EP2130950B1 - Method for pretreating reinforced steel, wrought iron or cast iron before galvanic coating - Google Patents

Abstract

Method for pretreating an electrolytic substrate, which is to be coated with zinc and/or zinc alloy, comprises cathodic etching in diluted acid, where: the substrate is hardened steel, wrought iron or cast iron; during the cathode etching, the cathode and anode are separated by an ion exchange membrane; the cathode comprises hydrochloric acid (catholyte); and the anode comprises an aqueous mineral acid, except hydrochloric acid, and/or alkyl sulfonic acid (anolyte). An independent claim is included for a method for electrolytic coating of the substrate with mono- or multi-layers of zinc and/or zinc alloy comprising pretreating the substrate by the above process, before coating.

Description

Workpieces made of iron or steel must be protected against corrosion by suitable coatings. Very good corrosion protection values can be achieved by galvanic deposition of zinc or zinc alloy layers. These coating systems have a greater negative potential compared to the base material and protect it in the sense of anodic corrosion protection. In the formation of such a corrosion element, the less noble coating forms the anode and thereby protects the base material. Pure zinc layers are very widespread for such coating systems. In addition to galvanic galvanizing, hot dip galvanizing is also used here.

Significantly improved corrosion protection is achieved by the electrodeposition of zinc nickel alloy layers having an alloy content of 12 to 16 wt .-% nickel. In the salt spray test according to DIN EN ISO 9227, these alloy coatings have resistances up to the occurrence of zinc corrosion of at least 120 hours compared to only 16 hours for zinc coatings (each with a passivation based on trivalent chromium compound).

The galvanic deposition of zinc or zinc alloy layers can be carried out from weakly acidic processes (pH 4.5-5.5) or strongly alkaline processes (pH 14). Both types of process initially differ in their cathodic efficiency. Weakly acidic processes have a cathodic efficiency of almost 100%, in alkaline processes, depending on the process type and applied cathodic current density, it is about 25-50%.

To achieve a certain corrosion protection value, a minimum layer thickness is crucial. In the case of geometrically complex shaped workpieces, there are areas such as recesses, undercuts, etc., where only low cathodic current densities are present during the galvanic coating. This results in less layer thickness growth at these sites.

Weakly acidic electrolytes have an approximately constant efficiency in the entire current density range. As a result, a large amount of metal is deposited on a workpiece in regions of high cathodic current density, but relatively little in regions of low cathodic current densities. Since the minimum layer thickness at the most unfavorable point of the workpiece is decisive for the required corrosion resistance, the deposition from a weakly acidic method causes significantly more metal to be deposited at points of high cathodic current density than is required for the corrosion protection, before the range defined in areas of low current density Minimum layer thickness is reached. The deposition of unnecessarily thick layers in areas of high current density is undesirable from an economic point of view. It may also be disadvantageous from a technical point of view, since thick layers are generally relatively brittle and can flake off even with slight mechanical stress.

In the case of alkaline zinc or zinc alloy electrolytes, the cathodic efficiency changes with the applied cathodic current density. In areas of high cathodic current density, the cathodic efficiency is low, but high at low cathodic current density. Overall, it results from this characteristic that in a complex-shaped component a much more uniform layer distribution is obtained than from a weakly acid zinc or zinc alloy electrolyte. Alkaline zinc or zinc alloy electrolytes are the preferred systems for coating geometrically complex components from this point of view.

But they have the disadvantage that various base materials can only be coated insufficiently. When depositing from an alkaline zinc or zinc alloy electrolyte, hydrogen separation always occurs in parallel to the zinc or zinc alloy deposition. Depending on the nature of the material, it may then happen that in areas of low current densities only hydrogen separation and no more metal deposition takes place. The expert then speaks of the fact that the metal deposition in these areas does not start.

