Precious alloy

Precious alloys (Au, Pt, Ag, Cu, and low amounts of other metals of the platinum group)... 

Non-precious alloys, i.e., stainless steel, Co-Cr and Ni-Cr alloys, some of which may also contain Cd and Be... 

Pd this is contained in semi-precious alloys, and some two years ago provoked a major controversy concerning its innox-... 

This relationship between allergy and metallic biomaterials has also been confirmed for alloys used in dentistry Ni, Cr and Co in non-precious alloys (Hildebrand et al. 1989a, 1989b), and Hg and Ag in dental amalgams (Veronetal. 1986 Horsted-Bind-slev et al. 1997 Sandborgh-Englund et al. 

Precious, semi-precious, most Pd-base alloys, Ti-base alloys and stainless steel also produce excellent biological responses with a survival rate of 98% to 70% for the highest concentration. Two semi-precious alloys with high Ag-content reflect the cytotoxic effect of pure silver (Figure 5.1). Ni-Cr alloys (14% and 23%) induce a strong cytotoxic action, and dental amalgams produce total cell death at very low concentrations. 

NY12590. USA Tel +1 845 226 3839 Fax + 1 845 226 3565 Email gbaker bopp.com Website ww wbopp.com The Art of Swiss Precision ISO years weaving expertise, including fabricated components, sintered mesh and precious alloys. 

Skiving is a variant in which the base metal surface oxides are mechanically removed foUowed immediately by pressure rolling of a precious metal or alloy strip. This is commonly used for inlays for electrical contacts and for jewelry fabrication. The common inlay materials include gold, silver, copper, brass, and solder. No heat is needed, and the coating is appHed only to designated areas so there is Htde waste (3,50). 

GoldJilloys, Wrought Type. Two types of wrought gold alloys were formerly recognized by the ADA specification no. 7 for the fabrication of orthodontic and prosthetic dental appHances, ie, type I, high-precious-metal alloys, and type II, low-precious-metal alloys (gold color). Alloys of this type are seldom used in the United States they have been replaced by stainless steels and nickel—titanium alloys. 

JS/oble Metals. Noble or precious metals, ie, Pt, Pd, Ag, and Au, are ftequendy alloyed with the closely related metals, Ru, Rh, Os, and Ir (see Platinum-GROUP metals). These are usually supported on a metal oxide such as a-alumina, a-Al202, or siUca, Si02. The most frequently used precious metal components are platinum [7440-06-4J, Pt, palladium [7440-05-3] Pd, and rhodium [7440-16-6] Rh. The precious metals are more commonly used because of the abiUty to operate at lower temperatures. As a general rule, platinum is more active for the oxidation of paraffinic hydrocarbons palladium is more active for the oxidation of unsaturated hydrocarbons and CO (19). 

Toxic heavy metals and ions, eg, Pb, Hg, Bi, Sn, Zn, Cd, Cu, and Fe, may form alloys with catalytic metals (24). Materials such as metallic lead, ziac, and arsenic react irreversibly with precious metals and make the surface unavailable for catalytic reactions. Poisoning by heavy metals ordinarily destroys the activity of a precious-metal catalyst (8). 

The precious metals are many times the cost of the base metals and, therefore, are limited to specialized applications or to those in which process conditions are highly demanding (e.g., where conditions are too corrosive for base metals and temperatures too high for plastics where base metal contamination must be avoided, as in the food and pharmaceutical industries or where plastics cannot be used because of heat transfer requirements and for special applications such as bursting discs in pressure vessels). The physical and mechanical properties of precious metals and their alloys used in process plants are given in Table 3.38. 

Hensten-Pettersen, A. and Jacobsen, N. Nickel Corrosion of Non-precious Casting Alloys and the Cytotoxic Effect of Nickel In Vitro , Journal of Bioengineering, 2, 419-425 (1978)... 

Bergman, M., Bergman, B and Soremark, R. Tissue Accummulation of Nickel Released due to Electrochemical Corrosion of Non-precious Dental Casting Alloys , Journal of Oral Rehabilitation 7, 325-300 (1980)... 

The copper-bearing aluminium alloys are more noble than most other aluminium alloys and can accelerate attack on these, notably in sea-water. Mercury and all the precious metals are harmful to aluminium. 

There are obviously situations which demand considerable over-design of a cathodic protection system, in particular where regular and efficient maintenance of anodes is not practical, or where temporary failure of the system could cause costly damage to plant or product. Furthermore, contamination of potable waters by chromium-containing or lead-based alloy anodes must lead to the choice of the more expensive, but more inert, precious metal-coated anodes. The choice of material is then not unusual in being one of economics coupled with practicability. 

Laister and Benham have shown that under more arduous conditions (immersion for 6 months in sea-water) a minimum thickness of 0-025 mm of silver is required to protect steel, even when the silver is itself further protected by a thin rhodium coating. In similar circumstances brass was completely protected by 0 012 5 mm of silver. The use of an undercoating deposit of intermediate electrode potential is generally desirable when precious metal coatings are applied to more reactive base metals, e.g. steel, zinc alloys and aluminium, since otherwise corrosion at discontinuities in the coating will be accelerated by the high e.m.f. of the couple formed between the coating and the basis metal. The thickness of undercoat may have to be increased substantially above the values indicated if the basis metal is affected by special defects such as porosity. 

Numerous proprietary electrolytes have been developed for the production of harder and brighter deposits. These include acid, neutral and alkaline solutions and cyanide-free formulations and the coatings produced may be essentially pure, where maximum electrical conductivity is required, or alloyed with various amounts of other precious or base metals, e.g. silver, copper, nickel, cobalt, indium, to develop special physical characteristics. 

In view of the high cost, when tarnish resistance of the surface is the only requirement it is customary to use the thinnest possible coatings of rhodium (0-000 25-0-000 5 mm). Since rhodium deposits in this thickness range, like thin electrodeposits of other metals, show significant porosity, readily corrodible metals, e.g. steel, zinc-base alloys, etc. must be provided with an undercoating deposit, usually of silver or nickel, which is sufficiently thick to provide a fairly high level of protection to the basis metal even before the final precious metal deposit is applied, and, in this way, to prevent accelerated electrochemical corrosion at pores in the rhodium deposit. 

Chemical reduction of metal salts in solution is the most widely used method of preparation of metal nanoparticles, especially in laboratories. In general, the reducing reagents are added into the solution of the precursor ions, but in some cases, a solvent works as a reductant. Various reducing reagents have been proposed to prepare metal nanoparticles. Ethanol or small alcohols can reduce precious metal ions such as Au, Pt", Pd, Ag, and so on [3j. Polymer-stabilized precious metal nanoparticles and their alloy particles can be used as good catalysts for various reactions. Polyols, such as ethylene glycol, were... 

Aluminum and aluminum alloys Copper and copper alloys Rare-earth and rare-earth-like metals and alloys Low-melting metals and alloys Miscellaneous nonferrous metals and alloys Nickel and nickel alloys Precious metals and alloys Reactive and refractory metals and alloys... 

Moving on from preparation of homogenous Pt alloy particles to tailoring of core-and-sheU alloy particles, targeting (i) further lowering of the mass of precious metal per unit power output and (ii) further boost of catalytic activity per square centimeter of catalyst area [Zhang et al., 2004]. 

---