Surface treated steel sheet for battery case, battery case and battery using the case

A surface treated steel sheet for a battery case comprising:

• a steel sheet having a first surface and a second surface, said first surface to be used as the inner surface of the battery case and the second surface to be used as the outer surface of the battery case;
• a first diffusion layer of a nickel-cobalt phosphorous alloy formed as an uppermost layer over said first surface of the battery case;
• a second diffusion layer of iron-nickel between said first diffusion layer and said steel sheet;
• a nickel layer formed as an upper layer over the second surface;
• and
• an iron-nickel diffusion layer between said nickel layer and said steel sheet,
• wherein the first diffusion layer contains Ni3P,
• the cobalt content in the first diffusion layer is within a range from 5 to 30% by weight,
• the phosphorus content in the first diffusion layer is within a range from 1 to 12% by weight,
• a thickness of the first diffusion layer is in the range of 0.1 to 2 μm, and
• a thickness of the nickel layer is in the range of 0.2 to 3 μm.