WO1995002463A1

WO1995002463A1 - Resin-finished metal surface and method of producing it

Info

Publication number: WO1995002463A1
Application number: PCT/DE1994/000828
Authority: WO
Inventors: Michael Stoz
Original assignee: Michael Stoz
Priority date: 1993-07-15
Filing date: 1994-07-13
Publication date: 1995-01-26
Also published as: AU7225694A

Abstract

The invention concerns a metal part, in particular an iron or a steel part with a treated, in particular finished, surface, the surface of the part, which may or may not be pretreated in some way, being finished using an aqueous resin solution containing less than 20 % by wt. of resin and the thickness of the resin coat not exceeding 5 νm at any point on the surface. A particularly high degree of protection against corrosion is ensured by virtue of the fact that the resin in the solution is a reactive cross linking low-molecular resin which cross links on the surface when heated at a temperature of more than 100 °C. As well as being environmentally particularly acceptable, the method proposed has the advantage of requiring only an extremely thin coat of resin on the metal surface and hence gives enormous cost and weight savings in the finished product. In addition, the method can be used with any metal surface and makes it possible to carry out a bath-in-bath treatment following degreasing and phosphatization of the metal part.

Description

Resin coated metal surface and process for its production

The invention relates to a metal part, in particular an iron or steel part with a treated, in particular tempered surface, in which the raw or otherwise pretreated metal surface is coated with an aqueous resin solution which has a resin concentration of less than 20% by weight of the resin The local covering thickness with resin on the entire surface is less than 5 μm.

Such a metal part is known from EP 0 319 018 A2.

Surface-treated, in particular phosphated, metal parts are generally known, for example from the book "The Phosphating of Metals" by Dr. Werner Rausch, published by Eugen G. Leuze Verlag, Saulgau / Württe berg, 1988. Metal surfaces, in particular iron surfaces, have been phosphated for preventing corrosion for more than 100 years. As early as 1864, Ch. De Bussy received an English patent for the treatment of red-hot iron with a mixture of coal dust and calcium dihydrogen phosphate to increase the resistance of the iron surface to corrosive influences. TW Coslett from Birmingham, according to his basic English patents from 1906 and the corresponding patents in France, the USA and Germany, is regarded as the actual founder of phosphating technology, cleaned objects made of iron and steel for rust protection with hot diluted phosphate phosphoric acid should be treated at temperatures close to the boiling point. The corrosion protection of the layers produced in this way against atmospheric influences was not too great, but Coslett tried to improve it by various measures, such as by post-treating the layers with oil or varnish or by rinsing with chromic acid solutions.

In the meantime, phosphating or zinc phosphating of metal parts, in particular steel sheets, is part of the standard treatment against corrosion, particularly in the automotive industry. In the conventional phosphating of steel, the treatment object is rinsed with deionized water after the phosphate has been applied and, in order to increase the corrosion resistance, either a chromate or, more recently, a zirconium-containing solution is added to the last rinsing bath as aftertreatment. After a metal surface treated in this way has dried, the parts must either be wetted with an oil layer, a conventional coating, an electro-dip coating (KTL), a powder for longer-lasting corrosion protection. lacquer or plastic coating. The coatings produced by this aftertreatment have a thickness of more than 15 μm to 30 μm even when applied very thinly. This of course makes the treated sheets correspondingly heavier.

In some of the aftertreatment processes, the surface treated in this way can no longer be overcoated with lacquer or powder coating. This applies in particular to the treatment of the phosphate surfaces with oils, waxes, etc.

Another disadvantage of the conventional aftertreatment processes for phosphated metal surfaces is that the corresponding processes can only be used with very specific phosphate layers. In the case of other phosphated surfaces, other post-treatment methods must then be used.

A disadvantage of most known aftertreatment processes is that no "bath-in-bath" treatment is possible. While the degreasing of the metal parts and the subsequent phosphating in aqueous solution take place, for example painting or applying an oil layer is not possible immediately afterwards, but the treated part must be thoroughly dried before the aftertreatment.

