Chromium (Cr)
Chromium forms a robust rust protection layer on steels and prevents corrosion. It increases hardness, toughness, and resistance of the material. However, too high chromium content can lead to brittleness.
You alloy steel with chromium to make it stainless – the chromium forms a dense, invisible oxide layer on the surface that keeps oxygen out. When welding, you have to be careful: the high heat can burn out the chromium in the weld and the corrosion protection layer is lost there. That's why you use a suitable filler material for such steels to compensate for the chromium loss. If you grind or brush the sheet after welding, you destroy the protective layer – but it forms again on its own as long as the surface is clean and free of impurities.