PASSIVATING LAYER FOR METAL-CONTAINING SUBSTRATES

A method for sequential production of metal-containing substrates which have a conversion layer including chromium and zirconium on at least part of their surface, the method including bringing into contact at least part of the surface of metal-containing substrates with an electrolyte solution including at least one trivalent chromium compound and tetravalent zirconium compound to form the conversion layer. The method further includes adding fresh trivalent chromium and tetravalent zirconium compounds to the electrolyte solution to compensate for chromium and zirconium deposited in the conversion layer and controlling or regulating a Cr:Zr mass ratio in the electrolyte solution or the addition of trivalent chromium compounds and tetravalent zirconium compounds to the electrolyte solution such that the Cr:Zr mass ratio in the conversion layer, based upon a sum of the metals Zr and Cr, is >0.7.

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Description
CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2023/052417, filed on Feb. 1, 2023, and claims benefit to German Patent Application No. DE 10 2022 105 844.1, filed on Mar. 14, 2022. The International Application was published in German on Sep. 21, 2023 as WO 2023/174611 A1 under PCT Article 21 (2).

FIELD

The present invention relates to a method for the sequential production of metal-containing substrates. The invention further relates to a metallic substrate treated by the method and to a passivating conversion layer produced by the method.

BACKGROUND

In order to protect metallic material surfaces from corrosive environmental influences, they are passivated, among other things. Passivation, also known as conversion treatment, increases the corrosion resistance of the unpainted metal surface and prepares the surface for a final coating, such as painting. A conversion layer is a very thin layer that is created by chemical or electrochemical reaction on and with the surface of metals. During conversion, either metal ions from the surface react with the solution used for treatment, or inorganic layers are deposited on the metal. These are then firmly connected to the metal underneath. The conversion layer serves to increase corrosion protection without any further treatment or coating. The formation of the conversion layer can also be an important step in surface pretreatment, ensuring good adhesion of a subsequent coating, and at the same time ensuring good corrosion protection of the coated surface.

Good corrosion resistance is obtained with passivation operations based upon hexavalent chromium. However, hexavalent chromium has a negative impact on health and the environment because it is highly toxic and known to cause cancer. Hexavalent chromium has therefore been replaced by chromium (III) compounds for several years now, as these are considered harmless.

WO 2018/146292 describes a method for the electrolytic passivation of an outer chromium or outer chromium alloy layer. A trivalent chromium electrolyte is produced by reducing hexavalent chromium ions in the presence of phosphoric acid and at least one reducing agent.

EP 3 456 865 describes an aqueous conversion solution for zinc and zinc alloys, which substantially contains chain-shaped, colloidal silicon nanoparticles in addition to trivalent chromium and zirconium compounds. The molar ratio of chromium ions:zirconium ions is 0.1 to 4.

EP 3 045 563 describes a method for applying a trivalent chromium conversion layer, which additionally contains a titanium compound (titanium lactate) and is used on zinc or zinc alloys.

WO 2017/194187 describes a method for improving the corrosion protection of metallic and in particular zinc-containing surfaces. The reaction solution contains chromium (III), zirconium and titanium ions, silicon oxide nanoparticles, and fluoride ions. The aqueous treatment solution contains Cr(III) ions in a concentration of 0.1 g/L to 8.0 g/L, zirconium and/or titanium anions in a concentration of 0.1 g/L to 15 g/L, organosilane-modified silicon oxide nanoparticles in a concentration of 0.1 g/L to 50 g/L, and fluoride ions in a concentration of 0.5 g/L to 10 g/L. It is further described that a weight fraction of Zr/Cr<1 and only in combination with silicon oxide nanoparticles can achieve particularly good corrosion protection. A mass ratio of Cr:Zr>0.7 in the conversation layer is not described.

WO 2018/178390 discloses a method for increasing the corrosion protection of electroplated chromium layers by bringing them into contact with an electrolyte containing Cr(III) ions, at least one conductive salt, and at least one reducing agent, and subjecting them to “reverse pulse plating,” wherein a trivalent chromium oxide layer is formed on the substrate.

WO 02/055758A2 describes a method for depositing corrosion-resistant films on aluminum surfaces using aqueous acidic media containing chromium (III) and zirconium. In addition to the chromium and zirconium salts, thickeners and surfactants are also used.

WO 2004/065642 describes a method for treating aluminum and aluminum alloys with aqueous acidic solutions containing trivalent chromium salts and alkali metal hexafluorozirconate salts. In addition, the treatment solution contains zinc compounds, alkali metal fluorine compounds, and effective amounts of water-soluble thickeners, surfactants, or wetting agents.

WO 2006/088519 discloses a method for producing chromium-zirconium coatings on aluminum, its alloys, anodized aluminum, and iron alloys. The coating solution contains trivalent chromium salts, fluorozirconate and zinc compounds, surfactants, and organic carboxyl compounds.

WO 2020/061427 describes solution-based corrosion inhibitors (containing chromium and zirconium sources) for aluminum-based coatings. Chromium sulfate is used at a concentration of 25 g/1,000 mL and hexafluorozirconate at a concentration of 20 g/1,000 mL. It is described, among other things, that the inhibition solution can be kept functional by measuring the ion concentrations and the pH value and, if necessary, by adding chromium ions and zirconium ions as well as a common acid or base.

