WO2005038095A2

WO2005038095A2 - Electrolytic method for phosphating metal surfaces and phosphated metal layer

Info

Publication number: WO2005038095A2
Application number: PCT/EP2004/052269
Authority: WO
Inventors: Juergen Hackenberg; Dirk Zimmermann
Original assignee: Robert Bosch Gmbh
Priority date: 2003-10-16
Filing date: 2004-09-22
Publication date: 2005-04-28
Also published as: BRPI0415520A; DE10348251A1; EP1675975A2; CN1867704A; TR200601814T1; CN1867704B; WO2005038095A3; JP2007508457A; US20070295608A1

Abstract

The invention relates to a method for phosphating metal layers by electrodeposition from acidic aqueous solutions containing at least zinc ions and phosphate ions, and by using direct current. Electrodeposition of zinc takes place in the same electrolytes and at the same time as deposition of the phosphating layer. The current density lies in the region of > 5 A/dmEs.

Description

Electrolytic process for phosphating metal surfaces and thus phosphated metal layer

The invention relates generally to an electrolytic method for phosphating metal surfaces according to the preamble of claim 1. It also relates to a phosphated metal layer produced using this method.

State of the art

Zinc phosphating is a widely used process for the corrosion protection of low-alloy steels. Here, zinc phosphate crystals (Hopeit) are deposited on the component surface in a pH-controlled precipitation reaction. To one

To be able to deposit the phosphate layer, the solubility product of the zinc phosphate must be exceeded. This is done by attacking (pickling) the base metal (eg Fe -> Fe ²⁺ +2 ^{e "} ). The electrons released in the process serve to reduce protons. The pH is shifted towards neutral to basic and the solubility product is exceeded. They form Usually layers with a thickness of 2-3 μm are formed, which have a degree of coverage of approximately 90 to 95% The corrosion protection is limited by the thin and porous layer that forms and is therefore usually combined with other coatings, for example corrosion protection oil or KTL Further developments aim to increase corrosion protection without such additional coatings.

One approach to achieving thicker layers is the use of electrolysis current. The electrolysis reaction leads to an influenceable pH shift and thus to the regulation of the layer growth.

JP-A-85/211080 relates to a method for producing corrosion protection layers Metal surfaces with the help of zinc phosphating solutions with temporary use of a cathodic current. Here, in particular, a corrosion-resistant protective layer is also produced on the edges of the metal surfaces to be treated.

A similar process is disclosed in EP-A-0 171 790. Here, the metal surfaces are treated with an acidic, aqueous solution which contains zinc, phosphate and chlorine ions following a customary zinc phosphatization, contacting the anodically switched metal surfaces applies a direct current at the same time.

DE-41 11 186 AI discloses a method for phosphating metal surfaces, preferably electrolytically or hot-dip galvanized steel strip surfaces, by treating in immersion or spray-immersion with acidic, aqueous phosphating solutions, the workpieces being simultaneously treated cathodically with a direct current. This involves working with phosphating solutions which contain the following components: Zn ²⁺ cations in the range from 0.1 to 5 g / 1; PO ^{3 "} anions in the range from 5 to 50 g / 1; NO ₃ ^~ anions in the range from 0.1 to 50 g / 1; and Ni ²⁺ cations in the range from 0.1 to 5 g / 1, and / or Co ²⁺ cations in the range from 0.1 to 5 g 1. The pH of the phosphating solutions is in the range from 1.5 to 4.5, and the temperature of the phosphating solutions is in the range from 10 to 80 ° C. The current density of the direct current with which the workpieces are treated cathodically during phosphating is between 0.01 and 100 mA / cm ² .

A disadvantage of the conventional phosphating processes is that they are limited to low-alloy steels, as well as Zn and Al, and that the layers produced do not have cathodic corrosion protection due to the fact that they consist of zinc phosphate crystals. In addition, an upstream activation is necessary in most cases.

Advantages of the invention The method according to the invention for phosphating metal surfaces has the advantage over the prior art that compact layers are formed whose thicknesses can be adjusted almost as desired.

Another advantage is that the layers produced have a significantly higher corrosion resistance.

It is also advantageous that the phosphating can be carried out without activation.

Advantageous developments of the invention result from the measures mentioned in the subclaims.

For example, the electrolysis can be carried out either potentiostatically or galvanostatically, or with a mixture of the two.

