WO2009016075A1

WO2009016075A1 - Process for the electrolytic zinc coating of aluminium alloys

Info

Publication number: WO2009016075A1
Application number: PCT/EP2008/059633
Authority: WO
Inventors: Sergio Vitella; Achille Vitella
Original assignee: Sergio Vitella; Achille Vitella
Priority date: 2007-07-27
Filing date: 2008-07-23
Publication date: 2009-02-05
Also published as: EP2185748A1; ITMI20071514A1

Abstract

There is disclosed a process (1) for the electrolytic zinc coating of aluminium alloys including a cathode protection (6) of a piece (2) obtained by placing the piece (2) made of aluminium alloy to be treated at a cathode (12) and at least one aluminium element (15) and at least one zinc element (16) at an anode (14) for the deposition of aluminium and zinc oxide on the piece (2) at the cathode (12) adapted to promote a subsequent acidic electrolytic galvanising (7) of the piece (2) made of aluminium alloy without the production of sludges.

Description

"Process for the electrolytic zinc coating of aluminium alloys"

* * * *

DESCRIPTION

The present invention relates to a process for the electrolytic zinc coating of aluminium alloys.

Different techniques are currently known for protecting a surface in order to increase the resistance to corrosion on aluminium alloys, these techniques being briefly summarised hereinafter to highlight the limits thereof. Both decorative and hard anodic oxidations aim to produce a layer of oxide of the metal itself, which is stable and protective for the underlying non-oxidized metal, on the surface of the pieces to be treated. They differ as far as the different thickness of the deposit is concerned, 15-40 microns for the first and 50-300 microns for the second, and due to the fact that the first may display different colours. The processing steps include: cleaning the piece, electro lytical passivation of the piece that is connected to the anode of the cell, colouring and fixing. The increase in the resistance to corrosion is obtained with several steps specific for the alloy that needs to be anodised. Treating again a material that has already been treated leads to vary the tolerances and the surface appearance thereof, and not always does the piece thus obtained still satisfy the processing specifications and it needs to be discarded. The pickling steps required for this method make the material more porous, and sealants need to be used to avoid pitting of the treated surface (aluminium staining). The baths employed need continuous regenerations and very strong and long suctions, and furthermore because of the oxidation of the metal, regeneration sludges are produced; accordingly, the maintenance and the use are expensive and polluting, and the costs are uneconomical for the client.

A further corrosion protective treatment for alloys of aluminium which is widespread nowadays is alodine, or chromium-plating, which is used as a grip for powder or epoxy paints, and is a strong oxidising agent with an effect similar to that seen for the previous method. The deposition occurs by contact and not by current flow. Also in this case the material must undergo a pre-treatment that depends on the aluminium alloy to be coated and consists of an acidic treatment, the porosity of the material increasing. The treatment requires suction because of the gas produced in the processing step, as well as a continuous regeneration of the electrolyte being required and plenty of sludges being produced.

Finally, an electrolytic galvanising treatment named zirconium, with zinc-cobalt or zinc-iron baths is considered. The preparatory treatments are the same, although they may not be carried out on all aluminium alloys. The process is laborious and implies pack-hardening and copper coating or nickel plating to provide an optimum grip for the following zinc-cobalt or zinc-iron baths. It requires a very good suction system, constant controls of the levels of zinc in the baths and a continuous regeneration of the same: in fact it produces more polluting sludges than other processes.

It is the object of the present invention to therefore provide a process for the electrolytic zinc coating of aluminium alloys, which is free from the above mentioned drawbacks, i.e. providing the galvanising of aluminium and its alloys without pre-treatments which corrode the material to be treated, without the use of carcinogenic substances, without producing regeneration sludges, and without compromising the tolerances in the case of repeated treatments.

According to the invention this object is achieved by a process for the electrolytic zinc coating of aluminium or aluminium alloys, characterized in that it includes a cathode protection of a piece obtained by placing the piece made of aluminium or aluminium alloy to be treated at a cathode and at least one aluminium element and at least one zinc element at an anode for the deposition of aluminium and zinc oxide on the piece at the cathode in a solution including at least 10% of sulphuric acid and surface- active agents, said cathode protection being adapted to promote a subsequent acidic electrolytic galvanising of the piece made of aluminium or aluminium alloy.

These and other features of the present invention will become more apparent from the following detailed description of a practical embodiment thereof, shown by no way of limitation in the accompanying drawings, in which:

Figure 1 shows a block diagram of an embodiment of the process for the electrolytic zinc coating which is the object of the invention; Figure 2 shows a diagrammatic view of a cathode protection tank;

Figure 3 shows a diagrammatic view of an acidic electrolytic galvanising tank.

