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guowei7980金虫 (小有名气)
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[交流]
翻译一段电化学方面的英文，拒绝翻译软件
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COLORING METALS: A PLEASANT PATINA When a piece of jewelry is fabricated, solder is the glue that holds together the individual pieces. After the object is assembled, a surface color may be desired. This surface color, or patina, may develop naturally upon exposure to the atmosphere or may be produced as the result of an anticipated chemical change. Usually, the patina will make the piece of jewelry more attractive than it was originally. After metals are fabricated into jewelry pieces, a patina may be added by the artisan.The patina is usually considered ornamental and not protective. However, the chemical reactions to produce patinas are similar to those reactions that cause metals to form protective coats. These are oxidation-reduction reactions in which the metal atoms change to ions by losing electrons: They undergo oxidation. These metal ions can react with available anions, such as carbonates (CO32–), oxides (O2–), sulfides (S2–), and sulfates (SO42–), to form colored compounds that, in small amounts, give the metal an attractive patina and, in larger amounts, provide a protective coating to the metal. However, if the oxidation proceeds to erode a large portion of the metal, the design of the original piece of jewelry is not protected but will be destroyed. We call this process corrosion. What are some examples of chemical combinations that produce metal patinas? • For an antique green patina, combine 5 g of ammonium chloride (NH4Cl) with 42 g of CuSO4•5H2O, in one liter of distilled water. Heat this solution to 68C. Heat the metal object to be plated to 79C. Wash the metal with cool water until it reaches a temperature of around 23C, about room temperature. Wash the coloring solution over the metal surface. Let it dry. Repeat this process until the desired shade of green is achieved. • For a golden yellow patina, combine 7 g of sodium thiosulfate (Na2S2O3•5H2O), with 56 g of iron III nitrate (Fe(NO3)3•9H2O), in 1 L of distilled water. Bring to a boil. Dip the object to be colored in this solution. Patinas, Protective Coatings, and Corrosion Patinas add color and interest to works of art on public display, along with those items, such as jewelry, that are kept in a more controlled environment. Henry Moore’s bronze Reclining Figure (1963–1965), displayed in front of the Center for Performing Arts in New York City, is a beautiful blue color because the copper combines with sulfate to form copper sulfate (CuSO4). The blue-green color of the Statue of Liberty comes from brochantite [CuSO4•3Cu(OH)2] and antlerite [CuSO4•2Cu(OH)2]. Copper can form a black patina as copper oxide (CuO) and also as copper sulfide (Cu2S), and can form an olive-green patina as copper chloride (CuCl2). In ornate silver jewelry, black silver sulfide (Ag2S) can form in crevices in the design. This can delineate and emphasize the design, adding to the beauty of the jewelry. Upon exposure to air, most metals tend to form an oxide coating. If colored, this oxide coating acts as a patina to enhance the appearance of the metal. The oxide coating can also act as protection against further chemical change with internal metal atoms. Particular types of steel are protected from rust formation by a coating of metal oxide. Aluminum forms aluminum oxide (Al2O3) or [Al2(OH)6], which provides a protective coating. The corrosion of metals is an electrochemical process involving oxidation-reduction, or redox, reactions. When corrosion is addressed, it is often in the context of structural materials for buildings, bridges, and transportation vehicles, such as steel, an alloy of iron (Fe) and carbon (C). However, steel is also used for necklace and bracelet findings, such as clasps, and is often used for the posts in earring. When corrosion of iron occurs, the iron atoms are oxidized to iron ions (Fe2+), and electrons are released. In a reduction reaction, the released electrons react with oxygen and water to form hydroxide ions (OH-). Another oxidation reaction can occur when iron atoms react with hydroxide ions, producing iron (II) hydroxide (Fe(OH)2) and releasing electrons. Finally, iron ions can react with oxygen and water