Dezincification plug type

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.

Figure 13.9 Stratified copper corrosion product in plug-type dezincification. Denickelification...

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).

Figure 13.10S A transverse cross section through the tube wall in Fig. 13.10A. Note the through-wall plug-type dezincification.

Two sections of utility condenser tubing were received. One of the sections had deep plug-type dezincification on internal surfaces (Fig. 13.5) the other showed only superficial corrosion on internal surfaces (Fig. 13.13). 

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. 

Figure 17. Light colored areas show plug-type dezincification in section of brass...

Figure 18. Thin cross-section of admiralty brass tube wall penetrated by plug-type dezincification. (Upper section) specimen polished, unetched. (Lower section) same specimen polished and etched. Magnification about 170 diameters.

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

Figure 20.4. Plug-type dezincification in brass pipe (actual size).

FIG. 1—Plug-type dezincification (cross section) In a yellow brass (alloy C26000) tube (magnification 15x). 

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. 

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. 

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. 

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. 

Selective leaching or dealloying is the selective removal of one element from an alloy by corrosion processes. The most common example is the selective removal (dezincification) of zinc in brass alloys. Dezincification may either be plug-type or uniform. In other alloy systems, aluminum, iron, cobalt, nickel, chromium, and other elements may be selectively removed [49]. Little work has been done in differentiating susceptibility of selective leaching of alloys in synthetic and naturtil seawater [6]. 

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

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

Brass alloys of copper can be subject to deallo)dng (dezincification), a type of corrosion in which the brass dissolves as an alloy and the copper constituent redeposits from solution onto the surface of the brass as a metal in a porous form. The zinc constituent may be deposited in place as an insoluble compound or carried away from the brass as a soluble salt. The corrosion can take place uniformly or locally. The latter, called plug-type... 

The two types of attack can be observed in Figs 4.9 and 4.10. In the uniform type of dezincification, the active area is leached out over a broad area of the surface and it is not localized to a certain point of the surface. On the other hand, the plug type of attack is localized, at a certain point on the surface and the surrounding area remains unaffected. Dezincification can occur on grain boundaries, such as a-fi brasses (Fig. 4.9). 

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

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