Figure 1 distinguishes the various processing steps of a process 1 for the electrolytic zinc coating of aluminium or aluminium alloys, which includes the known steps of chemical degreasing 3, activating 4, neutralising

5 on a piece 2 to be treated, and the innovative coupling of a cathode protection 6 of the piece by an electrolytic acidic galvanising 7; the piece 2 is then subjected to known steps such as a recovering step 8 and a passivation step 9, with related intermediate washing steps 10 between one step and the other and a final drying step 11 of the piece.

In detail, the chemical degreasing step 3 consists of a 10%

(hereinafter the percentage is to be considered a weight percentage per volume of solution) sodium hydroxide bath at a temperature of 40-50⁰C, for about 7-8 minutes. An emulsifying agent is added to roughly eliminate the organic residues, such as chemical oil.

A washing step 10 with demineralised water follows to avoid the deposition of limescale.

The following step relates to activation 4 which is a bath that contains a percentage of 10% sodium hydroxide, surface-active agents and demineralised water at a temperature of 20-30 degrees centigrade, for 6-7 minutes. The purpose is to completely eliminate the organic component from the surface of the piece 2. A washing step 10 by the disclosed modes follows.

The following step is the neutralising step 5 which is a bath consisting of demineralised water and a very low percentage (10-15%) of nitric acid in a solution at room temperature for 2-7 minutes. The purpose is to prepare the piece 2 for the following treatment by increasing the porosity of the material.

Referring now to figures 2 and 3, the following step includes the coupling of the cathode protection 6 to promote the following acidic electrolytic galvanising step 7.

The cathode protection in general may occur by two methods: by impressed current and by galvanic coupling. The method by impressed current provides that the piece to be treated is placed as a cathode at a given potential generated by an emf (electromotive force) impressed by a direct current power supply, such that the zinc at the anode is consumed by corrosion. The method for the galvanic coupling is disclosed hereinafter by way of mere example.

For the cathode protection 6, the piece 2 made of aluminium alloy is placed at the cathode 12, for instance a copper grid, in a solution 13 including sulphuric acid and surface-active agents, and it preferably consists of: sulphuric acid, preferably 80-100 g/1 (at least 10% of solution, preferably 25-35% of the solution, more preferably 30%); surface-active agents, weak acid or base, preferably 25-35 g/1, more preferably 30 g/1 (10-20% of the solution, preferably 15%); anti-dots agents (not pure anodes form dots), preferably 15- 25 g/1, more preferably 20 g/1 (5-15% of the solution, preferably 10%); and demineralised water (remaining).

The cathode protection step 6 occurs at room temperature (20-30°) for about fifteen minutes (12-18 minutes) for a deposit of 10-20 micron, for about eight minutes (5-10 minutes) for a deposit of 3-7 micron. PH is preferably comprises between four and six. Aluminium elements 15 and zinc elements 16 are placed at the anode 14.

Depolarized anodes of about 80 kg (70-90 kg) are used to speed up the coating and reduce the water absorption and micro-fractures. When current flows, zinc and aluminium ions pass into solution from the anode 14 and a layer of aluminium and zinc oxide with a thickness of about 1-20 micron deposits on the aluminium piece 2 at the cathode 12, according to the application required (see above the relation time-micron). This forms an optimal preparatory layer for the following acidic electrolytic galvanising 7, promoting in this step the deposition of zinc on aluminium.

The current employed is preferably comprised between 4,5-6 A/dm² and 7-9 A/dm². Under said current intensity the deposit is slowed down and occurs water absorption with micro-fractures of the base metal (for example for nuts and bolts). On the other hand too much ampere means an excessive energy consumption.

Advantageously the process claimed therein allows to obtain a high quality product with a lower overall energy consumption. It is not necessary a dehydrogenation treatment and the material resistance is optimal (third place of the hardness scale after diamond and corridone which cost much more).

Anyway the ampere values remain much lower than the known methods disclosed, as the solution 13 allows the anode 14 to more rapidly disperse than in the known art. The aluminium piece 2 at the cathode 12 hangs by means of titanium hooks (not shown) to prevent the metal of the hook from passing into solution. A carbon filter pump allows to limit the consumption of solution 13. The washing step 10 follows, as already disclosed. The following step of electrolytic acidic galvanising 7 provides for the piece 2 being placed at the cathode 12 and for only the zinc elements 16 being placed at the anode 14 in baths with a galvanising solution 17 of potassium chloride, zinc chloride and brightening base in hydrogen peroxide, the latter for the removal of zinc chloride.