Brief description of the drawing

An exemplary embodiment of the invention is shown in the drawing and explained in more detail in the following description. The figure shows a schematic diagram of the manufacturing method according to the invention.

embodiments

The aim of the present invention is to develop an electrolytic coating method for phosphating metal surfaces, in which the pores in the phosphate layer are filled with metallic zinc or zinc alloys. In the method according to the invention, the electrolytic zinc or zinc alloy deposition takes place simultaneously with the zinc phosphate crystal formation in the same electrolyte. The process according to the invention comes in contrast to conventional phosphating, in which after cleaning or pickling the workpiece was immersed in a titanium phosphate suspension (approx. 60 s at pH ~ 9) without an upstream activation process. The layer formation rate is around 3 to 20 μm / min Current densities from j = -10 to -50A / dm ² extremely fast. With the conventional methods used up to now, deposition was only at approx. 1 μm / min. In addition to low-alloy steels, the described process can also be used to directly coat stainless steels and other noble and base materials such as Al, Al alloys, Cu, Cu alloys, Ni, Ni alloys, etc. In the case of currentless processes, on the other hand, deposition can only be carried out on materials which permit a corrosive pickling attack, since otherwise the required pH shift described above cannot occur. The electrolysis can be controlled both potentiostatically and galvanostatically, or can be carried out with a mixture of the two parts.

In the method according to the invention, compact layers are formed, which are distinguished in that the spaces between the zinc phosphate crystals are filled up by a network of metallically deposited zinc or zinc alloy. Due to the simultaneous formation of the electrically conductive zinc or the zinc alloy, a pH shift induced by electrolysis can take place, i.e. the electrons are supplied from the outside, and an almost arbitrary layer thickness increase

Zinc (zinc alloy) / zinc phosphate layer can be achieved by reducing H * on the zinc surface.

The figure shows a schematic diagram of the coating method according to the invention. The zinc / zinc phosphate layer 14 is deposited on the base metal 11 in a conventional electrolysis cell 10 with a working electrode 11 made of the corresponding base metal and a counter electrode 12 through the electrolyte 13. As already indicated, in contrast to standard phosphating, the electrons required for pH shifting do not come from iron corrosion from the low-alloy steels (pickling attack on the base metal), but from an external power source 15. This protective current ensures, among other things, that the Base metal 11 is not attacked.

With the aid of the method according to the invention, closed, that is to say largely non-porous, mixed layers (with metal, for example zinc, zinc / nickel, filled phosphate layers without exposed base metal) of approximately 3 μm to approximately 500 μm can be deposited practically indefinitely. While a conventionally produced layer in Salt spray test shows a resistance of approx. 5 hours until red rust formation, with a 20-30 μm zinc / zinc phosphate mixed layer a resistance of more than 1000 hours could be achieved in the salt spray test. Even after a phosphating time of 30 seconds, the corrosion resistance is above 420 hours.

The coating method according to the present invention can be carried out in commonly used electrolytic cells. The counter electrode can consist of noble sheets such as platinum, Pt / Ti or gold, as well as less noble sacrificial anodes such as Zn, Ni, Fe, which ensure continuous transport of metal ions. As working electrodes on which the layers are deposited (base metal), stainless steels as well as bronze, Cu, Cu alloys, Ni, Ni

Alloys, etc. can be used. The electrolyte is essentially an electrolyte of the kind used in phosphating without external current. The electrolyte contains, for example: Zn ²⁺ : 5-50 g / 1 H ₂ PO ₄ ^" : 5-80 g / 1

In this context, it is important that a so-called high-zinc bath is used, the zinc content of which is above 5 g / 1, while in the normally conventional low-zinc baths the zinc content is only around 1 - 5 g / 1, with no elemental zinc or Zinc alloy deposition comes between the phosphate crystals.

In addition, the electrolyte can contain ions from elements which can form an alloy with zinc, so that when the phosphating layer is deposited, zinc alloys are deposited simultaneously. The addition of nanoparticles and organic molecules is also conceivable. Other possible bath additives for modifying the layers are polyphosphates, borates, organic polyhydroxy compounds, glycerophosphates and fluorides. The additional ions can be, for example, ions of a divalent metal M, the further divalent metal M being selected from the group consisting of Ni, Fe, Co, Cu, Mn and the like. The reaction can be carried out with or without the addition of an accelerator. Examples of suitable accelerators are urea, nitrates, nitrites, chlorates, bromates, hydrogen peroxide, ozone, organic nitro bodies, peroxy compounds, hydroxylamine or mixtures thereof. Nitrate ions in the range of 0-20 g / l are advantageous as accelerators. The pH of the bath is between 1.5 and 4, preferably between 2.5 and 3.5. Binary, ternary or even higher alloys can be deposited by adding Zn, Ni, Co, Fe or Mn salts. The metal ions can also be supplied to the electrolyte by anodic dissolution.

