EP0019897B1 - Process for obtaining conversion coatings on metallic surfaces by spraying - Google Patents

Abstract

In the process for the production of conversion layers continuously on a continuously moving band or sheet-like metal surfaces of aluminum, zinc or iron, by the application of a solution creating a layer on said metal surfaces selected from the group consisting of a chromating solution, an acid solution free of chromic acid and containing fluorides and compounds of titanium, zirconium or manganese, an acid solution free of chromic acid containing fluorides, and an acid solution free of chromic acid containing compounds of titanium, zirconium or manganese, by the stationary spray method, onto the cleaned and rinsed metal surface, and subjecting said metal surfaces to customary aftertreatments, the improvement consisting in that said solution creating a layer on said metal surfaces is sprayed through at least one two-component jet spray nozzle with the aid of an inert compressed gas where said solution and said compressed gas are fed separately each to an opening in said two-component jet spray nozzle.

Description

The invention relates to an improved method for producing conversion layers on continuously moving, strip-like or tabular metal surfaces made of aluminum, zinc or iron by applying a chromating solution or a chromic acid-free, fluoride and / or compounds of titanium, zirconium or manganese containing spray solutions.

The generation of conversion layers by spraying is known per se. For example, Ries reports in "Metallfläche - Angewandte Elektrochemie", 25th year (1971) pages 1 to 6, 53 to 56, 75 to 80, 132 to 134 and 153 to 159, in summary about process engineering and processes for the production of conversion layers on metal strips and also discusses corresponding spraying methods in this context. In the usual spraying processes, the respective treatment solution is sprayed through a nozzle system onto both sides of the cleaned and degreased metal strip or a metal plate, the spray pressure required for this being generated and controlled using a suitable pump. Spray pressures of approximately 0.7 to 1.5 bar are generally used here, but the pressure can also be increased to values of approximately 10 to 100 bar in the case of special high-pressure spraying processes.

All of these methods have in common that they require complex pump systems made of chemical-resistant materials which on the one hand can withstand the aggressive ingredients of the treatment solutions and on the other hand ensure a constant but controllable spray pressure. In addition, the known methods usually spray larger amounts of liquid per square meter of metal surface, which necessitate rinsing or squeezing off the excess treatment solution after the reaction has taken place.

British Patent GB-A-863 098 also describes a method for producing protective layers on metal surfaces by spraying on an aqueous solution, for example a chromating solution. The solution is atomized using a propellant gas. Of essential importance for this process, however, is the fact that the metal surfaces have to be heated on the one hand before the treatment solution is applied and on the other hand must be completely free of adhering water. Furthermore, the treatment solution is sprayed onto the stationary metal surface with moving application devices.

In contrast, the object of the present invention is to provide a spraying method for producing conversion layers which allows the spraying treatment only on one side of the band-shaped or tabular metal surfaces.

The invention accordingly relates to a process for producing conversion layers on continuously moving, strip-like or tabular metal surfaces made of aluminum, zinc or iron by applying a chromating solution or a chromic acid-free, fluoride and / or compounds of titanium, zircon or manganese containing spraying process , wherein the solution is sprayed onto the cleaned and flushed metal surface with the aid of inert compressed gas through one or more two-component nozzles in which the solution and the pressurized gas are fed separately to the nozzle outlet opening, and the metal surface is then post-treated, which is characterized in that the Spray solution on only one side of the band or plate-shaped metal surface and at the same time supply compressed air to the other side.

The method according to the invention offers a significant advantage over the conventional spraying methods. In the known processes, both sides - that is to say both the top and bottom - of the metal strip or sheet to be treated are usually sprayed with solution. Due to the better controllability of the amount of liquid by means of compressed air and - in connection therewith - the only small amount of sprayed treatment solution, the method according to the invention enables the treatment of only one side - for example the top side - of the metal strip or sheet. Accordingly, in the sense of the method according to the invention, solution is sprayed onto only one side of the strip-like or tabular metal surface and compressed air is simultaneously supplied to the opposite side. This simultaneous supply of compressed air on the one hand causes the metal strips or sheets to pass through the treatment zone in a straight line - that is, sagging or bending is avoided - and on the other hand prevents the treatment solution from spreading undesirably from one side to the other. In addition, if desired, the drying process can be initiated by supplying heated compressed air.

