THE ELECTROCHEMICAL THIOCYANATE POROSITY TEST FOR TINPLATE

A previous study has shown that the porosity of tinplate can be calculated from measurements of either the corrosion potential or polarisation conductance of the tinplate in ammonium thiocyanate solution. This paper proposes and discusses an explanation for the relationship which exists between porosity and these electrochemical parameters as measured on commercially produced tinplate. Interpretation of the results is of importance because of the need to convert raw electrochemical data into porosity values, i.e. the area of steel exposed at the tinplate surface. Surface roughness, the passivation film, and the presence of non-conducting overlayers, typically lacquers, affect the results. Their influence has therefore been assessed. In the case of lacquers, it is also possible to assess the areas of both the tin and steel exposed at pores in the lacquer overlayer coating.