NiCr-alloys

Aluminium also produces a passive oxide layer, aids in the bonding to the porcelain and strengthens the alloy due to the precipitation of AlNi3. Silicon lowers the melting temperature and, like manganese, acts as a deoxidizer. Molybdenum and niobium are added to improve corrosion resistance and, like iron, are used to adapt the thermal expansion coefficient to the coefficient of the porcelain. 

The wide composition range results in an equally wide range of physical (Table ld.20) and mechanical properties (Table ld.21). The high rigidity and strength of these alloys as compared to that of the precious metal alloys make them suitable for the production of small prosthetic devices. 

Mean coefficient of linear thermal expansion between 25-600 °C 

Elagli, K., Traisnel, M. and Hildebrand, H.F. (1993) Electrochemical Behaviour of Titanium and Dental Alloys in Artificial Saliva. Electrochimica Acta, 38(14), 1769-1774. 881. 

Berglund, A. (1993) An in vitro and in vivo study of the release of mercury vapor for different types of amalgam alloys. J. Dent. Res. 72 (5), 939-945. 

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There are no films or protective surface films on active metals, e.g., mild steel in acid or saline solutions. Passive metals are protected by dense, less readily soluble surface films (see Section 2.3.1.2). These include, for example, high-alloy Cr steels and NiCr alloys as well as A1 and Ti in neutral solutions. Selective corrosion of alloys is largely a result of local concentration differences of alloying elements which are important for corrosion resistance e.g., Cr [4],... 

The MCFC membrane electrode assembly (MEA) comprises three layers a porous lithiated NiO cathode structure and a porous Ni/NiCr alloy anode structure, sandwiching an electrolyte matrix (see detail below). To a first approximation, the porous, p-type semiconductor, nickel oxide cathode structure is compatible with the air oxidant, and a good enough electrical conductor. The nickel anode structure, coated with a granular proprietary reform reaction catalyst, is compatible with natural gas fuel and reforming steam, and is an excellent electrical conductor. As usual, the oxygen is the actual cathode and the fuel the anode. Hence the phrase porous electrode structure . 

Nickel alloys may be attacked by intergranular corrosion in certain very aggressive environments after incorrect heat treatment. In NiCr alloys, ehromium carbide is precipitated in the same temperature range as for the austenitic stainless steels. The NiCr alloys are primarily attacked by strong oxidizers such as hot nitric acid. The prevention measures are mainly the same as for the austenitic stainless steels. 

Table ld.l9 Chemical composition (wt%) of the NiCr-alloys used in dentistry Ref. 11)... 

Table ld.21 Mechanical properties of NiCr-alloys used in dentistry (Ref. 12)... 

Heat treatment and high temperature behaviour of uncoated or Y203-coated NiCr alloy... 

NiCr alloys with a sufficient Cr content are very resistant to corrosion and thus used for... 

For Ni-base alloys, the addition of Al to NiCr alloys has been done with a fair level of commercial success. Low levels of Al have been added to alloys... 

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