Method for coating metallic surfaces with a multi-component aqueous composition

Abstract

A method for coating metallic surfaces with aqueous compositions, wherein a silane-based aqueous composition containing at least one silane and/or a related silicon-containing compound and optionally additional components is treated further, for example, at temperatures above 70 C., in a pretreatment step without drying the coating, by using at least one aqueous rinse step after this pretreatment step and then performing an electrodeposition coating, in which at least one surfactant is added at least in the last rinse step of the aqueous rinse steps. Coated metallic surfaces are also described.

Claims

1. A method for improving the throwing power of an electrodeposition coating, the method comprising: applying to a metallic surface two aqueous treatment compositions having different contents of at least one iron compound dissolved in water prior to contacting the metallic surface with an aqueous silane-based pretreatment composition; contacting the metallic surface with the aqueous silane-based pretreatment composition that comprises: a) at least one compound selected from silanes, silanols, siloxanes, and polysiloxanes, of which at least one of these compounds is still condensable, and b) at least one titanium, hafnium, and zirconium compound, and c) at least one type of cation selected from cations of metals of Groups IB to IIIB and VB to VIIIB, including lanthanides, and of main group II, of the periodic table of the elements, and/or at least one corresponding compound c), and/or d) at least one organic compound selected from monomers, oligomers, polymers, copolymers, and block copolymers, and e) water, and optionally at least one organic solvent and/or at least one substance to adjust the pH, thereby forming a pretreatment coating; rinsing the pretreatment coating at least once with water optionally comprising a surfactant; and applying an electrodeposition coating after the rinsing, wherein the aqueous silane-based pretreatment composition has a pH of from 1.5 to 9, and wherein the pretreatment coating is not completely dried, so that the at least one compound a) is not highly condensed before the rinsing of the pretreatment coating with water and/or before the coating with the electrodeposition coating.

2. A method according to claim 1, further comprising applying an after-rinse solution following the application of the aqueous silane-based pretreatment composition to form a second conversion layer or a coating.

3. A method according to claim 1, wherein the aqueous silane-based pretreatment composition has a content of silane, silanol, siloxane, and polysiloxane in the range of 0.005 to 80 g/L, calculated on the basis of the corresponding silanols.

4. A method according to claim 1, wherein the aqueous silane-based pretreatment composition contains at least one silane, silanol, siloxane, and/or polysiloxane which contains at least one amino group, urea group, and/or ureido group.

5. A method according to claim 1, wherein the aqueous silane-based pretreatment composition has a content of compounds of b) selected from titanium, hafnium, and zirconium in the range of 0.01 to 50 g/L, calculated as the sum of the corresponding metals.

6. A method according to claim 5, wherein the aqueous silane-based pretreatment composition has at least one complex fluoride of titanium, hafnium, and/or zirconium.

7. A method according to claim 6, wherein the complex fluoride(s) of titanium, hafnium, and/or zirconium is in the range of 0.01 to 100 g/L, calculated as the sum of the corresponding metal complex fluorides calculated as MeF.sub.6.

8. A method according to claim 1, wherein the at least one type of cation c) is selected from cations of aluminum, iron, calcium, cobalt, copper, magnesium, manganese, molybdenum, nickel, niobium, tantalum, yttrium, zinc, tin, cerium and other lanthanides.

9. A method according to claim 1, wherein in the aqueous silane-based pretreatment composition, only types of cations or corresponding compounds c) selected from the group of aluminum, magnesium, calcium, yttrium, lanthanum, cerium, manganese, iron, cobalt, copper, tin, and zinc, or selected from the group of aluminum, magnesium, calcium, yttrium, lanthanum, cerium, vanadium, molybdenum, tungsten, manganese, iron, cobalt, copper, bismuth, tin, and zinc are present.

10. A method according to claim 1, wherein the aqueous silane-based pretreatment composition has a cation content from compounds c) in the range of 0.01 to 20 g/L, calculated as the sum of the metals.

11. A method according to claim 1, wherein organic compounds d) have a content in the range of 0.01 to 200 g/L, calculated as the sum of the corresponding compounds.

12. A method according to claim 1, wherein a mix of various metallic materials is coated with the aqueous silane-based pretreatment composition simultaneously.

13. A method according to claim 1, wherein the aqueous silane-based pretreatment composition forms a coating having a layer weight which, based on titanium and/or zirconium, is in the range of 1 to 200 mg/m.sup.2.

