EP1090160B1 - Method for controlling the coating weight for strip-phosphating - Google Patents

Abstract

A process for controlling the layer weight during the phosphating of steel strip zinc-coated on one or both sides, using a phosphating solution which contains 1 to 6 g/l zinc ions and 10 to 30 g/l phosphate ions, characterized in that a Fe(II) ion content within the range of 3 to 100 mg/l is established in the phosphating solution. The higher the Fe(II) content, the lower is the layer weight. An alteration in the Fe(II) content of 3 to 20 mg/l results in an alteration in the layer weight of about 0.1 g/m2.

Description

The invention relates to a method for controlling the layer weight in the phosphating of steel strip galvanized on one or both sides. As a result, the layer weights can be kept reliably in the desired range of about 1 to about 2 g / m ² even when the belt speed and thus the phosphating time or the change in other phosphating parameters are changed.

If in the sense of this invention of steel strip galvanized on one or both sides Is talked about, this includes both electrolytically galvanized as well to understand hot-dip galvanized steel strip. Furthermore, this will include Alloy galvanized steel strips understood. These contain the zinc layer additional alloy components such as iron, nickel and / or Aluminum.

The term layer weight is common in the field of phosphating metal surfaces. Instead of "layer weight" or more detailed "phosphate layer weight", the terms "layer overlay" or "area-related mass" are also used. This is understood to mean the mass, based on a unit area, of the metal phosphate layer produced on the metal surface by the phosphating. It is usually given in g / m ² . It can be determined by weighing a phosphated metal sheet with a known surface, detaching the metal phosphate layer and weighing the metal sheet again. The mass of the metal phosphate layer based on an m ² can be calculated from the determined weight difference, taking into account the surface of the metal sheet. A 0.5% by weight chromic acid solution can be used, for example, to detach the metal phosphate layer. The method of determining the layer weight is described in more detail in the German standard DIN 50942.

The layer weight represents an essential parameter for checking the phosphating result. Depending on the intended use of the phosphated metal parts, layer weights are sought in different areas. The present invention is preferably concerned with sheet metal used in automotive engineering. Layer weights of above 0.8 g / m ² , but at most about 4 g / m ^{2, are} aimed for. The layer weights should preferably be below 3 g / m ² and in particular be about 1 to about 2 g / m ² .

Process for phosphating surfaces made of iron, steel, zinc and the like Alloys as well as aluminum and its alloys have long been the Technology. The phosphating of the surfaces mentioned serves to increase the Adhesive strength of lacquer layers and to improve corrosion protection. The phosphating takes place by immersing the metal surfaces in the Phosphating solutions or by spraying the metal surfaces with the Phosphating solutions. Combined methods are also known. Shaped metal parts such as, for example, can be phosphated Automotive bodies, but also metal strips in high-speed conveyor systems. The The present invention is concerned with such a band phosphating. Band phosphating differs from partial phosphating in that due to the high belt speeds, the phosphating, d. H. the Growing a closed metal phosphate layer within a short Period of time from about 2 to about 20 seconds, for example.

Process for phosphating metal strips, in particular electrolytically galvanized or hot-dip galvanized steel strips are state of the art known. For example, WO 91/02829 describes a method for Phosphating of electrolytically and / or hot-dip galvanized steel strip through short-term treatment with acid phosphating solutions, which in addition to zinc and Phosphate ions Manganese and nickel cations as well as anions containing oxygen Contain acids with accelerator effect. Under the latter term are especially to understand nitrate ions. DE-A-35 37 108 also describes a process for the phosphating of electrolytically galvanized steel strips Treatment with acid phosphating solutions, which in addition to zinc, manganese and Phosphate ions other metal cations such as nickel ions and / or Anions of oxygen-containing acids with accelerating action, in particular Nitrate ions. The content of zinc cations lies in that relatively deep range from 0.1 to 0.8 g / l.

