Silver alloys, oxidation

A third group includes silver—nickel, silver—cadmium oxide, and silver—graphite combinations. These materials are characterized by low contact resistance, some resistance to arc erosion, and excellent non sticking characteristics. They can be considered intermediate in overall properties between silver alloys and silver or copper—refractory compositions. Silver—cadmium oxide compositions, the most popular of this class, have wide appHcation in aircraft relays, motor controllers, and line starters and controls. 

Eigure 11 illustrates the superior conductivity of P/M silver—nickel or silver—cadmium oxide contacts when compared with contacts made by standard melting techniques and formed from soHd-solution alloys. 

The most common form of corrosion is uniform corrosion, in which the entire metal surface degrades at a near uniform rate (1 3). Often the surface is covered by the corrosion products. The msting of iron (qv) in a humid atmosphere or the tarnishing of copper (qv) or silver alloys in sulfur-containing environments are examples (see also SiLVERAND SILVER ALLOYS). High temperature, or dry, oxidation, is also usually uniform in character. Uniform corrosion, the most visible form of corrosion, is the least insidious because the weight lost by metal dissolution can be monitored and predicted. 

The outstanding characteristics of the noble metals are their exceptional resistance to corrosive attack by a wide range of liquid and gaseous substances, and their stability at high temperatures under conditions where base metals would be rapidly oxidised. This resistance to chemical and oxidative attack arises principally from the Inherently high thermodynamic stability of the noble metals, but in aqueous media under oxidising or anodic conditions a very thin film of adsorbed oxygen or oxide may be formed which can contribute to their corrosion resistance. An exception to this rule, however, is the passivation of silver and silver alloys in hydrochloric or hydrobromic acids by the formation of relatively thick halide films. 

A commercial silver alloy in the form of wire or foil is suitable for this determination. Clean the alloy with emery cloth and weigh it accurately. Place it in a 250 mL conical flask, add 5 mL water and 10 mL concentrated nitric acid place a funnel in the mouth of the flask to avoid mechanical loss. Warm the flask gently until the alloy has dissolved. Add a little water and boil for 5 minutes in order to expel oxides of nitrogen. Transfer the cold solution quantitatively to a 100 mL graduated flask and make up to the mark with distilled water. Titrate 25 mL portions of the solution with standard 0.1 M thiocyanate. 

In support of that explanation, X-ray analysis of the catalyst after use indicated the presence of MgO. Hence, the catalytically active phase was finely divided copper in intimate contact with magnesia, quasi as carrier. The same phenomenon was observed with the Zintl-phase alloys of silver and magnesium. Such catalysts were then deliberately prepared by coprecipitation of copper and silver oxides with magnesium hydroxide, followed by dehydration and reduction. Table I shows that these supported catalysts had the same activation energies as those formed by in situ decomposition of copper and silver alloys with magnesium. 

ZDDP (zinc dialkyl dithiophos-phate or zinc diaryl dithiophos-phate)—widely used as an anti-wear agent in motor oils to protect heavily loaded parts, particularly the valve train mechanisms (such as the camshaft and cam followers) from excessive wear. It is also used as an anti-wear agent in hydraulic fluids and certain other products. ZDDP is also an effective oxidation inhibitor. Oils containing ZDDP should not be used in engines that employ silver alloy bearings. All car manufacturers now recommend the use of dialkyl ZDDP in motor oils for passenger car service. 

Tominaga, M., Shimazoe, T., Nagaashima, M., Kusuda, H., Kubo, A., Kuwahara, Y., and Taniguchi, I. 2006. Electrocatalytic oxidation of glucose at gold-silver alloy, silver and gold nanoparticles in an alkaline solution. Journal of Electroanalytical Chemistry 590, 37-46. 

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