EP1232295A1

EP1232295A1 - Method and means for corrosion preventive surface treatment of metals

Info

Publication number: EP1232295A1
Application number: EP00973332A
Authority: EP
Inventors: Hans Magnusson
Original assignee: Altitech AB
Current assignee: Altitech AB
Priority date: 1999-10-25
Filing date: 2000-10-24
Publication date: 2002-08-21
Also published as: AU1185401A; SE9903828L; SE9903828D0; SE515243C2; WO2001031084A1

Abstract

A method and a combined agent for surface treatment of metals, such as aluminium and zinc, by means of achieving an anti-corrosive surface layer on the metal surface. The method comprises exposure of the metal surface in a pre-treatment step and subsequent surface treatment for obtaining said anti-corrosive surface layer on the metal surface. The surface treatment is performed in two steps; in the first step through contact with a water solution containing sulphuric acid and an oxidation agent for the acid, such as alkali metal permanganate or ammonium persulphate; in the second step through contact with a water solution containing an alkali metal carbonate and an alkali metal permanganate. Hereby, a passivating surface is obtained on the metal surface, consisting of the elements manganese, oxygen, sulphur and carbon.

Description

Method and means for corrosion preventive surface treatment of metals

TECHNICAL FIELD The present invention relates to a method for surface treatment of metals, to a combined agent for accomplishing the method through treatment baths in water solutions, and to metal objects being treated by means of the method and preferably with the agent. The object of the invention is to provide the metal surface with a corrosion-protective layer that retains its electrical conductivity as well as makes it suitable for surface coating, such as through lacquering. A surface treatment of this kind is technically frequently termed passivation.

STATE OF THE ART

Aluminium and aluminium alloys are common construction materials, offering low weight and good workability. However, in most cases these materials have to be protected against corrosion.

A common method to this end is anodising, which provides an excellent corrosion protection, especially outdoors. The method is electrolytic and limited to electrically non-screened surfaces. The method provides an electrically insulating layer, and is therefore unsuitable if electrical conductivity is required.

Another common method for protecting aluminium and aluminium alloys against corrosion is chromating, which provides a good protection against self-corrosion, preferably in indoor environments where condensing humidity might occur. Chromating provides very thin layers, about 1 μm, substantially consisting of chromium oxides and chromate salts. This type of surface treatment is commonly called passivation. Chromate layers provide an electrical conductivity that is sufficient in most cases. This property is of increasing importance, as a good electromagnetic screening is sought after in modern electronic applications. Another important property of chromate layers is to provide a good lacquer adhesion, especially for outdoor exposure. A modern paint application method is powder coating, placing even higher demands on the pre-treatment, whereby chromating, preferably so-called yellow chromating, will provide a perfectly satisfactory result. Chromating is a surface treatment method for aluminium and aluminium alloys that has a major importance in technical applications.

Lately, however, the negative environmental properties of chromium combinations have attracted much attention. Such combinations are suspected to be allergenic and, in certain cases, carcinogenic. It is therefore pressing to find a replacement for chromating that has equally good technical properties whilst neither causing any health hazards or being detrimental to the exteriour environment.

Other surface treatment methods for especially aluminium has therefore been searched for, which are able to provide the same corrosion protection as chromating, but do not exhibit said adverse environmental properties. One suggested method is thereby to replace chromium with manganese, see the US Patent Docu- ment No 4 755 224 (to Bibber). Here, a treatment of the metal surface with alkali metal permanganate is suggested, for example potassium permanganate. According to the patent document, a corrosion protection equal to that obtainable through chromating will hereby be achieved, but with a treatment method and a result that are much more advantageous from an environmental viewpoint, as manganese does not exhibit the negative properties discussed above.

However, the surface treatment method according to that invention requires a cumbersome procedure with treatment in several baths at a temperature of about 100 °C. This makes the suggested process rather non-rational and thereby expensive, and it is disadvantageous from a work environment aspect.

DISCLOSURE OF INVENTION

The object of the present invention is to provide a method, through which an efficient corrosion protection can be achieved for objects made of aluminium, as well as for other corroding metals such as zinc, and for the alloys of such metals. This is achieved through a coating of the metal surface with a passivating layer containing manganese and without the use of chromium, said method being realised by a rational, simple process with only a few steps, using baths that can be held at room temperature. The invention also includes the combined agent necessary for realising the method. The invention also relates to metal objects having received the special passivation layer provided by the method. The method according to the invention, except for pre-treatment steps such as degreasing and removal of oxides and other contaminants, is characterised by consisting of two steps: Step 1 , consisting of treatment in an acidiferous bath containing sulphuric acid and an oxidation agent for the sulphuric acid. Potassium or sodium permanganate is preferably used as oxidation agents. In some cases, see the following Example 2 for further information, ammonium persulphate is preferred. Through the oxidation of the sulphuric acid, a film containing sulphur and oxygen is obtained on the metal surface.

