CA2057825A1

CA2057825A1 - Process of postrinsing conversion layers

Info

Publication number: CA2057825A1
Application number: CA 2057825
Authority: CA
Inventors: Thomas Kolberg; Joachim Heitbaum; Horst Gehmecker
Original assignee: Individual
Current assignee: GEA Group AG
Priority date: 1990-12-21
Filing date: 1991-12-17
Publication date: 1992-06-22
Also published as: PL292868A1; MX9102653A; DE4041091A1; PT99880A; HU914090D0; JPH04276087A; CS386691A3; HUT59728A; BR9105374A; EP0492713A1

Abstract

ABSTRACT

Conversion layers on metal surfaces, particularly of steel, galvanized steel, zinc alloy-coated steel and/or aluminum, are prepared for a pain-ting and/or for an application of adhesive in that they are rinsed with an aqueous solution which contains Ce(IV) and/or Ce(III) in a total amount of 0.01 to 1.0 g/l and which has a pH value of 3 to 6. A post-rinsing solution which is preferably employed contains Ce(III) and Ce(IV) in a weight ratio of 9 to 2 : 1 and optionally also contains molybdate and/or tungstate.
Preferred uses of the process serve to prepare metal surfaces provided with conversion layers for the suc-ceeding electro-dipcoating, particularly the cathodic electro-dipcoating, or for the postrinsing of metal surfaces provided with conversion layers which consist of phosphate layers or of layers formed by a chromium-free, acid process using Zr and/or Ti, F and optionally PO4, or of complex metal oxide layers.

Description

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The present invention relates to a process for the passivating postrinsing of conversion layers OD
metal surfaces, particularl~ of steel, galva~ized steel, 5 . zi~c allog-coated steel and/or ~luminum i~ praparation for a painting or an application of adhesive, which postrinsing is effected by means of chromium-~ree aqueous solutions, and also relates to the use of such process for the preparation for a subsequent electro-dipcoating, particularly a cathodic electro-dipcoating, and for the postrinsing of metal surfaces provided with certain con- ~:

version layers.
The phosphating process is industrially used on a large scale to prepare metal surfaces for a subsequ~nt paintingO The resulting phosphate layers effect, inter alia, an improved adhesion of the paint films on the metal, they increase the resistance to cor:~sion ~nd reduce migration under paint due to .... . . . ... ..... .....

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locational defects in the paint film. Similar effects are produced by other conversion layers, parti-cularly by the chromate layers formed by colorless, yellow or green chromating processes, and by the layers formed by means of Ti- or Zr-based solutions. ~he protection afforded by such conversion layers can further be improved by an aqueous passivating postrinsing.
The passivating postrinsing age~ts based on hexavalent and/or trivalent chromium ha~e good properties from the aspect of application technology but the toxicity of the trivalent and particularly of the hexavalent chromium compounds is undesirableO
US-A-4,37~,000 discloses a chromium-free postrinsing agent based on polyvinylphenol but that agent must be used in a comparatively high concentration so that its use will result in an undesired pollution of the sewage and will particularly give rise to a high oxygen demand for its decomposition.
It is known from US-A-3,695,942 to use a soluble zirconium compound for an aftertret~tment of conversion layers. In addition to zirconium, the post-rinsing agents contain cations consisting of alkali and ammonium. An explicit warning has been made against the presence of cations of alkaline earth metals. ~hese post-rinsing agents are used at a pH value from 3 to 805 but will not produce the same quality as the chromium-' ,", , , 7 ,, ~ ?~1 containing agents.
US-A-3,895,970 desdribes acid aqueous postrinsing agents for phospha-te layers, which agents are based on simple or complex fluorides, and out of the group of the zirconium compounds mentions chromium-zirconium fluoride and zirconium fluoride. With the ex-ception of chromium-zirconium fluoride, the products ~ :
mentioned in that patent specification will meet only moderate requirements. But chromium-zirconium fluori~e has the above-mentioned disadvantage that it is toxic~
In accordance with an earlier proposal, phosphated metal surfaces are rinsed with aqueous solu-tions which contain aluminum fluorozirconat with an Al:
Zr:F mole ratio of (0.15 to 0.67) : 1 : (5 to 7) whereas the total concentration of Al + Zr + ~ is 0.1 to 2.0 g/l and the pH value is adjusted to 3 to 5. That postrinsing . .
distinguishes by resulting in a very high quality pro~e.
It is an object of the invention to provide for a passivatin~ postrinsing o~conversion layers on metals before a painting or before an application of adhesives a process which is free of the disadvantages of the known processes and results in a further improve-ment over the earlier proposal and distinguishes by resulting in a high corrosion resistance a~'in a very strong adhesion of paint and adhesive and will virtually not pollute the environment.

