METHOD FOR THE SOL-GEL COATING OF A HEAT-TREATED OBJECT

FIELD OF THE INVENTION The invention relates to a method defined in the preamble of claim 1 for the sol-gel coating of a heat-treated object.

BACKGROUND OF THE INVENTION Known from the prior art is the sol-gel coating of various products. Sol-gel coating is usually provided as a separate manufacturing step in the production process of the product. One process step in providing a sol-gel coating normally comprises curing the coating, which is usually carried out e.g. in a furnace at a temperature of about 100 to 150 ² C as a separate process step. The heat treatment / curing step which is part of forming a sol-gel coating easily causes problems, in particular when applying the proc- ess in the already existing production plants. There may not necessarily be the required heat treatment apparatus or capacity it requires, one is not willing to make the investment required for acquiring one, or there is simply not enough room for the new equipment in the production area. In addition, heat treatment / curing of a sol-gel coating and the preceding and following work steps take time, which is why this particular work step may form a bottleneck for the production process.

OBJECTIVE OF THE INVENTION

The objective of the invention is to eliminate the drawbacks referred to above and to disclose a new type of method for providing a sol-gel coating.

SUMMARY OF THE INVENTION

The method according to the invention is characterized by what has been presented in the claims . The invention is based on a method for sol- gel coating a heat-treated object, wherein the object is heat-treated e.g. by hot galvanization or hot spraying, and the sol-gel coating is formed onto the object. According to the invention, the heat energy transferred and stored in the object in conjunction with the heat treatment is applied for curing the sol- gel coating in the formation of the sol-gel coating.

In this context, an object refers to any kind of two- or three-dimensional object. The object may have been formed from any material which is suitable for the purpose, e.g. metal or alloy.

The basic idea of the invention is that the heat energy transferred and stored in the object during a heat treatment process, e.g. hot galvanization process or hot spray coating performed before the sol- gel coating, is utilized in the sol-gel coating provided after the heat treatment, whereby the sol-gel coating cures and acquires its final properties without any separate heat treatment to provide the curing. Further, the basic idea of the invention encompasses the utilization of residual heat energy in the sol-gel coating, whereby the sol-gel coating is cured by utilizing residual heat energy generated e.g. during hot galvanization or hot spraying. In one embodiment of the invention, the heat treatment is hot galvanization, preferably hot galvanization of piece articles.

In one embodiment of the invention, the heat treatment is hot spraying. In one embodiment of the invention, the temperature of the object after hot galvanization, e.g.

as lifted from the hot galvanization kettle after immersion coating, is 430 to 470 °C, preferably about 450 ² C.

In one embodiment of the invention, the tem- perature of the object after hot spraying is 180 to 220 ⁰ C, preferably about 200 ² C.

In one embodiment of the invention, the object is cooled to a desired temperature after the heat treatment by air . In one embodiment of the invention, the object is cooled to a desired temperature after the heat treatment by water cooling.

In one embodiment of the invention, the object is cooled before sol-gel coating to a temperature of 100 to 200°C.

In one embodiment of the invention, the object is cooled before sol-gel coating to a temperature of 80 to 100 ⁰ C.

In one embodiment of the invention, the ob- ject is cooled before sol-gel coating to a temperature of 50 to 8O ⁰ C, which are lower temperatures of the material than what is usual.

In one embodiment of the invention, the sol- gel coating is provided onto the object substantially directly after heat treatment. Between heat treatment and sol-gel coating, the object can be cooled to a desired temperature.

The invention provides the advantage that a sol-gel coating provided after heat treatment, e.g. galvanization or hot spraying, does not require a separate heat treatment step to cure the coating, or any pretreatment step. Use of the method according to the invention saves both energy and processing costs in the formation of a sol-gel coating.

DETAILED DESCRIPTION OF THE INVENTION

In the following section, the invention will be described by means of detailed examples of its embodiments . Based on preliminary tests, it was discovered that it was possible to provide sol-gel coating onto hot-galvanized objects and cure the coating by the method according to the invention. In the tests, the objects were let cool after hot galvanization to dif- ferent temperatures which were 250, 200, 150, 100 and 50 ² C, after which the objects were coated with sol-gel coating and let cool further to room temperature. It was discovered that the sol-gel coating of the objects and curing of the coating were successfully accom- pushed by the method according to the invention.

Further to hot spraying, the preliminary tests comprised testing test samples coated by WC10Co4Cr coating and their corresponding coating by the sol-gel coating, the applied temperatures of the coated material being 170, 140 and HO ² C. The tests revealed that it is possible to use even lower temperatures of the material that is to be sol-gel coated, even temperatures between 50 and 80 ^a C. It was discovered that the sol-gel coating of objects that were treated by hot spraying, and curing of the coating were successfully accomplished by the method according to the invention.

Also subsequent coating of other coatings than the hot-spray coating presented above by the sol- gel coating is possible with the same method. The method can also be applied to other objects, products and surfaces in which a sufficient amount of heat energy has been stored during the preceding process steps . In summary of the invention, one can conclude that based on the conducted tests, sol-gel coating of

various products, e.g. metal products, and curing the coating are possible without a separate heat treatment step by utilizing the heat energy stored during some other process step or remaining from that step in the material that is to be coated. Compared to furnace curing in optimum conditions, this method provides equal properties in terms of corrosion prevention or water-repelling ability of the sol-gel coating, if the temperature of the substrate at the time of coating is suitable for the applied coating. At its best, the corrosion prevention ability is equal to a coating cured in optimum conditions, as is also the water contact angle, i.e. water repelling ability.

The method according to the invention is suitable in different embodiments for use in the most different kinds of sol-gel coating processes.

The invention is not limited merely to the examples referred to above; instead, many variations are possible within the scope of the inventive idea defined by the claims.