WO2015136353A1

WO2015136353A1 - Plant and process for the anodizing treatment of products made of aluminium or its alloys

Info

Publication number: WO2015136353A1
Application number: PCT/IB2015/000307
Authority: WO
Inventors: Carlo Calcaterra
Original assignee: Qualital Servizi S.R.L.
Priority date: 2014-03-11
Filing date: 2015-03-10
Publication date: 2015-09-17
Also published as: ES2772808T3; EP3117028B1; EP3117028A1

Abstract

A plant and a process are disclosed for the continuous anodizing treatment of products made of aluminium or alloys thereof. The plant comprises means for feeding the products along a treatment path, at least one tank for collecting and/or storing a liquid electrolyte solution and a plurality of pipes made of an electrically insulating material and adapted for supplying jets of the liquid electrolyte solution towards the products along the treatment path, the solution being pressurized by a pump. Cathode electrodes are arranged in electric contact with the liquid electrolyte solution, and anode electrodes are arranged in electric contact with the products.

Description

"PLANT AND PROCESS FOR THE ANODIZING TREATMENT OF

PRODUCTS MADE OF ALUMINIUM OR ITS ALLOYS"

* * *

Field of the Invention

The present invention relates to a plant and a process for the anodizing treatment of products made of aluminium or alloys thereof and, particularly, a plant and a process designed for the treatment of profiled sections made of such materials.

As is known, aluminium is a silvery metal having the unique feature of turning its surface into a thin layer of natural aluminium oxide which is hard, compact, and practically inert to atmospheric agents.

This property can be usefully exploited by appropriate electrochemical surface processing treatments of aluminium which allow obtaining an anodic oxide having a thickness much higher than the thickness of natural oxide.

Such electrochemical processes are referred to as "anodizing" processes because the material (aluminium) is connected to the anode (positive pole) of a suitable electrochemical system comprising a direct current generator.

The thickness of aluminium oxide is expressed in microns (symbol μηι), i.e. thousandths of a millimetre. For most applications of anodized aluminium, layer thickness is of great importance because it is a key factor controlling its corrosion resistance and its behaviour in the external environment. However, the choice of the thickness should be agreed with the user and it depends on the degree of aggressiveness of the environment in which the material is to be placed, the final appearance of the surfaces and the final size of the finished object.

Technical standards in force for architectural applications (doors, windows, curtain walls) provide five classes of thickness as indicated in Table 1 below.

20 20 16

25 25 20

For example, thicknesses of Classes 15 and 20 are generally used for external architectural applications. In the United Kingdom, as well as in some other countries of Northern Europe, thicknesses of Class 25 may be required, especially where industrial pollution is high or when used in industrial marine environments.

Internal architectural applications, as well as many general applications of anodized aluminium, may require thicknesses of Class 15, Class 10 and Class 5. Special applications, such as heat- or light-reflectors, may even require thicknesses less than 5 μπι.

Prior Art

The anodizing treatment of aluminium is generally carried out by dipping the products to be anodized into a tank containing an electrolyte solution, for example a solution of sulphuric acid (H₂S0₄). The products are then connected to the positive pole (anode), whereas the negative pole (cathode) consisting of a lead or aluminium electrode, is dipped into the electrolyte solution contained in the tank.

The quality and properties of the anodic oxide as obtained by these known types of processes depend on the characteristics of aluminium and alloys thereof, but especially on conditions and treatment variables which may influence the characteristics of the final product, namely:

1) Concentration of sulphuric acid;

2) Current density;

3) Treatment time;

4) Bath temperature;

5) Content of aluminium dissolved in the bath;

6) Stirring condition of the bath;

7) Addition of additives to the solution;

8) Presence of extraneous impurities.

In the products to be anodized, the current density, i.e. the current strength per unit surface, is generally maintained between 1.4 and 1.8 A/dm². However, the choice of current density to be used depends on certain factors, such as the maximum power of the current rectifier with respect to the amount of material to be treated and the cooling capacity of the bath into which the products are dipped.

From the practical point of view, the following empirical formula can be applied to calculate a priori the thickness obtainable for the oxide:

s = k * d * t

wherein:

s is the thickness of the oxide as expressed in microns;

d is the current density, expressed in A/dm²;

t is the treatment time, expressed in minutes

k is a proportionality factor dependent on the type of alloy subjected to treatment.

For most alloys suitable to be subjected to anodizing treatment, for example those of the 6xxx series, a value of k = 0.3 could be experimentally derived.

By assuming that treatment is carried out at a current density of about 1.5 A/dm², the above mentioned empirical formula can be used to determine treatment times as a function of the various thicknesses desired to be obtained. For example, when factor k is set to a value of 0.3, the following Table 2 shows the treatment times and the average thicknesses obtained.

However, since the aluminium anodizing process is carried out by passing an appropriate electric current through the products to be treated, utmost care should also be paid to the electrical connection of the products to the positive pole (anode) of the circuit. In the plants of the known type, electrical connection is obtained by means of suitable supports of conductive material (also referred to as "hangers") which support the products while connecting them to the positive pole of the circuit.

The supports can be made, for example, of an aluminium alloy or otherwise of a material having characteristics of high electrical conductivity, good workability, light weight and affinity for the material to be anodized.

The shapes of the supports depend on the type of products to be treated. For example, they can be rectangular, square, circular, pentagonal in cross-section or rack-shaped, or the like. In any case, the main features of the supports should be a good compromise among the following requirements:

- ability to remain tightly anchored to both the positive pole and the products to be oxidized throughout the entire treatment;

- ability to transfer the required amount of electric current to all the products connected thereto, i.e. having a useful cross-section allowing the required current strength to be transferred;

- if possible, establishing the contact with the material at points or edges rather than on flat surfaces so as to prevent a non-homogeneous distribution of the oxide from occurring in the areas of contact with the products.

