Surface-treated metal plate

1. A surface-treated metal plate comprised of a metal sheet on one surface or both surfaces of which two or more coating layers are provided,

the surface-treated metal plate is characterized in that

the two or more coating layers are formed by water-based dispersion coatings or emulsion coatings,

a diffusion layer is formed at an interface part between a first coating layer as a top layer and a second coating layer as a bottom layer,

wherein one of the first or the second layer contains rust preventing pigments and the other does not contain rust preventing pigments,

wherein the concentration of rust preventing pigments in the diffusion layer changes from 2 mass % to 98 mass % with respect to concentration of the rust preventing pigments in the one of the first and the second layer in the film thickness direction found by analysis of the rust preventing pigments in the coating depth direction by high frequency GDS, and in that

t/T is 0.2≦t/T≦0.8 where the film thickness of said diffusion layer is “t” and the total film thickness of said diffusion layer, said first coating layer, and said second coating layer is “T”.

2. A surface-treated metal plate as set forth in claim 1 , characterized in that said t/T is 0.4≦t/T≦0.6.

3. A surface-treated metal plate as set forth in claim 1 or 2 characterized in that said total film thickness T is 1.0 μm≦T≦20.0 μm.

4. A surface-treated metal plate as set forth in claim 1 , characterized in that said first coating layer has a film thickness of 0.2 μm or more.

5. A surface-treated metal plate as set forth in claim 1 , characterized in that said second coating layer has a film thickness of 0.2 μm or more.

6. A surface-treated metal plate as set forth in claim 1 , characterized in that said second coating layer contains one or more types of rust preventing pigments comprised of compounds including at least one element which is selected from Si, P, and V and in that said first coating layer does not contain any compound including an element included in said second coating layer among Si, P, and V.

7. A surface-treated metal plate as set forth in claim 1 , characterized in that said metal sheet is a zinc-based plated steel sheet.

8. A method of production of a surface-treated metal plate as set forth in claim 1 ,

said method of production of a surface-treated metal plate characterized by coating one surface or both surfaces of a metal sheet with a first coating which forms said first coating layer and a second coating which forms said second coating layer respectively by a wet-on-wet method or simultaneous multilayer coating method, and drying and baking, so as to form a diffusion layer at an interface part between said first coating layer and said second coating layer directly under the first coating layer of the front side of the sheet, wherein the ingredients of the first coating layer and the second coating layer are mixed in the diffusion layer so that the concentrations of ingredients of the first and the second layers continuously change and is characterized in that

said first coating and said second coating are a water-based coating,

a difference (Δσ=σ2−σ1) between a surface tension (σ1) at 20° C. of said first coating and a surface tension (σ2) at 20° C. of said second coating which are measured by a platinum ring method is 2 to 20 mN/m and

a difference (Δφ=φ2−φ1) between a viscosity (φ1) at 20° C. of said first coating and a viscosity (φ2) at 20° C. of said second coating which are measured by a B-type viscometer is −10 to 200 mPa·s.