Abstract
Oxidation in air has been studied on three nickel/20% chromium alloy wires having, respectively, lives of 35, 140 and 600 hours, as assessed by the ASTM accelerated life-test for electrical resistance materials (ASTM B76–65). Total oxygen consumption was measured during periods of continuous and of intermittent heating (2 minutes on–2 minutes off). Separate measurements were made of the oxygen used in forming chromic acid anhydride (CrO3), condensed on a water cooled tube surrounding the heated wires, and of the oxygen consumed in formation of adherent oxide. Comparison of the scale shedding properties of the wires was obtained by difference.
The wire having the shortest life showed the most rapid rate of total oxygen-consumption, the greatest tendency to shedding of scale, and the lowest rate of emission of chromic acid anhydride.
The longest-life wire consumed oxygen at the slowest rate in intermittent, but not in continuous, tests. On that wire the scale was less adherent than on the wire of intermediate life, and it is suggested that the greater uniformity of scale shedding from the long life wire was a favourable factor.
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