14. A method according to claim 1, wherein the coating formed from the aqueous silane-based pretreatment composition has a layer weight which, based only on siloxanes/polysiloxanes, is in the range of 0.2 to 1000 mg/m.sup.2, calculated as the corresponding polysiloxane.

15. A method according to claim 1, wherein prior to applying the aqueous silane-based pretreatment coating, a prerinse and/or a first silane coating aqueous composition is performed, wherein the first silane coating aqueous composition contains at least one silane, at least one compound selected from fluoride-free compounds of titanium, hafnium, zirconium, aluminum, and boron, at least one alkaline solution, and/or at least one complex fluoride.

16. A method according to claim 1, wherein the rinse water has at least two different surfactants which in combination improve the wetting and foam suppressant properties.

17. A method according to claim 1, wherein at least one rinse with an aqueous composition contains at least one surfactant for homogenizing the wet film.

18. A method according to claim 1, further comprising applying, after the electrodeposition coating, at least one primer, paint, or adhesive, and/or a paint-like organic composition, to form at least one further coating.

19. A method according to claim 1, wherein each aqueous treatment composition having at least one iron compound dissolved in water also has at least one complexing agent.

20. A method according to claim 1 wherein each aqueous treatment composition having at least one iron compound dissolved in water has a pH of 9 to 14.

21. A method according to claim 1 wherein each aqueous treatment composition having at least one iron compound dissolved in water has a total iron content in the range of 0.005 to 1 g/L.

22. A method according to claim 1 wherein each aqueous treatment composition having at least one iron compound dissolved in water contains gluconate and/or heptonate.

Description

EXAMPLES AND COMPARATIVE EXAMPLES

(1) The examples (E) and the comparative examples (CE) according to the invention as described below are presented to illustrate the subject matter of the invention in greater detail.

(2) According to Table 2, the aqueous bath compositions are prepared as mixtures using prehydrolyzed silanes. They each contain a silane and optionally also a small amount of at least one similar additional silane, and here again, for the sake of simplicity, when silane is mentioned, it is also understood to mean silane, silanol, siloxane and/or polysiloxane, and as a rule, this variety of compounds, to some extent similar compounds in even larger numbers, is also run through in the development of the coating, so that several similar compounds are frequently also present in the coating. The prehydrolysis may also last for several days at room temperature with vigorous stirring, depending on the silane, unless the silanes to be used are already present in prehydrolyzed form. To prehydrolyze the silane, the silane is added to water in excess and optionally catalyzed with acetic acid. Acetic acid was added in only a few individual embodiment variants merely to adjust the pH. In some embodiment variants, acetic acid is already present as the catalyst for hydrolysis. Ethanol is not added but it is formed by hydrolysis. The finished mixture is used as a freshly prepared mixture.

(3) Then for each test, at least three sheets of cold-rolled steel (CRS) are cleaned on both sides with an aqueous alkaline cleaning agent and rinsed with process water and then afterwards with deionized water as well as sheets of aluminum alloy Al6O16 and/or hot-dip galvanized or electrolytically galvanized steel and/or Galvanneal (ZnFe layer on steel) are brought in contact with the corresponding treatment fluid on both sides at 25 C. by spraying, dipping or Roll Coater treatment. Immediately thereafter, the sheets pretreated in this way are rinsed briefly with deionized water. The sheets from the comparative examples are then dried at 90 C. PMT and next painted by a cathodic automotive dip coating (CDC). However, after the aqueous silane-based pretreatment, the sheet metal in the examples according to the invention is rinsed and then immersed in the CDC bath immediately after rinsing. Next the sheets are provided with a complete commercial automotive paint coating (electro-dip coating, filler, top coat or clear coat; total thickness of the layer package including CDC approx. 105 m) and tested for their corrosion resistance and paint adhesion. The compositions and properties of the treatment baths as well as the properties of the coatings are summarized in Table 2.

(4) The organofunctional silane A is an amino-functional trialkoxysilane and has one amino group per molecule. Like all the silanes used here, it is present in the aqueous solution mostly or completely in hydrolyzed form. The organofunctional silane B has a terminal amino group and has one ureido group per molecule. The nonfunctional silane C is a bis-trialkoxysilane. The corresponding hydrolyzed molecule has up to 6 OH groups on two silicon atoms.