US 2004/024487 relates to a method for treating metal surfaces, wherein the treatment agent contains a zirconium-containing and a fluorine-containing compound. The treatment is carried out by cathodic electrolysis. The weight ratio of zirconium metal:fluorine is 0.2 to 1.0. The concentration of zirconium and fluorine can be adjusted and is determined by means of atomic absorption analysis.

U.S. Pat. No. 3,162,207 relates to chromate conversion coatings from liquid media and a method for automatically controlling the effectiveness of the conversion coating bath. If the conductivity was low, an additional liquid chromating concentrate was added to the bath in an amount sufficient to restore the original conductivity.

In summary, the prior art presents some methods for producing conversion layers on metal surfaces. However, the corrosion protection achieved with the layers described in the prior art is in need of improvement. There is therefore still a need for conversion layers/passivation layers that provide improved corrosion protection, in particular by increasing corrosion protection without further treatment or coating.

SUMMARY

In an embodiment, the present disclosure provides a method for sequential production of metal-containing substrates which have a conversion layer comprising chromium and zirconium on at least part of their surface, the method comprising bringing into contact at least part of the surface of metal-containing substrates with an electrolyte solution comprising at least one trivalent chromium compound and at least one tetravalent zirconium compound to form the conversion layer. The method further comprises adding fresh trivalent chromium and tetravalent zirconium compounds to the electrolyte solution to compensate for chromium and zirconium deposited in the conversion layer and controlling or regulating a Cr:Zr mass ratio in the electrolyte solution and/or the addition of trivalent chromium compounds and tetravalent zirconium compounds to the electrolyte solution such that the Cr:Zr mass ratio in the conversion layer, based upon a sum of the metals Zr and Cr, is >0.7. A limit value for the Cr:Zr mass ratio in the conversion layer, based upon the sum of the metals Zr and Cr, is ≤0.7. A limit value for the Cr:Zr mass ratio in the electrolyte solution, based upon the sum of the metals Zr and Cr, is ≤0.55.

BRIEF DESCRIPTION OF THE DRAWINGS

Subject matter of the present disclosure will be described in even greater detail below based on the exemplary figures. All features described and/or illustrated herein can be used alone or combined in different combinations. The features and advantages of various embodiments will become apparent by reading the following detailed description with reference to the attached drawings, which illustrate the following:

DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a Cr:Zr mass ratio in a deposited conversion layer as a function of different concentrations of chromium and zirconium in an electrolyte solution;

FIG. 2 illustrates the result of a neutral salt spray test according to DIN EN ISO 9227:2017-07 and after 336 h, carried out on EN AW 2024 T3 aluminum test panels; and

FIG. 3 illustrates the result of a neutral salt spray test according to DIN EN ISO 9227:2017-07 carried out on an aluminum alloy 2024 T6.

 DETAILED DESCRIPTION

In an embodiment, the present invention provides a method for producing a conversion layer which has better corrosion protection.

It has now surprisingly been found that this is achieved by a method according to the present disclosure for the sequential production of metal-containing substrates which have a conversion layer on at least part of their surface, the metal treated with the method according to the present disclosure, and the conversion layer according to the present disclosure.

The method according to the present disclosure and the conversion layer obtained by the method have the following advantages:

    • With the method according to the present disclosure, it is possible to obtain a higher chromium content in the conversion layer than with methods known from the prior art. This is associated with improved corrosion protection, as demonstrated in the salt spray test according to DIN EN ISO 9227:2017-07.
    • With the method according to the present disclosure, the desired mass ratio of Cr:Zr can be specifically monitored and, if necessary, adjusted. It is possible to regulate/control at least one of the following parameters during the production of the passivating conversion layer:
      • a) mass ratio of Cr:Zr in the conversion layer,
      • b) mass ratio of Cr:Zr in the electrolyte solution,
      • c) concentration of Cr in the electrolyte solution,
      • d) concentration of Zr in the electrolyte solution.
    • The method allows for a single-step and currentless dipping process for producing the conversion layer.
    • The method is free of harmful substances, such as in particular hexavalent chromium compounds.
    • The relatively higher proportion of Cr(III) in the layer leads to a slightly stronger coloration due to the body color that Cr(III) compounds bring with them, which improves visual recognition, allowing for simple, qualitative quality control.

A first subject matter of the present disclosure is a method for the sequential production of metal-containing substrates which have a conversion layer comprising chromium and zirconium on at least part of their surface, in which metal-containing substrates are brought with at least part of their surface into contact with an electrolyte solution comprising at least one trivalent chromium compound and at least one tetravalent zirconium compound to form the conversion layer, wherein fresh trivalent chromium and tetravalent zirconium compounds are added to the electrolyte solution to compensate for the chromium and zirconium deposited in the conversion layer, and the Cr:Zr mass ratio in the electrolyte solution and/or the addition of trivalent chromium compounds and tetravalent zirconium compounds to the electrolyte solution are controlled or regulated such that the Cr:Zr mass ratio in the conversion layer, based upon the sum of the metals Zr and Cr, is >0.7.

