If the welding or grinding process of stainless steel is heated to a certain temperature in air, the two sides of the weld, weld the lower surface and bottom are chromium oxide heat back to the fire. Heat back to the fire than protective oxide thin film, and visible. The color depends on the thickness, a rainbow color, blue, purple to pale yellow and brown. The thicker oxide is generally black. It is because of the long time high temperature or at a height of stay by. When there is any kind of this kind of oxide layer, the metal surface of the chromium content can be reduced, resulting in these regions reduces the corrosion resistance. In this case, not only to eliminate the back fire heat and other oxide, chromium depleted layer also deal with them the following cleaning.