Abstract:
A method for treating or pre-treating parts, profiles, strips, or wires comprising surfaces of aluminum, or other metal surfaces with an acidic, aqueous solution containing fluoride and phosphate.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION  
       [0001]     The invention relates to a method for the treatment or pretreatment of parts with aluminium surfaces.  
         [0002]     Phosphatizing methods for aluminium and aluminium alloys are known in principle. In industrial practice, fluoride-modified phosphatizing methods with at least 150 mg/l free fluoride have proved to be particularly successful. These methods are important in particular in the automotive industry and are chiefly used when a mix of substrates of various metals or alloys is passed through the plants.  
         [0003]     However, these methods have the serious disadvantages that due to the high fluoride content comparatively large amounts of cryolite (Na 3 AlF 6 ) and/or related precipitates are precipitated in the phosphatizing bath, and that parts of the precipitates are also precipitated on the phosphatized surface without being removable therefrom by means of simple rinsing operations, with an increased roughness being produced on the pretreated surface which, even after the subsequent application of lacquer, has a disturbing effect with its roughness.  
         [0004]     DE-A1-197 35 314 describes a method for the pretreatment of components with aluminium surfaces—if applicable in the presence of magnesium, steel and/or zinc surfaces—in a phosphatizing plant in which the components are degreased by means of a degreasing solution, are phosphatized by treatment with a phosphatizing solution containing fluoride, and are subsequently passivated by treatment with a passivating solution. The proportion of the aluminium and/or magnesium surface with respect to the entire surface of the components to be treated is at least 10% in this connection. The fluoride should be added to the phosphatizing solution exclusively as complex-bound fluoride, and the free-fluoride ion content formed therefrom in the phosphatizing solution for phosphatizing the steel and/or zinc surfaces without phosphatizing the aluminium and/or magnesium surfaces should be maintained at less than 100 mg/l. The passivating solution should also be composed in such a way that it passivates the phosphatized steel and/or zinc surfaces and forms a conversion layer on the aluminium and/or magnesium surfaces.  
         [0005]     However, this method has the disadvantage that it can only be used with comparatively small surface proportions in terms of aluminium surfaces, mostly only up to approximately 20% by surface of all the surfaces to be treated, with respect to the mix of substrates. In addition it has the disadvantages that cryolite and/or related precipitates are still formed in the pretreatment bath, and that with an increased aluminium content of the phosphatizing solution the layer-forming reactions, in particular on iron and steel surfaces, are impaired so that the entire mix of substrates of various metal and alloy surfaces can no longer be coated well in the bath in a uniform manner.  
         [0006]     It is an object of the invention to overcome the disadvantages of the prior art and propose in particular a method for phosphatizing aluminium and alloys containing aluminium, which method, even with increased proportions of aluminium-containing surfaces of the components to be treated or to be pretreated, allows the application of a good conversion layer that is also applicable on an industrial scale and/or a corresponding passivation layer on the surfaces of aluminium or alloys containing aluminium—if applicable in a mix of substrates of various metals or alloys.  
         [0007]     The object is achieved by means of a method for the treatment or pretreatment of parts, sections, strips or wires with surfaces of aluminium or alloys containing aluminium—if applicable in the presence of surfaces of further metals or alloys—with an acid aqueous solution containing fluoride and phosphate, which method is characterised in that the fluoride is at least partly present in the solution as free fluoride, and in that, in the bath of the phosphatizing solution 
        the free-fluoride content is maintained at a concentration in the region from 5 to 500 mg/l F free , and     the aluminium content is maintained at a concentration in the region of ≦100 mg/l Al ions (including complex-bound Al) 
 
 by virtue of the fact that increasing aluminium contents, in a precipitation tank outside the phosphatizing bath, are decreased to contents ≦100 mg/l Al ions in the bath by circulating the phosphatizing solution from the phosphatizing bath to the precipitation tank and back. 
       
 
         [0010]     Alternatively or at the same time, it can be ensured in the case of the method in accordance with the invention that in a separate zone of the phosphatizing bath increasing aluminium contents in the phosphatizing solution are decreased to contents ≦100 mg/l Al ions.  
         [0011]     As a result of the pickling attack, aluminium is solubilized at the metallic surface, and an aluminium content is absorbed into the phosphatizing solution. A certain aluminium content may, however, also get into the phosphatizing solution from other sources, for example from chemicals introduced into the rinsing solution. The aluminium content can mainly be decreased or completely be decreased by precipitation, but in part also by complex formation. The content of free fluoride in the phosphatizing solution is preferably maintained in a range from 6 to 120 mg/l, in particular preferably in a range from 10 to 80 mg/l, and especially preferably in a range from 20 to 50 mg/l. The aluminium content in the phosphatizing solution is preferably maintained at values ≦80 mg/l, in particular preferably ≦60 mg/l, and especially preferably ≦30 mg/l. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0012]      FIG. 1  is a flow diagram in which one possible principle out of several principles of the separate aluminium deposition is represented schematically. 
     
