GB2039538A

GB2039538A - Dip phosphating process and apparatus therefor

Info

Publication number: GB2039538A
Application number: GB7941998A
Authority: GB
Original assignee: Nippon Paint Co Ltd
Current assignee: Nippon Paint Co Ltd
Priority date: 1978-12-05
Filing date: 1979-12-05
Publication date: 1980-08-13
Also published as: GB2039538B; BE880442A; JPS5576076A; FR2443513A1; DE2948649A1; FR2443513B1

Abstract

A dip phosphating process, which comprises introducing a substrate to be phosphated into a phosphating bath while producing undulations or waves in the phosphating solution at the entrance section of the bath. An apparatus for carrying out the process comprises a phosphating bath 1 and a means 35 for producing at the entrance section of the bath undulations or waves in the phosphating solution when the bath is in operation. <IMAGE>

Description

SPECIFICATION Dip phosphating process and apparatus therefor The present invention relates to improvements in a dip phosphating process, which may be adopted for the pretreatment before coating of a metallic substrate, and to an improved apparatus for carrying out the pretreatment.

A phosphating treatment, particularly by spraying process, is usually adopted as the pre-treatment before coating of automobiles, home electric appliances or steel furnitures, for example. When however, the substrate includes spray pockets (i.e. pocket-like recesses), at least portions of the pocket walls cannot be sprayed directly, and only receive splashes of the phosphating solution, which are bounced or struck off the substrate surface, even when the direction of the spray nozzle is changed several times to ensure as uniform spraying as possible. As a result, only a temper-like phosphating film is formed on the surface. Such a phosphating surface has poor corrosion-resistance and inferior adhesion onto the coating which is subsequently formed, and is therefore insufficient as a base for coating.In the case of treatment by the dip process, the substrate may be sufficiently subjected to phosphating even including the pocket portions referred to above. However, the dip process suffers from the problem that, due to the irregular conveyance speed of the conveying device normally used for dipping the substrate in a phosphating solution, the phosphated surface involves a stepped unevenness of phosphating, and a uniform phosphating film is not obtainable. This problem has provided the greatest obstacle to the use of the dip process for the phosphating of large-sized objects such as automobiles. Furthermore, an article having a recess at its bottom surface, e.g. an automobile tyre housing, is apt to make an air pocket at the dipping stage, so that the phosphating solution is unable to make sufficient contact with the housing walls.Furthermore, the sludge floating in the phosphating solution tends to stick on the substrate and is only removed with difficulty by washing with water.

In recent years, there has come to be employed a spray-dip process has been developed which utilizes the strong points of both the spray process and the dip process. According to this latter process, a substrate is first subjected to spray treatment for a period of time of from 5 to 60 seconds and then to dip treatment. The process is therefore effective in preventing faulty phosphating of the pocket portions and also stepped unevennesses in the phosphating layer. Since, however, the crystal shape of the phosphating film is determined at an early stage of the phosphating treatment, a phosphating film with the crystal shape obtainable by the dip process, which shows better results than the spray process in both corrosion resistance and adhesion as a coating base, cannot unfortunately be formed when the spray-dip process is used.

We have sought to devise a dip phosphating process which would yield a phosphating film with a good corrosion resistance and high adhesion, and which would avoid the drawbacks of the conventional dip processes.

As a result of our work, we have now surprisingly found that if, at the entrance stage of the phosphating bath the phosphating solution is given a wave or undulation character or shape the occurrence of stepped unevenness in phosphating and the formation of air pockets are both avoided. We have also found that the spraying of water or of phosphating solution onto the phosphated substrate which is being taken from or has just been taken from the phosphating bath is effective in easily washing out any sludge which has adhered to the substrate.

According to the present invention, there is provided a dip phosphating process comprising introducing a substrate to be phosphated into a phosphating bath, dipping the substrate in the bath until the phosphating is accomplished at the surface of the substrate, and taking out the phosphated substrate from the bath, characterised in that the introduction of the substrate into the bath is effected while subjecting to undulation the phosphating solution in the bath at the entrance section of the bath.

