ZINC PHOSPHATING WITH INTEGRATED SUBSEQUENT PASSIVATION

A process for the phosphatizing of metal surfaces composed of steel, zinc-coated steel or steel coated with zinc alloy, of aluminum and/or of aluminum-magnesium alloys, characterized in that the phosphatizing solution contains: 0.2 to 3 g/l of zinc ions, 3 to 50 g/l of phosphate ions, calculated as PO 4 , 0.001 to 4 g/l of manganese ions, 0.001 to 0.5 g/l of one or more polymers selected from polyethers, polycarboxylates, polymeric phosphonic acids, polymeric phosphinocarboxylic acids and nitrogen-containing organic polymers and one or more accelerators. Preferred polymers are poly(vinylphenol) derivatives containing amino groups.

EXAMPLES The phosphatizing processes according to the present invention and comparison processes were tested on steel sheets ST 1405, which are used in automobile manufacture. The dipping process carried out was the following procedure, conventional in the manufacture of car bodywork: 1. Cleaning by means of an alkaline cleaner (Ridoline® 1501, Henkel KGaA), formulation 2% in tap water, 55° C., 4 minutes. 2. Rinsing with tap water, room temperature, 1 minute. 3. Activation by means of an activating agent containing titanium phosphate (Fixodine® 950, Henkel KGaA), formulation 0.1% in demineralized water, room temperature, 1 minute. 4. Phosphatizing using phosphatizing baths of the following composition: 1.0 g/l of Zn 2&plus; 1.0 g/l of Mn 2&plus; 0.1 g/l of Fe 2&plus; 14 g/l of PO 4 3− 0.95 g/l of SiF 6 2− 0.2 g/l of F − 1.7 g/l of (NH 3 OH) 2 SO 4 Polymers as in the Table. In addition to the cations listed above, the nitrate-free phosphatizing baths contained, if necessary, sodium ions for adjusting the free acid. The number of points of the free acid was 0.9 and that of the total acid was 23; the pH was 3.35. The number of points of the free acid means the required consumption in ml of 0.1 N sodium hydroxide solution to titrate 10 ml of bath solution until a pH of 3.6 is attained. Similarly, the number of points of total acid indicates the consumption in ml to attain a pH of 8.2. 5. Rinsing with demineralized water 6. Blowing dry using compressed air. The mass per unit of surface (“layer weight”) was determined by dissolving in 5 % chromic acid solution in accordance with DIN 50942. It is shown in the Table. The phosphated specimen sheets were coated with a cathodic dipping paint from the firm BASF (FT 85-7042). The anticorrosive action was tested in an alternating climate test by VDA 621-415 over 9 cycles. The result is included in the Table as the loss of the paint at the scribe (half gap width). 1 TABLE Phosphatizing baths and results of phosphatizing Loss of Polymer Layer weight paint (Half No. (concentration) (g/cm 2 ) gap width) Comp. 1 Without polymer 3.7 1.9 Ex. 1 Polyethylene glycol 3.2 1.7 Molecular weight 1000, 10 ppm Ex. 2 As Example 1, 50 ppm 3.9 1.5 Ex. 3 TD-1355-CW 3000*) 3.5 1.5 Ex. 4 As Example 3, 50 ppm 3.8 1.3 Ex. 5 TD-1355-CW 8600**) 3.7 1.4 Ex. 6 As Example 5, 50 ppm 3.2 1.1 *)Mannich reaction product of polyvinyl-phenol (molecular weight 3000, determined by gel permeation chromatography) with paraformaldehyde and glucamine (Parker Amchen, USA) **)as above, molecular weight of the polyvinylphenol: 8600.