The invention is a process for applying a nickel-free, copper-containing phosphate coating to a metal surface by contacting the metal surface with a phosphate solution containing 0.2 to 2.0 g/l zinc ions, 0.5 to 25 mg/l copper ions, and 5-30 g/l phosphate ions.

FIELD OF THE INVENTION
 This invention relates to a process for the production of copper-containing
 nickel-free phosphate coatings on metal surfaces and to the use of the
 process as a pretreatment of the metal surfaces before lacquering, more
 particularly before cataphoretic dip lacquering (CDL).
 BACKGROUND OF THE INVENTION
 The quality of phosphate coatings before cataphoretic dip lacquering (CDL)
 depends upon a number of parameters, including physical parameters, such
 as the shape and size of the crystals, their mechanical stability and, in
 particular, the free metal surface after phosphating, the so-called pore
 area. Among the chemical parameters, alkali stability during cataphoretic
 coating, the binding strength of the water of crystallization of the zinc
 phosphate crystals during stoving of the lacquers and the rehydration
 capacity are of particular interest.
 The weight of the coating can be controlled and, in particular, reduced by
 using activating agents before phosphating. Active centers from which
 crystal growth advances are formed on the metal surface by the polymeric
 titanium phosphates present in the activating agents. On the one hand,
 this results in smaller and mechanically more stable crystals, on the
 other hand the pore area is reduced in size which makes it more difficult
 for corrosive media to attack the lacquer coating in the event of damage.
 RELATED ART
 In the prior art, it has proved to be of advantage to provide a separate
 treatment bath in order optimally to influence the quality of the
 phosphate coating subsequently applied. However, the effective life of the
 activating baths is limited by carryover from the preceding cleaning
 baths. In particular, water hardness ions deactivate the polymeric
 titanium phosphates.
 Accordingly, a search has been made for ways of obtaining a dense,
 substantially nonporous phosphate coating with a low weight per unit area
 by other methods or of producing such a coating in the phosphating bath
 itself in addition to activation. Extensive basic works have been carried
 out to this end. Some of these works were carried out at the Institute for
 Crystallography of the University of Cologne and resulted in the discovery
 of a new crystal phase Ba.sub.3 (PO.sub.4).sub.2.H.sub.2 O (Z. fur
 Kristallographie 196, 312-313 (1991)). Although barium phosphate coatings
 do not contain any zinc, they have a number of positive properties,
 including in particular high thermal stability. Unfortunately, the coating
 weights obtainable are not sufficient to afford high protection against
 corrosion in combination with cataphoretic dip lacquering. Accordingly,
 barium phosphate coatings occupy an intermediate position between the
 "thin" iron phosphate coatings (0.3-0.5 g/m.sup.2) and the "thicker" zinc
 phosphating coatings (2.0-3.5 g/m.sup.2).
 Aluminium ions reduce the weights of the phosphate coatings to an even
 greater extent, so that so-called "passivation phenomena", i.e.
 disturbances to the formation of zinc phosphate coatings, occur beyond a
 concentration of only 5 ppm Al.sup.3+ ions in the phosphating bath.
 Additions of magnesium ions have also been investigated. The positive
 effects of these ions on performance were recognized at an early stage
 (DE-A-39 20 296) and are based on several factors. The high crystal
 stability on heating is crucial in this case, too. The release of water of
 crystallization, which weakens the crystal structure and hence the system
 as a whole, is displaced to higher temperatures with increasing
 incorporation of magnesium. On the other hand, the crystals become
 smaller, the phosphate coating becomes denser and the free metal surface
 after phosphating is minimized by additions of Mg.sup.+2 ions. The
 reduction in coating weight by magnesium ions is so considerable that
 other controlling factors which, normally, are also used for reducing
 coating weight, such as very low zinc concentrations (0.6 g/l Zn.sup.2+),
 high concentrations of accelerators, such as sodium nitrite or
 meta-nitrobenzenesulfonate/Na salts, do not have to be additionally used
 to produce a weight per unit area of 1.5 to 2.0 g/m.sup.2.