Various conventional baths may be used as the phosphating bath. However, a bath of ship shape is preferred, i.e. consisting of (1) an entrance section which is tapered and gradually deepened to the central section; (2) a central section which constitutes a horizontally flat bottom portion; and (3) an exit section which is tapered and gradually shallowed from the central section. A bath having a section of ship shape and a depressed portion between the entrance section and the central section, serving as a settling bath for sludge is particularly preferred.

A conveyor which can so work that the substrate is introduced into the phosphating bath, carried on in the phosphating bath under a dipping state and taken out from the phosphating bath may be used for transmitting the substrate to be phosphated.

In order to produce undulations in the phosphating solution, various means which can produce waves at the solution surface without material splash may be used. A typical procedure producing such undulation is to cause water andior phosphating solution to flow into the phosphating bath through nozzle apertures which open at, above or below the solution surface.

The phosphated substrate is preferably washed by spraying water or the phosphating solution onto the substrate to remove sludge adhering thereto. Such washing is effected on the phosphated substrate at the exit section, i.e. substrate which is being taken from or has been just taken from the phosphating bath.

In addition, in the phosphating bath, it is preferable to cause the phosphating solution to flow from the exit section to the central section, in particular to the settling bath, with or without the flow from the entrance section to the central section, e.g. the settling bath, so that the movement and settlement of the sludge into the settling bath are accelerated. These flows can be readily produced, for instance, by causing water and/or phosphating solution to spout through nozzle holes which are appropriately set in various places in the phosphating bath.

Usually, the phosphating solution in the phosphating bath is overflowed, and the overflowed phosphating solution is returned or recycled through the nozzle holes into the phosphating bath. One advantageous procedure is to set an overflow tank in front of the entrance section of the phosphating bath so that the overflowed phosphating solution is accumulated therein and is returned or recycled through appropriate pipe lines to the phosphating bath, whereby the desired flows of the phosphating solution are produced.

The present invention is now described in greater detail with reference to the accompanying drawings wherein: Figure 1 is a flow sheet of one embodiment of the apparatus according to the present invention; and Figure 2 is a perspective view of an essential part thereof.

The apparatus illustrated in the drawings comprises a phosphating bath 1, a settling bath 2 between the entrance section and the central section of the bath 1, and an overflow tank 3 in front of the entrance section of the bath 1. Alternatively, the overflow tank 3 may be provided behind the exit section or at one side of the bath 1.

At the bottom of the settling bath 2, there is provided a discharge pipe 4, which is branched into a pipe 4A having a valve 5A and a pipe 4B having a valve 5B. The pipe 4B is connected to a pump 6, which is connected to a filter 7 by a pipe 8. Valves 9 and 10 are provided at the front and rear ends of the filter 7. A pipe 12 having a valve 11 is connected to the pipe 8 without passing through the filter 7.

An outlet pipe 14 provided with a valve 13 exits from the bottom of the overflow tank 3, the pipe 14 being connected to a pump 15. A heat-exchanger 16 is connected to the pump 15 by means of a pipe 17, and valves 18 and 19 are provided at the front and rear of the heat-exchanger 16. A pipe 21 having a valve 20 as a detour is connected to the pipe 17.

The pipe 17 is connected to a main pipe 22 to which branch pipes 24 each having a valve 23 are also connected. A pipe 25 branched from the pipe 17 is connected to the main pipe 26. Branch pipes 31 having respectively valves 27, 28, 29 and 30 connected to the main pipe 26.

As will be apparent from Figure 2, the branch pipes 24 and 31 are arranged at a suitable distance on the bottom surface of the phosphating bath 1 and inside the lateral wall, and are connected to risers 32,33 and 34 respectively, which have nozzle apertures for spouting. A part of each branch pipe 31 is arranged at the entrance section of the bath 1 and connected to a riser 35 also provided with nozzle apertures for spouting. A further part of the branch pipe 31 is arranged at the exit section of the bath 1, and is connected with a riser 36 having further nozzle apertures for spouting. A device 37 for the conveyance of a substrate A to be phosphated is also provided. Pipes 38 and 39 supply water (which may be the water used in the washing step after the phosphating step).

