SURFACE TREATED, CORROSION PROTECTED, METAL OBJECT, AND METHOD FOR TREATMENT OF THE OBJECT FOR PROTECTION AGAINST CORROSION

Abstract: Surface treated, corrosion protected, metal object, the metal surface (2) of which presents a chosen surface structure and is coated with at least one coating layer (7) having a very even surface structure. The surface structure of the metal surface is profiled with a topography which presents uneven recesses (3) and protrusions (4) and which has also been provided with a surface having an area which is enlarged at least 4 times in relation to the very even surface (9) of the coating layer (7), by means of which a reduced velocity for the spreading of corrosion is obtained, as regarded in relation to the surface of the surface layer.

Description: TITLE: Surface treated, corrosion protected, metal object, and method for treatment of the object for protection against corrosion. TECHNICAL FIELD: The present invention relates to a surface treated, corrosion protected, metal object in accordance with the preamble of appended claim 1. The present invention also relates to a method for treatment of a metal object for protection against corrosion in accordance with the preamble of appended claim 8. BACKGROUND OF THE INVENTION: Corrosion protection of metal objects consisting of reducible metals, usually steel and aluminium, is constantly the object of technical development. However, this still constitutes a serious problem, particularly in marine environments. The basic principle which still generally applies is to seal the surfaces of the metal object by means of one or several coating layers so as to prevent the metal surfaces from coming into contact with oxidizing substances. In this regard, it is important to clean the metal surface carefully in order to remove corrosion which is already present and to achieve maximum adhesion of the coating layers. Organic as well as inorganic coatings, or often a combination thereof, are used as coating layers. Colour systems, plastic coatings, rust protection oil and the like are often used as organic coatings. As inorganic coatings, metal coatings made for example from zinc or nickel and surface conversion layers, for example of the phosphate or chromate type, are included. An often used combination of organic and inorganic coatings is one having a surface conversion layer arranged closest to the metal surface, a primer arranged outside the surface conversion layer and one or several coating layers of the laqcuer type arranged as a top layer. The above-mentioned cleaning of the surface is usually accomplished by means of blasting with round blasting particles, after which the surface is usually polished. The chosen blasting method and the subsequent polishing are intended to provide an optimal surface evenness so as to give the treated surface the best possible surface evenness after the final surface coating. Up to now, conventional surface coating technology using relatively thin layers has required this type of mechanical pre-treatment. The use of powder paints has increased due to their good properties regarding their application, both for spray applications and immersion using electrostatic charge, which gives a very high coverage, especially for objects having a complicated shape. Powder paints are generally applied in relatively thick layers and may give a very high surface evenness, even on relatively rough metal surfaces. A further advantage is the abscence of environmentally harmful solvents. However, the powder paints which are applied by means of known treatment methods present inferior properties as regards protection against corrosion. Attempts have been made to improve this by means of primer layers of the kind which, due to environmental reasons, also lack harmful solvents and for this reason are water-soluble. Due to the relatively thick layers of the powder paints, very high mechanical demands on the underlying primer layers exist. For example, most primers cause cohesive ruptures, especially during use of mat primers. This results in a breakthrough of the sealing surface treatment, which is a source of corrosion due to the metal surface's contact with the surrounding atmosphere. At a breakthrough, corrosion spreads in time in all directions, especially along the metal surface, which may take place at a relatively high spreading velocity particularly in environments containing chlorides, such as sea water. The propagation of corrosion is also affected by the adhesion of the coating layers to the metal surface. SUMMARY OF THE INVENTION: In contrast to prior known methods, the object of the present invention is to reduce the spreading of corrosion along a metal surface which is surface treated with a coating layers and to obtain a high adhesion between the coating layers and the metal surface. Said object is accomplished by means of a metal object in accordance with the present invention, the features of which are apparent from appended claim 1. Said object is also accomplished by means of a method for corrosion protective treatment of a metal object, in accordance with the characterizing part of claim 8. BRIEF DESCRIPTION OF THE DRAWINGS: The invention will be described in the following with an embodiment and with reference to the annexed drawings , in which Fig. 1 shows a partly cut-out perspective view of a surface which is mechanically surface treated according to the method of the invention, the view being greatly enlarged, and Fig. 2 also shows a greatly enlarged, partly cut-out cross-sectional view through a metal object which is completely treated by means of the method according to the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS: The perspective view according to Fig. 1 is a photo, magnified approximately 75 times, of a metal object according to the invention after a first treatment step including a mechanical pre-treatment of the metal surface 2. This pre-treatment comprises a profiling of the metal surface 2 to form uneven recesses 3 and sharply edged protrusions 4, by means of which a great surface enlargement in relation to a plan projection of the surface of the object is accomplished. The object 1 can be a sheet or a moulded object of aluminium, steel with a certain carbon rate or the like, which requires corrosion protection for technical and aesthetic reasons. If the object has a very high wall thickness, a surface treatment such as lacquering is made primarily for aesthetic reasons and in order to facilitate cleaning of the surface. In accordance with the invention, this profiling to the above- mentioned uneven topography shown in the figures to an optimum surface enlargement degree exceeding 4 times, preferably within the interval 6-12 times, is accomplished in order to reduce the spreading of corrosion with time in an optimum way and in order to increase the adhesion of applied coating layers, without reducing the demands of a surface evenness of high quality on the outer coating layer. This is accomplished by means of a blasting method which results in the required sharp-edged, uneven profile which is similar to an alpine landscape, i.e. blasting by means of sharp-edged blasting particles of hard material, for example aluminium oxide Al 2 0 3 having a particle size within the interval 0,2 - 1,2 mm. After the finished blasting, an organic coating such as a surface conversion layer 5 of e.g. phosphate or chromate type is applied, after which one or several primer layers 6, for example of the water-soluble type, is applied. After this surface conversion, washing by means of for example an alkaline liquid and subsequent drying are carried out. After masking areas which are not to be painted, priming takes place by means of the application of one or several primer layers 6, preferably of the water-soluble type. After air-drying of the primer layers, the coating layer or layers 7, which is preferably in the form of powder paints, is applied. The powder paints are for example applied by means of spraying or immersion. After hardening in an oven and subsequent cooling, the masking is removed and insufficiently treated surfaces can be touched-up by means of wet paint. To sum up, the surface treatment process can for example be carried out according to the following. 1. Blasting with Al 2 0 3 , 0,2-1,2 mm. 2. Washing with a liquid. 3. Surface conversion, for example chromating. 4. Washing with a liquid. 5. Drying. 6. Masking. 7. Applying primary coat 10-100 μm. 8. Drying. 9. Powder painting, 50-800 μm in one layer. 10. Hardening in an oven or in air . 11. Optional cooling . 12. Removal of masking. 13. Optional touch-up treatment. With reference to Fig. 2 which shows a cross-section through a surface-treated object in approximately 600 times enlargement, it will be apparent how the metal object, having been treated by means of the method according to the present invention, provides a reduction of the spreading of corrosion when a breakthrough 8 through all coating layers occurs. The breakthrough could be a crack, which means that the metal surface 2 is exposed to the surrounding corroding atmosphere, for example salt water, whereby corrosion is spread, i.e. the metal is reduced both vertically and along the metal surface. The major spreading of corrosion normally takes place along the metal surface 2, and it should be noted that the profiled surface according to the invention provides a sharp increase of the spreading path along the metal surface as compared with a very even surface, i.e. a surface having a very smooth surface. The increase in the propagation path can be expressed as a surface enlargement degree as compared with a plan projection of the metal surface or as compared with the very smooth surface 9 of the coating layer of the paint 7 through the use of modern paints, for example powder paints, and associated thick coating layers. By means of a suitable application method, a very high surface evenness can be obtained in the covering surface 9, even with a substantial profiling of the metal surface 2. From experience, an optimum interval has been found within which a high surface enlargement, and thereby also a long path for the spreading of the corrosion, can be obtained while still maintaining a high surface evenness in the outer surf ce 9 of the coating layer 7. Preferably, this degree of surface enlargement is within the interval 6-12 times or exceeds at least 4 times. The expression "surface enlargement" signifies the number of times the surface is enlarged in relation to a certain reference surface. In this case, the surface of the coating layer has been chosen, but alternatively a plan projection of the metal surface can thus be chosen as a reference, whereby a plane object is considered. A glass surface with a high degree of fineness can alternatively be regarded as a reference. The invention is not limited to the embodiments described above and shown in the figures, but may be varied within the scope of the subsequent claims. For example, it is possible to omit certain coating layers or to replace them with other types of coating layers.

