EP1230425A1

EP1230425A1 - Method and device for applying an anti-corrosive coating

Info

Publication number: EP1230425A1
Application number: EP00974632A
Authority: EP
Inventors: Didier Raymond Charles Pernes; Daniel Rouquier
Original assignee: Dacral SA
Current assignee: Dacral SA
Priority date: 1999-11-02
Filing date: 2000-11-02
Publication date: 2002-08-14
Also published as: AU1286201A; BR0015204A; WO2001032956A1; JP2003516841A; FR2800390A1; HK1049189A1; KR20020068527A; FR2800390B1

Abstract

The invention concerns a method for applying an anti-corrosive coating by spraying a treating liquid on a plurality of small-size metal parts, in particular screws, bolts, nuts, rivets and blind parts. The invention is characterised in that said parts are subjected to mutual agitation during the spraying process proper.

Description

METHOD AND DEVICE FOR APPLYING AN ANTI-CORROSION COATING

The present invention relates to the general field of anti-corrosion treatment of metal parts, and more particularly of small metal parts such as screws, bolts, nuts, rivets, blind parts, screws with captive washers, etc. In the context of the present description, small metal parts will be understood to mean parts whose largest dimension remains less than approximately 15 cm and which have a unit weight less than approximately 200 g.

These small metal parts must be coated with one or more deposits protecting them from corrosion. This deposit must necessarily be thin to avoid any dimension problem and therefore assembly.

The most commonly used method currently is to immerse a batch of parts in a bath consisting of an aqueous dispersion of chromium oxide, lamellar zinc, aluminum and organic matter. Such a corrosion treatment product is notably marketed by the applicant company under the name of DACROMET®.

In the context of such treatment, the basket in which the metal parts have been placed is immersed in the treatment liquid, then taken out and centrifuged in order to remove the excess product. The pieces are then poured onto a carpet or a tray and are subjected to a drying and cooking operation. In practice, this first protective layer can be followed by the deposition of additional successive layers, either based on mineral compositions, or based on entirely organic paints, and this according to the same method of application by dipping.

This type of immersion anti-corrosion coating application has a number of major drawbacks. he firstly has the consequence of the appearance of a greater retention of liquid in the confined areas of the parts to be treated. In particular, socket head screws, of the torx type, or any other metal part having a zone favoring the retention of liquid, would encounter a problem of localized excess thickness and consequently problems of dimension when mounting these parts. Since it is out of the question to present such parts, such as torx screws whose heads are totally or partially blocked, at the various stations of the assembly lines, in particular of motor vehicles, whether automated or manual , it is therefore in practice necessary to carry out a manual sorting of these parts beforehand to avoid problems of use on the assembly line. This type of immersion process also leads to other drawbacks, such as the appearance of peeling of the coating at the bottom of the threads of the screws.

Finally, this type of application process by immersion requires the use of large volumes of baths to ensure the immersion of all the parts. In addition, some anti-corrosion product formulations have a limited shelf life, due to polymerization reactions or reactions that can take place between the various components. Consequently, the renewal of the immersion bath by consumption is an important parameter in the economics of anti-corrosion treatment processes for this type of part. In fact, in practice, it turns out that the bath must be completely renewed in a time less than its normal lifespan. The present invention has made it possible to remedy these drawbacks by using a method of applying an anti-corrosion coating by spraying a treatment liquid onto a plurality of small metal parts. size, while all of these parts are subjected to shuffling.

The mixing of all the parts to be treated with each other must be carried out in such a way as to allow them to roll randomly over one another so as to occupy all the possible spatial positions during the operation of spray and therefore to present the maximum surface area facing the spray of the treatment liquid in a determined volume, namely in the volume of the spray cone.

According to a particular embodiment of the process which is the subject of the present invention, the stirring movement of said metal parts is slowed down immediately after the spraying operation. Advantageously, the stirring movement of said parts is even completely stopped at the end of the spraying operation.

