Slag is the tool of steel production. The lighter slag floats on the melt and can absorb and bind unwanted elements from the steel. By that, you can increase the so-called degree of purity of the steel. After the work being done the slag has different colorings (depending on the elements absorbed) and is nothing more than artificially produced rock.
Depending on its chemical composition, steel changes its properties very strongly. If it contains more than 12% chromium, it is coated with a chromium oxide layer only a few atoms thick, which protects it from corrosion. An example of such stainless steel is cutlery. By immersing stainless steel in saline solutions, the salt eats wholes into the protective layer. This can happen, for example, when too much salt is used in the dishwasher.