The German patent application DE-A-196 39 596 tries to provide a phosphating process which, on the one hand, solves the problem of speck formation and, on the other hand, makes it possible to also use galvanized steel strips or the non-galvanized side of one-side galvanized steel strips with the short phosphating times customary in strip systems with a to provide a closed crystalline phosphate layer. "Specks" are understood to be whitish corrosion spots on the metal surface that show a crater-like appearance in microscopic images. Such specks often occur on galvanized steel surfaces if the phosphating solution has too high levels of chloride ions and / or nitrate ions. According to the cited document, this object is achieved by a method for phosphating steel strip or steel strip galvanized on one or both sides or alloy galvanized by spraying or dipping treatment for a period of time in the range from 2 to 15 seconds with an acidic, zinc and manganese-containing phosphating solution at a temperature in the range from 40 to 70 ° C, characterized in that the phosphating solution 1 to 4 g / l Zinc ions, 0.8 to 3.5 g / l manganese ions 10 to 30 g / l phosphate ions 0.1 to 3 g / l Hydroxylamine in free, ionic or bound form and contains no more than 1 g / l nitrate ions, has a free acid content in the range of 0.4 to 4 points and a total acid content in the range of 12 to 50 points.

DE-A-197 40 953 describes a method for phosphating steel strip or steel strip galvanized on one or both sides or galvanized with alloys by spraying or dipping treatment for a period of time in the range from 2 to 20 seconds with an acid, zinc, magnesium and manganese-containing phosphating solution with a temperature in the range from 50 to 70 ° C, characterized in that the phosphating solution is free of nitrate ions and that it 1 to 4 g / l Zinc ions, 1.2 to 4 g / l manganese ions 1 to 4 g / l magnesium ions 10 to 30 g / l phosphate ions 0.1 to 3 g / l Hydroxylamine in free, ionic or bound form contains, has a free acid content in the range of 0.4 to 4 points and a total acid content in the range of 15 to 45 points.

If you add the zinc and manganese-containing phosphating solutions, additional nickel ions to, you get the so-called "trication-phosphating solutions".

The terms "free acid" and "total acid" are in the field of Commonly known phosphating. They are determined by looking at the acidic one Titrate the bath sample with 0.1 normal sodium hydroxide solution and measure its consumption. The Consumption in ml is given as a score. In this document, under the Score of free acid consumption in ml of 0.1 normal sodium hydroxide solution understood to 10 ml bath solution, diluted to 50 ml with deionized water was to titrate to a pH of 4.0. Similarly, the score gives the Total acid consumption in ml up to a pH of 8.2.

Various measures are known in the prior art, the layer weight to adjust to the desired range. For example, with otherwise same bath parameters by changing the belt speed. Usually, however, a certain belt speed is specified, so that the phosphating bath parameters must be set so that they predetermined belt speed layer weights in the desired range result. The belt speeds can fluctuate considerably for example in the range between about 20 and about 180 m / min. As So far, options for regulating the layer weight are known: Change in the temperature of the phosphating bath, change in the free acid, the total acidity and / or the concentration of the layer-forming ions. This However, changes only respond very slowly, so it takes considerable time takes until layer weights are obtained in the desired range. It is particularly problematic, the layer weight by changing the Adjust bath composition. These changes are often only with one undo considerable time delay. At least they are with an additional consumption of phosphating chemicals and thus with additional costs.

Therefore, there is a need for a method, particularly layer weight when the belt speed changes, as quickly as possible, reversibly and with the lowest possible chemical consumption in the desired area adjust.

EP-A-0 111 246 relates to a process for phosphating electrolytically galvanized steel strips with acid phosphating solutions which, in addition to zinc and phosphate ions, contain further metal cations, for example nickel and iron (II), and accelerators such as nitrate. The zinc (II) cation content is 1 to 2.5 g / l, the free acid content 0.8 to 3 points and the total acid / free acid weight ratio 5 to 10. The duration of the treatment should not be significantly longer than 5 sec. In one example, the phosphating solution contains 0.4 g / l nickel (II) and 3 mg / l iron (II) ions. Layer weights below 2 g / m ² and a constant quality of the phosphate coating are achieved.