As an alternative to the processes of the known type, which rely on dipping of the products into the electrolyte solution, international patent application No. WO2011/145033 proposes the use of jets of electrolyte solution directed towards the profiled sections to be anodized. The plant described in this document is provided with a cathode (negative pole) comprising a plurality of tubular conduits made of an electrically conductive material, for example aluminium or lead, and spraying means which are integral to the cathode. Once the profiled sections have been suitably connected to the positive pole of a direct current generator, they are huiig up only in a vertical position and translated through the treatment area where they are sprayed by opposite jets of electrolyte solution.

One of the main drawbacks of this approach is that, when the profiled sections are in a hung condition, they can easily swing within the treatment area and directly contact the cathodes, resulting in a short circuit. Under normal conditions, the circuit between the anode and the cathode is closed by the jets themselves of the electrolyte solution, which forms the load resistance of the circuit. The currents passing between the anode and the cathode are very high and, therefore, the direct contact between the anode and the cathode turns-on the protection systems of the generator, thereby blocking the production cycle or, in the worst case, potentially damaging the direct current generator.

Other drawbacks of the plant described in this document are that the profiled sections are fed along the treatment area only in a vertical position, i.e. hung up by means of a single hooking means. Accordingly, the maximum current which can pass through the profiled section is limited by the maximum size that a single hooking means can have.

In addition, since the profiled sections may also be of considerable lengths, for example up to about six/seven metres, the pump used to pressurize the electrolyte solution should be very powerful in order to ensure the thrust required to supply the electrolyte solution under an appropriate pressure up to the highest spraying means.

Summary of the Invention

From the foregoing, the task of the present invention is to provide a plant and a process for the continuous anodizing treatment of products made of aluminium or alloys thereof, which allow the drawbacks of the prior art to be solved.

In particular, an object of the present invention is to provide a plant of the type mentioned above in which the short circuit between the anode and the cathode is prevented from occurring.

Another object of the present invention is to provide a plant of the type mentioned above which can treat profiled sections of any length by effectively delivering the current along the entire length of the profiled sections.

A further object of the present invention is to provide a plant of the type mentioned above which can reduce the amount of power used by the pump for spraying the products, and particularly the profiled sections, with jets of an electrolyte solution. Another object of the present invention is to provide a plant of the type mentioned above which can even treat profiled sections having a complex cross-profiled section. Still another object of the present invention is to provide a plant and a process which can be easily integrated with other processes and plants for the continuous treatment of products made of aluminium or alloys thereof.

These objects are achieved by the present invention which provides a plant according to claim 1 and a process according to claim 9. Further characteristics and advantages of the present invention are set forth in the respective dependent claims.

Generally, a plant for the continuous anodizing treatment of products made of aluminium or alloys thereof comprises means for feeding the products along a treatment path, at least one tank for collecting and/or storing a liquid electrolyte solution, a plurality of pipes for supplying jets of the liquid electrolyte solution towards the products along the treatment path, a plurality of conduits for hydraulically connecting the tank to the pipes, and at least one pump for feeding the electrolyte solution under pressure to the pipes. The plant comprises cathode electrodes arranged in electric contact with the liquid electrolyte solution, and anode electrodes arranged in electric contact with the products.

According to the present invention, at least the pipes supplying the jets of electrolyte solution are made of an electrically insulating material. This helps to prevent dangerous short circuits from occurring between the anode and the cathode while the products are moved through the treatment area. Practically, the electric circuit is closed only and exclusively by means of the "liquid contact" of the jets connected to the cathode, so as to ensure the load resistance required in the circuit while preventing that any swinging movement of the products connected to the anode may bring them into direct contact with the cathode.

In a possible embodiment, the pipes supplying the electrolyte solution are arranged on dispensing devices to which the electrolyte solution is fed under pressure. At least one cathode electrode is advantageously housed in each of the dispensing devices. Actually, it was surprisingly found that the resistance of the electrolyte solution is not significantly altered compared to that of the solutions of the known type. In fact, the cathode is positioned very close to the jets, and any change in the distance between the ends of the jets and the products passing through the treatment area will result in negligible changes in resistance. In a possible embodiment of the present invention, an electrically insulating material which has proved to be suitable for the manufacture of the pipes is, for example, PVC. In addition to the pipes, the dispensing devices in which the cathodes are housed, as well as the collection and/or storage tank for the electrolyte solution and the relevant pipings hydraulically connecting all the components of the hydraulic circuit, may also be advantageously made of the same material. The material employed for these purposes allows the costs of the plant to be reduced and the maintenance of the cathode electrodes, which are made of aluminium or lead, to be carried out independently from that of the dispensing devices made of PVC.

In one embodiment of the plant according to the present invention, the pipes open onto opposite sides along the treatment path so that the entire surface of the products, particularly that of the profiled sections made of aluminium or alloys thereof, can be sprayed with the jets.

The pipes can also be equipped with nozzles, also made of an electrically insulating material, at their ends. Advantageously, the nozzles may be adjustable to allow even the treatment of products in the form of profiled sections having a particularly complex cross-section. This allows the nozzles to be appropriately oriented in such a way as to direct their jets towards areas which are difficult to be effectively reached with a horizontal jet.

While the products are moved along the treatment path, they are supported by appropriate supporting means which comprise the anode electrodes adapted to connect said products to the positive pole of a direct current generator. The anode electrodes may constitute a portion of the supporting means.

