Samples of tinplate were cathodically treated in sodium bichromate solutions with pH values of 1∼8 for 1∼90 seconds. Surface films formed by these treatments were investigated by means of electron microscopy, using a replication and separation technique, to clarify their growth mechanism, and then examined by electron diffraction. Also, the reduction value of surface oxide, cathodic polarization, porosity and surface chromium content were measured as a function of treatment time to study the growth rate of the film.
The results are summarized as follows:
(1) The passivation film formed on the surface of tinplate by the cathodic treatment is not uniform in thickness, and its formation is affected by the pH of treatment solutions. That which is produced in an acid solution covers a larger area.
(2) The film grows rapidly to some extent in less than 1 sec, after which its growth is made slowly. The growth rate, though decreases with increasing treatment time, does not follow a simple functional time dependence.
(3) A compound with a tetragonal crystal structure is present in the film.
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