Influence of Silica on Polymerization of Chromate Coating

The influence of the amount of silica added to dry-in-place chromate and of drying temperature were analysed by thermal analysis and mass analysis. The interaction between silica and chromium is estimated further. At temperatures of 80-200°C, two reaction were observed: dehydration condensation of the silanol group and polymerization of the chromate constituent. When silica was added, the polymerization enthalpy of the chromate constituent decreased. This was caused by a reduction in the chance of bonding between the ions of the chromate constituent due to the dilution effect of silica particles in dried coating. This demonstrates that polymerization of the chromate coating is obstructed by the addition of silica. Change in chromium fixation rate in aqueous solution is good agreement with the enthalpy and the polymerization behavior of chromate constituent. Silica in the coating forms coordination cross-linking, and the anions in the chromate constituent work as a bridging group in this case. Thus it is postulated that silica particles are combined with each other by the chromate constituent.