According to a particular embodiment of the method according to the invention, the treatment liquid consists of an aqueous dispersion, in an organic or mineral binder, of solid mineral particles. These particles are mainly constituted by particles of metals such as zinc and / or aluminum, but can also include pigments, in particular metal oxides such as manganese oxide. Such products are notably marketed by the applicant under the brands DACROMET®, GEOMET® and DACROKOTE®.

According to another characteristic of the present invention, the spraying operation can be carried out after a preliminary treatment operation, in particular a surface cleaning using treatment products, in the liquid or vapor state, or even after a shot blasting operation.

The present invention also relates to a device for implementing the method as previously stated. Such a device comprises a mixing enclosure for processing said parts, which has an axis of symmetry around which it is subjected to a continuous rotational movement and inside which opens at least one stationary nozzle for spraying the treatment liquid. These spray nozzles are fixedly oriented on the side of the zone or zones in which or which said parts are accumulated as a result of the rotational movement of said mixing and processing enclosure.

Various other technical characteristics and advantages of the present invention will appear on reading the detailed description given below, in particular with regard to two particular embodiments, and this with regard to the appended drawings in which:

- Figure 1 shows a first embodiment of the device according to the invention, consisting of a rotating basket tank, and;

- Figure 2 shows a second embodiment of a tubular continuous spraying device.

FIG. 1 represents a device comprising a mixing and processing enclosure produced in the form of a tank 10 containing a rotary cylindrical basket 12 containing the parts to be treated 14. This basket 12 has mesh walls to allow the liquid to flow excess processing. Of course, the size of the mesh openings of this basket 12 is chosen so as to prevent the metal parts to be treated 14 from coming to rest there and partially clogging the basket. The mesh opening must, however, be large enough to allow good flow of the treatment liquid at the end of the spraying operation. As shown in Figure 1, the tank 10 is arranged so as to be able to affect a stirring and spraying position inclined at an angle α from 40 ° to 60 ° relative to the vertical. On the other hand, it is clear that at rest, the tank 10 is arranged so as to have a position in which its axis of symmetry is substantially vertical.

In the case where it is desired to subject the metal parts 14 to an impregnation pretreatment, it is perfectly possible to keep the tank 10 in a vertical position and immerse all of the parts 14 in a sufficient volume of liquid of treatment. To carry out such an immersion operation in the tank 10 and the basket 12, it is of course necessary to close off the discharge of treatment product, located in the low position of said tank. When the immersion operation is finished, the evacuation of the product is opened and the entire immersion bath flows through the evacuation duct to return to the reserve tank. The parts are then centrifuged at very fast speed (30-700 rpm) to remove the product retained in the retention areas. In this specific case, the product used has a lower viscosity than in the traditional process. This low viscosity, combined with a high centrifugation speed, allows the deposition of a homogeneous pre-layer in distribution and having no negative influence on the plugging of the screw heads.

In accordance with the present invention, the additional application of the coating is then carried out by spraying the anti-corrosion treatment liquid. This operation will advantageously be carried out in accordance with the present invention by tilting the axis of the tank 10 and the basket 12 at an angle of about 40 ° to 60 ° relative to the vertical.

As stated previously, the spraying process according to the invention is carried out while the parts 14 are subjected to mixing between them. To do this, the basket 12 is rotated about its axis. In practice, a rotation speed of the order of 10 to 30 revolutions per minute for a basket with a diameter of about 300 mm has led to excellent results.

As shown in FIG. 1, the tank 10 is closed by a cover 16 at its upper part, said cover being provided with an anti-fog filter 18.