If the products are profiled sections, i.e. products having a prevailing dimension with respect to the other two, the supporting means support the profiled sections preferably by keeping them with their prevailing dimension parallel to the feeding direction along the treatment path, i.e. in a horizontal position. The supporting means support the profiled sections in at least two points in order to ensure the passage therethrough of high currents which can either facilitate the formation of an oxide layer of increased thickness for the same treatment time or, alternatively, reduce the treatment times for the same thickness of the oxide layer. Alternatively, the profiled sections can also be fed while keeping them with their prevailing dimension perpendicular to the feeding direction along the treatment path, i.e. in a vertical position, and with the use of a single supporting means to electrically connect the profiled sections.

The invention also relates to a process for the continuous anodizing treatment of products made of aluminium or alloys thereof. The process comprises the steps of:

a) feeding the products through a treatment plant having a plurality of pipes for supplying jets of a liquid electrolyte solution towards the products along a treatment path;

b) collecting the liquid electrolyte solution in a tank hydraulically connected to the pipes by means of a plurality of conduits, at least one pump being arranged along the conduits to feed the electrolyte solution under pressure to the pipes;

c) arranging cathode electrodes into electric contact with the liquid electrolyte solution

d) arranging anode electrodes into electric contact with the products. Advantageously, the process according to the invention envisages that the jets of liquid electrolyte solution directed towards the products along the treatment path are supplied by pipes made of an electrically insulating material.

The electrolyte solution is fed under pressure to dispensing devices in each of which at least one cathode electrode is housed.

The jets of electrolyte solution under pressure are supplied by pipes opening onto opposite sides along the treatment path.

According to one embodiment of the process of the present invention, the jets directed towards the products can be supplied through nozzles arranged at the ends of the pipes. The nozzles are also made of an electrically insulating material, and they can be of an adjustable type so as to appropriately direct the jets depending on the geometry of the profiled section.

While the products are moved along the treatment path, they are supported by appropriate supporting means which comprise the anode electrodes. Particularly, in the frequent case in which the products are profiled sections, i.e. products having a prevailing dimension with respect to the other two, the profiled sections are fed along the treatment path while keeping them hung up in at least two points with their prevailing dimension parallel to the feeding direction, i.e. in a horizontal position. Alternatively, the profiled sections can be fed along the treatment path while keeping them hung up in at least one point with their prevailing dimension perpendicular to the feeding direction, i.e. in a vertical position.

In a possible embodiment of the process according to the invention, the products can also be moved with a reciprocating motion along directions perpendicular to the axes of the pipes supplying the jets. Practically, regardless of the horizontal feeding movement, the products can be lowered and raised while they are moved through the treatment area, or they can be horizontally moved forward and backward to maintain them within the treatment area for all the time required to form the desired oxide layer.

Brief Description of the Drawings

Further features and advantages of the present invention will be more apparent from the following description which is given by way of illustration and not by way of limitation with reference to the accompanying schematic drawings, in which:

- Figure 1 is a cross-sectional view, taken along a plane perpendicular to the feeding direction of the products, of a portion of a treatment plant according to a possible embodiment of the present invention;

- Figure 2 is a longitudinal sectional view, taken along a plane parallel to the feeding direction of the products, of a treatment plant according to a possible embodiment of the present invention;

- Figure 3 is a longitudinal sectional view of a detail of the plant according to a possible embodiment of the present invention;

- Figures 4 A and 4B illustrate the thicknesses of the anodic oxide as obtained from a test of a first profiled section sample with a flat shape;

- Figures 5A and 5B illustrate the thicknesses of the anodic oxide as obtained from another test of a second profiled section sample with a flat shape;

- Figures 6A and 6B illustrate the thicknesses of the anodic oxide as obtained from another test of a third profiled section sample with a flat shape; - Figures 7 A and 7B illustrate the thicknesses of the anodic oxide as obtained from another test of a fourth profiled section sample with a flat shape;

- Figures 8A and 8B illustrate the thicknesses of the anodic oxide as obtained from another test of a fifth profiled section sample with a flat shape;

- Figure 9 schematically shows a treatment plant such as that of Figure 1 , which has been used to test a length of a door-and-window frame profiled section with a complex geometry;

- Figure 10 shows the cross-section of the door-and-window frame profiled section subjected to treatment in the plant of Figure 9;

- Figures 1 OA to 10D illustrate the thicknesses of the anodic oxide as obtained from a test of the profiled section sample of Figure 10; and

- Figure 11 schematically shows a plant such as that of Figure 1, which has been used to obtain the results for a pair of samples of extruded plates made of aluminium alloy EN AW 6060 which were simultaneously subjected to anodizing treatment.

Detailed Description

In Figure 1, a portion of a treatment plant 100 according to a possible embodiment of the present invention is schematically illustrated. The plant 100 comprises a containing body 101 having arranged therein two dispensing devices 9 to dispense an electrolyte solution, for example constituted by a sulphuric acid solution, which is fed from an inlet 4 through pipings 8 made of an electrically insulating material, such as PVC or the like.

The dispensing devices 9 face the two sides of an aluminium plate sample 2 to be subjected to the anodizing treatment. The dispensing devices 9 have arranged thereon pipes 3 made of PVC or a similar electrically insulating material, which supply jets of the electrolyte solution towards the sample 2 while it is being moved through the treatment area.

The pipings 8 are connected to a collection reservoir 7 for collecting the electrolyte solution, which is fed by means of a circulation pump 6 from a storage/collection tank 5 for storing/collecting sulphuric acid 5.