Another subject matter of the present disclosure is a treated metal produced by the method according to the present disclosure, as defined above and below.

Another subject matter of the present disclosure is a passivating conversion layer produced by the method according to the present disclosure, as defined above and below.

In the context of the present disclosure, “metal-containing substrates” are understood to mean components which contains at least one metal, which is preferably selected from copper, copper alloys, nickel, nickel alloys, aluminum, aluminum alloys, steels, tin alloys, zinc, and zinc alloys. Metal-containing substrates preferably contain or consist of aluminum or aluminum alloys. It may happen that the aluminum has an oxide layer. Aluminum forms an oxide layer when stored in air or when in contact with water. The oxide layer thickness in this case is 0.01-0.05 μm. Furthermore, an oxide layer can be formed on the metal through electrolytic oxidation (anodizing process) of aluminum. The oxide layer thickness is between 3-25 μm. In any case, a possible oxide layer on the aluminum has no negative influence on the method according to the present disclosure. Aluminum alloys in particular have an aluminum content of >60%.

According to the present disclosure, the conversion layer is produced by bringing metal-containing substrates into contact with an electrolyte solution which comprises at least one trivalent chromium compound and at least one tetravalent zirconium compound, wherein fresh trivalent chromium and tetravalent zirconium compounds are added to the electrolyte solution to compensate for the chromium and zirconium deposited in the conversion layer, and the Cr:Zr mass ratio in the electrolyte solution and/or the addition of trivalent chromium compounds and tetravalent zirconium compounds to the electrolyte solution are controlled or regulated such that the Cr:Zr mass ratio in the conversion layer, based upon the sum of the metals Cr and Zr, is >0.7.

The method according to the present disclosure is advantageously suitable for the sequential passivation of metal-containing substrates. This means that, according to the passivation method according to the present disclosure, a single substrate or a plurality of substrates at the same time are subjected to the same passivation method in a sequence that is in principle not limited. The method for measuring, controlling, or regulating according to the present disclosure ensures that the electrolyte solution used always has an optimal mass ratio of Cr:Zr. For this purpose, at least one of the aforementioned measurement, control, or regulation variables can be determined continuously or at regular intervals, e.g., after completion of each coating sequence or a certain number of coating sequences, and the Cr and/or Zr content of the electrolyte solution can be supplemented according to this value. In the case of continuous passivation, e.g., strip treatment, the passivated strip can be measured, and, if necessary, the content of Cr and/or Zr in the electrolyte solution can be adjusted.

Conveniently, the contact is brought into contact by immersion, i.e., the metal-containing substrate is partially or completely immersed in the electrolyte solution located in a container. For this purpose, the metal-containing substrate can, for example, be held on racks and immersed in the reaction solution with the racks, or it can be located in a drum or in a centrifuge or on a tray, and immersed in the reaction solution with the drum or tray.

In an alternative procedure, the metal-containing substrate is brought into contact with the electrolyte solution by spray dipping.

In another alternative procedure, the metal-containing substrate is brought into contact with the electrolyte solution by spraying.

In another alternative procedure, the metal-containing substrate can also be brought into contact with the electrolyte solution by flooding. In contrast to dipping, during flooding, the substrate or substrates are placed in a container which is then filled with the electrolyte solution, and the latter is removed from the container again after the treatment is completed.

Furthermore, the metal-containing substrate can also be sprayed with the electrolyte solution, e.g., by means of a nozzle, from which a spray of the electrolyte solution emerges. Yet another manner of treatment is to apply the electrolyte solution to the substrate surfaces by brushing, rolling, padding, or other application techniques. The treatment can take place in conventional plants in which the metal-containing substrates are treated in batches, or in continuous plants through which the substrates are continuously passed and treated.

In a preferred embodiment of the present disclosure, the electrolyte solution has a bath temperature of approximately 10 to 100° C., more preferably 15 to 80° C., even more preferably 20 to 60° C., in particular 25 to 45° C. If the metal-containing substrates are brought into contact with the electrolyte solution by immersion, the immersion time is preferably 5 to 900 seconds, more preferably 15 to 600 seconds, and in particular 30 to 300 seconds. Depending upon the technique used to bring the metal-containing substrates into contact with the electrolyte solution, longer or shorter treatment times can be used.

Before being brought into contact with the electrolyte solution, the metal-containing substrates to be treated are cleaned if necessary. For this purpose, the substrate can be treated mechanically and/or brought into contact with a suitable cleaning solution.

It may also be advantageous for the metal-containing substrate to undergo a surface pretreatment. Suitable surface pretreatment processes are selected from degreasing, pickling, polishing, desmutting, anodizing, and combinations thereof. Methods for pretreatment are known to a person skilled in the art.

After completion of the method according to the present disclosure, the metal-containing substrates are preferably dried—for example, with warm air. In addition, the substrates can also be rinsed before drying to remove excess electrolyte solution from the surface.

In a preferred embodiment of the present disclosure, the method is single-stage. However, it is also provided that the treated metal-containing substrate be treated with another electrolyte solution, e.g., a sealing solution, before and/or after the application of the conversion layer. The additional electrolyte solution can be based upon polymers, silane compounds, other metal salt solutions, and/or inhibitors.

