Electrodeposition Material, Process for Providing a Corrosion-Protective Layer of TiO2 on an Electrically Conductive Substrate and Metal Substrate Coated with a Layer of TiO2

Abstract

The present invention relates to electrodeposition material for the electrochemical deposition of a corrosion-protective layer of TiO.sub.2 on an electrically conductive substrate comprising a titanium compound, a complexing agent, an accelerator, water and optionally organic solvents, buffering agents and one or more additives, characterized in that the titanium compound is titanyl sulfate and/or titanyl oxalate, the complexing agent is selected from the group consisting of citric acid, citrates, tartaric acid, tartrates, lactic acid, lactates, gluconic acid, gluconates, polyhydroxy-polycarbonic acids, ethylenediaminetetraacetate, methylglycinediacetate, iminodisuccinate, nitrilotriacetic acid and nitrilotriacetate, triethanolamine, phosphonic acid and phosphonates, polyaspartic acid and polyaspartates, polyacrylic acid and polyacrylates and the accelerator is selected from the group consisting of H.sub.2O.sub.2 and organic peroxides. The invention further relates to a process for providing a corrosion-protective layer of TiO.sub.2 on an electrically conductive substrate and to a metal substrate coated with a layer of TiO.sub.2.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation under 35 U.S.C. .sctn. 365(c) and 35 U.S.C. .sctn. 120 of international application PCT/EP2006/005790, filed 16 Jun. 2006, and published 28 Dec. 2006 in English as WO 2006/136333, which is incorporated herein by reference in its entirety. This application also claims priority under 35 U.S.C. .sctn. 119 of EP 05013424.6 filed 22 Jun. 2005, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to an electrodeposition material for the electrochemical deposition of a corrosion-protective layer of TiO.sub.2 on an electrically conductive substrate comprising a titanium compound, a complexing agent, an accelerator, water and optionally organic solvents, buffering agents and one or more additives. Such TiO.sub.2 layer deposited electrochemically may serve as an appropriate primer layer for subsequent coating treatment (e.g. coating with organic materials, such as for instance lacquers, varnishes, paints, organic polymers, adhesives, etc.).

BACKGROUND OF THE INVENTION

[0003] A very common industrial task involves providing metallic or non-metallic substrates with a first coating, which has a corrosion-inhibiting effect and/or which constitutes a primer for the application thereon of a subsequent coating containing e.g. organic polymers. An example of such a task is the pre-treatment of metals prior to lacquer coating, for which various processes are available in the art. Examples of such processes are layer-forming or non-layer-forming phosphating, chromating or a chromium-free conversion treatment, for example using complex fluorides of titanium, zirconium, boron or silicon. Technically simpler to perform, but less effective, is the simple application of a primer coat to a metal prior to lacquer-coating thereof. An example of this is the application of red lead. An alternative to so-called "wet" processes are so-called "dry" processes, in which a corrosion-protection or coupling layer is applied by gas phase deposition. Such processes are known, for example, as PVD or CVD processes. They may be assisted electrically, for example by plasma discharge.

[0004] A layer produced or applied in this way may serve as a corrosion-protective primer for subsequent lacquer coating. However, the layer may also constitute a primer for subsequent bonding. Metallic substrates in particular, but also substrates of plastics or glass, are frequently pre-treated chemically or mechanically prior to bonding in order to improve adhesion of the adhesive to the substrate. For example, in vehicle or equipment construction, metal or plastic components may be bonded metal to metal, plastic to plastic or metal to plastic. At present, front and rear windshields of vehicles are as a rule bonded directly into the bodywork. Other examples of the use of coupling layers are to be found in the production of rubber/metal composites, in which once again the metal substrate is as a rule pre-treated mechanically or chemically before a coupling layer is applied for the purpose of bonding with rubber.

[0005] The conventional wet or dry coating processes in each case exhibit particular disadvantages. For example, chromating processes are disadvantageous from both an environmental and an economic point of view owing to the toxic properties of the chromium and the occurrence of highly toxic sludge. However, chromium-free wet processes, such as phosphating, as a rule, also result in the production of sludge containing heavy metals, which has to be disposed of at some expense. Another disadvantage of conventional wet coating processes is that the actual coating stage frequently has to be preceded or followed by further stages, thereby increasing the amount of space required for the treatment line and the consumption of chemicals. For example, phosphating, which is used virtually exclusively in automobile construction, entails several cleaning stages, an activation stage and generally a post-passivation stage. In all these stages, chemicals are consumed and waste is produced which requires disposal.

