In addition to chromium steel, we use XD-15 material for our special top end ceramic bearings. This is a very special nitrogen infused steel material and will not rust or corrode. It is more than 5 times the corrosion resistance of 440C stainless material. The process to make this steel involves remelting a very high grade of martensitic stainless steel from the end of an electrode. This occurs in a highly-controlled remelt chamber while introducing nitrogen gas, producing a homogenous steel free from course carbides and giving ultimate fatigue resistance. A super strong steel made for ceramic balls, XD-15 is the pinnacle in bicycle bearing technology.