A Process for Coating a Surface of a Substrate with a Metal Layer

Abstract

In a process for coating a surface of a substrate with a metal layer zinc is used as a coating agent. Zinc metal and said substrate are brought together at an elevated temperature in a liquid diffusion medium to allow a diffusion of zinc through said diffusion medium to said surface of said substrate. Said diffusion medium comprises a molten salt liquid, particularly molten salt bath, of at least one salt that is maintained at a bath temperature of between 200° C. and 800° C. Said substrate and zinc as a coating agent are heat treated in said bath to promote said diffusion of zinc to said surface of said substrate.

Claims

1. A process for coating a surface of a substrate with a metal layer, wherein a coating agent containing zinc and said substrate are brought together in a diffusion medium and are subjected to a heat treatment at elevated temperature to allow a diffusion of zinc through said diffusion medium to said surface of said substrate, a molten salt liquid, comprising at least one molten salt, is used as said diffusion medium, wherein a metallic zinc source is added to said molten salt liquid as a source for diffusion of zinc to said surface of said substrate, and wherein said heat treatment involves exposing said substrate to said molten salt liquid at elevated temperature to allow metallic zinc to diffuse to said substrate.

2. The process according to claim 1, wherein said molten salt liquid is maintained at an elevated bath temperature of between 200° C. and 800° C. during said heat treatment, while said substrate is submerged in said molten salt bath of said molten salt liquid.

3. The process according to claim 2, wherein said heat treatment is carried out in said salt bath at a temperature between 300° C. and 600° C., particularly below 450° C., and preferably at a temperature between 330° C. and 400° C.

4. The process according to claim 1, wherein the substrate comprises a zinc-alloyable metal, preferably at least one of iron, copper, nickel, aluminum and at least one of its alloys, such as steel and cast iron, and said metal layer comprises a corresponding zinc alloy layer on said substrate.

5. The process according to claim 1, wherein a majority of said diffusion medium consists of said at least one molten salt, preferably a combination of two or more molten salts, having a melting point below or equal to said elevated temperature and wherein said one or more salts are selected from a group that consists of halides, cyanides, cyanates and mixtures thereof.

6. The process according to claim 5, wherein said salts are selected from a group of halides, specifically from a group of halides that consists of chlorides, bromides and iodides.

7. The process according to claim 5, wherein said halides comprise one or more alkali metal halides or alkaline-earth metal halides.

8. The process according to claim 6, wherein said halides comprise one or more salts from a group consisting of zinc chloride, potassium chloride, barium chloride, calcium chloride, sodium chloride and aluminum chloride.

9. The process according to claim 1, wherein said metallic zinc source is added in the form of granules, chips, powders or mixtures to the process, preferably with a powder particle size of less than 100 microns and more preferably with a particle size of less than 50 microns.

10. The process according to claim 1, wherein the molten salt liquid is circulated during the process.

11. The process according to claim 1, wherein the substrate is moved during the process through the molten salt liquid.

12. The process according to claim 1, wherein after deposition of said metal layer on said surface, the substrate is post-treated by washing, passivating, painting, rubberizing or any combination thereof.

13. The process according to claim 1, wherein said substrate is subjected to a heat pre-treatment before being quenched in said molten salt liquid.

14. The process according to claim 13, wherein said molten salt liquid has an initial bath temperature below the process temperature.

15. The process according to claim 1, wherein a solid filler is added to said molten salt liquid, particularly in the form of an inert powder.

16. The process according to claim 15, wherein said solid filler has a density which does not exceed a density of said molten salt liquid by more than 25%.

17. The process according to claim 15, wherein said solid filler is chosen from a group of silicates and carbons, and particularly comprises silicon oxide.

Description

EXAMPLE

[0030] To carry out the process according to an embodiment of the present invention, a salt melt is prepared in a suitable heatable container, for example, from a mixture of about 0.6 mol % ZnCl.sub.2, about 0.2 mol % NaCl and ca. 0.2 mol % KCl. The handling of salt melts is not without risk, therefore, normal expert safety measure should be taken. These salts may be hygroscopic and in that case are first purified by sustained settling for a period of time to free and remove any crystal water and possible dissolved gases so that no bubbles will form and foaming of the melt is prevented. In order that the purification process does not proceed too fast, it is advantageous to start at moderate temperatures of e.g. 250° C. and slowly heat up the salt bath to the desired process temperature or even slightly beyond that temperature. This may take several hours, or longer, and depends on the degree of crystal water and outgassing.

[0031] Once the salt bath has settled and water or gasses have escaped. A gaseous reaction with residues may occur. Once this has ended, the salt bath is stable and ready for coating.

[0032] Metallic zinc is prepared and added as metallic zinc source to the melt. For this purpose, at least the amount of zinc necessary to achieve the desired layer weight (thickness) plus an excess amount of zinc of for instance at least 3 wt.-% is introduced in the molten salt. Besides to zinc grains (granules) or chips, also zinc powder or zinc dust may be used as this zinc-source. The finer the particle size, the easier the transition of zinc into the melt will be.