The hard-to-coat base iron-based materials include, for example, hardened steel, forged components, and cast iron. For hardened steel, there may be various alloying components in the steel that degrade light-off performance. In the case of forged components, iron oxide residues on the surface (eg haematite) often lead to poor start-up. Cast iron is a material that contains precipitates of graphite. The hydrogen overvoltage is very low on graphite. For a cast surface to be coated, this means that primarily hydrogen is deposited at the locations where such graphite precipitates are present. Because of the locally low hydrogen overvoltage, the more negative potential for zinc or zinc alloy deposition is not achieved there. This does not start. In addition to graphite, residues of foundry sand and solidified metal oxides are often present on the surface of cast iron. These impurities are typically mechanically removed by blasting prior to the entire electrolytic coating process. Impurities in easily accessible area can be removed very reliable. At In contrast, the residues can not always be completely removed from depressions or other areas that are difficult to reach, or they can only be removed over a long period of time, which is no longer justifiable in the context of economic production. These areas, which are difficult to reach due to the blasting, often also represent the area of low current density in the case of the electrolytic coating. The problem of poor light-off behavior during metal deposition is therefore intensified by the insufficiently pretreated surface.

In the prior art, therefore, processes for the pretreatment of hardened steel, wrought iron or cast iron have been developed, so that surfaces pretreated in this way can subsequently be coated without defects.

So will in DE 100 35 102 B4 describes a pretreatment process, characterized in that the hardened steel or cast iron substrate is electrolytically activated prior to the electrolytic coating in a solution comprising one or more mineral acids and / or one or more alkylsulfonic acids (cathodic pickling).

In the case of cathodic pickling, the evolution of hydrogen already present during chemical pickling (acid treatment) and the associated effect of the mechanical detachment of rust and scale are vigorously supported and increased many times over. The amount of hydrogen evolved in the electrolytic treatment according to this method depends only on the applied cathodic current density and is therefore independent of the type of acid used.

In practice, however, it has been found that cathodic pickling in dilute hydrochloric acid has advantages over other acids. The galvanizability of particular Forged parts or castings may be different due to the process despite the same material composition. For example, different thicknesses of haematite deposits can be present on forged parts. For cast iron, depending on casting conditions and cooling of the mold, different forms of graphite precipitates may occur. It is therefore explainable that, despite supposedly identical material properties, the suitability for a defect-free coating can be different. In this regard, it has been found that these component-related differences can be compensated if dilute hydrochloric acid is used for cathodic activation.

However, the use of hydrochloric acid also has disadvantages. While the cathodic reaction in electrolytic activation in dilute mineral acids and / or alkylsulfonic acids is always the same, namely the evolution of hydrogen, there are differences in the anode reaction. In the case of mineral acids such as phosphoric acid, sulfuric acid, tetrafluoroboric acid or alkylsulfonic acids, when using insoluble anodes (eg platinum-coated titanium or iridium-ruthenium mixed oxide anodes), the anodic reaction is an oxidation of water in accordance with equation [1]:

2 H ₂ O - 4 e ^- -> 4 H ⁺ + 0 ₂ [1]

When using hydrochloric acid, however, the main reaction at the anodes is the formation of chlorine corresponding to reaction [2]:

2 Cl ^- - 2 e - -> Cl ₂ [2]

The development of chlorine has procedural disadvantages: First, the chlorine gas formed at the anode must be separated by an effective suction and the exhaust air to be cleaned by a suitable scrubber.

Second, the chlorine partially dissolves in the hydrochloric acid. This may cause a chemical reaction with the pickling inhibitor, with which the hydrochloric acid is added to prevent hydrogen embrittlement. As pickling inhibitors are organic compounds, such as. e.g. Butindiol used. These compounds add chlorine very easily with the formation of organohalogen compounds. For discharging wastewater, there is a very low limit for such compounds. Here, the sum parameter AOX (activated carbon adsorbable organic halogen compound) is used.

