METHOD FOR ELECTROLYTIC DEPOSITION OF A PHOSPHATE LAYER ON ZINC SURFACES

Description

This application is a continuation of international application number PCT/EP2023/082652, filed Nov. 22, 2023, which claims priority to European patent application number EP 22211916.6 filed Dec. 7, 2022.

FIELD OF THE INVENTION

The present invention relates to a method for phosphating metal surfaces, preferably alloy-galvanized steel strip surfaces, by treating them by dipping or spray-dipping with acidic, aqueous solutions which may contain magnesium ions, phosphate ions and nitrate ions and, for further improvement of the layer formation, possibly further ions selected from ammonium ions, alkali metal ions and/or fluoride ions, wherein the workpieces are simultaneously treated cathodically with a direct current. This produces phosphate layers which have better corrosion properties than the well-known trication phosphating. The phosphate layers formed also show high abrasion resistance.

BACKGROUND

The use of electric current in phosphating methods is well known. For example, cathodic treatment results in an acceleration of the phosphating method (see M. H. Abbas, Finishing, October 1984, pages 30-31). Corrosion protection layers can be deposited on galvanized steel surfaces using acidic aqueous solutions based on aluminum phosphate and/or magnesium phosphate or polycondensed phosphoric acid and simultaneous application of cathodic currents (see JP-A-77/047 537, JP-A-75/161 429 and JP-A-89/219 193). Furthermore, phosphate layers with high abrasion resistance can be produced on metal surfaces using acidic phosphating baths containing phosphoric acid, manganese and copper ions and simultaneous application of cathodic currents (JP-A-87/260 073). JP-A-85/211 080 relates to a method for producing corrosion protection layers on metal surfaces using zinc phosphating solutions, with temporary application of a cathodic current. This creates a corrosion-resistant protective layer, particularly on the edges of the metal surfaces to be treated. A similar method is described in EP-A-0 171 790 where following a conventional zinc phosphating method, the metal surfaces are treated with an acidic aqueous solution containing zinc, phosphate and chlorine ions, wherein a direct current is simultaneously applied to the anodically connected metal surfaces.

On the other hand, it has been known to a person skilled in the art for some time that high nickel contents in phosphate layers result in particularly good corrosion protection. In this context, however, it is also known that in order to achieve high nickel contents in phosphate layers, high nickel contents in the phosphating solutions to be used are also required. On the one hand, this results in higher method costs due to the high price of nickel. On the other hand, large amounts of toxic nickel compounds from the used phosphating solutions have to be disposed of, since usually only approximately 2% of the nickel from the phosphating solutions is incorporated into the phosphate layers. For example, a high-nickel zinc phosphating method is known from WO-A-85/03 089. Extremely high nickel concentrations are used in this case for phosphating. It is generally pointed out that part of the nickel can in principle be replaced by a number of monovalent or divalent cations. These are selected, for example, from cobalt, manganese and magnesium. It is further stated that the nickel content of the solution to be used must be at least 1.0 g/l. The ratio to be used between low zinc and high nickel content is a substantial constituent of the technical teaching.

SUMMARY OF THE INVENTION

In contrast, it is the object of the present invention to provide a method for phosphating metal surfaces in which the presence of critical heavy metals such as zinc, nickel, copper and cobalt can be dispensed with.

Surprisingly, it was found that with a method as described below, good corrosion properties can be achieved even in the absence of critical heavy metals such as zinc, nickel, copper and cobalt, or even improved corrosion properties can be achieved compared to the prior art.

Accordingly, the present invention relates in Aspect 1 to: a method for phosphating metal surfaces, preferably electrolytically or hot-dip galvanized steel strip surfaces, by treating them by dipping or spray-dipping with acidic, aqueous solutions containing magnesium, phosphate and nitrate ions, which is characterized in that

• • a) phosphating solutions containing the following components are used:
    • PO₄³⁻ anions in a range of 2 to 50 g/l,
    • NO₃₋ anions in a range of 0.1 to 60 g/l,
    • Mg²⁺ cations in a range of 0.1 to 20 g/l,
  • b) wherein the following conditions are observed:
    • pH of the phosphating solutions in the range of 1.0 to 5.0,
    • temperature of the phosphating solutions in the range of 10 to 80° C.,
    • treatment duration in the range of 1 to 300 sec., and furthermore
  • c) wherein, during phosphating, the workpieces are treated cathodically with a direct current with a density in the range of 1 to 150 mA/cm².