The nozzle apertures may each open out in an appropriate direction so as to achieve the desired flows of the phosphating solution including the wave effect at the entrance section. The arrow marks in Figures 1 and 2 are examples of directions of the nozzle apertures which are applicable in practice.

The phosphating treatment with the use of the apparatus shown in Figures 1 and 2 may be carried out as hereinafter explained.

Firstly, the required amounts of a phosphating solution are supplied to the phosphating bath 1 and the overflow tank 3. Then, the valves 13, 18, 19 and 23 are opened, the pump 15 is actuated to spout the phosphating solution in the tank 3 into the bath 1 from the riser 32 through the heat-exchanger 16, whereby the phosphating solution in the bath 1 flows into the tank 3. The heat-exchanger 16 is a means of maintaining the temperature of the phosphating solution at the required level; when heating is not necessary, the valves 18 and 19 may be closed with opening of the valve 20.

When the phosphating solution comes to the required temperature, the valves 27,28,29 and 30 are opened, and the phosphating solution is spouted from the risers 33,34,35 and 36. Then, by means of the conveying device 37, the substrate A is brought into the bath 1, and phosphating treatment is started.

By the actions of the phosphating solution andior water spouted through the riser 35, the phosphating solution at the entrance section of the bath 1 is subjected to an undulating or wave effect. In this way, a uniform phosphating film can be formed on the surface of the substrate A without stepped unevenness of phosphating. Furthermore, by spouting the phosphating solution upwards from the riser 34 on to the bottom surface at the entrance section of the bath 1, the phosphating solution is allowed sufficient contact with portions of substrate which are otherwise not readily phosphated as a result of the development of air pockets, e.g. the recess portion on the bottom surface of the substrate A.

Finally, by spouting the phosphating solution from the risers 32 and 33, the phosphating solution in the bath 1 is stirred to run into the tank 3. By conveying the substrate A through such phosphating solution, a uniform film of phosphating can be formed.

After the phosphating treatment, the substrate A is carried to the water washing stage by means of the conveying device 37. The sludge formed as a by-product of the phosphating treatment floats in the bath 1, and the sludge so formed sticks to the substrate A. Before the sludge is firmly deposited, the phosphating solution or water is spouted from the riser 36 to wash and to remove the sludge.

In the phosphating treatment, a large quantity of sludge which mainly comprises phosphate compounds is produced in the bath 1. The sludge precipitated at the bottom of the bath 1 moves readily under the effect of its own gravity and under the spouting action of the phosphating solution from the risers 32 and 33 to fall into the settling bath 2. When the sludge in the settling bath 2 reaches a high concentration, the valves 5B, 9 and 10 are opened to operate the pump 6, so that the slurry is sent to the filter 7 for liquid-solid separation.

The sludge is then discharged from the system in a solid state. The filtrate is returned through the pipe 8 to the overflow tank 3. In an early stage of the treatment (the stage where the sludge has not yet accumulated in the settling bath 2), the discharge liquid from the settling bath 2 may be returned to the overflow tank 3 through the pipe 12 instead of being supplied to the filter 7.

In the phosphating treatment according to the present invention, the positions of the nozzle apertures of the riser 35 are appropriately selected; thus, whether the phosphating solution is spouted from at, above or below the solution surface, it should not produce material splash to stick to the substrate A. Usually, a range of from about 5 cm to about 20 cm both above and below the solution surface is suitable. Also, the spouting volume from the riser 35 may be appropriately adjusted by the valve 29 so as to produce waves in the phosphating solution at the entrance portion of the bath 1. Further, the spouting volume of the riser 34 is suitably adjusted by means of the valve 28 to a valve such that the substrate A does not float up as a result of upward spouting of the phosphating solution.Besides, the spouting volumes of the risers 32 and 33, respectively, which take part in the circulation of the phosphating solution in the bath and the movement of the sludge, are suitably adjusted with the valves 23 and 27, depending on the circulating conditions, the amount of sludge and the properties of the slurry in the bath. A draft tube system may be used as a means of achieving flows of the phosphating solution in the bath.

