CN1867704B - Electrolytic method for phosphating metal surfaces and metal layers thus phosphated - Google Patents

Abstract

The present invention relates to a method for phosphatizing a metal layer by electrolytic deposition of an acidic aqueous solution containing at least zinc ions and phosphate ions while simultaneously using direct current. Electrolytic deposition of zinc occurs simultaneously with the deposition of the phosphated layer in the same electrolyte. The current density is in the range of greater than -5 A/dmEs.

Description

Electrolytic method for phosphating metal surfaces and metal layers thus phosphated

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

Background technique

Zinc phosphating is a method widely used for corrosion protection of low alloy steels. In this case, zinc phosphate crystals (zincite) are deposited on the component surface in a pH-controlled precipitation reaction. In order to be able to deposit a phosphate layer, the solubility product of zinc phosphate must be exceeded. This is achieved by etching (pickling) the base metal (eg Fe>Fe ²⁺ +2 ^e− ). The electrons thus released are used to reduce protons. The pH shifts toward neutral to basic and exceeds the solubility product. This typically results in a 2-3 μm thick layer with a coverage of about 90-95%. Anti-corrosion is limited to the thin porous layer produced and is therefore often combined with other coatings such as anti-corrosion oil or KTL. Further research is aimed at improving corrosion protection while avoiding these additional coatings.

One technical solution for obtaining thicker layers is to use electrolytic current. An easily adjustable pH shift and thus adjustable layer growth is obtained by the electrolytic reaction.

JP-A-85/211080 relates to a method for producing an anti-corrosion coating on metal surfaces by means of a zinc phosphating solution with intermittent application of a cathodic current. In this case, a corrosion-resistant protective layer is also obtained, especially at the edges of the metal surfaces to be treated.

EP-A-0171790 discloses a similar method. Wherein the metal surface is treated with an acidic aqueous solution containing zinc ions, phosphate ions and chloride ions after conventional zinc phosphating, wherein a direct current is simultaneously applied to the anodized metal surface.

Known from DE-4111186A1 is a method for the phosphating of metal surfaces, preferably for the phosphating of electrolytically galvanized or melt dip galvanized steel sheet surfaces, by using Acidic aqueous phosphating solution treatment is carried out, wherein the workpiece is simultaneously anodized with direct current. wherein the treatment is carried out by means of a phosphating solution comprising the following components: 0.1-5 g/l Zn ²⁺ cation; 5-50 g/l PO ₄ ^3- anion; 0.1-50 g/l NO ₃ ^- Anions; 0.1-5 g/l of Ni ²⁺ cations and/or 0.1-5 g/l of Co ²⁺ cations. The pH value of the phosphating solution is 1.5-4.5, and the temperature of the phosphating solution is 10-80°C. The direct current used to anodize the workpiece during phosphating has a current density of 0.01-100 mA/cm ² .

A disadvantage of the conventional phosphating method is that it is limited to low-alloy steels as well as Zn and Al, and the resulting layer is not cathodic corrosion resistant because it is composed of zinc phosphate crystals. Additionally, pre-activation is required in most cases.

Advantages of the invention

Compared with the prior art, the method according to the invention for the phosphatization of metal surfaces has the advantage that a dense layer of almost arbitrarily adjustable thickness is formed.

Another advantage resides in the significantly higher corrosion resistance of the resulting layer.

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

Further advantageous developments of the invention are given by the measures mentioned in the dependent claims.

In this way, electrolysis can be performed, for example, at constant voltage or constant current or a mixture of the two.

Brief description of the drawings

Embodiments of the invention are depicted in the drawings and described in more detail in the description that follows. The accompanying drawing shows a schematic diagram of the preparation method of the present invention.

Example

The object of the present invention is to develop an electrolytic coating method for phosphated metal surfaces in which the pores in the phosphate layer are filled with metallic zinc or a zinc alloy. According to the method of the invention, the electrolytic deposition of zinc or zinc alloys is carried out in the same electrolyte simultaneously with the formation of zinc phosphate crystals. In contrast to conventional phosphating processes in which the workpiece is dipped in a titanium phosphate suspension (approx. 60 seconds at pH ~ 9) after cleaning or pickling, the process of the present invention does not require preactivation process. The layer formation speed is about 3-20 μm/min at the current density of j=-10--50 A/dm ² , which is very fast. In conventional methods used hitherto, deposition takes place only at approximately 1 μm/min. Using the methods described above, stainless steel as well as other noble and non-noble metal materials (eg Al, Al alloys, Cu, Cu alloys, Ni, Ni alloys, etc.) can be directly coated in addition to low alloy steels. In contrast to this, the electroless method can only be deposited on materials which are permissible for pickling attack, since otherwise the above-mentioned required pH shift cannot be achieved. The electrolysis can be regulated both at constant voltage and at constant current, or a mixture of these two parts can be carried out.

A dense layer is formed by the method according to the invention, which layer is characterized in that the spaces between the zinc phosphate crystals are filled by a metal-deposited zinc or zinc alloy network. Electrolysis-induced pH shift (i.e. electrons supplied from outside) can occur by simultaneous formation of conductive Zn or Zn-alloy and almost arbitrary layering of Zn(Zn-alloy)/Zn-phosphate layer by reduction of H ⁺ at the Zn surface thick growth.

The drawing shows a schematic diagram of the coating method according to the invention. Therein, a zinc/zinc phosphate layer 14 is deposited on a base metal 11 via an electrolyte 13 in a conventional electrolytic cell 10 with a working electrode 11 consisting of the corresponding base metal and a counter electrode 12 . As mentioned above, in contrast to standard phosphating, the electrons required for the pH shift here do not come from ferrocorrosion of the low alloy steel (pickling attack on the base metal), but from an external current source 15 . In particular, this protective current also serves to protect base metal 11 from corrosion.

