EP0366941A1 - Process for the electrophoretic coating of chromizable metal surfaces - Google Patents

Abstract

A process is described for the electrophoretic coating of chromisable metal surfaces. An improvement in adhesion of the coating is achieved by a chromising pretreatment of the metal surfaces, the chromised metal surfaces being kept wet in the period from chromising until introduction into the electrophoretic coating bath.

Description

The invention relates to a method for electrophoretic dip coating of chromatable metal surfaces. Chromatable metal surfaces are primarily surfaces of the metals zinc, cadmium, aluminum, magnesium, and their chromatable alloys. The method is therefore also suitable for the electrophoretic dip coating of iron and steel surfaces which are coated with such metals or their alloys, for example for coating galvanized iron or steel surfaces.

Metal surfaces are not only painted for decorative purposes, often - especially when painting iron and steel parts - the focus is on corrosion protection. A very good corrosion protection is achieved by painting iron and steel parts as long as the paint forms a complete coating. If the varnish layer has pores, holes, cracks or similar defects due to injuries, aging processes or due to incorrect manufacture of the varnish layer, these defects lead to rapid rust formation and a cauliflower-like lifting of the varnish layer infiltrated by corrosion processes ("cauliflower" -Corrosion and filiform corrosion). The corrosion damage is therefore not limited to the location of the defect in the paint layer, but spreads quickly.

Corrosion protection, which is retained even in the event of minor damage to the protective layer, can be provided by galvanizing. Due to the thick zinc layer of typically 50 to 200 µm, hot galvanizing of steel parts together with the iron-zinc alloy as a transition layer of zinc on the surface to the base material steel provides excellent corrosion protection.

With galvanizing, zinc layers are much less thick. In addition, galvanized steel sheets have no brittle intermediate layers made of iron-zinc alloys. Galvanizing is the only low-cost process a cathodic corrosion protection of the steel, which means that even in the case of scratches up to a width of approx. 0.7 mm, the corrosion of the steel is electrochemically suppressed by the zinc layer.

The zinc coating as the sole protection for steel is only a completely inadequate corrosion protection due to the small layer thicknesses that are applied in galvanic processes (average 10 µm). This applies in particular to shiny, smooth layers. These are not even close to hand. The corrosion protection found in the salt spray test according to DIN 50021 is only minimal; after about 6 to 8 hours the zinc layer is corroded away and brown rust appears.

A significant improvement in corrosion protection can be achieved by chromating or phosphating the zinc surface. In the salt spray test, it is customary for blue chromating to take 24 hours, and for yellow chromating to about 200 hours before the first white rust appears.

Phosphating on galvanic zinc layers also increases corrosion protection. However, these surfaces are rough and, if damaged, do not show the self-healing mechanism that is known from chromating. Therefore, phosphating is only used as a primer for subsequent painting. In the salt spray test, these layers are corrosion-resistant up to a test duration of around 150 hours.

However, these values only apply to smooth-walled parts and not to critical points, such as those represented by protruding angles (e.g. depressions, blind holes, threads). In these areas, too little zinc is deposited due to electrical field effects in order to achieve good corrosion protection.

The protective effect of chromating can be increased by dipping freshly chromated parts in special aqueous so-called sealing solutions, which has a significant effect in the salt spray test. Similarly, in the method known from DE-AS 20 46 449 for protecting corrosion-sensitive safety-relevant parts, such as brake lines, galvanized metal parts are chromated, rinsed and then rinsed wet in a dispersion of Dipped plastics, whereby an elastic, intimately interlocking composite layer of uneven thickness of chromating and plastic is obtained, which provides good protection against corrosion.

DE-AS 15 21 656 describes the chromating of zinc and zinc alloys. After rinsing and subsequent air drying, the chromated metal surfaces can be given a siccative coating based on so-called drying oils (unsaturated fatty acids), which crosslink with the absorption of oxygen from the air.

For environmental reasons, the combination of galvanizing and subsequent non-porous painting in large systems (e.g. automotive industry) is mostly water-borne paint, especially electro-dip paint.

Electrocoating is particularly suitable since this process produces layers of uniform thickness, so that high demands on the dimensional accuracy of the painted parts can be met. In contrast to the galvanic deposition of metals, critical cavities such as blind holes or inner pipe walls are also continuously painted. These inaccessible areas are usually well protected against injuries from external influences. Electrophoretic dip painting of galvanized steel parts can therefore lead to a significant improvement in corrosion protection. With this combination, however, the adhesion between zinc and the organically structured paints is very problematic. It has been shown that the paint flakes off within a short time under exposure to weather and / or mechanical stresses. Adhesion mediation is therefore required for the paint application. Adhesion mediation is also necessary in any case if metals with similar properties to zinc, for example cadmium or aluminum, are to be painted. An adhesion promoter is also frequently used for steel.