The use of two-component nozzles makes it possible to spray the respective treatment solutions onto the metal surfaces when carrying out the method according to the invention with the aid of compressed gas, the liquid quantities to be applied being controlled exclusively by regulating the pressure of the compressed gas used. The use of technically complex pressure and control pumps can thus be omitted.

The new process can be used advantageously to produce conversion layers - which are often also referred to in the literature as conversion coatings - on aluminum, iron, steel, zinc or galvanized steel surfaces insert surfaces. The metal surfaces to be treated can pass through the continuously operating systems in the form of «endless strips as well as in the form of sheets or plates.

Before the actual conversion treatment, the metal surfaces are subjected to cleaning or degreasing in a manner known per se and then rinsed thoroughly with water.

Alkaline baths based on sodium hydroxide solution, which may additionally contain polyphosphates, complexing agents and wetting agents, are usually used for degreasing. Depending on the degree of soiling of the metal surfaces, it may be advantageous to use rotating brush pairs to support the cleaning effect and to intensify the entire cleaning process by dividing it into several successive steps - pre-degreasing and post-degreasing - with cleaning solutions of the same or different composition in the circuit in the individual treatment zones can be sprayed onto the metal surfaces with or without pressure. In the interest of subsequent removal of the cleaning chemicals as completely as possible, multiple rinsing of the degreased metal surfaces is also appropriate, with service water being usually sprayed onto the metal surfaces with an overflow. It has proven to be advantageous to work with cold water - that is, water at room temperature - in the first rinsing zone and to use heated water at a temperature of, for example, 50 to 70 ° C. in the subsequent rinsing zone. After passing the last rinsing zone, the conversion layers are produced by spraying on the respective treatment solutions in the manner according to the invention.

For the conversion treatment, for example, the chromating solutions known per se can be used, which usually contain, in addition to chromic acid or alkali chromates, phosphoric acid, nitric acid and / or hydrofluoric acid and optionally polyvalent metal ions, free or complex-bound fluorides and modifying additives. However, chromic acid-free solutions based on, for example, phosphoric acid, which also contain fluorides and / or compounds of titanium, zirconium and manganese and modifying additives, can also be used for the process according to the invention.

For spraying the respective treatment solutions onto the metal surfaces, two-substance nozzles of known design can be used, in which solution and compressed gas are guided in separate channels up to the nozzle outlet opening, so that they only meet at the nozzle outlet opening. Figure 1 shows the basic structure of such a two-component nozzle in cross-section. Herein 1 means the compressed gas supply or the compressed gas channel and 2 the supply for the treatment solution and the corresponding channel. If necessary, the solution can also be supplied via channel 1 and the compressed gas via channel 2. In principle, all those materials from which the nozzles are also produced in conventional spraying systems for producing conversion layers, for example stainless steel, are suitable as materials for such two-component nozzles. The same also applies to the nozzle cross sections of the two-component nozzles to be used according to the invention; As a rule, nozzle cross sections in the range from 0.7 to 2.5 mm can be used.

The arrangement of the two-substance nozzles in the actual treatment zone can be seen in Figures 2 and 3.

Figure 2 shows a perspective view of the guidance of a metal strip 3, which passes through the treatment zone in the direction indicated by the arrows. Herein 4 mean a squeeze roller pair, 5 a height-adjustable support roller, 6 the two-substance nozzles arranged above and below the metal strip, 7 the storage container for the treatment solution and 8 the compressed gas supply.