In addition, most aftertreatment processes are not particularly environmentally friendly, since considerable amounts of solvent usually have to be used for the aftertreatment. The thick layers applied in the known aftertreatment processes not only increase the weight of the treated parts, as mentioned above, but also facilitate the wallet of the customer of these parts, since the application involves a cost-intensive use of materials. Especially when painting, plastic or powder coating, the material costs for applying the layer are by no means negligible.

In many cases it would often be possible to work with a thinner layer on the phosphate layer, but it has been shown that in this case the aftertreatment only leads to a very slight increase in the corrosion resistance of the phosphated metal part.

From DE 28 50 977 C2 a method is known in which a protective layer of an air-drying organic coating agent is applied to a phosphated metal surface by means of electrophoresis. However, such an electrophoretic coating is very complex and costly.

DE 37 21 017 AI describes an electrically conductive and corrosion-resistant steel sheet in which a coating of small thickness with resins or plastics is provided. The coating is applied with the aid of solvents, resulting in layer thicknesses of the order of 20 μm. Such methods are harmful to the environment because of the solvents used and are costly and yet do not lead to satisfactory corrosion protection.

Finally, EP 0 319 018 A2 cited at the beginning describes a method for coating metal surfaces Resins preferably have higher melocular weights by applying an aqueous resin solution which, according to the publication, can have a resin concentration between 0.001% and 80% by weight. A non-reactive, air-drying resin is used, in which no crosslinking of the resin can occur when the aqueous resin solution dries on the metal surface, which should take place in a temperature range between room temperature up to a maximum of 80 ° C. In addition, according to the method described in EP 0 319 018 A2, the surface can be rinsed optimally after the surface has been wetted with the resin solution, so that it is questionable whether a permanent resin coating can form on the metal surface treated in this way .

In contrast, it is an object of the present invention to achieve an inexpensive improvement in the corrosion resistance of metal parts with untreated or pretreated, in particular phosphated, surfaces by simple means, with no noticeable application of layer thickness and thus only an extremely low use of material on the treatment Treatment material is required, after the treatment of the surface any conceivable type of further processing of the metal part is possible, wherein the treatment process itself is particularly environmentally friendly, enables bath-in-bath treatment and generally on any type of metal surface, in particular phosphated Surface is applicable.

According to the invention, this difficult task is solved in an astonishingly simple and effective manner in that the resin in the resin solution is a low-molecular, reactive, crosslinking resin and on the surface by a temperature turbo treatment is cross-linked at more than 100 ° C.

Surprisingly, it has been shown that a metal surface prepared in this way does not have any signs of corrosion after more than 800 hours in a condensation test DIN 50017, whereas, for example, a phosphated test surface, on which the aftertreatment according to the invention with aqueous resin solution was not carried out, already after 2 to 3 hours of continuous corrosion on the entire surface shows. Completely untreated metal surfaces naturally corrode even faster. The importance of the innovation according to the invention for the entire metalworking industry, in particular for the automotive industry, should not be overestimated on the basis of these results.

In addition to the universal applicability, the environmental friendliness and the low cost of metal parts prepared according to the invention, there are further advantages of the possibility of inner coatings of thin lines, which is absolutely not possible, for example, due to the known cathodic dip coating processes. in particular, no change in the clear diameter can occur, since the coating thickness of the resin on the metal surface can hardly be measured. Furthermore, with the metal parts prepared according to the invention, there can be no problems with uneven application of a coating, as is often the case, for example, with painting or plastic coating.

Another considerable advantage of the metal parts prepared according to the invention is that subsequent deformation of the part does not damage the coating is expected. In contrast, a lacquer, plastic or powder coating that has been applied after phosphating breaks off in most cases during bending, so that phosphate or even the raw metal surface is released at the kink, which opens the door to a corrosion attack.