In the case of the base metals steel, zinc (as die casting or layer application, for example on steel), cadmium (as layer application) aluminum (or its alloys), as well as magnesium (or its alloys), the adhesion imparting usually consists of phosphating.

However, the phosphating process has some serious disadvantages:
-The entire process for promoting adhesion using phosphating is quite extensive:
-Activate (inoculate with TiO₂ particles in order to obtain fine-grained, even phosphating layers)
-Phosphating; do the washing up
-Cromic acid diving; Do the washing up.

It therefore requires corresponding plant investments and leads to long throughput times; the chemicals used are also expensive.
-The phosphating solution must be filtered continuously to remove any precipitated, insoluble tertiary phosphate; this would interfere with the phosphating process.
-The concentration of the phosphating solution is very high with 100 to 200 g / l phosphating salt; this requires a considerable amount of rinsing after phosphating. Both the filtration and the rinsing lead to a considerable amount of phosphate-containing sludge; This must be disposed of as special waste due to the heavy metal content.
-The analysis of the phosphating solution is complex and difficult to automate; however, the process should be automated to ensure consistent quality in continuous operation.

So far, the skilled worker had to assume that adhesion promotion for electrocoat materials by chromating is not possible. If metal surfaces, which were provided, for example, with chromated connecting parts, such as screws, were painted by electrophoretic immersion deposition, then these parts showed defective painting or paint flaking, such as in "Galvanotechnik 80 (1989), pages 1615-1621 and in" Sealing and painting of galvanized surfaces, lecture and discussion meeting on February 21 and 22, 1989, Düsseldorf ", German Research Association for Surface Treatment eV (DFO) together with the German Society for Electroplating and Surface Technology eV (DGO), pages 143-153.

The object of the invention is to provide a method by means of which electro-dipping lacquers can be deposited on chromatable metal surfaces with much better adhesion than corresponds to the state of the art and by which the disadvantages of phosphating for promoting adhesion can be avoided.

This object is achieved according to the invention by a process for electrophoretic dip coating of chromatable metal surfaces, which is characterized in that the metal surfaces are pretreated by chromating and the chromated metal surfaces are kept wet from chromating to introduction into the bath for electrophoretic dip coating. By keeping wet it is to be understood that the aqueous wet chromated metal surface obtained after the chromating is held until the electrodeposition is carried out under such conditions that no drying can occur.

Surprisingly, it has been shown in the context of the invention that chromating layers can be used as adhesion promoters for electrocoat materials, provided that these are kept wet after production until electrocoating. In the context of the invention, it has been shown that freshly produced, still wet chromating layers have a hydrophilic surface which is suitable for electrocoating. This suitability is maintained if the freshly prepared chromate layers are kept wet or stored in a moist state until electro-coating. After the chromated surfaces have dried, electro-dipping paints can only be deposited with poor adhesion. It has also been found that after drying, once again wetting the once dried chromating layer with water does not lead to an improvement in the adhesion of coating compositions applied by electrocoating. This applies in the same way to all chromate layers applied by customary procedures.

According to the invention, chromating is applied as an adhesive base before electrocoating. In contrast to the phosphating used up to now, such chromating has a smooth, shiny surface. If the chromated surface is kept wet according to the invention, the surface has such a high surface tension that when wetted with water at the interface, that is, at the edge of the water drop, a contact angle of the order of 0 degrees results. If drying takes place, this contact angle is greatly increased, for example to 20 to 50 degrees, which results in poor wettability.

The high-quality deposition of the electrocoating material on the chromating layer can be ensured in that the lacquer is deposited directly after the chromating without intermediate drying.

The high-quality deposition of the electrocoating material on the chromating layer can also be ensured by keeping the chromating layers permanently moist until the beginning of the lacquer deposition by spraying with water or by storing in air with a high relative humidity. Spraying with water is particularly suitable if the entire surface can be sprayed.

A high relative humidity is to be understood as a humidity which is required so that no water can evaporate from the chromated surface and drying out is prevented. It depends on the length of time that must be bridged before the paint deposition begins. In general, for example, a relative humidity of> 90% can be assumed; with long storage it can be up to 100%.

Storage of the fresh layers under water until the start of the electrocoating is, on the other hand, less preferred, since components of the chromating (in particular chromate ions) can dissolve and the chromating layer can bleed out.

The method according to the invention can improve the adhesion of coatings deposited by electrophoretic dip coating on surfaces of chromatable metals. Such metals are, for example, zinc, cadmium, aluminum, magnesium and their chromatable alloys. The method according to the invention is thus suitable, for example, for iron or steel surfaces, such as sheets, which are coated with such chromatable metals, such as zinc.

