JP2007508457A - Electrolytic method for phosphating metal surfaces and phosphated metal layers thereby - Google Patents

Abstract

The present invention relates to a method for phosphating a metal layer by electrodepositing simultaneously using direct current from an acidic aqueous solution containing at least zinc ions and phosphate ions. Simultaneously with the deposition of the phosphating layer, electrodeposition of zinc occurs in the same electrolyte. In this case, the current density is in the range of> -5A / dm ^2.

Description

  The present invention generally relates to an electrolysis process for phosphating a metal surface according to the superordinate concept of claim 1. The invention also relates to a phosphatized metal layer produced using the method.

Prior Art Zinc phosphating is a well-known method for corrosion protection of low alloy steels. In this case, zinc phosphate crystals (hopeite) are deposited on the surface of the structural member in a pH-adjusted precipitation reaction. In order to deposit the phosphate layer, the solubility product of zinc phosphate must be exceeded. This is done by attack (corrosion) on the base metal (for example, Fe → Fe 2 ⁺ 2 ^e− ). The electrons released at this time are used for proton reduction. The pH shifts from neutral to basic and exceeds the solubility product. In this case, a layer usually having a thickness of 2 to 3 μm is formed, which has a coverage of about 90 to 95%. Since corrosion protection is limited by the resulting thin and porous layer, it is usually combined with another coating, such as corrosion prevention oil or KTL. Further research is aimed at improving corrosion protection while omitting such additional coatings.

  The starting point for achieving a thicker layer is the use of an electrolytic current. The electrolytic reaction results in a sensitive pH shift and concomitant adjustment of layer growth.

  JP-A-85 / 211080 relates to a method for producing a corrosion protection layer on a metal surface by applying a cathodic current temporarily using a zinc-phosphate treatment solution. In this case, a corrosion-resistant protective layer is produced even at the edge of the metal surface to be treated.

  A similar method is disclosed in EP-A-0171790. In this case, the metal surface is treated with an acidic aqueous solution containing zinc ions, phosphate ions and chlorine ions, followed by a conventional zinc-phosphate treatment, simultaneously on the anode-connected metal surface. A direct current is applied.

A method for phosphating from DE 4111186A1 by treating a metal surface, preferably a surface of an electroplated or hot-plated steel strip, by dipping or spraying with an acidic phosphating solution. In which the unfinished product is simultaneously treated with direct current at the cathode. In this case, it is treated with a phosphating solution containing the following components: Zn ²⁺ cation in the range 0.1-5 g / l; PO ₄ ³⁻ anion in the range 5-50 g / l; NO ₃ ⁻ Within the range of 0.1-50 g / l of anions; and within the range of 0.1-5 g / l of Ni ²⁺ cations and / or within the range of 0.1-5 g / l of Co ²⁺ cations. In this case, the pH value of the phosphating solution is in the range of 1.5 to 4.5, and the temperature of the phosphating solution is in the range of 10 to 80 ° C. The DC current density for cathodic treatment of the unfinished product during phosphating is 0.01-100 mA / cm ² .

  The disadvantages of the conventional phosphating method are based on the fact that the phosphating method is limited to low alloy steel and Zn and Al and the resulting layer consists of zinc phosphate crystals. It is in the point that corrosion prevention is impossible. Furthermore, in most cases a pre-connected activation is necessary.

Advantages of the Invention The method according to the invention for phosphating metal surfaces has the advantage over the prior art that a dense layer is formed whose thickness can be adjusted almost arbitrarily.

  Another advantage is that the resulting layer has a clearly higher corrosion resistance.

  Furthermore, it is advantageous that the phosphating can be carried out without activation.

  Advantageous embodiments of the invention are apparent from the measures described in the dependent claims.

  For example, electrolysis can be performed at a constant voltage or a constant current, or a mixture of both.

BRIEF DESCRIPTION OF THE DRAWINGS Examples of the invention are illustrated in the drawings and will be described in detail in the following description. The figure shows the general outline of the production process according to the invention.