Cathode electrodes 1, which are made of a metal, are housed within each of the dispensing devices 9. Corresponding anode electrodes (not shown here) are arranged in electric contact with the sample 2.

In Figure 2, a plant 100 for the continuous anodizing treatment of profiled sections 20 made of aluminium or alloys thereof is schematically shown. The profiled sections 20 are fed along a treatment path in the direction indicated by arrows A. On the side shown in Figure 2, various devices 9 for dispensing the electrolyte solution, which is supplied through the pipes 3, are highlighted. The electrolyte solution supplied along the treatment path by the pipes 3 falls towards the bottom and is directed into the electrolyte solution collection/storage tank 5, for example by providing a hopper 50 below the containing body 101.

As also shown in Figure 1, the pump 6 feeds the electrolyte solution under pressure to the collection reservoir 7 and then again to the dispensing devices 9 through a hydraulic circuit comprising the conduits 8 and the inlets 4 of the devices 9. In the view of Figure 2, also the piping 8 is outlined by a dashed line and connects the opposite side of the dispensing devices 9 (not visible here) facing those shown.

In the embodiment shown herein, the profiled sections 20 are fed in a horizontal position, i.e. with their major dimension (length) parallel to the feeding direction, as indicated by the arrows A, along the treatment path. The profiled sections 20 are supported by at least two supporting means 40: as is known these can be, for example, grippers or else the so-called "hangers", i.e. filaments of aluminium or the like which are wrapped around the profiled sections to establish an electric contact between the profiled sections 20 and the positive pole 120 through the anodic bar 45. Along the anodic bar 45, which is shaped as a track or a rail, for example, wheels or sliding contact pads 42 coupled to conductors 43 electrically connecting the supporting means 40 to the anodic bar 45, are moved along direction A.

The negative pole 110 is electrically connected to the cathodes 1 housed in each of the dispensing devices 9. The poles 110 and 120 are connected to a direct current generator (not shown) which can supply the currents required to perform the treatment.

While the profiled sections 20 are moved along the treatment path, they can also be moved with a reciprocating motion along directions perpendicular to the axes of the pipes 3 supplying the jets of electrolyte solution. These movements, schematically referred to by double arrows M in Figure 2, are independent from the feeding movement and can be useful for increasing the spreading of the electrolyte solution on the profiled sections 20 as well as for complying with the residence times of the profiled sections 20 along the feeding path as a function of the oxide thickness to be obtained.

In the enlarged view of Figure 3, a supplying pipe 3 equipped with a nozzle 30 at the end thereof is shown. The nozzle 30, which is also made of a non-conductive material as the pipe 3 and the dispensing device 9 from which it protrudes, is preferably a nozzle of an adjustable type which can be tilted to various positions (as represented by dashed lines, for example) by an angle a with respect to the axis 3 a of the pipe 3.

In addition to being a surface treatment capable of conferring excellent aesthetic and weathering resistance properties to aluminium, anodizing can also be used as one of the possible treatment techniques preceding a subsequent painting process and as an alternative to other conventionally used processes which are based on chromium or titanates, zirconates, silanes, etc.

A plant 100 according to the present invention, which operates in a continuous manner, can be advantageously integrated into a plant for the continuous painting of profiled sections made of aluminium and alloys thereof. The process according to the present invention also allows for easily meeting the treatment requirements necessary for subjecting the profiled sections made of aluminium and alloys thereof to a subsequent painting step.

Indeed, anodizing, as well as other treatments which will be defined by the generic term "chemical conversion" in the following, allow the formation of a layer which is protective against possible corrosive phenomena as well as suitable to allow the final layer of paint to be adhered thereto.

All these types of pre-treatment for the following painting, including anodizing, steps are also certified by the mark of international quality QUALICOAT, which is specific for applications of aluminium and alloys thereof in the architecture field.

For architectural outdoor applications, anodizing represents an actual surface finishing process by which oxide layer thicknesses of 15, 20, 25 microns can be achieved. Anodizing, as pre-treatment for painting, envisages layers of anodic oxide even more reduced in thickness, for example having a thickness in the range from 3 to 5 microns. Anodizing treatments intended to reach thicknesses much higher than those indicated above, for example thicknesses even in excess of 50 microns, are also widespread; in this case, they are referred to as the so-called "hard" anodizing or "hard-coat".

In the prior art, the prevailingly used techniques of chemical conversion are different from anodizing, and they are integrated in the lines of industrial painting processes. Actual painting is carried out after the step of chemical conversion and it is performed by spray application of powder painting products, for example products with thermosetting characteristics, followed by treatment in an oven in order to cure the powders to such a degree as to form an even layer of paint adhered to the aluminium substrate.

The steps preceding the application of the powders, including the step of chemical conversion, can be carried out according to the following two techniques:

1) by dipping - a typical technique for the so-called "horizontal" plants because the final step of painting is carried out with the aluminium profiled sections arranged horizontally on special "tray conveyors". Before the material is painted, it is dipped into tanks containing the various pre-treatment solutions according to one of the following two dipping modes (one alternative to the other):

a. the material to be dipped is horizontally loaded into special baskets; once the pre-treatment is concluded, the material is discharged from the baskets and hooked by tray conveyors for the final step of painting;

b. the material to be dipped is hooked directly by the tray conveyors; once the pre-treatment is concluded, the material remains hooked on the tray conveyors for the final step of painting;

2) by spraying - a technique applied to both "horizontal" plants and "vertical" plants.

These latter are so defined because the aluminium profiled sections to be painted are hung vertically and, in this position, they are fed by means of a chain handling system for all the pre-treatment and painting processes.

