is then heated. A heated wire, a spark, a laser, or an oven may provide the heat. The exothermic reaction is initiated, and the resulting heat propagates the reaction throughout the rest of the sample. Uses Chromium carbide is useful in the surface treatment of metal components. Chromium carbide is used to coat the surface of another metal in a technique known as thermal spraying. Cr3C2 powder is mixed with solid nickel-chromium. This mixture is then heated to very high temperatures and sprayed onto the object being coated where it forms a protective layer. This layer is essentially its own metal matrix composite, consisting of hard ceramic Cr3C2 particles embedded in a nickel-chromium matrix. The matrix itself contributes to the corrosion resistance of the coating because both nickel and chromium are corrosion resistant in their metallic form. After over spraying the coating, the coated part must run through a diffusion heat treatment to reach the best results in matter of coupling strength to the base metal and also in matter of hardness. Another technique utilizes chromium carbide in the form of overlay plates. These are prefabricated chromium carbide-coated steel plates, which are meant to be welded onto existing structures or machinery in order to improve performance. Chromium carbide is used as an additive in cutting tools made of cemented carbides, in order to improve hardness by preventing the growth of large grains. The primary constituent in most extremely hard cutting tools is tungsten carbide. The tungsten carbide is combined with other carbides such as titanium carbide, niobium carbide, and chromium carbide and sintered together with a cobalt matrix. Cr3C2 prevents large grains from forming in the composite, which results in a fine-grained structure of superior hardness. Undesired formation of chromium carbides in stainless steel and other alloys can lead to intergranular corrosion. References External links National Pollutant Inventory - Chromium (III) compounds fact sheet Carbides Chromium(II) compounds Refractory materials Superhard materials