[0033] The substrates to be coated may be cleaned before the treatment, if necessary. The salt bath that is being used may also have a cleansing effect. In principle, any conventional method is suitable, like de-greasing and sand blasting. Non-metallic coatings, such as oxides or skins, should be removed. In order to avoid hydrogen embrittlement, preferably only blasting is used. Subsequently, the dry substrates can be placed in baskets or attached to appropriate carriers in or onto which the substrates are further processed.

[0034] Subsequently, the thus prepared substrates are completely submersed in the molten salt bath, in this example at a process temperature of about 380° C. for about 1 hour in the presence of said zinc source. This delivers a highly uniform zinc diffusion coating of the desired thickness even when applied to extremely complex substrate geometries. A comparable result is obtained in carrying out the process according to another embodiment of the present invention, when instead of the ternary salt bath, a salt bath is used with only ZnCl.sub.2.

[0035] It should be noted that thereby the zinc source resides in the melt at a temperature well below 420° C., i.e. still in its solid phase, the melting point of metallic zinc being 419.5° C. The transfer of zinc apparently is merely effected by zinc diffusion from the zinc source through the molten salt to the substrate surface, which triggers an inter-metallic phase reaction with the zinc at the substrate surface. The coating process takes place without the object or the bath being disturbed.

[0036] In an alternative embodiment of the invention, the diffusion medium is circulated during the process. Such a circulation of the melt is advantageous, particularly in the case of large substrate surfaces, so that an optimum local zinc supply as well as a uniform temperature distribution and coating are achieved.

[0037] In order that no contact points arise on the surfaces to be coated or that bubbles form in any cavities, a further embodiment of the process according to the invention is characterized in that the substrate is moved during the process through said molten salt liquid. Such movement of the substrate during the coating process ensures that all surfaces are sufficiently wetted and a uniform layer can grow everywhere on its surface.

[0038] A wide variety of molten salts may be used for the diffusion medium, however, a preferred embodiment of the process according to the invention is characterized in that salts are being used that are soluble in a convenient solvent, particularly in water. In that event, remaining salts that solidified or precipitated on the surface or in any substrate cavities, can be removed quite easily by rinsing or washing afterwards with the appropriate solvent.

[0039] In one embodiment of the process according to the present invention, the racks or baskets that are used to hold the substrate during the heat treatment and coating process can also conveniently be used for subsequent post-treatments, such as washing or passivating.

[0040] Depending on the later intended use of the coated substrate, it may be advantageous to passivate, coat or rubberize the coated substrate after the heat treatment. In principle, any desired post-treatment, such as passivating, sealing, painting or rubberizing of the surface may be applied.

[0041] Depending on the size of the salt bath and the substrate, the cost for constructing and filling the salt bath can be considerable. In order to reduce this initial investment, a further preferred embodiment of the process according to the invention is characterized in that a solid filler is added to said diffusion medium, particularly in the form of an inert powder. Such fillers reduce the active volume and can be supplied to the salt bath to save on its salt contents without adversely affecting the diffusion and coating behaviour of zinc. Although many filler materials are feasible within the framework of the present invention, especially silicates and more particularly a fumed silicate that is commercially available under the brand Aerosil® may be highly suitable for this purpose.

[0042] Particularly suitable for this purpose are inert powders or flakes whose density differs as little as possible from the density of the molten salt liquid and can therefore be easily moved and distributed in the melt. A particular embodiment is therefor characterized in that said solid filler has a density which does not exceed a density of said molten salt liquid by more than 25%. Based on the density of the above ternary melt of about 2.43 g/cm.sup.3 (250° C.), modifications of silicates, particularly silicon oxide (e.g. tridymite with 2.28 g/cm.sup.3 or quartz 2.65 g/cm.sup.3) are suitable for this purpose and also graphite dust (2.26 g/cm.sup.3).

[0043] Although the invention has been described in further detail with reference to merely a single explanatory embodiment only, it should be noted that the invention is by no means limited to this embodiment. On the contrary, many more embodiments and variations are feasible within the scope of the present invention to a person of ordinary skill without requiring him to exercise any inventive effort.

[0044] As such the aforementioned specific compounds, although very suitable within the context of the present invention, may be replaced by other compounds. The examples gives focussed of the formation of a pure zinc layer. The invention, however, is likewise suitable for forming layer comprising zinc in combination with traces of one or more other compounds, notably metals like chromium, nickel, magnesium and copper, to form ternary conformal metal layers covering and shielding the surface of the substrate.

[0045] More generally, the present invention offers an entirely new and inventive process for forming a protective layer on a substrate, specifically a metal substrate, said layer comprising zinc and optionally one or more other elements, based on metal diffusion from an appropriate source through a suitable liquid medium, notably a salt melt. This will open a door to a widespread variety of application in which the process according to the invention will easily outweigh traditional coating and deposition techniques, particularly electro-galvanizing and hot-dip galvanizing, in terms of layer thickness (control), conformability and economics.