Method for coating metal surfaces

Abstract

A method for coating a light metal alloy component to form a protective layer comprising Sn. First, a surface of the light metal alloy component is cleaned and passivated. A layer comprising Zn is formed on the surface, and a layer comprising Sn is deposited. An intermediate layer is preferably deposited between the Zn-containing layer and the Sn containing layer. The Sn-containing layer may additionally be varnished.

Description

BACKGROUND OF THE INVENTION

[0001] This invention relates to a method for coating a surface of a light metal alloy component. Light metal alloys include, but are not limited to, alloys that contain Al and/or Mg in an amount that contributes considerably to determining the chemical properties of the surface. Because of their low specific gravity, light metal alloys are of great interest for many different applications in which both high mechanical stability and the total weight of the component are important, for example, in aircraft construction, motor vehicles, or housings for high quality devices. Also, light metal alloy frame parts lend stability to portable metal telephones while burdening the user as little as possible. However, light metal alloys are sensitive to oxidation, so they require surface treatment to avoid corrosion problems. Typical treatment methods have the disadvantages of satisfying technical requirements only to a limited extent, being very costly, or unduly restricting the size or geometry of the parts that may be treated. Also, such surface treatments may have a negative effect on the appearance of light metal alloy components.

SUMMARY OF THE INVENTION

[0002] Among the several objects of the present invention, therefore, may be noted the provision of a method for coating a light metal alloy which is effective for corrosion protection. A further object of the present invention is the provision of a method of coating a light metal alloy which is capable of coating a wide variety of surfaces with regard to size and shape. A further object of the present invention is the provision of a protective coating for a light metal alloy which has an appealing appearance.

[0003] Briefly therefore, the present invention is directed to a method for coating a metallic surface which comprises cleaning and passivating a surface of a light metal alloy component and forming a first layer comprising Zn on the surface. A second layer comprising Sn is formed such that the first layer is located between the surface and the second layer. The present invention is further directed to a coating for a light metal alloy comprising a first layer which comprises Zn and a second layer which comprises Sn, wherein the first layer is located between the light metal alloy surface and the second layer.

[0004] Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWING

[0005] FIG. 1 is a view of the internal surface of a diecast Mg alloy AZ91 chassis for a mobile telephone housing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0006] In metallurgy and materials science, light-metal alloys are understood to encompass a variety of metal mixtures comprising “light metals,” such as Al, Be, Mg, and Ti. The most common light-metal alloys contain Al or Mg. Preferably, the method according to this invention is performed on light-metal alloys with a relatively high Al content or those with a relatively high Mg content.

[0007] A Sn-containing layer enables reliable finishing of a light metal alloy surface. It has been discovered that a protective layer comprising Sn adheres well to a light metal alloy surface which has first been cleaned and passivated and then coated with a Zn-containing layer.

[0008] Cleaning and Passivating

[0009] An alkaline degreasing of the light metal alloy surface is a useful initial step for cleaning and passivating. The degreased surface is preferably treated with a solution which is acidic or which comprises the salt of an acid to perform some etching of the light metal alloy surface and which also performs an oxidative passivation. The term oxidation is generally understood here to mean a valence electron transition and in particular implies the formation of oxides like Al2O3 and fluorides like MgF2.

[0010] In one embodiment, wherein the alloy comprises Mg, preferably having an Mg fraction of at least about 50 weight %, especially at least about 80 weight %, the cleaning and passivating may be performed in two steps. First, the surface is treated with relatively weak acidic solution having a pH from about 3 to about 5, preferably about 4. Then, the surface is treated with a relatively strong acidic solution comprising fluoride ions and having a pH in the range of about 0.5 to about 2, preferably about. During the etching of the surface, the fluoride ions form a passivating layer comprising MgF2. The weak acidic solution may comprise, for example, carboxylic, citric acid, malic acid, oxalic acid, lactic acid, a pyrophosphate, and combinations thereof. The strong acidic solution may comprise, for example, a mixture of phosphoric acid and ammonium bifluoride.

