EP0372591A1 - Process for phosphatizing metal surfaces - Google Patents

Abstract

During phosphatizing of metal surfaces comprising at least partly iron or steel by dipping or flooding by means of phosphatizing solutions containing film-forming cations and nitrate or equivalent accelerators, the iron content is limited through precipitation of iron phosphate by withdrawing discontinuously a portion of the phosphatizing solution from the bath tank (1) and contacting it, in a separate aeration unit (3), with oxygen or an oxygen-containing gas, and feeding the resultant solution, freed from iron phosphate sludge, back into the bath tank (1). According to the invention, the portion of phosphatizing solution is introduced from below into an aeration unit (3), which is fitted with a self-aspirating aerating stirrer (4) which narrows at least in the lower region, and is aerated. When aeration is complete, the iron phosphate formed is allowed to settle, and the phosphatizing solution, with a depleted iron(II) content, is sucked upward and fed back into the bath tank (1). <??>In a particularly advantageous procedure, the iron phosphate sludge deposited principally in the narrowed region of the aeration unit (3) is stirred up in water with the aid of a stirrer (8) projecting into the narrowing region, but this is not carried out until several batches have undergone precipitation treatment. <IMAGE>

Description

The invention relates to a method for phosphating metal surfaces at least partially consisting of iron or steel by dipping or flooding using layer-forming cations and nitrate or equivalent accelerators containing phosphating solutions, the iron content of which is limited by precipitation of iron phosphate by discontinuously removing a partial volume of the phosphating solution from the bath tank ( 1) is drawn off, is brought into contact with oxygen or oxygen-containing gas in a separate gassing device (3) and the resulting solution, freed from iron phosphate sludge, is returned to the bath tank (1).

In the manufacture of phosphate coatings on metal surfaces by treatment with e.g. In a zinc phosphate solution, it is generally customary to add one or more oxidizing agents to accelerate the layer formation of the phosphating solution. As far as the production of phosphate coatings on iron and steel surfaces is concerned, the associated dissolution of iron, which initially dissolves in the form of iron (II) ions, poses particular problems.

One category of phosphating processes uses phosphating solutions which contain oxidizing agents which convert iron (II) to iron (III), so that insoluble iron phosphate is formed. With increasing Throughput on the surface then creates considerable amounts of sludge that must be physically removed. In some processes, however, this sludge removal is difficult or difficult to carry out, so that it is more advantageous to conduct the phosphating process in such a way that practically no or only a little sludge is formed.

In this category of phosphating processes that work "on the iron side", the phosphating solution is formulated from chemicals that do not convert the iron in solution into the trivalent state. This can be achieved, for example, by using nitrate or similarly weak oxidizing agents as accelerators.

Although the sludge formation is largely suppressed in the phosphating processes working "on the iron side", they have other disadvantages. Due to the absence of strong oxidizing agents, the formation of the phosphate layer is slow. Due to the accumulation of iron (II) phosphate in the solution, the proportion of iron phosphate in the phosphate layer formed can also become undesirably high. The phosphate layers formed also tend to be coarse.

There has been no shortage of attempts to solve a wide variety of problems. Thus, in the process according to GB-A-996 418, urea is to be added to the phosphating bath, as a result of which the temperature during the phosphating can be increased without risking significantly increased sludge formation. Accelerated phosphate layer formation is associated with this, but the other disadvantages mentioned above remain.

In addition, there is the additional disadvantage that heating of the phosphating solution, which is not required per se and requires additional energy, is required.

Another bath which does not work "on the iron side" has tried to solve the sludge problem by increasing its solids content and consequently reducing the amount of sludge (GB-A-1 555 529). Although a compact sludge is obtained and, consequently, the intervals between the required desludging of the respective treatment device are larger, it has been found that it can be more difficult to remove compact sludge instead of light, voluminous sludge.

From EP-A-45 110 it is known to work in the manufacture of phosphate layers on iron or steel surfaces in the immersion or flooding process with solutions of a certain composition in which by suitable measurement of ClO₃ or an iron (II) having the same effect to iron (III) oxidizing accelerator an iron (II) content of 0.05 to 1 wt .-% is set.