In the case of vertical plants as well as in the above case b for horizontal plants, the material is hooked only once before being subjected to all the various steps of the entire process, i.e. before being sprayed with the pre-treatment solutions and being sprayed with the powder paints in the final step of painting.

Using anodizing instead of other types of chemical conversion results in the following changes to the painting plants:

- in the case of the dipping mode, the addition of an in-line anodizing tank;

- in the case of the spraying mode, the use of a spray anodizing technique to be integrated into the plant.

The first approach involves costly and complex modifications to the process.

The second approach can be easily implemented by means of a device and a process according to the present invention.

Indeed, the possibility of adopting the process according to the present invention has been successfully tested by using a pilot plant designed according to the present invention. For the various series of tests, the following process parameters were recorded:

• Cathode/anode resistance

• Electric voltage

· Electric current

• Type of electrolyte solution

• Concentration of the electrolyte solution

• Temperature of the electrolyte solution

• Size of the cell, cathode/anode distance

and the following features of the anodized pieces were recorded:

• Coating Ratio (C.R.)

• Unit Sealing Value (U.S.V.)

• Thickness of the anodic oxide

• Sealing quality (Scott's Method and Weight Loss)

· Abrasion resistance (Clark's test).

The above mentioned features have been compared with the corresponding features obtained with the use of traditional dipping-type anodizing processes.

Considering that a primary object of the invention is to be able to perform continuous surface treatments, the anodized samples were translated with a reciprocating motion (at a speed of about 1.2 m/min) in front of the jets, thus simulating the desired condition of integrating an anodizing step into continuous, full treatment lines, i.e. anodizing followed by painting.

As will be noted later, the movement of the pieces with a reciprocating motion allowed obtaining a thickness of the anodic oxide layer which is clearly superior in evenness compared to those obtained by keeping the pieces to be anodized stationary in front of the jets.

The present invention is a valid alternative to the current techniques of chemical conversion and, as such, it offers the possibility of integrating the present treatment into a continuous painting line without altering its layout and while increasing the production yield thereof compared to the traditional approach which provides the step of dipping-type anodizing in a separate plant.

The results of the tests carried out according to the present invention confirmed the soundness of the process and allowed evaluating the efficiency of the dispensing device 9 of electrolyte solution with the metal cathode 1 housed therein, in making the electric resistance of the jet independent from the cathode/anode distance, resulting in undoubted advantages especially from the point of view of safety (no risk of cathode/anode contacts and consequent short circuits) and plant simplicity.

The samples used for the tests were samples made of aluminium alloy EN AW 6060 with a chemical composition according to the requirements of standard EN 573-3. In order to prevent electric current leakages, the hooks used for supporting the test samples as well as the hidden faces of the test samples were suitably coated with a special resin which cannot be altered by the contact with the acidic solution of the electrolyte.

Before the anodizing step, the samples were mechanically brushed and pre-treated (degreased and pickled) by means of three successive steps (with washes between the steps) in 1) an acidic solution, 2) an alkaline solution), and 3) an acidic solution, with process parameters as given in Table 3 below. Table 3

Pre-treatment step and Approximate Time Temperature products formulation (min) (°C)

Room

1. Acid degreasing Inorganic acids 1

temperature

2. Alkaline pickling Na(OH)-based 2 40

Room

3. Acid neutralization Inorganic acids 1

temperature

Experimental activity was divided into a number of steps, each comprising various tests in order to derive meaningful conclusions from their results. The various steps are described below.

STEP NO. 1

At this step, the electrolyte solution dispensing device 9 comprising therein an aluminium cathode 1 was tested.

The system was constituted by two identical dispensing devices 9 opposing each other and equidistant from the anode, in this case constituted by the aluminium sample 2 to be anodized, as schematically shown in Figure 1.

The aim of the first step was to verify whether it was possible to avoid the use of cathodes in the vicinity of the pieces to be anodized as well as to replace the holes drilled on the cathodes with simple PVC pipes.

Test No. 1

The process parameters for Test No. 1 were as follows:

- Concentration of sulphuric acid: about 200 g/1;

- Free aluminium: about 7 g/1;

- Temperature of sulphuric acid solution: about 33°C;

- Aluminium sample size: 100 x 100 mm;

- Distance "d" between the free ends of the PVC pipes (Figure 1): about 100 mm;

- Total electric current: about 4A;

- Current density on test sample (total surface of the two faces): about 2 A/dm²;

- Electric voltage: about 30 V;

- Anodizing time: about 10 min.

From Test No. 1, which was essentially designed to evaluate only the functionality of the plant, a quite uniform thickness of anodic oxide of about 6 - 7 μηι was obtained on both the faces of test sample 2.

Test No. 2

The process parameters for Test No. 2 were as follows:

- Concentration of sulphuric acid: about 200 g/1;

- Free aluminium: about 7 g/1;

- Temperature of sulphuric acid solution: about 26°C;

- Aluminium sample size: 100 x 100 mm;

- Distance "d" between the free ends of the PVC pipes (Figure 1): about 100 mm;

- Total electric current: about 6 A;

- Current density on test sample (total surface of the two faces): about 3 A/dm²;

- Electric voltage: about 43 V;

- Anodizing time: about 10 min.

The results of Test No. 2 (Sample 2), in terms of thickness of the anodic oxide as expressed in microns, are shown in Figures 4A and 4B with the values recorded in the areas highlighted by a circle. The thicknesses obtained for the anodic oxide varied, for example with thicknesses in the range from 5.8 to 12.3 μιη on one face (Fig. 4A) and thicknesses in the range from 6.9 to 11.2 μπι on the other face (Fig. 4B).

