Copper-zinc alloys, dezincification

DEZINCIFICATION. A form of electrolytic corrosion observed in sume brasses where the copper-zinc alloy goes into solution with subsequent redepnsition of the copper. The small red copper plugs thus formed in the brass are usually porous and of low strength. In recent years, the term dcziticilicalion lias also been applied in a more general sense to signify any metallic corrosion process that dissolves one of the components from an alloy. 

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 Copper-zinc alloys containing more than 15% of zinc are... 

Dezincification is the selective corrosion of copper-zinc alloys (brass) with more than 15% zinc, in which porous copper deposits form on the corroded parts. Although less common, similar phenomena have also been observed in other copper alloys such as copper-aluminum (aluminum stripping), copper-tin (detinning), and copper-nickel (nickel stripping). 

Alloys of gold with copper or silver retain the corrosion resistance of gold above a critical alloy concentration called the reaction limit by Tammann [1]. Below the reaction limit, the alloy corrodes, for example, in strong acids, leaving a residue of pure gold either as a porous solid or as a powder. This behavior of noble-metal alloys is known as parting and is probably similar in mechanism to dezincification of copper-zinc alloys (see Section 20.2.2). 

Figure 20.3. Trends of dezincification, stress-corrosion cracking, and impingement attack with increasing zinc content in copper-zinc alloys (brasses).

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],... 

The most widely used group of copper alloys is the brasses, which are basically copper-zinc alloys. The resistance of brasses to corrosion by aqueous solutions does not change markedly as long as the zinc content does not exceed about 15 % above 15 % zinc, dezincification (or deaUoying) may occur. Selective removal of zinc leaves a relatively porous and weak layer of copper. 

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. 

Dezincification Selective leaching of zinc from copper-zinc alloys. 

Copper-zinc alloys containing more than 15% zinc are susceptible to dezincification. In the dezincification of brass, selective removal of zinc... 

In contrast, the selective dissolution or leaching-out by corrosion of one component of a single-phase alloy is of considerable practical importance. The most common example of this phenomenon, which is also referred to as parting , is dezincification, i.e. the selective removal of zinc from brass (see Section 1.6). Similar phenomena are observed in other binary copper-base alloys, notably Cu-Al, as well as in other alloy systems. 

In general, the rate of dezincification increases as the zinc content rises, and great care needs to be exercised in making brazed joints with copper/zinc brazing alloys, particularly if they are to be exposed to sea-water. Under these conditions, a properly designed capillary joint may last for some time, but it is preferable to use corrosion-resistant jointing alloys such as silver solders (e.g. BS 1845, Type AGJ or /4G5) . 

Copper alloys, such as brass, bronze, admiralty, and Muntz metals, can exhibit better corrosion resistance and better mechanical properties than pure copper. In general, high-zinc alloys should not be used with acids or alkalies owing to the possibility of dezincification. Most of the low-zinc alloys are resistant to hot dilute alkalies. 

Studies on samples exposed underground have shown that tough pitch coppers, deoxidized coppers, silicon bronzes, and low-zinc brasses behave essentially alike. Soils containing cinders with high concentrations of sulfides, chlorides, or hydrogen ions corrode these materials. In this type of contaminated soil, alloys containing more than 22 % zinc experience dezincification. In soils that contain only sulfides, corrosion rates of the brasses decrease with increasing zinc content and no dezincification occurs. 

Dezincification is a form of de-alloying. As the phenomenon was first observed in brass in which zinc separated by dissolution from copper, the term dezincification is still used. 

Selective leaching is found in solid solution alloys and occurs when one element or constituent is preferentially removed as a consequence of corrosion processes. The most coimnon example is the dezincification of brass, in which zinc is selectively leached from a copper-zinc brass alloy. The mechanical properties of the alloy are significantly impaired... 

Dezincification Dezincification is corrosion of a brass alloy containing zinc in which the principal product of corrosion is metallic copper. This may occur as plugs rilling pits (plug type) or as continuous layers surrounding an unattacked core of brass (general type). The mechanism may involve overall corrosion of the alloy followed by redeposition of the copper from the corrosion products or selective corrosion of zinc or a high-zinc phase to leave copper residue. This form of corrosion is commonly encountered in brasses that contain more than 15 percent zinc and can be either eliminated or reduced by the addition ox small amounts of arsenic, antimony, or ph osphorus to the alloy. 

Dezincification is explained by two theories. The first is that the alloy dissolves, with a preferential redeposition of copper. The other proposes a selective leaching of the zinc, leaving the copper behind. There is evidence that both mechanisms may operate, depending on the specific environment. 

Two theories have been proposed to explain dezincification, but since both have considerable support the precise mechanism remains unresolved. One theory proposes that the zinc is selectively leached from the alloy leaving a porous residue of metallic copper in situ (c/. parting of Ag-Au alloys), whilst the other proposes that the whole of brass dissolves and that the copper immediately redeposits at sites close to where the brass was dissolved. 

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