Electrolysis of Sodium through Alumina Arc Tubes

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Abstract
When the outer surface of a polycrystalline alumina (PCA) arc tube is biased negatively in an operating high pressure sodium lamp, sodium has been found to selectively electrolyze through the PCA wall and deposit primarily at the cold spot of the outer jacket. Electrolysis currents have been measured and found to be consistent with the quantity of sodium lost from the arc tube. Under the influence of the negative field, black spots form on the alumina tube that in appearance are similar to ones sometimes observed on the arc tube walls of aged high pressure sodium lamps.1 Arc tubes were compared in terms of resistance to sodium penetration with PCA variables including grain size, wall thickness and wall temperature. Sodium penetration rates were found to decrease with wall thickness and increase with wall temperature.

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