Step 2, which is performed in an alkaline bath containing an alkali carbonate and potassium or sodium permanganate. The proper treatment steps 1 and 2 can be performed at a temperature of 20-25 °C.

PREFERRED EMBODIMENTS

In the following, some examples are given of the realisation of the method, and the means used therefor. The stated examples relate partly to variants for application with objects of different kinds regarding the metal and its alloying, and how its surface is constituted, whereas other examples are stating conceivable variations within the scope of the invention and the appended claims. The following examples are based on treatment of aluminium.

Example 1

Pre-treatment in order to obtain a clean metal surface, through degreasing and removal of oxides and other contaminants, i.e. pickling. The pre-treatment stated below is preferably applied with such aluminium alloys as used in production by means of rolling and extrusion, that is for sheets and profiles, respectively:

- degreasing in alkaline detergent at an elevated temperature of about 50-60 °C,

- pickling in a sodium hydroxide water solution, about 5%, at an elevated tem- perature of about 50-60 °C,

- neutralisation in diluted nitric acid, about 20%,

- rinsing in cold or warm water. Surface treatment step 1 : to be performed in direct connection to the pre-treatment step, in a water bath containing:

- concentrated sulphuric acid, in an amount of 2-15 g/l of water, preferably 5 g/l of water,

- potassium permanganate, in an amount of 2-15, preferably 5 g/l of water. The bath to be held at a temperature of about 20-25 °C.

Treatment of the objects is performed by submerging them in the bath for a time span of about 5-10 minutes.

Surface treatment step 2: to be performed in a water bath containing:

- potassium carbonate, 5-20, preferably 10 g/l of water,

- potassium permanganate, 2-15, preferably 5 g/l of water. The bath to be held at a temperature of about 20-25 °C.

Treatment of the objects is performed by submerging them in the bath for a time span of about 5-10 minutes. Rinsing is then performed in warm water, about 50 °C, and in cold water.

Example 2

Pre-treatment:

Intended for pre-treatment of objects made from casting alloys. These normally contain high concentrations of silicon, which has to be removed from the surface.

The following active components are to be comprised in a water bath: - concentrated nitric acid, 65%, about 150 ml/l of water,

- concentrated phosphoric acid, 85%, about 800 ml/l of water,

- hydrofluoric acid, 40%, about 50 ml/l of water.

The bath to be held at a temperature of about 20-25 °C and the treatment time should be about 2-5 minutes. Finally, a careful rinsing is performed in warm water, about 50 °C, and in cold water.

Surface treatment step 1 :

The following components are to be comprised in a water bath:

- sulphuric acid, 98%, about 10 g/l of water, - ammonium persulphate, about 40 g/l of water.

The bath to be held at a temperature of about 20-25 °C and the treatment time should be about 5-10 minutes.

Then a careful rinsing is performed in warm water, about 50 °C, and in cold water.

Surface treatment step 2:

The following components are to be comprised in a water bath:

- potassium carbonate, about 5-20, preferably 10 g/l of water,

- potassium permanganate, about 2-15, preferably 5 g/l of water. The bath to be held at a temperature of about 20-25 °C. The treatment time should be about 5-10 minutes. Finally, a careful rinsing is performed in warm water, about 50 °C, and in cold water.

In the examples, potassium permanganate and potassium carbonate have been stated. Other alkali metals may also be used as a compound together with permanganate and carbonate, respectively, and preferably sodium. Such a change to another alkali metal than potassium does not entail any substantial modifications of the procedures, or of the composition of the used agents, as stated in the examples. Considering the availability and the handling in an industrial environment, potassium is however preferred.

The preliminary steps and the composition of the agents stated in the examples are not of vital importance for the realisation of the invention. Within the known art, there are examples of other methods and agents for the preparation of the metal surface before the following steps. The preliminary steps stated in the examples have been found to yield a good result, but the invention is thus not bound to the procedures stated in the examples, as far as the pre-treatment steps are concerned.

In the surface treatment step 1 , sulphur is present in the form of sulphuric acid, which is of major importance in this context, as this sulphur will activate the surface and will be included, after oxidation of the sulphuric acid in step 1, in the passivating layer created in step 2. In Example 1 , relating to common aluminium alloys for the manufacture of sheets and profiles, potassium permanganate is stated as the oxidation agent. In Example 2, relating to treatment of casting alloys in which alloying constituents such as silicon, copper and iron are included, ammonium persulphate is stated as the oxidation agent.

