In conjunction with the metal coating of aluminium with other metals, such as nickel, copper or tin, it is often necessary to apply a thin layer of zinc in conjunction with the surface treatment in order to prevent oxidation of the aluminium surface and to ensure good adhesion.

This is usually done in a so-called zincate pickle, which is a solution of sodium zincate in a strongly alkaline solution of sodium hydroxide.

By immersing aluminium components, after customary pre-treatment such as degreasing and pickling, a thin layer consisting of zinc is produced through ion exchange with the aluminium.

In the case of certain alloys, in particular those which contain high levels of silicon and copper or iron, this type of pre-treatment does not function satisfactorily, because the zinc layer is not sufficiently dense. This leads to impaired adhesion of the subsequent metal coating. Zinc that is dissolved in subsequent acidic treatment baths also has a negative effect on these.

The principal object of the present invention is thus, in the first instance, to make available a method which inter alia solves the aforementioned problems in a simple, effective and environmentally friendly fashion.

The aforementioned object is achieved by means of a method in accordance with the present invention, which is characterized essentially in that a surface layer of tin

and/or copper is achieved by immersing the object in an acidic bath solution containing an alkaline salt of trihydroxy benzoic acid and a tin and/or copper salt.

Means for carrying out the application of zinc previously comprised, as described above, a solution of sodium zincate in a strongly alkaline solution of sodium hydroxide, although this is not capable of providing a satisfactory coating of zinc on the object in question.

A further object of the present invention is thus to make available a means that is suitable for use as a method for the surface treatment of objects consisting of an alloy containing aluminium and copper before coating the object in question with metal.

The aforementioned further object is achieved by means of an arrangement in accordance with the present invention, which is characterized essentially in that a bath solution contains an alkaline salt of 3,4,5-trihydroxy benzoic acid and a tin and/or copper salt and preferably also alkaline fluoride.

The present invention relates to a method which eliminates the aforementioned inconveniences involving customary pre-treatment with a zincate pickle.

The method involves the use of a solution in which zinc is replaced by tin and/or copper.

One condition for the method is that the aluminium alloy concerned must contain copper and/or iron, which is very commonly used for both cast alloys and alloys for sheet and bar materials.

A further condition for the method is that the treatment solution must contain small quantities of fluorides.

A method in accordance with the present invention for the surface-treatment of objects which consist of an alloy containing aluminium and copper or iron, before coating the object in question with metal, involves producing a

surface layer of tin and/or copper on the object in question by causing the object to be immersed in an acidic bath solution which contains an alkaline salt of trihydroxy benzoic acid and a tin and/or copper salt.

The means used for carrying out the method consist of a bath solution which contains an alkaline salt of 3,4,5- trihydroxy benzoic acid and a tin and/or copper salt and preferably also alkaline fluoride.

3,4,5-trihydroxy benzoic acid is an organic acid, the alkaline salt of which is reducing and, in the application in question, reduces 2-valent copper to 1-valent copper.

It has emerged in the course of practical trials that the addition of, for example, 5 g of potassium 3,4,5- trihydroxy benzoic acid per litre to a solution containing tin and/or copper sulphate and sulphuric acid produces a finely crystalline deposit of tin and/or copper on the aluminium alloys in question by their immersion in a bath solution for 3-5 minutes.

The depositing of tin and/or copper takes place through a process of ion exchange between copper, aluminium and tin and/or copper. The layers of tin produced in this way are thin, ca. 0.5 Um, and they need to be supplemented as a rule with other electrolytically or chemically deposited layers, e. g. copper, nickel or tin.

A pre-treatment known as pickling, which removes oxides and silicon from the surface and exposes the copper, is required in order for the process described above to function.

A pickling fluid of this kind, which in the case in question has proved to be highly effective, has the following composition:

Phosphoric acid, conc. 120 grams per litre Nitric acid, conc. 30 grams per litre Hydrofluoric acid, 50% 10 grams per litre Bath temperature 20-25°C Treatment period 1-3 minutes Example 1 A component that has been manufactured by pressure die casting in an alloy, which, apart from aluminium, contains 3% copper, 9% silicon and 1% iron, must be surface- treated by tin coating.

An appropriate process by which to achieve a result of the kind described may be: A Degreasing in an alkaline wash.

B Pickling in a bath, as described above, for 1 minute.

C Deoxidizing in a bath with the following composition: Sulphuric acid 10 grams per litre Ammonium persulphate 40 grams per litre Bath temperature 20-25°C Treatment period 3 minutes D Treatment in a solution with the following composition: Sulphuric acid 20 grams per litre Tin and/or copper sulphate 20 grams per litre Potassium trihydroxy benzoate 4 grams per litre Potassium fluoride 5 grams per litre Bath temperature 20-25°C Treatment period 5 minutes E Electrolytic tinning in an acid bath to the specified thickness.

Rinsing carefully with water is performed between each stage in the treatment process.

Example 2 A component that has been manufactured by the stamping and folding of sheet metal in an alloy, which, apart from aluminium, contains 4% copper, must be surface-treated by nickel plating. The following process has been found to be highly suitable.

I Degreasing in an alkaline wash.

II Pickling in a solution in accordance with the example described above.

III Deoxidizing in a bath as described above.

Treatment period: 5 minutes.

IV Treatment in a bath in accordance with point D above.

Treatment period: 2 minutes.

V Electrolytic or chemical nickel-plating to the specified thickness.

The invention is naturally not restricted to the embodiment described above. Modifications are possible, especially with regard to the nature of the various methods and means, or by the use of equivalent technology, but without departing from the area of protection afforded to the invention as defined in the Patent Claims.