 The influence of Cu.sup.2+ ions has also been investigated. Additions of
 small quantities of copper ions to phosphating baths have been known for
 40 years. Thus, in U.S. Pat. No. 2,293,716, very small quantities of
 Cu.sup.2+ ions are added as "accelerators" or as "color neutralizers" to
 improve the whiteness of anodic electrodeposition lacquers. It was found
 that additions of copper increase the weight of the coating, particularly
 on steel.
 DE-A-40 13 483 describes a process for phosphating metal surfaces in which
 phosphating solutions substantially free from nickel are used. Zinc,
 manganese and small contents of copper are mentioned as key bath
 constituents. In addition, the concentration of Fe(II) is kept below a
 maximum value by oxygen and/or other equivalent oxidizing agents. The
 process in question is used in particular for the pretreatment of metal
 surfaces for subsequent lacquering, more particularly electrode-position
 lacquering, and for the phosphating of steel, galvanized steel,
 alloy-galvanized steel, aluminium and alloys thereof.
 EP-A-0 186 823 describes strongly acidic phosphating solutions with a pH
 value of 1.8 to 2.5 which contain 7.5 to 75 g/l of zinc ions, 0.1 to 10
 g/l of hydroxylamine and optionally up to 20 g/l of manganese ions and
 also 5 to 75 g/l of nitrate ions. The solutions tolerate an iron content
 of up to 25 g/l.
 A process for the zinc phosphating of iron-containing surfaces is known
 from EP-A-0 315 059. The desired morphology of the zinc phosphate crystals
 is established by the use of hydroxylammonium salts, hydroxylamine
 complexes and/or hydroxylamine. All the Examples contain nickel in
 addition to zinc as another layer-forming cation. The toxicological
 disadvantages of nickel are well-known.
 BRIEF SUMMARY OF THE INVENTION
 Accordingly, the problem addressed by the present invention was to provide
 a process for the production of nickel-free phosphate coatings which,
 despite the absence of nickel, would guarantee very firm lacquer adhesion
 and excellent corrosion protection on metal surfaces, such as cold-rolled
 steel, electrogalvanized steel and aluminium.
 According to the invention, this problem has been solved by a specially
 selected phosphating solution which contains hydroxylamine salts,
 hydroxylamine complexes and/or hydroxylamine in a quantity of 500 to 5,000
 ppm hydroxylamine, based on the phosphating solution, as the active
 component for modifying the crystal morphology ("accelerator"). With
 phosphating solutions such as these, it is possible to produce
 copper-containing phosphate coatings with a defined copper content and a
 defined edge length of the phosphate crystals.
 In a first embodiment, therefore, the present invention relates to a
 process for the production of copper-containing nickel-free phosphate
 coatings with a copper content of 0.1 to 5% by weight and an edge length
 of the phosphate crystals of 0.5 to 10 .mu.m on metal surfaces selected
 from steel, galvanized steel, alloy-galvanized steel, aluminium and alloys
 thereof by treatment of the surfaces by spraying, dipping or
 spraying/dipping with a phosphating solution containing the following
 components:
 zinc ions 0.2 to 2 g/l
 copper ions 0.5 to 25 mg/l
 phosphate ions 5 to 30 g/l (expressed as P.sub.2 O.sub.5)
 and hydroxylamine salts, hydroxylamine complexes and/or hydroxylamine in a
 quantity of 500 to 5,000 ppm of hydroxylamine, based on the phosphating
 solution.
 It has been found that, even in the absence of nickel, these phosphating
 solutions guarantee very firm lacquer adhesion and excellent corrosion
 protection on the metal surfaces mentioned above without the formation of
 any patches. The zinc phosphate coatings thus produced are made up of
 small (0.5 to 10 .mu.m), compact and densely grown crystals.
 DETAILED DESCRIPTION OF THE INVENTION
 More particularly, the investigation of phosphating baths containing copper
 ions has shown that only very small quantities of copper ions are required
 in the solution to establish the desired copper content of the phosphate
 coating of 0.1 to 5% by weight.
