A sheet-metal layer includes anti-diffusion structures made of a high-temperature-corrosion-resistant steel having a longitudinal direction, upper and lower surfaces, a thickness of 0.015 to 0.1 mm and discontinuous microstructures extending approximately in the longitudinal direction. The microstructures have a structure height (0.02 to 0.1 mm), a structure length (2 to 10 mm), a structure width (0.2 to 1 mm), a longitudinal spacing (greater than 2 mm), formed by interruptions, from the nearest microstructure aligned approximately in the longitudinal direction and a lateral distance (1 to 10 mm) from the nearest laterally adjacent microstructure. Some of the microstructures project out of the sheet-metal layer toward the upper surface and some toward the lower surface. The microstructures cause each straight theoretical line extending across the sheet-metal layer perpendicularly to the longitudinal direction to intersect at least two microstructures projecting toward the upper surface and two microstructures projecting toward the lower surface.