The chromating of the metal surfaces is carried out in the usual manner known to the person skilled in the art. Any known chromating process can be used, as described, for example, in the book T.W. Jelinek, "Galvanic galvanizing", Leuze Verlag 1982, is described.

The chromating is preferably carried out using a chromating solution which consists only of inorganic components. Such a chromating solution has, for example, a concentration of 1 to 10 g / l chromic acid (H₂CrO₄), in particular about 4g / l chromic acid at a pH of 0 to 3, preferably from 2.3 to 2.7. It is cheap, but not necessary, if the chromating solution contains one or more salts of the metal to be chromated. Examples of such salts are chlorides, nitrates and / or fluorides. The concentration of such salts which may be present is, for example, 0.001 to 0.1 mol / l, preferably 0.05 mol / l. The pH of a freshly prepared chromating solution can be adjusted, for example, with an oxide or hydroxide of the metal to be chromated. It can be checked during operation by measurement, for example with a glass electrode or by conductivity measurement, and set again to the desired value by adding acid, oxide or hydroxide.

The usual chromations represent the last layer, the so-called "finish" for chromatable metals and are optimized for this purpose; ie they offer some protection against corrosion (zinc: yellow and olive chromate) or improve the appearance (zinc: blue and black chromate). However, other aspects such as environmental friendliness, long service life, regenerability etc., which do not directly affect the quality of the chromating layer, are currently hardly considered.

In the process according to the invention, on the other hand, chromating is only required to impart adhesion, the requirements for appearance and, in part, also for corrosion protection are met by the paint or by the combination of galvanically deposited zinc layer / paint layer.

Any conventional chromating which guarantees a long service life, simple regenerability and a low chemical consumption of the chromating baths is therefore particularly suitable for the method according to the invention.

For example, chromating solutions that contain only inorganic components are particularly suitable, since they can be regenerated by the process known from DE-PS 31 38 503. According to the teaching of DE-PS 31 38 503, disruptive degradation products are removed from the chromating solutions with the aid of ion exchangers, electrodialysis, electrolysis or chemical oxidation, the pH or the conductivity are measured during operation, the Cr⁶⁺ - and Cr³⁺- Concentrations are determined photometrically and supplementary solutions are added in accordance with these analysis values and the flow through the ion exchangers or the separation, exchange or reaction devices are regulated in such a way that the composition of the chromating solution is kept in a predetermined concentration range. Very low-concentration chromating solutions are sufficient to promote adhesion between galvanically deposited zinc and electrocoat. Because the chromate solution is carried away with the chromated metal parts, undesired degradation products in the chromate solutions cannot be concentrated to disturbing values. On removal of the unwanted degradation products, as described in DE-PS 31 38 503 e.g. provided with the help of ion exchangers can therefore be dispensed with; it is sufficient to supplement the compensation of the chromate solution that has been carried out.

According to the invention, the known chromating processes can also be used which work without hexavalent chromium, that is to say without chromate. These methods are also familiar to the person skilled in the art and are described, for example, in the above-mentioned book by TW Jelinek. Such Chromating is badly suited for corrosion protection purposes, but it gives good adhesion and has the advantage that the baths used are environmentally friendly because they do not contain hexavalent chromium. Such baths contain chromium (III) salts, such as potassium chromium sulfate; they can contain acids such as nitric acid and salts such as fluorides, for example ammonium hydrogen fluoride. As described above, they can be regenerated simply by adding supplementary solution.
All conventional chromating processes are suitable for the process according to the invention. In the Federal Republic of Germany the chromating processes are standardized according to the regulation DIN 50960, part 1. A distinction is made between colorless chromating, blue chromating, yellow chromating, olive chromating and also black chromating. These chromates are suitable, for example, for the process according to the invention. According to TW Jelinek, "Galvanic galvanizing", Leuze Verlag 1982, page 140, layer thicknesses of up to 0.01 µm with a layer weight of 0.03 mg / dm² are used for colorless chromating, and layer thicknesses of up to 0.08 µm for a layer weight for blue chromating from 0.5 to 5 mg / dm², with yellow chromating layer thicknesses of up to 1 µm with layer weights of 5 to 20 mg / dm² and with olive chromating layer thicknesses of up to 1.25 µm with a layer weight of 20 mg / dm². All of these layer thicknesses (which in the stated state each relate to dry layer thicknesses) are suitable for the process according to the invention, it also being sufficient for the process according to the invention to form the thin colorless or blue chromates, which are normally only for decorative purposes, but not for the Corrosion protection, can be used.