 In another preferred embodiment of the invention, therefore, the
 phosphating solution contains 5 to 20 ppm of copper ions when the metal
 surface is contacted with the phosphating solution by dipping. Where the
 phosphating solutions are applied by spraying, they preferably contain
 from 1 to 10 ppm of copper ions to incorporate correspondingly high copper
 contents in the conversion coating.
 To guarantee satisfactory formation of the phosphate coating, it is known
 that the pH value of the phosphating solution can be adjusted to a value
 of 2.5 to 3.5. If necessary, other cations, for example alkali metal
 cations and/or alkaline earth metal cations, are used with corresponding
 anions known from the prior art to establish the pH value of the
 phosphating solution. Corrections to the pH value during the phosphating
 process may be made, for example, by additions of bases or acids.
 Fine crystals which have a much more compact granular morphology rather
 than the known acicular structure are formed by the addition of
 manganese(II) ions, particularly where the phosphating solutions are
 sprayed onto surface-treated materials. The use of manganese ions in
 addition to zinc ions in low-zinc phosphating processes improves corrosion
 protection, particularly where surface-treated fine plates are used. The
 incorporation of manganese in the zinc phosphate coatings leads to smaller
 and more compact crystals with increased alkali stability. Accordingly, in
 one particularly preferred embodiment of the present invention, the
 phosphating solution contains 0.1 to 5 g/l, 0.15 to 5 g/l and, more
 particularly, 0.5 to 1.5 g/l of manganese(II) ions.
 The quality of the copper-containing nickel-free phosphate coatings
 produced by the process according to the invention is not impaired if the
 phosphating solution contains alkaline earth metal cations in quantities
 of up to 2.5 g/l, more particularly magnesium and/or calcium ions.
 The process according to the invention may be applied in particular to
 steel, steel galvanized on one or both sides, steel alloy-galvanized on
 one or both sides, aluminium and alloys thereof. In the context of the
 invention, the term steel is understood to encompass soft, non-alloyed
 steels in addition to low-alloyed steels and also more highly alloyed and
 high-strength steels. A key feature of the invention is that the aqueous
 acidic phosphating solutions are free from nickel. However, this does mean
 that, under industrial conditions, a small quantity of nickel ions may be
 present in the phosphating baths. In consistency with the prior art
 (DE-A-40 13 483), however, this quantity should be less than 0.0002 to
 0.01 g/l and, more particularly, less than 0.0001 g/l.
 Where the phosphating process is applied to steel surfaces, iron passes
 into solution in the form of iron(II) ions. By addition of suitable
 oxidizing agents, iron(II) is converted into iron(III) and may thus be
 precipitated as iron phosphate sludge. According to the invention,
 therefore, the phosphating solution typically contains up to 50
 ppm--briefly even up to 500 ppm during production--of iron(II) ions.
 A number of oxidizing agents are known from the prior art for limiting the
 concentration of iron(II) ions. For example, the concentration of iron(II)
 ions may be limited by contacting the phosphating solution with oxygen,
 for example atmospheric oxygen, and/or by addition of suitable oxidizing
 agents.
 In a preferred embodiment of the invention, therefore, the phosphating
 solution contains oxidizing agents selected from peroxide compounds,
 chlorates, permanganates and organic nitro compounds.
 According to the invention, the oxidizing agents for the phosphating
 solutions are preferably selected from peroxide compounds, more
 particularly hydrogen peroxide, perborate, percarbonate and perphosphate,
 and organic nitro compounds, more particularly nitrobenzenesulfonate. The
 quantities of oxidizing agent to be used are known from the prior art, the
 following quantities being mentioned by way of example: peroxide compound
 expressed as hydrogen peroxide 0.005 to 0.1 g/l, nitrobenzenesulfonate
 0.005 to 1 g/l.
 Where the phosphating process is applied to galvanized steel,
 alloy-galvanized steel, aluminium and alloys thereof, the presence of
 iron(II) ions is not harmful. Accordingly, there is no need at all to add
 oxidizing agents in the phosphating of these materials by the process
 according to the invention.
