Brass, dezincification

In the tests described by Tracy, a high-tensile brass suffered severe dezinc-ification (Table 4.11). The loss in tensile strength for this material was 100% and for a non-arsenical 70/30 brass 54% no other material lost more than 23% during 20 years exposure. In Mattsson and Holm s tests the highest corrosion rates were shown by some of the brasses. Dezincification caused losses of tensile strength of up to 32% for a P brass and up to 12% for some of the a-P brasses no other materials lost more than 5% in 7 years. Dezinc-ification, but to a lesser degree, occurred also in the a brasses tested, even in a material with as high a copper content as 92%. Incorporation of arsenic in the a brasses consistently prevented dezincification only in marine atmospheres. 

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) ... 

In brasses, dezincification takes place either in localized areas on the metal surface, called plug type (Fig. 20.4), or uniformly over the surface, called layer type (Fig. 20.5). Brass so corroded retains some strength, but has no ductility. Layer-type dezincification in a water pipe may lead to splitting open of the pipe under conditions of sudden pressure increase and, for plug type, a plug of dez-incified alloy may blow out, leaving a hole. Because dezincified areas are porous. 

A corrosion mechanism similar to brass dezincification, known as dealuminification, can occur to the beta phase or eutectoid structure depending on environmental conditions. Proper quench and temper treatments produce a tempered beta structure with reprecipitated acicular alpha crystals, a combination often superior in corrosion resistance to the normal annealed structure. The nickel component in the more complex nickel aluminum bronzes alters the corrosion characteristics of the beta phase because of the nickel additive and gives greater resistance to deaUoying and cavitation-erosion in most liquids. 

In certain alloys and under certain environmental conditions, selective removal of one metal (the most electrochemically active) can occur that results in a weakening of the strength of the component. The most common example is dezincification of brass [164, 165]. The residual copper lacks mechanical strength. 

Antimony may be added to copper-base alloys such as naval brass. Admiralty Metal, and leaded Muntz metal in amounts of 0.02—0.10% to prevent dezincification. Additions of antimony to ductile iron in an amount of 50 ppm, preferably with some cerium, can make the graphite fliUy nodular to the center of thick castings and when added to gray cast iron in the amount of 0.05%, antimony acts as a powerflil carbide stabilizer with an improvement in both the wear resistance and thermal cycling properties (26) (see Carbides). 

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. 

Conditions that favor dezincification include stagnant solutions, especially acidic ones, high temperatures, and porous scale formation (2). Additions of small amounts of arsenic, antimony, or phosphoms can increase the resistance to dezincification. These elements are, however, not entirely effective in preventing the dezincification of the two-phase (cc—P) brasses because dezincification of the P-phase is not prevented (31). Another area of corrosion concern involves appHed or residual stresses from fabrication that can lead to EIC of brasses in the form of stress-corrosion cracking. 

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. 

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. 

Denickelification generally produces less wastage in cupronickels than dezincification in brasses. Wastage decreases as nickel content increases, becoming very slight in alloys containing 30% or more nickel. 

Figure 13.1 Layer-type dezincification on a brass casting. The red layers are uniformly corroded regions. The original yellow of the brass is visible in between.

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-... 

Flgure 13.2 Layer-type dezincification of a thin brass plate. The 0.019-in. (0.048-cm) plate is shown in cross section. The dezincified layers converge toward the plate edge. Note the porosity of the dezincified metal. 

Figure 13.5 Plug-type dezincification on the internal surface of a brass condenser tube. Note the extreme porosity of the copper plugs. Tube wall thickness was 0.040 in. (0.10 cm). Compare to Fig. 13.13. (Courtesy of National Association of Corrosion Engineers, Corrosion 89 Paper No. 197 by H. M. Herro.)...

Figure 13.8 A brass tube showing plug-type dezincification. The white patch above the plug on the external surface was caused by dissolved solids, concentrated by evaporation of water leaking through the porous dezincified plug.

In any specific environment, only certain alloys are affected. Substitution of more resistant materials does not always necessitate major alloy compositional changes. Adding as little as a few hundredths of a percent of arsenic, for example, can markedly reduce dezincification in cartridge brass. Antimony and phosphorus additions up to 0.1% are similarly efficacious. 

Figure 13.10 A hole emanating from the internal surface of a brass condenser tube. The hole was caused by plug-type dezincification (see Fig. 13.11).

The dezincification was caused by underdeposit corrosion. The fact that the brass was not an inhibited grade was a major contributing factor. Chemical cleaning had not been done since this exchanger was installed. No chemical treatment was used on either external or internal surfaces. 

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 principle the selective dissolution of the less noble component of a singlephase alloy would perhaps be expected and is in fact observed (dezincification of an a-brass, etc.) even though the details of the mechanism by which it occurs is not yet fully understood. In contrast, the preferential attack of the less noble phase of a two-phase alloy is not only expected and observed —the mechanism by which it occurs in practice is also quite clear. Selective dissolution of the more active phase of a two-phase alloy is best exemplified by the graphitic corrosion (or graphitisation) of grey cast iron. 

Unfortunately, there is no general theory that will explain all the forms of localised attack that occur with the variety of metal/environment systems encountered in practice, e.g. the mechanism of the pitting of stainless steels in Cl -containing solutions is quite different from the dezincification of brass in a fresh natural water. Nevertheless, many of the following factors play an important part in most forms of localised attack ... 

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. 

Fig. 1.60 Dezincification and impingement attack of copper-altoy tubes, (a) Uniform layer dezincification of a brass, (b) banded dezincification of a brass, (e) plug-type dezincification and...

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