The measurement of porosity in tin coatings on steel

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.]