(5) The complex fluorides of titanium and/or zirconium are used largely on the basis of an MeF.sub.x complex, for example, MeF.sub.6 complex. Manganese and optionally small amounts of at least one additional cation that is not mentioned in the table are added as metallic manganese to the respective complex fluoride solution and dissolved therein. This solution is added to the aqueous composition. If no complex fluoride is used, then manganese nitrate is added. The silylated epoxy polymer contains a small amount of OH.sup. and isocyanate groups and therefore 33333 can be crosslinked chemically even subsequently at temperatures above 100 C.

(6) The silanes contained in the aqueous compositionconcentrate and/or bathare monomers, oligomers, polymers, copolymers and/or reaction products with additional components based on hydrolysis reactions, condensation reactions and/or additional reactions. The reactions take place mainly in solution, during drying and/or optionally during curing of the coating, in particular at temperatures above 70 C. All concentrates and baths have proven to be stable over a period of a week and do not undergo any changes or develop any precipitates. No ethanol was added. Any ethanol content in the compositions originates only from chemical reactions.

(7) In most examples and comparative examples, the pH is adjusted, specific with ammonia in the presence of at least one complex fluoride or in other cases with an acid. All baths have a good quality of the solution and almost always good bah stability. There are no precipitates in the baths. After coating with the silane-containing solution, a brief rinsing is first performed once with deionized water in the examples according to the invention and in the comparative examples, immediately following the aqueous silane-based pretreatment. The freshly applied wet film could not be dried further because the samples were rinsed within 5 seconds after applying the silane-containing coating. Both the freshly coated substrate and the rinse water were at room temperature. Rinsing was necessary to prevent the entrainment of substances from the pretreatment solution into the downstream paint bath. The freshly rinsed coated substrate was then dipped immediately in the cathodic dip paint, so that no further drying could occur. However, the coated sheets of the comparative examples were dried for 5 minutes at 120 C. in the drying cabinet immediately after rinsing, but the examples according to the invention were coated immediately thereafter by immersion in a cathodic dip coating without intermediate drying.

(8) The visual test of the coatings can be performed significantly only with the coatings on steel because of the interference colors and this allows an evaluation of the uniformity of the coating. The coatings without any complex fluoride content are extremely uneven. Coating with titanium and zirconium complex fluoride has surprisingly proven to be much more uniform than if only if one of these complex fluorides had been applied. Addition of nitroguanidine, nitrate or nitrite also improves uniformity of the coating. In some cases, the layer thickness would increase with the concentration of these substances.

(9) TABLE-US-00002 TABLE 2 Compositions of baths in g/L based on solids contents, or in the case of silanes, based on the weight of the hydrolyzed silanes; residual content: water and in most cases a very small amount of ethanol; process data and properties of the coatings Example/Comparative example CE 1 E 1 CE 2 E 2 CE 3 E 3 CE 4 E 4 CE 5 E 5 CE 6 E 6 CE 7 E 7 CE 8 E 8 CE 9 E 9 Organo- 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.3 0.3 0.2 0.2 0.2 0.2 functional silane A H.sub.2TiF.sub.6 as Ti 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.3 0.3 0.2 0.2 0.2 0.2 H.sub.2ZrF.sub.6 as Zr 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.3 0.3 0.4 0.4 0.2 0.2 Mn 0.3 0.3 0.3 0.3 Silylated epoxy 1.0 1.0 polymer pH 10.5 10.5 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 Layer weight 10-20 10-20 20-50 20-50 20-50 20-50 20-50 20-50 20-60 20-60 10-40 10-40 30-80 30-80 30-80 30-80 50- 50- in mg/m.sup.2 100 100 from silanol and metal BMW cross-cut test: grade Steel 4 3 5 5 3 2 2 1 1 0 2 1 1 1 1 0 1 1 E-zinc on steel 3 3 4 3 4 3 1-2 1 1 0 1 1 1 1 1 1 0 0 Galvanized 2 2 4 3 4 3 1 0 0 0 1 0 0-1 0-1 0-1 0 0 0 zinc on steel Al 6016 2 2 2 2 2 2 1 1 1 0 2 1 1 1 1 0 1 1 Galvanneal 1 1 1 1 2 1 1 0 1 0 1 0 0 0 0 0 0 0 Ten cycles of VDA mm, migration beneath coating: Steel 8 6 7 5 4 3 3 2.5 2 1 3.5 2 1.5 1.5 1.5 <1 2.5 2 E-zinc on steel 5 4 3 2.5 4 4 2 1 1 <1 3 1.5 1.5 1 1 <1 1 <1 Galvanized 4 4 2.5 2 3.5 3 <1 <1 <1 <1 1.5 1 1 1 1 <1 <1 <1 zinc on steel Galvanneal 2 2 2 1 1.5 1.5 <1 0 <1 <1 1 1 <1 <1 <1 <1 0 0 Rock fall according to VDA loading, grade Steel 5 5 4 4 4 3 2-3 1 1 1 2 2 2 1 1-2 1 1 0-1 E-zinc on steel 5 4 3 2 4 3 2 1 1 1 2 1 1-2 0 1 0 1 0-1 Galvanized 5 4 3 2 4 3 1 0 1 0 1 1 1 0 1 0 0 0 zinc on steel Galvanneal 4 4 2 2 3 2 1-2 0 1 0 1 0-1 0 0 0 0 0 0 Salt spray test 1008 hours: Steel 7 6 4 4 3.5 3 2 1.5 1.5 <1 2.5 2 1.5 <1 1.5 <1 1 1 CASS test mm migration Al 6016 6 6 3.5 3 3.5 3 2.5 2.5 1.5 1 2.5 2 1.5 1 1.5 1 1.5 1 E = Example; CE = Comparative example