Furthermore, the method according to the present disclosure makes it possible to monitor, control, and/or regulate the mass ratio of Cr:Zr. The following is preferred:

    • a target value for the mass ratio of Cr:Zr, based upon the sum of the masses of the metals Zr and Cr, in the conversion layer is determined,
    • the actual value for the mass ratio of Cr:Zr, based upon the sum of the masses of the metals Zr and Cr, in the conversion layer is determined,
    • an actuator is provided for influencing a parameter to be controlled,
    • after reaching a limit value for the deviation of the actual value from the target value, the value of the control variable of the actuator (control value) is changed in order to influence the parameter to be controlled.

By definition, “regulation” or “control” refers to a process in which a variable—the control variable (actual value)—is continuously recorded, compared with another variable—the reference variable (target value)—and influenced in the sense of an adjustment to the reference variable. The control deviation as the difference between the actual value and the target value is added to the controller, which uses it to create a control variable. The control variable is the output variable (the position) of the actuator used, with the help of which a targeted intervention in the control system is carried out. The actuator can be part of the controller, but in many cases it is a separate device. The process is controlled by positioning or adjusting the actuator, e.g., by changing a mass or energy flow. The control variable in the method according to the present disclosure is the Cr:Zr mass ratio in the conversion layer. Examples of actuators are valves, switches, etc. An example of the control variable is the opening state of a valve. The control variable thereof is, for example, the position of the handwheel used to operate the valve.

During control, the process described above takes place automatically in a control loop of continuous measurement and control, i.e., the target value is compared with the actual value permanently or at regular intervals. In the control system, the path of action is open, i.e., an actuator is controlled from the actual value and a target value stored in the control system, but there is then no feedback to the control system as to whether the initial state corresponds to the target value. In the simplest case, the control can be done manually. In the event of a control deviation, a determined amount of Cr and/or Zr is defined as the control variable, which is added to the electrolyte.

It has now been found that conversion coatings on metal-containing substrates have improved corrosion protection compared to the prior art if the Cr:Zr mass ratio in the conversion layer, based upon the sum of the metals Zr and Cr, is >0.7. Furthermore, it was found that an improved conversion layer can be produced compared to the prior art if the Cr:Zr mass ratio in the conversion layer is used as a control variable.

In a preferred embodiment of the method according to the present disclosure, in addition to the Cr:Zr mass ratio in the conversion layer, at least one further control and/or regulating variable is determined, selected from

    • the Cr:Zr mass ratio in the electrolyte solution,
    • the concentration of Cr in the electrolyte solution,
    • the concentration of Zr in the electrolyte solution,
      a target value for each additional control and/or regulating variable is set, the actual value for each additional control and/or regulating variable in the electrolyte solution is determined, after reaching a lower limit value for the deviation of the actual value from the target value, the amount of trivalent chromium compound and/or tetravalent zirconium compound is added to the electrolyte solution required to compensate for the loss of the electrolyte solution due to deposited chromium and zirconium.

The Cr:Zr mass ratio in the conversion layer can be determined either offline or online. The Cr:Zr mass ratio in the conversion layer can be determined by conventional methods known to a person skilled in the art. Preferably, the Cr:Zr mass ratio in the conversion layer is determined/monitored by means of X-ray fluorescence analysis.

The Cr:Zr mass ratio in the electrolyte solution can be determined either offline or online. The Cr:Zr mass ratio in the electrolyte solution can be determined by conventional methods known to a person skilled in the art. Preferably, the Cr:Zr mass ratio in the electrolyte solution is determined/monitored sensorially, e.g., colorimetrically, spectroscopically, e.g., atomic absorption spectrometry (AAS) or inductively coupled plasma mass spectrometry (ICP), or by means of titration.

The concentration of Cr and/or Zr in the electrolyte solution can also be determined either offline or online. The concentration in the electrolyte solution can be determined by conventional methods known to a person skilled in the art. Preferably, the concentration of Cr and/or Zr in the electrolyte solution is determined/monitored sensorially, e.g., colorimetrically, spectroscopically, e.g., atomic absorption spectrometry (AAS) or inductively coupled plasma mass spectrometry (ICP), or by means of titration.

The concentration of Cr and/or Zr can be determined simultaneously or sequentially.

According to the present disclosure, the target value for the Cr:Zr mass ratio in the conversion layer is >0.7, preferably >2.5.

The limit value for the Cr:Zr mass ratio in the conversion layer is preferably ≤0.7.

Preferably, the target value for the Cr:Zr mass ratio in the electrolyte solution is >0.58, in particular >0.60.

Preferably, the limit value for the Cr:Zr mass ratio in the electrolyte solution is ≤0.55, in particular 0.58.

Preferably, the target value for the concentration of Cr in the electrolyte solution is from 0.02 to 2 g/L; in particular, the target value for the concentration of Cr in the electrolyte solution is from 0.02 to 0.8 g/L. The minimum concentration of Cr in the electrolyte solution is preferably 0.02 g/L.

Preferably, the limit value for the concentration of Cr in the electrolyte solution is <0.02 g/L and >2 g/L.