 In addition, it has proved to be of particular advantage to use
 nitrate-free phosphating solutions in the phosphating of galvanized metal
 surfaces in accordance with the invention.
 In another preferred embodiment of the invention, the phosphating solutions
 used are substantially free from nitrite ions. A major advantage of this
 variant of the invention is that no toxic decomposition products of
 nitrites, for example health-damaging nitrous gases, can be formed.
 The use of modifying compounds from the group consisting of surfactants,
 hydroxycarboxylic acids, tartrate, citrate, hydrofluoric acid, alkali
 metal fluorides, boron trifluoride, silicofluoride, is known in principle
 from the prior art. Whereas the addition of surfactants (for example 0.05
 to 0.5 g/l) leads to an improvement in the phosphating of lightly greased
 metal surfaces, it is known that hydroxycarboxylic acids, more
 particularly tartaric acid, citric acid and salts thereof in a
 concentration of 0.03 to 0.3 g/l contribute towards significantly reducing
 the weight of the phosphate coating. Fluoride ions promote the phosphating
 of metals which are relatively difficult to attack, leading to a reduction
 in the phosphating time and in addition to an increase in the surface
 coverage of the phosphate coating. The fluorides are known to be added in
 quantities of around 0.1 to 1 g/l. The controlled addition of fluorides
 also provides for the formation of crystalline phosphate coatings on
 aluminium and its alloys. Salts of boron tetrafluoride and silicon
 hexafluoride increase the aggressiveness of the phosphating baths which is
 noticeable in particular in the treatment of hot-galvanized surfaces, so
 that these complex fluorides may be used, for example, in quantities of
 0.4 to 3 g/l.
 Phosphating processes are typically applied at bath temperatures of 40 to
 60.degree. C. These temperature ranges are used both for spraying and for
 application by spraying/dipping and dipping.
 The metal surfaces to be phosphated are cleaned, rinsed and, if necessary,
 treated with activating agents, more particularly based on titanium
 phosphates, by methods known per se before the phosphate coatings are
 applied.
 The phosphating baths used to carry out the process according to the
 invention are generally prepared in the usual way known per se to the
 expert. Suitable starting products for the preparation of the phosphating
 bath are, for example, the following compounds: zinc in the form of zinc
 oxide, zinc carbonate and optionally zinc nitrate; copper in the form of
 acetate, sulfate or optionally nitrate; manganese in the form of the
 carbonate; magnesium and calcium in the form of the carbonates; phosphate
 preferably in the form of phosphoric acid. The fluoride ions optionally
 used in the bath are preferably used in the form of alkali metal or
 ammonium fluoride, more particularly sodium fluoride, or in the form of
 the complex compounds mentioned above. The compounds mentioned above are
 dissolved in water in the concentrations crucial to the invention. The
 phosphating solutions are then adjusted to the required pH value, as
 mentioned above.
 In the context of the invention, hydroxylamine may emanate from any source.
 According to the invention, therefore, it is possible to use any compound
 which yields hydroxylamine or a derivative thereof, for example a
 hydroxylamine salt or a hydroxylamine complex which is often present in
 the hydrate form. Useful examples include hydroxylamine phosphate,
 optionally hydroxylamine nitrate, hydroxylamine sulfate (also known as
 hydroxylammonium sulfate [(NH.sub.2 OH).sub.2.H.sub.2 SO.sub.4 ]) or
 mixtures thereof. Hydroxylamine sulfate and hydroxylamine phosphate are
 particularly preferred hydroxylamine sources.

EXAMPLES
 Process Sequence
 1. Degreasing with a commercial alkaline cleaner (Ridoline.RTM. 1558)
 Quantity: 2%
 Temperature: 55.degree. C.
 Time: 4 mins.
 2. Rinsing with process water
 Temperature: room temperature
 Time: 1 min.
 3. Activation with an activating agent containing oligo/polymeric titanium
 phosphates (FIXODINE.RTM.950)
 Quantity: 0.1% in deionized water
 Temperature: room temperature
 Time: 1 min.
 4. Phosphating with the solution mentioned in the Examples and Comparison
 Examples
 Quantities: see Examples and Comparison Examples
 5. Rinsing with process water
 Temperature: room temperature
 Time: 1 min.
