment solution which, as the water-soluble organic polymer, contains one based on polyacrylate or polyurethane.
• water-soluble, in particular cationic, polymers are also suitable, provided that they are stable in the aftertreatment solution containing chromium VI ions.
• cathodic electrolysis they are deposited on the phosphate layer and seal the phosphate layer. A certain caution is required when measuring the concentration of the cationic polymers and the choice of electrolysis conditions, since the deposited polymer has an electrically insulating effect which, if it is designed as an electro-dip coating, can impair the subsequent coating.
• a pH value below 1.5 can lead to an excessive dissolution of the phosphate layer.
• the aftertreatment solution becomes unstable, ie sludge is formed.
• Particularly favorable pH values are in the range from 3.5 to 4.5.
• chromic acid possibly also phosphoric acid, can be used without thereby exceeding the limits of 10 g / l or 2.0 g / l, or alkali metal hydroxide or ammonium hydroxide can be used.
• the aftertreatment is carried out with a solution containing chromium VI and chromium III ions, the temperature of which is set to a value in the range from ambient temperature to 50 ° C. A temperature of 20 to 40 ° C is particularly favorable.
• the treatment duration is set taking into account the selected current density.
• the treatment duration and current density or amount of current are selected such that a layer weight of 4 to 100 mg / m2 (calculated as Cr) results.
• a layer weight below 4 mg / m2 is practically not associated with an improvement in the corrosion resistance after painting and paint adhesion.
• Layer weights above 100 mg / m2 bring no additional increase in the improvement that has already been achieved.
• a layer weight in the range of 4 to 30 mg / m2 is particularly advantageous.
• the method according to the invention is intended in particular for the aftertreatment of phosphated automobile bodies.
• the bodies By appropriately guiding the bodies in relation to the anode arrangement, it can be achieved that the parts which are exposed to high corrosion stress receive a particularly intensive treatment in the chromium VI / chromium III solution. It should be taken into account that the effect that can be achieved is greatest in the body parts, that directly opposite and are close to the anode (s). In this respect, extensive influence can be exerted by the targeted arrangement of anodes.
• test panels obtained after the aforementioned complete or partial treatment course were subjected to the following tests:
• the bubble formation was determined in mm starting from the edges of the sample sheet.
• treated test panels were immersed in deionized water at 40 ° C. for a period of 10 days until the topcoat. Thereafter, the sheets were provided with a cross-cut extending to the metallic background, so that 100 fields each with an edge length of 2 mm were created. These fields were covered with an adhesive tape, which was then peeled off again. To assess the adhesion, the squares remaining on the metal surface were counted.
• test panels subjected to the aforementioned impact test were then subjected to the salt spray test described in more detail above for a period of 72 h. Then they were exposed to the free atmosphere. This alternating stress was carried out four times in total. Finally, the samples were again subjected to the salt spray test for 72 hours.
• test panels were then treated with a metal sponge to remove corrosion products and no longer adhering paint.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrochemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Mechanical Engineering (AREA)
• Application Of Or Painting With Fluid Materials (AREA)
• Chemical Treatment Of Metals (AREA)

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Citations (5)

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• 1986
  • 1986-04-26 JP JP61095909A patent/JPH0633465B2/ja not_active Expired - Lifetime
• 1987
  • 1987-04-20 US US07/040,432 patent/US4728401A/en not_active Expired - Fee Related
  • 1987-04-21 CA CA000535132A patent/CA1314511C/fr not_active Expired - Fee Related
  • 1987-04-22 DE DE19873713426 patent/DE3713426A1/de not_active Withdrawn
  • 1987-04-23 EP EP87200762A patent/EP0244022A3/fr not_active Withdrawn
  • 1987-04-24 PT PT84762A patent/PT84762B/pt not_active IP Right Cessation
  • 1987-04-24 AU AU71951/87A patent/AU586331B2/en not_active Ceased

Patent Citations (5)

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