The method according to Aspect 1, characterized in that phosphating solutions containing the following components are used:

• • PO₄³⁻ anions in the range of 10 to 40 g/l,
  • NO₃₋ anions in the range of 5 to 50 g/l,
  • Mg²⁺ cations in the range of 1 to 10 g/l.

The method according to either Aspect 1 or Aspect 2, characterized in that the following conditions are observed during the phosphating treatment of the workpieces:

• • pH value of the phosphating solutions in the range of 2.0 to 3.0,
  • temperature of the phosphating solutions in the range of 30 to 70° C.,
  • treatment duration in the range of 2 to 90 sec.

The method according to one or more of Aspects 1 to 3, characterized in that, during phosphating, the workpieces are cathodically treated with a direct current with a density of at least 5 mA/cm², particularly preferably at least 10 mA/cm², very particularly at least 20 mA/cm², but preferably below 100 mA/cm², very particularly preferably below 70 mA/cm².

The method according to one or more of Aspects 1 to 4, characterized in that phosphating solutions are used which additionally contain alkali metal ions in the range of 0.1 to 10 g/l, preferably 0.5 to 5 g/l.

The method according to one or more of Aspects 1 to 5, characterized in that phosphating solutions are used which additionally contain NH₄⁺ cations in the range of 0.1 to 10 g/l, preferably 1 to 7 g/l,

The method according to one or more of Aspects 1 to 6, characterized in that phosphating solutions are used which additionally contain simple or complex fluoride anions in the range of 0.01 to 2 g/l, preferably 0.1 to 1.5 g/l.

The method according to one or more of Aspects 1 to 7, characterized in that the workpieces to be phosphated are previously subjected to a known activation pretreatment, in particular with titanium-containing activation solutions.

The method according to one or more of Aspects 1 to 8 as a pretreatment for a subsequent painting or coating.

In another aspect, the present invention relates to a workpiece comprising at least one metal surface phosphated by means of a method as disclosed and described herein.

Further embodiments of the present invention are set out in the appended claims.

“At least two” as used herein includes, but is not limited to, 2, 3, 4, 5, 6, and more. “At least one” as used herein includes, but is not limited to, 1, 2, 3, 4, 5, 6, and more.

In the sense of the present invention, it is substantial that all of the parameters listed above are observed when carrying out the phosphating method. In other words, this means that the cathodic direct current treatment of the workpieces during phosphating only results in the desired result in appropriate, special phosphating solutions containing magnesium, phosphate and nitrate ions, as defined in detail above. When metal surfaces are mentioned in connection with the present invention, this refers to material surfaces made of iron, steel, zinc, aluminum and alloys of zinc or aluminum. Examples of aluminum surfaces and their alloys include pure aluminum, AlMg and AlMgSi materials. Examples of zinc alloy constituents are iron, nickel and cobalt. The term steel refers to unalloyed to low-alloyed steel, such as that used in the form of sheet metal for car body construction. This also includes alloy-coated steels, which are surface-treated with zinc/nickel alloys, for example. The method according to the invention is particularly suitable for phosphating electrolytically or hot-dip galvanized steel strip surfaces. The use of galvanized steel, especially electrolytically galvanized steel in strip form, has become increasingly important in recent years. The term “galvanized steel” includes both galvanization by electrolytic deposition and by hot-dip application and generally refers to so-called “pure zinc layers” as well as to well-known zinc alloys, in particular zinc/nickel alloys.

The method according to the invention is preferably carried out in the so-called dipping method; however, it is generally also possible to apply the phosphating solutions according to the invention to the substrate surfaces by spray dipping. The workpieces to be treated are connected cathodically for the phosphating treatment, wherein a stainless steel electrode is preferably used as the counter electrode. In general, a metal container of the phosphating bath can also serve as a counter electrode; furthermore, graphite electrodes or, in principle, all electrode materials known from the relevant prior art can also be used as a counter electrode.