On the lower part of the cover 16 of the inclined tank, is mounted a sprayer 20 which remains fixed in position during the mixing of the parts 14 obtained following the rotation of the basket 12 about its axis. Given the particular shape of the basket 12 having a frustoconical projection 22 in the bottom wall, and the inclination of the entire device, the rotation will lead to a zone of accumulation of parts 14 on which the cone spraying liquid treatment 24 must be precisely oriented. To complete the mixing of the metal parts 14 with one another, the inner surface of the rotating basket 12 is advantageously bristling with projecting elements, in particular produced in the form of rods or strips. The latter, not shown in Figure 1 attached, advantageously extend along generators of the rotary cylindrical basket 12 and, if necessary, even advantageously extend over all or part of the bottom of said basket. These rods can be simply welded to the mesh constituting the walls of said basket. To ensure the most efficient possible stirring, in practice we will have recourse to a plurality of such strips also spaced apart at the circumference of said basket.

In order to obtain good results, it has proved useful in practice to use a spray treatment by low pressure spraying, in particular less than approximately

1 bar and preferably between 0.7 and 1 bar. Such spraying can be obtained using an HVLP sprayer

(High Volume Low Pressure). With such a type of sprayer, a high volume of compressed air is obtained in the head of spraying by appropriate adjustment of the distribution of the compressed air in said head, respectively in the center of the cap and at the horns of the spraying head. The use of such a type of sprayer makes it possible to minimize the formation of mist.

In practice, the flow rate used with such a type of spraying corresponds to approximately 250 to 300 ml / min of anti-corrosion treatment liquid for approximately 100 kg of metal parts to be treated. This ensures an anti-corrosion coating deposit, of the DACROMET® type, of approximately 20 g / m ² with a duration of the order of 2 minutes. Of course, to the extent that one would like to obtain larger deposits, it is in particular possible to use several sprayers, or to increase the time. In practice, it has also proved useful to use a treatment liquid which has a viscosity of 15 to 30 seconds, determined by means of flow cups according to standard NF EN ISO 2431. Such a treatment liquid also advantageously has a dry extract greater than about 33%.

By applying the process of the invention, under these conditions, to DACROMETISATION®, it was possible to obtain perfectly satisfactory coatings, both from the point of view of their thickness and their continuity, as from the point of view of their anti-corrosion performance.

As indicated above, it is very easy to obtain coating thicknesses of the order of 20 g / m ² using a single sprayer. If it is desired to obtain larger layers, it is also possible to preheat the parts up to a temperature of approximately 50 ° C., while the normal spraying temperature is close to ambient temperature. By thus preheating the parts, it is possible to obtain a much thicker anti-corrosion coating since it goes from 20g / m ² to 30g / m ² . Alternatively, it is of course possible to obtain a given thickness, to reduce the spraying time by preheating the metal parts to be treated.

The schematic representation of Figure 1 illustrates how to arrange the sprayer in this type of treatment tank.

As already indicated above, the lower part of the tank 10 is provided with an evacuation orifice 26 coupled to an evacuation duct 28 which opens into a reserve tank 30. Through a pneumatic circuit comprising a regulator of air pressure 32, a pump 34, a circulation valve 36 and possibly a heater 38, this reserve tank supplies by a conduit 40 the stationary sprayer 20 placed in the cover 16 of the treatment device. This cover, as shown schematically by the arrow F in Figure 1, can be tilted around its hinge 42 to allow the introduction and unloading of the treated metal parts 14. For safety, it is possible to have the in the vicinity of the opening of this cover 16, the sheath 44 of a suction device intended to capture any fogs formed.

The second embodiment illustrated in FIG. 2 corresponds to a mixing and processing enclosure affecting the general shape of a rectilinear tube 46 open at its two ends 48 and 50. This tube is driven in a rotational movement around its axis. The circular cylindrical wall of this tube 46 is at least partially screened to also allow the flow of excess treatment liquid.

The inner surface of the tube has at least one helical projection 52, preferably continuous, ensuring both the advancement of the metal parts 54 to be treated along the tube 46 as well as their mixing. If necessary, other elements, in particular rods or strips, can advantageously be arranged between the turns of the helical projection (s) 52, preferably oriented in the direction of the axis of the straight tube to improve the efficiency of mixing. As illustrated in FIG. 2, this device is equipped inside the tube 46, with an axial ramp 56 provided with a plurality of spray nozzles 58, preferably oriented towards the areas of accumulation of the parts to be treated 54 .