[0011] In another embodiment, wherein the alloy comprises Al, preferably having an Al fraction of at least 60 weight %, especially at least 80 weight %, the treatment is preferably performed with a highly oxidizing solution that simultaneously etches the surface and produces a passivating layer comprising Al2O3. Examples of strongly oxidizing solutions are nitric acid, peroxomonosulfuric acid, and a potassium persulfate solution.

[0012] Cleaning and passivating may also be performed anodically with a solution comprising phosphoric acid and an alcohol as described in co-pending U.S. application Ser. No. 10/176,308, filed on Jun. 20, 2002, which is herein incorporated by reference in its entirety. Such cleaning provides effective degreasing and etching of the surface, and the anodic operation allows for flexible optimization via parameters such as anodic current density, voltage, and the like.

[0013] The alcohol may be, for example, methanol, ethanol, propanol, butanol, a polyhydric alcohol, or derivatives, such as isopropanol. Diols, polyethers and other alcohols are also useful, as are mixtures of alcohols. Preferably the alcohol comprises butanol or isopropanol.

[0014] Preferably, fluoride ions are used as described elsewhere herein to passivate the surface of a Mn-containing alloy. The fluoride ions may be in the form of, for example, ammonium bifluoride, an alkali fluoride, hydrofluoric acid, as well as other forms. The fluoride ions may be in a solution with the phosphoric acid, with the alcohol, or with the phosphoric acid and the alcohol. In a multi-step cleaning and passivation process, the step performed with fluoride ions is preferably performed last.

[0015] Treatments with fluoride ions, especially the two-step acid treatments and treatments with phosphoric acid and alcohol, are also useful when the light metal alloy comprises no or little Mg, but comprises Si, preferably at least about 0.1 weight %, and more preferably at least about 0.5, 1 or 2 weight % or higher. The fluoride ion concentration in this case is dependent upon the Si concentration.

[0016] The surface treatment with phosphoric acid, alcohol, and fluoride ions may further comprise an alkaline rinse step, preferably with an aqueous solution having a pH of at least about 10. However, an alkaline rinse step is less advantageous for a passivation surface dominated by Al2O3.

[0017] Anodic treatment of alloys comprising Al preferably employs treatment of the surface with an aqueous oxidation agent such as a persulfate solution or a solution of peroxomonosulfuric acid (Caro's acid). Oxidation is preferably performed after any fluoride treatment. An aqueous oxidation step at a pH of less than about 6 may be problematic if the alloy also has a high Mg fraction because the fluoride passivation can be damaged.

[0018] Useful anodic current densities have a lower limit of about 10, 30 or 50 A/m2 and an upper limit of about 1000 A/m2. Preferably, the light metal alloy surface is cleaned and passivated at a temperature of from about 10° C. to about 40° C. The solution used in the anodic cleaning steps comprises phosphoric acid in an amount which preferably ranges from about 30 to about 90 percent of the solution on a volumetric basis. Within this range of volume fractions, the phosphoric acid can measure from about 50 to about 95 percent H3PO4 by weight. The solution further comprises an alcohol and, optionally, fluoride ions. Useful fluoride solutions have a fluoride content of about 0.1, 0.3 or 0.5 weight % as a lower limit and about 30, 20 or 10 weight % as an upper limit.

[0019] Coating

[0020] After the cleaning and passivating pretreatments, a layer comprising Zn and an layer comprising Sb is applied to the light metal alloy surface. Preferably, the layer comprising Zn chemically metal plated. This metal plating can additionally contain the metals Cu and/or Ni. Preferably, the layer comprising Sn is electrolytically coated. The amount of Sn in this electrolytic layer is preferably at least about 40 weight %, more preferably at least about 50 weight %. This layer may also contain, for example, Zn, Bi and/or Pb in addition to Sb, in order to improve the corrosion properties.