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~o accomplish that object the process of the kind first hereinbefore is carried out in such a manner in accordance with the invention in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution which contains Ce(IV) and/or Ce(III) in a total amount of 0.01 to 1.0 g/l and has a pH value from 3 -to 6.
The process in accordance with the in-vention call be applied to conversion layers of all kinds which can be formed on metals, particu~ ly on steel, galvanized and zinc alloy-coated steel, alumi-nized steel, æinc, zinc alloys, aluminum and aluminum alloys. These conversion layers include layers based on phosphate, such as the phosphates of zinc, iron, manganese, calcium7 zinc-mang~nese and zinc-calcium and other mixed products containing two or more diva-lent cations. It is particularlg suitable for phosphate layers formed by low-zinc phosphating processes with and without an addition of further cations, such as Mn, Ni, Co, Mg~
~ he process in accordnce with the invention may also be used to postrinse conversion layers v~hich by means of solutions based on Ti or Zr have been formed on surfaces of aluminum or aluminum alloys.Such solutions may con-tain additional components consisting, e.g., o~
fluorides, phosphates, boron compounds and optional .
.
, , passivating components, such as tannins. ~he process may also be used to aftertreat cllromate coatings, such as those of the kind mentioned hereinbefore, and of conversion layers which are formed, e.g., on zinc or zinc alloys by means of a solution that contains at le~st two different polyvalent metal ions and complex-ing agents and preferably has a pH value above 11.
The formation of the conversion layers is succeedéd by a rinsing with water before the after-treatment by the process in accordance with the inven-tion, which is carried out, e.g., by dipping, spraying, floodi~g or application by a roller is performed.
According to all test results the passi-vating postrinsing in accordance with the invention im-proves the resistance to corrosion at least to the same .
degree as a passivating pos-trinsing with the solutions which contain Cr(III) and/or Cr(VI) and which are known for their excellent effect.
According to a particularly preferred embodiment of the invention the metal surfaces provided with conversion la~ers are rinsed with an aqueous solu-tion having a Ce concentration of 0.05 to 0.15 g/l.
~ ccording to a preferred feature of the invention the metal surfaces provided with conversion layers are rinsed with a solution which contains Ce(III) and Ce(IV) in a weight rati.o of 9 to 2 parts Ce(III) to 1 part Ce(IV). The use of a mixture of Ce(III) and Ce(IV) within said range of weight ratios will improve the resistance to corrosi.on to a h.igher d~gl~ae than the use of the Ce eompound in only one degree of oxidation.
~ further preferred feature of the invention resides in that the metal surfaces provided with conversion layers are rinsed with a solution into which Ce(III) and/or Ce(IV) has been introduced as a nitrate and/or as a sulfate. It will also be desirable to introduce the Ce compound at least in part as a fluoride, hex2~1uorozi~conate, hexaEluorotitanat~, hexafluorosilicate, hexafluoroaluminate, tetrafluoro-borate or as a salt of carboxylie aeids, hydroxyearboxylic aeids and/or aminocarboxylic acids. Suitable carboxylic acids are, e.g., acetie acid, oxylic aeid and malonic acid.
Sui.table hydro~yearboxylic acids are, e.~., glyeolie acid, lactie acid, tartarie aeid, eitrie acid. Suitable amino acids are, e.g.1 nitrilotriaeetie acid and ethylene- :~
diaminetetraacetic acid. The addition of Ce in the form of compounds of the above-mentioned acids will improve the solubility, particularly of Ce(IV).
hccording to a further desirable fea-ture of the invention the metal surfaces provided wlth conversion layers are rinsed with an aqueous solution which addil;i.onall;y contains molybda-l;e and/or tungstat~.