Figure 3 shows a cross section through the treatment zone; the numbers in this figure have the meaning given above.

The storage container for the treatment solution is expediently arranged over the treatment zone, as the illustrations show. No complex pressure and control pumps are required to fill this container; simple feed pumps, as are usually used for pumping aggressive liquids, are sufficient.

The number of two-substance nozzles arranged above and below the metal strip is ultimately determined by the width of the metal strip or sheet to be treated. In all cases, however, it must be ensured that every point on the metal surface passing the treatment zone "on the side to be sprayed" is hit by a spray jet of the treatment solution. In view of this and the different speeds at which the metal strips pass through the treatment zone, it can be advantageous to arrange several rows of nozzles one behind the other - possibly offset from one another. In general, the arrangement of the nozzles customary in conventional spraying systems can also be used for the process according to the invention.

In general, any compressed and - with regard to the chemicals used and the metal surfaces to be processed - inert gas can be used as the compressed gas for carrying out the method according to the invention. However, the treatment solutions are preferably sprayed with compressed air.

The compressed air used should preferably have a pressure of at least 0.1 bar, to ensure adequate spraying of the treatment solutions on the metal surface. However, lower pressures also lead to satisfactory results with regard to the conversion layers produced. With regard to optimal control of the liquid quantities to be sprayed and - in connection with this - an effective distribution of the treatment solution on the metal surfaces, it has proven to be particularly advantageous to set the pressure of the compressed air to a value in the range from 0.7 to 4 bar to vary in this area. The use of higher pressures is also possible within the scope of the method according to the invention, but generally does not lead to better results.

In the interest of a uniform formation of the conversion layers, liquid quantities of 1 to 50 ml of treatment solution per square meter of metal surface are preferably sprayed on, the liquid quantity being controlled by a corresponding pressure regulation of the compressed air. In general, however, the method according to the invention also permits spraying of smaller or larger amounts of liquid.

The temperature of the treatment solution when spraying is usually in the room temperature range. If desired, however, the solutions can also have higher temperatures - for example in the range from 30 to 70 ° C.

Within the scope of the preferred range of the amount of liquid specified above, the spraying of about 15 to 50 ml of treatment solution per square meter of metal surface requires an aftertreatment thereof in order to remove the portion of the solution which is not used for layer formation from the metal surface. After passing through the treatment zone, the metal strip first runs through a “reaction zone” within about 2 to 4 seconds, in which the sprayed-on treatment solution reacts with the metal surface to form the conversion layer or in which this reaction is completed. The excess treatment solution is then removed by squeezing with rollers and / or - if necessary several times - rinsing with water, the last rinsing preferably being carried out with demineralized water. If desired, however, rinsing with chromic acid solution can also be carried out in a manner known per se to passivate the metal surface. Following this, the metal surfaces are dried in the usual way.

Due to the pressure control in connection with the use of two-substance nozzles, the method according to the invention also enables the spraying of considerably smaller amounts of liquid. In this sense, spraying 3 to 7 ml of treatment solution per square meter of metal surface is particularly preferred. The advantage of this preferred procedure can be seen in the fact that the sprayed-on solution is completely used up to produce the conversion layer, and it is therefore not necessary to remove excess treatment solution from the metal surface. In this way, there is no wastewater contaminated with the treatment solution that requires subsequent treatment. In addition, the full use of the sprayed treatment solution for layer formation is of great interest with regard to the economy of the overall process.

Accordingly, it has proven to be particularly advantageous within the scope of the method according to the invention to dry the solution sprayed onto the metal surface directly after a reaction time of in particular 2 to 4 seconds without rinsing and / or squeezing, the metal surface being removed with warm air until the adhering water is removed is treated. This treatment is expediently carried out in a dry zone following the reaction zone, in which the temperature of the metal surfaces can rise to a maximum of 100 ° C.