WO-A-95/07370 describes a nickel-free phosphating process with phosphating solutions, the 0.3 to 2 g / l zinc (II), 0.3 to 4 g / l manganese (II), 5 to 40 g / l phosphate ions, 0.1 up to 5 g / l hydroxylamine and / or 0.2 to 2 g / l m-nitrobenzenesulfonate and at most 0.5 g / l Contain nitrate ions, the manganese content being at least 50% of the zinc content is. When the process is applied to steel surfaces, iron goes in the form of Iron (II) ions in solution, so that the phosphating solution iron (II) concentrations up to Can have 50 ppm. WO-A-90/12901 is cited as prior art here Phosphating solutions describes the addition of 0.3 to 1.5 g / l zinc (II), 10.0 to 20.0 g / l Phosphate ions, 0.1 to 2.0 g / l m-nitrobenzenesulfonate, inter alia also 0.01 to 0.8 g / l iron (II) included.

EP-A-0 315 059 relates to a phosphating solution which uses hydroxylamine as an accelerator contains to form Zn / Fe phosphate layers of certain crystal structure. These solutions preferably contain 0.02 to 0.2% by weight of zinc, 0.05 to 5.0% by weight Hydroxylamine and 0.001 to 0.5% by weight of iron (II).

In contrast, the present invention relates to a method for controlling the layer weight in the phosphating of steel strip galvanized on one or both sides with a phosphating solution which contains 1 to 6 g / l zinc ions and 10 to 30 g / l phosphate ions and contains a content of Fe (II) ions in the range from 3 to 100 mg / l, characterized in that between 3 and 20 mg / l Fe (II) ions are added to the phosphating solution to reduce the layer weight by 0.1 g / m ^2, or in that Phosphate solution to increase the layer weight by 0.1 g / m ² between 3 and 20 mg / l Fe (II) ions by oxidation to Fe (III) ions.

This process is based on the surprising observation that, with the process parameters remaining the same, the more iron (II) ions the phosphating bath contains, the lower the layer weight. It was observed that, in the case of layer weights in the introductory range, with otherwise the same phosphating parameters, the layer weight is reduced by 0.1 g / m ² when the phosphating bath is between 3 and 20 mg / l, in particular about 5 to about 10 mg / l Iron (II) ions added. The longer the treatment time, the less iron (II) is sufficient. The preferred procedure is to prepare a stock solution of a soluble iron (II) salt with a known iron concentration and add it to the phosphating bath if necessary. The soluble iron (II) salts used are preferably salts of anions which do not have a negative effect on the phosphating result and corrosion protection. Iron (II) sulfate is particularly suitable for this.

The process according to the invention therefore makes it possible to counteract the increase in the layer weight while reducing the belt speed by increasing the concentration of iron (II) ions in the phosphating bath by 3 to about 20 mg / l, depending on the treatment time, in order to reduce the Layer weight to achieve 0.1 g / m ² . When iron (II) ions are in the range from 3 to 100 mg / l, preferably between about 10 and about 100 mg / l and in particular between about 15 and about 55 mg / l, belt speeds in the range from about 20 to 1 are obtained about 180 m / min and resulting phosphating times of about 2 to about 15 seconds, reliable layer weights in the range between about 1 and about 2 g / m ² .

If the process weight is to be counteracted when the process parameters are changed again, for example an increase in the belt speed and a reduction in the phosphating time, the corresponding amount of iron (II) ions must be removed from the phosphating bath. In order to increase the layer weight by 0.1 g / m ² , depending on the treatment time, between 3 and 20 mg / l, in particular about 5 to about 10 mg / l, of iron (II) ions must be removed from the phosphating bath. The easiest way to do this is to add the calculated amount of an oxidizing agent to the phosphating bath in order to oxidize the desired amount of iron (II) ions to iron (III) ions. These precipitate out as iron (III) phosphate, so that their influence on the layer weight disappears.