DE 4122543 A1 relates to a process for the electrochemical coating of metal strips, preferably galvanized steel strips, with metals or metal alloys, in which an electrolyte laden with salts of the coating metals is circulated between the cathodic metal strip to be coated and insoluble anodes. In this case, a diaphragm is arranged between the anode and the metal strip to be coated to form separate electrolyte circuits, which at the anode forming gases, eg oxygen or chlorine, in the laden with the salts of the coating metals electrolyte circuit in the cathode compartment and the transfer of these salts prevents the cathode space in the not charged with the metal salts electrolyte circuit in the anode compartment. The resulting at the anode oxygen gas or chlorine gas is sucked out of the anode circuit.

task

The object of the present invention is therefore to provide an improved process for the pretreatment of hardened steel, wrought iron or cast iron so that surfaces pretreated in this way can then be electrolytically coated with zinc and / or a zinc alloy without error. This process is intended to have the above-described benefits of cathodic pickling with dilute hydrochloric acid, without, however, bringing about the disadvantages of chlorine evolution described above.

Description of the invention

According to the invention the above object is achieved by a pretreatment method by means of cathodic pickling, which is characterized in that the cathode and anode compartment are separated by a cation exchange membrane, wherein the cathode compartment contains hydrochloric acid (catholyte) and the anode compartment an aqueous mineral acid except hydrochloric acid and / or a Alkyl sulfonic acid contains (anolyte).

The ion exchange membrane is a cation exchange membrane, e.g. FTN-95017 of the company FUMA-Tech GmbH, St. Ingbert / Saar.

Dilute hydrochloric acid is used in the cathode compartment to achieve optimal activation of even critical material surfaces. Incidentally, the concentration of hydrochloric acid (i.e., the content of HCl) is preferably 5 to 36% by weight, more preferably 10 to 18% by weight, based on the weight of the catholyte.

In contrast, any other mineral acid and / or alkylsulfonic acid can be used in the anode compartment, because in this case the anode reaction is in each case the harmless evolution of oxygen and not the formation of chlorine. Preferred acids are sulfuric acid, phosphoric acid, tetrafluoroboric acid or alkylsulfonic acids, e.g. Methane sulfonic acid. The anode material used is, for example, titanium or zirconium, which is coated with platinum or an iridium-ruthenium mixed oxide coating.

It is surprising that in this separation of the cathode space, the same result is achieved as in the use of hydrochloric acid in an undivided cell. As stated above, when using hydrochloric acid or other mineral acids and / or alkylsulfonic acids, the cathode reaction is always the same. Since the use of hydrochloric acid in an undivided cell nevertheless a better result in terms of compensation component-related differences is achieved than with the use of other mineral acids, it can be assumed that this effect is due to the different anode reaction and consequently the presence of dissolved chlorine, which was formed in the anodic reaction.

Surprisingly, however, it has been found that even with a separation of anode and cathode space through a cation exchange membrane, ie where the formation of chlorine is not is possible given the improved effect of dilute hydrochloric acid in the cathodic activation.

In the pretreatment method according to the invention, the substrate to be coated is preferably cleaned before the actual cathodic pickling. For this purpose, in the prior art, conventional methods are suitable, for example, in DE 100 35 102 are described. For example, aqueous alkaline degreaser solutions can be used. Also suitable is an electrolytic degreasing, for example by anodic treatment in alkaline degreaser solution. All of these methods can also be used in combination.

Furthermore, the substrate may also be anodized after cathodic pickling and prior to electrolytic plating to remove possible pickle residues in an aqueous alkaline solution.

The substrate pretreated according to the invention can then be galvanically coated with zinc or a zinc alloy. The zinc alloy may be, for example, a zinc nickel alloy or a zinc cobalt alloy. In a zinc nickel alloy, the nickel content is usually 12 to 16% by weight.

The deposition of the zinc layer can be carried out, for example, from a weakly acidic zinc bath, while zinc nickel layers are usually deposited from an alkaline zinc nickel electrolyte. However, deposition from a weakly acidic electrolyte is also possible. Zinc cobalt alloys are typically deposited from weakly acidic electrolytes.

Also possible are multiple layers such as a duplex layer of a zinc layer and a zinc nickel layer.

Examples

The present invention will be further illustrated by the following Example of the present invention and Comparative Examples.