The conversion layers obtained by the method according to the invention correspond in terms of layer thickness and quality to the layers resulting from conventional spraying methods. The implementation of the method according to the invention is explained in more detail in the example below.

example Pretreatment of the metal surfaces

• a) Vorentfetten mit einer wäßrigen Lösung, enthaltend :
  
• b) Nachentfetten mit einer wäßrigen Lösung, enthaltend :
  
  Außerdem wurden bei der Entfettung der Stahloberflächen, zur Unterstützung der Reinigungswirkung, zusätzlich rotierende Bürsten eingesetzt.
• c) Spülen mit warmem Wasser durch Spritzen von oben und unten, 2 sec bei 50 °C und einem Spritzdruck von 1,5 bar, mit anschließendem Abquetschen des verbleibenden Flüssigkeitsfilms auf Ober- und Unterseite des Bandes mittels gummierten Abquetschwalzen.

In a continuous spraying machine, strip material made of steel and hot-dip galvanized steel was degreased and cleaned in the following way. The bandwidth of the material was 1.50 m; the belt speed 30 m / minute.

• a) pre-degreasing with an aqueous solution containing:
  
• b) subsequent degreasing with an aqueous solution containing:
  
  In addition, the degreasing of the Steel surfaces, additional rotating brushes are used to support the cleaning effect.
• c) Rinsing with warm water by spraying from above and below, 2 sec at 50 ° C and a spray pressure of 1.5 bar, with subsequent squeezing off the remaining liquid film on the top and bottom of the belt using rubberized squeeze rollers.

The strip material from the last rinsing zone is introduced into the following treatment zone after passing a free distance of approximately 0.8 m. This was equipped - both above and below the belt - with two nozzle strands arranged one behind the other, each with ten two-substance nozzles.

enthielt und eine Temperatur von 40 °C aufwies. Der Badansatz erfolgte gleichfalls mit vollentsalztem Wasser. Die versprühte Flüssigkeitsmenge betrug 5 ml/m² bei einem Druck von 2 bar (Druckluft). Der Unterseite des Bandmaterials wurde gleichzeitig nur Druckluft mit entsprechendem Druck zugeführt. Nach einer anschließenden Reaktionszeit von 4 sec wurde das Bandmaterial unmittelbar in eine Trockenzone eingeführt und mittels Warmumluft getrocknet.The top of the tape material was sprayed with an aqueous solution which

contained and had a temperature of 40 ° C. The bath was also made with deionized water. The amount of liquid sprayed was 5 ml / m ² at a pressure of 2 bar (compressed air). At the same time, only compressed air with the appropriate pressure was supplied to the underside of the strip material. After a subsequent reaction time of 4 seconds, the strip material was introduced directly into a drying zone and dried using warm air.

Claims (4)

1. A method of forming conversion coatings on moving strip-like or sheet-like metal surfaces of aluminium, zinc or iron by spraying on a chromating solution or an acidic solution free from chromic acid and containing fluorides and/or compounds of titanium, zirconium or manganese, the solution being delivered to and sprayed onto the cleaned and flushed metal surface by means of inert pressurised gas through one or more two-component nozzles in which the solution and the pressurised gas are separately delivered to the nozzle outlet opening,. after which the metal surface is aftertreated, characterised in that the solution is sprayed onto only one side of the strip-like or sheet-like metal surface and compressed air is simultaneously delivered to the other side.

2. A method as claimed in Claim 1, characterised in that the solution is sprayed on by means of compressed air under a pressure of at least 0,1 bar and preferably in the range from 0,7 to 4 bars.

3. A method as claimed in Claims 1 and 2, characterised in that from.1 to 50 ml and preferably from 3 to 7 ml of solution are sprayed on per square metre of metal surface.

4. A method as claimed in Claims 1 to 3, characterised in that the solution sprayed onto the metal surface is dried after a reaction time of, in particular, from 2 to 4 seconds using warm air, i. e. without rinsing and/or squeezing off, until the water has been removed.

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