Selective leaching is removal of one element from solid alloy by corrosion process. Parting is metallurgical term sometimes applied but selective leaching is preferred. The term dealloying is frequently used and is preferred by some corrosionists. Dimensional changes do not occur. The most common example is removal of Zinc in brass . Similar process occurs in other alloy systems in which Al, Fe, Co, Cr and other metals are removed.
Advantages of selective leaching
(1) Enrichment of silicon observed in the oxide film on stainless steels results in better passivity and resistance to pitting (2) Preparation of Raney nickel catalyst by selectively removing aluminium from Al-Ni alloy by action of caustic.
Dezincification
Two general types (i) uniform or layer-type (seems to occur in high brasses i.e. high Zn content) (2) localized or plung-type (seems to occur in low brasses i.e. low Zn content). The dezincified portion is weak, permeable, porous, brittle and possesses little aggregate strength. Hence addition of zinc to copper lowers the corrosion resistance of copper. Two mechanisms have been proposed (1) Zinc is dissolved, leaving vacant sites in the brass lattice structure (not proven) (2) Brass dissolves, zinc ions stay in the solution and copper plates back on (commonly accepted).
Prevention: (1) reducing aggressiveness of medium (i.e. oxygen removal) or by cathodic protection. but these not economical. (2) Addition of Sn to Admirality brass (70Cu-30Zn). (3) add inhibitors like As, Sb, P.
Graphitization Gray cast irons some times shows selective leaching in relatively mild environments. the name is given based on the surface layer has appearance of graphite. it is misnomer to call this as graphitization as graphite presents in the material before corrosion. This is also called as graphitic corrosion. selective leaching of Iron or steel matrix leaving the graphite network in gray cast iron (presence of graphite flakes). Graphite is cathodic to ferrite, (as amount of carbon is less in ferrite) and a galvanic cell exists, which results in dissolution of iron leaving a porous mass consisting graphite, voids and rust and loses its strength. Surface shows rusting This corrosion does not occur in nodular or malleable (absence of graphite flakes) and white cast iron (no free carbon).