The galvanizing solution 17 preferably consists of: potassium chloride, 100-140 g/1, preferably 120 g/1 (30-40% of the solution, preferably 35%); zinc chloride, 30-60 g/1, preferably 45 g/1 (15-25% of the solution, preferably 20%); base, 15-25 g/1, preferably 20 g/1 (5-15% of the solution, preferably 10%); - brightening, 15-25 g/1, preferably 20 g/1 (5-15% of the solution, preferably 10%); and hydrogen peroxide (remaining).

The solution 17 is advantageously free from additional substances serving as a buffer for the pH, as the piece 2 disperses aluminium oxide in electrolysis, which is considered the best buffer for the pH of acidic galvanising baths and is usually not employed because it is too expensive

(boric acid and ammonium chloride, which are more cost-effective, are usually used); the buffer substances are normally required to maintain the pH that tends to increase as the galvanising step proceeds, within the values of 4.5 and 5.2. In this step, which is about 20 minutes long, the effect is the deposition of 99.9% pure zinc on the piece 2 made of aluminium prepared with the cathode protection 6.

As for the cathode protection step 6, even during the galvanising step 7 is provided a carbon filter pump that allows to circulate again the solution 17. When the galvanising 7 is completed, the following step, designated recovering step 8, occurs in the galvanising tank and is carried out to recover the so-called spherules, i.e. the zinc spheres which tend to form on the piece. To detach them, pressurised air is introduced in the galvanising bath and the bath itself is aspirated with a filter pump to collect them. The treatment is a few tens of seconds long.

A washing step 10 by the disclosed modes follows.

A sealing passivation step 9 is then carried out in order to eliminate the porosity of the film and therefore confer better aesthetical features and protective features against corrosive agents. The surface irregularities of the zinc coating are filled and the whole of the galvanised surface is coated by placing it in a solution of tetrahedral dyes or with classical passivation agents for zinc, thus protecting it from oxidation in contact with the air that alters the colour thereof. It should be noted that no dyes but only passivation agents may be used in the electrolytic galvanising step 7 without the cathode protection step 6.

The subsequent and final washing step 10 is followed by the step of drying 11 in furnaces at 90⁰C.

In virtue of this treatment, optimum resistances to corrosion may be obtained even with a minimum coating thickness (3-7 micron), and unlike the treatments currently used, it does not employ carcinogenic substances, the processing of the material requires few steps and it may easily be treated again without compromising the tolerances of the treated piece.

The zinc coating is optimal as a grip for powder or epoxy paint, it may be highly conductive or insulating and may display different colours which emphasise the aesthetical appearance in virtue of passivation or colouring agents.

In virtue of the combination of cathode protection 6 and electrolytic galvanising 7, a very good hardening of the material is obtained, thus allowing it to be resistant to mechanical stresses such as torsions, rubbing, scratches, etc.

As the combination of cathode protection 6 and acidic galvanising 7 produces no sludges by dispersion of metals in the baths, no problems occur with the discharge or regeneration of the waters, and furthermore it does not produce gases during the treating step.

Finally, the energy saving (as already disclosed) and the time saving should be noted: the treatment suggested is about 1 hour 30 minutes long, as compared to the 2 hour and 30 minute long of traditional treatments.

In confirmation of the efficiency of the process according to the present invention, tests in conformity with the law UNI ISO 9227 about the formation of the white and red rust have been performed in a chemical analysis laboratory of Atotech Italia s.r.l. (analysis number 077/07).

The corrosion report in saline fog (normally used to verify the resistance to white rust and red rust) testify high resistance to corrosion with minimum zinc coating and high resistance of ferrous material during passivation step.

Specifically in known processes the black passivation (next to the coating with electrolytic zinc) needs a zinc thickness of 12 micron and has a white corrosion resistance of 48-72 hours long and a red corrosion resistance of 200 hours long.

A sample (aluminium base metal with cathode protection according to the present invention) subjected to black passivation with a thickness of

1,11 micron (less then a ten of said usual thickness), has a white corrosion resistance of 48 hours long and a red corrosion resistance of 312 hours long (more than 50%).

A further sample with a thickness of 3,3 micron (a quarter of usual thickness) has a white corrosion resistance of 48 hours long and a red corrosion resistance of 336 hours long (almost twice).

