Brass dealloying

Solid solution composition plays an important role in SeC propagation. In many cases, the composition determines the nature of See in austenitic stainless steels [67]. In the case of brasses, dealloying controls the See mechanism [68]. 

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. 

Zinc brasses containing phosphorus, antimony, and/or arsenic dezinc-ify much less readily than brasses free of these elements. Brasses containing less than 15% zinc are virtually immune to attack, and those containing more than 32% zinc are readily dealloyed. 

These galvanic corrosion processes take place when one or more elemental constituents of an alloy is leached, often leaving a weak, porous structure, although the component dimensions often are unchanged. Dealloying particularly affects equipment constructed of cupronickels, bronzes, brasses, and gunmetal, such as FW heaters, strainers, valves, and pump impellers. 

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... 

Corrosion by dealloying is common in brasses here the zinc component of the alloy is preferentially removed. Brasses with high proportions of the P phase are especially prone to this type of attack. The mechanism appears to be corrosion of both copper and zinc from the metal the zinc passes into solution but the copper is re-deposited with a porous structure of low strength. Aluminium bronzes also suffer dealloying of the aluminium component if incorrectly heat treated. Other metals which may be preferentially dissolved from their alloys are manganese from copper-manganese, nickel from copper-nickel, copper from either copper-silver or copper-gold, and tin from tin-lead (solders). It is evident from this list that it is the component which is anodic to the alloy which is removed. 

CDA(10) alloys 443, 444, and 445 (admiralty brass) are 30% zinc alloys inhibited with arsenic, antimony, and phosphorus respectively. Recent work indicates that a viable mechanism does not appear to exist for the role of arsenic as a dealloying suppressant however, the CDA 443 alloy remains the most popular of the three. 

Dealloying occurs when one component of an alloy is lost preferentially. Thus, brass is an alloy of zinc (a rather active metal) and copper (a rather noble metal). Consequently, the zinc tends to be lost in preference to the copper. Often the copper will form a seal over the surface, preventing further corrosion, but if conditions do not allow this, then the corrosion can penetrate into the component, removing most of the zinc. The result is a porous copper component, which has little mechanical strength, and the problem is often discovered when the component fractures. Similarly, one component of a two-phase alloy can... 

Any one of these mechanisms may apply in specific instances of dealloying. For example, twin bands in brass, visible in the completely or incompletely dezincified layer, constituted early evidence for a volume diffusion mechanism of zinc transport from the bulk alloy to the surface [26]. In the gold-copper alloy system, copper corrodes preferentially, without dissolution of gold, leaving a porous residue of gold-copper alloy or pure gold. 

Dealloying is normally detectable by a color change. Brasses turn from yellow to red. Cast irons become dark from silver gray as a result of enrichment of graphite. Gray iron that has suffered such corrosion is like a sponge with virtually no mechanical strength. 

In the dissolution and precipitation mechanism, proposed by various authors [12-14] working with brasses, both the LN and MN components dissolve, but the MN component is deposited. Several difficulties arose with this mechanism outside of the brass system in that ions of the MN component were not detected in the solution during dealloying, but instead alloys of intermediate compositions were found on the dealloyed surface. 

The brasses (Cu-Zn alloys) have a corrosion resistance similar to that of copper. Under certain conditions, selective corrosion of zinc may lead to dealloying however (Chapter 7). Adding small amounts of Sn, As, Sb or P to brass permits to reduce this kind of attack. In amine-containing environments, brass is sensitive to stress corrosion (Chapter 11). 

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