Four tinplate specimens with different corrosion properties were studied using secondary ion mass spectrometry (SIMS) image analysis. The continuity of the structure of the interfacial iron‐tin alloy was able to be examined using a volume rendition computer program, which generates a three‐dimensional representation of the stack of ion images. The surface coverage of tinplate was found to vary widely on some specimens; those exhibiting poor corrosion characteristics were found to have little or no elemental tin covering the raised regions between the rolling grooves.
Tinplate surfaces have been analysed on a microscopic scale for differences in composition which are related to their effectiveness in providing cathodic protection of the underlying steel. Secondary ion mass spectrometric images of a number of elemental and molecular ions have been collected and collated in stacks that represent the three-dimensional distribution of the ions in the tinplate. An iron-tin alloy structure at the interface of the tin and steel appears to be detectable by the measurement of the 176FeSn-molecular ion image. The localized corrosion behaviour of the tinplates studied has been correlated to the microscopic distribution of iron near the outer surface of the tinplate; the presence of such an iron-rich phase could increase the dissolution rate of surrounding tin and thus reduce the effectiveness of the cathodic protection provided to the steel substrate. The steel substrate was shown to have a low but detectable concentration of oxygen from its interface with tin down to a depth of 1-2 pm. INTRODIJCTIONSecondary ion mass spectrometry (SIMS) has demonstrated unique strengths when applied to the investigation of metallurgical structures at or near the surface.'-3 The high dynamic range of the secondary ion signals imparts a capability to measure phase composition over a wide range of elements, including light elements such as hydrogen, carbon and oxygen. Combined with the excellent depth resolution (< 5 nm) and very low detection limits (ppm for most elements and ppb for some), such measurements of phase composition are doubly useful if they can be carried out within microscopic areas of any domain, since the metallurgical phase structure is usually quite complex at gas/solid and layer boundaries.The structure of tinplate on steel is a good example of such complexity. Between the outer surface of pure tin (usually 0.15-2 pm thick) and the steel substrate (0.1-0.4 mm thick), one or more iron-tin alloys are proposed to form, particularly after the electrodeposited tin is subjected to a reflow treatment.4 Depending on the tin coating thickness and the reflow treatment used, the thickness of the iron-tin alloy layer varies from 0.15 to 2 pm. In general, the pure tin layer acts as a sacrificial electrode, thus protecting the steel substrate from corrosion. Further protection to the steel substrate, to some extent, is accorded by the iron-tin alloy layer, which acts as a barrier to solution exposure.The quality of the tinplate and its resistance to localized corrosion, to some extent, depend on the steel substrate. For example, the thickness and the uniformity of tin coating are affected by the rolling process prior to electroplating. After electroplating, the tin coating profile closely follows the rolling marks on the steel substrate. On subsequent reflow treatment, molten tin spreads evenly on the substrate, resulting in thicker tin layers at the troughs and thinner tin layers at the crests
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