Zinc brass dezincification

Layer-type dezincification is easy to recognize visually. The original component shape and dimensions are usually preserved, but the metal color changes from the golden yellow of zinc brass to the red of ele-... 

The most common example of selective corrosion is dezincification of brass, in which zinc is removed from the alloy and copper remains. After cleaning the surface, dezincification is easy to demonstrate because the Zn-depleted regions have a characteristically red copper colour in contrast to the original yellow brass. Dezincification occurs in two forms (see Figure 7.36) ... 

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. 

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... 

Brasses are susceptible to dezincification in aqueous solutions when they contain >15 wt% zinc. Stress corrosion cracking susceptibiUty is also significant above 15 wt % zinc. Over the years, other elements have been added to the Cu—Zn base alloys to improve corrosion resistance. For example, a small addition of arsenic or phosphoms helps prevent dezincification to make brasses more usefiil in tubing appHcations. 

Admiralty Brass and Naval Brass are 30 and 40% zinc alloys, respectively, to which a 1% tin addition has been added. Resistance to dezincification of Cu—Zn alloys is increased by tin additions. Therefore, these alloys are important for thein corrosion resistance in condenser tube appHcations. In these, as weU as the other higher zinc compositions, it is common to use other alloying additives to enhance corrosion resistance. In particular, a small amount (0.02—0.10 wt %) of arsenic (C443), antimony (C444), or phosphoms (C445) is added to control dezincification. When any of these elements are used, the alloy is referred as being "inhibited." For good stress corrosion resistance, it is recommended that these alloys be used in the fiiUy annealed condition or in the cold worked plus stress reHef annealed condition. 

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. 

Admiralty brass (70% Cu, 29% Zn, 1% Sn, 0.05% As or Sb) and arsenical aliuninum brass (76% Cu, 22% Zn, 2% Al, 0.05% As) are resistant to dezincification in most cooling water environments. In the recent past, heat exchangers have virtually always been tubed with inhibited grades of brass. Brasses containing 15% or less zinc are almost immune to dezincification. Dezincification is common in uninhibited brasses containing more than 20% zinc. Inhibiting elements include arsenic, antimony, and phosphorus. Without inhibiting elements. 

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 certain alloys and under certain environmental conditions selective removal of one metal (the most electrochemically active) can occur resulting in either localised attack, with the consequent possibility of perforation (plug type), or in a more uniform attack (layer type) that results in a weakening of the strength of the component. Although the selective removal of metals such as Al, Fe, Co, Ni and Cr from their alloys is known, the most prevalent form of de-alloying is the selective removal of zinc from the brasses —a phenomenon that is known as dezincification. 

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. 

Lucey concludes from his electrochemical studies that dezincification involves anodic dissolution of both copper and zinc followed by the cathodic deposition of copper, and on this basis he has explained why arsenic is capable of inhibiting dezincification of a-brass but not of a 3-brass. 

When dezincification occurs in service the brass dissolves anodically and this reaction is electrochemically balanced by the reduction of dissolved oxygen present in the water at the surface of the brass. Both the copper and zinc constituents of the brass dissolve, but the copper is not stable in solution at the potential of dezincifying brass and is rapidly reduced back to metallic copper. Once the attack becomes established, therefore, two cathodic sites exist —the first at the surface of the metal, at which dissolved oxygen is reduced, and a second situated close to the advancing front of the anodic attack where the copper ions produced during the anodic reaction are reduced to form the porous mass of copper which is characteristic of dezincification. The second cathodic reaction can only be sufficient to balance electrochemically the anodic dissolution of the copper of the brass, and without the support of the reduction of oxygen on the outer face (which balances dissolution of the zinc) the attack cannot continue. 

The three most corrosive sites were rifle peat (pH 2-6), cinders (pH 7 -6) and tidal marsh (pH 6-9). Corrosion of some of the alloys was particularly severe in the cinders. The behaviour of the brasses tested, particularly those high in zinc, was rather different from that of the other materials. In most cases dezincification occurred and the brasses were the worst materials in... 

Dezincification of brasses When dezincihcation occurs, regions of the brass become replaced by a porous mass of copper which, though retaining the shape of the original article, has virtually no strength. There has long been discussion as to whether there is selective corrosion of the zinc in the brass, which leaves the copper behind, or whether complete dissolution of the brass occurs, followed by re-deposition of copper. Possibly both processes occur in different circumstances. The mechanism has been investigated and discussed by Evans, Fink", Lucey , Feller" and Heidersbach , and is referred to in many other papers. ... 

With a single-phase brass the whole of the metal in the corroded areas is affected. Dezincification may proceed fairly uniformly over the surface, and this layer type takes much longer to cause perforation than the localised plug type that more often occurs . With a two-phase brass the zinc-rich 8 phase is preferentially attacked as shown in Fig. 4.12. Eventually the a phase may be attacked as well. The zinc corrosion products that accompany dezincification may be swept away, or in some conditions may form voluminous deposits on the surface which may lead to blockages, e.g. in fittings. 

Brass water fittings give no trouble except that dezincification may occur in acid waters or waters of high chloride content, especially when hot. This dezincification has three effects. Firstly, the replacement of brass by porous copper may extend right through the wall of the fitting and permit water to seep through. Secondly, the zinc which is dissolved out of the brass may form very voluminous hard corrosion products and eventually block the waterway —this is often the case in hot soft waters. Thirdly, and often the most important, the mechanical properties of the brass may deteriorate. For instance, a dezincified screwed union will break off when an attempt is made to unscrew it and a dezincified tap or ball-valve seat is readily eroded by the water. 

Dezincification preferential corrosion of zinc from brass resulting in a copper-rich residue on the surface of the alloy. The term also applies to preferential loss of the zinc component by evaporation at elevated temperature. 

Dezincification As either plug dezincification (localized corrosion) or layer dezincification (general corrosion) and refers to zinc (Zn) being selectively leached out of brass. It can be prevented either by reducing the Zn content to below 15% or by the addition of trace amounts of inhibiting elements such as arsenic (As)... 

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