Said compounds are particularly added in the form of their alkali salts.
The pH value of the postrinsing solu-tion is suitably adjusted with simple mineral acids, such as nitric acid or hydrofluoric acid. If additives which react alkaline are requirsd, it will be possible to use volatile bases, such as ammonia, ethanolamine, di- and triethanolamine. But according to a further de-sirable embodiment of the invention the pH value is adjusted with alkali hydroxides because alkali hydro-xides, particularly sodium hydroxides, have over volatile bases the advantage that a destabilisation o~ the postrinsing solutions will not occur.
The afterrinsing solution for use in the process in accordance with the invention is usu-ally prepared by a dilution of a concentra-te.
~ ccording to a fur-th-r desirable feature of the invention, the passivating postrinsing is suc-ceeded by a postrinsing with de-ionized water to remove any still adhering salts and/or salt solutions, which mi~ht adversely affect the adhesion of paint and the resistance -to corroslon.
The application of the passivating postrinsing solution to the metal surfaces provided with a conversion layer is carried out in a usual manner, e.g., by dipping, floodin~, s~aying and applicatiorl by ~ ~ ~ r~ 3 ~;

a roller. *he treating times lie between about 1 second and 2 minutes. The application temperature may lie bet-ween room temperature and about 80~. Temper~ures bet-ween 20 and 50 C are usually preferred. For t~ prepa-ration of -the postrinsing solutions it is preferable to use water which ~s the lowest possible salt content, pre`ferably de-ionized water. Water havin~ a hi~h salt content cannot be used to prepare a bath.
'~he process in accordance with the invention serves to prepare the metal surfaces coated with a conversion layers for painting or for an applica-tion of adhesives. It will improve the adhesion of the organic films, improve the resis~nce of the organic film against blistering under a corrosive action, and inhibit the subsurface corrosion from the locations of defects in the film. The process can be used for a prepa-ration for powder coating and paintin~ with low-solvent hight-solids paints and with paints comprising subst~ti-ally water as a solvent. A particularly desirabl~ use of the process in accordance with the invention is -the prepa-ration of metal surfaces provided with conversion layers for a subsequent elect,ro-dipcoating, particularly for a cathodic electro-dipcoating~
'~he process can be used to special advanta~e for the postrinsing of metal surfaces provided with conversion layers which consist of phos~ate layers or ~ ~ ~3 ~

of layers formed by a chromium-free, acid process using Zr and/or Ti~ F and optionally P04, or of complex metal oxide layers.
The process in accor~nce with the invention will be explained by way of example and more in detail hereinafter.

_XA~ E 1 In a manganese-modified low-zinc phosphating process, degreased sheets of steel~ electro-lytically galvanized steel and AlMgSi were sprayed at 55C for 2 minutes. rrhe phospha-ting solution had the following composition:

0.7 g/l Zn 0.04 g/l Fe(III) 1.0 g/l Mn 13 g/l Fe205 1.0 g/l Ni 2.1 g/l N0 2.9 g/l Na oO3 g/l F
0.15 g/l NH~ -7 g/b N02 .

Uni~ormly covering, finely crystalline phosphate layers having a wei~ht of 2.5 to ~ g/m3 were formed on the three metal substrates. The sheets were subsequently rinsed with water and were then subjected to a passivating postrinsing carried out by di~ping at 3~C for 1 minute. The final treatment consisted of a spraying with de-ionized water.
'~he sheets were painted with a cathodic electro-dipcoating primer, a filler and a top paint.

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paint -10-Each layer of the/coating was separately baked. The total -thickness of the paint coating was about 90 ~m.
~ The sheets were scratched to the metal substrate with a steel needle and were then subjected to various tests. The results have been compiled in ~e :
following Tables 1 to 3.
To prepare the postrinsing agent in ~ccor~ nce with the invention, 0.081 g Ce(III) (intro-duced as Ce(~03)3.6H20) and 0.020 g Ce(IV) (introduced as Ce(S04)2.4H20) per liter were dissolved. Ce(IV) was sta-bilized by an addition of 0.1 g/l fluoride in the form of HF to the bath. The pH value was subsequently ad-justed with ammonia -to 4.0 to 4O5 r~he slight turbidity resulting from the preparation of the bath solution does not affect the passivating properties of the postrinsing agent.
The solutions used for comparison con- ~ -tained:

1~ Cr(VI)/Cr(III) solution: 0.2 g/l CrO3 a~ :
0.0~7 g/l Cr(III) The pH value was ad~usted to 3.5 to 400.