In a preferred embodiment, the local covering thickness with resin on the entire surface is less than or equal to 1 μm. Due to the extremely low material thickness, there is practically no cost per post-treated with an aqueous resin solution for each tempered metal part.

A further development of this embodiment is particularly preferred in which the local resin coating on the surface is monomolecular. It has been shown that even with these extremely thin local coatings on the metal surface or a phosphate layer on a metal surface with resin molecules, the effectiveness of the corrosion protection is hardly impaired.

In order to save costs due to the low use of materials and above all with a view to increasing the weight of the treated surface as little as possible, in one embodiment of the invention the resin coating on the surface is less than 0.2 g / m ² , preferably about 0.1 g / m ² .

In preferred embodiments of the invention, the resin concentration of the aqueous resin solution is less than 5%, preferably between 3% and 4% by weight. This also guarantees that the surface coating according to the invention is particularly environmentally friendly. In addition, in the case of such dilute aqueous resin solutions, a bath-in-bath treatment of the metal surface after degreasing and phosphorous Phating possible without any problems.

An embodiment of the invention is particularly preferred in which the metal surface is already phosphated, preferably zinc-phosphated, before the aqueous resin solution is coated. As already mentioned at the beginning, the phosphating of metal surfaces is a particularly widespread type of corrosion treatment. The coating according to the invention of such a phosphated metal surface increases the corrosion protection in a simple manner once again by a multiple. The same naturally also applies to metallic surfaces treated in another way, for example anodized surfaces and the like.

An embodiment is preferred in which the aqueous resin solution is weakly alkaline, preferably in the range pH 8.5 to pH 9. Tests have shown that the corrosion protection in this case is considerably more effective than with a resin solution in the acidic range. In addition, in the case of aftertreatment of an already phosphated surface in an acidic environment, phosphate is redissolved from the surface into the treatment solution. The post-treatment would thus rather reduce the corrosion protection due to the phosphate layer.

A slightly increased corrosion resistance can be observed in an embodiment of the invention in which a phosphated surface is pretreated with chromium-6 rinsing or with zirconium rinsing before a resin bath aftertreatment is carried out.

In embodiments of the invention, the phosphated Have a low zinc phosphating surface with a layer thickness of up to about 4 μm. In other embodiments, the phosphated surface has normal phosphating with a layer thickness between 5 μm and 8 μm. However, the sealing of the phosphate surface according to the invention also increases the corrosion protection of a part coated with a thick layer of phosphate with a phosphate layer thickness between 10 μm and 40 μm. In the thick-layer phosphating, an amorphous phosphate layer is formed, which can only be post-treated by oiling. Other types of corrosion-inhibiting aftertreatment, such as painting, are not possible in the case of thick-layer phosphating, since the thick amorphous phosphate layer, together with the aftertreatment layer, breaks off from the surface. It is therefore particularly advantageous that the resin sealant according to the invention shows the corrosion-inhibiting effect described above even in the case of a thick-layer phosphated metal part.

In another embodiment of the invention, the metal surface is galvanized, nickel-plated or otherwise coated with metal before the aqueous resin solution according to the invention is applied. It has also been shown here that a substantial increase in the protection of the surface against corrosive attack is achieved.

Extraordinary corrosion protection is achieved with phenolic resin. The use of epoxy resin also leads to fairly good results, while a metal surface treated with acrylic resins, in particular pre-phosphated, is much more susceptible to corrosion attacks, but is still somewhat more resistant than an untreated surface. An embodiment of the invention is particularly preferred in which the resin solution contains a combination of synthetic resins, preferably an alkyd-phenolic resin mixture.

The coating according to the invention of a metal part with a thinned aqueous resin solution can be used with any type of metal. However, the metal part preferably consists of iron, steel, aluminum, copper, brass, zinc or alloys thereof.

In embodiments of the invention the metal part can be in the form of a rolled sheet, an extruded part or a casting, in other embodiments it can be in the form of an endol strip or a wire.

