Dealloying dezincification

Parting, or Dealloying, Corrosion This type of corrosion occurs when only one component of an alloy is removed by corrosion. The most common type is dezincification of brass. 

Dealloying is a prominent mode of decay of copper alloys. It is possible that one of the components of the alloy is preferentially leached out. Dealloying has been observed in the case of copper alloys in the form of preferential leaching of zinc from brasses, aluminum from aluminum bronzes and nickel from cupronickels. Brasses containing >15% zinc are susceptible to dezincification in aqueous medium. 

Dezincification. Copper-Zinc alloys containing more than 15% zinc are susceptible to dezincification. In the dezincification of brass, selective removal of zinc leaves a relatively porous and weak layer of copper and copper oxide. Corrosion of a similar nature continues beneath the primary corrosion layer, resulting in gradual replacement of sound brass by weak, porous copper. Uniform dealloying in admiralty brass is shown in Figure 6.25.5,7,53,54... 

Dezincification, as one type of deaUoying, was defined in Section 2.4. Other types of dealloying include the selective dissolution of copper in copper-gold alloys. 

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. 

If the test material is suspected of being subject to dealloying (such as dezincification) or intergranular corrosion, a cross section of the specimen should be microscopicaUy excimined. If embrittlement is suspect, simply bending the specimen may determine this susceptibiUty. 

Selective leaching or dealloying is the selective removal of one element from an alloy by corrosion processes. The most common example is the selective removal (dezincification) of zinc in brass alloys. Dezincification may either be plug-type or uniform. In other alloy systems, aluminum, iron, cobalt, nickel, chromium, and other elements may be selectively removed [49]. Little work has been done in differentiating susceptibility of selective leaching of alloys in synthetic and naturtil seawater [6]. 

Copper-zinc alloys (brasses) with more than 15 % zinc are susceptible to a type of dealloying CtiUed dezincification. Some aluminum bronze (Cu-Al) alloys, especially those containing more than 8 % aluminum, are also susceptible to dealloying. The dealloying in Cu-Al alloys can be prevented by adding more than 3.5 % nickel or by heat treating to obtain an a + P microstructure [79],... 

Conditions of the environment that favor dezincification are high temperatures, stagnant solutions, especially of acid, and porous inorganic scale formation. Other factors that stimulate the process are increasing zinc concentrations and the presence of both cuprous and chloride ions. As the dealloying proceeds a porous layer of pure or almost pure copper is left behind. This reaction layer is of poor mechanical strength. The dezincification process on copper-zinc alloys is therefore very detrimental. 

When one element of a solid alloy is removed by corrosion, the process is known as selective leaching, dealloying, or dezincification. The most common example is the removal of zinc from brass alloys that contain more than 15% zinc. When the zinc corrodes preferentially, a porous residue of copper and corrosion products remains. The corroded part often retains its original shape and may appear undamaged except for surface tarrush. However, its tensile strength, and particularly its ductility, are seriously reduced. 

Selective dissolution, where the less noble element dissolves selectively leaving behind a porous metal phase enriched in the more noble constituent, resulting in dealloying. The more common case is the selective dissolution of Zn (dezincification) from a-brass (Cu-Zn). Selective dissolution is in some instances associated with circumstances of stress corrosion cracking (SCC) [175]. 

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. 

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