Dezincification of brass

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. 

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. 

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 ... 

Many of the alloys of copper are more resistant to corrosion than is copper itself, owing to the incorporation either of relatively corrosion-resistant metals such as nickel or tin, or of metals such as aluminium or beryllium that would be expected to assist in the formation of protective oxide films. Several of the copper alloys are liable to undergo a selective type of corrosion in certain circumstances, the most notable example being the dezincification of brasses. Some alloys again are liable to suffer stress corrosion by the combined effects of internal or applied stresses and the corrosive effects of certain specific environments. The most widely known example of this is the season cracking of brasses. In general brasses are the least corrosion-resistant of the commonly used copper-base alloys. 

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. ... 

Dezincification of brasses This may occur, particularly in stagnant or slowly-moving warm or hot waters relatively high in chloride and containing little carbonate hardness. Dezincification of a brasses is inhibited by the usual arsenic addition (see Fig. 4.12), but two-phase brasses are liable to severe attack in some waters . In such waters the use of duplex-structure brass fittings should be avoided. 

It is hardly surprising that the preparation of surfaces of plain specimens for stress-corrosion tests can sometimes exert a marked influence upon results. Heat treatments carried out on specimens after their preparation is otherwise completed can produce barely perceptible changes in surface composition, e.g. decarburisation of steels or dezincification of brasses, that promote quite dramatic changes in stress-corrosion resistance. Similarly, oxide films, especially if formed at high temperatures during heat treatment or working, may influence results, especially through their effects upon the corrosion potential. 

A form of corrosion in which of one or more elemental constituents of an alloy is leached, often leaving a porous structure. Examples are dezincification of brass and denickelization of nickel alloys. 

Dezincification of brass fittings occurs when the /1-phase is present. Dezincification is accelerated by high temperature, high chloride concentration, low flow-rates and differential aeration. Dezincification can be inhibited by the addition of 1% tin and 0.04% arsenic. 

Dezincification. Copper-Zinc alloys containing more than 15% zinc are susceptible to dezincification. In the dezincification of brass, selective removal of zinc leaves a relatively porous and weak layer of copper and copper oxide. Corrosion of a similar nature continues beneath the primary corrosion layer, resulting in gradual replacement of sound brass by weak, porous copper. Uniform dealloying in admiralty brass is shown in Figure 6.25.5,7,53,54... 

Selective leaching applies to the situation where one element of an alloy is removed preferentially to another metallic constituent. The most common example of this phenomenon is the so-called dezincification of brass (see Section 10.2.1), but it may also occur through the selective removal of nickel from cupro nickel alloys and aluminium from aluminium bronze. 

Copper and brass may be included in the plant material of construction subject to corrosive aqueous environments. In general these metals have good corrosion resistance but under certain conditions, e.g. high pH and in the presence of high concentrations of chlorides and sulphates, corrosion of copper and dezincification of brass can occur. Under these conditions it is possible to use various heterocyclic compounds for example the sodium salt of 2-mercaptobenzothiazoleor benzotrizole, to limit corrosion by the formation of an insoluble chemical complex at the anodic areas. 

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) ... 

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

Evaluation of corrosive effects arising during the exposure can be evaluated by visual inspection, metaUographic examination of cross section under microscope with respect to pits, cracks, intergranular attack, and dezincification (of brass) and determination of loss in heat transmission capacity, a method which has direct relation to the function of automotive radiators. 

In addition to the corrosion penetration rates, one must be mindful of dezincification of brasses and selective attack on some bronzes as well as see of yellow brass or bronze (Fig. 6.46 in ehap. 6). These types of corrosion contribute to the failure of the material in mechanical respects without significant weight changes or losses in thickness. 

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.

Copper-zinc alloys containing more than 15% zinc are susceptible to dezincification. In the dezincification of brass, selective removal of zinc... 

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