The rust formed on the mild steel railing, which was used for 95 years in marine atmosphere, was investigated by means of infra-red absorption spectra, electron diffraction and of electron microscopy. The same measurements were carried out also about the corrosion products obtained from the test pieces of copper bearing low alloy steels, which were exposed in the seaside air for 6, 12, 42 or 64 months respectively. The effect of Cu2+ ion on the rust fomation on mild steels in aqueous solutions were studied and the electrochemical polarization measurement of the rusted steels was also performed.
So far as the composition of the rust, the relative quantity of its constituents and the microscopic size of its particles were concerned, no difference was observed between mild steels and low alloy steels. On the other hand, with the rust on the low alloy steels the grain size was much smaller and the quantity of adsorbed water was larger than with that on the mild steels.
While the Cu2+ ion in aqueous solution had no direct effect on the formation of FeOOH, it promoted the formation of Fe3O4. The best corrosion resisting film of rust was obtained in the solution containing about 10ppm of Cu2+ ion. The decrease, as well as the increase, in concentration of Cu2+ ion deteriorated the corrosion resistance of the film.
The obtained results indicate that the copper ingredient in steel accellerates rust formation in the early stages of corrosion, and the layer of rust thus formed prevents the oxygen supply to the metalrust boundary, resulting in the formation of Fe3O4. It is considered that the Fe3O4 layer having large quantity of adsorbed water may play the most important role in resisting the atmospheric corrosion.
