Inibition by Stannous Ions of the Corrosion of Mild Steel

Rates of corrosion were estimated by direct experiments and from polarisation conductance measurements. Comparison of the results obtained by the two methods showed that polarisation measurements acted as a guide to the inhibition by tin except in the case of solutions with a pH of 2·5. Inhibition reached a limiting value for a dis∼olved tin concentration of 10–30 ppm. For blackplate immersed in pH 2·5 citrate buffer, metallic tin appears to deposit on the blackplate surface in sufficient quantity for an exchange process to occur between dissolved and deposited tin. It is thought to be this exchange process which interferes with electrochemical measurements and gives rise to the lack of correlation between inhibition measured by direct and electrochemical experiments at pH 2·5. The presence of stannous tin not only inhibits dissolution of iron but also evens out sample to sample variations probably by depositing on the more electro-active sites on the blackplate surface.