For step 2, it is stated in both examples that the bath comprises potassium carbon- ate and potassium permanganate. Hereby, the passivating surface layer is finally created, in which manganese is included as the main component. The potassium carbonate contributes to the layer with the components carbon and oxygen. In step

1 , the surface is prepared, through oxidation, for formation of the final layer in step

2. Furthermore, sulphur is supplied by the sulphuric acid comprised in both exam- pies, such that the passivating layer will be constructed from the elements manganese, oxygen, carbon and sulphur. This composition of the surface layer will thus characterise the finished, treated object.

The surface layer created by the surface treatment according to the invention will provide a good protection against corrosion in an aggressive environment. The description of the invention has been directed towards corrosion protection of aluminium, and examples have been given for two types of aluminium alloy intended for objects produced by rolling or extrusion and through casting, respectively. Also other metals display corrosion properties similar to those of aluminium, this is espe- aily the case with zinc. The invention is thus not limited to application with aluminium, even if treatment of this material is the most important area of application. Within the scope of the invention will also lie its application for corrosion protection of other metals than aluminium and then primarily zinc, including some of its alloys.

Claims

1. A method for surface treatment of metals such as aluminium and zinc, by means of creating an anti-corrosive surface layer on the metal surface, said method comprising the steps of

- exposing the metal surface in a pre-treatment step, including such procedures as degreasing, removal of oxides and other non-metallic contaminants, and then

- treating the surface to obtain the anti-corrosive surface layer on the metal surface, through contact with a water solution containing concentrated sulphuric acid in a first step, and through contact between the surface and an alkali metal permanganate in a water solution in a second step, c h a r a c t e r i s e d i n that,

- in the first step, the 98% concentrated sulphuric acid is diluted to a concentration of 2-15 g/l of water, preferably 5 g/l of water, the water solution further being given a content of an oxidation agent for the sulphuric acid, thereby achieving a layer containing sulphur and oxygen on the metal surface; and that,

- in the second step, the water solution is given a content of an alkali metal carbonate in a concentration of 5-20 g/l of water, preferably 10 g/l of water, and of an alkali metal permanganate in a concentration of 2-15 g/l of water, preferably 5 g/l of water, the surface treatment preferably being performed at a water solution temperature in the first step, and in the second step, of about 20-25 °C, whereby a passivating surface layer is obtained on the metal surface, substantially consisting of the elements manganese, oxygen, sulphur and carbon.

2. The method according to claim ^ c h a r a c t e r i s e d i n that the pre-treatment step is performed through degreasing in an alkaline detergent at an elevated temperature of about 50-60 °C, subsequent pickling in a sodium hydroxide water solution with a concentration of 3-10%, preferably 5%, at about 50-60 °C, then neutralisation in nitric acid, diluted to 10-30%, preferably 20%, and concluded by rinsing in hot and cold water, said pre-treatment step being applied especially with aluminium alloys intended for rolling or extrusion.

3. The method according to claim ^ c h a r a c t e r i s e d i n that the pre-treatment step is performed through contact with a water solution containing - concentrated nitric acid, about 65%, diluted to about 150 ml/l of water,

- concentrated phosphoric acid, about 85%, diluted to about 800 ml/l of water,

- hydrofluoric acid, about 40%, diluted to about 50 ml/l of water, said pre-treatment being intended especially for the treatment of aluminium-based casting alloys.

4. A method according to claim 1 or claim 2, characterised in that, in the first surface treatment step, the metal surface is exposed to contact with a water solution containing, besides the sulphuric acid, an alkali metal permanga- nate as the oxidation agent, in a concentration of 2-15, preferably 5 g/l of water, at a water solution temperature of about 20-25 °C, said treatment being especially intended for aluminium alloys used for rolling or extrusion.

5. A method according to claim 1 or claim 3, characterised in that, in the first surface treatment step, the metal surface is exposed to contact with a water solution containing, besides the sulphuric acid, ammonium persulphate as the oxidation agent, in a concentration of 20-60, preferably 40 g/l of water, at a water solution temperature of about 20-25 °C, said treatment being especially intended for aluminium alloys used for casting.

6. A method according to any one of the preceding claims 1, 4, or 5, characterised in that the alkali metal of said compounds is potassium.

7. A combined agent for use with the method according to any one of the claims 1-6, characterised in that it comprises a first water solution for the first surface treatment step, containing 98% concentrated sulphuric acid, diluted to a concentration of 2-15 g/l of water, preferably 5 g/l of water and alkali permanganate or ammonium persulphate, and a second water solution for the second surface treatment step, containing an alkali metal carbonate in a concentration of 5-20 g/l of water, preferably 10 g/l of water, and of an alkali metal permanganate in a concentration of 2-15 g/l of water, preferably 5 g/l of water.

8. The combined agent of claim 8, characterised in that the alkali metal of said compounds is potassium.

9. An object, surface treated by the method according to any one of the claims 1-6, by means of the agent according to any one of the claims 7 or 8, characterised in that the surface layer created through the method and by the agent substantially contains manganese, oxygen, sulphur and carbon.