Concerning. Under the electrolysis conditions, the electrolyte can either rest or be moved during the process. The current densities are> -1 A / dm ² and preferably range from approximately j = -1 to approximately j = -100A / dm ² . Current densities in the range from -5 to -50 A / dm ² are particularly preferred. The temperature of the electrolyte is> 40 ° C and is preferably in the range between 40 and 80 ° C, particularly preferably between 60 and 70 ° C.

As already mentioned, the electrolysis process can be controlled both potentiostatically and galvanostatically, whereby either direct current or pulsed direct current can be used. The layer thickness distribution can be regulated by controlling the local current density, i.e. by shaping and / or current orifices between the anode and the workpiece. In this way, geometrically more demanding parts can also be coated.

Claims

Expectations

1. A method for phosphating metal layers by electrolytic deposition from acidic aqueous solutions which contain at least zinc ions and phosphate ions, with simultaneous use of direct current, characterized in that electrolytic deposition of zinc takes place in the same electrolyte simultaneously with the deposition of the phosphating layer, the Current density is greater than -5 A / dm ² .

2. The method according to claim 1, characterized in that the current density is in the range from -5 to -50 A / dm ² .

3. The method according to claim 1 or 2, characterized in that the temperature is in the range> 40 ° C and preferably between 40 and 80 ° C, in particular between 60 and 70 ° C.

4. The method according to any one of claims 1 to 3, characterized in that the electrolyte zinc ions in the range of> 5 g / 1, in particular in the range of 5-50 g / 1, and phosphate ions in the range> 10 g / 1, in particular in the Contains range of 10-80 g / 1.

5. The method according to any one of claims 1 to 4, characterized in that the acidic aqueous solutions additionally contain ions of elements which can form an alloy with zinc, so that zinc and / or zinc alloys are deposited simultaneously during the deposition of the phosphating layer ,

6. The method according to claim 5, characterized in that nanoparticles or organic molecules are used instead of the ions.

7. The method according to claim 5, characterized in that the additional ions are ions of a divalent metal M.

8. The method according to claim 7, characterized in that the further divalent metal M is selected from the group consisting of Ni, Fe, Co, Cu, Mn and the like.

9. The method according to any one of the preceding claims, characterized in that the metal layers are selected from the group consisting of stainless steel, bronze, Al,

Al alloys, Cu, Cu alloys, Ni, Ni alloys, etc.

10. The method according to any one of the preceding claims, characterized in that the pH of the electrolyte is between approximately 1.5 and approximately 4, preferably between approximately 2.5 and approximately 3.5.

11. The method according to any one of the preceding claims, characterized in that an accelerator is added to the electrolyte.

12. The method according to claim 11, characterized in that the accelerator is selected from the group consisting of urea, nitrates, nitrites, chlorates, bromates, hydrogen peroxide, ozone, organic nitro bodies, peroxy compounds, hydroxylamine or mixtures thereof.

13. The method according to claim 7, characterized in that the metal ions of the divalent metal M are supplied by anodic dissolution of the electrolyte.

14. The method according to any one of the preceding claims, characterized in that Zn, Ni, Co and / or Mn salts are additionally added to the electrolyte.

15. The method according to any one of the preceding claims, characterized in that the electrolyte has the following composition:

Zn ²⁺ 5-40 g / 1; M ²⁺ 0.5-10 g / 1; H ₂ PO ₄ ^" 10-40; and NO ₃ ^" 1-10 g / 1

16. The method according to any one of the preceding claims, characterized in that the electrolysis is carried out either potentiostatically or galvanostatically or with a mixture of both parts.

17. The method according to any one of the preceding claims, characterized in that the layer thickness distribution on the metal layers is regulated by the local current density.

18. The method according to any one of the preceding claims, characterized in that the direct current is pulsed.

19. The method according to any one of the preceding claims, characterized in that the layer formation rate is in the range of about 3 to about 20 microns / minute.

20. Metal layer with a porous zinc phosphate layer deposited thereon, characterized in that the pores of the zinc phosphate layer are filled with metallic zinc and / or zinc alloy.