Abstract: A protective layer for a metal substrate is metallurgically bonded to the substrate and contains (as a percentage by mass) 35 to 50% chromium, up to 10% of which can be replaced by molybdenium, and at least iron, the proportion of iron being at least 25%. The minimum hardness is 800 HVO.1, obtained by a structure having a minimum content of 5% by volume as sigma phase. The sigma phase is obtained by heat-treatment of the coated substrate. The protective layer is particularly resistant to corrosion and also has good resistance to erosion and wear.

Abstract: A substrate based on aluminum (Al) is given a surface coating of aluminum (Al) and silicon (Si) and of at least one element X from the group iron (Fe), nickel (Ni), cobalt (Co), manganese (Mn and chromium (Cr), the total proportion of elements X being 1 to 35 wt.%. Depending on the concentration (C) of elements X in wt.%, the minimum hardness over at least 50% of the coated surface, measured in HV 0.05, can be calculated from the formula;HV 0.05=134+2.6 . C+0.44. C.sup.2The surface coating with the required minimum hardness greatly improves the mechanical and/or chemical properties of the substrate.