(10) If the various metallic surfaces that were coated are considered as a whole, all the examples show a significant improvement in the properties of the aqueous silane-based composition in comparison with the respective comparative example, wherein the same bath composition was applied in one case with subsequent drying (as comparative example CE) and in one case without subsequent drying (as example E according to the invention). The examples presented here are then examples according to the invention if they are utilized with this composition over the entire process sequence up to electro-dip coating on components using wrap-around.

(11) It was surprising that this improvement, which actually brings only a limited improvement, in particular in cases where the coating results are already good, is systematically improved by not drying after application of the aqueous composition. Therefore, by omitting the drying, it is surprisingly possible to achieve a significant improvement, which is almost independent of the chemical composition of the aqueous bath. It was initially surprising that this improvement occurred with the solutions containing only silane as well as with the solutions containing silane and complex fluoride and/or optionally also manganese ions. It is therefore assumed that a similar steady improvement from drying to nondrying also occurs with solutions having a similar composition or with solutions based on silane or based on silane and complex fluoride and containing a few different substances. When more substances are present and when the low contents are higher, the corrosion resistance and paint adhesion may be better, as long as an optimum that might have occurred is not exceeded.

(12) The layer weight varies not only according to the amounts of the individual components of the aqueous solution but also according to the type of the respective metallic surface which is coated. By selecting the bath components and their amounts, a very definite improvement in corrosion resistance and paint adhesion can thus be achieved on the whole.

(13) The bath compositions all proved to be stables in the very short use time and could be applied well. There were no differences in behavior, in the visual impression or in the test results among the various examples and comparative examples that could be attributed to the treatment conditions, for example, application by spraying, dipping or roll coater treating. The resulting films are transparent and almost all of them are largely uniform. They do not show any pigmentation of the coating. The resulting films are transparent and almost all of them are largely uniform. The structure, gloss and color of the metallic surfaces appear to be only slightly altered by the coating. In the case of a titanium and/or zirconium complex fluoride content, iridescent layers are formed on steel surfaces in particular. The combination of several silanes has not yet resulted in any further significant improvement in corrosion protection in the experiment. However, this cannot be ruled out. In addition, an H.sub.3AlF.sub.6 content was found on aluminum-rich metallic surfaces due to corresponding reactions in the aqueous composition. However, the combination of two or three complex fluorides in the aqueous composition has surprisingly proven to be extraordinarily successful.

(14) The layer thickness of the coating produced in this wayeven depending on the method of application, which was initially varied in separate testsis in the range of 0.01 to 0.16 m, usually in the range of 0.02 to 0.12 m, often as little as 0.08 m, and it is definitely greater when an organic polymer was added.

(15) Based on the development of zinc-manganese-nickel phosphating of vehicle bodies, which had been developed over a period of several decades, such phosphate layers produced today are of an extremely high quality. Nevertheless, contrary to expectation, it was also possible to achieve the same high-quality results even with the coatings containing silane, although greater efforts were necessary in this regard, even with the aqueous compositions containing silane that have been in use for only a few years.