Examples The object of the present invention is the study of an electrolytic coating method for phosphating metal surfaces, in which the pores in the phosphate layer are filled with metallic zinc or a zinc alloy. In the process according to the invention, the electrodeposition of zinc or zinc alloy takes place in the same electrolyte simultaneously with the formation of zinc phosphate crystals. The method according to the present invention differs from the conventional phosphating treatment in which the unfinished product is immersed in a titanium phosphate suspension (at pH˜9 for about 60 seconds) after cleaning or corrosion. Can be successfully implemented without the conversion process. The layer formation rate is very rapid at about 3-20 μm / min with a current density j = −10−50 A / dm ² . In the case of conventional methods that have been used in the past, it was only deposited at about 1 μm / min. In the case of the above method, in addition to the low alloy steel, stainless steel and other expensive and inexpensive materials such as Al, Al alloy, Cu, Cu alloy, Ni, Ni alloy and the like can be directly coated. On the other hand, in the case of the non-energized method, it can be deposited only on a material that allows corrosion attack because otherwise the necessary pH shift cannot be produced. The electrolysis can in this case be controlled with a constant voltage or a constant current or can be carried out by mixing both components.

In the case of the method according to the invention, a dense layer is formed, which is characterized in that the space between the zinc phosphate crystals is filled with a network of metal-deposited zinc or zinc alloy. Simultaneous formation of conductive zinc or zinc alloy can cause pH shift induced by electrolysis, ie, electrons are supplied from the outside, almost any of zinc (zinc alloy) / zinc phosphate layer Layer thickness growth is achieved by reduction of H ⁺ on the zinc surface.

  The figure shows a general outline of the coating method according to the invention. In this case, a zinc / zinc phosphate layer 14 is deposited on the base metal 11 by the electrolyte 13 in a conventional electrolysis cell 10 having a working electrode 11 and a counter electrode 12 made of a corresponding base metal. As already suggested, unlike the standard phosphating, the electrons required for the pH shift here are not iron corrosion from low alloy steel (corrosion attack on base metal) but to external power source 15 Derived from. This protective current is also prepared not to corrode the base metal 11 in particular.

  Using the method according to the present invention, a closed or fully non-porous mixed layer (filled phosphate layer free of metals such as zinc, zinc / nickel, free base metal) from about 3 μm to about 500 μm substantially It can be deposited without limitation. Conventionally produced layers have a durability of about 5 hours to red rust formation in the salt spray test, whereas when a 20-30 μm zinc / zinc phosphate mixed layer is used, 1000 in the salt spray test. Durability exceeding the time was able to be achieved. Already after a phosphating time of 30 seconds, the corrosion resistance exceeds 420 hours.

The coating process according to the method of the invention can generally be carried out in conventional electrolysis cells. The counter electrode may in this case consist not only of a noble metal sheet, for example platinum, Pt / Ti or gold, but also of an inexpensive sacrificial anode, for example Zn, Ni, Fe, which is a continuous post-transport to metal ions. cause. Stainless steel, bronze, Cu, Cu alloy, Ni, Ni alloy, etc. can be used as the working electrode (base metal) on which the layer is deposited. The electrolyte is essentially an electrolyte as used during external unpowered phosphating. The electrolyte is in this case eg
Zn ²⁺ 5~50g / l
H ₂ PO ₄ ^- 5~80g / l
Containing.

  In this connection, it is important to use so-called high zinc baths with a zinc content of more than 5 g / l, while in conventional customary low zinc baths the zinc content is only about 1 to 5 g / l. In this case, no elemental zinc or zinc alloy deposition occurs between the phosphate crystals.

  In addition, since the electrolyte can contain ions of elements that can form an alloy with zinc, the deposition of the zinc alloy occurs simultaneously with the deposition of the phosphating layer. Addition of nanoparticles and organic molecules is also conceivable. Other possible bath additives for layer modification are polyphosphates, borates, organic polyhydroxy compounds, glyceryl phosphate and fluoride. The additional ions may be, for example, ions of a divalent metal M, where the other divalent metal M is 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 accelerators. Examples of the accelerator include urea, nitrate, nitrite, chlorate, bromate, hydrogen peroxide, ozone, organic nitro compound, peroxy compound, hydroxylamine or a mixture thereof. Nitrate ions in the range of 0 to 20 g / l are advantageous as accelerators. The pH value of the bath is 1.5-4, preferably 2.5-3.5. Binary, ternary or multi-component alloys can be deposited by the addition of Zn, Ni, Co, Fe or Mn salts. Metal ions can also be supplied to the electrolyte by anodic dissolution.