Such a device makes it possible to be integrated within a continuous processing chain. Such continuous treatment is illustrated in FIG. 2. The metal parts to be treated are brought to a drive belt 60 and can for example undergo a pre-treatment, in particular continuous shot blasting by passing through a shot blasting machine 62 , the end of the feed belt 60 arriving inside the tube 46 after having passed through the first end 50 of the latter. The rotating tube 46 makes it possible, thanks to its helical projection 52, to carry out the driving and mixing of the parts 58 subjected to the spraying operation.

The excess liquid for treating the metal parts passes through the lower part of the screened tube 46 to be recovered by a recovery chute 64 and brought into a reservoir 66. This reservoir, which can moreover be supplied with new product, is also coupled to a device for supplying the sprayer boom 56 via an appropriate pump 68. At the outlet of the spray tube 46, the parts thus coated fall onto another drive belt 70 which allows, after passing through a baking oven 72, to complete the anti-corrosion treatment. The general spraying conditions and the nature of the spray nozzles used can be strictly identical to those corresponding to the device illustrated in Figure 1 previously described.

As indicated above, these two devices allow the implementation of a process leading to a spraying in a chamber for treating the metal parts subjected to mixing, which makes it possible to reach all the areas to be coated without the risk of localized blockage of the blind parts of said parts.

By way of example, the comparative results observed on parts treated by a conventional quenching method will be indicated below, in comparison with a spraying treatment with stirring, which is the subject of the present invention.

The results indicated below illustrate first of all the fact that the process according to the invention has overcome the phenomenon of localized retention. The results indicated above correspond to the percentages of cases where the tool cannot penetrate into the hollow heads of Tork T40 screws. The experiment was carried out on a sample of 270 Tork T40 screws. Conventional two-layer immersion process with two firing of DACROMET® leading to a deposit of 24g / m ² : under these conditions, we observe 6% of non-conforming parts, that is to say, the tool is unable to penetrate sufficiently into the hollow heads of the Tork screws due to the presence of excess thickness of the processing layers.

Comparatively, with the process which is the subject of the present invention, that is to say by spraying with stirring of the Tork screws, a deposit of DACROMET® of 22 g / m ² is obtained for a layer and a cooking operation:

- in this case we observe 0% of non-conforming parts. In accordance with the invention, a deposit of 30 g / m ² of DACROMET® was also always carried out on a batch of 270 Tork screws, this time applied by spraying in two layers with two respective cooking operations:

- in this situation, we have always observed 0% non-compliance. Comparative tests to determine the anti-corrosion performance of a conventional dipping process and of the process according to the invention are also reported below.

This sulfur dioxide corrosion test corresponds to the Kesternich test as described in DIN 50018, suitable for an amount of 0.3 liters.

Finally, in the context of a salt spray resistance test, the performance achieved by the spraying method according to the invention by depositing a layer of 15 g / m ² made it possible to obtain a salt spray resistance time. more than 1,500 hours. This result is therefore strictly of the same order of magnitude as those obtained by a traditional immersion process by DACROMETISATION®.

It is therefore observed that the process which is the subject of the present invention makes it possible to obtain results which are identical in all respects by depositing a single layer by spraying with respect to a conventional application by immersion renewed twice to obtain two layers, which necessarily , are each followed by a separate cooking operation. This therefore results in a considerable economic gain in application.

Finally, it will be observed that for high protection treatments, it is possible to apply up to 36g / m ² of treatment liquid of the DACROMET® type. Such a treatment can be obtained using two layers and two baking operations without as far as it is accompanied by a plugging of the hollow heads of Tork screws. In comparison, such an application density can only be obtained by a conventional immersion process by depositing three successive layers accompanied by three baking operations. But in such a case, there is an almost complete systematic obturation of the Tork screw head, which makes its use impossible.