[0006] Although dry coating processes entail fewer waste problems, they have the disadvantage of being technically complex to perform (for example requiring a vacuum) or of having high energy requirements. The high operating costs of these processes are therefore a consequence principally of plant costs and energy consumption.

[0007] Further, it is known from the prior art that thin layers of metals compounds, for example oxide layers, may be produced electrochemically on an electrically conductive substrate. For example, the article by Y. Zhou and J. A. Switzer entitled "Electrochemical Deposition and Microstructure of Copper (I) Oxide Films", Scripta Materialia, Vol. 38, No. 11, pages 1731 to 1738 (1998), describes the electrochemical deposition and microstructure of copper (I) oxide films on stainless steel. The article investigates above all the influence of deposition conditions on the morphology of the oxide layers; it does not disclose any practical application of the layers.

[0008] According to Blandeu et al. in Thin Solid Film, 42, 147 (1997) (Abstract), TiO.sub.2-layers are obtained on a Ti-sheet from H.sub.2SO.sub.4 aqueous solution by anodic oxidation methods. This is obtained at potentials below 50 V. However, this process can produce TiO.sub.2 only on Ti-substrates by anodic oxidation.

[0009] According to Nogami et al. in J. Electrochem. Soc., 135, 3008 (1988) (Abstract), TiO.sub.2 is obtained on a Ti-sheet from an aqueous solution containing 0.5 mol/L H.sub.2SO.sub.4 and 0.03 mol/L HNO.sub.3 by an anodic oxidation method (titanium anodization). Constant current is 1 mAcm.sup.2. The oxidation is performed in a cooled bath of 278.degree. K. to 283.degree. K. However, this process can produce TiO.sub.2 only on a Ti-substrate by anodic oxidation.

[0010] EP 1 285 105 B1 discloses a process for producing a coating comprising at least two layers on an electrically conductive surface wherein in a first stage a chromium-free layer of at least one X-ray crystalline inorganic compound of at least one metal is electrochemically deposited on an electrically conductive surface from a solution containing the metal in dissolved form. Besides many other metals, titanium is disclosed.

[0011] According to the applicant's not yet published application PCT/EP2004/014140 corrosion-protective layers of TiO.sub.2 are electrochemically deposited on a metal substrate from an electrodeposition material comprising titanyl sulfate or titanyl oxalate as a titanium component, citrate or citric acid, tartaric acid and tartrates, lactic acid and lactates as chelating agents and hydroxylamines and their derivates or nitrates as accelerators.

SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide further electrodeposition materials for the electrochemical deposition of a corrosion-protective layer of TiO.sub.2 on an electrically conductive substrate which result in TiO.sub.2 layers providing excellent corrosion protection and which may serve as an excellent primer layer for subsequent coating treatment. Surprisingly this object can be achieved by the use of titanyl sulfate and/or titanyl oxalate as the titanium component combined with a special combination of complexing agents and accelerators.

[0013] Subject-matter of the present invention is an electrodeposition material as specified above characterized in that the titanium compound is titanyl sulfate and/or titanyl oxalate, the complexing agent is selected from the group consisting of citric acid, citrates, tartaric acid, tartrates, lactic acid, lactates, gluconic acid, gluconates, polyhydroxy-polycarbonic acids, ethylenediaminetetraacetate, methylglycinediacetate, iminodisuccinate, nitrilotriacetic acid and nitrilotriacetate, triethanolamine, phosphonic acid and phosphonates, poly-aspartic acid and polyaspartates, polyacrylic acid and polyacrylates and the accelerator is selected from the group consisting of H.sub.2O.sub.2 and organic peroxides.

[0014] Further, according to a second aspect of the invention, the present invention relates to a process for providing a corrosion-protective layer of TiO.sub.2 on an electrically conductive substrate by electrodeposition of an electrodeposition material comprising a titanium compound, a complexing agent, an accelerator, water and optionally organic solvents, buffering agents and one or more additives.

