DE3705633A1 - METHOD FOR PREPARING WORKPIECES FROM ALUMINUM OR ITS ALLOYS FOR PAINTING - Google Patents

Description

The invention relates to a method for preparing machined at least partially Workpieces with surfaces of aluminum and its Alloys for subsequent painting and its Application to the treatment of workpieces with Surfaces of alloys of the type AlMg or AlMgMn or of the thermosetting type AlMgSi.

It is known to pickle and / or aluminum surfaces Apply a conversion layer for a subsequent one Painting, for example electro-dip painting, prepare. In practice, however, was repeated found that liability and Infiltration resistance of the paint films, e.g. B. on Car bodies while driving or on test parts Stress, e.g. B. in the salt spray condensation cycle test or in the outdoor weathering intensified with NaCl Fluctuations. This was evident u. a. particularly pronounced with AlMgSi sheets, e.g. B. of the type AlMgO, 4Sil, 2. No method was previously known, this Fix fluctuations and the quality reliably to bring a sufficiently high level.

The work leading to the present invention first gave the knowledge that the bad Paint adhesion and infiltration resistance especially heavily on pre-grinded or similar machined aluminum surfaces occurs while the undisturbed rolled surface mostly  provides a much better quality profile.

The object of the invention is to provide a method for Preparation of at least partially metal-removing machined workpieces with surfaces of aluminum or its alloys for subsequent painting To provide that the disadvantages described having.

The task is solved by the procedure of the beginning mentioned type according to the invention is designed that the workpieces first by Annealing and / or pickling in oxidizing acid medium treated and then with a conversion layer be covered.

Under the term "metal removal Workpieces "are understood to mean those workpieces that a mechanical surface treatment such. B. Grinding, scratching, turning, milling, blasting, subjected have been.

The method according to the invention is a pretreatment for suitable for all known painting processes. Especially However, it proves advantageous before the order from Powder, high solid and anodic as well as cathodic Electrocoating.

The glow level can be in the temperature range of e.g. B. 150 to 500 ° C at times of e.g. B. 5 minutes to several hours Air access must be carried out. It is believed that the beneficial effect of glowing on the construction and the  Densification of a natural oxide layer is based.

Have to carry out the oxidizing acidic pickling yourself u. a. HNO₃, H₂SO₄ + peroxide, H₂SO₄ + FeIII and simple and complex hydrofluoric acid + FeIII or + Peroxide proven to be cheap. In a special one Embodiment are the oxidizing acid medium Control of the pickling attack simple and / or complex Fluorides, e.g. B. in amounts of 0.2 to 50 g / l, added. HNO₃ can, for. B. in concentrations of 10 to 500 g / l, preferably 200 to 450 g / l, H₂SO₄ z. B. in Concentrations from 10 to 150 g / l, F-containing acids e.g. B. present in concentrations of 5 to 100 g / l. The Oxidizing agent FeIII is e.g. B. in amounts of 5 to 50 g / l, peroxide e.g. B. in amounts of 0.5 to 30 g / l applied.

Proven pickling solutions have e.g. B. The following composition: 350 to 400 g / l HNO₃ + 5 to 15 g / l HF; 250 to 400 g / l HNO₃; 75 to 100 g / l H₂SO₄ + 10 to 25 g / l FeIII (as sulfate) +0.5 to 2 g / l HF. The pickling solutions can in the spray and immersion process in the temperature range of 15 up to 45 ° C and with exposure times from 20 sec to 5 min be used.

It can also be advantageous to follow one Pickling treatment with fluoride-containing solution only one use oxidizing fluoride-free stains. A typical one Example of this is the operation HNO₃ + HF / rinsing in Water / HNO₃.

Before treatment in an acidic oxidizing medium or  by annealing, the surface of disturbing amounts Oil, grease, dust and dirt are removed. This can by the action of organic-based cleaners Solvent or mildly acidic, neutral or alkaline aqueous detergent happen. Furthermore, for Cleaning and staining the surface a treatment in Soda or potash lye, if necessary with additives Complexing agents.

After the treatment in oxidizing acid medium with Rinsed and dried water or generating the Conversion layer from an aqueous medium directly connected. If it is dried and stored and the There is a risk of surface contamination the order of the conversion layers cleaned again will.

To generate the conversion layers, z. B. following procedures:

• Yellow chromating e.g. B. with acidic aqueous solutions, which in addition to the CrVI and F components as further Additions e.g. B. NO₃, complex cyanides, Mo, W, Ba, Mg contain.
• Green chromating, e.g. B. with acidic aqueous solutions based on CrVI, H₃PO₄, F.
• Colorless chromating as a variant of the yellow or Green chromating with low thickness Conversion layer.  
• Chromium-free treatments based on titanium and / or Zirconium hydrofluoric acid and other additives, e.g. B. Alkali, PO₄, F, B₂O₃, tannin, water soluble organic compounds.
• Fluoride activated phosphating for the formation of preferably based on crystalline phosphate layers of Zn and / or Ni and / or Mn and / or Mg and / or Ca phosphate.