The invention also encompasses a method for treating a raw or already pretreated, preferably phosphated, in particular zinc-phosphated, metal surface, in particular an iron or steel surface, in which the metal surface is in the raw state or, if appropriate, after another pretreatment such as Phosphating etc. is wetted with an aqueous resin solution with a resin concentration of less than 20% by weight of the resin. The above-mentioned object of the invention is achieved in such a method in that the aqueous resin solution is weakly alkaline, preferably in the pH 8.5 to pH 9 range, that the resin in the resin solution is a low molecular weight, reactive, crosslinking resin, and in that after the metal surface has been wetted with an aqueous resin solution, the metal surface is dried and the resin is crosslinked at temperatures above 100 ° C.

A variant of the invention has proven advantageous Proven method in which the wetting of the metal surface with resin solution takes place at a resin bath temperature between 20 ° C and 40 ° C, preferably between 25 ° and 35 °.

With regard to the treatment period, wetting the metal surface with aqueous resin solution between 10 sec and 60 sec, preferably about 30 sec, is advantageous.

In variants of the method according to the invention, drying and crosslinking temperatures between 120 ° C. and 180 ° C. over a period of 30 minutes down to 5 minutes are provided.

The drying temperatures can preferably be between 120 ° C and 140 ° C and the corresponding drying times between 20 minutes and 10 minutes.

As an alternative to this, a type of shock treatment can be carried out in special applications, for example in the treatment of endless belts or longer wires which are to be tempered at a processing speed of typically 200 m / min, the drying of the wetted metal surface and the crosslinking of the resin is caused by a shock by heating to a temperature above 200 ° C in a period of less than 2 min.

For example, the metal surface to be treated can be heated to approximately 240 ° C. after wetting with the aqueous resin solution, but the surface should be exposed to this high temperature for a maximum of about 1 minute, otherwise the resin particles burn instead of crosslinking.

A treatment solution also falls within the scope of the invention. solution for the tempering of a raw or already otherwise treated, preferably phosphated, in particular zinc-phosphated metal surface, which consists of an aqueous resin solution with less than 20% by weight of the resin, preferably less than 5%, in particular between 3% and 4% resin concentration, and is weakly alkaline, preferably in the pH 8.5 to pH 9 range.

The invention is described and explained in more detail below with reference to exemplary embodiments and the drawing. The features to be found in the description can be used in other embodiments of the invention individually, individually or in combination in any combination.

The figure shows a graphical representation (SEM image) of a zinc phosphate layer on steel (approx. 18 g / m ² ) according to the prior art without the coating according to the invention.

The phosphating treatment of steel takes place in different, precisely coordinated process steps. These are usually broken down into

- degreasing and cleaning

- rust removal and descaling

- activation

- phosphating

- post-treatment.

One or more rinse cycles are carried out between each treatment stage. Special aftertreatment processes can increase the corrosion resistance of phosphate layers. Such known aftertreatment processes are Post-treatment with inorganic salts, post-treatment with oils, waxes, etc., post-treatment with impregnation paints. Further possibilities for increasing the protection against corrosion of phosphated metal surfaces consist in a lacquer coating, a plastic coating, a powder coating or a cathodic dip coating.

In contrast, an aftertreatment of the raw or already phosphated metal surface with a strongly diluted aqueous resin solution is proposed according to the invention. In the process, water-dilutable, low molecular weight resin creeps into the "defects" on the metal surface or between the phosphate flakes as can be seen in the drawing in the scanning electron microscope image. To this end, an extremely low covering of these "defects" in the valleys between the phosphate flakes with the protective reactive resin is apparently sufficient to shield the metal surface which is exposed at these points or which is only covered with a relatively thin phosphate layer and is accordingly unprotected against corrosion attacks. The thinned resin may also "glue" the pointed phosphate structures protruding from the surface and thus contribute to mechanical and chemical solidification of the surface.