(16) The corrosion prevention grades in the cross-cut test according to DIN EN ISO 2409 after storage for 40 hours in 5% NaCl solution, corresponding to BMW specification GS 90011, were from 0 to 5, where 0 indicates the best value. In the salt spray condensed water alternating tests over 10 cycles according to the VDA test sheet 621-415 with a varying corrosion load between the salt spray test, the condensation water test and a drying pause, migration beneath the cut was measured on one side, starting from the scratch and reported in mm, where the under-migration was to be as low as possible. In the rock fall test according to DIN 55996-1, the coated sheets are bombarded with steel scrap following the VDA alternating load test for 10 cycles, as described above. The damage pattern is characterized by characteristic values from 0 to 5, where 0 indicates the best results. In the salt spray test according to DIN 50021 SS, the coated sheets were exposed to a corrosive sodium chloride solution by spraying for up to 1008 hours. Then the migration was measured in mm for the scratch, where the scratch was produced down to the metallic surface using a standardized gouge and where the migration beneath the film should be as minor as possible. In the CASS test according to DIN 50021 CASS, the coated sheets made of an aluminum alloy are exposed to a special corrosive atmosphere by spraying for 504 hours. Then the migration from the scratch is measured in mm and should be as small as possible.

(17) Additional experiments on vehicle body elements have shown that the electrochemical conditions of the CDC bath could possibly be adapted slightly to the different type of coating but otherwise the excellent properties observed in the laboratory experiments on sheet metal can also be applied to vehicle body elements in an industrial environment.

(18) The influence of additives on the spray water was investigated in additional experiments.

(19) TABLE-US-00003 TABLE 3 Comparison of coating methods with and without the use of at least one surfactant and optionally additional additives in the rinse water to improve the electro-dip coating results Example/Comparative example (a stands for the wet-wet process) CE 10 E 10 CE 11 E 11 CE 12 E 12 E 13 E 14 Additives to the rinse water: Total surfactant content in g/L 0.2 0.2 0.2 0.2 0.2 + 0.2 Added surfactant mixtures A A A B A + B Additional additives and content 1) 0.1 1) 0.1 2) 0.005 2) 0.005 in g/L Cross-cut after 240 hours in the KK 5 2 3 2 1 1 0 0 test: grade Salt spray test 1008 h in mm 4.5 2.3 4.0 2.8 3.0 3.0 2.5 2.5 Visual impression of the beading of good good good good good good very very the rinse fluid good good Visual impression of the layer good good good good good good good good containing silane after rinsing Layer thickness CDC in m 43.7 41.9 40.3 38.3 39.8 38.4 37.7 37.5 Layer thickness fluctuations of the 3.0 1.5 1.6 1.0 1.7 1.3 1.6 0.5 CDC d in m Visual homogeneity of the CDC very faint heavy faint heavy faint faint faint layer with respect to streaks heavy streaks streaks streaks streaks streaks streaks streaks streaks Visual: evenness of the CDC layer very somewhat somewhat very somewhat somewhat almost almost uneven uneven uneven uneven uneven uneven even even Examples/Comparative example CE 15 E 15 E 16 E 17 E 18 Additives to the rinse water: Total surfactant content in g/L 0.2 0.2 0.2 0.2 + 0.2 Added surfactant mixtures C D A A + B Visual impression of the flow of the rinse good good very good very fluid good good Visual impression of the layer containing good good good good Good silane after rinsing Visual: homogeneity of the CDC layer heavy faint faint faint faint with respect to streaks streaks streaks streaks streaks streaks Visually: evenness of the CDC layer very somewhat somewhat somewhat almost uneven uneven uneven uneven even Center layer thickness of CDC, outside, 16 18 19 19 20 m Center layer thickness of CDC, inside, m 5 16 17 17 19 Fluctuations in layer thickness of the CDC 11 2 2 2 1 d in m between the inside and the outside as a measure of throwing power E = Example CE = Comparative example

(20) All examples and comparative examples E 10 to E 18 and CE 10 to CE 12 as well as CE 15 to CE 18 were used in the wet-on-wet method with and without addition of a surfactant to the water after-rinse following the aqueous silane-based pretreatment and before immersion in the same electro-dip paint used for the manufacturing series. The compositions of the examples E 10 to E 18 and comparative examples and CE 10 to CE 12 and CE 15 to CE 18 were produced in the same way as the other examples and comparative examples and were used, except that only sheets of cold-rolled steel (CRS) were used in the second series and sheets of hot-dip galvanized steel were treated in the third series, and the sheets treated with the silane-containing composition were stored in room air at room temperature for 5 minutes to 30 minutes after rinsing before they were coated with a commercially available cathodic dip coating (electro-dip paint, e-coat, CDC) by immersing at 250 V (second series) or at 240 V (third series).