Example:

• Schritt 1:
  Alkalische Reinigung in einem wässrig alkalischen Reiniger mit 3 Gew.-% Entfettersalz SLOTOCLEAN AK 161 (Mischung aus Natriumhydroxid, Natriumsilikat, Natriumpolyphosphat)
  Behandlungszeit: 4 Minuten
  Temperatur: 60 °C
• Schritt 2:
  Anodische elektrolytische Entfettung
  10 Gew.-% Entfettersalz SLOTOCLEAN EL-DCG (Mischung aus Natriumhydroxid und Natriumsilikat)
  Stromdichte: 8 A/dm²
  Behandlungszeit: 4 Minuten
  Temperatur: 35 °C
• Schritt 3:
  Beizen mit elektrolytischer Unterstützung.
  Behandlungsdauer: 5 Minuten
  Temperatur: 40 °C
  Kathodische Stromdichte ca. 5 A/dm²
  Dabei wurden Anoden- und Kathodenraum durch eine Kationenaustauschermembran vom Typ FTN-95017 der Fa. FUMA-Tech GmbH, St. Ingbert/Saar abgetrennt.
• Im Kathodenraum:
  • 18 Gew.-% Salzsäure
  • 1 Gew.-% Beizinhibitor 0066 (Alkinol) und
  • 0,05 Gew.-% Beizentfetter SLOTOCLEAN BEF 30 (Mischung von Alkinol und aliphatischen nichtionogenen Tensiden)
• Im Anodenraum:
  • 10 Gew.-% Schwefelsäure
  • Anodenmaterial: platiniertes Titan
• Schritt 4:
  • Elektrolytische Beschichtung in schwach saurem Zinkbad SLOTANIT OT 120
  • Kathodische Stromdichte: ca. 3 A/dm²
  • Beschichtungsdauer: 12 Minuten
  • Temperatur: 25 °C
• Schritt 5:
  • Elektrolytische Beschichtung im alkalischen
  • Zinknickelelektrolyten SLOTOLOY ZN 80
  • Kathodische Stromdichte: ca. 3 A/dm²
  • Beschichtungsdauer: 35 Minuten
  • Temperatur: 35 °C
• Ergebnis:
  Das Bauteil zeigt eine gleichmäßige Beschichtung. Auch in den kritischen Stellen (Achseln des Bremsträgers, niedriger Stromdichtebereich) liegt eine geschlossene Metallisierung vor (Bild 1).

A substrate made of cast iron (brake carrier) was subjected to the following process steps:

• Step 1:
  Alkaline cleaning in an aqueous alkaline cleaner with 3% by weight degreaser salt SLOTOCLEAN AK 161 (mixture of sodium hydroxide, sodium silicate, sodium polyphosphate)
  Treatment time: 4 minutes
  Temperature: 60 ° C
• Step 2:
  Anodic electrolytic degreasing
  10% by weight of defatting salt SLOTOCLEAN EL-DCG (mixture of sodium hydroxide and sodium silicate)
  Current density: 8 A / dm ²
  Treatment time: 4 minutes
  Temperature: 35 ° C
• Step 3:
  Pickling with electrolytic support.
  Duration of treatment: 5 minutes
  Temperature: 40 ° C
  Cathodic current density approx. 5 A / dm ²
  Anode and cathode compartment were separated by a FTN-95017 cation exchange membrane from FUMA-Tech GmbH, St. Ingbert / Saar.
• In the cathode compartment:
  • 18% by weight of hydrochloric acid
  • 1 wt .-% Beizinhibitor 0066 (alkynol) and
  • 0.05% by weight of pickling degreaser SLOTOCLEAN BEF 30 (mixture of alkynol and aliphatic nonionic surfactants)
• In the anode compartment:
  • 10% by weight of sulfuric acid
  • Anode material: platinized titanium
• Step 4:
  • Electrolytic coating in weakly acidic zinc bath SLOTANIT OT 120
  • Cathodic current density: approx. 3 A / dm ²
  • Coating time: 12 minutes
  • Temperature: 25 ° C
• Step 5:
  • Electrolytic coating in alkaline
  • Zinc nickel electrolyte SLOTOLOY ZN 80
  • Cathodic current density: approx. 3 A / dm ²
  • Coating time: 35 minutes
  • Temperature: 35 ° C
• Result:
  The component shows a uniform coating. Even in the critical areas (armpits of the brake carrier, low current density range) there is a closed metallization (Figure 1).