The dilution of the resin in resin solution, expressed in percent by weight, is less than 5%, preferably between 3% and 4%. This results in layer thicknesses of less than or equal to 1 μm, in some cases even only local coverings of the treated phosphate surface with a monomolecular resin layer which only covers the “valley areas” between the phosphate tips protruding from the surface. After the treatment according to the invention, the weight per unit area of the resin applied to the metal part is between 0.1 and 0.2 g / m ² .

The quality of the resin coating is in line with the selection of an appropriate phosphate. Resin-coated thick-layer phosphating with phosphate layer weights of approx. 40 g / m ² can achieve corrosion protection values that are as good as the corresponding layer aftertreated with corrosion protection oil. Resin-coated thick-film phosphating of this kind is comparable in terms of its corrosion protection, even with galvanized or cadmium-plated and subsequently chromated steel parts. It is an advantage over galvanic treatment that Farday effects cannot occur. Thus, by means of resin-coated thick-film phosphate surfaces according to the invention, corrosion-protective layers or coatings can be applied in places where this is not possible with the galvanic processes.

As already mentioned above, the process according to the invention can also be used to temper a raw, galvanized, nickel-plated, copper-plated or otherwise pretreated metal surface.

Particularly good corrosion protection results can be achieved when using phenolic resins, in particular when combining different synthetic resins, preferably with alkyd-phenolic resin combinations. Epoxy resins also lead to good results, while a much lower corrosion resistance is observed when using acrylic resins, but this is still higher than that of untreated metal surfaces. Test results with condensation water tests according to DIN 50017 on phosphated sheets showed that only phosphated surfaces showed continuous corrosion over the entire surface after about 2 to 3 hours, while sheets treated according to the invention showed no signs of corrosion even after more than 800 hours.

The resin solution is preferably applied to the phosphate surface in the weakly alkaline range, in particular at pH 8.5 to pH 9. It is much more effective than in the acidic range, especially since at low pH values, phosphate would be redissolved from pre-phosphated surfaces in the treatment bath and the surface protection would thus be weakened.

A chrome 6 bath rinse or a zirconium bath rinse after phosphating and before the resin bath treatment proves to be favorable with regard to the corrosion properties. However, the corrosion protection cannot be increased too much in relation to the improvement achieved by the treatment according to the invention.

The bath temperature of the aqueous resin solution should be about 25 ° C to 35 ° C. Treatment times of approx. 10 sec to 60 sec, preferably 30 sec lead to excellent results.

A bath-in-bath treatment after the degreasing and phosphating of the metal surface which takes place in an aqueous environment is unproblematic since, as described above, treatment with strongly diluted aqueous resin solution is at the core of the invention. Since no aggressive solvents are used, the one according to the invention is Corrosion protection can be achieved particularly environmentally friendly.

After the resin bath treatment, the treated parts are dried between 10 minutes and 20 minutes at temperatures in the range from 120 ° C. to 140 ° C. At higher temperatures in the range of 180 ° C, the drying time can also be reduced to 5 minutes.

A particular advantage of the corrosion treatment of metal surfaces according to the invention is that even very thin hollow pipes can be coated internally without changing the inside diameter, since there is practically no coating thickness application with the resin used on the metal surface. Although such coatings are extremely thin (up to monomolecular), no damage to the coating occurs if the treated part is subsequently deformed.

Applications of the invention are conceivable in every area of metal processing, but in particular in the automotive industry, in the processing of metal sheets, but also in areas such as the manufacture of medical instruments or precision mechanical devices.

Claims

Metal part, in particular an iron or steel part with a treated, in particular tempered surface, in which the raw or already pretreated metal surface is coated with an aqueous resin solution which has a resin concentration of less than 20% by weight of the resin, the local Covering thickness with resin on the entire surface is less than 5 μm,

characterized,

that the resin in the resin solution is a low-molecular, reactive, crosslinking resin and is crosslinked on the surface by a temperature treatment at more than 100 ° C.