(21) However, a slightly different type of cold-rolled steel was used than in the first series for the experiments according to Table 2 (=first series). For the examples E 10 to E 14 and comparative examples and CE 10 to CE 12 (second series), however, a different electro-dip paint was used than that used for examples E 15 to E 18 and comparative examples CE 15 to CE 18 (series 3). An electro-dip paint of generation 6, MC3 of PPG, was used for the latter. The layer thicknesses of the electro-dip paint were measured using the VDA method.

(22) The half-hour waiting time simulates the cycle time of vehicle bodies coated in this way until the vehicle body is immersed in the CDC pool. The silane-containing coatings dry somewhat superficial here but not completely. The silane pretreatment of these examples and comparative examples is based on the compositions of example E 8 and comparative example CE 8, wherein aqueous silane-based pretreatment compositions, such as those in E 8 and CE 8, were used in the third series, except that they also contained 0.001 to 0.10 g/L Cu and 0.1 to 1 g/L Zn plus optionally also traces of Al and small amounts of Fe. The pH was also set at 4. The deionized water for the after-rinse was prepared with the addition of at least one surfactant in the examples according to the invention, where the surfactant or the surfactant mixture was added in the form of an aqueous solution. The surfactant mixture A contained a nonionic surfactant based on a fatty alcohol polyglycol ether. The surfactant mixture B contained a different type of nonionic surfactant and a solubilizer. The surfactant mixture B proved to be especially suitable for beading of the rinse water. The surfactant mixture C contained a nonionic surfactant based on an alkylamine. The surfactant mixture D contained a nonionic surfactant and a cationic surfactant. Additive 1) was a water-soluble diphosphonic acid with a longer alkyl chain. Additive 2) was a water-soluble tin compound.

(23) All the CDC layers of a series were applied at the same voltage, even if this resulted in great differences in layer thickness. Fundamentally, the CDC layers of the second series were slightly too thick. The layer thicknesses were formed not only according to the electric conductivity of the pretreated substrate but apparently also to a great extent depended on the quality of the remaining pretreatment layer, which evidently differed in uniformity due to the different rinse compositions. The conditions were selected, so that inhomogeneities in the electro-dip paint were readily visible and a differentiation in the quality of the CDC layer was possible.

(24) The additional investigations were performed on pretreated rinsed and CDC-coated sheet metal but they were different than in the first series of examples and comparative examples without the additional paint layers of a typical automotive paint structure: the corrosion resistance was determined in the salt spray test according to DIN EN ISO 9227 over a period of 1008 hours, and the paint adhesion was determined according to the cross-cut test method after a 240-hour constant climate test according to DIN EN ISO 6270-2 and according to DIN EN ISO 2409. In both test methods, the smaller values are better values.

(25) On the one hand, a surprisingly strong correlation of the results with respect to corrosion resistance, paint adhesion CDC layer thickness and presumed homogeneity of the CDC layer as well as a great dependence of the results on rinsing with and without surfactant was revealed, wherein additives to the rinse water containing surfactant to some extent also yielded a further improvement. On the other hand, it was found that the homogeneity of the CDC layer is better, the smaller the resulting CDC layer thickness. Although the CDC layers of examples E 13 and E 14 were the thinnest in this series, these coated metal plates nevertheless had a much better corrosion resistance than the thicker CDC layers. The differentiation in quality with regard to paint adhesion is also surprisingly strong over the total possible range of grades from 5 to 0.

(26) It has surprisingly been found that the quality of the silane-based pretreatment coating and the composition of the water for rinsing after the silane pretreatment had a substantial effect on the quality of the paint layers applied subsequently.

(27) It was surprising that addition of at least one surfactant would have a strong effect on the subsequent coating with the electro-dip paint despite the comparatively low surfactant content in the rinse water and due to a very thin surfactant film which is even monomolecular under some conditions and is thereby produced and that the addition of at least one surfactant in the after-rinse would have strong effect on the interface between the silane pretreatment coating and the electro-dip coating as well as on the layer formation of the electro-dip coating. The electro-dip paints selected in the second and third series are of a particularly high quality and it is known that they can be processed especially uniformly.

