Selective leaching

Selective leaching is the removal of an element from an alloy by corrosion. The most common example is dezincification, the selective removal of zinc in brasses. Many alloys are susceptible to selective leaching under certain conditions. The elements that are more resistant to the environment remain behind, provided they have a sufficiently continuous structure to prevent them from breaking away in small particles.

Two mechanisms have been described for selective leaching: (1) two metals in an alloy are dissolved, and one redeposits on the surface; and (2) one meta) is selectively dissolved, leaving the other metals behind. Dezincification of brasses occurs by the first mechanism; the loss of molybdenum from nickel alloys in the molten sodium hydroxide occurs by the second. In some alloys, selective leaching takes place by either mechanism, depending on temperature and on the type, concentration, and flow rate of the corrodent.

A special case of selective leaching is preferential attack on inclusions. The metal in the affected area becomes porous and loses much of its strength, hardness, and ductility. Failure may be sudden and unexpected because dimensional changes are not always substantial and the corrosion sometimes appears to be superficial, although the selective attack may bave left only a small fraction of the original thickness of the part unaffected.

Selective Attack on Inclusions

Selective attack on inclusions by an environment to which the body of metal is resistant, in which only small amounts of material (as compared to the massive attack usually encountered in selective leaching) are preferentially corroded away, is a special case of selective leaching. The inclusions provide small anodic areas surrounded by large cathodic areas. Where the inclusions are in the form of elongated stringers and there is end-grain exposure to the environment the attack is highly directional and deep penetration of the metal is possible.
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