[0015] Finally, according to a third aspect of the invention, the present invention relates to a metal substrate coated with a layer of TiO.sub.2 produced by the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] In one embodiment, the invention provides an electrodeposition material for the electrochemical deposition of a corrosion-protective layer of TiO.sub.2 on an electrically conductive substrate comprising a titanium compound, a complexing agent, an accelerator, water and optionally organic solvents, buffering agents and one or more additives, characterized in that the titanium compound is titanyl sulfate and/or titanyl oxalate, the complexing agent is selected from the group consisting of citric acid, citrates, tartaric acid, tartrates, lactic acid, lactates, gluconic acid, gluconates, polyhydroxy-polycarbonic acids, ethylenediaminetetraacetate, methylglycinediacetate, iminodisuccinate, nitrilotriacetic acid and nitrilotriacetate, triethanolamine, phosphonic acid and phosphonates, poly-aspartic acid and polyaspartates, polyacrylic acid and polyacrylates and the accelerator is selected from the group consisting of H.sub.2O.sub.2 and organic peroxides. Desirably, an electrodeposition material having a pH of 5 to 10, preferably 6 to 9, more preferably 7.5 to 8.0 is provided.

[0017] In one embodiment, the electrodeposition material characterized in that it comprises 0.05 to 0.3 mol/l titanium compound, 0.01 to 0.2 mol/l complexing agent and 0.02 to 0.2 mol/l accelerator.

[0018] In some embodiments, the electrodeposition material further comprises a polymeric cationic binder. It is a further object of the invention to provide an electrodeposition material that comprises polymeric cationic binder in an amount of 5 to 60% by weight based on the total weight of the electrodeposition material.

[0019] Another aspect of the invention is a process for providing a corrosion-protective layer of TiO.sub.2 on an electrically conductive substrate by electrodeposition of a electrodeposition material comprising a titanium compound, a complexing agent, an accelerator, water and optionally organic solvents, buffering agents and one or more additives, characterized in that the titanium compound is titanyl sulfate and/or titanyl oxalate; the complexing agent is selected from the group consisting of citric acid, citrates, tartaric acid, tartrates, lactic acid, lactates, gluconic acid, gluconates, polyhydroxy-polycarbonic acids, ethylenediaminetetraacetate, methylglycinediacetate, iminodisuccinate, nitrilotriacetic acid and nitrilotriacetate, triethanolamine, phosphonic acid and phosphonates, poly-aspartic acid and polyaspartates, polyacrylic acid and polyacrylates and the accelerator is selected from the group consisting of H.sub.2O.sub.2 and organic peroxides. It is a further object of the invention to provide such a process using the electrodeposition material as described above. Desirably, the electrodeposition is carried out under the following conditions: current density: 0.01 to 100, preferably 0.1 to 20, more preferably 0.5 to 10 mA/cm.sup.2; coating time: 0.15 to 20, preferably 0.5 to 10, more preferably 1 to 4 minutes; temperature: 0 to 100, preferably 20 to 60.degree. C.; and pH: 5 to 10, preferably 6 to 9, more preferably 7.5 to 8.0.

[0020] It is also an object of the invention to provide a process as described above wherein the electroconductive substrate is selected from the group consisting of steel, especially cold rolled steel and galvanized steel, and aluminium.

[0021] It is a further object of the invention to provide a process as described above, characterized in that the TiO.sub.2-layer is deposited on the electrically conductive substrate with an essentially uniform layer thickness, calculated as weight per unit area, in the range of from 0.01 to 3.5 g/m.sup.2, preferably in the range of from 0.5 to 1.4 g/m.sup.2.

[0022] Another aspect of the invention is a metal substrate coated with a layer of TiO.sub.2 produced by the process described herein. Desirably, the metal substrate is selected from the group consisting of steel, especially cold rolled steel and galvanized steel, and aluminium.

[0023] The electrodeposition material preferably comprises 0.05 to 0.3 mol/l titanium compound, 0.01 to 0.2 mol/l complexing agent and 0.02 to 0.2 mol/l accelerator.

[0024] The pH of the electrodeposition material preferably is 5 to 10, more preferably 6 to 9, most preferably 7.5 to 8.0.