Test No. 3

The process parameters for Test No. 3 were as follows:

- Concentration of sulphuric acid: about 200 g/1;

- Free aluminium: about 7 g/1 ;

- Temperature of sulphuric acid solution: about 22°C;

- Aluminium sample size: 200 x 100 mm; - Distance "d" between the free ends of the PVC pipes (Figure 1): about 100 mm;

- Total electric current: about 6A;

- Electric voltage: about 40 V;

- Anodizing time: about 10 min.

The results of Test No. 3 (Sample 21), in terms of thickness of the anodic oxide as expressed in microns, are shown in Figures 5A and 5B with the values recorded in the areas highlighted by a circle. The thicknesses obtained for the anodic oxide varied, for example with thicknesses in the range from 3.9 to 6.3 μιη on one face (Fig. 5 A) and thicknesses in the range from 3.6 to 13 μιη on the other face (Fig. 5B). STEP NO. 2

This second test step was designed to evaluate the influence of the distance of PVC pipes from the piece to be anodized while keeping the ends of the pipes equidistant from the sample to be anodized, i.e. the anode. Practically, the electric resistance between the cathode and the anode was measured.

Test No. 4

The process parameters for Test No. 4 were as follows:

- Concentration of sulphuric acid: about 200 g/1;

- Free aluminium: about 7 g/1 ;

- Temperature of sulphuric acid solution: about 26°C;

- Aluminium sample size: 100 x 100 mm;

- Distance "d" between the free ends of the PVC pipes (Figure 1): about 100 mm and 200 mm.

From this test it was found that, by varying the distance "d" between the free ends of the PVC pipes (Figure 1) from about 100 mm to about 200 mm, the resistance value of jets was not significantly changed. These measurements refer to the simultaneous action of opposite jets on both the faces of the sample 2 to be anodized.

STEP NO. 3

Once the above was verified, further anodizing tests were carried out at different electric voltage and current conditions to measure the resulting thickness values of the anodic oxide layers on the two faces of a sample.

Test No. 5

The process parameters for Test No. 5 were as follows:

- Concentration of sulphuric acid: about 200 g 1;

- Free aluminium: about 7 g/1;

- Temperature of sulphuric acid solution: about 26°C;

- Aluminium sample size: 100 x 100 mm;

- Distance "d" between the free ends of the PVC pipes (Figure 1): about 100 mm;

- Current density on the test sample (total surface of the two faces): about 3 A/dm².

By setting the distance "d" between the free ends of the PVC pipes to about 100 mm, the test sample 22 (Figures 6 A and 6B) was subjected to an anodizing cycle in which voltage and current were varied as shown in Table 4 below.

The results of Test No. 5 (Sample 22), in terms of thickness of the anodic oxide as expressed in microns, are shown in Figures 6A and 6B with the values recorded in the areas highlighted by a circle. The thicknesses obtained for the anodic oxide varied, for example with thicknesses in the range from 8.7 to 19.2 μπι on one face (Fig. 6A) and thicknesses in the range from 9.4 to 14.9 μπι on the other face (Fig. 6B).

Test No. 6

The process parameters for Test No. 6 were as follows:

- Concentration of sulphuric acid: about 200 g/1; - Free aluminium: about 7 g/1;

- Temperature of sulphuric acid solution: about 26°C;

- Aluminium sample size: 100 x 100 mm;

- Distance "d" between the free ends of the PVC pipes (Figure 1): about 100 mm;

- Electric voltage: about 27 V;

- Current density on test sample (total surface of the two faces): about 1.5 A/dm²;

- Anodizing time: about 15 min.

The results of Test No. 6 (Sample 23), in terms of thickness of the anodic oxide as expressed in microns, are shown in Figures 7A and 7B with the values recorded in the areas highlighted by a circle. The thicknesses obtained for the anodic oxide varied, for example with thicknesses in the range from 6.3 to 8.7 μπι on one face (Fig. 7A) and thicknesses in the range from 6.3 to 9.1 μιη on the other face (Fig. 7B). Test No. 7

The process parameters for Test No. 7 were as follows:

- Concentration of sulphuric acid: about 200 g/1;

- Free aluminium: about 7 g/1;

- Temperature of sulphuric acid solution: about 28°C;

- Aluminium sample size: 100 x 100 mm;

- Distance "d" between the free ends of the PVC pipes (Figure 1): about 100 mm.

Test sample was subjected to an anodizing cycle in which voltage and current were varied as shown in Table 5 below for a total time of 8 minutes.

40 5 2.5 1

50 7 3.5 2

60 10 5 2

70 12 6 1

The results of Test No. 7 (Sample 24), in terms of thickness of the anodic oxide as expressed in microns, are shown in Figures 8A and 8B with the values recorded in the areas highlighted by a circle. The thicknesses obtained for the anodic oxide varied, for example with thicknesses in the range from 7.1 to 13 μιη on one face (Fig. 8A), and thicknesses in the range from 6.1 to 12.5 μηι on the other face (Fig. 8B). STEP NO. 4

In this step, the test sample was a length of a door-and-window frame profiled section 25 made of aluminium alloy EN AW 6060 and having a length of 70 mm, which is representative of the extruded profiled sections widely used and always subjected to surface treatments of anodizing and/or painting. A diagram of the plant 100 with the profiled section 25 arranged along the treatment path is shown in Figure 9, whereas a cross-section of only the profiled section 25 is shown in Figure 10. The aim of this test step was to evaluate the anodizing capability for a profiled section, over its entire outer surface, being much more complex in the transverse direction, as precisely shown in Figure 10 compared to that of the flat extruded samples as used in the previous tests.