2. Al-zr-F solution: 0.014 g/l Al 0.14 G/l Zr, and 0.17 g/l F.
The pH value was also adjusted -to 3.5 to 4Ø

r7 r j ~, --ll--From the tables it is apparent that as regards resistance to corrosion the results produced by the process in accordence with the invention are at least equivalent and in part even superior to those produced particu~rly by the Cr(VI)/Cr(III) solution, which is recognized for its strong effectO

2~ .3 TABIE 1 Results on STEEL
Postrinsing Agent Cr(~ III) Aluminum- Ce(N0 ) /Ce(SOL~)2 zirconium adjus~e~ with fluoride NH3 to pH
.
pH value 3,5_4.0 3-5_L~o L~.o-4.5 ~later of c(onden-~ation test conditions DIN 50017 ~
cross-cutting ` .
DIN 53151 (Note) Stone throw according to VW, 12 cycles Test under changing conditions -:

Subsurface corrosion, mm : ~ Flaking of paint, /a (1) (1) (1) :
~ ~-,:-: GM-Scab Test (20 cycles) G~l 9511P
Subsurface corrosion 9 m~ 2 2 2 :

:

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q'ABLE 2 Results on GA~VANIZED Sl'EEL
Postri~sing A~ent Cr~VI/-III) Aluminum- ae(N0 ) /Ce($o4)2 zirconium adjus~e~ with fluoride NH3 to pH

pH value 3O5_400 '3.5_4.o 4 o_4 5 '~'a-ter of conden~
sation test conditions DIN 50017 ~
cross-cutting DIN 53151 ~Note) Stone throw according to VW, 12 c~cles ~est under changing conditions Subsurface corrosion, mm 5 6 2-5 Flaking of paint, % (1-2) (1-2) (1-2) GM-Scab Test (20 cycles) GM 9511p Subsurface corrosion, mm 1... 2 1... 2 1~oo2 ~ $~3 TABLE 3 Results on ALUMINUM
Postrinsing Agent Cr(VI/-III) Aluminum- Ce(N0 ) /Ce(S~ 2 zirconium ad,jus~e~ with fluoride ~H3 to pH
.. . . .. . . _ pH Value 3,5_400 305-4.0 4.0-405 Water of conden-sation test conditions DIN 50017 +
cross-cutting :
DIN 53151 (Note) 0.5 005 o .
Stone throw according -. :.
to VW, 12 cycles Test under changing conditions '-:;
~DA 621-415 Subsurface corrosion, mm 0 0 0 Flaking of paint, % (1) (1) (1) GM-Scab Test (20 cycles) Subsurface corrosion1 mm 0 0 0 , ~ ~3~ 2 -~

EXAMPI,E 2 Cleaned a~ degreased aluminum sheets to be provided with a conversion coating were dipped for lO seconds into a so].ution which was at a tempe.rat;ure o~
50C and had the following composition:

Ti O.lr~ g/l F lo24 g/l P205 0~09 ~/l N~I4 o9l g/l rTannin 0,11 g/l Na 0.003 g/l Biocide O.lO g/l The sheets were subseque~tly rinsed with water and subjected to a passivating postrin-lngO For that purpose the sheets were dipped into the postrinsing solution at 35C for 5 seconds and were subsequently squeegeed to remove surplus solution. After the sheets had been dried in a recirculating-air oven at 60C for 005 minute they were painted with a foodstuff-compatible two-layer paint t which in its first layer cons;sted of an epoxide-phenol re~in an-~n the second layer cGnsi^ted of an organosol. The to-tal thickness of the paint coati.ng was between lO and 15 ~mO
The treated sheets havinK a thickness of about 0.25 mnl were subse~;uen-tly punched tc lorm round disks 60 mm in diameter and by deep dr.lwing were formed to CUp9, which were 26 mm in diameter and had a he~ht of ' `:

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25 mmO-Said cups were subjected to a steri-li~ing -test, in which they were subjected in a pressure container for 40 minutes to the action of an aqueous solution of 3% common salt, 1,~ citric acid and 0.5%
lactic acid at 121C. The defects subsequently detected at the cups (spalling of paint, blisters) were rated in accordance with a scale ranging from 1 (paint spalled throughout the shell of -the cup = unusable) to 15 (no paint defects = excellent).
The postrinsing solution (A) which was employed contained 0.110 g/l Ce(III) (introduced as -Ce(N03)3.6H20) and 0.020 g/l Ce(IV) (introduced as Ce(S043204H20). The pH value had been adjusted to 4.0 to 4-5.
For comparison, postrinsing was also effected with an aqueous solution of 0.6 g/l polyvinyl~
phenol at a pH value of about 5 (postrinsing solution B) and with a solution which contained 0.014 gjl Al, 0.14 g/l Zr, 0.17 g/l F and 0.016 g/l r~4 and had a pE value of 3.5 to 4.0 (postrinsing solution C).

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Test results of the sterilizing test (Rating scale: 1 = unusable to 15 - excellent) Postrinsing solu~ion Ratirlg . _ . .. . . . . .
A (invention) 12-13 B (control) 6 C (control) 11 . .

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A comparison of the tabulated data shows that the process i~ accordance with the invention results in distinctly better values, particularly when compared with the control example in which a pos-trinsing solution based on polyvinylphenol was employed.

X~LE 3 Cleaned and degreased sheets of hot-galvanized steel were provided with a conversion coating in that they were dipped for 30 seconds into a solution which was at a temperature of 55C and had the following composition:

ao2+ 0.3 g/l Fe~+ 0.2 g/l N03- 1.3 g/l Sodium salt of hexahydrohepta-noic acid 202 g/l - ;
~aOH 27.4 g/l l'he sheets were subsequently rinsed with water and subjected to a passivating postrinsing.
For that purpose the sheets were dipped into the post-rinsing solution at ~C for 5 seconds and were subse-quently squeegeed to remove surplus solution. ~fter a drying at 75C in a recirculating-air oven for 0.5 minute the pre-treated sheets were painted with an epoxide primer and with an acrylate -top coat~ ~he total thickness of the paint coating amounted to about 25 ~um.
The treated sheets were subsequently sub-jected to the following tests:
The adhesion of the paint ~as determined in the ~ bend test, in which the sheets were bent through 180. ~he various radii of curvature were stated as n times the thickness of the sheet metal (n = O, 1, 2 ...) (Tn). The test result is stated as the percentage o~ the area of the flaked paint in the entire curved surfaceO
were Other treated sheets/provided by means of a sheet me-tal needle with scratches extending to the metal substrate and by means of sheet metal shears were formed with a cut edge. The sheets were subsequently subjected to the salt spray test in accordance with DIN 50021 SS for 1008 hours. The test result is stated as the subsurface corrosion under the paint in mm from the scratch or the cut edge.
he postrinsing solution (1~) employed in accordance with the inven-tion contained 0.110 ~/1 Ce(III) (introduced as Ce(N03)3~6ll20) and 0.020 g/l Ce(IV) (introduced as Ce(S04)2~4H20) and had a pH value of 4.0 to 4.50 For comparison, a postrinsin~ solution (B) was ernployed, which contained 0.014 g/1 Al, 0.14 ~/1 Zr, 0.17 g/l F and 0.016 ~ ~4 and had a pH value ~f 3.5 .

r~ J ~ -to 4.0~ and a postrinsing solution (C), ~hich contained 2.0 g/l Cr(VI), 0~8 g/l Cr(IIIj, 0.2 g/l F, and 0.3 g/l Zn.
The test resu].ts are stated in ~e following tables.
Adhesion of Paint in ~-bent test Flaked area (,0) in case of a Pos~trinsing solution radius of curvature Tn ,.
~1 ~2 T3 T4 : A (invention) 100 5 10 ~5 B (control) 100 55 15 5 ~ C (control) 100 80 30 5 :

';
Resistance to corrosion in salt spray test , postrinsing sol~utionSubsurface corroslon (mm)a ter ~-.. _ ~t the scr~tch At the edge A (invention) 0 - 1 6 - 8 B (control) <1 - 1 8 - 9 C (control) 1 - 3 9 - 10 A comparison of the tabulated data reveals that in each case the process in accordance with the invention gives at least equivalent or improved results, particularly when compared with the con-trol process in which a postrinsing solution based on Cr(VI)/Cr(III) ~20- .
was employed, which is recognized for its strong effect as re~;ards the resi~tance to corrosionO