[0025] The electrodeposition material preferably comprises a polymeric cationic binder in addition to the components specified above. As the cationic binder all electrodepositable resins known in the art may be used. Examples of such cationic film-forming resins include amine salt group-containing resins such as the acid-solubilized reaction products of polyepoxides and primary or secondary amines. Usually, these amine salt group-containing resins are used in combination with a blocked isocyanate curing agent. Besides amine salt group-containing resins, quaternary ammonium salt group-containing resins can also be employed. Examples of these resins are those which are formed from reacting an organic polyepoxide with a tertiary amine salt. Also, film-forming resins which cure via transesterification can be used. Further, cationic compositions prepared from Mannich bases can be used. From an electrodeposition material comprising the components of the present invention combined with a polymeric cationic binder, a layer of TiO.sub.2 and a resinous layer can be deposited simultaneously.

[0026] Preferably the electrodeposition material of the present invention comprises the polymeric cationic binder in an amount of 5 to 60% by weight based on the total weight of the electrodeposition material.

[0027] The present invention further relates to a process for providing a corrosion-protective layer of TiO.sub.2 on an electrically conductive substrate by electrodeposition of a electrodeposition material comprising a titanium compound, a complexing agent, an accelerator, water and optionally organic solvents, buffering agents and one or more additives, characterized in that the titanium compound is titanyl sulfate and/or titanyl oxalate, the complexing agent is selected from the group consisting of citric acid, citrates, tartaric acid, tartrates, lactic acid, lactates, gluconic acid, gluconates, polyhydroxy-polycarbonic acids, ethylenediaminetetraacetate, methylglycinediacetate, iminodisuccinate, nitrilotriacetic acid and nitrilotriacetate, triethanolamine, phosphonic acid and phosphonates, poly-aspartic acid and polyaspartates, polyacrylic acid and polyacrylates and the accelerator is selected from the group consisting of H.sub.2O.sub.2 and organic peroxides. Desirably the electro-deposition materials described herein may be used in the process.

[0028] The electrodeposition preferably is carried out under the following conditions current density: 0.01 to 100, preferably 0.1 to 20, more preferably 0.5 to 10 mA/cm.sup.2, coating time: 0.15 to 20, preferably 0.5 to 10, more preferably 1 to 4 minutes, temperature: 0 to 100.degree. C., preferably 20 to 60.degree. C., pH: 5 to 10, preferably 6 to 9, more preferably 7.5 to 8.0.

[0029] The electroconductive substrate preferably is selected from the group consisting of steel, especially cold rolled steel and galvanized steel, and aluminium.

[0030] The TiO.sub.2-layer is deposited on the electrically conductive substrate preferably with an essentially uniform layer thickness, calculated as weight per unit area, in the range of from 0.01 to 3.5 g/m.sup.2, more preferably in the range of from 0.5 to 1.4 g/m.sup.2.

[0031] Illustrating the invention are the following examples which, however, are not to be considered as limiting the invention to their details. All parts and percentages in the following examples as well as throughout the specification are by weight unless otherwise indicated.

EXAMPLES

Example 1

TABLE-US-00001 [0032] Ti Compound 0.14 mol/l Titanium potassium oxalate dihydrate (50 g/L) Complexing Agent 0.02 mol/L Citric acid monohydrate(5 g/L) Accelerator 0.04 mol/L H.sub.2O.sub.2 (5 g/L 30% by weight) Process Parameters pH = 7.5; Current density 0.5-1.8 mA/cm.sup.2; t = 5 min; T = 60.degree. C.; Substrate: galvanized steel

Example 2

TABLE-US-00002 [0033] Ti Compound 0.14 mol/l Titanium potassium oxalate dihydrate (50 g/L) Complexing Agent 0.03 mol/L L(+)-Tartaric acid (5 g/L) Accelerator 0.04 mol/L H.sub.2O.sub.2 (5 g/L 30% by weight) Process Parameters pH = 7.5; Current density 0.5-1.8 mA/cm.sup.2; t = 5 min; T = 60.degree. C.; Substrate: galvanized steel

Example 3

TABLE-US-00003 [0034] Ti Compound 0.14 mol/l Titanium potassium oxalate dihydrate (50 g/L) Complexing Agent 0.03 mol/L Gluconic acid (12 g/L 50% by weight) Accelerator 0.04 mol/L H.sub.2O.sub.2 (5 g/L 30% by weight) Process Parameters pH = 7.5; Current density 0.5-1 mA/cm.sup.2; t = 5 min; T = 60.degree. C.; Substrate: galvanized steel

