Recent experimental studies have shown that the porosity of thin coatings of tin on steel can be determined by the measurement of polarisation resistance and corrosion potentials. The form of this relationship is discussed and modified equations presented to improve the validity of the method.
A detailed description is given of the surface composition and morphology of D&I can surfaces. Surface analysis indicated that a protective film of tin was present over virtually the entire surface, albeit very thin in places. This finding is in contrast to the results of electrochemical porosity tests which normally indicate that the majority of can wall surfaces are exposed steel. Investigations, however, revealed that the widely used porosity tests cause substantial surface damage resulting in erroneously high exposed steel values. It is recommended that they not be used in heavily worked tinplate, such as that in D&I cans.
Existing methods for porosity testing are not suitable for accurate quantitative measurements on the very thin coatings produced by modern tinplate lines. Electrochemical tests for use on these coatings, in both the as plated and reflowed condition, have therefore been evaluated. The corrosion potential and polarisation resistance of tinplate in a 0.5M ammonium thiocyanate solution were both found to give an index of the porosity. In the simple case of non-reflowed plate simulated porous tin plates, made from iron wires cast into tin, were used to construct calibration curves for these two methods. [Continues.]
SummaryAn electrochemical porosity test for use on low coating mass tinplates has been developed. Both the corrosion potential and polarisation resistance of the tinplate in a 0.5M ammonium thiocyanate solution were found to give an index of the porosity. Equations derived from the Tafel equation were found to describe the results allowing calibration curves to be drawn comparing the porosity measurement to the amount of exposed substrate. The test method is also applicable to many other coating systems given a suitable electrolyte.
