The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, protection methods vary. When the addition of chromium reaches 10.5 percent , the atmospheric corrosion resistance of the steel increases significantly, but when the chromium content is very high, although the corrosion resistance can still be enhanced, it is not very large. The reason is that alloying steel with chromium changes the type of surface oxide to a surface oxide similar to that formed on pure chromium metal. This tightly adhered chromium-rich oxide protects the surface from further oxidation. This oxide layer is so thin that the natural luster of the steel surface can be seen through it, giving stainless steel a unique finish. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere to repair itself, re-form this "passivation film", and continue to play a protective role.
Felly, mae gan ddur di-staen nodwedd gyffredin, hynny yw, mae'r cynnwys cromiwm yn uwch na 10.5 y cant.
Nid yw'r term dur di-staen yn cyfeirio at un math o ddur di-staen yn unig, mae gan bob math o ddur di-staen berfformiad da yn ei faes cais penodol. Yr allwedd i lwyddiant yw deall y pwrpas yn gyntaf, ac yna dewis y radd dur cywir. Ar hyn o bryd, fel arfer dim ond chwe gradd dur sy'n gysylltiedig â maes cymhwysiad adeiladu adeiladau. Maent i gyd yn cynnwys 17-22 y cant o gromiwm, ac mae'r graddau gorau hefyd yn cynnwys nicel. Gall ychwanegu molybdenwm wella ymwrthedd cyrydiad atmosfferig ymhellach, megis ymwrthedd i atmosfferau sy'n cynnwys clorid.