The location of the riser 36 may be so arranged that the spouting solution is applied from the neighbourhood of the position where the substrate A is partly exposed from the phosphating solution by the conveying device 37. The spouting amount is appropriately adjusted by the valve 30 so that the sludge deposited on the substrate A can be eliminated. Usually, the spouting pressure may be from 0.8 to 2.0 kg/cm2. With regard to the nozzle of the riser 36, a pipe nozzle which does not make the spouting solution misty (e.g. a pipe of about 10 mm in diameter with its tip tapered) is preferred.

When the phosphating treatment is effected by the process and apparatus of the invention, it is possible to avoid any stepped unevenness of phosphating, defective phosphating of any air pocket portions, and coating defects resulting from dust or sludge which have adhered to the substrate, and thus to provide excellent phosphating as a coating base.

The present invention is further explained by the following Examples and Comparative Examples, wherein parts and percentages are by weight unless otherwise indicated.

A test plate of commercially available cold rolled steel (100 x 300 x 0.8 mm) was subjected to dip degreasing with a weak alkali degreasing agent (2%) ("RIDOLINE SD200" (trade mark) manufactured by Nippon Paint Co., Ltd) at a temperature of 60"C for 2 minutes. The plate was thereafter washed with water and subjected to a dipping treatment with a metal surface conditioning agent (0.1%) ("FIXODIN 5N-5" (trade mark) manufactured by Nippon Paint Co., Ltd) at room temperature for 30 seconds.Then, after a dipping treatment with a phosphating solution (Zn, 0.15% Ni, 0.04%; P04, 1.4%; NO3,0.5%; Cl03,0.1 Cl, 0.1%; NO2, 0.0065%) underthe conditions of a total acidity of 17 points, a free acid degree of 0.9 point, a toner value of 1.5 points and a temperature of 50"C for 2 minutes, the plate was washed with water and dried.

The conveyance speed of the substrate (in this case, an automobile body in an automobile coating line) is usually from 4 to 5 m/min, and dipping of the substrate in a phosphating solution is usually made with an inclination at an angle of 20 to 25 degrees to the solution surface. In practice, however, phosphating treatment is effected with the various dictated by experience adjustments, and the finished appearance of the phosphating film is judged by visual inspection.

The point of total acidity represents the volume in ml of 0.1 N NaOH required for colouration in red of a sampled 10 ml of the phosphating solution when phenolphthalein is used as an indicator. The point of free acid representsthevolume in ml of 0.1 N NaOH requiredforcolouration in blueofasampled 10 my phosphating solution when bromophenol blue is used an an indicator. The point of the toner value represents the volume in ml of 0.042 N KMnO4for colouration in red of a sampled 25 ml of phosphating solution with the addition of several drops of 50% H2SO4.

Comparative Example 1 By the use of a conventional zinc phosphating equipment of the dip type, a phosphating treatment was carried out.

Example 1 A zinc phosphating equipment of the dip type but different from the conventional equipment in the following respect was used: at the entrance section of the phosphating bath, the nozzle apertures of the riser 35 were provided slightly below the surface of the phosphating solution so that the phosphating solution was stirred by undulation under the spouting of the phosphating solution. Phosphating treatment was effected by the Roller system.

Example 2 In addition to the construction of Example 1, the riser 36 was laterally provided at the exit section of the phosphating bath so that the phosphating solution was spouted to the plate which had been just phosphated. A phosphating treatment was effected by this system.

Example 3 In addition to the construction of Example 2, the risers 32,33 and 34 were arranged at the bottom and at the side so that the phosphating solution was stirred by spouting. A phosphating treatment was effected by this system.

The results of the above are shown in Table 1 as follows: TABLE 1 Appearance of phosphating film Stepped unevenness Film quality Deposited sludge Example 1 No Fine Deposited Example 2 No Fine None Example 3 No Fine None Comparative Present Rough Deposited Example 1 Example 4 Apolyestertypeelectro-coating composition ("POWER COAT 9600 K" (trade mark) manufactured by Nippon Paint Co., Ltd) was applied to each of the plates treated under Comparative Example 1 and Examples 1 to 3 to a film thickness of 20 microns, and the resulting coated plate was baked at 170"C for 30 minutes.

