Finding an alternative to chromate passivation of galvanised steel is critical due to the detrimental health effects associated with hexavalent chromium. Organosilanes have been proposed as a promising alternative, as they have been shown to give comparable results to chromate passivation [1-2]. Commonly organic solvents such as ethanol are used as the solvent for organosilanes , therefore industrial applications are limited due to ethanol’s low flash point. Few water based systems incorporating organosilanes have been described in literature, but the stability of organosilanes in water over time is major issue with these systems . Surfactants have been suggested and as a means of increasing the stability . The stability of organosilanes is related to the rate of hydrolysis and condensation of this hybrid molecule since hydrolysed organosilanes are required for a corrosion resistant film to form. The effect of surfactants on the stability of organosilanes can be tested with 29Si-NMR spectroscopy to obtain information about the structure and amount of oligomers found [6-7].
The hydrolysis and condensation of organosilanes/surfactant systems was studied using NMR spectroscopy to characterise the system behaviour. Two surfactants were studied, a cationic surfactant (myristyltrimethylammonium bromide) and a small non-ionic surfactant (Pluronic L-35) with the silane 3-mercaptopropyltrimethoxysilane. Comparisons with observations of the emulsions and electrochemical testing were also drawn upon.
We found that hydrolysis and condensation occurs quickly in with the emulsion containing the non-ionic surfactant, however the rate of hydrolysis is hindered by the cationic surfactant. These results explain the other conflicting data, where the stable cationic surfactant containing emulsion is stable over time but produces a film that has poor corrosion protection. The NMR data indicates that the cationic surfactant is encapsulating the organosilanes preventing further hydrolysis and the production of a homogenous nonporous corrosion resistant film.
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