(28) Nevertheless, the unevenness in the electro-dip coating layer was so great in comparative example CE 11a that it must be assumed that marks would be visible up to the top coat in the subsequent coating with the paint layers that are typically used in automotive engineering. On the other hand, it has been observed in similar studies that clearly visible striations were formed in coating large-area vehicle body elements when they were rinsed without addition of a surfactant, but these striations could be prevented by adding a surfactant. A smoother CDC layer could be produced with the surfactant additive in the rinse liquid and would then in turn be partially responsible for the fact that even more uniform, smoother paint layers with fewer defects could be formed on the CDC layer. The throwing power of the paint in electro-dip coating was surprisingly also influenced to a great extent.

(29) In the after-rinse following the silane pretreatment with water alone, inhomogeneities in the electro-dip paint that was subsequently applied or observed repeatedly despite the adequate in some cases repeated rerinsing and despite rerinsing at least once with deionized water.

(30) In additional experiments not presented here in detail, it was also determined that fundamentally any surfactant can be added, wherein nonionic surfactants in particular are preferred, but it is necessary to select low-foaming surfactants or those with little or almost no foam production and/or surfactant-containing mixtures for rerinsing by spraying and these mixtures may additionally contain, for example, a foam suppressant and/or a solubilizer and may have a minor, very minor or almost no tendency to foam, for example, in spray processes when used individually or in any combination. The nonionic surfactants are advantageously selected from linear ethoxylates and/or propoxylates, preferably those with alkyl groups of 8 to 18 carbon atoms. The latter also includes the surfactants A, B and D. With such a combination of surfactants, the wetting and foam suppressing properties can be optimized at the same time but surprisingly a plurality of properties of the electro-dip paint and electro-dip coating have proven to be advantageously subject to influence by such a combination of surfactants.

(31) On a zinc-rich metallic substrate in particular, the quality of the silane pretreatment and the type of after-rinse with water have a very strong effect on the homogeneity or inhomogeneity of the electro-dip coating (e-coat, CDC) and consequently also on the subsequent paint layer such as the base coat (filler as color medium) and the subsequent top coat (clear enamel). In the case of rinse water containing no added surfactant, it has been found that inhomogeneities in the electro-dip paint such as streaks are hardly avoidable. Streaks and other inhomogeneities as well as unevenness then subsequently easily and frequently lead to plastic marks in the following paint layers. Basically there should not be any plastic marks in the base coat or in the top coat of vehicle bodies for automobiles because these usually necessitate intense mechanical reworking and repainting. If the paint layers in reworking are removed too greatly, e.g., in reworking, for example, down to or even into the metallic substrate, then a pretreatment should also be applied before applying the first paint layer, for example, a pretreatment composition based on at least one silane or based on at least one silane with a titanium and/or zirconium compound and/or with an organic polymer. Such reworking not only causes problems in the work sequence but also causes substantial costs in particular due to the manual labor.

(32) If at least one surfactant has been added to the rinse water and if the silane pretreatment has been processed well in the normal way, inhomogeneities were not observed anywhere in the electro-dip coating in any of the experiments and plastic marks were not found in any of the following paint layers. Plastic marks refer to inhomogeneities in the top paint layer, which are more or less distinctly visible to the naked eye due to height differences in the paint surface in particular. Only if the pretreatment composition itself was already extremely inhomogeneous were definitely inhomogeneous electro-dip coating layers formed even under extreme conditions after the after-rinse with rinse water containing surfactant and, following that, paint layers with only minor plastic markings were obtained.

(33) The electro-dip-coated substrates whose aqueous silane-based pretreatment coating was rinsed with water containing a surfactant showed a definitely better paint throwing ability than the electro-dip-coated substrates rinsed with water that did not contain a surfactant.

(34) Metallic components can be electro-dip coated with good results using the coating method according to the invention, even if problems had already occurred before the silane-based pretreatment, the water rinse contains no surfactant and no iron-containing treatment is performed before the silane-based pretreatment.

(35) Alternatively or in addition to the procedure with the aqueous rinse containing surfactant, an aqueous treatment with an iron compound dissolved in water can be performed before the pretreatment with the silane-based composition.