to produce iron oxide (Fe2O3), commonly known as rust. The following equations describe these pathways to the corrosion of iron in steel: Oxidation (anode reaction): Fe ——Fe2+ + 2e– or 2Fe + 4OH——2Fe(OH) 2 + 4e- Reduction (cathode reaction): O2 + 2H2O + 4e——4OH Formationof rust: 4Fe2+ + O2 + 6H2O —— 2Fe2O3 + 2H2O + 8H+ What is the composition of steel? • All steel contains iron and carbon. Sometimes other elements such as chromium, nickel, and molybdenum are added. • The percentage of carbon in steel ranges from just above 0% to 2%. • Steel with less carbon is more flexible but weaker than steel with more carbon. The Sacrificial Anode When it is desirable to prevent corrosion of iron, a new anode, such as a zinc (Zn) or magnesium (Mg) strip, is connected to the surface of the iron. These metals are stronger reducing agents than iron and will be more easily oxidized. As oxidation occurs, the zinc or magnesium, rather than the iron, will furnish electrons. These “sacrificial anodes” will erode instead of the iron. They must be replaced periodically, but the iron will remain intact. (See Figure 6.12.) In a piece of jewelry, such as an earring, the steel post is often placed into a silver or gold earring design. When the earring is worn, body fluids come into contact with the steel. Voila! We have a small electrochemical cell. The iron in the steel acts as the anode, the silver (Ag) or gold (Au) is the cathode, and the body fluids act as an electrolyte. Oxidation at the anode results in iron atoms converting to iron ions and releasing electrons. The anode is eroded. Some iron ions will travel to the gold or silver cathode region through skin moisture and react with oxygen in a reduction reaction, resulting in the formation of rust (Fe2O3), which can deposit on the silver or gold. Our earrings would not be very pretty, nor would they be wearable. Fortunately, steel compositions have been formulated so that the iron is tightly bonded and not available to participate in redox reactions. Steel posts are ideal for earring construction. Coloring Metals: In Nature and in the Laboratory Some metals will react with substances available in the environment, such as oxygen, sulfur, and water vapor, to form colors, highlights, and shadings that are attractive to the eye. However, a natural change in color can take years. We can speed the process by placing a piece of metal jewelry into an appropriate chemical solution that will cause a chemical change resulting in a new color on the surface of the metal. There are hundreds of coloring chemicals, such as copper sulfate and ammonium sulfide. CASTING: THE PLOT THICKENS Casting is used to produce jewelry that is too difficult or inappropriate for fabrication methods. There are two types of casting: vacuum casting, using a lost wax mold (described on page 256), and gravity mold casting, using pewter. In both methods, the metal is melted and poured into a prepared mold. Gravity Mold Casting: Gravity Pulls It Down A gravity mold casting uses the force of gravity to produce the piece. A variety of materials can be used for the mold, including plaster, charcoal, and cuttlefish bone. Pewter is a metal often used in gravity mold casting. It is an alloy consisting of about 84% tin and about 16% of another metal, such as copper. The copper adds strength to the alloy. Pewter is an easy metal to work with because of its relatively low melting point (500F,260℃). It is available in casting ingots or sheets from supply houses. |
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slowstar
木虫 (职业作家)
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18楼2009-10-22 09:27:02
slowstar
木虫 (职业作家)
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2楼2009-10-17 19:47:35
slowstar
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有色金属：令人愉快的色泽（涂层） 制作一串珠宝时，胶水是将各个单独部分连接起来的焊料（比喻用）。连好后，表面可能还得着色。这表层着色或涂层可以通过暴露在空气里让其自然形成，也可以通过通过预期的化学变化来实现。通常涂层后，珠宝会变得变得比原来更加吸引人。 当将金属嵌入珠宝后，工匠会对珠宝进行涂层（或叫抛光处理）。涂层通常只具有观赏性而无防护性。然而，化学涂层法与金属表面形成氧化物保护膜的原理是相似的 。金属失去电子变成离子发生了氧化还原反应：金属氧化了。这些金属离子能与一些接触到的阴离子比如碳酸根(CO32–),氧离子(O2–)，硫离子(S2–)，硫酸根离子(SO42–) 反应生成有色物质，量少的话，可以形成吸引人的涂层（光泽）；量多的话，则对金属形成保护膜。但是，一旦氧化过程腐蚀了过多的金属，则原先的珠宝不但不能被保护，反而会被破坏掉，我们称这个为侵蚀。 |
3楼2009-10-17 20:43:24
slowstar
木虫 (职业作家)
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4楼2009-10-17 21:06:47
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