    
     DETAILED DESCRIPTION  
       [0013]     The method in accordance with the invention is distinguished by virtue of the fact that in the precipitation tank or in the separate zone of the bath aluminium is precipitated in the phosphatizing solution by adding alkali ions, fluoride complexes and/or fluoride ions, in particular by means of Na or K ions or by means of at least one easily dissociating fluoride such as, for example, NaF, NH 4 F, NaHF 2  or KF. The AlF x  complex may be present in an already precomplexed form in this case. It is advantageous to control the method in accordance with the invention in such a way that despite the addition of F the content of free fluoride in the bath is not increased. The alkali ion contents in the phosphatizing bath preferably amount to 1 to 20 g/l and are preferably also maintained in this range, in particular in the range from 3 to 10 g/l. In principle, the alkali ion content may also lie far above the concentration of 20 g/l, for example at 30 g/l. In many cases, however, given such a high alkali concentration, instability of the bath may occur. In the case of a mixture of various alkali ions one beside the other, a dominant sodium and/or potassium ion content is preferred.  
         [0014]     This leads to the fact that the precipitated cryolite and the related precipitates, such as K 2 NaAlF 6 , for example, do not occur or hardly occur any more in the phosphatizing bath, but largely or (almost) exclusively in the separate zone of the bath or in the precipitation tank. As a result, the precipitates can no longer settle or hardly settle any more on the surface to be coated.  
         [0015]     Advantageously, the free-fluoride concentration of the phosphatizing solution in the phosphatizing bath is 8 to 80 mg/l and in particular is 10 to 50 mg/l, and the concentration in the precipitation tank or in the separate zone of the bath container is 5 to 500 mg/l free fluoride, in particular 20 to 200 mg/l, and especially preferably 30 to 120 mg/l. It is therefore also preferable to maintain these contents at these concentrations. Since below 5 mg/l usually there is no pickling attack against aluminium by free fluoride, and since this pickling attack is needed, as the subsequent passivation for example alone by means of chromate-containing compounds, titanium fluoride, zirconium fluoride, a soluble rare-earth compound—in particular a cerium-containing compound, where scandium, yttrium and lanthanum are included by the term rare-earth element—on the basis of silane, self-organizing molecules on the basis of phosphonate, a polymer soluble and/or dispersible in solvent is not sufficient for passivation, a small minimum free-fluoride content in the phosphatizing solution is required. On the other hand, in many cases a closed phosphate layer is formed on the metallic surfaces from the phosphatizing bath at contents above 120 mg/l free fluoride, if the content with respect to cations co-precipitated with the phosphate, such as Zn, Cu, Ni, Fe, Mn etc., is not very small. The formation of a phosphate layer on the aluminium surfaces for reasons of protection against corrosion is not absolutely necessary. Therefore, a free-fluoride content above 120 mg/l will not usually be chosen, although in accordance with the invention it is also possible to operate above this value, because it causes additionally a higher consumption of chemicals and a larger amount of precipitated sludge of cryolite and/or related precipitates. Moreover, a difference in the free-fluoride concentration between the phosphatizing bath and the precipitation tank or separate zone in the bath of 30 to 60 mg/l should preferably be adjusted and maintained.  
         [0016]     In many cases, the method in accordance with the invention will be utilized in such a way that the dwell time of the phosphatizing solution in the precipitation tank or in the separate precipitation zone is up to 1 h, often up to 0.5 h. The volume flow from the bath to the precipitation tank and back is adjusted in accordance with the chosen volumes or partial volumes as well as the desired aluminium content in the phosphatizing bath.  
         [0017]     The term “part” for the purposes of this application includes all kinds and shapes of sheets, strips and sections, moulded articles, semifinished products, components, assemblies etc.  
         [0018]     In the case of the method in accordance with the invention, the parts, sections, strips and/or wires to be treated or pretreated are usually cleaned, rinsed and, if appropriate and separately from the rinsing and cleaning stages, brought into contact with an activating solution, for example on the basis of colloidally dispersed titanium phosphate, prior to pickling/phosphatizing.  
         [0019]     After pickling/phosphatizing, the treated or pretreated parts, sections, strips and/or wires can be rinsed and/or passivated, in particular by means of a passivating solution on the basis of a chromate-containing compound, titanium fluoride, zirconium fluoride, a soluble rare-earth compound—in particular a cerium-containing compound, self-organizing molecules, for example on the basis of phosphonate, on the basis of silane, a polymer soluble and/or dispersible in solvent.  