Rinsing with water can be beneficial to remove excess chromating solution immediately after chromating. Whether such a rinsing process is carried out depends on the concentrations of the chemical compounds and ions used in the chromating solution and on the procedure. With electrophoretic paint deposition, as few ions as possible should be carried into the paint bath. If the chromated metal parts are to be introduced into the electrodeposition bath without delay, a rinsing process can be carried out before the, regardless of the composition of the chromating solution used Electrodeposition can be particularly cheap. If, on the other hand, the chromated metal parts are kept wet by spraying with water until they are introduced into the electrodeposition bath, an additional rinsing process can be dispensed with if the chromating solution is suitably composed.

The metal surfaces pretreated according to the invention by chromating and wet holding can be coated or painted by conventional electrophoretic dip coating. All customary coating compositions or electrophoretic dip coating processes which are familiar to the person skilled in the art are suitable. There is no restriction with regard to the coating agents that can be used or electrophoretic dip coating processes. Of the two principal possibilities of electrophoretic dip painting, anaphoresis and cataphoresis, cataphoresis, i.e. cataphoretic dip coating is particularly preferred. However, anodic deposition (anaphoresis) is also suitable.

The lacquer layers produced in the manner according to the invention are shiny, smooth and non-porous and provide excellent protection against corrosion. These lacquer layers can serve, for example, as primers, on which conventional further processing with, for example, fillers and topcoats can take place.

An example of the treatment of a galvanized steel sheet by the method according to the invention is given below.

EXAMPLE:

A steel sheet was treated according to the following process steps:
- degreasing (with solvent or aqueous alkaline); do the washing up
- pickling (with mixed acid or sulfuric acid); do the washing up
- electrolytic degreasing; do the washing up
- galvanizing (cyanide, alkaline or acidic); rinsing
- Brightening (10 s; 3g / l HNO₃); rinsing can be omitted
- chromating (1-3 min), rinsing; do not dry;
- cataphoretic dip painting with a commercially available electrodeposition paint; Rinse with water
- drying, baking

In this example, the chromating bath has the following composition:
1 - 5g / l H₂CrO₄
2 - 10 g / l Zn (NO₃) ₂
pH about 2.5; adjusted with ZnO or NaOH

The chromating layer is almost transparent and leads to very good adhesion of the lacquer layer. The paint layer is shiny, smooth, even, non-porous and provides good protection against corrosion.

The chromating solution shows no self-decomposition. In addition, since the dissolution of zinc (and iron in the non-galvanized areas) during the chromating process is very low, degradation products do not accumulate to a disruptive concentration; there is no need to clean the solution using a cation exchanger. For continuous operation, it is advisable to continuously add the bath components that have been towed out and to keep the pH constant (by hand after analysis or fully automatically and continuously in accordance with DE-PS 31 38 503). Example of a chromate-free chromating solution Ammonium hydrogen fluoride (NH₄) HF₂ 2.0 g / l nitric acid 4.0 g / l Potassium chromium sulfate KCr (SO₄) ₂x12H₂O 3.0 g / l

Claims (11)

1. Process for electrophoretic dip coating of chromatable metal surfaces,
characterized in that the
Metal surfaces are pretreated by chromating and the chromated metal surfaces are kept wet from chromating to introduction into the bath for electrophoretic dip coating. 2. The method according to claim 1, characterized in that the freshly chromated metal surfaces are introduced without delay with wet surfaces in the bath for electrophoretic dip coating, are sprayed with water until introduction into the bath for electrophoretic dip coating or in air with high relative humidity be stored. 3. The method according to claim 1 or 2, characterized in that the electrophoretic dip coating is carried out cataphoretically. 4. The method according to claim 1, 2 or 3, characterized in that the freshly chromated metal surfaces are rinsed with water directly after the chromating. 5. The method according to one or more of the preceding claims, characterized in that a chromating solution is used to produce the chromating layer, which contains only inorganic components. 6. The method according to claim 5, characterized in that an aqueous solution is used as the chromating solution, the chromic acid and optionally contains salts of the metal to be chromated. 7. The method according to claim 5 or 6, characterized in that the chromating solution contains low concentrations of the inorganic constituents, in particular less than 10 g / l H₂CrO₄ and less than 0.1 mol / l of salts of the metal to be chromated which may be present in the chromating solution . 8. The method according to claim 7, characterized in that the chromating solution is regenerated solely by adding supplementary solutions. 9. The method according to claim 8, characterized in that a supplementary solution of concentrated chromic acid to adjust the desired concentration of CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4th}}$

is used, and optionally a second supplementary solution, which contains hydrochloric acid, nitric acid and / or hydrofluoric acid, as well as chloride, nitrate and / or fluoride of the metal to be chromated, is used to adjust the desired pH. 10. The method according to any one of claims 1 to 5, characterized in that the chromating is carried out without the use of chromium (VI). 11. Use of chromating layers kept wet after production as an adhesion promoter in the electrophoretic dip coating of chromatable metal surfaces.