[0021] The Zn-containing layer is preferably electrolytically coated with an intermediate layer to protect the Zn-containing layer from damage by subsequent coating steps, for example, the electrolytic coating with the Sn-containing layer. The intermediate layer may comprise Cu and/or Ni. The specific process chosen for the intermediate coating is matched to the stability of the Zn-containing metal plating. The intermediate layer is preferably coated at a pH from about 7 to about 10 because the Zn-containing layer can be damaged by processes which are too acidic as well as process which are too alkaline. Processes at pH which may damage the Zn-containing layer may be desirable or unavoidable in the production of the Sn-containing layer. Preferred layer thicknesses for the intermediate layer lie are between about 5 and about 10: m. Preferred layer thicknesses for the Sn-containing layer are between about 5 and about 10: m.

[0022] The method according to the present invention provides stable and permanent electrolytic coatings on light metal alloy surfaces. Since the method can be carried out with wet chemical and electrolytic process steps, it is very flexible with regard to the usable part sizes and geometries and incidentally can be carried out inexpensively on a large scale. In the above-described procedures, a metallic conductive surface is achieved, which is desirable for many applications.

[0023] However, a particular appeal of the invention lies in the fact that a varnish may additionally be deposited on the Sn-containing electrolytic layer. This provides far-reaching freedom with regard to the visual design of the surface. For example, the varnish can be colored to be opaque or transparent. In this way many different kinds of decoration effects can be achieved. It can also have structures, for example, surface spattering, which can be applied with conventional varnishing machines in a standard way, in order to give the treated part an individual visual and tactile appearance. Furthermore, the varnished surface is characteristically electrically insulating, which can be desirable, depending on the application. Finally, better corrosion protection may be afforded by the varnish layer. Preferably, a two-component varnish is used. One-component varnishes are useful, but they generally have poorer technical performance.

[0024] The adhesion of the varnish is improved if the Sn-containing layer is passivated prior to applying the varnish. Passivation is preferably performed by alkaline anodic oxidation, for example, with a solution that contains phosphates and/or carbonates. This alkaline anodic oxidation can be supplemented by a subsequent cathodic treatment in a solution of hexavalent chromium, for example, chromic acid. This results in a coating of the surface with trivalent chromium. From the standpoint of health and environment the use of hexavalent chromium is, however, problematic (although not for the product itself), due to which varnishing the electrolytic surface that has only been pretreated by alkaline anodic oxidation is preferable.

[0025] In addition to the advantages already described, the varnished surface may be subsequently subsequently treated to return conductivity to partial areas. This can be useful, for example, in order to apply electrical contacts to the coated component at specific sites, but where the component is supposed to remain insulated or coated with varnish for visual appeal or for protection against chemical and mechanical stress.

[0026] In one embodiment, a laser is used to chip off or evaporate the varnish at selected parts of the surface and through a remelting brings these areas to a metallic conductivity. An exposed Sn-containing layer provides good electrical conductivity and provides stability for the region from which the varnish has been removed. Incidentally, laser treatment can also be advantageous in the case of nonvarnished parts coated in accordance with the invention in order to give some improvement to the already existing surface conductivity. Finally, the laser treatment can also be used if the surface treated in accordance with the invention is provided with other or additional insulating layers, for instance with sputtered oxides, nitrides and the like. It is preferable to apply a flowable metallically conductive substance, for example, an adhesive or another plastic-based hardening substance that contains metallically conductive particles, onto the laser-bombarded regions of the surface within a few hours or a few days. Silver particles or silver-coated particles are useful. The laser bombardment is preferably carried out at least two times in order to limit the thermal stress on the surface, and the laser bombardment be performed in an atmosphere of air with a conventional apparatus. An Nd:YAG laser, for example, a 90 W laser, has proven to be suitable. The process of laser treatment is described in detail in European Patent Application No. 01124434.0, filed on Oct. 11, 2001 and titled “Producing metallically conductive surface regions on coated light metal alloys” and filed on Oct. 11, 2001, which is herein incorporated by reference in its entirety.