Preferably, the target value for the concentration of Zr in the electrolyte solution is 0.03 to 2.8 g/L. In particular, the target value for the concentration of Zr in the electrolyte solution is 0.03 to 1.3 g/L. The minimum concentration of Zr in the electrolyte solution is preferably 0.03 g/L.

Preferably, the limit value for the concentration of Zr in the electrolyte solution is <0.03 g/L and >1.3 g/L.

In a preferred embodiment, this is a method in which

    • a1) a conversion layer comprising chromium and zirconium is deposited on at least part of the surface of a metal-containing substrate,
    • b1) the actual value for the Cr:Zr mass ratio in the conversion layer is determined,
    • c1) if the actual value is 0.7 or less, at least one of the following parameters is determined, selected from
      • the Cr:Zr mass ratio in the electrolyte solution,
      • the concentration of Cr in the electrolyte solution,
      • the concentration of Zr in the electrolyte solution,
    • d1) a target value for each additional selected parameter is set, and the actual value in the electrolyte solution is determined,
    • e1) the amount of trivalent chromium compound and/or tetravalent zirconium compound is added to the electrolyte solution required to compensate for the loss of the electrolyte solution due to deposited chromium and zirconium,
      wherein steps b1) to e1) can be carried out once or several times.

In the method according to the present disclosure, preferably

    • a2) a conversion layer comprising chromium and zirconium is deposited on at least part of the surface of a metal-containing substrate,
    • c2) at least one of the following parameters is determined, selected from
      • the Cr:Zr mass ratio in the electrolyte solution,
      • the concentration of Cr in the electrolyte solution,
      • the concentration of Zr in the electrolyte solution,
    • d2) a target value for each additional selected parameter is set, and the actual value in the electrolyte solution is determined,
    • e2) the amount of trivalent chromium compound and/or tetravalent zirconium compound is added to the electrolyte solution required to compensate for the loss of the electrolyte solution due to deposited chromium and zirconium,
      wherein steps c2) to e2) can be carried out once or several times.

In particular, the present disclosure relates to a method for producing a passivating conversion layer having a Cr:Zr mass ratio of >0.7, in which

    • a) an electrolyte solution is provided comprising trivalent chromium compounds and tetravalent zirconium compounds, wherein the Cr:Zr mass ratio in the electrolyte solution is >0.6,
    • b) the electrolyte solution from a) or a recycled electrolyte solution is brought into contact with a metallic substrate,
    • c) a passivating conversion layer is formed.

Specifically, the present disclosure relates to a method for producing a passivating conversion layer having a Cr:Zr mass ratio of >0.7, in which

    • d) an electrolyte solution is provided comprising trivalent chromium compounds and tetravalent zirconium compounds, wherein the Cr:Zr mass ratio in the electrolyte solution is >0.6,
    • e) a metal-containing substrate with at least part of its surface is brought into contact with the electrolyte solution from a) or a recycled electrolyte solution,
    • f) a passivating conversion layer is formed,
      wherein the Cr:Zr mass ratio in the electrolyte solution in the passivating conversion layer is determined, and the amount of the chromium source and/or zirconium source is controlled or regulated as a function of the Cr:Zr mass ratio.

A further preferred embodiment is a method for monitoring, controlling, and/or regulating a method for producing a passivating conversion layer having a Cr:Zr mass ratio of >0.7, in which

    • g) an electrolyte solution is provided comprising trivalent chromium compounds and tetravalent zirconium compounds, wherein the Cr:Zr mass ratio in the electrolyte solution is >0.6,
    • h) the electrolyte solution from a) is brought into contact with a metallic substrate,
    • i) a passivating conversion layer is formed,
      wherein the Cr:Zr mass ratio in the electrolyte solution a) or in the passivating conversion layer is determined, and the amount of the chromium source and/or zirconium source is controlled as a function of the Cr:Zr mass ratio.

Preferably, in the method according to the present disclosure, the addition of trivalent chromium compounds and of tetravalent zirconium compounds to the electrolyte solution is controlled or regulated independently of one another. In a first embodiment, the mass ratio of Cr:Zr according to the present disclosure is adjusted by adding further trivalent chromium compounds. The mass ratio of Cr:Zr according to the present disclosure can also be adjusted by adding further tetravalent zirconium compounds. It is also provided to adjust the mass ratio of Cr:Zr according to the present disclosure by adding further trivalent chromium compounds and tetravalent zirconium compounds.

The electrolyte solution is preferably an acidic aqueous solution comprising at least one trivalent chromium compound and at least one tetravalent zirconium compound. The preferred pH value of the electrolyte solution is in the range of 2 to 5, in particular in the range of 3 to 4.

Preferred are trivalent chromium compounds selected from Cr2(SO4)3, (NH4)Cr(SO4)2, Cr(NO3)·9H2O, KCr(SO4)2, Cr(OH)SO4, Cr2O3, and mixtures thereof. In particular, the chromium compound is Cr2(SO4)3.

Tetravalent zirconium compounds are preferred, selected from salts of H2ZrF6.

Alkali hydroxides, carbonates, ammonia, amines, or mixtures thereof are suitable for adjusting the pH value.

