Copper-zinc alloys mechanism

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). 

The ionization-redeposition mechanism, according to which the alloy corrodes and the more noble metal (copper in copper-zinc alloys) is then redeposited to form a porous outer layer [20]. 

The surface diffusion mechanism, in which only the less noble element (zinc in copper-zinc alloys) dissolves and the remaining more noble metal is rearranged by diffusion on the surface and nucleation of islands of almost pure metal [23]. 

Conditions of the environment that favor dezincification are high temperatures, stagnant solutions, especially of acid, and porous inorganic scale formation. Other factors that stimulate the process are increasing zinc concentrations and the presence of both cuprous and chloride ions. As the dealloying proceeds a porous layer of pure or almost pure copper is left behind. This reaction layer is of poor mechanical strength. The dezincification process on copper-zinc alloys is therefore very detrimental. 

Copper nickels or cupronickels are alloys that result from addition of 3 to 30% Ni to copper. Nickel silvers are copper-zinc alloys (brasses) that contain some nickel. Copper nickels and nickel silvers have an alpha or fee structure and good mechanical strength and forming characteristics. Cupronickels are the most corrosion-resistant copper-based alloys. For example, the long-... 

The selective net loss of a component such as zinc, aluminium or nickel from copper-base alloys sometimes occurs when these alloys corrode. Early studies of the phenomenon were done by simple immersion. More recently, however, the potential-pH dependence of de-alloying has been examined , and it appears that this approach can provide a much more detailed understanding of the mechanism. Future experimental work is expected to include potentiostatic and potentiodynamic techniques to a much greater extent. 

When this system was studied over time, it was found that the marker wires move toward each other. This shows that the most extensive diffusion is zinc from the brass (an alloy of zinc and copper) outward into the copper. If the mechanism of diffusion involved an interchange of copper and zinc, the wires would not move. The diffusion in this case takes place by the vacancy mechanism described later, as zinc moves from the brass into the surrounding copper. As the zinc moves outward, vacancies are produced in the... 

Cementation consists in the formation of a surface alloy with a less reactive metal. For zinc, alloying can be effected with mercury (amalgamation), copper, silver, nickel.12 The reactivity of a cemented metal can be explained considering that the supporting metal plays only the role of an electron reservoir, the true chemistry is effected by the superficial additional metal. Zinc can also be activated by washing with aqueous ammonium chloride,13 or by reacting the powder with trimethyl-chlorosilane.14 In this latter case, Barbier reactions were effected even at 0°C in short times. The activation mechanism was not determined. 

To increase the diphenyldichlorosilane content in the condensate, it is advisable to conduct the direct synthesis of phenylchlorosilanes not with copper-silicon alloy but with a mechanical mixture of silicon and copper powders, promoted by zinc oxide. The introduction of zinc oxide seems to inhibit the undesirable reactions of diphenyl and benzene formation, creating favourable conditions to attach phenyl radicals to the silicon atom, i.t. to form diphenyldichlorosilane. 

Copper alloys, such as brass, bronze, admiralty, and Muntz metals, can exhibit better corrosion resistance and better mechanical properties than pure copper. In general, high-zinc alloys should not be used with acids or alkalies owing to the possibility of dezincification. Most of the low-zinc alloys are resistant to hot dilute alkalies. 

Some SRB may cause loealized corrosion on stainless steels, nickel alloys, aluminium, zinc and copper alloys. Mechanisms of sulphur-assisted corrosion, with emphasis on Fe- and Ni-based materials, have recently been reviewed by Marcus [6.17]. The review includes the fundamentals of enhanced dissolution, retarding or blocking of passivation, and passivity breakdown. 

Copper-nickel-zinc alloys Copper-nickel-zinc alloys (also known as German silver ), which have a high mechanical and corrosion resistance, are mainly used for machining of optical mold inserts for injection molding of plastics. 

The effect of copper in zinc alloys is to increase slightly the corrosion resistance (Zinkberatung, 1975). Kehrer et al. (1982), however, show that castings have predictable lower corrosion rates (and mechanical properties) only if the copper content is above 0.7%, which is about the solubility limit of... 

Due to the rather low mechanical strength (/ = 90 MPa) of this material, alloys have been considered for this use. In general, the resistance of aluminium alloys at room temperature is slightly less good than that of unalloyed aluminium. Alloying elements such as silicon, copper, zinc or magnesium slightly increase the dissolution rate, especially at low acid concentrations. 

SMAs include copper-zinc-aluminum, copper-aluminum-nickel, and nickel-titanium (NiTi) alloys. NiTi alloys exhibit better mechanical properties than copper-based SMAs which were first developed in the early 1960s. SMAs have been widely used in retrofit and strengthening projects in the field of damping, active vibration control, and prestressing or posttensioning of structures with fibers and tendons, while recently, iron-based SMAs were developed that can reduce the cost. 

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... 

The properties of aluminum alloys (mechanical, physical, and chemical) depend on alloy composition and microstructure as determined by casting conditions and thermomechanical processing. While certain metals alloy with Al rather readily [9], comparatively few have sufficient solubility to serve as major alloying elements. Of the commonly used alloying elements, magnesium, zinc, copper, and silicon have significant solubility, while a number of additional elements (with less that 1% total solubility) are also used to confer important improvements to alloy properties. Such elements include manganese, chromium, zirconium, titanium, and scandium [2,10]. 

For example,copper has relatively good corrosion resistance under non-oxidizing conditions. It can be alloyed with zinc to yield a stronger material (brass), but with lowered corrosion resistance. Flowever, by alloying copper with a passivating metal such as nickel, both mechanical and corrosion properties are improved. Another important alloy is steel, which is an alloy between iron (>50%) and other alloying elements such as carbon. 

Copper and nickel can be alloyed with zinc to form nickel silvers. Nickel silvers are ductile, easily formed and machined, have good corrosion resistance, can be worked to provide a range of mechanical properties, and have an attractive white color. These alloys are used for ornamental purposes, as sHverplated and uncoated tableware and flatware in the electrical iadustry as contacts, connections, and springs and as many formed and machined parts (see Electrical connectors). 

---