Comparative Example 1 (hydrochloric acid, without separation)

The process steps 1, 2, 4 and 5 were carried out as described in Example 1. In process step 3, the same conditions with regard to time, temperature and current density as in example 1 were selected, but without separation of anode and cathode space through an ion exchange membrane was used. As the electrolyte, the same composition as used for the cathode compartment in Example 1 was used.

Result:

The component shows a uniform coating. Even in the critical areas (armpits of the brake carrier, low current density range) there is a closed metallization (Figure 2). This results in an equally advantageous result in terms of the coating of the workpiece as in the example according to the invention. However, in contrast to the example according to the invention in Comparative Example 1, chlorine is formed in the anode reaction.

Comparative Example 2 (sulfuric acid, without separation)

• 10 Gew.-% Schwefelsäure
• 1 Gew.-% Beizinhibitor 0066 (Akinol)
• 0,05 Gew.-% Beizentfetter SLOTOCLEAN BEF 30 (Mischung von Alkinol und aliphatischen nichtionogenen Tensiden)

The process steps 1, 2, 4 and 5 were carried out as described in Example 1. In process step 3, the same conditions with regard to time, temperature and current density as in example 1 were selected, but without separation of anode and cathode space through an ion exchange membrane was used. The electrolyte used was:

• 10% by weight of sulfuric acid
• 1% by weight pickling inhibitor 0066 (Akinol)
• 0.05% by weight of pickling degreaser SLOTOCLEAN BEF 30 (mixture of alkynol and aliphatic nonionic surfactants)

Result:

The coating was not complete. In areas of low current density, the base material was not completely covered (Figure 3, arrows).

Claims (12)

1. Process for the pre-treatment of a substrate to be coated electrolytically with zinc and/or a zinc alloy and made from hardened steel, wrought iron or cast iron by means of cathodic pickling in dilute acid, characterised in that during cathodic pickling, the cathode area and anode area are separated by a cation-exchange membrane, wherein the cathode area contains hydrochloric acid (catholyte) and the anode area contains an aqueous mineral acid apart from hydrochloric acid and/or an alkyl sulphonic acid (anolyte).
2. Process according to claim 1, characterised in that the catholyte contains hydrochloric acid in a quantity of 5 to 36 wt.% of HCl relative to the weight of the catholyte.
3. Process according to claim 2, characterised in that the catholyte contains hydrochloric acid in a quantity of 10 to 18 wt.% of HCl relative to the weight of the catholyte.
4. Process according to one or more of claims 1 to 3, characterised in that the anolyte contains as the mineral acid, sulphuric acid, phosphoric acid or tetrafluoroboric acid.
5. Process according to one or more of claims 1 to 4, characterised in that titanium or zirconium, which is coated in each case with platinum or iridium-ruthenium mixed oxide coating, is used as the anode material.
6. Process according to one or more of claims 1 to 5, characterised in that the substrate is cleaned before cathodic pickling in an aqueous-alkaline degreasing solution.
7. Process according to one or more of claims 1 to 6, characterised in that the substrate is treated anodically in an aqueous-alkaline solution after cathodic pickling and before electrolytic coating.
8. Process for the electrolytic coating of a substrate made from hardened steel, wrought iron or cast iron with one or more layers made from zinc and/or a zinc alloy, characterised in that the substrate is pre-treated before coating by a process according to one of claims 1 to 7.
9. Process according to claim 8, characterised in that a zinc layer is deposited from a weakly acid zinc bath.
10. Process according to claim 8 or 9, characterised in that a zinc-nickel alloy layer is deposited from an alkaline zinc-nickel electrolyte.
11. Process according to claim 10, characterised in that the deposited zinc-nickel layer has a nickel proportion of 12 - 16 wt.%.
12. Process according to one or more of claims 8 to 11, characterised in that a zinc-cobalt alloy is deposited from a weakly acid zinc-cobalt electrolyte.

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