Closed (ie as non-porous as possible) mixed layers of approximately 3 μm to approximately 500 μm (phosphate layers without bare base metal filled with metals such as zinc, zinc/nickel) can be deposited with the aid of the method according to the invention. Conventionally obtained layers have a resistance to rust formation in the salt spray test of approximately 5 hours, whereas a resistance of more than 1000 hours in the salt spray test can be achieved with a 20-30 μm zinc/zinc phosphate mixed layer. A corrosion resistance of more than 420 hours is already achieved after a phosphating time of 30 seconds.

The coating method according to the invention can be carried out in conventionally used electrolytic cells. The counter electrode can be composed of noble metal plates such as platinum, Pt/Ti or gold, or non-noble metal sacrificial anodes such as Zn, Ni, Fe, etc. The counter electrode is used for continuous transport of metal ions. As the working electrode (base metal) on which the layer is deposited, stainless steel as well as bronze, Cu, Cu alloy, Ni, Ni alloy, etc. can be used. The electrolytic solution is basically an electrolytic solution used for phosphating applications without external current. Wherein the electrolyte may for example contain:

Zn ²⁺ : 5-50g/l

H ₂ PO ₄ ^- : 5-80g/l

For this it is important to use so-called high zinc baths (Hochzinkbad) with a zinc content of more than 5 g/l, whereas in normal ordinary low zinc baths the zinc content is only about 1-5 g/l, which does not lead to Deposition of elemental zinc or zinc alloys between salt crystals.

In addition, the electrolytic solution may contain ions of an element capable of forming an alloy with zinc, so that the zinc alloy is deposited simultaneously with the deposition of the phosphating layer. Also conceivable is the addition of nanoparticles and organic molecules. Other possible bath additives for modifying the layers are polyphosphates, borates, organic polyols, phosphoglycerides and fluorides. For example, ions of divalent metals M can be considered as additional ions, wherein the further divalent metals M are selected from Ni, Fe, Co, Cu, Mn and the like.

The reaction can be carried out with or without the addition of accelerators. Possible accelerators include, for example, urea, nitrates, nitrites, chlorates, bromates, hydrogen peroxide, ozone, organic nitro compounds, peroxides, hydroxylamines or mixtures thereof. Nitrate ions in the range of 0-20 g/l are advantageous as accelerators. The pH of the bath is 1.5-4, preferably 2.5-3.5. Binary alloys, ternary alloys or higher order alloys can be deposited by adding Zn salts, Ni salts, Co salts, Fe salts or Mn salts. Metal ions can also be supplied to the electrolyte by dissolution from the anode.

In terms of electrolysis conditions, the electrolyte can be static or flowing in the process. The current density is greater than -1 A/dm ² and preferably between about j=-1 to about j=-100 A/dm ² . It is especially preferred that the current density is between -5 and -50A/dm ² . The temperature of the electrolyte is greater than 40°C, preferably 40-80°C, especially preferably 60-70°C.

As already mentioned above, the electrolysis process can be controlled both at constant voltage and at constant current, it being possible to use direct current or pulsed direct current. By adjusting the local current density, ie through shaping and/or mixing between the anode and the workpiece, the layer thickness distribution can be adjusted. Geometrically demanding components can also be coated in this way.

Claims (17)

1. the method for phosphated metal layer; It carries out through using under the galvanic situation electrolytic deposition by acidic aqueous solution at the same time; This acidic aqueous solution contains zine ion and phosphate anion at least; Said method is characterised in that, in same electrolytic solution, in deposition phosphatize layer, carries out the electrolytic deposition of zinc, and wherein current density is greater than-5A/dm ², and said acidic aqueous solution also contains nano particle or organic molecule.

2. the method for claim 1 is characterized in that, temperature is greater than 40 ℃.

3. the method for claim 2 is characterized in that, temperature be greater than 40 ℃～smaller or equal to 80 ℃.

4. the method for claim 2 is characterized in that, temperature is 60～70 ℃.

5. the method for claim 1 is characterized in that, the contained zine ion of said electrolytic solution is greater than 5g/l, and institute's phosphorus-containing acid ion is greater than 10g/l.

6. the method for claim 5 is characterized in that, the contained zine ion of said electrolytic solution is smaller or equal to 50g/l greater than 5g/l-.

7. the method for claim 5 is characterized in that, said electrolytic solution institute phosphorus-containing acid ion is smaller or equal to 80g/l greater than 10g/l-.

8. the method for claim 1 is characterized in that, said metal level is selected from stainless steel, Al, Al alloy, Cu, Cu alloy, Ni and Ni alloy.

9. the method for claim 1 is characterized in that, the pH value of said electrolytic solution is 1.5～4.

10. the method for claim 9 is characterized in that, the pH value of said electrolytic solution is 2.5～3.5.

11. the method for claim 1 is characterized in that, in said electrolytic solution, adds accelerator.

12. the method for claim 11 is characterized in that, said accelerator is selected from urea, nitrate salt, nitrite, oxymuriate, bromate, ozone, organic nitro-body, superoxide, oxyamine or their mixture.

13. the method for claim 1 is characterized in that, the salt of extra interpolation Zn salt, Ni salt, Co salt and/or Mn in electrolytic solution.

14. the method for claim 1 is characterized in that, said electrolysis constant voltage or constant current or two portions mixing are carried out.

15. the method for claim 1 is characterized in that, the layer thickness distribution on metal level is regulated through local current densities.

16. the method for claim 1 is characterized in that, said direct current is pulse.

17. the method for claim 1 is characterized in that, layer formation speed is 3-20 μ m/ minute.