         [0020]     After pickling/phosphatizing or after passivation, the treated or pretreated and/or passivated parts, sections, strips and/or wires may be dried. In some cases, for example in the case of immediately following electro-dipcoating, drying is not, however, required.  
         [0021]     The precipitation of the aluminium may be effected under normal pressure and at a temperature in the range from room temperature to 70° C., in particular at a temperature in the range from 40 to 60° C. In the case of the method in accordance with the invention, the formation of the conversion or passivation layer may be effected under normal pressure and at a temperature from room temperature to 70° C., preferably at 35 to 60° C. The pH value usually lies in the range from 2 to 4. In principle, the pH value of phosphatizing baths always lies in the range around pH 3. At values of pH ≧4.0, the bath is usually unstable, whilst at values of pH ≧2.0 the bath is so stable that usually there is no good formation of layers, because the displacement of the pH value at the freshly pickled metallic surface is not sufficient for deposition of the conversion layer.  
         [0022]     Finally, the treated or pretreated and/or passivated parts, sections, strips and/or wires can be coated with a lacquer, with another kind of organic coating, with a film and/or with an adhesive layer, if applicable printed and if applicable reshaped, where the metal parts coated in this way can in addition be bonded, welded and/or otherwise connected together with other parts.  
         [0023]     The products produced in accordance with the invention can be used in the automotive industry, in the aeronautical industry, in apparatus and machine construction, in the furniture industry, in the building trade, for household appliances, electrical appliances, measuring instruments, control devices, testing devices, construction elements, housings, panellings, shelf systems, racks, frames, dividers, partitions, trim panels, lighting fixtures, crash barriers, radiator or fence elements as well as small parts, in particular for car body parts or car bodies.  
         [0024]      FIG. 1  shows a flow diagram in which one possible principle out of several principles of the separate aluminium deposition is represented schematically.  
         [0025]     Compared with methods described and practised so far, the phosphatizing method in accordance with the invention has the advantage that the sludge with its cryolite content and/or related precipitates content is largely obtained in a separate precipitation zone or in a separate precipitation tank and can be disposed of from there. With the method in accordance with the invention it is also possible to treat or pretreat different metallic substrates in one mix, without the formation of layers, for example on steel, being impaired as a result. On account of the reduced free-fluoride contents in the phosphatizing solution, there is also a reduced pickling attack against aluminium-containing surfaces, which also gives rise to a correspondingly reduced formation of sludge. It has also been possible to ensure by means of the method in accordance with the invention that, on account of only few particles being co-precipitated and deposited on the phosphatized surface after the subsequent application of lacquer, no disturbing markings such as roughness, streaking or other irregularities could be detected.  
         [0026]     Surprisingly, largely stable bath conditions with respect to the free-fluoride content and aluminium content could be realized in the phosphatizing bath as well as in the separate zone or in the precipitation tank despite the very different concentrations of these contents.  
         [0027]     The subject-matter of the invention will be explained in greater detail in the following with reference to an exemplifying embodiment.  
       EXAMPLE  
       [0028]     The following experiments were performed in order to determine the suitable precipitation conditions for aluminium-containing phosphatizing solutions in a phosphatizing bath in accordance with the invention. Above all, the influences of the concentrations of free fluoride, sodium as well as complex-bound fluoride on the speed of the precipitation of aluminium were examined in this connection.  
         [0029]     1. Test Design  
         [0030]     A computer-assisted test design was employed in order to be able to recognize in a better way interactions of the significant parameters that might possibly exist. The Stavex 4.3 program was used.  
         [0031]     2. Analyzers  
                                                       Fluoride measurement:   Orion Model 960 with ion-               selective electrode           Aluminium   ICP.                      
 
