1. C: C content must be tested, carbon is the most basic element to distinguish steels, the lower it is, the less easy to occur inter-granular corrosion, so stainless steel is also called ultra-low carbon stainless steel. 2.Ni: It can be called Austenitic stainless steel only when Ni is added. It can take away the presence of ferrite. But the content of Ni can not more than the standard, otherwise it is not corrosion resistance.