In order to achieve a uniform and particularly adhesive conversion layer on the metallic substrate, inhibitors are preferably added to the electrolyte solution according to the present disclosure. Suitable inhibitors are selected from triazoles, thiazoles, oxazoles, and mixtures thereof. In particular, inhibitors are selected from benzotriazole, 3-mercapto-1,2,4-triazole, 2-aminothiazole, 2-mercaptobenoxazole, and mixtures thereof. The inhibitors are preferably present in the electrolyte in a concentration range of 0.001-2.0 g/L, in particular 0.02-1.0 g/L.

In addition, it can be advantageous if surfactants are added to the electrolyte solution according to the present disclosure. Suitable surfactants are selected from non-ionic surfactants, anionic surfactants, and cationic surfactants. The surfactants are preferably present in the electrolyte in a concentration range of 0-10.0 g/L, in particular 0.05-8.0 g/L.

In order to achieve improved resistance to filiform corrosion compared to the prior art, it is advantageous if the electrolyte solution according to the present disclosure is free of zinc compounds, zinc ions, or zinc-containing compounds.

In the context of the present disclosure, filiform corrosion is understood to mean a thread-like corrosion phenomenon. It is a special form of anodic infiltration, in particular in coatings on substrates containing aluminum. Existing zinc is normally incorporated into the conversion layer. The presence of zinc-containing compounds promotes filiform corrosion. In addition, the presence of zinc-containing compounds can have a negative influence on any subsequent coating, e.g., painting.

Preferably, therefore, the electrolyte solution according to the present disclosure is free of zinc compounds, zinc ions, or zinc-containing compounds, or contains these compounds in the range of 0 to 0.01 wt. %, based upon the total weight of the electrolyte solution.

It is also preferred that the electrolyte solution according to the present disclosure be free of organosilane-modified silicon oxide nanoparticles, in particular free of silicon oxide nanoparticles, or comprise organosilane-modified silicon oxide nanoparticles, in particular silicon oxide nanoparticles in the range of 0 to 0.01 wt. %, based upon the total weight of the electrolyte solution.

In organosilane-modified silicon oxide nanoparticles or silicon oxide nanoparticles, the silicon oxide is preferably present in the form of a nanoscale agglomerate. The agglomerate of organosilane-modified silicon oxide nanoparticles can be regarded as a core, the surface of which is silane-modified, i.e., on the surface of which organic silane compounds are arranged. Organosilane modification is understood in particular to mean that oxygen atoms are covalently bonded to silicon atoms of an organic silicon compound, at least on the surface of the silicon oxide nanoparticles. The organic silicon compound is preferably an epoxy, amido, ureido, amino, ester, mercapto, and/or isocyanate silane.

Nanoparticles generally have a size (average particle diameter) of 1 to 100 nm.

In addition, the electrolyte solution according to the present disclosure is preferably free of thickeners. The use of thickeners is disadvantageous, because the consumption of electrolyte is increased by carryover. In other words, the electrolyte adheres more strongly to the substrates/components to be coated and is carried away.

Thickeners according to the present disclosure are selected from cellulose, methylcellulose, polyurethane compounds, and mixtures thereof.

The electrolyte solution according to the present disclosure preferably contains thickeners in the range of 0 to 0.1 wt. %, based upon the total weight of the electrolyte solution.

Another subject matter of the present disclosure is a treated metallic substrate produced by the method according to the present disclosure, as defined above.

Another subject matter of the present disclosure is a passivating conversion layer produced by the method according to the present disclosure, as defined above.

The conversion layer can also contain other elements selected from Ni, Ti, Hf, Al, Fe, Co, Cd, Mn, and rare earths—for example, from the treated surface.

For example, the thickness of the conversion layer may vary as a function of the desired corrosion protection properties. For most applications, it has proven to be favorable to adjust the conversion layer with a layer thickness of 10 nm to 1,000 nm, preferably 20 nm to 600 nm, and in particular 50 nm to 300 nm. The layer thickness can be determined by measuring a fracture in the scanning electron microscope.

The passivating conversion layer according to the present disclosure preferably has a layer weight of 30 to 500 mg/m2, in particular of 50 to 400 mg/m2.

The passivating conversion layer according to the present disclosure preferably has a chromium content of the surface in the range of 3 to 15 cps, wherein the calibration factor is 8.5406. The passivating conversion layer according to the present disclosure preferably has a chromium content of the surface in the range of 25 to 130 mg/m2, in particular in the range of 35 to 100 mg/m2, especially in the range of 40 to 85 mg/m2.

The passivating conversion layer preferably has a surface zirconium content in the range of 1.5 to 7.5 cps, wherein the calibration factor is 23.258. The passivating conversion layer according to the present disclosure preferably has a zirconium content of the surface in the range of 35 to 175 mg/m2, in particular in the range of 45 to 140 mg/m2, especially in the range of 60 to 115 mg/m2.

Counts per second (cps) is a unit of measurement for the count rate (the average repetition rate of measurement signals counted as individual, temporally separated events).

As mentioned above, the conversion layer according to the present disclosure already provides the treated metallic substrate with excellent corrosion protection.

