A range of commercial nickel chromium alloys is based on this solid solution. Such nickel chromium alloys have excellent resistance to high temperature oxidation and corrosion and good wear resistance. Oxidation resistance can be attributed to the formation of a highly adherent protective scale.
The introduction of small amounts (<7%) of chromium to nickel increase the sensitivity of the nickel chromium alloy to oxidation. This is because the diffusion rate of oxygen in the scale is increased. This trend reverses after addition levels increase above 7% chromium and increases up to an addition level of approximately 30%. Above this level, there is little change.
A marked increase in electrical resistivity is observed with increasing chromium additions. An addition level of 20% chromium is considered the optimum for electrical resistance wires suitable for heating elements. This composition combines good electrical properties with good strength and ductility, making it suitable for wire drawing. Commercial grades include Nichrome and Brightray. Small modifications of to this composition may be made to optimise it for particular applications.
The 90/10 nickel chromium alloy is commonly used in thermocouples, in conjunction with a 95/5 Ni/Al alloy. This combination is called chromel-alumel, and similar to heating elements has a maximum operating temperature of 1100°C. This couple becomes susceptible to drift in the region of 1000°C due to preferential oxidation after prolonged usage. The addition of silicon has been found to overcome this effect.
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TAG: nickel chromium alloys
