Abstract
X–ray photoelectron spectroscopy (XPS)has been used to determine the variation of lead concentration with depth in three tinplate samples with bulk lead/tin concentration ratios (wt-%) of 0·17, 0·04, and 0·02. In addition, the extent and rate of segregation of lead to the free surface of the tinplate have been measured for the temperature range 20–125°C. It has been found that the binding energy for lead at a free surface is about –20 kJ mol−I, giving enrichment factors of about 2000:1 (surface/bulk concentrations) at ambient temperature. The lead contents observed at the tin/tin oxide interface in commercial tinplates show good agreement with the surface segregation studies. The results indicate that lead has significant mobility at these temperatures, resulting in considerable levels of lead enrichment in periods of 1–2 days at near ambient temperatures. The maximum lead content in canned foods is maintained below the prescribed statutory limits by restricting the bulk lead content of the tin used for plating to less than 0·05 wt–%. Anomalously high initial dissolution rates of lead relative to tin in malic acid based foodstuffs is explained by the segregation of lead to the free surface (or to the oxide/metal interface) and the high rates of diffusion of lead in tin. The results indicate that the tinplate thickness will also have a significant effect on the ultimate level of lead pick–up.
MST/124
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