The process according to the invention is preferably operated in such a way that the Phosphating solution supplemented with supplementary solutions that do not contain iron (II). This has the consequence that the Iron (II) content of the phosphating bath decreased over time, so that the Increase layer weights over time. This effect can be desirable as long as the layer weight is in the technically preferred range. An unwanted one further increase can then be counteracted by that Phosphating bath adds the appropriate amount of iron (II) ions.

Currently are for the phosphating of galvanized steel strips Phosphating solutions commonly used, which in addition to zinc ions also ions of one or contain several other divalent metals. In particular are currently Phosphating baths commonly used, which additionally include one or more of the following Cations contain: 1 to 5 g / l manganese ions, 1 to 4 g / l magnesium ions, 0.8 to 4.5 g / l nickel ions. The method according to the invention is also applicable to such baths applicable.

In addition to the layer-forming cations mentioned, the phosphating solutions contain Alkali metal and / or ammonium cations to determine the value of the free acid on the set the desired range.

Phosphating baths usually also contain so-called accelerators. These are substances associated with the pickling reaction on the metal surface resulting hydrogen react. This will prevent a so-called Polarization of the metal surface by covering with hydrogen. The Accelerators thereby improve the even occupancy of the Metal surface with finely divided phosphate crystals, which are usually one size have between about 1 and about 10 microns. The method according to the invention sets assuming that accelerators are not used which oxidize iron (II) to iron (III). A particularly useful accelerator that does not have an oxidizing effect on iron (II) Hydroxylamine. Accordingly, the method is used a phosphating solution preferred, which additionally 0.1 to Contains 3 g / l hydroxylamine in free, ionic or bound form.

Hydroxylamine can be used as a free base, as a hydroxylamine-releasing compound for example hydroxylamine complexes and ketoximes or aldoximes or in Form of hydroxylammonium salts can be used. One adds free Hydroxylamine is added to the phosphating bath or a phosphating bath concentrate it largely due to the acidic nature of these solutions Hydroxylammonium cation are present. When used as Hydroxylammonium salt, the sulfates and the phosphates are particularly suitable. In the case of phosphates, the acid salts are due to the better solubility prefers. To take account of economic aspects on the one hand and on the other hand, the phosphating baths with not too much sulfate ions can advantageously a combination of free hydroxylamine and Hydroxylammonium sulfate can be used. Hydroxylamine or its Compounds are added to the phosphating solution in such amounts that the calculated concentration of free hydroxylamine between about 0.1 to about 3 g / l, preferably between about 0.15 and about 1 g / l.

For the indication of the phosphate concentration, the total phosphorus content of the phosphating bath is considered to be present in the form of phosphate ions PO ₄ ^3- . Accordingly, the known fact that the pH values of the phosphating baths in the range from about 2.0 to about 3.6, which are in the acidic region, only a very small part of the phosphate is actually in the form is ignored when calculating or determining the concentration the triple negatively charged anions are present. At these pH values it is rather to be expected that the phosphate is present primarily as a single negatively charged dihydrogen phosphate anion, together with undisociated phosphoric acid and with smaller amounts of double negatively charged hydrogen phosphate anions.

The phosphating of hot-dip galvanized steel strips is caused by fluoride ions relieved and also for the phosphating of electrolytically galvanized steel strip can the presence of fluoride ions for an even layer formation be beneficial. Accordingly, there is a further preferred embodiment of the Invention to use phosphating solutions that up to about 0.8 g / l fluoride in contain free or complex bound form. For example, for the Phosphating of electrolytically galvanized steel strip the preferred Fluoride levels in the range of 0.0 to about 0.5 g / l, especially in the range of about 0.1 to about 0.2 g / l.