In the sense of the present invention, the term “direct current” is understood to mean not only “pure” direct currents, but also practically similar currents, for example those which can be generated by full-wave rectification of a single-phase alternating current or by rectification of a three-phase alternating current. So-called pulsating direct currents and chopped direct currents are also applicable in the sense of the invention. Of importance in the sense of the invention is only the current density of the direct current, which should lie in the range defined above. The specification of suitable voltage values for the direct current to be used in the sense of the present invention is deliberately omitted, since taking into account the different conductivities of the phosphating baths on the one hand and the geometric arrangement of the electrodes on the other hand, a different relationship between current and voltage can exist. In addition, concentration gradients, which are determined by the current density and not by the bath voltage, are crucial for the formation mechanism of the phosphating layers. In each individual case, a person skilled in the art will select suitable voltage values for carrying out the method according to the invention on the basis of the specified values for the current density.

In accordance with a preferred embodiment of the present invention, phosphating solutions containing the following components are used:

• • PO₄³⁻ anions in the range of 10 to 40 g/l,
  • NO₃₋ anions in the range of 10 to 50 g/l,
  • Mg²⁺ cations in the range of 1 to 10 g/l.

In accordance with a further preferred embodiment of the method according to the invention, the following conditions are observed during the phosphating treatment of the workpieces:

• • pH value of the phosphating solutions in the range of 2.0 to 3.0,
  • temperature of the phosphating solutions in the range of 30 to 70° C., for example 30, 35, 35, 40, 45, 50, 55, 60, 65 or 70° C.,
  • treatment duration in a range of 2 to 90 seconds, for example 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85 or 90 seconds.

In the sense of the present invention, it is further preferred that during phosphating, the workpieces are cathodically treated with a direct current with a density of at least 5 mA/cm², particularly preferably at least 10 mA/cm², very particularly at least 20 mA/cm², but preferably below 100 mA/cm², very particularly preferably below 70 mA/cm², for example with a density of 5 to 100 mA/cm², for example from 10 to 70 mA/cm², for example 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65 or 69 mA/cm².

According to a further embodiment of the method according to the invention, the phosphating baths can additionally contain alkali metal ions and/or ammonium ions. In various preferred embodiments, phosphating baths according to the invention contain NH₄³⁰ , Li⁺, Na⁺ and/or K⁺ cations, preferably NH₄⁺, Li⁺, Na⁺ and/or K⁺ cations, particularly preferably NH₄⁺ and/or K⁺ cations.

In this sense, it is preferred according to the invention to work with phosphating solutions which additionally contain ammonium and/or alkali metal ions in the range of 0.1 to 10 g/l, preferably 0.5 to 5 g/l, particularly preferably at least 1 g/l. The additional use of ammonium ions and/or alkali metal ions in the phosphating baths according to the invention results in an improvement in layer formation.

In a particularly preferred embodiment of the method according to the invention, the phosphating baths contain NH₄⁺ cations, preferably in the range of 0.1 to 10 g/l, particularly preferably in the range of 1 to 7 g/l. It turns out that in the presence of ammonium ions, particularly fine-crystalline phosphate layers are obtained.

In the case of phosphating of aluminum-alloyed hot-dip galvanized steel surfaces, for example hot-dip galvanized steel strip of types (Z), (ZM), (ZA), (AZ), (AS) or (ZF), the use of fluoride ions in the method according to the invention results in a more uniform degree of coverage of the phosphating layers. In this context, it is preferred according to the invention that phosphating solutions are used which additionally contain simple or complex fluoride anions in the range of 0.01 to 2 g/l, preferably 0.1 to 1.5 g/l. When phosphating surfaces of steel or zinc or galvanized steel strip which is not aluminum alloyed, for example electrolytically galvanized steel of the type (ZE), the presence of fluoride anions is not required, but the presence of fluoride anions does not interfere with the phosphating method according to the invention even in these cases. According to the invention, the fluoride anions can also be used in the form of complex fluorine compounds, for example tetrafluoroborate or hexafluorosilicate. In various embodiments, the use of phosphating baths is therefore preferred in which the amount of complex fluoride anions, preferably SiF₆²⁻ anions is in the range of 0.1 to 2 g/l, preferably in the range of 0.5 to 1.5 g/l. In various other embodiments, the use of phosphating baths is preferred in which the amount of F⁻ anions is in the range of 0.01 to 2 g/l, preferably in the range of 0.1 to 1.5 g/l.