In a further method for phosphating metal surfaces at least partially consisting of iron or steel by dipping or flooding by means of layer-forming, in particular zinc cations, and phosphating solutions containing nitrate or equivalent accelerators, a partial volume of the phosphating solution is branched off from the bath container in a separate device for the purpose of precipitation of iron phosphate with oxidizing agents and the obtained solution, freed from iron phosphate sludge, returned to the container. The iron phosphate precipitation takes place primarily by adding chlorate and / or hydrogen peroxide, the extent of the iron phosphate precipitation being expediently measured such that the iron content of the phosphating solution in the bath tank is at most equal to the content of the cation determining the layer type (DE-A-33 45 498).

The latter two methods work satisfactorily with regard to the regulation of the iron (II) content in the phosphating solution, but the handling of the phosphate sludge formed is not satisfactorily solved.

The object of the invention is to provide a process for the phosphating of surfaces consisting at least partially of iron or steel which does not have the known, in particular the aforementioned disadvantages, works without additional chemicals, yet is simple to carry out without loss of layer quality and does not require any significant equipment outlay and enables simple sludge handling.

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the partial volume of the phosphating solution is introduced from below into a gassing device (3) which is equipped with a self-priming gassing stirrer (4) and which narrows at least in the lower region , brings in and fumigates after completion of the Fumigation allows the iron phosphate formed to sediment, and the phosphating solution, which is depleted in terms of the iron (II) content, is sucked upward and returned to the bath tank (1).

The equipment of the gassing device with a self-priming gassing stirrer has the advantage that there is no need for other equipment-intensive entry of the required oxygen-containing gases.

With the narrowing of the gassing device, at least in the lower area, better sedimentation and easier removal of the iron phosphate sludge can be achieved.

With the suction of the phosphating solution depleted of iron (II) it is achieved that the regenerated solution introduces as little iron phosphate as possible into the bath container.

With regard to the dimensions of the gassing device, the volume of the phosphating bath to be treated and the load on the phosphating bath, i.e. the throughput, remarkable. The space available and accessibility during maintenance work must be taken into account.

The speed of the gassing stirrer depends to a certain extent on the size of the gassing device. It should be above 800 rpm if possible. lie so that the intake air volume and its distribution over the Fumigation device are sufficiently large. The use of gassing stirrers with a speed of approx. 1200 to 1500 rpm is particularly expedient. It is essential that the gassing stirrer distributes the oxygen-containing gas in the smallest possible bubbles in the phosphating solution.

The extent of iron removal or precipitation essentially depends on the type of layer that is to be produced during the phosphating. The minimum is given by iron contents, at which an beginning disturbance of the layer formation can be observed (approx. 12 to 13 g / l). As a rule, you will stay well below such concentrations.

In general, it will be considered expedient to measure the extent of the iron precipitation in such a way that the iron content of the phosphating solution in the bath tank is at most equal to the content of the cation determining the layer type, provided that this does not already exceed the aforementioned and permissible content.

In general, it is advisable to branch off and treat partial volumes of, for example, 5 to 10% of the total volume of the phosphating bath, which makes correspondingly large time intervals possible. However, comparatively small partial volumes of, for example, 3 to 7% can also be treated within shorter time intervals. In general, the type of operation depends on the total volume of the phosphating bath, ie ultimately on the throughput.

Of course, several phosphating baths can also be regenerated with the aid of the method according to the invention.

The process according to the invention is particularly suitable when using phosphating solutions which contain mainly zinc as a layer-forming cation, in addition to any additional manganese and / or calcium contents.

It is also advantageous to use phosphating solutions whose temperature is between 35 and 70 ° C., preferably between 45 and 55 ° C. Experience has shown that at these temperatures, nitrate or equivalent accelerators do not cause iron oxidation and therefore no iron phosphate precipitation. In addition, phosphating processes work more economically due to lower energy requirements at such temperatures.

A phosphating solution suitable for carrying out the phosphating process contains, for example, at least 0.3% by weight of Zn, at least 0.3% by weight of PO₄ and at least 0.75% by weight of NO₃ or an equivalent, iron (II) non-oxidizing accelerator and has a weight ratio of Zn: PO₄ greater than 0.8 and a ratio of total acid to free acid of at least 5. In particular, it contains a maximum of 2.2% by weight of Zn, a maximum of 2.2% by weight of PO₄ and a maximum of 5.5 Wt .-% NO₃ or an equivalent accelerator with a weight ratio Zn: PO₄ less than 4 and a ratio of total acid to free acid of a maximum of 30. Further details are described in EP-A-45 110.