         [0032]     A standard phosphatizing solution was prepared from p.a. chemicals to have following composition:  
                                                       Zn:   1.5 g/l           Mn:   1.0 g/l           Ni:   1.0 g/l           P 2 O 5 :   14.0 g/l            NO 3 :   3.0 g/l           SiF 6 :   1.0 g/l           FA:   2.0 free acid.                      
 
         [0033]     Proceeding from this solution, ions of SiF 6  and Na as well as the proportion of free fluoride were varied in further preparations.  
         [0034]     3. Practical Execution  
         [0035]     0.5 l of the phosphatizing solution described above was poured into a plastics beaker, and the possibly desired higher silicofluoride content was adjusted by means of a 24% H 2 SiF 6  solution, the correction of the free-acid content being effected by adding NaOH. The Na content was adjusted by means of NaNO 3 ; the corresponding free-fluoride content was adjusted by means of a dilute ammonium bifluoride solution. 20 mg/l aluminium were added to the bath by means of a dilute aluminium nitrate solution. After 0, 15, 30 and 60 minutes, the free fluoride was measured, and a sample of the solution was discharged for the determination of aluminium.  
         [0036]     To prevent postprecipitation of the aluminium in the sample drawn, the procedure was as follows: 5 ml of the sample filtered by means of a membrane filter was added to a strongly hydrochloric deionized-water solution and filled up to 50 ml by means of deionized water. This solution was examined for its aluminium content by means of ICP.  
         [0037]     The results are compiled in Table 1.  
                                                                                                                                         TABLE 1                           Precipitation tests                Content of F free  in the   Content of Al in the           phosphatizing solution (mg/l)   phosphatizing solution (mg/l)                F free     Na   SiF 6                 60                   5       Test   (ppm)   (g/l)   (g/l)   0 min   15 min   30 min   min   0 min   15 min   30 min   60 min   days                    1   40   3.0   1.00   39   25   27   27   20   20   20   20   20       2   80   3.0   1.00   78   53   52   58   20   20   19   19   3.5       3   40   7.0   1.00   39   28   27   28   20   19   19   19   4       4   80   7.0   1.00   80   50   46   44   20   1.5   1   0.5       5   40   3.0   3.00   46   44   44   44   20   19   19   19   20       6   80   3.0   3.00   79   19   36   54   20   19   18   19   2       7   40   7.0   3.00   40   16   17   22   20   19   20   19   2       8   80   7.0   3.00   79   54   48   148*   20   1   0.5   &lt;0.5       9   60   5.0   2.00   60   34   40   106*   20   10   4.5   2       10   43   5.0   2.00   46   25   34    80*   20   18   11   5       11   77   5.0   2.00   73   67   42   37   20   19   17   13       12   60   3.3   2.00   59   43   39   28   20   19   19   19   3       13   60   6.7   2.00   59   52   51   33   20   15   7.5   3.5       14   60   5.0   1.15   61   32   25   21   20   19   2   1       15   60   5.0   2.85   59   29   25   27   20   4   2   1                 *Measurement error, as no free fluoride was added during the test.             
 
         [0038]     In Test 1 no precipitation of aluminium was observed over the period of 5 days. Tests 4, 8 and 15 revealed a drastic reduction in the aluminium content even within 15 minutes. Increased alkali contents are preferred in conjunction with increased free-fluoride contents. SiF 6  was added because of the stability of the phosphatizing solution and for usability for a mix of various kinds of metallic surfaces, in particular to avoid stippling on zinc-plated or zinc-containing surfaces.  
         [0039]     Upon evaluation of the values found in Example 1, the following exemplifying calculation can be made:  
         [0040]     4. Exemplifying Calculation of the Volume Flows or of the Reaction Vessel  
                                                           Phosphatizing bath   200   m 3             Partial stream for precipitation   100   m 3             in a precipitation tank           Feed of Al per hour   10   ppm                      
 
         [0041]     This feed corresponds to about 40 car bodies per hour with an aluminium-surface proportion of 50% and a pickling attack of approximately 1 g/m 2  during the contact time.  
         [0042]     Calculation formula for the stationary concentration of aluminium in the phosphatizing solution: 
 
Al(ppm)=(((200 m 3 −partial stream)×conc. Al in ppm after 1 h+(partial stream×rest Al in ppm after precipitation))/200)+Al in ppm from feed via pickling attack/h. 
 
         [0043]     Hence it was concluded that it is necessary for the volume flow, which is to be treated, to be at least 0.5 basin volumes per hour in order to be able to detect a sufficiently high proportion of dissolved aluminium. With an average dwell time of 30 min, this results in a volume of the reaction bath of 50 m 3 . A residual aluminium content in the outlet of the treatment bath of approximately 5 ppm results in a theoretical stationary aluminium content in the treatment bath of approximately 25 ppm. However, since a partial precipitation of the aluminium can also be expected in the treatment bath (see for example Test 3 from Table 1), the actual content in terms of dissolved aluminium will adopt a lower value. Approximately 20% of all the precipitates are obtained in the phosphatizing bath and provided an Al content of approximately 20 ppm in the phosphatizing solution of the bath in the stationary condition. Approximately 80% of all the precipitates were precipitated in the precipitation tank.