Claims: CLAIMS: 1. Surface treated, corrosion protected, metal object, the metal surface (2) of which presents a chosen surface structure and is coated with at least one coating layer (7) having a very even surface structure, c h a r a c t e ¬ r i z e d i n that the surface structure in the metal surface is profiled with a topography which presents uneven recesses (3) and protrusions (4) and which has also been provided with a surface having an area which is enlarged at least 4 times in relation to the very even surface (9) of the coating layer (7), by means of which a reduced velocity for the spreading of corrosion is obtained, as regarded in relation to the surface of the surface layer. 2. Surface treated, corrosion protected, metal object according to claim 1, c h a r a c t e r i z e d i n that said profiled surface is enlarged 6-12 times. 3. Surface treated, corrosion protected, metal object according to claim 1, c h a r a c t e r i z e d i n that the coating layer (7) is one or several surface treatment layers. 4. Surface treated, corrosion protected, metal object according to claim 1, c h a r a c t e r i z e d i n that the metal surface (9) is blasted by means of sharp-edged blasting particles. 5. Surface treated, corrosion protected, metal object according to claim 1, c h a r a c t e r i z e d i n that the coating layer (7) is a powder paint. 6. Surface treated, corrosion protected, metal object according to claim 1, c h a r a c t e r i z e d i n that a primer layer (6) is applied between the coating layer (7) and the metal surface (2). 7. Surface treated, corrosion protected, metal object according to claim 5, c h a r a c t e r i z e d i n that the primer layer (6) is waterbased. 8. Surface treated, corrosion protected, metal object according to claim 1, c h a r a c t e r i z e d i n that the metal surface (2) is coated with a surface conversion layer (5), for example a chromate or phosphate layer. 9. Method for treatment of a metal object for protection against corrosion, comprising a mechanical pre-treatment and application of at least one coating layer (7) with a highly even surface structure, c h a r a c t e r i z e d i n that the mechanical surface treatment comprises a step including a profiling of the metal surface to promote uneven recesses and protrusions with a surface the area of which is enlarged at least 4 times in relation to the highly even surface of the coating layer (7). 10. Method according to claim 8, c h a r a c t e r i z e d i n that said profiling is accomplished by means of blasting with sharp-edged blasting particles. 11. Method according to claim 9, c h a r a c t e r i z e d i n that blasting particles of aluminium oxide having a particle size preferably within the interval 0,2-1,2 mm are used.

Inventors: ARVIDSSON LENNART, MARTINSON INGEMAR, ARVIDSSON, LENNART, MARTINSON, INGEMAR
Applicants: ARVIDSSON LENNART, MARTINSON INGEMAR, VOLVO PENTA AB, AB VOLVO PENTA, ARVIDSSON, LENNART, MARTINSON, INGEMAR
Application year: 1995
Main classification: B05D3
IPC codes: B05D3, B05D7, B32B15, C23C22