The Study Group for the Prevention of Atmospheric Corrosion of Steel Structure has been conducting atmospheric exposure tests at seven sites in Japan. After the presentation of the first report of these studies, additional exposure tests have been made on respective three kinds of low alloy steel and same numbers of selected steel for comparison. And moreover, tests have also been máde on exposure started in a changed season 6 months later than the former described in the first report.
As a result of these tests, average corrosion rates of the test specimens after one year were as follows:
Ni-Cr steel, No. 2......0.037mm/yr
Cr-Mo steel, No. 4......0.050mm/yr
Ni-Cr-Mo steel, No. 8......0.039mm/yr
Steels selected for comparison
Killed steel......0.056mm/yr
Cu-containing converter steel......0.049mm/yr
Atmospheric corrosion resistant
High strength steel......0.041mm/yr
As for the corrosion rates estimated after 6 months exposure, remarkable differences were observed (except at two sites) between the tests which started in spring and the ones which started in autumn. But both estimations after one year exposure became average. So, in the country having such season change in climate as in Japan, one round year exposure, as the unit, must be needed for comparison of corrosion rates of test specimens.

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Hot dipped tin plate is lustrous and very corrosion-resistive, and therefore, widely used for food cans which need corrosion resistance. But, when the plate is used for such a purpose, it is often observed that the cans become discolored purple or black in time. This phenomenon causes not only poor look of the cans but also spoiling of their contents, and seems to be one of serious problems in can manufacturing industry and in can packing industry. To prevent it, the plate is usually coated with special lacquers or is electrochemically treated.
Extent of the discoloration depends largely on the state of the microstructure of the tin plate surface, which is divided into two different types: one is susceptible to discoloration and the other is not.
Many investigations have been reported on the mechanism of the tin plate corrosion. But, we can't obtain any knowledge on the microstructure of tin plate surface from the majority of them, which can only provide electrochemical data. The present investigation was carried in order to clarify the relation between metallographical factors and corrosion, by measuring corrosion rate in acid solution, oxidation rate in the atmosphere and observation of surface microstructure by means of an optical microscope.
It was concluded that the surface which is susceptible to discoloration corresponds to rapid dissolution rate, rapid oxidation rate and large density of dislocations.

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