WO1996002330A1

WO1996002330A1 - Method for surface treatment of metal objects

Info

Publication number: WO1996002330A1
Application number: PCT/SE1995/000866
Authority: WO
Inventors: Ingemar Martinson; Lennart Arvidsson
Original assignee: Ab Volvo Penta
Priority date: 1994-07-19
Filing date: 1995-07-18
Publication date: 1996-02-01
Also published as: SE9402515L; SE9402515D0; SE503304C2; AU3089995A

Abstract

Method for surface treatment of metal objects, including applying a coating layer (13, 16) on at least one first metal surface (8, 9, 10) which is to be given a surface treatment which is different from another metal surface (11, 12) of the object. Said first and second metal surfaces (8-12) are surface-treated by means of applying at least one totally covering coating layer (13-16) over both metal surfaces, and said second metal surface (11, 12) is subjected to subsequent machining for removal of said coating layer.

Description

TITLE: Method for surface treatment of metal objects

FIELD OF THE INVENTION: The present invention relates to a method for surface treatment of metal objects according to the preamble of appended claim 1.

The present invention concerns a method in which a metal surface, after completed surface treatment, is to differ from at least one other metal surface with regard to its surface treatment.

BACKGROUND TO THE INVENTION:

With many types of surface coatings where surfaces are to receive different surface treatments, for example with surfaces which are not to be painted, masking of these surfaces is used, for example by taping with temporary covering sheets of paper, plastic or the like, or by attaching of specially designed protective covers of plastic for example. Masking is time-consuming and gives unclean transitions between surface-coated and non- surface-coated surfaces, which often requires after- treatments in the form of cleaning or touching up, or the like. One common surface treatment method is application of powder paint by means of spraying or immersion, which places high demands on the masking of the surfaces which are not to be surface treated. Immersion coating provides large advantages when surface treating objects with internal channels, such as castings. Immersion coating puts particularly high requirements on the masking operation, which is both material-consuming and labour- consuming. SUMMARY OF THE INVENTION:

The object of the present invention is to achieve a time- saving method which gives high quality transitions between surfaces which are to be given different surface treatments, without comprehensive after-treatment.

Said object is achieved by a method according to the present invention as defined in appended claim 1.

DESCRIPTION OF THE FIGURES:

The invention will now be described in more detail by means of an embodiment and with reference to the accompanying drawings, in which,

Figs. 1 and 2 show an example of the application of the method according to the invention with surface treatment of two parts of an object.

DESCRIPTION OF A PREFERRED EMBODIMENT: Fig. 1 and 2 show parts 1, 2 of a cast object, for example of aluminium, which is to be assembled so as to form a unit in the final product. The parts 1, 2 hereby present not only surfaces 3, 4, 5 which after completion of surface treatment are to remain surface-coated with a layer of paint/lacquer, but also surfaces 6, 7 which form assembly surfaces intended to be connected to each other in the assembled condition of the final product.

The surfaces which are to remain surface-coated in the final state are constituted by the outer surfaces 3, 5 which have to meet high requirements with regard to decorative finish, corrosion protection as well as being easily cleanable, and by surfaces 4 which are more or less not visible from the outside, in the shown embodiment a cavity, e.g. in the form of a drilling or the like, which primarily has to meet high requirements concerning corrosion protection and be easily cleanable.

In order to achieve a high grade of evenness and a maximum surface coating coverage of the outer surfaces 3, 5, as well as the cavity 4 according to a preferred embodiment of the invention, a surface coating by immersion of the object's parts 1, 2 in powder paint/lacquer is used, which can be applied in relatively thick layers and which permits a good working environment. Powder paints however require thorough preparation, preferably in that all the metal surfaces 8- 12 of the object's parts 1, 2, following a mechanical pre-treatment of the metal surfaces in order to remove rust and other impurities, are subjected to a chemical surface transformation by application of an inorganic surface transformation means of chromate or phosphate type.