 6. Passivation with a commercial passivation (DEOXYLYTE.RTM. 41)
 Quantity: 0.1% by volume
 Temperature: 40.degree. C.
 Time: 1 min.
 7. Rinsing with deionized water
 Example 1
 Starting out from an aqueous solution of a bath composition in step 4 of
 the above-mentioned process sequence with the following ion
 concentrations: Zn 1.1 g/l, Mn 0.8 g/l, Cu 0.015 g/l, PO.sub.4.sup.3- 17.5
 g/l, NO.sub.3.sup.- 2.0 g/l, SiF.sub.6.sup.2- 0.95 g/l, F.sup.- 0.2 g/l,
 accelerator (hydroxylammonium sulfate) 1.7 g/l, total acid 22.7 points,
 free acid 0.9 points, surfaces of steel plate (Sidca) (Example 1a) and
 electrogalvanized fine plate (ZE) (Example 1b) were phosphated for 3
 minutes at a temperature of 52 to 54.degree. C., the corrosion protection
 results set out in Table 1 being obtained.
 Comparison Example 1
 Starting out from an aqueous solution of a bath composition in step 4 of
 the above-mentioned process sequence with the following ion
 concentrations: Zn 1.0 g/l, Mn 1.4 g/l, PO.sub.4.sup.3- 16.9 g/l,
 NO.sub.3.sup.- 2.0 g/l, SiF.sub.6.sup.2- 1.0 g/l, F.sup.- 0.2 g/l,
 accelerator (hydroxylammonium sulfate) 1.8 g/l, total acid 21.8 points,
 free acid 0.9 points, surfaces of steel plate (Sidca) (Example 1a) and
 electrogalvanized fine plate (ZE) (Example 1lb) were phosphated for 3
 minutes at a temperature of 52 to 54.degree. C., the corrosion protection
 results set out in Table 1 being obtained.
 Comparison Example 2
 Starting out from an aqueous solution of a bath composition in step 4 of
 the above-mentioned process sequence with the following ion
 concentrations: Zn 1.0 g/l, Mn 0.7 g/l, Ni 0.9 g/l, PO.sub.4.sup.3- 17.3
 g/l, NO.sub.3.sup.- 3.5 g/l, SiF.sub.6.sup.2- 0.25 g/l, accelerator
 (NaNO.sub.2) 0.15 g/l, bath temperature 50 to 52.degree. C., total acid
 21.7 points, free acid 1.1 points, surfaces of steel plate (Sidca)
 (Example 2a) and electrogalvanized fine plate (ZE) (Example 2b) were
 phosphated for 3 minutes at a temperature of 52 to 54.degree. C., the
 corrosion protection results set out in Table 1 being obtained.
 Examples 2a and 2b and Comparison Examples 3a and 3b
 Starting out from an aqueous solution of a bath composition in step 4 of
 the above-mentioned process sequence with the following ion
 concentrations: Zn 1.0 g/l,Mn 0.8 g/l, Cu (see Table 2), NO.sub.3.sup.-
 (see Table 2), P.sub.4.sup.3- 13.7 g/l, SiF.sub.6.sup.2- 0.95 g/l, F.sup.-
 0.22 g/l, accelerator (hydroxylammonium sulfate) 2.0 g/l, total acid 20.0
 points, free acid 1.2 points, electrogalvanized fine plate was phosphated
 for 1 minute at a temperature of 53.degree. C. The test plates were then
 provided with a test paint of CDL and white finishing lacquer and
 subjected to the alternating climate test according to VDA 621-415. The
 results obtained after a test duration of 5 cycles are set out in Table 2.
 Test Methods
 The corrosion-inhibiting effect of the phosphate coating according to the
 invention was determined in accordance with the standards of the Verband
 der Automobilindustrie e.V. (VDA 621-414 (outdoor weathering) and VDA
 621-415 (alternating climate test)).