In various embodiments, methods are preferred in which phosphating solutions containing the following components are used:

• • PO₄³⁻ anions in a range of 5 to 50 g/l, particularly preferably 10 to 40 g/l, most preferably 20 to 40 g/l,
  • NO₃⁻ anions in a range of 0.1 to 60 g/l, particularly preferably 10 to 50 g/l, most preferably 20 to 40 g/l,
  • Mg²⁺ cations in a range of 0.1 to 20 g/l, particularly preferably 1.0 to 15 g/l, most preferably 2.0 to 10 g/l, and
  • alkali metal ions, preferably selected from Li⁺, Na⁺ and/or K⁺ cations, particularly preferably K⁺ cations, in the range of 0.1 to 10 g/l, preferably 0.5 to 5 g/l, most preferably 0.5 to 3.5 g/l, or
  • NH₄⁺ cations in the range of 0.1 to 10 g/l, preferably 1 to 7 g/l.

In various embodiments, methods are preferred in which phosphating solutions containing the following components are used:

• • PO₄³⁻ anions in the range of 5 to 50 g/l, particularly preferably 10 to 40 g/l, most preferably 20 to 40 g/l,
  • NO₃₋ anions in the range of 0.1 to 60 g/l, particularly preferably 10 to 50 g/l, most preferably 20 to 40 g/l,
  • Mg²⁺ cations in the range of 0.1 to 20 g/l, particularly preferably 1.0 to 15 g/l, most preferably 2.0 to 10 g/l,
  • simple or complex fluoride anions in the range of 0.01 to 2 g/l, preferably 0.1 to 1.5 g/l and
  • alkali metal ions, preferably selected from Li⁺, Na⁺ and/or K⁺ cations, particularly preferably K⁺ cations, in the range of 0.1 to 10 g/l, preferably 0.5 to 5 g/l, most preferably 0.5 to 3.5 g/l, or
  • NH₄⁺ cations in the range of 0.1 to 10 g/l, preferably 1 to 7 g/l.

In various embodiments, methods are preferred in which phosphating solutions containing the following components are used:

• • PO₄³⁻ anions in the range of 5 to 50 g/l,
  • NO₃₋ anions in the range of 0.1 to 50 g/l,
  • Mg²⁺ cations in the range of 0.1 to 10 g/l, as well as
  • SiF₆²⁻ anions, in the range of 0.1 to 2 g/l, preferably in the range of 0.5 to 1.5 g/l, and/or
  • F⁻ anions, in the range of 0.01 to 2 g/l, preferably in the range of 0.1 to 1.5 g/l, and
  • alkali metal ions, preferably selected from Li⁺, Na⁺ and/or K⁺ cations, particularly preferably K⁺ cations, in the range of 0.1 to 10 g/l, preferably 0.5 to 5.0 g/l, most preferably 0.5 to 3.5 g/l, or
  • NH4+cations in the range of 0.1 to 10 g/l, preferably 1 to 7 g/l.

As already stated, compliance with all of the above-mentioned parameters is substantial for the optimal implementation of the method according to the invention. This includes, among other things, the specified range of the pH value to be maintained. If the pH value of the phosphating bath is not within the specified range, it is necessary to adjust the pH value of the phosphating bath to within the specified range by adding acid, such as phosphoric acid, or by adding a base, such as caustic soda. If values for the free acid or total acid content of the phosphating solutions are given in the examples below, these were determined in the manner described in the literature. The “free acid score” is accordingly defined as the number of ml of 0.1 N NaOH required to titrate 10 ml of bath solution against dimethyl yellow, methyl orange or bromophenol blue. The total acid score is then given as the number of ml of 0.1 N NaOH required to titrate 10 ml of bath solution using phenolphthalein as indicator until the first pink color appears. The phosphating solutions according to the invention generally have free acid scores in the range of 0.5 to 4 and total acid scores in the range of 10 to 40.

The preparation of the phosphating baths for carrying out the method according to the invention is generally carried out in the conventional manner which is known per se to a person skilled in the art. The following compounds, for example, can be used as starting materials for the production of the phosphating bath: magnesium: in the form of magnesium nitrate, magnesium oxide, magnesium hydroxide or magnesium hydroxycarbonate; phosphate: preferably in the form of phosphoric acid; nitrate: in the form of the above-mentioned salts, optionally also in the form of the sodium salt; alkali metal ions: e.g., NaH₂PO₄, KH₂PO₄; ammonium: e.g., NH₄H₂PO₄. The fluoride ions which may be used in the bath are preferably used in the form of sodium fluoride or in the form of the complex compounds mentioned above (e.g., MgSiF₆, H₂SiF₆, NaF, NH₄HF₂).