After application of the surface transformation means which, for example, occurs by means of immersion whereby all the metal surfaces 8-12 of the object's parts 1, 2 are treated or, more precisely, the metal surfaces 8, 9 which will be outwardly facing in the finished product, and the metal surfaces 10 which delimit the cavity 4, as well as the metal surfaces 11, 12 which are to form assembly surfaces 6, 7 in the finished product.

All of the surfaces treated with surface transformation means are provided with a priming protection means, so- called primer, in a separate application step, which suitably occurs by immersion application of one or more priming layers 13, 14.

After application of the priming layer and subsequent drying, at least one layer of powder paint 15, 16 is applied over the same surfaces which were chromated and primed, said application advantageously occurring by immersion, whereby all the surfaces subjected to immersion (including the cavity surfaces) are well covered with a desired layer thickness.

The applied powder paint is oven-hardened, whereafter cooling occurs.

According to the invention, there is no masking of the surfaces which, in the final condition, are not to be surface-coated with primer or a paint layer, but instead these surfaces, i.e. the assembly surfaces 11, 12, are subjected to machining after completion of the aforementioned layer application, in order to remove all the coating layers on the mounting surfaces down to bare metal surface 11, 12. Machining occurs for instance by grinding.

After machining, the assembly surfaces 11, 12 are subjected to a renewed surface transformation by means of inorganic surface transformation means of chromate or phosphate type in order to give the surfaces a chemical corrosion protection. Since the surfaces are not subjected to mechanical influence, such as by impacts, no primer or painting is necessary, which itself cannot provide the surface finish which is required for assembly surfaces where in many cases a sealed connection, possibly using an intermediate gasket, is required.

Examples where sealing contact is required are contact between casing parts, engine parts and the like. Exa ple :

To summarise, the following account will be given for the various steps of an advantageous embodiment using the surface treatment procedure of the present invention:

1. Blasting with AL₂0₃. 0,2-1,2 mm.

2. Washing with liquid.

3. Surface transformation, for example chromating.

4. Washing with fluid.

5. Drying.

6. Chromating.

7. Application of primer 10-100 μm.

8. Drying.

9. Powder coating. 50-800 μm in one layer.

10. Oven-hardening or alternatively air-hardening.

11. Possible cooling.

12. Machining, e.g. milling of certain surfaces down to bare metal.

13. Renewed chromating of machined surfaces.

The invention is not limited to the embodiments described above and shown in the drawings, but can be varied within the scope of the appended claims. For example, one or more of the surface treatment stages can be omitted, whereby however at least one surface coating layer is applied on at least two of the object's surfaces and the coating layer, in a subsequent step, is removed from said one surface by means of machining.

Claims

1. Method for surface treatment of metal objects, including applying a coating layer (13-16) on at least one first metal surface (8, 9, 10) which is to be given a surface treatment which is different from another metal surface (11, 12) of the object, characterized in that said first and second metal surfaces (8-12) are surface- treated by means of applying at least one totally covering coating layer (13-16) over both metal surfaces, and in that said second metal surface (11, 12) is subjected to subsequent machining for removal of said coating layer.

2. Method according to claim 1, characterized in that a surface transformation layer, differing from said first coating layer, is applied on the second metal surface (11, 12) after removal of said coating layer (13, 14, 15, 16).

3. Method according to claim 1, characterized in that said first coating layer (15, 16) is applied in the form of a powder paint/lacquer.

4. Method according to claim 2, characterized in that said powder paint/lacquer is applied by immersion of the metal object.

5. Method according to claim 1, characterized in that the application of said differing surface layer is constituted by an inorganic chemical surface transformation means, for example chromating.

6. Method according to claim 1, characterized by the steps of: blasting, chromating of all of the metal surfaces (8-12), applying of at least one layer (13, 14) of primer to all the metal surfaces, applying of at least one layer (15, 16) of powder paint/lacquer on all the metal surfaces, machining of said second metal surface (11-12) in order to remove all of the applied coating layers (13-16) and renewed chromating on said machined surface.