In another process which can be used, the metal surfaces are brought into contact with a phosphating solution which contains at least 0.6 g / l, preferably 1 g / l, of manganese ions in the weight ratio at treatment temperatures of 50 to 98 ° C.
P₂O₅: NO₃ = 1: (0.3 to 3.0),
Total P₂O₅: Free P₂O₅ = 1: (0.25 to 0.70),
Mn: Zn = 1: (22 to 0.2),
preferably 1: (12 to 0.8),
and which has a total acid score of at least 20 points when incorporated.

The phosphating solutions can additionally contain simple and / or complex fluorides, such as NaF, NaHF₂ and / or Na₂SiF₆, and with regard to the components zinc, manganese, phosphate and nitrate ions in a weight ratio
P₂O₅: NO₃ = 1: (0.3 to 2.0),
Total P₂O₅: Free P₂O₅ = 1: (0.3 to 0.8),
Mn: Zn = 1: (2 to 80)
can be supplemented (EP-A-42 631).

Further, for example, suitable phosphating solutions are described in DE-C-22 41 798 and DE-B-11 84 592.

A preferred embodiment of the invention provides that only after the precipitation treatment of several inserts, primarily in the narrowed area of the gassing device Use a stirrer protruding into the constricting area to stir up the deposited iron phosphate sludge in water. This mode of operation has the advantage of a higher throughput of phosphating solution, since the sedimented phosphate sludge can be dispensed with after each individual sedimentation phase. In particular in this embodiment of the invention, the introduction of the phosphating solution to be treated has an advantageous effect from below, because it swirls up the iron phosphate sludge from the previous precipitation treatment or precipitation treatments and prevents solid caking on the container wall.

In principle, the stirrer for whirling up the sedimented iron phosphate sludge in water can be arranged as desired and separately, but it is particularly advantageous if, according to a further development of the invention, the phosphate sludge is whirled up with the aid of a stirrer with a coaxial shaft with respect to the gassing stirrer. In general, the iron phosphate slurry suspended in water is stirred for about 10 minutes. via the floor outlet of the gassing device into the sewage system of the neutralization system. If such a neutralization system is not available, the slurry formed can also be neutralized in the gassing device and then discharged in accordance with an expedient development of the invention.

With the aid of the method according to the invention, the iron content in Keep phosphating solutions that work on the iron side constant within narrow limits. Additional chemicals for the oxidation of iron (II) to iron (III) are not required. The skilful supply of the phosphating solution to be regenerated from the inside into the gassing device, in particular in the embodiment of the invention with multiple precipitation treatment, prevents a firmly adhering, difficult to remove iron phosphate crust from forming. In addition, the multiple precipitation treatment has the considerable advantage that the usual large fresh water requirement can be significantly reduced.

The invention is explained for example and in more detail with the aid of the flow diagram and the example.

The phosphating solution is discontinuously e.g. pumped from the phosphating bath (1) via the line (2) and the floor outlet into the gassing tank (3) by means of a compressed air membrane pump until the intended filling level is reached. Then the gassing stirrer (4) is activated and kept in operation until the desired precipitation of iron phosphate sludge has occurred.

After the gassing has ended, the iron phosphate sludge formed is sedimented, and the regenerated phosphating solution is then suctioned off via line (5) and introduced into the phosphating bath (1) via line (6). After preferably several precipitation treatments, fresh water is fed into the gassing device (3) via line (7) and the stirrer (8) is activated. If the all sedimented iron phosphate sludge is whirled up, the slurry is withdrawn via line (9) - if appropriate after a previous neutralization. An emergency overflow line is identified by (10).

example

• 1. Entfetten
• 2. Kaltwasserspülen (im Tauchen)
• 3. Beizen in Schwefelsäure, 20 Gew.-%
  Das Beizbad enhielt 0,5 g/l Beizinhibitor.
  Beiztemperatur 65°C
  Beizdauer ca. 20 min.
• 4. Kaltwasserspülen
• 5. Aktivierendes Vorspülen in einer Titanorthophosphatdispersion
• 6. Phosphatieren bei 50°C mit einer Behandlungsdauer von 10 min.
• 7. Kaltwasserspülen
• 8. Neutralisierende Spülung
• 9. Beseifen mittels einer Natriumseifenlösung Konzentration der Seifenlösung 5 Gew.-% Temperatur der Seifenlösung 75°C Behandlungsdauer 3 min.
• 10. Trocknen der Kaltstauchdrähte an der Luft.

