The adsorption-desorption behavior of Cr (VI
) ions on the surfaces of fine silica particles was examined by atomic absorption spectrometry and by electric conductivity measurement of solutions so as to analyze an anticorrosion function of silica particles in dry-in-place chromate coatings.
Among chromium species, Cr (VI
) ion shows high rate of adsorption due to coordination bonds formed with the Si atoms of silica particles. In an aqueous solution of less than pH5.5, Cr (VI
) ions are adsorbed on the surfaces of silica particles. At pH8.5-11, desorption of already-adsorbed Cr (VI
) ions occurs through a reaction that is mainly SN
1D ligand substitution of OH-
and Cr (VI
An anticorrosion mechanism of silica particles in dry-in-place chromate coatings was surmised to be as follows. In acidic regions, where metal is corroded, Cr (VI
) ions in the chromate coating are adsorbed and fixed on the silica particles. In basic regions, Cr (VI
) ions that were previously adsorbed are desorbed through the substitution reaction. These desorbed Cr (VI
) ions contribute to the corrosion resistance of metal substrates as an inhibitor.