(36) In a new series of experiments, a further improvement in the application of the cathodic electro-dip paint to metallic surfaces containing zinc was found in examples 20 to 23 and in similar process variants in comparison with the procedures using a water rinse with or without a surfactant content. This improvement was achieved due to the fact that with an otherwise similar treatment sequence and similar treatment conditions as in the examples listed in Table 3, in which the electro-dip coating layer often has a 5 to 15% smaller layer thickness even when the temperature is constant and the voltage is kept constant. For cleaning the sheet metal prior to the silane-based pretreatment, a two-step cleaning process was utilized in which the sheet metal was first sprayed and then was dipped. When two content values are listed in Table 4, the content on the left is based on the spray process and the content on the right is based on the dipping operation if different contents were utilized. In this procedure the electro-dip coating layer was applied by using silane-based pretreatment compositions in comparison with zinc phosphate-based pretreatments with a lower voltage, so the throwing power of the electro-dip coating paint is also lower accordingly. It is therefore desirable to be able to use a voltage higher than 250 V, for example, without exceeding a layer thickness of the dried and baked electro-dip coating layer of 20 m, for example. In these examples an ideal layer thickness of the dried and baked electro-dip enamel layer on the outside was obtained by using a voltage of approx. 250 V in electro-dip coating without employing the process steps according to the invention. The reduction in this layer thickness despite the use of a voltage of 250 V in electro-dip coating indicates the possibility of using a higher voltage which then also leads to a higher throwing power. The surfactant E added here is a nonionic surfactant based on an alkyl ethoxylate with one alkyl group and with end group capping in which a cationic compound was also added. The pH of the cleaning agent was in the range of 10 to 11. In cleaning in examples 20 to 23, a gluconate and/or a heptonate was added as a complexing agent in the total amount indicated there. Furthermore the cleaning agent contained at least one alkali compound which served to adjust the pH. Other variants that were not listed in detail in Table 4 relate to optional additional additives of boric acid or silicate as well as additional variation of all the cleaning agent ingredients, but all these process variants led to the same or similar results. In comparison with all these examples according to the invention, no cleaning step containing Fe was performed in comparative example 19, nor was there a rinse using a surfactant.

(37) It has now been found that the use of an aqueous composition containing iron before application of the silane-based pretreatment composition permits an increased voltage in electro-dip coating for the production of a dried and backed electro-dip coating layer of 20 m, for example. The voltage used here was often 5 to 15% higher, for example, 260 to 290 V. It was also found here that the throwing power achieved was also approx. 5% to 15% improved based on the increased voltage. Preliminary results also indicate improved paint adhesion and improved corrosion resistance for these variants according to the invention.

(38) TABLE-US-00004 TABLE 4 Comparison of coating methods with and without Fe-containing additive in two-step cleaning and with and without the use of at least one surfactant in the rinse water to improve the electro-dip coating Addition in g/L (cleaning: left equal first spraying; right equal Examples/Comparative example subsequent dipping) CE 19 E 20 E 21 E 22 E 23 Additives in cleaning: Surfactant E + cationic compound 2.0/3.0 2.0/3.0 2.0/3.0 2.0/3.0 5.0/8.0 Water-soluble Fe.sup.2+ compound sulfate Amount of Fe.sup.2+ additive 0 0 0 0.080 0 Water-soluble Fe.sup.3+ compound nitrate nitrate nitrate nitrate Amount of Fe.sup.3+ additive 0 0.056/0.084 0.056/0.084 0.056/0.084 0.056/0.084 Carboxylic acid(s) additive 0 0.8/1.2 0.8/1.2 0.8/1.2 0.8/1.2 Additives to rinse water: Total surfactant content in g/L 0 0 0.2 0 0 Surfactant added E Visual impression of the flow of good Good very good good rinse fluid on the silane-containing good layer Visual impression of the silane- good Good good good good containing layer after rinsing Visual: homogeneity of the CDC heavy faint faint faint faint layer with respect to streaks streaks streaks streaks streaks streaks Visual: evenness of the CDC very slightly almost slightly slightly layer uneven uneven even uneven uneven Average layer thickness of CDC 19.5 17 16 16 18 on the outside, m Average layer thickness of CDC 7 15 14 14 17 on the inside, m Fluctuations in layer thickness of 12.5 2 2 2 1 the CDC, d in m between the inside and the outside as a measure of throwing power E = Example CE = Comparative example