Dezincification graphitic corrosion

In principle the selective dissolution of the less noble component of a singlephase alloy would perhaps be expected and is in fact observed (dezincification of an a-brass, etc.) even though the details of the mechanism by which it occurs is not yet fully understood. In contrast, the preferential attack of the less noble phase of a two-phase alloy is not only expected and observed —the mechanism by which it occurs in practice is also quite clear. Selective dissolution of the more active phase of a two-phase alloy is best exemplified by the graphitic corrosion (or graphitisation) of grey cast iron. 

Fig. 10.15 Types of corrosion, (a) General, uniform attack corrosion, (b) Intergranular corrosion, (c) Selective corrosion, e.g. dezincification of brass, (d) Stress corrosion cracking, (e) Pitting, (f) Layer corrosion (exfoliation), (g) Graphitic corrosion, (h) Corrosion fatigue. Types (b)-(h) are initially associated with the nature of the metal.

In the dealloying process, typically one of two mechanisms occurs alloy dissolution and replating of the cathodic element or selective dissolution of an anodic alloy constituent. In either case, the metal is left spongy and porous and loses much of its strength, hardness, and ductility. Table 2 lists some of the alloy-environment combinations for which dealloying has been reported. By far the two most common forms of dealloying are dezincification and graphitic corrosion. 

Oxidation has been known since Sir Humphrey Davy electrolytically decomposed potash in 1806 and dissolution as a form of corrosion was investigated by the British Navy in 1823 to develop a method preventing the degradation of their copper-huUed ships. Dealloying has been known by other names snch as leaching, dezincification or parting, graphitization, and so on which refer to the removal of a reactive metal component from an alloy or the purification of a noble metal from an alloy, respectively. 

Selective Leaching Corrosion is a metal removal process from the base aUoy matrix, such as dezincification (Zn is removed) in Cu - Zn alloys and graphitization Fe is removed) in cast irons. 

---