Dezincification layer type

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. 

A pump impeller and a shaft bushing from a small cooling water pump assembly were generally corroded. Reddish surface discoloration revealed layer-type dezincification (Figs. 13.14 and 13.15). 

Figure 6.25 Selective corrosion by layer type (dezincification) of a bolt in brass7...

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. 

Figure 20.5. Layer-type dezincification in brass bolts (actual size).

FIG. 2—Layer-type dezincification (cross section) In a yellow brass (alloy C26000) threaded fastener (magnification 15x). 

A plug of dezincified brass may fall out, leaving a hole, whereas water pipe having layer-type dezincification may split open. 

Uniform layer type dezincification occurs in tooth of gear wheel. It also occurs on the inner surfece of admiralty brass heat exchanger tubes when exposed to water at pH = 8.0 and temperature range 31-49°C. 

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...

Dezincification is readily apparent, since the yellow colour of the brass is replaced by the characteristic red of copper, which may take the form of small plugs or of layers that in some cases can extend over the whole of the surface (Fig. 1.60). In plug-type dezincification a mechanically weak, porous residue of copper is produced, which may remain in situ or become removed by the pressure of water, leading to a perforation. In the layer type the transformation of the alloy into a mechanically weak layer of copper results in loss of strength, and failure may occur by splitting when the metal is subjected to water pressure or to external stress. 

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. 

Figure 7.36 a) Uniform (layer) dezincification and h) localized (plug-type) dezincification of brass. 

Intergranular corrosion, spongiosis, dezincification, and line and layer type corrosion are classified in this subgroup. Of particular importance is intergranular corrosion. 

What is the essential difference between the plug type and layer type of dezincification ... 

Common yellow brass consists of approximately 30% zinc and 70% copper. Dezincification is readily observed with the naked eye because the alloy assumes a red or copper color that contrasts with the original yellow. There are two general types of dezincification and both are readily recognized. One is uniform or layer-type, and the other is localized or plug-type dezincification. 

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 may be either plug-type (Fig. 1) or layer-tyjje (Fig. 2) as exhibited. 

Fig. 10.15 Types of corrosion, (a) General, uniform attack corrosion, (b) Intergranular corrosion, (c) Selective corrosion, e.g. dezincification of brass, (d) Stress corrosion cracking, (e) Pitting, (f) Layer corrosion (exfoliation), (g) Graphitic corrosion, (h) Corrosion fatigue. Types (b)-(h) are initially associated with the nature of the metal.

---