Test No. 8

The process parameters for Test No. 8 were as follows:

- Concentration of sulphuric acid: about 200 g/1;

- Free aluminium: about 7 g 1;

- Temperature of sulphuric acid solution: about 28°C;

- Aluminium sample: length of a profiled section made of aluminium alloy EN AW 6060;

- Distance "d" between the free ends of the PVC pipes (Figure 9): about 100 mm;

- electric voltage: about 35 V; - Current density on the test sample (total surface of faces): about 2 A/dm²;

- Anodizing time: about 10 min.

The results of Test No. 8 (Sample 25), in terms of thickness of the anodic oxide as expressed in microns, are shown in Figures 10A, 10B, IOC and 10D, corresponding to faces A-D as indicated in Figure 10, with the values recorded in the areas highlighted by a circle. The thicknesses obtained for the oxide were in the range from 5.1 to 7.9 μιη on face A (Fig. 10A), from 5.5 to 14 μπι on face B (Fig. 10B), from 2.5 to 6.9 μιη on face C (Fig. IOC) and of about 5.5 μιη in average thickness on face D (Fig. 10D).

STEP NO. 5

In this experiment step, whose operating conditions are shown in Figure 11, anodizing tests were carried out on samples which were previously pickled with an acid solution (5 min at room temperature) and sealed in demineralized water at 100°C for 15 min (practically at about 2.5 - 3 min/μπι) after anodizing.

After these cycles, characterization tests (Scott's test, Clark's method, C.R., U.S.V.) were performed to assess the quality of anodic oxide and sealing. Anodizing tests were performed on the plant shown in Figure 11, and they were carried out simultaneously on two samples, 26 and 27, obtained from extruded plates made of alloy EN AW 6060 with a size of 50 x 100 mm and a thickness of 2 mm.

Test No. 9

The process parameters for Test No. 9 were as follows:

- Concentration of sulphuric acid: about 200 g/1;

- Free aluminium: about 7 g/1 ;

- Temperature of sulphuric acid solution: about 28°C;

- Aluminium samples: two extruded plates made of aluminium alloy EN AW

6060;

- Distance "d" between the free ends of the PVC pipes (Figure 11): about 100 mm;

- electric voltage: about 43 V;

- Current density on test samples (total surface of faces): about 3 A/dm²;

- Anodizing time: about 6 min. The results of Test No. 9 for both samples 26 and 27 were on average equal to 6 μιη in terms of thickness of anodic oxide.

As for the characterization tests, the results obtained for sample 26 are as follows:

- Scott's test = 0÷l;

- Clark's method = 0 (no dust formed).

Weight loss tests were successively performed, whose values are reported in Table 6 below.

From these values, the following characteristic parameters can be calculated:

C.R. = (P2-P4)/(P1-P4) = 1.49;

U.S.V. = (P3-P2)/(P2-P4) * 1000 = 66.57;

Sealing quality according to ISO 3210:

pre-dip: 2,5 mg/dm²;

After phospho-chromatation: 12,6 mg/dm².

Test No. 10

The process parameters for Test No. 10 were as follows:

- Concentration of sulphuric acid: about 200 g/1;

- Free aluminium: about 7 g/1 ;

- Temperature of sulphuric acid solution: about 28°C;

- Aluminium samples: two extruded plates made of aluminium alloy EN AW 6060;

- Distance "d" between the free ends of the PVC pipes (Figure 11): about 100 mm; - Electric voltage: about 27 V;

- Current density on test samples (total surface of faces): about 1.5 A/dm²;

- Anodizing time: about 12 min.

The results of Test No. 10 for both samples 26 and 27 were on average equal to 5 μπι in terms of thickness of anodic oxide.

As for the characterization tests, the results obtained for sample 27 are as follows:

- Scott's test = 0*1;

- Clark's method = 0.5 μηι (little amount of dust formed).

Weight loss tests were successively performed, whose values are reported in Table 7 below.

From these values, the following characteristic parameters can be calculated:

C.R. = (P2-P4)/(P1-P4) = 1.39;

U.S.V. = (P3-P2)/(P2-P4) * 1000 = 69.73;

S ealing quality according to ISO 3210:

pre-dip: 3,0 mg/dm²;

After phospho-chromatation: 13,5 mg/dm².

The results obtained from Tests No. 9 and No. 10 were compared with those obtained for a sample which was anodized by dipping in a static bath with the following process conditions:

- Concentration of sulphuric acid: about 200 g/1;

- Free aluminium: about 7 g/1;

- Temperature of sulphuric acid solution: about 20°C; - Average current density on test samples (total surface of faces): about 1,8 A/dm²;

- Class (average thickness of about 24 μιη): 20;

- Sealing: in deionized water at 100°C at about 3 min/μηι (for about 75 min.) Table 8 below shows the results obtained for the two samples subjected to "jet" anodizing and, for comparison purposes, the results obtained for the sample subjected to in-tank anodizing.

The tests described above demonstrate the ability to anodize aluminium and alloys thereof with the use of a particular electrolyte dispenser cathode.

The use of this particular type of dispenser highlighted a series of applicative advantages, in terms of both processing and plant simplicity, compared to the currently used techniques.

The use of the apparatus according to the present invention offers several advantages, including:

- the ability to deliver cathode current along the jets in a simple and uniform manner; - the ability to operate with a variable distance between the ends of the pipes supplying the electrolyte solution and the anode without significant changes in the electric resistance of the jets;

- the fact that electrical conduction of jets is ensured by a simple negative electric contact on the cathode plate fitted within the device for delivery as jets of electrolyte solution;

- the elimination of potential impacts, electric contacts and short circuits due to cathode/anode contacts.