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A process for the passivating postrinsing of conversion layers on metal surfaces, which postrinsing is effected by means of chromium-free aqueous solutions, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution which contains Ce(IV) or Ce(III) in a total amount of 0.01 -to 1.0 g/l and has a pH value from 3 to 6.
2. A process according to claim 1, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution having a Ce concentration of 0.05 to 0.15 g/l.
3. A process according to claim 1, characterized in that the metal surfaces provided with conversion layers are rinsed with a solution which contains Ce(III) and Ce(IV) in a weight ratio of 9 to 2 parts Ce(III) to 1 part Ce(IV).
4. A process according to claim 1, characterized in that the metal surfaces provided with conversion layers are rinsed with a solution into which Ce(III) or Ce(IV) has been introduced as a nitrate or as a sulfate.
5. A process according to claim 1, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution in which Ce(IV) or Ce(III) has been introduced at least in part as a fluoride, hexafluorozirconate, hexafluorotitate, hexafluorosilicate, hexafluoroaluminate, tetrafluoroborate or as a salt of carboxylic acids, hydroxycarboxylic acids or aminocarboxylic acids.
6. A process according to claim 1, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution which additionally contains molybdate or tungstate.
7. A process according to claim 1, characterized in that the metal surfaces provided with convers ion layers are rinsed with an aqueous solution the pH value of which has been adjusted with simple mineral acids or with alkali hydroxides.
8. A process according to claim 1, 2, 3, 4, 5, 6 or 7, characterized in that the passivating postrinsing is succeeded by a postrinsing with de-ionized water.
9. A process for the passivating postrinsing of conversion layers on metal surfaces, which postrinsing is effected by means of chromium-free aqueous solutions, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution which contains Ce(IV) and Ce(III) in a total amount of 0.01 to 1.0 g/l and has a pH value from 3 to 6.
10. A process according to claim 1, characterized in that the metal surfaces provided with conversion layers are rinsed with a solution into which Ce(III) and Ce(IV) has been introduced as a nitrate and as a sulfate.
11. A process according to claim 1, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution in which Ce(IV) and Ce(III) has been introduced at least in part as a fluoride, hexafluorozirconate, hexafluorotitate, hexafluorosilicate, hexafluoroaluminate, tetrafluoroborate or as a salt of carboxylic acids, hydroxycarboxylic acids and amino-carboxylic acids.
12. A process according to claim 1, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution which additionally contains molybdate and tungstate.
13. A process according to claim 7, characterized in that the mineral acids are selected from nitric acid or hydrofluoric acid and in that said alkali hydroxides are sodium hydroxide.
14. A process according to claim 1, characterized in that the metal surfaces provided with convers ion layers are rinsed with an aqueous solution the pH value of which has been adjusted with nitric acid, hydrofluoric acid, or sodium hydroxide.
15. A process according to claim 9, 10, 11, 12, 13, 14 or 15, characterized in that the passivating postrinsing is succeeded by a postrinsing with de-ionized water.
16. A process according to claim 1, 2, 3, 4, 5, 6, 7, 9, 10, 11, 12, 13 or 14, characterized in that said metal surfaces are of steel, galvanized steel, zinc alloy-coated steel or aluminum in preparation for a painting or an application of adhesive.
17. The use of the process according to claim 1, 2, 3, 4, 5, 6, 7, 9, 10, 11, 12, 13 or 14, for the preparation of metal surfaces provided with conversion layers for a succeeding electro-dipcoating.
18. The use of the process according to claim 1, 2, 3, 4, 5, 6, 7, 9, 10, 11, 12, 13 or 14, for the postrinsing of metal surfaces provided with conversion layers which consist of phosphate layers or of layers formed by a chromium-free, acid process using Zr or Ti, F or of complex metal oxide layers.
19. The use of the process according to claim 17, characterized in that said electro-dipcoating is a cathodic electro-dipcoating.
20. The use of the process according to claim 18, for the postrinsing of metal surfaces provided with conversion layers which consist of phosphate layers or of layers formed by a chromium-free, acid process using Zr and Ti, F, PO4, or of complex metal oxide layers.