Cold heading wire of various steel qualities was treated according to the following procedure:

• 1. Degrease
• 2.Rinse cold water (in diving)
• 3. Pickling in sulfuric acid, 20% by weight
  The pickling bath contained 0.5 g / l pickling inhibitor.
  Pickling temperature 65 ° C
  Pickling time approx. 20 min.
• 4. Rinse cold water
• 5. Activating pre-rinse in a titanium orthophosphate dispersion
• 6. Phosphate at 50 ° C with a treatment time of 10 min.
• 7. Rinse cold water
• 8. Neutralizing rinse
• 9. soaping with a sodium soap solution concentration of the soap solution 5% by weight temperature of the soap solution 75 ° C. treatment time 3 min.
• 10. Air dry the cold heading wire.

Following this treatment, the cold heading wires were cold formed.

The phosphating according to stage 6 was carried out using a phosphating solution, the initial composition of which was as follows:
18.3 g / l zinc
15.0 g / l phosphate (calculated as P₂O₅)
33.8 g / l nitrate
The total acid score was 64.

In order to maintain the phosphating properties of the phosphating solution, the bath was cleaned using a supplementary solution, the
12.1% by weight zinc
24.4% by weight phosphate (calculated as P₂O₅)
11.3% by weight nitrate
contained, added to constancy of total acid points.

The container of the gassing device (3) had a total height of 1500 mm with a diameter of cylindrical part of 800 mm. The conical bottom of the container had an angle of inclination of 60 °. The filling volume of the gassing device up to the overflow was 470 l.

The fumigation container was equipped with an aeration stirrer (4), which had a speed of 1400 rpm. owned, equipped. It had an immersion depth of 500 mm and had a gassing capacity of approx. 7 m³ / h.

The phosphating bath contained 6 m³ of phosphating solution. After the iron content of the phosphating solution had reached a value of about 6 g / l, 450 l of this solution were introduced into the gassing device (3) and there during a gassing time of 30 min. brought into contact with atmospheric oxygen. Due to the aforementioned suction power, the air supply was 0.78 m³ / 100 l phosphating solution.

After fumigation, the iron phosphate sludge formed was 6 min. let sit down. The separation took place primarily in the conical part of the container.

After the settling process had ended, the phosphating solution was suctioned off by means of line (5) and returned to the phosphating bath (1) by means of line (6). The suction has the result that a residual volume of approx. 30 l phosphating solution remains in the gassing container (3).

After a five-time precipitation treatment, the gassing device (3) was fed with fresh water via line (7) filled in an amount of approx. 80 l and the stirrer (8) started. The gassing stirrer (4) is out of action during the operating time of the stirrer (8). After a stirring time of approx. 10 min. the iron phosphate sludge sedimented primarily in the conical part of the gassing device (3) was suspended and could be discharged for neutralization.

With the aid of the method according to the invention, it was possible to keep the iron content of the phosphating solution of approximately 6 m 3 at an average bath load of 800 m 2/8 h to a value in the range from 6 to 7 g / l.

Claims (4)

1. Process for phosphating metal surfaces at least partially made of iron or steel by dipping or flooding using layer-forming cations and nitrate or equivalent accelerators containing phosphating solutions, the iron content of which is limited by precipitation of iron phosphate by discontinuously removing a partial volume of the phosphating solution from the bath container (1) withdrawn, brought into contact with oxygen or oxygen-containing gas in a separate gassing device (3) and the solution obtained, freed from iron phosphate sludge, is returned to the bath tank (1), characterized in that the partial volume of the phosphating solution is fed from below into one with a gassing device (3) equipped with a self-priming gassing stirrer (4) which narrows, introduces and gasses at least in the lower region, and after the gassing has ended the iron phosphate formed can be sedimented, and with regard to the ice s (II) content of depleted phosphating solution is suctioned off and returned to the bath tank (1). 2. The method according to claim 1, characterized in that after the precipitation treatment of several inserts, the iron phosphate sludge deposited primarily in the narrowed area of the gassing device (3) is stirred up in water with the aid of a stirrer (8) projecting into the narrowed area. 3. The method according to claim 2, characterized in that the phosphate sludge with the aid of a stirrer (8) with - with respect to the gassing stirrer (4) - swirled coaxial shaft. 4. The method according to claim 2 or 3, characterized in that the slurry formed is neutralized in the gassing device (3).

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