A particularly preferred embodiment of the invention consists of the workpieces first in HNO₃ + HF and / or pickle HNO₃ and then by Yellow chromate or fluoride activated Zinc manganese phosphating with a conversion layer too cover.

The method according to the invention is for the treatment of Workpieces made of aluminum or aluminum alloys, e.g. B. with Si, Fe, Cu, Mn, Mg, Cr, Zn and Ti as Alloy component, suitable. It is particularly advantageous it on the treatment of workpieces with surfaces of Alloys of the type AlMg or AlMgMn or the thermosetting type AlMgSi applicable. The workpieces can e.g. B. as sheet metal, solid or hollow profile and Cast part available.

The following examples illustrate the invention and explained for example.

Examples

Cleaned sheets from AlMgO, 4Sil, 2 were cut in half  sanded, cleaned again, then different "preparations" and afterwards subjected to different "chemical treatments".

The following "preparations" were used:

"Without": no special preparation for the comparison "Glow": Glow in air at 370 ° C above 2 hours "NaOH / HNO₃ + HF": NaOH (25 g / l in H₂O), 60 ° C, 30 sec diving / rinsing water / HNO₃ (380 g / l in H₂O) + HF (10 g / l in H₂O), 20 ° C, 30 sec diving / Rinse / rinse in demineralized water (VEW) / dry "NaOH / HNO₃ + HF / HNO₃": As above, but before Rinsing in VEW additionally treated in: HNO₃ (400 g / l in H₂O), 20 ° C, 5 min diving / Rinse water

The following "chemical treatments" were used:

"Without": no special treatment for comparison "Green chromating": Green chromating in an acid aqueous solution based on CrVI + H₃PO₄ + HF, 45 ° C, 2 min diving / Rinsing water / Rinsing in VEW / dry  "Yellow chromating": yellow chromating in one acidic aqueous solution based CrVI + HNO₃ + HF + K₃FeCN₆, 20 ° C 20 sec diving / rinsing water / Rinse in VEW / dry "Krist. ZnMnph": treatment with an acid aqueous low zinc Phosphate solution based on Zn + Mn + Ni + Na + NO₃ + HNO₂ + F, 55 ° C, 2 min Spray / rinse / passivate by rinsing with acidic aqueous Solution based on CrVI + CrIII / Showering with VEW / drying

The sheets were then cathodized Electro dip coating and two-layer automotive paint system provided, with a hard metal stylus to the metallic Scratched underground and various tests subject (see tables 1 and 2).

Table 1

Test in salt spray-condensation alternating test; 10 weeks of paint infiltration in (mm) values in brackets: at a maximum of 4 points on the scratch mark

Table 1 shows the good results of the method according to the invention by strong border highlighted.  

Table 2

Testing in outdoor weathering tightened with NaCl; 3.5 months of paint infiltration in (mm) values in brackets: at a maximum of 4 points on the scratch mark

Table 2 shows the good results of the method according to the invention by strong border highlighted. The preparation "Without" clearly shows that harmful influence of a grinding treatment (+) on the Paint infiltration.

Claims (7)

1. A process for the preparation of at least partially metal-machined workpieces with surfaces of aluminum and its alloys for subsequent painting, characterized in that the workpieces are first treated by annealing and / or pickling in oxidizing acid medium and then coated with a conversion layer. 2. The method according to claim 1, characterized in that the workpieces with solutions, the HNO₃, H₂SO₄ + peroxide, H₂SO₄ + FeIII and simple or complex hydrofluoric acid + FeIII or peroxide in stained in one or more successive stages will. 3. The method according to claim 1 or 2, characterized characterized in that the workpieces with solutions that contain simple and / or complex fluoride, pickled will. 4. The method according to claim 1, 2 or 3, characterized characterized in that the workpieces by Green chromating, yellow chromating, Colorless chromating, chrome-free treatment based Titanium and / or zirconium hydrofluoric acid or a fluoride-activated phosphating based Alkali and / or zinc and / or nickel and / or Manganese and / or calcium and / or magnesium phosphate be covered with a conversion layer.   5. The method according to one or more of claims 1 to 4, characterized in that the workpieces first in a solution containing HNO₃ + HF and / or HNO₃, pickled and then by yellow chromating or fluoride-activated zinc manganese phosphating with a Conversion layer are coated. 6. The method according to one or more of claims 1 to 5, characterized in that the workpieces in Electro dipping process, powder coating process or High-solid painting processes can be painted. 7. Application of the method according to one or more of the Claims 1 to 6, on the treatment of workpieces with surfaces of alloys of the type AlMg or AlMgMn of the thermosetting type AlMgSi.