Dezincification examples

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. 

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. 

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. 

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. 

Chlorides have probably received the most study in relation to their effect on corrosion. Like other ions, they increase the electrical conductivity of the water so that the flow of corrosion currents will be facilitated. They also reduce the effectiveness of natural protective films, which may be permeable to small ions the effect of chloride on stainless steel is an extreme example but a similar effect is noted to a lesser degree with other metals. Turner" has observed that the meringue dezincification of duplex brasses is affected by the chloride/bicarbonate hardness ratio. 

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. 

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. 

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) ... 

Dezincification can occur, for example, in tap water, river water, seawater, and Uqnid containing chloride. The rate of this corrosion process is increased by plastic deformation of the material. Dezincification can occur by two mechanisms ... 

Alloys of gold with copper or silver retain the corrosion resistance of gold above a critical alloy concentration called the reaction limit by Tammann [1]. Below the reaction limit, the alloy corrodes, for example, in strong acids, leaving a residue of pure gold either as a porous solid or as a powder. This behavior of noble-metal alloys is known as parting and is probably similar in mechanism to dezincification of copper-zinc alloys (see Section 20.2.2). 

This form of corrosion corresponds to a process where one constituent of an alloy is preferentially removed, leaving behind an altered structure. The most common example for this type of corrosion is the selective removal of zinc from brass (Cu-Zn alloys), also referred to as dezincification. Selective leaching has been observed in other alloy systems containing Al, Fe, Co, and Cr. 

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]. 

When one element of a solid alloy is removed by corrosion, the process is known as selective leaching, dealloying, or dezincification. The most common example is the removal of zinc from brass alloys that contain more than 15% zinc. When the zinc corrodes preferentially, a porous residue of copper and corrosion products remains. The corroded part often retains its original shape and may appear undamaged except for surface tarrush. However, its tensile strength, and particularly its ductility, are seriously reduced. 

Aluminum brass is resistant to erosion-corrosion and dezincification. It is used as condenser heat exchanger tube material for marine applications. A good example is the use of Al-Brass (76-79 Cu, 1.8-2.5 Al, 0.005 As, 0.007 Pb) as in marine service. 

Selective leaching. Selective leaching refers to the selective removal of one element from an alloy by corrosion processes. A common example is the dezincification of unstabilized brass, whereby a weakened. 

Selective leaching is usually controlled by material selection. For example, brass is resistant to dezincification if traces of arsenic, phosphorous, or antimony are added to the alloy. Modem brass plumbing fixtures are made exclusively from these stabilized alloys. Brass with a low Zn content generally tends to be less susceptible. In more corrosive environments the use of cupro-nickel alloys has been advocated. 

Dezincification can be minimized by reducing the aggressiveness of the environment (i.e. oxygen removal) or by cathodic protection, but in most cases these methods are not economical. UsnaUy a less susceptible alloy is used. For example, red brass (15% Zn) is almost immune. Better brass is made by addition of 1% tin to a 70-30 brass (Admiralty metal). Further improvement is obtained by adding small amounts of arsenic, antimony or phosphorus as inhibitors. ... 

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... 

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