Metal part, in particular an iron or steel part with a treated, in particular tempered surface, in which the raw or otherwise pre-treated metal surface is coated with an aqueous resin solution which has a resin concentration of less than 20% by weight of the resin, the local covering thickness with resin on the entire surface is less than 5 μm, characterized in that the resin concentration in the aqueous resin solution is between 3% and 4% by weight.

3. Metal part according to claim 1 or 2, characterized gekenn¬ characterized in that the local covering thickness with resin on the entire surface is less than or equal to 1 micron.

4. Metal part according to claim 3, characterized gekennzeich¬ net that the local resin coating on the surface is monomolecular.

5. Metal part according to one of the preceding Ansprü¬ surface, characterized in that the resin coating on the surface is less than 0.2 g / m ² , preferably about 0.1 g / m ² .

6. Metal part according to one of the preceding claims, characterized in that the metal surface is phosphated, preferably zinc phosphated.

7. Metal part according to claim 6, characterized in that the phosphated surface is pretreated with chromium-6 flushing or with zirconium flushing.

8. Metal part according to one of claims 6 or 7, characterized in that the phosphated surface has a low zinc phosphating with a layer thickness of up to about 4 microns.

9. Metal part according to one of claims 6 or 7, • characterized in that the phosphated surface has a normal phosphating with a layer thickness between 5 microns and 8 microns.

10. Metal part according to one of claims 6 or 7, characterized in that the phosphated surface has a thick-layer phosphating with a layer thickness between 10 microns and 40 microns.

11. Metal part according to one of claims 1 to 5, characterized in that the metal surface is galvanized, nickel-plated or otherwise coated with metal.

12. Metal part according to one of claims 1 to 11, characterized in that the resin is a phenolic resin.

13. Metal part according to one of claims 1 to 11, characterized in that the resin is an epoxy resin.

14. Metal part according to one of the preceding claims, characterized in that the resin solution contains a combination of synthetic resins, preferably an alkyd-phenolic resin mixture.

15. Metal part according to one of the preceding claims, characterized in that the metal part consists of iron, steel, aluminum, copper, zinc, brass or alloys thereof.

16. Metal part according to one of the preceding claims, characterized in that the metal part is in the form of a rolled sheet, an extruded part or a cast part.

17. Metal part according to one of the preceding claims, characterized in that the metal casting is in the form of an endless belt or a wire.

18. A method for treating a raw or already pretreated, preferably phosphated, in particular zinc-phosphated, metal surface, for example an iron or steel surface, in which the raw or already pretreated metal surface is treated with an aqueous resin solution with a resin concentration of less than 20% by weight of the resin is wetted,

characterized,

that the aqueous resin solution is weakly alkaline, preferably in the pH 8.5 to pH 9 range, that the resin in the resin solution is a low-molecular, reactive, crosslinking resin, and that after the metal surface has been wetted with aqueous resin solution ¬ solution drying the metal surface and crosslinking of the resin at temperatures above 100 ° C, preferably between 120 ° C and 180 ° C.

19. The method according to claim 18, characterized in that the wetting of the metal surface with resin solution occurs at a resin bath temperature between 20 ° C and 40 ° C, preferably between 25 ° C and 35 ° C.

20. The method according to claim 18 or 19, characterized gekenn¬ characterized in that the wetting of the metal surface with resin solution over a treatment period of 10 sec to 60 sec, preferably about 30 sec.

21. The method according to any one of claims 18 to 20, characterized in that the drying of the wetting of the metal surface takes place over a period of between 30 min and 5 min.

22. The method according to claim 21, characterized in that the drying temperatures between 120 ° C and 140 ° C and the corresponding drying times are between 20 min and 10 min.

23. The method according to any one of claims 18 to 20, characterized in that the drying of the wetted metal surface takes place in a shock-like manner at a temperature of over 200 ° C. in a period of less than 2 minutes.