Zinc brass corrosion

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. 

Table 16 illustrates the property enhancements and tradeoffs seen when tin is added to a copper—zinc brass base composition. The most commonly used alloys for electrical connectors are the Cu—10 Zn—Sn brasses, such as C411, C422, and C425. These lower level zinc—tin alloys offer good corrosion resistance along with the good formabiHty, conductivity, and strength levels of brass. 

Bronzes are somewhat similar to brasses in mechanical properties and to high-zinc brasses in corrosion resistance (except that bronzes are not affected by stress cracking). Aluminum and silicon bronzes are very popiilar in the process industries because they combine good strength with corrosion resistance. 

Copper-alloy corrosion behavior depends on the alloying elements added. Alloying copper with zinc increases corrosion rates in caustic solutions whereas nickel additions decrease corrosion rates. Silicon bronzes containing between 95% and 98% copper have corrosion rates as low as 2 mil/y (0.051 mm/y) at 140°F (60°C) in 30% caustic solutions. Figure 8.2 shows the corrosion rate in a 50% caustic soda evaporator as a function of nickel content. As is obvious, the corrosion rate falls to even lower values as nickel concentration increases. Caustic solutions attack zinc brasses at rates of 2 to 20 mil/y (0.051 to 0.51 mm/y). 

Zinc brasses are corroded much more rapidly by ammonium hydroxide than by caustic solutions. Corrosion rates approaching 240 mil/y (6.1 mm/y) have been measured at room temperature in two normal ammonium-hydroxide solutions. Corrosion rates in hot, concentrated caustic solutions may be as high as 70 mil/y (1.8 mm/y). 

The resistance of a metal to erosion-corrosion is based principally on the tenacity of the coating of corrosion products it forms in the environment to which it is exposed. Zinc (brasses), aluminum (aluminum brass), and nickel (cupronickel) alloyed with copper increase the coating s tenacity. An addition of V2 to 1)4% iron to cupronickel can greatly increase its erosion-corrosion resistance for the same reason. Similarly, chromium added to iron-base alloys and molybdenum added to austenitic stainless steels will increase resistance to erosion-corrosion. 

The outstanding properties of copper-base materials are high electrical and thermal conductivity, good durabihty in mildly corrosive chemical environments and excellent ductility for forming complex shapes. As a relatively weak material, copper is often alloyed with zinc (brasses), tin (bronzes), aluminum and nickel to improve its mechanical properties and corrosion resistance. 

Silver items, however, are also relatively rare in the archaeological record. The most common metal found is either copper, usually alloyed with either tin (bronze) or, in the later periods, zinc (brass), or iron. The latter contains very little lead and, because of severe corrosion problems, its survival rate is often low (but see Degryse et al., 2007). Fortunately, copper can also be characterized from its lead isotope signature, since the primary ore of copper is chalcopyrite (CuFeS2), which often co-occurs with galena (PbS) and sphalerite (ZnS). Even if the ore used is a secondary mineral formed by the oxidation of the primary deposit, the copper smelted from such a deposit would normally be expected to... 

Season cracking of high zinc brasses is a severe form of embrittlement resulting in cracking or disintegration. Somewhat similar forms of stress-corrosion cracking occur in many other metals and alloys. Embrittlement of boiler plate, discussed below, may be considered a special case. 

Brass is basically a copper- zinc alloy bronze is a copper- tin alloy. In practice both often contain many other metals. Their high machinability, resistance to corrosion, and ease of soft soldering make them very useful in apparatus construction. Owing to the volatility of zinc, brass should not be used in vacuum components that must be baked out or operated hot. Certain bronzes such as phosphor bronze are useful for springs and diaphragms beryllium copper is also useful in these applications. 

Another type of solution involves one solid mixed with another solid. Examples include solid alloys, such as brass, bronze, and steel. Brass, shown in Figure 3, is a mixture of copper and zinc. Brass is widely used in musical instruments because it is harder and more resistant to corrosion than pure copper. 

Incompatibilities and Reactivities Strong oxidizers (including bromine), ethylene oxide, nitrosating agents (e g., sodium nitrite), mercury, strong acids [Note Corrosive to many metals (e g., zinc, brass, aluminum, copper).] ... 

In addition to ductile iron and PVC, copper and lead are used in pipes, and brass in fixtures and connections. Lead is released because of uniform corrosion. Copper is also released because of uniform corrosion, localized-attack cold water pitting, hot water pitting, MIC, corrosion fatigue, and erosion-corrosion. Lead pipes and lead-tin solder exhibit uniform corrosion. Brass corrosion includes erosion-corrosion, impingement corrosion, dezincification, and SCC. The direct health impacts are because of increased copper, lead, and zinc concentrations in the drinking water. Mechanical problems because of corrosion include leaks from perforated pipes, rupture of pipes, and the loss of water pressure because of blockage of pipes by corrosion products. 

High-zinc brasses (e.g., 45-50% Zn-Cu) having a P or (3 -1- y structure, stress-corrosion crack through the grains (i.e., transgranularly) and, unlike a brasses, only moisture is required to cause failure [35]. 

Brass (65%copper, 35% zinc) Very corrosion-resistant. Casts well, easily machined. Can be work hardened. Good conductor. Valves, taps castings, ship fittings, electrical contacts... 

Low Resistance Current Paths. An indirect use of zinc for corrosion protection is to provide a low resistance path for impressed current protection of concrete bridge structures with uncoated steel rebar. Electrochemical protection of steel rebar in concrete was developed as a repair technique but is now being promoted also for new structures. Current generated by impressed anodes on the outside of the concrete and a positive electrical contact with the rebar stops the latter from rusting. The primary anode is usually brass or copper. 

Alloys are compounds or solid solutions of more than one element in metallic form, but cannot be considered as mixtures of the metals. A broad spectrum of nickel-containing alloys are produced. Common examples are stainless steels (iron/nickel/ chromium), copper/nickel and nickel-silver (nickel/ copper/zinc). Brass (copper/zinc) and red gold (gold/ silver/copper) are examples of nickel-free alloys. Resistance to corrosion on skin contact varies widely among different nickel-containing alloys depending on their composition. 

Studies on samples exposed underground have shown that tough pitch coppers, deoxidized coppers, silicon bronzes, and low-zinc brasses behave essentially alike. Soils containing cinders with high concentrations of sulfides, chlorides, or hydrogen ions corrode these materials. In this type of contaminated soil, alloys containing more than 22 % zinc experience dezincification. In soils that contain only sulfides, corrosion rates of the brasses decrease with increasing zinc content and no dezincification occurs. 

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... 

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. 

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