[0027] FIG. 1 is an internal view of a diecast Mg alloy AZ91 frame part 1, or so-called chassis, of a mobile telephone housing. Frame part 1 is glued to other metallic or metallically coated housing parts along a strip 2. It is important that the frame part 1 have good long-term surface adhesion and a high grade appearance. Through frequent contact with hands and the resulting simultaneous effect of salts, weak acids and moisture, and also through the effects of weather and other circumstances in long-term use, a surface of frame part 1, not shown, can become unsightly if there is insufficient coating. Furthermore, corrosion of an inner surface, not shown, could lead to the formation of particles and thus to failure of electronic components.

[0028] In the gluing of the parts of a mobile phone, it is additionally important that the glued parts are bonded to each other while retaining good electrical conductivity in order to provide electromagnetic shielding for the telephone. Thus, a stable coating of the frame part 1 must provide good electrical surface conductivity along the strip 2 to which glue is applied. The same is true for flat parts of the indicated support dome 3 for a circuitboard, which likewise become conductive because of the necessary mass connection. Other details of the frame part 1 are not important for understanding of the invention.

[0029] The following examples illustrate the invention.

EXAMPLE

[0030] The frame part 1 is first conventionally degreased by alkali treatment and treated at pH 4 in a solution with citric acid and pyrophosphate, followed by passivation at pH 1 in a strongly acidic solution with phosphoric acid and ammonium bifluoride.

[0031] A chemical conversion layer of Zn and Cu is applied to the cleaned and passivated surface. Onto this layer a 7: m thick Cu layer is then be deposited by conventional electrolysis. Then an electrolytic layer of Sn and Zn is deposited on the electrolytic layer of Cu. The weight ratio of Zn:Sn is 70:30. The layer thickness is 8: m.

[0032] This still electrolytically conductive surface is now prepared for varnishing with an alkaline anodic oxidation in a phosphate solution. A treatment with hexavalent chromium is not employed. Instead, a commercial two-component varnish is applied directly onto the anodized surface and hardened. The surface of the Mg diecast frame part 1 has the final visual and technical quality so that it can be varnished with an absolutely transparent color, so that an attractive appearance results from the metal shining through the varnish.

[0033] This surface is then treated on the indicated strip 2 and support domes 3 with a commercial Nd:YAG laser. This laser is Q switched and has a power of 90 W at a lamp current of about 32 A. The strip 2 and domes 3 are traced two times, precisely setting point next to point.

[0034] Empirically, the point spacing, point size and energy per point can be determined so that a continuous strip of sufficient width results. The strip width should not be too small, in order to optimize the electrical contact resistance with the other part of the housing. On the other hand, the strip width should not be too great and should be completely covered by the subsequently applied bead of adhesive (1 mm wide in this example). Also, the coupled energy per shot should not be unnecessarily high, in order to avoid heating that is too great at greater depths. By two-fold bombardment the energy per shot can be kept small. In this case 15 W/mm2 is used per shot. The laser feed rate in this case is 400 mm/sec.

[0035] Then a bead of silicone glue mixed with silver particles can be applied to the thus remetallized surface regions 2 and 3, so that an electrically conductive glueing to another housing part, not shown, can take place. This other housing part is likewise metallic or metallically coated and is glued so that it obtains electrical contact to the adhesive. In this way an electrical contact to the adhesive is obtained, and a tight and electrically shielded housing can be produced.

[0036] In view of the above, it will be seen that the several objects of the invention are achieved.

[0037] As various changes could be made in the above material and processes without departing from the scope of the invention, it is intended that all matter contained in the above description be interpreted as illustrative and not in a limiting sense.

Claims

1. A method for coating a metallic surface comprising:

cleaning and passivating a surface of a light metal alloy component;

forming a first layer on the cleaned and passivated surface wherein the first layer comprises Zn; and

forming a second layer which comprises Sn such that the first layer is located between the surface of the light metal alloy component and the second layer.

2. The method according to claim 1 wherein the cleaning and passivating step comprises:

alkaline degreasing the surface of the light metal alloy component; and

performing an acid treatment of the surface to oxidatively produce a passivation layer on the surface, the acid treatment comprising contacting the surface with a solution which is selected from a group consisting of an acidic solution, a solution comprising the salt of an acid, and combinations thereof.