The phosphating solutions are generally prepared in the Known to those skilled in the art. For example, phosphate is in the form of Phosphoric acid introduced into the phosphating solutions. The cations are in the form of acid-soluble compounds such as the carbonates, the oxides or the hydroxides of phosphoric acid are added so that they are partially neutralized becomes. Further neutralization to the desired pH range takes place preferably by adding sodium hydroxide or sodium carbonate. As Sources of free fluoride anions are, for example, sodium or Potassium fluoride. For example, tetrafluoroborate or Hexafluorosilicate can be used.

For the production of phosphate layers with a layer weight in Desired range is preferably used phosphating solutions that one Free acid content in the range of 0.4 to 4 points and a content of total acid in the range of 15 to 45 points. The Terms "free acid" and "total acid" as well as their method of determination were used already explained above. The values of the free acid are preferably between about 1.5 and about 3.5 and especially between about 2.0 and about 3.0 Points. The total acid content is preferably in the range of about 25 up to about 35 points.

The temperature of the phosphating solution is in the process according to the invention preferably in the range from 50 to 70 ° C. and in particular between 53 and 65 ° C.

In the process according to the invention, the one or both sides is galvanized Steel strip with the phosphating solution for a period in the range from 2 to 30 seconds in contact by placing the phosphating solution on the sprayed galvanized steel strip or by inserting the galvanized steel strip into the Dipping phosphating solution. The spray treatment is technically simpler feasible and therefore preferred. Are particularly preferred Treatment times between 3 and 15 seconds. After the desired one The phosphating solution is treated with the galvanized steel strip Rinsed off water.

The process according to the invention assumes that iron (II) ions are not uncontrolled be entered into the phosphating solution. As already mentioned are therefore Prefer supplemental solutions that do not contain iron (II). Furthermore, at the phosphating of steel strip galvanized only on one side can be prevented that the non-galvanized steel side comes into contact with the phosphating solution and this get iron (II) ions into the phosphating solution by a pickling reaction. Accordingly, the process according to the invention is carried out in the case of Phosphating of one-sided galvanized steel strip in such a way that only the galvanized strip side in contact with the phosphating solution. You avoid through suitable technical measures such as a cover the non-galvanized side of the strip so that it comes into contact with the phosphating solution.

The process according to the invention is preferably used to produce phosphate layers with layer weights in the range from 1 to 2 g / m ² . In particular, the iron (II) ion content in the phosphating bath is adjusted so that layer weights of 1.5 ± 0.3 g / m ^{2 are} obtained. The iron (II) ion content can be checked using known analytical techniques and particularly simply by immersing corresponding commercially available measuring strips in the treatment solution.

Before applying the phosphating solution, the metal surface be completely water wettable. This is in continuous working Belt systems are usually given. If the belt surface is oiled this oil should be cleaned with a suitable cleaner before phosphating remove. The procedures for this are well known in the art. Before the Phosphating is usually carried out with activation in the prior art known activating agents. Usually solutions or Suspensions used that contain titanium phosphates and sodium phosphates. On the activation follows the application of the invention Phosphating process, which is advantageously a passivating rinse follows. This takes place between phosphating and passivating Rinsing is usually an intermediate rinse with water. For one passivating rinsing are far from chromic acid treatment baths common. For reasons of work and environmental protection as well as For reasons of disposal, however, there is a tendency to use chrome Replace passivation baths with chrome-free treatment baths. For this are pure inorganic bath solutions, in particular based on hexafluorozirconates, or also organic reactive bath solutions, for example based on substituted ones Poly (vinylphenols) known. Rinse solutions can also be used containing 0.001 to 10 g / l of one or more of the following cations: Lithium ions, copper ions, silver ions and / or bismuth ions.

