Copper alloys selective attack

Brass is only one strong phase of the copper and zinc dissolved. In certain circumstances one notes a preferential dissolution of brass however. This dezincification (Figure 6.24) can be localized (plug dezincification) or more uniformly distributed (layer dezincification).7 Figure 6.26 shows the selective attack of the rich phase of the alloy. 

Figure 7.24 Scanning electron micrographs of the surface of a duplex stainless steel after immersion in 10% HCl for 2 h showing selective attack of constituent phases (a). The micrograph (b) shows the same alloy after immersion in 0.1 M CUSO4. One observes selective deposition of copper on the more cathodic austenite phase [5].

It is a form of corrosion in which zinc is selectively attacked in zinc-containing alloys, like brasses. It mainly occurs in alloys containing less than 85% copper. De-alloying and selective leaching are broader terms which refer to the corrosion of one or more constituent of a solid solution alloy. 

F) Resistance to service conditions. Corrosive environments seriously shorten the performance of a material. The selection process requires materials compatibility with the environment. For instance, austenitic steels (304, 304L, 316, 316L) withstand the corrosive attack of polluted seawater much better than the copper alloys. 

Multiphase gold or palladium-based alloys never show dissolution of Au or Pd but often exhibit progressive surface ennoblement due to selective dissolution of copper or silver from the outer 2-3 atomic layers Heat treatment often decomposes multicomponent alloys into a Pd-Cu rich compound and an Ag-rich matrix with corrosion of the latter phase in deaerated artificial saliva and S -containing media . Au-Cu-rich lamellae have similarly been observed, again with preferential attack on Ag-rich phases or matrix. These effects presumably arise from the ability of the noble alloy phases to catalyse the cathodic reduction of oxygen . 

Monel 400, a nickel alloy containing 66.5% nickel, 31.5% copper and 1.25% iron, has a marked tendency for the initiation of pitting in chloride-containing environments where the passive film can be disturbed. Under stagnant conditions chlorides penetrate the passive film at weak points and cause pitting attack. Sulfides can cause either a modification of the oxide layer, as described for copper, or breakdown of the oxide film of nickel alloys. Pit initiation and propagation depend on depth of exposure, temperature and presence of surface deposits. Little and coworkers [30] reported selective dealloying of nickel in Monel 400 in the presence of SRB from an estuarine environment. 

Another possible adverse effect of inhibition is an increased rate of corrosion of a metal in the system other than the one for which the inhibitor was selected to protect. For example, some amines protect steel admirably but will severely attack copper and brass. Nitrites may attack lead and lead alloys such as solder. In some cases, the inhibitor may react in the system to produce a harmful product. An illustration of this is the reduction of nitrite inhibitors to form ammonia that causes stress corrosion cracking of copper and brass. The only way to avoid these problems is to know the metallic components of a system and be thoroughly familiar with the properties of the inhibitor to be used (Nasr-El-Din et al. 2002). 

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). 

Many materials experience several of the above forms of corrosion. Some forms of attack are more prevalent in certain materials, e.g., exfoliation is well known in aluminum alloys and selective leaching can occur in copper-zinc alloys. A more complete description of corrosion types is found elsewhere in this book and is discussed in the literature [i-3]. 

Another possible adverse effect of inhibition is an increased rate of corrosion of a metal in the system other than the one the the inhibitor was selected to protect. For example, some amines protect steel admirably, but will severely attack copper and brass. Nitrites may attack lead and lead alloys such as solder. 

This qualitative rule is well checked in the attack by oxygen of silver alloys containing a small proportion of some less noble metals (copper, cadmium, and aluminum) and indeed the solubility of oxygen in silver is relatively high and these alloys lead almost exclusively to a selective internal oxidation of the added element. 

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