Example 4

TABLE-US-00004 [0035] Ti Compound 0.14 mol/l Titanium potassium oxalate dihydrate (50 g/L) Additive: Urea 0.08 mol/L (5 g/l) Complexing Agent 0.1 mol/L Citric acid monohydrate Accelerator 0.04 mol/L H.sub.2O.sub.2 (5 g/L 30% by weight) Process Parameters pH = 7-7.5; Current density 0.5-1.8 mA/cm.sup.2; t = 1-5 min; T = 60.degree. C.; Substrate: galvanized steel

Example 5

TABLE-US-00005 [0036] Ti Compound 0.14 mol/l Titanium potassium oxalate dihydrate (50 g/L) Additive: Urea 0.08 mol/L (5 g/l) Additive: Phosphoric acid 5 g/L Complexing Agent 0.1 mol/L Citric acid monohydrate Accelerator 0.04 mol/L H.sub.2O.sub.2 (5 g/L 30% by weight) Process Parameters pH = 7-7.5; Current density 0.5-1.8 mA/cm.sup.2; t = 1-5 min; T = 60.degree. C.; Substrate: galvanized steel

Example 6

TABLE-US-00006 [0037] Ti Compound 0.14 mol/l Titanium potassium oxalate dihydrate (50 g/L) Additive: Urea 0.08 mol/L (5 g/l) Additive: Phosphoric acid 5 g/L Complexing Agent 0.1 mol/L Citric acid monohydrate Accelerator 0.04 mol/L H.sub.2O.sub.2 (5 g/L 30% by weight) Process Parameters pH = 7-7.5; Current density 0.5-1.8 mA/cm.sup.2; t = 1-5 min; T = 60.degree. C.; Substrate: galvanized steel

Example 7

TABLE-US-00007 [0038] Ti Compound 0.14 mol/l Titanium potassium oxalate dihydrate (50 g/L) Additive: Urea 0.08 mol/L (5 g/l) Additive: Hydroxybenzoic ca. 0.03 mol/L (5 g/L) acid: 3-; 3,5 Di- and 2,5 Di- Hydroxybenzoic acid Complexing Agent 0.1 mol/L Citric acid monohydrate Accelerator 0.04 mol/L H.sub.2O.sub.2 (5 g/L 30% by weight) Process Parameters pH = 7-7.5; Current density 0.5-1.8 mA/cm.sup.2; t = 1-5 min; T = 60.degree. C.; Substrate: galvanized steel

Example 8

TABLE-US-00008 [0039] Ti Compound 0.14 mol/l Titanium potassium oxalate dihydrate (50 g/L) Additive: Urea 0.08 mol/L (5 g/l) Additive: 5 g/L Gum arabic Complexing Agent 0.1 mol/L Citric acid monohydrate Accelerator 0.04 mol/L H.sub.2O.sub.2 (5 g/L 30% by weight) Process Parameters pH = 7-7.5; Current density 0.5-1.8 mA/cm.sup.2; t = 1-5 min; T = 60.degree. C.; Substrate: galvanized steel

Example 9

TABLE-US-00009 [0040] Ti Compound 0.14 mol/l Titanium potassium oxalate dihydrate (50 g/L) Additive: Urea 0.08 mol/L (5 g/l) Adhesion promoter: 0.1 g/L Formaldehyde resin modified with phenylsalicylic acid Complexing Agent 0.1 mol/L Citric acid monohydrate Accelerator 0.04 mol/L H.sub.2O.sub.2 (5 g/L 30% by weight) Process Parameters pH = 7-7.5; Current density 0.5-1.8 mA/cm.sup.2; t = 1-5 min; T = 60.degree. C.; Substrate: galvanized steel

Preparing of the Electrodeposition Material:

[0041] The titanium compound was dissolved in deionized water (accelerated by heating to 30 to 50.degree. C.) [0042] The complexing agent was added. [0043] Thereafter, the accelerator and optionally the additives were added. [0044] The pH was adjusted by the addition of KOH (0.5 to 1.5 mol/l) at a temperature of 45 to 60.degree. C.