The plate electrocoated as above was subjected to a 5% saline spray test (JIS Z-2371), and the results are shown in Table 2 below. A cellophane adhesive tape manufactured by Nichiban Company, Ltd. was used for a peeling offtest (JIS Z-1522).

TABLE 2 Peeling of coating film AFTER After After 240 hrs. 360 hrs. 500 hrs.

Example 1 (A) Within 1.5 - 2.0 2.0-3.0 1.5 mm mm mm (B) None 5 10 Example 2 (A) Within 1.5 - 2.0 2.0 - 2.5 1.5 mm mm mm (B) None None None Example3 (A) Within 1.5-2.0 2.0-2.5 1.5 mm mm mm (B) None None None Comparative (A) 1.5-2.0 2.0-3.0 Tape Example 1 mm mm width (B) 15 20 Whole surface Note: (A) Peeling from the cut part (B) Peeling in spot rust from

Claims

1. A dip phosphating process, which comprises introducing a substrate to be phosphated into a bath containing a phosphating solution while causing undulations or waves in the phosphating solution at the entrance section of the bath.

2. A process according to claim 1, wherein the substrate is washed by spouting a phosphating solution and/or water thereon when the substrate is being taken out from the phosphating solution in the bath at the exit section of the bath.

3. A process according to claim 1 or 2, wherein the phosphating solution in the bath is circulated.

4. A dip phosphating process comprising introducing a substrate into a phosphating bath; dipping the substrate into a phosphating solution in the bath until the substrate is phosphated; and taking out the characterised in that the introduction of the substrate into the phosphating bath is carried out while producing undulations or waves in the phosphating solution at the entrance section of the bath so as to prevent any stepped unevenness of phosphating.

5. A process according to claim 4, wherein the undulations of the phosphating solution are produced by spouting the phosphating solution from nozzle apertures.

6. A process according to claim 4 to 5, wheren the taking out of the phosphated substrate is carried out while spouting the phosphating solution and/or water onto the substrate so as to remove any sludge adherent thereto.

7. A process according to any of claims 4 to 6, wherein the dipping of the substrate is carried out while transferring the substrate from the entrance section of the bath to the exit section of the bath.

8. A process according to claim 7, wherein the transfer of the substrate is carried out while spouting the phosphating solution through nozzle apertures so as to move the sludge from the exit section of the bath to the central section of the bath.

9. An apparatus for dip phosphating, which comprises a phosphating bath and a means for subjecting the phosphating solution in the bath to undulation provided at the entrance section of the bath.

10. An apparatus according to claim 9, wherein the spouting means are provided along both sides of the bath.

11. An apparatus according to claim 9 or 10, wherein a means for spouting the phosphating solution and/or water on to the substrate taken out from the phosphating solution in the bath is provided at the exit section of the bath.

12. An apparatus according to any one of claims 9 to 11, wherein means are provided for ensuing flow of the phosphating solution in the bath by spouting the phosphating solution from the bottom section and the lateral side of the bath.

13. An apparatus for dip phosphating which comprises a phosphating bath comprising an entrance section, a central section and an exit section, and a conveyor provided thereon for introducing a substrate into the bath, transferring the substrate from the entrance section to the exit section and taking out the substrate from the bath, characterized in that a means is proved for achieving undulation in the phosphating solution at the entrance section of the bath.

14. An apparatus according to claim 13, wherein the bath has a section of ship shape, the entrance section is tapered and deepened to the central section, the central section constitutes a horizontal flat bottom and the exit section is tapered and shallowed from the central section, a depressed portion being provided between the entrance section and the central section.

15. An apparatus according to claim 13 or 14, wherein a means is provided for spouting the phosphating solution andíor water onto the substrate taken out from the bath at the exit section.

16. A process according to any of claims 1 to 8, substantially as herein described with reference to the accompanying drawings and/or of specific examples.

17. An apparatus according to any of claims 9 to 15, substantially as herein described with reference to the accompanying drawings and/or of the specific examples.