With respect to the electrolysis conditions, the electrolyte may be stationary or moving during the process. The current density is> -1 A / dm ² and preferably varies within a range of approximately j = −1 to approximately j = −100 A / dm ² . Current density in the range of -5~-50A / dm ² is particularly advantageous. The temperature of the electrolyte is> 40 ° C., preferably in the range 40-80 ° C., particularly preferably in the range 60-70 ° C.

  As already mentioned, the electrolysis process can be controlled with a constant voltage or a constant current, in which case direct current or pulsed direct current can be applied. The layer thickness distribution can be adjusted by controlling the local current density, ie by shaping between the anode and the unfinished product and / or by interrupting the current. In this way, geometrically demanding parts can also be covered.

The figure which shows the principle outline of the manufacturing method by this invention.

Explanation of symbols

  10 Electrolytic cell, 11 Working electrode (base metal), 12 Counter electrode, 13 Electrolyte, 14 Zinc / zinc phosphate layer, 15 External power supply

Claims (20)

In a method for phosphating a metal layer by simultaneously electrodepositing using direct current from an acidic aqueous solution containing at least zinc ions and phosphate ions, A method characterized in that electrodeposition occurs in the same electrolyte, wherein the current density is greater than −5 A / dm ² . The method of claim 1, wherein the current density is in the range of −5 to −50 A / dm ² .   3. A process as claimed in claim 1, wherein the temperature is in the range> 40 [deg.] C., preferably 40-80 [deg.] C., in particular 60-70 [deg.] C.   The electrolyte contains zinc ions in the range> 5 g / l, in particular in the range 5-50 g / l and phosphate ions in the range> 10 g / l, in particular in the range 10-80 g / l The method according to any one of claims 1 to 3.   5. The deposition of zinc and / or a zinc alloy simultaneously with the deposition of the phosphating layer, since the acidic aqueous solution additionally contains ions of elements that can form an alloy with zinc. The method of any one of Claims.   6. A method according to claim 5, wherein nanoparticles or organic molecules are used instead of ions.   6. The method of claim 5, wherein the additional ions are divalent metal M ions.   The method of claim 7, wherein the other divalent metal M is selected from the group consisting of Ni, Fe, Co, Cu, Mn, and the like.   The method according to any one of claims 1 to 8, wherein the metal layer is selected from the group consisting of stainless steel, bronze, Al, Al alloy, Cu, Cu alloy, Ni, Ni alloy and the like.   10. A process according to any one of claims 1 to 9, wherein the pH value of the electrolyte is about 1.5 to about 4, preferably about 2.5 to about 3.5.   The method according to claim 1, wherein an accelerator is added to the electrolyte.   12. The method of claim 11, wherein the accelerator is selected from the group consisting of urea, nitrate, nitrite, chlorate, bromate, hydrogen peroxide, ozone, organic nitro, peroxy compound, hydroxylamine or mixtures thereof. .   The method according to claim 7, wherein the metal ions of the divalent metal M are supplied by anodic dissolution of the electrolyte.   The method according to any one of claims 1 to 13, wherein a Zn salt, a Ni salt, a Co salt and / or a Mn salt is additionally added to the electrolyte. The electrolyte has the following composition:
Zn ²⁺ 5-40 g / l;
Mn ²⁺ 0.5-10 g / l;
^{_{· H 2 PO 4 - 10~40g /}} l; and _· ^NO 3 - _1~10g / l;
The method according to claim 1, comprising:   The process according to any one of claims 1 to 15, wherein the electrolysis is carried out at a constant voltage or a constant current or a mixture of both.   The method according to any one of claims 1 to 16, wherein the layer thickness distribution on the metal layer is adjusted by the local current density.   The method according to claim 1, wherein the direct current is pulsed.   19. A method according to any one of claims 1 to 18, wherein the layer formation rate is in the range of about 3 to about 20 [mu] m / min.   A metal layer having a porous zinc phosphate layer deposited thereon, wherein the pores of the zinc phosphate layer are filled with metal zinc and / or a zinc alloy.

Images