Claims

1. Method for applying an anti-corrosion coating by spraying a treatment liquid onto a plurality of small metal parts, in particular screws, bolts, nuts, rivets and blind parts, characterized in that said parts are subjected to mixing between them during the spraying operation using a treatment liquid consisting of an aqueous dispersion, in an organic or inorganic binder, of metallic particles comprising zinc and / or aluminum.

2. Method according to claim 1, characterized in that the stirring movement of said parts is slowed down immediately after the spraying operation.

3. Method according to claim 1, characterized in that the stirring movement of said parts is stopped immediately after the spraying operation.

4. Method according to one of claims 1 and 2, characterized in that the treatment liquid is sprayed by low pressure spraying, in particular less than about 1 bar and preferably between 0.7 and 1 bar, with volume d high compressed air in the spray head to minimize mist formation.

5. Method according to claim 4, characterized in that the spraying is carried out with a flow rate of 250 to 300 ml / mm for approximately 100 kg of parts to be treated.

6. Method according to one of claims 1 to 5, characterized in that the treatment liquid has a viscosity of 15 to 30 sec, determined by means of flow cups according to standard NF EN ISO 2431.

7. Method according to one of claims 1 to 6, characterized in that the treatment liquid has a dry extract greater than about 33%.

8. Method according to one of claims 1 to 7, characterized in that the spraying operation is carried out in the vicinity of room temperature, optionally after preheating the parts to a temperature of about 50 ° C.

9. Method according to one of claims 1 to 8, characterized in that the spraying operation is carried out after a pretreatment operation, in particular an impregnation using a treatment or shot blasting liquid.

10. Device for implementing the method according to one of claims 1 to 9, characterized in that it comprises an enclosure for mixing and processing said parts (14), which has an axis of symmetry around which it is subjected to a continuous rotational movement and inside which opens at least one stationary spray nozzle (20) for the treatment liquid.

11. Device according to claim 10, characterized in that the spray nozzle (s) (20) of the treatment liquid are fixedly oriented towards the side of the zone in which said parts (14) are accumulated as a result of the movement of rotation of said mixing and processing enclosure.

12. Device according to one of claims 10 and 11, characterized in that said mixing and processing enclosure is produced in the form of a tank (10) enclosing a basket (12) of cylindrical, rotary mesh screen containing said parts (14 ) and allowing the flow of excess treatment liquid.

13. Device according to claim 12, characterized in that said tank (10) is arranged so as to present at rest a position in which its axis of symmetry is vertical and a stirring and spraying position inclined from 40 ° to 60 ° with respect to the vertical.

14. Device according to one of claims 12 and 13, characterized in that said tank (10) is closed by a cover (16) provided with an anti-fog filter (18).

15. Device according to one of claims 12 to 14, characterized in that the inner surface of the rotary basket (12) is bristling with projecting elements to perfect the mixing of the parts, which are in particular in the form of rods or strips extending along generators of the rotating cylindrical basket.

16. Device according to one of claims 10 and 11 characterized in that said mixing and processing enclosure is produced in the form of a rectilinear rotary tube (46) open at its two ends and the wall of which is at least partially screened to allow excess treatment liquid to drain.

17. Device according to claim 16, characterized in that the inner surface of the tube (46) has at least one helical projection (52), preferably continuous, ensuring both the advancement of said parts (54) along said tube as well as their mixing.

18. Device according to one of claims 16 and 17, characterized in that the interior of the tube is equipped with an axial ramp (56) provided with a plurality of spray nozzles

(58), preferably oriented towards the parts accumulation zones (54).

19. Device according to one of claims 16 and 17 characterized in that the spaces between the turns of the helical projection (s) (52) are bristling with other projecting elements to perfect the mixing of the parts, in particular produced in the form of rods or strips co-axial with the rotary rectilinear tube (46).