The yellow trivalent passivation needs normally on ferrous material a zinc thickness of 8-12 micron and has a white corrosion resistance of 96 hours long and a red corrosion resistance of 200 hours long. A sample obtained according to the present invention and subjected to trivalent yellow passivation with a thickness of 3,3 micron has a white corrosion resistance of 432 hours long and a red corrosion resistance of 888 hours long. The white trivalent passivation needs normally on ferrous material a zinc thickness of 8-12 micron and has a white corrosion resistance of 24-48 hours long and a red corrosion resistance of 150-200 hours long. A sample obtained according to the present invention and subjected to trivalent white passivation with a thickness of 4,9 micron has a white corrosion resistance of 144 hours long and a red corrosion resistance of 600 hours long.

The green passivation needs normally on ferrous material a zinc thickness of 8-12 micron and has a white corrosion resistance of 150-200 hours long and a red corrosion resistance of 300-400 hours long. A sample obtained according to the present invention and subjected to trivalent white passivation with a thickness of 3,2 micron has a white corrosion resistance of 456 hours long and a red corrosion resistance of 912 hours long.

Moreover a thermic shock at 240° for 18 hours long has been performed with very good results.

Claims

1. A process (1) for the electrolytic zinc coating of aluminium or aluminium alloys, characterized in that it includes a cathode protection (6) of a piece (2) obtained by placing the piece (2) made of aluminium or aluminium alloy to be treated at a cathode (12) and at least one aluminium element (15) and at least one zinc element (16) at an anode (14) for the deposition of aluminium and zinc oxide on the piece (2) at the cathode (12) in a solution (13) including at least 10% of sulphuric acid and surface-active agents, said cathode protection (6) being adapted to promote a subsequent acidic electrolytic galvanising (7) of the piece

(2) made of aluminium or aluminium alloy.

2. A process (1) according to claim 1, characterized in that said solution (13) of sulphuric acid and surface-active agents includes also a anti-dots agent and demineralised water.

3. A process according to claim 2, characterized in that the solution (13) for the cathode protection (6) consists of: sulphuric acid, 80-100 g/1; surface-active agents, weak acid or base, 25-35 g/1; anti-dots agents, 15-25 g/1; and - demineralised water.

4. A process according to claim 3, characterized in that the solution (13) consists of: sulphuric acid, 90 g/1; surface-active agents, weak acid or base, 30 g/1; - anti-dots agents, 20 g/1; and demineralised water.

5. A process according to claim 2, characterized in that the solution (13) consists of: sulphuric acid, 25-35%; - surface-active agents, weak acid or base, 10-20%; anti-dots agents, 5-15%; and demineralised water.

6. A process according to claim 5, characterized in that the solution (13) consists of: - sulphuric acid, 30%; surface-active agents, weak acid or base, 15%; anti-dots agents, 10%; and demineralised water.

7. A process according to any of the preceding claims, characterized in that said electrolytic acidic galvanizing (7) occurs in a solution (17) consisting of: potassium chloride, 100-140 g/1; zinc chloride, 30-60 g/1; base, 15-25 g/1; - brightening, 15-25 g/1; hydrogen peroxide.

8. A process according to claim 7, characterized in that said electrolytic acidic galvanizing (7) occurs in a solution (17) consisting of: potassium chloride, 120 g/1; - zinc chloride, 45 g/1; base, 20 g/1; brightening, 20 g/1; and hydrogen peroxide.

9. A process according to any of the preceding claims, characterized in that said electrolytic acidic galvanizing (7) occurs in a solution (17) consisting of: potassium chloride, 30-40%; zinc chloride, 15-25%; base, 5-15%; - brightening, 5-15%; and hydrogen peroxide.

10. A process according to claim 9, characterized in that said electrolytic acidic galvanizing (7) occurs in a solution (17) consisting of: potassium chloride, 35%; - zinc chloride, 20%; base, 10%; brightening, 10%; and hydrogen peroxide.

11. A process according to any of the preceding claims, characterized in that the current intensity of the cathode protection (6) is comprised between

4,5 and 9 A/dm².

12. A process according to any of the preceding claims, characterized in that the anode (14) is depolarized.

13. A process according to any of the preceding claims, characterized in that the deposit time of the cathode protection (6) is about 8 minutes long for

3-7 micron.

14. A process according to any of the preceding claims, characterized in that the deposit time of the cathode protection (6) is about 15 minutes long for 10-20 micron.

15. A process according to any of the preceding claims, characterized in that the PH of the solution (13) of the cathode protection (6) is comprised between 4 and 6.

16. A process according to any of the preceding claims, characterized in that dyes are provided in the electrolytic acidic galvanizing step (7).

17. A process according to any of the preceding claims, characterized in that it is provided a passivation step (9) on treated pieces (2) with a zinc thickness of 8 micron.

18. A process according to any of the preceding claims, characterized in that it is provided with a passivation step (9) on treated pieces (2) with a zinc thickness of 5 micron.