On the effect of environment on spontaneous growth of lead whiskers from commercial brasses at room temperature

In this paper we report on a detailed statistics-based study of the effect of various environments – air, argon, vacuum, oil and water – on the room temperature spontaneous nucleation and growth of lead whiskers from hacksaw-cut surfaces of three leaded commercial brasses. The samples were initially held in the various environments for 166 h, before being stored in ambient air for a total of 1126 h. The environment was found to have a strong effect. The highest whisker density, ∼30,000 cm−2, was observed for the sample initially kept under a mechanical (∼1 Pa) vacuum; the lowest, ∼3000 cm−2, was observed for the sample initially immersed in water; the densities of the others were in between. The samples held in water only grew whiskers when removed from the water. Once exposed to air, a few whiskers grew at an average rate that, at 0.9 nm s−1, was the highest measured. When the samples were exposed to air, after the initial storage in the different environments, both nucleation and growth of the whiskers were accelerated, before ultimately ceasing to grow roughly 400 h after creating the surfaces from which the whiskers grew. These results are consistent with a scenario in which oxygen and/or moisture diffusion down the Pb/brass interfaces results in a volume expansion that provides the driving force for whisker growth. The results also indicate that the oxygen levels needed for whisker growth can be as low as a few parts per million.