The metal tapes phosphated according to the invention can be used directly with a organic coating. However, you can also start with unpainted condition after cutting, shaping and joining to components like Automobile bodies or household appliances are put together. The hereby associated forming processes are facilitated by the phosphate layer. Is the Corrosive stress on the finished components is low, such as at Household appliances can be made from the pre-phosphated metal assembled devices can be painted directly. For higher ones Corrosion protection requirements, such as those made in automotive engineering it is advantageous to do another one after assembling the bodies To follow phosphating treatment.

embodiments

3,5 g/l Zink,

3,0 g/l Mangan,

3,0 g/l Nickel,

17 g/l Phosphationen,

15 g/l Nitrationen

The method according to the invention for controlling the layer weight was tested in a belt system for the phosphating of steel galvanized on both sides. After galvanizing, the electrolytically galvanized metal strips were activated with an activation solution containing titanium phosphate (Fixodine ^R 950, Henkel KGaA, batch concentration 0.5% by weight) and phosphated under the conditions given in the table. In addition to the values given in the table, the phosphating bath in this example had the following composition:

3.5 g / l zinc,

3.0 g / l manganese,

3.0 g / l nickel,

17 g / l phosphate ions,

15 g / l nitrate ions

The values correspond to the commercially used phosphating process GRANODINE ^R 5854 (Henkel KGaA). Influence of the iron (II) content on the layer weight tape speed
m / min Treatment time seconds Free acid points Total acidity points temperature
° C Fe (II) content
mg / l layer weight
g / m ² Vergl.1 45 12 2.5 30 58 ≈0 2.0 Beisp.1 45 12 2.5 30 58 20 1.7 Beisp.2 45 12 2.5 30 58 35 1.4 COMP.2 90 6 2.5 30 58 ≈0 1.7 Beisp.3 90 6 2.5 30 58 20 1.5 Vergl.3 180 3 2.5 30 58 ≈0 1.2 Beisp.4 180 3 2.5 30 58 20 1.1

Claims (8)

1. Process for controlling the weight of the layer during phosphating of one or two sided galvanized steel tape by means of a phosphating solution, which contains 1 to 6 g/l of zinc ions and 10 to 30 g/l of phosphate ions and shows a content of Fe(II)-ions in the range of 3 to 100 mg/l, characterized in that between 3 and 20 mg/l of Fe(II)-ions have to be added to the phosphating solution for a reduction of the weight of the layer by 0.1 g/m² or in that between 3 and 20 mg/l of Fe(II)-ions have to be withdrawn from the phosphating solution through oxidation to Fe(III)-ions for an increase of the weight of the layer by by 0.1 g/m².
2. Process according to claim 1, characterized in that the phosphating solution additionally contains one or several of the following cations:

   1 to 5 g/l of manganese ions,

   1 to 4 g/l of magnesium ions,

   0.8 to 4.5g/l of nickel ions.
3. Process according to one or both of claims 1 and 2, characterized in that the phosphating solution additionally contains 0.1 to 3 g/l of hydroxylamine in free, ionic or bound form as an accelerator.
4. Process according to one or several of claims 1 to 3, characterized in that the phosphating solution shows a content of free acid in the range of 0.4 to 4 points and a content of total acid in the range of 15 to 45 points.
5. Process according to one or several of claims 1 to 4, characterised in that the phosphating solution has a temperature in the range of 50 to 70 °C.
6. Process according to one or several of claims I to 5, characterized in that the phosphating solution is brought in contact with a one or two-sided galvanized steel tape through spraying or dipping treatment for a period of time in the range of 2 to 30 seconds.
7. Process according to one or several of claims 1 to 6, characterized in that solely the galvanized side of the tape is brought in contact with the phosphating solution in case of phosphating of a one-side galvanized steel tape.
8. Process according to one or several of claims I to 7, characterized in that phosphate layers are produced with weights of layers in the range of 1 to 2 g/m².