Claims

1. An electrodeposition material for the electrochemical deposition of a corrosion-protective layer of TiO.sub.2 on an electrically conductive substrate comprising: a.) at least one titanium compound comprising titanyl sulfate and/or titanyl oxalate; b.) a complexing agent selected from the group consisting of one or more of citric acid, citrates, tartaric acid, tartrates, lactic acid, lactates, gluconic acid, gluconates, polyhydroxy-polycarbonic acids, ethylenediaminetetraacetate, methylglycinediacetate, iminodisuccinate, nitrilotriacetic acid, nitrilotriacetate, triethanolamine, phosphonic acid, phosphonates, polyaspartic acid, polyaspartates, polyacrylic acid and polyacrylates; c.) an accelerator selected from the group consisting of H.sub.2O.sub.2 and organic peroxides; d.) water; and e.) optionally organic solvents, buffering agents and one or more additives.

2. The electrodeposition material according to claim 1, comprising 0.05 to 0.3 mol/l titanium compound, 0.01 to 0.2 mol/l complexing agent and 0.02 to 0.2 mol/l accelerator.

3. The electrodeposition material according to claim 2, wherein the electrodeposition material has a pH of 5 to 10.

4. The electrodeposition material according to claim 1, comprising: a.) 0.05 to 0.3 mol/l titanyl oxalate; b.) 0.01 to 0.2 mol/l complexing agent selected from one or more of citric acid, citrates, tartaric acid, tartrates, lactic acid, lactates, gluconic acid, and gluconates; and c.) 0.02 to 0.2 mol/l accelerator.

5. The electrodeposition material according to claim 4, wherein the electrodeposition material further comprises urea.

6. The electrodeposition material according to claim 1, wherein the electrodeposition material has a pH of 6 to 9 and further comprises a polymeric cationic binder.

7. The electrodeposition material according to claim 6, comprising the polymeric cationic binder in an amount of 5 to 60% by weight based on the total weight of the electrodeposition material.

8. The electrodeposition material according to claim 7, wherein the polymeric cationic binder comprises at least one electrodepositable resin.

9. The electrodeposition material according to claim 8, wherein the at least one electrodepositable resin is selected from amine salt group-containing resins, and quaternary ammonium salt group-containing resins.

10. A process for providing a corrosion-protective layer of TiO.sub.2 on an electrically conductive substrate comprising electrodepositing on an electrically conductive substrate a corrosion-protective layer of TiO.sub.2 by contacting the electrically conductive substrate with the electrodeposition material according to claim 1 at a current density of 0.01 to 100 mA/cm.sup.2 and a temperature of 0 to 100.degree. C., for 0.15 to 20 minutes.

11. The process of claim 10 wherein the electrodeposition material has a pH of 6 to 9 and further comprises a polymeric cationic binder.

12. The process of claim 10 wherein the electrodeposition material comprises: 0.05 to 0.3 mol/l titanium compound; 0.01 to 0.2 mol/l complexing agent; and 0.02 to 0.2 mol/l accelerator.

13. The process of claim 10, wherein the electroconductive substrate is selected from the group consisting of steel and aluminium.

14. The process of claim 13, comprising the polymeric cationic binder in an amount of 5 to 60% by weight based on the total weight of the electrodeposition material.

15. The process of claim 12, wherein the electrodepositing step is carried out by contacting the electrically conductive substrate with the electrodeposition material at a current density of 0.1 to 20 mA/cm.sup.2 and a temperature of 20 to 60.degree. C., for 0.5 to 10 minutes.

16. The process according to claim 15, wherein the electroconductive substrate is selected from the group consisting of steel and aluminium.

17. The process of claim 10, wherein the electrodeposition material comprises: a.) 0.05 to 0.3 mol/l titanyl oxalate; b.) 0.01 to 0.2 mol/l complexing agent selected from one or more of citric acid, citrates, tartaric acid, tartrates, lactic acid, lactates, gluconic acid, and gluconates; and c.) 0.02 to 0.2 mol/l accelerator.

18. The process of claim 10, wherein the electrodeposition material further comprises urea.

19. The process of claim 10, wherein the corrosion-protective layer of TiO.sub.2 is deposited on the electrically conductive substrate such that said layer has a uniform layer thickness in the range of from 0.01 to 3.5 g/m.sup.2.

20. A metal substrate coated with a layer of TiO.sub.2 produced by the process of claim 10.