Therefore, it is not necessary to provide it with an additional layer, such as a sealing layer. Nevertheless, the conversion layer according to the present disclosure is excellently suited as a basis for further inorganic and/or organic layers. Objects or articles that have a conversion layer according to the present disclosure can thus be permanently and therefore particularly advantageously protected against corrosion. Objects or articles that have a conversion layer according to the present disclosure are also a subject matter of the present disclosure.

Embodiments of the invention shall be explained with reference to the following examples, without, however, being restricted to the specifically described embodiments.

EXAMPLES Example 1

An EN AW 2024 T3 aluminum test panel was degreased with mild alkaline cleaner and immersion surfactant (5 vol. %, 0.5 vol. %) and then rinsed with distilled water. This is followed by pickling with caustic soda and complexing agent (5 vol. %) and subsequent rinsing with distilled water. The test sheet is then subjected to desmutting with nitric acid and iron (III) compounds (20 vol. %) and rinsed again with distilled water. Passivation is carried out for 5 minutes in an electrolyte solution with a pH of 3.8 and a temperature of 30° C. The concentrations of zirconium and chromium are given in Table 1. After passivation, the test panel is rinsed with distilled water and dried in an oven at 60° C. for 20 min.

TABLE 1 Bath 1 2 3 4 5 6 Bath concentration Cr [g/L] 0.2 0.2 0.3 0.4 0.6 0.8 Bath concentration Zr [g/L] 0.5 0.25 0.5 0.5 0.5 0.5 Cr/Zr mass ratio in the layer 0.5 0.73 0.86 1.07 1.08 1.18 Cr/Zr mass ratio in the 0.4 0.8 0.6 0.8 1.2 1.6 electrolyte solution

The round measuring points in FIG. 1 indicate the mass ratio of chromium and zirconium on the surface of the aluminum test sheets treated as described above, measured by X-ray fluorescence (Fischerscope X-Ray XDV, Helmut Fischer). It can be seen that, with increasing Cr:Zr ratio in the electrolyte, more chromium is deposited on the aluminum surface, which leads to an increase in the Cr:Zr ratio on the surface of the aluminum sheets.

In FIG. 1, the concentration of chromium and zirconium on the surface of an EN AW 2024 T3 aluminum sheet was measured by X-ray fluorescence analysis (Fischerscope X-Ray XDV, Helmut Fischer). The measured value CPS (counts per second) was calibrated using test surfaces with known Cr or Zr concentrations.

Example 2

An EN AW 2024 T3 aluminum test sheet was treated as in Example 1 (degreased, pickled, desmutted, and passivated) and subsequently subjected to a neutral salt spray test according to DIN EN ISO 9227:2017-07 for 336 h. The results are shown in FIG. 2. It can be seen that, with increasing Cr:Zr ratio on the surface of the test panels (a-d), fewer corrosion points (KP) occur. This is considered proof of better corrosion protection.

Example 3

Alloys as listed in Table 2 were treated as in Example 1 (degreased, pickled, desmutted, and passivated). The results for corrosion protection are also shown in Table 2.

TABLE 2 Cr Zr Cr:Zr in the (mg/L) (mg/L) electrolytes Comment Electrolyte 1 200 500 0.4 Cf. WO 02/055758A2 Electrolyte 2 400 500 0.8 NSS time Cr:Zr in to >10 Alloy Electrolyte the layer KP [h] Comment EN AW 2024 T3 1 0.30 336 Very significant 2 0.87 504 increase in corrosion protection in alloys by using the electrolyte solution according to the present disclosure EN AW 7475 1 0.54 504 Significant increase 2 0.70 672 in corrosion protection by using the electrolyte solution according to the present disclosure AlSi9Cu3(Fe) 1 0.42 672 Significant increase 2 0.74 >672 in corrosion protection for Cu- containing cast alloys by using the electrolyte solution according to the present disclosure

Example 4

An aluminum sheet alloy 2024T6 was treated as in Example 1 and then subjected to a neutral salt spray test according to DIN EN ISO 9227:2017-07. The electrolyte solution compositions and parameters are listed in Table 3.

TABLE 3 WO2006/ Solution A Solution B WO2006/ 088519A2 According to According to 088519A2 Example the present the present Ingredient/parameters Example 1 (TCPI) disclosure disclosure Cr-sulfate basic [g/L] 6 0.6 2.3 1.4 Cr [g/L] 1.0 0.10 0.38 0.23 K-hexafluorozirconate [g/L] 8.0 0.8 1.6 1.0 Zr [g/L] 2.6 0.26 0.51 0.31 pH 3.95 3.80 3.80 3.80 Treatment time [s] 30 300 300 300 Temperature [° C.] 40 30 30 30 Results cps Cr 2.5 3.5 6.8 5.9 cps Zr 1.8 3.1 3.4 3.1 cps Cr:Zr 1.4 1.1 2.0 1.9 Cr (mg/m2) calibrated 22 30 58 51 Zr (mg/m2) calibrated 43 72 79 71 Cr:Zr (mass ratio) 0.5 0.4 >0.7 >0.7 Corrosion protection ISO <360 <360 >360 >360 9227 neutral salt spray [h]

The results are shown in FIG. 3. It can be seen that the examples according to the present disclosure have significantly better corrosion protection compared to the prior art, despite longer salt spray treatment.