3. The method according to claim 2 wherein the light metal alloy comprises Mg.

4. The method according to claim 3 wherein the acid treatment step comprises:

contacting the surface of the light metal alloy component with a weak acidic solution; and

contacting the surface with a strong acidic solution which comprises fluoride ions.

5. The method according to claim 4 wherein the light metal alloy has a Mg composition of at least about 50 weight %.

6. The method according to claim 5 wherein the weak acidic solution has a pH of from about 3 to about 5.

7. The method according to claim 5 wherein the weak acidic solution comprises a carboxylic acid and a pyrophosphate.

8. The method according to claim 5 wherein the strong acidic acid solution has a pH of from about 0.5 to about 2.

9. The method according to claim 5 wherein the strong acidic acid solution comprises phosphoric acid and ammonium bifluoride.

10. The method according to claim 5 wherein the weak acidic solution has a pH of from about 3 to about 5 and the strong acidic acid solution has a pH of from about 0.5 to about 2.

11. The method according to claim 5 wherein the weak acidic solution comprises a carboxylic acid and a pyrophosphate and wherein the strong acidic acid solution comprises phosphoric acid and ammonium bifluoride.

12. The method according to claim 5 wherein the weak acidic solution has a pH of from about 3 to about 5 and comprises a carboxylic acid and a pyrophosphate and wherein the strong acidic acid solution has a pH of from about 0.5 to about 2 and comprises phosphoric acid and ammonium bifluoride.

13. The method according to claim 2 wherein the light metal alloy comprises Al.

14. The method according to claim 13 wherein the acid treatment step comprises contacting the surface of the light metal alloy component with a strong oxidizing solution.

15. The method according to claim 14 wherein the light metal alloy has an Al composition of at least about 60 weight %.

16. The method according to claim 15 wherein the strong oxidizing solution comprises an oxidizer selected from the group consisting of nitric acid, peroxomonosulfuric acid, and a persulfate solution.

17. The method according to claim 1 wherein the cleaning and passivating of the surface of the light metal alloy component comprises:

anodically connecting the surface to an electrical source; and

contacting the surface with a solution which comprises phosphoric acid and an alcohol.

18. The method according to claim 17 wherein the light metal alloy comprises a metal selected from the group consisting of Mg, Si, and combinations thereof.

19. The method according to claim 18 wherein the light metal alloy has a Mg composition of at least about 50 weight %.

20. The method according to claim 18 wherein the light metal alloy has a Si composition of at least about 0.1 weight %.

21. The method according to claim 18 wherein the cleaning and passivating step comprises contacting the anodically connected surface with a solution that comprises phosphoric acid and fluoride ions.

22. The method according to claim 17 wherein the light metal alloy comprises Al.

23. The method according to claim 22 wherein the light metal alloy has an Al composition of at least about 60 weight %.

24. The method according to claim 22 wherein the cleaning and passivating step comprises contacting the surface of the light metal alloy component with an aqueous oxidation agent.

25. The method according to claim 1 wherein the first layer is formed by chemical metal plating.

26. The method according to claim 1 wherein the first layer further comprises a metal selected from the group consisting of Cu, Ni, and combinations thereof.

27. The method according to claim 1 wherein the second layer is formed by electrolytic deposition.

28. The method according to claim 1 wherein the second layer further comprises a metal selected from the group consisting of Zn, Bi, Pb, and combinations thereof.

29. The method according to claim 1 further comprising forming an intermediate layer such that the intermediate layer is located between the first layer and the second layer.

30. The method according to claim 29 wherein the intermediate layer is formed by electrolytic deposition.

31. The method according to claim 29 wherein the intermediate layer comprises a metal selected from the group consisting of Cu, Ni, and combinations thereof.

32. The method according to claim 1 further comprising depositing varnish layer on the second layer wherein the varnish layer comprises a varnish.