The above-mentioned compounds are dissolved in water in the concentration ranges substantial to the invention; subsequently, as also already mentioned above, the pH of the phosphating solutions is adjusted to the desired value. Before the actual phosphating treatment, the metal surface to be treated must be completely wettable with water. For this purpose, it is generally necessary to clean and degrease the metal surfaces to be treated using methods known per se and sufficiently described in the prior art. In various embodiments, it is further preferred that the workpieces to be phosphated are previously subjected to a known activation pretreatment, in particular with titanium-containing activation solutions. In the sense of the present invention, it is further preferred, after rinsing the cleaned and degreased workpieces with water, preferably with deionized water, to subject the workpieces to be phosphated to a known activation pretreatment. In particular, titanium-containing activation solutions are used in this context, as described, for example, in DE-A-20 38 105 or DE-A-20 43 085. Accordingly, the metal surfaces to be subsequently phosphated are treated with solutions which substantially contain titanium salts and sodium phosphate as activating agents, optionally together with organic components, for example alkyl phosphonates or polycarboxylic acids. Soluble titanium compounds, such as potassium titanium fluoride and in particular titanyl sulfate, are preferred as titanium components. Disodium orthophosphate is generally used as sodium phosphate. Titanium-containing compounds and sodium phosphate are used in such proportions that the titanium content is at least 0.005 wt. %, based on the weight of the titanium-containing compound and the sodium phosphate.

Following this activation treatment, the actual phosphating method takes place; the phosphated metal surfaces are then rinsed again with water, in turn preferably with deionized water. In specific cases, it may be advantageous to passivate the phosphate layers thus produced in a subsequent method step. Such passivation is always useful and advantageous when the metal surfaces phosphated by the method according to the invention are subsequently painted or otherwise coated with organic materials. Typical suitable passivations are chromium-free acidic aqueous compositions based on water-soluble complex fluorides of the elements Zr, Ti, Hf and/or Si. However, if the phosphated substrates are subsequently subjected to a mechanical deformation process and then phosphated again, as is the case in car body construction, for example, passivation treatment should be omitted.

The phosphating layers produced by means of the method according to the invention can be used in all fields in which phosphate coatings are used. A particularly advantageous application is the preparation of metal surfaces for painting, for example by spray painting or electrocoating, or for coating with organic films.

In addition to the method described above, a workpiece comprising at least one metal surface phosphated by means of a method as disclosed and described herein represents a further aspect of the present invention.

All aspects, objects and embodiments described for methods according to the invention are also applicable to this subject matter of the invention. Therefore, reference is expressly made at this point to the disclosure at the corresponding point with the note that this disclosure also applies to the above workpieces according to the invention.

In the subsequent practical example, the agent according to the invention is described in a non-limiting manner.

EXAMPLES

Example 1

• • Substrate: Hot-dip galvanized (Z) sheet (specification: DX56+Z140MC)
  • Cleaning: 30 g/l BONDERITE® C-AK 2011+3 g/l BONDERITE® C-AD 1270, each commercially available cleaners from Henkel AG & Co. KGaA for 30 seconds by spraying (pressure: 2 bar) at 50° C.
  • Activation: 1 g/l BONDERITE® M-AC 50 CF (commercially available from Henkel AG & Co. KGaA) for 5 seconds by dipping at 20° C.
  • Mg electrolyte: 25 g/l PO₄³⁻
    • 4 g/l Mg²⁺
    • 6 g/l NO₃₋
    • 1 g/l SiF₆²⁻
    • pH adjusted with NH₃ to 2.8
  • Cathodic polarization: 30 mA/cm²
  • Anode: Stainless steel sheet of specification 1.4571
  • Duration: 60 seconds
  • Cathodic dip coating: 20 μm (Cathoguard® 800 from BASF Coatings GmbH)
  • Corrosion: Ritz according to Clemen and stone chip corrosion test (PV 1210)

The values given are mean values obtained from three parallel samples.