While subject matter of the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Any statement made herein characterizing the invention is also to be considered illustrative or exemplary and not restrictive as the invention is defined by the claims. It will be understood that changes and modifications may be made, by those of ordinary skill in the art, within the scope of the following claims, which may include any combination of features from different embodiments described above.

The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.

Claims

1. A method for sequential production of metal-containing substrates which have a conversion layer comprising chromium and zirconium on at least part of their surface, the method comprising:

bringing into contact at least part of the surface of metal-containing substrates with an electrolyte solution comprising at least one trivalent chromium compound and at least one tetravalent zirconium compound to form the conversion layer;
adding fresh trivalent chromium and tetravalent zirconium compounds to the electrolyte solution to compensate for chromium and zirconium deposited in the conversion layer; and
controlling or regulating a Cr:Zr mass ratio in the electrolyte solution and/or the addition of trivalent chromium compounds and tetravalent zirconium compounds to the electrolyte solution such that the Cr:Zr mass ratio in the conversion layer, based upon a sum of the metals Zr and Cr, is >0.7,
wherein a limit value for the Cr:Zr mass ratio in the conversion layer, based upon the sum of the metals Zr and Cr, is ≤0.7, and
wherein a limit value for the Cr:Zr mass ratio in the electrolyte solution, based upon the sum of the metals Zr and Cr, is ≤0.55.

2. The method according to claim 1, comprising:

determining, in addition to the Cr:Zr mass ratio in the conversion layer, at least one further control and/or regulating variable, the control and/or regulating variable being selected from a Cr:Zr mass ratio in the electrolyte solution, a concentration of Cr in the electrolyte solution, and a concentration of Zr in the electrolyte solution;
setting a target value for each additional control and/or regulating variable;
determining an actual value for each additional control and/or regulating variable in the electrolyte solution; and
after reaching a lower limit value for a deviation of the actual value from the target value, adding an amount of trivalent chromium compound and/or tetravalent zirconium compound to the electrolyte solution required to compensate for the loss of the electrolyte solution due to deposited chromium and zirconium.

3. The method according to claim 2, wherein the target value for the Cr:Zr mass ratio in the electrolyte solution, based upon the sum of the metals Zr and Cr, is >0.58.

4. (canceled)

5. (canceled)

6. The method according to claim 2, wherein the target value for the concentration of Cr in the electrolyte solution is 0.02 to 2 g/L.

7. The method according to claim 2, wherein the limit value for the concentration of Cr in the electrolyte solution is <0.02 and >2 g/L.

8. The method according to claim 2, wherein the target value for the concentration of Zr in the electrolyte solution is 0.03 to 2.8 g/L.

9. The method according to claim 2, wherein the limit value for the concentration of Zr in the electrolyte solution is <0.03 and >2.8 g/L.

10. The method according to claim 1, further comprising:

a1) depositing a conversion layer comprising chromium and zirconium on at least part of the surface of a metal-containing substrate;
b1) determining the actual value for the Cr:Zr mass ratio in the conversion layer;
c1) determining, based on if the actual value being 0.7 or less, at least one parameter selected from: the Cr:Zr mass ratio in the electrolyte solution, the concentration of Cr in the electrolyte solution, and the concentration of Zr in the electrolyte solution;
d1) setting a target value for each additional selected parameter, and determining the actual value in the electrolyte solution; and
e1) adding the amount of trivalent chromium compound and/or tetravalent zirconium compound to the electrolyte solution required to compensate for the loss of the electrolyte solution due to deposited chromium and zirconium,
wherein steps b1) to e1) can be carried out once or several times.

11. The method according to claim 1, further comprising:

a2) depositing a conversion layer comprising chromium and zirconium on at least part of the surface of a metal-containing substrate;
c2) determining at least one parameter selected from: the Cr:Zr mass ratio in the electrolyte solution, the concentration of Cr in the electrolyte solution, and the concentration of Zr in the electrolyte solution;
d2) setting a target value for each additional selected parameter, and determining the actual value in the electrolyte solution; and
e2) adding the amount of trivalent chromium compound and/or tetravalent zirconium compound to the electrolyte solution required to compensate for the loss of the electrolyte solution due to deposited chromium and zirconium,
wherein steps c2) to e2) can be carried out once or several times.

12. The method according to claim 1, wherein the metal-containing substrate is aluminum or contains aluminum.

13. A treated metallic substrate produced according to the method of claim 1.

14. A passivating conversion layer produced according to the method of claim 1.

15. The method according to claim 1, wherein the Cr:Zr mass ratio in the conversion layer is determined either offline or online.

16. The method according to claim 3, wherein the target value for the Cr:Zr mass ratio in the electrolyte solution, based upon the sum of the metals Zr and Cr, is >0.6.

Patent History
Publication number: 20260258569
Type: Application
Filed: Feb 1, 2023
Publication Date: Sep 3, 2026
Inventors: Peter VOLK (Zwingenberg), Yang LIU (Ladenburg), Sebastian ETSCHEL (Erlangen)
Application Number: 18/846,687
Classifications
International Classification: C25D 3/06 (20060101);