33. The method according to claim 32 wherein the varnish is a two-component varnish.

34. The method according to claim 32 further comprising performing a passivating treatment on the second layer prior to depositing the varnish layer.

35. The method according to claim 34 wherein the passivating treatment comprises an alkaline anodic oxidation.

36. The method according to claim 35 wherein the passivating treatment comprises contacting the second layer with a solution comprising a compounds selected from the group consisting of phosphates, carbonates, and combinations thereof.

37. The method according to claim 35 wherein the passivating treatment further comprises a cathodic treatment wherein the second layer is contacted with a solution comprising hexavalent chromium ions.

38. The method according to claim 32 further comprising removing a portion of the varnish layer to expose a portion of the second layer.

39. The method according to claim 38 wherein the portion of the varnish layer is removed by bombarding the varnish layer with a laser beam.

40. The method according to claim 39 wherein the varnish layer is bombarded by the laser beam at least twice to remove the portion of the varnish layer.

41. A method for coating a metallic surface comprising:

alkaline degreasing the surface of a light metal alloy component, wherein the light metal alloy component has a Mg composition of at least about 50 weight %;

performing an acid treatment of the surface to oxidatively produce a passivation layer on the surface, the acid treatment comprising:

contacting the surface of the light metal alloy component with a weak acidic solution having a pH of from about 3 to about 5; and

contacting the surface with a strong acidic solution which has a pH of from about 0.5 to about 2 which comprises fluoride ions;

forming a first layer on the cleaned and passivated surface wherein the first layer comprises Zn; and

forming a second layer which comprises Sn such that the first layer is located between the surface of the light metal alloy component and the second layer.

42. The method according to claim 41 further comprising forming an intermediate layer such that the intermediate layer is located between the first layer and the second layer.

43. The method according to claim 42 wherein the intermediate layer is formed by electrolytic deposition.

44. The method according to claim 42 wherein the intermediate layer comprises a metal selected from the group consisting of Cu, Ni, and combinations thereof.

45. A method for coating a metallic surface comprising:

contacting a surface of a light metal alloy component with an aqueous oxidation agent, wherein the light metal alloy component has an Al composition of at least about 60 weight %;

forming a first layer on the surface wherein the first layer comprises Zn; and

forming a second layer which comprises Sn such that the first layer is located between the surface of the light metal alloy component and the second layer.

46. The method according to claim 45 further comprising forming an intermediate layer such that the intermediate layer is located between the first layer and the second layer.

47. The method according to claim 46 wherein the intermediate layer is formed by electrolytic deposition.

48. The method according to claim 46 wherein the intermediate layer comprises a metal selected from the group consisting of Cu, Ni, and combinations thereof.

49. A coating for a light metal alloy comprising:

a first layer comprising Zn wherein the first layer is deposited on a surface of the light metal alloy component; and

a second layer comprising Sn,

wherein the first layer is located between the surface of the light metal alloy component and the second layer.

50. The coating according to claim 49 wherein the light metal alloy comprises Mg.

51. The coating according to claim 49 wherein the light metal alloy comprises Al.

52. The coating according to claim 49 wherein the first layer is formed by is formed by chemical metal plating.

53. The coating according to claim 49 wherein the first layer further comprises a metal selected from the group consisting of Cu, Ni, and combinations thereof.

54. The coating according to claim 49 wherein the second layer is formed by electrolytic deposition.

55. The coating according to claim 49 wherein the second layer further comprises a metal selected from the group consisting of Zn, Bi, Pb, and combinations thereof.

56. The coating according to claim 49 further comprising an intermediate layer wherein the intermediate layer is located between the first layer and the second layer.

57. The coating according to claim 56 wherein the intermediate layer is formed by electrolytic deposition.

58. The coating according to claim 56 wherein the intermediate layer comprises a metal selected from the group consisting of Cu, Ni, and combinations thereof.

59. The coating according to claim 49 further comprising a varnish layer which comprises a varnish wherein the second layer is located between the first layer and the varnish layer.