The cathode used for the described tests was an extruded aluminium cathode made of alloy EN AW 6060, however, it is clearly possible to use any other metal having improved characteristics of corrosion resistance (for example Al SI 316 L) when dipped into an electrolyte solution.

Another important advantage is connected with the possibility to implement various forms of the jet dispensing device with the use of inert materials (e.g. PVC) which also lead to a significantly reduced maintenance time.

In practice, it has been found that the invention achieves the intended task and objects.

In fact, a system for anodizing treatment of aluminium and alloys thereof is provided which comprises a particular metallic cathode positioned within the electrolyte dispensing tank. The dispenser is perforated and equipped with pipes and/or nozzles to allow the electrolyte solution to be supplied as a jet capable of anodizing objects made of aluminium and alloys thereof when these objects are positioned at the anode of a direct current electric circuit.

Placing the cathode within the containing body highlighted a particularly interesting phenomenon of electrolytic conduction.

In fact it was observed that, when the cathode (aluminium) is housed within said containing body (entirely made of PVC, an electrically insulating material) and when appropriate pipes (also made of an insulating material) are arranged on the wall facing the anode to form the jet of the electrolyte solution, the resistance between the anode and the cathode does not change substantially as the length of such pipes changes, provided that the distance between the ends thereof and the anode remains unchanged.

In any case, even by increasing such a distance of about 10-15 cm, the resistance increases in a fairly negligible manner.

Obviously, the materials as well as the size employed may be changed as needed.

Claims

1. A plant for the continuous anodizing treatment of products made of aluminium or alloys thereof, comprising means for feeding said products along a treatment path, at least one tank for collecting and/or storing a liquid electrolyte solution, a plurality of pipes for supplying jets of said liquid electrolyte solution towards said products along said treatment path, a plurality of conduits for hydraulically connecting said tank to said pipes, at least one pump for feeding the electrolyte solution under pressure to said pipes, wherein cathode electrodes are arranged in electric contact with said liquid electrolyte solution and anode electrodes are arranged in electric contact with said products, characterized in that at least said pipes are made of an electrically insulating material.

2. The plant according to claim 1, wherein said pipes are arranged on dispensing devices to which said electrolyte solution is fed under pressure, and wherein at least one cathode electrode is housed in each of said dispensing devices.

3. The plant according to claim 1, wherein said pipes open onto opposite sides along said treatment path.

4. The plant according to claim 1 , wherein said pipes are provided with nozzles at their ends, and wherein said nozzles are made of an electrically insulating material.

5. The plant according to claim 4, wherein said nozzles are adjustable.

6. The plant according to claim 1, wherein supporting means are provided to support said products while they are moved along said treatment path, and wherein said anode electrodes constitute a portion of said supporting means.

7. The plant according to claim 1, wherein said products are profiled sections with a prevailing dimension with respect to the other two, and wherein supporting means are provided to support said profiled sections in at least two points while keeping them with their prevailing dimension parallel to the feeding direction along said treatment path.

8. The plant according to claim 1, wherein said products are profiled sections with a prevailing dimension with respect to the other two, and wherein supporting means are provided for supporting said profiled sections in at least one point while keeping them with their prevailing dimension perpendicular to the feeding direction along said treatment path.

9. A process for the continuous anodizing treatment of products made of aluminium or alloys thereof, comprising the steps of:

a) feeding said products through a treatment plant having a plurality of pipes for supplying jets of a liquid electrolyte solution towards said products along a treatment path;

b) collecting said liquid electrolyte solution in a tank hydraulically connected to said pipes by means of a plurality of conduits, at least one pump being arranged along said conduits to feed the electrolyte solution under pressure to said pipes;

c) arranging cathode electrodes into electric contact with said liquid electrolyte solution;

d) arranging anode electrodes into electric contact with said products, characterized in that the jets of said liquid electrolyte solution directed towards said products along said treatment path are supplied by pipes made of an electrically insulating material.

10. The process according to claim 9, characterized by arranging said pipes on dispensing devices and feeding said electrolyte solution under pressure to said dispensing devices, and wherein at least one cathode electrode is housed in each of said dispensing devices.

11. The process according to claim 9, characterized in that the jets of electrolyte solution are supplied by pipes opening onto opposite sides along said treatment path.

12. The process according to claim 9, characterized by supplying the jets towards said products through nozzles arranged at the ends of said pipes, and wherein said nozzles are made of an electrically insulating material.

13. The process according to claim 12, characterized by adjusting said nozzles in order to direct the jets towards said products.

14. The process according to claim 9, characterized by supporting said products while they are moved along said treatment path with the use of supporting means comprising said anode electrodes.

15. The process according to claim 9, wherein said products are profiled sections with a prevailing dimension with respect to the other two, characterized in that said profiled sections are fed along said treatment path while keeping them hung up in at least two points with their prevailing dimension parallel to the feeding direction.

16. The process according to claim 9, wherein said products are profiled sections with a prevailing dimension with respect to the other two, characterized in that said profiled sections are fed along said treatment path while keeping them hung up in at least one point with their prevailing dimension perpendicular to the feeding direction.

17. The process according to claim 9, characterized by moving said products with a reciprocating motion along directions perpendicular to the axes of the pipes supplying said jets.

18. A plant for the continuous painting of products made of aluminium, characterized by comprising at least one plant for the anodizing pre-treatment of aluminium according to any one of claims 1 to 8.

19. A process for the continuous painting of products made of aluminium, characterized by comprising a step of anodizing pre-treatment of aluminium according to any one of claims 9 to 17.