Selective corrosion is observed in alloys in which one part or impurity is clearly less noble than the other parts of the material. The corrosion mechanism implies that the less noble element is removed from the material. A porous material with very low strength and ductility is the result.
Selective corrosion is rare in stainless steels but may occur if there is a large difference in the corrosion resistance between, for example, the ferrite phase and austenite phase in the important alloy elements responsible for the corrosion resistance. A balanced material remains corrosion resistant.
Problems with selective corrosion in duplex stainless steels only occur when recommended safe service limits have been exceeded and where the corrosion rates typically are more than 0.3 mm/year.