 Testing of the corrosion inhibiting effect of motor vehicle lacquers by
 outdoor weathering is used to determine the corrosion inhibiting effect of
 motor vehicle lacquers under the influence of natural weathering for the
 total multilayer lacquer finish as in the Example with no protection
 against light and with the additional burden of spraying with salt
 solution.
 Test paints consisting of a typical automotive layer sequence of CDL,
 filler, white finishing lacquer (according to the Ford specification) are
 provided parallel to the longitudinal side with a straight score
 penetrating under control to the metal substrate. The test paints are
 stored on suitable frames. They are liberally sprayed once a week with a
 dilute sodium chloride solution.
 In the present case, the test duration was 6 months.
 For final evaluation, the test paints are rinsed with clear running water,
 optionally blown surface-dry with compressed air and inspected for visible
 changes. The visible creepage of rust from both sides of the score line is
 observed. The width of the metal surface damaged by rust adjacent the
 score line is generally easy to see on the paint surface. For evaluation,
 the average total width of the rust zone is measured in mm. To this end,
 the width is measured at several places and the arithmetic mean value is
 formed.
 The object of testing the corrosion inhibiting effect of motor vehicle
 lacquers under cyclically varying load is to evaluate the corrosion
 inhibiting effect of motor vehicle lacquers by an accelerated laboratory
 process which produces corrosion processes and corrosion patterns
 comparable with those formed under actual driving conditions. The
 accelerated test simulates in particular the creepage of rust from damaged
 paint and also the margin and edge rusting of special corrosion test
 plates or components with known weak spots in the paint finish and also
 surface rust.
 As in the outdoor weathering tests, test plates were again provided
 parallel to their longitudinal side with a straight score line penetrating
 to the metal substrate.
 The test plates were set up at angles of 60.degree. and 75.degree. to the
 horizontal in the test apparatus.
 One test cycle lasts 7 days and consists of
 1 day=24 h salt spray mist testing-SS DIN 50 021
 4 days=4 cycles condensation/alternating climate-KFW DIN 50 017 and
 2 days=48 h room temperature (18 to 28.degree. C.)-DIN 50 014.
 The test duration comprises 10 cycles corresponding to 70 days.
 On completion of the test, the test plates are rinsed with clear running
 water, optionally blown surface-dry with compressed air and inspected for
 visible changes. The visible creepage of rust from both sides of the score
 line is observed.
 In general, the width of the metal surface damaged by rust adjacent the
 score line is readily visible in the form of blisters or traces of rust on
 the lacquer surface. In addition, the paint film with rust underneath can
 be carefully removed up to the firmly adhering zone with a blade, for
 example an erasing knife, held at an oblique angle.
 For evaluation, the average total width of the rust creepage zone is again
 measured in mm. To this end, the width is measured at several places and
 the arithmetic mean value is formed.
 TABLE 1
 Corrosion test results
 3-Layer lacquer system
 Outdoor weather-
 ing Alternating climate
 VDA 621-414 test
 6 Months VDA 621-415
 Creepage Creepage Chipping
 mm mm value
 Example 1
 (a) Steel 0.4 0.6 0.4 0.6 0.6 0.5 1-2 1 1
 (b) ZE 0 0 0 0.9 0.8 1.0 1 1 1
 Comparison
 Example 1
 (a) Steel 0.5 0.6 0.8 0.5 0.8 0.9 1 1-2 1
 (b) ZE 1.0 0.8 0.7 2.8 3.3 2.5 6 6 6
 Comparison
 Example 2
 (a) Steel 0.3 0.3 0.3 0.3 0.6 0.8 1 1 1
 (b) ZE 0 0.3 0 1.6 1.0 1.3 1 1 1
 TABLE 2
 Ion concentrations
 in the bath
 Cu NO.sub.3.sup.- Creepage under lacquer
 Example [ppm] [g/l] [mm]
 2a 3 -- 1.1-1.7
 2b 8 -- 1.6-1.9
 Comp. 3a 3 2 2.6-4.6
 Comp. 3b 8 2 2.8-2.5
 These Examples clearly show the positive influence of nitrate-free
 phosphating solutions in the phosphating of galvanized metal surfaces.